US Patent # 6,776,837
Formation of Chemically Bonded Ceramics
with Magnesium Dihydrogen Phosphate Binder
US Cl. 106/690 // Intl Cl. C04B 012/02
Abstract ~ A new method for combining magnesium oxide,
MgO, and magnesium dihydrogen phosphate to form an inexpensive
compactible ceramic to stabilize very low solubility metal
oxides, ashes, swarfs, and other iron or metal-based additives,
to create products and waste forms which can be poured or dye
cast, and to reinforce and strengthen the ceramics formed by the
addition of fibers to the initial ceramic mixture.
References Cited
U.S. Patent Documents: 5645518 ~ 6133498
CONTRACTUAL ORIGIN OF INVENTION
The United States Government has rights in this invention
pursuant to Contract No. W-31-109-ENG-38 between the U.S.
Department of Energy and the University of Chicago, representing
Argonne National Laboratory.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for forming compactible
ceramics and for forming ceramics with improved compression,
flexural and fracture strength, and more specifically, this
invention relates to a method for using a phosphate binder with
enhanced binding characteristics in high waste loading scenarios
which is compactible in the paste stage and which can be used
with fibers to improve the ceramics' strength.
2. Background of the Invention
The effective sequestration and disposition of waste oils,
bulky waste forms and other unwieldy objects continues to elude
disposal researchers. Typical concretes and ceramics, the later
of which are described in U.S. Pat. Nos. 5,846,894, 5,830,815,
5,645,518, and 6,204,214, and incorporated herein by reference,
have high compression strengths. However, these materials
exhibit comparatively poor flexural and fracture properties.
This leads to crack propagation, particularly when attempting to
macroencapsulate large-size objects. Compactibility of these
materials also is lacking.
To macroencapsulate large-size objects, better fracture
toughness is needed to avoid crack propagation. For cementing
lateral junctures in multilateral oil well completions, improved
flexural strength is needed. To arrest surface crack
propagation, fiber reinforcement is needed.
The use of glass fiber as a strengthening additive in cement
causes problems. Cement is very alkaline and glass fibers
deteriorate in such alkaline environments. This leads to
weakening of the composite structure. Some common solutions are
to over-engineer the composite to compensate for the eventual
degradation and loss of strength, to use alkali resistant glass
fibers, and to use coatings such as polyvinyl chloride (PVC)
over the glass fiber to protect it from the alkaline
environment. All of these solutions lead to higher costs.
Ceramic systems leading to highly ductile waste forms remain
elusive. For example, in the ceramic formation reaction
disclosed in U.S. Pat. No. 5,830,815, and given by Equation 1,
infra.
MgO+H.sub.3 PO.sub.4 +2H.sub.2
O.fwdarw.MgHPO.sub.4.multidot.3H.sub.2 O Eq. 1
102.44 Kilojoules per mole of heat are released. This high
amount of heat results in too rapid ceramic product formation
(leading to brittleness) for any practical use. Also, some
materials for disposal prove too soluble in the very low pH
environs in which the phosphate ceramics disclosed in the '815
patent operate. Alternatively, solids of low solubility
(pK.sub.s.about.15 to 25) are not soluble enough in the
solutions utilized in the '815 patent.
U.S. Pat. No. 5,846,894 issued to Singh et al. on Dec. 8, 1998
discloses a method to produce phosphate bonded structural
products from high volume benign wastes.
U.S. Pat. No. 5,678,234 issued to Colombo, et al. on Oct. 14,
1997 discloses an encapsulation method utilizing a modified
sulfur cement at elevated temperatures, and glass or other
fibers for enhancement of the compressive and tensile strength.
None of the aforementioned patents teaches a method for the
reduction of volume during stabilization of solid powdered
wastes. In addition, none of these patents provides a method for
the stabilization of near insoluble oxides.
None of the aforementioned patents teaches a method for
enhancing the flexural and fracture toughness of the structural
products via a truly homogeneous ceramic-fiber composite.
None of the aforementioned patents even contemplates using a
dispersant for fiber additives for strength enhancement.
A need exists in the art for a method to produce superior
compactible structural products with enhanced flexural and
fracture toughness. The method should result in a ceramic which
can be compressed while it is still putty-like, i.e., before it
sets completely. The method also should result in the formation
of a durable and chemically stable ceramic which can be utilized
to sequester hard-to-contain wastes. Finally, the method should
utilize inexpensive and commonly available reactants at ambient
temperatures to produce low cost ceramics.
SUMMARY OF INVENTION
An object of the present invention is to provide a method for
producing chemically bonded phosphate ceramics (CBPCs) that
overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a ceramic
capable of encapsulating very low solubility metal oxides. A
feature of the invention is that the oxides do not need to be
calcined prior to encapsulation. An advantage is that an
encapsulation process utilizing a ceramic uses much less energy
and, accordingly, is less expensive.
Still another object of the present invention to provide a
method for producing ceramics which have enhanced flexural and
fracture toughness. A feature of the invention is that fibers
being evenly dispersed throughout the ceramic binder enhances
the flexural and fracture toughness of the binder. An advantage
of this feature is that it minimizes leakage of encapsulated
hazardous wastes.
Yet another object of the present invention is to provide a
method for producing ceramics which can encapsulate wastes that
contain nonpolar, or oil-based fluids. A feature of this
invention is that a magnesium dihydrogen phosphate (MHP)-based
binder can effectively encapsulate wastes having trace amounts
of oil. An advantage of this feature is that, at present, there
is not any effective means for encapsulating petroleum
fluid-tainted wastes.
Another object of the present invention is to provide a method
for producing glass fiber-reinforced ceramics. A feature of the
invention is that phosphate-based ceramics are homogeneously
mixed with the fibers to produce a ceramic structure containing
fibers dispersed throughout the structure. An advantage of the
method is that it provides an acidic- to neutral-pH environ
favorable to the glass fibers. Therefore, corrosion of the glass
fibers is minimized, and structural integrity of the resulting
structures is maximized for periods of time heretofore not
attainable.
Still another object of the present invention is to provide a
method for the effective dispersal of fiber additives in a
ceramic binder. A feature of the invention is that monopotassium
phosphate is utilized as a dispersant to prevent aggregation of
fibers into strands and bunches. An advantage of this feature is
that the flexural strength of the resulting structure is
typically twice that of cements.
Yet another object of the present invention is to provide a
method which produces compactible ceramics. A feature of the
invention is the use of magnesium dihydrogen phosphate as a
binder. An advantage of the method is that it allows for
compression of the ceramic to a volume 40% less than the
starting volume of the reaction slurry.
Briefly, the invention provides a room temperature process for
producing a compactible ceramic from powders, the process
comprising combining MgO and magnesium dihydrogen phosphate
dihydrate to create a dry homogeneous mixture; and contacting
the mixture with water to form a slurry.
Also provided is a process for strengthening phosphate
ceramics, the process comprising adding fibers to the initial
ceramic mixture to create a homogeneous composite substrate.
BRIEF DESCRIPTION OF THE DRAWING
The invention together with the above and other objects and
advantages will be best understood from the following detailed
description of the preferred embodiment of the invention shown
in the accompanying drawing, wherein:
FIG. 1. is a schematic representation of the X-ray
diffraction of a swarf before encapsulation, in accordance with
features of the present invention;
FIG. 2 is a schematic representation of the X-ray
diffraction of a swarf after encapsulation, in accordance with
features of the present invention;
FIG. 3 is a schematic representation of the X-ray
diffraction of yttrium phosphate ceramic, in accordance with
features of the present invention;
FIG. 4 is a schematic representation of the X-ray
diffraction of magnetite phosphate ceramic, in accordance with
features of the present invention;
FIGS. 5(a-b) is a schematic representation of
compressive strength of fiber-reinforced ceramicrete
as a function of fiber content for 40 wt. % ash and 60 wt. % ash
composites, in accordance with features of the present
invention;
FIGS. 6(a-b). is a schematic representation of flexural
strength of fiber-reinforced ceramicrete as a
function of fiber content for 40 wt. % ash and 60 wt. % ash
composites, in accordance with features of the present
invention;
FIGS. 7(a-b) is a schematic representation of fracture
toughness of fiber-reinforced ceramicrete as a
function of fiber content for 40 wt. % ash and 60 wt. % ash
composites, in accordance with features of the present
invention; and
FIG. 8 is a schematic representation of a scanning
electron micrograph of a fractured surface of a fiber-reinforced
MKP-based ceramicrete, in accordance with
features of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
A process is provided to facilitate macro-encapsulation of
bulk-waste, oil waste, and similar unwieldy types of waste forms
using ceramic materials and fiber-enforced ceramic materials.
The same compactible phosphate binder may be used in "neat"
formulations to make ceramics.
The inventors also have developed a ceramic-based waste binder
for utilization with compaction technologies. Use of the binder
in a compaction mode also accommodates very high waste loadings.
The binder has enhanced binding characteristics which allow
treatment of more difficult wastes such as those containing
oils.
The invented method and binder enable the production of
structural products such as conventional bricks and blocks and
at lower costs than typical processes and materials.
The inventors have determined that phosphate-based ceramics,
such as magnesium dihydrogen phosphate (MHP), homogeneously
mixed with reinforcing fibers form compactible, high strength
waste sequestration matrices. The phosphate ceramics utilized in
the reinforcing fiber-encapsulation method include, but are not
limited to magnesium potassium phosphate hexahydrate, magnesium
ammonium phosphate, magnesium sodium phosphate, magnesium
phosphate, aluminum phosphate, iron phosphate, zinc phosphate,
and phosphates of all rare earths such as, but not limited to,
phosphates of lanthanum, cerium, yttrium, and neodymium.
The resulting ceramics have wide-ranging utility, including the
ability to encapsulate metal wastes that require a more durable
ceramic which will withstand the shock of impacts and will not
crack over time. Such metal wastes include hazardous materials
(e.g. chromium and arsenic), and fission products such as
technetium wastes and low-level radioactive materials. Low-level
radioactive materials suitable for encapsulation in the instant
ceramic include pyrophoric uranium chips that are stored in
mineral oils. It is these oil-tainted materials cannot be easily
encapsulated using state-of-the-art technology.
The general process for formulating the ceramic starting
material comprises mixing MgO, and magnesium dihydrogen
phosphate dihydrate (MHP) to produce a dry mixture; and then
combining the mixture with water at room temperature to produce
Newberyite, MgHPO.sub.4.3H.sub.2 O. That reaction is illustrated
in Equation 2, infra.
MgO+Mg(H.sub.2 PO.sub.4).sub.2.multidot.2H.sub.2 O+3H.sub.2
O.fwdarw.2MgHPO.sub.4.multidot.3H.sub.2 O Eq. 2
MHP can be produced by reacting MgO and phosphoric acid
according to Equation 3:
MgO+H.sub.3 PO.sub.4 +H.sub.2 O.fwdarw.Mg(H.sub.2
PO.sub.4).sub.2.2H.sub.2 O Eq. 3
Both reactions given by Eqs. 2 and 3 are exothermic and produce
56.5 and 108.07 Kilojoules of energy per mole, respectively. As
such, the addition of heat is not necessary to produce the
binder or resulting composite.
The magnesium oxide, MgO (calcined), and MHP are present in a
molar ratio of MgO to MHP which varies from 3:7 to 1:1. An
equimolar mixture of MgO and MHP is the preferred mode used in
all MHP binder work mentioned herein. The MgO/MHP mixture to
water weight ratio varies from 10:1.85 (stoichiometric
composition) to 10:3.7. The inventors found that MgO should be
calcined as described in U.S. Pat. No. 6,204,214, issued to
Singh, et al. on Mar. 20, 2001 incorporated herein by reference,
as the preferred mode for the instant invention.
Crystalline MHP is made by mixing 16 wt. % of MgO, 77 wt. % of
H.sub.3 PO.sub.4, and 7 wt. % H.sub.2 O. Those weight
percentages reflect the stoichiometry of the reaction. Only the
MgO can be added in excess. The H.sub.3 PO.sub.4 is dissolved in
water with subsequent slow addition of the MgO. The MgO must be
added slowly as the reaction is very exothermic and to maintain
a temperature increase of approximately 5-10.degree. C. from the
ambient temperature.
The mixture described supra, without the addition of any other
materials (i.e., "neat MHP), provides a compactible and durable
ceramic. The fiber enhanced ceramic made with the MHP process is
also compactible.
Waste Detail
For optimal utility, the MHP binder mixture is loaded with any
one of a myriad of waste powders derived from high volume
wastes. These wastes include, but are not limited to, the group
consisting of any inorganic oxides and metals, such as hazardous
and radioactive wastes, low solubility metal oxides, ceramic
powders, ashes, red mud, sand, swarfs, lateritic soils, mineral
wastes, and the same with traces of oils or greases, or
combinations thereof. The waste powder loading into the ceramic
mixture comprises up to 85 wt. % of the final ceramic.
The hazardous and radioactive waste is material selected from
the group consisting of high level radioactive wastes, low-level
radioactive wastes, low-level radioactive and hazardous waste
called "mixed waste," heavy metals, fission products, uranium
and any other radioactive and pyrophoric metals stored in
mineral oil, or combinations thereof. Aside from products
tainted with mineral oil, other oils or greases also can be
accommodated by the instant binder, including petroleum-based or
vegetable-based nonpolar compounds, and any other hydrocarbons.
The low solubility metal oxides (pK.sub.sp in the range of
.about.15 to 25) as waste encapsulation candidates include, but
are not limited to, oxides of cobalt, copper, dysprosium,
erbium, europium, holmium, neodymium, palladium, samarium,
tellurium, ytterbium, yttrium, and zinc.
The inventors found that the MHP formulations can be compacted
or compressed to a smaller volume as much as 40% of the original
volume. The slurry is compacted at temperatures between 0 and
30.degree. C. Compression process temperatures above 30.degree.
C. cause the formulation to lose water by evaporation and the
ceramic loses strength. Compaction methods employed can include,
but are not limited to, uniaxial presses, harmonic presses,
adobe presses, and cold or hot isostatic presses. The ceramic
mixture also facilitates dye casting.
Monopotassium phosphate can be used as the sole initial
phosphate reactant to produce a ceramic mixture. Alternatively,
monopotassium phosphate can be added to any initial phosphate
binder-waste mixture up to 20 wt. % of the initial mixture as a
dispersant.
Fiber Detail
The phosphate-based ceramic systems utilized herein have acidic
to neutral environments. Those pH ranges are favorable for glass
fibers which deteriorate in environments of high alkalinity
(i.e., high pH), as noted supra. As such, the inventors have
found that the alkaline degradation problem of glass fiber in
concrete systems, is resolved when cement is supplanted by
ceramic material, and particularly low-pH ceramic formation
systems.
Glass has a very low solubility in moderately acidic and
neutral environments. The invented ceramic paste, of the type
derived from equation 3, has an initial pH of 4.3, and when
setting is complete, it has a pH of 8. This is much lower than
the pH of cement, which is typically 12 to 13.
Fibers are added to the initial binder mixture to reinforce
phosphate-based ceramics and arrest cracks and crack
propagation. The addition of fibers increases the flexural
strength up to 2000 psi and the fracture toughness up to 0.8
Megapascal.multidot. meter (Mpa.multidot.m.sup.1/2). The
compressive strength of the product is up to five times that of
conventional bricks and blocks for which the compressive
strength is 2000 psi.
To effectively enhance the strength characteristics of the ceramicrete,
the fibers are dispersed evenly throughout the ceramic binder to
create a composite mixture that is homogenous throughout. The
inventors found that monopotassium phosphate is a good
dispersant of cut fibers. This is advantageous in that fibers
can be added as bunches and strands, or any combination thereof,
but they disperse throughout the binder to be encapsulated as
individual fibers by MKP. Monopotassium phosphate can be added
to any initial phosphate binder-waste mixture as a dispersant up
to 10 wt. % of the initial mixture of any phosphate binder.
Fibers are comprised of materials selected from the group
consisting of ceramics, glass fibers, organic polymers, carbon,
metal fibers, and natural substances. Fibers come from natural
substances selected from the group consisting of coconut, corn,
bagasse, jute, sisal, wood, and any cellulosic material.
Polymers are organic compounds selected from the group
consisting of nylon, polyethylene, polypropylene, and polyvinyl
chloride (PVC).
The fibers can be added to the ceramic mixture as weaved mat,
short cut fibers, long cut fibers, oriented strands, or simply
as cut fibers that are not oriented in any way, or any
combination thereof, and are added as 1 to 10 wt. % of the
substrate. To obtain fiber loadings above 2 wt. %, fiber to the
extent of 2 wt. % must first be added to the binder mixture with
subsequent stirring to allow the binder mixture to dissolve,
then addition of fiber up to the desired higher wt. % when the
mixing slurry becomes thin due to dissolution of the phosphate
binder.
The fiber-reinforced ceramic paste can be cast, molded, and
used to dip-coat, paint, or spray surfaces, and to cement
lateral junctures in oil and gas wells.
Addition of fibers increases the viscosity of the setting
slurry making it difficult to pour the slurry into suitable
molds. It is easier to dye cast the forms, but dye casting needs
a modified Ceramicrete binder. MHP serves this
need very well.
Process Detail
A salient feature of the invented process is the ability to
produce sequestration matrices, up to 80 weight percent of which
is comprised of waste, and without the addition of heat.
In one instance, as much as 73 wt. % powder with 18 wt. %
MgO/MHP binder (equimolar amounts of MgO and MHP) and 9 wt. %
water is utilized to form a slurry of putty-like consistency.
The resultant paste can then be pressed at a pressure of
.about.1000 pounds per square inch (psi) to form a dense
monolith. Initial setting takes place within ten minutes.
Alternatively, more water can be added to make a thinner paste
or slurry and pour this slurry into a mold. Within an hour, the
slurry sets into a hard ceramic.
Boric acid can be added to retard the reaction and reduce the
reaction rate. This provides more time to transfer the mixture
into the mold and apply pressure for the purpose of compacting
the slurry. The iron examples infra are present to serve as
illustrations. Other metals can be encapsulated by the invention
disclosed herein.
EXAMPLE 1
Pelletizing Steel Industry Waste or Swarfs
Swarfs are machining wastes containing iron in them. These
wastes also contain oils and as such, conventional cements
cannot be used to solidify them. The presence of metal also
makes swarfs unsuitable for encapsulation/incorporation into
ceramics generated from a phosphoric acid solution, as disclosed
in U.S. Pat. No. 5,830,815, because reaction of metal with
phosphoric acid generates large amounts of heat and boils the
slurry.
In storage, swarfs oxidize and form magnetite and haematite.
While they are pyrophoric wastes and hence are a liability, they
are ideal raw materials for forming iron phosphate ceramics.
This is because the wastes contain a significant amount of
elemental iron that has not rusted, and they also contain
different iron oxide forms that include haematite and magnetite.
To recover metal values from these wastes, it is necessary that
they be pelletized.
Swarf waste powder was pelletized by encapsulation pursuant to
the procedure given supra in the "Process Detail" with a swarf
waste loading of 73 wt. % in the final dry mixture, the
remaining 23 wt. % being the dry mixture described on p. 6.
During mixing and pressing, the mixture did not generate heat.
Each sample formed was a briquette of dimensions
2".times.2".times.1", and was placed under pressure for 2
minutes at a pressure of 1000 psi, thus reducing its volume 40%.
Each briquette was stored in a polyethylene bag for 3 weeks for
complete curing. Within a day all samples appeared hard and were
unscratchable, but continued to release water in the bag
indicating a continued reaction.
FIG. 2 shows an X-ray diffraction pattern of a typical swarf
before encapsulation in ceramic. The diffraction pattern shows
that the swarf contains iron (Fe), carbonized iron (Fe.sub.3 C)
and haematite (Fe.sub.2 O.sub.3) as its main constituents. FIG.
3 shows a typical X-ray diffraction pattern of the encapsulated
swarf. Apart from the unreacted haematite ("#"), unreacted iron
(".star-solid."), and unreacted carbonized iron ("+"), FIG. 3
discloses the presence of additional compound, magnetite
(Fe.sub.3 O.sub.4), designated as @. Magnetite may form via the
reactions according to equations 4 and 5:
Fe.sub.2 O.sub.3 +Fe+2e.sup.-.fwdarw.3FeO Eq. 4
FeO+Fe.sub.2 O.sub.3.fwdarw.Fe.sub.3 O.sub.4 Eq. 5
Some of the FeO that forms via the reaction given in Equation 4
may react with MHP to form FeHPO.sub.4 according to Equation 6.
FeO+Mg(H.sub.2 PO.sub.4).sub.2.multidot.2H.sub.2
O.fwdarw.MgHPO.sub.4.multidot.3H.sub.2 O+FeHPO.sub.4 Eq. 6
The water that was added to dissolve the MHP binder does not
participate in the reaction, but is released in the polyethylene
bag. Thus at the end, 80 wt. % swarf and 20 wt. % binder all
reacted to form a hydrophosphate compound.
The briquettes' properties are given in TABLE 1.
TABLE 1
Properties of solidified swarf samples.
Property Measured values
Density (g/cm.sup.3) 2.16
Open Porosity (vol. %) 10
Compressive strength (psi) 2345 .+-. 345
Given the fact that the iron content was high in the original
swarf material, the density of the briquette is relatively low.
The lower than anticipated density is most likely due to the
formation of hydrated iron compounds that generally have lower
densities. The open porosity of 10% is lower than found in
solidified concrete and hence lower water absorption.
The compressive strength of the iron-encapsulating ceramic is
very similar to that of conventional bricks (.about.2000 psi).
The samples are hard enough to withstand being dropped from a
height of 12 feet onto a hard floor. The data in TABLE 1 clearly
shows that the swarf pellets can withstand rough transportation
and rough handling. The pellets are also light weight and
therefore can easily be picked up by an electromagnet for
feeding into blast furnaces, without being broken into pieces.
These observations show that the MHP binder can be very
suitable in recycling metal wastes and for stabilizing uranium
chips, which have been stored in mineral oils.
The binder metal wastes and radioactive materials are easily
incorporated into monolithic waste forms for long-term storage.
EXAMPLE 2
Compacting Ashes
A number of different ashes were used which included high
chloride content fly ash, bottom ash and low chloride content
Class F fly ash. These ashes were radioactively contaminated
chloride-containing ashes from various U.S. Department of Energy
(DOE) sites. In each case, a small amount of water was added to
moisten the mixture. Briquettes of dimensions
2".times.2".times.1" were made using these ashes along with the
MHP binder. As with the swarf, each sample formed was a
briquette of dimensions 2".times.2".times.1", and was placed
under pressure for 2 minutes at a pressure of 1000 psi, thus
reducing its volume. Each briquette was stored in a polyethylene
bag for 3 weeks. The samples were cured for one full week and
were then cut into 1".times.1".times.1" cubes for which
compression strengths were measured. The results of the strength
measurements are given in TABLE 2 which also contains results
for briquettes made with MKP.
TABLE 2
Compressive strengths of ash briquettes produced with MHP and MKP.
Wt. % of ash components
High Cl High Cl Low Cl Class Wt. % of Compressive
Bottom Ash Fly Ash Fly Ash Binder Strength (psi)
35 -- 35 MHP, 30 4421
50 -- 20 MHP, 30 3038
27.5 7.5 35 MHP, 30 2495
35 -- 35 MKP, 30 2402
50 -- 20 MKP, 30 2327
27.5 -- 35 MKP, 30 1843
-- -- 80 MHP, 20 4056
-- -- 85 MHP, 15 2059
-- -- 90 MHP, 10 600
As noted in the last three rows of the table, high waste
loadings can be attained with ash of low Cl content. The
compressive strengths values are 4056 psi, 2059 psi, and 600 psi
for low Cl waste loadings of 80%, 85%, and 90%, respectively.
The minimum strength requirement for land disposal of hazardous
and radioactive waste is 500 psi.
As a result of compressing the MHP binder briquettes, their
volumes decreased by as much as .about.40%. The MKP binder
briquettes do not undergo any volume reduction whatsoever when
compressed. For waste treatment, this compaction reduces
disposal costs by 40% which is a distinct advantage over waste
encapsulation processes utilizing MKP, and is estimated to be
the lowest cost process for treating high volume radioactive
waste.
EXAMPLE 3
Yttrium Oxide Containing Waste Forms
Yttrium oxide was thoroughly mixed with pre-mixed MHP dry
mixture in a weight ration of 1:2. Water was then added to the
powder mixture at a weight ratio of powder to water of 3:1. The
resultant slurry, viscosity of 200 cp, was mixed for 15 minutes
until it warmed slightly to .about.40.degree. C. due to the
exothermicity of the process, and subsequently set into a hard
ceramic within 10 minutes. As such, the slurry provides an
exothermic reaction, which aids in the setting reaction.
The ceramic formed in this Y.sub.2 O.sub.3 protocol has a
density of 1.78 grams per cubic centimeter (g/cc) and its open
porosity is .about.5% giving the same density-porosity
characteristics of MKP binder with various encapsulated wastes.
However, because Y.sub.2 O.sub.3 is less than sparsely soluble,
the invented MHP process is the best means to form a ceramic of
this oxide and other rare earth oxides with similar
solubilities.
FIG. 3 shows a X-ray diffraction pattern of the yttrium
phosphate ceramic. In addition to Newberyite ("*"), Y.sub.2
O.sub.3 ("o"), and yttrium phosphate trihydrate,
YPO.sub.4.multidot.3H.sub.2 O (".circle-solid.") are present.
EXAMPLE 4
Magnetite-Containing Ceramic Waste Forms
Magnetite based ceramics have been made in the past by the
direct reaction of magnetite and aqueous phosphoric acid
solution. The reaction is rapid and very exothermic which
creates difficulties when attempting to form large monoliths.
This problem may be overcome by using MHP as the binder.
Magnetite and MHP powder were mixed thoroughly in a weight
ratio of 1:2 of magnetite to MHP. Water was added in a weight
ratio of 1:3 of water to mixture. The resultant slurry,
viscosity of 200 cp, was mixed for .about.15 minutes until the
mixture's temperature began to increase. The slurry was then
poured into molds after which it set within an hour and formed a
dense and hard ceramic. The ceramic has a density of 1.71 g/cc
and an open porosity of 4.6%. This particular magnetite ceramic
is a lightweight material.
FIG. 4 shows the X-ray diffraction pattern of the ceramic
formed. Peaks are directly observed only for Fe.sub.3 O.sub.4
("x") and MgHPO.sub.4.3H.sub.2 O ("T") are seen to be present.
Although no peaks of iron phosphate are visible, the large humps
indicate a significant amount of a glassy phase. The magnetite
ceramic sample also looked very glassy and scanning electron
microscopy showed large portions of featureless or glassy
material. Thus, the iron phosphate formed an amorphous or glassy
solid structure. The solid is free of open porosity with almost
zero water absorption.
EXAMPLE 5
Strength Enhancement by Fiber Addition
As taught supra, reinforcing substrates such as fibers may be
directly incorporated into the phosphate powders. In this
example, the binder powder mixtures comprised magnesium
potassium phosphate hexahydrate, and Class F fly ash. Two
different chopped glass fiber lengths were used, 0.25 inch and
0.5 inch. Fibers were added into the powder blend in a
proportion of 1 to 3 wt. % of the total mixture. Water was added
and as the powder dissolved, more fibers could be added, if
desired or needed to obtain a particular wt. %. A dye-casting
process can be used to form composites with a greater amount of
fiber, up to 10 wt. %.
FIGS. 5(a-b) shows the compressive strength as a function of
fiber content in composites of 40 and 60 wt. % Class F fly ash
with two different fiber lengths. As shown in FIG. 5a, with a
waste loading of 40 wt. % ash and loading with 0.25 inch chopped
glass fiber, the compressive strength increased, from a value of
6500 psi for MHP binder ceramic without fibers, to 10,800 psi
when fiber was added to the extent of 1 and 2 wt. % of the total
composite. With a fiber loading of 3 wt. %, the compressive
strength decreased to 9,400 psi. The results for 0.5 inch
chopped glass fiber were somewhat lower than those for 0.25 inch
fiber and dropped to 9800 psi at 3 wt. % fiber.
In the case of 60 wt. % ash, the compressive strength profiles
are exactly the opposite of those for 40 wt. % ash, as shown in
FIG. 5b. The compressive strength without fiber is 10,600 psi.
With 1 wt % of 0.25 inch fiber, the compressive strength is
12,000 psi; then inversely, the compressive strength of the
composite with 0.5 inch fibers consistently decreases with
increasing fiber content.
FIGS. 6(a-b) shows the flexural strength as a function of fiber
content in composites of 40 wt. % ash and 60 wt. % ash. FIG. 6a
shows the results for 40 wt. % ash and FIG. 6b the results for
60 wt. % ash. For both ash wt. %'s and both fiber lengths,
flexural strength increases as the fiber wt. % increases.
FIGS. 7(a-b) shows the fracture toughness of these same
composites. FIG. 7a shows the results for 40 wt. % fiber and
FIG. 7b the results for 60 wt. % fiber. The fracture toughness
of the ceramicrete samples without any fibers is
0.22 and 0.35 Mpa.multidot.m.sup.1/2. Adding fiber to the extent
of 3 wt. % for both 0.25 and 0.5 inch fibers increases the
fracture toughness of 40 wt. % ash composite to 0.65
Mpa.multidot.m.sup.1/2. That figure is approximately twice the
fracture toughness of MKP formulations, neat, i.e., without any
fiber additive.
FIG. 8 shows the scanning electron micrograph of a fractured
surface of a MKP-Class F fly ash composite sample, 60 wt. %
Class F fly ash and 2 wt. % of 0.25 inch glass fiber. The fibers
were added as strands or bunches of fibers; yet, the fibers
dispersed and became encapsulated as individual fibers in the ceramicrete
matrix. The micrograph clearly shows each fiber is surrounded by
the matrix. MKP serves as an effective dispersant. As is to be
expected in an acidic or neutral pH environment, there is no
corrosion on the fibers' surfaces. This indicates a
compatibility between the glass fibers and the matrix.
MKP is a rugged binder, applicable to a wide variety of wastes
and for specialized structural products where the strength
requirements are very high. On the other hand, MHP allows for
high waste loading, is much less expensive, and may be useful
for development of structural products of high volume waste
streams.
A myriad of wastes can be encapsulated by the invented methods
and ceramics. As such, hazardous metals are good candidates,
including, but not limited to arsenic, cadmium, chromium, lead,
nickel, and zinc. Low solubility-oxides are also good waste
substrate candidates, as are fission products, including
technetium, strontium, barium and cesium. Low level wastes, such
as biomedical materials and other slightly radioactive
substrates are suitable encapsulation candidates. Wastes
containing difficult to encapsulate oils are particularly good
candidates for the instant invention. Even heterogeneous wastes
and mixed phase wastes are suitable.
While the invention has been described with reference to
details of the illustrated embodiment, these details are not
intended to limit the scope of the invention as defined in the
appended claims.
US Patent # 6,518,212
Chemically Bonded Phospho-Silicate
Ceramics
Arun WAGH, et. al.
US Cl. 501/111; Intl Cl. C04B 035/447
Abstract: A chemically bonded phospho-silicate
ceramic formed by chemically reacting a monovalent alkali metal
phosphate (or ammonium hydrogen phosphate) and a sparsely
soluble oxide, with a sparsely soluble silicate in an aqueous
solution. The monovalent alkali metal phosphate (or ammonium
hydrogen phosphate) and sparsely soluble oxide are both in
powder form and combined in a stochiometric molar ratio range of
(0.5-1.5):1 to form a binder powder. Similarly, the sparsely
soluble silicate is also in powder form and mixed with the
binder powder to form a mixture. Water is added to the mixture
to form a slurry. The water comprises 50% by weight of the
powder mixture in said slurry. The slurry is allowed to harden.
The resulting chemically bonded phospho-silicate ceramic
exhibits high flexural strength, high compression strength, low
porosity and permeability to water, has a definable and
bio-compatible chemical composition, and is readily and easily
colored to almost any desired shade or hue.
References Cited
U.S. Patent Documents:
2687967 ~ 3078186 ~ 3821006 ~ 3960580 ~ 4036655 ~ 4066471 ~
4375516 ~ 4504555 ~ 4792359 ~ 4872912 ~
4956321 ~ RE33366 ~ 4978642 ~ 5002610 ~ 5518541 ~ 5645518 ~
5718757 ~
Other References:
Semler, Charles "A Quick-Setting Wollastonite Phosphate Cement"
Ceramic Bulletin vol. 55, No. 11 (1976).
Sugama and Allan "Calcium Phosphate Cements Prepared by
Acid-Base Reaction" J. Am. Ceram. Soc. (Aug. 1992).
Fukase et al. "Setting Reactions and Compressive Strengths of
Calcium Phosphate Cements" J. Dent. Res. vol. 69 No. No. 12
(Dec. 1990).
Brown and Chow "A New Calcium Phosphate, Water-Setting Cement"
pp. 352-379 (1986).
C.E. Semler "A Quick-Setting Wollastonite Phosphate Cement"
American Ceramic Society Bulletin, vol. 55, No. 11, (No Date
Available).
Goverment Interests
The United States Government has rights in this invention
pursuant to Contract No. W-31-109-ENG-38 between the U.S.
Department of Energy and University of Chicago operators of
Argonne National Laboratory.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to ceramics, and more
particularly to a chemically bonded phospho-silicate ceramic
that exhibits high flexural strength, high compression strength,
low porosity and permeability to water, sets rapidly at room
temperature, has a definable and bio-compatible chemical
composition, is easily colored, and a method of producing the
same.
2. Description of the Related Art
There is an acute need for a rapid setting pore-free high
strength binding material for use in the construction and waste
management industries. Traditional cements and ceramics used in
these industries have many drawbacks that make those traditional
materials less than ideal. For example, traditional cement, such
as Portland cement, lacks fracture toughness, is extremely
porous and permeable to water, and is very slow in setting. The
open porosity of these traditional cement materials makes these
materials susceptible to deterioration during the freezing and
thawing which occurs in many climates across the United States,
Europe and beyond. The expansion and contraction of water within
the open pores of these traditional cement materials causes them
to break down as they are exposed to extreme temperature
fluctuations. Additionally, traditional cements, such as
Portland cement, are slow in setting, requiring continuous
hydration and attention until the cement material has been
properly set, thus adding considerable labor costs to any given
project.
The open porosity and thus high water permeability of
traditional cement materials also limits the practical use of
these materials in waste management and waste encapsulation
projects. Highly porous cements are permeable to ground water
and allow wastes and toxins to leach out from the encapsulated
cement material.
Slightly soluble silicate minerals such as Wollastenite
(CaSiO.sub.3) and serpentinite (Mg.sub.6 Si.sub.4 O.sub.10
(OH).sub.8), have been used to develop phosphate cements. These
phosphate cements are produced by using phosphoric acid,
partially neutralized with zinc and aluminum, and then reacted
with Wollastenite or serpentine. In spite of the neutralization
step, the acid solutions are highly acidic, making them hard to
transport to a construction site as a raw material and requiring
rigorous safety training for employees in the construction
industry, who are used to just adding water to powdered cement.
Additionally, the high acidity of these phosphate cements
corrodes conventional construction and concrete equipment.
Ceramics are typically less porous than traditional cement,
however, traditional ceramics must be fired at extremely high
temperatures in order to solidify and cure the ceramic material
for practical use. Fired ceramic construction products are
expensive, especially if there are large size components. The
firing process is not suitable for waste management purposes
because waste components volatilize during firing. Resins and
other polymer products used as binding materials also provide a
less porous product than traditional ceramic materials, however
resins are typically expensive to manufacture, their fumes are
toxic, and resulting resin products are flammable.
One ceramic material that has had some success as a binding
material is the ceramicrete binder. Ceramicrete
binders disclosed in our U.S. Pat. Nos. 5,645,518; 5,830,815 and
5,846,894, include compounds such as magnesium potassium
phosphate (MgKPO.sub.4.6H.sub.2 O) and newberyite
(MgHPO.sub.4.3H.sub.2 O) ceramics. These ceramicrete
binders are considerably less porous than conventional cements,
are not toxic or flammable, set at a controllable rate, and are
a low cost alternative to polymer resins. These ceramicrete
binders provide a compression strength comparable to the
compression strength exhibited by Portland cement.
It is also known to combine ash with ceramicrete
binders, as disclosed in our U.S. Pat. No. 5,830,815, to
increase the compression strength to a level two to three times
that of the compression strength of Portland cement, The
porosity of the ceramicrete ash product is quite
low reducing its susceptibility to freeze thaw deterioration and
increasing its practical usefulness as a suitable waste
encapsulation material that resists permeation of ground water
and the leaching of wastes out of the encapsulated ceramicrete
ash product. The ceramicrete ash product,
however, is not often suitable for architectural uses where many
true and subtle colors and shades are desired because the ash
product cannot be easily dyed or colored. The ceramicrete
ash product is gray or beige depending upon whether fly ash or
bottom ash is used. This gray or beige starting color prevents
many common architectural colors such as red, yellow, blue,
etc., from being achieved, regardless of how much dye or pigment
is added to the ash-containing product. Additionally, ash is a
mixture of many oxides and silicates and may contain components
that are not bio-compatible.
The lack of bio-compatibility in ash containing products,
limits the use of those products in the bio-material industries
which also have a great need for rapid setting, pore free, high
strength binding materials which are also bio-compatible. Only
bio-compatible components can make up the binding materials used
in dentistry and orthopedics etc. For example, zinc phosphate
cements have been used as dental cements because they are dense,
hard and also bio-compatible. Zinc phosphate cements, however,
are expensive to manufacture and set rapidly, within minutes,
making them difficult to work with and produce in any sort of
large quantity. For these reasons zinc phosphate cements are not
practical for use in construction or waste encapsulation
projects as well. Zinc phosphate cements also do not contain
calcium phosphates or hydroxyapatite, which are desirable
elements for bone tissue growth.
None of the previous binding materials provide a high strength,
low porosity, rapid setting, easily colored, bio-compatible
chemical composition needed for use in the construction,
waste-management, and bio-material industries.
BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to provide a chemically
bonded phospho-silicate ceramic that exhibits high flexural and
compression strength. The high strength phospho-silicate ceramic
of the present invention can reduce the size of load-bearing
structures in the construction industry, provide very strong
waste encapsuation matrix and provide a high strength
biomaterial for use in prosthetics and dentistry.
Another object of the present invention is to provide the new
chemically bonded phospho-silicate ceramic exhibiting low
porosity and permeability to water, providing a desirable
construction material that is resistant to freeze-thaw
deterioration during temperature fluctuations, as well as
providing an excellent material for waste encapsulation that is
resistant to permeation of ground water and leaching from the
encapsulation material.
It is another object of the present invention to provide a
phospho-silicate ceramic that sets rapidly at room temperature,
without the continuous hydration and attention required by
traditional cement materials, thus reducing labor costs.
Additionally, the low temperature manufacture of the present
invention makes the ceramic suitable for the construction, waste
management and bio-material industries.
It is another object of the present invention to provide a new
chemically bonded phospho-silicate ceramic made from non-toxic,
readily available, easily transportable and inexpensive
compounds.
It is another object of the present invention to provide a
phospho-silicate ceramic having a definite and definable
chemical composition, suitable in the bio-material industry
where the chemical components must be known to ensure their
bio-compatibility before introduction into the human body.
It is yet another object of the present invention to provide a
phospho-silicate ceramic that is easily and readily colored to
true colors in any variety of shades or hues.
It is yet another object of the present invention to provide a
kit for the simple and easy manufacture of the new chemically
bonded phospho-silicate ceramic at an industrial site or for
home use.
Yet another object of the present invention is to provide a
simple and quick method for manufacturing the new chemically
bonded phospho-silicate ceramic of the present invention.
According to one aspect of the present invention, the above
objects are realized in a phospho-silicate ceramic formed by
chemically reacting a monovalent alkali metal phosphate and a
sparsely soluble oxide, with a sparsely soluble silicate in an
aqueous solution. The preferred sparsely soluble oxide is
magnesium oxide, and the preferred sparsely soluble silicate is
calcium silicate.
In one embodiment, the monovalent alkali metal phosphate, the
sparsely soluble oxide and the sparsely soluble silicate are all
in powder form and are combined to form a mixture. The mixture
is comprised of 60% sparsely soluble silicate.
According to one aspect of the invention, the above objects are
realized in a method of producing a phospho-silicate ceramic
comprising the steps of (a) combining a monovalent alkali metal
phosphate powder with a sparsely soluble oxide powder in a
stochiometric molar ratio of 1:1 to form a binder powder; (b)
adding a sparsely soluble silicate powder in a range of 1-80% by
weight to the binder powder, to form a mixture;
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is an X-ray diffraction pattern of
phospho-silicate ceramic of Example 1.
FIG. 2 is a scanning electron microphotograph of a
phospho-silicate ceramic sample.
DETAILED DESCRIPTION OF THE INVENTION
The present invention teaches a new chemically bonded
phospho-silicate ceramic that will benefit the construction,
waste management, and biomaterial industries, as well as a
method for producing the new chemically bonded phospho-silicate
ceramic. The phospho-silicate ceramic of the present invention
exhibits high flexural strength, high compression strength, low
porosity and permeability to water, has a definable and
bio-compatible chemical composition and is readily and easily
colored to almost any desired shade or hue. The phospho-silicate
ceramic of the present invention is simply manufactured in large
or small quantities, sets rapidly at room temperatures in only a
few hours, and continues to cure over a period of time, The
phospho-silicate ceramic of the present invention can also be
easily made on site away from the manufacturing plant as its
separate components are very safely and easily transportable.
The phospho-silicate ceramic of the present invention is formed
by chemically reacting a monovalent alkali metal phosphate and a
sparsely soluble divalent oxide, with a sparsely soluble
silicate in an aqueous solution. The phospho-silicate ceramic of
the present invention can alternatively be formed by replacing
the monovalent alkali metal phosphate with ammonium hydrogen
phosphate, aluminum hydrophosphate or a phosphoric acid
solution. The monovalent alkali metal phosphate (or ammonium
hydrogen phosphate, aluminum hydrophosphate or phosphoric acid
solution) and sparsely soluble oxide form a ceramicrete
binder as disclosed in our earlier patents, namely U.S. Pat.
Nos. 5,645,518, 5,830,815 and 5,846,894, incorporated herein by
reference. U.S. Pat. Nos. 5,645,518, 5,830,815 and 5,846,894,
disclose ceramicrete binders such as magnesium
potassium phosphate (MgKPO.sub.4.6H.sub.2 O) and newberylite
(MgHPO.sub.4.3H.sub.2 O), etc. Ceramicrete
binders are inexpensive to manufacture in large scale, because
their components are widely available and generally inexpensive.
Monovalent alkali metal phosphates suitable for forming ceramicrete
binders include dihydrogen phosphates of all Group 1A elements
in the periodic table and suitable ammonium hydrogen phosphates
include, ammonium dihydrogen phosphate ((NH.sub.4)H.sub.2
PO.sub.4) and diammonium hydrogen phosphate ((NH.sub.4).sub.2
HPO.sub.4). Sodium dihydrogen phosphate (NaH.sub.2 PO.sub.4);
lithium dihydrogen phosphate (LiH.sub.2 PO.sub.4); and potassium
dihydrogen phosphate (KH.sub.2 PO.sub.4) are preferable
monovalent alkali metals for forming the ceramicrete
binders. In addition, aluminum dihydrogen phosphate may also be
used.
Suitable sparsely soluble oxides include oxides of Group IIA
elements that have a solubility constant between 5 and 25.
Preferably, solubility constants between 5 and 12 are desired,
and magnesium oxide (MgO); Barium oxide (BaO); and Calcium oxide
(CaO) are the most preferred oxides for reacting with the
monovalent alkali metals to form ceramicrete
binders. Zinc oxide (ZnO) of group IIB elements may also be
used.
Sparsely soluble silicates suitable for forming the
phospho-silicate ceramics of the present invention include
silicates of Group IIA and IA elements that have a solubility
constant between 5 and 25. Once again silicates with a
solubility constant between 5-12 are more preferred and
silicates such as calcium silicate (CaSiO.sub.3); talc or
magnesium silicate (MgSiO.sub.3) barium silicate (BaSiO.sub.3);
sodium silicate (NaSiO.sub.3); lithium silicate (LiSiO.sub.3);
and serpentinite (Mg.sub.6 Si.sub.4 O.sub.10 (OH.sub.8) are
preferred. Calcium silicate has a solubility product constant of
approximately 8, which is similar to magnesium oxide, and thus
would be a compatible material with the ceramicrete
binder, and the most preferred sparsely soluble silicate for
forming the phospho-silicate ceramics of the present invention.
Calcium silicate, or Wollastonite as it is known in its mineral
form, is an inexpensive product, typically 5-10 cents per pound,
and is widely available in large amounts. Wollastonite is also
available in powder form, is not toxic, and is thus easy to
handle and transport. Wollastonite as referred to in the present
patent application is defined according to Dana's Manual of
Mineralogy, Revised by Cornells Klein and Cornelius S. Hurlbut,
Jr. 20.sup.th ed, pub. John Wiley and Sons, New York (1977) pp.
406-408.
The addition of Wollastonite to the ceramicrete
binders produced exciting and unexpected results, as the
Wollastonite unexpectedly modified the ceramicrete
binders significantly. The Wollastonite crystals greatly
increased the fracture toughness of the ceramicrete
matrix, and amorphous silicate released from the Wollastonite in
solution, greatly increased the compression and flexural
strength of the matrix while at the same time reducing its
porosity and permeability to water.
Individual Wollastonite crystals are acicular or elongated in
structure, and when combined within the ceramicrete
matrix, the Wollastonite crystals act as whiskers to resist
crack propagation. The Wollastonite crystals either stop or
divert propagating cracks requiring more energy for the crack to
continue, and thus increasing the fracture toughness of the
resulting product. Wollastonite also increases the viscosity of
the mixture, and overall strength of the resulting
phospho-silicate ceramic.
Amorphous silica released from Wollastonite in an aqueous
solution chemically reacts with the phosphates of the ceramicrete
binder to form a glassy phase within the ceramicrete
matrix. This chemical reaction was completely unexpected because
typically silicates and silicas, i.e. sand, are stable
materials. Silicates and silicas, even do not dissolve in acidic
solutions and do not react in an aqueous environment. However,
it was observed that the addition of a sparsely soluble silicate
such as Wollastonite to phosphate ceramicrete
binders provided the very unexpected chemical reactions as
outlined below.
Sparsely soluble silicates such as Wollastonite, talc, and
serpentinite are slightly alkaline and when combined with water
they become ionized, releasing the metal cations. For example,
Wollastonite dissolved in acidic water such as solutions of
H.sub.3 PO.sub.4, KH.sub.2 PO.sub.4, Al(H.sub.2 PO.sub.4).sub.3
etc., released cations Ca.sup.++ and silicate SiO.sub.3.sup.--.
The calcium cations reacted with the phosphates to form calcium
phosphates. The silicate anion formed metasilicic acid (H.sub.2
SiO.sub.3) which further reacted with available cations to form
K.sub.2 SiO.sub.3 as seen in equation 5. ##EQU1##
The first and third equations demonstrate that the addition of
a sparsely soluble silicate such as calcium silicate is a good
method for introducing metasilicic acid to a phosphate slurry.
The acid reacts subsequently with other available cations, such
as 2K.sup.+ as shown in equation 5 to form silicate glass. For
example, if sodium dihydrogen phosphate or potassium dihydrogen
phosphate was used instead of phosphoric acid water as the
provider of phosphate anions, the metasilicic acid will react
with either Na.sup.+ or K.sup.+ ions to form alkali metal glass.
This alkali metal glass formed within the phospho-silicate
ceramic of the present invention is believed to fill the voids
between particles of the ceramic and produce a dense solidified
non-porous ceramic product. Additionally, the glassy phase
within the ceramic product is also believed to bind particles of
the product together to produce a hard ceramic, thus increasing
both the compression and flexural strength of the resulting
product.
After the chemical reaction between the silicate and the ceramicrete
binder, at least three products are produced, namely, magnesium
potassium phosphate binder (MgKPO.sub.4.6H.sub.2 O), calcium
hydrophosphate (CaHPO.sub.4.2H.sub.2 O), and potassium silicate
(K.sub.2 SiO.sub.3). The magnesium potassium phosphate provides
the bulk strength for the new phospho-silicate ceramic, and the
potassium silicate produces a glassy phase that fills the voids
between the bulk compounds, resulting in a product that is
almost completely dense. This glassy phase provides the benefits
of reducing or even eliminating the porosity of the resulting
ceramic and smoothing its surface.
The compounds of the phospho-silicate ceramic of the present
invention are not as acidic as phosphate cement, nor as alkaline
as Portland cement. The compounds are more neutral, less
corrosive, and weather better over time. However, similar to
Portland cement, the phospho-silicate ceramic of the present
invention can be easily made on site by just adding water to a
blend of powders. Thus, current construction equipment can be
easily used, without extensive modifications, to make the
phospho-silicate ceramic of the present invention on a
construction site.
The phospho-silicate ceramic of the present invention is
manufactured through a unique but relatively simple process of
combining a monovalent alkali metal phosphate powder and a
sparsely soluble oxide powder, with the sparsely soluble
silicate powder in an aqueous solution. The powders are simply
blended together. The sparsely soluble silicate powder comprises
1-80% of the powder mixture and preferably 50-60% of the powder
mixture by weight. The monovalent alkali metal phosphate and the
sparsely soluble oxide powder are combined in the molar ratio
range of (0.5-1.5):1 in the widest range, (0.8-1.2):1 in a
preferred range, and 1:1 in the most preferred ratio range.
Alternatively, an ammonium hydrogen phosphate powder, aluminum
hydrophosphate powder or phosphoric acid solution can replace
the monovalent alkali metal phosphate powder in the same
concentration.
Water is then stirred into the powder blend to form a slurry. A
suitable water to powder weight ratio is (1-1.5):2 in the widest
range, (1-1.2): 2 in a preferred range and 1:2 in the most
preferred range. The slurry is stirred for 10 to 25 minutes at
room temperature and left to harden, or alternatively, poured
into molds and left to harden. The ceramic material will harden
within two hours, and then continue to cure for at least 3
weeks.
Coloring the phospho-silicate ceramic of the present invention
can also be achieved by simply adding an inorganic powder
pigment to the powder blend before the water is added. Since
Wollastonite crystals are white, the phospho-silicate ceramic of
the present invention can easily be colored to any desired shade
or hue. This attribute can be particularly useful in the
construction industry where colored binding materials can
provide substantial cost savings and provide great architectural
freedom and creativity in designing structures utilizing binding
materials of various color schemes.
EXAMPLE 1
In a preferred method, 60% by weight Wollastonite powder, 10%
by weight magnesium oxide powder, and 30% by weight potassium
dihydrogen phosphate powder were combined to form a powder
blend. Water was added to the powder blend in a weight ratio of
1:5 respectively, and stirred for 15-25 minutes at room
temperature to form a slurry. The slurry was then poured into
plastic syringes of one inch diameter and filled to a volume of
60 cc's and left to cure. The slurry set into a hard ceramic
within two hours. The resulting chemically bonded
phospho-silicate ceramic was dense, non-porous and homogenous.
EXAMPLE 2
In another embodiment of the invention, 12.5% by weight
magnesium oxide powder, 37.5% by weight potassium dihydrogen
phosphate powder, and 50% by weight Wollastonite power was mixed
to form a powder blend. Water was added to the powder blend in a
weight ratio of 1:4 respectively, and stirred for 15-25 minutes
at room temperature to form a slurry. The slurry was then poured
into plastic syringes having a one inch diameter and filled to a
volume of 60 cc's, and left to cure. The slurry set into a hard
ceramic within two hours. The resulting chemically bonded
phospho-silicate ceramic was dense, non-porous and homogenous.
The following Table compares the mechanical properties of
Examples 1 and 2, with a phospho-silicate ceramic/sand sample, a
ceramicrete binder sample, a ceramicrete/ash
sample, and high strength concrete.
TABLE 1
Cure Fracture
Composition (wt. %) Time Strengths (psi) Toughness Water
Absorption
Binder* Wollastonite Other Days Compres Flexural MPa. m Wt. %
40 60 Nil 21 8,426 1,474 0.66 2
50 50 Nil 14 7,755 1,236 0.63 2
30 30 Sand 40 11 6,264 1,255 0.63 3.0
100 Nil Nil 21 .apprxeq.3,500 .apprxeq.1,100 n/a
.about.15
40 Nil Ash 60 14 11,507 1,474 0.19 1.78
High Strength concentrate 28 8,000 940 n/a .apprxeq.10-20
(literature value)
*Binder is defined as the mixture of potassium dihydrogen phosphate
(monopotassium phosphate) and magnesium oxide.
As shown in Table 1, the phospho-silicate ceramics of Examples 1
and 2 have a low water absorption, and thus reduced porosity,
compared to high strength concrete and ceramicrete
binder. The weight percent water absorption of the
phospho-silicate ceramics of Examples 1 and 2 is 1/5-1/10 the
weight percent water absorption of high strength concrete, and
1/2-1/8 the weight percent water absorption of ceramicrete
binder. It appears that the addition of Wollastonite to the ceramicrete
binder reduced the porosity of the resulting phospho-silicate
ceramic by a significant margin, thus resulting in a much denser
end product. Typically, water absorption is a direct indication
of the porosity of a material, however, in the phospho-silicate
ceramics as shown in Examples 1 and 2, it is believed that at
least some of the water absorption is due to the formation of
hydroxides of magnesium and calcium rather than due to the
porosity of the ceramic. The actual porosity of the
phospho-silicate ceramic of the present invention is believed to
be less than that indicated by the water absorption test in
Table 1. As a result, it is expected that the phospho-silicate
ceramics of the present invention would not experience
freeze/thaw deterioration during the temperature fluctuations
experienced in cold climates because the phospho-silicate
ceramics of the present invention appear fully dense.
As shown in Table 1 above, the phospho-silicate ceramic of
Examples 1 and 2 have a flexural strength far superior to the
flexural strength of high strength concrete and ceramicrete
binder. The phospho-silicate ceramics of Examples 1 and 2 had a
flexural strength 30-50% higher than the flexural strength of
high strength concrete. The phospho-silicate ceramics of
Examples 1 and 2 have a flexural strength 20-40% greater than
the flexural strength of ceramicrete binder.
Additionally, as shown in Table 1, the phospho-silicate ceramics
of Examples 1 and 2 have a compression strength two times the
compression strength of ceramicrete binder.
Table 1 also compares the fracture toughness of the
phospho-silicate ceramics of Examples 1 and 2, with the ceramicrete
binder/ash product. The MPa.m fracture toughness values for
phospho-silicate ceramic Examples 1 and 2 are 0.66 and 0.63
respectively, and the fracture toughness value for ceramicrete
binder/ash is 0.19. Both phospho-silicate ceramic Examples 1 and
2 exhibit a fracture toughness 3 times greater than the fracture
toughness of the ceramicrete/ash product. This result further
supports Applicant's theory that the acicular or elongated
crystals of Wollastenite act as whiskers to resist crack
propagation and thus increase the overall fracture toughness of
the resulting product. Ash does not contain elongated crystal
structures, and as a result, ash is poor in resisting crack
propagation and increasing the overall fracture toughness of the
resulting product.
The X-ray diffraction pattern of the phospho-silicate ceramic
of Example 1 shown in FIG. 1 exhibited a broad hump in the
center of the pattern. This broad hump is believed to be due to
the formation of potassium silicate glass (K.sub.2 SiO.sub.3)
formed by the chemical reaction between potassium cations and
metasilic acid, as shown in equation 5 above. It is believed
that this glassy phase fills in the voids and pores in the
resulting ceramic of the present invention, and is important for
both reducing the open porosity of the ceramic of the present
invention, which appears very dense and has a smooth surface,
and for increasing the compression and flexural strength of the
ceramic of the present invention.
Also shown in the X-ray diffraction analysis, are major peaks
that were identified as unreacted calcium silicate, magnesium
potassium phosphate binder (MgKPO.sub.4.6H.sub.2 O) and calcium
hydrophosphate (CaHPO.sub.4.2H.sub.2 O). Calcium hydrophosphate
is likely to be absorbed into the human body and can regenerate
body tissues when the phospho-silicate ceramic of the present
invention is used in bio-materials. For this reason, the
phospho-silicate ceramic of the present invention may be one of
the most suitable materials for bio-material purposes such as
orthopedic and dental applications.
A scanning electron micrograph of fracture surface of samples
of the phospho-silicate ceramic of the present invention is
shown in FIG. 2. The micrograph shows the crack propagation of
the ceramic is intergranular, with the cracks running around the
elongated crystals of the Wollastonite. Such crack deflection by
the Wollastonite crystals increases the fracture energy and
improves the fracture toughness of the phospho-silicate ceramic
of the present invention.
In another embodiment, the phospho-silicate ceramic of the
present invention can be sold as separate components grouped
into kits for forming phospho-silicate ceramic structures for
home as well as industrial use. For example, a typical kit would
include a bag or drum, depending on the volume needed, of
suitable monovalent alkali metal phosphate powder, suitable
sparsely soluble oxide, and suitable sparsely soluble silicate.
Each powder is combined in an appropriate amount as described
above. The kit could also include an optional bag or drum of
inorganic pigment powder to dye the final product to a desired
color, or a bag or drum of an aggregate, such as granite, if a
particular texture is desired in the final product. Easy to
follow instructions would direct the user to combine the
powders, add the appropriate amount of water to form a slurry,
and add any desired aggregate. The slurry is then poured into a
mold, for example a countertop mold for home use, and allowed to
set and cure.
Alternatively, the phospho-silicate ceramic can be sprayed onto
the surface of a structure for fireproofing, water proofing,
etc. The phospho-silicate ceramic will chemically bond to the
substrate, making this product far superior to a laminate
applied with an adhesive to a substrate.
Other ways of making Wollastonite containing phospho-silicate
ceramics include reacting Wollastonite with phosphoric acid
solution, or aluminum hydro phosphate solution, but a small
amount of boric acid needs to be added as a retardant because
phosphoric acid is too reactive. It is also possible to
neutralize the phosphoric add with hydroxides or carbonates of
an alkali metal such as sodium or potassium, and react it with
Wollastonite.
EXAMPLE 3
1. 40 g of Wollastonite, and 67 g of 50 wt. % concentrated
phosphoric acid solution neutralized with 15% of sodium
carbonate were reacted for 20 min. The slurry warmed up and set
into a hard ceramic. We believe, sodium carbonate reacted with
phosphoric acid solution to form sodium dihydrogen phosphate
which reacted with Wollastonite to form the ceramic. The ceramic
contained some glassy phase, probably sodium phosphate and
sodium silicate, unreacted Wollastonite, and calcium
hydrophosphate.
2. 40 g of Wollastonite, 67 g of 50% concentrated phosphoric
acid solution when mixed together reacted instantaneously and
formed a precipitate. With addition of 3 g of boric acid,
however, the slurry set into a hard ceramic. Thus, it is
possible to produce ceramics of Wollastonite without
neutralizing phosphoric acid or adding magnesium oxide as done
in the previous case studies. The product contained calcium
hydrophosphate and unreacted Wollastonite.
3. 80 g of Wollastonite, 100 g of 50% concentrated phosphoric
acid solution and 15 g of potassium carbonate were mixed for 10
min. Initial setting was in one hour and complete setting was in
3 days.
4. 100 g of Wollastonite was mixed with 100 g of sodium
dihydrogen phosphate and 100 g of water. The slurry was mixed
for 30 min. It warmed up and set in two days.
5. 25 g of Wollastonite, 50 g of sodium dihydrogen phosphate,
112.5 g of ash and 80 g of water were mixed for 10 min. The
slurry set in two days. The set product contained unidentifiable
glassy phase.
All these tests demonstrate that phosphates may be added in
different forms to produce Wollastonite containing chemically
bonded phospho-silicate ceramics.
The foregoing description has been provided to clearly define
and completely describe the present invention. Various
modifications may be made without departing from the scope and
spirit of the invention which is defined in the following
claims.
WAGH PATENTS
COMPOSITION AND APPLICATION OF NOVEL SPRAYABLE PHOSPHATE
CEMENT THAT BONDS TO STYROFOAM
WO 2006001891
1-05-2006
WAGH ARUN S (US); PAUL JAMES W JR (US)
Classification: - international: B05D3/02; B05D7/00; C04B2/00;
C04B7/00; C04B9/00; C04B12/02; C04B14/00; C04B14/38; C04B18/06;
C04B28/30; C04B28/34; B05D3/02; B05D7/00; C04B2/00; C04B7/00;
C04B9/00; C04B12/00; C04B14/00; C04B14/38; C04B18/04; C04B28/00;
(IPC1-7): C04B28/34; C04B14/06; C04B14/30; C04B18/08; C04B22/00;
C04B28/34; - european: C04B28/34
Application number: WO2005US13451 20050418
Priority number(s): US20040868062 20040615
Also published as: US2005274290 (A1)
Cited documents: WO03031367 // WO0066878 // WO9734848 // EP0203658
// WO0006519
Abstract: A dry mix particulate composition of a
calcined oxide of Mg and/or Ca, an acid phosphate, and fly ash
or equivalent, wherein the calcined oxide is present in the
range of from about 17% to about 40% by weight and the acid
phosphate is present in the range of from about 29% to about 52%
by weight and the fly ash or equivalent is present in the range
of from about 24% to about 39% by weight when sand is added to
the dry mix, it is present in the range of from about 39% to
about 61 % by weight of the combined dry mix and sand. A method
of forming a structural member is also disclosed wherein an
aqueous slurry of about 8-12 pounds of water is added to dry mix
and sand.
PERMAFROST CERAMICRETE
WO 2005073145
8-11-2005
WAGH ARUN S (US); NATARAJAN RAMKUMAR (US); FISHER BRANDON
Classification:- international: C04B28/34; C09K8/46; C04B28/00;
C09K8/42; (IPC1-7): C04B28/34; - european: C04B28/34; C09K8/46
Application number: WO2005US00485 20050107
Priority number(s): US20040538818P 20040123; US20040941592
20040914
Also published as: WO 2005073145 // US 2005160944
Cited documents: US6136088 // WO0066878 // WO0006519 //
US2003131759 // EP0203658
Abstract: A dry mix of a calcined oxide of Ca and/or Mg
and an acid phosphate and fly ash with or without insulating
extenders useful in permafrost conditions. Calcined oxide is
present at about 12% to about 40% by weight and the acid
phosphate is present at about 35% to about 45% by weight. The
fly ash is present at about 10% to about 50% by weight with the
fly ash being between about 50% to about 100% class F with the
remainder class C. Insulating extenders are present in the range
from 0% to about 15% by weight of the combined calcined oxide
and acid phosphate and fly ash. 0.1% to about 0.5% boric acid
and/or borate by weight of the dry mix is present.
METHOD OF WASTE STABILIZATION WITH DEWATERED CHEMICALLY
BONDED PHOSPHATE CERAMICS
WAGH ARUN S (US); MALONEY MARTIN
EC: IPC: A62D3/00; B09B3/00; G21F9/00 (+7)
CA 2540293
9-10-2004
CERAMICRETE STABILIZATION OF U-AND PU-BEARING MATERIALS
WAGH ARUN S (US); MALONEY DAVID D
EC: C04B28/34 IPC: G21F9/04; C04B28/34; G21F9/16
(+5)
CA 2540287
9-02-2004
METHOD AND PRODUCT FOR PHOSPHOSILICATE SLURRY FOR USE IN
DENTISTRY AND RELATED BONE CEMENTS
WAGH ARUN S (US); PRIMUS CAROLYN
EC: A61K6/00 IPC: A61K6/00; A61K6/00; (IPC1-7):
A61K6/06 (+1)
EP 1651172
5-03-2006
CHEMICALLY BONDED PHOSPHATE CERAMIC SEALANT FORMULATIONS FOR
OIL FIELD APPLICATIONS
WAGH ARUN S (US); JEONG SEUNG-YOUNG
EC: IPC: C04B28/00; C04B28/00
US 2006048682
3-09-2006
CONSTRUCTION MATERIAL AND METHOD
WAGH ARUN S (US); ANTINK ALLISON L
US 2006003886
1-05-2006
METHOD & PRODUCT FOR PHOSPHOSILICATE SLURRY FOR USE IN
DENTISTRY...
WAGH ARUN S (US); PRIMUS CAROLYN
EC: A61K6/00 IPC: A61K6/00; A61K6/00; (IPC1-7):
A61C13/08
US 2005028705
2-10-2005
CANISTER- SEALING METHOD & COMPOSITION FOR SEALING A
BOREHOLE
BROWN DONALD W (US); WAGH ARUN S
EC: C04B28/34; C09K8/46; (+2) IPC: C04B28/34;
C09K8/46; E21B27/02 (+12)
US 2003150614
8-14-2003
CHEMICALLY BONDED PHOSPHATE CERAMICS OF TRIVALENT OXIDES OF
IRON AND MANGANESE
WAGH ARUN S (US); JEONG SEUNG-YOUNG
EC: C04B28/34A; C04B32/00; (+1) IPC: C04B28/34;
C04B32/00; G21F9/16 (+4)
WO 02058077
7-25-2202
FORMATION OF CHEMICALLY BONDED CERAMIC...
WAGH ARUN S (US); JEONG SEUNG-YOUNG
EC: C04B28/34A IPC: C04B12/02; C04B35/447; C04B12/00
(+2)
US 2003092554
5-15-2003
CORROSION PROTECTION
BROWN DONALD W (US); WAGH ARUN
EC: C23C22/73; C23C22/74 IPC: C23C22/73; C23C22/74;
C23C22/73 (+1)
US 2002179190
12-05-2002
DOWNHOLE SEALING METHOD AND COMPOSITION
BROWN DONALD W; WAGH ARUN
EC: C04B28/34; C09K8/46; (+2) IPC: C04B28/34;
C09K8/46; E21B27/02 (+7)
WO 0066878
11-09-2000
PUMPABLE/INJECTABLE PHOSPHATE-BONDED CERAMICS
SINGH DILEEP; WAGH ARUN
EC: B09B1/00; B09C1/08; (+2) IPC: B09B1/00;
B09C1/08; C04B28/34 (+9)
WO 0006519
2-10-2000
METHOD FOR PRODUCING CHEMICALLY BONDED PHOSPHATE CERAMICS...
SINGH DILEEP (US); WAGH ARUN S
EC: A62D3/00M10D; A62D3/00K4; (+9) IPC: A62D3/00;
C03C1/00; C03C10/00 (+14)
US 6133498
10-17-2000
POLYMER COATING FOR IMMOBILIZING SOLUBLE IONS...
SINGH DILEEP (US); WAGH ARUN
EC: A62D3/00; A62D3/00E4; (+5) IPC: A62D3/00;
B09B3/00; C04B41/48
US 6153809
11-28-2000
PUMPABLE/INJECTABLE PHOSPHATE-BONDED CERAMICS
SINGH DILEEP (US); WAGH ARUN
EC: B09B1/00; C04B28/00; (+3) IPC: B09B1/00;
C04B28/00; C04B28/34 (+9)
US 6204214
3-20-2001
METHOD OF WASTE STABILIZATION...
WAGH ARUN S; JEONG SEUNG-YOUNG
EC: IPC: B09B; C02F; C04B (+9)
ZA 9708254
6-10-1998
METHOD OF WASTE STABILIZATION VIA CHEMICALLY BONDED
PHOSPHATE CERAMICS, STRUCTURAL MATERIALS INCORPORATING
POTASSIUM PHOSPHATE CERAMICS
WAGH ARUN S (US); SINGH DILEEP
EC: B09B1/00; C04B28/00; (+3) IPC: B09B1/00;
C04B28/00; C04B28/34 (+10)
WO 9734848
9-25-1997
PHOSPHATE-BONDED STRUCTURAL PRODUCTS FROM HIGH VOLUME WASTES
SINGH DILEEP (US); WAGH ARUN S
EC: C04B28/00; C04B28/34; (+1) IPC: C04B28/00;
C04B28/34; C04B35/63 US 5846894
12-08-1998
QUICK-SETTING CONCRETE & METHOD FOR MAKING
WAGH ARUN S (US); SINGH DILEEP
EC: C04B7/36; C04B22/00H; (+1) IPC: C04B7/36;
C04B22/00; C04B40/00
US 5624493
4-29-1997
METHOD FOR STABILIZING LOW-LEVEL MIXED WASTES AT ROM
TEMPEPRATURE
WAGH ARUN S (US); SINGH DILEEP
EC: A62D3/00; C04B28/34; (+3) IPC: A62D3/00;
C04B28/34; G21F9/16
US 5645518
7-08-1997
CERAMICRETE STABILIZATION OF U-AND PU-BEARING MATERIALS
CA 2540287
9-02-2004
WAGH ARUN S (US); MALONEY DAVID D (US); THOMPSON GARY H (US)
Classification: - international: G21F9/04; C04B28/34; G21F9/16;
G21F9/28; C04B28/00; G21F9/04; G21F9/16; G21F9/28;- european:
C04B28/34
Application number: CA20042540287 20040218
Priority number(s): US20030448792P 20030218; WO2004US04885
20040218
Also published as: WO2004075207 // WO2004075207 // EP1597736 //
EP1597736 //
US Patent # 5,830,815
Method of Waste Stabilization via
Chemically Bonded Phosphate Ceramics
Arun S. WAGH, et al.
US Cl. 501/155
Abstract: A method for regulating the reaction
temperature of a ceramic formulation process is provided
comprising supplying a solution containing a monovalent alkali
metal; mixing said solution with an oxide powder to create a
binder; contacting said binder with bulk material to form a
slurry; and allowing the slurry to cure. A highly crystalline
waste form is also provided consisting of a binder containing
potassium and waste substrate encapsulated by the binder.
References Cited:
U.S. Patent Documents
3093593 ~ 3383228 ~ 3879211 ~ 3985567 ~ 5302565 ~ 5502268 ~
5645518
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for stabilizing large
volumes of waste, and more specifically, this invention relates
to a ceramic material to stabilize large volumes of low-level
radioactive and mixed wastes and a method for producing the
ceramic material.
2. Background of the Invention
Low-level mixed wastes contain hazardous chemical and low-level
radioactive materials. Generally, mixed waste streams contain
aqueous liquids, heterogeneous debris, inorganic sludges and
particulates, organic liquids and soils. The projected volume
over the next five years of the mixed waste generated by the
U.S. Department of Energy alone is estimated at approximately
1.2 million cubic meters.
Stabilization of these mixed wastes requires that both phases
of contaminants are stabilized effectively.
Typical approaches to stabilization and storage of these mixed
wastes include vitrification. For example, one process (Crowe,
U.S. Pat. No. 5,302,565) requires firing temperatures of at
least 1,850.degree. C. for at least 12 hours to produce ceramic
containers. However, such processes, associated with high
temperatures are costly. In addition, vitrification of waste
streams often result in the lighting off of volatile components
contained in the waste stream. This lighting off results in the
unwanted generation of secondary waste streams.
One system for producing cements having ceramic type properties
does not require high temperatures for final crystallization
(Sugama et al. U.S. Pat. No. 4,436,555, assigned to the instant
assignee). However, that process results in ammonia being
liberated during processing and storage, which leads to
container corrosion, and also explosive compositions if wastes
contain nitrates.
The inventors also have developed ceramic fabrication methods
to both stabilize and encapsulate waste. These methods offer a
number of advantages over typical portland cement grout-,
polymer- and ceramic-encapsulation techniques. Ceramic
encapsulation systems are particularly attractive given that the
bonds formed in these systems are either ionic or covalent, and
hence stronger than the hydration bonds in portland cement.
Since waste stabilization using ceramics is due to chemical
stabilization as well as physical encapsulation, the leaching
characteristics of these final waste forms are superior to the
above-identified waste forms which are mainly dependent on
physical encapsulation. Also, unlike prior vitrification
requirements, the exothermic ceramic formulation process needs
no thermal treatment or heat input, resulting in waste
stabilization being done economically on site and without
capital intensive equipment and transportation procedures.
However, exothermic ceramic formulation processes are not
suitable for the economic encapsulation of large amounts of
waste. The inventors have found that the production of large
amounts of heat during reaction causes the reacting solution to
boil, leading to flaws (i.e. pores) in the final ceramic form,
short workability time, and fast, uneven curing. While reaction
temperatures may be partially controlled by circulating cold
water around the slurry container or mold in which the sample is
setting, sufficient heat conduction is not present as sample
sizes increase.
Another drawback to typical ceramic waste production processes
is that such systems foster low pH conditions. For example,
acid-base ceramic encapsulation reactions begin in severe acidic
conditions, near pH 0. Such severe conditions destabilize HgS to
a leachable form prior to its physical encapsulation. Low pH
conditions also lead to CaCO.sub.3 decomposition.
A need exists in the art for a high volume waste stabilization
and solidification method that does not generate high amounts of
heat during the encapsulation process. The process must also be
operational at moderate pH conditions so as to facilitate
stabilization of wastes which are unstable at low pH. The final
product must exhibit low leachability and high durability in
aqueous systems.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome many of the
disadvantages of the prior art in the encapsulation and
stabilization of low-level, radioactive, mixed and other wastes.
Another object of the present invention is to provide a
temperature-controlled ceramics formation process to encapsulate
and stabilize wastes. A feature of the invention is the
utilization of readily available compounds to regulate the
acid-base reactions associated with the formation of ceramics
waste forms. An advantage of the invention is maintaining a low
temperature during the formation process.
Yet another object of the present invention is to provide a low
temperature reaction liquor in a process to stabilize mixed
waste using chemically bonded phosphate ceramics. A feature of
the present invention is the moderation of the pH of the
reaction liquor. An advantage of the present invention is that
the lower reaction temperatures facilitate the formation of more
dense waste forms. Another advantage is that certain waste
materials, which decompose or destabilize in low-pH environs,
are more completely stabilized.
Still another object of the present invention is to provide a
ceramic waste form high in potassium. A feature of the invention
is a high amount of crystalline phase in the final waste form.
An advantage of the invention is a more dense, less porous waste
form.
Briefly, the present invention provides for a method for
regulating the reaction temperature of a ceramic formulation
process comprising supplying a solution containing a monovalent
alkali metal; mixing said solution with an oxide powder to
create a binder; contacting said binder with bulk material to
form a slurry; and allowing the slurry to cure.
The invention also provides for a ceramic waste form comprising
a potassium containing ceramic binder and waste substrate
encapsulated by the binder.
BRIEF DESCRIPTION OF THE DRAWING
These and other objects and advantages of the present invention
will become readily apparent upon consideration of the following
detailed description and attached drawing, wherein:
FIG. 1 is a temperature graph showing the effects of the
addition of a carbonate solution to the ceramic processing
liquor, in accordance with the features of the present
invention;
FIG. 2 is a graph showing the compression strength of an
exemplary waste form, in accordance with the features of the
present invention; and
FIG. 3 is a graph depicting the porosity of an exemplary
ceramic form, in accordance with the features of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
This invention teaches two processes for chemically controlling
the reaction temperature in ceramic formulation processes. These
two processes allow for the formation of large final waste forms
for a wide variety of waste streams, said waste streams
containing ash, cement, silica, Bayer process wastes (red mud),
potliner residue, pyrophorics, salt mixtures, volatiles, such as
mercury, lead, cadmium, chromium, and nickel, and unstable
compounds which cannot be treated by conventional high
temperature techniques such as vitrification. The invention is
also applicable to stabilize secondary waste streams resulting
from thermal treatment processes, such as vitrification and
plasma hearth processes.
Radioactive materials are also stabilized by this method, such
materials including uranium, plutonium, thorium, americium,
fission products, and any other radioactive isotopes. Irradiated
lead, hazardous metals, flue-gas desulfurization residues are
also stabilized and/or encapsulated by the invented method.
The invention also can be used to stabilize certain RCRA
organics. The inventors have found that certain of these
organics do not retard the setting of phosphate ceramics. In one
scenario, organics such as naphthalene and dichlorobenzene are
trapped in activated carbon which in turn is stabilized in the
phosphate matrix by the method claimed herein. This method of
stabilization can be utilized in situations wherein mixed waste
contains trace amounts of organics such as polychlorinated
biphenyls, dioxin, dichlorobenzene, naphthalene, among others.
As such, the invented method is superior to encapsulation
methods wherein cement is utilized, in that cement cannot
stabilize in the presence of organics.
The method may also be used to stabilize and solidify wastes
containing salts, such as chlorides, nitrates, nitrides,
sulfites and sulfates. Conventional cement technology cannot
stabilize these waste streams.
Ash waste may be consolidated by this process to 80 volume
percent of its original volume. Experiments by the inventors
show good reaction and bonding between amorphous and reactive
silica from fly ash and bottom ash with phosphate matrix.
Formation of hard silico-phosphate bonds via this reaction can
be used for the stabilization of hazardous silica compounds such
as asbestos. The invention also encapsulates and stabilizes
silica based filter aids, such as vermiculites and perlites,
which are used in the removal of contaminants from liquid waste
streams.
The two invented temperature control processes yield
superior-strength final forms having uniform high density
throughout and improved microstructure compared to typical
methods of ceramics formation.
A salient feature of the low-temperature ceramic-waste
formulation processes is an acid-base reaction, such as that
depicted in Equation 1, below. Typically, the reaction produces
phosphate of MgO (Newberyite).
The acid base reaction results in the reaction of the waste
components with the acid or acid-phosphates. These reactions
lead to chemical stabilization of the waste. In addition,
encapsulation of the waste in the phosphate ceramics formed by
the reaction products results in physical encapsulation of the
waste components.
As noted supra, a problem with the above-disclosed reaction
sequence is the extremely low pH that exists in the reaction
liquor as a result of the presence of the phosphoric acid. This
low pH leads to destabilization of some waste materials during
encapsulation, and higher reaction temperatures which ultimately
renders weak final waste forms.
The two processes for minimizing the exothermicity of the
acid-base reactions are disclosed as follows: Process #1 deals
with pretreating phosphoric acid with a carbonate, bicarbonate
or hydroxide of a monovalent metal prior to mixing with an oxide
or hydroxide powder so as to buffer the acid. An exemplary
reaction for process #1 is illustrated in Equation 2, below:
whereby M is a monovalent metal which can be selected from the
group consisting of potassium, sodium, lithium. M'oxide
designates the oxide powder, whereby M' is a metal which can be
selected from the group consisting of Mg, Al, Ca, and Fe. As
noted above, M' also can be supplied as an hydroxide.
Process #2 discloses a method for bypassing the use of acid
altogether and mixing the oxide powder with a dihydrogen
phosphate to form a ceramic at a higher pH. Illustrations of
process #2 are Equations 3-5, below:
Solid Waste Preparation Detail
Solid wastes first can be manipulated in powder form by
grinding the waste to an average, preferable approximate
particle size of 8 to 10 micrometers (.mu.m). However, particles
can range in size from between approximately 5 .mu.m to several
millimeters.
Ash and cement wastes can be first mixed with the starter oxide
or hydroxide powders using a vibratory shaker, or any
conventional agitator. Weight percentages of the mixture varies
at this juncture, but can range from between approximately 15
percent oxide to 50 percent oxide. Typically, an even weight
percent (50:50) of oxide to solid waste is sought. However, the
inventors have successfully encapsulated and stabilized
single-component fly ash at weight percents as high as 85
percent ash to 15 percent MgO powder, which makes this technique
particularly attractive for utilities where single-component fly
ash is a major land-filling problem.
The above mixture of powders is then added to pretreated
phosphoric acid solution (process #1) or to the dihydrogen
phosphate solution (process #2) to form a reaction slurry. The
slurry is mixed using a mixer for 10 minutes to 30 minutes
during which it forms a viscous paste. The paste sets in a few
hours once poured into a mold. Typically, no pressure is applied
to the now-molded slurry. The slurry gains full strength in
approximately one day.
Mold shapes can vary, depending on the configuration of the
ultimate deposition site, and can be selected from a myriad of
geometrical shapes including cuboid, pyramidal, spherical,
planar, conical, cylindrical, trapezoidal, rectangular, and the
like. Generally, molds having the shape and size of a typical 55
gallon drum are used for waste management applications.
Liquid Waste Processing Detail
In dealing with liquid waste, the invented temperature
regulated encapsulation method provides a simplified approach
for an end user compared to more typical encapsulation methods.
For example, acid phosphates systems are made by adding said
phosphate to the liquid on site, a process similar to that
practiced in the cement industry. As such, liquid wastes, such
as tritiated water, are easily and economically encapsulated
with this procedure.
Either process #1 or process #2 can be used if solely liquid is
being encapsulated and stabilized. In process #1, the waste
liquid is first combined with acid to form a pH modified
solution. This modified solution is then mixed with oxide
powder. Alternatively, the waste liquid can be added to oxide
powder, to form a slurry, and then mix the slurry with acid.
In process #2, the liquid waste is mixed with dihydrogen
phosphate solution. Then, oxide powder is added. As above, an
alternative procedure is to first combine the liquid waste with
oxide powder and then add the dihydrogen solution.
The inventors have found that the ratio of acid to water,
selected from a range of between approximately 37:63 to 50:50,
produces good results. An acid:water ratio of 50:50 is most
preferred. If the liquid waste contains more than the required
amount of water, then correspondingly less water is added to the
acid to bring the water weight percent of the liquid waste-acid
mixture up to 50 percent.
In situations involving liquid-solid waste streams, the liquid
fraction of the waste stream can be prepared as outlined
directly above. The resulting liquid waste-acid mixture is then
mixed with a mixture of solid waste and oxide powder in weight
percent ranges similar to those outlined above for solid waste
processing. When using powder mixtures containing MgO and
dibasic phosphate, weight percent ratios of the oxide to the
phosphate selected from the range of approximately 87:13 to
77:23 produce good results.
Phosphate and Oxide Reactant Detail
Several phosphate systems can be used for the stabilization of
the target chemical, radioactive and mixed waste streams. Some
final phosphate-ceramic forms include, but are not limited to
phosphates of Mg, Mg--Na, Mg--K, Al, Zn and Fe, whereby the
metals are derived from starter oxide powders and hydroxide
powders (such as in process #1). In process #2, the metals in
the final phosphate ceramic forms are derived from both the
starter powders and the dihydrogen phosphates. Exemplary
dihydrogen phosphates used in process #2 include, but are not
limited to, phosphates of potassium, sodium and lithium. The
acid component may be concentrated or dilute phosphoric acid or
acid phosphate solutions such as dibasic or tribasic sodium or
potassium, or aluminum phosphates. The setting times for the
pastes formed by the reaction ranges from a few hours to a week.
The phosphates attain their full strength in approximately three
weeks.
Oxide powders can be pretreated for better reactions with the
acids. One technique includes calcining the powders to a typical
temperature of between approximately 1,200.degree. C. and
1,500.degree. C. and more typically 1,300.degree. C. The
inventors have found that the calcining process modifies the
surface of oxide particles in a myriad of ways to facilitate
ceramic formation. Calcining causes particles to stick together
and also form crystals; this leads to the slower reaction rates
that foster ceramic formation. Fast reactions tend to form
undesired powdery precipitates.
Another reaction enhancement technique is washing the powders
with dilute nitric acid and then water.
A myriad of oxide and hydroxide powders can be utilized to
produce the ceramic system, including but not limited to MgO,
Al(OH).sub.3, CaO, FeO, Fe.sub.2 O.sub.3, and Fe.sub.3 O.sub.4.
MgO and Al(OH).sub.3 powders are available through any
commercial supply house, such as Baxter Scientific Products,
McGaw Park, Ill. The myriad iron oxides enumerated above could
actually be supplied as part of some waste streams such as those
generated in conjunction with soil and also in low-temperature
oxidation systems which destroy organics using iron compounds.
Process #1--pH Modification of Acid Solution
Surprisingly and unexpectedly, the inventors have found that
when carbonate, bicarbonate, or hydroxides of monovalent metals
(such as K, Na, Li, and Rb) are used to pretreat the acid prior
to the acid-base reaction, a decrease in reaction temperature
results. Also unexpectedly, the inventors have found that the
addition of potassium containing alkali compounds (such as
K.sub.2 OO.sub.3) result in a more crystalline waste form that
is impervious to weathering, compressive forces and leaching.
Furthermore, and as can be determined in FIGS. 1-3, the higher
the concentration of potassium containing compounds (such as
K.sub.2 OO.sub.3, KHCO.sub.3, and KOH) in the pre-reaction
mixture, the more crystalline the final product. This high
crystallinity correlates to higher compression strength and
lower porosity.
The carbonate in the pretreatment process decomposes into
hydroxide, with an evolution of CO.sub.2. This results in a
partial neutralization of the acid, which in turn reduces the
rate of reaction and the rate of heat evolution. Typically, pH
of the reaction slurry is raised from zero to between
approximately 0.4 and 1.
Overheating of the slurry is thus avoided by this pH adjustment
mechanism. Second, and as more thoroughly disclosed infra, the
use of potassium carbonate generates more crystalline, and
therefore more stable, phosphate complexes.
EXAMPLE 1
K.sub.2 OO.sub.3 Buffer
5, 10 and 15 weight percent of potassium carbonate K.sub.2
OO.sub.3 was added to a 50 weight percent dilute solution of
phosphoric acid. The resulting solution was allowed to
equilibrate for several hours. In the equilibration process, the
pH of the solution raised from near zero to 0.4, 0.6 and 0.9,
respectively. After equilibration, 100 grams of the solution was
mixed with 50 grams of an oxide powder. The oxide powder was a
combination of calcined MgO and boric acid in a 85 weight
percent MgO to 15 weight percent boric acid ratio.
While adding the MgO and boric acid mixture to the acid
solution, the temperature of the slurry, for phosphate
concentrations ranging from 0 to 10 weight percent, was
monitored. FIG. 1 depicts the temperature rise in each case.
System A was a simulation of a process wherein no K.sub.2
OO.sub.3 was added. The maximum temperature reached in this
system was 45.degree. C. in a 50 cc volume sample. For samples B
and C made with 5 and 10 weight percent of K.sub.2 OO.sub.3, the
temperature rise was 8.degree. C. and 2.degree. C.,
respectively. No temperature increase was noted when 15 weight
percent of K.sub.2 OO.sub.3 was added to the acid prior to
reaction.
X-ray diffraction analysis of the samples showed high
crystallinity with samples made with 15 weight percent of
K.sub.2 OO.sub.3. Samples made with 5 and 10 weight percent of
K.sub.2 OO.sub.3 were more glassy. As can be noted in Table 1,
below, the X-ray diffraction studies of the samples identified
unique mineral phases that are responsible for the desired low
solubility product constant of the final product. This superior
final product, a chemically bonded composite ceramic, is
designated hereafter as MKHP.
TABLE 1 ______________________________________ Mineral
composition of Ceramic Developed Via K.sub.2 CO.sub.3 Addition
Mineral Phase Chemical formula weight %
______________________________________ Magnesium potassium
phosphate MgKPO.sub.4.6H.sub.2 O 52 Lunebergite Mg.sub.3 B.sub.2
(PO.sub.4).sub.2 (OH).sub.6.6H.sub.2 24 Newberyite
MgHPO.sub.4.3H.sub.2 O 14 Residual Magnesium Oxide MgO 10
______________________________________
Surprisingly and unexpectedly, magnesium potassium phosphate
(MKP) is a new component in the material that formed exclusively
by the addition of K.sub.2 OO.sub.3. MKP represents a superior
phase for waste form matrix materials, given its solubility
constant of 10.sup.-11, which is five magnitudes lower than that
of newberyite which is 10.sup.-6. All of the phases depicted in
Table 1 have very low solubilities in ground water, and
lunebergite and newberyite are natural minerals which are hence
stable in ground water environments.
Porosity characteristics of the samples varied widely. In the
K.sub.2 OO.sub.3 5- and 10-weight percent samples, the glass
phase of the samples was abundant, with a concomitant higher
amount of cracking and therefore porosity. By comparison, the
K.sub.2 OO.sub.3 15 weight percent samples showed an open
porosity of approximately 6.1 percent. Density was 1.77 g/cc,
and closed porosity was 10.2 volume percent. Compression
strength was approximately 3,700 psi.
EXAMPLE 2
K.sub.2 OO.sub.3 Buffer+Fly Ash
The matrix material disclosed in Example 1 was used in Example
2. Starter powder composition was 70 weight percent fly ash,
25.5 weight percent calcined MgO, and 4.5 weight percent boric
acid. The solution used was a 50 weight percent diluted H.sub.3
PO.sub.4 buffered with K.sub.2 OO.sub.3. The solution was poured
into a mixer, such as a cement mixer, and the powder was slowly
added until all the powder was mixed with the solution in
approximately 48 minutes. A cylindrical sample of 1,000 ml was
made.
The maximum temperature during mixing and setting ranged from
between approximately 50.degree. C. and 60.degree. C. These
temperatures did not increase, even when smaller weight percents
of K.sub.2 OO.sub.3 were used. For example, when 10 weight
percent of K.sub.2 OO.sub.3 was used, even at higher volumes
(1,200 cc), maximum temperatures attained were between
56.degree. C. and 58.degree. C.
The inventors have found that in the absence of K.sub.2
OO.sub.3, the concentration of MgO in the final product is high,
with Newberyite as the main crystalline phase in the material.
Data on compression strength and porosity of the materials made
in Example 2 are shown in FIGS. 2 and 3. These figures show that
as the content of K.sub.2 OO.sub.3 increases, the strength
increases and the porosity drops. When K.sub.2 OO.sub.3 is 15
weight percent in the solution, the compression strength is
8,750 psi (which is more than twice that of portland cement)
while porosity is reduced to 7.5 percent.
EXAMPLE 3
Sodium Carbonate
5, 10, and 15 weight percent of sodium carbonate (Na.sub.2
OO.sub.3) was added to 50 weight percent of a dilute solution of
phosphoric acid and the resulting solution was allowed to
equilibrate for several hours. The pH of the solution was raised
in the process from near zero to approximately 2.3. 100 grams of
this solution was reacted with 30 grams of a mixture of calcined
MgO and boric acid (85 weight percent MgO and 15 weight percent
boric acid) and 70 grams of fly ash.
The properties of the 5 weight percent Na.sub.2 OO.sub.3 sample
were measured. Density was 1.7 g/cc and its open porosity was
8.6 volume percent. Microstructural analysis of the samples
revealed that the sample was primarily glassy except for the fly
ash particles. This process shows that completely glassy phase
material can be made by the process described above.
EXAMPLE 4
Hazardous Material+MKHP
Two different hazardous material waste streams were treated. An
iron oxide-iron chloride waste stream (95 weight percent
Fe.sub.2 O.sub.3 +5 weight percent FeCl.sub.3) was spiked with
0.5 weight percent of Ce.sup.3+ and Ce.sup.4+ as surrogates of
U.sup.3+,.sup.4+ and Pu.sup.3+,.sup.4+, incorporated as oxide.
Also added was 0.5 weight percent of Ce.sup.4+ as a surrogate of
U.sup.4+ and Pu.sup.4+, incorporated as oxide.
The second waste stream was iron phosphate waste stream
(FePO.sub.4) spiked with 0.5 weight percent of Pb to represent
hazardous component, introduced as soluble nitrates.
Both waste streams were stabilized via the carbonate
modification method of Process #1. Containment of Ce.sup.3+,
Ce.sup.4+ and Pb was 8.7 ppm, <0.09 ppm and <0.2 ppm,
respectively. In as much as the 5 ppm regulatory limit on Pb is
due to be revised downward to 0.37 ppm, the results show that
the invented encapsulation procedure provides an acceptable
method of containment.
Process #2--Dihydrogen Phosphate
Instead of adding carbonate to reactants to reduce reaction
temperatures, the inventors have devised a simplified method to
achieve the same results. This second process reacts dihydrogen
phosphates of potassium, sodium, lithium, or any other
monovalent alkali metal with an oxide to form a phosphate
ceramic. This method forms a ceramic at higher pH while
minimizing heat generation. An exemplary ceramic formed via this
process is magnesium potassium phosphate hexahydrate (MKP),
which is formed via the reaction mechanism depicted in Equation
5, above.
The inventors found that with the avoidance of acid in the
initial reaction slurry, initial pH values are approximately
6.2. Consistent with the fact that the dissolution of KH.sub.2
PO.sub.4 is an endothermic process, the inventors found that at
initial mixing, the temperature of the slurry slightly
decreases. As the dissolution and reaction of MgO progresses,
however, slurry temperatures increase to approximately
30.degree. C.
EXAMPLE 5
MKP Ceramic Fabrication
One mole of calcined and ground MgO was mixed with one mole of
ground potassium dihydrophosphate (KH.sub.2 PO.sub.4) crystals.
The mixture was slowly added to 5 moles of water to form a
paste. When the paste was well mixed, it was poured into
cylindrical molds, of 1 cm in diameter and 20 cc volume. Hard
ceramic forms developed in approximately 1 hour.
X-ray diffraction analysis revealed that all major peaks were
MKP. No peaks of the potassium dihydrophosphate were noted,
indicating that it all reacted.
Open porosity, measured by the water intrusion method, was
calculated as 2.87 volume percent. Density was 1.73 g/cc. Given
a theoretical density of 1.88 g/cc, the total porosity is
calculated to be 8.19 volume percent. Thus, closed porosity
(i.e., that porosity that is not accessible from outside the
sample) was 5.33 g/cc.
These values show that MKP is much denser than Mg-phosphate
ceramic, wherein total porosity is approximately 30 percent.
EXAMPLE 6
MKP+Fly Ash
MKP ceramic synthesized in Example 5 was used to develop waste
forms of fly ash. Samples were made using three different
powders which are mixtures of calcined MgO and KH.sub.2 PO.sub.4
in mole ratios of 1:1, 1.5:1 and 2:1. These powders were mixed
with fly ash in equal weight proportions using a hopper and
feeder mechanism. The final mixtures of the powders were
combined at a slow but constant rate with 5 moles of water in a
cement mixer to form a slurry.
The slurry was poured into 1.5 gallon molds as well as 1 liter
molds. Smaller samples were made by stirring the powders in
water and using 1 cm diameter, 20 cc cylindrical molds. All
samples set in approximately 1 hour and hardened fully after one
week.
Unlike the material described in Examples 1 and 2, the
temperature of the slurry does not rise during mixing, but only
during setting. This eliminates the prior art problem of
evaporation of contaminants that occurs as a result of heat
generation during the mixing stage. The inventors found that
temperatures of the slurry before setting generally do not
exceed 30.degree. C. As such, no evaporation of either the water
fraction or the components of the waste occur. Once the slurry
starts setting, the temperature rises. However, maximum
temperatures (approximately 75.degree. C.) are reached after the
sample sets into a hard monolith, thereby not resulting in any
detrimental effect on the final waste form. Furthermore, the
inventors have found that the temperature rise is not
proportional to the size of specimens but in fact tapers off as
the specimen size is increased. This facilitates the target
waste encapsulation sizes of 55 gallons.
The invented process utilizing MKP generates superior final
ceramic forms. Open porosity values of the waste forms was found
to be approximately 4.18 volume percent. Measured density was
1.8 g/cc. Given the estimated theoretical density of 2.05 g/cc,
the total porosity is 8.9 volume percent, which is much lower
than Mg-phosphate ceramic found in the prior art. Closed
porosity was calculated as 4.72 volume percent. Compression
strength of the sample was 6,734 psi, which is more than 50
percent stronger than portland cement concrete.
EXAMPLE 7
Boric Acid+MKP+Ash
Samples of fly ash waste forms were made with MKP matrix and
calcined MgO powder in which from 0-5 weight percent boric acid
was added. The addition of boric acid delayed the temperature
rise of the reacting slurry. Therefore, the addition of boric
acid facilitates the large scale processing of waste streams
where more time is needed to mix and pour the slurry.
EXAMPLE 8
CaCO.sub.3 Stabilization
As noted supra, CaCO.sub.3 decomposes in low pH environs. As a
result, if waste streams contain this compound, carbon dioxide
is produced which bubbles from the reaction slurry. Such
bubbling makes the set product porous and hence permeable to
ground water. Strength is also compromised.
Cement sludge, typical of cement-containing waste streams was
prepared. The composition of the waste stream is depicted in
Table 2, below:
TABLE 2 ______________________________________ Cement Waste
Stream Composition Component Weight Percent
______________________________________ Activated Carbon 10 Fly
Ash 10 Water 10 Concrete 50 Plaster of Paris 10 Haematite
(Fe2O3) 3 Alumina 3 Perlite 1.5
______________________________________
Samples were made by two methods. In the first method, slurry
was formed with H.sub.3 PO.sub.4 as the reacting acid and waste
forms containing approximately 30 volume percent of waste were
fabricated. During this first process, the slurry formed tiny
bubbles of CO.sub.2, which made the samples porous.
When samples were made with the MKP process disclosed in
Examples 5 and 6, supra, wherein KH.sub.2 PO.sub.4 was used as
the acid phosphate, no evolution of CO.sub.2 occurred.
Comparison of the sample values are presented in Table 3, below:
TABLE 3 ______________________________________ Physical
properties of chemically bonded waste forms. Parameter H.sub.3
PO.sub.4 Stabilization KH.sub.2 PO.sub.4 Stabilization
______________________________________ pH of acid soln. 0.2 4
Open porosity 28-33 6.2 (volume percent) Density 1.2-1.3 1.77
______________________________________
Table 3 shows that the waste forms generated via the MKP
process are denser and contain relatively small amounts of open
porosity, thereby illustrating the superiority of the invented
process compared to processes whereby large amounts of acid are
utilized.
EXAMPLE 9
Red Mud+MKP
The refining of bauxite to produce aluminum oxides results in
the production of large amounts of residue, known as red mud.
Red mud consists of 50 percent inorganic oxides, other compounds
and hazardous metals. Tremendous amounts of red mud are
generated annually.
Large volumes of red mud are easily stabilized when combined
with the invented phosphate ceramic binder. If the reaction
slurry, loaded with red mud, is poured as a barrier layer, it
not only bonds with the substrate soil but also enters fissures
in the soil and quickly hardens to form a nonporous ceramic
layer. For example, red mud ceramics produced by the invented
process exhibit low porosities (.apprxeq.2 volume percent) and
high compression strengths (4,944 psi). The materials display a
low porosity and high durability in a range of acid and basic
environments, thereby making them ideal for mining industry
applications, pond liners, tailing liners, waste pond dikes, and
quick-setting grouts.
A myriad of red mud waste can be utilized in producing final
structural forms. Red mud waste used by the inventors was
produced from gibbsitic bauxite. Essentially, it was dry mud
collected from the periphery of a red mud waste pond. Its
contents were .apprxeq.50 weight percent iron oxide (Fe.sub.2
O.sub.3), .apprxeq.16.5 weight percent alumina (Al.sub.2
O.sub.3), .apprxeq.3 weight percent silica (SiO.sub.2),
.apprxeq.5.7 weight percent calcium oxide (CaO), and
.apprxeq.6.8 weight percent titania (TiO.sub.2). X-ray
diffraction analysis identified haematite (.alpha.-Fe.sub.2
O.sub.3), goethite (.alpha.-FeOOH), calcite (CaCO.sub.3),
boehmite (.gamma.-AlOOH), anatase (TiO.sub.2), and bayerite
(.beta.-AlOOH) as the major crystalline phases. Surprisingly and
unexpectedly, the inventors have learned that the hydrated
phases, i.e., boehmite, bayerite and goethite, facilitate the
development of phosphate bonds in the binding process.
The amorphous characteristics of the alumina and silica
components of red mud, which is discussed above, plays a major
role in the ceramic bonding mechanisms. The inventors have found
that it is the characteristically smaller particles of amorphous
material that readily participates in the acid-base reaction and
therefore facilitates the setting reaction during ceramic
formation.
As discussed supra, MgO, when reacted with phosphoric acid or
an acid phosphate solution, forms magnesium phosphate
precipitate in an exothermic reaction. This reaction can be
controlled by use of calcined MgO and also by adjusting the feed
rate of the solid phase (i.e., the red mud powder+oxide+boric
acid) to the solution.
In one embodiment, calcined MgO first is mixed with red mud
powder in a specific weight percent, disclosed in Table 4,
below. Crushed dry red mud is a super-fine material with more
than 60 weight percent of the particles finer than 10 mm.
Particle sizes ranging from between 1 and 5 mm provide good
results, so that grinding of the mud may not always be necessary
prior to combining with the MgO.
TABLE 4 ______________________________________ Physical
Properties of Red Mud Ceramics Waste Maximum Open Compression
loading particle size Density Porosity Strength (wt %) (mm)
(g/cm.sup.3) (%) (psi) ______________________________________ 40
5 2.19 0.82 4944 40 1 2.1 1.09 4294 50 5 2.26 2.98 2698 55 5
2.29 1.94 2310 ______________________________________
The dry mixture is then reacted with the phosphoric acid or an
acid phosphate solution via constant stirring. This results in a
low-viscosity paste which thickens as the reaction proceeds. The
paste is then poured into cylindrical molds of 1.9 cm in
diameter. Dense ceramics form in approximately 15 minutes, with
complete hardening occurring in 2 to 24 hours. Prior to testing,
the samples were stored for three weeks.
Density was measured by weighing the samples and measuring the
dimensions and determining the volume. Open porosity was
determined by water immersion in which the pre-weighed samples
were immersed in water at 70.degree. C. for 2 hours. The samples
were then cooled in the water and then removed from the water.
Excess water was wiped from the surface of the samples and the
samples were weighed again to determine the amount of water that
filled the open pores. This higher weight (compared to
pre-immersion weights) yielded the volume of the open pores in
the samples, thereby allowing for calculation of the open
porosity.
Compression strength was measured with an Instron.TM. machine
used in compression mode.
Waste loadings of the samples ranged from 40 to 55 weight
percent. As an example, a 40 weight percent waste loading means
that 40 grams of a sample is red mud and 60 grams is both binder
and water. Densities of samples with red mud are slightly higher
than that of pure matrix (binder) material, which is 1.73
gm/cm.sup.3. Red mud density is approximately 3.3 g/cm.sup.3.
As can be determined in Table 2, the open porosity of the
red-mud-loaded samples was low compared to the .apprxeq.20
percent value seen in cement. Furthermore, the compression
strength of the samples with 40 weight percent loading was found
to be higher than the 4,000 psi value for portland cement
concrete.
Scanning electron microscopy (SEM) and energy dispersive X-ray
(EDX) analyses of a fractured red-mud sample revealed a glassy
region and a granular region. Both of these regions are well
bonded. The glassy phase was cracked everywhere while the
granular phase displayed only those cracks emanating from the
glassy phase. Table 5, below, provides the general elemental
composition of each phase. The values contained therein are
averages of three measurements taken at three different
locations of each phase.
The granular phase is attributable to the red mud and the
glassy phase is mostly the phosphate matrix. As shown, the
granular phase was found rich in red-mud elements such as Fe and
Al, while the glassy phase is rich in Mg phosphate elements such
as Mg and P.
TABLE 5 ______________________________________ Elemental
distributions of glassy and granular phases of red-mud ceramics
Elements Phase Fe Al Mg P Other
______________________________________ Glassy 5.78 4.7 23.53
34.23 31.76 Granular 23.66 18.6 5.3 16.2 36.2
______________________________________
That significant amounts of phosphate and some magnesium are
also in the granular phase indicates that phosphate binding
occurred here with Fe and Al as the cations.
The relatively few cracks seen in the granular phase (red mud)
portion of the samples indicates that the strength of the
invented material is due to this phase. Improving the strength
of the ceramic therefore entails reducing the amount of the
glassy phase or reinforcing the glassy phase with particulates.
One method for such reinforcement is to incorporate finer red
mud in the starter powder so as to facilitate more consistent
distribution and better particle reinforcement. Grinding the red
mud prior to mixing with the oxide powder is one way to obtain
this finer red mud material.
While the invention has been described with reference to
details of the illustrated embodiment, these details are not
intended to limit the scope of the invention as defined in the
appended claims.
For example, in as much as process #2 utilizes KH.sub.2
PO.sub.4, and in as much as KH.sub.2 PO.sub.4 has components of
common fertilizer, i.e. potash (K.sub.2 O) and phosphate
(P.sub.2 O.sub.5), process #2 makes it possible to use high
potash and high phosphate fertilizer to stabilize soils
containing contaminants.
Also, given that red mud contains high concentrations of
oxides, it is feasible to mix red mud with MKP to generate the
stable waste forms otherwise generated using process #2. This
modification precludes the need for supplying and pretreating
oxides, such as MgO in red mud stabilization procedures.
US Patent # 5,645518
Method for Stabilizing Low-Level Mixed
Wastes at Room Temperature
July 8, 1997
Abstract
A method to stabilize solid and liquid waste at room
temperature is provided comprising combining solid waste with a
starter oxide to obtain a powder, contacting the powder with an
acid solution to create a slurry, said acid solution containing
the liquid waste, shaping the now-mixed slurry into a
predetermined form, and allowing the now-formed slurry to set.
The invention also provides for a method to encapsulate and
stabilize waste containing cesium comprising combining the waste
with Zr(OH).sub.4 to create a solid-phase mixture, mixing
phosphoric acid with the solid-phase mixture to create a slurry,
subjecting the slurry to pressure; and allowing the now
pressurized slurry to set. Lastly, the invention provides for a
method to stabilize liquid waste, comprising supplying a powder
containing magnesium, sodium and phosphate in predetermined
proportions, mixing said powder with the liquid waste, such as
tritium, and allowing the resulting slurry to set.
US Cl. 588/318; 501/155; 588/18; 588/256; 588/404;
588/405; 588/406; 588/408; 588/409
International Class: A62D 3/00 (20060101); C04B 28/34
(20060101); C04B 28/00 (20060101); G21F 9/30 (20060101); G21F
9/16 (20060101
References Cited
U.S. Patent Documents
4049462 ~ 4351749 ~ 4436555 ~ 4460500 ~ 4620947 ~ 5198190 ~
5202033 ~ 5246496 ~ 5302565
Other References:
Low-Temperature-Setting Phosphate Ceramics for Stabilizing DOE
Problem Low-Level Mixed Waste (Part I -Material & Waste Form
Development -Dileep Singh, Arun S. Wagh and Lerry Knox) and Part
II Low-Temperature-Setting Phosphate Ceramics for Stabilizing
DOE Problem Low-Level Mixed Waste (Performance Studies on Final
Waste Foms) -Arun S. Wagh, Dileep Singh, Manish Sutaria, and
Sara Kurokawa -Proceedings of Waste Management 94
Conference-Tucson, AZ -Feb. 17-Mar. 3 1994-26 pages. .
Stabilization of Low Level Mixed Waste In Chemically Bonded
Phosphate Ceramics -Arun s. Wagh; Dileep singh and J. Cunnane
-Spectrum 1994 "Nuclear and Hazardous Waste Management
International Topical Meeting"-Atlanta, GA.-Aug. 14-18 -4 pages.
.
Phosphate-Bonded Ceramics as Candidate Final-Waste-Form
Materials -D. Singh, A.S. Wagh, J. Cunnane, M. Sutaria & S.
Kurokawa -Proceedings of 96th Annual Meeting of the American
Ceramic Society, Indianapolis, IN, -April 24-28, 1994 -11 pages.
.
Low-Temperature-Setting Phosphate Ceramics for Mixed Waste
Stabilization -Arun S. Wagh & Dileep Singh -Proceedings of
Second International Symposium and Exhibition on Environmental
Contamination in Central and Eastern Europe, Budapest, Hungary,
Sep. 20-23 1994 -12 pages. .
Conner, Jesse R., Chemical Fixation And Solidification of
Hazardous Wastes, Van Nostrand Reinhold, 1990, p. 299-303.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a material to stabilize waste and a
method of producing the material, and more specifically, this
invention relates to a ceramic material to stabilize low-level
mixed wastes and a method for producing the ceramic material.
2. Background of the Invention
Low-level mixed wastes contain hazardous chemical and low-level
radioactive materials. Generally, mixed waste streams contain
aqueous liquids, heterogeneous debris, inorganic sludges and
particulates, organic liquids and soils. The projected volume
over the next five years of the mixed waste generated by the
U.S. Department of Energy alone is estimated at approximately
1.2 million cubic meters.
Stabilization of these mixed wastes requires that both phases
of contaminants are stabilized effectively.
Typical approaches to stabilization and storage of these mixed
wastes include vitrification. For example, one process (Crowe,
U.S. Pat. No. 5,302,565) requires firing temperatures of at
least 1850.degree. C. for at least 12 hours to produce ceramic
containers. However, such processes, associated with high
temperatures are costly. In addition, vitrification of waste
streams often result in the lighting off of volatile components
which often are contained in the waste stream. This lighting off
results in the unwanted generation of secondary waste streams.
One system for producing cements having ceramic type
properties, does not require high temperatures for final
crystallization (Sugama et al. U.S. Pat. No. 4,436,555, assigned
to the instant assignee). However, that process results in
ammonia being liberated during processing and storage, which
leads to container corrosion.
A need exists in the art for a low level waste encapsulation
technology that connotes relatively high strength and low
porosity to the final product, and which also sets up at low
temperatures. The final product must exhibit low leachability
and high durability in aqueous systems.
SUMMARY OF THE INVENTION
An object of the present invention is to overcome many of the
disadvantages of the prior art in the encapsulation and
stabilization of mixed low-level wastes.
Another object of the present invention is to provide a ceramic
to encapsulate and stabilize mixed low-level wastes. A feature
of the invention is the utilization of readily available
materials. An advantage of the invention is the low cost of
production of these ceramics.
Yet another object of the present invention is to provide a
method for using chemically bonded phosphate ceramics to
stabilize mixed waste forms. A feature of the present invention
is the fabrication of these ceramics at room temperatures. An
advantage of the present invention is that the low temperature
setting characteristics of these ceramics makes them suitable
for stabilization of mixed wastes containing volatile compounds,
without the generation of secondary waste streams.
Still another object of the present invention is to provide a
method for producing phosphate ceramics for use as waste
stabilizers. A feature of the present invention is the
fabrication of these ceramics via acid-base reactions between an
inorganic oxide and a phosphate-containing acid solution. An
advantage of the present invention is the ability to treat both
solid and liquid wastes, while also obviating the need for high
temperature vitrification processes, and therefore reducing the
costs of final waste forms production.
Another object of the present invention is to provide for a
method to minimize bulk of final waste forms. A feature of the
present invention is the utilization of liquid and/or solid
waste fractions to produce chemically bonded ceramic forms. An
advantage of the invention is the economic and environmental
savings of bulk reduction.
Briefly, the present invention provides for a method to
stabilize solid and liquid waste at room temperature comprising
grinding the solid waste to a predetermined particle size,
combining the now ground solid waste with a starter oxide to
obtain a powder, contacting the powder with an acid solution to
create a slurry, said acid solution containing the liquid waste,
mixing the slurry while maintaining the slurry below a
predetermined temperature, shaping the now-mixed slurry into a
predetermined form, and allowing the now-formed slurry to set.
The invention also provides for a method to encapsulate and
stabilize waste containing cesium comprising combining the waste
with Zr(OH).sub.4 to create a solid-phase mixture, grinding said
solid-phase mixture, mixing phosphoric acid with the solid-phase
mixture to create a slurry, shaping the now-mixed slurry into a
predetermined form, subjecting the now-shaped slurry to
pressure; and allowing the now pressurized slurry to set.
In addition, the invention provides for a method to stabilize
liquid waste comprising supplying a powder containing magnesium,
sodium and phosphate in predetermined proportions, mixing said
powder with the liquid waste to produce a slurry, forming the
slurry into a predetermined shape, and allowing the now-shaped
slurry to set.
BRIEF DESCRIPTION OF THE DRAWING
These and other objects and advantages of the present invention
will become readily apparent upon consideration of the following
detailed description and attached drawing, wherein:
FIG. 1 is a schematic diagram of a method for producing
ceramic waste forms, in accordance with the features of the
present invention;
FIG. 2 is a graph depicting weight changes in an
exemplary waste form during immersion, in accordance with the
features of the present invention;
FIG. 3 is a graph depicting variation of pH in immersion
water for an exemplary waste form, in accordance with the
features of the present invention; and
FIG. 4 is a graph depicting variation in compression
strength for an exemplary waste form before and after immersion,
in accordance with the features of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
This invention addresses the need to develop benign final waste
forms for a wide variety of waste streams, said waste streams
containing pyrophorics, volatiles, such as mercury, lead,
cadmium, chromium, and nickel, and other unstable compounds
which cannot be treated by conventional high temperature
techniques such as vitrification. The invention is also
applicable to stabilize secondary waste streams resulting from
thermal treatment processes, such as vitrification and plasma
hearth processes.
The invention also can be used to stabilize certain RCRA
organics. The inventors have found that certain of these
organics do not retard the setting of phosphate ceramics. In one
scenario, organics such as naphthalene and dichlorobenzene are
trapped in activated carbon which in turn is stabilized in the
phosphate matrix by the method claimed herein. This method of
stabilization can be utilized in situations wherein mixed waste
contains trace amounts of organics such as polychlorinated
biphenyls, dioxin, dichlorobenzene, naphthalene, among others.
As such, the invented method is superior to encapsulation
methods wherein cement is utilized, in that cement cannot
stabilize in the presence of organics.
Ash waste may be consolidated by this process to 80 volume
percent of its original volume. Experiments by the inventors
show good reaction and bonding between amorphous and reactive
silica from fly ash and bottom ash with phosphate matrix.
Formation of hard silico-phosphate bonds via this reaction can
be used for the stabilization of hazardous silica compounds such
as asbestos. The invention also encapsulates and stabilizes
silica based filter aids, such as vermiculites and perlites,
which are used in the removal of contaminants from liquid waste
streams.
The inventors have identified a number of phosphate systems,
which form into hard ceramics via chemical bonding, that can
stabilize contaminants by both chemical and physical means. The
process needs no thermal treatment, resulting in waste
stabilization being done economically on site and without
capital intensive equipment and transportation costs.
The room-temperature setting phosphate ceramic waste forms are
formulated by using a route of acid-base reactions. Oxides or
hydroxides of various elements are used as starter powders for
this purpose. Said oxides and hydroxides chemically react with
phosphoric acid or soluble acid phosphates to form ceramics.
When waste is mixed with these powders or acid components, the
waste also participates to form various stable phases in the
final reaction product which then may be set into ceramic waste
forms.
In one instance, the acid-base reaction results in the
formation of the phosphate of MgO (Newberyite) via the following
equation:
The acid base reaction also results in the reaction of the
waste components with the acid or acid-phosphates. These
reactions lead to chemical solidification of the waste. In
addition, encapsulation of the waste in the phosphate ceramics
formed by the reaction products results in physical
encapsulation of the waste components.
Several advantages in phosphate waste forms exist viz portland
cement grout or polymer encapsulation techniques. Since the
stabilization in chemically bonded ceramics is due to chemical
solidification as well as physical encapsulation, the leaching
characteristics of these final waste forms is superior to the
above-identified waste forms which are dependent on only
physical encapsulation. The setting in chemically bonded
ceramics can occur in a wide pH range and hence is not very
sensitive to pH of the waste. Further, setting reactions are not
sensitive to ambient temperatures and hence open-field
stabilization in cold climates is possible.
The bonds are either ionic or covalent in phosphate ceramics,
and hence they are stronger than the hydration bonds in portland
cement thereby providing better strength to the final product.
The results on chemical stabilization presented infra are very
general, given that in these acid-base systems, the acid
phosphates seem to react with the contaminants irrespective of
the bulk composition of the waste stream. This translates into a
wide application in the stabilization of different types of
waste streams.
Several phosphate systems can be used for the stabilization of
chemical, radioactive and mixed waste streams. These include,
but are not limited to, phosphates of Mg, Mg--Na, Al, Ca, Fe and
Zr. The acid component may be concentrated or dilute phosphoric
acid or acid phosphate solutions such as dibasic or tribasic
sodium or aluminum phosphates. The reactions are exothermic and
require no external heat treatment. However, the paste-setting
reactions can be controlled either by the addition of boric acid
to reduce the reaction rate, or by controlling the rate of
addition of powder to the acid while concomitantly controlling
the temperature of the reaction vessel. The setting times for
the pastes formed by the reaction ranges from a few hours to a
week. The phosphates attain their full strength in approximately
three weeks.
The various invented systems include those outlined in Table 1,
below:
TABLE 1 ______________________________________ Phosphate
Systems and Processing Details Starting System Materials
Solution Curing Time ______________________________________ Mg
phosphate Calcined MgO Phosphoric >8 days acid-water (50/50)
Mg--NH.sub.4 phos- Crushed dibasic Water 21 days phate NH.sub.4
phos- phate crystals mixed w. cal- cined MgO Mg--Na phos-
Crushed dibasic Water 21 days phate Na phosphate crystals mixed
with calcined MgO Al phosphate Al(OH).sub.3 Phosphoric acid
Reacted pow- powder (.apprxeq.60.degree. C.) der, pressed Zr
phosphate Zr(OH).sub.4 Phosphoric acid 21 days powder
______________________________________
The five systems disclosed in Table 1 were chosen for ready
availability of materials, low cost, and availability of
literature on the materials.
Three waste streams were selected for treatment: ash waste,
contaminated cement sludge, and salts dominated by carbonates.
Each of the waste streams differs in their bulk composition,
with the bulk compositions forming nearly 91 weight percent of
the total waste streams. Representative bulk constituents for
the ash waste stream sample include activated carbon, fly ash,
coal ash and vermiculite. 3Representative bulk constituents for
the salt waste include activated carbon, Na.sub.2
(CO.sub.3).sub.2, widely used cation or anion exchange resins
(such as Purolite), water, NaCl, Na(NO.sub.3).sub.2, Na.sub.3
PO.sub.4, Na.sub.2 SO.sub.4. Representative bulk constituents
for the cement sludge waste include activated carbon, fly ash,
water, concrete, Plaster of Paris, Haematite (Fe.sub.2 O.sub.3),
alumina and perlite.
Each of these waste streams was spiked to a level of
approximately 0.5 weight percent, with heavy metals, said metals
added in the form of soluble nitrates. Total nitrate content was
approximately 7 weight percent of the total waste. The heavy
metals included Cr, Ni, Pb, and Cd. The invented method can
stabilize and encapsulate high heavy metal concentrations (for
example, exceeding 2 percent); however, most metal
concentrations encountered are one percent or less, with
economics dictating the reclamation of anything above one
percent.
While a myriad of salts are encountered during waste
processing, the above identified metals were added for
demonstration purposes via solubilization to the following RCRA
nitrates:
and
RCRA organics added to the three waste streams included
naphthalene (C.sub.10 H.sub.8), and dichlorobenzene (C.sub.6
H.sub.4 Cl.sub.2). These organics were added to see their effect
on the stabilization process. A radionuclide surrogate CsCl was
also added to each of the three waste mixtures.
The heavy metals, organics and radionuclide examples disclosed
above are meant to serve merely as representative of the variety
of such compounds that could be encapsulated and stabilized,
given the myriad of mixed waste scenarios to which the invented
product and method could be applied.
Oxide and Hydroxide Preparation
Oxide powders can be pretreated for better reactions with
acids. One technique includes calcining the powders to a typical
temperature of between approximately 1,200.degree. C. and
1,500.degree. C. and more typically 1,300.degree. C. Another
reaction enhancement technique is washing the powders with
dilute nitric acid and then water. A myriad of oxide and
hydroxide powders can be utilized to produce the ceramic system,
including but not limited to MgO, Al(OH).sub.3, CaO, FeO,
Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 and Zr(OH).sub.4.
MgO and Al(OH).sub.3 powders are available through any
commercial supply house, such as Baxter Scientific Products,
McGaw Park, Ill. Zr(OH).sub.4 is obtained through Atomergic
Chemetals Corporation, Farmingdale, N.Y. The myriad iron oxides
enumerated above could actually be supplied as part of the waste
stream to be stabilized and encapsulated.
Waste Processing Detail
Both solid and liquid wastes are treated by the invented
process of using acid-base reactions. An exemplary process is
depicted in FIG. 1., designated generally as numeral 10. Solid
waste 12, such as contaminated ash, cement, medical waste, may
first be crushed and mixed with the starter powder, 14, (Table
1, "Starting Materials"), via a vibratory mixer, 16. The
resulting mixture is then reacted with the solution, 18, (Table
1, "Solution") to form a slurry or paste. If the waste is a
liquid, 20, said liquid 20 is mixed with the solution 18 in a
standard splash mixer 22 and then the mixture is reacted with
the powder-mix in a reaction chamber 24.
After reaction, the reaction liquor is allowed to stabilize in
a stabilization unit 26. The setting or stabilization step 26
takes from a few hours to a week. Final waste forms 28 are
obtained in approximately 3 weeks.
Solid Waste Processing:
In the solid waste processing scenarios depicted in Table 1,
the waste streams were manipulated in powder form by grinding
the waste to an average, preferable approximate particle size of
8 to 10 micrometers (.mu.m). However, particles can range in
size from between approximately 4-75 .mu.m.
Ash and cement wastes can be first mixed with the starter oxide
or hydroxide powders using a vibratory shaker, or any
conventional agitator. Weight percentages of the mixture varies
at this juncture, but can range from between approximately 15
percent oxide to 50 percent oxide. Typically, an even weight
percent (50:50) of oxide to solid waste is sought. However, the
inventors have successfully encapsulated and stabilized
single-component fly ash at weight percents as high as 85
percent ash to 15 percent MgO powder, which makes this technique
particularly attractive for utilities where single-component fly
ash is a major land-filling problem.
Unlike ash and cement waste, salt waste is reacted first with
phosphoric acid to consume the CO.sub.2 that is formed via the
reaction depicted in equation 2, below, and then mixed with the
starting powder. Otherwise, the evolution of CO.sub.2 gas during
stabilization results in very porous ceramic waste forms being
produced.
In all three processing scenarios, the resulting powders are
slowly added to the respective acid solution and thoroughly
mixed using a mixer. Mixing can occur at ambient temperatures.
Typically, the rate of powder addition to the acid solution
should result in the reaction liquor being maintained at less
than 100.degree. C. Typical times required for controlled mixing
range from 30 minutes to 1 hour. Mixing times can be shortened
if the heat from resulting exothermic reactions, associated with
the above method, is dissipated via reaction vessel cooling. The
inventors found that reaction vessel cooling is more likely to
be necessary when the resulting oxide powder-solid waste mixture
contains less than 50 weight percent of waste.
Alternatively, the reaction can be slowed with the addition of
from 5 to 25 weight percent of boric acid in the powder, and
preferably 15 weight percent.
Upon temperature equilibration, the reacted paste is a liquid
slurry which sets in a few hours once poured into a mold.
Typically, no pressure is applied to the now-molded slurry.
However, in some processing scenarios, such as when
zirconium-based powders are utilized, discussed infra, pressures
in the range of 1,000 to 2,000 pounds per square inch are used.
The slurry gains full strength in approximately one week.
Mold shapes can vary, depending on the configuration of the
ultimate deposition site, and can be selected from a myriad of
geometrical shapes including cuboid, pyramidal, spherical,
planar, conical, cylindrical, trapezoidal, rectangular, and the
like. Generally, molds having the shape and size of a typical 55
gallon drum are used.
In the case of zirconium phosphate, the slurry is allowed to
thicken first and then pressed into uniform shapes in a mold at
low pressures. Set times for zirconium systems are approximately
3 weeks and requires initial pressure to achieve a dense form.
While this additional processing time may seem to detract from
using zirconium phosphate in the invented encapsulation method,
surprisingly and unexpectedly the inventors have found that this
phosphate provides orders of magnitude better encapsulation of
cesium viz-a-viz the other phosphates. For example, while cesium
leaching results from Mg-Phosphate and Mg--Na phosphate waste
forms yielded low levels of 11.1 parts per million and 23 parts
per million respectively, leaching levels obtained when using Zr
phosphate waste forms were 0.26 parts per million.
Liquid Waste Forms:
The invented liquid encapsulation method provides a simplified
approach for an end user compared to more typical encapsulation
methods. For example, Mg--Na phosphates systems are made by
adding said phosphate to the liquid on site, a process similar
to that practiced in the cement industry. As such, liquid
wastes, such as tritiated water, are easily and economically
encapsulated with this procedure.
If solely liquid is being encapsulated and stabilized by the
invented method and product, the liquid is first combined with
an acid, such as phosphoric acid. The inventors have found that
the ratio of acid to water, selected from a range of between
approximately 37:63 to 50:50, produces good results. An
acid:water ratio of 50:50 is most preferred. If the liquid waste
contains water, then correspondingly less water is added to the
acid to bring the water weight percent of the liquid waste-acid
mixture up to 50 percent. The resulting liquid waste-acid
mixture is then mixed with oxide powder in weight percent ranges
similar to those outlined above for solid waste processing. When
using powder mixtures containing MgO and dibasic phosphate,
weight percent ratios of the oxide to the phosphate selected
from the range of approximately 87:13 to 77:23 produce good
results.
Physical and Mechanical Properties
The physical properties most relevant to the final waste forms
are density, porosity and compression strength. Density of the
samples was measured by determining the mass and geometrical
volume. Porosity values were obtained by water immersion.
Compression strength was measured using an Instron.TM. machine
in compression mode, or other similar compression strength
machine. The results of these measurements are depicted in Table
2, below.
Overall, the product waste forms are light weight materials
having very low density. Very low porosity in Mg phosphate waste
forms with ash waste was attained. High loading, in the range of
approximately 50-80 percent was observed for the Magnesium
systems.
Strength of the waste forms in the Mg phosphate system with ash
waste does not depend on waste loading, and in fact is higher
than other room temperature setting materials, such as Portland
Cement (approx. 6,500 vs. 5,750 psi at 50 percent weight loading
and 7000 vs. 2000 psi at 70 percent). All of these values well
exceed the minimum statutory land disposal compression value of
500 psi for final waste forms.
TABLE 2 ______________________________________ Physical
Properties and Compression Strength of Waste Forms Waste Matrix
Loading Density Porosity Compression Material (Wt %)
(g/cm.sup.3) (Vol %) Strength (psi)
______________________________________ Mg phosphate 50-70
1.706-1.756 <5 6223-6787 w. ash waste Mg phosphate 50-70
1.26-1.32 29.4-38.7 2224-5809 w. cement waste Mg phosphate 50-70
1.239-1.319 29.4-34.3 2224-5809 w. salt waste Mg--Na phos- 50-70
1.285-1.436 36.8-49.6 1172-1573 phate w. ash waste Zr phosphate
20 16 7572 w. ash waste ______________________________________
Leaching Detail
All of the invented phosphate systems are very effective in
stabilizing heavy metal contaminants. As Table 3, depicts below,
a comparison between the leaching levels for the untreated waste
and the stabilized samples shows that the leaching levels in the
stabilized waste are by an order of magnitude lower than those
for the untreated waste. The data also show that the leaching
values, even at a high loading of 70 weight percent, are from
one to two orders of magnitude below the regulatory limits
established for these metals.
The superior immobilization is due to the chemical
stabilization of the contaminants in the matrix. This chemical
stabilization results from the reaction between contaminant
metal salts and the acid solution, followed by the physical
encapsulation within the dense phosphate matrix. The nitrates of
heavy metals are converted to insoluble phosphates by chemical
reactions and hence they do not leach into the acidic leachate
water used in the Toxicity Characteristics Leaching Procedure
(TCLP) of the EPA. The physical encapsulation also immobilizes
the contaminants in the matrix, thereby providing an excellent
final waste form for long term storage.
TABLE 3 ______________________________________ TCLP Data on
Waste Streams Treated With Invented Ceramic Materials
Contamination levels (ppm) Sample Specification Cd Cr Ni Pb
______________________________________ Ash waste (neat) 40.4 196
186 99.7 Ash w H.sub.3 PO.sub.4 1.5 0.12 57.5 <0.5 MgP w. 50%
ash 0.09 <0.05 0.21 <0.2 MgP w. 60% ash 0.12 <0.05 1.27
<0.2 MgP w. 70% ash 0.06 <0.05 3.71 <0.2 MNP w. 50% ash
0.03 0.12 0.04 <0.2 MNP w. 60% ash 0.06 0.11 0.05 <0.2 MNP
w. 70% ash 0.13 0.12 0.08 <0.2 MgP w. 50% cement 0.03
<0.05 0.13 <0.2 MgP w. 60% cement 0.04 <0.05 0.26
<0.2 MgP w. 70% cement 0.06 <0.05 0.74 <0.2 MgP w. 60%
salt <0.01 <0.02 <0.05 <0.5 MgP w. 70% salt <0.01
<0.02 <0.06 <0.5 ZrP w. 20% ash <0.02 0.04 0.55
<0.1 Regulatory Limits 1 5 -- 5
______________________________________
Immersion Detail
Immersion studies indicate that the phosphate waste forms are
durable in aqueous environments. Samples were immersed in
distilled water and periodically the water was replenished to
compensate for evaporation loss. Periodic measurements were made
whereby the samples were removed from the water, dried and
weighed to observe any weight loss. The pH of the water was also
measured. At the end of 90 days, specimens were tested for
compression strength. Results of weight change, pH and
compression strength of Mg-phosphate ash waste forms at the end
of 90 days are shown in FIGS. 2-4, respectively.
FIG. 2 shows the weight change of Mg-phosphate specimens with
70 weight percent ash waste loading after 90 days immersion
compared to the weight of the waste form at the beginning of the
immersion study. After an initial weight loss, probably due to
the release of free phosphoric acid and unreacted MgO, the waste
form stabilized.
FIG. 3 depicts pH changes, with the curve depicted therein
similar to that of FIG. 2. The initial drop in pH is again due
to phosphoric acid release, with a later rebound due to the
slower release of MgO. Once the excess acid and unreacted MgO
were completely released, the pH and weight of the waste form
stabilized.
FIG. 4 shows the variation in the compression strength of the
Mg-Phosphate waste forms as a function of waste loading. For
comparison purposes, the strength of just-fabricated
Mg-phosphate waste forms that were not exposed to water is
presented. FIG. 4 shows that all of the Mg-phosphate waste forms
exhibited excellent strength viz the 500 psi requirements for
land-fill final forms. The resultant strength of all of the
waste forms was exceptionally good and satisfied the regulatory
requirements after a 90-day exposure in an aqueous environment.
Microstructure and Reaction Mechanism
The structure of the final waste forms is complex. Scanning
Electron Microscope (SEM) analysis reveals that the
room-tempera-ture stabilized Mg-phosphate ash waste forms are
both crystalline and noncrystalline. Both crystalline and
noncrystalline phases contain Mg phosphates and silicates,
perhaps evidence of the presence of phospho-silicate structures.
X-ray diffraction analysis done on ash waste forms of Mg and Zr
phosphate suggest that Mg phosphate waste forms contain
unreacted MgO while the reaction in Zr phosphate is complete.
The formation of phosphate phases is due to a MgO surface
reaction with phosphoric acid, wherein the core portion of the
MgO grains remains unscathed.
The major crystalline phase observed in the Mg phosphate system
is Newberyite (MgHPO.sub.4.3H.sub.2 O), an insoluble stable
phase. The major phase in Zr phosphate is either zirconium
phosphate or a hydrophosphate of zirconium.
The fact that the compression strength of the ash waste forms
in Mg phosphate systems does not depend on the extent of ash
loading indicates that ash waste itself participates in the
stabilization process. NMR studies reveal that ash loading
modifies the mineralogy of the Mg phosphate matrix. Such a
result makes the Mg phosphate system very suitable for
stabilization of ash waste streams.
SEM analysis of Mg phosphate ash waste forms indicates that
contaminates in the final waste forms are well dispersed both in
the crystalline and well as noncrystalline phases. These data
coincide with the results obtained by the inventors whereby any
variations in the formulations of crystalline or noncrystalline
phases due to variations in waste stream composition will not
effect contaminant distribution. Generally, the inventors found
that the phosphate waste resins microencapsulate contaminants
very effectively in a homogeneous distribution in a complex
matrix.
EXAMPLE 1
Magnesium Phosphate Ceramic
Magnesium oxide powder of approximately 8 micron size was first
calcined for one hour at 1,300.degree. C. 15 weight percent of
boric acid was added to this powder and mixed well. The mixture
was then reacted with 50 weight percent dilute phosphoric acid.
The addition was done slowly by constantly stirring the powder
into the solution. The resulting paste was either put in a mold
to set or was pressed into a hard ceramic during its setting.
To make a waste form, ash surrogate waste in powder form (with
particle size of approximately 8 microns) was added to the oxide
powder. The ratio of powder to the waste was 65:35 by weight in
this test. The surrogate waste used in these experiments was a
mixture of fly ash (Class F), coal cinder ash and vermiculite as
the major component, and activated carbon as a minor component.
The fly ash content was 40 weight percent, coal cinder ash 33
weight percent and vermiculite content was 20 weight percent.
Thus, these three constituents formed 93 weight percent of the
total surrogate composition. Trace contaminants used were RCRA
metal nitrates and organics. The concentrations of the metal
nitrates were such that the metal concentration in the surrogate
waste was 0.5 weight percent of each metal. Each of the organics
was also 0.5 weight percent of the final waste form.
The samples were cured for at least one week. They were
subjected to various tests. They were found stable in water
immersion tests. Their strength was 2,923 psi and concentrations
of heavy metals in TCLP test leachates were 1.64 ppm of Cd, 0.05
ppm of Cr, 9.63 ppm of Ni and <0.1 ppm of Pb. These values
are well below the EPA's pass/fail tests for Cd, Cr and Pb of 1
ppm, 5 ppm and 5 ppm, respectively. The levels for organics were
below the detection limit of 5 ppm for both naphthalene and
dichlorobenzene. In as much as the minimum compression strength
for a waste form is 500 psi, the magnesium phosphate waste forms
generated herewith the invented method exceeds such requirement.
EXAMPLE 2
Magnesium Sodium Phosphate
The starter MgO powder was washed with 0.28M nitric acid
solution, then by distilled water and dried. Considerable
amounts of hydroxide was formed in the powder as a result of the
washings and this hydroxide reacted with acids to form
phosphates. Beyond this, the procedure followed herein was the
same as that in Example 1. The compressive strength observed was
2561.4 psi and TCLP results were 0.03 ppm for Cd, 0.05 ppm for
Cr. 0.05 ppm for Ni and <0.1 ppm for Pb. The levels of
organics in the leachate were below <5 ppm for both
dichlorobenzene and naphthalene. These results show that the
magnesium sodium phosphate waste form meets regulatory
requirements.
EXAMPLE 3
Zirconium Phosphate
Zirconium hydroxide was the starter powder used. It was reacted
with 90 wt % concentrated (or 10 wt % diluted) phosphoric acid.
The reaction yielded a paste which took approximately 3 weeks to
set into a hard ceramic. The waste form was prepared by the
method given in Example 1 and the same test procedures were
followed. Compression strength was 6781.6 psi, the levels in the
leachate for Cd, Cr, Ni and Pb were 0.07 ppm, 0.16 ppm, 11 ppm
and <0.1 ppm. The levels for organics were <5 ppm. This
shows that Zr-phosphate waste forms meet regulatory
requirements.
While the invention has been described with reference to
details of the illustrated embodiment, these details are not
intended to limit the scope of the invention as defined in the
appended claims.
US Patent # 5,846,894
Phosphate Bonded Structural Products from
High Volume Wastes
Dileep SINGH & Arun WAGH
Abstract: A method to produce structural products from
benign waste is provided comprising mixing pretreated oxide with
phosphoric acid to produce an acid solution, mixing the acid
solution with waste particles to produce a slurry, and allowing
the slurry to cure. The invention also provides for a structural
material comprising waste particles enveloped by an inorganic
binder.
US Cl. 501/155; 252/62; 588/10; 588/15; 588/249; 588/252;
588/256; 588/901
References Cited
U.S. Patent Documents: 4432666 ~ 5482550 ~ 5502268 ~
5580378 ~ 5645518
Other References
CA 78:88360, "Waste solidification program, Evaluation of
solidified waste products", nuclear Science abstracts, 26(23).
(No Month) 1972. .
Low-Temperature-Setting Phosphate Ceramics for Stabilizing DOE
Problem Low-Level Mixed Waste (Part I--Material & Waste Form
Department--Dileep Singh, Arun S. Wagh and Lerry Knox) and Part
II Low-Temperature-Setting Phosphate Ceramics for Stabilizing
DOE Problem Low-Level Mixed Waste (Performance Studies on Final
Waste Forms)--Arun S. Wagh, Dileep Singh, Manish Sutaria, and
Sara Kurokawa--Proceedings of Waste Management 94
Conference-Tucson, AZ--26 pages, Mar. 1994..
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for producing structural
materials and, more specifically, this invention relates to a
method for producing structural products by binding benign
wastes with a ceramic binder. The method is also applicable to
producing a near term containment material.
2. Background of the Invention
The amount of available landfill space continues to dwindle. To
preserve remaining space, recycling programs have been
implemented to separate out reusable waste materials from
materials that cannot be recycled, often at considerable
expense. Despite these efforts, tipping fees continue to
escalate, particularly because tremendous amounts of
unrecyclable waste are still generated. Some examples of this
waste include lumber waste, styrofoam, various kinds of
cellulose fiber, automobile tires, ashes, used carpet backing,
mineral wastes, and plastics. Ash, typically generated from
incinerators, has extremely high disposal costs, partially due
to the presence of heavy metals. Inked substrates, such as
colored paper, colored fabrics, and synthetic fabrics pose
recyclability problems. In addition to not being recyclable,
many waste forms, such as plastics, or polymeric materials, also
are not biodegradable.
A myriad of applications exist to convert many of these waste
forms into usable products. However, many of the current methods
incorporate organic compounds, such as formaldehyde, in
polymeric binders. For example, organic binders are flammable,
give off noxious fumes during setting, and have limited
long-term stability. They are also expensive. Such methods are
therefore unsuitable for housing applications.
Methods for encapsulating small amounts of low-level mixed
wastes using ceramic binders also has been considered. However,
those processes also are not suitable, primarily because they
require high weight ratios of ceramic binder to waste forms. The
inventors have found that high concentrations of binder leads to
undesirably fast curing times, and therefore reduces the
flowability characteristics of the slurry that are required for
application in structural component and insulation substrate
applications. Also, a high weight ratio of binder to waste
particles connotes higher costs in that the space and economic
advantages of disposing large volumes of benign waste with small
amounts of binder are not realized.
A method for encapsulating waste using phosphate-containing
material also is known in the art (U.S. Pat. No. Re. 32,329 to
Paszner et al.). However, that process is relegated to porous
vegetable matter, such as sugarcane, plant stalks and wood. The
process also is designed to facilitate rapid setting of the
final product, which is the antithesis of rendering a blowable
or flowable mixture for use as a structural support or
insulation product.
A need exists in the art for a method to utilize or otherwise
dispose of nonrecyclable and non-biodegradable, benign waste
without generating secondary waste streams. The method must be
economical in providing structural materials for use in housing.
A need also exists for an inexpensive structural product which
is partially comprised of benign waste.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome many of
the disadvantages of the prior art in the utilization of benign
waste forms.
It is another object of the present invention to provide a
method for utilizing benign waste. A feature of the invention is
encapsulating and shaping the benign waste using a binder
material. An advantage of the invention is conserving valuable
landfill space.
Still another object of the present invention is to provide a
method for producing a structural material. A feature of the
invention is encapsulating benign waste using a nontoxic binder
material. An advantage of the invention is utilizing the now
encapsulated benign waste as safe insulative material and
fire-proof material in housing and other structures. Another
advantage of the invention is that the method does not emit
noxious materials and therefore is safe for operators and end
users.
Another object of the present invention is to provide a method
for producing light-weight structural materials. A feature of
the invention is the room-temperature encapsulation of large
amounts of widely available wastes with relatively smaller
amounts of an inorganic binder. An advantage of the invention is
that it is an inexpensive process to utilize nonrecyclable waste
material in blowable or pumpable preparations for ultimate use
as housing materials.
Yet another object of the present invention is to provide a
structural material partially comprised of benign waste. A
feature of the invention is a high volume percent of waste to
binder material. An advantage of the invention is the production
of light weight, strong structural materials that can supplant
traditional materials.
Another object of the present invention is to provide a method
for producing a near-term containment material. A feature of the
invention is using high weight ratios of the containment
material to the invented binder. An advantage of the invented
method is the ability to confine the near-term containment
material to desired mold shapes or structures until most
activity is reduced or dissipated.
Briefly, the invention provides for a method to produce
structural products from benign waste comprising calcining an
inorganic oxide, mixing the now-calcined oxide with a powdered
acid to produce a mixture, contacting the mixture with
phosphoric acid to produce an acid solution, mixing the acid
solution with waste particles to produce a slurry or a wet mix,
and allowing the slurry or mix to cure.
The invention also provides for a structural material
comprising waste particles, and an inorganic binder enveloping
the waste particles.
BRIEF DESCRIPTION OF THE DRAWING
These and other objects and advantages of the present invention
will become readily apparent upon consideration of the following
detailed description and attached drawing, wherein:
Fig. 1 is a schematic diagram of a method for
producing ceramic bonded waste forms using benign material.
DETAILED DESCRIPTION OF THE INVENTION
The method described herein produces ceramically bound benign
wastes for use as structural materials. Unlike previous attempts
to produce stable structural materials from bulk waste, the
instant process produces an amorphous, more flowable material by
utilizing lower volume percents of binder to formulate the final
forms. The resulting ceramic formulation is sufficiently
amorphous and low in crystalline properties to insure good flow
of the material and extended work time. The amorphous phase
mimics polymeric formulations by facilitating the encapsulation
of waste particles with binder during formation of the final
slurry.
A myriad of benign wastes are utilized to produce the
structural materials, including, but not limited to, lumber
wastes, styrofoam, various cellulose fibers (including those
fibers having colored ink), tires, textile wastes, ashes, carpet
backing, mineral wastes, plastics and other solid materials that
cease to be useful. These wastes are used in powder or shredded
forms and are solidified by using a phosphate binder to form
desired shapes for use in the construction industry. Products
produced from the method include blowable insulation, particle
boards, packaging materials, bricks, tiles, wall-forms and
engineered barrier and shield systems.
The room-temperature setting phosphate ceramic waste forms are
formulated by using a route of acid-base reactions. Oxides or
hydroxides of various elements are used as starter powders for
this purpose. Said oxides and hydroxides chemically react with
phosphoric acid or soluble acid phosphates to form ceramics.
When waste is mixed with these powders or acid components, the
waste also may participate to form various stable phases in the
final reaction product which then may be set into ceramic waste
forms.
In one instance, the acid-base reaction results in the
formation of the phosphate of MgO (Newberyite) via the following
equation:
The acid base reaction also results in the reaction of the
waste components with the acid or acid-phosphates. These
reactions lead to chemical solidification of the waste. In
addition, encapsulation of the waste in the phosphate ceramics
formed by the reaction products results in physical
encapsulation of the waste components.
Oxide and Hydroxide Preparation
Oxide powders can be pretreated for better reactions with
acids. One technique includes calcining the powders to a typical
temperature of between approximately 1,200.degree. C. and
1,500.degree. C. and more typically 1,300.degree. C. Another
reaction enhancement technique is washing the powders with
dilute nitric acid and then water. A myriad of oxide and
hydroxide powders can be utilized to produce the ceramic system,
including but not limited to MgO, Al(OH).sub.3, CaO, FeO,
Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 and Zr(OH).sub.4, ZrO, and
TiO.sub.2 and crushed dibasic sodium phosphate crystals mixed
with MgO.
MgO and Al(OH).sub.3 powders are available through any
commercial supply house, such as Baxter Scientific Products,
McGaw Park, Ill. Zr(OH).sub.4 is obtained through Atomergic
Chemetals Corporation, Farmingdale, N.Y.
A generic embodiment of the invented method is depicted in FIG.
1 as numeral 10. First, a supply of oxide 12 is subjected to
either or both a calcining pretreatment step 14 and a boric acid
addition step 16. Both steps serve to slow down the reaction
mechanism. Generally, the boric acid is incorporated when a
slower reaction is required, for example when extended
workability of the material is desired. The inventors have found
that the boric acid forms a glassy phase that coats the oxide
particles so that the oxide cannot as readily react with
phosphoric acid.
As discussed supra, a myriad of oxides can be used in the
invented method. Any mixing of the oxide with the boric acid is
strictly controlled to maintain an optimum weight percent of
constituents of the resulting dry mixture. This weight percent
is selected from a range of between approximately 5 weight
percent boric acid to the oxide to 15 percent boric acid to the
oxide. A preferable weight percent is 10 percent boric acid to
oxide, e.g., 10 grams of boric acid for every 90 grams of oxide,
to form the dry mixture.
The resulting dry mixture 18 is then mixed with between
approximately 50 to 60 weight percent dilute phosphoric acid 20
to form an acid solution or binder 22. A preferable weight
percent is at or below 55 percent, i.e., 55 grams of dry mixture
to 45 grams of 50 percent dilute phosphoric acid, so as to
facilitate flowability of the resulting solution. Concentrated
acid tends to make the reaction more intense. This results in a
thick slurry developing which is not conducive to coating the
particles.
Waste particles, 24, which may be subjected to a pretreatment
sizing process 23, are then thoroughly mixed with the binder.
The mixing step 26 ensures that the waste particles are
completely encapsulated or coated with binder. The resulting
slurry is molded into desired shapes 28 under no pressure or
under small pressure (approximately 1,000 pounds per square
inch), depending on the waste material being bonded. For
example, processes for encapsulating wood waste often requires
the aid of pressurization, primarily because wood surfaces
participate less in the ceramic formation reaction. Rather,
bonding in such cases is purely from the phosphate phase
encapsulating the wood particles. Compression also may be
required to attain desired strengths of final products, such as
in particle board manufacture.
Solid Waste Processing:
In solid waste processing scenarios, the waste streams are
often manipulated in powder form by grinding the waste to an
average, preferable approximate particle size of 8 to 10
micrometers (.mu.m). However, particles can range in size from
between approximately 4-75 .mu.m.
Ash and cement wastes can be first mixed with the starter oxide
or hydroxide powders using a vibratory shaker, or any
conventional agitator. Weight percentages of the mixture varies
at this juncture, but can range from between approximately 15
percent oxide to 50 percent oxide. Typically, an even weight
percent (50:50) of oxide to solid waste is sought. However, the
inventors have successfully encapsulated and stabilized
single-component fly ash at weight percents as high as 85
percent ash to 15 percent MgO powder, which makes this technique
particularly attractive for utilities where single-component fly
ash is a major land-filling problem.
Typically, the rate of powder addition to the acid solution
should result in the reaction liquor being maintained at less
than 100.degree. C. Typical times required for controlled mixing
range from 30 minutes to 1 hour. Mixing times can be shortened
if the heat from resulting exothermic reactions, associated with
the above method, is dissipated via reaction vessel cooling. The
inventors found that reaction vessel cooling is more likely to
be necessary when the resulting oxide powder-solid waste mixture
contains less than 50 weight percent of waste.
Alternatively, the reaction can be slowed with the addition of
from 5 to 25 weight percent of boric acid in the powder, and
preferably 10 weight percent.
Upon temperature equilibration, the reacted paste is a liquid
slurry which sets in a few hours once poured into a mold.
Typically, his slurry is mixed with shredded waste and put into
a mold.
Mold shapes can vary, depending on the configuration of the
ultimate deposition site, and can be selected from a myriad of
geometrical shapes including cuboid, pyramidal, spherical,
planar, conical, cylindrical, trapezoidal, rectangular, and the
like.
Liquid Waste Forms
Sludges and benign wastes with high water content can be
incorporated or bound using this binder system by appropriate
modification to the water content in the acid.
The inventors have found that the volume of the loading of the
wastes in the final product can range from between approximately
50-90 volume percent. For insulation and building reinforcement
applications, the composition of the mixture is adjusted to
convert it into a pumpable slurry (50 volume percent waste) or a
blowable particle mixture (80-90 volume percent waste) so as to
facilitate the filling of cavities.
The acid-base reaction between the oxide and phosphoric acid
results in the formation of phosphates on the surface of the
particles thereby encapsulating individual particles with a thin
layer of impermeable phosphate binder. This results in a
structural product in which particles of the waste are protected
by the binder to provide not only product strength but also
confers resistance to fire, chemical attack, humidity and other
weathering conditions.
Several advantages of the resulting embodiment exist over
commercially available polymer-based binders. Unlike polymer
binders, phosphate binders are nonflammable. Also, several
polymer ingredients are occupational hazards, whereas inorganic
phosphate binders are comparatively safe. No toxic chemicals or
vapors are released during production of phosphate bonded
products. Lastly, phosphate based binder improves the rigidity
and long-term stability to the structural product, compared to
currently used organic binders.
EXAMPLE 1
Styrofoam Insulation
The inventors have found that utilizing the above-disclosed
method, styrofoam particles are completely coated with a thin,
impermeable layer of the phosphate binder phase. The uniform
coating of the styrofoam particles not only provides structural
stability but also confers resistance to fire, chemical attack,
humidity and other weathering conditions. As shown in Table 1,
below, these characteristics are superior to more typical
insulation materials.
TABLE 1 ______________________________________ Comparison of
ceramic-bonded Styrofoam insulation to Fiber Glass- and
Cellulose-insulation Bonded Key Features Styrofoam Fiberglass
Cellulose ______________________________________ Density
(lb/ft.sup.3) 2.0 0.4-1.0 2-3.5 R Values 4.5 2-3 3-3.5 (1 in.
thicknesses) Fire Resistance noncombustible noncombustible
noncombustible Water absorption <4% 1-2% 5-20% Dimensional
Stability .apprxeq.2% settling noted .apprxeq.20% Health Hazards
minimum high minimum Material Costs Low/blown or high low pumped
______________________________________
As depicted in Table 1, the resulting binder-covered styrofoam
material provided superior R values. For example, thermal
conductivity measurements, utilizing a modified radial hot-wire
technique (established by Anter Laboratories, Pittsburgh, Pa.)
showed that the thermal resistance of the material produced via
the invented method was approximately 4.5 hour square foot
degrees Fahrenheit per British Thermal Unit (h.ft..sup.2
.degree.F./BTU, compared to 2-3 h.ft..sup.2 .degree.F./BTU for
fiber glass and 3-3.5 h.ft..sup.2 .degree.F./BTU for cellulose.
This superior R value indicates that phosphate ceramic
binder-covered styrofoam provides superior energy savings when
used as an insulation product.
Insulation products often are susceptible to humidity and tend
to sag, thereby loosing their structural integrity over time.
The invented material was subjected to an aging test pursuant to
ASTM D 2126 (ASTM=American Society for Testing and Materials),
whereby the material is exposed to severe environments for
extended periods of time with dimension changes of the material
closely monitored. The material was exposed to 38.degree. C.
temperatures at 98 percent humidity for 3 weeks. Specimen volume
change was shown to be approximately two percent after a two
week period. This compares to 20 percent for cellulose
insulation material and is also superior to that seen in
fiberglass material.
Generally, a wide range of waste particle sizes can be utilized
when producing structural products using the invented method.
When using styrofoam materials, optimal results are obtained
when particle sizes ranging from 2 millimeters to 5 millimeters
are used, and when the particles are mixed with binder in a
weight ratio of 1:2. Optimal weight loading of the styrofoam in
the final product is approximately 7.5 weight percent, which
corresponds to approximately 85 to 90 volume percent of the
final product.
EXAMPLE 2
Wood Waste
The inventors have found that when wood waste is subjected to
the invented method, particle board having superior flexural
strength values is produced. For example, samples containing 50
weight percent of wood and 50 weight percent of binder display
approximately 1,500 psi in flexural strength. Samples containing
60 weight percent and 70 weight percent of wood exhibit flexural
strengths of 400 psi and 300 psi, respectively.
Generally, suitably sized wood particles range from between
approximately 1 and 5 millimeters (mm) long, 1 mm thick and 2 to
3 mm wide.
In addition, once the wood and binder is thoroughly mixed, the
samples are subjected to pressurized molding on the order of
approximately 2650 psi, and for approximately 30 to 90 minutes.
The disclosed process is not be construed as limited to the
above examples. Also, aside from the myriad of wastes listed
above for which this process can be used to encapsulate, other
waste streams are also sufficiently stabilized herewith. For
example, potliner residue, Bayer sands, ashes generated at plant
sites and any other mining refuse can be stabilized by this
process and utilized as structural components. Potliner residue,
when combined with magnesium phosphate hexahydrate in a 50:50
weight proportion, yields a ceramic form having a density of 2.9
grams per cubic centimeter, a porosity of 2.17 percent and a
compressive strength of 4,210 psi, the last of which is
comparable to portland cement forms. Consistent with data
disclosed supra, when higher proportions (60 weight percent) of
waste material (in this instance potliner residue) is used,
desired values degraded slightly from those values obtained when
50 weight percent loadings were used. Density decreased to 2.0
grams per cubic centimeter, porosity increased to 2.6 percent
and compression strength decreased to 3,402 psi.
While the invention has been described with reference to
details of the illustrated embodiment, these details are not
intended to limit the scope of the invention as defined in the
appended claims.
`
United States Patent # 6,133,498
Method for Producing Chemically Bonded
Phosphate Ceramics and for Stabilizing Contaminants
Encapsulated therein Utilizing Reducing Agents
A. Wagh, et al.
October 17, 2000
Abstract: Known phosphate ceramic formulations are
improved and the ability to produce iron-based phosphate ceramic
systems is enabled by the addition of an oxidizing or reducing
step during the acid-base reactions that form the phosphate
ceramic products. The additives allow control of the rate of the
acid-base reactions and concomitant heat generation. In an
alternate embodiment, waste containing metal anions are
stabilized in phosphate ceramic products by the addition of a
reducing agent to the phosphate ceramic mixture. The reduced
metal ions are more stable and/or reactive with the phosphate
ions, resulting in the formation of insoluble metal species
within the phosphate ceramic matrix, such that the resulting
chemically bonded phosphate ceramic product has greater leach
resistance.
Current U.S. Class:588/319 ; 264/.5; 501/111; 501/112;
501/155; 588/10; 588/252; 588/320; 588/4; 588/408; 588/413
Current International Class:A62D 3/00 (20060101); C03C
10/00 (20060101); C03C 14/00 (20060101); C03C 1/00 (20060101);
C04B 20/02 (20060101); C04B 18/04 (20060101); C04B 20/00
(20060101); C04B 28/34 (20060101); C04B 28/00 (20060101); G21F
9/16 (20060101)
References Cited
U.S. Patent Documents:5645518 ~ 5830815 ~ 5846894 ~
5939039
Other References
Modified Phosphate Ceramics for Stabilization and
Solidification of Salt Mixed Wastes, authored by Dileep Singh,
Kartikey Patel, Arun S. Wagh, Seung-Young Jeong, published in
the Proceedings of Spectrum '98, International Conference on
Decommissioning and Decontamination and on Nuclear and Hazardous
Waste Management, Denver, Colorado, Sep. 13-18, 1998. .
Modified Phosphate Ceramics for Stabilization of Salt Mixed
Wastes, thesis authored by Kartikey Patel, submitted for review
on or after May 7, 1998 and defended on Jun. 26, 1998. .
U.S. Patent Application Ser. No..sub.-- /124,822,
Continuation-in-part of U.S. Patent Nos. 5,846,894 and
5,830,815. .
U.S. Patent Application Serial No..sub.-- /617,284,
Continuation-in-part of U.S. Patent No. 5,830,815. .
PCT Patent Application No. PCT/US97/04132.
Description
TECHNICAL FIELD
This invention relates to the use of chemically bonded
phosphate ceramic (CBPC) waste forms for immobilizing large
volumes of low-level, radioactive and/or hazardous waste, and,
in particular, to an improved process and CBPC product.
BACKGROUND OF INVENTION
Low-level mixed wastes contain hazardous chemical and low-level
radioactive materials. Of particular concern are low-level mixed
waste streams that contain heavy metals, such as lead, cadmium,
copper, zinc, nickel, and iron, among others, and waste streams
from nuclear materials processing applications that contain
technetium-99, chromium, and antimony. The U.S. Environmental
Protection Agency classifies waste as hazardous, under the
Resource Conservation and Recovery Act (RCRA), if excessive
amounts of heavy metals leach from the waste during the Toxicity
Characteristic Leaching Procedure (TCLP). Land disposal of
leachable heavy metal waste is very expensive and strictly
regulated, and therefore cost-effective, safe, leach resistant
methods for encapsulating heavy metal waste is of current
environmental importance.
Stabilization of low-level mixed waste requires that the
contaminants, including soluble heavy metals ions, are
effectively immobilized. No single solidification and
stabilization technology is known to successfully treat and
dispose of low-level mixed waste, due to the physical and
chemical diversity of the waste streams. Conventional
high-temperature waste treatment methods (e.g., incineration,
vitrification) are largely unsuitable for the treatment of
low-level mixed waste streams, because their reliance on high
temperature risks the release of volatile contaminants and they
generate undesirable secondary waste streams. A low-temperature
approach is to stabilize hazardous waste by using inorganic
(e.g., pozzolanic) binders, such as cement, lime, kiln dust,
and/or fly ash. Disadvantages of this approach include a high
sensitivity to the presence of impurities, high porosity, and
low waste loading volume. Organic binders (e.g., thermosetting
polymers) are used even less frequently, because of cost and
greater complexity of application. Organic binders are not
compatible with water-based wastes, unless the waste is first
pre-treated and converted to an emulsion or solid, and organic
binders are subject to deterioration from environmental factors,
including biological action and exposure to ultraviolet light.
Recently, an alternative low-temperature approach has been
developed at Argonne National Laboratory for stabilizing and
solidifying low-level mixed waste by incorporating or loading
the waste into a phosphate ceramic waste form. This technique
immobilizes the waste by solidification, such that the waste is
physically micro-encapsulated within the dense matrix of the
phosphate ceramic waste form, and stabilizes the waste by
converting the waste into their insoluble phosphate forms.
Ceramic encapsulation systems are particularly attractive given
that the bonds formed in these systems are ether ionic or
covalent, and hence stronger than the hydration bonds in cement
systems. Also, the ceramic formulation process is exothermic and
economical.
Phosphates are particularly good candidates for stabilization
of radioactive and hazardous waste, because phosphates of radio
nuclides and hazardous metals are essentially insoluble in
groundwater. A salient feature of the low-temperature ceramic
phosphate formulation process is an acid-base reaction. For
example, magnesium phosphate ceramic waste forms have been
produced by reacting magnesium oxide (MgO) with phosphoric acid
to form a phosphate of magnesium oxide, Newberyite, as
represented in Equation (1), below.
The acid-base reaction results in the reaction of the waste
components with the acid or acid-phosphates, leading to chemical
stabilization of the waste. In addition, encapsulation of the
waste in the phosphate ceramic results in physical containment
of the waste components. The reaction represented by Equation
(1) above occurs rapidly and generates heat, and upon
evaporation of the water, a porous ceramic product results.
U.S. Pat. No. 5,645,518 issued to Wagh, et al., incorporated
herein by reference, describes in detail the process steps for
setting liquid or solid waste in CBPC products using acid-base
reactions. Accordingly, the process involves mixing ground solid
waste, including salt waste spiked with heavy metals, with a
starter powder of oxide and hydroxide powders of various
elements; slowly adding the waste-powder mixture to an acid
solution of phosphoric acid or soluble acid phosphates;
thoroughly mixing the waste-powder-acid mixture for about a half
hour to an hour at ambient temperatures (less than 100.degree.
C.), such that the components of the
mixture chemically react to form stable phases and a reacted
viscous slurry or paste results; and allowing the slurry or
paste to set for a few hours into the final CBPC product. Liquid
waste is similarly stabilized by mixing the liquid waste with
the acid solution (preferably 50:50), and then reacting the
waste-acid mixture with the starting powders. The maximum
temperature for the process is about 80.degree. C. The CBPC
products attain full strength in about three weeks, and exhibit
a complex structure, including a major crystalline phase, e.g.,
Newberyite (MgHPO.sub.4.3H.sub.2 O), and an insoluble, stable
phase. The waste components are generally homogeneously
distributed within the phosphate ceramic matrix. Unfortunately,
however, the porous product (Newberyite) is unsuitable for waste
treatment on a large scale.
U.S. Pat. No. 5,830,815 issued to Wagh, et al., incorporated
herein by reference, describes improving the CBPC fabrication
process by incorporating two temperature control processes for
both reducing heat generation during the encapsulation
(reaction) steps and moderating pH conditions (some wastes are
unstable at a low pH). The first temperature control process
involves pre-treating the phosphoric acid with a carbonate,
bicarbonate or hydroxide of a monovalent metal (e.g., K, Na, Li,
Rb) prior to mixing with an oxide or hydroxide powder so as to
buffer the acid. In particular, potassium containing alkali
compounds (e.g., K.sub.2 O, KHCO.sub.3, KOH) result in a more
crystalline waste form, and the higher the concentration of
potassium in the potassium containing compound, the more
crystalline the final product, resulting in a higher compression
strength, lower porosity, and greater resistance to weathering,
compressive forces, and leaching. The second temperature control
process involves bypassing the use of the acid and mixing the
oxide powder with dihydrogen phosphates of potassium, sodium,
lithium, or other monovalent alkali metal, to form a ceramic at
a higher pH.
Neutralizing the phosphoric acid solution in equation (1) by
adding potassium hydroxide (KOH), as represented in the chemical
equation (2) below, reduces the reaction rate and heat
generation, and results in the formation of a superior magnesium
potassium phosphate (MKP) mineral product, MgKPO.sub.4.6H.sub.2
O (magnesium potassium phosphate hexahydrate), as represented in
chemical equation (3) below.
The chemically bonded ceramic phosphate (CBPC) waste form (e.g,
MgKPO.sub.4.6H.sub.2 O) is a dense, hard material with excellent
durability and a high resistance to fire, chemicals, humidity,
and weather. The low-temperature (e.g., room-temperature)
process encapsulates chloride and nitrate salts, along with
hazardous metals, in magnesium potassium phosphate (MKP)
ceramics, with salt waste loadings of up to between
approximately 70 weight percent and approximately 80 weight
percent. This durable MKP ceramic product has been extensively
developed and used in U.S. Department of Energy waste treatment
projects.
Phosphates in general are able to bind with hazardous metals in
insoluble complexes over a relatively wide pH range and most
metal hydroxides have a higher solubility than their
corresponding phosphate forms. In addition to the magnesium and
magnesium-potassium phosphate waste products discussed above,
known waste encapsulating phosphate systems include, but not
limited to, phosphates of magnesium-ammonium, magnesium-sodium,
aluminum, calcium, iron, zinc, and zirconium (zirconium is
preferred for cesium encapsulation). A non-exclusive summary of
known phosphate systems and processing details is provided in
Table I below, selected according to the ready availability of
materials and low cost. It is also known to add other materials
to either the waste or ceramic binder ingredients, such as fly
ash.
TABLE I ______________________________________ Phosphate
Systems and Processing Details Curing System Starting Materials
Solution Time ______________________________________ MKP Ground
MgO, ground K Water 1 hour dihydrophosphate crystals Mg
phosphate Calcined MgO Phosphoric acid- >8 days water (50/50)
Mg-NH.sub.4 Crushed dibasic NH.sub.4 Water 21 days phosphate
phosphate crystals mixed with calcined MgO Mg-Na phosphate
Crushed dibasic Na Water 21 days phosphate crystals mixed with
calcined MgO Al phosphate Al(OH).sub.3 powder Phosphoric acid
Reacted (.apprxeq. 60.degree. C.) powder, pressed Zr phosphate
Zr(OH).sub.4 Phosphoric acid 21 days
______________________________________
Iron oxides including either iron oxide (FeO) itself or
magnetite (Fe.sub.3 O.sub.4) have also been used in the
formation of phosphate ceramic products, however, these
materials are uncommon and expensive. Haematite (Fe.sub.2
O.sub.3) is a very unreactive powder and efforts to form a
chemically bonded phosphate ceramic (CBPC) product using
haematite have been unsuccessful. When mixed with phosphoric
acid, and even highly concentrated phosphoric acid, the
haematite either does not react or reacts at such a slow rate
that the reaction is impractical for the development of CBPC
products. The slow rate of reaction is due to the insolubility
of haematite, which is in its highest oxidation state.
Appropriate oxide powders include, but are not limited to,
oxides or hydroxides of aluminum, calcium, iron, magnesium,
titanium, and zirconium, and combinations thereof. The oxide
powders may be pre-treated (e.g., heated, calcined, washed) for
better reactions with the acids. While no pressure is typically
applied to the reacted paste, about 1,000 to 2,000 pounds per
square inch may be applied when zirconium-based powders are
used.
The acid component may be dilute or concentrated phosphoric
acid or acid phosphate solutions, such as dibasic or tribasic
sodium, potassium, or aluminum phosphates, and the paste-setting
reactions are controllable either by the addition of boric acid
to reduce the reaction rate, or by adding powder to the acid
while concomitantly controlling the temperature. Examples of
appropriate phosphates include phosphates of aluminum,
beryllium, calcium, iron, lanthanum, magnesium,
magnesium-sodium, magnesium-potassium, yttrium, zinc, and
zirconium, and combinations thereof. Salt waste may be reacted
with phosphoric acid to consume any carbon dioxide (CO.sub.2)
present, prior to mixing the salt waste with the oxide powders
or binding powders, as the evolution of CO.sub.2 results in very
porous final ceramic products.
Unfortunately, the acid-base reactions involved in the
phosphate ceramic systems described above occur very rapidly,
resulting in the generation of considerable exothermic heat that
prevents the formation of homogeneous large scale phosphate
ceramic monoliths. The rapid setting of the CBPC products also
hinders the proper conversion of hazardous or radioactive
contaminants into stabilized phosphate forms. As a result, the
CBPC products formed by methods known in the art have very poor
density and strength.
Encapsulation of waste containing heavy metals in known CBPC
systems is also of limited practical use. Although heavy metals
in the form of soluble nitrates (e.g.,
Cr(NO.sub.3).sub.3.9H.sub.2 O, Ni(NO.sub.3).sub.2.6H.sub.2 O,
Pb(NO.sub.3).sub.2, and Cd(NO.sub.3).sub.2.4H.sub.2 O) are
reportedly converted to insoluble phosphates by the CBPC forming
chemical reactions, there is a critical need to improve their
leach resistance and to provide greater stabilization for the
metal anions of technetium-99, chromium, and antimony. Efforts
to encapsulate heavy metal waste in phosphate ceramic products
are further hampered by low maximum waste loading capacities,
because of interference of the metal anions with ceramic-setting
reactions, leaching of soluble metal anions from the resulting
highly porous ceramic product (especially in aqueous
environments), and rapid structural degradation of the ceramic
product caused by the high leach rates. Also, environmental
stresses degrade the integrity of known CBPC waste forms over
time. For example, exposure to repeated cycles of wetting,
drying and/or freezing, or acidic or other conditions conducive
to leaching may affect the long term effectiveness of waste
encapsulated CBPC waste forms.
A need exists for improved phosphate ceramic systems and
improved methods for disposing of wastes containing metal anions
in phosphate ceramic products.
The present invention is a surprisingly effective process step
that significantly improves known phosphate ceramic
formulations, enables the production of iron-based phosphate
ceramic systems, and critically increases the stabilization of
wastes containing heavy metals within CBPC composites. The
invention involves adding oxidants or reductants to the ceramic
phosphate formulations to retard or accelerate the acid-base
reactions and thereby control the exothermic temperature of the
reactions. In addition, the use of reducing agents significantly
improves the stabilization of the metal anions within the
phosphate ceramic composition, and thus the leach resistance of
the encapsulated metals, by changing the valence of the metal to
a lower oxidation state, such that the metal is more stable in
the presence of the phosphate ions and/or the metal is more
reactive with the phosphate ions.
Therefore, in view of the above, a basic object of the present
invention is to control the reactions rates and heat generation
in phosphate ceramic processes to allow homogeneous large scale
phosphate ceramic monoliths.
Another object of the present invention is to significantly
improve the density and strength of phosphate ceramic products
formulated from methods known in the art.
Another object of the present invention is to form chemically
bonded phosphate ceramic products from inexpensive iron-based
materials, such as haematite.
Yet another object of the present invention is to provide an
improved method for stabilizing waste containing metal anions in
a phosphate ceramic composite having increased loading capacity
and improved leach resistance.
A further object of the invention is to provide an improved,
safe, low temperature, economical method for stabilizing large
volumes of waste containing metal anions in a durable, long term
storage phosphate ceramic product.
Additional objects, advantages, and novel features of the
invention are set forth in the description below and/or will
become apparent to those skilled in the art upon examination of
the description below and/or by practice of the invention. The
objects, advantages, and novel features of the invention may be
realized and attained by means of instrumentation and
combinations particularly pointed out in the appended claims.
BRIEF SUMMARY OF THE INVENTION
Briefly, this invention is a surprisingly effective method for
significantly improving phosphate ceramic formulations and
enabling the production of iron-based phosphate ceramic systems.
The invention involves the addition of an oxidizing or reducing
step to known phosphate ceramic formulations during the
acid-base reactions between the oxide powders and phosphoric
acid or acid phosphate solutions. The additives allow control of
the rate of the acid-base reactions and concomitant heat
generation. As a result, phosphate ceramic systems incorporating
iron-based materials are practical, including the formation of
iron phosphate ceramic products from haematite, a readily
available, inexpensive material. The CBPC products may be
crystalline ceramics and/or glass (non-crystalline).
In an alternate embodiment, the addition of reducing agents to
the ceramic phosphate system significantly improves the
stabilization of heavy metal waste encapsulated within
chemically bonded phosphate ceramic (CBPC) waste forms. Addition
of the reducing agent, preferably a stannous salt, changes the
valence of the metal to a lower oxidation state, such that the
metal is more stable in the presence of the phosphate ions
and/or the metal is more reactive with the phosphate ions.
Importantly, the reduced metal ions are more stable and/or more
reactive with the phosphate ions, resulting in the formation of
insoluble metal species within the final phosphate ceramic
matrix.
BRIEF DESCRIPTION OF THE DRAWINGS
The appended claims set forth those novel features which
characterize the invention, however, the invention itself, as
well as further objects and advantages thereof, will best be
understood with reference to the following detailed description
of a preferred embodiment, in conjunction with the accompanying
drawings, where like reference characters identify like elements
throughout the various figures, in which:
FIG. 1 shows an scanning electron microscopy (SEM)
micrograph of an iron-based CBPC product made from haematite;
FIG. 2 shows an SEM micrograph of an iron-based CBPC
product made from magnetite; and
FIG. 3 graphically illustrates Eh-pH values for
magnesium potassium phosphate (MKP) ceramic products fabricated
by the present improved method .
DETAILED DESCRIPTION OF THE INVENTION
The present invention modifies known methods for encapsulating
waste in chemically bonded phosphate ceramic (CBPC) products,
described in detail in the Background Section above, by
incorporating a new and unique oxidation or reducing step that
controls the rate of the acid-base reaction in the formation of
phosphate ceramic systems. The addition of the oxidation or
reducing agent to the CBPC binder mix aids in controlling the
rate of the acid-base reactions and, importantly, changes the
oxidation state and/or reactivity of CBPC ingredients, e.g.,
starter oxide powders. Altering the oxidation state of the
compound may allow CBPC compounds to become more reactive and
encapsulated waste to become more reactive and less soluble.
Iron Phosphate Ceramic Products Formed from Haematite
As discussed above, haematite (Fe.sub.2 O.sub.3), an
inexpensive and very common ingredient in lateritic soils and
several mineral wastes, including iron waste tailing and red
mud, has not been successfully used to form a chemically bonded
phosphate ceramic (CBPC) product, because of its high oxidation
state, insolubility, and slow reaction rate with phosphoric
acid. Using known CBPC formulations, the haematite remains
unaltered in the oxidation environment of the phosphate solution
during the acid-base reactions.
The present invention enables the formation of an iron
phosphate ceramic product made from haematite by converting the
haematite into a slightly lower oxidation state, thereby
increasing its reactivity. For example, adding reductants, such
as tin chloride or iron sulfide, during the acid-base reaction
results in the conversion of Fe.sub.2 O.sub.3 to Fe.sub.2
O.sub.3-.delta.. Preferably, the haematite is reduced prior to
contacting the haematite with the acid phosphate solution. The
reduction of haematite is alternatively accomplished by heating
haematite powder in a reducing atmosphere that contains, but is
not limited to, nitrogen, high carbon, low oxygen, carbon
monoxide, and/or iron, or by calcining the haematite powder in a
vacuum.
EXAMPLE 1
An iron-based chemically bonded phosphate ceramic product was
formed using haematite and tin chloride as a reductant. First,
100 g of haematite having a particle size of between 10-50 .mu.m
was mixed with 5 wt % of tin chloride, in a powder form. The
mixture was thoroughly stirred in 50 wt %
dilute phosphoric acid solution, in a solution to powder ratio
of 1:1, for 30 minutes, forming a thick, pourable paste. The
paste hardened after 24 hours and was completely set after 3
weeks. The resulting iron-based CBPC product was dense with
negligible porosity, and had the red color of haematite. The
surface appeared very glassy, and upon fracturing, the fracture
surfaces were smooth and glass-like with no visible pores. X-ray
diffraction showed the iron-based CBPC product to be mainly
glassy, and, therefore, the newly discovered iron-based CBPC
product is identified as a glass-crystalline ceramic. FIG. 1
shows an SEM micrograph of the iron-based CBPC product made from
haematite.
EXAMPLE 2
An iron-based chemically bonded phosphate ceramic product was
formed using haematite and iron sulfide as a reductant. 100 g of
haematite having a particle size of between 10-50 .mu.m was
mixed with 5 wt % of iron sulfide, in a powder form, and the
mixture was thoroughly stirred in 60 wt % dilute phosphoric acid
solution, in a solution to powder ratio of 1:1, for 30 minutes,
forming a thick, pourable paste. The paste hardened after 24
hours and was completely set after 1 week. The resulting
iron-based CBPC product was red in color, dense with negligible
porosity, and was also identified as a glass-crystalline
ceramic.
In an alternate embodiment, the rate of reaction in the
formation of iron phosphate ceramics from iron oxide or
magnetite is retarded by the addition of boric acid. It is known
to control paste-setting reactions by the addition of boric
acid.
EXAMPLE 3
An iron-based chemically bonded phosphate ceramic product was
formed from magnetite. First, 100 g of magnetite having a
particles size of between 10-50 .mu.m was mixed with 10 wt % of
boric acid (retardant). The resultant mixture was thoroughly
stirred in 40 to 50 wt % dilute phosphoric acid solution in a
1:1 ratio of solution to powder for 30 minutes, forming a thick,
pourable paste. The paste hardened after 1 hour and was
completely set in 24 hours. The iron-based CBPC product had a
black color, and was dense with negligible porosity. X-ray
diffraction indicated the existence of glassy and crystalline
phases. FIG. 2 shows an SEM micrograph of the iron-based CBPC
product made from magnetite. A similar material was also made
using 50 wt % class F fly ash in the binder powder. Analysis of
the iron-based CBPC products made from magnetite indicated that
a considerable amount of unreacted magnetite remained in the
CBPC product.
In another embodiment, the reaction rate of the acid-base
reaction in the formation of an iron-based CBPC product made
from magnetite was controlled by adjusting the concentration of
the phosphoric acid solution and the pH. Although it is known to
use phosphoric acid (H.sub.3 PO.sub.4) having a concentration of
50 wt %, magnetite or FeO may be reacted with a dilute H.sub.3
PO.sub.4 solution that is between about 30 and about 40 wt %.
The H.sub.3 PO.sub.4 solution may also be partially neutralized
using oxides, hydroxides, carbonates, or anhydrous phosphates
prior to the acid-base reaction with FeO or Fe.sub.3 O.sub.4 to
reduce the rate of the reactions for forming the iron-based CBPC
products.
EXAMPLE 4
An iron-based chemically bonded phosphate ceramic product was
formed from magnetite and a pH adjusted phosphoric acid. 50 wt %
concentrated phosphoric acid was mixed with 5 to 15 wt %
potassium carbonate (K.sub.2 CO.sub.3). 100 g of magnetite
having a particle size of between 10-50 .mu.m was thoroughly
stirred in the pH adjusted phosphoric acid solution, in a ratio
of powder to solution of 1:2, forming a thick, pourable paste.
The paste hardened after 1 hour and was completely set in 24
hours. The iron-based CBPC product had a black color, and was
dense with negligible porosity. X-ray diffraction indicated the
existence of glassy and crystalline phases. A similar material
was also made using 50 wt % class F fly ash in the binder
powder. Analysis of the iron-based CBPC products made from
magnetite indicated that a considerable amount of unreacted
magnetite remained in the CBPC product.
Stabilization of Metal Anions
In this embodiment, radioactive and/or hazardous waste
materials containing metal anions are stabilized in CBPC
products by the addition of a reducing agent to the waste and/or
phosphate ceramic ingredients. The waste is generally waste
containing nitrates, chlorides, sulfates, silicates, salts,
heavy metals, any type of inorganic waste, and/or combinations
thereof. Addition of the reducing agent to the metal anions,
oxide or hydroxide powders, and/or phosphoric acid or soluble
acid phosphates reduces the valency of the metal anions to a
lower oxidation state during formation of the CBPC product.
Incorporation of the reducing agent into CBPC formulations
solves the problems experienced in the art due to the presence
of metal anions in the waste stream by stabilizing the metal
anions within the CBPC product in an insoluble form.
The reducing agent is preferably selected from a group
including, but is not limited to, sodium monosulfide (Na.sub.2
S), potassium monosulfide (K.sub.2 S), calcium sulfide (CaS),
iron sulfide (FeS), iron sulfate (FeSO.sub.4.7H.sub.2 O), sodium
thiosulfate (Na.sub.2 S.sub.2 O.sub.5), sulfur dioxide
(SO.sub.2), sodium borohydride (NaBH.sub.4), hydrazine, sodium
bisulfite (NaHSO.sub.3), calcium hydroxide (Ca(OH).sub.2),
sodium hydroxide (NaOH), sodium carbonate (Na.sub.2 CO.sub.3),
sulfuric acid (H.sub.2 SO.sub.4), and formic acid (HCOOH), among
others. Preferably, the reducing agent is a stannous salt, such
as tin chloride (SnCl.sub.2). Table II below provides a summary
of preferred reducing agents depending upon the content of the
metal waste. More than one reducing agent may be used where the
waste is known to contain various metal contaminants.
TABLE 11 ______________________________________ Appropriate
Reducing Agents for Heavy Metal Waste Metal Waste Reducing Agent
______________________________________ Arsenic SnCl.sub.2
Chromate SnCl.sub.2, Na.sub.2 S Mercury Na.sub.2 S Selenium
SnCl.sub.2 Technetium SnCl.sub.2
______________________________________
The reducing agent may be added to the heavy metal waste,
starter powder, and/or acid solution, in any combination.
Preferably, the reducing agent is initially added to the heavy
metal waste, resulting in the precipitation of the hazardous
metals, and subsequently mixed with the CBPC powder and acid
solution. The addition of the reducing agent to the waste and/or
ceramic binder ingredients is largely dependent upon the type of
reducing agent and its reactivity with the phosphates. For
example, if a reducing agent is very strong, it is preferably to
add the reducing agent to the waste-ceramic slurry early in the
mixing step. In general, two to three times more than the
stochiometric amount of the reducing agent is used, depending on
the amount of metal present in the waste. The addition of the
reducing agent results in reduction of the metal anions to their
lower oxidation states, and in some cases to cations, such that
the reduced metal ions are more stable and/or more readily react
with the phosphate ions to form insoluble metal species,
including oxides and hydroxides of the metals.
EXAMPLE 5
In this non-limiting example, the stabilization of chromium
anions was improved in magnesium potassium phosphate (MKP)
ceramics by incorporating waste containing chromate (Cr.sub.2
O.sub.7.sup.2-) into an MKP binder powder including a tin
chloride (SnCl.sub.2) reducing agent. Addition of the reducing
agent results in the reduction of the valency of the chromium
anions from +6 to +3, thus decreasing the leachability of the
chromate from the MKP ceramic product. In addition, the
reduction of the chromate may increase the reactivity of the
chromium ions with the phosphate ions in the acid solution,
promoting the formation of insoluble chromium phosphate.
MKP ceramic composites loaded with 58 wt % and 70 wt % nitrate
waste were fabricated by incorporating nitrate waste containing
chromium into MKP binder materials, both with and without the
presence of a tin chloride reducing agent.
The 58 wt % waste loaded MKP ceramic product was fabricated by
first adding 50 g of the waste to 0.17 g of the reducing agent
sodium monosulfide (Na.sub.2 S) and 12 g of water. About 0.5 g
of boric acid may also be added as a retarder. Next, 0.86 g of a
second reducing agent, tin chloride (SnCl.sub.2), was added to
the waste slurry and mixed for 5 minutes. Fractions of the
ceramic binder ingredients, including 4.67 g of water and a
mixture of 7.69 g of magnesium oxide (MgO) and 25.64 g of
potassium dihydrogen phosphate (K.sub.2 HPO.sub.4), were then
added to the waste slurry in 5 minute intervals.
The 70 wt % loaded MKP ceramic waste product was fabricated by
the same steps, except the amounts of ceramic binder ingredients
used were 12 g water, 4.95 g MgO, and 16.48 g KH.sub.2 PO.sub.4.
Table III below provides the results of the Toxicity
Characteristic Leaching Procedure (TCLP) applied to the
fabricated MKP ceramic products. The results show dramatic
improvements in leach resistance in MKP ceramic products
fabricated in accordance with the present invention. Without the
reducing agent, the MKP ceramic product failed the leaching
test, while addition of the reducing agent produced an MKP
ceramic product that is well below allowable EPA regulatory
limits. Thus, addition of the SnCl.sub.2 reducing agent was
critical to the successful stabilization and containment of the
chromium.
TABLE III ______________________________________ Phosphate
Systems and Processing Details CBPC Product Fabricated Nitrate
Leach Resistance Waste Chromium EPA TCLP Loading Concentration
Addition of Regulatory Results (wt %) (ppm) SnCl.sub.2 Limit
(ppm) (ppm) ______________________________________ 58 300 NO
0.86 10.3 58 300 YES 0.86 0.02 70 360 NO 0.86 16.3 70 610 YES
0.86 0.04 ______________________________________
EXAMPLE 6
Technetium-99 (.sup.99 Tc) is present in some high-level wastes
(HLW), in addition to other volatile fission products, including
cesium-137 (.sup.137 Cs) and strontium-90 (.sup.90 Sr). Under
oxygen-containing conditions, the predominant form of technetium
is the pertechnetate anion, TcO.sub.4.sup.-, which is highly
soluble in water and readily mobile in the environment.
Immobilization of technetium-99 is of critical concern, because
of its high leachability and long half life (e.g.,
2.13.times.10.sup.5 years). Technetium-99 was successfully
stabilized in the MKP ceramic product, in accordance with the
present invention, in that the addition of stannous chloride
reduced the oxidation state of technetium-99 from +7 to +4. The
waste solution used in this example was a stripping solution
generated by a complexation and elution process developed at the
Los Alamos National Laboratory (LANL) to separate technetium-99
from HLW, and contained approximately 20 ppm to 150 ppm of
technetium-99.
In a first approach, eluted aqueous waste was directly
stabilized, such that the water in the waste was used in the
CBPC fabrication process. CBPC products were fabricated with and
without the addition of the reducing agent, tin chloride
(SnCl.sub.2). MKP ceramic products were formed from 19.973 g of
LANL stripping solution containing about 40 ppm .sup.99 Tc by
adding the stripping solution to a binder mixture including 2.48
g SiO.sub.2, 8.38 g of MgO, and 28.28 g of KH.sub.2 PO.sub.4,
and mixing the mixture for 20 minutes, resulting in a fine
slurry. The reducing agent, 1.16 g of tin chloride (SnCl.sub.2)
was added to the mixtures after 18 minutes of mixing. No water
was added during the process. MKP ceramic products were
similarly formed, by the same process, from 1.712 g of LANL
stripping solution that did not contain any .sup.99 Tc.
The resulting fine slurries were transferred into molds, and
allowed to set. The typical temperature rise during setting was
between about 55.degree. C. to about 70.degree. C. The slurries
hardened into a dense monolithic MKP ceramic products in about 2
hours. After at least 14 days of curing, the resulting MKP
ceramic products fabricated directly from the LANL elution
solution had a density of 1.8 g/cm.sup.3, a very low open
porosity of 4%, and a compression strength of 30.+-.6.7 MPa, a
compression strength significantly higher than the land disposal
compression strength requirement of 3.4 MPa, demonstrating the
MKP ceramic product's superior dense, hard, high-strength
structure. The MKP ceramic products were tested for strength,
leaching, and water immersion, evidencing that the addition of
the reducing agents helped to maintain the .sup.99 Tc in its
relatively insoluble cationic form, Tc.sup.+4. The optimal
loading of the elution solution in the MKP ceramic product was
35%, and the concentrations of the .sup.99 Tc in the MKP ceramic
products were in the range of between about 20 to about 150 ppm.
The leachability of .sup.99 Tc is highly dependent upon its
oxidation state, and, therefore, it is important to establish
redox conditions in the phosphate slurry during fabrication of
the MKP ceramic product. FIG. 1 is a graphical illustration of
an Eh/pH diagram for both a Re--O--H and a Tc--O--H system in
the pH range of 5 to 10. Also shown are experimentally
determined Eh values as a function of pH for the MKP slurries
containing eluted waste, measured at various elution solution
loadings, wherein rhenium was added to the elution solution as a
substitute for .sup.99 Tc. For example, FIG. 3 shows that the Eh
and pH values of the slurry with 36% .sup.99 Tc elution solution
loading under the normal setting conditions are +225 mV and 6.5,
respectively. In the pH range of 5 to 10 pH, Eh values of Re are
in the highly soluble heptavalent oxidation state. For a pH of
less than 7, Eh values of .sup.99 Tc are in the insoluble
TcO.sub.2 (Tc.sup.+4) oxidation state, while for a pH of greater
than 7, .sup.99 Tc is present as TcO.sub.4.sup.- (Tc.sup.+7).
The MKP slurry and setting conditions prescribed by the present
invention (e.g., 36% loading, 6 to 7 pH, Eh +225 mV) are highly
conducive to maintaining .sup.99 Tc in the insoluble +4
oxidation state. Importantly, the addition of a reducing agent
to the slurry critically aids in the reduction of
TcO.sub.4.sup.- (Tc.sup.+7) to its stable and insoluble +4
oxidation state.
In a second approach, technetium-99 was precipitated from the
eluted solution by heating in the presence of zinc and 4 M
hydrochloric acid, and the precipitated technetium-99
(TcO.sub.2.2H.sub.2 O) was incorporated into the MKP ceramic
product. The loadings of the technetium-99 in the MKP ceramic
products were as high as 900 ppm. Since it is well known that
TcO.sub.2.2H.sub.2 O is highly insoluble, with a solubility of
10.sup.-7 to 10.sup.-8 mol/L in water under mildly reducing
conditions, precipitation of the technetium-99 as the highly
insoluble TcO.sub.2.2H.sub.2 O, followed by encapsulation in the
MKP matrix, yields a superior, stabilized phosphate ceramic
product, with a higher loading than that accomplished by the
direct elution method of the first approach.
Technetium-99 is generally precipitated from LANL stripping
solutions by adding zinc to the LANL stripping solution, adding
HCl to the mixture, and boiling the mixture at about 70.degree.
C. for about 45 minutes. This process results in the
precipitation of TcO.sub.2.2H.sub.2 O with about a 40% recovery
of .sup.99 Tc.
Table IV below provides results of diffusivity and leachability
testing of the MKP ceramic products loaded with .sup.99 Tc,
fabricated with and without the reducing agent step. These
results demonstrate that the MKP ceramic products fabricated
with the addition of the reducing agent (SnCl.sub.2) provide
significantly improved retention of contaminants within the MKP
matrix.
TABLE IV ______________________________________ ANS 16.1
Results for MKP Ceramic Products Containing .sup.99 Tc CBPC
Product Fabricated Test Results .sup.99 Tc Effective
Concentration Diffusivity Leachability Composition (ppm)
(cm.sup.2 /s) Index ______________________________________ MKP +
Eluted Waste 20 1.20E-09 8.92 MKP + Eluted Waste 40 2.95E-09
8.53 MKP + SnCl.sub.2 + Eluted Waste 20 2.9E-12 11.54 MKP +
SnCl.sub.2 + Eluted Waste 40 5.4E-12 11.27 MKP + SnCl.sub.2 +
Eluted Waste 124 3.8E-15 14.42 MKP + SnCl.sub.2 + Precipitated
41 2.2E-14 14.6 .sup.99 Tc MKP + SnCl.sub.2 + Precipitated 16 4
2.3E-13 13.3 .sup.99 Tc MKP + SnCl.sub.2 + Precipitated 903
7.2E-15 14.6 .sup.99 Tc ______________________________________ *
The Nuclear Regulatory Commission (NRC) requires a leachability
index o at least 6.0.
Table V below provides results of Product Consistency Test
(PCT) conducted on the MKP ceramics fabricated according to the
second, precipitation approach. Normalized leaching rates of
.sup.99 Tc, after a 7-day test period at room temperatures,
e.g., 25.degree. C., were reported as low as 1.times.10-3
g/m.sup.2 -d. At an elevated temperature, e.g., 90.degree. C.,
the dissolution rate of the matrix increases, and, therefore,
the normalized leaching rate for .sup.99 Tc also increased to
the 10.sup.-2 to 10.sup.-1 g/m.sup.2 -d range. Significantly,
for both the room temperature and elevated temperature testes,
the MKP ceramics with the highest .sup.99 Tc loadings
demonstrated the lowest normalized leaching rate. The PCT test
was initially designed to evaluate chemical durability of
crushed borosilicate glass. A comparison between PCT test
results at 90.degree. C. for high-temperature encapsulation of
.sup.99 Tc in borosilicate glass, resulting in a leach rate as
low as 10.sup.-2 g/m.sup.2 -d, versus the low-temperature
encapsulation of .sup.99 Tc within MKP ceramics stabilized with
a reducing agent, as reported in Table IV, show that the present
invented low-temperature encapsulation is a viable and
competitive approach.
TABLE V ______________________________________ PCT Results for
MKP Ceramic Products Containing .sup.99 Tc Test .sup.99 Tc
Temper- Concen- Normalized ature tration Leaching Composition
(.degree. C.) (ppm) Rate (g/m.sup.2 -d)
______________________________________ MKP + SnCl.sub.2 +
Precipitated .sup.99 Tc 25 40 3.9E-3 MKP + SnCl.sub.2 +
Precipitated .sup.99 Tc 25 164 8.5E-3 MKP + SnCl.sub.2 +
Precipitated .sup.99 Tc 25 903 1.1E-3 MKP + SnCl.sub.2 +
Precipitated .sup.99 Tc 90 40 7.2E-2 MKP + SnCl.sub.2 +
Precipitated .sup.99 Tc 90 164 1.1E-1 MKP + SnCl.sub.2 +
Precipitated .sup.99 Tc 90 903 3.6E-2
______________________________________
The foregoing description of a preferred embodiment of the
invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and obviously many
modifications and variations are possible in light of the above
teaching. The embodiments described explain the principles of
the invention and practical applications and should enable
others skilled in the art to utilize the invention in various
embodiments and with various modifications as are suited to the
particular use contemplated. While the invention has been
described with reference to details of the illustrated
embodiment, these details are not intended to limit the scope of
the invention, rather the scope of the invention is to be
defined by the claims appended hereto.
US Patent # 6,153,809
Polymer Coating for Immobilizing Soluble
Ions in a Phosphate Ceramic Product
Singh, et al.
Abstract: A polymer coating is applied to the surface
of a phosphate ceramic composite to effectively immobilize
soluble salt anions encapsulated within the phosphate ceramic
composite. The polymer coating is made from ceramic materials,
including at least one inorganic metal compound, that wet and
adhere to the surface structure of the phosphate ceramic
composite, thereby isolating the soluble salt anions from the
environment and ensuring long-term integrity of the phosphate
ceramic composite.
References Cited
U.S. Patent Documents: 4315831 ~ 5318730 ~ 5402455 ~
5645518 ~ 5830815 ~ 5846894
Other References: Modified Phosphate Ceramics for
Stabilization and Solidification of Salt Mixed Wastes, authored
by Dileep Singh, Kartikey Patel, Arun S. Wagh, Seung-Young
Jeong, published in the Proceedings of Spectrum '98,
International Conference on Decommissioning and Decontamination
and on Nuclear and Hazardous Waste Management, Denver, Colorado,
Sep. 13-18, 1998. .
Modified Phosphate Ceramics for Stabilization of Salt Mixed
Wastes, thesis authored by Kartikey Patel, submitted for review
on or after May 7, 1998 and defended on Jun. 26, 1998. .
U.S. Patent Application Serial No. .sub.-- /124,822,
Continuation-in-part of U.S. Patent Nos. 5,846,894 and
5,830,815. .
U.S. Patent Application Serial No. .sub.-- /617,284,
Continuation-in-part of U.S. Patent No. 5,830,815. .
PCT Patent Application No. PCT/US97/04132.
Description
TECHNICAL FIELD
This invention relates to the use of chemically bonded
phosphate ceramics (CBPCs) for immobilizing large volumes of
low-level mixed waste material, and, in particular, to a
polymeric coating that increases the leach resistance in CBPCs
encapsulating waste containing salt anions.
BACKGROUND OF INVENTION
Low-level mixed waste streams are composed of aqueous liquids,
heterogeneous debris, inorganic sludge and particulates, organic
liquids, and soils. Of particular concern are low-level mixed
waste streams that are high in salt content, especially those
salt waste streams generated as sludge and solid effluents in
nuclear processing applications. For example, the extraction of
plutonium and uranium from their ore matrices by the use of
strong acids or precipitation techniques produces nitrate salt
and heavy metal waste. Chemical compositions typically found in
salt waste streams, either high in chloride or high in nitrate,
include aluminum trihydroxide, sodium phosphate, MicroCel E
(CaSiO.sub.3), water, sodium chloride, trichloroethylene,
calcium sulfate, sodium nitrate, and oxides of lead, chromium,
mercury, iron, cadmium, and nickel, among other compounds.
Stabilization of salt waste requires that the contaminants and
soluble salt anions are effectively immobilized. No single
stabilization and solidification technology is known to
successfully treat and dispose of salt waste, due to the
physical and chemical diversity of salt waste streams.
Generally, stabilization refers to the conversion of the waste
into a less soluble form, while solidification refers to the
micro-encapsulation of the waste in a monolithic solid of high
structure integrity. Conventional thermal waste treatment
methods, such as incineration or vitrification, are expensive
and largely unsuitable for the treatment of salt waste streams
because of their reliance on high temperature steps that risk
the release of volatile contaminants and the generation of
undesirable (e.g., pyrophoric) secondary waste streams. In
addition, thermal treatments produce hot spots that affect the
quality of a solidified final waste form.
A low-temperature approach is to stabilize hazardous waste by
using inorganic (e.g., pozzolanic) binders, such as cement,
lime, kiln dust, and/or fly ash. Disadvantages of this approach
include a high sensitivity to the presence of impurities, high
porosity, and low waste loading volume. Organic binders (e.g.,
thermosetting polymers) are used even less frequently, because
of cost and greater complexity of application. Organic binders
are not compatible with water-based wastes, unless the waste is
first pre-treated and converted to an emulsion or solid, and
organic binders are subject to deterioration from environmental
factors, including biological action and exposure to ultraviolet
light.
Recently, an alternative non-thermal, low-temperature approach
has been developed at Argonne National Laboratory for
stabilizing and solidifying low-level mixed waste by
incorporating or loading the waste into a phosphate ceramic
waste form having a high structural integrity. This technique
immobilizes the waste by solidification, such that the waste is
physically micro-encapsulated within the dense matrix of the
phosphate ceramic waste form, and/or stabilizes the waste by
converting the waste into their insoluble phosphate forms.
Ceramic encapsulation systems are particularly attractive given
that the bonds formed in these systems are ether ionic or
covalent, and hence stronger than the hydration bonds in cement
systems. Also, the ceramic formulation process is exothermic and
economical.
Phosphates are particularly good candidates for stabilization
of radioactive and hazardous waste, because phosphates of radio
nuclides and hazardous metals are essentially insoluble in
groundwater. A salient feature of the low-temperature ceramic
phosphate formulation process is an acid-base reaction. For
example, magnesium phosphate ceramic waste forms have been
produced by reacting magnesium oxide (MgO) with phosphoric acid
to form a phosphate of magnesium oxide, Newberyite, as
represented in the chemical equation (1), below.
The acid-base reaction results in the reaction of the waste
components with the acid or acid-phosphates, leading to chemical
stabilization of the waste. In addition, encapsulation of the
waste in the phosphate ceramic results in physical containment
of the waste components. The reaction represented above in
Equation (1) occurs rapidly and generates heat, and upon
evaporation of the water, a porous ceramic product results.
U.S. Pat. No. 5,645,518 issued to Wagh, et al., incorporated
herein by reference, describes in detail the process steps for
setting liquid or solid waste in CBPC products using acid-base
reactions. Accordingly, the process involves mixing ground solid
waste, including salt waste spiked with heavy metals, with a
starter powder of oxide and hydroxide powders of various
elements; slowly adding the waste-powder mixture to an acid
solution of phosphoric acid or soluble acid phosphates;
thoroughly mixing the waste-powder-acid mixture for about a half
hour to an hour at ambient temperatures (less than 100.degree.
C.), such that the components of the mixture chemically react to
form stable phases and a reacted viscous slurry or paste
results; and allowing the slurry or paste to set for a few hours
into the final CBPC product. Liquid waste is similarly
stabilized by mixing the liquid waste with the acid solution
(preferably 50:50), and then reacting the waste-acid mixture
with the starting powders. The maximum temperature for the
process is about 80.degree. C. The CBPC products attain full
strength in about three weeks, and exhibit a complex structure,
including a major crystalline phase, e.g., Newberyite
(MgHPO.sub.4.3H.sub.2 O), and an insoluble, stable phase. The
waste components are generally homogeneously distributed within
the phosphate ceramic matrix. Unfortunately, however, the porous
product (Newberyite) is unsuitable for waste treatment on a
large scale.
U.S. Pat. No. 5,830,815 issued to Wagh, et al., incorporated
herein by reference, describes improving the CBPC fabrication
process by incorporating two temperature control processes for
both reducing heat generation during the encapsulation
(reaction) steps and moderating pH conditions (some wastes are
unstable at a low pH). The first temperature control process
involves pre-treating the phosphoric acid with a carbonate,
bicarbonate or hydroxide of a monovalent metal (e.g., K, Na, Li,
Rb) prior to mixing with an oxide or hydroxide powder so as to
buffer the acid. In particular, potassium containing alkali
compounds (e.g., K.sub.2, KHCO.sub.3, KOH) result in a more
crystalline waste form, and the higher the concentration of
potassium in the potassium containing compound, the more
crystalline the final product, resulting in a higher compression
strength, lower porosity, and greater resistance to weathering,
compressive forces, and leaching. The second temperature control
process involves bypassing the use of the acid and mixing the
oxide powder with dihydrogen phosphates of potassium, sodium,
lithium, or other monovalent alkali metal, to form a ceramic at
a higher pH.
Neutralizing the phosphoric acid solution in equation (1) by
adding potassium hydroxide (KOH), as represented in the chemical
equation (2) below, reduces the reaction rate and heat
generation, and results in the formation of a superior magnesium
potassium phosphate (MKP) mineral product, MgKPO.sub.4.6H.sub.2
O (magnesium potassium phosphate hexahydrate), as represented in
chemical equation (3) below.
The chemically bonded ceramic phosphate (CBPC) waste form (e.g,
MgKPO.sub.4.6H.sub.2 O) is a dense, hard material with excellent
durability and a high resistance to fire, chemicals, humidity,
and weather. The low-temperature (e.g., room-temperature)
process encapsulates chloride and nitrate salts, along with
hazardous metals, in magnesium potassium phosphate (MKP)
ceramics, with salt waste loadings of up to between
approximately 70 weight percent and approximately 80 weight
percent. This durable MKP ceramic product has been extensively
developed and used in U.S. Department of Energy waste treatment
projects.
Phosphates in general are able to bind with hazardous metals in
insoluble complexes over a relatively wide pH range and most
metal hydroxides have a higher solubility than their
corresponding phosphate forms. In addition to the magnesium and
magnesium-potassium phosphate waste products discussed above,
known waste encapsulating phosphate systems include, but not
limited to, phosphates of magnesium-ammonium, magnesium-sodium,
aluminum, calcium, iron, zinc, and zirconium (zirconium is
preferred for cesium encapsulation). A non-exclusive summary of
known phosphate systems and processing details is provided in
Table I below, selected according to ready availability of
materials and literature about the materials, in addition to low
cost.
TABLE I ______________________________________ Phosphate
Systems and Processing Details Curing System Starting Materials
Solution Time ______________________________________ MKP Ground
MgO, ground K Water 1 hour dihydrophosphate crystals Mg
phosphate Calcined MgO Phosphoric >8 days acid-water (50/50)
Mg--NH.sub.4 phosphate Crushed dibasic NH.sub.4 Water 21 days
phosphate crystals mixed w. calcined MgO Mg--Na phosphate
Crushed dibasic Na Water 21 days phosphate crystals mixed w.
calcined MgO Al phosphate Al(OH).sub.3 powder Phosphoric Reacted
acid powder, (.apprxeq.60.degree. C.) pressed Zr phosphate
Zr(OH).sub.4 Phosphoric 21 days acid
______________________________________
Appropriate oxide powders include, but are not limited to, MgO,
Al(OH).sub.3, CaO, FeO, Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4,
Zr(OH).sub.4, ZrO, and TiO.sub.2, and combinations thereof. The
oxide powders may be pre-treated (e.g., heated, calcined,
washed) for better reactions with the acids. While no pressure
is typically applied to the reacted paste, about 1,000 to 2,000
pounds per square inch may be applied when zirconium-based
powders are used.
The acid component may be dilute or concentrated phosphoric
acid or acid phosphate solutions, such as dibasic or tribasic
sodium, potassium, or aluminum phosphates, and the paste-setting
reactions are controllable either by the addition of boric acid
to reduce the reaction rate, or by adding powder to the acid
while concomitantly controlling the temperature.
Representative bulk constituents for salt waste include, but
are not limited to, activated carbon, Na.sub.2 (CO.sub.3).sub.2,
widely used cation or anion exchange resins, water, NaCl,
Na(NO.sub.3).sub.2, Na.sub.3 PO.sub.4, and Na.sub.2 SO.sub.4.
The salt waste may be reacted with phosphoric acid to any
consume carbon dioxide (CO.sub.2) present, prior to mixing the
salt waste with the oxide powders or binding powders, as the
evolution of CO.sub.2 results in very porous final ceramic
products.
Unfortunately, however, encapsulation of low-level mixed waste
into CBPC products is currently of limited practical use for
waste that is predominantly comprised of salts, such as
chlorides, nitrates, and sulfates. Efforts to encapsulate salt
waste in phosphate ceramic products are hampered by low maximum
waste loading capacities, because of interference of the salt
anions with ceramic-setting reactions, leaching of soluble salt
anions from the resulting highly porous ceramic product
(especially in aqueous environments), and rapid structural
degradation of the ceramic product caused by the high leach
rates. Also, environmental stresses degrade the integrity of
known CBPC waste forms over time. For example, exposure to
repeated cycles of wetting, drying and/or freezing, or acidic or
other conditions conducive to leaching may affect the long term
effectiveness of waste encapsulated CBPC waste forms.
A need in the art exists for a method for disposing of salt
waste that involves a low-temperature stabilization process and
improves resistance to leaching, without degrading the integrity
of the ceramic phosphate product.
The present invention is a process and product for safely
containing radioactive and/or hazardous waste comprised of salt
anions in a phosphate ceramic product, involving a new and
surprisingly effective immobilization technique. The invented
process and product involves the application of a specific
polymer coating to the exterior surface of a phosphate ceramic
composite encapsulating waste, such that the polymer coating
infiltrates the surface structure and adheres to and/or bonds to
the phosphate ceramic composite matrix, effectively isolating
the waste from the environment and improving the leach
resistance of the phosphate ceramic composite. The polymer
coating contains at least one inorganic metal compound,
preferably an inorganic metal oxide of magnesium or silicon.
Therefore, in view of the above, a basic object of the present
invention is to provide an improved process and product for
immobilizing hazardous, radioactive, and/or mixed salt waste in
phosphate ceramic composites.
Another object of the invention is to provide a safe, low
temperature, economical process and product for immobilizing
salt waste in a phosphate ceramic product that increases the
loading capacity and improves the leach resistance of the salt
waste within the phosphate ceramic product.
A further object of the invention is to provide process and
product for immobilizing large volumes of salt waste in a
durable, long term storage phosphate ceramic product.
Additional objects, advantages, and novel features of the
invention are set forth in the description below and/or will
become apparent to those skilled in the art upon examination of
the description below and/or by practice of the invention. The
objects, advantages, and novel features of the invention may be
realized and attained by means of instrumentation and
combinations particularly pointed out in the appended claims.
BRIEF SUMMARY OF THE INVENTION
Briefly, the present invention is a surprisingly effective
process and product for immobilizing waste having a high
concentration of salt in chemically bonded phosphate ceramic
(CBPC) products. The invention involves a new coating step,
wherein a select polymer coating is applied to the surface of a
fabricated salt waste loaded CBPC product, such that the coating
infiltrates the surface structure of the CBPC product and
adheres to the phosphate ceramic matrix, thereby isolating
soluble salt anions from the environment and ensuring long-term
integrity of the phosphate ceramic system. The fabricated salt
waste loaded CBPC product is formulated by methods known in the
art.
A critical feature of the invention is the selection of the
polymer coating, which contains at least one inorganic metal
compound. Preferably, the polymer coating is a polymer resin
comprised of fine powders of magnesium oxide and/or silicon
oxide. The powders of the coating material act as wetting agents
that apparently cause mechanical and/or chemical bonding between
the phosphate ester in the surface structure of the CBPC product
and the polymer coating composition. The polymer coating
infiltrates and macro-encapsulates the CBPC product to improve
durability and leach resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
The appended claims set forth those novel features which
characterize the invention, however, the invention itself, as
well as further objects and advantages thereof, will best be
understood with reference to the following detailed description
of a preferred embodiment, in conjunction with the accompanying
drawings, where like reference characters identify like elements
throughout the various figures, in which:
FIG. 1 is a schematic diagram of a method for
fabricating magnesium potassium phosphate (MKP) ceramic waste
products loaded with surrogate salt waste;
FIG. 2 is a scanning electron microscopy (SEM)
photomicrograph of a fractured surface of an MKP ceramic waste
product loaded with 58% surrogate salt waste;
FIG. 3 is a high magnification (2000.times.) scanning
electron microscopy (SEM) photomicrograph of the surface of a
polymer coated MKP ceramic waste product loaded with surrogate
salt;
FIG. 4 is a very high magnification (7500.times.)
scanning electron microscopy (SEM) photomicrograph of the
surface of a polymer coated MKP ceramic waste product loaded
with surrogate salt waste;
FIG. 5 is a low magnification (350.times.) scanning
electron microscopy (SEM) photomicrograph of the interface
between a MKP ceramic waste product loaded with surrogate salt
waste and a polymer coating applied thereon;
FIG. 6 is a high magnification (2000.times.) scanning
electron microscopy (SEM) photomicrograph of the interface
between a MKP ceramic waste product loaded with surrogate salt
waste and a polymer coating applied thereon; and
FIG. 7 is a graphical illustration of cumulative nitrate
leaching for MKP ceramic products with and without the invented
polymer coating.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to an improved process and
product for immobilizing waste in a chemically bonded phosphate
ceramic (CBPC) waste form. As described in detail in the
background section above, although methods for fabricating CBPC
products encapsulating waste materials are well known, the known
CBPC encapsulation methods are ineffective for containing wastes
having a high concentration of salt.
The present invention modifies known CBPC encapsulation methods
and products to include a unique immobilization step that
specifically addresses problems experienced in the art due to
the presence of soluble salt anions in the waste stream.
According to the present invention, a polymer coating is applied
to the exterior surface of the CBPC product to infiltrate the
complex surface structure of the CBPC product and bond and/or
adhere thereto, such that salt waste is effectively
macro-encapsulated with in phosphate ceramic matrix and isolated
from the environment. Advantageously, the polymer surface
coating protects the CBPC waste form from environmental stresses
by providing a greater resistance to air, water, organic
liquids, acids, and alkalis, among other conditions. The polymer
surface coated CBPC waste form also has improved mechanical
properties, such as greater hardness and high abrasion
resistance.
The polymer coating has three main components: the binder, the
pigment, and the solvent. The binder provides adhesion and
cohesion between the coating and the CBPC surface, the pigment
is a fine powder that provides the coating with color and
hardness, and the solvent is a volatile liquid for dissolving
solid or semi-solid binders. The pigment has considerable
influence on the consistency of the properties of the polymer
coating and contributes to its abrasion and weather resistance.
A feature of the invention is the inclusion of at least one
inorganic metal compound in the binder component of the polymer
coating. Preferred inorganic metal compounds are inorganic metal
oxides, such as magnesium oxide (MgO) and/or silicon oxide
(SiO.sub.2). These inorganic metal compounds may be in the form
of magnesite (MgCO.sub.3), talc (Mg.sub.2 (Si.sub.2
O.sub.5).sub.2.Mg(OH).sub.2), or borosilicate glass (i.e.,
silicate glass with at last 5% boron oxide). These ceramic
materials provide an excellent interface adhesion between the
polymer coating and the surface and infiltrated structure of the
CBPC product, apparently caused by mechanical and chemical
interactions between the phosphate ester comprising the CBPC
product and the ceramic coating composition. Polymer coating
materials that do not contain ceramic, inorganic metal compounds
peel off of the surface of the phosphate ceramic product after
curing.
The most preferred polymer material is a commercially available
thermoset polyester resin that is comprised of a polyester resin
binder, magnesite, talc, or soda-lime glass pigment, a styrene
monomer solvent, and also a benzoyl peroxide initiator.
Generally preferred polymer coatings are comprised of
unsaturated polyester resins that are straight-chain polymers
having reactive double bonds at intervals along the chain. In
their popular form, unsaturated polyester resins are supplied as
solutions in vinyl monomer (e.g., styrene), and copolymerization
is activated by the addition of an initiator (e.g., organic
peroxides or hydroperoxides) and promoters (e.g., metallic
dryers, cobalt octoate, naphthenate). Copolymerization results
in the cross-linking of polyester chains by the formation of
polmerized vinyl monomors.
According to the preferred method of the present invention, the
polymer material is applied to the exterior surface of a
phosphate ceramic product as a thin film by adding the initiator
to the pigment and the binder, mixing the
initiator-pigment-binder composition for a few minutes to form a
slurry, uniformly coating the exterior surface of the phosphate
ceramic product with the slurry, and chemically drying the
coating by allowing sufficient time for the slurry to infiltrate
the phosphate ceramic product surface, such that the slurry
completely wets and adheres to the surface. Although the polymer
coating hardens in about ten minutes, a curing time of 24 hours
is preferred. The polymer coating is subjected to a chemical
drying step, e.g., curing, a process in which the molecules of
the binder chemically react with one other to form bonds within
the film by primary valences. These bonds are very strong and
not susceptible to dissolution by the action of solvents. Thus,
a feature of the invention is the subjection of the surface
coated CBPC product to a chemical drying step that converts the
coating from a fluid to a solid state, wherein chemical
reactions occur to anchor the thin film coating to the CBPC
surface.
Table II below provides the results of the American Nuclear
Society's ANS 16.1 Standard Test for nitrate and chloride loaded
polymer coated MKP ceramic products. Generally, the ANS 16.1
Standard Test studies leachability of contaminants contained in
matrices in an aqueous environment over time and evaluates
retention rates by calculating a leachability index value from
the test data. (The leachability index is the negative logarithm
of the effective diffusivity coefficient). Sample polymer coated
salt loaded MKP ceramic products were placed in the leaching
solution for a fixed period of time, after which the leaching
solution was analyzed for specific ions. As shown in Table II,
the chloride leaching was excessively low, with the chloride ion
reading below the detection limit even after a cumulative 96
hours of exposure. The nitrate leaching was relatively higher.
TABLE II ______________________________________ Cumulative
Leaching of Chloride and Nitrate Ions from Polymer Coated MKP
Ceramic Products Cumulative Chloride Ion (Cl.sup.-) Nitrate Ion
(NO.sub.3.sup.-) Time (hours) (ppm) (ppm)
______________________________________ 2 ND 3.96 7 ND 5.28 24 ND
2.20 48 ND 3.08 72 2.64 96 ND 2.20 456 3.4 13.20 1128 * 43.12
2136 * 176.00 ______________________________________ ND
indicates None Detected; *indicates test in progress.
Salt waste is generally highly reactive and therefore its
flammability is of concern, in view of transportation and
storage issues. Department of Transportation (DOT) oxidation
tests conducted on polymer coated salt loaded phosphate ceramic
products demonstrated that because phosphate ceramics are
inorganic ceramic-type materials, they advantageously inhibit
the spread of flames and are an excellent solidification medium
for flammable salt waste.
The resulting phosphate ceramic materials may be used to
produce building and construction materials, e.g., engineering
barrier systems.
EXAMPLE
Nitrate Loaded Polymer Coated MKP Ceramic Product
Surrogate waste having the composition listed below in Table
III was prepared in the laboratory and mixed for 72 hours using
mixing rollers. The surrogate waste was chemically treated by
mixing the surrogate waste first with an aqueous solution
containing a small amount of sodium monosulfide (Na.sub.2 S) for
about 8 to 10 minutes to efficiently convert mercury (Hg) into
its most stable form of mercury sulfide (HgS), and next treating
the surrogate waste with tin chloride (SnCl.sub.2) for about 5
minutes to reduce the valency of chromium from +6 to a less
toxic, less water soluble oxidation state of +3.
TABLE III ______________________________________ Surrogate
Waste Composition Constituent wt % Contaminant ppm
______________________________________ Fe.sub.2 O.sub.3 6.0 PbO
1000 Al.sub.2 (OH).sub.3 4.0 CrO.sub.3 1000 Na.sub.3 PO.sub.4
2.0 HgO l000 Mg(OH).sub.2 4.0 CdO 1000 CaSiO.sub.3 8.0 NiO 1000
Portland Cement 2.0 H.sub.2 O 14.0 NaNO.sub.3 (nitrate salt)
60.0 ______________________________________
Magnesium potassium phosphate (MKP) ceramic waste products
incorporating the surrogate waste were fabricated by methods
generally shown in FIG. 1 for waste loadings of 58% and 70%.
Accordingly, a binder was formed by spontaneously reacting a
stoichiometric amount of well mixed, calcined magnesium oxide
(MgO) powder and monopotassium phosphate (KH.sub.2 PO.sub.4),
under aqueous conditions and constant stirring, in four
successive batches at one minute intervals, to produce magnesium
potassium phosphate (MgKPO.sub.4.6H.sub.2 O), according to
Equation (3) above. The resulting binder has a highly
crystalline ceramic structure and a solubility product constant
as low as 10.sup.-12.
The chemically treated surrogate waste and binder were combined
to form a slurry that initially experienced a few degrees
decrease in temperature due to the dissolution of the phosphate
crystals in the water. Upon dissolution of the phosphate, the
temperature increased to about 35.degree. C., and the slurry
having a pH of about 6 to 7 was stirred thoroughly for about 18
to 20 minutes, or until the slurry started to set. The slurry
was hardened in molds for about 2 to 5 hours, resulting in
dense, monolithic, chemically bonded phosphate ceramic (CBPC)
waste products. After 14 days of curing, the CBPC waste products
were subjected to variance performance tests, including
strength, leaching and characterization.
FIG. 2 is a high magnification (2000.times.) scanning electron
microscopy (SEM) photomicrograph of a fractured surface of a
magnesium potassium phosphate (MKP) ceramic waste product loaded
with 58% surrogate salt waste. The photomicrograph shows a very
dense, crystalline structure with a small amount of pores. Pores
allow water to penetrate the waste form, causing nitrates to
(e.g., NaNO.sub.3) to dissolve and leach into the environment.
According to the present invention, a select number of the CBPC
waste products were coated with an unsaturated polyester resin
system to further immobilize the surrogate waste within the CBPC
waste products. FIGS. 3 and 4 show high (2000.times.) and very
high (7500.times.) magnification SEM photomicrographs,
respectively, of the polymer coated surface of a CBPC waste
product. The photomicrographs show a very smooth, substantially
pore free surface structure, demonstrating a very low
possibility for water to penetrate into the polymer coated CBPC
waste product through its surface structure, and the prevention
of nitrate dissolution and subsequent leaching. FIGS. 5 and 6
show low (350.times.) and high (2000.times.) magnification SEM
micrographs of the interface between a CBPC waste product loaded
with surrogate waste and a polymer coating applied thereon. As
shown in FIGS. 5 and 6, the polymer coating has completely wet
and adhered to the phosphate ceramic surface, resulting in a
CBPC waste product having superior leaching performance. The
polymer coating-CBPC waste product interface also appears to be
essentially free of cracks demonstrating high compression
strength and excellent compatibility between the polymer coating
and the CBPC waste product.
Table IV below provides the results of density and compression
strength tests conducted on the uncoated and polymer coated
magnesium potassium phosphate (MKP) ceramic products loaded with
58 weight percent and 70 weight percent nitrate salts. The
compression strength of the waste forms are well above of the
Nuclear Regulatory Commission (NRC) minimum requirement of 500
psi.
TABLE IV ______________________________________ Structure
Properties of MKP and Nitrate Waste Products Uncoated Uncoated
Polymer Coated 58 wt % Salt 70 wt % Salt 58 wt % Salt Property
Waste Waste Waste ______________________________________ Density
(g/cc) 1.893 2.000 1.691 Compression Strength 1400 .+-. 160 1900
.+-. 180 1970 (PSI) ______________________________________
FIG. 7 is a graphical illustration of cumulative nitrate
leaching for nitrate loaded MKP ceramic products with and
without the polymer (unsaturated polyester resin) coating. As
depicted, the polymer coated nitrate loaded MKP ceramic product
immobilized the nitrate ions significantly more effectively than
the uncoated nitrate loaded MKP ceramic product. A comparison of
the leachability index for the polymer coated nitrate loaded MKP
ceramic product versus an uncoated nitrate loaded MKP ceramic
product is provided in Table V, below. The calculated
leachability index for the polymer coated nitrate loaded MKP
ceramic product was greater than 12, substantially above the ANS
16.1 standard leachability index of at least 6.0. Generally, the
leachability index is related to the effective diffusivity in
that the higher the leachability index, the lower is the
effective diffusivity, resulting in a more favorable retention
of a contaminant within a matrix. These results demonstrate that
the essentially pore free surface structure of the polymer
coated salt waste loaded MKP ceramic product provides superior
immobilization of the waste salts than uncoated salt loaded
phosphate ceramic products currently known in the art.
TABLE V ______________________________________ ANS 16.1 Results
for Various Waste Containment Products NO.sub.3.sup.- in Waste
Fraction Waste Containment of NO.sub.3.sup.- Effective
Containment Product Leached Diffusivity Leachability Product
(ppm) Out (cm.sup.2 /s) Index (LI)
______________________________________ Uncoated, 58 wt % 218700
0.33 6.31 .times. 10.sup.-8 7.20 Loaded Uncoated, 70 wt % 260600
0.35 5.82 .times. 10.sup.-8 7.24 Loaded Polymer Coated 218700
0.0169 6.87 .times. 10.sup.-13 12.16 58 wt % Loaded
______________________________________
Alternative coating systems were tested, including fly ash
coatings, epoxy resins, and rubber derivatives. The fly ash
coating system exhibited excellent film integrity and good waste
form compatibility, while the epoxy resin and rubber derivative
coating systems demonstrated very poor film integrity and waste
form compatibility.
The foregoing description of a preferred embodiment of the
invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and obviously many
modifications and variations are possible in light of the above
teaching. The embodiments described explain the principles of
the invention and practical applications and should enable
others skilled in the art to utilize the invention in various
embodiments and with various modifications as are suited to the
particular use contemplated. While the invention has been
described with reference to details of the illustrated
embodiment, these details are not intended to limit the scope of
the invention, rather the scope of the invention is to be
defined by the claims appended hereto.
US Patent # 6,204,214
Pumpable/injectable phosphate-bonded
ceramics
S. Dileep, et al.
Abstract -- A pumpable ceramic composition is provided
comprising an inorganic oxide, potassium phosphate, and an oxide
coating material. Also provided is a method for preparing
pumpable ceramic-based waste forms comprising selecting
inorganic oxides based on solubility, surface area and
morphology criteria; mixing the selected oxides with phosphate
solution and waste to form a first mixture; combining an
additive to the first mixture to create a second mixture; adding
water to the second mixture to create a reactive mixture;
homogenizing the reactive mixture; and allowing the reactive
mixture to cure.
References Cited
U.S. Patent Documents
4347325 ~2391493 ~ 3357843 ~ 3392037 ~ 3540897 ~ 3647488 ~
3920464 ~ 3923534 ~ 3960580 ~ 4160673 ~ 275091 ~ 4298391 ~
4444594 ~ 4459156 ~ 4836854 ~ 4843044 ~ 4921536 ~
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a pumpable/injectable ceramic, and
more particularly this invention relates to a ceramic
composition that maintains low viscosity for extended periods of
time to allow the composition to be pumped or injected into hard
to reach geologic or manmade locations.
2. Background of the Invention
Disposal of hazardous waste, low-level radioactive waste or
benign waste continues to present problems. Landfill space is
becoming more scarce so that only nonrecyclable material or
nonbiodegradable material is often considered the only
candidates for land filling.
When hazardous material is land filled, care must be taken to
prevent destabilization of the material, so that leaching will
not occur.
Aside from land filling, efforts have been made to combine
to-be-disposed-of-material with cement so as to form solid
monoliths or waste forms for burial or for use as structural
products. For example, U.S. Pat. No. 4,432,666 to Frey et al.,
discloses using cement and other water repellant binders to
dispose of waste thought to be damaging to the environment.
However, cement is unstable in many situations, for example when
attempts are made to encapsulate halogenated materials.
U.S. Pat. No. 3,093,593 to Arrance discloses a method for
vitrifying silicate materials to encapsulate radioactive waste.
However, temperatures of up to 1,400.degree. C. are required to
produce final waste forms.
In U.S. Pat. No. 5,645,518, awarded to the instant Assignee, a
method to stabilize low-level mixed wastes, such as radioactive
medical wastes and other such materials, is provided, wherein
phosphate ceramics physically and chemically stabilize the waste
at ambient temperatures. However, in such endeavors, final waste
forms rapidly set. This rapid-setting feature causes various
degrees of unworkability to the process, particularly in
situations where low viscosity and long operational times are
required. Such situations include where the material is to be
blown, poured or injected into deep wells or crevices, in-situ
stabilization of buried wastes, remediation at nuclear-accident
and waste spillage sites, and pumpable refractory applications.
In addition to the drawbacks of the above-mentioned processes,
commercially supplied materials for use as components
(particularly the oxides) of the above processes are in forms
adverse to formulating flowable mixtures. As a result,
exothermic reactions become uncontrollable, leading to thicker
slurries, with the final product lacking homogeneity. Also, the
resulting rapid setting time of the ceramic does not provide
adequate working time.
A need exists in the art for a formulation and a waste
encapsulation process to accommodate stabilization of a myriad
of waste materials in a myriad of deposition scenarios. The
formulation and process should provide waste liquors having
protracted, workable consistencies or viscosities to accommodate
currently available pump- injection-, or spray
application-equipment. The process should also provide a
protocol for selecting and preparing components of the
formulation so as to tailor the formulation for situations
requiring varying degrees of viscosity.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a
formulation comprising waste and waste-encapsulation material
that overcomes many of the disadvantages of the prior art.
It is another object of the present invention to provide a
method for using waste as a bulk-component in pumpable hardening
agents. A feature of the invention is the use of additives to
slow the reaction between components of the hardening agents. An
advantage of the invention is the extension of time of
workability. Another advantage is the employ of heretofore
unusable waste.
Still another object of the present invention is to provide a
means for converting commonly available oxides into a component
in pumpable hardening agents. A feature of the invention is
calcining the oxides and then mixing the calcined oxides with
coating agents. An advantage of the invention is the reduction
in reaction rate of the calcined oxide in relation to the other
components of the hardening agents, leading to extended
workability time of the hardening agent prior to curing.
Yet another object of the present invention is to provide
selection criteria for oxides to be used in the production of
pumpable ceramic hardening formulations. A feature of the
invention is the use of solubility, porosity and morphology
characteristics of the oxides to differentiate acceptable oxides
from unacceptable oxides. An advantage of the invention is that
the characteristics are determinable prior to mixing or forming
of waste forms. Another advantage is that oxides having the
desired characteristics can be produced via calcination.
Briefly, the invention provides pumpable ceramic composition
comprising an inorganic oxide, potassium phosphate, and an oxide
coating material.
Also provided is a pumpable ceramic-based waste formulation
comprising 7 to 14 weight percent MgO, 25 to 40 weight percent
KH.sub.2 PO.sub.4, 15 to 50 weight percent ash, 1 to 4 weight
percent boric acid, 0.5 to 2 weight percent lignosulfonate, and
15 to 25 weight percent water.
The invention also provides a method for preparing pumpable
ceramic-based waste forms comprising selecting inorganic oxides
based on solubility, surface area and morphology criteria;
mixing the selected oxides with phosphate solution and waste to
form a first mixture; combining an additive to the first mixture
to create a second mixture; adding water to the second mixture
to create a reactive mixture; homogenizing the reactive mixture;
and allowing the reactive mixture to cure.
BRIEF DESCRIPTION OF THE DRAWING
The present invention together with the above and other objects
and advantages may best be understood from the following
detailed description of the embodiment of the invention
illustrated in the drawings, wherein:
FIG. 1 is a schematic diagram of a process for producing
pumpable ceramic waste forms, in accordance with features of the
present invention;
FIG. 2 is an x-ray diffraction analysis of the invented
pumpable ceramic waste form;
FIGS. 3A and 3B are photomicrographs depicting
acceptable and unacceptable oxide components, respectively, of a
ceramic formulation, in accordance with features of the present
invention;
FIGS. 4A and 4B are x-ray diffraction analysis of
acceptable and unacceptable oxide components for use in the
invented pumpable formulation;
FIG. 5 is a decision tree for determining suitable
oxides, in accordance with features of the present invention;
and
FIG. 6 is a solubility curve of various oxides, in
accordance with features of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The inventors have found a method for incorporating wastes into
pumpable or sprayable mixtures which in turn are used to fixate
the wastes in hard-to-reach locations. Also, solely the binders
from such mixtures can be utilized and combined with waste in
situ for encapsulation, particularly in situations wherein the
waste is located in inaccessible or dangerous areas such as the
sarcophagus of a contaminated nuclear power plant, or waste
spillage sites. Generally, the method retards the setting action
of phosphate ceramics and also reduces the overall viscosity of
the phosphate ceramic slurry or the phosphate ceramic/waste
slurry long enough to allow the slurry to be pumped, sprayed or
otherwise transported to final points. Ultimately, the method
produces a hard, impenetrable waste form rivaling the durability
of concrete forms, and without the addition of heat.
The inventors also disclose herein a process for selecting
appropriate oxides for use as a component of the free flowing
binder. Also disclosed is a process for pretreating common
oxides for subsequent use as a component of the free-flowing
binder, as mentioned supra.
As to the first aspect of the invention, pumpable ceramic,
waste-encapsulating binder has been developed. The inventors
have found that the addition of certain compounds coats the
components of waste binders and serves to slow the
setting-reaction to the point where setting or curing is
retarded long enough for the reacting ceramic-waste liquor to be
pumped, sprayed, or otherwise deposited in its final resting
place. Once deposited, the ceramic waste liquor is allowed to
cure or set up to its very dense, final form. Despite the use of
coating materials to slow reaction speeds, the final product
proves to be harder than those produced using Portland cement.
Compression strengths of more than 4,000 pounds per square inch
are typical.
Fabrication of the pumpable waste-encapsulation material is
shown schematically as numeral 10 in FIG. 1. Briefly, oxide
powder 12 is mixed with phosphate powder/solution 14 and
previously sized waste material 16 in a first mixing process 17
and in various proportions.
Waste material of a size less than 200 microns is suitable. A
myriad of phosphate solutions can be utilized including
phosphates having cation moieties selected from the group
consisting of potassium, sodium, calcium, zirconium, iron,
magnesium-ammonium, and combinations thereof. A suitable
concentration of phosphate solution is one which creates a
slurry which when combined with other ingredients enumerated
below, provides a liquor with a centipoise value that
facilitates easy dispersion. Once such phosphate concentration
is where the weight ratio of water to KH.sub.2 PO.sub.4 is
approximately 0.66.
The mixing of the oxide powder, the phosphate solution and the
waste creates a first mixture or slurry 18. Optionally, the
oxide powder is subjected to a pretreatment process 11 prior to
mixing with the phosphate solution and waste material. Details
of the oxide pretreatment process are disclosed infra.
Suitable oxide: phosphate: waste weight proportions range from
1:3.4:1.45 to 1:3.4:6.6. When MgO is utilized as the oxide,
KH.sub.2 PO.sub.4 as the phosphate and ash as the waste
material, a particularly suitable MgO:KH.sub.2 PO.sub.4 :Ash
weight ratio is 1:3.4:1.45.
In one exemplary procedure, the slurry 18 is contacted with
additives 20 comprising various coating agents and water
"getters" to create a second mixture 21. However, the additives
could be added at separate points in the mixing process. For
example, coating agents, including but not limited to citric
acid or boric acid, can be added directly to the oxide powder 12
prior to the formation of mixing of the oxide with the phosphate
solution and waste material. Then, a water getter such as a
lignosulphonate can be added after the above three components
are thoroughly mixed together.
After integration of the additives, water 22 is added to the
resulting mixture and the now-hydrated liquor is mixed in a
second mixing stage 23 to create a homogenous, reacting liquor
25. Mixing times of approximately 20 minutes to 60 minutes are
typical.
The addition of water begins the reaction process 24. It is
during the reaction process 24 that the now-reacting liquor 25
can be pumped, jet grouted or otherwise manipulated 26 prior to
final setting occurring. Setting, stabilization or curing 28 of
the liquor 25 occurs no earlier than 2 hours after the water
addition step 22, discussed above.
A salient feature of the invented process is that no
externally-applied heat is required to effect reaction, pumping,
transporting and ultimately curing of the ceramic-waste liquor.
Despite the fact that additives are used to fluidize the
reaction liquor, the resulting final, cured waste form exhibits
superior qualities. For example, and as depicted in FIG. 2, the
process yields a waste form primarily comprising magnesium
potassium phosphate crystalline phase, whereby the crystalline
phase is represented by sharp peaks on the graph.
Oxide Preparation Detail
A myriad of oxides are suitable powders for the invention.
Calcium oxide, sodium oxide, zirconium oxide, iron oxide,
magnesium oxide and combinations thereof are all appropriate
starter powders.
The inventors have found that pretreatment 11 of the oxide
often enhances the flowability of the resulting ceramic
binder-waste mixture. However, pretreated oxide as supplied by
commercial suppliers does not provide the desired enhancements
to flowability. Rather, the invented calcining process disclosed
herein yields a binder with optimal properties such as high
strength and low porosity. Setting rates of approximately two
hours, low exothermicity (i.e., low heat generation) even during
setting, and a very homogenous structure result from employing
the invented oxide pretreatment process 11.
To produce suitable ceramic-binder, the oxide component should
have the following properties:
1. Average oxide particle sizes should be approximately 8 to 10
microns. Each particle should be dense, crystalline, and free
from any amorphous coating.
2. The oxide particles used should not result in substantial
heating of the ceramic binder-waste mixture during mixing. Also,
the oxide particle selection should not result in slurry
thickening.
3. The final product should be a homogenous ceramic and should
not result in precipitates forming in the slurry. Ultimate
compression strength should be at least 3,500 pounds per square
inch. Open or connected porosity of the product should be
approximately less than 10 percent by volume of the entire
monolith as formed and cured.
The inventors have found that oxides which do not satisfy the
above enumerated parameters are not suitable for use in the
stabilization of radioactive waste, or in applications for
producing high volume structural products. As such, the
following three test criteria have been developed for
determining the suitability of oxides. The criteria can be
applied to oxides provided by typical suppliers, or can be
applied to determine the efficacy of oxide pretreatment methods
employed on site by end users or others.
As illustrated in the decision tree depicted in FIG. 5, all
three of the following criteria should be satisfied to qualify
an oxide as a suitable component in a flowable ceramic binder
for waste:
A.) Surface area: Surface area of the powder should be in the
range of 0.3 to 0.55 m.sup.2 /g. Otherwise, the inventors have
found that lower surface area results from agglomeration of the
particles which in turn inhibits reaction between the oxide and
other reactants. Surface areas above the desired range results
in an accelerated reaction rate, leading to the slurry over
heating, in some cases causing the slurry to boil. Surface areas
of commercially available oxides are approximately 4 m.sup.2 /g.
B.) Morphology: Under a high resolution microscope (i.e.,
approximately 1500.times.), oxide particle surfaces should
appear smooth and with striations indicating crystalline
structure. Examples of acceptable and unacceptable morphologies
are depicted as FIGS. 3A and 3B, respectively.
Aside from visual, albeit microscopic inspection, of oxide
candidates, x-ray diffraction analysis also allows determination
of sufficient crystalline structure in the oxides. For example,
suitable oxides (FIG. 4A) exhibit sharp peaks, indicative of
crystalline structure, whereas unsuitable oxides (FIG. 4B)
exhibit broad peaks, which is indicative of the presence of
amorphous materials. Amorphous materials react fast and generate
unwanted excess heat in the slurry.
C.) Solubility: Solubility of the oxide in a solution of
phosphoric acid also distinguishes between acceptable and
unacceptable oxide. Generally, the less soluble the oxide is,
the more suitable it is for producing a flowable ceramic
material, which will solidify into an impenetrable mass. Lower
oxide solubility results in more MgO particles present after
reaction and therefore more nucleation sites existing in the
reaction liquor. These nucleation sites help form individual
centers of hardness which adds to the overall strength of the
final waste form.
FIG. 6 depicts a graph showing two acceptable oxides (dotted
lines) and two unacceptable oxides (solid lines). As
illustrated, 20 percent more (by weight) of the acceptable
oxides are necessary to bring an acid-oxide solution to pH 8
compared to the amount required of unacceptable oxides. Initial
impurity levels of the suitable oxides do not effect their
solubilities. For example, the more soluble oxides (i.e., the
acceptable oxides) depicted in FIG. 5 had varying levels of
calcium contaminants with one oxide (analytical grade)
containing 0.05 weight percent calcium and the other acceptable
oxide (technical grade) containing 0.45 weight percent calcium.
Solubility testing of oxide candidates are conducted as
follows: A 30 milliliter solution of 5 percent H.sub.3 PO.sub.4
in deionized water is kept in constant agitation, via a magnetic
stirrer or other means. One gram of MgO is added to the solution
and the resulting mixture is allowed to equilibrate, with pH
recorded. Additional MgO is added, one gram at a time, with
equilibration allowing to occur. The procedure is continued
until the pH reaches 8.
The inventors have found that one method for obtaining oxide
satisfying criteria A-C stated above is through a calcining
process. If commercial MgO is to be used, the oxide first must
be sized to between 8 and 10 microns, as noted supra. Calcium
content of the oxide should not exceed 0.5 weight percent.
Once the calcium content and particle size of the oxide is
determined to be suitable, the oxide is placed in a furnace
maintained at 1,300.degree. C. The oxide could be placed in a
"preheated" furnace or present during temperature ramp-up.
Soaking or heating time of the oxide will vary depending on the
source (and therefore impurity) levels of the oxide. Highly pure
(analytical grade) oxide is soaked for approximately one hour
while technical grade is heated for approximately three hours.
After heating, the samples remain in the furnace during cool
down. The resulting, now-cooled oxide usually presents as an
agglomerated mass, which must be reground or sized to 8 to 10
micron levels.
Additive Agent Detail
As noted supra, a salient feature of the invention is the use
of additives to stymie reaction and therefore extend workability
of the reaction liquor 25. The additives can comprise two
components: a coating agent and a water "getter" or water
eliminating agent. As mentioned supra, suitable coating agents
are selected from the group consisting of boric acid, citric
acid, and combinations thereof. The coating component of the
additives work by coating and therefore isolating the oxide
particle from a complete exposure or contact by other reactants.
Suitable water getters are derived from the class of polymeric
organic compounds comprising the lignophosphonates. The
lignophosphonate compounds serve to keep water from quickly
reacting with the phosphates. In essence, these ligno-compounds
scavenge up any water to prevent rapid hydration of the
phosphate. Alkaline lignophosphonate compounds are preferable
getters, particularly those selected from the group consisting
of lignosulphonate, lignophosphonates and other hydroxylated
organic compounds and combinations thereof. A suitable group of
lignosulphonate compounds consists of sodium lignosulphonate,
calcium lignosulphonate, potassium lignosulphonate and
combinations thereof. One exemplary lignosulphonate compound is
DARATARD 17.TM., available through Grace Construction Products
of Cambridge, Mass.
Weight percents of the additive agents to the final reaction
liquor can vary, with values of between 0.5 percent and 3.5
percent suitable. Generally, the additive is combined with the
first mixture in an additive to oxide to phosphate weight ratio
of between 1.83:10.3:35 to 2.1:10.3:34
The inventors have found that the water-isolating effect of
lignosulphonate compounds is enhanced with the presence of boric
acid, wherein boric acid is present in weight percents similar
to those utilized for the lignophosphonates.
The additive agent can be added anytime before or at the
initiation of the reaction process 24, with exemplary results
obtained when the additive is introduced as designated in FIG.
1, that is, prior to the addition of water 22.
In one embodiment, proportions of each ingredient of the final,
pumpable product are as follows: MgO (12 weight percent),
KH.sub.2 PO.sub.4 (40 weight percent), fly ash (17 weight
percent), boric acid (1 weight percent), lignosulfonate (1
weight percent), and water (29 weight percent). This embodiment
results in the slurry having the consistency of milk, and
therefore easily pumped or grouted. After two to three hours,
the slurry sets into a hard and dense ceramic. X-ray diffraction
analysis of the final form, as depicted in FIG. 2. Along with
hydrated magnesium potassium phosphate, also present is residual
magnesium oxide.
As can be determined in Table 1 below, a myriad of different
slurry consistencies result from varying concentrations of the
additives. Table 1 shows the viscosities in centipoises (cp) for
additive concentrations of between 0.5 and 3.5 weight percent to
the total slurry weight.
TABLE 1 Viscosities of ceramic-waste liquors depending on
variations in additive concentrations. Additive Initial Waste
Particles Concen. Viscosity <200 .mu.m Set Time Final
Hardness (Wt %) (Centipoise) Wt % (Min) (Psi) 0.0 27,000 60 20
>8000 2.0 300 15 >190 >3000 2.5 3000 28 190 >3000
Inasmuch as spraying and injection applications are enhanced at
viscosity values less than 1,500 cp, formulations containing
between 0.5 and 3 weight percent coating agent of additive
(which comprises both a coating agent and a water "getter") are
suitable.
Several different types of waste material can be incorporated
with the pumpable ceramic binder, either separately or combined,
to form a waste form. Such wastes include, but are not limited
to ash, saw dust, clay, soils, red mud, dust from metal
industries, slags, and combinations thereof. Sizing, grinding or
other pretreatment of the material may be necessary. Suitable
micron sizes of the waste range from 10 microns to 200 microns.
The resulting slurry, as described supra, can be utilized
either neat to dispose of the already-incorporated waste
particles in the pumpable liquor, or used as a sealant agent to
stabilize other waste. For example, a slurry formulated with ash
can be subsequently injected into contaminated soil for
stabilization of the latter. Another application is the use of
the invented waste-containing slurry as a pumpable refractory.
While the invention has been described with reference to
details of the illustrated embodiment, these details are not
intended to limit the scope of the invention as defined in the
appended claims.
