Note: Descriptions are shown in the official language in which they were submitted.
0
IMMOBILIZATION OF LEAD AND CADMIUM IN SOLID RESIDUES
FROM THE COMBUSTION OF REFUSE LIME AND PHOSPHATE
~ACRGROUND OF THE INVENTION
An increasing world population leads to a continually increasing
amount of refuse. Additionally, an increased level of civilization
appears to generate an increased amount of refuse on a per capita
basis. Both factors in combination lead to mounting pressure to devise
methods of waste disposal which are economically, energetically, and
environmentally sound.
In recent years, especially in urban areas, the increased demand
for usable land and other concerns has caused one to turn from a
landfill as the major mode of refuse disposal to other options,
especially the use of raw refuse as an energy source. One variant of
the latter is the mass burning approach, where all the refuse in its
raw state is burned without any preliminary treatment such as
separating the noncumbustible from combustible material. Quite
briefly, in this method raw garbage is dumped into storage where it is
homogenized and dried to some degree. Refuse from the storage area is
fed into a combustion zone where the heated gases often are used to
generate steam. Flue gases then pass from the combustion zone to a
separation zone, often an electrostatic precipitator, where dust and
ash are removed. The ash so removed from the glue gas, called fly ash,
is then mixed with the ash collected in the combustion zone, called
bottom ash, and the combined ash used for landfill, in road
construction, and so forth.
It is well known that some of the more volatile compounds of
certain metals tend to accumulate in the fly ash. Especially where the
latter is to be used as landfill, leaching of toxic metals, especially
d~
cadmium and lead, constitutes a potential hazard to the ecosystem, for
example, both surface water supplies and aquifers. The Environmental
Protection Agency (EPA) has promulgated a procedure to determine the
toxicity of solid wastes, and where residues exceed the toxicity as
stated in the Federal Register Code 40, No. 26124, the waste is
classified as a hazardous waste requiring control under the Hazardous
Waste Management System. A recent report prepared for the Office of
Solid Waste, U.S. Environmental Protection Agency, which was a limited
survey of several kinds of solid waste, seems to suggest that levels of
cadmium and lead in fly ash pose perhaps the most serious environmental
threat, and that such fly ash alone would need to be treated as a
hazardous waste; EP Toxicity Test Results on Residues from Eight
Resource Recovery Facilities, SYSTECH Corporation, February, 1981.
The environmental hazard of fly ash containing amounts of cadmium
and lead greater than the toxic levels specified by the EPA is somewhat
diminished by mixing such ash with heavy ash, such that the resulting
landfill mixture is within the toxic levels for the cited metals.
Nonetheless, it is highly desirable to reduce the amount of cadmium and
lead leached from fly ash and other solid waste to an amount below the
toxic levels specified by the EPA. The invention herein is a solution
to this problem. More specifically it is a method of treating dry,
solid residues, especially fly ash, and mixtures containing fly ash, so
as to reduce the amounts of cadmium and lead leached from such residues
to a level below the toxic level specified by the EPA. Stated
differently, the invention herein is a method of immobilizing, or
insolubilizing, cadmium and lead in solid waste, especially over a wide
pH range. The method is convenient, quite simple, very efficient,
applicable over a wide pH range, and relatively low cost. The method
is, therefore, commercially extraordinarily attractive as well as being
environmentally beneficial.
The problem we have addressed is not new; only our solution to
this problem is new. Prior solutions have relied on transforming
metal-laden ash into a solid, hardened, often brick-like consistency to
immobilize lead and cadmium. Such solutions are based on producing a
product largely impermeable to water, thereby reducing, if not
eliminating, metal transport by diffusion. In contrast, our invention
o
retains the powdery (particulate) nature of the ash-containing residues
while immobilizing lead and cadmium; the treated residue remains a
particulate, non-hardened solid which does not harden to a brick-like
consistency and this characteristic serves as a distinguishing feature
of our invention.
The precipitation of heavy metals, including cadmium and lead, at
high pH is a well-known analytical technique, and the use of lime as
the basic agent is a common procedure. For example, solid wastes
containing cadmium and lead were treated with 3-15% calcium hydroxide
and/or magnesium sulfate, the pH was adjusted to 8-10.5, and the solid
coated with asphalt to prevent the leaching of cadmium and lead.
Chemical Abstracts, 92; 185414d. The preceding method is a mixture of
coagulation-flocculation followed by encapsulation in a hydrophobic,
petroleum-based solid.
In U.S. 4,049,462 Cocozzo treated industrial desulfurization
residues resulting from removal of sulfur oxides from effluent gas with
alkaline calcination stack dust and water under acidic conditions to
form a solid, hardened, leach-resistant product. The patentee
recognized that the cement-like product resulted from the reaction of
calcium oxide and silicate in the stack dust with acid anions, whose
nature was not significant so long as the mass reacted under acidic
conditions to provide a hardenable mass which upon drying became
cementitious solid.
Pichat describes a process to transform strongly acidic liquid
wastes containing relatively high metals content, including cadmium,
into solid materials by mixing wastes with coal fly ash, adjusting the
pH to about 7, adding a lime-containing substance and a binder, such as
Portland cement, with the mixture setting to a petrified mass; U.S.
4,375,986. As the patentee recognized, coal ash is pozzolanic, i.e.,
in the presence of lime it agglomerates into a hard, compact, mortar
type product. Clearly, Pichat's invention describes a method to treat
acidic liquid wastes and uses coal fly ash as an additive. The
patentee also recognizes that coal fly ash does not contain sufficient
amounts of Pb and Cd to present an environmental concern. Although
Schneider-Arnoldi et al. in U.S. 3,676,165 teach that phosphorous
furnace slag can be substituted for lime as a binder, and
1~3-~ ~0
that such slag contains phosphorous compounds in the amount of 0.5-2.0%
reported as P20s, the slag is a hard vitreous mass which fails to
furnish soluble phosphate, an essential element of our invention. In
fact, such slag contains phosphorous chiefly as calcium phosphate,
which we show to be inoperative in immobilizing lead and cadmium.
In all instances reported in which a cement-like material is
fabricated from fly ash, the inventors use coal fly ash which due to
chemical composition, surface composition and morphology, and size
distribution is pozzolanic. However, for these same reasons,
incinerator fly ash is not pozzolanic and cannot form a stable cement
in the absence of ordinary portland cement. The invention described
here does not require ordinary portland cement and neither requires nor
utilizes solidification or agglomeration for its successful
application. Methods applicable to agglomeration or fixation of coal
fly ashes are simply not readily applicable to incinerator fly ashes.
A base course for pavement construction can be made from
incinerator ash reacted with lime and water prior to compaction; U.S.
4,496,267 European patent 34-389, directed toward the agglomeration of
coal fly ash into pellets, discloses some phosphorous compounds in the
ash and reports the total phosphorous content as P20s, but as with
Schneider-Arnoldi et al. this phosphorous source does not furnish
soluble phosphates.
We have discovered a method of immobilizing lead and cadmium in
refuse-to-energy combustion residues effective over a broad pH range to
reduce the leaching of the aforementioned heavy metals to a level below
the maximum dictated by the EPA. Quite simply, the method involves
treatment of the solid residues with lime followed by addition of a
water soluble phosphate. Using this method levels of lead and cadmium
are reduced to less than 5 and 1 ppm, respectively. It is also
desirable to immobilize the toxic metals to pass the regulatory limits
with a typical acid rain to water extraction. This requires an
immobilization system which is effective over the entire pH range above
about 5.0; the method we have discovered meets this requirement. Our
method does not change the particulate nature of the untreated solid
U
residue; it generates no cement-like mass. Our method
does not generate calcium phosphate as the metal binder;
substitution of calcium phosphate for our soluble
phosphate fails to immobilize lead and cadmium.
Whatever may be the detailed mechanism of metals
immobilization in our method, it appears that our
immobilizing materials of lime and soluble phosphate
remain quiescent and inactive in the dry solid residue,
but when water -the extractant- perfuses through the
solid the immobilizers raise a barrier to dissolution
and/or diffusion of the metals into the liquid phase.
SUMMARY OF THE INVENTION
Various aspects of this invention are as
follows:
A method of immobilizing lead and cadmium in a
free flowing particulate dry solid residue which
maintains its free flowing particulate nature after the
immobilizing treatment, said dry solid residue
comprising fly ash and mixtures of fly ash with bottom
ash resulting from the incineration of municipal waste,
comprising contacting the dry solid residue with at
least one water soluble phosphate in an amount
equivalent to about 1 to about 8~ by weight of
phosphoric acid based on the total residue in the
presence of a free lime source selected from the group
consisting of lime, hydrated lime, flue gas scrubber
product, and combinations thereof, in an amount
sufficient to furnish from about 1 to about 25 parts by
weight calcium hydroxide per 5 parts by weight of fly
ash whereby the leaching of cadmium and lead is reduced
to a level no more than 1 ppm cadmium and 5 ppm lead as
determined in an EPA test performed on the resulting dry
treated residue.
1~3'7 ~0
A method of immobilizing lead and cadmium as a
free flowing particulate mass in a free flowing dry
particulate mass of a fly ash and bottom ash mixture
where each said ash results from the incineration of
municipal waste in a mass burning facility comprising
contacting the dry ash mixture with at least one water
soluble phosphate in an amount equivalent to about 1 to
about 8 percent by weight of phosphoric acid based on
the total ash mixture in the presence of a free lime
source selected from the group consisting of lime,
hydrated lime, flue gas scrubber products, and
combinations thereof, in an amount sufficient to furnish
from about 1 to about 25 parts by weight of calcium
hydroxide per 5 parts by weight of fly ash whereby the
leaching of cadmium and lead is reduced to a level no
more than 1 ppm cadmium and 5 ppm lead as determined in
an EPA test performed on the resulting treated ash
mixture.
By way of added explanation, the purpose of
this invention is to increase the immobilization of lead
and cadmium in solid residues from combustion plants.
In one embodiment fly ash is treated with lime, mixed
with bottom ash, and the resulting mixture treated with
a source of water soluble phosphate. In a more specific
embodiment the lime originates from flue gas scrubber
product. In a still more specific embodiment the water
soluble phosphate is added in an amount from about 1% to
about 8% by weight of the ash-lime mixture. Other
embodiments will become apparent from the following
description.
DESCRIPTION OF THE FIGURE
The Figure shows the final pH of the extract
in an EPA test of various solid residues from the
burning of solid wastes. The cross-hatched part
7~0
represents the sole region where the EPA Toxicity limits
for both lead and cadmium are met in the residues.
THE PROBLEM
Flue gas resulting from the combustion
of refuse often is passed through lime to remove
such materials as hydrogen chloride, sulfur dioxide,
sulfuric acid, carbon dioxide, nitrogen oxide, and
other acidic compounds normally found in flue gas to
afford a solid called flue gas scrubber product.
Fly ash also frequently is mixed with lime
5b
7~0
in part to immobilize (insolubilize) heavy metals found therein,
including lead and cadmium. Where flue gas scrubber product is
available it is used either as the sole source of lime or as lime
make-up for treatment of the fly ash. The fly ash-lime/flue gas
scrubber product mixture is then admixed with bottom ash for uses as
mentioned above. However, the ratio of bottom ash to fly ash varies
considerably, as does the ratio of flue gas scrubber product to fly ash
and the extent to which the lime is neutralized in flue gas scrubber
product, according to the source of refuse, the operational
characteristics of the plant, and so forth. The resulting mixture
containing flue gas scrubber product, fly ash, and bottom ash has an
alkalinity which can vary considerably and additionally displays a
broadly varying buffering power. As the data of Example 1 show, such
mixtures often fail the EPA test for lead and/or cadmium, essentially
because cadmium precipitates at a pH greater than about 7.5 but lead,
being amphoteric, begins to redissolve at a pH greater than about 12.
Consequently, only in those mixtures whose final pH after extraction in
the EPA test (vide infra) is between about 7.5 and about 12.0 are lead
and cadmium immobilized sufficiently well for the mixture to be within
the stated regulatory limits.
The practical aspects of refuse burning dictate a broad range of
flue gas scrubber product-fly ash-bottom ash solid waste mixtures with
an accompanying range of alkalinity. The regulatory aspects of solid
wastes dictate that leaching of lead be limited to less than S ppm and
leaching of cadmium to be no more than 1 ppm. The technical aspects of
the aforementioned solid waste mixtures demonstrate an enormous
variation in the leaching of lead and cadmium depending upon pH. The
problem, simply stated, is to make the practical, regulatory, and
technical aspects compatible. That is, what can be done to immobilize
lead and cadmium in the broad range of solid waste mixtures of flue gas
scrubber product-fly ash-bottom ash normally produced in refuse burning
plants so as to conform to EPA regulations?
7 ~0
THB 80LUTION
The solution, simply stated, is to add water soluble phosphate.
Stated somewhat more extensively, we have discovered that addition of
water soluble phosphate to flue gas scrubber product-fly ash-bottom ash
solid waste residues of a broad compositional range insolubilizes lead
and cadmium to an extent as to make the residue in total compliance
with EPA regulations, notwithstanding a broad variation in alkalinity
of such residues. The solution is remarkable in that it cures a vexing
problem with an extraordinarily simple treatment. The remainder of
this exposition is devoted to a more complete description of our
invention.
Even more generally, our invention takes a particulate (powdery or
granular) dry solid residue arising from the burning of solid waste in
a mass burning plant, and from which lead and cadmium are leached at
levels of more than 5 and 1 ppm, resp., and treats the residue with
lime, especially that arising from a flue gas scrubber product of a
mass burning plant, and one or more water soluble phosphates, to obtain
a particulate residue which maintains its particulate nature but from
which leaching of the aforementioned is below the stated levels.
DE8CRIPTION OF THE INVENTION
The solids being treated in our invention are residues resulting
from the burning of solid wastes, generally in commercial mass burning
facilities, and from which cadmium and/or lead are leached at levels in
excess of 1 and 5 ppm, resp., as determined by an EPA test. Initially
such solids are a free flowing particulate mass, and a virtue of our
invention is that after treatment to immobilize lead and cadmium the
solids remain a free flowing particulate mass, even after water
percolation, and maintain this characteristic. The solids treated
generally are fly ash, in whole or in part, since lead and cadmium tend
to be concentrated in the fly ash. In one variant of our invention the
solid residue treated is a mixture of fly ash and bottom ash, usually
containing between about 2 and 25% by weight of fly ash, even more
often
X 7
lZ~37 ~0
between 5 and 20% fly ash. The following description of our invention
is couched in terms of the fly ash first being treated with lime or a
lime source, with this mixture subsequently being combined with bottom
ash prior to addition of a water soluble phosphate. This corresponds
to the most convenient way of carrying out our invention, but the
choice of this particular description is for expository convenience
only. It is to be clearly understood that variants such as treatment
of fly ash alone with lime and phosphate prior to mixing with bottom
ash, or treating a mixture of fly and bottom ash with lime and
phosphate, are intended to be subsumed under our invention as claimed,
as are other permutations which one skilled in the art will recognize.
Using fly ash as an example of the solid residue to be treated,
the fly ash is mixed with lime. By lime we mean calcium oxide (dry
lime), calcium hydroxide (hydrated lime), a lime source or any mixture
thereof. Where flue gas is scrubbed with lime, the flue gas scrubber
product (FGSP) may be either the sole source of lime or may be used
only in part as the lime source. In addition to containing calcium
hydroxide, the FGSP typically will contain such materials as calcium
sulfate, calcium sulfite, calcium chloride, and calcium carbonate. The
percentage of calcium hydroxide in the FGSP is itself subject to broad
variation, and the amount of FGSP used will depend in part on the
amount of calcium hydroxide present. In the successful practice of
this invention, lime or FGSP will be added to fly ash in an amount from
1 to about 25 parts by weight of lime, based on its calcium hydroxide
content, per 5 parts by weight of fly ash.
The fly ash-lime mixture is then mixed with bottom ash in the
normal, commercial practice of this invention. The relative amounts of
these two components often is expressed as a ratio of bottom ash to fly
ash, and normally varies from perhaps 3:1 to 49:1, i.e., the mixture
contains from about 2 to about 25% by weight fly ash, most often being
in the range of 5-20% by weight fly ash. The lime-fly ash-bottom ash
mixture is then treated with a source of water soluble phosphate to
complete the immobilization of lead and cadmium. It is, perhaps, most
convenient merely to spray the mixture with the phosphate source and
then agitate the mixture to ensure the dispersion of phosphate.
However, merely dispersing a good source of water soluble phosphate
X 8
12~37 ~V
through the mixture also may be performed, although not necessarily
with equivalent results.
Any convenient source of water soluble phosphate may be used in
the practice of this invention. By a water soluble phosphate is meant
a phosphate soluble in water at about 20C at least to the extent of
about five weight-volume percent. Phosphoric acids, including
orthophosphoric acid, hypophosphoric acid, metaphosphoric acid and
pyrophosphoric acid, can be conveniently used in this invention.
Sometimes it is desirable to use a less acidic source of phosphate, and
in fact it is essential that the phosphate source and use level be such
that a substantial part of the lime is not neutralized. Other less
acidic sources of phosphates include phosphate, monohydrogen phosphate,
and dihydrogen phosphate salts, such as trisodium phosphate, disodium
hydrogen phosphate, sodium dihydrogen phosphate, potassium phosphate,
dipotassium hydrogen phosphate, potassium dihydrogen phosphate, lithium
phosphate, lithium hydrogen phosphate, and lithium dihydrogen
phosphate. Quite generally, the salts of the various phosphoric acids
may be utilized, and among these the alkali metal salts are most
frequently employed.
The amount of water soluble phosphate source to be added to the
solid residue to ensure adequate immobilization of lead and cadmium
will depend on such variables as alkalinity of the solid residue, its
buffering capability, the amount of lead and cadmium initially present,
and so on. It has been found generally that an amount of the water
soluble phosphate source equivalent to between about 1% and about 8% by
weight of phosphoric acid, H3PO4, based on total solid residue is
sufficient, but it is not intended to preclude yet higher usage of an
water soluble phosphate if needed.
The examples below are merely illustrative of this invention and
are not intended to limit it thereby in any way.
The following procedure, based on an EPA method as described in
the Federal Register V. 45, No. 98, May 19, 1980, pp 33099 et ff., was
used to screen various methods. The EPA test was modified only as to
scale, i.e., the test used by us was a scaled-down version of the
standard EPA procedure. Experiments were performed by mixing an
immobilizing material with 10 g dry fly ash in a 500 ml Erlenmeyer
lZ93~ ~0
flask. Water (160 ml) was added and the mixture was agitated
thoroughly on a wrist action shaker. After one hour the pH was
recorded and adjusted to 5.0 + 0.2 by addition of 0.5N acetic acid.
Agitation was continued with hourly adjustment of pH to 5.0 + 0.2 until
a stable pH of 5.0 was reached or the maximum allowed amount (40 ml) of
0.5N acetic acid was used. The total mixing time on the standard test
was 24 hours. Solids were separated on a vacuum Millipore filter
XX1004700 using an AP type prefilter and an HA type 0.45 micron fine
filter. If less than 40 ml acetic acid was used, the final volume was
adjusted with water in an amount determined by the following equation:
V = (20)~W) - 16(W) - A
where:
V = ml distilled water to be added.
W = weight in g of solid charged to extractor
A = ml of 0.5N acetic acid added during extraction
Ultrapure concentrated nitric acid in an amount of 1 ml per 100 ml
leachate was added after filtration to stabilize the solution. The
modified EPA toxicity reference test itself is carried out without the
addition of immobilizing material. Levels of cadmium and lead in
leachate were determined by atomic absorption spectroscopy.
The bottom ash-fly ash mixtures used in our studies contained
about 0.5 weight percent phosphorous, which is equivalent to 1.1%
reported as P205. This shows that the phosphorous-containing materials
present in the ash residue is not a source of soluble phosphate
necessary for immobilization.
EXAMPLE 1
Solid residues exemplifying a broad spectrum of flue gas scrubber
product-fly ash-bottom ash compositions were tested for lead and
cadmium content using the EPA test as described above. The FGSP
typically had a calcium hydroxide content between 40% and 60%. The
final pH after extraction in the EPA test is plotted for various
compositions in the figure. It was observed that the EPA limits for Pb
X 10
1~37 ~0
were met only within the pH range 6.7-12.0, and the EPA limits for Cd
were met only at a pH above 7.5. As can be seen from that figure, only
a limited number of such compositions afforded a final pH between 7.5
and 12.0, the range within which the EP~ test for both lead and cadmium
are met.
EXANPLE 2
Solid residues were prepared using a ratio of bottom ash to fly
ash of 19:1. To this was added flue gas scrubber product containing
about 57% free calcium hydroxide in different weight ratios. The EP
toxicity test was then run on this mixture of FGSP-fly ash-bottom ash
as well as one containing 4.25% phosphoric acid. The results are
tabulated below.
Table 1. Effect of ~.25% H3PO~ in Modified EP Toxicity Test
FGSP:Fly Ash 4:1 4:1 1:1 1:1 3:7 3:7
%H3PO4 o 4.25 0 4.25 0 4.25
EP Toxicity Test
Initial pH 12.62 12.24 - 7.40 12.46 5.43
Final pH 12.38 10.21 5.38 5.05 4.99 5.11
Extract m~/L
Pb 5.6 0.1 11.8 0.23 8.46 0.1
Cd 0.014 0.01 1.27 0.45 1.33 0.29
As can be seen, the EPA test limits for lead and cadmium are met
over the pH range from 5.05 to 10.2, whereas in the absence of
phosphate acceptable limits of leaching were not met.
X 11
l~t37 ~0
EXAMPLE 3
In this example solid residues of varying bottom ash:fly ash and
FGSP:fly ash ratios were subjected to the EP toxicity test with and
without the addition of 4.25% phosphoric acid. The following table
again demonstrates the efficacy of phosphoric acid in immobilizing both
lead and cadmium over the quite broad pH range from 5.2 to 12.6.
12~37~
Table 2. Effect of 4.2~% HbPC4 With Various Botto~
~sh:Fly Ash and FGSP:Fly Ash ~atios
.. . .. . . . . . . . .. ..
Botto~ Ash:Fly Aah 7:1 7:1 7:1 7:1 9:7 9:7 4:1 4:1
FGSP:Fly ~sh 4:1 4:1 3:7 3:7 2:1 2:1 1:1 1:1
% HbPOb - 4.25 - 4.25 - 4.25 - 4.25
EP Toxicity Test
Initial pH 12.63 12.60 - 7.0712.60 12.67 12.6012.68
Final p~ 12.43 12.60 5.60 5.1812.43 10.19 12.6Q11.00
Extract mg/L
Pb 17.0 1.2 12.0 0.3113.5 0.062 14.0 0.063
Cd 0.090 0.01 2.82 0.700.01 0.01 0.01 0.01
12~37~
EXAMPLE ~
In this example an ash composite was extracted with synthetic acid
rain. A blend of nitrates, sulfates, and chlorides was made to
simulate acid rain representative of the Northeastern U.S. The
following compounds were dissolved in a total solution of four liters
to prepare an acid rain concentrate.
Compound g
NaNO3 O.llSO
KNO3 0.2196
NH4NO3 0.0648
MgCl2 0.0821
H2SO4 0.1755
CaS04 0.1057
The pH of the concentrated solution was 2.88. A solution was
prepared for use in the acid rain extraction tests by diluting this
mixture by a factor of 10; the resulting pH was 3.93. This dilute
solution, which should be representative of a typical acid rain, was
used as a replacement for 0.5N acetic acid to test blends of FGSP, fly
ash, and bottom ash. Otherwise, the extraction was identical to the EP
Toxicity Test. As the data of Table 3 demonstrate phosphoric acid
addition again was quite effective in red~cing the levels of lead
leached from such a composite.
1~3740
Table 3
Acid Rain. Extraction of Aqh Composite; Effect of 4.2% H3P0~
FGSP:Fly Ash 1:1 1:1 4:1 4:1 3:7 3:7
%H3P04 - 4.25 - 4.25 - 4.25
Acid Rain Extraction
Initial pH 12.58 8.10 12.7012.6712.54 5.47
Final pH 12.66 7.29 12.7312.7812.50 5.79
Extract mg/L
Pb 2.8 0.1 3.50.71 1.5 0.1
Cd 0.01 0.01 0.010.01 0.01 0.063
EXAMPLE 5
Various ash composites were extracted w~th water alone to
determine the effect of added phosphate on heavy metal leaching. The
experiments were performed in a 500 ml Erlenmeyer flask with 10 g of a
FGSP:fly ash blend and 200 ml H2O with agitation by a wrist action
shaker for 24 hours.
Table 4. H20 Extraction of Ash Composite: Effect of 4.2~ H3P0
FGSP:Fly Ash 1:1 1:1 2:1 2:1 4:1 4:1
%H3PO4 0 4.25 04.25 0 4.25
H2O Extraction Test
Initial pH 12.42 7.30 12.6011.8812.61 12.51
Final pH 12.70 8.07 12.6610.4812.67 12.57
Extract mg/L
Pb 14.9 0.1 6.50.19 5.8 0.93
Cd 0.01 0.01 0.010.02 0.01 0.01
1~3~7 ~U
As in the prior examples, addition of phosphoric acid substantially
reduces the amount of lead leached under the conditions of this test.
EXA~PLE 6
Composites containing a 19:1 ratio of bottom ash:fly ash were
tested with either phosphoric acid or disodium hydrogen phosphate,
Na2HPO6 as the source of water soluble phosphate. In both cases the
solid residues easily met the EPA toxicity test.
Table 5. Comparison of H3PO~ With Na2HPO4 Modified EP Test
FGSP:Fly Ash 4:1 4:1 1:1 1:1 3:7 3:7
%H3PO4 4.25 - 4.25 - 4.25
N HPO - 5.0 - 5.0
EP Toxicity Test
Initial pH 12.70 12.69 6.27 12.30 5.50 11.88
Final pH 6.50 11.62 5.11 5.18 5.07 5.10
Extract mg/L
Pb 0.1 0.075 0.1 0.24 0.1 0.15
Cd 0.036 0.015 0.34 0.33 0.19 0.50
EXAMPLE 7
A composite containing a ratio of bottom ash:fly ash of 19:1 with
varying ratios of FGSP:fly ash were tested using from 1% to 4.25%
phosphoric acid. As can be seen, even 1% phosphoric acid was generally
effective in reducing leaching of lead and cadmium to an acceptable
level except with a FGSP:fly ash ratio of 1:1.
16
1~3~7 ~0
Table 6 Effective H3PO4 Content
Immobilization of Pb and Cd in FGSP:Fly Ash:Bottom A~h Blends
9.5 g Bottom Ash + 0.5 g Fly A~h
FGSP:Fly Ash 4:1 4:1 4:1 4:1 1:1 1:1 1:1 1:1 3:7 3:7 3:7 3:7
XH3P04 04.25 2.11.0 0 4.25 2.1 1.0 04.25 2.1 1.0
EP Toxicity Test
Initial pH12.6Z12.24 12.6012.64 - 7.0412.Z5 12.42 lZ.46 5.43 7.03 12.04
Fina~ pH 12.3810.21 12.4512.24 5.38 5.05 5.14 5.08 4.99 5.11 5.16 5.11
Extract mg/L
Pb 5.6 0.10.46 0.3611.80.23 0.1 0.49 8.46 0.1 0.1 0.38
Cd 0.0140.010.01 0.011.270.45 0.51 1.2 1.33 O.Z9 0.24 0.83
B~AMPLE 8
The following study was performed to show that the initial
leaching results in the EPA test were not merely temporary, and that
the lead and cadmium in the treated material remained immobilized. A
mixture of fly ash and flue gas scrubber product (Ca. 30:70) was
sprayed with water containing a variety of phosphates to afford a
mixture with 20% moisture and containing various levels of phosphates,
reported as weight percent phosphorous. (A level of 2.~ weight percent
phosphorous is equivalent to 8.0 weight percent phosphate as phosphoric
acid.) This mixture was aged in a closed bottle and subjected to the
EPA leach test at intervals for lead and cadmium. In all cases the
leachate contained <0.01 ppm cadmium. Results are reported in Table 7.
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37 ~0
Table 7. 8tability of Immobilization
Phosphate Lead in Leachate
Level~ Source Day Level
none 0 44
2.6 Na2HP04 o 3.6
4 3.1
7 3.0
28 2.1
85% H3P04 o 3.9
4 2.7
Na4P207 10 H20 6.0
3.2
Na4P207 . 10 H20b o 6.5
5.1
NasP301o 6.8
3 5.3
2.0 (NaP03)6 0 7.2
7.5
NaH2P04 . 2H20 o 5.7
5.5
NasP301o 0 4.5
4.7
Na2HP04 o 7.g
4 6.5
NasP3010-85%H3po4(1 1) o 4.4
6.4
a. in wt. %
b. added as solid, water subsequently sprayed on to 20% moisture
level.
X 18
7~t~)
EXAMPLE 9
A 19:1 bottom ash-fly ash composition was mixed with an equal
amount of flue gas scrubber product and treated with various acids and
the anions of these acids. The data in Table 8 shows unequivocally
that phosphate is unique; neither sulfuric nor nitric acids immobilize
lead and cadmium, nor do their salts.
Table 8 Effectivane~ of Various Acid~ ~nd
Their 8slt-~ in Immobilization
AddendNone H3PO4 H2SO4 HNO3 NazHPO4 NazSO4 NaNO3
Concentration 4.3 4.3 4.3 3.5 3.5 3.5
(meq/g residue)
EP Toxicity Test
Initial pH 12.16 7.40 10.48 11.88 12.30 12.21 12.14
Final pH 5.38 5.05 5.01 5.18 5.18 5.12 5.21
Extract mg/ml
Pb 11.8 0.23 4.7 6.6 0.24 14 8.9
Cd 1.27 0.45 1.2 1.39 0.33 0.93 0.82
EXAMPLE 10
To he ash-flue gas scrubber product mixture of the prior example
was added 4.25% by weight phosphoric acid. This mixture was placed in
a column and three volumes of water was percolated through the
particulate mass to simulate landfill conditions. The mass was
removed, air dried, and subjected to a particle size distribution
analysis whose results appear in Table 9.
X 19
1~37 ~)
~able ~ - Particle ~ize Di~tribution of Ash Before
and After Immobilization by Phosph~te
Particle Size, mmPercent of Total
Untreated Ash Treated Ash
<0.074 3.9 3.7
0.074-0.42 8.2 8.9
0.42-2 23.6 21.3
2-9.5 64.3 66.1
These data show that the particulate nature of the mass
remains virtually unaffected by the immobilization treatment of this
invention.
EXAMPLE 11
The same mixture of fly ash and flue gas scrubber product as
described in Example 8 was treated with .6% phosphorous as phosphate
from insoluble Ca3(P04)2 and then subjected to the EPA leach test. The
leachate had 19 ppm lead, showin~ that calcium phosphate is ineffective
as a phosphate source in the immobilization of lead by our method.
~ 20
