Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FLY ASH AND FLY ASH LEACHATE TREATMENT
FIELD OF THE INVENTION
[0001] The present invention is directed to a process of treating fly ash
and/or fly
ash leachate to immobilize heavy metals contained in such fly ash and/or fly
ash
leachate, which process comprises treating such fly ash and/or fly ash
leachate with
a soluble ferrous compound under alkaline conditions.
BACKGROUND
[0002] The generation of power from coal can result in a number of undesirable
pollutants being placed into the environment. These pollutants may be released
in a
number of forms, including flue gases and fly ash. Many of the gases which may
be
produced as a result of coal combustion, such as oxides of nitrogen and
sulfur, will
react with water in the environment to produce acid rain. Such gases may also
contain oxides of heavy metals including selenium, arsenic, vanadium and
chromium which can cause problems in the environment. Fly ash, which
constitutes
fine solid particles which rise with such flue gas, typically contains oxides
of such
heavy metals as well.
[0003] Pursuant to environmental requirements, in the United States fly ash
must be
removed from flue gas before its discharge into the environment. Fly ash is
typically
removed from flue gas employing electrostatic precipitators or other particle
filtration equipment. Such captured fly ash is generally stored at coal power
plants
or placed in landfills. Indeed, as is noted by Donahoe et al, Chemical
Fixation of
Trace Elements in Coal Fly Ash, 2007 World of Coal Ash, May 7-10, 2007,
Covington, Kentucky, USA, more than two-thirds of such coal combustion
products
in the US are stored in dry landfills or wet lagoons; most of the older ash
deposit
sites are unlined and many are unmonitored. Therefore, heavy metals contained
in
such combustion products can create environmental concerns if it is leached
through
contact with rain water or other similar means.
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[0004] In order to treat flue gases so to remove acid forming compounds such
as
SO2 and S03, many power plants treat flue gas with carbonate-containing
materials
such as trona, bicarbonate or limestone. Thus, for example, US Patents
7,531,154
and 7,854,911 disclose a process for removing SOx gases from a flue gas stream
employing trona. While such systems are effective for removing sulfur oxides
from
flue gas, they can result in the production of fly ash which has increased
amounts of
heavy metals, particularly selenium, due to the extraction of such material
from the
flue gas stream as a consequence of the use of such an alkaline sorbent
material.
[0005] The selenium present in such fly ash deposits is generally in the form
of
selenite (Se032-; or Se(IV)) and selenate (5e042-; or Se(VI)). As is discussed
in US
Patent Application 2009/0130013, both selenite and selenate are soluble in
water;
however, selenite can be removed from wastewater by co-precipitation with iron
hydroxide at a pH in the 5.5 to 6.5 range. [Paragraph 00071. This publication
discloses that the addition of iron to a limestone slurry flue gas
desulfurization
("FGD") system, particularly a forced oxidation FGD system, may reduce the
formation of selenate, and may result in the absorption or precipitation of
reduced
forms of selenium with iron hydroxide -- a reaction which favorably occurs at
the
pH at which FGD scrubbers typically operate (between approximately 5.5 and 6).
In
this regard, it is noted that Disney et al, FGD Forced Oxidation Mechanism A
Pilot
Plant Case Study, 2005 World of Coal Ash (WOCA), April 11-15, 2005, Lexington,
Kentucky, USA state that the presence of unreacted lime or limestone in the
feed
slurry is a cost factor which must be controlled ... Depending upon feed
chemistry, a
pH increase (above 5.5) can quickly impede the oxidation chemistry." As they
are
used to desulfurize emissions as they are produced in a combustion reaction,
FGD
processes are conducted at elevated temperatures (ranging from 1400 to 153 C
in
Disney et al).
[0006] Thus, processes which are effective to reduce selenate to selenite
under the
acidic pH conditions and high temperatures at which FGD processes are employed
to treat flue gas may not be practical to treat alkaline fly ash deposits,
particularly
those which are highly alkaline due to treatment with trona or similar high
carbonate
materials. Specifically, such processes would require the addition of large
amounts
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of acid and heat to such fly ash deposits, which additions themselves are
expensive
and environmentally unfavorable.
[0007] US Patent Application 2010/0145130 proposes a method of stabilizing
selenium in coal combustion products which comprises mixing such product with
a
sulfide compound (including FeS, an insoluble compound) followed by treatment
with soda ash, nahcolite, trona, sodium sulfite and/or sodium hydroxide.
Although
details of this treatment are not provided, it is apparent the addition of
insoluble FeS
would require substantial physical mixing; while the addition of strongly
basic
materials to fly ash stored in landfills or ponds would be environmentally
undesirable.
[0008] Donahoe et al, Chemical Fixation of Trace Elements in Coal Fly Ash,
2007
World of Coal Ash, May 7-10, 2007, Covington, Kentucky, USA discloses a
process
for chemically fixing heavy metals contained in fly ash; however, such process
requires a drying step (to permit oxidation to occur). Such a drying step is
not
practical in treating fly ash stored in wet ash lagoons or in landfills which
are subject
to periodic rainfall, dew condensation or other forms of moisture addition.
[0009] Consequently, there is a need for a process to reduce the leaching of
heavy
metals from alkaline fly ash, particularly fly ash deposited in wet ash
lagoons or
landfills which does not require the addition of pH modifiers which could be
costly
and environmentally undesirable, and which does not require commercially
impractical mixing, drying or heating steps.
SUMMARY OF THE INVENTION
[0010] The present invention is directed to a process of treating fly ash
and/or fly
ash leachate to immobilize heavy metals contained in such fly ash and/or fly
ash
leachate, which process comprises treating such fly ash and/or fly ash
leachate with
a soluble ferrous compound under alkaline conditions. This process may be
conducted in the absence of any pH modification, mixing (in the sense of a
physical
blending with a solid material), drying or heating steps, making it practical
for
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treatment of alkaline fly ash (and other coal combustion by-products) which is
currently stored in landfills or wet ash lagoons, particularly fly ash which
has been
recovered from flue gas streams treated with highly alkaline materials such as
trona,
bicarbonate or limestone and the like.
DETAILED DESCRIPTION OF THE INVENTION
[0011] The present invention is directed to a process of treating fly ash
and/or fly
ash leachates to immobilize heavy metals contained in such fly ash and /or fly
ash
leachate, which process comprises treating such fly ash and/or fly ash
leachate with
a soluble ferrous compound under alkaline conditions without a subsequent
drying
step..
[0012] As is employed herein, the term "soluble ferrous compound" refers to an
iron
(II) compound having a solubility in water of at least 0.02 mole/L at 25 C;
and
preferably having a solubility in water of at least 0.2 mole/L at 25 C.
Particularly
preferred soluble ferrous compounds include ferrous chloride and ferrous
sulfate,
including hydrated forms of these compounds such as FeSO4.7H20 and FeC12.4H20.
[0013] Further, as is employed herein, the term "fly ash leachate" refers to
water
which has come into contact with fly ash and which contains dissolved heavy
metals
as a result of such contact. The term "immobilize" refers to the complexing of
a
heavy metal such that it is no longer soluble in aqueous solutions.
[0014] As is employed herein, the term "heavy metal" means transition metals,
and
other metals and metalloids in Period 4 or higher of the Periodic Table. Heavy
metals which are environmentally undesirable and which may be immobilized by
the process of this invention include selenium, arsenic, vanadium, chromium,
cadmium, lead, nickel and mercury. The process is particularly useful for the
immobilization of selenium, arsenic, vanadium, and chromium; and is especially
useful for the immobilization of selenium.
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[0015] The fly ash to be treated may be located in storage areas including dry
landfills or wet ash lagoons, or it may be present at combustion locations
after
collection by electrostatic precipitators or other means. The fly ash may be
mixed
with other coal combustion products. Although the fly ash to be treated may
have
any pH above 7, the process of this invention is particularly suitable for the
immobilization of highly alkaline fly ash (typically having a pH of 8 or more;
or
even as high as 10 or more) produced by the desulfurization systems which
employ
highly basic materials such as trona, bicarbonate, limestone or the like.
[0016] The soluble ferrous compound may be added to the fly ash in solid form
where practical, such as in the treatment of ash lagoons; or may be added in
liquid
form (dissolved in an aqueous solution) to treat dry landfills or similar
locations.
With respect to the treatment of ponds, one preferred embodiment is to add a
solution of the soluble ferrous material to the slurry containing the fly ash
as it
enters the holding pond, as this would provide desirable mixing of the
solution into
the pond water.
[0017] It has been surprisingly found that the addition of a soluble ferrous
compound will immobilize heavy metals present in fly ash such that they do not
leach out into ground water, without the need for pH adjustment or heating or
drying
steps. Further, because soluble compounds are employed, dry landfills
containing fly
ash can be treated without the need for the extensive physical mixing required
if
non-soluble compounds were employed. The properties render the present process
suitable for the in situ treatment of both fly ash and fly ash leachate.
[0018] The amount of soluble ferrous compound added will depend upon the
amount of heavy metal present in the fly ash and/or fly ash leachate to be
treated. In
general, when leachate is treated, between 0.5 grams of Fe(II) per liter of
leachate
and 15 grams of Fe(II) per liter of leachate will be employed; with amount of
from 2
to 9 more typically being used. In general, when fly ash is to be treated,
generally
between 0.1 weight percent and 15 weight percent Fe(II) is employed (based
upon
the weight of the fly ash to be treated); typically between 0.5 weight percent
and 10
weight percent of Fe(II) is applied.
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[0019] The following Examples are intended to further illustrate the
invention, but
are not intended to limit the scope of the invention in any manner.
EXAMPLES
Example 1
[0020] 1000 grams of deionized water was added to 100 grams of fly ash
(containing 88 weight percent coal ash, 6.4 weight percent Na2SO4, 1.9 weight
percent Na2CO3, and 3.7 weight percent NaHCO3) and stirred for 24 hours. The
final solution was at pH 10.3. 11 grams of FeSO4.7H20 (i.e., 2.2 g Fe/L) was
added
dry and the mixture was stirred for a few minutes until all of the Fe salt was
dissolved, then stirring was stopped. Aliquots of the solution were withdrawn
at the
time intervals indicated in the table, filtered, then analyzed for selenium,
arsenic and
vanadium content. The results of such testing are shown in Table 1 below:
Table 1
Treatment Time Percent Se Percent As Percent V
(Days) Removal from removal from Removal from
Solution Solution Solution
0 0 0 0
0.083 60 100 100
1 60 100 100
2 60 100 100
3 60 100 100
4 60 100 100
61 100 100
7 61 100 100
14 64 100 100
21 68 100 100
28 74 100 100
35 79 100 100
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[0021] The above results indicate that a significant amount of Se is removed
from
the leachate within a few hours of treatment and additional removal occurs
over
time. As and V are completely removed from solution within a few hours.
Example 2
[0022] 1000 grams of deionized water was added to 100 grams of fly ash
(containing 51.5 weight percent bituminous coal ash, 10.7 weight percent
Na2SO4,
30.6 weight percent Na2CO3, 7.2 weight percent NaHCO3) and stirred for 24
hours.
The final solution was at pH 10.1. The leachate was separated from the solids
by
filtration, then 30 grams of FeC12.4H20 (i.e., 8.4 g Fe/L) was added dry and
the
mixture was stirred for a few minutes until all of the Fe salt was dissolved,
then
stirring was stopped. Aliquots of the solution were withdrawn at the time
intervals
indicated in the table, filtered, then analyzed for selenium, arsenic,
vanadium and
chromium content. The results of such testing are shown in Table 2 below:
Table 2
Treatment Percent Se Percent As Percent V Percent Cr
Time (Days) Removal
from Removal from Removal from Removal from
Solution Solution Solution Solution
1 40 100 Not measured Not
measured
7 44 100 100 100
14 48 100 100 100
21 50 100 100 100
28 49 100 100 100
35 52 100 100 100
42 58 100
49 66 100
[0023] The above results indicate that a significant amount of Se is removed
from
the leachate within 1 day of treatment and additional removal occurs over
time. As,
V, and Cr are completely removed within 1 week.
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