Patent Application
20130315804
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.
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.
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, Ky., 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.
In order to treat flue gases so to remove acid forming compounds such as SO2 and SO3, many power plants treat flue gas with carbonate-containing materials such as trona, bicarbonate or limestone. Thus, for example, U.S. Pat. Nos. 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.
The selenium present in such fly ash deposits is generally in the form of selenite (SeO32−; or Se(IV)) and selenate (SeO42−; 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 0007]. 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), Apr. 11-15, 2005, Lexington, Ky., 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 140° to 153° C. in Disney et al).
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 of acid and heat to such fly ash deposits, which additions themselves are expensive and environmentally unfavorable.
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.
Donahoe et al, Chemical Fixation of Trace Elements in Coal Fly Ash, 2007 World of Coal Ash, May 7-10, 2007, Covington, Ky., 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.
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.
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 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.
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.
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 FeSO4.7H2O and FeCl2.4H2O.
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.
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.
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.
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.
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.
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.
The following Examples are intended to further illustrate the invention, but are not intended to limit the scope of the invention in any manner.
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 FeSO4.7H2O (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:
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.
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 FeCl2.4H2O (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:
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.
| Number | Date | Country | |
|---|---|---|---|
| 61650228 | May 2012 | US |
