WO1998024938A1

WO1998024938A1 - Extraction of metals from heavy metal-bearing wastes

Info

Publication number: WO1998024938A1
Application number: PCT/US1997/023149
Authority: WO
Inventors: Keith E. Forrester; Arokiasamy J. Francis
Original assignee: Brookhaven Science Associates Llc
Priority date: 1996-12-05
Filing date: 1997-12-04
Publication date: 1998-06-11

Abstract

Methods for extracting and recovering heavy metals from a heavy metal bearing material are described, as well as methods for further treatment of the heavy metal after it has been extracted. The method can be used to cleanse the surface and improve the quality of ferrous and non-ferrous containing scrap metal. The methods utilize a unique combination of carboxylic acid (e.g., citric acid) treatment, dewatering, rinsing and/or bioconversion and/or precipitation, to produce a waste that can pass toxicity characteristic leaching procedures (TCLP) set forth by the EPA, or to meet standards for lower metal content before it can be further disposed of or used.

Description

EXTRACTION OF METALS FROM HEAVY META -BEARING WASTES

Background of the Invention

The leaching of heavy metal bearing wastes and human and biological exposure to heavy metal content has long been of concern to environmental regulators and waste producers. Under the Resource Conservation and Recovery Act (RCRA) solid waste is classified by the U.S. Environmental Protection Agency (EPA) as hazardous waste if excessive amounts of heavy metals leach from the waste when tested under the Toxicity Characteristic Leaching Procedure (TCLP) . The EPA also regulates the land disposal of certain heavy metal bearing wastes depending on the content of the heavy metals regardless of the leaching potential. In addition, several state governments require that solid wastes having elevated levels of heavy metals be disposed of as hazardous waste. Disposal of waste at a hazardous waste landfill is typically more expensive than disposal at non-hazardous waste landfills.

To reduce the expenses associated with landfill disposal of heavy metal leachable waste, particularly lead bearing waste, various methods to control heavy metal leaching and reduce heavy metals have been developed. These methods include the stabilization of lead bearing waste with, for example, portland cement, silicates, sulfates, phosphates and combinations thereof as well as acid digestion and subsequent extract metals recovery. involve complex handling equipment and operations. Additionally, some of these methods use chemicals, such as high molarity acids, which in the amounts used, are corrosive to the waste generation and/or treatment process equipment. These methods also mostly do not reduce the level of Pb and/or the bioavailability of Pb and other regulated metals and thus fail to reduce the risks of direct exposure to the waste after treatment and fail to approach the objective of heavy metal content reduction as put forward by the EPA and various state regulators.

Thus a need exists for means of reducing leaching and heavy metal content from heavy metal bearing waste, which are less expensive and less damaging to waste processing equipment.

Summary of the Invention

This invention describes methods for extracting and recovering heavy metals, such as lead (Pb) , cadmium (Cd) , copper (Cu) , zinc and other heavy metals, from ash residues, other industrially generated residues and ferrous and non-ferrous containing scrap metal, as well as soils contaminated with heavy metals, collectively referred to herein as "heavy metal bearing waste(s)" or "heavy metal containing waste (s)". The methods described herein reduce heavy metal content in these materials and thus improve their ability to be reused or disposed of in accordance with permissible regulations and criteria. An advantage of the invention is to remove a sufficient amount of heavy metal (s) from heavy metal bearing waste such that the treated waste can be reused as well as pass TCLP criteria. Heavy metals removed from the waste can be further processed into insoluble precipitates that will not leach under normal leaching conditions, or the metal complexes can be biodegraded and then precipitated. In the case of ferrous and non-ferrous scrap metal, the ethod can be used to cleanse the surface of the scrap metal and improve its reuse value.

According to the invention, the heavy metal content in a heavy metal bearing waste can be reduced to levels which improve reuse potential or allow for reuse and pass TCLP test requirements by first contacting heavy metal bearing waste, such as incinerator bottom ash, with hydroxycarboxylie acid (preferably citric acid) , salts thereof or mixtures of salt and acid in an aqueous environment. The amount of acid should be an effective amount to solubilize at least a portion, depending upon product end use disposal, of the heavy metal contained in the waste, whereby aqueous-soluble chelated metal complexes are formed. The type of complexes formed are variable based on the metals present and the mineralogical association of such as presented in U.S. Patent No. 5,292,456; the teachings of which are incorporated herein by reference in its entirety. At least a portion of the aqueous hydroxycarboxylic acid solution and solubilized chelated metal complexes, depending again upon product end use or disposal objectives, are then removed from the acid treated waste, e.g., by centrifugation, gravity drainage, gravity filtration or other dewatering methods. The waste can then be treated with (1) rinse water to extract a portion of remaining heavy metals and acid from the waste,

(2) Pseudomonas fluorescens to consume a large portion of residual citric acid in the waste and convert aqueous soluble metal complexes into insoluble organic bound compounds and precipitates and/or (3) precipitating agent to reduce water solubility of residual chelated metal complexes contained in the waste. Alternatively or in combination with the steps described above, a heavy metal precipitating agent can be used to convert aqueous soluble chelated metals removed from the waste and free soluble metals after citric acid consumption by Pseudomonas fluorescens into solid precipitates. The election of this post-citric acid extraction rinsing, Pseudomonas fluorescens seeding and/or precipitation step can vary depending on the desired waste product metals content, compound character and TCLP leachable content. Methods for recycling the citric acid are also described.

Brief Description of the Drawings

Figures 1 and 2 are schematic representations of embodiments of the present invention.

Detailed Description of the Invention

The materials being treated in the present invention are heavy metal containing wastes or contaminated materials. Examples of heavy metal containing wastes include bottom ash obtainable by solid waste combustion, ferrous and non-ferrous metals normally produced by shredding or along with the bottom ash, flyash and flyash scrubber residues, as well as heavy metal bearing soils resulting from industrial contamination, foundry sands and shredder residues from wire chopping and auto shredding operations. Such wastes, materials and residues contain high levels of lead and cadmium which can be leached at levels in excess of 5.0 ppm (5 mg/1) and 1.0 ppm (1 mg/1) as determined by the USEPA TCLP leaching test and copper and zinc in excess of 25.0 ppm and 200.0 ppm as determined by the California state leaching test method. Such wastes also often contain total compositional heavy metals such as lead, cadmium, barium, chromium, copper, magnesium, manganese, nickel, strontium, tin, and zinc at levels which are higher than those allowed by the USEPA, state regulators and local officials for ash use in engineered products such as cement blocks and/or application to the environment such as roadbase aggregate, landfill cover and plastics substitutes in products such as timber posts. When dry, the initial physical character of such solid residues and ash is a free flowing particulate mass. An advantage of the present invention is that after treatment to extract and/or immobilize these metals, the solid ash residues retain their initial free flowing character, which is important in the handling thereof, i.e., there is no need to change handling equipment in part because the weight, density and adhesion character of the solid residue has not been significantly modified. However, the present invention may modify the drainability of the solid residue and reduces the fines within the residue depending on the aggressiveness of extraction, rinsing and wastewater processing used. These are important features which facilitate extraction and recovery since the materials are more suitable for drainage, handling, landfilling and reuse.

According to the methods of this invention, the heavy metal content of a heavy metal bearing waste can be reduced to levels which improve reuse and soluble metals within the waste reduced or precipitated to levels which pass TCLP test requirements. Heavy metal bearing waste is contacted with hydroxyca boxylie acid in an aqueous environment in an amount that is effective to solubilize at least a portion of the heavy metal contained in the waste, thereby forming aqueous-soluble chelated metal complexes. This step can be performed in a batch or continuous mode depending upon equipment and waste type. Hydroxycarboxylic acids such as citric acid and/or salt solutions thereof, such as calcium or ferric citrates, can be used but citric acid is preferred. Any convenient source of citric acid can be used in the practice of this invention. The citric acid can be applied to the waste either in a wet or dry form, but in any event, the contacting step should be performed in an aqueous environ- ment in order to allow the citric metal complexes to form in solution. The aqueous environment can be achieved using an aqueous solution of citric acid which is applied onto the waste or it can be added in dry form such that an acid solution is formed in situ by virtue of the presence of liquid in the waste. In one embodiment, the wet contact would preferably be completed in a water tight vessel in which bottom ash is wetted with the citric solution.

The concentration of acid, contact time, mixing aggressiveness and pH of the mixing chamber will be that which is suitable to solubilize an appropriate amount of heavy metal so that the waste can be used for intended purposes and/or pass TCLP test conditions upon completion of the entire process. This can be assessed by the skilled artisan based upon knowledge of the type and amount of heavy metal in the waste. Typically, the concentration of the citric acid solution is from about 0.01 M to about 20.0 M, preferably from about 0.05 M to about 5.0 M and more preferably from about 0.05 M to about 0.50 M. The required molarity of the acid can change depending upon factors such as temperature, mixing aggressiveness and pH employed in the acid extraction step. The adjustment of pH was found to alter the degree of metals extraction. pH levels of 1.0 to 9.0 were observed effective for Pb control. It is desirable to use an elevated pH compared to the pH of the citric acid solution. This will increase the efficiency of metal extracted and will reduce the amount of acid required.

In one embodiment, the citric acid solution used has a concentration of less than about 0.10 M, preferably from about 0.04 M to about 0.10 M. The pH of the citric acid solution can be unmodified or the pH can be increased compared to the original citric acid solution. A citric acid solution having a desired pH can be readily prepared by one of skill in the art, for example, by adding a suitable amount of a base, such as sodium hydroxide, potassium hydroxide and others which are well known in the art, to a citric acid solution or by adding a citric acid salt, in a suitable amount to achieve the desired pH and citrate concentration. For example, the pH of the citric acid solution can be greater than about 4 , preferably from about 4 to about 6, and, more preferably, from about 4.5 to about 5.0. It has been found that methods using citric acid solutions of less than about 0.10 M concentration and elevated pH have increased selectivity for the extraction of certain heavy metals, including regulated metals, such as arsenic, silver, barium, cadmium, chromium, lead, mercury and selenium, as well as copper and zinc. Thus, such methods are particularly advantageous when it is desired to remove only certain metals from a sample and retain other metals, for example, iron and calcium.

The acid extraction step can also be performed at ambient conditions or at elevated conditions. It has been shown that elevated temperatures in the range of from about 25°C to about 85°C will cause more of the heavy metals to solubilize. The required acid contact time and contact mixing aggressiveness will also depend upon factors such as acid concentration, pH and temperature but in any event, should be sufficient to remove a desired level of heavy metals, that upon completion of the entire process of this invention permits the waste to meet total metal content requirements and pass TCLP tests. In most instances, a contact time of from about 1 to about 30 hours (with about 1 to about 8 hours being preferred) will be adequate, to remove a desired percentage of metal(s) .

After acid treatment, a large fraction of the aqueous acid solution is removed from the acid extracted waste, as well as solubilized metal chelated complexes formed during the acid extraction step. This step is typically referred to in the industry as dewatering. Methods for dewatering are well known to the skilled artisan and include, but are not limited to, centrifugation, decanting, gravity drainage, gravity filtration, overhead drainage and the like. A certain amount of acid solution and metal chelated complexes will remain in the pores and surfaces of the dewatered waste.

The dewatered, acid treated waste is then further subjected to rinsing, biodegradation, photodegradation, precipitation or combinations of these. It may be neces- sary to rinse the waste after dewatering in order to reduce residual acid residing on and within the pore spaces of the waste. Residual citric acid can also be removed at various degrees from the waste by contacting it with bacteria which are capable of consuming citric acid as its carbon source. A particularly suitable bacteria is Pseudomonas fluorescens which is known to consume citric acid. Pseudomonas is also capable of converting citric metal complexes that remain in the waste into insoluble organic bound compounds and consume citric acid, thus making metals available for precipitation. Another possible application is to contact the dewatered waste with a metal precipitating agent such as phosphate or sulfide so that chelated metals may become insoluble. The term "insoluble" is intended herein to mean that the compounds will not leach under normal or induced leaching conditions, and that the wastes containing the insoluble organic complexes will pass the TCLP test. The extent and intensity of precipitation, rinsing and/or biodegradation will depend upon the nature of the waste, the heavy metal type and content. Pseudomonas contact times of from about 24 to 48 hours should be adequate to this end. Rinsing at a 10/1 rinse to waste weight ratio is also often adequate.

Depending upon the nature of the waste and the type of waste treatment facility, it may be desirable to further process the liquid removed from the waste by the dewatering step as well as the rinse waters. The aqueous- soluble chelated metal complexes that are removed from the waste by dewatering can be treated with heavy metal precipitating agents to convert the complexes into insoluble compounds that can be recovered and properly disposed. Suitable precipitating agents include phosphates (e.g., hydroxyapatite) , sulfides (e.g., sodium or calcium sulfide) , carbonates and silicates. Other known precipitating agents can be used. Citric acid can also be recycled and reused according to the methods of this invention.

The following description of the present invention is couched in terms of bottom ash resulting from the combustion of solid waste. This corresponds to a convenient way of carrying out the present 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 a mixture of bottom ash with other solid residues such as incinerator flyash or other solid residues, treatment of other solid wastes or solid materials containing heavy metals such as heavy metal contaminated soils, auto shredder residue, wire chopping insulation waste, foundry sands, ferrous and non-ferrous scrap metal, sandblast wastes or independently treating bottom ash and then combining it with another inert solid residue are contemplated for use in the present invention, as are other permutations which one skilled in the art will recognize.

Incinerators for burning trash and other solid wastes are well known to those skilled in the art. Bottom ash produced from incineration is what remains on the grate of the furnace after combustion of the waste. The bottom ash is often a granular character and somewhat glassy in nature, and also includes ferrous and non-ferrous metals which are often recovered from the bottom ash by mechanical and electromechanical means such as screening, electromagnetic and eddy current separation. The other inert material resulting from combustion of solid wastes is flyash, a finer material which becomes airborne in the furnace and is captured in various forms of air pollution control units such as fabric filters, electrostatic precipitators and cyclones. The bottom ash produced during the combustion of waste is sometimes mixed with the flyash in order to allow for a mixed combined ash disposal system. The bottom ash discharges from the incinerator at high temperatures ranging from 1500°F to 2500°F, and thus requires cooling by air and/or water prior to further handling. The most common method of ash cooling is by water quenching in a drag tank or ram discharging tank which also acts as a seal to restrict air flow into the base of the furnace. The bottom ash is expelled from the wet quenching tank by means of either a pushing ram or drag flights, both which allow a controlled period of time for ash to be wetted and cooled in the bath of water prior to discharging to a removal conveyor. Both of these common ash quench expelling methods incorporate inclined drainage after the bath which allows quenching waters not retained, adsorbed or otherwise captured by the bottom ash to be returned to the quench tank. The water retained by the bottom ash and water also lost to evaporation due to the water seal exposure to heat is made up by new process water called make-up water.

This process can be more particularly illustrated by the Figure which shows a schematic diagram of an incinerator which allows the application of the water soluble citric acid therein to a bottom ash contact tank 10 followed by ash gravity dewatering, second stage combined rinsing and residual citric consumptive seeding of Pseudomonas fluorescens followed by gravity dewatering, and a third stage post-rinsed ash biological retention step prior to discharge of such ash to a receiving container, such as an open-top dumpster 21. The schematic also shows an optional process supporting wastewater treatment facility 17 for chelated metals precipitation, metals recovery and free citric acid recirculation connected to the primary ash citric acid contact chamber 10. The process supporting wastewater treatment facility 17 provides removal of citric chelated metals and solid fines from the primary citrate contacting tank 10 and recirculation of valuable citric acid solution back to the contact tank for reuse as the chelating agent for heavy metals such as lead and cadmium.

Acid extraction can be performed in the ash primary contact tank 10. Accordingly, citric acid 11 is added to the tank 10 through a water makeup supply 12 at a rate sufficient to maintain a desired molarity of citric acid in solution. The bottom ash 13 can be removed by dischargers 14 at a rate which can regulate ash mass release from the incinerator, as well as allowing for a desired retention time of ash in contact with the citrate solution. The citrate solution molarity and retention time selection can vary depending on the desired reduction of heavy metal content in the ash as well as the need for reduction of soluble metals as evaluated under TCLP and other state regulatory test methods.

Alternatively, the extraction of heavy metals from the ash can be performed in a tank reactor separate from the quench tank. This arrangement permits the process to be conducted in a batch mode as compared to the continuous flow mode of the ash quenching tank. After contact with the citric acid solution tank 10 for a controlled period of time, the bottom ash is subjected to a dewatering step, preferably by gravity drainage on an inclined drag chain bed 15 or an inclined ram or by overhead drainage if the ash is placed into a batch reactor separate from the furnace ash wet quench tank 10. The drainage of citric solution back to the citric solution contact tank 10 allows for recycling of the non-ash retained portion of the citric acid solution as well as return of the chelated metal complexes to the contact tank 10 where the complexes are subjected to recirculation 16, recovery by precipitation 17 and return 18 to the contact tank 10.

In an alternative embodiment, the acid treated bottom ash can be subjected to a separate batch drainage tank or centrifuge type mechanism, where dewatering liquids and soluble complexes can be discharged directly to on-site or off-site wastewater treatment processes. It is most probable that the ash bath and batch dewatering of ash separate from the ash quenching tank and precipitation of complexes from the dewatering liquids will be the preferred methods, as this allows for a closed loop and zero contaminant discharge method most easily permitted in todays highly regulated industrial wastewater management field as well as allows for a high degree of process variability given that the ash batch operations are separated from the ash discharging process.

After gravity dewatering, the ash can be subjected to further chelated metals removal and recovery by a water rinsing step 19. The purpose of this additional rinsing step is to produce a washed ash for reuse or to further reduce the chelated metals content on the surface of the ash.

The acid treated ash can further be contacted with Pseudomonas fluorescens 20 to biodegrade residual citric acid that has not been removed in the dewatering step. Although other forms of bacteria may provide a certain degree of citric acid consumption, the Pseudomonas fluorescens method is preferred. The use of Pseudomonas fluorescens and precipitants can act as a final polishing step under which chelated citric heavy metal complexes can be converted from a relatively water soluble state to a lesser water soluble state of organic bound metal species through biodegradation and precipitation. Optionally, the application of Pseudomonas fluorescens to the ash residue can be performed without the rinsing step because

Pseudomonas fluorescens alone is capable of reducing residual ash bearing citric acid and residual heavy metal complexes can be precipitated to levels acceptable to the end user and under associated regulations. The amount of water soluble citric acid to be added to the bottom ash wetting vessel or other solid residues combined with bottom ash to ensure adequate TCLP immobilization and reduction of metals content will depend on such variables as bottom ash alkalinity, heavy metal content, surface character and desired reduction and TCLP leaching levels for heavy metals. It is believed that a citric acid solution having a concentration of from about 0.05 M to about 0.10 M with a contact time of 1 to 30 hours followed by either ash rinsing of 50% of ash wet weight basis (wwb) or ash 50% wwb rinsing in combination with Pseudomonas seeding will be sufficient to extract enough of the more readily soluble heavy metals from the ash such that the compositional levels are reduced below regulatory limits and to remove and convert enough of the heavy metals such that TCLP levels are below regulatory limits of 5.0 ppm for Pb and 1.0 ppm for Cd. However, the foregoing is not intended to preclude yet higher or lower usage of citric acid, contact time, mixing aggressiveness, rinsing water, precipitation and/or Pseudomonas seeding as found reasonable or necessary given regulatory criteria or differences due to ash initial metals content or character.

It has been found that the TCLP and composition character of bottom ash varies widely from facility location and ash production type, thus causing uncertainty among regulators and potential ash users as to the leaching and total metals character of the ash. The present invention provides a means to reduce the TCLP variability of bottom ash and reduce the total metals content and content variability, thus improving the reusability and disposal options for bottom ash.

The examples below are merely illustrative of the invention and are not intended to limit it thereby in any way.

EXAMPLE 1

Effect of Acid Concentration on Metal Extractability

Bottom ash from a solid waste combustion facility using an ash quench tank was first subjected to baseline total metals and TCLP analyses in order to define the character of the ash prior to citric digestion and processing. Total metal content in the ash was determined by acid digestion after grinding of an ash composite sample of 0.5 grams with a mortar and pestle. The composited bottom ash sample was placed in a 30-ml platinum crucible. Five ml of concentrated HN0₃ was added to each crucible. The mixture was then heated on a hot plate to almost dryness. An additional 3 ml of HN0₃ plus 2 ml of HC10₄ was added to the crucible. The mixture was then heated until fumes of HC10₄ appeared. The crucibles were cooled and then 5 ml of 30% HF was added. The mixture was then reheated and evaporated to dryness. Finally, 5 ml of concentrated HN0₃ and 5 ml of deionized water were added, heated until the solution boiled gently, transferred into volumetric flasks, and diluted to 100-ml with deionized water. Metals were then analyzed by ICP-MS.

The bottom ash baseline sample was also subject to the Toxicity Characteristic Leaching Procedure (TCLP) as set forth in the Federal Register Vol. 55, No. 61 (Mar. 29, 1990) which corresponds in pertinent part to the procedure set forth in Federal Register, Vol. 55, No. 126, pp 2698526998 (June 29, 1990), both of which are hereby incorporated by reference. This test procedure is also referenced in EPA SW 846, 3rd Edition. The TCLP test produces an aliquot filtered solution containing soluble metals which were analyzed by ICP-MS.

The bottom ash was subjected to three separate citric acid molarity solutions of 0.10 M, 0.20 M and 0.50 M, at an ash to solution ratio of 1:20 for 48 hours using a wrist action shaker to maintain mixing contact. During extraction, 5 ml aliquot samples were taken in order to assess the time variance of the citric extraction efficiency. After the 48 hour extraction period, the ash was dewatered by centrifuge means (12,000 rpm, 20 min.) and then subjected to a post-extraction TCLP and total metals analyses. These extractions were also conducted at 25°C and 85°C in order to assess the impact of temperature on the extraction of metals, given that the bottom ash incinerators often quench the bottom ash at elevated temperatures due to the thermals remaining in ash after combustion.

Treatment of Waste and Liquid with Pseudomonas fluorescens

The citric acid extract was subjected to biodegradation to recover the metals from solution. For this purpose, Pseudomonas fluorescens biovar II (ATCC 55241) capable of degrading citric acid as a sole carbon source was used. The bacterium was grown in modified Simmon's citrate medium containing: 2 g citric acid, 0.2 g MgS0₄, 1 g K₂HP0₄ and 5 g NaCl in one liter of deionized water. The pH of the medium was adjusted to 6.2 with 2 N NaOH.

The bacterium was grown at 26±1°C in the dark in a rotary shaker .

Duplicate diluted citric acid extract solutions, each at approximately 18 mM citric acid, were amended with nutrients consisting of 0.1% of NH₄C1, K₂HP0₄ and KH₂P0₄. The pH of the extract was adjusted to 6.3. One hundred millimeters of each sample were placed in 250 ml flasks and then inoculated with 4 ml of an 18 hour old culture of Pseudomonas fluorescens . One control flask for each sample was prepared but not inoculated in order to define a baseline. The samples were then incubated on a shaker at 26±1°C and 150 rpm. All experiments were carried out in the dark to minimize possible photodecomposition of the metal-citrate complexes. Following incubation, 3 ml aliquots from each flask were removed periodically and filtered through a 0.22 micron filter. The filtered samples were analyzed for pH, citric acid and metals. Three separate sets of biodegradation experiments were carried out.

After biodegradation, the citric extract samples were centrifuged (12,000 rpm, 20 min.) to separate the solids biomass and metal precipitates from solution. The solid phase was dried overnight at 70°C then weighed. The solids were dissolved in 5 ml concentrated Ultrex HN0₃ diluted to 200 ml with deionized water and analyzed for metals by ICP-MS. Metal recovery was calculated as a percentage of the initial metal concentration of the citric acid extract. The ash baseline and post-48 hour citrate extracted heavy metals concentrations and percent metal extracted are presented in Table 1 in rounded average values. The results clearly confirm that citric acid extraction can remove major metals such as aluminum, iron and magnesium, as well as trace metals such as cadmium and lead at all molarities evaluated. The evaluation indicated that a majority of the metals recovered from the ash were removed within the first 5 hours. Thereafter, inflection of diffusion rate occurred at 5 to 10 hours with a gradual to flat slope of extraction after 20 hours. Lead was found to have a 85% exhaustion percentage at 5 hours with a very steep rate from 0 to 5 hours, 100% of total extracted amount at 25 hours with a 10 degree slope from 5 to 25 hours and a flat extract-to-time relationship after 25 hours. This time-effect observation has commercial importance, as long-term extractions such as 48 hours would likely (1) require separate ash handling facilities due to the high tonnage production of ash; (2) require storage facilities to house ash generated during the 48 hour period when the batch bath is in use; or (3) require larger batch production system to handle batch reaction times. Shorter extraction periods may allow for extraction and ash processing in-line or along side existing ash production facilities.

TABLE 1

Effects of Temperature on Extraction Efficiency

The ash extractions were also found to vary with temperature. Most metals showed an increase in extraction efficiency at 85°C as compared to 25°C, with the exception of Fe, Sr and Ti. This temperature effect could prove useful when applying the citric acid to ash quenching tanks in-line with the furnace, or when timing the bath reactor loading such that hot ash is subjected to extraction. A hot batch extraction process may also prove very useful in post-combustion ferrous and non-ferrous scrap metal polishing and cleaning prior to sale, as the removal of surface oxidized metals and contaminants from the combustion process such as bottom ash or ash fines will increase the value of the scrap metal. Table 2 presents results of temperature effect on ash metals extraction.

TABLE 2

Effectiveness of Citric Acid Extraction on Metal Removal

The ash TCLP baseline and post-citric extraction, postcentrifuge dewatering results presented in Table 3 also confirm that citric acid removes the more readily soluble forms of metals such as Pb and Cd as determined under the TCLP extraction method, thus reducing the residual metals available after ash dewatering for leaching within the buffered acetic acid solution used within the TCLP test. TABLE 3

Effectiveness of Citric Acid Removal by Biodegradation

Biodegradation of residual free citric acid remaining in the extract solution after ash was removed was found to occur rapidly and at a somewhat linear rate within 43 hours. The initial degradation rate of citric acid by application of Pseudomonas fluorescens was 80 mg per liter per hour and the pH was observed to increase from 6.3 to 9.0. The citric concentrations were observed by taking samples from Pseudomonas fluorescens inoculated solution batches over a period of 70 hours. The non-inoculated solutions retained the initial citric acid content over time with little to slight reduction due to hydration and precipitatious reactions. Several evaluations of citric degradation rate versus pH confirmed that metals conversions from solution to precipitate were enhanced at higher pH conditions as the citric content reduced.

Coprecipitation of metals at higher pH levels is well documented in environmental chemistry. The removal of metals from solution and conversion of such to a precipitate at a pH to 9.0 was observed to be very high for metals as shown in Table 4. The rate of metals removal varied, yet generally remained linear during the 48 hour period up to the point of reduction to levels less than 0.5 ppm where the metal concentration to time relationship flattened. Lead removal rates of 0.045 mg per liter per hour were observed. These results confirm that the residual metals in solution, and/or within the pores and on surfaces of the processed ash remaining after the ash citric batch bath, can be converted through biodegradation to insoluble precipitates within a liquid or solid ash matrix by addition of Pseudomonas fluorescens , which converts the citric acid and adjusts pH 1eve1s upward.

TABLE 4

It is further possible that an addition of common heavy metal precipitating agents such as phosphates, sulfides, carbonates and silicates could reduce the remaining citric chelates solution within the ash. An advantage of combining Pseudomonas fluorescens with any such precipitating agents is that the Pseudomonas fluorescens will degrade residual citric acid within the waste and thus assist the precipitation process. Evaluation of Ability of Residual Citric Acid to Change Solubility of Heavy Metals

It is desired to reduce the citric acid residue to a low level within the ash, as remaining citric acid in solution could increase the potential for metals to become water soluble at some time in the future, thus increasing the potential of metals release to the environment. In order to confirm the potential of free residual citric acid creating an increase in soluble Pb, one TCLP testing series was conducted on a bottom ash baseline and after a 0.1 and 0.5 molar citric acid bath with liquid to solid ratio of 10:1 and a bath time of 20 minutes followed by gravity drainage alone. The TCLP results of the post extracted and post gravity drainage ash increased over the baseline of 7.41 ppm to 33 ppm at the 0.1 molar bath, and 63 ppm at the 0.5 molar bath. Such an increase in soluble metals production is counter to one objective of this process which is to reduce the environmental hazards of the ash residue, thus confirming the need to aggressively extract and/or convert free water soluble bearing metals from the post-extracted ash by rinsing, precipitation and/or Pseudomonas seeding.

Recycling Citric Acid Using Heavy Metal Precipitating Agents Under Variable pH Conditions The use of Pseudomonas fluorescens for degradation of citric acid in solution and/or within ash liquids will provide useful control of residual citric acid and reduction of metals when recycling of citric acid is not considered necessary. As an alternative method, recycling of citric acid from the ash batch reactor, ash drainage or ash rinsing solutions may be considered valuable to operators using high molarity solutions of citric acid. Several evaluations of post-batch extraction citric acid content revealed that the citric acid consumption from a 0.05M to 0.5M bath resulted in only 0.5 to 1.0 percent reduction of available citric acid for bath durations of 5 minutes to 48 hours. Such a low consumption rate of citric acid presents the opportunity to include citric acid recycling in a process where loss of citric acid by solution biodegradation or disposal is not desirable.

Consequently, the heavy metal precipitating agents, sodium sulfide and hydroxyapatite, were evaluated in order to determine their capability to recover heavy metals from citrate recycle wastewaters and ability to return free citric acid in solution to the acid batch reactor tank. In addition to evaluating these agents for metals recovery, the effects of citric acid solution pH on the recoverability of metals was conducted to determine the most suitable conditions for precipitation, as well as an evaluation of the suitability of recycled citric acid for reuse as the primary acid tank solution. Both sodium sulfide and hydroxyapatite at concentrations ranging from about 0.01 to about 1.0% solution were found to precipitate over 99% of Pb out of solution within several minutes of contact and both precipitating agents were found to be efficient using solutions having a pH ranging from about 3.5 to about 8.0. Of the two precipitating agents, sodium sulfide was found to be less costly. The recovery of citric acid after addition of 1% sodium sulfide to citric acid extract of ash was found to be 98.7%. The comparison of fresh citric acid against recycled citric acid in the batch ash reactor, as shown in Table 5, also indicates that recycled citric acid provides similar and often superior metal extraction efficiencies. The improved extraction efficiency of recycled acid is partially due to the increased pH of the extraction solution. This observation provides insight into the possible intentional pH adjustment of citric acid batch solutions to more alkaline conditions in order to improve specific metals extraction.

TABLE 5

EXAMPLE 2 Evaluation of Extraction Time, pH and Molarity on Extraction Efficiency

In this example ash was subjected to various extraction times ranging from 5 minutes to 60 minutes with citric acid solutions of 0.1M to 0.5M in order to evaluate the suitability of a short-duration, non-mixed batch extraction reactor as compared to the initial bench scale evaluations using an aggressive mixed long-term citric acid extractor as presented in Example 1. Metals extractions were measured at 5, 10, 15, 20, 25, 30, 45 and 60 minutes for Ag, Al, As, Ba, Ca, Cd, Co, Cr, Cu, Fe, Mg, Mn, Ni, Pb, Se, Sr, Ti and Zn. Table 6 presents select results for the 0.1 and 0.5 molarity solutions extractions revealing the suitability of the lower molarity, adjusted pH range of about 5.0 to about 6.0 up to 9.0 citric acid solution on certain extractions such as Cd and Pb and the higher molarity, pH 6.0 solution on other metals such as Al and Fe. From this example it is apparent that specific recipe solutions of pH level and citric content can be engineered to extract each specific metal or combinations of metals. In the case of Pb, the lower molarity and 5.0 to 6.0 pH conditions would be preferred given lower cost of extraction and more efficient content extraction.

TABLE 6

EXAMPLE 3

Use of Pseudomonas fluorescens as a Stabilizing Agent

In this example bottom ash was first subjected to a tumbled extraction for 10 hours at a citric acid molarity solution of 0.05, liquid to solid ratio of 10:1, and a pH solution extraction adjusted to 5.0.

After extraction and gravity dewatering, the ash was split and one sample was held as a baseline and the other was seeded with Pseudomonas fluorescens in order to measure the ability of Pseudomonas fluorescens to act as a citric conversion and TCLP lead stabilizing agent.

Table 7 presents results revealing the suitability of Pseudomonas fluorescens as a Pb TCLP stabilizing agent in a post-extracted bottom ash. The post-extracted ash baseline TCLP Pb was relatively high as expected due to the residual water soluble Pb citrate complexes remaining on and between the wetted surfaces of the ash.

TABLE 7

EXAMPLE 4 Process for Polishing/Cleansing Post-Incinerator Ferrous and Non-ferrous Scrap Metals

In this example ferrous and non-ferrous metals within the bottom ash samples were first separated from the aggregate-like bottom ash by a hand magnet and simple screening of the ash/non-ferrous mix after hammer crushing the ash to size reduce non-crushable metal from crushable ash clinker and grit. The non-ferrous metal recovered was primarily aluminum. After separation from the non-metal ash, ferrous and non-ferrous samples were subjected to digestion and total metals analyses as shown in Table 8. The post-incinerator ferrous and non-ferrous metals were then subjected to a 33 rpm tumbled extraction for one hour in a 0.5M citric acid solution (10:1 liquid to solid ratio) at ambient temperature. After extraction, the ferrous and non-ferrous metals were subjected to a 2:1 liquid to solid ratio spray rinsing. The mixed samples were then subjected to metals digestion analyses.

TABLE 8

The ash extractions were previously found to vary with temperature. Most metals showed an increase in extraction efficiency at 85°C as compared to 25°C, with the exception of Fe, Sr and Ti. This temperature effect could prove useful when applying the citric acid to post- combustion ferrous and non-ferrous scrap metal polishing and cleaning prior to sale, as the removal of additional surface oxidized metals and contaminants from the combustion process such as bottom ash or ash fines would likely increase the value of the scrap metal.

Example 5 Treatment of Auto Shredder Fluff

The ability of citric acid extraction methods to remove lead from Auto Shredder Fluff was also evaluated. A sample of Auto Shredder Fluff was obtained from

Wiederkehr AG, Sweden, and determined to have a baseline lead content of 4280 mg/kg dry sample weight. This sample was treated for 1 hr with 1 M citric acid at a 1:5 solid to liquid ratio. Following the extraction procedure, the lead content of the sample was determined to be 2710 mg lead/kg dry weight, a 37% reduction. This result establishes the ability of citric acid extraction to substantially reduce the lead content of Auto Shredder Fluff.

Example 6 Treatment of Electric Arc Furnace Dust

The total metal content of a sample of electric arc furnace dust was determined via the method described in Example 1. Five 15 g portions of this sample were each treated for one hour with a citric acid solution (300 mL) ranging in concentration from 0.05 M to 3.0 M and ambient temperature. The results are presented in Table 9 and show that, in general, the amount of each metal extracted increased with increasing citric acid concentration. Citric acid extraction was particularly effective for removing lead, with, for example, 83% and 99% of total lead extracted from the sample following treatment with 0.75 M and 3.0 M citric acid, respectively.

The effect of increasing citric acid concentration at constant pH on the extraction of lead from incinerator ash. At pH 5, little difference in extracted lead was observed for citric acid solutions ranging in concentration from 0.05 M to 5.0 M. In contrast, the extraction efficiency of several other metals, including iron, aluminum and calcium was substantially lower at the lower citric acid concentrations. Table 9

Example 7 Short Duration Treatment of Bottom Ash

The total lead content of a sample of bottom ash obtained from Energy Answers Corporation SEMASS incinerator was determined to be 3150 mg lead/kg dry sample weight using methods described in Example 1. A portion of this sample was then fractionated with a 16 mesh sieve. The total lead content of the <16 mesh fraction was determined to be 4780 mg/kg dry sample weight. The original bottom ash sample and the <16 mesh fraction were then treated with 0.5 M citric acid solution for 1 hour at a 5:1 liquid: solid ration and ambient temperature. Following the citric acid extraction, the lead content of each sample was determined and found to be significantly reduced. The total lead content of the original bottom ash sample was reduced by citric acid treatment to 1630 mg/g dry weight ash, a reduction of almost 50%. The lead content of the <16 mesh fraction following treatment was 1680 mg/kg dry ash weight, a 65% reduction. These results demonstrate that significant reductions in the lead content of particulate wastes such as bottom ash can be achieved in a relatively short time.

Example 8 Extraction of Copper and Lead from Sand Blast Grit

Two samples of sand blast grit containing copper and lead were analyzed for total copper content as described previously. Samples 1 and 2 had baseline copper contents of 2600 mg/kg dry weight grit and 3580 mg/kg dry weight grit, respectively. Each sample was treated with 0.5 M citric acid solution for one hour. Following the citric acid treatment, the total copper content of Sample 1 was 1660 mg/kg dry sample weight, a 36% reduction. The post-treatment copper content of Sample 2 was 1670 mg/kg dry sample weight, a reduction of 53%. These results establish that short duration citric acid treatment of materials such as sand blast grit can be used to remove substantial amounts of copper from such materials.

Example 9 Extraction of Arsenic from Fly Ash

A sample of fly ash having a baseline arsenic content of 418 mg/kg dry sample weight was treated with citric acid solution at a 10:1 liquid: solid ratio for 0.5 hr. Following this extraction step, the arsenic content of the sample was determined to be 8.5 mg/kg dry sample weight, a 98% reduction. This result establishes that a citric acid extraction process can significantly reduce the arsenic content of a particulate sample, such as fly ash.

The foregoing results readily establish the operability of the present process to reduce heavy metal content and reduce leachable soluble metals in waste residues and materials. The degree of reduction of heavy metal content and leachable soluble metals can be regulated by the operator and would likely depend on the baseline character of the waste as well as regulatory criteria relative to the waste reuse or disposal options. The above described process allows for a wide range of operations options and is highly flexible given ones ability to adjust process variables such as citric acid solution pH molarity, contact method, mixing aggressiveness, dwell time, recirculation rates, precipitation election and dosages, rinsing degree, Pseudomonas seeding dosages, and degradation holding period. It is expected that the most common element of regulatory concern, Pb, will be reduced and controlled to suitable content and TCLP levels using a 0.01M to 0.05M citric acid solution adjusted from pH5 to 6, followed by mixing, waste rinsing, metals precipitation, Pseudomonas fluorescens seeding or combinations thereof. It will be apparent from the foregoing that many other variations and modifications can be made in the methods and the compositions herein before described, by those having experience in this technology, without departing from the concept of the present invention. Accordingly, it should be clearly understood that the methods and compositions referred to herein in the foregoing description are illustrative only and are not intended to have any limitations on the scope of the invention.

Equivalents

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.

Claims

CLAIMSWhat is claimed is:

1. A method of reducing heavy metal content or surface contaminants in a heavy metal bearing waste, comprising the steps of: a) contacting heavy metal bearing waste with hy- droxycarboxylic acid and/or salt thereof in an aqueous environment, in an effective amount to solubilize at least a portion of heavy metal contained in the waste and form aqueous- soluble chelated metal complexes; b) removing aqueous hydroxycarboxylic acid and/or salt thereof and the chelated metal complexes which are soluble therein from the product of step (a) ; and c) contacting the waste obtained in step (b) with rinse water, Pseudomonas fluorescens , precipitating agent or combination thereof to extract remaining heavy metals and residual acid from the waste, thereby yielding a waste having a reduced heavy metal content compared to the waste before step (a) is performed.

2. The method of Claim 1 wherein the heavy metal bearing waste is selected from the group consisting of incinerator bottom ash, incinerator flyash, heavy metal contaminated soil, auto shredder residue, wire chopping insulation waste, foundry sands, sandblast waste, and ferrous and/or non- ferrous scrap metal from shredding or incinerator residue.

3. The method of Claim 1 wherein the hydroxycarboxylic acid or salt thereof is selected from the group consisting of citric acid, calcium citrate and ferric citrate.

4. The method of Claim 3 wherein the hydroxycarboxylic acid is citric acid.

5. The method of Claim 4 wherein citric acid is present in a concentration of from about 0.01 M to about 20.0 M.

6. The method of Claim 5 wherein citric acid is present in a concentration of from about 0.05 M to about 5.0 M.

7. The method of Claim 6 wherein citric acid is present in a concentration of from about 0.05 M to about 0.50 M.

8. The method of Claim 1 wherein step (a) is performed in a batch mode or continuous flow mode.

9. The method of Claim 1 wherein step (b) is performed by centrifugation, gravity filtration, decanting, gravity drainage or overhead drainage.

10. The method of Claim 1 wherein step (a) is performed under agitating conditions and at an elevated pH compared to the pH of the acid solution.

11. The method of Claim 1 wherein step (a) is performed at a temperature that is at least above ambient temperature .

12. The method of Claim 1 further comprising the step of contacting the aqueous soluble chelated metal complexes removed from the waste with a heavy metal precipitating agent.

13. The method of Claim 12 wherein the precipitating agent is selected from the group consisting of phosphates, sulfides, carbonates and silicates.

14. The method of Claim 13 wherein the precipitating agent is sodium sulfide or hydroxyapatite .

15. The method of Claim 1, further comprising contacting the waste with a precipitating agent after step (c) is performed to reduce water solubility of residual metal complexes contained in the waste.

16. A method of extracting and recovering heavy metal from a heavy metal bearing waste, comprising the steps of: a) contacting heavy metal bearing waste with hydroxycarboxylic acid and/or salt thereof in an aqueous environment, in an effective amount to solubilize at least a portion of heavy metal contained in the waste and form aqueous- soluble chelated metal complexes; b) removing aqueous hydroxycarboxylic acid and/or salt thereof and the chelated metal complexes which are soluble therein from the product of step (a) ; c) optionally contacting the waste obtained in step (b) with rinse water, Pseudomonas fluorescens , precipitating agent or combination thereof to extract remaining heavy metals and residual acid from the waste, thereby yielding a waste having a reduced heavy metal content compared to the waste before step (a) is performed; and d) converting the chelated soluble heavy metal compounds removed from the waste to an aqueous insoluble precipitate, thereby recovering the heavy metal as a precipitate that is not susceptible to natural or inducing leaching conditions.

17. The method of Claim 16 wherein the heavy metal bearing waste is selected from the group consisting of incinerator bottom ash, incinerator flyash, heavy metal contaminated soil, auto shredder residue, wire chopping insulation waste, foundry sands, sandblast waste, and ferrous and/or non- ferrous scrap metal from shredding or incinerator residue.

18. The method of Claim 16 wherein the hydroxycarboxylic acid or salt thereof is selected from the group consisting of citric acid, calcium citrate and ferric citrate.

19. The method of Claim 18 wherein citric acid is present in a concentration of from about 0.01 M to about 20.0 M.

20. The method of Claim 16 wherein step (d) is performed using a precipitating agent.

21. The method of Claim 19 wherein the precipitating agent is selected from the group consisting of phosphates, sulfides, carbonates and silicates.

22. The method of Claim 15 wherein step (d) is performed by contacting the chelated heavy metal compounds with Pseudomonas fluorescens .

23. A method for cleansing the surface of ferrous and/or non-ferrous metal, comprising the steps of: a) contacting ferrous and/or non-ferrous metal with hydroxycarboxylic acid and/or salt thereof in an aqueous environment, in an effective amount to solubilize at least a portion of heavy metal contained on the metal and form aqueous-soluble chelated metal complexes; b) removing aqueous hydroxycarboxylic acid and/or salt thereof and the chelated metal complexes which are soluble therein from the product of step (a) ; and c) contacting the metal obtained in step (b) with rinse water, Pseudomonas fluorescens , precipitating agent or combination thereof to remove containments, oxidized metals and residual acid from the metal, thereby yielding a metal having a surface which is cleansed compared to the metal before step (a) is performed.

24. The method of Claim 23 wherein the hydroxycarboxylic acid or salt thereof is selected from the group consisting of citric acid, calcium citrate and ferric citrate.

25. The method of Claim 23 wherein step (a) is performed at a temperature at least above ambient temperature .