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'Reference 46.

'Reference 46.

Pretreatment Requirements

Pretreatment is generally not necessary before the leaching of a sludge. Qewatering of the sludge to 1 percent solids, however, may provide a higher acidification efficiency while still extracting most of the metals content.

Posttreatment Requirements

The leaching procedure used for metals removal from sludge is intimately related to the procedures chosen for recovery of the metals from the resulting solutions. Leaching is not a stand-alone method for metals recovery unless the leaching extract (liquid stream concentrated with the leached metals) is directly reused 1n a process; i.e., leaching may meet tho goal of rendering the sludge nonhazardous, but it usually will not meet the goal of recovering the metals in a directly reusable form. Complete recovery of metals will usually involve a process train of which leaching is only the first step. Often, the leached solutions can be electrolyzed to recover pure metals. Chemical precipitation can also be used to attain high concentrations of the metallic constituents of interest, which can be further treated in metals recovery processes.

A leaching process will produce a secondary sludge that must be disposed. Because rinse baths from electroplating operations usually contain sulfates, the addition of lime in the wastewater treatment plant will create a calcium sulfate precipitate. This sludge is insoluble in an acid leach and will therefore be the secondary sludge resulting from an acid leaching procedure.

Performance Data

Several leaching technologies have been proposed for the recovery of metals from various kinds of sludges. Leaching has been implemented on an industrial scale, and several 1aboratory-scale tests have been conducted. Some of the leaching processes are discussed in the following subsections. Brief descriptions of the posttreatment requirements are also included. The posttreatment requirements are generally metals-recovery techniques that are discussed in other sections of this report,

Metals Recovery from Hydroxide Sludges--

ln an EPA-sponsored project conducted by the Montana College of Mineral Science and Technology, the use of well-established metallurgical techniques to recover metals from metal-finishing hydroxide sludges was investigated. The process train consisted of sulfuric acid leaching; iron removal by jarosite [KFej(0H)t(S0Jj] precipitation (for iron concentrations from S to 20 percent) or solvent extraction (for iron concentrations below S percent); copper removal by solvent extraction and copper recovery by electrowinning or copper sulfate crystallization; zinc recovery by solvent extraction; chromium oxidation and recovery by lead chromate precipitation; nickel removal by sulfide precipitation or nickel sulfate crystallization;

and final solution purification of low concentration residual ions by ion exchange. Pilot-scale tests were conducted to investigate solvent reagent degradation, to develop mass balances, and to determine what operational problems may occur in full-scale operations.

The equipment required for the process consisted of leach vessels, settlers, a filter press, solvent extraction mixer-settlers, chlorine or electrochemical oxidizer, pH monitors and controllers, precipitating vessels, crystalllzers, and an ion exchange column. No problems were encountered with mechanical control of the system. Chemicals were necessary to maintain the pH.

The sulfuric acid leaching solution was effective in dissolving metals from a sludge containing 23 weight percent solids under relatively mild conditions; i.e., a residence time of 30 minutes, a temperature of 40° to 50SC, a sludge-to-added-liquid ratio of 0.8, sufficient acid content to maintain a pH of O.S to 1.5, and enough agitation to suspend particulates in the liquid phase. Approximate removal rates obtained were 56 to 95 percent for Iron, 94 to 95 percent for copper, 91 to 96 percent for zinc, 8S to 98 percent for nickel, 97 percent for chromium, 93 to 100 percent for calcium, and 90 to 97 percent for aluminum.

A Z70 L leach vessel was capable of processing more than a ton of sludge per 8-hour day. Because the fllterability of the leach residue was difficult, the jarosite precipitation operation was conducted in the same vessel as the leach process. The resulting precipitate was easy to filter. Fllterability of this leach residue can also be improved by using filter aids. In this process scheme, the precipitation rather than the leaching step was the rate-limiting factor.

For every 100 pounds of metal-finishing hydroxide sludge {23 percent solids content) that was treated, 15 pounds of leach residue was generated. This residue was EP Toxic for cadmium and chromium; however, the leaching process was effective in achieving a reduction in the total quantity of hazardous metal-bearing sludges. In this study, 45 pounds of jarosite were produced per 100 pounds of hydroxide sludge. The precipitate from the jarosite precipitation process contained 10 percent of the copper, 6 percent of the zinc, 18 percent of the chromium, and 6 percent of the nickel. Subsequent leaching of the jarosite-based precipitate may be desirable to recover additional metals. Higher iron concentrations created larger amounts of sludge and resulted in greater losses of chromium.

This process was also used to investigate the removal of metals from a hydroxide sludge generated at an electrochemical machining facility. The primary metals in the sludge were iron, chromium, and nickel; however, significant amounts of niobium, titanium, and cobalt were also present. Sulfuric acid leaching at I8*C and a pH of 0.7 dissolved 97 percent of the sludge (with a solids concentration of 33 percent), 93.5 percent of the iron, 95.4 percent of the chromium, 95.1 percent of the nickel, 100 percent of the cobalt, 73 percent of the titanium, and none of the cobalt. Some of the nickel present with niobium as metallic particles did not readily dissolve.

The leach residue contained 19.3 percent nickel, 43.1 percent niobium, and 7.2 percent titanium; this metal content is high enough to be a marketable product. The leach residue contained all of the niobium and 27 percent of the titanium in the original sludge. By selective phosphate precipitation at various pH values, 87 percent of the chromium and 94,6 percent of the nickel were recovered as separate (and therefore reusable or marketable) sludges. The combined leach residue and ferric phosphate sludge was not EP Toxic for chromium, but the EP extract contained nickel at concentrations more than 100 times the drinking water standard. The sludges were combined because the available filter apparatus was not appropriate for separating the leach residue from the sulfuric acid solution.

AMAX Extractive Research and Development, Inc., under contract to the U.S. Army Toxic and Hazardous Materials Agency (USATHAKA), conducted a study aimed at identifying alternatives for recovering metals from Army depot electroplating wastewater treatment sludges. The researchers considered three processes: 1) an ammonia leach followed by an acid or caustic leach, 2) a caustic leach, and 3) an acid leach. Sulfuric acid was chosen for laboratory-scale testing because of the advantages listed earlier.

For laboratory testing, a synthetic sludge similar to sludges expected from Army depot electroplating operations was prepared. This sludge contained 10 percent chromium hydroxide; 7 percent each of copper hydroxide, iron (II) hydroxide, nickel hydroxide and zinc hydroxide; 1 percent cadmium hydroxide; and 61 percent calcium sulfate. The sludge was prepared at three conditions: 1) no aging, 2) 7-day aging at 65°C, and 3) 6-week aging at room temperature. The third condition is the most representative of actual wastewater treatment sludges. The sludges were leached with sulfuric acid for 90 minutes at a pH of 1.5. More than 99 percent of each of the metals was removed except cadmium, which had a metal-removal efficiency near 98 percent. Most of the metals were leached within 30 minutes. The age of the sludge had little effect on the efficiency of metals removal.

The sludge remaining after the leaching procedure was found to be EP Toxic for cadmium and chromium. The EP extract from the sludge that was aged for 7 days at 65°C had the highest metals concentrations—nearly an order of magnitude higher than those in the sludge that was not aged. The sludge that was aged for 6 weeks also had higher concentrations of metals in the extract than the sludge that was not aged. Photomicrographs of the aged sludge showed a different crystalline structure than the nonaged sludge, which could perhaps trap some heavy metals by chemisorption, mechanical occlusion, or lattice substitution. As a result, the sulfuric acid was less effective in the removal of all the heavy metals under the test conditions, although the metals were leached under the conditions (24-hour leach by an aqueous solution maintained at a pH of 5 with acetic acid) of the EP Toxicity test. One sample of the sludge that was aged for S weeks was leached 1n a blender, which apparently broke down the crystalline structure and resulted in an EP extract with lower metal concentrations than all other results. After the filter cake was leached, it was made nonhazardous by mixing 10 percent by weight of lime with the solids.

Selective sulfide precipitation was used to remove the metals from the leaching solution. Copper and cadmium Mere precipitated from the solution with the least amount of sulfide addition, and line was nearly quantitatively removed by the additional sulfide. Nickel and iron were only partially removed, whereas more than 90 percent of the chromium stayed in solution. After the pH was raised, chromium and iron precipitated and the nickel remained in solution. The precipitate was then leached with three organic solvents, which concentrated the chromium in the aqueous phase and the iron in the organic phase.

Bfohydrametallurgy--

Biohydrometallurgy or bacterial leaching is emerging as a metals recovery technology. A mixed bacterial culture containing Thiobacillus ferrooxidans and Thiobacillus thioxldans has been used to extract Zn, Cu, Pb, and Cd from waste sludges.These bacteria use elemental sulfur to grow. The metals in the waste are solub111zed by the sulfuric acid produced by the bacteria, as shown in the following equation:

bacteria

This process has shown removal rates as high as 80 to 90 percent for Cd. Leptosperrillum ferroxidans and a thermophilic Sulfolobu; species have also shown promise as bacterial leaching cultures.1J

Ammonium Carbonate Leaching--

In a 1977 study conducted by EPA, the use of amnonium carbonate was investigated to remove copper and nickel from hydroxide sludge produced during treatment of electroplating wastewater.4 The sludge also contained chromium. Upon the addition of ammonium carbonate, copper and nickel formed water-soluble amine complexes. These complexes could then be separated from the chromium, which does not form complexes with amoonia. The leach residue was then treated by roasting it with sodium carbonate and leaching the fused mass with water to return soluble chromates and dichromates into solution, from which they could be recovered as anhydrous chromic acid. The process was successful in reducing copper, nickel, and chromium from levels of 10 to 20 percent in the sludges to a level of 1 percent in the leach residue. The experiments removed more than 90 percent of the copper, 60 percent of the nickel, and less than 10 percent of the chromium in the leaching stages. The recovery of nickel was less than desired; additional optimization could produce better results.

Because amnonium carbonate leaching Is more metal-specific than acid leaching, chromium can be separated from copper and nickel In the leaching step itself rather than in the posttreatment phase. Removal of chromium is necessary because it interferes with the electrodeposition of copper and nickel.

Leaching studies were also conducted to evaluate the Influence of certain variables on the ability of the process to dissolve copper and nickel arid yet leave chromium in the sludge, A summary of the conclusions of these studies is provided here.

* Sludge pretreatment. Four forms of the sludge were examined:

1) the raw sludge containing 20 percent solids; 2) sludge dried at 110°C and then pulverized and blended; 3} dried sludge that had been bal1-millad with water to create a slurry suspension; and A) dried sludge washed with water to remove 10 percent of its weight as water-soluble material, followed by filtering, redrying, and grinding. The data showed that conditioning the sludge was not advantageous for copper and nickel recovery. Creating a slurry suspension seemed to enhance chromium solubility, which was undesirable.

Choice of leachant. Three leaching agents were studied: anmonium carbonate, aumonium hydroxide, and amnonium sulfate. Ammonium carbonate at a concentration of 10 weight percent of ammonia provided the best removal rates for copper and nickel.

Temperature. 50®C provided the most rapid extractions.

Number of leaching stages. A two-stage leach was required because the dissolution of nickel is Inhibited by high concentrations of copper in solution. The reason for this is that the leaching of copper proceeds best at a pH of about 10, whereas the leaching of nickel proceeds best at a pH of 8 or 9. Most of the copper is removed in the first stage, and most of the nickel Is removed In the second stage.

Leaching time. Extending the leaching time beyond 3 hours improved the efficiency of extraction only marginally—not enough to justify the additional costs. Extended leaching times at elevated temperatures volatilized some of the armonia values and resulted in low leaching efficiencies.

Aeration. Bubbling air and carbon dioxide through the leaching solution was ineffective in improving copper and nickel dissolution.

Metal form. Metals in wastewater treatment sludges are present in both hydroxide and oxide forms. Experiments were conducted to compare the amount of dissolution of oxide versus hydroxide sludges. Approximately SO percent of the copper was recovered from the oxide, whereas all of the copper was recovered from the hydroxide. The oxides of chromium and nickel were not leached to any significant degree. All of the nickel was recovered from its hydroxide, whereas the hydroxide of chromium was not affected.

Aluminum-finishing Sludges--

Sulfuric acid leaching of aluminum sludges from water treatment plants has often been used for the recovery of aluminum as aluminum sulfate for reuse in the treatment plant. The process has been practiced since as early as 1903 and has been widely applied in Japan, Great Britain, and Poland. Alum sludge is gravity-thickened, and the underflow sludge from the thickener is mixed with sulfuric acid in a rapid-mix tank. Acidified sludge is transferred to a separator, where supernatant liquid (i.e., the leachate) is recovered for reuse as a coagulant in the water treatment process. Sludge from the underflow of the separator 1s neutralized with lime, dewatered, and disposed.

Because the primary constituent of most sludges from the aluminum-finishing industries is aluminum hydroxide, the sulfuric acid leaching process may be applicable to aluminum-finishing sludges. A 1987 research study sponsored by EPA and conducted by the Georgia Institute of Technology focused on producing cotnaercial-strength solutions of aluminum sulfate (liquid alum) from aluminum-finishing sludges. Three sludges were investigated: 1) a gelatinous aluminum hydroxide suspension produced by conventional lime neutralization of dilute aluminum anodizing rinse waters in a wastewater treatment plant; 2) a crystalline aluminum hydroxide sludge produced from neutralizing spent caustic etch with spent finishing acid (referred to as segregated neutralization); and 3) a high-solids sludge produced by recovering aluminum trlhydrate crystals from caustic etch solutions.

Commercial-strength solutions of liquid alum require concentrations of 8 weight percent aluminum oxide. Obtaining this strength requires a minimum sludge solids content of at least 20 percent. Sludges obtained from an actual aluminum-finishing plant were therefore dewatered before leaching. After dewaterirg, the sludges were leached separately with stoichiometric quantities of sulfuric acid (based on the sludge aluminum content) at leaching times of 30 to SO minutes, an initial temperature greater than 9S°C, and a maintained temperature of SO® to 90°C for the remainder of the experiments.

Conventional neutralization sludge filter cakes with solids contents after dewatering of 17.4 to 18.1 percent were extracted to produce liquid alum with concentrations of 7.4 to 8.8 percent aluminum oxide, A total of 93 to 97 percent of the aluminum was leached, and 95 to 99 percent of the initial suspended solids were destroyed.

Segregated neutralization sludge filter cakes with solids contents of 35.8 percent were extracted to produce liquid alum with concentrations of 8.1 to 9.0 percent aluminum oxide by the addition of water equal to 80 to 100 percent of the mass of wet sludge extracted. The etch recovery sludge with a solids content of 91,6 percent produced a liquid alum with a concentration of 8.3 to 9.2 percent aluminum oxide by the addition of water equal to 200 to 370 percent of the wet sludge extracted. A total of 70 to 8S percent of the aluminum was extracted, and 54 to 85 percent of the suspended solids were destroyed. The purity of the final liquid alum was comparable to other commercial products except that the concentrations of nickel and tin were high. These metals may have been the result of drag-out from the anodizing process and the use of nickel in seal tanks; segregation of these wastewaters would prevent these metals from contaminating the aluminum sludges.

Zinc Sulfate Sludge-

Leaching has also been applied to the recovery of zinc sulfate from viscose manufacturing. More than 100,000 tons of zinc sulfate is used in the manufacture of viscose rayon in the United States. Because zinc sulfate is not consumed in the production reactions, this quantity represents a loss of nonrenewable resources. Currently, the zinc is usually precipitated with lime at a pH of 10 and is not recovered. In a two-stage system developed by American Enka Co., the pti is raised to 1.0 in the first stage, which results in precipitation of most of the iron, calcium sulfate, and other constituents, but not zinc hydroxide. The pH is ttlefl raised to 9.5 to 10 in a separate vessel, the zinc solution is contacted with a slurry of previously precipitated zinc hydroxide crystals, and additional zinc hydroxide precipitates onto the surface of the crystals. The dense sludge is first allowed to settle and is then leached with sulfuric acid. The resulting zinc sulfate solution can be directly reused in the rayon manufacturing process.

FMC Corporation recovers zinc from zinc hydroxide sludge generated during the manufacture of rayon fiber. Sludge containing Z to 6 percent solids is heated to approximately 150°C to make the sludge more amenable to filtration. The cake solids after filtration contain approximately 34 percent zinc. The cake is leached with sulfuric acid, and a 25 to 30 percent zinc sulfate solution is produced. To remove iron contamination, ferrous iron 1s converted to ferric iron with hydrogen peroxide and precipitated out at a pH of 4.5. After the iron is filtered out, the zinc sulfate solution is recycled to the rayon plant.

Another manufacturer of vulcanized fiber treats a wastewater containing 100 to 300 mg/L zinc by raising the pH to 8.5 to 9.5. The precipitate is then leached with hydrochloric acid, which results in a zinc chloride solution. After It is concentrated 1n an evaporator, the zinc solution can be reused in the manufacturing process.

Plating Sludge Ponds Remediatlon--

In late 1988-early 1989, Oavy Hckee conducted a feasibility and treatability study for the remediation of a large electroplating sludge pond. These studies focused on utilizing metals recovery techniques to produce a copper-nickel carbonate for sale, and a caomtua-zinc sulfide for further processing. A multi-step dissolution-precipitation process using a sulfuric acid leach followed by lime soda softening was selected for recovery of nixed copper and nickel values as their carbonates; a sulfide precipitation step was evaluated for recovery of cadmium and zinc values.

Lead Wastes from Superfund Sites-The Bureau of Mines Is developing a technique for recovery of lead from battery casings and contaminated soils from several Superfund sites. Crushed ebonite casing material containing 3,000 to 4,000 ppm lead is the major source of lead contamination at these sites. The leaching technique consists of prewashing in an ammonium carbonate solution followed by leaching with fluosilicic and/or nitric acid. The lead can then be recovered from the leached solution by electrowinning.

Av«mb1HtY

American Enka Co. recycles zinc sul/ate sludge, and FHC recovers zinc from hydroxide sludges as zinc sulfate. The leaching of aluminum sludges from water treatment plants for the recovery of aluminum as aluminum sulfate has been widely practiced. The ammonium carbonate leaching process, which was developed by a Swedish company, KX-Processor, has been used for metals recovery at several companies, some of which have gone out of business as a result of the falling prices of some metals.

Most of the leaching procedures discussed in previous sections have not been implemented at full scale. The technology, however, consists of relatively simple equipment for the leaching step itself. The basic unit operations have been practiced widely in metallurgical industries, and adaptations to metals recovery from sludges would be easy if the value of the recovered metals Is determined to be sufficient to justify the expense of the operation.

At the Recontek waste recycling facility, zinc-bearing solutions are leached with alkaline solutions, whereas non-zinc sludges are treated with acidic solutions. Zinc-bearing sludges are digested at approximately 80°C with sodium hydroxide for a sufficient period of time, cooled and filtered. The filtrate is processed in a zinc cementation tank to precipitate metals more electro-negative than zinc {e.g., lead, cadmium) and then pumped to a zinc electrowinning system. The non-zinc sludge waste from the digester (primarily copper and nickel) is digested with sulfuric acid, filtered to produce a residue containing precious metals (e.g., gold, silver), and the filtrate sent to the copper electrowinning system for production of copper cathodes. The solution leaving the copper electrowinning system is sent to a crystallizer for nickel sulfate recovery."

Table 39 presents the potential applicability of leaching for the various RCRA waste codes discussed in Section Z. Performance data do not exist for all of the waste codes, and bench- and pilot-scale studies need to be conducted to determine the viability of leaching as a metals recovery process for these wastes. Table 40 presents the solubility of various metal compounds in different solvents. The compounds listed are those that result from precipitation processes as well as those that may be present in RCRA waste sludges. Table 39 can be used to select a solvent for leaching particular metals from a specific sludge.

TABLE 39. POTENTIAL APPLICABILITY OF LEACHING FOR REPRESENTATIVE RCRA WASTES'

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