United States Environmental Protection Agency Risk Reduction Engineering Engineering Laboratory Cincinnati OH 45268 Research and Development EPA/600/S2-91/041 Sept. 1991 iSrEPA Project Summary Recovery of Metals from Sludges and Wastewaters This report presents information on the state-of-the-art of metals recovery technologies to assist in Identifying waste-management options for metal- bearing sludges and wastewaters that may be regulated under the Resource Conservation and Recovery Act (RCRA). Only a few of the technologies ad- dressed In this report (e.g., electrowin- ning, high-temperature metals recovery [HTMR]) are directly applicable to the recovery of metals from wastes; other technologies treat the wastes to a physi- cal form that may be amenable to even- tual metals recovery. Wastewaters can be treated effec- tively by several methods. Precipitation processes have been widely used to remove arsenic, cadmium, chromium (+3), copper, iron, manganese, nickel, lead, and zinc from metal-bearing waste- waters. For economic reasons, electro- winning is a commercial technology that has normally been restricted to the treat- ment of wastewaters containing noble metals such as gold and silver. After appropriate pretreatment, slud- ges can be effectively treated by HTMR processes. These processes allow for the direct recovery of metals from slud- ges. The economic feasibility depends on the amount of sludges treated and the amount of metals contained In the sludges. Membrane separation pro- cesses such as microfiltratlon (MF) and ultra filtration (UF) can be used in com- bination with chemical treatment for the physical separation of metal sludges. Leaching may be used to extract cad- mium, chromium, copper, lead, nickel, and zinc directly from sludges by using various process trains. This Project Summary was devel- oped by EPA's Risk Reduction Engi- neering Laboratory, Cincinnati, OH, to announce key findings of the research project that Is fully documented In a separate report of the same title (see Project Report ordering Information at back). Introduction Section 3004 of the Resource Conser- vation and Recovery Act (RCRA), as amended by the Hazardous and Solid Waste Amendments of 1984, prohibits plac- ing untreated RCRA-regulated hazardous wastes in or on the land. Waste manage- ment options are needed to help recyclers comply with these regulations. In the full report, summarized here, we address the following processes that are amenable for recovery of metals from hazardous wastes: chemical precipitation, electrolytic recov- ery, HTMR, membrane separation, leach- ing, ion exchange and evaporation. For each of these technologies, the following parameters are summarized: (1) design specifications of applicable processes, (2) waste characteristics affecting perfor- mance, (3) pretreatment/posttreatment re- quirements, (4) available performance data, and (5) availability of the technology and feasibility for treating various hazardous waste categories. Waste Chacterizatlon This report covers nine major metal- waste-producing industries: Printed on Recycled Paper ------- (1) metal coatings; (2) smelting and refining of nonferrous metals; (3) paint, ink, and associated products; (4) petroleum refining; (5) iron and steel manufacturing; (6) photographic industry; (7) leather tan- ning; (8) wood preserving; and (9) battery manufacturing. Waste streams from each of these industries have unique character- istics; however, the wastes also contain common metals, such as aluminum (Al), arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), silver (Ag), and zinc (Zn). Table 1 presents the number of metal- waste generators (as of 1983) by Standard Industrial Classification (SIC) code for the major industry categories discussed in the report. Table 2 indicates the amount and num- ber of generators of metal-bearing wastes by D (wastes which are hazardous be- cause they exhibit a particular hazardous characteristic), F (wastes from non-spe- cific sources), and K (wastes from specific sources) EPA hazardous waste codes. Until very recently, only about half of the indus- tries that generate metal-bearing wastes recovered the metals from wastewaters and sludges. Table 3 presents brief descriptions of the hazardous wastes generated from the major industry categories included in the report. Metals Recovery Technologies Chemical Precipitation Precipitation of metal-laden wastewa- ters involves adding chemicals to alter the physical state of the dissolved or sus- pended metals and to facilitate their re- moval through sedimentation. These precipitates may then be processed fur- ther for metals recovery. Chemicals used to effect precipitation include: caustic soda, lime, ferrous and sodium sulfide, soda ash, sodium borohydrkJe, and sodium phos- phate. Some wastewater constituents, e.g., hexavalent chromium, cannot be effectively precipitated without first chemically reduc- ing the metal to a more favorable form for precipitation. Reducing agents typically used by industry include sulfur dioxide, sodium bisulfite, sodium metabisulfite, and ferrous sutlate. Coagulation chemicals may be needed to enhance settling times of the precipitated metal particles. Examples of coagulants currently used by industry in- clude lime, alum, and synthetic polyelec- trotytes. Chemical precipitation is commonly used to treat metal-bearing wastewaters from electroplating, pigment manufacture, Table 1. Number of Major Metal-Waste Generators, by SIC Code, in 1983 SIC Code No. 3471 2851 3479 3714 2819 3341 3400 9711 3721 3900 3356 2893 3312 3321 4911 2869 2821 3662 3679 3711 3545 SIC Description Plating and surface finishing Paints and allied products Metal coating and allied products Motor vehicle parts and accessories Industrial inorganic chemicals Metals, nonferrous, secondary Fabricated metal products National security Motors and generators Miscellaneous manufacturing industries Metal, nonferrous, rolling, drawing Printing ink Blast furnaces, steel mills Foundries, gray iron Electric services Industrial organic chemicals Plastics material Radio and TV communication equipment Electronic components Motor vehicle bodies Machine tool accessories No. of Facilities 4,287 2,145 2,902 4,151 2,183 876 55,380 393 966 32,867 384 609 1,229 1,229 2,614 1,160 1,529 4,656 5,392 1,040 3,432 Tablo 2. Nationwide Metal-Waste-Generation Data by Waste Group D Wastes F Wastes K Wastes Waste Volume, 10*gal/yr 3685 3920 219 Percent of Total Metals 46.9 49.9 2.8 Number of Generators 3860 2091 402 the photographic industry, leather tanning, wood preserving, the electronics industry, battery manufacture, and nonferrous metal production. Approximately 75% of all elec- troplating facilities use precipitation in the treatment of their wastewaters. The pro- cess is several decades old, and chemical feed reagents are being improved to yield better metal removals from the aqueous phase. Specific waste characteristics that af- fect the performance of chemical precipita- tion systems include (1) the concentration and type of metals, (2) the concentration of total dissolved solids, (3) the concentration of complexing agents, and (4) the concen- tration of oil and grease. Pretreatment of wastewaters before metals precipitation can involve segrega- tion, removal of large solids, flow equaliza- tion, cyanide destruction (if applicable), chrome reduction, oil separation, neutral- ization, and/or waste treatment of the indi- vidual process streams. Sand filtration is a common post-pre- cipitation/sedimentation effluent treatment technique. If concentrations in the effluent do not meet discharge standards, addi- tional metal treatment technologies (e.g., ion exchange, reverse osmosis) may be needed. Electrolytic Recovery Electrolytic processes are used exten- sively to recover metals from industrial wastewaters. The electrolytic cell is the basic device used in electrolytic recovery operations. The cell consists of an anode and a cathode immersed in an electrolyte. When current is applied, dissolved metals in the electrolyte are reduced and depos- ited on the cathode. Because the metal(s) removed from solution can be reused, the technology, termed "electrowinning," is con- ------- Table 3. Metal-Bearing Hazardous Wastes From Major Industry Categories EPA Hazardous Listed Waste No. Hazardous Waste Description Constituent(s) F006 F007 F008 F009 F019 K002 K003 K004 K005 K006 Wastewater treatment sludges from electro- plating operations except the following: (1) sulfuric acid anodizing of aluminum; (2) tin plating on carbon steel; (3) zinc plating (segregated basis) on carbon steel; (4) aluminum or zinc-aluminum plating on carbon steel; (5) cleaning/stripping asso- ciated with tin, zinc, and aluminum plating on carbon steel; and (6) chemical etching and milling of aluminum. Spent cyanide plating bath solutions from electroplating operations. Plating sludges from the bottom of plating baths from electroplating operations where cyanides are used in the process. Spent stripping and cleaning bath solutions from electroplating operations where cya- nides are used in the process. Wastewater treatment sludges from the chemi- cal conversion coating of aluminum. Wastewater treatment sludge from the pro- duction of chrome yellow and orange pigments. Wastewater treatment sludge from the pro- duction ofmolybdate orange pigments. Wastewater treatment sludge from the pro- duction of zinc yellow pigments. Wastewater treatment sludge from the pro- duction of chrome green pigments. Wastewater treatment sludge from the pro- duction of chrome oxide green pigments Cadmium, hexa- valent chromium, nickel, cyanide (complexed) Cyanide/salts Cyanide/salts Cyanide/salts Cadmium, hexavalent chromium, cyanide (complexed) Hexavalent chromium, lead Hexavalent chromium, lead Hexavalent chromium Hexavalent chromium, lead Hexavalent chromium K007 K008 K048 K049 K050 K051 K052 K060 K061 K062 K064 (anhydrous and hydrated). Wastewater treatment sludge from the pro- duction of iron blue pigments. Oven residue from the production of chrome oxide green pigments. Dissolved air flotation (DAF) float from the petroleum refining industry. Slop oil emulsion solids from the petroleum refining industry. Heat exchanger bundle-cleaning sludge from the petroleum refining industry. API separator sludge from the petroleum refining industry. Tank bottoms (leaded) from the petroleum refining industry. Ammonia still lime sludge from coking operations. Emission control dust/sludge from the primary production of steel in electric furnaces. Spent pickle liquor generated by steel- finishing operations of facilities within the iron and steel industry. Acid plant blowdown slurry/sludge resulting from the thickening of blowdown slurry from primary copper production. Cyanide (complex), hexavalent chromium Hexavalent chromium Hexavalent chromium, lead Hexavalent chromium, lead Hexavalent chromium Hexavalent chromium, lead Lead Arsenic Hexavalent chromium, lead, cadmium Hexavalent chromium, lead Lead, cadmium (continued on p.4) sidered a recovery process. If a membrane is used between the cathode and the an- ode for the selective transport of some ions, the process is called electrodialysis. Electrowinning is most effective for recov- ery of noble metals such as gold and sil- ver. These metals have high electrode potentials and are easily reduced and de- posited on the cathode. Metals such as cadmium, copper, chromium, lead, tin, and zinc can be removed, but a greater amount of current is required. Electrowinning is very effective for plating solutions used in printed circuit boards; these contain che- lated metals that are difficult to remove by other means. Electrowinning of metals is a particu- larly attractive process because it com- pletely eliminates the generation of a metal-bearing sludge. Its applicability, how- ever, is limited to waste streams contain- ing metals in solution such as cadmium, copper, chromium, gold, lead, silver, tin, or zinc. For dilute solutions, electrowinning can be difficult because of the low mass- transfer rates; however, mass transfer rates can be enhanced both by agitating the solution and by increasing the effective surface area of the cathode. The principal area of application of elec- trodialysis is the recovery of metals from electroplating bath rinse waters. In many cases, the wastewater must be filtered before it is fed through the elec- trolytic reactor. Adjustment of pH is a nec- essary pretreatment measure because the waste pH affects metal speciation. Metal recoveries of up to 98% from plating rinse waters have been demon- strated with the use of high-surface area (HSA) electrodes. Several vendors are currently manu- facturing electrodialysis systems for treat- ment of wastes from gold, chromium, silver, and zinc cyanide plating operations and from nickel plating operations. Other suc- cessful electrodialysis applications include recovery of metals from tin and trivalent chromium baths and the recovery of chro- mic acid and sulfuric acid from spent brass etchants. Electrowinning and electrodialy- sis systems have both been used exten- sively in industrial applications. High-Temperature Metals Recovery (HTMR) Several types of HTMR processes are currently available or under development for the recovery of metals from sludges generated either directly by industrial pro- cesses or from the treatment of industrial wastewaters. These HTMR processes may involve plasma-based or high-temperature fluid-wall reactor systems (which use elec- ------- Table 3. (Continued) EPA Hazardous Waste No. Hazardous Waste Description (continued on p.4) Listed Constituents) K065 K066 K069 K086 K090 K100 D004 D006 D007 D008 D009 D011 Surface impoundment solids contained in and dredged from surface impoundments at primary lead smelting facilities. Sludge from treatment of process wastewater and/or acid plant btowdown from primary zinc production. Emission control dust/sludge from secondary lead smelting. Solvent washes and sludges, caustic washes, and sludges from cleaning tubs and equipment used in the formulation of ink from pigments, driers, soaps, and stabilizers containing chromium and lead. Emission control dust or sludge firom ferro chromium silicon production. Waste leaching solution from add leaching of emission control dust/sludge from secondary lead smelting. Characteristic waste based on concentrations. Characteristic waste based on concentrations. Characteristic waste based on concentrations. Characteristic waste based on concentrations. Characteristic waste based on concentrations. Characteristic waste based on concentrations. Lead, cadmium Lead, cadmium Hexavalent chromium, lead, cadmium Lead, hexavalent chromium Chromium Hexavalent chromium, lead, cadmium Arsenic Cadmium Chromium Lead Mercury Silver trterty as the energy source) or coal/natu- ral- gas-based technologies. HTMR processes are applicable only for the processing of sludges, not for waste- waters. One significant advantage of the HTMR processes is that other toxic con- stituents in the wastes, such as complexed cyanides/organics, would also be destroyed at the high temperatures (>1100°C) pre- vailing in the furnaces. Important waste characteristics affect- ing the performance of HTMR processes include: (1) concentrations of undesirable volatile metals, (2) boiling points of the metal constituents, and (3) thermal con- ductivity of the waste. Pretreatment re- quirements for HTMR processes vary with the type of process. This may include op- erations such as drying of feed sludges or pelletizing with special additives. The crude metallic oxides produced in certain HTMR processes must be further treated for sepa- ration and recovery of metals. Gases from the high temperature furnaces must be treated before atmospheric release. The INMETCO Plant in Ellwood City, PA (which utilizes a rotary hearth/electric furnace) and the Horsehead Waelz Kiln in Palmerton, PA, have processed hazard- ous wastes (sludges) under an Interim Per- mit status. The INMETCO Plant has processed the following waste codes: F006, K061, K062, D006, D007, and D008. The Horsehead Waelz kiln has processed F006, F019, K061, D006, and D008. Horsehead has two operating Waelz plants in the United States—one at Palmerton, PA, and one at Calumet City, IL. The Palmerton plant has three Waelz kilns with a total capacity of 270,000 tons/ y r; the Calumet plant has one kiln with a capacity of 80,000 tons/yr. The INMETCO plant is capable of treating 50,000 tons of wastes per year. Both of the Horsehead Waelz plants as well as the INMETCO plant are operated primarily to treat steel- making electric arc furnace dust (K061); however, as previously mentioned, they are capable of treating other sludges. A third Horsehead Waelz plant with a capac- ity of 60,000 tons/yr is planned for Rockwood, TN. Membrane Separation The commercially available membrane processes for removal of metals from in- dustrial wastewaters are microfiltration, ul- trafiltration, reverse osmosis, and electrodialysis. Microfiltration (MF) and ul- trafiltration (UF) are used in combination with chemical treatment for the physical separation of metal sludges. Reverse os- mosis (RO) and electrodialysis (ED) are used to recover plating compounds from rinse water and to enable reuse of rinse waters. MF and UF membranes cannot be ap- plied directly to recover metals present as dissolved solids in wastewaters. UF, how- ever, can be used as a pretreatment method for RO units to avoid fouling of the RO membranes. When applied to heavy metal wastes with appropriate pretreatment chemistry, the metal content of the effluent can be extremely low. Each application requires treatability studies to integrate the chemi- cal pretreatment and the MFAJF mem- brane system. If oil and grease are present, addi- tional pretreatment of the waste stream will be required. Well-run precipitation/ UF and MF systems can achieve metal removals greater than 99%. Over 150 full-scale industrial systems, ranging in size up to 400 gal/min, have been in- stalled in the electroplating, printed cir- cuit board manufacturing, battery manufacturing, and photographic process- ing industries. RO systems consist of several mod- ules connected in series, or parallel, or a combination of both. The application of RO to the treatment of metal-containing wastes is limited by the pH range in which the membrane can operate. Cellulose acetate membranes cannot be used on waste streams where the pH is much above 7. The amide or polysutfone membranes, how- ever, have a pH range of 1 to 12. Collodial matter, low-solubility salts, and dissolved organics can seriously inhibit the effective- ness of RO. Pretreatment steps such as pH adjustment, carbon adsorption, chemi- cal precipitation, or filtration are therefore recommended to ensure extended service life of RO systems. Systems are being used commercially to recover brass, hexavalent chromium, copper, nickel, and zinc from metal-finishing solutions. Leaching Leaching is a process in which a solid material is contacted with a liquid solvent for selectively dissolving some components of the solid into the liquid phase. Leaching can sometimes be used to extract various metals from sludges. The goals of this process are: (1) to dissolve the metals in a liquid phase to produce a solution that can be reused directly in a process or from which the metal can be recovered by other techniques, such as electrowinning; and (2) to produce a secondary sludge that is nonhazardous or from which additional metals can be reclaimed by other pro- cesses. Several leaching agents can po- tentially be used, including sulfuric acid, ------- ferric sutfate, ammonia or ammonium car- bonate, hydrochloric acid, sulfur dioxide, ferric chloride, nitric acid, or a caustic solu- tion. Selection of a suitable solvent and unit process depends on the chemical state and physical environment of the metals. Sludges that contain only one metal often can be sent directly to a refiner for reclamation; however, in some operations (e.g., electroplating), all metals are precipi- tated from solution in the same wastewa- ter treatment plant, usually as hydroxides. A process train with numerous unit opera- tions, therefore, is necessary to separate each metal. Complete recovery of the met- als typically includes electrowinning of the leachate. At the Recontek waste recycling facility in Newman, IL, zinc-bearing solutions are leached with alkaline solutions, whereas non-zinc sludges are treated with acidic solutions. Zinc-bearing sludges are di- gested at approximately 80*C whh sodium hydroxide for a sufficient period of time, cooled, and filtered. The filtrate is pro- cessed in a zinc cementation tank to pre- cipitate metals more electronegative than zinc (e.g., lead, cadmium) and then pumped to a zinc electrowinning system. The non- zinc sludge waste from the digester (pri- marily copper and nickel) is digested with suffuric acid and filtered to produce a resi- due containing precious metals (e.g., gold, silver). The filtrate is then sent to the cop- per electrowinning system for production of copper cathodes. Ion Exchange Ion exchange is a treatment technol- ogy applicable to (1) metals in wastewa- ters where the metals are present as soluble ionic species (e.g., Cr3 and CnV); (2) nonmetallic anions such as halides, sul- fates, nitrates, and cyanides; and (3) wa- ter-soluble, ionic organic compounds including (a) acids such as carboxylics, sulfonics, and some phenols, at a pH suffi- ciently alkaline to yield ionic species, (b) amines, when the solution acidity is suffi- ciently acid to form the corresponding acid salt, and (c) quaternary amines and aklyl- sulfates. Ion exchange is a reversible chemical reaction in which an ion from solution is substituted for a similarly charged ion at- tached to an immobile solid particle. The use of this process is practical only on wastewaters and sludge leachates. In con- ventional ion exchange, metal ions from dilute wastewater solutions are exchanged for ions electrostatically held on the sur- face of the exchange medium. Ion ex- change systems have proven to be effective in the removal of barium, cadmium, chro- mium (VI), copper, lead, mercury, nickel, selenium, silver, uranium, and zinc. Evaporation Evaporation is a simplified recovery sys- tem for the separation of substances based on volatility differences. Although the tech- nology is established, recent advancements have made mechanical evaporation a more viable cost-efficient method for metals re- covery. The four basic types of evapora- tors used in the electroplating industry today are rising-film, flash, submerged-tube, and atmospheric. Conclusions Only a few of the technologies ad- dressed in this report (e.g., electrowinning, HTMR) are directly applicable to recover- ing metals from wastes; other technologies treat the wastes to a physical form that may be amenable to eventual metals re- covery. Commercial waste recycling facilities render services that are important if metals are to be recovered as opposed to being treated and disposed. Technologies used at the metals recovery facilities include chemical precipitation, leaching, electro- winning, and evaporation. Current information on metals recovery technologies show that combinations of technologies may often be required to re- cover metals from wastewaters and slud- ges. Additional studies are needed to determine the specific combinations of methods that will most effectively recover metals from different types of wastes. The full report was submitted in fulfill- ment of EPA Contract No. 68-03-3413, Work Assignment 2-63, by IT Corporation, Cincinnati, OH (formerly PEI Associates, Inc.), under the sponsorship of the U.S. Environmental Protection Agency. 6l).S. GOVERNMENT PRINTING OFFICE: 1991 - S4H-OU/40080 ------- ------- ------- This Project Summary was prepared by the staff of IT Corporation, Cincinnati, OH 45246. Ronald J. Turner is the EPA Work Assignment Manager (see below). The complete report, entitled "Recovery of Metals from Sludges and Wastewaters," (Order No. PB91-220384/AS; Cost: $23.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Work Assignment Manager can be contacted at: Risk Reduction Engineering Laboratory U.S. Environmental Protection Agency Cincinnati, OH 45268 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati, OH 45268 BULK RATE POSTAGE & FEES PAID EPA PERMIT NO. G-35 Official Business Penalty for Private Use $300 EPA/600/S2-91/041 ------- |