As anodes As anodes
Sold Sold Sold
As anodes 1.37 1,096
As anodes 10.12 15,180
* Reference 18.
b ISA ■ low-surface area; HSA - high-surface area. c Source: Reference 19.
Cadmium recoveries of up to 98 percent from plating rinse waters have been demonstrated with the use of high-surface area (HSA) electrodes.
Figure 5 compares the performance of a high-mass-transfer carbon electrode with a conventional electrowinning unit.
Brush Wellman, Inc., which produces beryllium and berryllium alloys, has operated an on-site facility In Elmore, Ohio, since 1987 to recover I million pounds of copper and 80,000 pounds of beryllium per year from spent acid liquors used in pickling operations on beryllium-copper alloy products. In the process, a copper hydroxide by-product precipitate typically containing approximately 51 percent copper and 0.8 percent beryllium 1s leached with sulfuric acid, "and the filtrate is electrowon to produce cathode copper with a specification of greater than 99.95 percent purity. The yield of copper is reported to be approximately 95 percent. The recovered copper is used as a feedstock for beryllium-copper alloy production. The beryllium is subsequently recovered in the form of beryllium fluoride using solvent extraction for recycle into the process stream for production of beryllium.
The Bureau of Mines has initiated a research project in Fiscal fear 1990 to determine the feasibility of using line-bearing wastes from steel plants (e.g., K061) as a partial raw material replacement for primary zinc used for eletrogalvanlzing. Because of the operating parameters of high current density and short plating times, the electrogalvanizing process is less sensitive to impurity effects. Indeed, some impurities have been shown to be beneficial; cobalt and chromium, at low concentrations, have produced more corrosion resistant coatings. Purification of leach solutions, however, is necessary to remove elements such as copper, chlorine and manganese, that are detrimental to the electrogalvanizing process. A 100 liter pilot electro-galvanizing cell is being operated to produce specimens in large quantities for evaluating the properties of formabiltty and corrosion.
Availability-Several different types of electrolytic reactors are currently manufactured and marketed by numerous vendors. The Recontek metals recovery facility in Newman, Illinois, which began operation in July, 1990, plans to electrowin 210 tons/month copper and 140 tons/month zinc from various categories of hazardous waste. Copper-bearing sludges are leached with sulfuric acid prior to electrowinning whereas zinc-bearing sludges are leached with sodium hydroxide. Sulfate solutions containing copper, nickel and minor metallic impurities are electrolyzed to recover copper in electrowinning cells. During the electrowinning process, copper is plated onto the cathodes. After seven days of operation, the cathodes are lifted out of the cells, rinsed off, and sold as a product. The anodes remain in the cell and can be reused. Zinc-bearing sludges are converted to sodium zincate solutions and pumped into electrowinning cells comprised of insoluble nickel anodes and insoluble magnesium cathodes. Zinc crystals are loosely deposited on the cathodes, and are removed from the cells by periodically vibrating the cathodes. The crystals are centrifuged and dried, and the resulting zinc powder is sold as a product.
CE - Corrcrt etfciancy ■ HMT
Electrowinning of metals 1s a particularly attractive process because it completely eliminates the generation of a metal-bearing sludge. Its applicability, however, is limited to waste streams containing metals in solution such as cadmium, copper, chromium, gold, lead, silver, tin, or zinc.
In electrowinning operations, an acid mist may be generated in the immediate vicinity of the electrowinning cells. The generation of mist can be reduced to the maximum extent feasible by reducing the exposed surface of solution in the cells, for example, this may be accomplished by floating plastic balls on the surface. Scrubbers should be employed for removal of any acid mists from electrowinning operations.
The major cost associated with electrowinning is usually the capital cost of the reactor Itself which can range from Î3500 for a reactor with a 1-ft2 stainless steel cathode to i89,000 for a reactor that has a higher surface area and uses a carbon fiber cathode.u Additional capital costs are incurred for items such as a rectifier and electrical connections, pumps and plumbing, and installation labor. These items may represent 15 to 25 percent of the cost of the reactor. Operating costs for electrowinning include electricity, maintenance or replacement of electrodes, and labor. Anode designs are often proprietary (e.g., dimensionally stable anodes) because of high replacement costs.
Tables 27 and 28 present comparisons of average costs for electrowinning versus chemical precipitation. Table 27 shows the monthly costs of using a carbon fiber electrolytic reactor versus the costs of alkaline chlorinatlon and precipitation for a waste stream containing cadmium and cyanide. The capital costs were estimated to be 177,000 for electrowinning and $160,000 for chemical precipitation.
Table 28 presents a comparison of the costs of using electrolytic treatment versus sulfide precipitation to treat a copper-bearing waste stream generated from the manufacture of printed circuit boards. In this case, two electrolytic reactors are required to treat the 10 gal/mln waste stream, which contains 200 mg/L capper. Again, both the capital costs and the overall operating costs are much less for electrowininning than those for sulfide precipitation.
Pomano, a division of General Dynamics Corp., has Installed a highmass- transfer copper-plating machine for the recovery of copper from the process solutions generated during the manufacture of printed circuit boards. The electrowinning process recovers 30 lb of copper slab per batch. In its old sludge form, this would have cost Î1200 to dispose of this material; whereas the recovered copper has a value of about S50/slab. Annual operating cost savings are estimated to be Î130,000.a
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