Electrolytic recovery

Electrolytic recovery converts the toxic metals in solution into their nonhazardous elemental form. The positively charged metal ions, attracted to the negatively charged cathodes, are reduced at the cathodes where they plate out as sheet metal or pcwder. At Aeroscientific, planar cathodes are used, producing metal sheets that can be easily removed by simply flexing the cathodes. The sheets of metal can then be conveniently stored, transported, and disposed of. The cathodes can be reused. 

Concentrated bath dumps (with metal concentrations above 1 g/L) are treated directly by electrcwinning. Other metal bearing waste streams are first directed to ion-exchange and the resulting eluate is treated by electrcwinning. 

TWo types of electrcwinning cells are used non-membrane cells for most applications, and membrane cell for applications in which anodic oxidation processes would otherwise interfere with the metal recovery process. 

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A. K. Suri and C. K. Gupta, Electrolytic Recovery of Molybdenum from Molybdic Oxide and Molybdenum Sesquisulfide, Metall. Trans. B, Vol. 6B, p. 453,1975. 

Utilizing electrolytic recovery, customized resins, selective membranes, and adsorbents to separate metal impurities from plating baths, acid/caustic dips, and solvent cleaning operations. 

Primary metals manufacturing operations have experienced source reduction and recycle/reuse benefits similar to those available to metal finishing operations, including conserving waters through countercurrent rinsing techniques, and utilizing electrolytic recovery, customized resins, selective membranes, and adsorbents to separate metal impurities from acid/caustic dips and rinsewaters to thereby allow for recycle and reuse. 

One approach to waste reduction is to recover process materials for reuse. Materials used in metal finishing processes can be effectively recovered using available technologies such as dragout, evaporation, reverse osmosis, ion exchange, electrodialysis, and electrolytic recovery.22-26... 

Electrolytic recovery (ER) is the oldest metal recovery technique. Metal ions are plated-out of solution electrochemically by reduction at the cathode.34 There are essentially two types of cathodes used for this purpose a conventional metal cathode and a high surface area cathode (HSAC). Both cathodes can effectively plate-out metals, such as gold, zinc, cadmium, copper, and nickel.22... 

Electrolytic recovery systems work best on concentrated solutions. For optimal plating efficiency, recovery tanks should be agitated ensuring that good mass transfer occurs at the electrodes. Another important factor to consider is the anode/cathode ratio. The cathode area (plating surface area) and mass transfer rate to the cathode greatly influence the efficiency of metal deposition. 

Electrolytic recovery can be used with most plating baths. The amount of metal to be plated per square meter of cathode determines the electrolytic recovery unit s design capacity. Therefore, the volume and concentration of plating dragout greatly influences system design and size.22 35... 

There are advantages to the electrolytic recovery process. For instance, ER units can operate continuously, and the product is in a metallic form that is very suitable for reuse or resale. Electrolytic units are also mechanically reliable and self-operating. Very importantly, contaminants are not recovered and returned to the plating bath. Thus, electrolytically recovered metals are as pure as virgin plating raw material. 

The major disadvantage to electrolytic recovery is high energy cost. Energy costs will vary, of course, with cathode efficiencies and local utility rates.22... 

In addition to these three treatments, there are several alternative treatment technologies applicable to the treatment of common metals wastes. These technologies include electrolytic recovery, electrodialysis, reverse osmosis, peat adsorption, insoluble starch xanthate treatment, sulfide precipitation, flotation, and membrane filtration.1516... 

Precious metal wastes can be treated using the same treatment alternatives as those described for treatment of common metal wastes. However, due to the intrinsic value of precious metals, every effort should be made to recover them. The treatment alternatives recommended for precious metal wastes are the recovery techniques—evaporation, ion exchange, and electrolytic recovery. 

Lithium metal is produced commercially by electrolysis of a fused eutectic mixture of hthium chloride-potassium chloride (45% LiCl) at 400 to 450°C. The eutectic mixture melts at 352°C in comparison to the pure LiCl melting at 606°C. Also, the eutectic melt is a superior electrolyte to LiCl melt. (Landolt, P.E. and C. A. Hampel. 1968. Lithium. In Encyclopedia of Chemical Elements.C. A. Hampel, Ed. Reinhold Book Corp. New York.) Electrolysis is carried out using graphite anodes and steel cathodes. Any sodium impurity in hthium chloride may be removed by vaporizing sodium under vacuum at elevated temperatures. All commercial processes nowadays are based on electrolytic recovery of the metal. Chemical reduction processes do not yield high purity-grade metal. Lithium can be stored indefinitely under airtight conditions. It usually is stored under mineral oil in metal drums. 

The annual operating cost for a tin mill plant with a new waste treatment plant is more than 1,450,000, with more than 700,000 in sludge disposal costs. Lewis asserts that, in comparison, the ENVIRO-CLEAN process treatment cost includes chromium recovery valued at more than 190,000 and water savings valued at more than 50,000 per year. According to Lewis, a large-scale ENVIRO-CLEAN system complete with electrolytic recovery has an annual operating cost of less than 800,000 a year (D135035, p. 1). 

Many process and rinse solutions can be recycled in some way if operators and plant engineers fully understand the chemistry of their waste streams. Rinse solutions too contaminated for their original purpose can often be used as rinses elsewhere. Metals can be recovered from spent process solutions and wastewater using technologies such as reverse osmosis, ion exchange, electrolytic recovery, and evaporation. 

Recycling of wastes is the preferable waste management method after source reduction opportunities have been exhausted. Recycling can be performed within the process itself, within the plant, or off-site, and can involve reuse of the entire waste stream, or recovery of a part of it. Recovery of the stream s metal content can be achieved through operations such as electrolytic recovery, reverse osmosis, and ion exchange. 

Several basic principles well known to the electroplating industry are employed in electrolytic recovery expanded cathode surface area, close spacing between cathode and anode, and recirculation of the rinse solution. Electroplaters can design their own units by... 

A good summary of electrolytic recovery potential is included in Cal-Tech (1987, pp 57-60). 

Recovery of metals from treatment sludges by off-site commercial plants is an option that is employed at the present time to some extent in California, and that will likely see rapid growth in the near future. Types of off-site recovery services that presently exist in California or elsewhe. in the United States include those offered by smelters, hydrometallurgical plants, and ion exchange/electrolytic recovery plants. 

The most popular method of silver recovery is electrolytic deposition. In an electrolytic recovery unit, a low voltage direct current is created between a carbon anode and stainless steel cathode. Metallic silver plates onto the cathode. Once the silver is removed, the fixing bath may be able to be reused in the photographic development process by mixing the desilvered solution with fresh solution. Recovered silver is worth about 80% of its commodity price. Used silver films also constitute a significant quantity of waste. The film can be sold for silver recovery to many small recyclers. 

Another method used for silver recovery is ion exchange. Small ion exchange columns can be employed for approximately the same cost as electrolytic recovery (Hart 1987). Effluent from some ion exchange systems can have silver concentrations as low as 0.1 ppm. High-performance units can process as much as 500 gallons per hour. 

Electrolytic Recovery of Mercury Metal from a Mercuric Chloride-Containing Waste... 

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