Electrolytic Recovery Electrowinning

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). 

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Electrowinning from Aqueous Solutions. Electrowinriing is the recovery of a metal by electrochemical reduction of one of its compounds dissolved in a suitable electrolyte. Various types of solutions can be used, but sulfuric acid and sulfate solutions are preferred because these are less corrosive than others and the reagents are fairly cheap. From an electrochemical viewpoint, the high mobiUty of the hydrogen ion leads to high conductivity and low ohmic losses, and the sulfate ion is electrochemicaHy inert under normal conditions. 

Zinc. The electrowinning of zinc on a commercial scale started in 1915. Most newer faciUties are electrolytic plants. The success of the process results from the abiUty to handle complex ores and to produce, after purification of the electrolyte, high purity zinc cathodes at an acceptable cost. Over the years, there have been only minor changes in the chemistry of the process to improve zinc recovery and solution purification. Improvements have been made in the areas of process instmmentation and control, automation, and prevention of water pollution. 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

Metals are important resources and have a wide range of applications. Metals are often extracted from ores. Once the ore is mined, the metals must be extracted, usually by chemical or electrolytic reduction. Pyrometallurgy uses high temperatures to convert ore into raw metals, while hydrometalluigy employs aqueous chemistry for the same purpose. The methods used depend on the metal and their contaminants. Most metals are obtained by hydrometallurgical processes such as aqueous acids or alkalis are predominantly used to dissolve the metal oxides, sulfides, or silicates. Electrowinning and solvent extraction are frequently used to recover and concentrate the metals. A limited number of high-temperature molten salts have also been used for the recovery of refractory metals, such as titanium and aluminum, from their ores... 

Preliminary studies have shown that ionic liquids have potential as solvents and electrolytes for metal recovery, and the feasibility of these solvents has been demonstrated for the extraction of gold and silver from a mineral matrix [7], the recovery of uranium and plutonium from spent nuclear fuel [8], and the electrodeposition and electrowinning of metals (especially, for active metals such as Li, Na, Al, Mg, and Ti) from ionic liquids [9-11], Ionic liquids as green solvents and electrolytes have shown important and potential application in extraction and separation of metals. In this chapter, the new applications and the important fundamental and appUed studies on the extraction and separation of metal in ionic liquids including metal oxides and minerals or ores processing, electrodeposition of metals (mainly for active metals), and extraction and separation of metal ions are described. 

Electrowinning from zinc sulfate solution is the usual final stage in the recovery of zinc. Approximately 80% of total world production is electrowon. Two typical methods for zinc electrolysis were previously used in industry. Either the electrolytes were slightly acidic and current densities were about 325 A m or they were strongly acidic and current densities were close to 850 A m. The process parameters widely used today are current densities in the range 400-600 A m and a process temperature of 30-40 C [5]. 

Since the focus of this paper is on pollution control applications of metal recovery, the complex and as yet incompletely told story of the early development of electroplating, surface finishing and early electrowinning techniques will not be discussed further. The development of electrolytic cells for pollution control applications of metal recovery dates from the mid-1960 s when several major advances in electrochemical engineering took place. Advances in potential and current distribution theory, mass transfer processes, coupled with the introduction of new materials, created a stimulus for the introduction of novel cathode designs with improved mass transfer characteristics. 

Dutra, A. J. R, G. P. Rocha, and E. R. Pombo, Copper Recovery and Cyanide Oxidation by Electrowinning from a Spent Copper-Cyanide Electroplating Electrolyte, Journal of Hazardous Materials 152, 648-655 (2008). 

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