Electrowinning of gold

Zadra cell — Electrolytic cell employed in -> electrowinning of gold. The design is based on placing into a perforated cylindrical polymer container steel wool as - cathode, a feeder tube, and a -> current distributor. The anode is a stainless steel mesh outside the container. Gold-rich solution is fed at low feed rate through the feeder at the bottom of the steel wool-filled compartment, gold is reductively extracted, and the extracted solution passes the polymer container. 

Elges CH III, Wroblewski MD, Eisela JA (1984) Direct electrowinning of gold. Proc Electrochem Soc 84 501-512... 

Cementation, the process by which a metal is reduced from solution by the dissolution of a less-noble metal, has been used for centuries as a means for extraction of metals from solution, and is probably the oldest of the hydrometallurgical processes. It is also known by other terms such as metal displacement or contract reduction, and is widely used in the recovery of metals such as silver, gold, selenium, cadmium, copper and thallium from solution and the purification of solutions such as those used in the electrowinning of zinc. The electrochemical basis for these reactions has been well established414 and, as in leaching reactions, comprises the anodic dissolution of the less-noble metal coupled to the cathodic reduction of the more-noble metal on the surface of the corroding metals. Therefore, in the well-known and commercially exploited44 cementation of copper from sulfate solution by metallic iron, the reactions are... 

Electrowinning on titanium or stainless steel cathodes yields high current efficiencies for high gold concentration ( 1400 ppm). At low concentrations, the current efficiency can be less than 10%, but the recovery of gold is still nearly 100% [64],... 

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. 

Electrolysis of solutions containing Au(CN)2 is widely used to recover gold from solution (electrowinning) [17]. The process is also used to deposit gold coverings for electrons (e.g. printed circuit boards, electrical connectors) and most recently for hip and shoulder joint replacement surgery. 

Porous electrodes have the same advantages as particle bed reactors, but they have the difficulty of becoming plugged as the metal deposit builds up. Sometimes this is not important if the value of the deposited metal is high and the electrode may be sacrificed. An example of such a system is the Zadra cell used in gold electrowinning, where the cathode is a steel mat that can be melted away when gold is recovered. 

Metallic copper is then obtained either by electrowinning or hydrogen reduction. At least 98% copper recovery, gold and silver capture equivalent to conventional smelting, and recovery of 60-90% of the sulfur in elemental form has been demonstrated [51]. 

The current efficiency in most gold electrowinning operations is only 5-10%. The majority of current is used in oxygen reduction. As the anode reaction consumes OH ions, the local pH lowers. On the cathode, metal is deposited and hydroxide ions are formed. As a net result, the metal concentration and pH of the electrolyte decrease. The decrease of pH is critical especially in cyanide solutions as anode corrosion increases and at pH <9.0, the formation of lethal HGN gas begins [63]. 

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