Zinc electrorefining

The anode material for silver clectrorelining is mainly derived from anode slimes from copper. Lead, nickel and zinc electrorefining processes together with diverse contributions from the secondary recovery and scrap-recovery processes. 

Air emissions for processes with few controls may be of the order of 30 kilograms lead or zinc per metric ton (kg/t) of lead or zinc produced. The presence of metals in vapor form is dependent on temperature. Leaching processes will generate acid vapors, while refining processes result in products of incomplete combustion (PICs). Emissions of arsine, chlorine, and hydrogen chloride vapors and acid mists are associated with electrorefining. 

Electrorefining can be used to purify a metal by using alternate electrodes of a pure and impure metal. Impurities oxidized at the anode, which is made of the impure metal, travel into solution. By arranging the cell appropriately, the ion of the metal to be purified is reduced on the pure metal cathode. For example, copper metal that contains lead and iron may be used as one electrode and pure copper as the other electrode in a cell. When the proper voltage is applied, copper, lead, and zinc will be oxidized and move into the electrolyte. Because copper is more easily reduced compared to zinc and lead, it will be plated out at the pure copper cathode. Therefore, this process effectively removes the zinc and lead impurities from the copper. 

The metal deposition rate from aqueous solutions can be expressed as a current density (A m-2 of electrode surface) as the rate is linked to the current density by Faraday s law (1). In modern copper electrorefining and electrowinning, the current density is 300-350 Am-2, while in zinc operations it is greater, 450-500Am-2 or more. According to Faraday s law, enormous amounts of electricity are needed for the tens or hundreds of kilotons of metal produced in a typical electrolysis plant per year. 

High purity dihydrogen is likewise produced by electrolysing water, it is also worth mentioning here the purification of certain metals, such as copper, zinc and aluminium, by an electrorefining process involving anodic dissolution and cathodic deposition by a selective electrolysis. 

At the time of writing this paper, three plant trials are currently underway with promising early signs. The sites selected for the trial are spread between Australia and the United States of America in the production of copper in electrowinning and electrorefining plants. Following the completion of these trials in early 2013, it is expected that commercial units will be available for sale by the end of Ql, 2013. All trials to date have been for copper with the next step to include commercial trials in nickel, cobalt and zinc operations. 

Electrochemical processes, primarily in the refining of copper, lead, and zinc, yield certain rare metals as by-products. The value of these by-product metals should be included within the electrochemical industry. In Table 7, the major metals derived as byproducts of electrorefining operations are listed. The total value of shipments for each of these metals was based on data contained in the Bureau of Mines Minerals Yearbook Since copper refining... 

Zinc is another common impurity in copper. Using standard reduction potentials, determine whether zinc will accumulate in the anode sludge or in the electrol) c solution during the electrorefining of copper. 

Kubota, H, Kusakabe, T, Takei, K and Takewaki, M, 1995. Current operations of Sumitomo Metal Mining s Betts lead electrorefining, in Proceedings Zinc and Lead 95 Symposium, Sendai, pp 353-366. 

Electrorefining in aqueous solution is used for purifying metals and for electroplating, for example, for corrosion protection by zinc coating of steel. Electrorefining of metals is only possible for those metals with a positive standard potential (Ag, Au, Cu, Table 6.19.1) or if H2 formation at the cathode (which competes with deposition of the metal) is hindered by a high overpotential (Zn, Ni). For electrochemically very non-noble metals such as aluminum with a very negative standard potential, fused salt electrolysis is needed (Section 6.19.4.2). 

Anode quality is vital in particular, insoluble particulate matter (from soluble Cu anodes) in copper electrorefining or PbSO (from inert Pb- Ag or Pb-Sb) in copper or zinc elect rowinning may cause deleterious deposits. Metallurgically homogeneous, fine-grained soluble anodes are usually preferred via, for example, rolled fabrication techniques. 

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