Metal extraction and refining

This chapter will describe those electrolytic processes that are used in the manufacture of metals from their ores, i.e. electrowinning, and for the purification of metals, i.e. electrorefining. 

In terms of scale of production (around 2x10 ton yr V worldwide) aluminium electrolysis is second in importance only to the chlor-alkali industry. This is because aluminium is both light and strong and therefore suitable for many engineering and construction applications, may readily and cheaply be treated by anodizing (see Chapter 7) to retard corrosion and is the principal alternative to copper as a conductor of electricity. Moreover, the known reserves of aluminium ores are relatively high. 

Aluminium is normally produced from the ore, bauxite, a hydrated aluminium oxide containing silica and other metal oxides, particularly iron. It is converted to a pure alumina using the equilibrium 

The ore is first treated with caustic soda under pressure. The aluminium largely dissolves as the aluminate, the iron oxide is insoluble and the silica also remains in the form of a sodium aluminium silicate, which leads to a loss of aluminium. Hence the best bauxites are those low in silica. After filtration, the hydrated aluminium oxide is reprecipitated by seeding and the caustic soda solution may be re-used. The alumina is washed and then heated at 1200°C to remove water. The final step in the production of aluminium metal has to be electrolytic since the reduction of alumina with carbon is only possible at very high temperatures and the reverse reaction occurs on cooling. Moreover, because of the chemistry of aluminium, the electrolysis medium cannot be water in fact almost all commercial production of aluminium during the last ninety years has used an electrolysis in molten cryolite (Na3AlF5). 

The carbon anode is consumed in a stoichiometric amount but the free energy for 

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Carbon monoxide Chlorinated hydrocarbons G Coke ovens Incomplete combustion generally Smelting Vehicle exhausts Metal extraction and refining... 

Sulphur oxides G Sulphur dioxide Metal extraction and refining Coal distillation... 

T. K. Mukherjee and C. K. Gupta, Fused Salt Electrolysis for Refractory Metal Extraction and Refining, Trans SAEST, Vol. 11, No. 1, p. 127,1976. 

Use of Solid State Galvanic Cells In Metal Extraction and Refining... 

Metal extraction and refining Industries Penoles S.A. de C.V., Grupo Industrial Minero de Mexico S.A. de C.V., Industriales Nacobre S.A. de C.V., Mexicana de Cobre S.A. de C.V., Cobre de Mexico S.A., Compania Minera Autlan, S.A. de C.V., Aluminio S.A. de C.V., etc. It is worth mentioning that Mexico has been the first silver productor in the capitalist world, for several years. 

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