The extraction, refining and production of metal

This chapter will describe those electrolytic processes which are used in the manufacture of metals from their ores (i.e. electrowinning) and for the purification of metals, (i.e, electrorefining) and the production of metals in powder form. 

Many metals are now electrorefined but perhaps the most important are copper, tin, lead, nickel, cobalt, aluminium, silver and gold. 

In terms of scale of production (around 2 x 10 ton year 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 by treated by anodizing (section SJ.l) 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. 

Alumimum is normally produced from the ore, bauxite, which is 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 sodium hydroxide 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 repredpitated by seeding and the sodium hydroxide solution may be reused. The alumina is washed and then healed 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 90 years has used an electrolysis in molten cryolite, NajAlF.  

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The extraction, refining and production of metal 4.3J Precious metals... 

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