The production of Aluminium
This is the most plentiful metal in the earth's crust. The increasing demand for lightness and strength has also now made it by far the largest, in terms of production output, in this group.
The only commercial source is bauxite, a hydrated form of aluminium which is found in the USA, France, Guyana, Australia and parts of Africa.
Unfortunately it is difficult to decompose and no cheap chemical is available for reduction purposes. Therefore, an expensive electrolytic process, Electrolysis, is needed for production, which consumes large quantities of electrical energy. As much as 5 times more electrical energy is needed to produce 1 tonne at aluminium compared with that required to produce 1 tonne of steel
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1. Bauxite is obtained by open-cast mining and then crushed and dried 2. Refining is done in two stages. First, it is treated by mixing with caustic soda and heated under pressure, where it dissolves. 3. It is then filtered and washed to remove impurities. 4. The precipitate is heated (roasted) to 1100°C in a rotary tubular kiln (calciner). This produces alumina, a white powder of aluminium oxide. 5. The reduction of alumina to oxygen and aluminium is achieved by an electrolytic process. A powerful electric current is passed though the mixture causing aluminium to be liberated. It then sinks and is deposited on the carbon lining of the furnace. Periodically the very pure (99%) aluminium is syphoned off and cast into ingots for further processing and shaping. In order to improve the properties of hardness and strength, a large proportion of this pure metal is used in alloyed forms containing other metals (e.g. copper, manganese and magnesium). Like steel, it is available in a variety of forms. |