Aluminium

Aluminium was first produced in 1825. Presently it is produced in quantity second only to steel. It is the most abundant metallic element on the crust of the earth easily comprising about 8% of the crust. Bauxite, a hydrous oxide of aluminium and several other oxides, is the principal ore of aluminium. Aluminium is extracted from its ore mainly through electrolytic process. The ore is first washed off to remove clay and dirt, the ore is crushed into powder and treated with hot sodium hydroxide (caustic soda) to remove impurities. Alumina (the oxide of aluminium) extracted from this solution is dissolved in molten sodium fluroride and aluminium fluoride bath at 940-980oC. This mixture is then subjected to direct current electrolysis by passing direct current between carbon anode and cathode. The metallic aluminium forms in liquid state and sinks to bottom of the cell. This liquid aluminium is tapped off from time to time. The aluminium so obtained is 99.5 to 99.9% pure with iron and silicon as the major impurities.

Aluminium, then is taken to large refractory lined furnaces for refining before casting. The chlorine gas is used as purging agent to remove the dissolved hydrogen gas, and the liquid metal surface is skimmed off to remove oxidized metal. The molten metal is then cast into ingots for remelting or rolling.