Aluminium is extracted from bauxite in two stages:
- Purification of the ore to produce alumina (Al2O3)
- Reduction of alumina by electrolysis
9.1.1 Purification of the ore to produce alumina (Al2O3)
The first process is called Bayer process, in which the alumina in the bauxite is dissolved in a solution of caustic soda, filtered and reprecipitated to obtain aluminium hydroxide and calcinated to form pure Al2O3.
9.1.2 Reduction of alumina by electrolysis
In the next process, called Hall-Hiroult’s process, a steel tank is used, which is lined with refractory material with a second lining of carbon, which forms the cathode. Electric current is supplied through carbon electrodes with their lower ends dipped in the solution derived from Bayer process. These electrodes serve as anode. When a current of 8,000–20,000 amperes is switched on, the alumina, which is dissolved in the solution, is separated from the oxygen and deposits at the bottom of the steel tank in a molten state. The oxygen is liberated from carbon monoxide with carbon and burns off.
Pure aluminium is too soft for practical purposes. Hence, it is alloyed with copper, manganese, magnesium, zinc, silicon and nickel, which increases its tensile strength and hardness, while retaining the characteristics of lightness and durability. Aluminium is highly resistant to corrosion and is a very good conductor of heat and electricity. The commercial forms of aluminium are aluminium sheets, aluminium plates, aluminium foils, aluminium powder used as a pigment in the manufacture of paints, aluminium rods, aluminium bars and other aluminium structural parts.