Write the main steps in the extraction of aluminium. 1(b) What is the melting point of alumina and how is the melting point reduced ? 2(c) Why anodes need to be replaced from time to time during the electrolysis of alumina ? 2

Answer: (a) Concentration of bauxite to obtain pure alumina, and electrolytic reduction of alumina; (b) melting point of alumina is about 2050 ºC; it is lowered to about 1000 ºC by adding cryolite and fluorspar; (c) the carbon anodes get oxidised to CO₂ by the oxygen liberated at them.

(a) Main steps: (1) Concentration of the ore — bauxite is concentrated by leaching (Bayer’s process/Hall’s process) to obtain pure alumina (Al₂O₃); (2) Electrolytic reduction of molten alumina in a steel tank with graphite lining as cathode and graphite rods as anodes; aluminium is deposited at the cathode and collected from the bottom.

(b) The melting point of alumina is very high, about 2050 ºC (≈2000 ºC), so electrolysis of pure molten alumina would need a lot of energy. Cryolite (Na₃AlF₆) and fluorspar (CaF₂) are added to the alumina; this lowers the melting point of the mixture to about 1000 ºC, saving energy (cryolite also increases the conductivity).

(c) During electrolysis oxygen gas is liberated at the anodes. At the high temperature this oxygen reacts with the carbon (graphite) of the anodes to form carbon dioxide: \(\mathrm{C + O_2 \rightarrow CO_2}\). The anodes are thus oxidised and gradually burnt away, so they have to be replaced from time to time.