Question

Explain in brief electrolytic reduction of alumina with a neat labelled diagram.

Answer 1

1. The pure alumina obtained from bauxite ore by Bayer’s process or Hall’s process is electrolysed to obtain metallic aluminium.
2. Alumina has a very high melting point (>2000 °C).
3. Electrolysis of alumina is carried out in an electrolytic cell at a much lower temperature by dissolving it in molten cryolite (Na₃AlF₆) and fluorspar (CaF₂). Addition of cryolite and fluorspar reduces the melting point to about 1000 °C.
   
4. The electrolytic cell is a large steel tank having a lining of carbon (graphite) on the inner side. The carbon lining acts as a cathode. A set of carbon (graphite) rods dipped in the molten electrolyte acts as an anode.
5. On passing electric current, electrolysis of alumina takes place.
6. Aluminium is formed at the cathode and oxygen gas is liberated at the anode.
7. The electrode reactions are as follows:
   Cathode: \[\ce{Al^3+ + 3e^- -> Al_{(l)}}\] 
   (Reduction)
   Anode:
   \[\ce{2O2- -> O2 + 4e^-}\] (Oxidation)
8. The molten aluminium is heavier than the electrolyte used and hence, it sinks to the bottom of the tank from where it is removed periodically.
9. The oxygen gas liberated at the anode reacts with the carbon anode and forms carbon dioxide gas.
   \[\ce{C_{(s)} + O_{2(g)} -> CO_2(g)}\]
10. As the anode gets oxidized during the electrolysis of alumina, it has to be replaced from time to time.