Following two concepts are there for the reaction taking place during electrolysis:
- According to the first concept which is more famous, ionisation of alumina occurs firstly:
Hence, corrosion of graphite rods (anode) occurs and they have to be replaced periodically. For 1 kg of aluminium produced, about 0.5 kg of anode (graphite) is consumed.
- According to the second concept, ionisation of ‘Cryolite’ occurs firstly on passing electric current.
At cathode: Al3+ + 3e– → Al(l).
At anode: F– → F + e–
3C + 2Al2O3 → 4Al + 3CO2
At anode, Al2O3 is changed into AlF3 and the process of electrolysis continuously goes on when Al2O3 is again ionised.
The whole reaction may be written in the following way:
2Al2O3 + 3C → 4Al + 3CO
Molten aluminium thus obtained is collected at the bottom of cell which is taken out with the help of tap. The aluminium metal. obtained by this method is about 99.5% pure. The complete process is known as electrometallurgy.
