Question

Write chemical reaction of Hall-Heroult process of bauxite ore and draw labelled diagram of elecrtrolytic cell.

Answer 1

Following two concepts are there for the reaction taking place during electrolysis :
1. 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.

2. 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.