Question

Explain the following process:

Hall-Heroult’s process

Answer 1

Hall-Heroult’s process is the electrolytic process used for the extraction of aluminium from purified alumina (Al₂O₃).

Electrolyte: Purified alumina is dissolved in molten cryolite (Na₃AlF₆) and a small amount of fluorspar (CaF₂). Cryolite lowers the melting point of alumina and increases the conductivity of the electrolyte.

Cathode: The carbon lining of the electrolytic cell acts as the cathode. Aluminium ions are reduced to molten aluminium:

\[\ce{Al^{3+} + 3e- -> Al}\]

Anode: Carbon rods act as anodes. Oxide ions are discharged and the oxygen formed reacts with the carbon anodes:

\[\ce{2O^{2-} -> O2 + 4e-}\]

\[\ce{C + O2 -> CO2}\]

The molten aluminium collects at the bottom of the cell and is removed periodically.

Overall reaction:

\[\ce{2Al2​O3​ + 3C -> 4Al + 3CO2​}\]