Explain the Hall-Heroult process.

(N/A) In the electrolytic reduction of aluminium,purified $Al_{2}O_{3}$ is mixed with $Na_{3}AlF_{6}$ (cryolite) or $CaF_{2}$ (fluorspar),which lowers the melting point of the mixture and increases its electrical conductivity.
The fused matrix is electrolysed in a steel vessel with a carbon lining that acts as the cathode,while graphite rods act as the anode.
The overall reaction is:
$2 Al_{2}O_{3} + 3 C \rightarrow 4 Al + 3 CO_{2}$
The oxygen gas liberated at the anode during electrolysis reacts with the carbon of the anode to form $CO$ and $CO_{2}$. Consequently,for each $1 \ kg$ of aluminium produced,about $0.5 \ kg$ of carbon anode is consumed. Since the anodes are consumed in the reaction,they need to be replaced periodically.
At Cathode: $Al^{3+} (melt) + 3 e^{-} \rightarrow Al (l)$
At Anode:
$C (s) + O^{2-} (melt) \rightarrow CO (g) + 2 e^{-}$
$C (s) + 2 O^{2-} (melt) \rightarrow CO_{2} (g) + 4 e^{-}$