In the metallurgy of aluminium ……

In the metallurgy of aluminium, electrolysis is performed in a steel vessel with lining of carbon acts as cathode and graphite acts as anode. During this process graphite anode is oxidised to CO and ${\mathrm{CO}}_2$.

Chemical reaction occuring in this process is as follows 

$2{\mathrm{Al}}_2{\mathrm{O}}_3+3\mathrm{C}\to 4\mathrm{Al}+3{\mathrm{CO}}_2$

This process is known as Hall-Heroult process. The electrolytic reactions are 

At cathode: ${\mathrm{Al}}^{3+}\left(\text{melt}\right)+3{\mathrm{e}}^{-}\to \mathrm{Al}\left(\mathrm{l}\right)$

At anode: $\mathrm{C}\left(\mathrm{s}\right)+{\mathrm{O}}^{2-}\left(\mathrm{melt}\right)\to \mathrm{CO}\left(\mathrm{g}\right)+2{\mathrm{e}}^{-}$

                  $\mathrm{C}\left(\mathrm{s}\right)+2{\mathrm{O}}^{2-}\left(\text{melt}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+4{\mathrm{e}}^{-}$