Following two concepts are there for the reaction taking place during electrolysis:
1. According to the first concept which is most famous, ionisation of alumina occurs firstly:
At cathode $\left\{\begin{array}{cc}{\displaystyle \underset{\mathrm{Molten}}{{\mathrm{Al}}_2{\mathrm{O}}_3}}& \stackrel{\mathrm{Electrolysis}}{\to }2{\mathrm{Al}}^{3+}+3{\mathrm{O}}^{2-}\\ {\mathrm{Al}}^3+3{\mathrm{e}}^{-}& \to \mathrm{Al}\left(\mathrm{l}\right)\end{array}\right\}$
At anode $\left\{\begin{array}{ccc}{\mathrm{O}}^{2-}& \to & \mathrm{O}+2\mathrm{e}\\ \mathrm{O}+\mathrm{O}& \to & {\mathrm{O}}_2\\ 4\mathrm{C}+3{\mathrm{O}}_2& \to & 2\mathrm{CO}+2{\mathrm{CO}}_2\end{array}\right\}$
2C + O₂ → 2CO
2CO + O₂ → 2CO₂
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.
3NaF + AlF₃
AlF₃Al³⁺ + 3F^–
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.

Hall Heroult process : Electrolytic cell for molten Alumina