Elemental aluminium can't be prepared by the electrolysis of an aqueous aluminium salt because hydronium ions easily oxidize elemental aluminium. Although a molten aluminium salt could be used instead of it, aluminium oxide has a melting point of 2072ºC so electrolysing it is impractical. In the Hall–Héroult process, alumina, Al₂O₃, is dissolved in molten synthetic cryolite, Na₃AlF₆, to lower its melting point for easier electrolysis.

Hall-Heroult Cell

Anode:
(positive electrode) C(s) + 2O^2–  (I)  →  CO₂(g) + 4e 

oxidation at anode

Cathode:
(negative electrode) Al³⁺  (I)  +  3e →  Al(I)
reduction at cathode

Overall Reaction : 2Al₂O₃ (l) + 3C(s) → 4Al(I) + 3CO₂(g)