A theoretical way to obtain pure aluminium is to electrolyse molten Al2O3 (alumina). However, the melting point of Al2O3 is 2072 oC, making the idea impractical. A French and an American scientist named Paul Héroult and Charles Hall respectively, developed a better method in the 1800s called the Hall-Heroult process where Al2O3 is first dissolved in a solvent mixture of Na3AlF6 (cryolite) and AlF3 at about 1000 oC before being electrolysed to give pure aluminium.
Dissolution reaction:
Al2O3 (s) + 4[AlF6]3- (l) → 3[Al2OF6]2- (l) + 6F– (l)
Solid alumina is dissolved in cryolite and AlF3 at about 1000 oC to give an oxyflouridealuminate complex.
At the anode:
2[Al2OF6]2- (l) + 12F– (l) → 4[AlF6]3- (l) + O2 (g) + 4e–
O2 (g) + C (s) → CO2 (g)
The oxyflouridealuminate complex reacts with excess fluoride ions to give the hexafluoroaluminate complex and oxygen, which then oxidises the carbon anodes to give carbon dioxide (note that at such high temperatures, the carbon anodes are no longer inert). The overall anode reaction is:
2[Al2OF6]2- (l) + 12F– (l) + C (s) → 4[AlF6]3- (l) + CO2 (g) + 4e–
At the cathode:
[AlF6]3- (l) + 3e– → Al (l) + 6F– (l)
The hexafluoroaluminate complex is reduced to give pure molten aluminium.
The overall redox reaction is
2Al2O3 (s) + 3C (s) → 4Al (l) + 3CO2 (g)
As the carbon anodes are oxidised to carbon dioxide, they have to be replaced over time.