Aluminium is refined by an electrolytic method. The electrolysis is carried out in a carbon lined iron tank, which is filled with three molten layers of different specific gravity one over the other. The top layer consists of pure molten Aluminium and acts as cathode. The middle layer consists of fused mixture of fluorides of Sodium, Aluminium and Barium (Na3 AlF6 + Ba F2) and acts as the electrolyte. The bottom layer consists of molten impure aluminium containing the impurities. This layer acts as anode.
When electric current is passed, Al+3 ions from the middle layer go to the top layer and are deposited as pure Aluminium. At the same time an equivalent amount of Al from the bottom layer passes into the middle layer. The impurities remain in the bottom layer as they do not dissolve in the electrolyte. The pure aluminium containing 99.99% Al is removed from time to time from the top layer whereas impure aluminium is added into bottom layer.