The iron remains behind as a solid. When CO 2 is blown through the resulting solution, the sodium silicate stays in solution while the aluminium is precipitated out as aluminium hydroxide. The hydroxide can be filtered off, washed, and heated to form pure alumina, Al 2 O 3 . The next stage is formation of pure aluminium. This is obtained from the pure Al 2 O 3 by an electrolytic method.
Electrolysis is necessary as aluminium is so electropositive.
It seems these days that electrolysis of the hot oxide in a carbon lined steel cell acting as the cathode with carbon anodes is most common.