Electrolysis of Alumina (hall’s process):
The pure and dry alumina obtained from either of the above processes is fused with cryolite, Na3AIF6. This molten mass is electrolysed in a steel tank (8 ft x 6 ft) lined with blocks of carbon which serve as the cathode (Figure 1.1). The anode is rods of carbon hanging in the molten mass.
The resistance of the electrolyte to the current produces enough heat which keeps the mass in the molten state. Liquid aluminium obtained at the cathode sinks to the bottom off occasionally. Fresh amounts of bauxite are added when required.
The raw material for this process is Al2O3 but Na3AlF6 provides AlF3 first. Aluminium trifluoride liberates F2 at the anode which reacts with Al2O3 forming ALF3 again. Thus Al2O3 gets consumed in the process.
The liberated oxygen attacks carbon anode forming some CO and CO2 Therefore, in the modified method a mixture of composition, 2ALF3. 6NaF. 3CaF2 is used.
Al metal produced in this method is about 99% pure but contains trace impurities of Fe, Si, Al2O3 etc.