Electrolysis of Alumina (hall’s process):
The pure and dry alumina
obtained from either of the above processes is fused with cryolite, Na3
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 Al2
but Na3AlF6 provides AlF3
first. Aluminium trifluoride liberates F2
at the anode which reacts with Al2
again. Thus Al2
gets consumed in the process.
The overall reaction may be shown as:
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.