Extraction of Aluminium Metal from the Electrolysis of Fused Alumina Chemistry Notes
Extraction of Aluminium Metal from the Electrolysis of Fused Alumina (Hall-Heroult’s Process) :
→ The chief ore of aluminium is bauxite (Al2O3 . 2H2O). First of all, bauxite ore is concentrated by Baeyer’s Process. Now the pure alumina, lbtained from Baeyer’s process. is used to extract aluminium by Hall Heroult’s process. Pure alumina (Al2O3) is a bad conductor of electricity. Therefore, some amount of cryolite (Na3AlF6) and fluorspar (CaF2) are also added to it. These compounds not only make alumina good conductor of electricity but also reduce the fusion temperature of the mixture of about 1140 K.
→ The electrolysis of molten mixture is carried out in an electrolysis tank. This tank is made up of iron and have carbon electrodes. Here carbon lining on iron tank behave as cathode while graphite rods as anode. The molten mixture of alumina, cryolite and fluorspar behave as electrolyte. The molten electrolyte is covered with a layer of powdered coke to prevent oxidation and loss of heat due to radiation. The temperature of the both is maintained around 1173 K.
→ During electrolysis, aluminium metal is deposited over cathode while oxygen at anode. Oxygen gas reacts with graphite rod and convert into CO and CO2. For each kilogram of aluminium produced, 0.5 kg of carbon anode is burnt away. Hence, the anodes have to be replaced from time to time.
→ The aluminium metal which is obtained at the cathode melts at the high temperature (1173 K) of the tank. The reactions related to above given two concepts during electrolysis are as follow:
According to the first concept, which is more famous, ionisation of alumina occurs as.
According to second concept, ionisation of cryolite occurs on passing electric current.
→ At anode Al2O3 is changed into AlF3 and the process of electrolysis continuously goes on when Al2O3 is again ionised. The whole reaction may be written in the following way:
2Al2O3 + 3C → 4Al + 2CO2
→ Molten aluminium thus obtained is collected at the bottom of cell which is taken out with the help of top. The aluminium metal obtained by this is about 99.5% pure. The complete process is known as electrometallurgy.