Chapter : 1. Metals & Non Metals
2. Electrolytic reduction of alumina: Hall-Heroult process
Alumina is mixed with cryolite and the mixture is melted in an iron cell.
Alumina melts at 2303 K. It is a bad conductor of electricity. But when mixed with cryolite and some calcium fluoride, the mixture becomes a good conductor of electricity, and melts at 1173-1223 K. The cryolite thus considerably reduces the' energy cost. The iron cell is lined inside with gas carbon, which serves as cathode. Carbon rods act as anode. The electrolyte, thus, contains Na+, Al3+, P– and O2– ions.
On passing electric current, AP3+ ions are discharged at the cathode and the 022ions, at the anode. O2–
Al3+ + 3e → Al (at cathode)
2O2– → O2 + 4e (at anode)
Some quantity of oxygen formed in the reaction escapes and some reacts with the anode to form CO2 Thus, carbon anode burns away due to its reaction with oxygen.
C + O2 → CO2
Hence, the anode has to be replaced from time to time. This increases the cost of production of aluminium. Molten aluminium sinks to the bottom, and is taken out from there.
Alumina is mixed with cryolite and the mixture is melted in an iron cell.
On passing electric current, AP3+ ions are discharged at the cathode and the 022ions, at the anode. O2–
Al3+ + 3e → Al (at cathode)
2O2– → O2 + 4e (at anode)
Some quantity of oxygen formed in the reaction escapes and some reacts with the anode to form CO2 Thus, carbon anode burns away due to its reaction with oxygen.
C + O2 → CO2
Hence, the anode has to be replaced from time to time. This increases the cost of production of aluminium. Molten aluminium sinks to the bottom, and is taken out from there.
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