Question

The major role of fluorspar which is added in small amount in the electrolytic reduction of $A{l}_2{O}_3$ dissolved in fused cryolite is :

• A. as a catalyst
• B. to make the fused mixture very conducting
• C. to lower the temperature of melting
• D. to decreases the rate of oxidation of carbon at the anode

Answer 1

Answer: (B), (C)

to make the fused mixture very conducting
to lower the temperature of melting

The electrolysis of alumina is carried out in a steel tank lined inside with graphite. The graphite lining serves as a cathode. The anode is also made of graphite rods hanging in the molten mass.The electrolyte consists of alumina dissolved in fused cryolite(${Na}_{3}Al{F}_6)$ and fluorspar ($Ca{F}_2$).

Cryolite lowers the melting point of alumina to ${950}^{o}C$ and fluorspar increases the fluidity of the mass, so that the liberated aluminium metal may sink at the bottom of the cell.

Therefore, it makes the fused mixture very conducting and lowers the fusion temperature of the melt. When an electric current is passed through this mixture, the aluminium is collected at the cathode in the molten state and sinks at the bottom and is tapped off.

Hence, options B & C are correct.