Question

During extraction of aluminium from bauxite,

• A. the concentration of ore is done by gravity separation method
• B. molten mixture of aluminium oxide, cryolite and fluorspar is electrolysed
• C. impure aluminium is refined by liquation
• D. molten aluminium is obtained at cathode while fluorine is liberated at anode.

Answer 1

Answer: (B)

Purified $Al_2O_3$ is mixed with $Na_3AlF_6$ and $CaF_2$ to improve conductivity of $Al_2O_3$ and to lower the melting point of the mixture.
$Al_2O_3 \xrightarrow{\text{Electrolysis}} 2Al^{3+} + 3O^{2-} $
At cathode : $Al^{3+} + 3e^- \rightarrow Al$
At anode : $C + O^{2-} \rightarrow CO + 2e^-$
$ C + 2O^{2-} \rightarrow CO_2 + 4e^-$