Question

How electrolysis can be used in extraction of aluminium? Why aluminium cannot be reduced by conventional reducing agents?

Answer 1

The method of electrolysis is used to extract aluminium from its oxide ore. In this method, the electrolyte used is a combination of fused pure alumina $\left({\mathrm{Al}}_2{\mathrm{O}}_3\right)$ mixed with cryolite $\left({\mathrm{Na}}_3{\mathrm{AlF}}_6\right)$ and fluorspar $\left({\mathrm{CaF}}_2\right)$. Cryolite and fluorspar mixture is added as they raise the electrical conductance of alumina and reduce its melting point.

Electrolysis is carried out in an iron tank coated with a layer of gas carbon acting as the cathode. Carbon rods immersed in electrolyte are taken as anode. When electricity is passed through the electrode, the following reactions occur:

Dissociation of alumina: $2{\mathrm{Al}}_2{\mathrm{O}}_3\to 4{\mathrm{Al}}^{3+}+3{\mathrm{O}}_{}^{2-}$

Reaction at cathode: ${\mathrm{Al}}^{3+}+3{\mathrm{e}}^{-}\stackrel{}{\to }\mathrm{Al}$

Reaction at anode: $$\begin{gathered} {\mathrm{O}}_{}^{2-}\stackrel{}{\to }\mathrm{O}+2{\mathrm{e}}^{-} \\ \mathrm{O}+\mathrm{O}\stackrel{}{\to }{\mathrm{O}}_2 \end{gathered}$$
Oxygen reacts with carbon anode and forms CO and CO₂. Due to this oxidation reaction at the anode, it is replaced regularly.

Aluminium is a highly reactive metal, as it has greater affinity for oxygen. Therefore, aluminium oxide cannot be easily reduced by conventional reducing agents like carbon, hydrogen, carbon monoxide, etc. and is extracted by the process of electrolysis only.