Question

In electrometallurgy of aluminium, why is the graphite rod used?

Answer 1

In the electrometallurgy of aluminium, a fused mixture of purified alumina ($A{l}_2{O}_3$), cryolite$\left(N{a}_{3}Al{F}_6\right)$ and fluorspar $\left(Ca{F}_2\right)$ is electrolysed. In this electrolysis, graphite is used as the anode and graphite-lined iron is used as the cathode. During the electrolysis, Al is liberated at the cathode, while $COandC{O}_2$are liberated at the anode

$Cathode:{Al}^{3+}\left(melt\right)+3e^{-}\to Al\left(l\right)$

$Anode:C+O^{2-}\left(melt\right)\to C{O}_2\left(g\right)+2e^{-}$

$C\left(s\right)+2O^{2-}\left(melt\right)\to {CO}_2\left(g\right)+4e^{-}$

If a metal is used instead of graphite as the anode, then $O_2$ will be liberated. This will not only oxidize the metal of the electrode but also convert some of the $Al$ liberated at the cathode back into $A{l}_2{O}_3$. Hence, graphite is used for preventing the formation of $O_2$ at the anode. Moreover, graphite is cheaper than other metals.