Question

Explain the method of obtaining pure aluminium from alumina?

Answer 1

Electric current cannot pass through a solid form of alumina. Also, a melting point of alumina is very high at $2348$K. Electrolysis at such a high temperature is very expensive. Hence, cryolite$\left(N{a}_{3}Al{F}_6\right)$ is added so that electrolysis can be carried out easily. This mixture works as a better electric conductor than melted alumina. The melting point can be brought still lower by an addition of feldspar$\left(Ca{F}_2\right)$. The mixture of alumina, cryolite and feldspar is electrolyzed in a vessel of iron having inner surface layered with carbon. In this cell, the rods of carbon are joined by a copper clamp as anode and carbon layered graphite is taken as a cathode. On passing electric current, molten aluminium is deposited on cathode and dioxygen gas is produced at the anode.

The molten aluminium collected at the bottom of the cell is taken out. Cathode: $2A{l}^{3+}\left(l\right)+6e^{-}\to 2Al\left(l\right)$ Anode: $6O^{2-}\left(l\right)\to 3O_2\left(g\right)+12e^{-}$

The dioxygen gas produced at the anode reacts with the rod of carbon and forms carbon dioxide.

$C\left(s\right)+O_2\left(g\right)\to C{O}_2\left(g\right)$