Question

In the extraction of aluminium ;
(i) name the process of concentration of bauxite.
(ii) write the cathode reaction in electrolytic reduction of alumina.
(iii) write the function and chemical formula of cryolite.
(iv) write a chemical equation for the action of heat on aluminium hydroxide.
(v) why is it necessary to replace anodes time to time?

Answer 1

(i) The concentration of ore is done by Baeyer's process using chemical method.

(ii) The electrolytic reduction of alumina is done by Hall Heroult's process. The reaction at cathode is : $A{l}^{3+}+3e^{-}\to Al$

Thus, pure aluminium metal is formed at the cathode.

(iii) Cryolite is sodium aluminium fluoride ($N{a}_{3}Al{F}_6$). It is an ore of aluminium. Cryolite and fluorspar are added to the molten mixture of alumina to reduce the melting point and increase the electrical conductivity.

(iv) $2Al(OH{)}_3\to A{l}_2{O}_3+3H_{2}O$

(v) The oxygen evolved at the anode escapes as gas or reacts with the carbon anode. The anode is thus oxidized to carbon monoxide which either burns giving $C{O}_2$ or escapes. Thus, anode has to be replaced as it is consumed periodically.