Question

Purification of aluminium by electrolytic refining is called as:
A. Serpeck’s process
B. Hoopes’s process
C. Hall’s process
D. Baeyer’s process

Answer 1

Aluminium is a highly reactive metal and is found as oxide in most of its ores. Electrolysis is basically using electricity to break down electrolytes to form elements.

Complete Solution:
- On industrial level, aluminium is extracted from bauxite by electrolysis. The process is widely known as Hall’s process. As a first step, the ore is crushed or pulverized. This is also called dressing of the ore. Removal of impurities from the ore is called concentration of the ore. In case of aluminium it is done by gravity separation method. The ore is then also subjected to magnetic separation to remove ferric oxide impurities. Alumina does not dissolve in water but it easily dissolves in the molten form a compound called cryolite.
- Pure alumina i.e. ${\text{A}}{{\text{l}}_2}{{\text{O}}_3}$ is mixed with cryolite i.e. ${\text{N}}{{\text{a}}_3}{\text{Al}}{{\text{F}}_6}$ and another compound called as fluorspar i.e. ${\text{Ca}}{{\text{F}}_2}$. This lowers the melting point and enhances the conductivity of the solution. This entire mixture is then electrolysed in a steel vessel. The vessel has a lining of carbon which acts as a cathode and a graphite rod which acts as an anode.

Reactions occurring on cathode and anode are –
At cathode - ${\text{A}}{{\text{l}}^{{\text{ + 3}}}}_{{\text{(melt)}}}{\text{ + 3}}{{\text{e}}^{\text{ - }}}{\text{ }} \to {\text{ A}}{{\text{l}}_{{\text{(l)}}}}$

At anode - $\begin{gathered} {{\text{C}}_{{\text{(s)}}}}{\text{ + }}{{\text{O}}_{{\text{(melt)}}}}^{{\text{ - 2}}}{\text{ }} \to {\text{ C}}{{\text{O}}_{{\text{(g)}}}}{\text{ + 2}}{{\text{e}}^{\text{ - }}} \\\ {{\text{C}}_{{\text{(s)}}}}{\text{ + 2}}{{\text{O}}_{{\text{(melt)}}}}^{{\text{ - 2}}}{\text{ }} \to {\text{ C}}{{\text{O}}_2}_{{\text{(g)}}}{\text{ + 4}}{{\text{e}}^ - } \\\ \end{gathered}$
Overall reaction is given as - $2{\text{A}}{{\text{l}}_2}{{\text{O}}_3}{\text{ + 3C }} \to {\text{ 4Al + 3C}}{{\text{O}}_2}$.

So, the correct answer is “Option C”.

Note: All the processes mentioned above are used for extraction of aluminium. However, electrolytic refining with the use of cryolite is a widely used method and is called Hall's process.