Question

Describe the role played by cryolite in the metallurgy of aluminium.

Answer 1

Hint: The extraction of aluminium from its ore (bauxite) involves two essential steps. One is to convert the ore to pure aluminium oxide (called alumina $A{l_2}{O_3}$ ) by the process called Bayer’s process and second is to convert alumina to aluminium by electrolytic reduction (this process is called smelting). In this process the alumina is reduced at a very high temperature by the passage of electricity.

Complete step by step answer:
Cryolite is sodium hexafluoroaluminate ($N{a_3}Al{F_6}$ ). During smelting the alumina is dissolved in cryolite due to following reasons:
(i) It acts as a solvent as well as electrolyte for the passage of electricity.
(ii) Alumina has a very high melting point of $2323K$ but cryolite has a lower melting point. So, dissolution of alumina in cryolite lowers down the temperature of the mixture.
(iii) Addition of alumina to the cryolite also increases its electrical conductivity for the reduction process.
Additional information: The electrolytic process is also known as Hall-Heroult process and the reaction taking place can be written as:
$A{l^{3 + }} + {e^ - } \to Al$
A smelter is equivalent to a large container having a large number of cells where the electrolysis is taking place. On an average a single smelter can have $300$ to $750$ cells producing tons of aluminium. It is also very interesting to know that cryolite is probably the only mineral on earth that has been mined to its commercial extinction owing to its very uncommon occurrence at few places on earth only.

Note:
Cryolite is used as a solvent for bauxite in the electrolytic production of aluminum and has various other metallurgical applications, and it is used in the glass and enamel industries, in bonded abrasives as a filler, and in the manufacture of insecticides. A large amount of synthetic cryolite is made from fluorite.