121.

Discuss the extraction of lithium.

It involves the following steps:
(i) Preparation of lithium chloride: The silicate mineral is crushed to fine powder and then boiled with dilute H₂SO₄ and filtered to remove insoluble silica (SiO₂). The mother liquor is then treated with calculated amount of Na₂CO₃ to precipitate aluminium and iron as carbonates which are filtered off. The filtrate is then treated with an excess of Na₂CO₃ to precipitate lithium as Li₂CO₃. The precipitate is dissolved in HCl to form LiCl.

(ii) Electrolysis of lithium chloride. A mixture of dry lithium chloride and potassium chloride is fused and electrolyzed in an electrolytic cell (Down’s cell). Potassium chloride is added to increase the electrical conductivity and also to lower the melting point from 883K to 723K. The cell is operated at a temperature of about 675-775K and voltage of 8-9 volts is applied.
As a result of electrolysis, the following reactions take place:



Chlorine gas is liberated at the anode while molten lithium rises to the surface of the fused electrolyte and collects in the cast iron enclosure surrounding the cathode. The metal thus obtained is 99% pure.