EXTRACTION AND REFINING OF TIN :

The different varieties of glasses and their special constituents are given below,

(1) Important ore : Cassiterite (tin stone) SnO₂

(2) Extraction of tin from tin stone.

(i) Concentration : The powdered tin stone is concentrated by gravity separation and the magnetic impurities like wolframite etc., are separated from tin stone by magnetic separators.

(ii) Roasting : The concentrated ore is heated in a current of air when impurities like s and As are oxidised to volatile SO₂ and As₂O₃ . Iron pyrites change to their oxides and sulphates.

(iii) Leaching and washing : The roasted ore is treated with water when CuSO₄ and FeSO₄ are washed away from the main ore. Further lighter ferric oxide is washed away leaving behind heavier ore particles known as black tin containing 60 to 70% .

SnO₂.SnO₂ + 2C ———> Sn + 2CO ;

Molten tin is drawn into blocks. It contains 99.5 percent of tin metal and is called block tin.

Refining of tin : It is purified by liquation, poling and electrolytic refining.

For very high purity, it is purified by electrolytic method. The electrolyte consists of tin sulphate containing a small amount of hydrofluorosilicic acid (H₂SiF₆) and sulphuric acid. Impure tin makes anode while pure tin sheet serves as cathode.