Q. Explain about Roasting process?

Roasting is the method of heating ores in the presence of excess air and involves oxidation. It is mainly applied to sulphide ores, which are converted lo oxides or sulphates. Various impurities like sulphides of antimony and arsenic also get oxidised' and volatilised. For example,

4FeS₂+ 11O₂--------------> 2Fe₂SO₃+ 8SO₂

ZnS + 2O₂------------------> ZnSO₄

2ZnS + 3O₂-------------------> 2ZnO+ 2SO₂

2As₂S3+ 9O₂--------------> 2As₂O₃+ 6 So₂

When cuprous sulphide is roasted in a partial supply of air, it is partially oxidised to Cu₂0,

Which is then  reduced to copper by the residual cuprous sulphide:

2CuS₂+ 3O₂-------------> 2Cu₂O + 2SO₂

Cu₂+ 2Cu₂O-----------> 6CU + SO2

Sometimes, the oxides formed throughout roasting are unstable and decompose into elements at a moderately high temperature. For Illustration, in the roasting of cinnabar, the red sulphide ore of mercury, the oxide formed decomposes to provide the metal: