Question

Why we use calcination for carbonates but roasting for sulphides?

Answer 1

Roasting is the process of converting an ore into its oxide by heating strongly in the presence of an excess of air (oxygen), so that oxygen reacts to forming the corresponding oxide. It is done for sulphide ores in order to remove sulphur as sulphur gets volatilised in the form of gas (SO2SO2)

2ZnS(s)+3O2(g)→2ZnO(s)+2SO2(g)2ZnS(s)+3O2(g)→2ZnO(s)+2SO2(g).

In case of carbonate ores, we need to eliminate the carbonate and moisture. So, the ore is heated to high temperatures in the absence of air. Carbon, which is in its highest oxidation state, cannot be oxidised further. This process is called calcination.

ZnCO3(s)→ZnO(s)+CO2(g)ZnCO3(s)→ZnO(s)+CO2(g).

So, roasting is not done for carbonate ores.