Explain the difference between roasting and calcination in the context of metal extraction. When is each process used?

Both roasting and calcination are processes used to convert metal ores into their oxides before reduction.

Roasting: This process involves heating a sulphide ore strongly in the presence of excess air. The sulphide ore is converted into its metal oxide and sulphur dioxide gas is released. It is used for sulphide ores (e.g., 2ZnS(s) + 3O2(g) → 2ZnO(s) + 2SO2(g)).

Calcination: This process involves heating a carbonate ore or hydrated oxide ore strongly in a limited supply of air or in the absence of air. The carbonate ore decomposes to form its metal oxide and carbon dioxide gas. It is used for carbonate ores 

(e.g., ZnCO3(s) → ZnO(s) + CO2(g)) and hydrated oxides.