Roasting is a process of heating the concentrated ore to a high temperature in presence of air. Generally, sulphide ores are roasted.
Example
$$
2 \mathrm{ZnS}+3 \mathrm{O}_{2} \rightarrow 2 \mathrm{ZnO}+2 \mathrm{SO}_{2}
$$
Calcination is a process of heating ore in the absence of air to a high temperature, but insufficient to melt the ore. Generally, carbonates and hydrated ores are calcined.
Example:
(i) \(\mathrm{ZnCO}_{3} \rightarrow \mathrm{ZnO}+\mathrm{CO}_{2}\)
(ii) Anhydrous alumina is left behind when bauxite is calcined and water is removed.
\(\mathrm{Al}_{2} \mathrm{O}_{3} \cdot 2 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{Al}_{2} \mathrm{O}_{3}+2 \mathrm{H}_{2} \mathrm{O}\)
