Roasting is done generally in the case of sulphide ores.
Roasting is a metallurgical process in which ore is heated below its melting point in the presence of excess air to turn it into its oxide. Sulphide ores are converted using the roasting process. Moisture and non-metallic impurities are emitted as volatile gases during roasting. The roasting method is made up of solid-gas thermal reactions including oxidation, reduction, sulfation, chlorination, and pyro hydrolysis.
An example of roasting is when Zinc sulphide is converted into zinc oxide.
2ZnS+3O2 —> 2ZnO + CO2