Roasting of ores is done

in the presence of an excess of oxygen

Roasting is a process of metallurgy where ore is converted into its oxide by heating it below its melting point in the presence of excess air. While calcination is mostly used in the oxidation of carbonates, roasting is a method that is used for converting sulphide ores. During roasting, moisture and non-metallic impurities are released in the form of volatile gases. The process of roasting consists of a solid-gas thermal reaction which includes oxidation, reduction, sulfation, chlorination and pyro hydrolysis.

However, roasting which involves sulphides is a major source of air pollution and the main drawback of this process is that it releases a large amount of metallic as well as toxic and acidic compounds which cause harm to the environment.

An example of roasting is when Zinc sulphide is converted into zinc oxide.

\(2ZnS + 3O_2 −−−−→  2ZnO + 2SO_2\)