Why is it advantageous to roast a sulphide ore to the oxide before reduction ?

Explanation

The standard free energy of formation `(Delta_f G^@)` of most of the sulphides are larger than those of `CS_2` and `H_2 S ` . Therefore, these sulphides are more stable than those with carbon or hydrogen. Hence neither carbon nor hydrogen can reduce metal sulphides to metal. In constrast the standard free energies of formation of oxides are much lower than those of `SO_2` and therefore oxidation of metal sulphides to metal oxides is thermodynamically favourable . Therefore , it is a common practice to roast sulphide ore to oxide ore before reduction.