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

Explanation

The free energies of formation `(Delta_(f) G^(Ө))` of most sulphides are greater than those for `CS_(2)` and `H_(2) S`. Carbon disulphide is, in fact, an endothermic compound. So neither carbon nor hydrogen is a suitable reudcing agent for metal sulphides. Moreover, the roasting of a sulphide to the oxide is quite advantageous thermodynamically. Hence, the common practice is to roast sulphide ore to the oxide prior to reduction.