Roasting involves heating of an ore in the presence of excess of air while calcination involves heating of the ore in the presence of limited supply of air. Roasting of chalcocite is necessary because it not only removes the impurities of S, P, As, Sb, etc., present in it as their volatile oxides but also partially oxidises Cu2S to Cu2O.
Calcination, on the other hand will not be able to convert Cu2S to CurO.
