2Cu2S(s) + 3O2(g) -> 2Cu2O(s) + 2SO2(g)
When a good amount of copper(I) oxide is formed, then the supply of air for roasting stopped. In the absence of air, copper(I) oxide reacts with remaining copper(I) sulphide to form copper metal and sulphur di oxide.
2Cu2O(s) + Cu2S(s) -> 6Cu(s) + SO2(g)