: 220
Copper extraction from copper pyrites (CuFeS2) is unique because it involves self-reduction (auto-reduction) — no external reducing agent is needed.
The sequence: (1) Concentration by froth flotation (sulphide ore). (2) Roasting in air: 2CuFeS2 + O2 -> Cu2S + 2FeS + SO2 (partial oxidation). (3) Smelting with SiO2 flux: FeS + SiO2 -> FeSiO3 (iron slag removed), leaving Cu2S (copper matte). (4) Bessemerisation (self-reduction): 2Cu2S + 3O2 -> 2Cu2O + 2SO2; then Cu2S + 2Cu2O -> 6Cu + SO2. The sulphide reduces the oxide without external help.
Product: blister copper (98.5% pure, blistered surface from escaping SO2). (5) Electrolytic refining: impure Cu = anode, pure Cu = cathode, acidified CuSO4 = electrolyte. Result: 99.99% pure Cu. Anode mud contains Ag and Au (valuable by-product).
The self-reduction concept is a JEE favourite. The key insight is that Cu2S simultaneously acts as the reducing agent (for Cu2O) in the final step. This is possible because copper's position in the reactivity series allows its sulphide and oxide to react directly.