From copper pyrites CuFeS2: (1) Concentration by froth flotation. (2) Roasting in air: 2CuFeS2 + O2 → Cu2S + 2FeS + SO2 (partial). (3) Smelting with SiO2 flux: FeS + SiO2 → FeSiO3 (iron slag removed). Cu2S remains (matte). (4) Self-reduction (bessemerisation): 2Cu2S + 3O2 → 2Cu2O + 2SO2; then Cu2S + 2Cu2O → 6Cu + SO2. No external reducing agent needed! (5) Product: blister copper (98.5% pure, blisters from escaping SO2). (6) Electrolytic refining: impure Cu = anode, pure Cu = cathode, CuSO4/H2SO4 = electrolyte. Impurities (Ag, Au) settle as anode mud (valuable).