From zinc blende ZnS: (1) Froth flotation (sulphide ore). (2) Roasting: 2ZnS + 3O2 → 2ZnO + 2SO2 (converts sulphide to oxide). (3) Carbon reduction: ZnO + C → Zn + CO at 1673 K. Zinc vaporises (b.p. 1180 K) and is condensed. (4) Refining: Distillation (Zn is volatile) or electrolytic (ZnSO4 solution, Zn deposited on Al cathode). By-product SO2 from roasting can be used to make H2SO4 (Contact process). The roasting step is critical — sulphide ores must first be converted to oxides because carbon cannot directly reduce sulphides efficiently.