Roasting: Heating ore in excess air/oxygen. Converts sulphides to oxides: 2ZnS + 3O2 → 2ZnO + 2SO2; 2PbS + 3O2 → 2PbO + 2SO2. Removes volatile impurities (As, S, P as oxides). Calcination: Heating ore in limited/no air. Decomposes carbonates to oxides: CaCO3 → CaO + CO2; ZnCO3 → ZnO + CO2. Also removes moisture and volatile matter. Key difference: Roasting = with air (for sulphides); calcination = without air $\frac{for carbonates}{hydrated ores}$. After roasting/calcination, the oxide is ready for reduction.