$word_{count}$: 180

Both roasting and calcination are pre-treatment steps that convert ores to oxides before reduction, but they differ fundamentally.

Roasting: Heating ore in excess air/oxygen. For sulphide ores. Reactions: 2ZnS + 3O2 -> 2ZnO + 2SO2; 2PbS + 3O2 -> 2PbO + 2SO2. Also removes volatile impurities (As, S, P as their oxides). By-product SO2 can be used to manufacture H2SO4 (Contact process).

Calcination: Heating ore in limited or no air. For carbonate and hydrated ores. Reactions: CaCO3 -> CaO + CO2; ZnCO3 -> ZnO + CO2; Al2O3.2H2O -> Al2O3 + 2H2O. Removes moisture and volatile matter.

The critical distinction: roasting = with air (sulphides); calcination = $\frac{without}{limited}$ air (carbonates). After either process, the resulting oxide is ready for reduction by carbon, aluminium, or electrolysis depending on the metal's reactivity.