Roasting is a process in which sulphide ores are converted to oxide ores by heating the ore in excess of oxygen in a reverberatory furnace below the melting point of the metal.
eg: Galena when roasted gets converted to lead monoxide.
2PbS + 3O2 → 2PbO + SO2
During roasting volatile and non-metallic impurities are removed. The ore is rendered porous so that further steps in the extraction process becomes easy.
