The concentrated ore is converted into metal oxide by calcination or roasting.
Calcination involves heating of the concentrated ore in a limited supply of air so that it loses moisture, water of hydration and gaseous volatile substances. The ore is heated to a temperature so that it does not melt.
Removal of water of hydration
Al2O3.2H2O → Al2O3 + 2H2O
Expulsion of CO2 from carbonate
ZnCO3 → ZnO + 2CO2
Roasting is a process in which the concentrated ore is heated in a free supply of air at a temperature insufficient to melt it.
Calcination and roasting are generally carried out in a reverberatory furnace or in a multiple hearth furnace.