1. Conversion to oxide. 2. Reduction of the oxides to metal.
Conversion to Oxides
( i) Calcination It is the process of converting an ore into its oxides by heating it strongly, below its melting point in a limited supply of air or in absence of air.
During calcination, volatile impurities as well as organic matter and moisture are removed.
Calcination is used for metal carbonates and hydroxides and is carried out in reverberatory furnace.
( ii) Roasting It is the process of converting an ore into its metallic oxide by heating it strongly. below its melting point m excess of air. This process is commonly used for sulphide ores and is carried out in blast furnace or reverberatory furnace. Roasting helps to remove the non-metallic impurities and moisture.
The furnaces used in calcination and roasting employ refractory materials which resist high temperature and do not become soft.
Acidic refractories: SiO2 and SiO2 + Al2O3
Basic refractories: CaO and MgO Neutral refractories: Graphite, chromites. etc.