The concentrated ore must be converted into a form which is suitable for reduction. Oxides are easier to reduce, thus non-oxide ores are first converted into an oxide ore and then it reduce to metal by suitable agent.
a) Conversion to oxide:
i) Roasting
Roasting is the process in which the ore is heated strongly, below its melting point, in presence of excess of air. This process is used for the conversion of sulphide ores to their respective metal oxides (de-electronation of ores).
2ZnS + 3O2 → 2ZnO + 2SO2 ;
ZnS + 2O2 → ZnSO4
ii) Calcination
Calcination is the process in which the ore is heated strongly, below its melting point, either in absence or in a limited supply or air. This process is used for the conversion of the carbonates and hydrated oxide ores to their respective oxides.
CaCO3 → CaO + CO2
b) Reduction of oxide to the metal:
i) Reduction by Carbon-Smelting
Smelting is the process of extraction of a metal from its roasted or calcined ore by heating it with powdered coke in presence of a flux.
ZnO + C → Zn + CO
ii) Flux is a substance which combines with gangue (earthy impurities) still present in the roasted or the calcined ore to form a fusible product called slag.
Flux + Gangue → Slag
iii) Reduction by Aluminium—Goldschmidt Aluminothermic Process
Aluminothermic process involves the reduction of oxides (Fe2 O3 , Cr2 O3 , Mn3 O4 , etc.) by aluminium power.
The mixture of metallic oxide such as Fe2 O3 and aluminium powder in the ratio of 3 : 1 is known as thermite.
iv) Reduction by Precipitation (Hydro-metallurgy)
Hydrometallurgy is the process of dissolving the metal or its ore by the action of a suitable chemical reagent followed by recovery of the metal either by electrolysis or by the use of a suitable precipitating agent (displacement method). For example,
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v) Reduction by Mg
This method is applicable for the reduction of TiCl4 (Krolls process).
TiCl4 + 2 Mg → 2 MgCl2 + Ti
