(a) Smelting : The reduction of a metal from its ore by a process involving melting is known as smelting. It is generally done in a reverberatory furnace or a blast furnace in a controlled supply of air. Several reducing agents such as sodium, magnesium and aluminium are used for reduction. The calcinated or roasted ore is mixed with carbon (coal or coke) and heated in a reverberatory or a blast furnace. Carbon and carbon monoxide produced by incomplete combustion of carbon reduce the oxide to the metal.
Carbon Reduction Process :
PbO + C → Pb + CO
PbO + CO → Pb + CO2
SnO2 + 2C → Sn + 2 CO
Aluminium Reduction Process (Gold -Schimidt alumino thermic Process)
Cr2O3 + 2 Al → 2 Cr + Al2O3
3Mn3O4 + 8 Al → 9 Mn + 4 Al2O3
(b) Flux : The ores even after concentration contain some earthy matter called gangue which is heated combine with this earthy matter to form an easily fusible material. Such a substance is known as flux and the fusible material formed during reduction process is called slag. Slag is usually lighter and floats on the surface of the molten metal.
Concentrated ore + gangue + reduction agent + flux →(heated) → Metal + slag + gases
Fluxes are of the following two types:
(i) Acidic fluxes like silica, borax etc. These are used when the gangue is basic such as lime or other metallic oxides like MnO, FeO, etc. The chemical reaction which takes place in removing a basic gangue is as follows:
CaO + SiO2 → Ca SiO3
(basic gangue) + (Acidic Flux) → (Slag)
(ii) Basic fluxes like CaO, lime stone (CaCO3), magnesite (MgCO3), haematite (Fe2O3) etc. These fluxes are used when the gangue is acidic like silica, P4O10 etc. For example
SiO2 + MgCO3 → MgSiO3 + CO2↑
(Acidic gangue) + (basic flux) → Slag
P4O10 + 6 CaO → 2 Ca3 (PO4)2
(Acidic flux) + (basic gangue) → Slag
The other methods used for the reduction of metals are electrolytic reduction, hydro metallurgy and amalgamation method.