REDUCTION OF METAL OXIDE TO METAL
The process of converting metal oxides into metals is called reduction. For reduction, different types of reducing agents are used depending on reactivity of ores. The commonly used methods for reduction are :
1. Reduction by Heat (pyrometallurgy)
The oxides of metals which are present at the bottom of reactivity series can be reduced to metals by action of heat alone e.g. mercury oxide can be reduced to mercury metal by heating it to a temperature of about 300°C.
2HgO 2Hg + O2
Mercury oxide Mercury metal
2. Reduction by Coke (smelting)
The Oxides of Metals like Zn, Fe, Cu, Ni, Sn and Pb are usually reduced by using carbon as reducing agent. In this process, coke is mixed with roasted ore and heated to a high temperature in a furnace. Coke reduces the metal oxides into free metal. For example,
ZnO + C Zn + CO
Zinc oxide Carbon Zinc Carbon monoxide
PbO + C Pb + CO
Lead oxide Carbon Lead Carbon monoxide
3. Reduction by Aluminium (aluminotherapy)
Oxides of manganese and chromium metals are reduced to metals with the help of Aluminium. The process of reduction of a metal oxide to the metal with the help of aluminium is called aluminotherapy.
3MnO2 + 4Al 3Mn + 2Al2O3
Manganese dioxide Manganese metal Aluminium oxide
4. Electrolytic Reduction
The Oxides of metals which are quite high in reactivity series can be reduced to metals by electrolytic reduction. For example, sodium and magnesium metals are obtained by electrolytic reduction of their chloride solutions in molten state.
2NaCl 2Na + Cl2
Sodium chloride Sodium metal Chlorine gas
MgCl2 Mg + Cl2
Magnesium chloride Magnesium metal Chlorine gas
During this process, chlorine gas is liberated at anode while metals (sodium or magnesium) deposit at cathode.