(1) Some important ores
Galena; –PbS(Main); Cerussite – PbCO3
Anglesite –PbSO4, lararkite PbO.PbSO4
(2) Extraction from galena
(i) Concentration : The finely powdered ore is concentrated by froth floatation process.
(ii) Reduction process
(a) Self reduction process
Roasting : The concentrated ore is heated in air. lead sulphide is partially converted into lead oxide and lead sulphate.
Δ
2PbS + 3O2 ——> 2 PbO + 2SO2
Δ
PbS + 2O2 ——> PbSO4
Self reduction : The supply of air is cut off and the temperature is raised to melt the charge. The galena reduces bothPbO and PbSO4to metallic lead.
Δ
Δ
PbS + PbSO4 ——> 2Pb + 2SO2
(b) Carbon reduction process
Roasting : The powdered ore is mixed with lime and roasted in excess of air. the impurities likeS and Asare oxidised to their volatile oxides while PbSis converted toPbO.
Δ Δ
S + O2 ——> SO2; 4AS + 3O2 ——> 2As2O3
Δ
2PbS + 3O2 ——> 2PbO + 2SO2
Smelting : Roasted ore is mixed with coke and lime (flux) and smelted in a blast furnance. PbOis reduced to Pb.
Δ Δ
PbO + C ——> Pb + CO; PbO + CO ——> Pb + CO2
Δ Δ
CaCO3 ——> CaO + CO2; CaO + SiO2 ——> CaSiO3
Lime Slag
(iii) Purification : It is purified electrolytically. The electrolyte consists of lead silicofluoride(PbSiF6) and hydrofluosilicic acid. Impure lead is made anode and sheet of pure lead serves as cathode.
Properties of Lead
With oxygen lead form oxides, with chlorine it forms chloride PbCl2, with sulphur it gives sulphide PbSand with H2SO4the corresponding sulphate PbSO4. With NaOHit forms plumbate.
Pb + 2NaOH ——> Na2PbO2 + H2
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