Chemistry EXTRACTION OF LEAD

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EXTRACTION OF LEAD

EXTRACTION OF LEAD

`text(Occurrence of lead)` : Lead does not occur in the free state. The important ores of lead are :

(i) Galena (`PbS`) (ii)Anglesite (`PbSO_4`) and (iii) Cerussite (`PbCO_3`).

`text(Extraction of lead)` :

(i) `text(Self reduction process)` : Galena, after concentration by Froth-floatation process, is roasted in air to form `PbO` and `PbSO_4`. The unchanged galena then brings about the reduction of `PbO` and `PbSO_4` to `Pb` metal (auto reduction).

`3PbS +SO_2 undersettext(Roasting)oversettext(Heat)(->) 2PbO + PbSO_4 + 2SO_2`

`2PbO + PbS understtext(Auto reduction)oversettext(Heat)(->) 3Pb + SO_2`;

`PbSO_4 + PbS understtext(Auto reduction) oversettext(Heat)-> 2Pb + 2SO_2`

(ii) `text(Carbon reduction process)` : The roasted ore is reduced to metal by heating with coke.

`PbO + C undersettext(Coke) oversettext(Heat)-> Pb +CO`.

The metal obtained by above methods contain `Ag`, `Cu`, `Fe`, `Sn` and `Sb` as impurities. It is refined by either liquation, softening, desilverisation or electrolytic refining processes.

In electrolytic refining the electrolyte taken consists of a solution of lead fluorosilicate (`PbSiF_6`) and hydrofluorosilicic acid (`H_2SiF_4`) with a little gelatin.

`text(Occurrence of lead)` : Lead does not occur in the free state. The important ores of lead are :

(i) Galena (`PbS`) (ii)Anglesite (`PbSO_4`) and (iii) Cerussite (`PbCO_3`).

`text(Extraction of lead)` :

(i) `text(Self reduction process)` : Galena, after concentration by Froth-floatation process, is roasted in air to form `PbO` and `PbSO_4`. The unchanged galena then brings about the reduction of `PbO` and `PbSO_4` to `Pb` metal (auto reduction).

`3PbS +SO_2 undersettext(Roasting)oversettext(Heat)(->) 2PbO + PbSO_4 + 2SO_2`

`2PbO + PbS understtext(Auto reduction)oversettext(Heat)(->) 3Pb + SO_2`;

`PbSO_4 + PbS understtext(Auto reduction) oversettext(Heat)-> 2Pb + 2SO_2`

(ii) `text(Carbon reduction process)` : The roasted ore is reduced to metal by heating with coke.

`PbO + C undersettext(Coke) oversettext(Heat)-> Pb +CO`.

The metal obtained by above methods contain `Ag`, `Cu`, `Fe`, `Sn` and `Sb` as impurities. It is refined by either liquation, softening, desilverisation or electrolytic refining processes.

In electrolytic refining the electrolyte taken consists of a solution of lead fluorosilicate (`PbSiF_6`) and hydrofluorosilicic acid (`H_2SiF_4`) with a little gelatin.

Properties of Lead :

Lead is a bluish grey lustrous metal but when exposed to air, it acquires dull appearance due to the forming of a thin layer of `Pb(OH)_2` and `PbCO_3` on its surface. It dissolves in hot concentrated alkali evolving `H_2`.

`Pb + 2NaOH -> H_2 + Na_2PbO_4` (Sod. plumbite)

Other reactions with acids, `S`, `Cl_2`, are similar to those of `Sn`.

Lead forms two series of compounds i.e. `Pb (II)` and `Pb(IV)` compounds but `Pb(II)` compounds are more stable than `Pb (IV)` compounds. `Pb(II)` compounds are essentially ionic while `Pb (IV)` compounds are covalent in nature.

Lead used for making cable coverings, bullet shots, lead accumulators, tetraethy llead (antiknocking agent) and a number of pigments such as red lead (`Pb_3O_4`); white lead [`2Pb(OH)_2*PbCO_3`], chrome yellow (`PbCrO_4`) and chrome red (`PbO*PbCrO_4`).

Lead is a bluish grey lustrous metal but when exposed to air, it acquires dull appearance due to the forming of a thin layer of `Pb(OH)_2` and `PbCO_3` on its surface. It dissolves in hot concentrated alkali evolving `H_2`.

`Pb + 2NaOH -> H_2 + Na_2PbO_4` (Sod. plumbite)

Other reactions with acids, `S`, `Cl_2`, are similar to those of `Sn`.

Lead forms two series of compounds i.e. `Pb (II)` and `Pb(IV)` compounds but `Pb(II)` compounds are more stable than `Pb (IV)` compounds. `Pb(II)` compounds are essentially ionic while `Pb (IV)` compounds are covalent in nature.

Lead used for making cable coverings, bullet shots, lead accumulators, tetraethy llead (antiknocking agent) and a number of pigments such as red lead (`Pb_3O_4`); white lead [`2Pb(OH)_2*PbCO_3`], chrome yellow (`PbCrO_4`) and chrome red (`PbO*PbCrO_4`).

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