Metallurgy of Lead: Ores, Properties, Extraction, Purification, Uses

Metallurgy of Lead 

Ores

1.      Galena    PbS

2.     Cerrusite PbCO3

3.     Anglesite PbSO4

4.     Lead ochre PbO

Extraction: Lead is mainly extracted from the sulphide ore galena. Galena contains lead sulphide and small quantities of silver.

1.    Concentration: The ore is concentrated by froth floatation process.

2. Smelting in a Reverberatory furnace: The concentrated ore is roasted in a reverberatory furnace at a moderate temperature. The temperature of furnace is controlled by regulating the air supply. During roasting, galena is partly oxidized to lead monoxide and partly to lead sulphate.

2PbS + 3O2    ®    2 PbO + 2SO2

PbS + 2O2       ®     PbSO4

More of galena is then added. The temperature is raised and simultaneously the air supply is reduced. Lead sulphide reacts with the two oxidised products giving lead.

PbS+2PbO ® 3Pb+SO2

PbS+PbSO4   ® 2Pb+2SO2

Thus in this process roasting and smelting are carried out in the same furnace,

at two different temperatures.

About 90% of lead is obtained as metal, the rest passes into slag. Lead is

recovered from the slag by heating with lime and powdered coke.

Purification of Lead

Lead extracted by the above method contains impurities such as silver, copper, tin, bismuth, gold and iron. It is refined by the following processes.

a. Liquation

The impure metal is heated on a sloping hearth. Lead melts and flows down the slope. The infusible impurities remain on the hearth.

b. Desilverisation

Silver is removed by either Pattinson's process or Park's process.

c. Electrolytic refining

Very pure lead is obtained by this process.

Anode - Impure lead

Cathode - Very pure lead

Electrolyte - Lead fluosilicate + Hydrofluosilicic Acid

(PbSiF6)      (H2SiF6)

The metallic impurities which are more electropositive than lead, such as iron and tin, go into the solution while the rest of the impurities are thrown down as anode mud.

Physical properties

1.           Lead is a bluish grey metal with a bright luster.

2.           It is soft and can be cut with a knife and drawn into a wire and rolled into a sheet.

3.           It is not a good conductor of heat and electricity. It marks paper.

Chemical properties

1. Action of air

       i.            It is unaffected by dry air but in moist air a layer of lead carbonate or lead hydroxide is deposited on its surface which protects it from further action of air.

     ii.            When heated in air or oxygen, lead is oxidized to litharge (PbO) and red

lead (Pb3O4)

2Pb + O2 ® 2PbO 3Pb + 2O2 ® Pb3O4

2. Action of water

Lead is not attacked by pure water in the absence of air, but water containing dissolved air has a solvent action on it due to the formation of lead hydroxide (a poisonous substance). This phenomenon is called Plumbo solvency.

2Pb + O2 + 2H2O         ®      2Pb(OH)2

3.    Action of acids

i) Dilute H2SO4 and HCl have no action on lead.

ii) Hot Conc. H2SO4 liberates SO2 but the reaction is retarded by the formation

of an insoluble layer of lead sulphate.

Pb + 2H2SO4® PbSO4 + 2H2O + SO2­

iii) Concentrated HCl evolves hydrogen and also forms Chloroplumbic acid

Pb + 2HCl    ®      PbCl2 +H2­

PbCl2 + 2HCl --- > < ---     H2PbCl4 (chloroplumbic acid)

Uses: Lead is used

       i.            For making lead pipes,

     ii.            For making telegraph and telephone wires,

  iii.            In making bullets and lead accumulators,

  iv.            In lead chambers, for the manufacture of sulphuric acid,

     v.            For making alloys like solder, pewter and type metal,

       vi.                        For preparing tetraethyl lead (Pb(C2H5)4) which is used as an additive to petrol to prevent knocking

Problem

An element A belongs to 14th group and occupies period number 6. A reacts with conc. HCl. to give B an acid. A is used to prepare C which is used as an antiknock in automobiles. Identify the element A and the compounds B and C Write the reactions.

Solution

1.    As per the position in the periodic table, the element A is lead. 2. Lead with Conc. HCl gives B

Pb + 4 HCl ® H2PbCl4 + H2

∴ Compound B is chloroplumbic acid.

3.   Compound C is tetraethyl lead.