Lead: Meaning, Properties, Classification and Uses | Metallurgy

In this article we will discuss about:- 1. Meaning and Properties of Lead 2. Classification of Lead 3. Uses.

Meaning and Properties of Lead:

Lead is obtained from its ores, by concentration, floatation, and reduction in blast furnaces. The crude lead is purified by washing with molten zinc (Parkes process), and the resulting lead is cast into pigs.

Lead is a soft and weak metal. Its tensile strength is about 150 kg/cm². It is very malleable and ductile. It is very heavy and resists corrosion. It has a high density, low melting point and high boiling point. It can be easily melted (melting point 327°C), cast, rolled and extruded. It is highly malleable and pliable. So it is easily handled during fabrication and installation.

It has low strength and due to that its ductility is also low. It has got low elastic limit, high coefficient of thermal expansion and has get very high anti-frictional properties. It is a good insulator against nuclear radiation. Impurities present in lead are very small but have profound effect chemically.

Classification of Lead:

Based on the content of impurities lead can be classified into the following groups:

i. Corroding Lead:

It is pure and is called corroding lead as white lead is prepared out of it.

ii. Chemical Lead:

Such a lead contains 0.04 to 0.8% copper, 0.002 to 0.02% silver and less than 0.005% bismuth. Presence of these impurities in lead makes lead resistant to corrosion, particularly to sulphuric acid. Such a lead is very commonly used. Its copper content confers added stiffness to it.

iii. Tellurium Lead:

Small quantities of tellurium in lead (between 0.5 and 0.005 per cent) will make lead have finer grain size, higher tensile strength and greater fatigue strength than ordinary lead. Tellurium lead has greater resistance to corrosion than pure lead.

Precisely tellurium will have the following effects on lead:

a. Grain is refined in a remarkable way.

b. Temperature of crystallisation is raised appreciably.

c. Work toughening properties are imparted.

d. Tensile strength is doubled.

e. Resistance to fatigue increases there times at ordinary temperature and four times at 100°C when compared with lead without tellurium.

Tellurium lead is many times resistant to corrosion. It has much greater resistance to the solvent action of water than ordinary lead. Tellurium lead toughens when strained, and strain is evenly distributed. So tellurium lead is resistant to hydraulic bursting.

iv. Antimonial Lead: 

Presence of antimony improves mechanical properties of lead. Antimonial lead has got better mechanical properties than chemical or tellurium lead but at high temperature this difference decreases. With 6.7% antimony it is used for storage battery plates. Creep or long-time tensile strength of lead is quite low.

Uses of Lead:

(I) Lead is used in the manufacture of a number of chemicals. For example, it is used for lining in the tank, when ground bauxite is treated with sulphuric acid for manufacturing alum, etc. The heating coil is also made of lead.

(ii) Lead resists corrosion due to brine solutions, salt sprays and sea coast atmospheres. So lead is used for waste pipes conveying sea water aboard ships, for lining refrigerator roofs and for lining large aquariums.

(iii) In the manufacture of dyes. The equipment where halogenation, sulphonation, hydrolysis, oxidation, esterification, reduction, extraction and condensation, etc., has to take place, is lined with lead.

(iv) It is resistant to corrosion by sulphuric acid upto 9.5% concentration of sulphuric acid. So lead chambers are used in chamber process of manufacturing sulphuric acid. Lead lining is also used in tanks where pickling of steel and the manufacture of nitroglycerine, titanium dioxide and ethers is carried out.

(v) Lead can be used in contact with sodium hydroxide up to 30% concentration at 25°C and upto 10% at 90°C. So it is used in petroleum refining where sulphuric acid treatment is followed by caustic wash.

(vi) Lead can resist wet sulphur dioxide gas. So it is used in lead covered anodes and for the lining of electrostatic precipitators used to remove sulphuric acid mist from sulphur dioxide gas.

In paper and pulp industry lead pipes are used to cool sulphur dioxide gas, for bleaching with hydrogen peroxide or zinc hydrosulphite, and for taking away the discharge liquors from the pulp digesters.

(vii) Lead can be safely used in various processes where it comes in contact with the following chemicals:

a. Solvents such as alcohols, acetone, trichloro- ethylene, etc.

b. Acids such as sulphuric acid, chromic acid, hydrofluoric acid, etc.

c. Alkalies such as ammonium hydroxide, sodium hydroxide etc.

(viii) Lead having density as high as 11.3 is used in X- ray protection and is also used as a protection against deadly rays from nuclear fission and radioactive isotopes.

Due to high density and ease of casting it is used as a counter-weight in various equipment’s and in keels of ships.

(ix) Due to its softness it is used as impression lead for reproducing halftone plates and electrotypes, etc. Due to the softness and low melting point of lead, it is readily extruded and pipes can be formed.

(x) Lead is used under machines and buildings for reducing vibrations. For such cases it is generally used in conjunction with asbestos. 25 mm thickness of asbestos is covered with 3 mm thick 6% antimonial lead.

(xi) Lead can be easily rolled into any shape so it is used for making ideal gasket material because of its pliability and low creep strength.

(xii) Lead sheets are used for flooring in chemical plants, acid works in the explosive manufacturing plants. Lead is also used for covering laboratory benches.

(xiii) Lead is used in battery plates.

(xiv) In paint industry lead is used as oxide of basic carbonate as pigment.

(xv) It is introduced into alloys to produce free-cutting characteristics.

Bureau of Indian standards have published following standards, relating to lead:

IS: 25 – Antifriction bearing alloys.

IS: 404 – Lead Pipes.

IS: 405 – Lead sheets and strips.

IS: 1654 – Antimony lead alloys.

IS: 8475 – Antifriction bearing alloys for heavy duty.