Engineering 

Production Process and Classification of Pig Iron

Ferrous metals are iron based metals which include all varieties of iron and steels. Ferrous basically refers to iron. Iron is the name given to pure ferrite (Fe), as well as to mixtures of this ferrite with about 1.7% Carbon also known as Pig Iron. Pig iron is the basic raw material which is produced from iron ore in the blast furnace. Cast iron, wrought iron and steels are made from Pig iron. Cast iron is produced in cupola furnace. Cast iron can be cast but it cannot be forged. Wrought iron can be forged but it cannot be cast. Steels can be cast as well as forged.

Iron ores

The starting point in the commercial production of iron and steel is the smelting of iron ore to produce pig iron. The principal iron ores are given in the table.

SL NO. Name of the ore Chemical formula Colour Iron  percentage
1 Magnetite Fe3O4 Black 72.5%
2 Hematite Fe2O3 Red 65-70%
3 Limonite 2Fe2O3,  3H2O Brown 60-65%
4 Siderite FeCO3 Brown 40- 45%

Pig Iron

Pig iron is the basic material from which, wrought iron and steels are produced. It is obtained by smelting (chemical reduction) of iron ore in the blast furnace. In addition to iron, Pig iron contains varying quantities of other elements such as carbon, silicon, manganese, sulphur and phosphorus. These may amount to as much as 10% of the weight and 25% of the volume of pig iron.

Production of Pig Iron                 

Pig iron is obtained by smelting (chemical reduction) of iron ore in the blast furnace.

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The basic materials used for the manufacture of pig iron are iron ore, coke, and limestone. The coke is burned as a fuel to heat the furnace; as it burns, the coke gives off carbon monoxide, which combines with the iron oxides in the ore, reducing them to metallic iron. This is the basic chemical reaction in the blast furnace; it has the equation: Fe2O3 + 3CO = 3CO2 + 2Fe. The limestone in the furnace charge is used as an additional source of carbon monoxide and as a “flux” to combine with the infusible silica present in the ore to form fusible calcium silicate. Without the limestone, iron silicate would be formed, with a resulting loss of metallic iron. Calcium silicate plus other impurities form a slag that floats on top of the molten metal at the bottom of the furnace. Ordinary pig iron produced by blast furnaces contains about 92 % Iron, 3 to 4 % carbon, 0.5 to 3 % silicon, 0.25 to 2.5 % manganese, 0.04 to 2 % phosphorus  and a trace of sulphur.

Uses of Pig Iron.

(a) For production of casting Iron.

(b) As raw material for production of wrought iron and steel.

Classification (On the Basis of Chemical Composition)

Basic Pig Iron – Basic pig iron has low content of sulphur (0.04%), carbon varies from 3.5 to 4.5%, phosphorus is normally less than 1 % and manganese varies from 1 to 1.5%.  It is used for steel making. Due to low Silicon percentage it prevents the attack of refractory lining of refining furnaces and to control slag formation.

Foundry Pig Iron – Foundry pig iron contains, Carbon – 3 to 4.5%, Phosphorus – 0.03% to 0.9%, Silicon – 0.5 to 3.5%, Sulphur – upto 0.05% , Manganese (Mn) – 0.4 to 1.2% and rest Iron (Fe). It is used for the production of iron castings.

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Ferroalloys – These are alloys of pig iron, each rich in one specific element. Ferroalloys are used as additives in iron and steel industries to control or change the properties of iron and steel.

(a) Ferromanganese – Pig iron that contains 74 to 82% manganese.

(b) Ferrosilicon – Pig iron with 5 to 17% of silicon content.

Classification ( On the Basis of  Free and Combined Carbon %)

Grey Pig Iron (Grade 1, 2 and 3) – These grades of pig iron contain almost all carbon in free form (graphite). The percentage of free carbon is more than 3 % and combined carbon is less than 1 %.

Mottled Pig Iron (Grade 4) – It contains equal proportions of free and combined carbon. It is between grey and white variety of pig iron

White Pig Iron ( Grade 5, 6 and 7) – These grades of pig iron contain almost all carbon in the combined state. The percentage of free carbon is less than 1 percent and combined carbon is more than 3 %.

WATCH PIG IRON PRODUCTION PROCESS VIDEO HERE

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