Pig iron was originated in the early days by reduction or iron ores in blast furnace and when the total output of the blast furnace was sand cast into pigs which is a mass of iron roughly resembling a reclining pig. It is roughly of 20″ × 9″ × 4″ in size. It is produced in a blast furnace and is the first product in the process of converting iron ore into useful ferrous metal. The iron ore on initial refining and heating in blast furnace becomes pig iron when the impurities are burnt out in a blast furnace. Pig iron acts as the raw material for production of all kinds of cast iron and steel products. It is obtained by smelting (chemical reduction of iron ore in the blast furnace. It is of great importance in the foundry and in steel making processes. It is partly refined in a cupola furnace that produces various grades of cast iron. By puddling processes, wrought iron is produced from pig iron. Steel is produced from pig iron by various steel making processes such as bessemer, open-hearth, oxygen, electric and spray steel making. The charge in the blast furnace for manufacturing pig iron is
(a) Ore Consisting of iron oxide or carbonate associated with earth impurities.
(b) Coke A fuel
(c) Limestone A flux
In addition to iron, pig iron contains various other constituents in varying form of impurity such carbon, silicon, sulphur, manganese and phosphorus etc. It has the following approximate composition which is as given as under.
|Carbon||—||4 to 4.5%||Phosphorus||—||0.1 to 2.0%|
|Silicon||—||0.4 to 2.0%||Sulphur||—||0.4 to 1.0%|
|Manganese||—||0.2 to 1.5 %||Iron||—||Remainder|
Carbon exists in iron in free form (graphite) and/or in combined form (cementite and pearlite). Pig iron is classified on the basis of contents of free and combined carbon as follows. These classifications are also termed as grades.
1. Grey pig iron (Grades 1, 2 and 3)
Grey pig iron contains about 3% carbon in free form (i.e., graphite form) and about 1% carbon in combined form. This is a soft type of pig iron.
2. White pig iron (Grades 4)
White pig iron is hard and strong. It contains almost all of the carbon in the combined form.
3. Mottled pig iron (Grade 5)
This type of pig iron is in between the grey and white variety. It has an average hardness and molted appearance. The free and combined forms of carbon are in almost equal proportion in mottled pig iron.