Production Process of Various Types of Irons

Introduction

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. (Refer table 1 & fig 1)

Table 1 Iron Ores

SL NO.Name of the oreChemical formulaColourIron  percentage
1MagnetiteFe3O4Black72.5%
2HematiteFe2O3Red65-70%
3Limonite2Fe2O3,  3H2OBrown60-65%
4SideriteFeCO3Brown40- 45%
Different Iron Ores
Fig 1 Different Iron Ores

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.

Pig Iron
Fig 2 Pig Iron

Production of Pig Iron

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

Blast Furnace
Fig. 3 Blast Furnace

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

  • For production of casting Iron
  • As raw material for production of wrought iron and steel.

Classification of Pig Irons (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.

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.

  • Pig iron that contains 74 to 82% manganese.
  • 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%.

Cast Iron

Cupola Furnace
Fig 4 Cupola Furnace

Cast iron is an alloy of iron and carbon. It is obtained by re-melting pig iron with coke, limestone and steel scrap in a furnace known as ‘‘cupola”. The carbon percentage varies from 1.7 to 4.5%. It also contains small amount of silicon, manganese, phosphorus and sulphur.

Production of Cast Iron

Cast irons are normally produced by melting Pig iron in cupola furnace with coke, lime stone and other fluxes. The limestone and fluxes aid in separating the impurities from the pig iron. The limestone also acts as an agent to carry off the oxides and ash from the fuel. Cupola is fired with coke to produce high temperatures required for melting pig iron and scrap.

Properties of Cast Iron

  • It is brittle.
  • It is very weak in tension, so it cannot be used for making bolts and machine parts which are liable to tension.
  • It is having good casting characteristics.
  • It has high compressive strength and high wear resistance.
  • It is having excellent machinability.
  • Its tensile strength is 100 to 200 MPa. (Mega Pascal)
  • Its compressive strength is 400 to 1000 MPa.
  • Its shear strength is 120 MPa.
  • Density of cast iron is – 6.8 to 7.8 gm/cm3.
  • Melting point of cast iron is 1150 to 1200°C.

Types of Cast Iron

Cast Iron may be classified as follows:

  • Grey Cast Iron
  • White Cast Iron
  • Mottled Cast Iron
  • Nodular Cast Iron
  • Malleable Cast Iron

Wrought Iron

Wrought iron is virtually pure iron, containing a large number of minute threads of slag lying parallel to each other, thereby giving the metal a fibrous appearance when broken. It contains negligible carbon percentage and therefore, does not harden when quenched in water. Wrought iron is produced by puddling process in reverberatory furnace.

Wrought Iron Anvil
Fig. 5 Wrought Iron Anvil

Puddling Process

Wrought iron is produced in reverberatory furnace by puddling process which is similar to open hearth furnace with combustion taking place at one end only. The furnace is lined with iron oxide. (in the form of mill scale or ore).  Pig iron is   charged   into the furnace. The carbon present in pig iron is melted and eliminated by the iron oxide lining. When practically all of the carbon and other impurities have been eliminated, the metal has a higher melting point and begins to form into a pasty mass. This pasty mass of metal and slag is well stirred (puddling) and formed into a bar, which is then removed from the furnace. The bulk of the slag is then squeezed from this mass by means of a mechanical squeezer and the iron is then rolled into bars. Now the material is a mixture of high purity iron and some slag (about 0.7%).

Reverberatory Furnace
Fig 6 Reverberatory Furnace

Table 2 Composition of Different Types of Iron

Type of IronCarbon%Sulphur%Silicon%Manganese%Phosphorus%Iron%
Pig Iron1-4.5<11-20.1-1.50.5-295-96
Grey Cast iron2.5-3.80.11.1-2.80.4-10.1595-96
White Cast Iron4.50.10.5-1.50.2-0.80.1894
Mottled Cast Iron.3.500.750.750.750.7593.5
Nodular cast iron3.2-4.1<0.031-2.780.3-0.8<0.1094.5
Malleable cast iron2.2-2.80.03- 0.10.7- 1.10.3 – 0.40.1097.5
Wrought Iron0.02-0.030.08-0.020.02-0.010.020.05-0.2598.5-99.5
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