Pig Iron – Composition

It is a crude form of iron and used as a raw material for the production of various other ferrous metals, such as cast iron, wrought iron, and steel. The pig iron is obtained by smelting iron ores in a blast furnace.

The iron ores are found in various forms,…

Iron ore1) As oxides 2) As carbonates 3) As sulphides.

  1. As oxides- a: Magnetite(Fe3O4), b: Haematite(Fe2O3), c: Limonite(Fe2O3H2O).
  2. As carbonates-  Siderite(FeCO3)
  3. As sulphides-  Pyrite(FeS2)

 

Metallic contents in iron ores:

IRON ORE COLOUR IRON CONTENT(%)
Magnetic Black            o 72
Haematite Red               0 70
Limonite Brown          0 60 – 65
Siderite Brown          0 48

 

The haematite is widely used for the production of pig iron. Since pyrite contains only 30-40% iron, therefore it is not used for manufacturing pig iron.

The pig iron is obtained from the iron ores in the following steps,….

  1. Concentration:- It is the process of removing the impurities like clay, sand, etc. From the iron ore by washing with water.
  2. Calcination or roasting:- It is the process of expelling moisture, Co2, So2, and arsenic from the iron ore by heating in shallow kilns.
  3. Smelting:- It is the process of reducing the iron ore with carbon in the presence of a flux. The smelting is carried out in a large tower called a blast furnace.

The blast furnace is a chimney-like structure made of heavy steel plates lined with fire bricks to a thickness of 1.2 to 1.5 m. It is about 30 m high maximum internal dia. of 9 m at its widest c/s. The portion of the furnace above its widest c/s is called a stack. The top most portion of the stack is called the throat through which the charge is fed into the furnace. The portion of the Furnace above its widest cross-section is called a furnace. The portion of the Furnace below its widest cross-section is known as Bosh are the burning zone( or zone of fusion). The boss is provided with holes for a number of water-jacketed iron blowing pipe known as tuyers. The tuyers are 12 to 15 in number and are connected to bustle pipe surrounding the Furnace.

In the lower part of the Furnace( call did zone of fusion), the temperature is 12000c to 13000c. In the middle part of the Furnace( cal dead zone of absorption), the temperature is 800c to 1000c. In the upper part of the Furnace( cal dead zone of reduction), the temperature is 4000c to 7000c.At the bottom of the Furnace, the molten iron sinks down while above floats the fusible slag which protects the molten iron from oxidation. The molten iron thus produced is known as pig iron. The slag from the blast furnace consists of calcium, aluminium and ferrous silicates. It is used as a blast for railroads, mixed with tar for Road making and in cement manufacturing.

The pig iron from the blast furnace contains 90 to 92 %  of iron. The various other elements present in pig iron are carbon(1- 5)%, silicon(1 to 2)%, MN(1- 2)%, sulphur and phosphorus(1-2)%.

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