Pig Iron: Meaning and Impurities | Metals | Industries | Metallurgy

In this article we will discuss about:- 1. Meaning of Pig Iron 2. Impurities in Pig Iron and their Effects 3. Distinction of Pig Iron from Cast Iron.

Meaning of Pig Iron:

All iron produced in blast furnace whether in molten state or cast into pigs is called pig iron. Iron ore, coke and limestone are charged into a blast furnace to produce pig iron. The composition of pig iron varies with the quality of ore and operation of the blast furnace. Due to the reducing conditions in the furnace, the impurities in the ore get reduced and incorporated with iron.

These impurities are mainly phosphorus, silicon, manganese, sulphur and carbon. The composition of slag, percentage of fuel and regulation of the blast furnace determine the quantities of phosphorus, sulphur, silicon, manganese and carbon.

The presence of these impurities in the pig iron effects the properties of iron and these must be removed to improve the quality of iron. Pig iron can be regarded as an impure form of cast iron and it is the raw material for practically all iron and steel products.

Impurities in Pig Iron and their Effects:

1. Phosphorus:

The quantity of phosphorus present in pig iron varies from 0.1 to 2.0%. It combines with iron to form Fe₃P which embrittles cast iron. Phosphorus is introduced into iron from phosphate in the ore. Phosphates get reduced in the reducing atmosphere in the blast furnace and phosphorus is formed.

This phosphorus combines with iron to form Ee₃P. Some of the phosphorus may get oxidised too, into P₂O₅. This oxidation of phosphorus to P₂O₅ is an exothermic reaction and takes place at low temperature. P₂O₅, if formed, is removed by the slag which is basic in character. In this way the quantity of phosphorus in iron is reduced. Its amount is kept as low as possible.

Phosphorus when present in pig iron increases the fluidity of molten iron and thus makes the filling of the moulds in a better way. However, wrought iron or steel made from phosphate iron is brittle when cold.

2. Sulphur:

Sulphur is introduced from coke used in production of pig iron. The quantity of sulphur present in ore varies from 0.4 to 1.0% Presence of manganese in iron lowers the percentage of sulphur by combining with sulphur in iron and forming MnS. MnS formed thus combines with CaO and is removed. In this way the percentage of sulphur in iron is reduced.

The reaction of sulphur with manganese is as follows:

It may be noted that FeS has a low melting point and it forms between the grains that make up the alloy. Iron sulphide being very brittle, whole alloy becomes brittle.

Presence of sulphur tends to make iron hard and produces unsound castings. Wrought iron and steel produced from iron containing sulphur makes wrought iron and steel to be brittle when heated. It makes steel cold-short as well as hot-short and such steel can neither be cold worked nor hot worked.

3. Silicon:

Percentage of silicon present in pig iron varies from 1.0 to 4.0%. Source of silicon in iron is from its presence in any of the raw materials. At low temperatures some silicon is oxidised into silica and basic slag removes it. In this way there may occur some elimination of silicon from iron.

Silicon affects the hardness and strength of iron. Both of these properties worsen by the presence of silicon in iron. Presence of silicon in steel increases its electrical resistance. Silicon when present in steel promotes the decomposition of cementite to form graphite.

4. Manganese:

The quantity of manganese varies from 0.2 to 1.5%. The source of this impurity in iron is from the ore. Manganese dioxide in ore is reduced at higher temperature as the reduction reaction is endothermic in nature. Manganese thus formed gets mixed up with iron. Some of the manganese is removed by acid slag.

Presence of manganese in iron reduces the sulphur content of iron by forming MnS thereby improving the quality of alloy since MnS is not as harmful as FeS. Manganese increases the tensile strength of iron. Since Mn promotes combined carbon, it increases hardness of cast iron.

5. Carbon:

The quantity of carbon varies from 4 to 4.5 per cent. Source of carbon in steel is coal. Carbon is present in pig iron either in free state as graphite or in combined state as iron carbide. Carbon in pig iron increases its hardness. Due to presence of impurities in pig iron, pig iron is too brittle and possesses very little strength and ductility. So most of the pig iron is converted into steel.

Distinction of Pig Iron from Cast Iron:

Pig Iron is the direct metallic product from the blast furnace without any change in it while cast iron is prepared from pig iron with or without refining or alloying treatments, or it may be prepared from mixtures of pig iron with steel or scrap or alloying agents.