Iron

Aug 26, 2022, 16:45 IST

About Iron 

The pig iron is remelted and cast or poured into moulds. This is known as cast iron. Hence, after remelting the pig iron becomes cast iron.When the molten pig iron is cooled at once, the iron is called white cast Iron, which contains carbon in the form of cementite, Fe and when the molten pig iron is cooled slowly and slowly, the iron is called as grey cast iron, which contains carbon in the form of graphite.

Symbol of Iron 

According to IUPAC the symbol of iron is Fe. Which is represented in periodic table 

Types of iron:

There are three commercial varieties of iron depending on their carbon content.( Cast iron: It is the most impure form of iron and contains the highest percentage of carbon from 2.5 to 5 per cent and about 2 per cent of other impurities like Si, P, Mn and S. Cast iron is of two types:

(a) White cast iron: Carbon is present in the form of cementite, Fe

(b) Grey cast iron: Carbon is present in the form of graphite. Cast iron melts at about 1250°C (due to presence of impurities) whereas pure iron melts at 1530°C. The molten cast iron expands on solidification and hence it produces good castings. Various articles such as stoves, pipes, radiators, railway sleepers, gutter pipes, toys, etc., are prepared from cast iron. Cast iron does not rust easily and neither be tempered. Due to high carbon content, it is hard and brittle and cannot be welded. It has very little ductility and thus cast iron is not suitable for forging.

(if) Wrought iron: It is the purest form of iron. It contains the lowest percentage of carbon from 0.1 to 0.25 per cent and 0.3 per cent other impurities. It is manufactured from cast iron by puddling process.

Wrought iron is manufactured in a special type of reverberatory f called puddling furnace, the hearth of which is lined with haematite, Fe The cast iron is melted on the hearth of the furnace by the hot gases and stirred with long iron rods. The impurities of cast iron are rapidly oxidised by oxygen of haematite (lining). Oxides of carbon and sulphur being volatile escape while those of Mn, P and Si form slag.

3C + Fe2O3  2Fe + 3CO

3Si + 2Fe2O3  3SiO2 + 4Fe

3Mn + fE2O3  3MnO + 2Fe

MnO + SiO2  MnSiO3 (slag)

6P + 5Fe2O3  3P2O5 + 10Fe

P2O3 + Fe2O3  2FePO4 (slag)

With the removal of impurities, the melting point of the metal rises and it becomes a semi-solid mass. The semi-solid mass is taken out in the form of balls and is beaten under steam hammers to squeeze out as much of slag as possible. This produces almost pure iron known as wrought iron.

Properties of Iron

(i) Wrought iron is extremely tough, highly malleable and ductile. (ii) It softens at about 1000°C and then it can be forged and welded. (iii) On account of the presence of very small percentage of slag, it has fibrous structure and thus, can withstand high stresses. (iv) Wrought iron is resistant towards rusting and corrosion.

Uses of Iron 

 It is used to make chains, nails, hooks, bolts, agricultural implements, electromagnets, bars, wires, etc.

Passivity of Iron

The following are the common properties of iron.

(a) It evolves hydrogen gas, when made to react with dilute HCI or dilute H2SO4.

(b) It precipitates silver from silver nitrate solution and copper from copper sulphate solution.

But if a piece of iron is first dipped in concentrated nitric acid for sometime and then made to react with the above reagents, neither hydrogen is evolved nor silver or copper are precipitated. Thus, iron by treatment with concentrated nitric acid has lost its usual properties or it has been rendered inert or passive. Such behaviour is not only shown by iron but also by many other metals like Cr, Co, Ni, Al, etc. This phenomenon is known as passivity and the chemical substances which bring passivity are called passivators.

“The inertness exhibited by metals under conditions when chemical activity is to be expected is called chemical passivity.” Iron can be rendered passive by other oxidising agents like chromic acid, KMnO4 conc. H etc. The passivity of the iron is believed to be due to formation of an extremely thin film (invisible) of oxide on the surface of iron. Passive iron can be made active by scratching or heating in a reducing atmosphere of H2 or CO2 or heating in HNO3 upto 75°C.

Compounds of Iron

Ferrous sulphate (Green vitriol), FeSO4)

This is the best known ferrous salt. It occurs in nature as copperas and is formed by the oxidation of pyrites under the action of water and atmospheric air.

2FeS2 + 7O2 + 2H2O  2FeSO4 + 2H2SO4

It is commonly known as harakasis.

Uses : (i) Ferrous sulphate is used for making blue black ink. The ink is prepared by mixing a solution of tannin and ferrous sulphate. A colourless iron salt of tannic acid is formed. A blue dye (usually indigo) is added. The writing with this ink is blue but soon on exposure to air, it becomes black on account of oxidation of colourless ferrous salt into black ferric salt.

(ii) It is used as a mordant in dyeing.

(iii) It is also used as an insecticide in agriculture.

(iv) It is employed as a laboratory reagent and in the preparation of Mohr’s salt.

About Steel

This is the most important commercial variety of iron. The percentage of carbon in this form of iron is midway between that of cast iron and wrought iron, i.e., 0.25 to 2 per cent. There are many varieties of steel depending on the amount of carbon present in it.

(a) Mild steels: These contain low percentage of carbon. Such steels show the properties of wrought iron along with elasticity and hardness.

(b) Hard steels: These contain high percentage of carbon. They are hard and brittle.

(c) Special steels or alloy steels: Steel mixed with small amount of nickel, cobalt, chromium, tungsten, molybdenum. manganese, etc., acquires special properties. Such products are called special steels or alloy steels. Some important alloy steels are listed below