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IRON

Iron is a silvery, malleable and a ductile metallic transition element. It is characterised by

1. Atomic number 26
2. Relative atomic mass 55.847
3. Relative density 7.87
4. Melting point 1535^OC
5. Boiling point 2750^OC
6. Is highly magnetic, but loses this property if heated above 766^OC

OCCURRENCE

Iron is the fourth most abundant element and the second most abundant metal. Because of its high reactivity iron rarely occurs in the free state. Iron occurs free (in meteorites) and chemically combined oxides, carbonates, silicates and sulphides. Iron mainly occurs in ores as, magnetite (Fe₂O₄), Haematite (Fe₂O₃), Siderite (FeCO₃) and limonite (Fe₂O₃.3H₂O). Of all these ores, magnetite is the most important as it contains about 70% iron.

EXTRACTION

Iron is extracted from haematite. One of the main impurities in this ore is sand (SiO₂). In order to remove sand the ore is reduced in a blast furnace to form pig iron or cast iron.

The charge in the furnace consists of hematite, coke (which acts as a fuel and reducing agent, limestone (flux, which removes the sand (acid) impurities as slag) and air blasts to provide for the combustion.

The reactions taking place in the blast furnace include the following :

C + O₂ è CO₂

₁₃₀₀^O_C

CO₂ + C è 2CO

₁₂₅₀^O_C

FeO + CO è Fe + CO₂

₈₀₀^O_{C - 1000}^O_C

CaCO₃ è CaO + CO₂

CaO + SiO₂ è CaSiO₃

CO₂ + C è 2CO

Fe₃O₄ + CO è 3FeO + CO₂

₂₅₀^O_C

3Fe₂O₃ + CO è 2Fe₃O₄ + CO₂

NOTE: Iron saturated with carbon melts at temperatures ranging from 1150^OC to 1250^OC.

CAST IRON

The iron produced by re-melting the pig iron as it comes from the blast furnace is known as cast iron. Two types of cast iron can be produced. Rapid cooling gives white cast iron while on slow cooling we get grey cast iron which is the commercially important variety.

Cast iron contains the following impurities :

1. Carbon (2-5%)
2. Silicon (0.7-3%)
3. Sulphur (0.1-0.3%)
4. Phosphorus (0-0.3%)
5. Manganese (0.2-1%)

The white variety of cast iron is hard, brittle, and crystalline, and melts at or near 1150^OC, whereas the grey cast iron melts at or near 1150^OC.

Cast iron is used for making castings, for pipes, and for radiators used in heating building.

WROUGHT IRON

When the pig iron is melted in reverberatory furnace in an oxidizing atmosphere, the variety of iron obtained is known as wrought iron.

Fe₂O₃ + 3C è 2Fe + 3CO

In this process, the charge melts at about 1150^OC, and as the impurities burn out, the melting point rises and lumps of iron are formed. These are raked together into lumps of 75 to 100 pounds each and then removed from the furnace. The enclosed slag is then squeezed out by hammering or passing the lumps of iron through suitable rolls.

The wrought iron is fibrous, malleable and ductile. It is soft and gray in colour. It melts between 1450^OC - 1500^OC depending upon the carbon content.

Wrought iron is used for making pipes, horse-shoes, chains, and bolts and in the manufacture of crucible steel.

STEEL

Steel is produced when the iron containing about 0.3% to 1.7% of carbon, is heated to a suitable temperature and then quenched. In this process it is hardened and toughened. Varieties of steel can be manufactured using different methods of heating and quenching.

Steels can be are classified as follows

1. BESSEMER STEEL

It is manufactured by the following method. Molten pig iron is poured into a movable egg-shaped furnace ( called the converter). A blast of air entering through tuyeres in the bottom is forced through the charge, as a result of this the carbon, silicon and certain other impurities are burned out. After the process is completed spelegeleisen, an alloy of iron, manganese, and carbon, is added to furnish the desired amount of carbon and manganese.Bessemer Steel is used in the manufacture of rails, wire bars, axles, reinforcements for concrete, etc.

1. BASIC OPEN-HEARTH STEEL

In order to produce this, pig iron, scrap iron and some iron oxide are heated in an open-hearth furnace lined with fire bricks. The heat for smelting the charge is supplied by a gas flame on either side of the furnace, which is reflected from the low-arched-ceiling on to the charge. The time required for each charge is determined by analysis of samples taken from the furnace ( it usually comes to be 10 to 15 hours)

NOTE For many uses open hearth steel is superior to Bessemer steels because its composition is more uniform.

Open-Hearth Steel is used in the manufacture of wire, nails, axles, rails, etc.

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