Iron (Fe) has been known since prehistoric times. It makes up 5% of the Earth's crust and is second in abundance to aluminium among the metals and fourth in abundance behind oxygen, silicon, and aluminium among the elements. It is also found in the sun and many types of stars in considerable quantity. Iron is found native only in the form of meteorites known as siderites. Its common ores are magnetic pyrites, magnetite, hematite (Fe2O3) and carbonates or iron.
Iron is obtained from its ores by fusing to drive off the oxygen, sulphur, and impurities. Melting is carried out generally in a blast furnace, directly in contact with the fuel and with the limestone as a flux. These combine with the quartz and clay, forming a slag, which is readily removed. The product is crude pig iron, and further remelting and refining produces commercially pure iron.
Iron metal is greyish in appearance and is very ductile. Small amounts of carbon will significantly alter the properties of iron. Iron containing 0.15% of chemically combined carbon is termed ‘steel’. It is very reactive chemically (it is attacked by most acids), corrodes readily which is accelerated by the presence of moist air or elevated temperatures.
Iron in its pure state is allotropic, existing as a solid in two different crystal forms. These forms and occurrence are outlined below:
- From subzero to 700°C iron has a body centred cubic crystal structure, identified as alpha (α) iron, and is magnetic
- From 700°C to 928°C iron changes from alpha (α) iron to beta (β) iron, the crystal structure remains unchanged but it loses its magnetism
- From 928°C to 1530°C iron changes to a face centred cubic crystal structure identified as gamma (γ) iron
- From 1530°C upwards the structure changes back to body centred cubic crystal structure, identified as delta (δ) iron
Common iron is a mixture of four isotopes, while ten other isotopes are known to exist.
Iron in the metal form is used in:
- As the primary constituent of ferrous metals/alloys and steels
- When alloyed with carbon, nickel, chromium and various other elements, to form cast iron or steel, it is the most versatile and popular metal used by mankind
- Heavy construction and building
- Fabricated metal products
- Industrial machinery
- Transportation equipment
- Toys, sport goods
- Carbonyl iron (Fe(CO)5) powder used for magnet core for high frequency equipment, medical applications
- Ferrocene or dicyclopentadienyl iron ((C5H5)2Fe), is used as a combustion control additive in fuels. It is also used as a heat stabiliser in lubricants and plastics and for radiation resistance
- Iron carbide is used in high wear applications, such as mine processing equipment
- Iron shot, peening shot, steel grit, steelblast, tru-steel shot, kut-steel, are used as a replacement for sand in sand blasting operations tumbling operations and for metal cleaning operations