Combined iron is found most notably as hematite (Fe2O3), limonite (Fe2O3.H2O), magnetite (Fe3O4), and taconite, and also as siderite, goethite, chromite, and pyrite. It is extracted by smelting oxide ores in a blast furnace to produce pig iron which may be refined to produce cast iron or wrought iron, or converted to steel in the Bessemer process (now obsolete), open-hearth process, or Linz-Donawitz process.
Iron is nearly always used in the form of alloys, which are harder and stronger than the pure metal. Cast iron, wrought iron, and steel are the chief alloys made by adding carbon and small quantities of other elements to the iron. Except in special steels, such as stainless steel and tool steels, the percentage of iron in these alloys is very high – usually well over 95 percent. But it was only in the 19th century that any fairly cheap method of producing these alloys in large quantities was discovered.
Pure iron is very little used; it is chemically reactive, and oxidizes to rust in moist air. It has four allotropes (see allotropy). The stable oxidation states of iron are +2 (ferrous) and +3 (ferric), though +4 and +6 states are known. The ferrous ion (Fe2+) is pale green in aqueous solution; it is mild reducing agent, and does not readily form ligand complexes.
Compounds of ironIron (II) sulfate (FeSO4.7H2O), also known as green vitriol or copperas, is a green crystalline solid, made by treating iron ore with sulfuric acid, used in tanning, in medicine to treat iron-deficiency anemia, and to make ink, fertilizers, pesticides, and other iron compounds. Melting point 64°C. Iron (III) sulfate is used in solution to stop bleeding, for example in pulpotomy of primary teeth.
The ferric ion (Fe3+) is yellow in aqueous solution; it resembles the aluminum ion, being acidic and forming stable ligand complexes, especially with cyanides.
Iron oxide is any of three compounds that exist in three different oxidation states: iron (II) oxide (ferrous oxide, FeO); iron (III) oxide (ferric oxide, Fe2O3); and ferrosoferric oxide (Fe3O4), which has iron in both oxidation states. Iron (III) oxide is a red-brown powder used as a pigment and as jewelers' rouge; it occurs naturally as hematite. Melting point 1,565°C.
Iron and lifeIron is required as a trace element by terrestrial living organisms. It is the most abundant metal in humans with healthy adults possessing some 3–4 g. Most of this occurs in the oxygen-carrying pigment hemoglobin found in red blood cells. It is also a constituent as cytochromes.
Iron is contained in a variety of foods, such as liver, meat, cereals (especially whole-grain), fish, green leafy vegetables, nuts, and beans. During pregnancy, iron supplements may be necessary for the healthy development of the baby.
Iron as a construction material
The first cast-iron bridge, built over the River Severn at Coalbrookdale, England, in 1779, represented the beginnings of iron as a construction material. The key to cheap iron was the development of blast furnaces and a smelting process using coke instead of coal. This was introduced from 1709, also at Coalbrookdale, by the Englishman Abraham Darby.
Related category INORGANIC CHEMISTRY
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