iron, metallic chemical element; symbol Fe [Lat. ferrum]; at. no. 26; at. wt. 55.845; m.p. about 1,535°C; b.p. about 2,750°C; sp. gr. 7.87 at 20°C; valence +2, +3, +4, or +6. Iron is biologically significant. Because iron is a component of hemoglobin, a red oxygen-carrying pigment of the red blood cells of vertebrates, iron compounds are important in nutrition; one cause of anemia is iron deficiency. For the history of the use of iron, see Iron Age.
Iron is a lustrous, ductile, malleable, silver-gray metal found in Group 8 of the periodic table. It is known to exist in four distinct crystalline forms (see allotropy). The most common is the α-form, which is stable below about 770°C, and has a body-centered cubic crystalline structure; it is often called ferrite. Iron is attracted by a magnet and is itself easily magnetized (see magnetism). It is a good conductor of heat and electricity. It displaces hydrogen from hydrochloric or dilute sulfuric acid, but becomes passive (loses its normal chemical activity) when treated with cold nitric acid.
Iron forms such compounds as oxides, hydroxides, halides, acetates, carbonates, sulfides, nitrates, sulfates, and a number of complex ions. It is chemically active and forms two major series of chemical compounds, the bivalent iron (II), or ferrous, compounds and the trivalent iron (III), or ferric, compounds. Ferrous sulfate heptahydrate, FeSO4·7H2O, sometimes called green vitriol, is a compound formed by the reaction of dilute sulfuric acid (formerly called oil of vitriol) with metallic iron; it is used in the manufacture of ink, in dyeing, and as a disinfectant. Ferric chloride hexahydrate, FeCl3·6H2O, is a yellow-brown crystalline compound used as a mordant in dyeing and as an etching compound. Ferric oxide, Fe2O3, is a reddish-brown powder used as a paint pigment and in abrasive rouges. Prussian blue, KFe2(CN)6, is a pigment containing the ferrocyanide complex ion. Iron rusts readily in moist air, forming a complex mixture of compounds that is mostly a ferrous-ferric oxide with the composition Fe3O4.
Iron is an abundant element in the universe; it is found in many stars, including the sun. Iron is the fourth most abundant element in the earth's crust, of which it constitutes about 5% by weight, and is believed to be the major component of the earth's core. Iron is found distributed in the soil in low concentrations and is found dissolved in groundwaters and the ocean to a limited extent. It is rarely found uncombined in nature except in meteorites, but iron ores and minerals are abundant and widely distributed.
The principal ores of iron are hematite (ferric oxide, Fe2O3) and limonite (ferric oxide trihydrate, Fe2O3·3H2O). Other ores include siderite (ferrous carbonate, FeCO3), taconite (an iron silicate), and magnetite (ferrous-ferric oxide, Fe3O4), which often occurs as a white sand. Iron pyrite (iron disulfide, FeS2) is a crystalline gold-colored mineral known as fool's gold. Chromite is a chromium ore that contains iron. Lodestone is a form of magnetite that exhibits natural magnetic properties.
Production and Refining
Iron is produced in the United States chiefly from oxide ores. For many years rich hematite ores were produced by open-pit mining in the Mesabi Range near Lake Superior. However, these ores have been largely depleted, and iron is now produced from low-grade ores that are treated to improve their quality; this process is called beneficiation. Iron ores are refined in the blast furnace. The product of the blast furnace is called pig iron and contains about 4% carbon and small amounts of manganese, silicon, phosphorus, and sulfur. About 95% of this iron is processed further to make steel, often by the open-hearth process or the Bessemer process, but more recently in the United States and other countries by the basic oxygen process or by an electric arc furnace. The balance is cast in sand molds into blocks called pigs. It is further processed in iron foundries (see casting).
Cast iron is made when pig iron is remelted in small cupola furnaces (similar to the blast furnace in design and operation) and poured into molds to make castings. It usually contains 2% to 6% carbon. Scrap iron or steel is often added to vary the composition. Cast iron is used extensively to make machine parts, engine cylinder blocks, stoves, pipes, steam radiators, and many other products. Gray cast iron, or gray iron, is produced when the iron in the mold is cooled slowly. Part of the carbon separates out in plates in the form of graphite but remains physically mixed in the iron. Gray iron is brittle but soft and easily machined. White cast iron, or white iron, which is harder and more brittle, is made by cooling the molten iron rapidly. The carbon remains distributed throughout the iron as cementite (iron carbide, Fe3C). A malleable cast iron can be made by annealing white iron castings in a special furnace. Some of the carbon separates from the cementite; it is much more finely divided than in gray iron. A ductile iron may be prepared by adding magnesium to the molten pig iron; when the iron is cast the carbon forms tiny spherical nodules around the magnesium. Ductile iron is strong, shock resistant, and easily machined.
Wrought iron is commercially purified iron. In the Aston process, pig iron is refined in a Bessemer converter and then poured into molten iron silicate slag. The resulting semisolid mass is passed between rollers that squeeze out most of the slag. The wrought iron has a fibrous structure with threads of slag running through it; it is tough, malleable, ductile, corrosion resistant, and melts only at high temperatures. It is used to make rivets, bolts, pipes, chains, and anchors, and is also used for ornamental ironwork.
See W. H. Dennis, Metallurgy of the Ferrous Metals (1963) and Foundations of Iron and Steel Metallurgy (1967).