Iron

Iron is a metallic chemical element of atomic number 26. Its symbol is Fe, atomic weight is 55.847, specific gravity is 7.874, melting point is 2,795°F (1,535°C), and boiling point is 4,982°F (2,750°C).

Iron is one of the transition metals, occurring in group 8 of the periodic table . Four naturally occurring isotopes exist with atomic weights of 54 (5.8%), 56 (91.7%), 57 (2.2%), and 58 (0.3%). In addition, six radioactive isotopes have been prepared, with atomic weights of 52, 53, 55, 59, 60, and 61. The element was originally known by its Latin name ferrum, from which its chemical symbol is derived.

General properties

Iron is a silver-white or gray metal that is malleable and ductile. In a pure form, it is relatively soft and slightly magnetic. When hardened, it becomes much more magnetic. Iron is the most widely used of all metals. Prior to its use, however, it must be treated in some way to improve its properties or it must be combined with one or more other elements to form an alloy . By far the most common alloy of iron is steel .

One of the most common forms of iron is pig iron, produced by smelting iron ore with coke and limestone in a blast furnace. Pig iron is approximately 90% pure iron and is used primarily in the production of cast iron and steel.

Cast iron is a term used to describe various forms of iron that also contain carbon and silicon ranging in concentrations from 0.5-4.2% of the former and 0.2-3.5% of the latter. Cast iron has a vast array of uses ranging from thin rings to massive turbine bodies. Wrought iron contains small amounts of a number of other elements including carbon, silicon, phosphorus , sulfur , chromium, nickel, cobalt, copper , and molybdenum. Wrought iron can be fabricated into a number of forms and is widely used because of its resistance to corrosion .

Sources of iron

Iron is the fourth most abundant element in the earth's crust and the second most abundant metal, after aluminum . It makes up about 6.2% of the crust by weight. In addition, iron is thought to be the primary constituent of the earth's core as well as of siderite meteorites. Soil samples taken from the Moon indicate that about 0.5% of lunar soil consists of iron.

The primary ores of iron are hematite (Fe2O3), magnetite (Fe3O4), limonite (FeO(OH) • nH2O), and siderite (FeCO3). The element also occurs as the sulfide, iron pyrite (FeS), but this compound is not used commercially as a source of iron because of the difficulty in reducing the sulfide to the pure element. Iron pyrite has a beautiful golden appearance and is sometimes mistaken for elemental gold. This appearance explains its common name of fool's gold. Taconite is a low-grade ore of iron that contains no more than about 30% of the metal.

In nature, oxides, sulfides, and silicates of iron are often converted to other forms by the action of water . Iron(II) sulfate (FeSO4) and iron(II) bicarbonate (Fe(HCO3)2) are the most commonly found of these.

How iron is obtained

Iron is one of the handful of elements that was known to ancient civilizations. Originally it was prepared by heating a naturally occurring ore of iron with charcoal in a very hot flame. The charcoal was obtained by heating wood in the absence of air. There is some evidence that this method of preparation was known as early as 3,000 b.c., but the secret of ore smelting was carefully guarded within the Hittite civilization of the Near East for almost two more millennia.

When the Hittite civilization fell in about 1200 b.c., the process of iron ore smelting spread throughout eastern and southern Europe . Ironsmiths were soon making ornamental objects, simple tools, and weapons from iron. So dramatic was the impact of this new technology on human societies that the period following 1200 b.c. is generally known as the Iron Age.

A major change in the technique for producing iron from its ores occurred in about 1773. As trees (and therefore the charcoal made from them) grew increasingly scarce in Great Britain, the English inventor Abraham Darby (1678?-1717) discovered a method for making coke from soft coal . Since coal was abundant in the British Isles, Darby's technique insured a constant supply of coal for the conversion of iron ores to the pure metal. The modern production of iron involves heating iron ore with coke and limestone in a blast furnace, where temperatures range from 392°F (200°C) at the top of the furnace to 3,632°F (2,000°C) at the bottom. Some blast furnaces are as tall as 15-story buildings and can produce 2,400 tons of iron per day.

Inside a blast furnace, a number of chemical reactions occur. One of these involves the reaction between coke (nearly pure carbon) with oxygen to form carbon monoxide . This carbon monoxide then reacts with iron ore to form pure iron and carbon dioxide . Limestone is added to the reaction mixture to remove impurities in the iron ore. The product of this reaction, known as slag, consists primarily of calcium silicate. The iron formed in a blast furnace exists in a molten form known as pig iron that can be drawn off at the bottom of the furnace. The slag is also molten but less dense than the iron. It is drawn off from taps just above the outlet from which the molten iron is removed.

Efforts to use pig iron for commercial and industrial applications were not very successful. The material was quite brittle and objects of which it was made tended to break easily. Cannons made of pig iron, for example, were likely to blow apart when they fired a shell. By 1760, inventors had begun to find ways of toughening pig iron. These methods involved remelting the pig iron and then burning off the carbon that remained mixed with the product. The most successful early device for accomplishing this step was the Bessemer converter, named after its English inventor Henry Bessemer (1813-1898). In the Bessemer converter, a blast of hot air is blown through molten pig iron. The process results in the formation of stronger forms of iron, cast and wrought iron. More importantly, when additional elements, such as manganese and chromium, are added to the converter, a new product—steel—is formed.

Later inventions improved on the production of steel by the Bessemer converter. In the open hearth process, for example, a charge of molten pig iron, hematite, scrap iron, and limestone is placed into a large brick container. A blast of hot air or oxygen is then blown across the surface of the molten mixture. Chemical reactions within the molten mixture result in the formation of either pure iron or, with the addition of alloying metals such as manganese or chromium, a high grade of steel.

An even more recent variation on the Bessemer converter concept is the basic oxygen process (BOP). In the BOP, a mixture of pig iron, scrap iron, and scrap steel is melted in a large steel container and a blast of pure oxygen is blown through the container. The introduction of alloying metals makes possible the production of various types of steel with many different properties.

How we use iron

Alloyed with other metals, iron is the most widely used of all metallic elements. The way in which it is alloyed determines the uses to which the final product is put. Steel, for example, is a general term used to describe iron alloyed with carbon and, in some cases, with other elements. The American Iron and Steel Institute recognizes 27 standard types of steel. Three of these are designated as carbon steels that may contain, in addition to carbon, small amounts of phosphorus and/or sulfur. Another 20 types of steel are made of iron alloyed with one or more of the following elements: chromium, manganese, molybdenum, nickel, silicon, and vanadium. Finally, four types of stainless and heat-resisting steels contain some combination of chromium, nickel, and manganese alloyed with iron.

Steel is widely used in many types of construction. It has at least six times the strength of concrete , another traditional building material, and about three times the strength of special forms of high-strength concrete. A combination of these two materials, reinforced concrete, is one of the strongest of all building materials available to architects. The strength of steel has made possible some remarkable feats of construction, including very tall buildings (skyscrapers) and bridges with very wide spans. It has also been used in the manufacture of automobile bodies, ship hulls, and heavy machinery and machine parts.

Metallurgists have also invented special iron alloys to meet very specific needs. Alloys of cobalt and iron (both magnetic materials themselves) can be used in the manufacture of very powerful permanent magnets. Steels that contain the element niobium (originally called columbium) have unusually great strength and have been used, among other places, in the construction of nuclear reactors. Tungsten steels are also very strong and have been used in the production of high-speed metal cutting tools and drills. The alloying of aluminum with iron produces a material that can be used in AC magnetic circuits since it can gain and lose magnetism very quickly.

Metallic iron also has other applications. Its natural magnetic properties make it suitable for both permanent magnets and electromagnets. It is also used in the production of various types of dyes, including blueprint paper and a variety of inks, and in the manufacture of abrasives .

Biochemical applications

Iron is essential to the survival of all vertebrates . Hemoglobin, the molecule in blood that transports oxygen from the lungs to an organism's cells, contains a single iron atom buried deep within its complex structure. When humans do not take in sufficient amounts of iron in their daily diets, they may develop a disorder known as anemia . Anemia is characterized by a loss of skin color , a weakness and tendency to faint, palpitation of the heart , and a general sense of exhaustion.

Iron is also important to the good health of plants. It is found in a group of compounds known as porphyrins that play an important role in the growth and development of plant cells. Plants that lack iron have a tendency to loose their color, become weak, and die.

Chemistry and compounds

Iron typically displays one of two valences in forming compounds, 2+ and 3+. According to the older system of chemical nomenclature, these classes of compounds are known as the ferrous and ferric salts, of iron respectively. Because of the abundance of oxygen in the atmosphere, most naturally occurring iron compounds tend to be in the higher (3+) oxidation state .

One of the most widely used of iron compounds is iron(III) (or ferric) chloride, FeCl3. When added to water, it reacts with water molecules forming a thick, gelatinous precipitate of iron(III) hydroxide. The compound is used in the early steps of water purification since, as the precipitate settles out of solution , it traps and carries with it organic and inorganic particles suspended in the water. Iron(III) chloride is also used as a mordant, a substance used in dyeing that binds a dye to a textile. In gaseous form the compound has still another use. It attacks and dissolves metal and can be used, therefore, for etching. Printed circuits, for example, are often first etched with iron(III) chloride.

Iron(II) (ferrous) compounds tend to oxidize rather easily and are, therefore, less widely used than their 3+ cousins. Iron(II) (ferrous) sulfate is an important exception. In solid form, the compound tends not to oxidize as readily as other Fe2+ compounds and is used as an additive for animal feeds, in water purification, in the manufacture of inks and pigments, and in water and sewage treatment operations.

From a commercial standpoint, probably the most important chemical reaction of iron is its tendency to oxidize. When alloys of iron (such as the steels) are used in construction, a major concern is that they tend to react with oxygen in the air, forming a coating or iron oxide, or rust. The rusting process is actually a somewhat complex process in which both oxygen and water are involved. If one or the other of these materials can be prevented from coming into contact with iron, oxidation will not occur. But if both are present, an electrochemical reaction is initiated, and iron is converted to iron oxide.

Each year, billions of dollars are lost when iron-containing structural elements degrade or disintegrate as a result of oxidation (rusting). It is hardly surprising, therefore, that a number of techniques have been developed for reducing or preventing rusting. These techniques include painting, varnishing, galvanizing, tinning, and enameling.

Resources

books

Greenwood, N. N., and A. Earnshaw. Chemistry of the Elements. 2nd ed. Oxford: Butterworth-Heinneman Press, 1997.

Hawley, Gessner G., ed. The Condensed Chemical Dictionary. 9th ed. New York: Van Nostrand Reinhold, 1977.

Joesten, Melvin D., et al. World of Chemistry. Philadelphia: Saunders, 1991.

Knepper, W. A. "Iron." Kirk-Othmer Encyclopedia of ChemicalTechnology. 4th ed. Suppl. New York: John Wiley & Sons, 1998.

Seely, Bruce Edsall, ed. Iron and Steel in the Twentieth Century. New York: Facts on File, 1994.

David E. Newton

KEY TERMS

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Blast furnace

—A structure in which a metallic ore (often, iron ore) is reduced to the elemental state.

Ductile

—Capable of being drawn or stretched into a thin wire.

Isotopes

—Two molecules in which the number of atoms and the types of atoms are identical, but their arrangement in space is different, resulting in different chemical and physical properties.

Malleable

—Capable of being rolled or hammered into thin sheets.

Transition metal

—An element found between groups IIA and IIIA in the periodic table.