Iron

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Iron (IPA: /ˈaiə(ɹ)n/) is a chemical element with the symbol Fe (Latin: ferrum) and atomic number 26. Iron is a group 8 and period 4 metal. Iron is a lustrous, silvery soft metal. Iron and nickel are notable for being the final elements produced by stellar nucleosynthesis, and thus are the heaviest elements which do not require a supernova or similarly cataclysmic event for formation. Iron and nickel are therefore the most abundant metals in metallic meteorites and in the dense-metal cores of planets such as Earth.

Iron is one of the few ferromagnetic elements.

[edit] Characteristics

Iron is believed to be the tenth most abundant element in the universe, and the fourth most abundant in the Earth's crust. The concentration of iron in the various layers in the structure of the Earth ranges from high (probably greater than 80%, perhaps even a nearly pure iron crystal) at the inner core, to only 5% in the outer crust. Iron is second in abundance to aluminium among the metals and fourth in abundance in the crust. Iron is the most abundant element by mass of our entire planet, making up 35% of the mass of the Earth as a whole.

Iron is a metal extracted from iron ore, and is almost never found in the free elemental state. In order to obtain elemental iron, the impurities must be removed by chemical reduction. Iron is the main component of steel, and it is used in the production of alloys or solid solutions of various metals, as well as some non-metals, particularly carbon. The many iron-carbon alloys, which have very different properties, are discussed in the article on steel.

Nuclei of iron have some of the highest binding energies per nucleon, surpassed only by the nickel isotope ⁶²Ni. The universally most abundant of the highly stable nuclides is, however, ⁵⁶Fe. This is formed by nuclear fusion in stars. Although a further tiny energy gain could be extracted by synthesizing ⁶²Ni, conditions in stars are unsuitable for this process to be favoured, and iron abundance on Earth greatly favors iron over nickel, and also presumably in supernova element production. ^{[citation needed]} When a very large star contracts at the end of its life, internal pressure and temperature rise, allowing the star to produce progressively heavier elements, despite these being less stable than the elements around mass number 60, known as the "iron group". This leads to a supernova.

Iron (as Fe²⁺, ferrous ion) is a necessary trace element used by almost all living organisms, the only exceptions are a few prokaryotic organisms which live in iron-poor conditions (such as the lactobacilli in iron-poor milk) which use manganese for catalysis, instead. Iron-containing enzymes, usually containing heme prosthetic groups, participate in catalysis of oxidation reactions in biology, and in transport of a number of soluble gases. See hemoglobin, cytochrome, and catalase.

[edit] Applications

Iron is the most used of all the metals, comprising 95% of all the metal tonnage produced worldwide. Its combination of low cost and high strength make it indispensable, especially in applications like automobiles, the hulls of large ships, and structural components for buildings. Steel is the best known alloy of iron, and some of the forms that iron can take include:

• Pig iron has 4% – 5% carbon and contains varying amounts of contaminants such as sulfur, silicon and phosphorus. Its only significance is that of an intermediate step on the way from iron ore to cast iron and steel.
• Cast iron contains 2% – 4.0% carbon , 1% – 6% silicon , and small amounts of manganese. Contaminants present in pig iron that negatively affect material properties, such as sulfur and phosphorus, have been reduced to an acceptable level. It has a melting point in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together. Its mechanical properties vary greatly, dependent upon the form carbon takes in the alloy. 'White' cast irons contain their carbon in the form of cementite, or iron carbide. This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken carbide, a very pale, silvery, shiny material, hence the appellation. In grey iron the carbon exists free as fine flakes of graphite, and also renders the material brittle due to the stress-raising nature of the sharp edged flakes of graphite. A newer variant of grey iron, referred to as ductile iron is specially treated with trace amounts of magnesium to alter the shape of graphite to spheroids, or nodules, vastly increasing the toughness and strength of the material.
• Carbon steel contains between 0.4% and 1.5% carbon, with small amounts of manganese, sulfur, phosphorus, and silicon.
• Wrought iron contains less than 0.2% carbon. It is a tough, malleable product, not as fusible as pig iron. It has a very small amount of carbon, a few tenths of a percent. If honed to an edge, it loses it quickly. Wrought iron is characterised, especially in old samples, by the presence of fine 'stringers' or filaments of slag entrapped in the metal. Wrought iron does not rust particularly quickly when used outdoors. It has largely been replaced by mild steel for "wrought iron" gates and blacksmithing. Mild steel does not have the same corrosion resistance but is cheaper and more widely available.
• Alloy steels contain varying amounts of carbon as well as other metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc. They are used for structural purposes, as their alloy content raises their cost and necessitates justification of their use. Recent developments in ferrous metallurgy have produced a growing range of microalloyed steels, also termed 'HSLA' or high-strength, low alloy steels, containing tiny additions to produce high strengths and often spectacular toughness at minimal cost.
• Iron(III) oxides are used in the production of magnetic storage media in computers. They are often mixed with other compounds, and retain their magnetic properties in solution.

The main drawback to iron and steel is that pure iron, and most of its alloys, suffer badly from rust if not protected in some way. Painting, galvanization, plastic coating and bluing are some techniques used to protect iron from rust by excluding water and oxygen or by sacrificial protection.

Iron is believed to be the critical missing nutrient in the ocean that limits the growth of plankton. Experimental iron fertilization of areas of the ocean using iron(II) sulfate has proven successful in increasing plankton growth^[1][2][3]. Larger scaled efforts are being attempted with the hope that iron seeding and ocean plankton growth can remove carbon dioxide from the atmosphere, thereby counteracting the greenhouse effect that is generally agreed by climatologists to cause global warming^[4].

[edit] Iron compounds

See also iron compounds.

• Iron(III) acetate (Fe(C₂H₃O₂)₃ is used in the dyeing of cloth.

• Iron(III) ammonium oxalate (Fe(NH₄)₃(C₂O₄)₄) is used in blueprints.

• Iron(III) arsenate (FeAsO₄) is used in insecticide.

• Iron(III) chloride (FeCl₃) is used: in water purification and sewage treatment, in the dyeing of cloth, as a coloring agent in paints, as an additive in animal feed, and as an etching material for engravement, photography and printed circuits.

• Iron(III) chromate (Fe₂(CrO₄)₃) is used as a yellow pigment for paints and ceramic.

• Iron(III) hydroxide (Fe(OH)₃) is used as a brown pigment for rubber and in water purification systems.

• Iron(III) phosphate (FePO₄) is used in fertilizer and as an additive in human and animal food.

• Iron(II) acetate (Fe(C₂H₃O₂)₂ is used in the dyeing of fabrics and leather, and as a wood preservative.

• Iron(II) gluconate (Fe(C₆H₁₁O₇)₂) is used as a dietary supplement in iron pills.

• Iron(II) oxalate (FeC₂O₄) is used as yellow pigment for paints, plastics, glass and ceramic, and in photography.

• Iron(II) sulfate (FeSO₄) is used in water purification and sewage treatment systems, as a catalyst in the production of ammonia, as an ingredient in fertilizer and herbicide, as an additive in animal feed, in wood preservative and as an additive to flour to increase iron levels.

• Iron-Fluorine complex (FeF₆)^3- is found in solutions containing both Fe(III) ions and fluoride ions.

[edit] Historical aspects

Main article: History of ferrous metallurgy

The first iron used by mankind, far back in prehistory, came from meteors. The smelting of iron in bloomeries probably began in Anatolia or the Caucasus in the second millennium BC or the latter part of the preceding one. Cast iron was first produced in China about 550 BC, but not in Europe until the medieval period. During the medieval period, means were found in Europe of producing wrought iron from cast iron (in this context known as pig iron) using finery forges. For all these processes, charcoal was required as fuel.

Steel (with a smaller carbon content than pig iron but more than wrought iron) was first produced in antiquity. New methods of producing it by carburizing bars of iron in the cementation process were devised in the 17th century AD. In the Industrial Revolution, new methods of producing bar iron without charcoal were devised and these were later applied to produce steel. In the late 1850s, Henry Bessemer invented a new steelmaking process, involving blowing air through molten pig iron, to produce mild steel. This and other 19th century and later processes have led to wrought iron no longer being produced.

[edit] Occurrence

Iron is one of the most common elements on Earth, making up about 5% of the Earth's crust. Most of this iron is found in various iron oxides, such as the minerals hematite, magnetite, and taconite. The earth's core is believed to consist largely of a metallic iron-nickel alloy. About 5% of the meteorites similarly consist of iron-nickel alloy. Although rare, these are the major form of natural metallic iron on the earth's surface.

The reason for Mars's red colour is thought to be an iron-oxide-rich soil.

See also Iron minerals.

[edit] Production of iron from iron ore

Main article: Blast furnace

Industrially, iron is produced starting from iron ores, principally haematite (nominally Fe₂O₃) and magnetite (Fe₃O₄) by a carbothermic reaction (reduction with carbon) in a blast furnace at temperatures of about 2000 °C. In a blast furnace, iron ore, carbon in the form of coke, and a flux such as limestone (which is used to remove impurities in the ore which would otherwise clog the furnace with solid material) are fed into the top of the furnace, while a blast of heated air is forced into the furnace at the bottom.

In the furnace,(hot/oven) the coke reacts with oxygen in the air blast to produce carbon monoxide:

2 C + O₂ → 2 CO

The carbon monoxide reduces the iron ore (in the chemical equation below, hematite) to molten iron, becoming carbon dioxide in the process:

3 CO + Fe₂O₃ → 2 Fe + 3 CO₂

The flux is present to melt impurities in the ore, principally silicon dioxide sand and other silicates. Common fluxes include limestone (principally calcium carbonate) and dolomite (calcium-magnesium carbonate). Other fluxes may be used depending on the impurities that need to be removed from the ore. In the heat of the furnace the limestone flux decomposes to calcium oxide (quicklime):

CaCO₃ → CaO + CO₂

Then calcium oxide combines with silicon dioxide to form a slag.

CaO + SiO₂ → CaSiO₃

The slag melts in the heat of the furnace, which silicon dioxide would not have. In the bottom of the furnace, the molten slag floats on top of the more dense molten iron, and apertures in the side of the furnace are opened to run off the iron and the slag separately. The iron once cooled, is called pig iron, while the slag can be used as a material in road construction or to improve mineral-poor soils for agriculture.

Pig iron is not pure iron, but has 4-5% carbon dissolved in it. This is subsequently reduced to steel or commercially pure iron, known as wrought iron, using other furnaces or converters.

In 2005, approximately 1,544Mt (million tons) of iron ore was produced worldwide. China was the top producer of iron ore with at least one-fourth world share followed by Brazil, Australia and India, reports the British Geological Survey.

[edit] Isotopes

Main article: isotopes of iron

Naturally occurring iron consists of four isotopes: 5.845% of radioactive ⁵⁴Fe (half-life: >3.1×10²² years), 91.754% of stable ⁵⁶Fe, 2.119% of stable ⁵⁷Fe and 0.282% of stable ⁵⁸Fe. ⁶⁰Fe is an extinct radionuclide of long half-life (1.5 million years).

Much of the past work on measuring the isotopic composition of Fe has centered on determining ⁶⁰Fe variations due to processes accompanying nucleosynthesis (i.e., meteorite studies) and ore formation. In the last decade however, advances in mass spectrometry technology have allowed the detection and quantification of minute, naturally-occurring variations in the ratios of the stable isotopes of iron. Much of this work has been driven by the Earth and planetary science communities, although applications to biological and industrial systems are beginning to emerge.^[5]

The isotope ⁵⁶Fe is of particular interest to nuclear scientists. A common misconception is that this isotope represents the most stable nucleus possible, and that it thus would be impossible to perform fission or fusion on ⁵⁶Fe and still liberate energy. This is not true, as both ⁶²Ni and ⁵⁸Fe are more stable, being the most stable nuclei. However, since ⁵⁶Fe is much more easily produced from lighter nuclei in nuclear reactions, it is the endpoint of fusion chains inside extremely massive stars and is therefore common in the universe, relative to other metals.

In phases of the meteorites Semarkona and Chervony Kut a correlation between the concentration of ⁶⁰Ni, the daughter product of ⁶⁰Fe, and the abundance of the stable iron isotopes could be found which is evidence for the existence of ⁶⁰Fe at the time of formation of the solar system. Possibly the energy released by the decay of ⁶⁰Fe contributed, together with the energy released by decay of the radionuclide ²⁶Al, to the remelting and differentiation of asteroids after their formation 4.6 billion years ago. The abundance of ⁶⁰Ni present in extraterrestrial material may also provide further insight into the origin of the solar system and its early history. Of the stable isotopes, only ⁵⁷Fe has a nuclear spin (−1/2).

[edit] Iron in organic synthesis

The usage of iron metal filings in organic synthesis is mainly for the reduction of nitro compounds.^[6] Additionally, iron has been used for desulfurizations^[7], reduction of aldehydes^[8], and the deoxygenation of amine oxides^[9].

[edit] Iron in biology

Main article: human iron metabolism

Iron is essential to nearly all known organisms. In cells, iron is generally stored in the centre of metalloproteins, because "free" iron -- which binds non-specifically to many cellular components -- can catalyse production of toxic free radicals.

In animals, plants, and fungi, iron is often incorporated into the heme complex. Heme is an essential component of cytochrome proteins, which mediate redox reactions, and of oxygen carrier proteins such as hemoglobin, myoglobin, and leghemoglobin. Inorganic iron also contributes to redox reactions in the iron-sulfur clusters of many enzymes, such as nitrogenase (involved in the synthesis of ammonia from nitrogen and hydrogen) and hydrogenase. Non-heme iron proteins include the enzymes methane monooxygenase (oxidizes methane to methanol), ribonucleotide reductase (reduces ribose to deoxyribose; DNA biosynthesis), hemerythrins (oxygen transport and fixation in marine invertebrates) and purple acid phosphatase (hydrolysis of phosphate esters).

Iron distribution is heavily regulated in mammals, partly because iron has a high potential for biological toxicity. Iron distribution is also regulated because many bacteria require iron, so restricting its availability to bacteria (generally by sequestering it inside cells) can help to prevent or limit infections. This is probably the reason for the relatively low amounts of iron in mammalian milk. A major component of this regulation is the protein transferrin, which binds iron absorbed from the duodenum and carries it in the blood to cells.^[10]

[edit] Nutrition and dietary sources

Good sources of dietary iron include red meat, fish, poultry, lentils, beans, leaf vegetables, tofu, chickpeas, black-eyed peas, potatoes with skin, bread made from completely whole-grain flour, molasses, teff and farina. Iron in meat is more easily absorbed than iron in vegetables.^[11]

Iron provided by dietary supplements is often found as iron (II) fumarate, although iron sulfate is cheaper and is absorbed equally well. Elemental iron, despite being absorbed to a much smaller extent (stomach acid is sufficient to convert some of it to ferrous iron), is often added to foods such as breakfast cereals or "enriched" wheat flour (where it is listed as "reduced iron" in the list of ingredients). Iron is most available to the body when chelated to amino acids - iron in this form is ten to fifteen times more bioavailable than any other, and is also available for use as a common iron supplement. Often the amino acid chosen for this purpose is the cheapest and most common amino acid, glycine, leading to "iron glycinate" supplements.^[12] The RDA for iron varies considerably based on age, gender, and source of dietary iron (heme-based iron has higher bioavailability)^[13]. Infants will require iron supplements if they are not breast-fed. Blood donors are at special risk of low iron levels and are often advised to supplement their iron intake.

[edit] Regulation of iron uptake

Excessive iron can be toxic, because free ferrous iron reacts with peroxides to produce free radicals, which are highly reactive and can damage DNA, proteins, lipids, and other cellular components. Thus, iron toxicity occurs when there is free iron in the cell, which generally occurs when iron levels exceed the capacity of transferrin to bind the iron.

Iron uptake is tightly regulated by the human body, which has no physiological means of excreting iron, so controls iron levels solely by regulating uptake. Although uptake is regulated, large amounts of ingested iron can cause excessive levels of iron in the blood, because high iron levels can cause damage to the cells of the gastrointestinal tract that prevents them from regulating iron absorption. High blood concentrations of iron damage cells in the heart, liver and elsewhere, which can cause serious problems, including long-term organ damage and even death.

Humans experience iron toxicity above 20 milligrams of iron for every kilogram of mass, and 60 milligrams per kilogram is a lethal dose.^[14] Over-consumption of iron, often the result of children eating large quantities of ferrous sulfate tablets intended for adult consumption, is one of the most common toxicological cause of death in children under six^[14]. The DRI lists the Tolerable Upper Intake Level (UL) for adults as 45 mg/day. For children under fourteen years old the UL is 40 mg/day.

Regulation of iron uptake is impaired in some people as a result of a genetic defect that maps to the HLA-H gene region on chromosome 6. In these people, excessive iron intake can result in iron overload disorders, such as hemochromatosis. Many people have a genetic susceptibility to iron overload without realizing it or being aware of a family history of the problem. For this reason, it is advised that people should not take iron supplements unless they suffer from iron deficiency and have consulted a doctor. Hemochromatosis is estimated to cause disease in between 0.3 and 0.8% of Caucasians. ^[15]

The medical management of iron toxicity is complex, and can include use of a specific chelating agent called deferoxamine to bind and expel excess iron from the body.

[edit] Bibliography

• Los Alamos National Laboratory — Iron
• H. R. Schubert, History of the British Iron and Steel Industry ... to 1775 AD (Routledge, London, 1957)
• R. F. Tylecote, History of Metallurgy (Institute of Materials, London 1992).
• R. F. Tylecote, 'Iron in the Industrial Revolution' in J. Day and R. F. Tylecote, The Industrial Revolution in Metals (Institute of Materials 1991), 200-60.
• http://www.webelements.com/webelements/elements/text/Fe/xtal.html