Iron

iso	NA	half-life	DM	DE M eV	DP
⁵⁴Fe	5.8%	Fe is stable with 28 neutrons
⁵⁵Fe	{syn.}	2.73 y	ε capture	0.231	⁵⁵Mn
⁵⁶Fe	91.72%	Fe is stable with 30 neutrons
⁵⁷Fe	2.2%	Fe is stable with 31 neutrons
⁵⁸Fe	0.28%	Fe is stable with 32 neutrons
⁵⁹Fe	{syn.}	44.503 d	β	1.565	⁵⁹Co
⁶⁰Fe	{syn.}	1.5E⁶ y	β^-	3.978	⁶⁰Co

SI units & STP are used except where noted.

Iron is a chemical element in the periodic table that has the symbol Fe and atomic number 26. Iron is group 8 and period 4 metal.

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Contents

1 Notable characteristics

Notable characteristics

Iron is the most abundant metal, and is believed to be the tenth most abundant element in the universe. Iron is also the most abundant (by mass, 34.6%) element making up the Earth; the concentration of iron in the various layers of the Earth ranges from high at the inner core to about 5% in the outer crust; it is possible the Earth's inner core consists of a single iron crystal although it is more likely to be a mixture of iron and nickel; the large amount of iron in the Earth is thought to contribute to its magnetic field. Its symbol Fe is an abbreviation of ferrum, the Latin word for iron.

Iron is a metal extracted from iron ore, and is hardly ever found in the free (elemental) state. In order to obtain elemental iron, the impurities must be removed by chemical reduction. Iron is used in the production of steel, which is not an element but an alloy, a solution of different metals (and some non-metals, particularly carbon).

The nucleus of iron has the highest binding energy per nucleon, so it is the heaviest element that is produced exothermically through fusion and the lightest through fission. When a star has sufficient mass to produce iron, it does so. Soon afterward, the star will no longer produce sufficient energy in its core, and a supernova will ensue.

Cosmological models with an open universe predict that there will be a phase where as a result of slow fusion and fission reactions, everything will become iron.

Applications

Iron is the most used of all the metals, comprising 95 percent 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 takes 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% – 3.5% carbon and small amounts of manganese. Contaminants present in pig iron that negatively affect the 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. It is extremely strong, hard and brittle. Working cast iron, even white hot cast iron, tends to break the object.
Carbon steel contains between 0.5% and 1.5% carbon, with small amounts of manganese, sulfur, phosphorus, and silicon.
Wrought iron contains less than 0.5% 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.
Alloy steels contain varying amounts of carbon as well as other metals, such as chromium, vanadium, molybdenum, nickel, tungsten, etc.
Iron (III) oxides are used in the production of magnetic storage in computers. They are often mixed with other compounds, and retain their magnetic properties in solution.

History

The first signs of use of iron come from the Sumerians and the Egyptians, where around 4000 BC, small items, such as the tips of spears and ornaments, were being fashioned from iron recovered from meteorites. Because meteorites fall from the sky some linguists have conjectured that the English word iron, which has cognates in many northern and western European languages, derives from the Etruscan aisar which means "the gods".

By 3000 BC to 2000 BC, increasing numbers of smelted iron objects (distinguishable from meteoric iron by the lack of nickel in the product) appear in Mesopotamia, Anatolia, and Egypt. However, their use appears to be ceremonial, and iron was an expensive metal, more expensive than gold. In The Illiad, weaponry is mostly bronze, but iron ingots are used for trade. Some resources (see the reference What Caused the Iron Age? below) suggest that iron was being created then as a by-product of copper refining, as sponge iron, and was not reproducible by the metallurgy of the time. By 1600 BC to 1200 BC, iron was used increasingly in the Middle East, but did not supplant the dominant use of bronze.

In the period from the 12th to 10th century BC, there was a rapid transition in the Middle East from bronze to iron tools and weapons. The critical factor in this transition does not appear to be the sudden onset of a superior ironworking technology, but instead the disruption of the supply of tin. This period of transition, which occurred at different times in different parts of the world, is the ushering in of an age of civilization called the Iron Age.

Concurrent with the transition from bronze to iron was the discovery of carburization, which was the process of adding carbon to the irons of the time. Iron was recovered as sponge iron, a mix of iron and slag with some carbon and/or carbide, which was then repeatedly hammered and folded over to free the mass of slag and oxidise out carbon content, so creating the product wrought iron. Wrought iron was very low in carbon content and was not easily hardened by quenching. The people of the Middle East found that a much harder product could be created by the long term heating of a wrought iron object in a bed of charcoal, which was then quenched in water or oil. The resulting product, which had a surface of steel, was harder and less brittle than the bronze it began to replace.

In China the first irons used were also meteoric iron, with archeological evidence for items made of wrought iron appearing in the northwest, near Xinjiang, in the 8th century BC. These items were made of wrought iron, created by the same processes used in the Middle East and Europe, and were thought to be imported by non-Chinese people.

In the later years of the Zhou Dynasty (ca 550 BC), a new iron manufacturing capability began because of a highly developed kiln technology. Producing blast furnaces capable of temperatures exceeding 1300 K, the Chinese developed the manufacture of cast, or pig iron.

If iron ores are heated with carbon to 1420–1470 K, a molten liquid is formed, an alloy of about 96.5% iron and 3.5% carbon. This product is strong, can be cast into intricate shapes, but is too brittle to be worked, unless the product is decarburized to remove most of the carbon. The vast majority of Chinese iron manufacture, from the Zhou dynasty onward, was of cast iron. Iron, however, remained a pedestrian product, used by farmers for hundreds of years, and did not really affect the nobility of China until the Qin dynasty (ca 221 BC).

Cast iron development lagged in Europe, as the smelters could only achieve temperatures of about 1000 K. Through a good portion of the Middle Ages, in Western Europe, iron was still being made by the working of sponge iron into wrought iron. Some of the earliest casting of iron in Europe occurred in Sweden, in two sites, Lapphyttan and Vinarhyttan, between 1150 and 1350 AD. There are suggestions by scholars that the practice may have followed the Mongols across Russia to these sites, but there is no clear proof of this hypothesis. In any event, by the late fourteenth century, a market for cast iron goods began to form, as a demand developed for cast iron cannonballs.

Early iron smelting (as the process is called) used charcoal as both the heat source and the reducing agent. In 18th century England, wood supplies ran down and coke, a fossil fuel, was used as an alternative. This innovation by Abraham Darby supplied the energy for the Industrial Revolution.

Occurrence

Iron is one of the more 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.

Industrially, iron is extracted from its ores, principally hematite (nominally Fe₂O₃) and magnetite (Fe₃O₄) by 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 are fed into the top of the furnace, while a blast of heated air is forced into the funace at the bottom.

In the furnace, the coke reacts with oxygen in the air blast to produce carbon monoxide:

6 C + 3 O₂ → 6 CO

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

6 CO + 2 Fe₂O₃ → 4 Fe + 6 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 (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 then 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 liquid iron, and spouts in the side of the furnace may be opened to drain off either the iron or the slag. 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.

Approximately 1100Mt (million tons) of iron ore was produced in the world in 2000, with a gross market value of approximately 25 billion US dollars. While ore production occurs in 48 countries, the five largest producers were China, Brazil, Australia, Russia and India, accounting for 70% of world iron ore production. The 1100Mt of iron ore was used to produce approximately 572Mt of pig iron.

Compounds

Common oxidation states of iron include:

the Iron(II) state, Fe²⁺, previously ferrous is very common.
the Iron(III) state, Fe³⁺, previously ferric, is also very common, for example in rust.

the Iron(IV) state, Fe⁴⁺, previously ferryl, stabilized in some enzymes (e.g. peroxidases).
the Iron(VI) state, Fe⁶⁺ is also known, if rare, in potassium ferrate.

iron carbide Fe₃C is known as cementite.

Biological role

Iron is essential to all organisms, except for a few bacteria. It is mostly stably incorporated in the inside of metalloproteins, because in exposed or in free form it causes production of free radicals that are generally toxic to cells. To say that iron is free doesn't mean that it is free floating in the bodily fluids. Iron binds avidly to virtually all biomolecules so it will adhere nonspecifically to cell membranes, nucleic acids, proteins etc.

Animals incorporate iron into the heme complex, an essential component of cytochromes, which are proteins involved in redox reactions (including but not limited to respiration), and of oxygen carrying proteins hemoglobin and myoglobin. Inorganic iron involved in redox reactions is also found in the iron-sulfur clusters of many enzymes, such as nitrogenase (involved in the synthesis of ammonia from nitrogen and hydrogen) and hydrogenase. A class of non-heme iron proteins is responsible for a wide range of functions within several life forms, such as 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). When the body is fighting a bacterial infection, the body sequesters iron in the transporter protein transferrin so it cannot be used by bacteria.

Iron distribution is heavily regulated in mammals. The iron absorbed from duodenum binds to transferrin, and carried by blood it reaches different cells. There it gets by an as yet unknown mechanism incorporated into target proteins. [1] (http://www.plosbiology.org/plosonline/?request=get-document&doi=10.1371%2Fjournal.pbio.0000079).

Good sources of dietary iron include meat, fish, poultry, lentils, beans, spinach, tofu, chickpeas, black-eyed pea, strawberries and farina.

Iron provided by dietary supplements is often found as Iron (II) fumarate. The RDA for iron varies considerably based on the age, gender, and source of dietary iron (heme-based iron has higher bioavailability)[2] (http://www.iom.edu/Object.File/Master/7/294/0.pdf). Also note the section below on precautions.

Isotopes

Iron has four naturally-occurring stable isotopes, ⁵⁴Fe, ⁵⁶Fe, ⁵⁷Fe and ⁵⁸Fe. The relative abundances of the Fe isotopes in nature are approximately ⁵⁴Fe (5.8%), ⁵⁶Fe (91.7%), ⁵⁷Fe (2.2%) and ⁵⁸Fe (0.3%). ⁶⁰Fe is an extinct radionuclide which had a long half-life (1.5 Myr). 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. The isotope ⁵⁶Fe is of particular interest to nuclear scientists as it represents the most stable nucleus possible. It is not possible to perform fission or fusion on ⁵⁶Fe and still liberate energy. This does not hold true for any other element. In phases of the meteorites Semarkona und 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 time formation of 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). For this reason, ⁵⁷Fe has application as a spin isotope in chemistry and biochemistry.

Precautions

Excessive dietary iron is toxic, because excess ferrous iron reacts with peroxides in the body, producing free radicals. When iron is in normal quantity, the body's own antioxidant mechanisms can control this process. In excess, uncontrollable quantities of free radicals are produced.

The lethal dose of iron in a two year old is about three grams of iron. One gram can induce severe poisoning. There are reported cases of children being poisoned by consuming between 10-50 tablets of ferrous sulfate over a few hour period. Overconsumption of iron is the single highest cause of death in children by unintentional ingestion of pharmaceuticals. 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.

If iron intake is excessive a number of iron overload disorders can result, such as hemochromatosis. For this reason, people shouldn't take iron supplements unless they suffer from iron deficiency and have consulted a doctor. Blood donors are at special risk of low iron levels and are often recommended to supplement their iron intake.

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Iron

Notable characteristics

Applications

History

Occurrence

Compounds

See also

Biological role

Isotopes

Precautions

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