Encyclopedia
Iron is a
chemical element with the symbol
Fe and atomic number 26. Iron is a group 8 and period 4
metal. Iron are notable for being the final elements produced by
stellar nucleosynthesis, and thus 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 the Earth.
Notable characteristics
Iron is the second most abundant metal on
Earth , and is believed to be the tenth most abundant
element in the
universe. Iron is also the second most abundant element by mass, making up 34% of the mass of 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 create its
magnetic field.
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 used in the production of
steel, an
alloy or
solid solution of different metals, and some non-metals, particularly
carbon. The many iron-carbon
allotropes, 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 nucleides is, however,
56Fe. This is formed by nuclear fusion in the stars. Although a further tiny energy gain could be extracted by synthesizing
62Ni, conditions in stars are not right for this process to be favoured, and iron abundance on Earth greatly favors iron over nickel, and also presumably in supernova element production. 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. Some 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.
Iron is a necessary trace element used by all living organisms. Iron-containing enzymes, usually containing
heme prosthetic groups, participate in cataysis of oxidation reactions in biology, and in transport of a number of soluble gases. See
hemoglobin and cytochrome.
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:
| Critical temperature [i] || 994 K [i]
...
. 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 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. 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 sheroids, 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
| Critical temperature [i] || 994 K [i]
...
, 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 oxide
...
s 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 blueing are some techniques used to protect iron from rust by excluding
water and
oxygen or by sacrificial protection.
History
The first signs of use of iron come from the
Sumerians and the
Egyptians, where around 4000 BCE, a few items, such as the tips of
spears,
daggers 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". Even if this is not the case, the word is likely a loan into pre-
Proto-Germanic from
Celtic or
Italic . The meteoric origin of Iron in its first use by humans is also alluded to in the
Quran : "and We sent down Iron, in which is mighty war, as well as many benefits for mankind" .
Ancient Greeks considered Halybes to be "the inventors of iron". The people of the Caucasian Isthmus, Khaldi people were one of the oldest west-
Georgian tribes .
By 3500 BCE to 2000 BCE, increasing numbers of smelted iron objects 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
Iliad, weaponry is mostly
bronze, but iron ingots are used for trade. Some resources suggest that iron was being created then as a by-product of
copper refining, as sponge iron, and was not reducible by the metallurgy of the time. By 1600 BCE to 1200 BCE, 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 BCE, 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 iron working 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. Classical authors ascribe the first invention of ironsmithing to peoples of the Caucasus and eastern Anatolia, such as the Khaldi and the Khalib .
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 archaeological evidence for items made of wrought iron appearing in the northwest, near Xinjiang, in the 8th century BCE. 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 , 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.
Iron was used in India as early as 250 BCE. The famous iron pillar in the
Qutb complex in
Delhi is made of very pure iron and has not rusted or eroded till this day.
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 .
Cast iron development lagged in Europe, as the smelters could only achieve temperatures of about 1000 C; or perhaps they did not want hotter temperatures, as they were seeking to produce blooms as a precursor of
wrought iron, not
cast iron. Through a good portion of the Middle Ages, in Western Europe, iron was thus still being made by the working of iron blooms 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 CE.
Cast iron was then made into
wrought iron by the osmond process. Some scholars have speculated the practice 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 used
charcoal as both the heat source and the reducing agent. In 18th century England, wood supplies became inadequate to enable the industry to expand and coke, a fossil fuel, began to be used an alternative. This innovation is associated with Abraham Darby at
Coalbrookdale in 1709, but it was only later in the century that economically viable means of converting
pig iron to
bar iron were devised. The most successful such process was Henry Cort's
puddling process, patented in 1784. Those processes permitted the great expansion in the production of iron that constitutes the
Industrial Revolution for that industry.
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.
See also .Production of iron from iron ore
Industrially, iron is produced starting from iron
ores, principally
hematite and
magnetite by a carbothermic reaction 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 furnace 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 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 and dolomite . 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 :
- 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 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.
Pig iron is later reduced into
steel, using convertitors.
Approximately 1100Mt 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 state, Fe2- * the Iron state, Fe242-.
- the Iron state, Fe5, Fe5.
- the Iron state, [Fe5NO]2+.
- the Iron state, Fe2+, previously ferrous is very common.
- the Iron state, Fe3+, previously ferric, is also very common, for example in rust.
- the Iron state, Fe4+, previously ferryl, stabilized in some enzymes .
Note that despite the chemical formula, the iron in the common
pyrite FeS
2 is
not in the +4 oxidation state; the sulfur is in the -1 oxidation state.
Iron carbide Fe
3C is known as cementite.
See also .Isotopes
Naturally occurring iron consists of four isotopes: 5.845% of radioactive
54Fe , 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 . Much of the past work on measuring the isotopic composition of Fe has centered on determining
60Fe variations due to processes accompanying
nucleosynthesis and ore formation.
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.
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 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 . For this reason,
57Fe has application as a spin isotope in chemistry and biochemistry.
Iron in biology
Iron is essential to all known
organisms. 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.
Many animals incorporate iron into the
heme complex, an essential component of cytochromes, which are proteins involved in
redox reactions , 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 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 ,
ribonucleotide reductase ,
hemerythrins and purple acid phosphatase . When the body is fighting a bacterial infection, the body sequesters iron inside of cells so that it cannot be used by bacteria.
Iron distribution is heavily regulated in mammals, as a defense against bacterial infection and also because of the potential biological toxicity of iron. The iron absorbed from the duodenum binds to transferrin, and is carried by
blood to different cells. There it gets by an as yet unknown mechanism incorporated into target proteins. . A lengthier article on the system of human iron regulation can be found in the article on
human iron metabolism.
Nutrition and dietary sources
Good sources of dietary iron include
meat,
fish,
poultry,
lentils,
beans,
leaf vegetables,
tofu,
chickpeas,
black-eyed pea,
strawberries and farina.
Iron provided by dietary supplements is often found as Iron fumarate. Iron sulfate is as well absorbed, and less expensive. The most bioavailable form of iron supplement is iron amino acid chelate. The RDA for iron varies considerably based on the age, gender, and source of dietary iron . Also note the section below on precautions.
Precautions
Excessive iron is toxic to humans, because excess ferrous iron reacts with peroxides in the body, producing
free radicals. Iron becomes toxic when it exceeds the amount of transferrin needed to free bound iron. In excess, uncontrollable quantities of free radicals are produced.
Iron uptake is tightly regulated by the human body, which has no physiologic means of excreting iron and regulates iron solely by regulating uptake. However, too much ingested iron can damage the cells of the
gastrointestinal tract directly, and may enter the bloodstream by damaging the cells that would otherwise regulate its entry. Once there, it causes damage to cells in the
heart,
liver and elsewhere. This can cause serious problems, including the potential of death from overdose, and long-term organ damage in survivors.
Humans experience iron toxicity above 20 milligrams of iron for every kilogram of weight, and 60 milligrams per kilogram is a lethal dose. Over-consumption of iron, often the result of children eating large quantitities of ferrous sulfate tablets intended for adult consumption, is the most common toxicological cause of death in children under six. The DRI lists the Tolerable Upper Intake Level for adults as 45
mg/day. For children under fourteen years old the UL is 40 mg/day.
If iron intake is excessive iron overload disorders can sometimes result, such as
hemochromatosis. Iron overload disorders require a genetic inability to regulate iron uptake; however, many people have a genetic susceptibility to iron overload without realizing it and without knowing a family history of the problem. For this reason, people should not 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.
The medical management of iron toxicity is complex. One element of the medical approach is a specific
chelating agent called deferoxamine, used to bind and expel excess iron from the body in case of iron toxicity.
References
General references
- H. R. Schubert, History of the British Iron and Steel Industry ... to 1775 AD
- R. F. Tylecote, History of Metallurgy .
- R. F. Tylecote, 'Iron in the Industrial Revolution' in J. Day and R. F. Tylecote, The Industrial Revolution in Metals , 200-60.
Inline references
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See also
External links
