Iron
Iron is a
chemical element with the symbol
Fe (
L.: Ferrum) and
atomic number 26. Iron is a
group 8 and
period 4 metal. Iron is notable for being the final element produced by
stellar nucleosynthesis, and thus the heaviest element which does not require a
supernova or similarly cataclysmic event for its formation. It is therefore the most abundant
heavy metal in the universe.
Iron is the 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.
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. 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 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 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 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, 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
blueing are some techniques used to protect iron from rust by excluding
water and
oxygen or by
sacrificial protection.
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 (OE
īsern), 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 (Krahe
IF 46:184f. compares
Old Irish,
Illyrian,
Venetic and
Messapic forms). 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 (material for) mighty war, as well as many benefits for mankind" (57:25).
Ancient Greeks considered Halybes to be "the inventors of iron". The people of the Caucasian Isthmus, Khaldi people (or Khalib/Halyb and Halisones by
Strabo) were one of the oldest west-
Georgian tribes (4th to 2nd millennia BC).
By 3500 BCE to 2000 BCE, 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
Iliad, 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 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 (
Chaldei) and the
Khalib (
Chalybes).
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 (ca 550 BCE), 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 (98%) and has not rusted or eroded till this day.
|
This blast furnace in eastern Missouri consumed up to 11,000 tons of ore and 16,000 cords of wood annually from 1827 to 1891. |
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 BCE).
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.
|
The red appearance of this water is due to iron in the rocks. |
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 iron minerals. |
How Iron was extracted in the 19th Century |
Industrially, iron is extracted from its
ores, principally
hematite (nominally Fe
2O
3) and
magnetite (Fe
3O
4) 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 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
COThe 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
CO2The 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 +
CO2Then calcium oxide combines with silicon dioxide to form a
slag.
CaO +
SiO2 '
CaSiO3The 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 worldin 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.
Common
oxidation states of iron include:
* the
Iron(-II) state, Fe
2- (e.g. Fe(CO)
42-,Fe(CO)
2(NO)
2.
* the
Iron(-I) state, Fe
2(CO)
42-.
* the
Iron(0) state, Fe(CO)
5, Fe(PF
3)
5.
* the
Iron(I) state, [Fe(H
2O)
5NO]
2+.
* the
Iron(II) state, Fe
2+, previously
ferrous is very common.
* the
Iron(III) state, Fe
3+, previously
ferric, is also very common, for example in
rust.
* the
Iron(IV) state, Fe
4+, previously
ferryl, stabilized in some enzymes (e.g.
peroxidases).
Note that despite the chemical formula, the iron in the common
pyrite is
not in the +4 oxidation state; the sulfur is in the -1 oxidation state.
* the
Iron(VI) state, Fe
6+ is also known, if rare, in
potassium ferrate.
Iron carbide Fe
3C is known as
cementite.
See also Iron compounds.Naturally occurring iron consists of four
isotopes: 5.845% of radioactive
54Fe (half-life: >3.1×10
22 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.
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 (−1/2). For this reason,
57Fe has application as a spin isotope in chemistry and biochemistry.
Iron is essential to all known
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. Iron can also help prevent lack of air in the lungs.
Many animals incorporate iron into the
heme complex, an essential component of
cytochromes, which are proteins involved in
redox reactions (including but not limited to
cellular 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 inside of cells (mostly stored in the storage molecule
ferritin) 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.
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 (II) fumarate. Iron sulfate is somewhat better absorbed, but the sulfur can upset the stomach. The most bioavailable form of iron supplement (ten to fifteen times more bioavailable than any other) is iron amino acid chelate.
. Also note the section below on
precautions.
Metallic iron filings are added to some
breakfast cereals and listed in the ingredients as "reduced iron" ("reduced" referring to
redox chemistry). If the cereal is crushed, the iron filings can be separated with a magnet.
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.
* WebElements.com – Iron
* It's Elemental – Iron
* The Most Tightly Bound Nuclei
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