Iron ( ) is a
metallic
chemical element with the symbol
Fe ( ) and
atomic
number 26. Iron is a
group 8 and
period 4 element and is therefore
classified as a
transition metal.
Iron and iron
alloys (
steels) are by far the most common
metals and the most common
ferromagnetic materials in everyday use. Fresh
iron surfaces are lustrous and silvery-grey in color, but oxidize
in air to form a red or brown coating of
ferric oxide or
rust. Pure
single crystals of iron are soft (softer than
aluminium), and the addition of minute amounts of
impurities, such as carbon, significantly strengthens them.
Alloying iron with appropriate small amounts (up to a few per cent)
of other metals and carbon produces
steel,
which can be 1,000 times harder than pure iron.
Iron-56 is the heaviest stable isotope
produced by the
alpha process in
stellar nucleosynthesis;
heavier elements than iron and nickel require a
supernova for their formation. Iron is the most
abundant element in the core of
red
giants, and is the most abundant metal in
iron meteorites and in the dense metal
cores of planet such as
Earth.
Characteristics
Pure iron is a
metal but is rarely found in
this form on the surface of the
earth because
it oxidizes readily in the presence of oxygen and
moisture. In order to obtain metallic iron,
oxygen must be removed from naturally
occurring
ores by chemical
reduction – mainly of the
iron ore hematite ( ) by
carbon at high temperature. The properties of
iron can be modified by
alloying it with
various other metals (and some
non-metals,
notably carbon and
silicon) to form
steels.
Nuclei of iron atoms 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. Elemental distribution on Earth greatly
favours iron over nickel, and also presumably in supernova element
production.
Iron (as Fe
2+,
ferrous ion)
is a necessary
trace element used by
almost all living organisms. The only exceptions are several
organisms that live in iron-poor environments and have evolved to
use different elements in their metabolic processes, such as
manganese instead of iron for catalysis, or
hemocyanin instead of hemoglobin. 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.
Mechanical properties
Mechanical properties of iron and its alloys are evaluated using a
variety of tests, such as the
Brinell
test,
Rockwell test, or
tensile strength tests, among others; the
results are so consistent that tests of iron are often used to
relate the results of one test to another. Those measurements
reveal that mechanical properties of iron crucially depend on
purity: Purest research-purpose single crystals of iron are softer
than aluminium. Addition of only 10
parts per million of carbon doubles their
strength. The hardness increases rapidly with carbon content up to
0.2% and saturates at ~0.6%. The purest industrially produced iron
(about 99.99% purity) has a hardness of 20–30 Brinell.
Allotropes
Iron represents perhaps the best-known example of
allotropy in a metal. There are three allotropic
forms of iron, known as α, γ and δ.
As molten iron cools down it crystallizes at 1538 °C into its
δ allotrope, which has a
body-centred cubic (bcc) crystal
structure. As it cools further its
crystal structure changes to
face-centred cubic (fcc) at
1394 °C, when it is known as γ-iron, or
austenite. At 912 °C the crystal structure
again becomes bcc as α-iron, or
ferrite, is formed, and at 770 °C (the
Curie point, T
c) the iron
becomes
magnetic. As the iron passes
through the Curie temperature there is no change in crystalline
structure, but there is a change in "domain structure", where each
domain contains iron atoms with a particular electronic spin. In
unmagnetized iron, all the electronic spins of the atoms within one
domain are in the same direction; however, in neighbouring domains
they point in various directions and thus cancel out. In magnetized
iron, the electronic spins of all the domains are all aligned, so
that the magnetic effects of neighbouring domains reinforce each
other. Although each domain contains billions of atoms, they are
very small, about 10 microns across.
Iron is of greatest importance when mixed with certain other metals
and with carbon to form steels. There are many types of steels, all
with different properties; and an understanding of the properties
of the
allotropes of iron is key
to the manufacture of good quality steels.
Alpha iron, also known as ferrite, is the most stable form of iron
at normal temperatures. It is a fairly soft metal that can dissolve
only a small concentration of carbon (no more than 0.021% by mass
at 910 °C).
Above 912 °C and up to 1400 °C α-iron undergoes a
phase transition from body-centred
cubic to the face-centred cubic configuration of γ-iron, also
called
austenite. This is similarly soft
and metallic but can dissolve considerably more carbon (as much as
2.04% by mass at 1146 °C). This form of iron is used in the
type of
stainless steel used for
making cutlery, and hospital and food-service equipment.
Occurrence
Iron is the sixth most
abundant element in the
Universe, formed as the final act of
nucleosynthesis, by
silicon fusing in massive stars.
While it makes up about 5% of the
Earth's crust, the
Earth's core is believed to consist largely of
an iron-
nickel alloy constituting 35% of the
mass of the Earth as a whole. Iron is consequently the most
abundant element on Earth, but only the fourth most abundant
element in the Earth's crust. Most of the iron in the crust is
found combined with oxygen as
iron oxide
minerals such as
hematite and
magnetite.
About 1 in 20
meteorites consist of the
unique iron-nickel minerals
taenite (35–80%
iron) and
kamacite (90–95% iron). Although
rare,
iron meteorites are the major
form of natural metallic iron on the Earth's surface.
The red color of the surface of
Mars is thought
to derive from an iron oxide-rich
regolith.
Isotopes
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 centred 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.
The most abundant iron 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
56Ni is much more
easily produced from lighter nuclei in the
alpha process in
nuclear reactions in supernovae (see
silicon burning process),
nickel-56 (14
alpha particles) is the
endpoint of fusion chains inside
extremely massive stars,
since addition of another alpha would result in zinc-60, which
requires a great deal more energy. This nickel-56, which has a
half-life of about 6 days, is therefore made in quantity in
these stars, but soon decays by two successive positron emissions
within supernova decay products in the
supernova remnant gas cloud, to first
radioactive cobalt-56, and then stable iron-56. This last nuclide
is therefore common in the universe, relative to other stable
metals of approximately the same
atomic weight.
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).
Chemistry and compounds
Iron forms compounds mainly in the +2 and +3
oxidation states. Traditionally, iron(II)
compounds have been called
ferrous, and
iron(III) compounds
ferric. There are many
compounds in each of the oxidation states; representative examples
would include
iron sulfate
(FeSO
4) and
iron
chloride (FeCl
3). There are also numerous examples
of compounds that contain iron atoms in both of these oxidation
states, such as
magnetite and
prussian blue. The
ferrate anion contains an iron(VI) centre, its
highest known oxidation state, and is present, for example in
potassium ferrate
(K
2FeO
4). There are numerous
organometallic compounds (such as
iron pentacarbonyl) that contain
formally zerovalent (or lower) iron.
History
The first
wrought iron used by mankind
during
prehistory came from meteors. The
smelting of iron in
bloomeries began in the second millennium BC.
Artifacts
from smelted iron occur in India from
1800–1200 BC, and in the Levant from about
1500 BC (suggesting smelting in Anatolia
or the
Caucasus).
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 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.
Industrial production
The production of iron or steel is a process unless the desired
final product is cast iron. The first stage is to produce pig iron
in a blast furnace. The second is to make wrought iron or steel
from pig iron by a further process.
Blast furnace
Ninety percent of all
mining of metallic
ores is for the extraction of iron.
Industrially, iron is produced starting from iron 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 (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.
Iron output in 2005
In the furnace, 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. In the bottom of the
furnace, the molten slag floats on top of the denser 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.
How Iron was extracted in the 19th
century
This heap of iron ore pellets will be
used in steel production.
In 2005, approximately 1,544 million
metric tons of iron ore were 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.
Further processes
Pig iron is not pure iron, but has 4–5% carbon dissolved in it with
small amounts of other impurities like sulfur, magnesium,
phosphorus and manganese. As the carbon is the major impurity, the
iron (pig iron) becomes brittle and hard. This form of iron is used
to cast articles in
foundries such as
stoves, pipes, radiators, lamp-posts and rails.
Alternatively pig iron may be made into steel (with up to about 2%
carbon) or wrought iron (commercially pure iron). Various processes
have been used for this, including
finery
forges,
puddling furnaces,
Bessemer converters,
open hearth furnaces,
basic oxygen furnaces, and
electric arc furnaces. In all cases,
the objective is to oxidize some or all of the carbon, together
with other impurities. On the other hand, other metals may be added
to make alloy steels.
The hardness of the steel depends upon its carbon content, the
higher the proportion of carbon, the greater the hardness and the
lesser the ductility. The properties of the steel can also be
changed by tempering it. To harden the steel, it is heated to red
hot and then cooled by quenching it in the water. It becomes harder
and more brittle. This steel is then heated to a required
temperature and allowed to cool. The steel thus formed is less
brittle.
Applications
Elemental iron
Iron is the most widely used of all the metals, accounting for 95%
of worldwide metal production. Its low cost and high strength make
it indispensable in engineering applications such as the
construction of machinery and
machine
tools,
automobiles, the
hull of large
ships, and
structural components for
buildings. Since
pure iron is quite soft, it is most commonly used in the form of
steel. Some of the forms in which iron is produced commercially
include:
- Pig iron has 3.5–4.5% 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% 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.
- Wrought iron contains less than
0.25% carbon. It is a tough, malleable product, but not as fusible
as pig iron. If honed to an edge, it loses it quickly. Wrought iron
is characterized by the presence of fine fibres of slag entrapped in the metal. Wrought iron is more
corrosion resistant than steel. It has been almost completely
replaced by mild steel for traditional
"wrought iron" products and blacksmithing. Mild steel corrodes more readily
that wrought iron, but is cheaper and more widely available.
- Carbon steel contains 2.0% carbon
or less, with small amounts of manganese,
sulfur, phosphorus,
and silicon.
- Alloy steels contain varying amounts
of carbon as well as other metals, such as chromium, vanadium,
molybdenum, nickel, tungsten, etc. Their alloy content raises their
cost, and so they can usually only be justified for specialist
uses. 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.
The main disadvantage of iron and steel is that pure iron, and most
of its alloys, suffer badly from
rust if not
protected in some way.
Painting,
galvanization,
passivation, plastic coating and
bluing are some techniques used to protect
iron from rust by excluding
water and oxygen
or by sacrificial protection.
Iron compounds
Iron chloride hexahydrate
- Iron oxides (FeO, Fe3O4, and
Fe2O3) are ores used for iron production (see
bloomery and blast
furnace). They are also used as a catalyst in the Space Shuttle Solid Rocket
Boosters, Solid rocket boosters and in the production of
magnetic storage media in
computers. They are often mixed with other compounds, and retain
their magnetic properties in solution.
- Iron(II) acetate (Fe(CH3CO2)2
(ferrous acetate) is used as a
mordant in the dyeing of
cloth and leather, and as a wood preservative.
- Iron ammonium citrate
(C6H5+4yFexNyO7)
is used in blueprints.
- Iron(III) arsenate (FeAsO4) is used in insecticides.
- Iron chloride
(FeCl3) is used in water
purification and sewage treatment,
in the dyeing of cloth, as a colouring agent in paints, as an additive in animal feed, and as an
etchant for copper in the manufacture of printed circuit boards.
- Iron(III) chromate
(Fe2(CrO4)3) is a yellow pigment
for paints and ceramics.
- Iron hydroxide
(Fe(OH)3) is used as a brown pigment for rubber and in
water purification systems.
- Iron phosphate
(FePO4) is used in fertilizers
and as an additive in human and animal food.
- Iron 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 ceramics, and in photography.
- Iron sulfate (FeSO4)
is used in water purification and sewage treatment systems, as a
catalyst in the production of ammonia, as an ingredient in fertilizer, herbicide, and moss killer, as
an additive in animal feed, in wood preservative, and as an additive to
flour to increase nutritional iron levels.
Experimental iron fertilization
of areas of the ocean using iron
sulfate has proven successful in increasing plankton growth.
The use of iron compounds in
organic
synthesis is mainly for the
reduction of
nitro compounds.
Article Additionally, iron has been used for
desulfurizations,
reduction of
aldehydes,
Article and the
deoxygenation of
amine
oxides.
Biological role
Iron is essential to nearly all known
organisms. In
cell,
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.
Iron deficiency can lead
to
iron deficiency
anemia.
In animals, plants, and fungi, iron is often the metal ion
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 invertebrate) 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
sequester 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.
Dietary sources
Good sources of dietary iron include
red
meat,
fish,
poultry,
lentils,
beans,
leaf vegetables,
tofu,
chickpeas,
black-eyed peas, fortified bread, and
fortified
breakfast cereals. Iron
in low amounts is found in
molasses,
teff and
farina.
Iron in meat (haem iron) is more easily absorbed than iron in
vegetables, but heme/
hemoglobin from
red meat has effects which may increase the
likelihood of
colorectal
cancer.
Iron provided by
dietary
supplements is often found as
iron
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. The
RDA for iron varies
considerably based on age, gender, and source of dietary iron
(heme-based iron has higher
bioavailability). Infants may require iron
supplements if they are bottle-fed cow's milk.
Blood donors and pregnant women are at
special risk of low iron levels and are often advised to supplement
their iron intake.
Regulation of uptake
Iron uptake is tightly
regulated by the human body, which has no regulated physiological
means of excreting iron. Only small amounts of iron are lost daily
due to mucosal and skin epithelial cell sloughing, so control of
iron levels is mostly by regulating uptake.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 do 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.
MRI finds that iron accumulates in the
hippocampus of the brains of those with
Alzheimer's disease and in the
substantia nigra of those with
Parkinson disease.
Precautions
Large amounts of ingested iron can cause excessive levels of iron
in the blood. High blood levels of free ferrous iron react 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. Damage to the cells of the
gastrointestinal tract can also
prevent them from regulating iron absorption leading to further
increases in blood levels. Iron typically damages cells in the
heart,
liver and
elsewhere, which can cause significant adverse effects, including
coma,
metabolic
acidosis,
shock,
liver failure,
coagulopathy,
adult respiratory distress
syndrome, 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 considered a
lethal
dose. Overconsumption 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 causes of death in children under six.
The
Dietary Reference
Intake (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.
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.
See also
References
Books
- Doulias PT, Christoforidis S, Brunk UT, Galaris D. Endosomal
and lysosomal effects of desferrioxamine: protection of HeLa cells
from hydrogen peroxide-induced DNA damage and induction of
cell-cycle arrest. Free Radic Biol Med. 2003;35:719-28.
- 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.
External links
_chloride_hexahydrate.jpg/180px-Iron(III)_chloride_hexahydrate.jpg)
, where 20% of the world's accessible iron and
magnesium is located.
- A historic landmark in Omaha, Nebraska
utilizing an iron facade.
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