Iron

Notable Characteristics

A typical iron atom has 56 times the mass of a typical hydrogen atom. Iron is believed to be the 10th most common element in the universe. Fe is an abbreviation for 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 that has sufficient mass to produce iron does so, it can no longer produce energy in its core and a supernova will ensue.

Iron is the most common metal in the universe.

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 sulphur 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.
• Ferric 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.

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. 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, 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 word, 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.

Iron is extracted from the ore hematite, which is mainly Fe₂O₃. The iron is separated from the ore by reduction with carbon - a more reactive element. This process takes place in a blast furnace at temperatures of about 2000°C.

First coke, mainly carbon, is entered in to the furnace and reacts with the oxygen to form carbon dioxide:

C + O₂ → CO₂

Then the carbon dioxide reacts with more carbon to form carbon monoxide:

CO₂ + C → 2CO

Carbon monoxide goes on to actually react with the haematite ore:

3CO + Fe₂O₃ → 2Fe + 3CO₂

The iron is tapped off as a liquid. However it is still not pure, the main impurity being sand (silicon dioxide). This is reacted with limestone (CaCO₃) to form slag First the limestone (CaCO₃) is decomposed to calcium monoxide[?] and carbon dioxide:

CaCO₃ → CaO + CO₂

Then the calcium monoxide reacts with the silicon dioxide forming slag, which can be used for road-building or fertiliser:

CaO + SiO₂ → CaSiO₃

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 ferrous state, Fe⁺²,
• the ferric state, Fe⁺³,
• the ferryl state, Fe⁺⁴, stabilized in some enzymes (e.g. peroxidases).
• Iron (VI) is also known, if rare. In the form of potassium-ferrate (K₂FeO₄) it is a selective oxidator for primary alcohols. The solid is only stable under vacuum and dark purple in both (caustic) solution and as a solid.
• iron carbide Fe₃C is known as cementite.

Essential in animals, where iron is the central atom in the heme molecule, and therefore an essential component of all hemoproteins. Inorganic iron is also found in the iron-sulfur clusters of many enzymes.

Bacteria often use iron. When the body is fighting a bacterial infection, the body "hides" iron so it cannot be used by the bacteria.

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 abundance of ⁶⁰Ni (daughter product of ⁶⁰Fe) present in extraterrestrial material may provide 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 2 year old is about 3 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.

If iron intake is uncontrollably high, a disease results, called hemochromatosis. The iron excess accumulates in the liver, with siderosis[?] and organ damage as a result. For this reason, people shouldn't take iron supplements unless they suffer from iron deficiency.

External links

• WebElements.com - Iron (http://www.webelements.com/webelements/elements/text/Fe/index)
• EnvironmentalChemistry.com - Iron (http://environmentalchemistry.com/yogi/periodic/Fe)
• It's Elemental - Iron (http://education.jlab.org/itselemental/ele026)