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Iron (lat. Ferrum) is a chemical element in the Periodic Table of Elements that has the symbol Fe and a proton number of 26. Iron is a relatively soft, silvery-gray metal. It belongs to the transitional elements. Chemically, it is relatively reactive, it reacts with acids to release hydrogen. Under the action of atmospheric moisture and oxygen, it oxidizes to form a mixture of hydrated oxides and hydroxides of iron, called rust. This process is a big problem for the industry, as iron is one of the most used metals. Various coatings, passivation by the action of phosphoric acid (formation of a solid layer of phosphates), plating, etc. are used as protection.





The most common oxidation state of iron compounds is +2 and +3. Of the compounds, these are mainly iron oxide (FeO), iron oxide (Fe2O3), and double iron-iron oxide (Fe3O4). They are mainly used for the production of pure iron.

As a transitional element, iron forms a number of complexes, in which it occurs most often in coordination numbers 4 and 6. Iron complexes are relatively stable. Among the most well-known are potassium hexacyanoferrate (yellow blood salt) K4[Fe (CN)6], potassium hexacyanoferrate (red blood salt) K3[Fe (CN)6] – used in analytical chemistry, and iron pentacarbonyl Fe (CO)5, used for laboratory preparation of pure iron.

Other compounds used are ferrous sulfate (FeSO4), ferric chloride (FeCl3).

The occurrence of oxidation states +4, +5, +6 (K2FeO4) is also known. These compounds are very strong oxidizing agents.

Iron is one of the most abundant elements in the earth's body (5% by mass). However, its distribution is not uniform, its content increases from the surface to the core, the core itself is formed by an alloy of nickel and iron in the ratio of ~85% Fe: ~6% Ni. In nature, iron occurs in various, most often mafic rocks - basalt and minerals - pyroxenes, amphiboles, olivine. Important iron ore minerals from which iron is industrially produced are: hematite, limonite, v magnetite, siderite [1]. Many other minerals also contain iron, but its extraction from them is not profitable or energy-intensive, e.g. pyrite. The world's most important iron ore deposits are the so-called banded iron formations.

The occurrence of pure iron is rare (some deposits in Greenland and Sweden). There is also the so-called iron meteorites, in the form of alloys with nickel.

More information in the main article: pig iron


Heap of iron ore.
Iron has been known to mankind since ancient times (Hallstad and Lateen times), its larger industrial production only started in the 18th century. The above-mentioned ores (hematite, magnetite, siderite, limonite) are used for production.

Production is based on redox reactions with carbon monoxide, or carbon in the so-called blast furnaces. Treated ores, coke and limestone are dosed into the blast furnace, or silicon dioxide. The temperature in the furnace is around 1800°C and several processes take place there:

burning coke with the formation of carbon dioxide, or coal
C + O2 → CO2
2C + O2 → 2CO
reduction of iron with carbon monoxide
3 CO + Fe2O3 → 2 Fe + 3 CO2
possibly carbon
3 C + Fe2O3 → 2 Fe + 3 CO
formation of slag (protection of molten iron from reverse oxidation)
CaO + SiO2 → CaSiO3

The whole process takes place continuously, ore, coke, slag-forming ingredients are added to the upper part, and molten iron (iron tapping) is discharged from the lower part at regular intervals.

Pyrite is not used as a raw material for iron production, as it contains sulphur, which forms undesirable sulphides.

The use
Iron is the most used metal. In addition to carbon, pig iron also contains various impurities, so it does not have suitable properties for direct use. Various types of cast iron are produced by modifying it. Cast iron is relatively hard, but brittle, with little possibility of further processing. However, it has very good casting properties. Therefore, it is used for products where high machining accuracy is not required and which can be produced by casting (e.g. hatches, radiators). However, there are also types of cast iron with modified properties.

By removing carbon from pig iron, steel is obtained, which is incomparably better processable, malleable, flexible. Additions of other metals (manganese, chromium, vanadium, tungsten, cobalt, etc.) modify various mechanical properties of steel (strength, hardness, chemical resistance, and many others). We encounter steel in everyday life (transport, industry, machinery, construction).

You can find more information about the use of iron in the categories Cast Iron and Steel

Biological significance
Iron is an important biogenic element. As the central atom of the organometallic complex heme, embedded in the protein hemoglobin, it plays an important role in the organism of warm-blooded animals in the transport of oxygen from the lungs to the cells.

Hemoglobin is bound in red blood cells. Oxygen is bound to hemoglobin by a coordination bond. The problem arises if a ligand with a higher binding energy (carbon monoxide) gets into the lungs, displacing oxygen and blocking its transport.

The main source of iron in food is offal (liver, heart) and meat, but also legumes and leafy vegetables.

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