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Pig iron, also known as crude iron, is an intermediate product of the
iron industry Ferrous metallurgy is the metallurgy of iron and its alloys. The earliest surviving prehistoric iron artifacts, from the 4th millennium BC in Egypt, were made from meteoritic iron-nickel. It is not known when or where the smelting of iron from o ...
in the production of steel which is obtained by
smelting Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and ...
iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the ...
in a
blast furnace A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric p ...
. Pig iron has a high carbon content, typically 3.8–4.7%, along with silica and other constituents of
dross Dross is a mass of solid impurities floating on a molten metal or dispersed in the metal, such as in wrought iron. It forms on the surface of low- melting-point metals such as tin, lead, zinc or aluminium or alloys by oxidation of the metal. ...
, which makes it
brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. ...
and not useful directly as a material except for limited applications. The traditional shape of the molds used for pig iron
ingot An ingot is a piece of relatively pure material, usually metal, that is cast into a shape suitable for further processing. In steelmaking, it is the first step among semi-finished casting products. Ingots usually require a second procedure of sha ...
s is a branching structure formed in sand, with many individual ingots at right angles to a central channel or "runner", resembling a litter of piglets being nursed by a sow. When the metal had cooled and hardened, the smaller ingots (the "pigs") were simply broken from the runner (the "sow"), hence the name "pig iron". As pig iron is intended for remelting, the uneven size of the ingots and the inclusion of small amounts of sand cause only insignificant problems considering the ease of
casting Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a ''casting'', which is ejected ...
and handling them.


History

Smelting and producing
wrought iron Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which give it a wood-like "grain ...
was known in ancient Europe and the Middle East, but it was produced in bloomeries by direct reduction. Pig iron was not produced in Europe before the Middle Ages. The Chinese were making pig iron by the later
Zhou dynasty The Zhou dynasty ( ; Old Chinese ( B&S): *''tiw'') was a royal dynasty of China that followed the Shang dynasty. Having lasted 789 years, the Zhou dynasty was the longest dynastic regime in Chinese history. The military control of China by ...
(which ended in 256 BC).Wagner, Donald. ''Iron and Steel in Ancient China''. Leiden 1996:
Brill Publishers Brill Academic Publishers (known as E. J. Brill, Koninklijke Brill, Brill ()) is a Dutch international academic publisher founded in 1683 in Leiden, Netherlands. With offices in Leiden, Boston, Paderborn and Singapore, Brill today publishes 2 ...
Furnaces such as
Lapphyttan Lapphyttan or Lapphyttejarn in Norberg Municipality, Sweden, may be regarded as the type site for the Medieval Blast Furnace. Its date is probably between 1150 and 1350. It produced cast iron, which was then fined to make ferritic wrought iron ...
in Sweden may date back to the 12th century; and some in Mark (today part of
Westphalia Westphalia (; german: Westfalen ; nds, Westfalen ) is a region of northwestern Germany and one of the three historic parts of the state of North Rhine-Westphalia. It has an area of and 7.9 million inhabitants. The territory of the regi ...
, Germany) to the 13th. It remains to be established whether these northern European developments derive from Chinese ones. Wagner has postulated a possible link via Persian contacts with China along the Silk Road and Viking contacts with Persia, but there is a chronological gap between the Viking period and Lapphyttan. The phase transition of the iron into liquid in the furnace was an ''avoided'' phenomenon, as decarburizing the pig iron into steel was an extremely tedious process using medieval technology.


Uses

Traditionally, pig iron was worked into
wrought iron Wrought iron is an iron alloy with a very low carbon content (less than 0.08%) in contrast to that of cast iron (2.1% to 4%). It is a semi-fused mass of iron with fibrous slag inclusions (up to 2% by weight), which give it a wood-like "grain ...
in finery forges, later puddling furnaces, and more recently, into steel.R. F. Tylecote, ''A history of metallurgy'' (2nd edition, Institute of Materials, London, 1992). In these processes, pig iron is melted and a strong current of air is directed over it while it is stirred or agitated. This causes the dissolved impurities (such as silicon) to be thoroughly oxidized. An intermediate product of puddling is known as ''refined pig iron'', ''finers metal'', or ''refined iron''. Pig iron can also be used to produce gray iron. This is achieved by remelting pig iron, often along with substantial quantities of steel and
scrap Scrap consists of recyclable materials, usually metals, left over from product manufacturing and consumption, such as parts of vehicles, building supplies, and surplus materials. Unlike waste, scrap has monetary value, especially recovered ...
iron, removing undesirable contaminants, adding alloys, and adjusting the carbon content. Some pig iron grades are suitable for producing ductile iron. These are high purity pig irons and depending on the grade of ductile iron being produced these pig irons may be low in the elements silicon, manganese, sulfur and phosphorus. These types of pig iron are used to dilute all the elements (except carbon) in a ductile iron charge which may be harmful to the ductile iron process.


Modern uses

Until recently, pig iron was typically poured directly out of the bottom of the
blast furnace A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. ''Blast'' refers to the combustion air being "forced" or supplied above atmospheric p ...
through a trough into a ladle car for transfer to the
steel mill A steel mill or steelworks is an industrial plant for the manufacture of steel. It may be an integrated steel works carrying out all steps of steelmaking from smelting iron ore to rolled product, but may also be a plant where steel semi-finish ...
in mostly liquid form; in this state, the pig iron was referred to as ''hot metal''. The hot metal was then poured into a
steelmaking Steelmaking is the process of producing steel from iron ore and carbon/or scrap. In steelmaking, impurities such as nitrogen, silicon, phosphorus, sulfur and excess carbon (the most important impurity) are removed from the sourced iron, an ...
vessel to produce steel, typically an
electric arc furnace An electric arc furnace (EAF) is a furnace that heats material by means of an electric arc. Industrial arc furnaces range in size from small units of approximately one-tonne capacity (used in foundries for producing cast iron products) up to a ...
, induction furnace or
basic oxygen furnace Basic oxygen steelmaking (BOS, BOP, BOF, or OSM), also known as Linz-Donawitz steelmaking or the oxygen converter processBrock and Elzinga, p. 50. is a method of primary steelmaking in which carbon-rich molten pig iron is made into steel. Blowing ...
, where the excess carbon is burned off and the
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductilit ...
composition controlled. Earlier processes for this included the finery forge, the puddling furnace, the
Bessemer process The Bessemer process was the first inexpensive industrial process for the mass production of steel from molten pig iron before the development of the open hearth furnace. The key principle is removal of impurities from the iron by oxidation ...
, and the
open hearth furnace An open-hearth furnace or open hearth furnace is any of several kinds of industrial furnace in which excess carbon and other impurities are burnt out of pig iron to produce steel. Because steel is difficult to manufacture owing to its high ...
. Modern steel mills and direct-reduction iron plants transfer the molten iron to a ladle for immediate use in the steel making furnaces or cast it into pigs on a pig-casting machine for reuse or resale. Modern pig casting machines produce stick pigs, which break into smaller 4–10 kg piglets at discharge.


References

{{Authority control Ancient Egyptian technology Ancient Roman technology Chinese inventions Ferrous alloys Iron Metalworking Smelting Steelmaking