Pig iron

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For other uses, see Pig iron (disambiguation).
Pig iron of a type used to make ductile iron, stored in a bin

Pig iron is the intermediate product of smelting iron ore into an transportable ingot of impure high carbon-content iron as an ingredient for further processing steps.[1] It is the molten iron from the blast furnace, which is a large and cylinder-shaped furnace nowadays normally charged with iron ore, coke, and limestone. Charcoal and anthracite have also been used as fuel; the former being the traditional fuel from classical times into the mid-1830s, but difficult to use in blast furnaces[a] while Anthracite iron was being developed concurrently in France, Pennsylvania, and Wales using the newly patented Scottish hot blast technology and the new blast furnace smelting structures, in large part because each principality was experience a fuel shortage, the first U.S.A. energy crisis was like those in industrially developed Europe—close-in stands of forest had been exhausted so because transportation options were so limited, both firewood and charcoal for heating and furnaces was growing scarce and expensive. This situation also inspired the American canal age with investors motivated to build the dawdling Schuylkill Canal[b] and the Lehigh Canal, the movement initially

Pig iron has a very high carbon content, typically 3.5–4.5%,[2] along with silica and other constituents of dross, which makes it very brittle and not useful directly as a material except for limited applications.

The traditional shape of the molds used for pig iron ingots was a branching structure formed in sand, with many individual ingots at right angles[3] to a central channel or runner, resembling a litter of piglets being suckled 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.[4] As pig iron is intended for remelting, the uneven size of the ingots and the inclusion of small amounts of sand caused only insignificant problems considering the ease of casting and handling them.


Casting pig iron, Iroquois smelter, Chicago, between 1890 and 1901.

Smelting and producing pig iron and other iron products was known to the Ancient Egyptians and gradually spread around the Eastern Mediterranean as far as Ancient Greece.[5] The Roman Empire and later the Muslim caliphates of the Middle Ages inherited and refined these technologies. Because of the collapse of the Western Roman Empire, Western Europe did not rediscover the process until the Late Middle Ages (1325–1500).[6] 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.

The Chinese were also making pig iron by the later Zhou Dynasty (which ended in 256 BC).[7]


Traditionally pig iron was worked into wrought iron in finery forges, later puddling furnaces, and more recently into steel.[8] 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.[9]

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 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 irons are used to dilute all the elements in a ductile iron charge (except carbon) which may be harmful to the ductile iron process.

Modern uses[edit]

Until recently, pig iron/slag was typically poured directly out of the bottom of the blast furnace through a trough into a ladle car for transfer to the steel mill 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 vessel to produce steel, typically an electric arc furnace, induction furnace or basic oxygen furnace, where the excess carbon is burned off and the alloy composition controlled. Earlier processes for this included the finery forge, the puddling furnace, the Bessemer process, and the open hearth furnace.

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.

See also[edit]


  1. ^ Charcoal was crumbly, so would break down into powder and pebbles blocking air flow so inhibiting combustion in the large stacks of fuel, air, and fluxes (limestone) to absorb non-volitile impurities.[1]
  2. ^ On the Schuylkill Canal 'Dawdling' and the synergy of building on success by others:
    The Schuylkill's board, including Josiah White and partner Erskine Hazard, who favored a fast expensive project, could not agree on methods and financing arrangements so only modest improvement work was funded and accomplished each year. After a couple of years suffering from want of fuel, this led to Hazard and White (who'd blazed the way to knowing how to burn anthracite) to seek sources of Anthracite instead via the Lehigh River from the unreliable deliveries of the Lehigh Coal Mine Company — even though their factories were using water power at the Falls of the Schuylkill River.
     • In March 1818 their petition to make improvements along the Lehigh was granted and after founding two companies sent teams of men into the field to construct improvements and by December had succeeded in shipping small quantities of Anthracite through the improvements made in essentially one season.
     • By December 1820 they saturated the Philadelphia market demand delivering nearly 300 tons to Philadelphia—then began regular deliveries proving the new wonder fuel was available as a reliable fuel.
     • This generated 1821's investment surge in the still incomplete Schuylkill Canal and a host of other coal canals over the next few years. The other canal projects fund raising was invigorated as well when 'Clinton's Folly', the Erie Canal often thought to be too difficult in news coverage, joined the Lehigh and began initial operations in spring of 1821.
     • Suddenly America was canal happy. By 1826 Pennsylvania had passed enabling and funding legislation to connect virtually the whole state by canals (and a few railroads) to the Delaware River and Philadelphia, including the Allegheny Portage Railroad to Pittsburgh and the new states growing from the Northwest Territory defined ca. 1891. Other projects were launched to connect the Potomac and the Chesapeake to the Delaware and even west through Ohio and the Great Lakes.


  2. ^ Camp, James McIntyre; Francis, Charles Blaine (1920). The Making, Shaping and Treating of Steel (2nd ed.). Pittsburgh: Carnegie Steel Co. p. 174. OCLC 2566055. 
  3. ^ Glossary of Metalworking Terms. Industrial Press. 2003. p. 297. 
  4. ^ The Making, Shaping, and Treating of Steel: Ironmaking volume (PDF). AISE Steel Foundation. 1999. p. 18. 
  5. ^ Waldbaum, Jane C. From Bronze to Iron. Göteburg: Paul Astöms Förlag (1978): 56–58.
  6. ^ Several papers in The importance of ironmaking: technical innovation and social change: papers presented at the Norberg Conference, May 1995 ed. Gert Magnusson (Jernkontorets Berghistoriska Utskott H58, 1995), 143-179.
  7. ^ Wagner, Donald. Iron and Steel in Ancient China. Leiden 1996: Brill Publishers
  8. ^ R. F. Tylecote, A history of metallurgy (2nd edition, Institute of Materials, London, 1992).
  9. ^ Rajput, R.K. (2000). Engineering Materials. S. Chand. p. 223. ISBN 81-219-1960-6.