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Pig iron is the intermediate product of smelting iron ore with a high-carbon fuel such as coke, usually with limestone as a flux. Charcoal and anthracite have also been used as fuel. Pig iron has a very high carbon content, typically 3.5–4.5%, 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 these ingots was a branching structure formed in sand, with many individual ingots at right angles to a central channel or runner. Such a configuration is similar in appearance to a litter of piglets suckling on a sow. When the metal had cooled and hardened, the smaller ingots (the pigs) were simply broken from the much thinner 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 caused only insignificant problems considering the ease of casting and handling them.
The vast majority of PIG IRON is produced and consumed within integrated steel mill complexes. In this context the term “pig iron” is something of a misnomer: within integrated steel mills, blast furnace iron –
MERCHANT PIG IRON is cold iron, cast into ingots and sold as ferrous feedstock for the steel and metal casting industries. It falls into the category of ferrous metallics, of which iron and steel scrap comprises by far the largest volume, others being direct reduced iron [DRI], hot briquetted iron [HBI] and various other “alternative iron” materials. Merchant pig iron is, by definition, produced by dedicated merchant plants all of whose production is sold to external customers. Some integrated steel mills produce blast furnace iron that is surplus to their internal requirements and this is also cast into ingots and sold as merchant pig iron.
Merchant pig iron comprises three main types: BASIC PIG IRON, used mainly in electric arc steelmaking, HAEMATITE PIG IRON [also known as FOUNDRY PIG IRON], used in mainly in the manufacture of grey iron castings in cupola furnaces, and NODULAR PIG IRON, used in the manufacture of ductile [also known as nodular or spheroidal graphite –
- BASIC PIG IRON 3.5-
4.5% carbon, <1.5% silicon, 0.5- 1.0% manganese, <0.05% sulphur, <0.12% phosphorus
- HAEMATITE PIG IRON 3.5-
4.5% carbon, 1.5- 3.5% silicon, 0.5- 1.0% manganese, <0.05% sulphur, <0.12% phosphorus
- NODULAR PIG IRON 3.5-
4.5% carbon, <0.05% manganese, <0.02% sulphur, <0.04% phosphorus
Most merchant pig iron is produced through the reduction of iron ore in blast furnaces, using either coke or charcoal as reductant and energy source. Some merchant pig iron, principally nodular pig iron, is produced through the smelting of ilmenite in electric furnaces, as a by-
With its defined and closely controlled specification and the absence of metallic impurities, pig iron is a reliable and consistent charge material for both electric steelmaking and ferrous castings production. It also contains valuable alloying elements and reduces the energy consumption of a melt.
The Chinese were making pig iron by the later Zhou Dynasty (1122–256 BC). In Europe, the process was not invented until the Late Middle Ages (1350–1500). Actually, 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.
More information on the History of Pig Iron can be found here.
Traditionally pig iron was worked into wrought iron in finery forges, later puddling furnaces, and more recently into steel. 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 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.
Until recently, pig iron/slag is typically poured directly out of the bottom of the Blast Furnace (BF) through a trough into a ladle car for transfer to the steel mill in mostly liquid form; in this state, the pig iron is referred to as hot metal. The hot metal is then poured into a steelmaking vessel to produce steel, typically with an Electric Arc Furnace (EAF) (Pig Iron Production By Electric Arc Furnace), induction furnace or Basic Oxygen Furnace (BOF), by burning off the excess carbon in a controlled fashion and adjusting the alloy composition. 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 pieces at discharge.
Pig iron was used as ballast on the NASA Boeing 747 Shuttle Carrier Aircraft.
1 Camp, James McIntyre; Francis, Charles Blaine (1920). The Making, Shaping and Treating of Steel (2nd ed. ed.). Pittsburgh: Carnegie Steel Co. p. 174. OCLC 2566055.
2 Wagner, Donald. Iron and Steel in Ancient China. Leiden 1996: Brill Publishers
3 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.
4 R. F. Tylecote, A history of metallurgy (2nd edition, Institute of Materials, London, 1992).
5 Rajput, R.K. (2000). Engineering Materials. S. Chand. p. 223. ISBN 81-219-1960-6.
6 Gravel Haulers: NASA’s 747 Shuttle Carriers