Pig iron

For other uses, see Pig iron (disambiguation).

Pig iron of a type used to make ductile iron, stored in a bin

Pig iron is an intermediate product of the iron industry. it is made by smelting iron ore into a transportable ingot of impure high carbon-content iron as an ingredient for further processing steps. It is the molten iron from the blast furnace, which is a large, and cylinder-shaped, furnace charged with iron ore, coke, and limestone. In the past, charcoal, or anthracite coal have also been used as fuel; charcoal was the traditional fuel from classical times into the mid-1830s, but was difficult to use in blast furnaces. 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 technology; in large part because each state was experiencing a fuel shortage. The first United States energy crisis was like those in industrially developed Europe — close-in stands of forest had been exhausted because transportation options were limited, so both firewood, and charcoal, for heating, steam production for stationary steam engines in mills, and iron smelting furnaces, was growing scarce and expensive. This situation also inspired the American canal age with investors motivated to build the Schuylkill Canal.

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 pig iron ingots was 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 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. 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.

History

See also: History of ferrous metallurgy

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. 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). 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).

Uses

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 (except carbon) in a ductile iron charge which may be harmful to the ductile iron process.

Modern uses

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

• Hot Metal Bridge