pig iron

Pig iron is the intermediate product of smelting iron ore with coke, usually with limestone as a flux. Pig iron has a very high carbon content, typically 3.5–4.5%,^[1] 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 inclusion of small amounts of sand was insignificant compared to the ease of casting and of handling.

The Chinese were making pig iron by the later Zhou Dynasty (1122–256 BC). In Europe, the process did not become common until the 14th century^{[citation needed]}.

Uses

Traditionally pig iron would be worked into wrought iron in finery forges^{[citation needed]}.

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 and phosphorous.

Modern uses

Today, pig iron is typically poured directly out of the bottom of the blast furnace through a trough into a ladle car for transfer to the steel plant in mostly liquid form, referred to as hot metal. The hot metal is then charged into a steelmaking vessel to produce steel, typically with an electric arc furnace or basic oxygen furnace, by burning off the excess carbon in a controlled fashion and adjusting the alloy composition. Earlier processes for this included the Bessemer process, open hearth furnace, finery forge, and the puddling 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.

Refining

Pig iron is melted and a strong current of air is directed over it while it is being stirred or agitated. This causes the dissolved impurities (such as silicon) to be thoroughly oxidized. The metal is then cast into molds or used in other processes. This is known as refined pig iron, finers metal, or refined iron.^[2]

See also

• Steelmaking
• Wrought iron

References