Pig iron

Pig iron is the raw iron that comes out of a blast furnace. It contains about 4% carbon plus some silicon, manganese and other impurities, which make it hard but brittle. Almost all of it is refined into steel. The name comes from the old way of casting it: molten iron ran into a main channel with side molds branching off, which looked like piglets feeding from a sow.

A blast furnace is a tall tower fed from the top. Iron ore supplies the iron. Coke, a nearly pure form of carbon baked from coal, is the fuel. Limestone is the cleaner. Hot air is blown in near the bottom. The coke burns to carbon monoxide, which rises through the load and pulls oxygen away from the ore, leaving metallic iron. Lower down, at about 1,500 °C, the iron melts and soaks up carbon. The limestone breaks down to lime, which grabs sand and clay from the ore to form a liquid slag that floats on the iron.

Iron and slag are tapped off separately. A large furnace runs nonstop for years and can make about ten thousand tonnes of iron a day.