The Blast Furnace
Iron ore, coke, and limestone are added at the top of the furnace, and air is blown in at the bottom. The oxygen of the air reacts with the hot coke to form carbon monoxide. This gas passes up the furnace, changing the iron oxide to iron as it moves down. Liquid iron collects at the bottom of the furnace, covered by a layer of molten slag. The iron is tapped off at the bottom of the furnace and allowed to solidify, forming pig iron. Pig iron is hard but very brittle. The molten slag is run off separately.
Basic Oxygen Furnace for the Production of Steel
Because pig iron is hard but quite brittle, it is not very useful in this state. To produce steel, most of the impurities are removed by blowing hot oxygen through the molten pig iron for a short time in a basic oxygen furnace. This process oxidizes the impurities which escape as gases or form slag. A typical basic oxygen furnace is charged with about 200 tons of molten pig iron,100 tons of scrap iron, and 20 tons of limestone. The product of the basic oxygen furnace is carbon steel, which is strong and malleable and therefore much more useful than pig iron.