Hot air (at 1200°C) is injected into the lower part of the blast furnace, where the carbon in the coke is gasified with the oxygen in the air to produce the reducing gas carbon monoxide, generating temperatures of up to 2,200°C. This gas rises, binding the oxygen in the iron oxides and produce carbon dioxide, thus reducing the ore. The rising gases heat the charge lying above them. The accompanying elements of the input material form a liquid slag and can thus be separated out. Hot metal and slag gather in the lower area of the blast furnace and leave the furnace at a temperature of about 1500°C via a tap hole there (which must be opened). The hot metal and slag are separated via a refractory lined runner system and directed to the hot metal torpedo ladle and the slag ladle.
Other carbon carriers, such as coal, oil, gas or specially prepared old plastic are also injected into the blast furnace as an alternative to coke in order to optimise the process and reduce production costs. Operation of a blast furnace entirely without coke, however, is not possible. The coke retains its solid structure in the area of the blast furnace in which the ores become soft and melt, thus ensuring an even gas flow and acting as a supporting matrix for the solid charge column above.
Large blast furnaces have hearth diameters of about 15 m and a total volume of approximately 6,000 cubic metres. They produce about 13,000 tonnes of hot metal per day or more than 4 million tonnes annually. This necessitates the movement and feeding-in of very large material flows every day, e.g. 20,800 tonnes of iron ores, 4,300 tonnes of coke, 1,900 tonnes of coal for injection, and 12 million cubic metres of air that is heated to over 1200°C in hot-blast stoves. In addition, 3,600 tonnes of slag are produced every day, largely used as a raw or building material in the cement industry or for road construction, and 18 million cubic metres of blast furnace gas is also produced which, after cleaning, is used to generate energy.
The campaign life of a blast furnace, i.e. the length of time until its refractory lining needs to be completely renewed, is currently 15 to 20 years.