From the Department of Energy:
Coal is used to make coke for the iron and steel industry, foundries, and other industries. The presence of large domestic deposits of coking coal, or metallurgical coal, played an important role in the development of the U.S. iron and steel industry. Coke is used chiefly to smelt iron ore and other iron bearing materials in blast furnaces, acting both as a source of heat and as a chemical reducing agent, to produce pig iron, or hot metal. Coke, iron ore, and limestone are fed into the blast furnace, which runs continuously. Hot air blown into the furnace burns the coke, which serves as a source of heat and as an oxygen reducing agent to produce metallic iron. Limestone acts as a flux and also combines with impurities to form slag.
Foundries use coke as a source of heat for producing metal castings. Other industrial uses of coke include the smelting of phosphate rock to produce elemental phosphorous and the production of calcium carbide. Small sizes of coke, termed breeze, are used as fuel in sintering finely sized particles of iron ore and other iron bearing material to produce an agglomerate that can be used in a blast furnace.
Coke is made by baking a blend of selected bituminous coals (called metallurgical coal or coking coal) in special high temperature ovens without contact with air until almost all of the volatile matter is driven off. The resulting product, coke, consists principally of carbon. A short ton of coal yields about 1,400 pounds of coke and a variety of byproducts such as crude coal tar, light oils, and ammonia, which are refined to obtain various chemical products. About 1,100 pounds of coke are consumed for every short ton of pig iron produced.