Sponge iron production is a technique of purifying iron ore by reacting it with gas mixtures to remove oxygen via a reduction process. Iron ore from the ground is a chemical compound of iron and oxygen called ferric oxide, which is similar to rust. At temperatures above 1800°F (10007deg;C), molecular oxygen can be removed from the iron by passing mixtures of hydrogen and carbon monoxide gas through the ore, which reduces the ore to a purified iron.
High temperatures are needed to chemically unlock the oxygen from the iron, and allow it to react with the hydrogen and carbon monoxide to form water and carbon dioxide. Sponge iron production can occur in different types of reactors, but a typical method is to use a horizontal rotary kiln. This equipment is a horizontal heated drum placed at a slight angle from level, which allows the iron ore to slowly move from one end to the other as the drum rotates. The drum is heated and gases are passed through the center of the drum, allowing the oxygen reduction to occur.
Sponge iron production can use iron ore of varying sizes, with most ore feeding the kiln being sized from chunks to pellets. Temperature control of the kiln is important, because the iron must not be heated to melting temperatures, only to levels needed to release oxygen. The iron leaving a sponge iron production process contains only a few percent impurities, including some small quantities of other metals found in the iron ore.
High purity and relatively low operating costs make sponge iron an important raw material for steel production. Blast furnaces and other steel production equipment require high temperatures and other chemicals to remove impurities, making sponge iron production important to supply the blast furnace. The efficiency of the blast furnace is improved by reducing the oxygen content of the ore prior to adding it, making sponge iron a more efficient raw material than unprocessed ore.
Sponge iron can be purified further after the reduction reaction step, by passing the cooled iron through magnetic separators. Iron can be attracted to a magnet, so once the ore leaves the reactor, magnets can pull the sponge iron away from any impurities. This step can be important when coal is used as the reducing medium, because the reacted coal can contain non-metallic charcoal and other impurities. Removing them will further improve the efficiency of the blast furnace and require less processing in later steps.