Sponge iron
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Sponge iron is the product created when iron ore is reduced to metallic iron, usually with some kind of carbon (charcoal, etc), at temperatures below the melting point of iron. This results in a spongy mass, sometimes called a bloom, consisting of a mix of incandescent wrought iron and slag.
[edit] Use of sponge iron
Sponge iron is not useful in itself, but must be processed further to create wrought iron. The sponge is removed from the furnace, called a bloomery, and repeatedly beaten with heavy hammers and folded over to remove the slag, oxidise any carbon or carbide and weld the iron together. This treatment usually creates wrought iron with about three percent slag and a fraction of a percent of other impurities. Further treatment may add controlled amounts of carbon, allowing various kinds of heat treatment (e.g. "steeling").
Today, sponge iron is created by reducing iron ore without melting it. This makes for an energy-efficient feedstock for specialty steel manufacturers which used to rely upon scrap metal.
[edit] History
Producing sponge iron and then working it was the earliest method used to obtain iron in the Middle East, Egypt, and Europe, where it remained in use until at least the 16th century. There is some evidence that the bloomery method was also used in China, but China had developed blast furnaces to obtain pig iron by 500BC.
The advantage of the bloomery technique is that iron can be obtained at a lower furnace temperature, only about 1100 C or so. The disadvantage, relative to using a blast furnace, is that only small quantities can be made at a time.
