A blast furnace is a chemical reactor that is used for metallurgy. It is used on producing metals from ores by reducing oxygen off the ores by means of carbon monoxide reduction. It is usually used in steelmaking, but can be used on the reduction of other metals as well.

In steelmaking, the blast furnace is used to make raw iron from ore. Chemically it is a countercurrent tube reactor. Ore, coke and limestone are fed at the top of the furnace while hot air ("wind") is blasted with a compressor through nozzles called tuyeres at the bottom. The hot blast ignites the coke, which burns into carbon monoxide, which will rise up (countercurrent) in the furnace. The carbon monoxide and any unburnt coke (which is almost pure carbon) will snatch the oxygen off the ores, reducing it into pure metal. The limestone will react with the side rock of the ore, producing slag, which is used on making bricks and road topping.

The iron melts in the high temperature of the furnace and forms an eutectic mixture with the carbon called "pig iron", which accumulates at the hearth (the bottom) of the furnace. The pig iron and slag are tapped every four hours. The pig iron is further refined into steel in a basic oxygen furnace (BOF) where it is mixed with scrap and additives.

Contrary to usual belief, scrap is not used on blast furnaces. It is melted down on BOF furnaces and electric arc furnaces.

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