A blast furnace derives its name from the way in which it's heated. In most types, preheated air of about 982 degrees Celsius is blasted into the furnace through a collection of nozzles at its base. Blast furnaces are primarily used in the making of different types of iron. Through a series of chemical reactions, all that hot air can smelt iron ore down to real iron in a short amount of time.
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Blast furnaces produce iron through a process known as smelting. It's actually smelting that allows any ore to be reduced down to its actual metal. In most furnaces, the combination of extreme heat and a reducing chemical agent such as coke does the job. The carbon or carbon monoxide produced when the coke is superheated displaces the oxygen in the ore. What results at the end is an actual metal such as iron.
In a blast furnace, the fuel and the ore are introduced through the top while superheated air is introduced at the bottom. Often the air is enriched with pure oxygen. As the ore and the fuel move down to the bottom of the furnace's heating chamber, the transformation from ore to iron occurs. At the bottom, it emerges as molten iron while the gases in the furnace are vented at the top.
In many cases, limestone is added during the process. Once heated sufficiently, it decomposes into calcium oxide. The oxide from the limestone removes much of the raw heated ore's acidic impurities. These impurities take the form of a liquid slag that floats to the top of the molten iron. Once at the top, it's collected as the purified molten iron moves out of a tap hole at the furnace's bottom.
Most blast furnaces can be operated continuously for about 10 years before their interior linings have to be replaced. They're made up of refractory firebricks that do an excellent job of deflecting heat back onto the iron ore rather than absorbing the heat. Also, blast furnaces are built so that they recycle much of the carbon monoxide-laden gases they produce. They're reused to heat the furnace's chamber or in other iron manufacturing processes.
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