blast furnace, structure used chiefly in smelting. The principle involved in this means of extracting metals is that of the reduction of the ores by the action of carbon monoxide, i.e., the removal of oxygen from the metal oxide in order to obtain the metal. Blast furnaces differ in construction. The one used in the production of iron consists of a chimneylike structure (usually 80–100 ft/24–30 m high) made of iron or steel and lined with firebrick. It is narrow at the top, increasing in diameter downward, but narrowing again suddenly almost at the bottom, to form the hearth or crucible. There the fine molten products are caught. The furnace is fed from the top with a charge of definite quantities of ore, coke, and a flux, mostly limestone. Preheated compressed air is introduced at the bottom through pipes (tuyères) entering just above the hearth. The air passes upward through the charge. The coke is oxidized to carbon dioxide, which changes to carbon monoxide at the high temperature. The carbon monoxide then reduces the ores and, taking on oxygen, reverts to carbon dioxide. This gas, together with unused carbon monoxide, nitrogen, and other constituents of the air originally introduced, is led off through a pipe from the top of the furnace and, being still at a high temperature, is employed to heat the stoves into which fresh air for the process is brought. As the operation proceeds, the mass in the furnace becomes molten and descends into the crucible. The iron sinks to the bottom; impurities, called the slag, being lighter, float on top. The slag is drained through a pipe in the upper portion of the crucible. The iron is tapped from below and run into sand molds to harden. The product is known as pig iron or cast iron (see iron). Efforts to increase production rates have led to the addition of pure oxygen and steam and the sizing of ore to obtain better gas-solid contact. Flux and ore are sometimes combined into pellets. Pig iron prepared in the blast furnace is converted into steel by the Bessemer process. Copper ore treated in a blast furnace yields a copper matte, from which only a part of the impurities are removed. It is usually further refined by electrolytic methods (see copper).
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