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What is a Basic Oxygen Furnace?

By Lea Miller
Updated May 17, 2024
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A basic oxygen furnace (BOF) is the machinery used for purification of pig iron by oxidizing impurities into slag and converting the rest into molten steel with an injection of oxygen. It is the primary method used for steel production worldwide. The steel produced is chemically tested and may be further modified by the addition of alloys. The process is called a basic oxygen furnace due to the alkalinity of the refractory and the addition of alkaline materials during production.

A heat, or batch, begins with the furnace structure itself, i.e., an elongated vessel. The container is lined with alkaline refractory materials such as magnesite and burnt lime. The refractory is necessary for the vessel to withstand the extreme temperatures and high level of oxidation that takes place in the basic oxygen furnace.

The furnace is tilted at an angle and charged, or filled, with about one-third steel scraps and two-thirds molten pig iron that has been liquefied in a blast furnace. Fluxes of lime or dolomite are added. A computer program uses the chemical analysis of the pig iron to determine the optimum proportions of iron, steel scraps, and fluxes and to program the oxygen blowing time.

The mixture is exposed to a stream of pure oxygen blown in through a water-cooled tube for about 20 minutes. The height of the tip of the oxygen stream in the vessel may be adjusted during blowing. The process is controlled and monitored by a computer.

The carbon and silicon in the mixture oxidize and create a huge volume of heat that melts the scrap. This chemical reaction quickly raises the temperature in the basic oxygen furnace to about 3,000° Fahrenheit (1,649° Celsius). Oxidized silicon becomes silica and combines with the fluxes to form slag. Sulfur and phosphorus content from the iron are absorbed by the slag. Carbon monoxide and carbon dioxide created by the process are exhausted through pollution control equipment.

After oxygen blowing, the furnace contains layers of molten steel and slag. The vessel is tilted to allow sampling and testing of the purified material. A preheated container called a ladle is moved into place below the basic oxygen furnace. The molten steel is tapped, or poured, into the ladle. Detectors function to prevent any slag from mixing with the steel. The steel may be altered with iron, silicon, or manganese alloys for specific uses or be sent for casting without further modification.

After all the steel is tapped, the slag is poured off into a separate container. The slag is cooled and left to stabilize for a period of months. After this time, the slag can be crushed for further use, often as an aggregate for construction projects.

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