Basic Oxygen Furnace: Process Description

 

Courtesy of “The Making, Shaping, and Treating of Steel”
Association of Iron and Steel Engineers.

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Process Description
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The oxygen steelmaking process converts the molten iron from the blast furnace – with up to 30% steel scrap – into refined steel. High purity oxygen is blown through the molten bath to lower carbon, silicon, manganese, and phosphorous content of the iron, while various fluxes are used to reduce the sulfur and phosphorous levels. The impurities and a small amount of oxidized iron are carried off in the molten slag that floats on the surface of the hot metal.

BOF Facility

The actual furnaces are a small part of the facility as the schematic drawing shows. Gas cleaning devices and materials handling equipment occupy most of the space.

Scrap Charging

The first step for making a heat of steel in a BOF is to tilt the furnace and charge it with scrap. The furnaces are mounted on trunnions and can be rotated through a full circle.

Molten Iron Charging

Hot metal from the blast furnace accounts for up to 80% of the metallic charge and is poured from a ladle into the top of the tilted furnace.

Furnace Operation

The charged furnace is returned to an upright position and a water cooled oxygen lance is lowered from the top; oxygen is blown into the bath at supersonic speeds causing rapid mixing and heat from the oxidation of iron and impurities. Fluxes (burnt lime, burnt dolomite, and fluorspar) are added to help carry off the impurities in the floating slag layer. This step requires only about 15 minutes of an overall 45 minute tap to tap cycle time.

Tapping

After the steel has been refined, the furnace is tilted (opposite to the charging side) and molten steel is poured out into a preheated ladle. Alloys are added to the ladle during the pour to give the steel the precise composition desired. In some steelmaking applications, further refining is conducted in the ladle to remove oxygen and sulfur from the molten steel.