Basic Oxygen Furnace (BOF)

More than half the world’s steel is produced in the basic oxygen process (BOP), which uses pure oxygen to convert a charge of liquid blast-furnace iron and scrap into steel. The basic oxygen furnace (BOF) is a refractory-lined, tiltable converter into which a vertically movable, water-cooled lance is inserted to blow oxygen through nozzles at supersonic velocity onto the charge. The use of pure oxygen at high flow rates results in such fast oxidation of the elements contained in blast-furnace iron that only about 20 minutes are required per heat—i.e., to refine one charge. Converters vary in size and are operated for heats ranging from 30 to 360 tons.

• The basic oxygen steel-making process is as follows:

  • • • Molten pig iron (sometimes referred to as "hot metal") from a blast furnace is poured into a large refractory-lined container called a ladle.

      • The metal in the ladle is sent directly for basic oxygen steelmaking or to a pretreatment stage. High purity oxygen at a pressure of 700–1,000 kilopascals (100–150 psi) is introduced at supersonic speed onto the surface of the iron bath through a water-cooled lance, which is suspended in the vessel and kept a few feet above the bath. Pretreatment of the blast furnace hot metal is done externally to reduce sulphur, silicon, and phosphorus before charging the hot metal into the converter. In external desulphurising pretreatment, a lance is lowered into the molten iron in the ladle and several hundred kilograms of powdered magnesium are added and the sulphur impurities are reduced to magnesium sulphide in a violent exothermic reaction. The sulfide is then raked off. Similar pretreatments are possible for external desiliconisation and external dephosphorisation using mill scale (iron oxide) and lime as fluxes. The decision to pretreat depends on the quality of the hot metal and the required final quality of the steel.

      • Filling the furnace with the ingredients is called charging. The BOS process is autogenous, i.e. the required thermal energy is produced during the oxidation process. Maintaining the proper charge balance, the ratio of hot metal from melt to cold scrap is important. The BOS vessel can be tilted up to 360° and is tilted towards the deslagging side for charging scrap and hot metal. The BOS vessel is charged with steel or iron scrap (25%-30%),if required. Molten iron from the ladle is added as required for the charge balance. A typical chemistry of hotmetal charged into the BOS vessel is: 4% C, 0.2–0.8% Si, 0.08%–0.18% P, and 0.01–0.04% S, all of which can be oxidised by the supplied oxygen except sulphur.(which requires reducing conditions)

      • The vessel is then set upright and a water-cooled, copper tipped lance with 3–7 nozzles is lowered into it and high purity oxygen is delivered at supersonic speeds. The lance "blows" 99% pure oxygen over the hot metal, igniting the carbon dissolved in the steel, to form carbon monoxide and carbon dioxide, causing the temperature to rise to about 1700 °C. This melts the scrap, lowers the carbon content of the molten iron and helps remove unwanted chemical elements. It is this use of pure oxygen (instead of air) that improves upon the Bessemer process, as the nitrogen (an undesirable element) and other gases in air do not react with the charge, and decrease efficiency of furnace.

      • Fluxes (burnt lime or dolomite) are fed into the vessel to form slag, to maintain basicity above 3 and absorb impurities during the steelmaking process. During "blowing", churning of metal and fluxes in the vessel forms an emulsion, that facilitates the refining process. Near the end of the blowing cycle, which takes about 20 minutes, the temperature is measured and samples are taken. A typical chemistry of the blown metal is 0.3–0.9% C, 0.05–0.1% Mn, 0.001–0.003% Si, 0.01–0.03% S and 0.005–0.03% P.

      • The BOS vessel is tilted towards the slagging side and the steel is poured through a tap hole into a steel ladle with basic refractory lining. This process is called tapping the steel. The steel is further refined in the ladle furnace, by adding alloying materials to impart special properties required by the customer. Sometimes argon or nitrogen is bubbled into the ladle to make the alloys mix correctly.

      • After the steel is poured off from the BOS vessel, the slag is poured into the slag pots through the BOS vessel mouth and dumped.

Earlier converters, with a false bottom that can be detached and repaired, are still in use. Modern converters have a fixed bottom with plugs for argon purging. The Energy Optimization Furnace (EOF) is a BOF variant associated with a scrap preheater where the sensible heat in the off-gas is used for preheating scrap, located above the furnace roof.

The lance used for blowing has undergone changes. Slagless lances, with a long tapering copper tip, have been employed to avoid jamming of the lance during blowing. Post-combustion lance tips burn the CO generated during blowing into CO₂ and provide additional heat. For slag-free tapping, darts, refractory balls and slag detectors are employed. Modern converters are fully automated with auto blowing patterns and sophisticated control systems.