Basic Oxygen Furnace
Basic oxygen steelmaking (BOS, BOP, BOF, and OSM), also known as Linz-Donawitz-Verfahren steelmaking or the oxygen converter process is a method of primary steelmaking in which carbon-rich molten Pig Iron (MPI) is made into steel. Blowing oxygen through molten pig iron lowers the carbon content of the alloy and changes it into low-carbon steel. The process is known as basic due to the type of refractories—calcium oxide and magnesium oxide—that line the vessel to withstand the high temperature of the molten metal.
The process was developed in 1948 by Robert Durrer and commercialized in 1952–1953 by Austrian VOEST and ÖAMG. The LD converter, named after the Austrian towns Linz and Donawitz (a district of Leoben) is a refined version of the Bessemer converter where blowing of air is replaced with blowing oxygen. It reduced capital cost of the plants, time of smelting, and increased labor productivity. Between 1920 and 2000, labor requirements in the industry decreased by a factor of 1,000, from more than 3 worker-hours per tonne to just 0.003. The vast majority of steel manufactured in the world is produced using the basic oxygen furnace; in 2000, it accounted for 60% of global steel output. Modern furnaces will take a charge of iron of up to 350 tons and convert it into steel in less than 40 minutes, compared to 10–12 hours in an open hearth furnace.
Basic oxygen steelmaking is a primary steelmaking process for converting the molten Pig Iron (MPI) into steel by blowing oxygen through a lance over the molten pig iron inside the converter. The converter used for steel making is called as Basic Oxygen Furnace because of the exothermic heat generated by the oxidation reactions during blowing.
The basic oxygen steel-making process is as follows:
1. Molten Pig Iron (MPI) (sometimes referred to as “hot metal”) from a blast furnace is poured into a large refractory-lined container called a ladle;
2. The metal in the ladle is sent directly for basic oxygen steelmaking or to a pretreatment stage. High purity oxygen at a pressure of 100-150 psi (pounds per inch square) 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 at 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.
3. 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 therefore very important. 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 (requires reducing condition).
4. The vessel is then set upright and a water-cooled, copper tipped lance with 3-7 nozzles is lowered down 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, and 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 (a particularly undesirable element) and other gases in air do not react with the charge.
5. Fluxes (burnt lime or dolomite) are fed into the vessel to form slag, to maintain basicity more than 3 and absorbs 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.
Tapping of steel from BOF:
1. The BOS vessel is tilted towards the deslagging 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 give the it special properties required by the customer. Sometimes argon or nitrogen is bubbled into the ladle to make the alloys mix correctly.
2. 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.
Basic Oxygen Furnace Documents
1 Wikipedia.org – Basic Oxygen Steelmaking
2 McGannon, Harold E. editor (1971). The Making, Shaping and Treating of Steel: Ninth Edition. Pittsburgh, Pennsylvania: United States Steel Corporation.
3 Smil, Vaclav (2006). Transforming the twentieth century: technical innovations and their consequences, Volume 2. Oxford University Press US. ISBN 0-19-516875-5.