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    Bos Bos Presentation Transcript

    • Gulfam Hussain e_gulfam@yahoo.com +923336275670
    •  Basic Oxygen Furnace (BOF) is a pear shaped vessel where the pig iron from blast furnace, and ferrous scrap, is refined into steel by injecting a jet high-purity oxygen through the hot metal. In BOF,  The carbon content of pig iron, which is typically 4-5%, is reduced to varying levels below 1% depending on the product specifications.  Unwanted impurities are removed.  Concentration of desired is brought to product specifications.  The Basic Oxygen Process (Basic Oxygen Furnace, Basic Oxygen Steelmaking, Basic Oxygen Converter) is the most powerful and effective steel making method. About 67% of the crude steel in the world is made in the Basic Oxygen Furnaces (BOF).  Basic oxygen furnace is also termed as L.D converter.
    •  Typical basic oxygen furnace has a vertical vessel lined with refractory lining. Only 8-12% of the furnace volume is filled with the treated molten metal. The bath depth is about 4-6.5 ft (1.2-1.9 m). The ratio between the height and diameter of the furnace is 1.2-1.5. The typical capacity of the Basic Oxygen Furnace is 250-400 t.  The vessel consists of three parts: spherical bottom, cylindrical shell and upper cone. The vessel is attached to a supporting ring equipped with trunnions.  The supporting ring provides stable position of the vessel during oxygen blowing.  The converter is capable to rotate about its horizontal axis on trunnions driven by electric motors. This rotation (tilting) is necessary for charging raw materials, sampling the melt and pouring the steel out of the converter.
    •  The top blown basic oxygen furnace is equipped with the water cooled oxygen for blowing oxygen into the melt through 4-6 nozzles. Oxygen flow commonly reaches 200-280 ft3/(min*t) (6-8 m3/(min*t)). The oxygen pressure is 150-220 psi (1-1.5 MPa). Service life of oxygen lance is about 400 heats. The bottom blown basic oxygen furnace is equipped with 15-20 tuyeres for injection of oxygen (or oxygen with lime powder). The tuyeres are cooled by either hydrocarbon gas (propane, methane) or oil supplied to the outer jacket of the tube.
    •  The refractory lining of basic oxygen furnaces work in severe conditions of high temperature and oxidizing atmosphere. The lining wear is fastest in the zone of contact with the oxidizing slag (slag line).  Refractory bricks for lining basic oxygen furnaces are made of either resin bonded magnesite or tar bonded mixtures of magnesite (MgO) and burnt lime (CaO). The bonding material (resin, tar) is coked and turns into a carbon network binding the refractory grains, preventing wetting by the slag and protecting the lining the from chemical attack of the molten metal.
    •  The following measures allows to prolong the service life of the lining:  Control of the content of aggressive oxidizing oxide FeO in the slags at low level.  Addition of MgO to the slags.  Performing “slag splashes” - projecting residual magnesia saturated slag to the lining walls by Nitrogen blown through the lance.  Repair the damaged zones of the lining by gunning refractory materials.
    •  The basic oxygen furnace uses no additional fuel. The pig iron impurities (carbon, silicon, manganese and phosphorous) serve as fuel. Iron and its impurities oxidize evolving heat necessary for the process.  Oxidation of the molten metal and the slag is complicated process proceeding in several stages and occurring simultaneously on the boundaries between different phases (gas-metal, gas-slag, slag-metal). Finally the reactions may be presented as follows:  (square brackets [ ] - signify solution in steel, round brackets ( ) - in slag, curly brackets {} - in gas).
    •  1/2{O2} = [O] [Fe] + 1/2{O2} = (FeO) [Si] + {O2} = (SiO2) [Mn] + 1/2{O2} = (MnO) 2[P] + 5/2{O2} = (P2O5) [C] + 1/2{O2} = {CO} {CO} + 1/2{O2} = {CO2}  Most oxides are absorbed by the slag.
    •  Gaseous products CO and CO2 are transferred to the atmosphere and removed by the exhausting system. Oxidizing potential of the atmosphere is characterized by the post-combustion ratio:  {CO2}/({CO2}+{CO}).  Basic Oxygen Process has limiting ability for desulfurization. The most popular method of desulfurization is removal of sulfur from molten steel to the basic reducing slag. However the slag formed in the Basic Oxygen Furnace is oxidizing (not reducing) therefore maximum value of distribution coefficient of sulfur in the process is about 10, which may be achieved in the slags containing high concentrations of CaO.
    •  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. Pretreatment of the blast furnace metal is used to reduce the refining load of sulfur, silicon, and phosphorus. In desulfurising pretreatment, a lance is lowered into the molten iron in the ladle and several hundred kilograms of powdered magnesium are added. Sulfur impurities are reduced to magnesium sulfide in a violent exothermic reaction. The sulfide is then raked off. Similar pretreatment is possible for desiliconisation and dephosphorisation using mill scale (iron oxide) and lime as reagents. The decision to pretreat depends on the quality of the blast furnace metal and the required final quality of the BOS steel.
    •  Filling the furnace with the ingredients is called charging. The BOS process is autogenous: the required thermal energy is produced during the process. Maintaining the proper charge balance, the ratio of hot metal to scrap, is therefore very important. The BOS vessel is one-fifth filled with steel scrap. Molten iron from the ladle is added as required by the charge balance. A typical chemistry of hot metal charged into the BOS vessel is: 4% C, 0.2–0.8% Si, 0.08%–0.18% P, and 0.01–0.04% S.  The vessel is then set upright and a water-cooled lance is lowered down into it. The lance blows 99% pure oxygen onto the steel and iron, igniting the carbon dissolved in the steel and burning it 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 oxygen instead of air that improves upon the Bessemer process, for the nitrogen (and other gases) in air do not react with the charge as oxygen does. High purity oxygen is blown into the furnace or BOS vessel through a vertically oriented water-cooled lance with velocities faster than Mach 1.
    •  Fluxes (burnt lime or dolomite) are fed into the vessel to form slag, which absorbs impurities of the steelmaking process. During blowing the metal in the vessel forms an emulsion with the slag, facilitating the refining process. Near the end of the blowing cycle, which takes about 20 minutes, the temperature is measured and samples are taken. The samples are tested and a computer analysis of the steel given within six minutes. A typical chemistry of the blown metal is 0.3–0.6% C, 0.05–0.1% Mn, 0.01– 0.03% Si, 0.01–0.03% S and P.  The BOS vessel is tilted again and the steel is poured into a giant ladle. This process is called tapping the steel. The steel is further refined in the ladle furnace, by adding alloying materials to give the steel special properties required by the customer. Sometimes argon or nitrogen gas is bubbled into the ladle to make sure the alloys mix correctly. The steel now contains 0.1–1% carbon. The more carbon in the steel, the harder it is, but it is also more brittle and less flexible.  After the steel is removed from the BOS vessel, the slag, filled with impurities, is poured off and cooled.
    •  The Water-Cooled Oxygen Lance: The Water-Cooled Oxygen Lance provides the oxygen to the Furnace so that the temperature in the Furnace will increase. The Oxygen that comes through the Lance is extremely hot after coming through special heating ovens. The Lance has to be Water-Cooled so that it will not melt in the Furnace.  The Slagging Hole: The Slagging Hole is like a spout where the slag can be poured out when necessary. During the manufacture of the Steel the Slagging Hole is "plugged" so that no heat can escape.  The Steel Shell: The main body of the Basic Oxygen Furnace is made from Steel, as the material is strong and durable, or tough. The Steel Shell does not melt because of the Refractory Lining.  The Refractory Lining: The Refractory Lining is a special type of cement that has the ability to reflect heat. You are more familiar with what a Refractory Lining is than you might think. If you look at the back of an open fireplace, the cement you see on the back wall is a Refractory material, although it would not be of the same quality as the Refractory lining in a Furnace. The Refractory Lining has two purposes. The first is to keep the heat from the furnace in so that less energy is required to keep the Furnace at operating temperature. The second reason is to protect the Steel Shell of the Furnace.
    •  The Molten Metal: The Molten Metal at the bottom of the Furnace is the Steel. The Steel is below the Slag as it is heavier or denser. The Molten Steel is removed from the Furnace when the Steel is of the correct consistency, through the Tapping Hole.  The Slag: The Slag which sits on top of the Molten Metal, because it is less dense, is the waste material from the process of creating Steel. It consists of the impurities, that is most materials other than Iron and Carbon which were put into the Furnace at the start when the Furnace was being Charged. The Slag is removed from the Furnace when the time is ready through the Slagging Hole.  The Tapping Hole: The Tapping Hole is used to remove the Molten Steel from the Furnace when it is of the right consistency. During the process of manufacturing the Tapping Hole is "plugged" so as not to allow heat to escape from the Furnace.  The Gas Offtake Hood: The Gas Offtake Hood has two purposes. The first is to trap the dangerous gases that the Basic Oxygen Process produces so that they cannot escape into the atmosphere to poison people or create Acid Rain. The gases are "cleaned" or put to other uses. One important use of the gases is to heat the Oxygen that is going through the Water-Cooled Oxygen Lance. The second purpose of the Gas Offtake Hood is to reduce the amount of heat loss in the Furnace.