Hot metal quality is a of key importance to the manufacturing of steel . Hot metal quality is described using four criteria, the temperature, and the compositional chemistry of the hot metal, focusing on the levels of sulphur, silicon, and manganese. The operational conditions of the blast furnace, as well as the quality and properties of the burden materials plays a key role in the manufacturing of hot metal with the desired temperature and chemical composition.
Sulphur, in elevated levels, can be detrimental to the mechanical properties of steels. In some instances, increased concentrations of sulphur is used to improve machinability, but that is the exception and not the rule. Desulphurization is an expensive process to undertake before steel manufacturing, so it is preferable to produce hot metal with low levels of sulphur. Sulphur is transferred to the hot metal from the metallurgical coke addition to the blast furnace.
[S] + CaO + C = CaS + CO
Sulphur content can be controlled through the use of low sulphur coke as a burden material. High basicity of the slag promotes the transfer of sulphur into the slag. Temperature in the surroundings can also impact the rate of sulphur transfer to the slag. The slag basicity can be controlled using flux agents, like lime. It can become difficult to remove sulphur from the hot metal when alkalis are present, as the control of alkalis requires an acidic slag to promote transfer and reduction of the alkalis in the hot metal composition.
Silicon content in blast furnace hot metal comes from both the metallurgical coke and the iron ore pellets that are added to the blast furnace as charge material. The majority of the silicon added into the blast furnace is removed as slag, and only a small portion is found in the hot metal. Higher silicon content in the hot metal requires more energy generation in the furnace. The silicon content of the hot metal will determine if it is suitable for steel manufacturing or if it is turned into foundry grade pig iron and sold for remelting to produce cast irons.
SiO2 + C = SiO + CO
SiO + C = [Si] + CO
Silicon content in the hot metal can be controlled through chemical screening of the burden materials, utilizing materials that have a lower content of SiO2. Silicon content can also be controlled through blaster furnace operation, a reduction in the raceway adiabatic flame temperature (RAFT) can decrease the reduction rate of SiO2 to SiO. This same effect can also be achieved through a lower cohesive zone height which decreases the temperature at the tuyere level.
Manganese is added into the blast furnace in the burden material, primarily in the iron ore and recycled BOF slag. Manganese is added into the furnace in the form of MnO. The manganese transfers to the hot metal through a smelting reaction which forms CO gas. MnO can only be reduced through the use of carbon. The reduction of MnO to Mn will be prevented when the MnO reacts to combine with silica instead of the carbon.
MnO + C = [Mn] + CO
SiO2 + MnO = MnSiO3
Manganese content can be controlled through the temperature conditions of the blast furnace. The smelting reduction reaction is a strong endothermic reaction and can cause a chilled hearth condition if not properly controlled. This is why temperature control during the smelting reaction is key, ensuring the smelting occurs in locations with enough heat energy available to be consumed in the reaction. To prevent the MnO from reacting with the silica, deliberate control of the flux agents to create a more basic slag is desirable, as an acidic environment will promote the combination. Adding flux agents is not without concern, as it can lead to an increase in the levels of slag generated and lower the amount of retained manganese in the hot metal.
The temperature of the hot metal upon tapping is of serious concern when the next process steps are taken into consideration. The hot metal is tapped into a transportation car and taken to the basic oxygen furnace and used to manufacture steel. If the temperature is too great it can damage the transportation, and if it is too cold it could begin to solidify within the car.
Controlling the temperature within the blast furnace can be partially accomplished with using the appropriately sized burden material. If the metallurgical coke added to the furnace is too fine then it impedes the movement of the hot metal, molten slag, and various gases in the furnace. This impediment slows the operation of the blast furnace and can cause difficulties in managing the reduction reactions of the oxides in the different zones of the blast furnace.
The temperature can also be controlled through the gaseous additions of hot blast and injected fuels through the raceways. The bosh gas ratio of the sensible heat to the available quantity of reductants, needs to be controlled with the RAFT to ensure the zones above are working efficiently to break down the iron ore and create the desired hot metal. When one looks at the different types of reduction reactions occurring within the blast furnace, it is imperative to control these reactions, and in some instances limit endothermic reactions from occurring in the inappropriate zones or locations.
Temperature control is one of the most important factors in controlling the output from the blast furnace. Managing the different zones within the furnace and the various inputs into the blast furnace allows for the temperature to be controlled and maintained at the desired levels. This tight process control produces a high-quality hot metal of the desired temperature and chemical composition.
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- D’Allesio, J. (2019). MET449: Primary Production of Steel, week 4 Blast Furnace Fundamentals notes [Word document]. Retrieved from https://avenue.cllmcmaster.ca/d2l/le/content/283876/viewContent/2329763/View
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