Development and evolution of hot metal pretreatment
Hot metal desulfurization Hot metal desulfurization started with ladle refining, adding sodium hydroxide or soda to the hot metal container. Subsequently, top-adding calcium carbide, bottom-blowing nitrogen stirring and shaking hot metal ladle processes were adopted. However, due to the deviation in reaction efficiency, the generation of acetylene gas and the difficulty in using it on large-tonnage hot metal containers, these technologies were gradually phased out. In the torpedo car injection method, the desulfurizing agent is dispersed in the hot metal and reacts. The reactants float on the hot metal. The desulfurizing agent is calcium carbide or sodium carbonate powder. Due to the weak stirring ability of this process, the desulfurization slag cannot be reused. Kawasaki Steel (now JFE Steel) developed a calcium oxide powder containing surfactant as a substitute for calcium carbide.
In 1965, Fuji Steel (now Nippon Steel) developed a mechanical stirring method called KR (Kanbara reactor). In the KR process, the desulfurizing agent enters the hot metal through the vortex generated by the rotating impeller and reacts with the hot metal interface. The reaction efficiency of the KR method is higher than that of the injection method. At 1400℃, the KR method of CaO-10%CaF2 can reduce the sulfur content in molten iron to below 5 ppm. The KR method has become a popular standard technology since the 1970s. On the other hand, the problem of the KR process is to reduce the amount of desulfurizing agent. Without CaF2, desulfurization to below 50 ppm can be achieved by using only CaO. The following will discuss newer technologies. In addition, Mg-CaO molten iron desulfurization was also industrialized during this period.
Dephosphorization of molten iron and differences between functions The double slag method is a technology that uses the converter to dephosphorize molten iron. However, the high phosphorus slag produced by desiliconization and dephosphorization requires intermediate slag discharge, which has problems such as large slag volume, slag splashing, poor molten iron recovery rate and low production efficiency. In response to the increasing demand for low phosphorus steel, a molten iron dephosphorization process was developed to reduce the dephosphorization burden of the converter. Desiliconization is used before dephosphorization to reduce the unit consumption of dephosphorizing agent CaO. Besides oxygen as an oxidant for Ca, rolled iron oxide scale, sintered ore powder, and iron ore powder can also be used as oxygen sources.
In torpedo cars, dephosphorization occurs as a short-lived reaction, with slag floating above the molten iron. Desulfurization, on the other hand, is a long-term process occurring between high-basicity slag and molten iron. This process has been commercialized since the early 1980s. During this process, the dephosphorized slag needs to be discharged, and the dephosphorized molten iron undergoes decarburization in the converter.
Dephosphorization using soda ash has also been investigated. Soda ash has shown good dephosphorization effects, but problems exist regarding the severe corrosion of refractory materials by Na₂O, the difficulty in handling Na₂O slag, and the evaporation and refining costs of Na(g). Due to these issues, the application of soda ash has been limited, while CaO-based fluxes have been widely used.
Reduction dephosphorization using Ca-CaF2 or CaC2-CaF2 fluxes has also been studied, but this process has not yet been commercialized due to the need for stabilization of the reduction product Ca3P2 and other issues.