Investigations On Fundamental Theory Of CO₂ Applied In Steelmaking Processes

During BOF steelmaking process, dephosphorization, decarburization and the rise of bath temperature mainly depend on oxygen supply. With the speeding up of the smelting rhythm the intensity of oxygen supply intensity also increase, which easily leads to instability of dephosphorization, and difficulty control of decarburization, increase of metal consumption and so on. Therefore, it is possible to use CO₂ in steelmaking processes to realize the dephosphorization, decarburization and purifying the liquid steel et al, and the fundamental theory of this process will be systematically studied.Based on researches of thermodynamics and kinetics during the oxidation reaction between CO₂ and elements in molten iron, the analysis of CO₂ effect on the change of material and energy in dephosphorization furnace was studied in this paper. It was found that when the ratio of scrap was 8% and the utilization ratio of CO₂ was 85%, the smelting demand of dephosphorization furnace would be satisfied when CO₂ blowing ratio was less than 28%. Compared with injecting the pure oxygen, the oxygen consumption reduced by 17% and CO content in furnace gas increased by 8.1%.Based on the research of reaction mechanism on CO₂ involved in the dephosphorization in the molten steel, the dephosphorization slag and equilibrium state of molten pool was studies and optimized when CO₂ was taking as oxidant during steelmaking by using Factsage. Besides, it was validated that CO₂ could be used in dephosphorization converters effectively by high temperature experiments. Experimental results showed:the dephosphorization rate increased by 6.99%, the decarburization rate reduced by 0.75%, LP?phosphorus distribution coefficient? increased by 32, iron losses of slag lowered by 0.64%. Blowing CO₂ is beneficial to dephosphorization and C yield in dephosphorization converters.Meanwhile, the decarburization kinetics of CO₂ in the steelmaking process was studied through high temperature experiments. It was found that the utilization ratio of CO₂ had a positive correlation with carbon content in molten steel and the bath temperature. When the molten steel was in the process of medium and high carbon, namely, the carbon content is 1.0% to 4.0%, the average utilization ratio of CO₂ reached 89.8%. In this stage, decarburization reaction was a zero order, and a mixed-control mechanism involving gas-phase mass transfer and dissociative adsorption of CO₂ was verified. When the molten steel was in the process of low carbon, namely, the carbon content is 0.1% to 0.5%, the average utilization ratio of CO₂ was reduced from 83.5% to 40.8%. In this stage, first order kinetics was confirmed, and the carbon mass transfer was the rate-determining step. The utilization ratio of CO₂ is inferior to O2, which indicates that the decarburization capacity of CO₂ is weaker than that of O2, and the oxidation ability of CO₂ can be used to control the reaction rate during different smelting stage.Based on the analysis of decarburization mechanism of CO₂, metallurgical effect by bottom blowing different gases was studied in induction furnace. It was found that the average decarburization rate of bottom blowing with O2 and CO₂ was basically identical. It was also found that the nitrogen content of bottom blowing with Ar/N2 was 2 to 6 times than that of CO₂/O2, and when the bottom blowing gas was CO₂, the terminal oxygen content is much lower than O2. The kinetics of dissociation or absorption of nitrogen with different bottom gases was analyzed. The results showed that when bottom blowing CO₂/O2/Ar, the denitrification reaction in molten steel was in accordance with the second order kinetics.The results of this thesis will provide data support for the industrial application of CO₂, and the utilization of CO₂ as resource in steelmaking processes will be realized.