| Using limestone instead of lime for slagging during the converter steelmaking process has a significant cost advantage,and it can also reduce the environmental pollution caused by the calcination and transportation of lime.In addition,it has been paid widespread attention at home and abroad with the demand for reducing cost and increasing economy and environmental protection.However,it is easy to slow the slagging rate at the initial blowing stage and influence the hot metal dephosphorization,which is mainly due to that the limestone would be decomposed before dissolving,and the decomposition of limestone is a strong endothermic reaction.In recent years,many steel plants have carried out a lot of production practices about using limestone instead of lime for slagging,but these application effects are different.Considering the characteristics of limestone decomposition before dissolution,there is a lack of systematic study about the effect of decomposition characteristics of limestone on the dissolution behavior of in-situ generated lime and dephosphorization of hot metal.There also exist great divergences about the understanding of using limestone instead of lime for slagging in converter steelmaking process.Therefore,combined with the development trend of energy conservation,emission reduction and environment-friendly metallurgy,to understand the metallurgical effect of using limestone instead of lime slagging as the research target,the decomposition characteristics and dissolution behavior of limestone in the converter slag were systematically studied in this paper by simulating the converter slagging condition in the initial blowing stage under the laboratory conditions.Moreover,the effects on the dephosphorization of hot metal were also investigated.The following conclusions have been drawn:(1)There was no obvious explosion phenomenon observed during the limestone decomposition in the slag at 1300?1400?,but some cracks across the limestone sample were generated at high temperature.The decomposition rate of limestone increased with increasing the temperature or decreasing the particle size of limestone.In order to complete the decomposition of limestone during the steelmaking process,it is necessary to control the particle size of limestone,and properly increase the bath temperature or oxygen supply intensity.Moreover,CaO grains arranged regularly and plump,and the porosity of lime decomposed from limestone was higher with the temperature increased from 1300? to 1400?.After the limestone had been decomposed completely,the average porosity of the in-situ generated lime was about 42.0%?43.7%and the grain size of CaO was less than 1 ?m.The porosity of the in-situ generated lime at 1300?1400? was equivalent to the calcined lime by the traditional technology and the grain size of CaO was less.(2)The results of the limestone decomposition kinetics showed that the limestone decomposition in early converter slag was a mixed control of chemical reaction and heat conduction through the lime layer.Furthermore,the reaction front temperature and the kinetic parameters including the effective thermal conductivity and the chemical reaction rate constant were obtained based on a nonisothermal kinetic model.The effective thermal conductivities of the lime layer ?e were 0.26?0.50W/(m·K),0.56-1.32W/(m·K),1.17?2.51W/(m·K),respectively and chemical reaction rate constants kr were 7.97×10-4?2.92×10-3m/s,1.60×10-3?4.02×10-3m/s,3.30×10?36.34×10-3m/s,respectively at 1300?,1350? and 1400?.(3)The dissolution process of limestone in converter slag can be divided into three stages:stagnation stage,coupling stage and sole dissolution stage.Higher slag temperature was conducive to reduce the duration time of the stagnation stage.During the coupling stage,the decrease of dissolution rate of the lime was due to the temperature drop caused by the limestone decomposition.The formed 2CaO·SiO2 layer hindered the dissolution of the generated lime during the sole dissolution stage.In addition,the CO2 generated from decomposition reaction could improve the stirring ability of bath and provide a certain enhancement of generated lime dissolution at the coupling stage.(4)The results of the limestone dissolution kinetics showed that the coupling stage of limestone dissolution was controlled by the chemical reaction.However,during the sole dissolution stage,boundary layer dissolution was the rate controlling step in the slag with basicity of 0.5,whereas a combination of controlling step including diffusion through the product layer and diffusion through the boundary layer was the rate controlling step in the slag with basicity of 1.0.With increasing the temperature from 1300? to 1400?,the chemical reaction rate constant increased from 8.39×10-6m/s to 3.49×10-5m/s in the slag with basicity of 0.5 and from 6.43×10-6m/s to 3.25×10-5m/s in the slag with basicity of 1.0 respectively.On the other hand,the diffusion coefficient through boundary layer increased from 1.43×10-7m2/s to 2.59×10-7m2/s in the slag with basicity of 0.5 and from 1.66×10-7m2/s to 2.35×10-7m2/s in the slag with basicity of 1.0 respectively.Moreover,the effective diffusion coefficient through product layer increased from 5.66×10-9m2/s to 2.27×10-8m2/s in the slag with basicity of 1.0.The activation energies of limestone dissolution during the coupling stage in the slag with basicity of 0.5 and 1.0 were 315kJ/mol and 376kJ/mol respectively,and on the other hand,the activation energies for the sole dissolution stage were 139kJ/mol and 284kJ/mol respectively.(5)With increasing the substitution ratio of lime from 0 to 50%,the oxidation rates of[C],[Si],[Mn]all increased and the oxidation rate of[P]decreased at the initial blowing stage,and the oxidation rates of[C]and[P]both decreased at the intermediate blowing stage.After blowing 5 minutes,adding limestone instead of 25%mass of lime for slagging could avoid a big temperature drop of bath caused by the endothermic decomposition reaction,which could increase the oxidation rates of[C]and[P]at the initial and intermediate blowing stages.The endpoint[C]and[P]contents decreased from 0.053%and 0.028%to 0.027%and 0.022%respectively.The dephosphorization ratio of hot metal increased from 90.70%to 92.70%.(6)When using all lime slagging,the concentration of CO was 39.3%?40.6%at the intermediate blowing stage.Compared with the slagging with all lime,when the substitution ratio of lime were 25%and 50%,the temperature drop of bath were 9?49? and 17?104?,respectively at the whole blowing process.The concentration of CO decreased by 2.7%?2.9%and 4.4%?4.6%,respectively at the intermediate blowing stage.After blowing 5 minutes,limestone instead of 25%mass of lime was added for slagging.The bath temperature increased by 10?26? and the concentration of CO increased by 3.4%?3.6%at the intermediate blowing stage.(7)The dephosphorization ratio of hot metal decreased with increasing the substitution ratio of lime from 0 to 50%at 1350?.However,the practice with substitution ratio of 25%achieved higher dephosphorization ratio with the rest at 1400?.Increasing the temperature of hot metal was conducive to facilitate the dephosphorization reaction rate for slagging with using limestone instead of lime,and this facilitation was enhanced with the increase of temperature and substitution ratio of lime from 25%to 50%.Therefore,it is necessary to determine the substitution ratio of lime according to the hot metal temperature for using limestone instead of lime for slagging.(8)The dephosphorization reaction rate was a mixed control of phosphorus transfer in the slag and hot metal,and the phosphorus transfer in the slag was obvious.This influence was enhanced with the increase of substitution ratio of lime from 25%to 50%.The comprehensive mass transfer coefficient of phosphorus decreased from 2.92×10-5m/s to 1.61×l0-5m/s with the increase of substitution ratio of lime from 0 to 50%at 1350?.However,it increased from 1.82×10-5m/s to 2.13×10-5m/s with the increase of substitution ratio of lime from 0 to 25%and decreased to 1.65×10-5m/s with the increase of substitution ratio of lime to 50%at 1400?. |
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