Fundamental Research On The Competitive Oxidation Reactions Of O₂-CO₂ And Fe-C Melts

CO2,as a greenhouse gas,has affected human development and environment if the emissions are not properly controlled.In the steel industry,it has very important practical significance to study the use of CO2 as a resource,and ways to reduce its emissions for promoting the sustainable development of society,economy and environment and it plays a necessary supporting role in the national strategy of "carbon peaking and carbon neutrality".In the converter steelmaking process,when CO2 was introduced as an oxidizing gas to substitute part of O2,multiple chemical reactions happened during the O2-CO2 decarburization process,accompanied by a certain propotion of post combustion.The complex transformation process has restricted the technological development and wide application of CO2 smelting.Therefore,exploring the mechanism of the competitive oxidation reactions of O2-CO2 and solving the key parameters such as the utilization ratio is of great significance to promote the process development of CO2 application in converter steelmaking.In this thesis,18O2-13CO2 dual isotope gas mass spectrometry method is firstly proposed,using different reactions in the decarburization process to generate isotope gas molecules of different mass numbers,distinguishing the different chemical reactions involved in the precipitation of O2-CO2 mixed gas.The decarburization reaction mechanism was studied by combing with sampling method,chemical analysis,thermodynamic calculation and so on.The main research results are as follows:The decarburization process was dynamically analyzed in real time using 18O213CO2 dual isotope gas mass spectrometry.In the present experiment conditions,the O2-CO2 mixed gas reached the molten surface and undergoes decarburization reactions.For O2,it can react with[C]to generate CO or CO2.As the reaction proceeds,due to the post combustion reaction of O2 with the generated CO,the proportion of O2 reacting with[C]to directly generate CO2 at the reaction interface decreases,on the contrary,the proportion of O2 to generate CO increases,and finally reach a dynamic equilibrium.If the gas supply is sufficient,O2 can generate CO or CO2 at the interface,otherwise O2 only generates CO.For CO2,regardless of whether the gas supply intensity is sufficient,CO2 can participate in the reaction,and the participation ratio is still larger than 97%.The decarburization competitive of O2 and CO2 to Fe-C molten was analyzed.From the decarburization rates,when the injection mixed gas ratio was PO2/PCO2=4:1,regardless of whether the gas supply was sufficient,the vO2-C/vCO2-C ratio was approximately 4:1.It indicated that the decarburization capacity of O2 and CO2 was basically the same,and a partial substitution of 20%of O2 with CO2 had the similar decarburization effect.Moreover,due to the high utilization of CO2 and the characteristics of being less affected by the injection height,it is beneficial to ensure the utilization of mixed gas,so as to ensure a certain decarburization rate.The rate-controlling mechanism of the O2-CO2 decarburization process was studied.When the carbon content was larger than 0.5 wt%,the influence of carbon in liquid phase transfer can be ignored.Therefore,the decarburization rate of O2 was controlled by the mixture control of gas-phase mass transfer and interfacial chemical reaction;as the increase of CO2 partial pressure,the rate-controlling step of CO2 decarburization reaction changed from the mixture control of gas-phase mass transfer and interfacial chemical reactions to gas-phase mass transfer control only.The apparent activation energy of the O2-CO2 decarburization process was calculated to be 21.5 kJ·mol-1.The kinetic of O2-CO2 decarburization rate controlling model was established.Through thermodynamic calculations and gas-phase mass spectrometry experiments,the effects of O2-CO2 mixed injection pattern in different decarburization stages on decarburization rate and furnace gas composition were investigated.It was found that the decarburization process was divided into first,middle and final stages according to carbon content and temperature.The effect of the gas supply pattern of injecting O2-CO2 mixture in any stage and pure oxygen in other stages was better than the pattern of O2-CO2 mixed injection in the whole process.The injection pattern of the mixture in the first and middle stages was conducive to adjusting the temperature of the molten pool,while the injection pattern of the mixture in the middle and final stages was more conducive to improving the quality of furnace gas save more oxygen.It was proposed that the optimal mixed gas injection pattern was to use 70%O2-30%CO2 mixed gas in the middle stage,and inject pure O2 in the other stages,which can save 6.3%O2 and increase the proportion of CO in the furnace gas by 1.2%.