Fundamental Study On Indirect Carbonation And Iron Enrichment Of Modified Steel Slag

Steel slag is an alkaline industrial solid waste generated during the steelmaking process.Finding ways to utilize this waste as a resource is an urgent problem for promoting green and low-carbon development of the iron and steel industry.The carbonation of steel slag is one of the crucial technologies for effectively utilizing steel slag resources and reducing CO2 emissions,this technology represents a critical means of promoting green and low-carbon development of the iron and steel industry.This study focused on basic oxygen furnace steel slag and proposes a twostep leaching-indirect carbonation process with the aim of utilizing the alkalinity of the slag,which can serve as a reference for the indirect carbonation of high-alkaline steel slag.Furthermore,this dissertation proposed a process of high-temperature modification-indirect carbonation for separating and collaboratively utilizing the calcium and iron elements in steel slag.By employing crystal structure theory,the leaching activity of various minerals of the steel slag were explained at the ion coordination level.Moreover,thermodynamic calculations were conducted to investigate the separability of the calcium and iron elements in the slag.The impact of high-temperature modification on the leaching of calcium and the enrichment of iron of steel slag was also studied via adjustments in SiO2 addition,firing temperature,holding time,and solid waste addition.Building upon this foundation,the leaching behavior of modified slag was studied using the Drozdov equation,and the kinetic equation of the leaching reaction of the modified slag was obtained by calculating the apparent activation energy and pre-exponential factor through linear fitting of lnkm with 1/T.Subsequently,the carbonation precipitation reaction of the modified slag was studied to comprehensively evaluate its CO2 fixation capacity.The specific research content is as follows:Experimental research on the two-step leaching-indirect carbonation of steel slag was initially conducted in this dissertation.The results indicated that the twostep leaching process can increase the pH of the leachate and the Ca2+ leaching rate,providing references into the indirect carbonation of high-alkaline steel slag.However,the Ca2+leaching rate was limited by the content of alkaline minerals in the slag.To address this limitation,high-temperature modification can be employed to modify the mineral composition of the slag,thereby improving the Ca2+leaching rate and the total iron content of the leaching residue.Moreover,to theoretically explain the impact of high-temperature modification on the leaching of calcium and the enrichment of iron of steel slag,prior to the modification experiments,this study conducted computational analysis of the crystal structure and leaching activity of common minerals in steel slag using Pauling’s Second Rule.The results indicated that the O2-valences of β-Ca2SiO4 and Ca3Mg(SiO4)2 were not fully saturated,resulting in their highly leaching activity.The O2-valence of Fe3O4 and MgFe2O4 were fully saturated,resulting in the low reactivity in their chemical properties.The stability of Ca2Fe2O5 falls between the two types of minerals mentioned above.During the investigation of the effect of SiO2 addition on the leaching of calcium and the enrichment of iron of steel slag,it was observed that with a decrease in alkalinity,the content of Ca2Fe2O5 gradually decreased while the content of Ca2SiO4 gradually increased.At an alkalinity of 2,the Ca2+leaching rate and the total iron content of the leaching residue reached their maximum values of 84.46%and 48.39%,respectively.As the alkalinity was further reduced,Ca3Fe2Si3O12 was generated in the modified slag,resulting in a gradual decrease in the Ca2+leaching rate and the total iron content of the leaching residue.After determining the optimal SiO2 addition amount,this study investigated the effects of firing schedule.The results show that a firing temperature of 1500℃was favorable for the nucleation and growth of Ca2SiO4,and ST1500 modified slag exhibited the highest Ca2+leaching rate and the total iron content of the leaching residue,reaching 94.62%and 53.94%,respectively.Extending the holding time was beneficial for the generation of Ca2SiO4,and the relationship between the grain size of Ca2SiO4 and the holding time was well fitted by the function of y=0.2344x0.4298.As the holding time increased,the Ca2+ leaching rate and the total iron content of the leaching residue gradually increased.Based on determining the optimal firing temperature and holding time,the impact of solid waste addition on the leaching of calcium and the enrichment of iron of steel slag was further investigated.Due to the formation of the low-reactivity compound Ca2Al2SiO7 in the solid waste modified slag,there is a loss in the Ca2+leaching rate and the total iron content of the leaching residue.The experimental results showed that using coal gangue for modification achieves better results,and the Ca2+ leaching rate and the total iron content of the leaching residue of MR2.5 modified slag reach 88.93%and 41.46%,respectively.After obtaining the optimal solid waste modified slag,the leaching reaction kinetics of the modified slag was studied.The results show that the Drozdov equation,which includes the self-inhibition coefficient,can well describe the leaching behavior of steel slag.High-temperature modification can increase the rate constant of the leaching reaction of steel slag.As the temperature of leaching reaction increased,the reaction rate constant gradually increased.The apparent activation energy indicated that internal diffusion was the limiting step of the leaching reaction.Finally,the carbonation precipitation reaction of the modified slag was studied.The Ca2+carbonation rate increased with the increase of pH.An increase in temperature could enhance the Ca2+carbonation rate,but excessively high temperature was unfavorable for the dissolution of CO2 and the progress of the carbonation reaction.The Ca2+carbonation rate increased with the increase of stirring rate and CO2 flow rate.This study improved the Ca2+leaching rate,the total iron content of the leaching residue,and the CO2 sequestration capacity of steel slag by modifying its mineral composition,and realized the separation and recovery of calcium and iron elements,which has positive implications for better utilizing steel slag as a major industrial solid waste and for achieving green and low-carbon development of the iron and steel industry.