Study On The Formation And Evolution Of The Slag Phase In The Cohesive Zone Of Blast Furnace

Cohesive zone of the blast furnace is closely related to the blast furnace conditions and not only governs the flow state of the gas-solid-liquid multiphase in the blast furnace,but also significantly affects the heat exchange in the blast furnace.A systematic and comprehensive study of the evolution of the slag phase in the blast furnace cohesive zone is the key to respond to the new situation of blast furnace raw material adjustment and to achieve efficient low carbon smelting.In this paper,based on the analysis results of actual samples taken from the cohesive zone of the first blast furnace with a volume of over 1000 m3 in China,the composition of the slag phase in cohesive zone is systematically analyzed,the slag phase system is established and the charge behaviour in cohesive zone is simulated via load-free experiment,to clarify the formation of slag phase in the cohesive zone.Diffusion coupling experiments combined with molecular dynamics simulations are conducted to analyze the mechanism of slag phase formation in cohesive zone.Classical softening&melting(S&M)experiment are carried out with ore type,distribution and proportion as variables to establish the relationship between the S&M behaviour of the burden and the formation of the slag phase.XRD,ICP,SEM-EDS analysis and thermodynamic calculations are used to systematically analyze the real material phase of the actual cohesive zone.The experimental parameters were selected to simulate the behaviour of the cohesive zone under laboratory conditions and to provide a closer match to the actual blast furnace working.The reduction of charge along the cohesive zone is 48.61%,and the temperature is 1100℃.The sinter and pellet bonding surface along the cohesive zone is dominated by Fe-Si system containing small amounts of CaO and MgO and silica-aluminate slag containing alkali metals,in which the highest enrichment of K and Na is 12 and 8 times that of the charge,respectively.The slag phase in the middle and upper part of the cohesive zone is Fe-Ca-Si with a small amount of MgO and Al2O3 and the slag phase containing alkali metals.,and multiple kinds of slag systems co-existing in this region.In the lower part,the various slag systems fuse with each other to form a low-magnesium Ca-Mg-Al-Si quaternary slag phase.The formation process of the slag phase in typical charge is analyzed in detail.The concept of high temperature interaction of the burden is refined into "formation interaction" and "composition interaction".Propose "initial slag","endogenous slag" and "interface slag " concept to distinguish the slag phase in the formation process of cohesive zone.The initial slag phase of individual ore is FeO-CaO-SiO2(1100℃),the endogenous slag in individual ore and the interface slag between homogeneous ore is of the same type,both being FeO-CaO-MgO(Al2O3)-SiO2.The interface slag phase of a homogeneous ore reacts with each other to promote the production of the liquid phase.The interface slag phases produced between dissimilar ores are mainly FeO-CaO-SiO2 and FeO-CaO-MgO(Al2O3)-SiO2,where the formation of FeO-CaO-SiO2 involves both formation interaction and composition interaction.FeO-CaO-SiO2 and FeO-CaO-Al2O3-SiO2 are the slag phases that mainly produce the liquid phase,and the cohesive zone slag phase exhibits high Al2O3 and low MgO characteristics.The diffusion experiments,combined with molecular dynamics simulations,are designed to clarify the mechanism of the slag phase formation,in which the diffusion behaviour of CaO-2FeO-SiO2 and FeO-2CaO-SiO2 is analyzed to clarify the pathway for the formation of the initial slag phase in individual ore.The poor diffusion ability of Mg2+to the initial slag result in the low content of MgO in cohesive zone slag.Ca2+ enhances the diffusion ability of original system and Fe2+enhances the ion diffusion capacity of the MgO-containing system.So Mg2+ and Al3+ diffuse more easily into FC2S system,reflecting the path selection of the endogenous slag phase formation process in individual ore.The interaction of the interface slag phase is further explained.The F2S and FC2S structures are more compatible with Al2O3 and therefore form interface slag phase dominated by FeOCaO-Al2O3-SiO2.The mechanism for the better permeability of the ore under mixed distribution is analyzed in terms of microscopic slag phase formation by classifying the reaction of the furnace charge during softening&melting into three types:P-P,S-S and SP.The softening&melting performance of the burden is directly related to its slag phase formation,and the main influence on the permeability of the cohesive zone is the melting interval.The softening and melting intervals of the burden correspond to the liquid phase formation and the viscosity change temperature intervals,respectively.The comprehensive influence of the change in charge ratio on the composition of the typical slag system(FeO-CaO-SiO2,FeO-CaO-MgO-SiO2 and FeO-CaO-Al2O3-SiO2)is analyzed.