Crystallization And Reduction Behavior Of Calcium Ferrites

As the main bonding phase of fluxed sinters,calcium ferrite plays an important role in solid–state reaction,liquid–phase formation and crystallization process in iron ore sintering.The mechanism on solid phase formation in calcium ferrites has been extensively studied and obtained delightful achievements since 1950s,whereas researches on the crystallization behavior after formation from liquid phase were insufficient.The crystallization process of calcium ferrite directly affects mineral composition of sinters and subsequent sinter properties in a blast furnace.In the previous work on mineralization mechanism and reduction process of sinters,industrial raw materials were usually adopted as the research samples,which should be beneficial to obtain direct results guiding the manufacturing practice,but not comprehensive to explore reaction mechanism between chemical components.This paper adopted thermal analysis and kinetics method to clarify the effect of gangue on crystallization and reduction processes of calcium ferrites.In the crystallization process of binary calcium ferrite system,the crystallization kinetics of CaO·Fe₂O₃?CF?was analyzed using non–isothermal method.Results showed that the crystallization process mainly includes two reaction stages,namely,liquid–solid transition of 2CaO·Fe₂O₃?C₂F?and peritectic crystallization of CaO·Fe₂O₃?CF?.Avrami model analysis indicated that the growth of C₂F and CF crystallization processes are controlled by a fibril–like mechanism,followed by a interdendritic–type mechanism.The activation energy of C₂F and CF crystallization was determined using Ozawa where the value of E_?lies at–382.38 and–373.83 kJ/mol,respectively,and the same model function:f???=?1–??² for the two processes was determined according to Malek analysis.The CF crystallization is promoted and C₂F crystallization is inhibited as the cooling rate increases.With the addition of 2wt.%SiO₂ into CaO–Fe₂O₃–SiO₂ternary system?CF2S system?,increasing cooling rate can also promote CF crystallization and inhibit C₂F crystallization.However,model functions of C₂F and CF crystallization remain unchanged compared with C–F system.With further increase of SiO₂ into CaO–Fe₂O₃–SiO₂ system,the crystallization of 2CaO·SiO₂?C₂S?enhances and the CaO in the liquid phase decreases,leading to the skipping of C₂F and CF crystallization stages in CF4S and CF8S system.The crystallization processes of CaO–Fe₂O₃–Al₂O₃ ternary calcium ferrite system include two crystallization stages by the order of C₂?A,F?and C?A,F?.Increasing Al₂O₃ can decrease the crystallization temperature of CaO–Fe₂O₃–Al₂O₃ system and clearly smooth both of C₂?A,F?and C?A,F?crystallization.In the crystallization process of CaO–Fe₂O₃–MgO ternary calcium ferrite system,increasing MgO can increase the crystallization temperature of CaO–Fe₂O₃–MgO system,obviously inhibit CF crystallization and even push crystallization process to occur during higher temperature range.In the reduction process of binary calcium ferrite system,Results indicated that the reduction degree and rates increase by the order of C₂F,CF,CF₂ and Fe₂O₃.Sharp analysis implied that the C₂F,CF,CF₂ and Fe₂O₃ reduction is described initially by plane–like Avrami–Erofeev?A–E?function when?<0.5 and subsequently by a cylinder–like A–E function when?>0.5.In the CaO–Fe₂O₃–SiO₂ system,CF2S and CF4S present a slight degrader in reduction degree but activate the reaction faster than CF reduction.The reduction of the samples with 8%silica is not only highly accelerated but also proceeded easily.CF8S reduction is expressed by plane–like A–E function in the whole process,whereas CF2S and CF4S reduction is expressed initially by plane–like A–E function and then by plane–like phase boundary–controlled function.In the CaO–Fe₂O₃–Al₂O₃ system,reduction degree slightly increases by the order of CF2A,CF4A and CF8A,which caused by the increasing of Fe₂O₃ in reduced samples.The reduction of CF2A and CF4A is expressed initially by plane–like A–E reaction and then by phase boundary controlled reaction,and that of CF8A is controlled first by plane–like A–E reaction then by cylinder–like A–E reaction.In the CaO–Fe₂O₃–MgO system,reduction degree decreases by the order of CF2M,CF4M and CF8M as more MgO·Fe₂O₃ contained in the reduced samples.CF8M reduction is expressed by plane–like A–E function in the whole process,whereas CF2M and CF4M reduction is expressed initially by plane–like and then by cylinder–like A–E function.A new kinetics model is proposed to elucidate the reduction of C₂F,CF,and CF₂with powder samples.This model is also suitable for powder reduction of ternary calcium ferrites.Compared with shrinking core model,the proposed kinetics model based on A–E function has the following characteristics,uniformity:powder samples receive equal concentrations of reduction agent and gas diffusion in and outside samples is homogeneous because of the micron–level size particles adopted;independence of reaction stages:reduction rates of powder reduction appear overlapping multi–peaks whereas that of pellet reduction shows monotone decreasing trend.