Development Of Assimilation Mechanism Model For Iron Ore Sintering Process

Iron ore sintering is a high-temperature physical chemistry process that depends on liquid phase generation to bond the ore particles together to form artificial lump ore with certain metallurgical properties.The sintering process involves solid reaction,liquid phase formation,dissolution and crystallization.The assimilation is the process in which iron ore reacts with lime to produce liquid phase and bond the solid phase,and assimilation is the core step of iron ore sintering and has a decisive influence on the quality of sinter.Based on the sintering mechanism,this thesis developed a solid reaction model for particles,dissolution model for particles and iron ore sintering assimilation model.The concept of initial calcium ferrite was proposed,and the solid reaction of calcium ferrite and TiO2 was studied.The digital analysis for whole sintering process was carried out.The thesis aims to provide a basis theoretical for the sintering process under complex raw material conditions.The main research contents and conclusions are as follows:? Development a solid reaction model for particels.Considering the particle shape,particle size distribution and the diffusion rate of reactant components,a solid reaction kinetic model for particles was developed based on reaction interface area calculation,and the solid reaction process is predicted based on this model.The particle shape is divided into primary shape and secondary shape,the specific surface area of the particle increases with the particle size decreases.The solid reaction process for particels is divided into three steps:1)the rapid reaction on the contact surface,2)the longitudinal development of the product on the contact surface and 3)the diffusion of the reactant.This model introduced the particle surface utilization coefficient a*to indicate the contribution rate of the particle surface to the reaction interface.The model includes the particle size distribution function f(r),the particle morphology function S(r)and the diffusion concentration function C(x),Get the reaction kinetic equation:m=xC/xBMc?a*?0+??0+? C(x)S(r)f(r)drdx.A large number of reported experimental data were used to verify this model,and the results shown that the model had good predictive effect on the solid reaction of particles.?Development of a dissolution model of the particles liquid phase.Particle dissolution has three situations:dissolution reaction controlled,diffusion controlled,and dissolution reaction and diffusion together controlled.Based on the steady-state process,the total dissolution rate equation is derived.The total dissolution rate equation can be expressed as the ratio of the dissolution driving force to the sum of the resistance of each step.For the diffusion-controlled dissolution process for particles,considering the particle size of the solid phase,the amount of the initial liquid phase,the diffusion of the components in the liquid phase,etc.,a diffusion-controlled particle dissolution kinetics model was developed,and the factors affecting the dissolution rate were explored.For the dissolution of spherical particles,theoretically,the liquid phase volume fraction has a minimum value of ?liq(min)=8.37%.As the dissolution process progresses,the average concentration of solute in the liquid phase increases with time and the dissolution rate decreases.The normalized diameter of the particles first decreases rapidly and then decreases slowly with the time.?Based on the solid phase reaction,liquid phase formation and cooling crystallization process in the sintering process,the iron ore sintering assimilation mechanism model was developed.Linear equations and polynomial equations are used in present model to describe the temperature changes of solid phase reaction stage and liquid phase formation stage.The sintered solid-phase reaction model is based on the aforementioned particle solid-solid reaction model,and proposes a calculation method suitable for the solid-phase reaction contact area of multiple particles,that is,the mutual contact surface between two particles can be calculated by the contact probability of the two particles.Based on the particle dissolution model developed above,a sintering liquid phase generation model was developed,and the phase transition equation of the sintering liquid phase cooling and crystallization process was proposed.and the phase distribution relationship of the sintering liquid phase cooling and crystallization process was calculated based on this.For calculating the fluidity of the sintering liquid phase,a twostep fitting method was proposed to calculate the viscosity of the calcium ferrite slag system.This method had a good predictive effect for predicting the viscosity of the calcium ferrite slag.This model calculates and predicts the solid phase reaction,liquid phase formation and dissolution,and sintered mineral phase composition during the sintering process.?The concept of initial calcium ferrite is proposed,that is,calcium ferrite formed by solid-phase reaction during the sintering process.The initial calcium ferrite content in the sinter is between 7%and 12%.The initial calcium ferrite content increases with the increase of the sintering temperature,and decreases with the increase of the particle size of the ore powder.The diffusion couple experiment and in-situ XRD were used to study the solid-phase reaction behavior of calcium ferrite and titanium dioxide at different temperatures and different reaction times.The reaction mechanism,the microstructure of the reaction interface and its formation mechanism were explored.The starting temperature of the solid-phase reaction between calcium ferrite and titanium dioxide is 1123K.In the solid-phase state,TiO2 will not combine with Fe2O3 to form Fe2TiO5.After the reaction,the interface forms a three-layer micro structure.Layer ? near the TiO2 side is composed of dense CaTiO3 and TiO2;layer ? is composed of CaTiO3,CaFe2O4,Ca2Fe2O5,Fe2O3,and layer ? is composed of CaTiO3,CaFe2O4,CaFe4O7 and Fe2O3.Cracks are formed at the boundary between layer ? and layer ?,and a large number of Kirkendall pores are formed in layer ?.The diffusion of Ca2+ and vacancies in the CaTiO3 layer during the reaction is the limiting link of the reaction.?Analysis of digital model of iron ore sintering process.According to the constructed iron ore sintering mechanism model,the sintering raw materials of a certain factory are used for simulation calculation research.Explore the influence of different raw material composition,different raw material average particle size and different temperature on the initial calcium ferrite formation,the initial liquid phase formation,the change of the liquid phase quantity,the liquid phase properties and the final phase composition of the sinter during the sintering process.The model calculation shows that the composition of the sintering liquid phase is approximately:?80%Fe2O3,?11%CaO,?5%SiO2,?2%Al2O3 and?1%MgO.The fluidity of the sintering liquid phase is mainly related to the sintering temperature,and the composition of the mixture and the particle size distribution of the raw materials have little effect on the fluidity of the liquid phase.In the liquid phase sintering stage,the amount of liquid phase increases rapidly at first and then slowly increases with time.The composition of the sintered mineral phase is approximately:?40%recycled hematite,?40%composite calcium ferrite,?4%calcium silicate,?16%unmelted hematite and?1%unmelted other gangue components.The amount of sintering liquid phase increases with the increase of the sintering temperature,and increases with the increase of the initial calcium ferrite content.With the increase of sintering temperature,the content of Al2O3,MgO,Fe2O3 in the liquid phase increases,and the content of CaO and SiO2 in the liquid phase decreases.As the sintering temperature increases,the content of calcium silicate in the sinter increases,the content of composite calcium ferrite increases,the content of recycled hematite increases,and the content of unmelted hematite decreases.The initial calcium ferrite content in the sintered ore increases with the increase in the basicity of the raw material.The amount of the sintered liquid phase increases first and then slightly decreases with the alkalinity.The content of CaO in the sintering liquid phase increases with the increase in alkalinity.The content of SiO2 in the liquid phase increases with the alkalinity.The temperature increases and decreases,and the remaining content in the liquid phase,such as Fe2O3,Al2O3,does not change much.As the alkalinity increases,the content of calcium silicate,unmelted hematite and recycled hematite in the sinter decreases,and the content of composite calcium ferrite increases.The particle size of the raw material has a significant impact on the sintering assimilation process.When the average particle size of the raw material is reduced from 70?m to 35?m,the initial calcium ferrite content increases from 7.2%to 14.5%,and the amount of liquid phase increases from 81.1%to 89.2%.At the same time,the sinter is compounded.The content of calcium ferrite increased from 37.2%to 41.5%.