Research On Sintering Technology Of Vanadium Titanomagnetite With The Preformed Calcium Ferrite

The iron and steel metallurgy process based on“sintering-blast furnace-converter”process is dominant in China’s iron and steel production,and this state will last for a long time.China is rich in vanadium titanomagnetite resources.At present,a complete process system of iron and steel metallurgy and vanadium metallurgy based on“blast furnace-converter”process and titanium metallurgy based on electric furnace process has been built with vanadium titanomagnetite as raw material,realizing the extraction and utilization of iron,vanadium and titanium.Vanadium titanomagnetite is a typical refractory iron ore with high TiO₂ content.In order to ensure the sinter quality,it is usually need to increase the fuel content to increase the amount of liquid phase,however,which also increases the production costs and environmental pressure.In order to improve the enrichment rate and recovery rate of vanadium titanomagnetite resources,it is of great practical significance to increase the vanadium titanomagnetite ratio in sintering process.Therefore,how to increase the sintering ratio of vanadium titanomagnetite while ensuring the sinter quality and reducing the fuel content has become an urgent problem for enterprises.This thesis took vanadium titanomagnetite as the research object and proposed a new sintering technology of preformed calcium ferrite,in order to strengthen the sintering process of vanadium titanomagnetite,improve the sintering theory of vanadium titanomagnetite,and provide theoretical basis for low carbon sintering with high vanadium titanomagnetite ratio.The main research contents and conclusions are as follows:（1）Thermodynamic calculation results showed that the addition of preformed calcium ferrite will not significantly affect the solidus and liquidus temperature of sintering mixture,but the amount of sintering liquid phase increases slightly.In the roasting process of calcium ferrite and vanadium titanomagnetite mixture,the amount of liquid phase and the extent of solid-liquid assimilation increase with the increasing of roasting temperature and time.Increasing the roasting temperature,the proportion of unreacted vanadium titanomagnetite decreases,and the grains of magnetite tend to be regular;increasing the roasting time,the apparent viscosity of the system decreases,the density of matrix increases,and the granular structure transfers to the melting corrosion structure.When increasing the oxygen partial pressure from 0.005 atm to 0.05 atm,acicular calcium ferrite and secondary hematite begin to appear,and the melting corrosion structure is widely distributed in the sintering matrix;with the further increase of oxygen partial pressure,the morphology of calcium ferrite transfers from acicular to block,and the content of hematite increases significantly.The addition of preformed calcium ferrite can effectively increase the amount of liquid phase,strengthen the solid-liquid assimilation reaction,promote the development of melting corrosion structure,increase the amount of SFCA,and increase the density of sintering matrix.In the heating process of the mixture,there are three endothermic peaks corresponding to the formation of calcium ferrite liquid phase:CF+CF₂→L,melting of SFCA-I and SFCA;the addition of preformed calcium ferrite can reduce the generation temperature of the initial liquid phase,promote the melting of iron oxide,increase the amount of sintering liquid phase,and increase the content of calcium ferrite precipitated during crystallization.The residual slag in vanadium titanomagnetite-calcium ferrite wetting system are composed of Ca O·Fe₂O₃,Ca₃TiFe₂O₈ and（Fe,Mg）₃O₄;compared with magnetite,TiO₂ and Mg O in vanadium titanomagnetite changes the product composition and inhibites the wetting behavior of calcium ferrite on vanadium titanomagnetite substrate.（2）Andradite titanium(Ca₃Fe₂Si_1.58Ti_1.42O₁₂),an important titanium-containing phase in vanadium titanomagnetite sinter,and titanium calcium ferrite（Ca₃TiFe₂O₈）,a newly discovered phase in vanadium titanomagnetite-calcium ferrite wetting system were prepared by solid state reaction method,and their metallurgical properties were tested.The compressive strength of andradite titanium is 11.35～16.27 Mpa and the melting point is 1229℃;the non-isothermal reduction results showed that andradite titanium is reduced to Fe,perovskite and wollastonite in a single step;its initial reduction temperature is about 500℃,when the temperature rises to 900℃,the reduction degree is only 25%,and the complete reduction temperature is 1150℃.The initial reduction temperature of titanium calcium ferrite is about 700℃,and the reduction degree can reach 64%when the reduction temperature is 900℃,the reduction products are Fe,Ca O,and Ca O·TiO₂;titanium calcium ferrite is close to CF and CF₂in terms of the medium temperature reducibility.The medium temperature（900℃）reducibility of typical phases in sinter is in the order of hematite>magnetite>hemicalcium ferrite>monocalcium ferrite>sharyginite>dicalcium ferrite>kirschsteinite>andradite titanium.The high temperature heat capacities of andradite titanium and titanium calcium ferrite were obtained.Vanadium titanomagnetite substrate will not be wetted by andradite titanium,the reason is that TiO₂ diffuses into the melt and precipitates a large number of perovskite crystals with micron size（1-2μm）,which limites the spread of melting slag on the substrate.After the wetting of andradite titanium on vanadium titanomagnetite substrate or Fe₃O₄ substrate,macroscopic cracks appeare in the slag-base interface.（3）When the vanadium titanomagnetite ratio is increased,the sinter yield and tumbler strength are stable at first and then decrease,the turning point is 70%.The basicity increases from 2.0 to 2.8,the yield increases steadily;the content of perovskite and calcium ferrite in sinter increases.With the coke ratio decreasing from 5.5 wt.%to3.4 wt.%,the sinter yield decreases linearly;After the decrease of coke ratio,the reducing atmosphere in sintering bed is inhibited and the sintering temperature is decreased;the content of magnetite and perovskite in sinter decreases gradually.With the proportion of preformed calcium ferrite increasing from 0 wt.%to wt.4.64%,the yield and tumbler strength tend to stabilize first and then increase;The content of calcium ferrite in sinter increases,and calcium ferrite mainly distributes in the dense matrix in the form of melting corrosion structure jointly composed with magnetite;with the application of preformed calcium ferrite technology,the reducibility and low temperature reduction degradation of sinter can be improved.Reducing coke particle size to increase coke combustion rate and adopting return fine-mosaic embedding to improve feed layer permeability can effectively reduce sintering time,strengthen preformed calcium ferrite sintering technology,ensure the utilization coefficient,yield and tumbler strength,and decrease the coke ratio by 30 wt.%.（4）The core mechanism of preformed calcium ferrite sintering technology is to increase the content of initial calcium ferrite,promote the formation of initial liquid phase and the development of solid-liquid assimilation,increase the amount of sintering liquid phase,decrease the proportion of unreacted iron ore,inhibit the generation of perovskite,improve the bonding effect of sintering liquid phase,optimize the micro morphology of sinter,and develop acicular calcium ferrite and melting corrosion structure.Based on the theory of sintering thermal regime,the linear relationship between the melt quantity index and the sinter yield,as well as the coke ratio and the melt quantity index were clarified,and the concept of equivalent heat of preformed calcium ferrite was proposed;through the calculation of material balance and heat balance,the specific value of equivalent heat of preformed calcium ferrite is 13846.96k J/kg.The thermal regime model under traditional conditions and preformed calcium ferrite sintering conditions was established,and the relationship between melt quantity index and yield under different coke ratio,preformed calcium ferrite proporation and coke particle size was analyzed;The proportioning model of coke ratio and preformed calcium ferrite proporation was established,and the prediction results of this model can be expressed by a quadratic function:M_CF=0.55483·M²_Coke-6.63821·M_Coke+20.92557,which improves the basic theory of preformed calcium ferrite sintering.