The Characteristics Of The Lime Calcined Rapidly At Super High Temperature And Its Dissolution Behavior In Converter Slag

As an important industrial raw material,lime is widely used in metallurgy,construction,power supply and other industries.It is applied for desulfurization and dephosphorization especially in metallurgical industry.Its reactivity directly influences the melting speed of slag and consequently,slag basicity,slag viscosity and chemical reactivity between slag and steel.The use of limestone instead of active lime can utilize the physical heat of molten steel directly to decompose the lime,promote the dissolution of the lime in the converter slag,reduce the cost of the steel-making slagging and CO₂emission of the rotary kiln and solve the problems of moisture and carbonation during cooling and transportation of lime.Studying the feasibility of using limestone instead of lime steel-making in converter,limestone decomposition has the high activity under what conditions and how to improve the slagging effect in steelmaking by limestone have gradually become the focus of people's attention.Based on the material balance and heat balance of converter,this paper establishes the relationship between surplus heat of converter and the theoretical substitution ratio of CaO and analyzes the influence of hot metal composition and temperature,the amount of scrap added and the tapping temperature of converter on the theoretical substitution ratio of CaO.The changes of physical and chemical properties,such as activity,porosity,specific surface area,bulk density,were studied under the condition of different particle size and time.Limestone calcined product in smelting temperature 1200°C?1500°C of BOF,and the micro-structure of samples were investigated with SEM+EDS.Putting the limestone into the converter for slag-making after the slag splashing protection furnace and blowing were simulated and the restriction link was deduced and fitted with the experimental data in order to determine the applicable model of limestone decomposition.The dissolution of limestone in the converter slag was studied,the change of dissolution rate and morphology during dissolution were discussed and the overflow behavior of CO₂from lime to converter slag and the random dissolution behavior of limestone in converter slag were analyzed.The following conclusions are drawn:?1?The heat income of the converter mainly comes from the physical heat of molten iron and the oxidation exothermic of elements,increasing the hot metal temperature and carbon content of molten iron can significantly improve the CaO theoretical substitution ratio.The effect of Si content in molten iron on theoretical substitution ratio of CaO is related to C content and temperature of molten iron.When the temperature of hot metal entering the furnace is lower than 1300°C,increasing the content of Si in hot metal can increase the theoretical substitution ratio of CaO;when the temperature of hot metal entering the furnace is higher than 1300°C,the theoretical substitution ratio of CaO decreases with the increase of Si content in hot metal instead.When the final slag alkalinity of the converter and the tapping temperature are fixed,the heat required for the melting of the scrap steel is the largest heat expenditure term.Therefore,educing the amount of scrap can greatly improve the theoretical substitution ratio of CaO for converter steel-making with limestone instead of active lime.?2?When calcined at 1200°C?1500°C,the limestone forms a three-layer structure with dense sintered layer,porous reaction layer and undecomposed limestone layer.When the particle size is the same,the higher the temperature,the greater the limestone conversion rate;when the temperature is the same,the smaller the particle size,the greater the limestone conversion rate.Limestone with small grain size and less impurity content,large porosity,large number of micro-pores and specific surface area,has the best reactivity and suit for limestone steel-making.?3?With the increase of calcination temperature,the CaO grain size and pore size become larger,while the number of micro-pores becomes smaller.As the calcination time is prolonged,the recrystallization of CaO grains grows very fast,the micropores disappear rapidly,and the structure becomes very dense,resulting in a serious phenomenon of‘overburning'.The porosity of lime increases first and then decreases with the increase of calcination time,and the porosity of limestone reaches its peak when it is completely decomposed,and the peak porosity decreases with the increase of calcination temperature.The variation of bulk density is opposite to that of porosity.The specific surface area of lime decreases continuously with the increase of calcination time,and decreases with the increase of calcination temperature.?4?The activity of lime is closely related to its porosity and specific surface area,the best lime reactivity can be obtained with high porosity and high surface area.The maximum activity of 12mm-15mm particle size lime calcined at 1400°C,1450°C and1500°C is 406 mL,410 mL and 403 mL respectively.?5?The high temperature decomposition of limestone with large particle size putting into converter was simulated after slap-splashing furnace-protecting,and its rate-limiting link is the interfacial chemical reaction.Using thermal analysis kinetics to explain the interfacial chemical reaction control rate,and derive it is satisfied with random nucleation and subsequent growth pattern?A-E?,the integral formula of the most probable function is:G???=[-ln?1-??]ⁿ?n=3/4???6?The high temperature decomposition of limestone with large particle size putting into the converter was simulated after blowing,and its rate-limiting link is the heat transfer in product layer and CO₂ outward diffusion.The following expression indicates the relationship between time and conversion:1?-a?ln?1-a??10?a?28?^k_r0²t?7?There are three-layer structures of slag layer,lime layer and limestone layer in the process of dissolving limestone in converter slag.The primary CaO grains of limestone react with materials in slag and become elliptical.Product?CaO?layer heat transfer result in internal limestone decomposition which produce CO₂.CO₂ intense external diffusion,coupled with the rapid dissolution of CaO,leads to the transformation of CaO grains from ellipse to flake,the pores are further expanded,and the limestone decomposition rate and dissolution rate are further improved.Due to the decomposition rate>dissolution rate of limestone,some primary CaO are not dissolved,which will nucleate and grow up and form 2CaO·SiO2 and 3CaO·SiO2 with the slag to block the pores and reduce their decomposition rate and dissolution rate.However,the dissolution of limestone has the random dissolution phenomenon,which is combined with the CO₂ outward diffusion to disintegrate the limestone and accelerate the dissolution of limestone as a whole.?8?The dissolution rate of limestone in slag is very fast at 1500°C because of the combination of three factors,namely,the outer CaO reacts with slag to form soluble materials,the CO₂ outward diffuses to form large pores and the random reaction interface accelerates dissolution.Limestone instead of lime for slag-making can have better effect in converter.