Iron Recovery And Preparation Of Water-quenched Slag With High Cementitious Performance From BOF Slag By Means Of Reduction

Basic oxygen furnace?BOF?slag is the main industrial waste of bulk quantifies,discharged during the steelmaking process.BOF slag accounts for 15%-20%of the weigh of raw steel.In 2017,China's raw steel output was 830 million tons,while BOF slag was over 130million tons.Due to the low stability,poor reactivity,and large fluctuations in composition of BOF slag,only about 10%of BOF slag can be reused now,which is far behind 79%-an expected value prescribed in national plan of"Made in China 2025".The National Eleventh Five-Year Research Program of China“Technology and equipment for steel slag reconstruction and performance optimization”proposed to add adjustment materials into molten slag during the slagging process and use the sensible heat of molten slag to reconstitute the steel slag.After reconstitution,the minerals of high reactivity,such as C₃S,increased.Thus the cementitious performance of the reconstituted slag was enhanced.It was found that after adding reductant into the adjustment materials,iron was recovered from BOF slag and high cementitious water-quenched slag,similar to blast furnace slag,was formed.This study provides a new reference for steel slag reconstitution.This work was supported by the National Natural Science Foundation of China[grant number 51372088].The feasibility of recovering iron from BOF slag by means of reduction and preparation of cementitious water-quenched slag by reconstitution method was estimated by means of thermodynamic calculation.At a high temperature,the composition of molten slag included liquid phases of CaO,Si O₂,MgO,FeO and solid phases of C₂S,RO and C₂F.These components can react with solid C,mullite?A₃S₂?,and quartz?SiO₂?that were brought into the system with component-adjusted materials.The Gibbs free energy??G?for various reactions showed that among the reaction products,the most thermodynamically stable products were metal Fe,C₃MS₂,and C₂AS.C₃MS₂ and C₂AS were the main components of blast furnace slag melt before water quenching.Therefore,it is completely feasible to reduce iron oxides?FeOx?to recover Fe by adding reducing agents into adjustment materials and prepare a high cementitious water-quenched slag mainly composed of C₃MS₂,and C₂AS by using the sensible heat of molten BOF slag.Kinetic study of the iron reduction reaction during the process of BOF slag reconstitution was carried out as well.The results showed that the FeOx reduction was a first order reaction mainly governed by the gasification reaction of solid C and the diffusion of FeO_x to the slag-air reaction interface.When the basicity was 1.0,the activation energy Ea of the reduction reaction was 154 kJ/mol,and the rate equation of FeOx reduction reaction was v=51758.53exp?-18484/T?·C.The effects of basicity?1.0².0?,temperature?1450¹550°C?and FeO_x initial content?15²6.9%?on the rate of reduction of FeO_x and apparent reaction rate constant k*were further investigated.The results showed that the higher the basicity was,the higher the reduction rate of FeO_x was,while the steel slag could not melt if the basicity was too high.As the temperature increased,the reduction rate of FeOx increased correspondingly.When the initial content of FeOx increased,the FeO_x reduction within the first40 min was significantly promoted,but the final reduction efficiency hardly changed.The apparent reaction rate constant k*of reduction increased with the increase of the basicity R,the reaction temperature T,and the initial content of FeOx C₀.Based on the kinetics of BOF slag reconstitution,experiments were performed to investigate the relationships between basicity and temperature,melt structure and properties,iron recovery and cementitious performances of the obtained water-quenched slag.The results showed that with the decrease of basicity R,the separation effect of slag and iron became better.The glassy phase content in water-quenched slag gradually increased,and smaller amount of crystals such as C₃MS₂,C₂AS,and C₂S,existed in the obtained water-quenched slag.This results are consistent with the thermodynamic analysis results.At 1500°C,when R?1.5 and the steel slag content was greater than 60%,the iron reduction rate was above 95%and the activity index of the water-quenched slag ranged between 75%and 108%,indicating an efficient recovery of iron and reutilization of slag.When the reaction temperature is higher than the melting point of the sample by 50?,the sample was melted well,which is beneficial for recovering iron and preparing cementitious water-quenched slag.The effect of water quenching temperature on the reaction is less than the effect of reaction temperature.As long as the sample can maintain good fluidity at the preset water quenching temperature,the glass-based water-quenched slag can always be prepared successfully.According to the study on molten slag performance,it can be concluded that when the basicity decreases,the melting point of the slag decreases.As the NBO/T value of the melt decreases,the melt polymerization degree and viscosity increase and the crystal crystallization becomes more difficult during water quenching.As a result,a glass-based water-quenched slag is more easily formed.For the melt with determined composition,an higher temperature leads to a more uniform distribution of the particles in the melt and a smaller viscosity and better free-flowing properties.Therefore,the contact area between the slag and the water during the water quenching treatment is large,and it is easy to obtain the glass-based water-quenched slag.Finally,experiments on reduction of iron and reconstitution of slag were carried out by using different sources of steel slag and different adjustment materials,in order to verify the universality of the proposed method for steel slag reduction and reconstitution.The results showed that the recovery rate of iron in steel slag can reach 74%to 97%.The composition of water-quenched slag prepared was mainly composed of glassy phase,accounting for 43%to97%of total weigh.The density of glassy phase was 2880 kg?m^-3 to 3100 kg?m^-3.The grindability of water-quenched slag was greatly improved compared with the original steel slag,with activity index reaching 77%to 103%.The cementitious performance was derived from the excitation of the glassy structure by cement.The hydration product was C-S-H gel.The hydration exothermic characteristics and the hydration reaction were similar to those of blast furnace slag.Therefore,the temperature of 1500°C,R?1.5,and the steel slag content of more than 60%can be universally adopted for steel slag reduction and reconstruction method.