Gas-solid Reduction Behaviour Of In-flight Fine Iron Ore Particles

With the structure changes in the metallurgical resources and the improvement of people's consciousness of environmental protection in our country,more and more attention has been paid to the ironmaking processes that could directly use iron ore fines.However,there are a few studies on the reduction behavior of iron ore fines at high temperature.A further analysis in theory and experiment on this aspect is still needed.Up to now,several ironmaking processes using iron ore fines are relatively mature and have greater development potential.According to the characteristcs of gas-solid reduction behavior of iron ore fines in these processes,this work has focused on the gas-solid reduction behavior and the kinetic mechanism of the in-flight fine hematite ore particles at high temperature.The experiments have been carried out with an improved High Temperature Drop Tube Furnace.The diameter of the hematite ore fines were between 75-90 ?m When the reductant was CO,the experimental temperature was between 1450-1600K and the compositions of the gas mixtures were 40%CO+60%CO2?80%CO+20%CO2.100%CO and 80%CO+20%N2.When the reductant was H2,the experimental temperature was between 1450-1550K and the compositions of the gas mixtures were 40%H2+60%N2?80%H2+20%N2 and 100%H2.The reduction degree,the composition and the variation law of the iron-containing phases of the partially reduced samples were detected with the chemical titration method and X-Ray Diffraction?XRD?analysis.The morphology change of the iron ore particles was observed by the optical microscope and Scanning Electron Microscope?SEM?.Based on the current experimental conditions and experimental results,the kinetic model of the gas-solid reduction of iron ore fines at high temperature was established,and then the kinetic parameters were determined.The reduction degree and reaction rate of iron ore fines increased with the increase of the experimental temperature as well as the partial pressure of the reduction gas.When the reduction degree was less than 11.1%,the main iron-c.ontarning phases in the iron ore fines were Fe2O3 and Fe₃O₄.When the reduction degree was between 11.1%-33.3%,the main iron-containing phases in the iron ore fines were Fe₃O₄ and FeO.In addition,a small amount of metallic iron was also detected at this situation.When the reduction degree was higher than 33.3%,the main iron-containing phases in the iron ore fines were FeO and metallic iron.The observation of the cross-sectional area of the partially reduced iron ore particles shows that the phases of Fe2O3 and Fe₃O₄ can be distinguished by color,while the phases of Fe2O3 and Fe₃O₄ can not be distinguished by color easily.On the other hand,it was no more than two iron-containing phases in the same partially reduced particle,and the reaction took place at the interface between the two phases.Therefore,the one-interface unreacted core model was determined for the mathematical description of the gas-solid reduction behaviour at the studied experimental conditions.When the reductant was CO,the activation energy of the gas-solid reaction of iron ore fines was 112.78 kJ mol-1 and the pre-exponential factor was 113.10.When the reductant was H2,the activation energy of the gas-solid reaction of iron ore fines was 111.68 kJ mol-1 and the pre-exponential factor was 1021.52.From the point of view macro-kinetics,the rate controlling step of the gas-solid reaction was chemical reaction.A further analysis on the microscopic mechanism of chemical reaction showed that the rate controlling step was solid-state diffusion or mixed control which contains solid-state diffusion.