Experimental And Numerical Simulation Of Reduction Behaviour Of In-flight Fine Iron Ore Particles

With the lack of metallurgical resources,environmental pollution in China's steel industry and other issues that are increasingly prominent,more and more attention has been paid to the ironmaking processes that could directly use iron ore fines.At present,many studies have been carried out on the kinetics of gas-solid reduction of iron ore fines at low temperatures,but few studies could be found on the reduction kinetics of iron ore fines at high temperatures.In this paper,a method of combining high-temperature experiment and numerical simulation was used to investigate the gas-solid reduction kinetics of the in-flight fine hematite ore particles at high temperature.The experiments have been done with a High Temperature Drop Tube Furnace.The experimental temperature was between 1450-1550K and the compositions of the gas mixtures were 20%H2+80%N2?40%H2+60%N2 and 60%H2+40%N2.The average particle sizes of the hematite ore fines were 62 and 82 ?m,respectively.Through the kinetic experiments that on the gas-solid reduction of the in-flight fine hematite ore at high temperature,the reduction degree and the variation law of the ironcontaining 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 results,the one-interface unreacted core model was determined for the mathematical description of the gas-solid reduction behaviour at the studied experimental conditions.The rate controlling step of the gas-solid reaction was chemical reaction.Under the experimental operating conditions,the gas-solid two-phase flow in the High Temperature Drop Tube Furnace was numerically simulated.The Realizable k-? model was used for the gas-phase turbulence flow and the DPM model was used for the discrete phase.The results indicated that:a jet of carrier gas was formed in the region that was adjacent to the outlet of the water-cooled feeding tube and under this region,the gas flow reached a steady state.Due to the effects of the carrier gas jet and water-cooled feeding tube,there is a low temperature zone below the water-cooled feeding tube.The length of the low temperature zone slightly increased with the increase of the carrier gas flow rate,but the incensement of carrier gas flow rate had no effect on the temperature distribution at the central axis of the reaction zone.Through the analysis of the particle phase in the reactor,it was found that under the studied particle sizes,the hematite ore fines could move downward with a small deviation from the axis.For example,all the hematite ore particles of 62?m deviated from the axis within 10mm in the reaction zone,and 99%of theses particles deviated from the axis within 4mm.Therefore,the temperature and velocity of the gas phase at the axis of the reaction zone were used as the temperature and the velocity of the reducing gas during the reaction process.Based on the high temperature experiment and the numerical simulation of gas-solid two-phase flow in the reactor,the kinetics parameters of gas-solid reduction of iron ore was further calculated.The 4th-order Runge-Kutta method has been used to solve the equations of motion,heat transfer and reduction rate under the Lagrangian coordinate with the MATLAB.In the upper part of the reaction zone,the temperature of the fines was relatively low and almost no reaction took place.The fine hematite ore particles was not in uniform motion all the time during the flight.It was found that the particles decelerated first,and then reached the terminal falling velocity at a certain distance from the outlet of feeding tube.The position where the particles reached the terminal falling velocity was related to the carrier gas flow rate,the experimental temperature and the gas composition.The actual residence time of hematite ore fines was less than the estimated values used in the experiments.The corrected pre-exponential factor and activation energy of the gas-solid reaction of iron ore fines was 9.2×106 and 223 kJ·mol-1,respectively.