The Study Of Kinetics Of Micro/Nanoscale Iron Ore Reduction With Gases

Since the development of traditional ironmaking process will be restricted from resource and environmental problems, blast furnace process must be further improved. Meanwhile non-blast furnace process like direct reduction and smelt reduction should be properly developed, especially including the new technique based on direct utilization of fine ore and coal powder as raw material. In order to get a deeper comprehension of mechanism and kinetics of extra fine iron ore reduction with gases, a method of combining numerical simulation with laboratory experiment was used to investigate the kinetics behavior of micro-nanometer iron ore reduction with gases at medial and low temperature. The reduction process of iron ore with microscale was numerically simulated and validated from experiments. The non-classical transfer effect on reduction process was also investigated. Above investigations may provide theoretical accordance for application of fine iron ore reduction at medial and low temperature..Weight loss method was used to study hydrogen reduction of fine iron ore in micro scale at temperature between 450 to 600℃.The structure and different phase changes of oxide ore during the reduction were examined with scanning electron microscopes (SEM) and X-ray diffraction technique (XRD). The results indicated that at initial stage a suddenly weight loss appeared, which reached its maximal value at 550℃.The slope of the curves were large at initial stage, and then became gentler during medial and final stages. In all temperature ranges, concentration of vapor at reaction interface went up, also did the thickness of product layer and densification with the increase of reduction degree. These caused the decreased reduction rate. Apparent activation energies in different stages were determined with dates from reduction experiment.Considering the effect of gaseous concentration and reaction heat, a non-isothermal kinetics model was developed to simulate the kinetics behavior of fine oxide ore. The model included coupled equations of heat transfer, mass transfer and chemical reaction between gas and solid phases. Finite Volume Method (FVM) with fully implicit form was applied for solving the governing parabolic equations. Temperature, gaseous concentration, reduction degree and reduction rate at every time were calculated through stepwise iteration. By means of numerical simulation, reduction rate and entirely reduction time at condition of different temperature, initial gaseous concentration and particle size could be predicted.The calculation result indicated that at the initial the temperature of the particle descended quickly where an endothermic reaction existed. With rapid supplement of surrounding heat, the temperature reached rapidly to the bulk temperature. The gaseous concentration within the particle decreased from surface to inside, and the diffusion rate of gas decreased as the thickness of product layer increased. Results of simulation also gave contributions and variations of temperature and gas concentration inside the particle of fine iron oxide. The model was validated with experimental results from reference and present work.Because of limit velocity of heat- and mass- transfer within microscale, a "bi-layered sphere" model was proposed in this paper to describe the non-classical transfer phenomenon in reduction process of iron ore with micro- or nano-scale. The model hypothesized that a 'very thin layer' effect would exist in the medium near the position of heat and mass disturbance source. In the thin layer, heat and transfer processes were depicted with non classical transfer laws, outside the thin layer of the medium, the behavior of heat and mass transfer were governed with classical laws. The heat and mass transfer at the boundary surface between two parts was satisfied to the continuous boundary condition. The calculation result indicated that the thermal and mass transfer propagated as nature of wave, and the waves just existed transiently. The thickness of non-classical effect was determined through numerical simulation, and relevant factors influencing the effect were also analyzed. According to theoretical and experimental results, the mechanism of chemical reaction kinetics may be divided into two parts: the heat and mass transfer was governed with non classical law at transient stage, and as in larger scales of time and space, the transfer of the reaction continued into classical region. When the surface of ore particle was subjected to a sudden temperature change or a concentration disturbance, the wave essence of heat and mass transfer caused values of temperature and gaseous concentration much larger than that on the boundary transiently, which caused the reaction rate much higher than that on routine condition.