【作者】 李秋菊；

【导师】 洪新；

【作者基本信息】 上海大学 ， 钢铁冶金， 2008， 博士

【摘要】 由于传统高炉炼铁生产的发展受到资源、环保等方面的制约,因此我国在继续完善和改进高炉炼铁工艺的同时,应适度发展直接还原和熔融还原等技术,尤其是直接利用粉矿、粉煤为原料的非高炉炼铁新技术。为了深入揭示微细铁矿粉的还原反应机理及动力学行为,本文采用数学建模与实验研究相结合的方法研究了微/纳米级氧化铁矿粉在中、低温下气体还原动力学行为,对矿粉在微尺度下还原过程进行数值模拟和实验验证,并考察了矿粉还原过程中非经典传输行为对还原过程的影响,为微细矿粉在中、低温度下还原的应用提供理论依据。本文运用失重法研究了450℃～600℃条件下,H₂还原微/纳米级铁矿粉的动力学过程。应用扫描电镜和X-射线衍射分析了还原过程中的结构与物相变化。结果表明,铁矿粉在还原初始会发生瞬间失重现象,瞬间失重率在550℃时最大;还原曲线初始斜率很大,后趋于平缓。在整个温度范围内,随着还原率增加,反应界面周围H₂O汽浓度逐渐提高,反应产物层厚度和致密度也逐渐增加,还原速率相减小。根据实验结果可确定还原过程初期、中期及后期的反应活化能值。针对气体浓度及反应热对微尺度矿粉还原过程的影响,本文提出了非等温反应动力学模型模拟氧化铁粉气相还原过程。模型中包含耦合的气、固相间传质方程、传热方程和化学反应动力学方程。采用差分格式的全隐式控制容积法对控制方程进行离散化处理,通过迭代逐步求出处在不同时刻、不同节点处的温度和气体浓度值,进而可计算出该时刻、该节点处的还原速率及还原率值。运用数值模拟可以预测在不同温度、不同初始气体浓度及不同矿粉粒径条件下反应速率和完全还原所需时间。模拟计算结果表明,当还原反应为吸热反应时,颗粒温度反应初始瞬间下降,随后周围热量迅速补上,使颗粒温度升高至矿粉整体温度;气体浓度由外向内逐渐减小;气体的扩散速率是随着厚度增加而降低。此外,模拟计算还给了出温度和气体浓度在微颗粒内部的分布及变化。数值模拟与实验结果基本吻合。鉴于微尺度条件下的热、质传递是以有限速率传播的,本文提出了“双层球形”模型来描述微纳米级铁矿粉气相还原过程中的非经典传输现象。该模型假设紧靠介质内受扰动的位置存在一瞬间“薄层”区域,在该薄层内的传输行为用非经典传输定律描述;而在薄层外的传输行为则近似满足经典的传输定律;“薄层”区域边界上的传输满足连续性边界条件。通过数值模拟计算,发现在极端传递条件下热、质传播具有波的性质,热、质波仅在扰动过后的极短瞬时存在。通过数值模拟确定了非经典效应的瞬间“薄层”厚度,并分析了各相关因素对非经典效应的影响。上述理论与实验研究结果表明,微尺度铁矿粉在气相还原过程中,反应动力学机理分为两部分,反应初始瞬间传热和传质受非经典传输定律控制;随着时间及空间尺度的增大,反应进入经典传输区域,传热和传质行为用经典传输定律来描述。当表面受到温度及气体浓度的扰动时,热量、质量传递的波动机制导致矿粉颗粒表面的温度和气体浓度会在瞬间远大于表面上附加的扰动源,导致反应初始速率也会远高于常规条件下的速率值。更多还原

【Abstract】 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.更多还原