Evolution of Porosity Profiles of Magnetite Phase during High Temperature Reduction of Hematite
Evolution of Porosity Profiles of Magnetite Phase during High Temperature Reduction of Hematite
复制标题
赤铁矿高温还原过程中磁铁矿相孔隙度分布的演变
DOI：
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发表时间：
1996
期刊：
影响因子：
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通讯作者：
A. Sadowski
中科院分区：
文献类型：
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作者：
J. Janowski;A. Baranski;A. Sadowski
在850℃的温度下，用CO+CO2（3:97）的混合物对初始无孔赤铁矿还原成多孔磁铁矿进行了热重监测。对于一系列的六次动力学运行，使用直径约为1.5 mm的颗粒。最终还原度从13%到100%不等。在每次动力学运行后，对晶粒的中心截面进行了定量的显微观察。观测得到了局部孔隙度的值。发现了经验方程，描述了局部孔隙率随距晶粒外表面距离的连续指数下降。汞孔法测定的总孔隙度值与微观数据吻合较好。将经典收缩核模型（SCM）拟合到动力学数据中。该模型既考虑了磁铁矿层内发生的孔隙扩散现象，又考虑了发生在明确界面处的气固反应。模型也进行了修改。将孔隙度的局部值引入有效扩散系数（Deff）的定义中。通过这种方式，允许Deff随距离颗粒外表面的距离而变化。修正后的三参数单片机产生了稍差的结果。这意味着Deff除了空间变化外，还应考虑时间变化。事实上，已还原层的局部孔隙率也随时间变化。然而，数据不够精确，不能将Deff的时间变化包括在模型计算中。
The reduction of initially nonporous hematite to porous magnetite by CO+CO2 (3:97) mixture was monitored thermogravimetrically at 850°C. For the series of six kinetic runs the grains of diameter ca. 1.5 mm were used. The final reduction degree varied from 13 to 100%. After each kinetic run the microscopic observations of the central cross-section of grains were done in quantitative way. The observations yielded the values of local porosity. The empirical equations were found describing the continuous exponential decrease of local porosity with the distance from the external surface of the grain. The value of total porosity obtained by mercury porosimetry agrees in a reasonable way with microscopic data. The classical shrinking core model (SCM) was fitted to kinetic data. The model took into account the gas-solid reaction occurring at sharp defined interface as well as the pore diffusion phenomena occurring inside the magnetite layer. The model was also modified. The local value of porosity was introduced to the definition of effective diffusivity (Deff). In this way Deff was allowed to vary with the distance from the external surface of the grain. The corrected three parameter SCM yielded slightly worse results. It implies that apart from the spatial variation of Deff the temporal one should be also considered. Indeed, it was found that the local porosity of the already reduced layer varied also with time. However, the data are not accurate enough to permit the temporal variation of Deff to be included in the model calculation.