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Evolution of Porosity Profiles of Magnetite Phase during High Temperature Reduction of Hematite

机译:赤铁矿高温还原过程中磁铁矿相孔隙分布的演变

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The reduction of initially nonporous hematite to porous magnetite by CO + CO_2 (3:97) mixture was monitored thermogravimetrically at 850℃. 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 (D~(eff)). In this way D~(eff) 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 D~(eff) 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 D~(eff) to be included in the model calculation.
机译:在850℃进行热重监测CO + CO_2(3:97)混合液将最初的无孔赤铁矿还原为多孔磁铁矿。对于一系列的六个动力学运行,直径为ca的晶粒。使用1.5毫米。最终还原度从13%到100%不等。每次动力学运行后,以定量方式对晶粒的中心截面进行显微镜观察。观察得出了局部孔隙度的值。发现了经验方程,描述了随着距晶粒外表面的距离,局部孔隙率连续指数下降。通过水银孔隙率法获得的总孔隙率值与微观数据合理地吻合。经典的收缩核心模型(SCM)适合动力学数据。该模型考虑了在明确定义的界面处发生的气固反应以及在磁铁矿层内部发生的孔隙扩散现象。该模型也进行了修改。将孔隙度的局部值引入到有效扩散率(D〜(eff))的定义中。以这种方式,允许D_(eff)随距晶粒外表面的距离而变化。校正后的三个参数SCM产生的结果稍差。这意味着除了D〜(eff)的空间变化外,还应考虑时间上的变化。实际上,已经发现已经减少的层的局部孔隙率也随时间变化。但是,数据不够准确,无法将D〜(eff)的时间变化包括在模型计算中。

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