微尺度氧化铁粉的低温还原机理

Reduction mechanism of micron-scale hematite powder at low temperature

用热重分析法研究低温条件下(450、500、550和600℃),氢气还原微尺度氧化铁的还原动力学行为。结果表明:随氧化铁粉粒径减小和反应温度升高,初始反应速率加快,后期反应速率减慢。这是因为反应后期生成大量铁须,铁须之间形成搭桥,导致还原后的粉末严重烧结并致密化,阻碍气体的扩散,致使反应速率减慢。且随着粉体粒径减小,粉体表面吸附能增大,粉体致密程度提高,反应后期的粘结现象更加严重,反应速率相应减慢。采用Hancock-Sharp方法分析微尺度氧化铁粉恒温还原的动力学过程,发现前期阶段Fe2O3→Fe3O4,在500℃以下,相界面化学反应的阻力所占的比例较大,表明此阶段的反应控速环节为界面化学反应,温度超过500℃时,则由界面化学反应机理和相转变机理共同控制,点阵结构由Fe2O3的斜方六面体结构转变为Fe3O4的立方结构;后期阶段Fe3O4→Fe,由于粉体发生粘结,还原反应的控速环节转变为扩散控速。

The reduction kinetics of hematite with various particle sizes by hydrogen at low temperature was studied through the thermogravimetric analysis.With the decrease of the hematite size and the increase of the reaction temperature,the initial reaction rate increases while the latter reaction rate decreases.This is because a huge number of iron whisker forms at Fe3O4→Fe stage,which leads the grains join to form bridges and then forms dense iron layer to hinder the diffusion of the reduction gases.At the same time,the smaller the particle size is,the bigger the surface adsorption energy,the more serious the binding phenomenon and the more slowly the reduction rate is.The reduction mechanism based on the Hancock-Sharp method indicates that the initial reaction is a surface-controlled process below 500 ℃,while a complex mechanism of path-boundary-controlled and phase change over 500 ℃（transformation of the crystallographic lattices from rhombohedral to cubic）.But at Fe3O4→Fe stage,because of the dense iron layer,the mechanism shifts to diffusion control.