The Effect of Pre-oxidation on the Hydrogen Reduction of Magnetite Ores at 973 K: Role of Particle Size and Gangue Chemistry

This study systematically investigates the effects of controlled pre-oxidation on hydrogen-based reduction of three industrial magnetite ores, distinguished by their gangue chemistries (CaO, negligible gangue, and MgO), and examines two particle size fractions. Pre-oxidation behavior showed a strong particle size dependence, with particles of the fine fraction achieving the target oxidation degree significantly faster than the relatively coarse fraction. Pre-oxidation universally improved reducibility, with the enhancement magnitude inversely correlated to the residual magnetite content. Kinetic analysis using absolute and normalized reduction rates, along with novel kinetic descriptors, revealed that pre-oxidation fundamentally altered reduction pathways by broadening the active reduction window and delaying diffusion limitations of the later part of wüstite reduction. Microstructural characterization revealed that pre-oxidized magnetite samples developed a network of pores during reduction, which helps in decreasing effective diffusion lengths and reducing the formation of dense Fe shells that impede the reduction of raw magnetite ores. The effectiveness of pre-oxidation on reduction varied distinctly by ore type, where ore with minimal gangue content showed the highest improvement, MgO-containing ore exhibited moderate enhancement in coarse fraction, which amplified in finer fractions, and CaO-containing ore showed limited improvement due to inherently favorable reduction characteristics even in the raw magnetite state. These findings show that the effectiveness of pre-oxidation is governed by the coupled roles of particle size and gangue chemistry, which determine the extent to which ore-specific kinetic and microstructural limitations can be alleviated during subsequent hydrogen reduction.