The reduction of magnetite-based iron ore fines in a hydrogen-induced fluidized bed becomes an attractive fossil-free ironmaking route. Our previous study showed that a prior oxidation treatment of magnetite was helpful to improve its fluidization and reduction behavior. However, the underlying oxidation mechanisms of magnetite ore fines remained unclear and required further investigations. In this study, two magnetite ore brands were analyzed viain situ high-temperature X-ray diffraction (HT-XRD) during oxidation, to investigate the thermal transformation of Fe ₃O ₄ to α-Fe ₂O ₃ at crystal scale. The lattice constants and crystallite sizes of both phases and oxidation degree were evaluated at different temperatures based on the HT-XRD patterns. The lattice constants of Fe ₃O ₄ and α-Fe ₂O ₃ increased with an increase in temperature due to the thermal expansion and can be successfully fitted with temperature by second-order polynomials. With Fe ₃O ₄ being oxidized into Fe ₂O ₃, the Fe ₂O ₃ crystallite grew and showed a certain growth habit. The Fe ₂O ₃ crystallite grew faster along the a/b axis than the c axis. The oxidation kinetics followed the parabolic law as shown by the sigmoid-shaped oxidation degree curve, suggesting that the solid diffusion of ions was the rate-limiting step.

Fossil-free ironmaking is indispensable for reducing massive anthropogenic CO ₂ emissions in the steel industry. Hydrogen-based direct reduction (HyDR) is among the most attractive solutions for green ironmaking, with high technology readiness. The underlying mechanisms governing this process are characterized by a complex interaction of several chemical (phase transformations), physical (transport), and mechanical (stresses) phenomena. Their interplay leads to rich microstructures, characterized by a hierarchy of defects ranging across several orders of magnitude in length, including vacancies, dislocations, internal interfaces, and free surfaces in the form of cracks and pores. These defects can all act as reaction, nucleation, and diffusion sites, shaping the overall reduction kinetics. A clear understanding of the roles and interactions of these dynamically-evolving nano-/microstructure features is missing. Gaining better insights into these effects could enable improved access to the microstructure-based design of more efficient HyDR methods, with potentially high impact on the urgently needed decarbonization in the steel industry.