Kinetics of Direct Iron Reduction Using Hydrogen in Steelmaking

Steelmaking contributes to 7% of the total annual CO₂ emissions globally. One approach to decarbonize steelmaking is to shift from the conventional coal-based reduction of iron ores in blast furnaces to hydrogen-based direct reduction, changing the emissions from CO₂ to H₂O. Despite its opportunity, hydrogen-based steelmaking, has been slow to commercialize because iron ore reduction with hydrogen is slow and energy-intensive (i.e. endothermic). While extremely important, the chemistry in these reactors is often complex, as the native ore includes particles from ~7-nm to 1-mm sizes, which exhibit different kinetics and mass transfer that are poorly understood. In this work, we explore the kinetics in the H₂ reduction chemistry of magnetite (Fe₃O₄), from the industrial Fe₃O₄ to the lab-synthesized nanoscale Fe₃O₄. Using <i>in situ</i> synchrotron X-ray diffraction, we demonstrate the kinetics of this 2-step reaction; we identify that the initial magnetite to wustite (FeO_x) reaction occurs quickly, while the subsequent wustite to metallic iron is the rate-limiting step. A closer look at the temperature dependence reveals that the quantitative picture of these kinetics must account for the reaction’s facet-dependence. Beyond the kinetics, we also observed mesoscopic structures that evolved during the reaction. The initial 10-nm Fe₃O₄ nanoparticles appear to self-assemble and ultimately form elongated and irregular structures (i.e. “nano-worms”), which we confirm with <i>in situ</i> SAXS. Our findings suggest that the (220) facets preferred by the chemistry may instigate hierarchical structuring, indicating key challenges for mass-transport in this system. Our deep understanding of the direct reduction of Fe₃O₄ using hydrogen sheds light on strategies to improve the performance of direct iron reduction, further commercializing hydrogen steelmaking.