Phase transformation mechanisms and multi-stage reduction kinetics of hematite at different hydrogen concentrations

Hydrogen-based iron ore reduction is essential for low-carbon metallurgy, and understanding the role of hydrogen concentration is key to optimizing kinetics. Using TGA, this study examines hematite’s stepwise reduction under varying hydrogen levels. Higher temperature or hydrogen concentration accelerates each stage and causes reaction overlap, which is deconvoluted using an asymmetric double-sigmoidal model. Each step follows a nucleation-and-growth mechanism. SEM shows a shift from dispersed to rapid parallel nucleation with increased sintering at higher hydrogen concentrations. The activation energy of the FeO → Fe step remains nearly constant, while the pre-exponential factor scales linearly with hydrogen concentration. Kinetic parameters derived from narrow hydrogen ranges can be reliably extended, enabling a universal kinetic model.