Multiscale Evaluation of the Direct Reduction of Mill-Scale Iron Industry Wastes with Green Hydrogen

Abstract The direct reduction of iron oxides using green hydrogen offers a sustainable route for the valorization of iron-rich industrial waste and the mitigation of CO2 emissions in steel production. In this work, a multiscale approach is used to examine the hydrogen-based reduction of mill-scale iron oxide pellets to direct reduced iron. The reduction mechanisms, assessed for both green and thermally pretreated materials, were first studied at the microscale via in situ X-ray diffraction to monitor real-time phase evolution, revealing that the conversion of wüstite (FeO) to metallic iron is the intrinsic rate-limiting step. The process was then scaled up in two steps: a fixed-bed laboratory reactor, where under optimized conditions the reaction rate was found to be limited primarily by the availability of hydrogen, followed by a kg-scale pilot plant moving-bed reactor. Under optimized conditions, more than 92% of the pellets were completely reduced, demonstrating the process scalability and technical feasibility. This study offers a promising approach to circular resource utilization and low-emission steelmaking through the hydrogen-based reduction of iron industry residues.