The reduction behavior and kinetic study of different manganese ores in a hydrogen-based reduction process

The decarbonization of manganese ferroalloy production is essential to reduce the industry’s reliance on fossil carbon. This PhD research explores an alternative low-carbon route using hydrogen to pre-reduce manganese ores before smelting, an approach aligned with the emerging HAlMan (Hydrogen and Aluminum use for Manganese and its alloys production) process. The study investigates how different manganese ores behave during calcination and hydrogen-based reduction, with a focus on reaction mechanisms, kinetics, and changes in structure and physical properties. Five industrially relevant ores were examined: Nchwaning, UMK, Comilog, Gloria, and Zambian through controlled laboratory experiments in a vertical thermogravimetric furnace (TG) under a 100% H₂ atmosphere, combined with advanced material characterization. The results show that hydrogen is highly effective in reducing both manganese and iron oxides. The reduction proceeds stepwise, ultimately producing manganese monoxide (MnO) and metallic iron (Fe). However, the rate and efficiency of reduction vary significantly depending on each ore’s mineral composition, structure, and carbonate content. Among the ores studied, Comilog and Zambian ores exhibited the fastest reduction rates due to their high content of reactive manganese phases, whereas Nchwaning ore showed slower reduction because of its dense structure and the presence of more stable minerals. Ores rich in carbonates (UMK and Gloria) benefited significantly from calcination, which increased porosity and improved hydrogen accessibility. The research also highlights how pore structure evolves during processing. Calcination creates additional pores and cracks, enhancing gas diffusion, while higher temperatures during reduction can lead to partial sintering, reducing porosity and slowing the reaction. These structural changes play a key role in controlling reduction kinetics. Kinetic analysis revealed that the reduction process initially depends on chemical reactions but gradually becomes limited by gas diffusion through the product layer. Overall, this work provides important scientific insights into hydrogen-based manganese ore pre-reduction and demonstrates its feasibility as a sustainable pretreatment step. Calcination plays a key role in this process by decomposing carbonate phases and increasing porosity, which improves gas accessibility and enhances the efficiency of the subsequent hydrogen reduction. The findings support the development of cleaner production routes for manganese alloys, contributing to the broader transition toward low-carbon metallurgy.