Sustainable Pre-reduction of Ferromanganese Oxides with Hydrogen: Heating Rate-Dependent Reduction Pathways and Microstructure Evolution

TL;DR

This study explores how heating rate influences the reduction pathways and microstructure evolution during hydrogen-based pre-reduction of ferromanganese ores, providing insights for more sustainable manganese alloy production.

Contribution

It demonstrates the impact of heating rate on reduction mechanisms, phase formation, and microstructure, advancing understanding for optimizing hydrogen-based pre-reduction processes.

Findings

Slow heating enables complete MnO and Fe reduction

Fast heating causes Fe- and Mn-oxides intermixing

Microstructure varies with heating rate, affecting particle dispersion

Abstract

The reduction of ferromanganese ores into metallic feedstock is an energy-intensive process with substantial carbon emissions, necessitating sustainable alternatives. Hydrogen-based pre-reduction of manganese-rich ores offers a low-emission pathway to augment subsequent thermic Fe-Mn alloy production. However, reduction dynamics and microstructure evolution under varying thermal conditions remain poorly understood. This study investigates the influence of heating rate on the hydrogen-based direct reduction of natural Nchwaning ferromanganese ore and a synthetic analog. Non-isothermal thermogravimetric analysis revealed a complex multistep reduction process with overlapping kinetic regimes. Isoconversional kinetic analysis showed increased activation energy with reduction degree, indicating a transition from surface-reaction to diffusion-controlled reduction mechanisms. Interrupted X-ray diffraction experiments suggested that slow heating enables complete conversion to MnO and metallic Fe, while rapid heating promotes Fe- and Mn-oxides intermixing. Thermodynamic calculations for the Fe-Mn-O system predicted the equilibrium phase evolution, indicating Mn stabilized Fe-containing spinel and halite phases. Microstructural analysis revealed that slow heating rate yields fine and dispersed Fe particles in a porous MnO matrix, while fast heating leads to sporadic Fe-rich agglomerates. These findings suggest heating rate as a critical parameter governing reduction pathway, phase distribution, and microstructure evolution, thus offering key insights for optimizing hydrogen-based pre-reduction strategies towards more efficient and sustainable ferromanganese production.