Contrasting the Mineralogical and Morphological Changes During Direct Reduction of a Hematite Iron Ore in Hydrogen and Ammonia at Different Temperatures

This study examined the mineralogical and morphological changes of hematite iron ore during solid-state reduction in H2 and NH3 at 500–900 °C. Reduction experiments were performed using a thermogravimetric analyzer and a fixed-bed reactor to obtain weight loss data. Samples collected at different reduction times were characterized by XRD for mineralogy and degree of metallization, SEM-EDS for morphology and elemental composition, nitrogen physisorption for porosity, and gas chromatography for NH3 exit-gas composition analysis. At 700 and 900 °C, reduction in both gases followed the conventional pathway (hematite → magnetite → wustite → iron), achieving full metallization and producing comparable dense iron morphologies, indicating rapid NH3 dissociation and hydrogen-dominated reduction behavior. At 500 °C, a clear mechanistic divergence was observed. H2 reduction proceeded via a two-stage pathway (hematite → magnetite → iron), forming porous metallic iron. In contrast, NH3 reduction was slower and incomplete, proceeding via a coupled reduction-nitridation mechanism and producing highly porous iron nitrides (Fe3N and Fe4N). Thermal treatments confirmed these nitrides are metastable, decomposing to metallic iron in argon or transforming to Fe2N in NH3 during cooling. These findings demonstrate a temperature-dependent transition in NH3 reduction, highlighting its dual role as both a reductant and a nitrogen source.