Phase Evolution and Proposed Mechanism of Hematite Pellet Reduction Using Ammonia

Abstract Ammonia has been proposed as a potential reductant in ironmaking processes as part of broader efforts to reduce carbon emissions in the steel industry. Despite existing studies, critical aspects regarding phase and microstructure evolution, and the reaction mechanism involved in ammonia-based ironmaking, especially within pellet-based processes, remain inadequately understood. In this study, synthetic hematite pellets were reduced using Ar/NH 3 gas mixtures with 40% and 100% NH 3 at temperatures ranging from 450 to 850 °C and gas flow rates of 0.4 L/min. Based on X-ray diffraction and chemical analysis results, the phase transformations occurring during reduction at T < 650 °C, proceeded through the sequential formation of Fe 3 O 4 → FeO → Fe → Fe 3 N. The Fe 3 N was metastable and gradually decomposed to Fe 4 N and then to Fe in the later stages of reduction. At 750 °C and 850 °C, the reaction sequence was Fe 3 O 4 → FeO → Fe. Fe 4 N was also formed as an intermediate product resulting from the nitriding of freshly reduced metallic iron. The Fe 4 N then decomposed due to its thermodynamic instability after 1.5 h at 750 °C and 30 min at 850 °C using 100% of NH 3 . The iron nitride phases were formed from simultaneous iron oxide reduction and ammonia decomposition processes, where the outer layer of the pellet catalyzed nitride formation. The reduction of the hematite pellet with NH 3 followed a topochemical mechanism as the reaction occurs at interfaces between solid product layers.