Thermodynamics and Kinetics of Ammonia‐Based Reduction of Natural Hematite

Abstract

Using hydrogen gas for iron ore reduction offers carbon‐neutral ironmaking, but scaling up faces safety and energy challenges. Ammonia is a safer carbon‐free hydrogen carrier with higher energy density and ease of liquefaction. This study investigates the characteristics of ammonia‐based direct reduction (AmDR) of natural iron oxide using thermodynamic modeling and thermogravimetric analysis (TGA) combined with quadrupole mass spectrometry (QMS). Particle sizes of 6, 10, and 14 mm in diameter were reduced at 650–850 °C, for 4–6 h in ammonia‐flow (180 mL min −1 ). AmDR characterization process utilized X‐ray diffraction (XRD), energy‐dispersive X‐ray spectroscopy (EDS), and light/electron microscopy (LOM/SEM). Temperature and particle size significantly influence reaction kinetics and apparent reduction degree (Rd a ). At 850 °C, all particle sizes reached >92% Rd a . Larger particles (10 and 14 mm) reduce slowly due to internal diffusion limitations. Gas evolution profiles indicate temperature‐dependent ammonia dissociation, hydrogen generation, and transient nitrogen‐oxide formation. Microstructural analyses reveal that AmDR produced porous metallic iron containing locally interconnected pore features and surface cracking. Cross‐sectional analyses of partially reduced particles revealed progressive growth of the product layer and inward advancement of the reaction front. Kinetic analysis shows agreement with contracting‐sphere (R3) and Ginstling–Brounshtein (D4) models ( R 2 > 0.97), with R3 fitting best, indicating predominant interfacial chemical‐reaction control for AmDR.