Columbite, often present in polymetallic iron ores, has uncertain phase transition behaviors during hydrogen metallurgy. This study provides thermodynamic parameters for columbite, including ΔfHθ(298), Sθ298 and Cp,m(298), using a two-parameter empirical model. A dominant region map, illustrating the states of columbite in an H2/H2O atmosphere, was developed. Critical thermodynamic conditions for phase transformation were also identified. Columbite was synthesized via solid-state sintering, and its hydrogen reduction at different temperatures was examined. The study further analyzed the evolution of metal valence states and phase transformation behaviors during hydrogen reduction. Thermodynamic analysis indicates that when \(\varphi_{{{\text{H}}_{2} }}\) ≥ 99.999%, Fe (II) in FeO·Nb2O5 is preferentially reduced to metallic iron at temperatures above 600 °C, while niobium oxide requires temperatures above 900 °C to be reduced from Nb (V) to Nb (IV). Experimental results show that at 800 °C, no phase transitions or changes in the valence states of Fe and Nb were observed. However, at 900 °C, the diffraction peak of FeO·Nb2O5 diminished, while metallic iron and NbO2 appeared. At this temperature, the valence state of iron transitioned from Fe (II) to Fe (0), and niobium from Nb (V) to Nb (IV). When the reduction temperature increased to 1000 °C, the diffraction peaks of NbO2 and Fe intensified, while those of FeO·Nb2O5 diminished. At 1100 °C, the diffraction peak of FeO·Nb2O5 disappeared, and transmission electron microscopy further confirmed the presence of NbO2. At this temperature, the proportion of Fe (0) in iron increased to 25.19%, while Nb (IV) in niobium rose to 16.25%. The research results provide a theoretical basis for developing hydrogen metallurgy technology for polymetallic associated iron ore.
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