Softening-melting behavior and phase evolution mechanism of vanadium-titanium magnetite burden under oxygen blast furnace conditions

The hydrogen-enriched oxygen blast furnace (OBF) process offers a promising route for efficient and low-carbon smelting of vanadium-titanium magnetite (VTM). In this study, softening-melting experiments combined with thermodynamic calculations were conducted to compare the softening-melting behaviors, element migration, and phase evolution of VTM pellets, sinter, and mixed burden under OBF and traditional blast furnace (TBF) conditions. Results indicate that OBF conditions lower the softening start and end temperatures of pellets and mixed burden but raise their melting start and dripping temperatures, leading to a wider softening interval and, narrower melting interval. Meanwhile, the maximum pressure drop and permeability index decrease, resulting in significantly improved overall permeability; however, the softening temperature of VTM sinter increases. In addition, the permeability improvement does not increase monotonically with decreasing N 2 content; when the N 2 fraction falls below 20 %, further reduction in nitrogen becomes counterproductive, leading to a decline in permeability. Phase and microstructural analyses reveal that OBF conditions promote the reduction of titanium oxides from higher to lower chemical valence states, leading to the formation of low-valence phases such as Ti 2 O 3 and TiO. Moreover, Ti exhibits strong spatial overlap with N and V, indicating that V is mainly hosted in Ti-containing phases, whereas Mg, Al, Si, and Ca are enriched in the silicates and magnesium-aluminum spinel phases. • First comprehensive study on the softening-melting behavior of VTM burden under OBF conditions. • Hydrogen-enriched OBF widens VTM burden softening interval, narrows melting interval, and improves burden permeability. • Permeability improvement under OBF is non-monotonic; N 2 fraction below 20 % reduces burden permeability. • Titanium oxides are partially reduced to Ti 2+ , while vanadium is mainly hosted in Ti-bearing phases.