Pure-Hydrogen Reduction of Vanadium–Titanium Magnetite Pellets: Competition Between Reaction Driving Force and Product-Layer Densification over 0.15–3 MPa

Raising hydrogen pressure does not monotonically accelerate the reduction of vanadium–titanium magnetite (VTM) pellets, and the pressure that maximizes performance shifts with temperature. Using a self-developed high-pressure thermogravimetric apparatus with buoyancy correction, VTM pellets were reduced in pure hydrogen at 850–1050 °C and 0.15–3 MPa under a constant H2 flow rate of 10 L/min. The pressure effect was strongly temperature-dependent and non-monotonic, with the relative performance of different pressure conditions varying markedly with temperature. SEM characterization identified the origin of this behaviour: the Fe-rich phase underwent a compact-porous-compact transition as pressure increased. Moderate pressurization accelerated oxygen removal, phase-transformation shrinkage, and water-vapour escape, generating an open and interconnected pore network, whereas excessive pressure drove rapid surface metallization, Fe-particle coalescence, and sintering, contracting the pores and raising gas-transfer resistance. Kinetic fitting with a three-step unreacted shrinking-core model reproduced the reduction curves (R2 > 0.996) and showed that rate constants did not vary monotonically with pressure, confirming that the pressure response is not governed by hydrogen partial pressure alone. Temperature remained the dominant kinetic variable, with the maximum reduction rate at 1.5 MPa rising from about 0.10 min−1 at 850 °C to 0.38 min−1 at 1050 °C. These results establish that pressurized hydrogen reduction of VTM is governed by a competition between reaction driving force and product-layer densification, and that temperature and pressure must be matched rather than independently maximized.