High-Pressure H2–CO Reduction of Vanadium–Titanium Magnetite Pellets: Reduction Behavior, Microstructure, and Kinetics

Pressurized gas-based reduction is central to modern shaft furnace operation, yet the available pressure-dependent kinetics are derived almost exclusively from conventional iron-ore pellets reduced in pure H2, and no systematic dataset exists for vanadium–titanium magnetite under H2–CO co-reduction. This work provides the first systematic measurement of the coupled pressure–temperature–atmosphere response of vanadium–titanium magnetite pellets, using a high-pressure thermogravimetric apparatus at 0.15, 1, and 3 MPa, over 850–1050 °C, and at H2/CO ratios of 1.5 and 6. Interrupted reduction experiments, SEM–EDS characterization, and a three-interface unreacted shrinking core model were further employed to analyze the reduction kinetics over the temperature range of 850–1050 °C. The results show that increasing the temperature and H2/CO ratio markedly promotes reduction, with the enhancement becoming more pronounced at elevated pressure and temperature. At 3 MPa, 1050 °C, and H2/CO = 6, the reduction degree approached 1.0. The pressure effect was not simply linear: increasing pressure provided only limited enhancement at 850 °C, whereas high pressure effectively accelerated the reaction at 1050 °C. The maximum reduction rate occurred mainly within a reduction degree range of 0.10–0.25. Once the reduction degree exceeded 0.33, the rate decreased substantially during the FeO → Fe stage because of product layer diffusion and microstructural evolution. SEM observations showed a relatively uniform reduced product structure at 950 °C, whereas iron-phase coalescence and pore coarsening became pronounced at 1050 °C. The kinetic model yielded R2 values of 0.9482–0.9917 and satisfactorily described the reduction process. K3 was generally lower than K1 and K2, indicating that FeO → Fe was the principal rate-limiting stage. These findings clarify the synergistic roles of reaction driving force, mass transfer resistance, and microstructural evolution during high-pressure H2–CO co-reduction, and they establish that pressurization is beneficial for vanadium–titanium magnetite only above a temperature threshold at which intrinsic kinetics and thermodynamic driving force are high enough to exploit the higher reductant concentration despite sintering of the iron layer —a design constraint that is essential to pressurized shaft furnace operation.