Effect of induration and gangue content on the strength and pore morphology of iron ore pellets for hydrogen-based direct reduction

Iron ore pellets for hydrogen-based direct reduction must combine sufficient strength with a pore morphology that allows efficient gas access, and both are set during induration. This work examined how induration temperature (1150-1300°C) and holding time (10-40 min) control the compressive strength and pore morphology of pellets made from a low-gangue iron ore concentrate, with a high-gangue concentrate used for comparison. Both mixtures contained 10 wt.% mill scale, incorporated as an iron-rich steelmaking by-product. Strength was mapped over the full temperature-time matrix, the microstructure was followed by optical and scanning electron microscopy, and the pore morphology was quantified by image analysis; selected pellets were then reduced in a hydrogen-rich gas at 950°C. Compressive strength increased from about 5 to 43 MPa, with temperature the more effective variable. Stronger induration lowered the total porosity only slightly and mainly closed the finest pores, while the gas-accessible network was preserved. All pellets reached a high final reduction degree; a higher gangue content and stronger induration each slowed reduction by limiting gas-accessible porosity. These results identify induration conditions that provide a practical balance between strength, reducibility, and mill-scale recycling.