Consolidation Mechanism and Performance Regulation of Medium-Silica Fluxed Pellets: Effects of SiO2 Content and Basicity

Medium-silica fluxed iron ore pellets are a key burden material for blast furnace ironmaking, offering advantages of high iron grade and low energy consumption. However, their industrial application is constrained by low strength and fragmentation due to difficulties in controlling liquid phase formation during roasting. This study systematically investigates the effects of SiO2 content (3.2–4.2%) and basicity (1.18 ± 0.05) on the preparation process (preheating at 950–1050 °C, roasting at 1215–1275 °C, and soaking at 925–1000 °C), roasting behavior, and metallurgical properties of medium-silica fluxed pellets, using high-silica iron ore fines (SiO2 > 6%) as the main raw material. The consolidation mechanism is elucidated through mineral phase analysis (SEM-EDS) and pore structure characterization (mercury intrusion porosimetry). The results show that optimal green pellet performance (compressive strength ≥10 N/pellet; drop strength ≥4 times/0.5 m) is achieved with 22% ore A, 35% ore B, 18% ore C, 25% ore D, 6.54% hydrated lime, and 0.75% bentonite. The compressive strength of fired pellets first increases and then decreases with increasing SiO2. Under the optimized roasting regime (preheating at 1000 °C for 10 min and roasting at 1260 °C for 10 min), the maximum compressive strength of 4117 N/pellet was achieved at 3.6% SiO2 (blend No. 4); a further increase to 4.2% leads to excessive CaSiO3 glassy phases, higher porosity (25.15%), and a ~10.7% strength reduction (from 4117 to 3675 N/pellet). Metallurgical evaluation shows that 3.6% SiO2 pellets exhibit the best-balanced performance: RI (reducibility) of 88.03%, RDI+3.15 (reduction disintegration index) of 94.04%, and RSI (reduction swelling index) of 14.20%, meeting the requirements of GB/T 27692-2024 (RI ≥ 80%, RDI+3.15 ≥ 85%, and RSI ≤ 15%). This work demonstrates the feasibility of utilizing high-silica iron ore fines for producing high-quality medium-silica fluxed pellets (green and fired pellets of 10–12.5 mm), providing theoretical guidance for industrial production and liquid phase regulation.