Process and Dephosphorization Mechanism for Producing Low-Phosphorus Steel via Direct Reduction–Electric Furnace Smelting Separation of Alkaline Briquettes from Refractory High-Phosphorus Oolitic Magnetite Concentrate

High-phosphorus oolitic iron ores (HPOIOs) possess abundant reserves but are incompatible with conventional blast furnace ironmaking, as phosphorus migrates into hot metals during carbothermic reduction, preventing the production of low-phosphorus clean steel. To overcome this limitation, an innovative approach integrating alkaline briquette direct reduction and smelting separation was proposed. Briquettes were prepared from oolitic magnetite concentrate (52.01 wt% Fe, 0.29 wt% P, 0.11 wt% S) with a basicity (R) of 2.0 and 5 wt% MgO added as a desulfurizer. After direct reduction and smelting separation, the resulting metallic iron exhibited a content of 98.56 wt% Fe, with 0.036 wt% P and 0.046 wt% S, achieving an Fe recovery of 87.63%. The dephosphorization and desulfurization efficiencies reached 94.67% and 90.56%, respectively, meeting the clean steel requirements. Phosphorus was effectively stabilized within the gehlenite and merwinite phases as a solid solution of Ca3(PO4)2, inhibiting its transfer to iron. Thermodynamic analyses confirmed that high basicity (R ≥ 2.0) significantly suppressed P2O5 activity, preventing phosphate reduction. The formation of a Ca3(PO4)2–Ca2SiO4 solid solution further obstructed phosphorus migration. This dual mechanism of “chemical fixation and thermodynamic stabilization” enables efficient dephosphorization, offering a sustainable pathway for utilizing HPOIOs.