Mechanism-Guided Transformation of Hematite to Battery-Grade FePO4 via Mild Carbothermal Reduction and Mixed-Acid Leaching

A novel low-carbon route was developed to synthesize high-purity FePO₄ from natural hematite (α-Fe₂O₃). To overcome the inherently low solubility of α-Fe₂O₃, a mild carbothermal reduction (0.8 wt % carbon, 650 °C, 15 min) was applied before leaching. This pretreatment partially reduced Fe₂O₃ to Fe₃O₄, disrupted the lattice, and generated microcracks, significantly enhancing structural reactivity. Thermodynamic analysis confirmed progressive reduction and in situ CO formation under carbon-rich conditions. The reduced samples were leached using an HCl-H₃PO₄ system (H₃PO₄/Fe = 1.05, H₂O₂/Fe = 0.3), where proton attack and ligand complexation promoted Fe³⁺ dissolution. Under identical leaching conditions, the reduced sample achieved 85.82% Fe³⁺ leaching within just 2 h, matching the nonreduced counterpart at 8 h, and reached 90.88% at 3 h, demonstrating good leaching efficiency. Kinetic analysis showed that the leaching behavior followed the Valensi-Carter model, with a high correlation (R² = 0.9968) and an apparent activation energy of 21.89 kJ/mol, indicating a mixed control mechanism governed by both solid-state diffusion and surface chemical reaction. Subsequent precipitation at pH 2.0, 60 °C for 20 min yielded FePO₄ with Fe/P = 0.989 and >99.60 wt % purity, meeting HG/T 4701-2021 standards. This work establishes a highly efficient, structurally driven hydrometallurgical route for producing battery-grade FePO₄ from refractory iron ores under low-carbon conditions.