Integrated biomass-assisted carbothermal reduction and staged hydrometallurgical extraction for selective multi-element recovery from red mud

This study integrated biomass-assisted carbothermal reduction with hydrometallurgical framework to enable comprehensive valorization of red mud. Circumscribed central composite design experiments systematically evaluated the combined effects of temperature (∼600–1000 °C), residence time (∼1–5 h), and red mud-to-biomass ratio (RMB, ∼0.3–1.2) on phase evolution, iron speciation, and magnetic separation performance. Regression analysis supported with XRD-Rietveld refinement and XPS characterizations demonstrated that temperature dominantly governed the Fe³ ⁺→Fe²⁺→Fe⁰ sequence, with hematite transforming to magnetite and subsequently to α-Fe above 850 °C. The RMB regulated reductant-oxide stoichiometry, whereas residence time promoted approach to thermodynamic equilibrium. Coupled TGA-DTG-MS identified three thermal regimes: devolatilization (30–250 °C), intermediate reductive gasification (250–500 °C) associated with partial hematite-to-magnetite conversion, and high-temperature deep reduction (>500 °C), during which sustained CO-rich conditions enabled magnetite metallization and concurrent carbothermal conversion of SiO₂ to SiC-6H. Multi-response optimization uncovered optimal conditions at 954.6 °C, 4.68 h, and RMB = 0.71, achieving ∼90% Fe recovery and ∼89% Fe purity following magnetic separation due to ferromagnetic Fe²⁺ (25%) and Fe⁰ (59%) proportions of total Fe. The non-magnetic tailings were subsequently processed via staged hydrometallurgical extraction: (i) primary acid leaching, (ii) Fe removal using Aliquat 336-based solvent extraction, (iii) selective precipitation of trivalent cations, and (iv) secondary acid leaching to partition transition metals and rare earth elements. This integrated approach recovered 88% Fe, 60% Tb, 55% Cr, and 95% Si into chemically distinct product streams. The combined thermochemical-hydrometallurgical strategy provides an efficient and scalable route for iron metallization and systematic fractionation of valuable elements from red mud.