Integrated caustic cracking and supercritical CO2 extraction for scandium and rare earths recovery from refractory silicate ores

The recovery of scandium and rare earth elements from refractory silicate ores remains a major technological challenge due to their predominantly lattice-bound occurrence in Fe–Mg–Ca silicates and aluminosilicates. This study develops and evaluates an integrated extraction pathway that couples caustic cracking with supercritical fluid extraction using CO 2 solvent and a tri-n-butyl phosphate–nitric acid adduct. Comprehensive mineralogical characterization (X-ray diffraction, Scanning Electron Microscopy/Energy Dispersive Spectroscopy, and Quantitative Evaluation of Minerals by SCANning electron microscopy) confirmed that the feed ore comprises a tightly intergrown fayalite–amphibole–hedenbergite framework, with scandium entirely hosted in structurally encapsulated sites. Low-temperature sodium hydroxide treatment at 500 °C produced limited structural modification and modest scandium extraction (33%). In contrast, high-temperature alkali cracking routes, i.e., pure NaOH at 1000 °C and a mixed NaOH–KOH eutectic at 500 °C, induced extensive depolymerization of the silicate lattice. The higher temperatures facilitated the formation of Na- and K-aluminosilicate phases, and generation of fine Fe-rich reaction products. Compared with NaOH-only cracking at 1000 °C, mixed NaOH–KOH eutectic cracking at 500 °C achieved equivalent scandium recovery by forming a low-melting molten alkali phase that enhances wetting, ionic mobility, and solid–liquid contact during mineral decomposition. Alkali cracking restructures the silicate matrix into porous Fe-rich and alkali-aluminosilicate domains that enhance internal permeability and ion diffusion, enabling efficient extraction of lattice-bound scandium. Under optimized supercritical fluid extraction parameters (65 °C, 34.5 MPa, adduct-to-solid ratio 5 mL g⁻ 1 ), both cracking pathways achieved ∼88% Sc extraction and ∼90% total rare earth elements recovery, representing a substantial improvement over untreated or mildly cracked feed. Quantitative Evaluation of Minerals by SCANning electron microscopy liberation and association metrics confirmed the preferential removal of alkali-activated phases during supercritical fluid extraction and leaving residues enriched in quartz and garnet, while mica phases such as biotite persist relative to the more reactive ferromagnesian silicates. A technoeconomic assessment comparing the two cracking strategies demonstrated that the NaOH–KOH eutectic route at 500 °C offers a significantly lower thermal energy requirement (∼60% reduction), lower capital and operating cost, and superior economic performance (higher net present value, internal rate of return, and shorter payback period) despite delivering comparable metallurgical outcomes. The technoeconomic analysis was conducted only for the two high-temperature caustic cracking routes, NaOH cracking at 1000 °C and NaOH–KOH eutectic cracking at 500 °C, which demonstrated high scandium extraction efficiencies. Low-temperature NaOH treatments were excluded from the TEA because their extraction performance was insufficient to support an economically viable process. This work establishes a mechanistically validated and economically compelling basis for integrating moderate-temperature eutectic cracking with supercritical fluid extraction to recover scandium and rare earth elements from refractory silicate ores. The results highlight a promising pathway for developing cleaner, more energy-efficient critical mineral extraction technologies suitable for industrial scale-up.