Mechanochemical dephosphorization and ultrasound-intensified leaching of rare earths from low-thorium monazite ore

• Combined ultrasound-assisted and mechanochemical leaching saves time, energy • Dry milling monazite with NaOH converts REEPO₄ to REE(OH)₃, >90% REE yield • Ultrasound enhances cavitation, mass transfer, and leaching efficiency • Sustainable, intensified pathway for REEs recovery from monazite ore The recovery of rare earth elements (REEs) from monazite ore was studied using ultrasound-assisted leaching (UL) and compared with silent leaching (SL). The UL experiments were conducted using a 40-mm diameter Monel alloy horn at 20 kHz, with power densities of 142 - 232 W/L. SL with H₂SO₄ yielded <3% recovery for all REEs, whereas UL enhanced dissolution within 10 min, (8.5% Ce, 7.0% Nd, 8.8% La) via cavitation-induced particle breakdown, confirmed by Scanning Electron Microscopy (SEM). Using 62% H₂SO₄, optimization achieved maximum recoveries: 20.5% Ce, 17.2% Nd, 22.0% La, 16.3% Pr, and 17.9% Sm. Recovery increased with ultrasound power density up to 206 W/L (26.2% La) and decreased at higher values due to cavitation shielding and nuclei removal. NaOH pretreatment via dry ball milling removed ⁓ 95% of phosphate with minimal REE loss. Among acids tested under SL (6 mol L -1 , 60°C, 30 min), HNO₃ achieved > 95% recovery from phosphate-free monazite, compared to <10% for crude ore. Applying ultrasound to phosphate-free monazite accelerated leaching, yielding 55-92% recovery of Ce-Sm in 10 min and 72-94% after 30 min. Enhanced kinetics arising from improved ore disintegration, phosphate removal, and optimized acid conditions provide a rapid, scalable REEs recovery strategy.