Mechanisms of ultrasonic enhanced co-leaching of U and REEs from low-grade uranium resources

Uranium (U) and rare earth elements (REEs) are both strategic metallic elements. With the consumption of high-grade mineral resources, it is necessary to enhance the utilization of complex low-grade resources. This study proposes an ultrasonic-enhanced alkali pretreatment - acid leaching process for the recycling of uranium tailings associated with REEs. The ultrasonic-assisted alkaline pretreatment effectively disrupted the gangue structure by dissolving a portion of silicon (3.66%) and aluminum (5.59%). This enhanced the accessibility of metals encapsulated within the gangue minerals and altered the speciation of residual REEs, thereby facilitating the subsequent acid leaching of both U and REEs. The advantageous regions for acid leaching of U and REEs were identified through thermodynamic analysis. Effects of ultrasonic-assisted acid leaching conditions on the leaching efficiencies of U and REEs were investigated. The optimized acid leaching conditions are as follows: HCl concentration of 2.5 mol/L, leaching temperature of 70 °C, leaching time of 2 h, L/S ratio of 10:1, H₂O₂ addition of 5%, and ultrasonic power of 400 W with frequency of 40 kHz. The leaching efficiencies of U and REEs reached 90.43% and 72%, which increased by 17.09% and 12.33% respectively, compared to conventional leaching. The characteristics of the residue before and after ultrasonic leaching were compared to analyze the leaching mechanism. During acid leaching, the free radical capture experiment found that the ultrasonic effect promoted the decomposition of hydrogen peroxide to produce more free radicals, oxidizing insoluble U(IV) to dissolved U(VI). The ultrasonic effects strengthened the leaching kinetics for both U and REEs. The results provide an enhanced method for the efficient utilization of low-grade encapsulated mineral resources.