Recovery of rare earths from rare earth molten salt electrolytic slag using atmospheric pressure alkaline leaching enhanced by ultrasonic

Abstract

Rare earth molten salt electrolytic slag (RMES) contains abundant rare earth elements (REEs) and is a valuable secondary resource. However, conventional approaches to recovering REEs from RMES are hindered by high energy consumption, environmental pollution, and lengthy processing times. This study presents an ultrasonic enhanced (USE) atmospheric pressure alkaline leaching process, which integrates ultrasonic cavitation with concentrated NaOH to realize efficient phase reconstruction of fluorinated rare earth compounds. Under optimized conditions (60 wt% NaOH initial concentration, NaOH-to-slag mass ratio of 3:1, reaction temperature of 130 °C, ultrasonic power of 1440 W, reaction time of 70 min, and stirring speed of 100 r/min), the leaching recovery of neodymium (Nd) and praseodymium (Pr) reached 96.22 % and 95.21 %, respectively, representing a 60 % improvement compared to conventional heating (COH) process. Mechanistic analysis shows that ultrasound helps reduce particle size, increase local reaction temperature (up to 172 °C), and generate reactive oxygen components. These effects collectively accelerate the conversion of fluorinated rare earth compounds (NdF₃, PrOF) to hydroxides (Nd(OH)₃, Pr(OH)₃). This method can eliminate the emission of toxic gases (HF) and reduce energy input, providing a green and efficient approach for recovering rare earth elements from secondary resources.