Ultrasound-assisted leaching of rare earth elements from phosphogypsum

Phosphogypsum (PG) has stood out as an alternative source of rare earth elements (REE) since it has a promising content and is generated in high amounts. Although the literature has already demonstrated the possibility of using leaching to recover REE from PG, the leaching efficiencies are generally relatively low, or drastic conditions are required. Ultrasound-assisted leaching is a promising alternative to overcome these problems and has not been sufficiently explored. This work investigated the REE leaching from PG using an ultrasound probe system and optimized it through a central composite rotational design. The temperature of the system was maintained at around 40 degrees C. A leaching efficiency of 84% was obtained at the optimized conditions of 0.6 mol L-1 H2SO4, ultrasonic amplitude of 77%, and pulse of 93.6%. The same leaching efficiency was not observed using conventional leaching with mechanical stirring (silent condition), around 68%. In addition, the kinetic study showed a considerable reduction in the leaching time with the assistance of ultrasound, reaching the equilibrium in about 20 min. The improvement in the REE leaching was attributed to the acoustic cavitation effects, which mainly led to a considerable reduction in the PG particle size, as demonstrated by SEM images. Overall, ultrasound proved to be a suitable alternative, allowing high REE recovery from PG with a diluted acid solution, low leaching time, and relatively low temperature, thus representing savings in reagents and energy.