Process investigation of the acid leaching of rare earth elements from phosphogypsum using HCl, HNO3, and H2SO4

Abstract

The increasing demand for some critical rare earth elements (REEs), which are crucial in the production of green and technologically advanced products, has sparked the development of novel processes to recover REEs from secondary resources, such as mine tailings, metallurgical slags, and phosphogypsum. These resources have low concentration of REEs, but are available in large volumes. Phosphogypsum (PG) is the main by-product of the production of phosphoric acid by sulphuric acid digestion of phosphate rock. The current study put the emphasis on the thorough characterization of phosphogypsum as well as investigation of the kinetics of the leaching process. The results show the leaching efficiency is controlled by the solubility limit of PG. Chemical modelling indicated that the systems are at the saturation level with respect to Ca. The instantaneous and saturation concentrations of Ca were used to model the dissolution kinetics. The apparent activation energy was determined to be 10.4kJ/mol. Furthermore; systematic leaching processes using three acids under various operating conditions were performed. The results indicate the optimal operation conditions are 1.5M, 80°C and solid-to-liquid ratio (S/L) of 1/8 and 20min residence time for all three acid, but the leaching efficiency using nitric and hydrochloric acids are higher than the sulfuric acid. This novel work investigates the feasibility of acid leaching of REEs from PG from both theoretical and practical standpoints.