Microstructural transformation and recovery of rare earth elements during hydrochloric acid leaching of apatite concentrate

Abstract

Hydrochloric acid concentration plays a pivotal role in governing the dissolution behavior of apatite and the recovery of rare earth elements (REEs) during hydrometallurgical processing. In this study, the influence of hydrochloric acid concentration on the leaching of cerium (Ce), lanthanum (La), and neodymium (Nd) from an apatite concentrate derived from iron ore processing waste was comprehensively investigated by correlating microstructural evolution with leaching performance. The effects of acid concentration, solid-to-liquid (S/L) ratio, leaching time, and temperature were systematically evaluated using Response Surface Methodology (RSM). Statistical analysis identified acid concentration as the most significant parameter controlling REE extraction, followed by leaching temperature, whereas leaching time exhibited only a marginal influence. Under the optimum conditions of 37 wt.% HCl, a solid-to-liquid ratio of 1:9, a leaching time of 60 min, and a temperature of 65 °C, the recoveries of Ce, La, and Nd reached 94.19%, 86.33%, and 76.34%, respectively. SEM analysis of the leaching residues revealed a progressive transition from relatively smooth, well-preserved crystalline particles at low acid concentrations to severely corroded, fragmented, and porous structures at higher acid concentrations. These microstructural transformations provide direct evidence of enhanced apatite decomposition and increased REE liberation, explaining the substantial improvement in extraction efficiency. The strong correlation between acid-induced microstructural evolution and REE recovery demonstrates that hydrochloric acid concentration is the primary driving force governing leaching performance. This study provides new mechanistic insight into acid-assisted apatite dissolution and establishes an optimized, economically attractive strategy for the efficient recovery of valuable REEs from apatite-rich iron ore tailings.