Efficient Leaching and Kinetic Analysis of Rare Earth Elements from Rare-Earth-Enriched Residues of Associated Rare Earth Phosphate Ores

The efficient recovery of rare earth elements from associated rare-earth-bearing phosphate ores is of paramount importance for expanding the supply of rare earth resources. In contrast to conventional studies that focus on extracting rare earths either from phosphate concentrates or from phosphogypsum generated during the sulfuric acid wet-process, this study takes as its subject the rare-earth-enriched residue—an intermediate product obtained after the selective leaching of phosphorus via the hydrochloric acid route—from a rare-earth-bearing phosphate ore in Zhijin, Guizhou Province. The occurrence states, leaching behavior, and kinetic mechanisms of rare earth elements within this residue were systematically elucidated. Analyses using scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS) and aberration-corrected scanning transmission electron microscopy (STEM) reveal that rare earth elements are hosted in residual fluorapatite and newly formed CaF2 through isomorphic substitution. The substitution of REE3+ for Ca2+ induces lattice contraction in CaF2, with the interplanar spacing decreasing from 0.27 nm to 0.26 nm. Through single-factor experiments and response surface methodology (RSM) optimization, the optimal leaching conditions were determined to be a temperature of 80 ◦C, a leaching time of 120 min, a hydrochloric acid dosage of 160% of the theoretical requirement, a solid–liquid ratio of 1:6, and a agitation speed of 500 r·min−1.