Mineralogy characterization and recovery of rare earth elements from the roof and floor materials of the Guxu coalfield

Abstract

Three segments of a core sample collected from the Guxu coalfield (Sichuan Province, China) were evaluated to determine the rare earth element (REE) leachability and mineralogy. Elemental analysis of the three samples showed that the roof and floor strata contained as high as 2087 ppm of total REEs, a value much higher than other coal-based materials reported in the literature. For the floor samples, 47% to 65% of the light REEs (LREEs) were leached within 5 min of contact time using 1 M mineral acid, and prolonged contact, up to 120 min, increased the LREE recovery to as high as 75%. For the roof sample, only 25% of the LREEs were leached at extended contact times. Unlike the LREEs, heavy REE (HREE) recovery from both the roof and floor materials was less than 25%. The relatively high REE content of the feed materials facilitated detailed SEM-EDX and TEM-EDX analyses of the samples. These studies confirmed the presence of several rare earth carbonate minerals, including bastnaesite and parasite, as well as rare earth phosphate minerals such as florencite and apatite. Given the prevalence and rare earth content of the particles analyzed via SEM-EDX, the data suggests that the LREEs present in the floor and roof samples were primarily associated with carbonates and phosphates, respectively. Further SEM-EDX analysis of the leaching residues showed that florencite was the only LREE-bearing species remaining after prolonged acid leaching, indicating that florencite is less soluble relative to the other rare earth minerals. The leaching results corroborate the SEM-EDX and TEM-EDX findings, while providing critical fundamental information on the relationship between REE content, REE mineralogy, and REE leachability in coal refuse materials.