Transformation and modes of occurrence of rare earth elements in coal fly ash from a power plant in Chongqing, southwestern China: Implications for REE recovery

Coal fly ash (CFA) is an abundant industrial byproduct and an underexploited source of rare earth elements (REEs). This study uses multiple analytical techniques, including electron probe microanalysis (EPMA) and high-resolution transmission electron microscopy (HRTEM) to quantitatively characterize REE-bearing phases in a REE-rich CFA from southwestern China and to investigate the transformation pathways of REE-bearing minerals during coal combustion. In the feed coal, REEs occur mainly as REE phosphates (monazite and xenotime) and REE-bearing aluminophosphates (florencite and goyazite). During combustion, monazite remains largely unaltered, whereas the less refractory REE-bearing aluminophosphates decompose and react with aluminosilicates, producing a REE-enriched melt that partially devitrifies into REE silicate (cerite-like), REE oxide, and REE-bearing glass. EPMA Ce-mapping of 11 randomly selected regions identified 67 REE-bearing particles, showing that 80.9% and 19.1% of the total REE mass were associated with aluminosilicate glass and crystalline phases, respectively. Within the aluminosilicate glass, REE concentration is heterogeneous, being higher in Al-rich than Si-rich compositions. These results identify both REE-bearing crystalline phases and Al-rich aluminosilicate glass as priority targets for selective recovery from coal combustion residues. Micro-submicron REE phases indicate fine grinding and liberation of these phases could facilitate more selective and efficient recovery strategies.