Enrichment and occurrence form of rare earth elements during coal and coal gangue combustion

Coal ash has emerged as an important alternative source for rare earth elements (REEs). The enrichment and occurrence form of REEs among coal combustion products are of great significance for both technical design and economic evaluation of recovering REEs from the coal ash. Here, the enrichment and occurrence form of REEs in the ash were investigated. Compared with ashes from muffle furnace, coal fly ash (CFA) from power plants involved higher enrichment ratio of REEs, which was explained by the fractionation of coal ashes to concentrate REEs in finer CFA, higher combustion temperature to vaporize more volatile elements, and longer residence time of fly ash to absorb REEs in the gas. In addition, CFA samples were analyzed by sequence chemical extraction procedure (SCEP) and scanning electron microscope with an energy dispersive spectrometer (SEM–EDX), which revealed the important role of aluminum in the occurrence form of REEs compared with Si in aluminosilicates of CFA. This conclusion was further confirmed by thermodynamic equilibrium calculation, which also agreed qualitatively with the observation that REEs mainly existed in the solid phase. Both experimental and computational results of this work provided insights to understand the distribution of REEs in CFA and optimize their extraction processes.