Speciation transformation of rare earth elements (REEs) during heating and implications for REE behaviors during coal combustion

The recovery of rare earth elements (REEs) from coal ash has gained growing attention as a promising resource recovery strategy. Compared to extensive studies on REE occurrence in coal and coal ash, few studies have investigated REE behaviors during coal combustion processes. This study investigated the mineralogy, oxidation state, and structure changes of twelve representative REE-bearing phases during heating up to 1500 °C using complementary thermal analysis, in-situ X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray absorption spectroscopy (XAS), in order to better understand and predict REE behaviors during coal combustion processes. It was found that: (1) REE-lignin and REE-chitosan complexes, hydrated Y2(CO3)3 and Ce2(CO3)3, bastnasite, and REE-doped calcite transformed to corresponding oxides after heating, accompanied by Ce(III) oxidation; (2) hydrated YPO4 and CePO4 formed corresponding anhydrous REE phosphates after heating with no evident Ce(III) oxidation; and (3) apatite, monazite, xenotime, and zircon barely altered during heating, although Ce(III) in apatite was partially oxidized and Y local structure became more disordered in apatite, monazite, and zircon. These results are consistent with previous studies on REE speciation in coal and coal ash and provided an in-depth understanding on REE behaviors during coal combustion, including mineral transformations and retention of REE-bearing phases. XAS analysis revealed that due to the co-existence of different REEs, mineralogical changes, and local structural distortion of REE-bearing phases during combustion, quantification of REE speciation in coal ash using XAS should be interpreted with caution and cross-validated by other techniques.