The Distribution of Rare Earth Elements in Coal Fly Ash Determined by LA-ICP-MS and Implications for Its Economic Significance

Coal fly ash represents a potential resource of some critical elements, including rare earth elements (REEs), which are retained and concentrated during coal combustion. Understanding the distribution and modes of occurrence of REEs within fly ash is vital to developing effective recovery methods and enhancing their economic value. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) was applied to investigate the in situ elemental constituents of coal fly ash phases, including aluminosilicates, Ca-(Fe)-enriched aluminosilicates, Fe-oxides, and SiO2/Quartz, in order to explore the distribution of REEs in combustion products. LA-ICP-MS results show that V, Cr, and Nb are mainly enriched in Ca-Ti-enriched aluminosilicates with trace element concentrations referenced to the original fly ash composition. Lithium is primarily enriched in SiO2 glassy grains, followed by Ca, (Fe)-enriched aluminosilicates. Co, Ni, and Cu present a concomitant distribution in the Fe-enriched phases, such as Fe-oxides and Fe-enriched aluminosilicates. The chondrite normalized REE distribution patterns show characteristics of LREE enrichment and Eu-negative anomalies in most phases, while the REE patterns of SiO2 glassy grains have a distinct positive anomaly in Sm, Gd, and Dy, coupled with a deficiency in LREEs. Compared to feed coal, elements such as Li, V, Cr, Co, Ni, and Nb and REEs are enriched 2 similar to 10 times in various phases of fly ash, with REEs notably concentrated six times higher in aluminosilicates and Ca-Ti-enriched aluminosilicates than the original coal. This study further discusses the feasibility, calibration principles, and advantages of using LA-ICP-MS to determine REE distribution, as well as the economic implications of REE extraction from coal fly ash.