Distribution of rare earth elements in coal combustion fly ash, determined by SHRIMP-RG ion microprobe

Abstract

During coal combustion for electric power generation, rare earth elements (REE) are strongly retained in the ash fraction leading to significant enrichment compared to the REE content of the respective feed coals. Knowing how the REE are distributed in coal ash is essential to devising effective approaches for REE extraction from this potential resource. To better understand the distribution of REE in coal ash, we used the Stanford-USGS SHRIMP-RG ion microprobe to determine grain-scale REE partitioning in a suite of 19 U.S. and international coal fly ash samples. SHRIMP-RG microanalysis used a nominal spot size of 15μm and an oxygen negative-ion primary beam. NIST multi-element glass standards SRM 611 and SRM 613 were used for REE calibration. Prior to SHRIMP-RG analysis, samples were characterized by backscattered electron imaging and wavelength-dispersive elemental mapping to delineate constituents of interest for REE microanalysis. Results confirm the occurrence of REE in aluminosilicate glasses consistent with previous results that showed a strong positive correlation between Al and REE content in a larger group of U.S. and international coal fly ash samples. Among these glasses, aluminosilicates consisting of Al and Si and lacking other major constituents tend to have REE contents similar to, or slightly lower than the bulk sample REE content, whereas Ca- and/or Fe-enriched aluminosilicate glasses are similar to, or more REE-enriched than the bulk sample. Co-occurring quartz and/or high-silica glass is almost always the most REE depleted fly ash constituent, having much lower REE contents than the aluminosilicate glasses. Fe-oxide magnetospheres have REE content with relative proportions of light (LREE) to heavy (HREE) rare earths that are more variable than in the aluminosilicate glasses. Results suggest that extraction of REE from the aluminosilicate glass fraction in coal ash will yield a significant portion of the REE present and therefore, this fraction should be targeted in ongoing efforts to recover REE from coal fly ash.