Taggart, and 6 co-authors, reported in 2018 that coal combustion ash is a promising alternative source of rare earth elements (REE; herein defined as the 14 stable lanthanides, yttrium, and scandium). Efforts to extract REE from coal ash will depend heavily on the location and speciation of these elements in the ash. This study sought to identify the major chemical forms of yttrium (Y), as a representative REE in coal fly ash samples selected from major coal sources in the United States. Y speciation was evaluated using both bulk scale analyses (sequential extractions, Y K-edge X-ray absorption near-edge spectroscopy – XANES) and complementary analyses at the micron scale (micro-focus X-ray fluorescence and micro-XANES). Sequential selective extractions revealed that the REE were primarily in the residual (unextracted fraction) of coal fly ash samples.
Extraction patterns for yttrium resembled those of the lanthanides, indicating that these elements were collectively dispersed throughout the aluminosilicate glass in fly ash. Bulk XANES analysis indicated that Y coordination states resembled a combination of Y-oxides, Y-carbonate, and Y-doped glass, regardless of ash origin. However, in the microprobe analysis, we observed “hotspots” of Y (10–50 mm) in some samples that included different Y forms (e.g., Y-phosphate) not observed in bulk measurements. Overall, this study demonstrated that yttrium (and potentially other REEs) are entrained in the glass phase of fly ash and that microscale investigations of individual high-REE regions in fly ash samples do not necessarily capture the dominant speciation.
Rare earth elements (REE) are critical materials in the defense, electronics, energy, optics, and automotive industries. Coal fly ash is one potential alternative source for REE, with some ashes exceeding 0.1% total REE content by weight. For many coal fly ashes produced in areas including the U.S., China, the U.K., Poland, Russia, and South Africa, REE are moderately enriched relative to upper continental crust (UCC) values. Given the vast quantities of coal fly ash produced each year, this waste stream represents a significant mass of REE if they can be efficiently extracted. The degree of REE enrichment in fly ash corresponds to the geological origin of the coal feedstock. In the U.S., ashes derived from central Appalachian and Illinois Basin (i.e. eastern U.S.) coals have significantly higher REE contents than those derived from Powder River Basin (i.e. western U.S.) coals. However, the mass of acid-extractable REE is often similar for eastern and western U.S. coal ashes due to the high leachability of REE in Powder River Basin coal ashes. Appalachian- and Illinois Basin-derived ashes require more aggressive methods to decompose the aluminosilicate glasses (such as alkaline extractions) for effective leaching of the REE into solution. The disparate REE recoveries from different ash samples highlight the need to better understand rare earth speciation and modes of occurrence in fly ashes. Information on the dominant REE-bearing phases of fly ash and the general and localized speciation of the REE can help inform REE extraction techniques for recovery purposes.
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