Comprehensive Understandings of Rare Earth Element (REE) Speciation in Coal Fly Ashes and Implication for REE Extractability

In recent years, recovery of rare earth elements (REEs) from coal fly ashes (CFAs) has been considered as a promising resource recovery option. Yet, quantitative information on REE speciation in CFAs and its correlation with REE extractability are not well established. This study systematically investigated the REE speciation-extractability relationship in four representative CFA samples by employing multiple analytical and spectroscopic techniques across the micro to bulk scale and in combination with thermodynamic calculations. A range of REE-bearing phases are identified, such as REE oxides, REE phosphates, apatite, zircon, and REE-bearing glass phase. REEs can occur as discrete particles, as particles encapsulated in the glass phase, or distribute throughout the glass phase. Although certain discrepancies exist on the REE speciation quantified by X-ray adsorption spectroscopy and acid leaching due to intrinsic limitations of each method, both approaches show significant fractions of REE oxides, REE phosphates, apatite, and REE-bearing Fe oxides. This study contributes to an in-depth understanding of the REE speciation-distribution-extractability relationship in CFAs and can help identify uncertainties associated with the quantification of REE speciation. It also provides a general methodology for future studies on REE speciation in complex environmental samples and a knowledge basis for the development of effective REE recovery techniques.