Distribution of rare earth elements in the pilot-scale processing of fly ashes derived from eastern Kentucky coals: Comparisons of the feed and processed ashes

Beneficiated fly ash from the combustion of Central Appalachian high volatile bituminous coals was extracted with HNO3 in a pilot-scale processing plant. Several major oxides (notably CaO and SO3, but also including Fe2O3, MgO, K2O, and P2O5) and minor elements (Mn, As, Sr, Ba, and Pb) are depleted in the post-HNO3extraction spent ash. The total lanthanides, Y, and Sc concentration is reduced by about 20% in the spent ash, with Gd showing the greatest decrease. Along with Gd, Nd and Dy are also well differentiated between the feed and spent ashes, with La and Sm showing minimal partitioning. The Gd decrease is correlated with the depletion of Fe2O3. The heavy rare earth elements (REE heavier than Eu) and Y are disproportionately concentrated in the HNO3-leachate compared to the light REE. For the ashes studied, Sc did not partition between the feed and spent ashes. Pozzolanicity tests show that the compressive strength and strength activity indices of the spent ash + ordinary Portland cement (OPC) mixes are comparable to 100% OPC, indicating that the spent ashes produced in the pilot-scale runs have the potential to be sold as a Class F fly ash. Ultimately, the beneficiated ash chemistry influences the chemistry of the post-HNO3-extraction spent ash and the HNO3-leachate. A 500-ppm-REE fly ash will presumably be a more economically favorable feedstock than an ash with a significantly lesser concentration.

Automated summary

Beneficiated fly ash from the combustion of Central Appalachian high volatile bituminous coals was extracted with HNO3 in a pilot-scale processing plant. The post-HNO3-extraction spent ash showed depleted major oxides and minor elements, with a significant reduction in total lanthanides, Y, and Sc concentration. Heavy rare earth elements and Y were disproportionately concentrated in the HNO3-leachate compared to light REE, while Sc did not partition between the feed and spent ashes. The spent ash + ordinary Portland cement mixes exhibited comparable compressive strength and strength activity indices to 100% OPC, indicating potential for commercial use as a Class F fly ash. Ultimately, the chemistry of the beneficiated ash influenced the composition of the post-HNO3-extraction spent ash and HNO3-leachate.