Addendum: Electron microbeam investigations of the spent ash from the pilot-scale acid extraction of rare earth elements from a beneficiated Kentucky fly ash (2025, Int. J. Coal Geol. 303, 104738): Further consideration of particle-size reduction from feed to processed fly ash

As part of the scanning electron microscopy (SEM)/ transmission electron microscopy (TEM)/ energy dispersive x-ray spectroscopy (EDS) investigation of the pre- and post-pilot-scale HNO₃-based extraction of rare earth elements from a beneficiated eastern Kentucky coal-derived fly ash, 2- to 3-μm-thick rims were observed in cross sections of post-processing Al-Si fly ash spheres. Compared to the cores of the particles, the rims are depleted in all measured elements except Si. As an aside, Berti et al. (2025) noted an apparent depletion, if not absence of <4-μm particles in the HNO₃-processed ash. In this addendum, particle-size analysis demonstrates that the processed ash is finer than the feed ash. Some of the fineness may be attributed to dissolution or partial dissolution of the finest grains. The latter trend might be offset, in part, by the attrition of fine flakes of material from the rims of coarser particles.