Electron microbeam investigations of the spent ash from the pilot-scale acid extraction of rare earth elements from a beneficiated Kentucky fly ash

Abstract

Fly ash derived from the combustion of an eastern Kentucky high volatile bituminous coal blend was, as discussed in previous studies, beneficiated to yield a − 75-μm product with a reduction in the carbon and spinels. The beneficiated fly ash was reacted with a strong acid in a pilot-scale unit to extract rare earth elements (REE) and other major and minor elements. In this study, polished, epoxy-bound pellets of the beneficiated and acid-extracted ashes were examined via scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Selected slices of the polished section were extracted using a focused ion beam (FIB) technology and further examined via transmission electron microscopy (TEM) and EDS. The bulk chemistry of the original feed ash (not studied by microbeam methods), the beneficiated ash, and the acid-extracted ash exhibit similar Upper Continental Crust-normalized REE patterns. The bulk chemistry indicates that the concentrations of the major oxides (aside from SiO₂), REE, V, Cr, Mn, Ni, As, Rb, Sr, Zr, Ba, and Pb are less in the processed ash compared to the beneficiated ash. The fly ashes have H-type distributions (La_N < Lu_N) with a strong M-type (medium type: La_N/Sm_N < 1, Gd_N/Lu_N > 1) contribution. Among the REE and Y, SEM-EDS and TEM-EDS showed the presence of zircon with Y (and Ce, Nd, and Gd, perhaps from an adjacent monazite); xenotime; a Dy-rich xenotime-structure mineral: and monazite with light REE (La, Ce, Pr, Nd, and Sm), Gd, Dy, Er, and Th. Glassy Al-Si-rich fly ash spheres showed the presence of Ce, Nd, Sm, and Dy but no discernable minerals at the scale of the SEM view. Certain spheres showed 2- to 4-μm amorphous rims surrounding nano-crystalline cores. As with the depletion of elements in the bulk fly ash, the amorphous rims showed a relative increase in Si and a depletion or apparent elimination of other elements, including the REE. It is possible that the apparent absence of 2- to 4-μm particles may mean that these particles were totally or largely dissolved by the acid or as a consequence of multiple wash-filtration cycles to remove leachate from the spent ash. Processing of a combination of a finer size than processed in this pilot-scale investigation and/or more fractured particles, perhaps via a pre-processing step, would permit a greater penetration of the acid into the interior of the fly ash particles, leading to a greater recovery of REE.