Discussion: “Effects of acid concentration on the recovery of rare earth elements from coal fly ash” [Mokoena et al., 2022, Int. J. Coal Geology, volume 259, article 104037]

The contribution by Mokoena et al. (2022, volume 259, article 104037) is flawed in the fundamental characterization of the fly ash. The TESCAN Integrated Mineral Analyzer (TIMA) is a tool designed for use with several-micron-scale crystalline particles, not for a fly ash largely composed of glass with nm-scale to micron-scale minerals. The Energy Dispersive X-ray Spectroscopy analysis is flawed by the selection of rare earth peaks that are either concealed by the stronger Ti and Fe peaks or cannot be distinguished from the background in the energy regions outside of the Ti and Fe peaks. Consequently, many of the minerals “identified” in the TIMA analysis were mistakenly considered to have observable concentrations of rare earth elements.