The distribution and occurrence of rare earth elements (REEs) in coal and industrial scale boiler and gasifier ash

Coal and coal ashes are being investigated as secondary sources of REEs due to scarcity, high demand, and elevated prices of these elements. The aim of this study was to evaluate the occurrences, associations, and distributions of REEs in South African feed coal, gasification ash, and boiler bottom ash and fly ash. The ashes were generated from industrial-scale gasifier and combustion boiler systems, both fed the same feed coal examined in this study. X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) analyses were employed to determine the oxide and mineralogical compositions of each sample. The mapping of the REE-bearing minerals within the samples was done using Field Emission Scanning Electron Microscope (FE-SEM) and quantification of REEs using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis. The feed coal is an inertinite-rich medium rank C bituminous coal. Mineralogical analyses revealed that primary REE-bearing mineral phases were REE-phosphates (monazite and xenotime) and REE silicates, with davidite (enriched in Ce) and zircon (enriched in Y) determined in the boiler ashes and gasification ash, respectively. Total REE concentrations were 161 ppm (feed coal), 238 ppm (gasification ash), 207 ppm (bottom ash) and 282 ppm (fly ash), with at least 76 % as LREEs, mainly Ce and La. Observations made from the mineralogical properties and the concentrations of REEs suggest that direct extraction may not be viable.