MINERAL LIBERATION ANALYZER USED TO ASSESS THE EXTRACTABILITY OF RARE EARTH ELEMENTS FROM COAL ASH

Rare earth elements (REEs) are the cornerstone of modern technologies. Due to their unique properties, they are highly in demand in developing the latest technologies, leading to the exploration and research of alternative sources. Coal combustion by-products such as coal ash have been identified as potential alternative sources. However, coal ash as an alternative source for critical materials such as REEs has not been fully exploited, especially in South Africa. The study aimed to extract REEs from the power station and laboratory-derived coal ash. Samples were characterized using X-Ray Fluorescence, X-Ray Diffraction, and Inductively Coupled Plasma–Mass Spectrometry. A particular focus was made on using Mineral Liberation Analyzer (MLA) to identify the association of rare earth-bearing mineral phases. The aqueous acid leaching preceded by two different pre-treatment methods was chosen as the extraction technique for this study.

The samples were characterized by high ash, low moisture, low volatility, and low fixed carbon. The most abundant oxides determined in all the samples were silica, aluminum oxides, calcium oxide, and ferric oxide, with sulfur trioxide, magnesium oxide, titanium oxide, and potassium oxide as minor constituents. The mineral compositions of the ash samples commonly consisted of quartz, mullite, amorphous materials, kaolinite, hematite, and magnetite.

MLA imaging determined four groups of rare earth-bearing mineral phases: monazite-REE, xenotime-REE, perrierite, and cerium-oxide grouped as REE-cerium and REE-silicate. The total concentration of REE ranged from 522 to 977 ppm in the ash samples. Moreover, the ash samples had higher concentrations of critical REEs ranging from 203 to 406 ppm compared to excessive at 179 to 348 ppm and uncritical REEs between 129 to 256 ppm. The extraction process results were expressed in terms of the leaching efficiency. Results demonstrated leaching efficiency ranging from 78% to 92%, varying by ash type.