Modes of occurrence of rare earth-bearing minerals in South African coal and ash samples using electron microscopy

Rare earth elements and yttrium (REY) are crucial for modern technologies. This study investigated the modes of occurrence, mineral hosts, and distribution in coal and laboratory-derived coal-ash samples from the Soutpansberg Coalfield (South Africa) using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), mineral liberation analysis (MLA), X-ray diffraction (XRD), and inductively coupled plasma mass spectrometry (ICP-MS). The mineral composition of the studied coal primarily comprises of silicates, predominantly kaolinite (avg. 24.1 wt%; whole coal basis), quartz (avg. 13.6 wt%), alongside carbonates (avg. 12.9 wt%) and pyrite (∼0.1 wt%). Although the coal-ash samples also have silicates (avg. 78.9 wt%), phosphates (monazite, xenotime), titanium oxides, and zircon. Average REY concentrations are 241.3 mg/kg in the coal and 468.3 mg/kg in the coal-ash, reaching a maximum of 957.4 mg/kg with multi-day multi-acid digestion, suggesting enhanced liberation. Backscattered electron (BSE) images obtained via SEM-EDS and MLA reveal monazite and xenotime as the major REY-bearing minerals in both the coal and coal-ash samples, with some REY hosted by carbonates. In the coal, micron-scale REY-bearing minerals are adsorbed onto the surfaces of major and minor minerals such as quartz, kaolinite, zircon, and Ti-oxides. The REY-bearing minerals in the coal-ash are encapsulated within new phases, likely amorphous or aluminosilicates. The LREY and MREY are associated with monazite, quartz, carbonate, zircon, and Ti-oxide phases, and HREY with xenotime. Particles ∼ 1 μm in size show better liberation, which may potentially enhance extractability. MLA offers higher-resolution imaging than SEM-EDS, enabling better identification of micron-size REY particles. These findings underscore the importance of understanding REY mineralogical associations and liberation dynamics to optimize extraction and enhancing resource recovery from coal and its by-products.