The Occurrence, Distribution, and Concentration levels of Critical Raw Materials from the Coal Discard of the Witbank Coalfield

Abstract

This study investigates the modes of occurrence, distribution, and concentration levels of critical raw materials (CRMs) within coal discard dumps from the Witbank Coalfield, South Africa. Traditionally regarded as waste, these coal discards have been found to contain economically significant concentrations of CRMs, including Ba, Ga, Sr, Zr, and other valuable elements. Two contrasting coal discard dumps an older, decommissioned dump (CD1) and a younger, active dump (CD2) were selected for sampling and analysed using a suite of analytical techniques such as thermogravimetric analysis (TGA), bomb calorimetry, Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), automated mineralogy (TIMA), X-ray fluorescence (XRF), and inductively coupled plasma mass spectrometry (ICP-MS). Sampling at CD1 was conducted systematically by depth profiling, while CD2 samples were collected unsystematically at the surface. Samples were categorized into four lithological groups: bright coal discard, moist coal discard, clay-rich coal discard, and reddish oxidized coal discard. Results indicate that both CD1 and CD2 contain high-volatile bituminous coal discards, with CD1 exhibiting average values of 4.49 wt% moisture, 21.91 wt% volatile matter, 38.53 wt% fixed carbon, 34.78 wt% ash, and a calorific value of 18.11 MJ/kg. CD2 showed slightly different values: 6.46 wt% moisture, 20.29 wt% volatile matter, 38.14 wt% fixed carbon, 35.11 wt% ash, and 17.55 MJ/kg calorific value. FTIR analysis revealed characteristic absorption bands including O-H, C H, C=C, C-O, C-C, M-O, Si-O, and Si-O-Si in samples from both dumps. Mineralogically, quartz and kaolinite dominate, accompanied by minor amounts of barite, zircon, apatite, illite, muscovite, and ilmenite. Major oxides in both dumps are dominated by SiO2 and Al2O3, however, Al2O3 content remains below 40%, classifying these as low-alumina coal discards. CD1 is comparatively enriched in Fe2O3, CaO, MnO, MgO, Ba, Ga, Sr, Cu, Ni, and rare earth elements plus yttrium (REY), whereas CD2 displays higher concentrations of SiO2, Fe2O3, P2O5, SiO2, Na2O, Be, Co, Li, Nb, Mo, Pb, Rb, Zr, V, and Zn. On average, CRM concentrations are higher in CD1 compared to CD2. Vertical variability of CRMs in CD1 was clearly evident, with depth-dependent trends noted for Ba and Sr, especially within 0–1 m and 3–4 m depth intervals. Sr is also widely distributed throughout CD2. Ba predominantly occurs in barite, while Sr exists mostly in barite in CD1 and is primarily associated with phosphate and carbonate minerals in CD2. Zr associates strongly with zircon, Ga occurs within kaolinite and organic matter, and Li shows intimate organic associations. REYs are hosted by Fe-bearing and phosphate minerals in CD1 but by Ca-rich aluminosilicates in CD2. Nb associates with both aluminosilicate and Ti-bearing minerals in CD1, but shows organic associations in CD2. Mo is mainly found in Ti-bearing minerals in CD1 and organic matter in CD2. Co occurs in Mn-, Ca-, Na- and Ti-bearing minerals in CD1 but is limited to Ca, Fe, Mn carbonate in CD2. Cu exhibits strong organic affinity in CD1 and occurs in phosphate and chromite minerals in CD2. Pb associates with organic matter in CD1 and is found in apatite, chromite, Ti-, and Mg-bearing minerals in CD2. Ni, Be, and Zn show organic affinity in both dumps, while Rb is mainly hosted by illite and muscovite. The study highlights the influence of hydrogeochemical processes and igneous intrusions on CRM enrichment and mineral associations within coal discards. The findings underscore the potential for coal discard dumps to serve as secondary sources of CRMs, offering promising opportunities for sustainable resource recovery and economic value addition. Furthermore, CRM extraction from coal discards could help mitigate the environmental risks posed by stockpiled waste materials. Based on results, it is recommended that further research focus on optimizing flotation and selective leaching techniques to enhance CRM recovery potentials.