Abstract
The increase in demand for rare earth elements (REE) has grown exponentially in recent years due to their use in high-technology products that improve the standard of living. Due to the high demand, conventional deposits of REE such as carbonatites, alkaline granites, and weathered crust are not able to meet the global supply. Therefore, alternative sources of REE are currently being investigated to bridge the supply gap. Researchers are looking into coal and coal by-products (discards and ash) as potential sources for REE. Before extraction of these elements, their concentrations and modes of occurrence need to be investigated.
South Africa has extensive coal reserves, which are located in various basins (coalfields) and are mainly used for power generation. Currently, there is limited understanding regarding REE concentration and mode of occurrence in these coalfields. This study sampled coal and related clastic partings from a wide-diameter drill core in Makhado, on the Seam 6 of Madzaringwe Formation to understand the concentrations, stratigraphic distribution, and modes of occurrence of REE in the Tshipise Sub-basin of the Soutpansberg Coalfield, South Africa. A total of forty-one (41) samples were split and a portion was ground to 106 μm, digested by microwave digestion, and analysed using inductively coupled plasma mass spectrometry (ICP-MS). The concentration of REE, including yttrium (Y) and scandium (Sc) (abbreviated to REY+Sc) in the studied Tshipise samples ranges between 47.33 and 349.52 μg/g. In addition, the concentrations of REY+Sc for the studied sub-seams and partings shows a general increase with depth. The REY+Sc concentrations for the samples from Seam Upper (SU) (58.62 to 113.48 μg/g) and Parting 1 (P1) (87.81 to 133.37 μg/g) are comparable to the average for United State coal (US = 68 μg/g) and World coal (72 μg/g), but lower than the average for Chinese coal (138 μg/g) and the Upper Continental Crust (UCC = 168 μg/g). The concentrations of REY+Sc for samples from Seam Middle Upper (SMU), Parting 2 (P2), Seam Middle Lower (SML), Parting 3 (P3), Seam Bottom Upper (SBU), Parting 4 (P4), Seam Bottom Middle (SBM), Parting 5 (P5), Seam Bottom Lower (SBL) to Parting 6 (P6) are higher than the average values for US, World and Chinese coal, and comparable or higher than the UCC, except for two samples (sample 13 = 47.33 μg/g and 31 = 103.82 μg/g). The samples are
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generally dominated by light REY (LREY), with outlook coefficient between 0.7 and 1.9, the studied Tshipise samples are promising for REY recovery.
Twenty (20) samples with varying concentrations of REY+Sc were selected to understand their organic and mineralogical composition. These samples were analysed using a combination of proximate analysis, organic petrography, X-ray diffraction (XRD), and X-ray fluorescence (XRF). Only five out of the twenty samples, were classified as coal based on their ash content, whereas the others have ash yields in excess of 50 wt% and are thus termed non-coal. The mean random vitrinite reflectance (%RoVmr) values for the coal samples range between 0.82 and 0.84 %RoVmr, classifying them as medium-rank C bituminous. The five coal samples are dominated by vitrinite (25 - 65 vol%), followed by inertinite (5 - 25 vol%), with minor liptinite (1 vol% in sample 26). The mineralogy of most samples (both coal and non-coal) is dominated by kaolinite and quartz, except for two samples (samples 13 and 31) which are dominated by carbonate minerals. These samples deviate from the norm regarding REY+Sc distribution in the core.
To determine the modes of occurrence and associations of REY+Sc, direct and indirect techniques were applied; these include statistics analysis, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), mineral liberation analysis (MLA), and sequential leaching/sequential chemical extraction procedure (SCEP). The REY+Sc in the studied samples may be associated with clay minerals, owing to a positive relationship between kaolinite contents and ash yields. The SEM-EDS results show that clay and quartz are syngenetic, whereas pyrite and siderite appear to be epigenetic infilling cracks and pores of the organic matter. The MLA analysis identified REY-bearing minerals including monazite, xenotime and thorite as well as a REE-Al-Si phase. In addition, the SCEP results indicated that the REY+Sc is associated with both organic and inorganic matter. For the organic-bound elements step, the samples were microwave digested with HNO3 and HF prior to analysis and the results show a recovery of between 25 and 56% of REY+Sc is associated with the organic matter. A recovery of 18 to 69% for the carbonate-associated elements step, leached using HCl, which can also leach phosphates, carbonates, and sulphates-associated elements. When leaching using HF, a recovery between 2 and 34% was observed for silicate-associated elements. Therefore, the REY+Sc in the studied
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Tshipise samples are interpreted to be associated with both organic and inorganic matter. For the organic matter, the REY+Sc shows a positive relationship with inertinite, whereas these elements show a positive relationship with clay (kaolinite) when considering the inorganic component. The clay likely reflects detrital input of sediments weathered and eroded from a felsic to an intermediate source region.
*(In this study, REE refers to the sum of lanthanides, REY if yttrium is included and REY+Sc if both Y and Sc are included).