Abstract
In recent years, the demand for rare earth elements (in this study, REE refers to the sum of lanthanides, REY refers to lanthanides plus yttrium, and REY+Sc represents lanthanides as well as yttrium and scandium) has increased for use in technology, health care, renewable energy, oil refining, and electronics due to global supply shortages. This demand has led to increased interest and research in coal deposits being considered an alternative source for these critical raw materials. Several countries, including South Africa, are considering coal and its by-products for the extraction of REEs. However, it is essential to understand the concentration of REEs in the host rock before extraction. South Africa has extensive coal resources, but the understanding of the concentration, mode of occurrence, and distribution of REEs in the coals and associated sediments is limited. The Waterberg Coalfield is estimated to contain 40 to 50 % of South Africa’s coal resources, but this coalfield is underexplored, and there is currently no information available pertaining to REE concentrations. The recovery methods for critical elements (including REEs) include preconcentration, activation, extraction, enrichment, and purification. Among these recovery methods, preconcentration has a direct effect on extraction efficiency and energy consumption. Density fractionation of coal is considered a preconcentration method.
The study aimed to assess density fractionated coal samples from the Waterberg Coalfield to determine the concentration and mode of occurrence of REEs, and to establish the behavior of REEs during density fractionation. Thirty density-fractionated coal samples selected from zones 8H, 8I, 7B, 4B, and 3C, sampled from an exploration core, were characterized using proximate analysis, petrography, X-ray diffraction (XRD), and X-ray fluorescence (XRF). The concentration of REEs in the density fractionated coals was determined using ICP-MS (after microwave digestion). The mode of occurrence of REEs was determined using Pearson’s correlation (indirect) and Tescan Integrated Mineral Analyzer (TIMA) (direct). The coals are classified as medium-rank C bituminous coals. The dominant minerals detected by XRD are kaolinite and quartz. The REY concentrations range from 45.1 to 389.2 ppm. The values are higher than that of world coals, except for sample 8H F1.30 (REY concentration of 45.1 ppm) and generally
iv
higher than the average reported for Chinese coals. Float products obtained at 1.40 to 1.80 densities show a greater variation of light REY (LREY) compared to medium REY (MREY) and heavy REY (HREY). Density fractions F1.30 and sink 1.80 show no distinct type of REY enrichment, as the LREYs are variable between the different zones. The results suggest a relationship between the concentration of LREY with float densities of 1.40 to 1.80. Pearson’s correlation identified positive correlation coefficients of REY with both organic matter and mineral matter, suggesting that the REY in the density fractionated coal samples have a mixed organic and inorganic affinity (i.e., association). The TIMA image analysis revealed that REEs in these samples have organic, intimate organic, and inorganic associations particularly associated with phosphates, silicates, aluminosilicates, iron-bearing, and sulphur-bearing minerals. The REE occurring in these coal samples may be due to inputs of hydrothermal solutions and terrigenous sediment input from the Bushveld Igneous Complex. Preconcentration by density fractionation may not be the best REY concentration process for these coals. Further investigations are required to confirm the potential of the Waterberg Coalfield coal ash for economic viability for REE extraction.