Exploring rare earth elements in coalmine overburden: nanoscale insights from FESEM, TEM and XPS analysis

Abstract

Rare earth elements and yttrium (REY) are critical for various advanced technologies, particularly in electronics, and play a vital role in the economic growth of any country. Coal and its by-products could be potential precursors of these commodities and other natural resources. While coal and coal fly ash have been assessed for their REY content, the coal mine overburden (OB) or waste remains unexplored as a potential source of rare earth elements. The coal and coalmine OB samples of the Makum coalfield from the Northeastern region (NER) of India are examined and found to be promising sources of REY. This study presents the existence, distribution, and depositional conditions of REY in the coal and coalmine OB using various advanced analytical techniques, such as nanoscale morphology, Field Emission Scanning Electron Microscopy (FE-SEM), and High-Resolution Transmission Electron Microscopy (HR-TEM) to provide light on the geochemical behaviour and potential commercial viability of REY. The average value of REY in the study area is 167.66 mg kg^-1 on a whole sample basis, in which the mean light (LREY) to heavy (HREY) ratio is 37.67. The average values of the europium anomaly (δEu), cerium anomaly (δCe), and Gadolinium anomaly (δGd) are 3.20, 0.71, and 5.30, respectively. The coal-forming conditions are characterized by slightly oxidizing and highly reducing environments, dominated by M-type enrichment, which are favourable for the weathering process. These conditions, marked by the absence of anaerobic microbial activity, facilitated the preservation of organic matter. Thus, the OB and coal deposits in this region present an opportunity for further exploration and assessment regarding the potential future recovery of REY.

Fig. 1. Study area map of the Makum coalfield mining area in Assam, Northeast India.

Fig. 2. Qualitative XRD-analysis of coal and OB samples showing the presence of mineral phases (Q: quartz; He: haematite; P: pyrite; Ca: calcite; K: kaolinite; M: magnetite).

Fig. 3. Correlation between δCe/δEu and ∑REY (modified after Mu et al., indicates that the environment during the coal depositional period was inclined to reduction).

Fig. 4. Scatter distribution of Ce/La and δCe (modified after Mu et al. indicate that all the coal and coalmine OB samples lie in the reducing environment).

Fig. 5. UCC normalized REY distribution plot for raw coal and OB samples showing higher enrichment of OB (BOB1, BOB2, BOB3, and BOB4) sample compared to coal (BAC1, BAC2, BAC3, BAC4, and BAC5) samples. UCC normalisation values are taken from Taylor and McLennan (1985).

Fig. 6. Chondrite normalized REY distribution plot for raw coal and OB samples showing higher enrichment of OB (BOB1, BOB2, BOB3, and BOB4) sample compared to Coal (BAC1, BAC2, BAC3, BAC4, and BAC5) samples (chondrite normalization values after Anders and Grevesse, 1989).

Fig. 7. (a) FE-SEM images of coal (BAC2 and BAC4) samples along with the EDS analysis with an elemental concentration of some REY. (b) FE-SEM images of OB (BOB1 and BOB2) samples along with the EDS analysis with the elemental concentration of some REY.

Fig. 8. (a) HR-TEM image showing the presence of mixed-layer minerals at 200 nm (A). (B) High-resolution image at 20 nm, revealing disordered crystal fringes. Insets (C) and (D) display d-spacing values of 3.57 Å and 1.25 Å, respectively. (E), (F), and (G) show the reverse FFT, FFT, and gray value scale for d-spacing calculation. (H) The selected area electron diffraction (SAED) pattern confirms the polycrystalline nature of the sample. (b) Transmission Electron Microscopy-Energy Dispersive X-ray Spectroscopy (TEM-EDS) investigation and elemental mapping of carbon (C), oxygen (O), aluminum (Al), silicon (Si), and titanium (Ti).

Fig. 9. The XPS survey spectra of the coal [BAC2 (Er_4d, Sc_2p), BAC4 (Er_4d, Sc_2p, Gd_4d)] and OB [BOB1(Er_4d, Sc_2p), BOB2 (Er_4d, Sc_2p, Gd_4d)] samples indicating the presence of Er (4d), Gd (4d), and Sc (2p).

Fig. 10. (a) The XPS deconvoluted spectra of the coal (BAC2, and BAC4) samples show the presence of Sc_2p, Er_4d, and Gd_4d. (b) The XPS deconvoluted spectra of the OB (BOB1, BOB2) samples show the presence of Sc_2p, Er_4d, and Gd_4d at different types of bonding.