India’s industrial heritage was built on coal mined indigenously, and coal will continue to be one of the most viable fossil fuel reserves, supplemented by continued development of renewable energy sources (Indian Bureau of Mines, 2019). Apart from understanding the importance of coal in combustion studies or its suitability for metallurgical applications, greater attention has recently been directed towards potential utilization of chemical constituents commonly found in coal. Trace elements (TEs) and rare earth elements (REEs) have gained significant importance owing both to associated environmental concerns and to their increased need in modern day devices, respectively. The impact of trace elements in combustion and gasification studies has been widely investigated (e.g., Wang 2023; Lapidus 2022; Xiong 2022; Vejahati et al. 2010; Clark, 1993; Querol, 1996). There is motivation to explore the potential to isolate REEs from coal by-products owing to three important factors, (a) meeting the increasing demand in modern day uses; (b) utilization of a waste which is generated in huge quantities; and (c) reduction in primary mining activities for these constituents (Eterigho-Ikelegbe et al. 2021; Fu et al. 2022). The distribution and association of TEs or REEs in coal vary considerably based on geologic setting and geographic distribution.

Trace elements in coal are primarily localized in the inorganic matter (Kolker et al. 2021; Finkelman et al. 2019; Chelgani 2019), although, partial organic associations are also reported (Wang et al. 2003; Qin et al. 2018; Dai et al. 2020). Where present in an organic association, trace elements can be bound with the organic matter or present as chelates, or organometallic complexes within the organic matter (Ward 2016). Particularly for low rank coals, trace elements dissolved in the pore water become adsorbed within organic components (Li et al., 2010). The TEs which are most commonly organically bound are B, Be, Ga, and Ge, implying their intimate association with the coal matrix (Vejahati, 2010). Other elements, including As, Cr, Ni, Sb, Se, Ti, U, and V, are known to be either inorganically or organically bound (Dai et al. 2020; Ikelegbe et al. 2021; Davidson 2000; Querol et al. 1998; Vejahati, 2010). The knowledge of chemical associations of trace elements is instrumental in determining; (a) if certain elements need to be reduced from the coal prior to utilization (Norton et al., 1991); (b) to estimate the fate (emission, soil, or sediment) of the constituent elements following coal utilization (Clarke 1993); (c) possible ways to mitigate any potential environmental and health issues an element may have in the utilization process (Clarke 1993); and (d) control release of metals from coal and coal combustion materials to ground water (Wang, 2003; Twardowska, 2003; Georgakopoulos, 2019).

The strategic importance of REEs has enhanced the global demand for these elements significantly in the past few years (Fu et al. 2022). To diversify supply of REEs, exploration of indigenous resources is of central importance in every country which possesses natural abundance. India possesses about 6% of the total global reserve of REEs, nevertheless, it produces only 1% of the global supply (Indian Bureau of Mines, 2019). One of the primary reasons behind the lower-than-expected production is insufficient knowledge of the distribution of REEs in the Indian sub-continent. Only a few recent exploratory studies have been reported from India (Mishra et al. 2019; Saha et al. 2016, 2018; Saikia et al. 2021; Kumari et al. 2023; Kumar et al., 2023). REE deposits are commonly found in (a) carbonatites (Wang et al. 2020), (b) magmatic magnetite-hematite bodies (Frietsch and Perdahl 1995), (c) peralkaline igneous systems (Chakhmouradian and Zaitsev 2012), (d) xenotime-monazite accumulations in mafic gneiss (Engi 2017), (e) ion-absorption clay deposits (Borst et al. 2020), and (f) monazite-xenotime-bearing deposits (Engi 2017).

According to the Geological Survey of India (GSI), the total reserves of coal in Rajmahal Coalfield in Jharkhand State were estimated to be 13.13 billion tonnes (metric tons). The Rajmahal coalfield, belonging to Eastern Coalfields Limited, possesses the third highest coal reserve in Jharkhand, producing thermal grade coals at shallow depth (up to 300 m). The Rajmahal Basin has been recently explored with respect to the origin of organic matter, depositional environment and paleoclimatic variations of Barakar formation (Table 1) which contains coaly matter (Mathews et al., 2020). Subsequently, presence of early Permian wildfire activities in the sediments was shown by Murthy et al. (2021) in the same area, followed by a detailed study of the pore and fractal attributes of the Barakar Shales belonging to Rajmahal Basin (Mishra et al. 2021). To complement these investigations, the present study delineates the geochemistry of the same geogrhic area, focusing on the strategically important elemental composition and their respective associations.

The present study aims to contribute to the understanding of distribution and quantification of both trace elements and rare earth elements from Bhalukasba Surni block of Rajmahal coalfield, Jharkhand, India. Coal samples were collected with varying depths of the selected borehole and converted to ash under laboratory conditions (815 °C), to be used for geochemical and elemental analysis. Furthermore, the coal ash samples were analyzed for their mineralogical composition and morphological properties using various analytical techniques.

Rajmahal Basin is located within latitudes 24º 00´´:25º 30´´ N and longitudes 87º 00´´:88º 00´´ E, and comes under Rajmahal-Purnea graben extending over the Jharkhand and West Bengal States of India (Fig. 1). The basin is oriented in a N–S direction bounded by easterly dipping step-fault systems and covered by soft sediments or alluvium (Ghose et al. 2017; Murthy et al. 2021). The total area of the basin includes four districts within Jharkhand, i.e., Sahibganj, Pakur, Godda and Dumka, comprising five coalfields in total (Ghose et al. 2017). Geographically, the basin is bounded by the river Ganges (Ganga) in the north, Rajmahal volcanics in the east, metamorphic rocks in the west, and by laterite and alluvium to the south (Singh and Singh 1996). In the western peripheries of the basin, the Early Permian Barakar coal seams are present as a thin and irregular stretch (Fig. 1). The lithostratigraphic succession of the Rajmahal Basin (Table 1) shows Archean basement separated by a well-defined unconformity with the overlying Lower Gondwana sediments, succeeded upward by the Karharbari Formation. The thick coal and shale horizons of the Barakar Formation spread conformably over the Archean basement (Singh and Singh 1996). The Permian sediments are overlain by the Dubrajpur and Rajmahal Formations which primarily consist of sandstones, siltstones, and clays (Murthy et al. 2021). A total of nine coal samples were collected for the present study at varying depths from the borehole in the Rajmahal basin.

a The location of Rajmahal basin in India; b The geological map of the sample area of Bhalukasba Surni (Singh and Singh 1996)

Nine coal samples taken at varying depth (124 m to 409 m) from cores penetrating the Rajmahal coalfield, Jharkhand, India were acquired in 2018 for the present investigation. The coal samples were pulverized to a particle size of -212 μm (-72 mesh) prior to analysis. Proximate analysis of the coal samples was performed using proximate analyzer as per IS 1350 (Part 1): RA 2019. The Gross Calorific Value (GCV) of coal samples was determined using Parr 6200 bomb calorimeter (Parr, USA) as per ASTM D 5865:2019. The coal ash sample was prepared as per IS: 1350 (Part 1): RA 2019 where the coal samples were heated in air at 815 °C for 1 h in a muffle furnace. Lithium metaborate and lithium tetraborate were added in sample fusion digestion. Nitric acid (reagent grade, 65%), was used to digest fused sample materials. Ashed sample was used in X-ray fluorescence analysis (XRF; major elements such as Si, Al, Fe, K, Ca, Ti, Mg, P, S, Na), X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS; REEs such as La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc and Y) and inductively coupled plasma-optical emission spectrometry (ICP-OES; trace elements such as Be, Cr, Mn, Ni, Cu, Ge, Cd, Pb, V, Zn, Zr, Mo). Scanning electron microscopy was employed in morphological studies of coal ash samples which was operated at 20 kV and a working distance of 14.6 mm, at a magnification of either 7000x or 10,000x.

2.1 Determination of total REE concentration

The total REE concentration was determined in accordance with the protocol published previously by Banerjee et al. (2021). Aliquots of laboratory coal ash (0.1 g) were mixed with a fusion mixture of lithium metaborate (0.2 g, LiBO₂) and lithium tetraborate (0.15 g, Li₂B₄O₇). The fusion mixture was thoroughly homogenized and then fused at 815 °C in a muffle furnace for 1 h. The temperature of the crucible was first lowered to 300 °C and subsequently reduced to room temperature. The fused sample was dissolved using 10% nitric acid solution along with constant agitation to ensure complete dissolution. All the dilutions were performed using high purity laboratory water which was produced using a Milli Q water purification system (Millipore). The resultant acidified extract thus obtained was then transferred into a 250 ml volumetric flask to be analyzed by ICP-MS and ICP-OES. This fusion approach ensures digestion of REE-bearing trace minerals such as zircon and monazite, which may not be completely broken down using standard multi-acid digestion approaches.

2.2 Solid phase analysis

The whole coal or coal ash prepared by low temperature ashing (LTA) was not available for XRD prior to ashing for chemical analysis. An X-ray fluorescence (XRF) spectrophotometer (S8 Tiger, Bruker) was employed to determine the major elemental composition of coal ash samples, using the pressed pellet method. XRD of the ash was recorded over a 2θ range of 5°-90°, with a step size of 0.03° using Ni-filtered Cu Kα radiation. The qualitative analyses of the mineralogical phases present were determined using High Score plus software based on the JCPDS database. Scanning Electron Microscopy (SEM) (FEI NOVA NANOSEM 430) was used to examine the morphology and the mineralogical analysis of coal ash samples.

3.1 Characterization of coal and coal ash

Characterization of coal samples prior to ashing, including proximate and ultimate analysis, is tabulated in Table 2. The average result for all the crucial parameters is summarized in Table 3. There was no trend in coal properties, in particular, with respect to depth. The sulphur content was typically low in all the coal samples (< 0.6%). The fixed carbon and mineral matter content are consistent with the calorific values (≤4000 cal/g), which demonstrated these to be low grade coals (Table 3). The volatile matter content in the examined coal samples was in the range of 18% – 28%. The equilibrated moisture determined at 60% Relative Humidity and 40 °C (as per IS: 1350 Part I-1984) showed that the moisture content in coals varied from 3.5% to 8% and the ash yield in the range of 28% – 50%.

3.2 Major element concentrations in coals

Major elements for the coal samples were performed using XRF analysis of the ash. Among the major oxides the concentration ranges were as follows- SiO₂ (62% – 72%), Al₂O₃ (17% – 23%) and Fe₂O₃ (2.0% – 9.5%) (Table 4). The ratio of SiO₂/Al₂O₃ for all nine samples varied from 2.5 to 4.3, indicative of the formation of coal under stable conditions of deposition, likely involving slow but steady subsidence and a low degree of tectonic activity (Ameh 2019). A notable increase in the concentration of Fe₂O₃ is present in samples B2 and B3 (> 9%), which is a relatively higher value compared to that of the typical concentrations found in coal ash samples elsewhere (≤ 8%; for example (Mishra et al. 2019; Kumari et al. 2023; Kumar et al., 2023). The quantity of SO₃ retained in the selected coal ashes varied from 0.23 wt% to 2.18 wt%. Sulphur in coal is from three sources, pyrite, inorganic sulphates, or from the organically-bound sulphur-rich compounds (Chou 2012). Enrichment in Fe₂O₃ in samples B2 and B3 ash does not show commensurate enrichment in SO₃ in ash or S in whole coal, suggesting that Fe₂O₃ is unlikely to be derived from pyrite alone. X-ray diffraction (XRD) patterns of the coal ash samples revealed the presence of quartz (SiO₂), keatite (SiO₂), anatase (TiO₂), zircon (ZrSiO₄), hematite (Fe₂O₃) and orthoclase (KAlSi₃O₈), with minerals having high melting temperatures (e.g. quartz, zircon) likely to be primary (Oreskes et al., 1990; Abaka-Wood, 2022; Kolker et al. 2017). Qualitative analysis of coal ash samples using SEM-EDX revealed the omnipresence of constituents derived from clay minerals (Al, Si, and in some cases, also K, Fe, Ca) in all the depths (data corresponding to B3 is only shown in Fig. 2). The morphology of the coal ash samples demonstrated the existence of (a) unfused detrital minerals, (b) irregular-spongy particles derived from partly-fused clay minerals (Al₂O₃ and SiO₂) and very locally, colourless vesicular glass, in irregular particles, which could be derived from viscous melting (Ramsden and Shibaoka 1982).

SEM images of coal ash sample from B3 which demonstrated the irregular spongy particles obtained by partial melts of clay minerals, likely derived from kaolinite (Al, Si) and/or illite-smectite (K, Ca, Fe, Al, Si) in spectra)

3.3 Rare earth elements

The concentration of rare earth elements in the coal ash samples varied considerably with depth. However, this variation did not show a distinct trend with depth, similar to relatively random variation seen in several past studies of Indian coal blocks (Mishra et al. 2019; Kumari et al. 2023). The average concentrations of all the major light rare earths (LREEs) across the various depths were- La (105 ppm), Ce (180 ppm), Nd (76 ppm), Pr (17ppm), Gd and Sm (nearly 13 ppm each) (ash basis; Table 5). In the present study, REE(s) refers to the lanthanide REEs, and REY or ΣREY is used to designate the lanthanides plus Y and Sc (Table 5).

On the other hand, the average concentrations of all heavy rare earths (HREEs) (on an ash basis) were in the range of 1–5 ppm except Dy (12 ppm). Of note, the concentrations of Sc and Y were considerably high in the whole coal block averaging to 23 ppm and 71 ppm, respectively. In particular, the two depths (B3 and B8) showed significant enrichment in critical HREEs (Dy and Y). On an ash basis, the ΣREY concentrations varied from 417 ppm to 697 ppm from B1-B9, with an average concentration of 528 ppm (Table 5). This is a significantly higher concentration than the estimated world average for coal ash (404.5 ppm) as reported by Ketris and Yudovich (2009) which was later revised to 435 ppm upon inclusion of samples from more countries (Fu et al. 2022). Upon comparing the data obtained with that of world coal ash, the concentrations of La, Ce, Nd, and Y were found to be prominently higher (La by 60%, Ce by 44%, Nd by 19% and Y by 39%) in the examined coal block of Bhalukasba Surni (Fig. 3).

Comparison of REE content between World Coal Ash (Ketris and Yudovich 2009) and representative coal ash sample from Bhalukasba Surni (this study). Error bars indicating uncertainty by height of the bar are shown at the top of each histogram

Such a higher concentration is indicative of retention of REY during coal combustion, and the coal ash thus produced could be a potential source for isolation of REY (Fu et al. 2022). Theoretically, the retention of REY at elevated temperatures is only possible if the association in coal is primarily with the inorganic matter a conclusion supported by sequential leaching studies (Finkelman et al. 2018). Kolker et al. (2017) showed that REEs are partitioned into the glass phase (the melt at high temperature) in fly ash, and this is the primary host of REEs in plant-generated fly ash in addition to any trace phases such as monazite and xenotime that persist due to their high melting temperature. Although the proportion of REY associated organically or inorganically varies based on location, low rank coals with low ash content are more likely to have organic associations, whereas for higher rank coals, REEs are primarily inorganically associated (Finkelman et al. 2018; Fu et al. 2022). Total REEs in the ash derived from coals of Bhalukasba Surni are notably high. Due to mass balance, the greatest enrichment in ash relative to the coal is in coals having a low ash yield, assuming a high proportion of REEs from the coal are retained in a smaller mass fraction of the ash (Kolker et al. 2024).

3.4 Distribution of REEs

The (La/Yb)_N ratio, where N reflects normalization to Upper Continental Crust (UCC; McLennan 2001) was calculated to quantify the relative proportions of LREE and HREE (Table 6). The results obtained demonstrated that the ratio varied between 1.9 and 0.9 and the value of (La/Yb)_N for most of the depths were greater than ≥ 1, indicating LREE enrichment. The coal samples demonstrated an absence of Ce anomalies as Ce_N/Ce_N* values ranged from 0.95 to 1.07 (Table 6), indicating that the depositional conditions during the formation of the coaly mass had prominent influence of anoxic marine water (Chen et al. 2015; Dai et al. 2016). UCC-normalized plots of all nine samples are given in Fig. 4, which essentially demonstrates that the samples originated from similar crustal sources.

Upper Continental Crust (UCC)-normalized plots of samples examined from the coal block of Bhalukasba Surni, Jharkhand, India (UCC after McLennan 2001)

Unusually high average concentrations of ΣREY were observed in coal ash samples for two depths in particular, B8 (660 ppm) and B3 (586 ppm) (Table 5). Overall, Ce, La, Nd and Y exhibited the highest concentrations of all the REEs, with, Ce being the most abundant lanthanide. A closer look at the concentrations of LREEs and HREEs with respect to the two important depths (B3 and B8) revealed interesting differences. While B3 was found to be HREE enriched {Gd (16 ppm), Dy (16 ppm), Ho (3 ppm), Er (9 ppm), Tm (1.5 ppm), Yb (8.6 ppm), Lu (2 ppm), Sc (28 ppm) and Y(100 ppm)}, B8, on contrary, was found to be especially enriched in LREEs (La, Ce, Pr, Nd and Eu). Thus, with respect to utilization, B3 possesses an abundance of HREEs which are critical in numerous modern applications (Liu and Chen 2021). The increasing demand for HREEs has led to global interest in identifying new sources of these elements. As such, the horizon represented by sample B3 of Bhalukasba Surni is promising for further investigation (Fig. 5). On the other hand, sample B8 was found to be significantly enriched with respect to La (143 ppm), Ce (233 ppm) and Nd (95 ppm), which when combined with that of Y (80 ppm), made this depth, in particular, enriched in REY (660 ppm) by 40% relative to the world coal ash average (404 ppm) (Fu et al. 2022). The outlook co-efficient (C_outl), a measure of economic potential (Seredin and Dai 2012) ranged from 0.8 to 1.0 (Table 5). The average REY of the coal block is 528 ppm. As such, while C_outl is above the 0.7 minimum for promising recovery, REY is somewhat below the 1000 ppm promising threshold proposed by Seredin and Dai (2012). A plot of REY vs. C_outl, demonstrating the promising and non-promising regions is illustrated in Fig. 5. Nonetheless, exploration of Bhalukasba Surni, enriched in REYs, particularly in the depth range of 181–183 m and 396–400 m, and recovery of the corresponding combustion ash may offer some benefit over extraction from conventional ores which typically involves mining.

A graphical presentation of promising and unpromising regions for extraction of REY on the basis of C_outl, excluding REY (after Seredin and Dai 2012) (Orange colour coded sample numbers represent moderately promising depths for extraction; Blue colour coded sample numbers belong to the promising depths and lastly, the green colour coded sample number represents the most promising depth for extraction of valuable constituents

3.5 Statistical correlation of REEs with major oxides

Statistical correlation of REEs with the major oxides in coal ash can be helpful to understand mineralogical occurrence of the REEs and to devise strategies for their isolation from the mineralogical framework. On the basis of multiple exploratory projects performed by Geological Survey of India (GSI; https://www.gsi.gov.in), coal-related studies elsewhere (Kolker et al. 2024) and characteristics of the UCC in general (McLennan 2001), it is expected that Indian coals, soils and sediments are primarily enriched in Light Rare Earth Elements (LREE). In the present study of 9 coal ash samples of Bhalukasba Surni, out of the total mean REE content (excluding Y; 435 ppm), 393.3 ppm was LREEs (La, Ce, Pr, Nd, Sm and Eu) while only about 112.2 ppm was found to be HREEs (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), a ratio of ∑LREE/∑HREE of 3.5 (Table 5). This proportion indicates that the primary association of the REEs in this coal block is with the inorganic matter, as HREEs have a greater affinity for organic portions of the coal (Dai et al. 2020). The inorganic affinity of LREEs was substantiated when correlation co-efficients with major oxides were considered, such as Al₂O₃ (0.6 < r > 0.8), TiO₂ (0.6 < r > 0.9), Fe₂O₃ (0.1 < r > 0.6) and P₂O₅ (0.4 < r > 0.9). These elemental oxides showed much lower coefficient values for the HREEs. More specifically, the affinity of the full range of LREEs with Al₂O₃ (0.4 < r > 0.9) and P₂O₅ (0.4 < r > 0.9) supports the enrichment of LREEs in the chosen coal block and their possible association with phosphate minerals (for example monazite, xenotime) (Wang and Liang 2015; Table 7). The affinity of REEs with TiO₂ is noteworthy, which in the present study is expected to be with the mineral phase anatase, which is supported by the XRD data obtained (Fig. 6; Table 7).

A thorough understanding of the geological occurrence of trace elements is required to determine the mobility of environmentally sensitive constituents during coal combustion and ash handling and ascertain the implications of potentially toxic elements towards human health and the environment. The coal block from Bhalukasba Surni demonstrated notable enrichment in certain trace elements (TE). Trace element concentrations expressed on a dry whole coal basis are presented in Table 8. However, considering the differences in their respective volatility, the subsequent discussion has been confined to only those elements which are least likely to be lost on ashing at 815 °C. Past work on trace elements in coal suggests that Ge and Be are most likely to be organically bound (Vassilev et al. 1995; Hower et al. 2002), whereas elements with an inorganic affinity (most likely the host being pyrite) include Mo, Mn, Ba, Pb, Ni, Zn, Cu, Cr and Co (Kolker et al. 2021; Dai et al. 2020). Zr and V can have mixed associations in zircon and illite, respectively (Dai et al. 2020; Vejahati 2010). A statistical correlation of trace elements with the major oxides in coal was examined in order to gain knowledge of their possible association in the mineralogical matrix of coal (Table 9).

4.1 Elements of economic significance

The two trace elements which are considered to be of high economic significance and present in considerable concentration in the chosen coal block are zirconium (Zr) and germanium (Ge). The enrichment of Zr (on dry whole coal basis) in B8 (168 ppm) and B9 (159 ppm) in particular, is noteworthy, and Zr was fairly high (90 ppm–150 ppm) throughout the coal block. The occurrence of Zr in the coal ash samples of Bhalukasba Surni coal block may indicate the influence of volcanic ash during coal deposition, as such coals are typically enriched in Zr along with other critical elements and REY (Zhao et al. 2019). There are three possible modes of occurrence of Zr, (a) as absorbed ions on clay minerals (Zhao et al. 2019; Dai et al. 2020); (b) embedded within the Ti-bearing phases such as anatase/rutile (Zhao et al. 2016); or (c) associated with zircon mineral. XRD analysis of the coal ash samples demonstrated the presence of zircon in all the depths. The XRD of sample B9 with the highest concentration of Zr is shown in Fig. 6.

X-ray diffractogram XRD traces for coal ash sample from the depth of B9 where Q = Quartz; K = Keatite; A = Anatase; H = Hematite; Z = Zircon

Given that Zr and REY are strongly retained in the ash fraction, their ash-basis data were considered together with major oxides expressed on an ash basis. Statistical results demonstrate significant correlation of Zr with TiO₂ and P₂O₅ (Figs. 7a, b). This further indicates the possibility of Zr existing either as the REE bearing mineral (zircon (ZrSiO₄) Zhang et al. 2015), or closely associated with the oxides which have a strong correlation with REEs (TiO₂ and P₂O₅). Of note, Ge failed to show any noticeable association with neither TiO₂ nor P₂O₅ (data not shown), which largely suggests it to be organically associated.

Correlation of zirconium (Zr) with the oxides (TiO₂ and P₂O₅), which demonstrates an association of Zr, likely as zircon, with phosphorus bearing minerals such as monazite, xenotime, and apatite, all of which are REE-bearing, and have an affinity for Zr and TiO₂. From Table 9, correlation between Zr and TiO₂ is 0.68, and between Zr and P₂O₅ is 0.70

Germanium is typically associated with the organic structure in coals (Wei et al. 2020). The highest concentration of Ge (55 ppm on dry whole coal basis) was found in B6 (275–277 m depth), which is three times the concentration found in ash from world hard coals (18 ppm; Ketris and Yudovich 2009; Dai et al. 2020). With respect to commercial importance as one of the critical elements, Ge presents the most successful example of commercial extraction from coal for more than five decades, which is commonly performed by employing hydrometallurgical processes consisting of leaching and solution purification (Dai et al. 2020; Nguyen and Lee 2021).

4.2 Elements of moderate concern

The TEs which are listed as hazardous in the Mercury and Air Toxics (MATS) standards by the U.S. Environment Protection Agency are Cr, Mn, Ni, As, Se, Cd, Sb, Hg, Mo and Pb (U.S. EPA, 2011). Those which pose some of the greatest health concerns, such as Se and Hg, are highly volatile and are best determined on whole coal without ashing. Apart from that, a large group of TEs likely show minimal volatility at the 815 ̊C ashing temperature, for example As, Cr, Ni, Sb, V and Mo. Nonetheless, values obtained for these elements can be considered minima, with substantial enrichment shown in the Bhalukasba Surni coal block for Mo (90–153 ppm on dry whole coal basis, B2 and B8 being the highest, and other important trace elements of interest. These include Ni (30–154 ppm), V (27–60 ppm), Cr (60–115 ppm), Cu (10–36 ppm), Cd (0.3-1.0 ppm), Ba (65–167 ppm), Co (9–36 ppm), Mn (80–540 ppm) and Zn (10–40 ppm) (Table 8). The highest concentrations of individual elements, with respect to depth, were Mn (540 ppm, B3), Ni (154 ppm, B3), V (60 ppm, B6) and Zn (40 ppm, B2) and Ba (167 ppm, B8).

The distribution of REEs and TEs was systematically investigated using a series of complementary analytical techniques for coal ash from a coal block of Indian origin, prepared under laboratory conditions (at 815 °C). The average total REY of the whole coal block is 528 ppm and C_outl was 0.9. As such, REY content and its distribution approaches that considered promising for extraction. The average concentration of LREE was found to be 394 ppm and that of HREE was 112 ppm (including Sc and Y). Statistical correlation of REEs with major oxides demonstrated significant correlation with P₂O₅ and TiO₂, likely indicating primary associations with common REE-bearing phosphate minerals such as xenotime, monazite, and apatite, and for TiO₂, possibly anatase. Correlation of REEs with Al₂O₃ indicates that a portion of the REE budget is derived from clay minerals which likely made up a significant portion of the mineral matter in coal prior to ashing, but could not be quantified, as raw coal and/or low temperature ash was not available for XRD prior to ashing at 815 °C. Examination of the morphology of coal ash using SEM − EDX showed an irregular-spongy texture suggestive of clay minerals (Al₂O₃ and SiO₂) undergoing partial melting during ash formation. The concentration of elemental zirconium was considerably high in all the depths (90–160 ppm on dry whole coal basis) which was concluded to be primarily present as the mineral zircon, which was confirmed by XRD. Zirconium shows a strong correlation with TiO₂ and P₂O₅, which also demonstrated strong correlation with the REEs, further indicating the association of zircon with other REE-bearing phases. In addition to Zr and the REEs, some samples (B6- 55.1 ppm, and B7- 29.5 ppm) showed significant enrichment in Ge, and together, these elements may present attractive exploration targets in the section investigated.