Modes of Occurrence of Critical Elements (Li-Ga-Nb-Zr-REE) in the Late Paleozoic Coals from the Jungar Coalfield, Northern China: An Approach of Sequential Chemical Extraction

Abstract

In recent years, recovering critical elements from coal has attracted considerable interest due to their significant potential and resulting advantages. A prime example is the coal-hosted Al-Ga-Li-REE deposit within the Jungar Coalfield of Inner Mongolia, northern China, where lithium (Li), gallium (Ga), and aluminum (Al) are successfully extracted from coal ash. However, the specific forms in which these elements exist, crucial for developing effective extraction methods, remain unquantified. This research investigated the distribution of Li, Ga, Nb, Zr, and rare earth elements (REEs) within the coal. The study employed a combination of analytical techniques, including inductively coupled plasma mass spectrometry (ICP-MS), sequential chemical extraction (SCE), scanning electron microscopy coupled with energy-dispersive X-ray spectrometry (SEM-EDS), and X-ray powder diffraction analysis (XRD). The analyzed coals exhibited enriched levels of Li, Ga, Zr, Nb, and REEs. Kaolinite and boehmite were the primary mineral constituents, along with minor amounts of calcite, pyrite, rutile, goyazite, and chlorite. Sequential chemical extraction revealed that Li and Ga are primarily associated with aluminosilicate phases (71.84%–84.39%) and, to a lesser degree, organic matter (12.15%–25.09%). Zirconium and Nb were also predominantly found within aluminosilicates (68.53%–95.96%). REEs occur mainly in carbonate (28.28%–60.78%), aluminosilicate (11.6%–33.08%), and organic (22.04%–29.42%) fractions.

1. Introduction

Coal plays a vital role in the energy landscape of numerous nations [1]. China, possessing substantial coal reserves, currently holds the position of top coal producer and consumer and is projected to maintain this dominance [2,3]. Beyond its primary application in electricity generation, coal serves as a crucial component in various industrial processes [4]. Significant amounts are utilized for gasification, steel manufacturing, and as feedstock for producing activated carbon and other chemical compounds [1,5]. Furthermore, coal represents a potential source of critical elements, including lithium (Li) [6,7], aluminum (Al) [8,9], scandium (Sc) [10], vanadium (V) [11], gallium (Ga) [12], germanium (Ge) [13,14,15,16,17], selenium (Se) [18], zirconium (Zr) [19,20,21], niobium (Nb) [22], rare earth elements (REEs) [23], yttrium (Y) [23], gold (Au) [24], silver (Ag) [24], rhenium (Re) [24], and uranium (U) [25,26].

Notably, a large coal-hosted aluminum and gallium deposit was discovered in the Heidaigou Mine in Jungar Coalfield, Inner Mongolia [27]. Subsequent investigations in neighboring mines, such as Haerwusu [28] and Guanbanwusu [29], and neighboring coalfields (i.e., Daqingshan Coalfield [30,31,32,33,34] and Ningwu Coalfield [35,36,37,38,39]) have corroborated these findings. Fly ash from this coal deposit exhibits average concentrations of more than 50% Al₂O₃ and 100 ppm Ga, comparable to those found in traditional bauxite deposits [40,41]. The Heidaigou Mine alone has confirmed reserves of 150 million tons of Al₂O₃ and 49,000 tons of Ga [42]. Various extraction techniques, such as acid/alkali leaching and salt activation, have been developed for recovering these metals from Jungar’s fly ash, and research efforts have also focused on characterizing the mineralogical and chemical changes that occur during aluminum extraction [43].

The distribution of elements within coal is a direct result of the complex processes involved in peat formation, followed by diagenetic and epigenetic changes [44,45,46]. Understanding how these elements occur offers valuable insights into their origin, the geochemical history of the precursor peat, and subsequent coal, and crucially, their behavior during coal cleaning, utilization, leaching, and waste disposal [5,46,47]. This knowledge is also fundamental to recovering critical elements from coal and its ash [48]. While previous research has explored the modes of occurrence of Li, Ga, and REEs in Jungar coals using techniques such as scanning electron microscope-energy dispersive spectrometry (SEM-EDS), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and statistical methods [49,50,51], these studies lacked quantitative data regarding the specific amounts of these elements associated with different coal fractions (e.g., aluminosilicates, organic matter, carbonates, or sulfides). Therefore, this study employed sequential chemical extraction (SCE), a reliable method for determining the associations of trace elements in coal by exploiting their differential solubility in various reagents. This approach was used to quantitatively analyze the modes of occurrence of Li, Ga, Nb, Zr, and REEs in coals from the Jungar Coalfield.

2. Geological Setting

Situated within the center of the Jungar Coalfield, the Heidaigou Mine ranks among China’s largest surface coal mines [27]. The Jungar Coalfield itself occupies the northeastern periphery of the Ordos Basin (Figure 1 [27]), a basin positioned west of the Northern China Platform. Extending 65 km north–south and 26 km east–west, the coalfield encompasses an area of 1700 km². With coal reserves reaching a staggering 26.8 Gt, the Jungar Coalfield stands as one of the Ordos Basin’s most coal-rich coalfields [28,49].

The Jungar Coalfield exhibits significant spatial variability in its sedimentary facies [27]. A gradual transition from limestones to terrigenous clastics occurs within the coalfield as limestone thicknesses diminish. Coal-bearing strata within the Jungar Coalfield encompass the Benxi and Taiyuan formations (Figure 2). The Benxi Formation, ranging from 15 to 35 m in thickness, unconformably overlies the limestones of Middle Ordovician Majiagou Formation. The Taiyuan Formation has a total thickness of 35 to 70 m and predominantly consists of gray and grayish-white quartzose sandstone, siltstone, mudstone, and coal beds, interspersed with mudstone, siltstone, limestone, and quartzose sandstone [27,28,29].

The Taiyuan Formation demonstrates notable lateral lithological variations. In the southern portion of the coalfield, it is primarily composed of detrital coal-bearing sediments, with sandstones constituting a significant proportion. This region hosts five coal beds, i.e., Nos. 1, 2, 3, 4 and 5, but they are currently not economically viable for mining. Overlying the coal-bearing sequences, a series of non-coal-bearing formations are present, including the Upper Shihezi, Lower Shihezi and Shiqianfeng, and Liujiagou formations [9,10].

3. Samples and Methods

Three coal seam channel samples of the No. 6 Coal (Figure 2), designated HDG-1, HDG-2, and HDG-3, were collected from working faces within the Heidaigou Surface Mine, located in the Jungar Coalfield. A channel sample is a continuous, thickness-weighted coal sample taken perpendicular to the bedding plane of a coal seam. Because it incorporates all coal types in their natural proportions, it provides a highly representative and reliable characterization of the entire seam’s quality and composition. All the three samples were obtained as channel samples from distinct sites across the coalfield (the distance between each of the two channel samples is ~250 m), and thus collectively reflect the spatial, stratigraphic, and coal rank variability within the No. 6 coal seam. Samples were carefully selected to exclude any partings exceeding 1 cm in thickness. The collected samples were immediately stored in bags to avoid any oxidation and contamination.

3.1. Proximate and Ultimate Analysis and Vitrinite Reflectance Determination

Proximate analyses of the collected coal samples were conducted in accordance with ASTM Standards D3173-11, D3175-11, and D3174-11 [52,53,54]. Total sulfur content was determined using ASTM Standard D3177-02 [55], while sulfur forms were analyzed according to ASTM Standard D2492-02 [56]. Elemental analyses for carbon (C), hydrogen (H), and nitrogen (N) were performed using a VarioMACRO elemental analyzer. Prior to microscopic analysis, each coal sample was crushed and prepared as grain mounts. Microscopic analysis was carried out under reflected light, adhering to ASTM Standard D2797/D2797 M-11a [57]. Vitrinite random reflectance was subsequently measured using a Leica DM-4500P microscope (Leica Camera AG, Wetzlar, Germany) with an installed spectrophotometer Craic QDI 302™ (Craic, San Dimas, CA, USA), following method described by Dai et al. [30].

3.2. Quantitative Analysis of Minerals

Powder X-Ray diffraction (XRD) was employed to characterize the mineralogy of the raw coal samples. Using a D/max-2500/PC diffractometer equipped with a Cu-Kα radiation source and a scintillation detector, XRD patterns were collected over a 2θ range of 2.6° to 70° in 0.01° steps. Quantitative mineralogical compositions were subsequently determined using Siroquant™ software 3.0, as detailed in studies by Ward et al. [58] and Ruan and Ward [59]. The reliability of Siroquant™ analytical method for quantitative analysis of minerals in coal and coal-related materials has been approved by a number of studies [60,61,62,63,64].

3.3. Concentration Determination of Critical Element in Coal

Concentrations of critical elements in both raw coal and sequentially extracted solid products were determined using inductively coupled plasma mass spectrometry (ICP-MS) on a ThermoFisher instrument (ThermoFisher, Waltham, MA, USA). Before ICP-MS analysis, all samples were digested by an UltraClave Microwave High Pressure Reactor (Anton Paar, Graz, Austria) [65]. A tuning solution containing Li, Co, In, and U (THMTS-1, Inorganic Ventures, Christiansburg, VA, USA) at a concentration of 1 μg/L was utilized for instrument optimization. Calibration of the ICP-MS was achieved using multi-element standards (CCS-1, CCS-4, CCS-5, and CCS-6) from Inorganic Ventures. Calibration curves exhibited excellent linearity (R² > 0.9999) across the 0–100 μg/L concentration range [66]. The method detection limit was calculated as three times the standard deviation of ten blank measurements, and was consistently below 0.1 μg/L. The recovery of the internal standard (¹⁰³Rh) ranged from 95.70 to 108.15%, demonstrating robust and reliable trace element determination by the ICP-MS method.

3.4. Sequential Chemical Extraction

A six-step SCE procedure was employed and more details were described by Liu et al. [66]. This approach enabled the identification of six distinct elemental occurrence forms in coal, namely water-soluble, ion-exchangeable, carbonate-bound, organic-bonded, silicate-bound, and sulfide-bound fractions [67,68,69,70]. For each SCE experiment, a ~5 g coal sample was used. First, 60 mL of distilled water was used to extract the water-soluble elements. The remaining solid was then treated with 60 mL of NH₄AC to extract the ion exchangeable elements. Next, organic matter and mineral fractions were separated using CHCl₃ at a density of 1.47 g/cm³. Following this, the organic- and carbonate-associated elements were dissolved with HNO₃ + CHClO₄ and HCl, respectively. Finally, the silicate-and sulfide-associated fractions were extracted from the remaining mineral fraction (after carbonate removal) using CHBr₃ at a density of 2.89 g/cm³ and subsequently digested in a mixture of 65% HNO₃ and 40% HF.

The 6-step sequential chemical extraction method has been widely adopted by many researchers [71,72,73,74,75,76,77,78,79,80] and its validation and efficiency of each step for the target elements have been well documented.

4. Results and Discussion

4.1. Coal Chemistry and Coal Rank

Coal samples from the Heidaigou Mine in the Jungar Coalfield were analyzed for proximate and ultimate analysis, sulfur forms, and vitrinite reflectance. Ash yields ranged from 18.5 to 32.43%, classifying them as medium- to high-ash coals according to Chinese Standard GB/T 15224.1-2010 [81] (medium-ash: 16.01%–29%; high-ash: >29%) (

Table 1. Proximate and ultimate analyses (%), total and forms of sulfur (%), and vitrinite random reflectance (%) for the coal samples from the Heidaigou Mine, Jungar Coalfield.

Sample ID	Mad	Ad	Vdaf	St	So	Sp	Ss	Ro
HDG-1	3.12	18.50	33.56	0.52	0.42	Bdl	0.10	0.56
HDG-2	4.25	32.43	31.85	0.43	0.31	Bdl	0.12	0.55
HDG-3	3.85	20.42	34.50	0.48	0.33	Bdl	0.15	0.57

Notes: M—moisture; A—ash yield; V—volatile matter content; S_t—total sulfur; S_p—pyrite sulfur; S_s—sulfate sulfur; S_o—organic sulfur; Rr—vitrinite random reflectance. Ad—as received basis; d—dry basis; daf—dry and ash-free basis.

). Volatile matter content and vitrinite reflectance (Rr) indicate a high-volatile bituminous coal rank based on ASTM Standard D388-12 [82]. Notably, these coals exhibit low total sulfur content, primarily consisting of organic sulfur.

4.2. Minerals Found in Coal

Table 2. Minerals determined by XRD and Siroquant (%; LTA, low-temperature ash yield).

Sample ID	LTA	Kaolinite	Boehmite	Calcite	Pyrite	Rutile	Goyazite	Chlorite
HDG-1	19.10	51.2	45.9	2.3	0.2	0.2	bdl	0.2
HDG-2	33.25	44.2	50.3	5.4	bdl	bdl	0.1	bdl
HDG-3	22.11	46.8	48.7	3.5	bdl	0.3	0.4	0.3

presents the mineral composition of the coal samples, determined through XRD and Siroquant analysis. Figure 3 presents the XRD spectrum of sample HDG-1. Kaolinite and boehmite are the dominant minerals, accompanied by varying minor amounts of calcite, pyrite, rutile, goyazite, and chlorite. Kaolinite exhibits two modes of occurrence with corresponding distinct origins: as cell-fillings (inertinite macerals, Figure 4A) with authigenic origin and as being distributed in collodetrinite (Figure 4B) with an origin of detrital-input from sediment source regions. Boehmite, primarily occurring as cell-fillings (Figure 4C), also suggests an authigenic origin, consistent with findings by Dai et al. [27]. Pyrite and calcite, predominantly found as fracture-fillings (Figure 4D,E), indicate epigenetic formation. Rutile, primarily distributed within the kaolinite matrix (Figure 4F), points to a detrital origin. Interestingly, neither XRD nor SEM-EDS detected quartz in the analyzed samples, contrasting with the observations of Dai et al. [27] who reported quartz presence in the upper portion of the coal seam.

4.3. Concentrations of Critical Elements in Coal and SCE Results

Coal samples from the Heidaigou Mine exhibit significantly enriched concentrations of Li, Al, Ga, and REEs compared to Chinese coals reported by Dai et al. [83] and global hard coals documented by Ketris and Yudovich [84] (

Table 3. Concentrations of major-element oxides and trace elements in samples investigated and their concentration coefficients (CC).

Elements	HDG-1	HDG-2	HDG-3	World	CC-HDG-1	CC-HDG-2	CC-HDG-3
Al2O3	13.54	8.76	6.21	5.98	2.26	1.46	1.04
SiO2	10.22	8.45	5.21	8.47	1.21	1.00	0.62
CaO	0.65	0.25	6.02	1.23	0.53	0.20	4.89
K2O	0.21	0.082	0.094	0.19	1.11	0.43	0.49
TiO2	0.61	0.51	0.34	0.33	1.85	1.55	1.03
Fe2O3	0.65	0.55	1.05	4.85	0.13	0.11	0.22
MgO	bdl	bdl	0.001	0.22	nd	nd	0.005
Na2O	0.058	0.062	0.055	0.16	0.36	0.39	0.34
MnO	0.006	0.005	0.011	0.015	0.40	0.33	0.73
P2O5	0.022	0.011	0.017	0.092	0.24	0.12	0.18
Li	232	283	319	14	16.57	20.21	22.79
Be	1.63	1.55	2.21	2	0.82	0.78	1.11
F	454	189	195	82	5.54	2.30	2.38
Sc	5.21	5.45	4.85	3.7	1.41	1.47	1.31
V	26.4	20.4	16.8	28	0.94	0.73	0.60
Cr	13.4	5.86	5.57	17	0.79	0.34	0.33
Co	1.31	0.68	1.21	6	0.22	0.11	0.20
Ni	3.25	0.56	2.34	17	0.19	0.03	0.14
Cu	12.4	9.85	6.84	16	0.78	0.62	0.43
Zn	23.8	18.5	35.7	28	0.85	0.66	1.28
Ga	15.4	18.6	16.5	6	2.57	3.10	2.75
Ge	0.38	0.22	0.54	2.4	0.16	0.09	0.23
As	1.02	0.65	0.12	9	0.11	0.07	0.01
Se	5.74	12.2	3.1	1.6	3.59	7.63	1.94
Rb	2.22	1.85	1.35	18	0.12	0.10	0.08
Sr	90.4	21.5	331	100	0.90	0.22	3.31
Y	10.1	13.2	20.8	8.2	1.23	1.61	2.54
Zr	214	352	158	36	5.94	9.78	4.39
Nb	19.4	16.4	10.5	4	4.85	4.10	2.63
Mo	1.05	1.45	1.34	2.1	0.50	0.69	0.64
Cd	0.034	0.065	0.063	0.2	0.17	0.33	0.32
In	0.03	0.044	0.038	0.04	0.75	1.10	0.95
Sn	2.11	2.35	1.54	1.4	1.51	1.68	1.10
Sb	1.55	0.42	0.25	1	1.55	0.42	0.25
Cs	0.054	0.087	0.064	1.1	0.05	0.08	0.06
Ba	23.4	18.4	22.9	150	0.16	0.12	0.15
La	13.5	4.21	49.5	11	1.23	0.38	4.50
Ce	23.1	12.1	85.1	23	1.00	0.53	3.70
Pr	2.46	1.55	9.24	3.4	0.72	0.46	2.72
Nd	8.69	6.21	31.2	12	0.72	0.52	2.60
Sm	2.05	2.04	5.63	2.2	0.93	0.93	2.56
Eu	0.43	0.37	0.91	0.43	1.00	0.86	2.12
Gd	1.89	2.21	4.85	2.7	0.70	0.82	1.80
Tb	0.34	0.41	0.72	0.31	1.10	1.32	2.32
Dy	2.12	2.85	4.21	2.1	1.01	1.36	2.00
Ho	0.41	0.58	0.81	0.57	0.72	1.02	1.42
Er	1.21	1.69	2.31	1	1.21	1.69	2.31
Tm	0.16	0.27	0.33	0.3	0.53	0.90	1.10
Yb	1.24	1.74	2.14	1	1.24	1.74	2.14
Lu	0.18	0.25	0.31	0.2	0.90	1.25	1.55
REE	67.88	49.68	218.06	68.41	0.99	0.73	3.19
Hf	6.54	9.85	3.74	1.2	5.45	8.21	3.12
Ta	1.54	1.4	0.63	0.3	5.13	4.67	2.10
W	2.22	1.52	1.55	0.99	2.24	1.54	1.57
Re	0.002	0.001	0.002	0.001	2.00	1.00	2.00
Hg	0.08	0.04	0.07	0.1	0.80	0.40	0.70
Tl	0.34	0.38	0.27	0.58	0.59	0.66	0.47
Pb	25.4	70.1	18.5	9	2.82	7.79	2.06
Bi	0.62	0.74	0.51	1.1	0.56	0.67	0.46
Th	19.01	33.52	12.1	3.2	5.94	10.48	3.78
U	3.22	4.12	3.25	1.9	1.69	2.17	1.71

). Particularly, the concentration coefficient (CC), which was defined as the ratio of concentrations of elements in investigated samples vs. corresponding elements in world coals [85], for Li in samples HDG-1,-2, and-3 is as high as 16.57, 20.21, and 22.79, respectively. The CC of Ga for the three samples is 2.57, 3.10, and 2.75, respectively. The CC of Zr and Nb are also high (Table 3) although their enrichment in these coals have not attracted much attention. Although elevated concentrations of REE in the coals from the Jungar Coalfield have been reported by previous studies [27,28,29], REE concentration is high only in sample HDG-3 (CC = 3.19). Previous researchers have demonstrated that late Paleozoic coals from the Jungar Coalfield [27,29,50,86,87] and its neighboring Daqingshan [30] and Ningwu Coalfields [35,36] are characterized by notably high concentrations of these critical elements. Concentrations of other trace elements in the Heidaigou Mine coals generally align with the global averages for hard coals [84]. The SCE results on the studied coals are shown in

Table 4. Mass fraction (μg) and leached percentage (%) of high-Li-Ga-Zr-Nb-REE samples. T-MoO, types of modes of occurrence; MF, mass fraction; LP, leached percentage.

Samples & T-MoO	Li		Ga		Zr		Nb		REE
	MF	LP	MF	LP	MF	LP	MF	LP	MF	LP
HDG-1
Water soluble	bdl	bdl	bdl	0	bdl	0	bdl	0	20.5	5.71
Ion exchangeable	bdl	bdl	5.8	7.44	5.65	0.52	bdl	0	15.2	4.23
Carbonate	35.6	3.46	bdl	bdl	bdl	bdl	4.58	4.22	112.3	31.26
Organic	125	12.15	8.5	10.9	15.4	1.43	9.76	9	105.7	29.42
Silicate	868	84.39	58.2	74.62	1035.3	95.96	85.6	78.94	105.6	29.39
Sulfide	bdl	0	5.5	7.05	22.5	2.09	8.5	7.84	bdl	bdl
HDG-2
Water soluble	bdl	0	bdl	0	bdl	0	bdl	0	8.55	3.21
Ion exchangeable	5.87	0.41	5.86	6.08	21.5	1.15	bdl	0	10.22	3.83
Carbonate	12.5	0.88	6.54	6.78	22.2	1.19	5.45	6.7	75.4	28.28
Organic	358.1	25.09	22.2	23.03	45.5	2.44	6.44	7.92	78.6	29.48
Silicate	1025.4	71.84	58.2	60.37	1655.2	88.72	65.8	80.9	88.2	33.08
Sulfide	25.5	1.79	3.6	3.73	121.2	6.5	3.65	4.49	5.68	2.13
HDG-3
Water soluble	bdl	0	bdl	0	4.52	0.59	bdl	0	21.5	2.1
Ion exchangeable	10.5	0.68	bdl	0	21.5	2.81	bdl	0	35.6	3.48
Carbonate	35.5	2.28	5.8	6.9	14.2	1.86	5.64	10.09	621.1	60.78
Organic	287.5	18.5	21.4	25.48	115.2	15.07	3.52	6.3	225.2	22.04
Silicate	1205.2	77.56	52.6	62.62	524	68.53	41.1	73.55	118.5	11.6
Sulfide	15.2	0.98	4.2	5	85.2	11.14	5.62	10.06	bdl	bdl

and Figure 5. The highly elevated concentrations of trace elements Li, Ga, Zr, Nb, and REE in the three coal samples are, however, very low or below detection limit in water-soluble, ion-exchangeable, and sulfide fractions.

Lithium and Ga mainly occur in aluminosilicate (71.84%–84.39%) and to a lesser extent, organic fractions (12.15%–25.09%); a small proportion occurs in carbonates (0.88%–3.46%). Due to its low atomic number of Li, direct analytical methods for determining its modes of occurrence in coal are challenging and hinder comprehensive investigation [29]. Consequently, researchers have relied primarily on indirect techniques such as statistical analysis, density fractionation, selective leaching, and float/sink experiments to identify its primary carriers [67,75]. For example, selective leaching studies by Finkelman et al. [67] suggested that approximately 90% of Li is bound to clays and micas, with the remaining portion either organically bound or associated with acid-insoluble phases like tourmaline. Similarly, Wang et al. [88], using selective leaching, observed that 96.6%–99.4% of Li in the analyzed coals was extracted by HF, indicating its predominant presence in clay minerals, specifically kaolinite and illite. Existing research indicates a general association of Li with clay minerals in coal [89,90,91]. While less common, organic associations have also been documented [92,93]. Dai et al. [29] identified a chlorite mineral, intermediate in composition between cookeite and chamosite, as the principal carrier of the elevated lithium concentrations in the coal they studied. The major modes of occurrence of Li in this study are consistent with the minerals identified.

The presence of Ga in coal has been observed in both mineral and organic forms, though mineral associations seem to be the more prevalent [29,93,94,95,96,97,98,99]. The geochemical similarity between Ga and Al allows for isomorphic substitution of Ga for Al in aluminum-bearing minerals, especially aluminosilicates [29]. While several other Ga-containing minerals, such as boehmite, goyazite, and diaspore, have been identified in coal, no primary gallium minerals have been discovered to date [5]. Statistical analysis of coal samples from the Haerwusu Mine (Jungar Coalfield, Inner Mongolia, northern China, China) suggests that Ga is primarily concentrated within boehmite and organic matter [28]. In contrast, Ga in coals from the Adaohai Mine (Daqingshan Coalfield, Inner Mongolia, China) is predominantly linked to diaspore and kaolinite [30]. Further studies in the nearby Jungar Coalfield revealed different Ga associations: boehmite in Heidaigou Mine coals and goyazite in Guanbanwusu Mine coals [27,29]. Analysis of coals from the Buertaohai-Tianjiashipan mining district (Jungar Coalfield, Inner Mongolia, China) indicates that kaolinite is the primary gallium-bearing mineral, with svanbergite playing a less significant role [100].

Zirconium and Nb dominantly occur in aluminosilicate (68.53%–95.96%), most likely zircon and rutile as well. As a ubiquitous accessory mineral in coal, zircon is the primary carrier of Zr, and often Hf as well [101,102]. While generally acid-insoluble, its small size (often just a few micrometers) and susceptibility to metamictization can make it vulnerable to acid attack [46]. Numerous in situ microanalysis studies, such as those using SEM-EDS [27,86,87], have confirmed zircon’s dominance as the Zr-bearing phase [27,103,104]. However, its relatively low abundance in coal makes XRD detection impossible. In rare instances, SEM-EDS has revealed trace Zr in anatase and REE-phosphates [33]. Niobium is commonly found in coal within the minerals such as zircon, anatase, and rutile [86,105,106]. However, the concentration of Nb, as well as other trace elements, can vary significantly in zircon depending on its origin [90]. For instance, authigenic zircon found in the Late Permian coals of the Huayingshan Coalfield, Southwest China, did not exhibit detectable levels of Nb [107]. Dai et al. [27,28] have found zircon and rutile in the Jungar coals, which originated from the sediment-source region, and this is consistent with the sequential chemical extraction data present in this study.

Seredin and Dai [23] established a set of criteria for evaluating the economic potential of rare earth elements and yttrium (REE+Y) in coal ash, which include both a threshold concentration and the specific distribution of individual elements. A concentration of 1000 ppm REE+Y oxides in the ash is regarded as the industrial cut-off for economically viable recovery. When this threshold is met, the “outlook coefficient” (C_outl) serves as the other metric to assess the industrial prospects of REE+Y extraction [108]. This coefficient is defined as the ratio between the relative proportion of critical REY and that of excessive REY in the total REE+Y content, and it is calculated using Equation (1) [23]. According to this criterion, C_outl values greater than 2.4 indicate a highly promising resource, values between 0.7 and 1.9 suggest moderate potential, and values below 0.7 are considered economically unattractive [108]. These criteria for assessing the economic potential of REE+Y in coal ash have been widely adopted and validated by researchers worldwide [109,110,111,112,113,114,115,116,117,118].

$${\mathrm{C}}_{\mathrm{outl}}={\displaystyle \frac{(\mathrm{N}\mathrm{d}+\mathrm{E}\mathrm{u}+\mathrm{T}\mathrm{b}+\mathrm{D}\mathrm{y}+\mathrm{E}\mathrm{r}+\mathrm{Y})/(\mathrm{R}\mathrm{E}\mathrm{E}+\mathrm{Y})}{(\mathrm{C}\mathrm{e}+\mathrm{H}\mathrm{o}+\mathrm{T}\mathrm{m}+\mathrm{Y}\mathrm{b}+\mathrm{L}\mathrm{u})/(\mathrm{R}\mathrm{E}\mathrm{E}+\mathrm{Y})}}$$

(1)

Based on the ash yield and REE concentrations in the coal samples, the calculated REE+Y oxide contents for HDG-1, HDG-2, and HDG-3 are 506 ppm, 233 ppm, and 1403 ppm, respectively (on an ash basis). Among these, HDG-3 exceeds the 1000 ppm industrial threshold proposed by Seredin and Dai [23], indicating strong potential for economic recovery. The corresponding outlook coefficients for HDG-1, HDG-2, and HDG-3 are 1.48, 3.51, and 1.13, respectively. These results suggest that HDG-1 and HDG-3 fall within the “promising” range, while HDG-2 demonstrates a “highly promising” potential for REE+Y extraction from coal ash.

REE mainly occurs in minerals (carbonate, 28.28%–60.78%; aluminosilicate, 11.6%–33.08%), and to a lesser extent, organic fraction (22.04%–29.42%). The fractions of REE in these coals are consistent with mineral compositions (kaolinite, boehmite, and carbonates). Early research has indicated that rare earth elements and yttrium (REY) in coal are typically found in association with various minerals, including authigenic minerals like APS minerals, carbonates, fluoro-carbonates, and water-containing phosphates [23,119,120]. Additionally, REY are also linked to detrital minerals such as zircon, monazite, apatite, and xenotime, as well as organic matter [67,121,122]. In general, it is understood that REY in coal, especially in higher-rank coals like bituminous coal and anthracite, are predominantly connected to minerals, with only a small fraction existing within the organic components [123]. It seems that the proportion of organically associated REE (22.04%–29.42%) in the studied coal samples is a bit higher than expectation, but it is not surprising for high-volatile bituminous coal. This is consistent with the findings of Dai et al. [27,28], who studied coals from the same coalfield and reported that a certain proportion of REEs was organically associated, although they did not quantitatively determine the organic fraction of REEs. Furthermore, a recent study by Jiu et al. [124] demonstrated that Pr, Nd, and Sm can be associated with organic matter, including vitrinite and liptinite, and that medium REEs and heavy REEs are highly enriched in vitrinite in subbituminous coal. These findings further support the organic association of REEs in low-rank coals, including high-volatile bituminous coal. The high proportion of REE in these coals are probably attribute to the high organic affinity of light REE in these coals as reported by Dai et al. [27,125], although it is commonly considered that heavy REEs in general have a high organic affinity than light REEs [46,126,127,128,129,130,131,132].

Overall, the XRD and SEM-EDS data, i.e., the identification of minerals in coal, primarily including kaolinite and boehmite, with minor amounts of calcite, pyrite, rutile, goyazite, and chlorite, are consistent with the quantitative distribution of critical elements in the specific forms in which they occur, i.e., in addition to organic matter, within aluminosilicate and carbonate phases. Such consistency between mineralogical and geochemical datasets enhances confidence in interpreting the host phases of these elements and could provide a solid basis for evaluating their extractability and economic potential.

5. Conclusions

The Ga-Al-Li-REE ore deposit in coal has been found in the Jungar Coalfield, Inner Mongolia, northern China. This study quantitatively investigated the specific forms in which these elements exist, which is crucial for developing effective recovery technology for critical elements from coal and coal ash. Kaolinite and boehmite were the primary mineral constituents, with minor amounts of calcite, pyrite, rutile, goyazite, and chlorite. Sequential chemical extraction revealed that Li and Ga are primarily associated with aluminosilicate phases (71.84%–84.39%) and, to a lesser degree, organic matter (12.15%–25.09%). Zirconium and Nb were also predominantly found within aluminosilicates (68.53%–95.96%). REEs occur mainly in carbonate (28.28%–60.78%), aluminosilicate (11.6%–33.08%), and organic (22.04%–29.42%) fractions.

The quantitative modes of occurrence of critical elements in Jungar coals may provide valuable insights into their forms in coal ash, which is beneficial for the commercial-scale recovery of these metals. The next steps in research include investigating the transformation of critical elements from coal to ash; determining the quantitative modes of occurrence of critical elements in coal ash; and identifying the factors that control the modes of occurrence of critical elements in both coal and coal ash. These studies would enhance the efficient and environmentally friendly recovery of critical elements from coal ash.