The results of proximate analysis, total sulfur content and calorific value of Linchang coal are given in Table 1. The average moisture content of the coal sample is 15.44%, which belongs to medium-high moisture coal. Linchang coals exhibit a high ash yield (46.15% on average), medium volatile content (21.69% on average) and extra-low gross calorific values (10.34 MJ/kg on average). Coal samples have an average of 1.39% total sulfur, corresponding to medium sulfur coal.

As presented in Table 1, the huminite reflectance (R_o) value is 0.40%, indicating

that the Linchang coal is lignite. The maceral composition is dominated by huminite (55.19% - 69.94%, 61.63% on average), followed by liptinite (11.39% - 21.89%, 16.66% on average) and little inertinite (1.90% - 4.91%, 2.78% on average). The content of inorganic matter in coal is 13.52% - 22.86%, with an average value of 18.93%.

The composition of minerals in the Linchang coal determined by XRD and Siroquant^TM software is presented (Table 2 & Figure 2). The coal LTAs mainly consisted of clay minerals such as illite (33.78%) and kaolinite (33.31%), followed by quartz (19.48%), as well as a lesser extent, pyrite (6.04%), basanite (3.77%), anhydrite (1.87%) and siderite (1.18%). The content of calcite is very tiny (0.58%), which may be due to the dissolution of a large amount of calcite under acidic conditions. Yan et al. (2019) found a great deal of eroded calcite in the roof and floor of the Zhoujing mine in Baise basin, and little calcite existed in the coal, further proving that acidic coal-forming environment.

The morphological characteristics of the main minerals in the Linchang coal are shown (Figure 3). Kaolinite is mainly clastic with a stratified structure (Figure 3(a)), which is derived from the detrital materials produced by weathering and denudation of the parent rock in the provenance. Quartz is mostly angular (Figure 3(b)), belonging to a near-source input type (Yan et al., 2019), and often associated with kaolinite. The morphology of pyrite is mainly framboidal (Figure 3(c))

and euhedral (Figure 3(d), Figure 3(e)), indicating that pyrite was formed in the peat development stage (Yan et al., 2019). The framboidal pyrite in the coal indicates that the coal-forming environment of Linchang coal is an acidic reducing

environment, mainly because in an acidic environment, sulfate-reducing bacteria reduce sulfate in the solution to H₂S, which combines with Fe²⁺ to generate pyrite (Ward, 2002). There is a close relationship between sphalerite and pyrite (Figure 3(f)). The displacement reaction of Zn to Fe results in the substitution of pyrite to sphalerite (Figure 3(h)).

Table 3 lists the content of major element oxides in Linchang coal, mainly SiO₂, Al₂O₃ and Fe₂O₃. The average content of SiO₂, Al₂O₃ and Fe₂O₃ are 24.64%, 14.18% and 2.16%, respectively. The major element content in coal is higher

Notes: Ash composition parameter in coal C = (Fe₂O₃ + CaO + MgO)/(SiO₂ + Al₂O₃); A/T= Al₂O₃/TiO₂; Si/Al = SiO₂/Al₂O₃.

than average values in China coal, except for Fe₂O₃, P₂O₅ and MnO. The ratio of SiO₂/Al₂O₃ is 1.39 - 2.26, with an average of 1.70, which is higher than the mean value of China coal (1.42) and theoretical value of kaolinite (1.18), except LC-1 with SiO₂/Al₂O₃ between these two values (1.39). This may be due to the presence of quartz and illite in most coal samples. However, illite was not found in LC-1 (Table 2), so the SiO₂ in the LC-1 sample was the only source from kaolinite with a little amount of quartz.

Table 4 lists the content, average value and enrichment coefficient (CC) of 14 trace elements in Linchang coal. According to the content level index of trace elements in coal suggested by Dai et al. (2012), the enrichment characteristics are depleted (CC < 0.5), normal (0.5 - 2), slight enrichment (2 - 5), enrichment (5 - 10) and high enrichment (10-100), the enrichment characteristics of valuable trace elements in Linchang coal are shown (Figure 4).

The average concentration of Li, V and Ag in coal are 74.60 μg/g (20.34 - 113.63 μg/g), 151.54 μg/g (117.80 - 197.72 μg/g) and 0.53 μg/g (0.22 - 0.83 μg/g), respectively (Table 4). The content of Li, V, and Ag in coal is higher than their average values in the upper crust, Chinese coals and world low-rank coals. For Li in Linchang coal, some of them exceed the suggested industrial cut-off grade of associated lithium in coal (80 μg/g) (Sun et al., 2012).

Notes: Data about Chinese coals are from (Dai et al., 2012); the upper crust data are from (Taylor & Mclennan, 1985); world coals data are from (Ketris & Yudovich, 2009); CC = the content of trace elements in Linchang coal/world coals.

The content of Be is 1.74 - 4.98 μg/g, with an average value of 2.60 μg/g, which is higher than the Chinese coals and the world low-rank coals, but lower than the upper crust (Table 4), showing slight enrichment (Figure 4). The Be content in Permian coal in South China is 0.6 - 3 μg/g with an average of 2 μg/g (Zhao et al., 2002), and its content in Linchang coal is mostly higher than this value.

The average content of Ga and Se in coal is 21.43 μg/g (17.08 - 30.04 μg/g) and 3.43 μg/g (1.04 - 6.33 μg/g), respectively (Table 4). Both of them are higher than the upper crust value, the Chinese coals and the world low-rank coals, showing slight enrichment (Figure 4). The concentration of Ga in coal and coal gangue is generally 0 - 20 μg/g in Baise basin (Zhang et al., 2019), and the average value of Ga in Linchang coal is higher than this value. Although the content of Ga in Linchang coal does not exceed the industrial utilization grade

(30 μg/g) (Ren & Dai, 2009), considering that the Ga content is not low, especially Ga will be further enriched in coal ash, which may reach the value of exploitation and utilization. Therefore, it is necessary to conduct an in-depth survey of the Ga content in this area. The Se content in Linchang coal is higher than that of Chinese lignite (2.22 μg/g) and Paleogene coal (1.57 μg/g) (Zhang et al., 2007).

The content of U is 10.35 - 70.25 μg/g, with an average value of 31.01 μg/g, which is higher than the Chinese coals, the world low-rank coals and the upper crust value (Table 4). Compared to world coals, the above trace elements are characterized by high enrichment (Figure 4). U is generally enriched in Guangxi coal, and its content can reach up to several hundred ppm (Lauer et al., 2017). Compared with coals of other geological ages, the content of U in Paleogene and Neogene coal is significantly higher (Chen et al., 2017), and the result of this study is consistent with previous studies. Sun et al. (2014) proposed that the comprehensive recycling value of coal associated U element is 40 μg/g, and the content of U in samples LC-8, LC-11 and LC-12 are 56.57 μg/g, 45.54 μg/g and 70.25 μg/g, respectively, which all exceed the threshold value.

SPSS software was used to analyze the correlation between trace elements and major elements, ash and total sulfur content in Linchang coal at the 95% confidence level (n = 13, the critical value of correlation coefficient r is 0.553), and the correlation coefficients are shown in Table 5. According to the correlation analysis, combined with the results of XRD, SEM-EDS and polarizing microscope, the occurrence of valuable trace elements in coal is inferred.

The correlation coefficient of Li, V, Ga, and Ag is 0.647 - 0.903, showing moderate or high positive correlation with each other, and their correlation coefficient with ash content is 0.737 - 0.939, indicating that these elements may have similar inorganic mineral sources and occurrence modes. The Li in coal is mainly related to clay minerals (kaolinite and chlorite), boehmite and other inorganic components, partly combined with organic matter (Qin et al., 2015b). Recent leaching experiments have shown that about 90% of Li in most high-rank coals is related to aluminosilicates (clay and mica), while about 50% of low-rank coals is related to organic matter (Finkelman et al., 2017). Although Linchang coal is low-rank coal, Li is highly correlated with ash yield (0.917), Al₂O₃ (0.914) and SiO₂ (0.891), indicating that Li mainly occurs in clay minerals, which is due to the high ash content of Linchang coal. Clay mineral usually has a negative charge and interlayer structure, and is easy to undergo cation exchange with metal ions (Finkelman et al., 2019), which is conducive to the adsorption of Li. Therefore, the occurrence mode of Li is adsorbed on the surface of clay minerals such as illite and kaolinite in Linchang coal. V in coal may be related to clay, and exist organically in low-rank coals (Finkelman et al., 2017). In this case, V has a moderate correlation with ash yield (0.737) and Al₂O₃ (0.695) in Linchang coal, illustrating that most of V are present in clay minerals, and partly in combination with organic matter. It may be that the organically bound V was released during coalification and subsequently absorbed by clay minerals (Finkelman et al., 2017). There are three occurrence modes of Ga in coal: inorganic, organic and mixed. The primary inorganic carrier is clay mineral (Qin et al., 2015a). This is because Ga and Al have similar chemical properties, and Ga can replace Al in aluminous phases by isomorphism (Dai et al., 2008). Ren et al. (2006) have shown that Ga in coal results form isomorphism in clay minerals. Ga is positively correlated with ash yield (0.813) and Al₂O₃ (0.900) in Linchang coal, and it may enter kaolinite and illite in the form of isomorphism. Qin et al. (2018) found that Ag in Zhongliangshan coal in Chongqing is mainly related to sulfide, followed by silicate. In Linchang coal, Ag is correlated highly with ash yield (0.939), Al₂O₃ (0.823) and SiO₂ (0.960), but is not correlated or negatively correlated with Fe₂O₃ (0.018) and total sulfur (−0.614), indicating that its main carrier may be kaolinite or illite.

The occurrence mode of Be in coal is complex, which may be combined with organic matter and clay minerals. When the content of Be is high, the organic affinity is dominant, and when the content is low, it is mainly present in clay minerals (Eskenazy, 2006; Finkelman et al., 2017). Be has a low correlation with ash yield (0.379), Al₂O₃ (0.338) and SiO₂ (0.356) in Linchang coal, indicating that carriers of Be are inorganic minerals, together with organic matter. Although Be occurs in organic components in low-rank coals rich in humic acid, the ash content in Linchang coal is relatively high, and the organically bound Be turns to be bound with clay minerals during the coalification process (Finkelman et al., 2017). This is due to that Be and Al are both amphoteric elements, and they