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Article

Distribution, Occurrence and Enrichment Causes of Sodium in Middle Jurassic Coal from Zhundong Coalfield, Xinjiang

by
Yulong Wang
1,2,
Wenfeng Wang
1,2,3,*,
Wenlong Wang
1,2,
Piaopiao Duan
2,
Xin He
4 and
Qingfeng Lu
5
1
Key Laboratory of Coalbed Methane Resources & Reservoir Formation Process, Ministry of Education, China University of Mining & Technology, Xuzhou 221008, China
2
School of Resources and Geosciences, China University of Mining & Technology, Xuzhou 221116, China
3
School of Geology and Mining Engineering, Xinjiang University, Urumqi 830047, China
4
School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, China
5
Carbon Neutrality Institute, China University of Mining & Technology, Xuzhou 221116, China
*
Author to whom correspondence should be addressed.
Minerals 2024, 14(2), 146; https://doi.org/10.3390/min14020146
Submission received: 2 January 2024 / Revised: 24 January 2024 / Accepted: 25 January 2024 / Published: 29 January 2024
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Abstract

:
The coal found in the Zhundong Coalfield is highly suitable for power generation and gasification. However, the high sodium content within the coal leads to severe boiler slagging and contamination. Additionally, sodium disperses into the gaseous phase to form haze, adversely affecting the local atmospheric environment. This study delved into the distribution and occurrence characteristics and enrichment causes of sodium in the coal, employing sequential extraction experiments and testing methods such as XRF, ICP-OES and SEM-EDS. The findings of this research indicate the following: (1) With the increasing burial depth of coal seams, there is a noticeable decrease in the sodium content within the coal. Sodium is primarily distributed within the coal seam, with higher concentrations observed in the upper portions of the same coal seam. Furthermore, the distribution of sodium within the epipedon, overlying rocks and coal seam also exhibits a decreasing trend. (2) Sodium primarily exists in a water-soluble state within coal seams, with H2O-Na accounting for over 70% of its composition. The ion-exchangeable sodium is higher than that in the roof, floor and gangue. Sodium exists in coal seams in both ionic and hydrated ionic forms. It is worth noting that the minerals within coal are not the primary carriers of sodium. (3) These coal seams were formed in a warm and humid shallow lake peat swamp environment, which is not significantly influenced by terrestrial or marine sources. The enrichment of sodium is primarily influenced by later hydrodynamic conditions.

1. Introduction

The study of alkali metals in coal began in the 1950s. Due to the widespread distribution of high-sodium coal worldwide, there are many studies on sodium in coal [1,2,3,4]. A universally accepted definition for high-sodium coal has not yet been established. However, research indicates that coal ash with a Na2O content exceeding 3% should be examined for its fouling and slagging properties [5,6]. It is generally accepted that high-sodium coal refers to coal with a sodium content (on an ash basis) exceeding 2%. Such coal varieties can be found in various countries, including China, Australia, the United Kingdom and others. In high-alkali coal, sodium compounds are generally classified into two categories: inorganic and organic. Inorganic alkali metal compounds mainly exist in the form of chlorides, hydrated ions and silicate-aluminates. Organic alkali metals are distributed in carboxylate salts and on nitrogen-containing or oxygen-containing functional groups that are combined in a coordinated form within the coal structure [7,8,9]. Water-soluble sodium represents the predominant occurrence of sodium in coal, where it exists in the pores and cracks of coal in the form of sodium salts or hydrated ions [10]. During the coalification process, coal seams influenced by seawater exhibit a higher sodium content, while those unaffected by seawater display a lower sodium content. In the investigation conducted by Sripada et al., it was discovered that the content of Na2O in the coal ash of Australia’s brown coal (Loy Yang coal) amounts to 6.33% [11]. Similarly, Yudovich et al. discovered in their study on the Novomoscovsk Coalfield (East Donbas) that the Na2O content in the coal ash reaches 16% [12]. Furthermore, Spears et al. examined 24 coal samples from the United Kingdom and found that the maximum sodium content in the coal was 0.22% [13]. Zhou et al., in their research on the chemical elements present in the coal from China’s Zhundong Coalfield, observed that the sodium content in the coal ranged from 0.2% to 0.4% [4]. In all of the aforementioned coals, sodium primarily exists in the form of water-soluble sodium. Except for Zhundong coal, the higher sodium content in the remaining coals is influenced by the seawater.
Within the borders of our nation, the Xinjiang region stands as a repository of abundant coal resources, waiting to be unearthed and harnessed [4,9]. The coal reserves in this area possess favorable natural conditions, with a lower ash yield, low sulfur, low phosphorus, medium–high moisture, medium–high volatile matter, good reactivity, etc., and have the advantages of thick coal seams and low mining costs [14,15]. However, due to the unique coal-forming environment and the influence of groundwater in the Zhundong Coalfield, the sodium content in the coal is relatively high. Typically, the Na2O content in Zhundong coal and coal ash exceeds 0.4% and 2%, respectively, significantly surpassing the average Na2O content in Chinese coal (0.16% [16]) and Chinese thermal coal ash (below 1% [17]). When the high-sodium coal from the Zhundong area in Xinjiang is utilized as a raw material for power generation or gasification, there will be significant slagging and fouling and other phenomena in boilers [18,19]. The mechanism of the slagging and fouling caused by alkali and alkaline earth metals in coal involves two aspects [20]: (1) the condensation and deposition of alkali and alkaline earth metals and (2) the deposition of sulfate aerosols enriched in Na and Ca, as well as the condensed sulfates. Moreover, during the process of combustion and pyrolysis, a large amount of highly corrosive sodium diffuses into the gaseous phase [21], subsequently condensing with aerosols to form haze. Sodium exists in various occurrence forms in coal, but not all forms of sodium will deposit with the ash [22,23]. The culprits were identified as the water-soluble and ion-exchangeable forms of sodium, which easily vaporize during the rapid devolatilization of coal at high temperatures [10]. Currently, most of the studies have focused on the mode of sodium storage and release behavior during the combustion of high-sodium coals [24,25,26], and various scholars have proposed methods such as pretreatment, coal blending and adding additives to mitigate the detrimental effects of sodium during coal utilization [27,28,29,30,31]. However, due to insufficient research on the distribution and occurrence characteristics of sodium in coal, the removal of sodium from coal remains incomplete. Furthermore, there is also a lack of comprehensive understanding regarding the enrichment causes of sodium in Zhundong coal.
The present study focuses on the high-sodium coal from the Yihua coal mine in Zhundong Coalfield, Xinjiang. It aims to investigate the distribution and occurrence characteristics of sodium, explore the enrichment mechanism of sodium and provide a theoretical basis for coal pretreatment. Additionally, it aims to offer scientific evidence for the rational utilization of high-sodium coal in Zhundong Coalfield, Xinjiang.

2. Geological Setting

The Zhundong area refers to the area east of the Wucaiwan–Fukang line in the Junggar Basin. It is a first-order tectonic unit in the Junggar Basin, comprising 15 secondary tectonic units, which were formed in the Late Permian–Triassic Periods. During the Jurassic period, the Zhundong area underwent an evolution stage of inland basin subsidence. The Zhundong area entered a phase of equilibrium sedimentation, during which the Badaowan Formation (J1b), Sangonghe Formation (J1s), Xishanyao Formation (J2x) and Toutunhe Formation (J2–3t) were successively deposited following the Indosinian orogeny. The Xishanyao Formation witnessed the remarkable development of marsh coal facies, characterized by substantial sediment thickness, forming the Zhundong Coalfield. From west to east, the Zhundong Coalfield is divided into five distinct mining areas: the Wucaiwan Mining Area, Dajing Mining Area, Jiangjunmiao Mining Area, Xiheishan Mining Area and Laojunmiao Mining Area.
The Yihua coal mine is located in the Wucaiwan Mining Area (Figure 1) within the Shazhang fault–fold Belt and west of the Zhangpenggou anticline. The primary mineable coal seam is the B1 seam, which is located in the Xishanyao Formation (J2x) of the Middle Jurassic, with an altitude of 260°∠11°. The average thickness of the coal seam at the mining points is 61 m, with certain areas reaching thicknesses exceeding 70 m. The overlying rock of the coal seams primarily consists of sandstone, carbonaceous mudstone, siltstone and mudstone. The underlying rock is predominantly composed of fine sandstone, carbonaceous mudstone, carbonaceous sandstone and sandstone. The gangue mainly consists of high-carbon mudstone, carbonaceous mudstone, as well as sporadic occurrences of pyrite and calcite. The depositional environment of the Xishanyao Formation is characterized as a shallow lake–marsh environment.

3. Materials and Methods

3.1. Collection and Preparation of Sample

In accordance with Chinese National Standard GB/T 482-2008 [32], a total of 69 samples were collected vertically from the working face of the Yihua coal mine in the Wucaiwan Mining Area of Zhundong Coalfield, Changji Hui Autonomous Prefecture, Xinjiang, China. Additionally, 16 samples were equidistantly collected from the overlying rock strata. A selection of 22 samples was made, including 12 coal samples (C3, C4, C9, C11 and C19: dull coals; C5: bright coal; C6, C8, C12, C15 and C16: semibright coals; and C18: semidark coal), 2 roof samples (C20: sandstone with coal clasts, C21: sandstone), 2 floor samples (C1: sandstone, C2: sandstone with coal clasts), 5 parting samples (C7, C13 and C14: pyrite marker layers; C10 and C17: calcite layers) and 1 through-layer crack sample (C22 was taken from a quartz vein that longitudinally penetrated the C18 and C19 coal seams). The sampling locations are illustrated in Figure 1. Among the 16 samples taken from the overlying rock strata, S1, S2, S4, S8, S12 and S14 are sandstones; S3, S5, S6, S9, S10, S13 and S16 are mudstones of various colors; S11 and S15 are muddy siltstones; and S7 is a grey mudstone with coal lines.
Each sample, weighing approximately 200 g, was placed in a vacuum-drying oven and dried at a constant temperature of 60 °C until reaching a constant weight. Subsequently, the dried samples were thoroughly ground into powder and preserved for future use. The samples were screened to a size of 18–40 mesh (0.38–0.85 mm) and 200 mesh (~71 μm) for experiments and tests, mainly including a proximate and ultimate analysis, observation under a scanning electron microscope, sequential extraction experiments and the testing of major elements.

3.2. Analytical Procedures

Proximate analyses (including the moisture content, ash yield and volatile matter) and a total sulfur analysis of the samples were conducted according to Chinese National Standards GB/T 212-2008 [33] and GB/T 214-2007 [34], respectively. Ultimate analyses (C, H, O and N) adopt national standards GB/T 19227-2008 [35] and GB/T 476-2008 [36]. The major elemental oxides (SiO2, Al2O3, Fe2O3, TiO2, CaO, Na2O, MgO, K2O, P2O5 and MnO) of 38 samples were determined by using the Axiosmax X-ray fluorescence spectrometer (S8 Tiger, Bruker, Germany) in accordance with the Chinese National Standard GB/T14506.28-2010 [37].

3.3. Sequential Extraction Experiment

The sequential extraction experiment method, widely employed for the quantitative determination of various alkali metals [38,39,40], was utilized in this study. Four sequential extraction solutions, namely deionized water, ammonium acetate (NH4OAc), hydrochloric acid (HCl) and a mixed digestion acid, as described by Benson et al. [41], were employed for the sequential extraction experiments on the selected samples. The specific procedures are outlined in Figure 2.
Step 1 (H2O-Na): Firstly, 1.5 (±0.01) g of raw coal and 60 mL of deionized water were added to a 100 mL centrifuge tube. The tube was then placed in a thermostatic oscillator and subjected to persistent oscillation for 12 h at 60 °C and 120 rpm. Then, the tube was centrifuged at 4000 rpm for 10 min, and the supernatant liquid was collected. The remaining solids in the centrifuge tube were washed twice. A total of 10 mL of deionized water was added to each wash. After each wash, the tube was centrifuged, and the supernatant liquid was collected and mixed with the previously collected supernatant liquid in a 100 mL volumetric bottle. Then, the H2O-Na solution was obtained after constant volume process, while the solid was dried at a constant temperature of 60 °C for 2 h to prepare for the next step.
Step 2 (NH4OAc-Na): The residual solid and 60 mL of 1 mol/L ammonium acetate solution were added to the centrifuge tube. Repeat Step 1 to obtain the NH4OAc-Na solution.
Step 3 (HCl-Na): The residual solid and 60 mL of 1 mol/L hydrochloric acid were added to the centrifuge tube. Repeat Step 2 to obtain the HCl-Na solution.
Step 4 (insoluble Na): The residual solid was placed in a high-temperature, high-pressure digestion tank and digested with a digestion solution consisting of nitric acid, hydrofluoric acid and perchloric acid in a ratio of 2:6:1. The volume was fixed at 100 mL to obtain the insoluble Na solution.
The solutions obtained from Steps 1 to 4 were analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES, iCAP-7400, Thermo Fisher Scientific, Waltham, MA, USA). Ion chromatography (ICS-Aquion, Thermo Fisher Scientific, US) was used to determine the anions, mainly Cl− and SO42−, in the aqueous solutions.

4. Results and Analysis

4.1. Proximate and Ultimate Analysis, Total Sulfur

The proximate and ultimate analysis and total sulfur of the samples from the Yihua coal mine are listed in Table 1. As shown in the table, the average moisture content (Mad) in the coal is 12.64%, indicating that it falls within the range of medium-to-high-moisture coal (MHM). The coal ash content (Ad) ranges from 2.93% to 7.92%, classifying it as ultralow-ash coal (ULA) according to GB/T15224.1-2018 [42]. The volatile matter (Vdaf) is between 28.54 and 37.96%, with an average value of 31.51%, which is medium–high-volatile-matter coal (MHV). The average fixed-carbon content (FCd) is 65.40%, categorizing it as medium-to-high fixed-carbon coal (MHFC). The average sulfur content is 0.49%, classifying it as special low-sulfur coal (SLS) according to GB/T15224.2-2021 [43]. The variation ranges in the element content (carbon, hydrogen, oxygen and nitrogen) in the raw coal are small, indicating that the concentrations of these elements do not vary significantly within the coal seam and remain relatively stable. In summary, the Yihua coal is classified as lignite, characterized by medium-to-high moisture, ultralow ash, medium-to-high volatile matter, medium-to-high fixed carbon and special low-sulfur content.

4.2. Mineralogical Characteristics

The mineral species in the coal, gangue, roof, floor and overlying rock samples from the Yihua coal mine were determined by using X-ray diffraction (XRD). The mineral morphology in the coal seam samples was determined by using scanning electron microscopy (SIGMA, Carl Zeiss, Germany) coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) to identify the types and occurrence forms of the minerals. By analyzing the occurrence characteristics of the minerals in the sampled coal seam, we can gain insights into the origin of these minerals and the geological environment in which they formed.
According to the XRD data, the main inorganic minerals in coal samples C5, C9 and C16 are kaolinite, bassanite and pyrite, followed by calcite and illite. These samples may also contain quartz, marcasite and smectite (Figure 3). The floor samples C1 and C2 are predominantly composed of kaolinite, with the presence of quartz, anatase and rutile. The roof samples C20 and C21 are mainly composed of quartz, with the presence of kaolinite, anatase and rutile. The through-seam fissures sample C22 is primarily composed of quartz, with small amounts of calcite and trace amounts of pyrite and anatase (Figure 4). The gangue samples C10, C13, C14 and C17 are mainly composed of calcite, with the presence of kaolinite and pyrite. They may also contain gypsum, illite, bassanite and marcasite. Sample C7 is mainly composed of marcasite, pyrite and calcite, with the presence of gypsum, bassanite, boehmite, illite and kaolinite (Figure 5).
The overlying rock of the coal seam mainly consists of sandstone and mudstone, and the minerals include quartz, albite, anorthite and kaolinite (Figure 6). S2 contains a significant amount of siderite, S8 contains calcite, S10 contains hematite and S12 contains an abundant amount of goethite and minor bassanite. The presence of these minerals results in significant differences in the content of SiO2, Al2O3, TiO2, Fe2O3, CaO and MgO in these stratums compared to other rock strata.
Based on the observations using SEM-EDS, the main minerals observed in the samples from the coal mine include quartz, pyrite, calcite, kaolinite and anatase (Figure 7 and Figure 8).
The occurrence forms of quartz in the samples are mainly as follows: (1) Semiautomorphic granular form distributed in the clay matrix (Figure 7A), with clear boundaries with surrounding minerals and poor psephicity, indicating a terrestrial clastic origin and proximity to the source area. Quartz is occasionally found coexisting with anatase. (2) It is irregularly dispersed in the clay matrix or organic matter (Figure 7A,C), indicating an authigenic origin, possibly formed by the crystallization of SiO2-rich hydrothermal fluids from the source area with other minerals enclosed within the quartz. (3) Quartz with good crystallization is present in fractures in a semiautomorphic-to-automorphic granular form (Figure 7B). Mechanical transport cannot form fine-grained automorphic quartz with a particle size smaller than 10 μm [44], so this type of quartz may have originated from mineral-rich fluids and is considered to be of an epigenetic origin.
Pyrite primarily forms in four stages: the synsedimentary diagenetic stage, early diagenetic stage, late diagenetic stage and postdiagenetic stage [45]. Automorphic pyrite with a grain size of approximately 30 μm is mainly distributed in organic matter fractures and commonly coexists with calcite (Figure 7D). Framboidal pyrites consist of multiple smaller-sized automorphic pyrite grains aggregated into spherical clusters (Figure 7F) and form during the peat-accumulation stage. Automorphic pyrite and framboidal pyrites are considered to be synsedimentary in origin. Pyrite filling in cell cavities (Figure 7E) forms during the late stage of coal formation and precipitates from fluid rich in sulfur (S) elements [46,47], indicating localized strong reducing conditions.
In the samples from the Yihua coal mine, fine vein-like calcite fills epigenetic fractures (Figure 7C and Figure 8A). One type of calcite is distributed in quartz fractures, while another type is distributed in organic matter fractures. The vein-like calcite is considered to be of epigenetic origin and may have formed through the precipitation of Ca from organic matter and CO2 in pore water [48,49]. Its formation occurred later than the minerals and organic matter. Calcite filling in cell cavities (Figure 7E and Figure 8B) is considered to be authigenic in origin, with its outer edge being enveloped by pyrite (Figure 7E). The formation sequence is from early to late: organic matter, calcite and pyrite. Automorphic calcite with cleavage (Figure 8B) may have a terrestrial clastic origin. Automorphic calcite contains trace amounts of Mg, while the calcite filling in cell cavities does not contain Mg.
The kaolinite filling in the cell cavities of coal is formed through precipitation in a low-pH environment. Aluminum (Al) can be leached and transported from terrestrial clastic material to peat bogs and then precipitate, and this type of kaolinite is commonly found as fillings in cell cavities or pores [50]. This is common in many coals and their closely associated strata, indicating that they have undergone authigenic processes [51]. The cell-filling pattern of kaolinite suggests that it is not derived from terrestrial clastic material but formed from solutions or colloids. Kaolinite filling in fractures (Figure 7F and Figure 8C) indicates its epigenetic origin, formed through precipitation from epigenetic solutions. Kaolinite as a matrix with other minerals and as discrete grains (Figure 7A and Figure 8D) suggests its origin from terrestrial clastic material. The poor psephicity and growth edges suggest short-distance transport and that the deposition of terrestrial clastic material and epigenetic solutions deposited and grew on its surface. Kaolinite derived from terrestrial clastic material may contain trace amounts of Ti, while epigenetic kaolinite may contain trace amounts of Fe.
Anatase mainly occurs as automorphic grains and irregular grains distributed in clay minerals (Figure 7A and Figure 8D,E) and organic matter. It has a larger grain size, approximately 20 μm. The irregularly rounded anatase in authigenic quartz (Figure 7C) with grain sizes generally less than 10 μm suggests that it originated from the transport of terrestrial-sourced detritus.
Additionally, minerals such as barite (Figure 8F), gorceixite (Figure 8D), apatite, galena and sphalerite were observed in the samples, but their abundance is very low. Among these minerals, barite coexists with calcite in cell cavities, indicating an epigenetic origin. Other minerals are mainly irregularly distributed in terrestrial clastic material and matrices, suggesting that these minerals were transported from terrestrial clastic material.

4.3. Analysis of Major Element

The average values of the oxide content of major elements in samples are listed in Table 2. The study of the content and occurrence of major oxides (SiO2, Al2O3, TiO2, Fe2O3, CaO, MgO, K2O, Na2O, MnO and P2O5) in coal can reflect the coal-forming environment and various geological processes that the coal seam has undergone [48]. Compared to the average values of Chinese coals [16], the coal samples from the Yihua coal mine show an enrichment in CaO, MgO and Na2O and a loss in SiO2, Al2O3, TiO2, Fe2O3, K2O and P2O5, while the MnO content is close to the average values of Chinese coals.
Among the 12 coal samples from the coal mine, the major constant element oxides are CaO and MgO (Figure 9), accounting for 47.8% and 18.1%, respectively. They are followed by SiO2, Fe2O3, Na2O and Al2O3, while the relative contents of TiO2, K2O, MnO and P2O5 are all below 0.6%. In the five gangue samples, the main oxides are Fe2O3 and CaO, accounting for 30.85% and 65.36%, respectively, while the other element oxides account for only 0.019%–1.56%. The roof and floor samples are mainly composed of SiO2 and Al2O3, with a total proportion exceeding 92%.
The content of SiO2, Al2O3 and Fe2O3 in the coal ash of the Yihua coal is much lower than the average values of Chinese coals, which is consistent with the characteristic of Yihua coal having very low ash content. The content of Na is approximately 2.5 times higher than the average value of Chinese coals, and the content of CaO and MgO is also significantly higher than the average content of Chinese coals. Therefore, Yihua coal exhibits a pronounced alkali-rich characteristic. The alkalinity index, AI = Ad × [(Na2O + K2O + CaO+ MgO + Fe2O3)/(SiO2 + Al2O3)], of the ash composition can indicate the content of minerals, and the larger the alkalinity index, the less mineral content in the coal. The alkalinity index of the ash composition of the Yihua coal is 25.66, which indicates that the mineral content in the coal is less.
Upon comparing the Loy Yang coal from Australia with the Gascoigne Wood coal in the Yorks coal field from the UK, it is evident that the Yihua coal exhibits significantly lower levels of SiO2, Al2O3, TiO2, Fe2O3 and K2O in comparison to these two coals. Conversely, the Yihua coal displays notably higher levels of CaO. The contents of MgO and Na2O in the Yihua coal fall between the two, with the order being Loy Yang coal > Yihua coal > Yorks coal, from highest to lowest.
From the profile diagram (Figure 10), it can be observed that the ash yield in the coal is relatively low (3.09%–5.72%), and the content of major elements shows a small variation range. The SiO2, Al2O3 and TiO2 in the coal exhibit similar trends, with ranges of 0.173%–0.958%, 0.084%–0.721% and 0.016%–0.086%, respectively. The floor samples C1 and C2, as well as the roof samples C20 and C21, have higher SiO2, Al2O3 and TiO2 contents compared to the coal seam, which is due to the fact that the roof and floor are sandstones rich in quartz and kaolinite and contain Ti-bearing minerals. The SiO2 content is higher in the roof than in the floor, while the Al2O3 and TiO2 contents are lower in the roof compared to the floor, possibly due to the dominance of quartz in the roof and kaolinite in the floor. Fe2O3, CaO and MnO show similar trends, which are opposite to the trends of SiO2, Al2O3, TiO2, MgO and Na2O. In the gangue samples, Fe2O3, CaO and MnO have higher contents, which may be related to the presence of minerals such as pyrite and calcite in the gangue.
The element content in the overlying rock stratums of the coal seam is shown in Figure 11. The rock samples exhibit similar trends in the SiO2, Al2O3 and TiO2 content, as well as in the Fe2O3 and CaO content. MgO, K2O and Na2O show a decreasing trend with increasing depth in the vertical direction. In S2, the high content of Fe2O3 and MgO is mainly influenced by siderite, with Mg being present as an isomorphous substitution in the siderite. S8 contains calcite, leading to a higher CaO content compared to the upper and lower stratums. The Fe2O3 content in S10 is influenced by hematite. S12 shows significant variations in element content because it is a weathered layer with coal seams, containing minerals such as goethite and bassanite.

4.4. Distribution of Sodium

4.4.1. Distribution of Na in Coal Seams

The average Na2O content in the Yihua coal seam samples ranges from 0.1% to 1.07%, with an average value of 0.37%. The Na2O content in the coal ranges from 0.27% to 1.07%, with an average value of 0.49%. This indicates that the Na2O content in the coal is higher than that in the roof, floor and gangue. Overall, the Na2O content tends to decrease with increasing depth in the coal seam (Figure 12). The coal seam can be divided into five seams based on the gangue: Seam1 (C3–C6), Seam2 (C8 and C9), Seam3 (C11 and C12), Seam4 (C15 and C16) and Seam5 (C18 and C19). The Na2O content in Seam4 and Seam5 is generally higher than that in Seam1, Seam2 and Seam3, with the order from highest to lowest being Seam4 > Seam2 > Seam5 > Seam3 > Seam1 > gangue > roof > floor. Within the coal seam, the upper part also exhibits higher Na2O content than the lower part, indicating that sodium is more likely to accumulate in the upper part of the coal seam.
The sodium content in the coal seam may be influenced by the gangue. Sample C7 contains minerals such as pyrite, marcasite and gypsum. Samples C13 and C14 mainly contain minerals such as calcite, pyrite and marcasite. Samples C10 and C17 contain mainly calcite and kaolinite. Gangue containing pyrite and marcasite have higher water resistance compared to gangue containing calcite and kaolinite. Therefore, the sodium content is higher in the coal seam situated above the gangue containing pyrite and marcasite. Samples C9, C15 and C16 have higher sodium content, which may be related to the organic matter in the coal.

4.4.2. Distributional of Na from Epipedon to Coal Seams

The roof of the Yihua coal seam is composed of gray–white sandstone. The overlying rock stratums consist mainly of mudstone and sandstone, with localized occurrences of argillaceous siltstone. The upper portion reveals sedimentary cycles of sandstone–mudstone deposition. The Na2O content in the Yihua coal seam, overlying rock stratums and epipedon is presented in Table 3. The sodium content in the overlying rock stratums and epipedon is relatively high but exhibits significant fluctuations. There is a significant decrease in the sodium content from the epipedon to the coal seam, as depicted in Figure 13.

4.5. Occurrence of Sodium

4.5.1. Sequential Extraction Results

The selected 22 samples were processed by using a sequential extraction method, and the resulting four solutions were subjected to an ICP-OES analysis. The obtained results are illustrated in Figure 14. The results indicate that in the vertical distribution of coal samples from the Yihua coal seam, sodium is primarily present in the form of H2O-Na, with the following descending order: Seam4 > Seam2 > Seam5 > Seam3 > Seam1 > gangue > roof > floor. The sequence of the sodium content is the same as the total sodium content in coal, suggesting that sodium predominantly exists in a water-soluble state within the coal. Under the influence of groundwater migration, sodium from the roof, overlying rock stratums and epipedon infiltrates into the coal seam in various forms, such as sodium chloride solutions, sodium sulfate solutions and hydrated sodium ions, becoming the primary source of water-soluble sodium in the Yihua coal. NH4OAc-Na and HCl-Na are the result of long-term interactions between water-soluble sodium and organic matter in the coal seam. NH4OAc-Na belongs to the ion exchange form of sodium, which is connected to carboxyl and hydroxyl functional groups. The NH4OAc-Na content in the coal seam (average 6.96 mg/L) is higher than that in the roof, floor and gangue. HCl-Na exists in the form of a ligand on the oxygen-containing or nitrogen-containing functional groups, and the sodium content in different stratigraphic positions shows little variation. The HCl-Na content exhibits significant variations, with the Seam4 coal displaying a HCl-Na content of 7.23 mg/L. This signifies a substantially higher sodium content in Seam4 coal compared to other coal seams. Sodium is primarily found bound to organic functional groups in the coal structure, and the sodium on these functional groups cannot be replaced by ammonium acetate. The insoluble Na content in the roof and floor (5.42 mg/L and 3.87 mg/L, respectively) is significantly higher than in the coal seam and gangue.
In the Yihua coal, sodium is primarily present in the form of H2O-Na, accounting for over 70% of the total sodium content. In the gangue, H2O-Na accounts for more than 80% of the total sodium content, while the absolute content of NH4OAc-Na, HCl-Na and insoluble Na is relatively low. Therefore, the gangue exhibits a higher percentage of H2O-Na content. The insoluble Na content in the roof can reach 5.42 mg/L, which represents approximately 27% of the total sodium content.

4.5.2. Correlation Analysis

In the Yihua coal, there is a certain correlation between SiO2 and Al2O3 (Figure 15A), with a correlation coefficient (R2) of 0.44. The ratio of w(SiO2) to w(Al2O3) is approximately one, which is consistent with the results from the SEM-EDX analysis. The correlation coefficients between SiO2, CaO and the ash content all are 0.38, indicating a moderate correlation (Figure 15C,D). The correlation coefficients between SO2 and Fe2O3 and Fe2O3 and the ash content are 0.86 and 0.75, respectively (Figure 15E,F), indicating a strong correlation. This suggests that the main components in coal ash are SiO2, Al2O3, CaO and sulfides of Fe, and the main mineral components in the Yihua coal are kaolinite, calcite and sulfides of Fe. There is no significant correlation between the Na2O content and ash content in the coal (Figure 15B), indicating that minerals are not the primary carriers of sodium in the Yihua coal.

4.5.3. Relationship between Sodium and the Inertinite

The maximum reflectance of vitrinite (Ro,max) in the 12 coal samples ranges from 0.30 to 0.49, with an average value of 0.39, indicating that the coal samples belong to the low-rank coal. The inertinite group is the dominant maceral, with content ranging from 50.54% to 82.93% and an average content of 73.10%. The average value of the vitreous group stands at 23.32% (ranging from 14.94% to 43.40%), while the exinite group exhibits an average content of 0.86% (ranging from 0.35% to 1.21%). The average sodium content and inertinite content for each seam are listed in Table 4. The sodium content in Seam4 is the highest, reaching 0.92% with an inertinite content of 82.23% (Figure 16). In conjunction with the sequential extraction results, it can be concluded that the sodium content in Seam4 is influenced by the content of inertinite and sodium on organic functional groups that cannot be replaced by ammonium acetate. The presence of sodium bound to organic functional groups in these coal structures is believed to originate from coal-forming plants. In Seam2, the sodium content is 0.6%, and the inertinite content is 72.23%. Combined with a mineralogical analysis, the sodium content in sample C9 may be related to the presence of smectite in the coal besides the inertinite.

5. Discussion

Mineral and elemental anomalies in coal are influenced by various factors, including the lithology of the source rocks, influence of seawater, intrusion of hydrothermal fluids, groundwater movement and input of volcanic ash [16]. In the case of the Yihua coal samples, mineral and elemental anomalies are primarily influenced by the supply from the source area and groundwater.

5.1. Sources of Sediments

The Al2O3/TiO2 ratio serves as a significant indicator for the provenance of sedimentary rocks [52,53] and sediment associated with coal reserves [54]. This is primarily due to the similarity in the proportions between the sedimentary rocks and their parent rocks. A typical Al2O3/TiO2 ratio of 3–8, 8–21 and 21–70 represents the sediment derived from mafic, intermediate and felsic igneous rocks, respectively [52].
The Al2O3/TiO2 ratio of the coal seam, roof and gangue are all greater than 21 (Figure 17) in this study, except for some coal samples. This suggests that the sedimentary source area of the Jurassic coal-bearing strata in the Yihua coal mine is composed of felsic igneous rocks. On the other hand, the Al2O3/TiO2 ratio of the overlying rock stratums (14.1–23.6) indicates that the sedimentary source area of these rocks is primarily composed of intermediate-to-felsic igneous rocks. Therefore, the parent rock of the Xishanyao Formation in the Zhundong Coalfield is a medium acidic igneous rock.

5.2. Environment of Sedimentation

The sulfur content in coal can reflect the coal-forming environment. Coal formed in shallow lake environments typically has lower sulfur content, while coal formed with the involvement of seawater tends to have higher sulfur content. It is generally believed that the sulfur in low-sulfur coal (<1%) primarily originates from coal-forming plants [55]. The average total sulfur content of the Yihua coal samples is 0.49%. The characteristics of ultralow sulfur and ultralow ash indicate the predominance of shallow lake environments during coal formation, without significant influence from seawater. The mineral content in the coal is minimal, mainly consisting of detrital minerals transported from terrestrial sources during deposition and chemically precipitated minerals introduced into fractures by epigenetic solutions. The presence of pyrite in the gangue primarily indicates codeposition during the peat-accumulation period, suggesting a strong reducing environment. Reducing conditions are the main cause for the preservation of organic matter in coal [56]. The ash composition index [57] K = w(Fe2O3 + CaO + MgO)/w(SiO2 + Al2O3) can reflect the strength of reducing conditions during coal formation. A higher ash composition index indicates stronger reducing conditions, which are favorable for the preservation of organic matter [58]. The Yihua coal samples exhibit a relatively high ash composition index (Figure 18A), with an average K value of 6.4 (ranging from 2.6 to 16.4), indicating an overall strong reducing environment during coal formation. Furthermore, the reducing conditions intensify with the sedimentation of coal seams and increase in the depth of overlying water, reaching their peak during the formation of the C15 coal seam, possibly due to geological movements causing the subsidence of the strata.
According to the ash component analysis method [59], plotting the ash components SiO2-Al2O3, CaO-MgO and Fe2O3-SO3 as vertices on a triangular diagram (Figure 18B), it can be observed that the SiO2-Al2O3 content in the coal is relatively low, indicating a lower presence of silicates and aluminosilicate minerals. This suggests a stronger influence of chemical and biochemical sedimentation processes compared to mechanical transport sedimentation. The coal samples are closer to the CaO-MgO vertex, indicating the presence of minerals such as calcite, indicating active chemical precipitation and strong postdepositional processes. The Fe2O3-SO3 vertex is primarily influenced by calcium-rich seawater, leading to the formation of abundant pyrite in the coal seams. However, the Yihua coal samples are located far away from the Fe2O3-SO3 vertex, indicating a sedimentary environment characterized by shallow lake peat swamps that were not directly influenced by seawater.
During the Middle Jurassic, the Zhundong Coalfield exhibited a paleogeographic pattern of higher elevation in the northeast and lower in the southwest. Therefore, the sedimentary source was mainly provided by Cara Hill, located in the northeastern part of the Zhundong Coalfield. The Zhundong Coalfield developed plain swamps, delta swamps and shallow lake swamps from northeast to southwest. The sedimentary environment of the Xishanyao Formation, overall, belongs to a warm and humid shallow-lake peat-swamp environment. High-sodium coal in the Zhundong Coalfield is mainly distributed in the lower and middle parts of the Xishanyao Formation. The slow subsidence rate of the peat swamp and the stable sedimentary environment indicate a relatively low input of terrestrial detrital material during the coal-forming period [4].

5.3. Hydrological Conditions

The enrichment of elements in coal might result from many causes. Some researchers [1,60] suggest that the enrichment of sodium in coal seams is primarily influenced by groundwater and surface water. Ion concentrations in groundwater were simulated by using ion concentrations in the aqueous washing solution of coal samples. Ion concentrations in aqueous solutions determined using ICP-OES and ICS-Aquion are listed in Table 5. The main ions in the aqueous washing solution of the Yihua coal samples are Na+, Cl− and SO42−, which are consistent with the groundwater in the Zhundong Coalfield [1]. This further indicates that the Na in the roof and overlying rock strata of the Yihua coal seams is transported and deposited with groundwater, serving as the main source of Na in the coal seams. There is a strong correlation between Na+ and Cl− (R2 = 0.95) in the coal samples (Figure 19), suggesting that Na in the coal samples mainly exists in ionic or hydrated ionic with water, and it is closely related to Cl−.

5.4. Patterns of Sodium Transport and Enrichment

The study area is located in the southern part of Cara Hill, between the Shaqi Uplift and Cara Hill. It is influenced by the Indochinese and Yanshan movement, resulting in the development of a series of uplifts and localized fractures in this area [61]. The Zhangpenggou Anticline is spreading in an NE–SW direction, while the Cara piedmont fault consists of multiple parallel faults and is spreading in an E–W direction and has a compressive nature. As a result, the study area is subjected to geological stresses from both the east and the north, which can easily generate structural fractures.
Due to tectonic movements, in the Shazhang fault–fold belt, the Xishanyao Formation (J2x) of the Middle Jurassic has been uplifted and subsequently eroded, resulting in the preservation of only the lower part of the formation. These stratums of the formation mainly consist of sandstone, mudstone, siltstone and coal seams. The rocks in this formation have low water content and permeability and are weak aquifers with fissure pore space.
The main sources of groundwater recharge in the study area are atmospheric precipitation and surface water. The intense evaporation in the epipedon leads to higher salinity in the surface water. Surface water with sodium salts can recharge groundwater through structural fractures and rock pores. During the groundwater flow process, sodium salts in the stratums can be carried away. Feldspar minerals in the overlying water-bearing strata undergo weathering and eluviation, releasing soluble sodium ions into the groundwater. The above processes provide a rich source of Na+ for groundwater. The sodium of the weathered roof, overlying strata and epipedon are the main sources of sodium in the coal.
The Yihua coal is mainly composed of inertinite, which has more abundant cavities compared to vitrinite macerals. These cavities provide more pathways and space for the transportation and occurrence of sodium. Sodium-rich groundwater enters the coal seam with low metamorphism and high inertinite content through structural fractures. After a long period of geologic progress, sodium accumulates in the coal seam pores in ionic form, hydrated ionic and organic-matter-bound states (Figure 20).
In summary, the Jurassic coal seams in the Zhundong Coalfield were formed in an inland shallow lake environment, and the coal-formation period was not significantly influenced by sediment sources or seawater. Later, mineralized surface water brought Na+, Cl− and SO42− into the groundwater. The evaporation and slow alternation of groundwater led to a gradual increase in groundwater mineralization, resulting in high salinity Cl-type water that comes into the coal. The low degree of metamorphism, high porosity and high water absorption of the Zhundong coal can enrich the water-soluble sodium from groundwater in the form of ionic, hydrated ionic and organic-matter-bound states in the pores and fissures in the coal.

6. Conclusions

In order to investigate the distribution patterns, occurrence characteristics and enrichment causes of sodium, samples from the Yihua coal mine in the Zhundong Coalfield of Xinjiang were selected. Utilizing testing methods such as XRD, SEM-EDS, XRF and ICP-AES, in conjunction with a proximate and ultimate analysis, the genesis of high-sodium content in the Yihua coal of the Zhundong Coalfield was explored. The primary conclusions are as follows:
(1)
The Yihua coal exhibits relatively high sodium content, with sodium primarily distributed within the coal seams. In descending order, the distribution of sodium within the coal seam samples is as follows: Seam4 > Seam2 > Seam5 > Seam3 > Seam1 > parting > roof > floor. Overall, the sodium content tends to decrease with the increasing depth of the coal seams. Additionally, the coal seam samples exhibit a characteristic where the sodium content is higher in the upper part compared to the lower part of the same coal. In addition, the sodium content in the epipedon, overlying rock stratums and coal seams also showed a decreasing trend with depth.
(2)
The sequential extraction experiment revealed that H2O-Na constitutes the primary component of sodium in the Yihua coal, accounting for over 70% of its composition. NH4OAc-Na in the coal seams is significantly higher than in the roof, floor and gangue. Among the different coal seams, Seam4 exhibits higher HCl-Na content, which is influenced by the sodium on organic functional groups in the coal structure. Insoluble Na, on the other hand, is relatively higher in the roof and floor. Although the sodium content in some seams is affected by sodium on organic functional groups in the coal structure or in minerals, the dominant forms of sodium in coal are ionic and hydrated ionic states. The primary mineral content in coal is minimal, mainly consisting of postgenetic minerals such as kaolinite, calcite and gypsum. Therefore, minerals are not the primary carriers of sodium in coal.
(3)
The coal was deposited in a warm, humid, shallow-lake peat-swamp environment. There was no significant influence from terrestrial or marine sources. The enrichment of sodium is primarily influenced by hydrodynamic conditions. The sodium generated from the weathering of the epipedon and overlying rock strata is dissolved by atmospheric precipitation and surface water, entering groundwater through structural and weathering fissures, as well as rock pores. Sodium-rich groundwater is transported downward and enters the coal seam, where, over the course of lengthy geological processes, sodium accumulates in the form of ions, hydrated ions and organic-matter-bound states within the coal seam pores.

Author Contributions

Conceptualization, Y.W.; methodology, Y.W.; validation, W.W. (Wenfeng Wang) and P.D.; formal Analysis, Y.W.; investigation, W.W. (Wenlong Wang); resources, W.W. (Wenfeng Wang); data Curation, W.W. (Wenfeng Wang) and Q.L.; writing—original draft preparation, Y.W.; writing—review and editing, W.W. (Wenfeng Wang) and X.H.; visualization, Y.W. and P.D.; supervision, W.W. (Wenfeng Wang); project administration, W.W. (Wenfeng Wang); funding acquisition, W.W. (Wenfeng Wang). All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (no. U1903207 and no. 41972176), the Third Xinjiang Scientific Expedition Program (no. 2022xjkk1003), the Major Science and Technology Special Project of Xinjiang Uygur Autonomous Region (no.2022A03014) and the Major Science and Technology Special Project of Xinjiang Uygur Autonomous Region (no.2022A01002).

Data Availability Statement

The original contributions presented in the study are included in the article material, further inquiries can be directed to the corresponding author.

Acknowledgments

We are very grateful to all the editors and reviewers who helped us improve and publish this paper.

Conflicts of Interest

The authors declare no conflicts of interest.

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Minerals 14 00146 g001
Figure 1. Location of the Yihua coal mine in Zhundong Coalfield and its stratigraphic column and sampling profile.
Figure 1. Location of the Yihua coal mine in Zhundong Coalfield and its stratigraphic column and sampling profile.
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Minerals 14 00146 g002
Figure 2. Process of the sequential extraction experiment.
Figure 2. Process of the sequential extraction experiment.
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Minerals 14 00146 g003
Figure 3. XRD patterns of Yihua coal samples C5, C9 and C16 (ash basis/%, K—kaolinite: 29-1488.pdf-Kaolinite-1Md, 14-0164.pdf-Kaolinite-1A; Q—quartz: 46-1045.pdf; P—pyrite: 42-1340.pdf; C—calcite: 47-1743.pdf; I—illite: 43-0685.pdf; S—smectite: 29-1499.pdf; M—marcasite: 21-1276.pdf; Bs—bassanite: 41-0224.pdf).
Figure 3. XRD patterns of Yihua coal samples C5, C9 and C16 (ash basis/%, K—kaolinite: 29-1488.pdf-Kaolinite-1Md, 14-0164.pdf-Kaolinite-1A; Q—quartz: 46-1045.pdf; P—pyrite: 42-1340.pdf; C—calcite: 47-1743.pdf; I—illite: 43-0685.pdf; S—smectite: 29-1499.pdf; M—marcasite: 21-1276.pdf; Bs—bassanite: 41-0224.pdf).
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Minerals 14 00146 g004
Figure 4. XRD patterns of the floor sample C1, roof sample C21 and through-seam fissures sample C22 of the Yihua coal seam (K—kaolinite: 29-1488.pdf-Kaolinite-1Md, 14-0164.pdf-Kaolinite-1A; Q—quartz: 46-1045.pdf; P—pyrite: 42-1340.pdf; A—anatase: 21-1272.pdf; C—calcite: 47-1743.pdf; R—rutile: 21-1276.pdf).
Figure 4. XRD patterns of the floor sample C1, roof sample C21 and through-seam fissures sample C22 of the Yihua coal seam (K—kaolinite: 29-1488.pdf-Kaolinite-1Md, 14-0164.pdf-Kaolinite-1A; Q—quartz: 46-1045.pdf; P—pyrite: 42-1340.pdf; A—anatase: 21-1272.pdf; C—calcite: 47-1743.pdf; R—rutile: 21-1276.pdf).
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Minerals 14 00146 g005
Figure 5. XRD patterns of gangue samples C7, C10, C13, C14 and C17 of the Yihua coal seam (K—kaolinite: 29-1488.pdf-Kaolinite-1Md, 14-0164.pdf-Kaolinite-1A; Q—quartz: 46-1045.pdf; P—pyrite: 42-1340.pdf; A—anatase: 21-1272.pdf; C—calcite: 47-1743.pdf; I—illite: 43-0685.pdf; R—rutile: 21-1276.pdf; Gy—gypsum: 21-0816.pdf; M—marcasite: 21-1276.pdf; Bs—bassanite: 41-0224.pdf; Gi—gibbsite: 33-0018.pdf).
Figure 5. XRD patterns of gangue samples C7, C10, C13, C14 and C17 of the Yihua coal seam (K—kaolinite: 29-1488.pdf-Kaolinite-1Md, 14-0164.pdf-Kaolinite-1A; Q—quartz: 46-1045.pdf; P—pyrite: 42-1340.pdf; A—anatase: 21-1272.pdf; C—calcite: 47-1743.pdf; I—illite: 43-0685.pdf; R—rutile: 21-1276.pdf; Gy—gypsum: 21-0816.pdf; M—marcasite: 21-1276.pdf; Bs—bassanite: 41-0224.pdf; Gi—gibbsite: 33-0018.pdf).
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Minerals 14 00146 g006
Figure 6. XRD patterns of the overlying rock samples S2, S4, S8, S10, S12 and S14 of the Yihua coal seam (K—kaolinite: 14-0164.pdf-Kaolinite-1A; Q—quartz: 46-1045.pdf; P—pyrite: 42-1340.pdf; A—anatase: 21-1272.pdf; C—calcite: 47-1743.pdf; Al—albite: 09-0466.pdf; An—anorthite: 09-0465.pdf; Go—goethite: 29-0713.pdf; Bs—bassanite: 41-0224.pdf; H—hematite: 33-0664.pdf; Si—siderite: 29-0696.pdf).
Figure 6. XRD patterns of the overlying rock samples S2, S4, S8, S10, S12 and S14 of the Yihua coal seam (K—kaolinite: 14-0164.pdf-Kaolinite-1A; Q—quartz: 46-1045.pdf; P—pyrite: 42-1340.pdf; A—anatase: 21-1272.pdf; C—calcite: 47-1743.pdf; Al—albite: 09-0466.pdf; An—anorthite: 09-0465.pdf; Go—goethite: 29-0713.pdf; Bs—bassanite: 41-0224.pdf; H—hematite: 33-0664.pdf; Si—siderite: 29-0696.pdf).
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Minerals 14 00146 g007
Figure 7. Minerals in Yihua coal mine samples ((A): quartz, kaolinite and anatase in sample C21; (B): quartz in sample C22; (C): quartz and calcite in sample C22; (D): pyrite and calcite in sample C14; (E): pyrite and calcite in sample C17; (F): pyrite and kaolinite in sample C21.
Figure 7. Minerals in Yihua coal mine samples ((A): quartz, kaolinite and anatase in sample C21; (B): quartz in sample C22; (C): quartz and calcite in sample C22; (D): pyrite and calcite in sample C14; (E): pyrite and calcite in sample C17; (F): pyrite and kaolinite in sample C21.
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Minerals 14 00146 g008
Figure 8. Minerals in Yihua coal mine samples (A): calcite in sample C17; (B): calcite in sample C10; (C): kaolinite in sample C16; (D): kaolinite, anatase and gorceixite in sample C1; (E): quartz, kaolinite and anatase in sample C21; (F): barite and calcite in sample C14).
Figure 8. Minerals in Yihua coal mine samples (A): calcite in sample C17; (B): calcite in sample C10; (C): kaolinite in sample C16; (D): kaolinite, anatase and gorceixite in sample C1; (E): quartz, kaolinite and anatase in sample C21; (F): barite and calcite in sample C14).
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Minerals 14 00146 g009
Figure 9. Relative content of oxides content of major elemental in Yihua coal mine samples.
Figure 9. Relative content of oxides content of major elemental in Yihua coal mine samples.
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Minerals 14 00146 g010
Figure 10. Profile of oxides content of major elements in coal seam samples from Yihua coal mine (ash basis, %).
Figure 10. Profile of oxides content of major elements in coal seam samples from Yihua coal mine (ash basis, %).
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Minerals 14 00146 g011
Figure 11. Oxides content of major elements in samples from the overlying rock strata of the Yihua coal mine.
Figure 11. Oxides content of major elements in samples from the overlying rock strata of the Yihua coal mine.
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Minerals 14 00146 g012
Figure 12. The vertical distribution of Na2O in Yihua coal seam.
Figure 12. The vertical distribution of Na2O in Yihua coal seam.
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Minerals 14 00146 g013
Figure 13. Distribution of sodium in epipedon, overlying rock and coal seams.
Figure 13. Distribution of sodium in epipedon, overlying rock and coal seams.
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Minerals 14 00146 g014
Figure 14. Occurrence of the Na in the samples ((A): content of H2O-Na; (B): content of NH4OAc-Na; (C): content of HCl-Na; (D): content of insoluble Na; (E): content of total Na; (F): percentage of sodium in the four occurrence forms).
Figure 14. Occurrence of the Na in the samples ((A): content of H2O-Na; (B): content of NH4OAc-Na; (C): content of HCl-Na; (D): content of insoluble Na; (E): content of total Na; (F): percentage of sodium in the four occurrence forms).
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Minerals 14 00146 g015
Figure 15. Correlation of components in coal seam samples. (A) Correlation of SiO2 and Al2O3; (B) correlation of Na2O and ash yield; (C) correlation of SiO2 and ash yield; (D) correlation of CaO and ash yield; (E) correlation of SO2 and Fe2O3; (F) correlation of Fe2O3 and ash yield.
Figure 15. Correlation of components in coal seam samples. (A) Correlation of SiO2 and Al2O3; (B) correlation of Na2O and ash yield; (C) correlation of SiO2 and ash yield; (D) correlation of CaO and ash yield; (E) correlation of SO2 and Fe2O3; (F) correlation of Fe2O3 and ash yield.
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Minerals 14 00146 g016
Figure 16. Biaxial diagram of Na2O content and inertinite content.
Figure 16. Biaxial diagram of Na2O content and inertinite content.
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Minerals 14 00146 g017
Figure 17. Al2O3/TiO2 ratio of the samples.
Figure 17. Al2O3/TiO2 ratio of the samples.
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Minerals 14 00146 g018
Figure 18. Ash composition index (A) and triangular diagram of analysis method based on ash composition (B) in coal.
Figure 18. Ash composition index (A) and triangular diagram of analysis method based on ash composition (B) in coal.
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Minerals 14 00146 g019
Figure 19. Correlation of Na+ and Cl−.
Figure 19. Correlation of Na+ and Cl−.
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Minerals 14 00146 g020
Figure 20. Transport and enrichment pattern of sodium (F: geological stress).
Figure 20. Transport and enrichment pattern of sodium (F: geological stress).
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Table 1. Proximate and ultimate analysis, total sulfur of samples (wt.%).
Table 1. Proximate and ultimate analysis, total sulfur of samples (wt.%).
Proximate AnalysisSt,dUltimate Analysis
MadAdVdafFCdOdafCdafHdafNdaf
Max16.267.9237.9668.921.8020.2679.593.930.90
Min9.282.9328.5459.610.0915.2975.802.760.56
Average12.644.5131.5165.400.4917.6777.833.290.70
O, oxygen; C, carbon; H, hydrogen; N, nitrogen; St, total sulfur; M, moisture; A, ash yield; V, volatile matter; FC, fixed carbon; daf, dry and ash-free basis; ad, air-dry basis; d, dry basis; FCd = 100 − Ad − Vd.
Table 2. Main components of Yihua coal (wt%).
Table 2. Main components of Yihua coal (wt%).
SamplesSiO2Al2O3TiO2Fe2O3CaOMgOK2ONa2OMnOP2O5
Coal-av0.440.240.030.422.200.830.020.400.010.02
parting-av0.260.180.0110.8723.020.550.010.150.160.01
Roof64.0014.560.670.050.310.371.280.190.010.04
Floor34.2023.622.640.270.630.310.360.180.010.03
China 18.475.980.334.851.230.220.190.160.020.09
Loy Yang 25.415.150.310.790.181.100.080.89n.d0.04
Yorks 310.554.810.182.770.420.370.720.300.05n.d
1, average concentrations of elements in common Chinese coals [16]; 2, calculated from the results of Sripada et al. [11]; 3, calculated from the results of Spears et al. [13]; n.d, no data.
Table 3. The content of Na2O in different samples (wt.%).
Table 3. The content of Na2O in different samples (wt.%).
SamplesNa2O (wt.%)
MaxMinAverage
Epipedon3.672.052.96
Overlying rock2.970.281.45
Coal1.070.270.49
Table 4. Na2O content (wt.%) and inertinite, vitrinite and exinite (%) in the different coal seams.
Table 4. Na2O content (wt.%) and inertinite, vitrinite and exinite (%) in the different coal seams.
SamplesSeam1Seam2Seam3Seam4Seam5
Na2O (wt.%)0.310.600.360.920.44
Inertinite (%)66.7472.2375.1082.2375.55
Vitrinite (%)28.6923.6022.5015.5220.95
Exinite (%)0.960.920.801.050.50
Table 5. Concentration of major ions in samples (mg/L).
Table 5. Concentration of major ions in samples (mg/L).
SamplesCa2+K+Mg2+Na+Cl−SO42−
Seam54.45n.d4.69526.556.56542.125
Seam43.040.3552.7756.651.0531.65
Seam32.24n.d2.49522.458.12529.8
Seam22.64n.d1.97532.6525.3128
Seam113.480.374.372518.2256.7625110.925
Zhundong 14.50.41.166.144.322.9
1, data from Bai et al., 2015 [1].
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Wang, Y.; Wang, W.; Wang, W.; Duan, P.; He, X.; Lu, Q. Distribution, Occurrence and Enrichment Causes of Sodium in Middle Jurassic Coal from Zhundong Coalfield, Xinjiang. Minerals 2024, 14, 146. https://doi.org/10.3390/min14020146

AMA Style

Wang Y, Wang W, Wang W, Duan P, He X, Lu Q. Distribution, Occurrence and Enrichment Causes of Sodium in Middle Jurassic Coal from Zhundong Coalfield, Xinjiang. Minerals. 2024; 14(2):146. https://doi.org/10.3390/min14020146

Chicago/Turabian Style

Wang, Yulong, Wenfeng Wang, Wenlong Wang, Piaopiao Duan, Xin He, and Qingfeng Lu. 2024. "Distribution, Occurrence and Enrichment Causes of Sodium in Middle Jurassic Coal from Zhundong Coalfield, Xinjiang" Minerals 14, no. 2: 146. https://doi.org/10.3390/min14020146

APA Style

Wang, Y., Wang, W., Wang, W., Duan, P., He, X., & Lu, Q. (2024). Distribution, Occurrence and Enrichment Causes of Sodium in Middle Jurassic Coal from Zhundong Coalfield, Xinjiang. Minerals, 14(2), 146. https://doi.org/10.3390/min14020146

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