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Article

Occurrence Mode of Sodium in Zhundong Coal, China: Relationship to Maceral Groups

by
Xin He
1,
Kexin Che
2,
Jinhe Pan
1,
Hao Sun
1,
Changchun Zhou
1 and
Wenfeng Wang
2,3,*
1
Key Laboratory of Coal Processing & Efficient Utilization, Ministry of Education, School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, 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
*
Author to whom correspondence should be addressed.
Minerals 2023, 13(9), 1155; https://doi.org/10.3390/min13091155
Submission received: 4 April 2023 / Revised: 25 July 2023 / Accepted: 14 August 2023 / Published: 31 August 2023
(This article belongs to the Section Mineral Deposits)
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Versions Notes

Abstract

:
The occurrence and separation relationships of Na and maceral groups in Zhundong coals were investigated in this study. The sequential extraction results indicate that the total Na content of all samples decreased with increasing sampling depth, and the level in inertinite-enriched samples (inertinite content 81.0%–84.0%) was significantly higher than that in corresponding raw coals and vitrinite-enriched samples (vitrinite content 90%). Additionally, H2O-Na (soluble salt species) and insoluble Na (acid-insoluble residues) were found to be concentrated in the inertinite-enriched samples. In combined SEM–EDS and microscope observations, local Na enrichments were detected in all raw coal and inertinite-enriched samples except for the vitrinite-enriched samples, but only inertinite-enriched samples were found to generally have over 10 wt% of Na enrichments, all of which occurred as NaCl. Moreover, Na is mostly filled or associated with cell-filling minerals in cells of fusinite. The maceral separation and Na removal of Zhundong coal were simultaneously achieved using triboelectrostatic separation. The vitrinite content in concentrates increased up to 60%, along with a reduced Na level, while the inertinite and sodium levels were both evidently raised in tailings. The obvious positive occurrence and separation correlation between sodium and inertinite offers new insight into, and a technical reference for, the sodium removal and maceral processing of Zhundong coal.

1. Introduction

The Zhundong coalfield, with the 39 Gt reserve, has become an essential coal base as the energy center of China strategically moves west [1,2]. Zhundong coal has proven to be of high overall quality due to its low ash yield and high volatile matter and reactivity [3,4,5]. With its satisfying quality indexes, Zhundong coal is not only a suitable feed for thermal power generation, but also a critical raw material for clean coal deep-processing technologies such as gasification and liquefaction. However, severe fouling and slagging issues have frequently been reported for Zhundong coal in boilers during combustion and gasification [6,7,8]. Zhundong coal is known as being abundant with typical alkali metals (Na, K) and alkali earth metals (Ca, Mg). Of these elements, Na is believed to be the leading cause of fouling and slagging problems. Numerous reports [9,10,11] have described the behavior of Na during the early pyrolysis stage of the combustion, gasification, and liquefaction of Zhundong coal, and water-soluble Na is regarded as the dominant type that causes slagging [11,12]. Water-soluble Na is readily volatilized and forms clusters of NaCl or Na2SO4 during combustion [13]; these are captured by SiO2 and Al2O3 in the bottom ash [14,15], leading to ash and causing issues [16]. To solve the slagging problem, it is crucial to explore the occurrence characteristics of different Na species, particularly water-soluble Na. Techniques including hydrothermal treatment [17,18], dynamic leaching [5], and ion-exchange methods [19,20] have been adopted to remove Na from Zhundong coal at the laboratory scale, but actual production is limited by poor efficiency and high costs. Meanwhile, the utilization of Zhundong coal has also been under pressure from emerging coal-staged utilization industries such as liquefaction [21] and gasification [22], which creates an increased demand for the maceral separation and individual processing of coal. For instance, it is widely recognized that liptinite and vitrinite are active components in coal liquefaction, while inertinite is regarded as an inert component. A previous study reported [23,24] that a rise in the inertinite content of feed significantly reduces the oil yield and conversion rate during coal liquefaction, and that the conversion continues to deteriorate when the inertinite level increases to over 60%. In view of this, the simultaneous realization of macerals separation and Na removal is highly important for the utilization of Zhundong coal. Notably, while numerous studies focus on the occurrence of Na in Zhundong coal, the authors of the present work found a rather limited number of published studies on the occurrence relationship between Na and maceral groups (vitrinite and inertinite); likewise, little information is available regarding the relevant separation methods. Lin et al. reported [25] that the inertinite-enriched fraction has a similar water-soluble Na level to raw coal, whereas the value in the vitrinite-enriched fraction was significantly decreased. This implies a possible occurrence relationship between Na and maceral groups in Zhundong coal. Despite the potential for development, the currently available relevant studies are markedly insufficient, and the relationship between maceral groups and other occurrence modes of Na is still unclear.
To this end, the work herein focuses on the occurrence and separation relationship of Na and typical maceral groups (vitrinite and inertinite) in high alkali coal sampled from the Yihua Coal Mine of the Zhundong coalfield, located in Xinjiang. The objectives of this research were as follows: (a) to investigate the distribution characteristics of Na in coal seams by determining different occurrence modes of Na in raw coals from various sampling depths; (b) to identify the occurrence status of Na in relationship to the organic matter; and (c) to evaluate the Na removal and maceral separation performance of Zhundong raw coals by employing triboelectrostatic separation.

2. Geological Setting

The Zhundong coalfield, Junggar Basin (Figure 1), is located in Jimusaer County, Changji Hui Aotonomous Prefecture, Xinjiang Province, Northwest China [26,27]. It includes the Upper Permian Xiachangfanggou group (P2cha), the Middle-Upper Triassic Xiaoquangou group (T2-3xq), the Lower Jurassic Badaowan (J1b) and Sangonghe formations (J1s), the Middle Jurassic Xishanyao formation (J2x), the Middle-Upper Jurassic Shishugou group (J2-3sh), and the Pliocene Dushanzi formation (N2d). As the major workable coal seam of this basin, the Middle Jurassic Xishanyao formation (J2x) contains the upper and lower members [28,29]. In this study, the sample section (Yihua Coal Mine) lies in the lower members, which is mainly composed of sandstone, siltstone, silty mudstone, mudstone, and only one extremely thick coal seam (40–87 m) [30].

3. Experimental Section

3.1. Sample Preparation

A total of 69 layered samples (including 2 roofs, 2 floors, 6 rock partings, and 59 coal layers) were collected from the thick coal seam (~60 m) in the Yihua Mine. In total, five raw coals (thickness ~0.8 m) with different sampling depths (sample 63, depth 7 m; sample 41, depth 20 m; sample 31, depth 32 m; sample 21, depth 41 m; and sample 11, depth 51 m) were selected and named YH1, YH2, YH3, YH4, and YH5, respectively. The corresponding vitrinite-enriched and inertinite-enriched factions of the five raw coals were also extracted manually; vitrain and fusain were separately set as the preliminary enrichment targets for vitrinite and inertinite. The final high-purity vitrinite and inertinite samples were obtained by crushing and sorting vitrain and fusain particles, respectively. The above operation was described in detail in our previous study [31] and the whole procedure was chemical-free. Liptinite was excluded from this investigation considering its severe scarcity (<3%) in Zhundong coal [32,33,34]. Moreover, owing to the varied characteristics of coal deposits, it was difficult to find both vitrain and fusain in each raw coal. In this study, fusain was found in all five raw coals, while vitrain was only collected in YH5. The collected maceral-enriched samples were named YH1I, YH2I, YH3I, YH4I, YH5V, and YH5I.

3.2. Sequential Extraction

Sequential extraction is a procedure commonly used for the determination of sodium content in coal [35,36]. A four-step sequential extraction is undertaken in most existing studies [8,37,38,39,40]. This technique employs deionized water, ammonium acetate (NH4OAc), hydrochloric acid (HCl), and acids for digestion (e.g., HNO3 and HF) as four sequential solutions [41], and the extracted Na can be categorized as ‘‘H2O-Na”, ‘‘NH4OAc-Na”, ‘‘HCl-Na”, and ‘‘insoluble-Na” in sequence [42]. To avoid the potential error of some organically associated Na being mistakenly classified as ‘‘HCl-Na” [43,44,45,46], a new five-step sequential extraction procedure was used in this study. Deionized water, 0.1 mol/L NH4Cl solution (buffered to pH 8.5 with NH3·H2O), 0.1 mol/L NH4-EDTA solution (buffered to pH 9 with NH3·H2O), 1 mol/L HCl, and mixed acids (HCl, HNO3, and HClO4) were set as solutions in five sequential steps. According to the extraction steps, Na in coal can be classified into five types, i.e., ‘‘H2O-Na”, ‘‘NH4Cl-Na”, ‘‘HCl-Na”, ‘‘NH4-EDTA-Na”, and ‘‘insoluble-Na”, which refer to inorganic sodium salts, Na associated with carboxyl, Na occurring in chelate complexes or attached to functional groups other than carboxylic groups, Na-containing acid-soluble minerals, and Na-containing acid-insoluble residues, respectively [47]. The complete procedure and relevant operational parameters are shown in Figure 2, and the approach was thoroughly described in our previous study [48].
All suspensions collected from steps one to four and the digestion in step five were tested using an inductively coupled plasma optical emission spectrometer (ICP-OES, ICAP 6200, Thermo Fisher, Waltham, MA, USA) for Na determination. In addition, another 0.8 g sample was sent for direct digestion as a comparison for the total Na content. The mean of the three repeated tests was taken as the final result for accuracy.

3.3. Sample Characterization

Proximate analyses (moisture, ash, and volatile matter yields) and ultimate analyses (contents of carbon, hydrogen, oxygen, nitrogen, and total sulfur) were performed on 11 samples (raw coal and corresponding maceral-enriched samples) according to Chinese national standards GB/T212-2008, GB/T19227-2008, GB/T476-2008, and GB/T214-2007.
An X-ray fluorescence spectrometer (XRF, S8 Tiger, Bruker, Germany) and an inductively coupled plasma mass spectrometer (ICP-MS, iCAP RQ, Thermo Scientific, Bremen, Germany) were used for the oxide and element analyses of the samples. A scanning electron microscope (SIGMA, Carl Zeiss, Taufkirchen, Germany) coupled with energy dispersive X-ray spectroscopy (EDS), and an optical microscope (Imager M1m, ZEISS, Pittsburgh, PA, USA) in conjunction with a microphotometer (MPV-SP, CRAIC Technologies, Inc., San Dimas, CA, USA), were separately employed for the observation of sodium-containing particles and the maceral identification of collected samples based on the ICCP System 1994 [49,50,51,52]. It should be noted that the kerosene was used as a lubricating agent to avoid the potential influence brought by water given the water-soluble Na was reported as the dominant Na type.

3.4. Triboelectrostatic Separation of Zhundong Coal

A laboratory-scale rotary triboelectrostatic separation system (Figure 3) was used in this investigation; it mainly comprised a rotating tribo-charger, a free-fall separator, a high-voltage power supply, and a centrifugal fan. During separation, coal particles under 74 μm in size are transported by air sourced from a centrifugal fan and charged in a rotating tribo-charger. Vitrinite and inertinite are positively and negatively charged, respectively, due to their different surface properties. Charged particles then enter the separator, and vitrinite and inertinite are attracted to the negative and positive electrodes, respectively. Consequently, concentrates enriched with vitrinite and tailings enriched with inertinite are collected from the negative and positive poles, respectively. The products of all five raw coals were tested by microscopy and sequential extraction to assess the effects of maceral separation and sodium removal.

4. Results and Discussion

4.1. Assessment of Coal Quality

Proximate analyses, and ultimate analyses results of all 11 samples, are shown in Table 1. All five raw coal samples were below the 0.50% level in total sulfur. The inertinite-enriched samples have higher C levels than the corresponding raw coals, whereas the vitrinite-enriched samples exhibited an inverse pattern. The O distribution of the samples is the converse of that observed for C. There was almost no difference in H contents between the raw coal and the inertinite-enriched sample, but the vitrinite-enriched sample had a significantly higher level of H than the former two. In the proximate analyses, both raw coal and the inertinite-enriched samples were characterized by poor ash contents (<5%) and elevated fixed carbon content (>55%), which is consistent with their high C and low sulfur levels. In addition, the high moisture and volatile contents of all samples indicated the low coalification degree of the Zhundong coal.
The oxides and elements detected in the five raw coals are summarized in Table 2. The results showed that, in addition to Na, the contents of Ca- and Mg-bearing oxides were also higher than the average levels. However, the K-bearing oxides had a lower value than the mean. Moreover, the levels of common ash-forming oxides such as SiO2, Al2O3, and Fe2O3 were clearly below the average for Chinese coals, which is in keeping with the low ash yield of the Zhundong samples. The previous literature [5] notes a positive correlation between Na and ash yields in Zhundong samples for ash yields below 10 wt%. This observation was also made in the present study based on the proximate analyses and XRF results. The ash yield of the five raw coal samples followed this order: YH4 > YH1 > YH3 > YH2 > YH5. This was similar to the order of the Na-containing oxide level in the XRF test: YH4 > YH3 > YH1 > YH2 > YH5. A reasonable explanation was offered previously [44,45,46,47,48,49,50,51,52,53,54,55]: the ash-forming substances in Zhundong coal with low ash content are mainly from non-mineral inorganics (organically associated Na and soluble salts), which leads to the development of a close relationship between Na and ash yields. In addition, there are very few trace elements in the five raw coals. REY, Li, Ga, etc., have received a great deal of attention; they occur at extremely low levels, distinctly below the average levels found in Chinese coal. Only YH2 and YH4 have obviously high Cr contents, a finding which should be investigated in the future.
Table 3 summarizes the maceral composition and vitrinite random reflectance results of the samples. Other than YH5, the raw coals are typical inertinite-abundant samples (>55%). Six extracted maceral-enriched samples were adequately represented, with purities exceeding 80%. Telinite and collodetrinite occupied the first and second positions, respectively, in the vitrinite group of YH1–YH4 and their corresponding maceral-enriched samples. However, this portion order was inverted in YH5 and its maceral samples, especially YH5V, with the highest vitrinite group content (90%), and collodetrinite occurred as the dominant portion (76.5%). All raw coals and their derived inertinite-enriched samples are dominated by fusinite and semifusinite. Additionally, all samples are characterized by a tiny amount of minerals.

4.2. Sequential Extraction Results for Sodium

The contents of different sodium species in samples, as determined by the above five-step sequential extraction methods, are presented in Figure 4. It is evident that the total sodium in raw coal samples decreases with increasing depth, and the values in inertinite-enriched samples also follow the same pattern. The results are largely related to the mode of occurrence of Na in the studied samples. The Na in Zhundong coal mainly originated from the percolation of highly salinized surface water [5]. The upper part of the coal seam easily reserves more Na due to its preferential access to underground water. The total Na levels in all extracted inertinite-enriched samples are higher than those in the raw coals from which they are derived, while the opposite happens to YH5 and its extracted vitrinite-enriched sample. This might be caused by the porous structure of inertinite, which is more receptive to Na-containing substances than vitrinite. The distribution of the five occurrence modes of Na is similar for all samples: H2O-Na and NH4Cl-Na take the first and second places with ~50% and 10%–20% proportions, respectively, whereas the other three Na modes have a share of approximately 10% each, without a significant difference.
However, the changing trends are diverse in terms of different Na occurrence modes. The content and percentage of H2O-Na decrease with increasing depth in all the raw coal samples except YH1, and the same rule holds for all five extracted inertinite-enriched samples. The vitrinite-enriched sample has the lowest H2O-Na content, which may be caused by the presence of fewer pores in vitrinite. It is fairly clear that H2O-Na (soluble Na salts) gradually infiltrates along coal layers, and YH1 is meant to have the highest H2O-Na content but does not in this case. Sample bias could be one possible cause for this phenomenon. The NH4Cl-Na content gradually decreases with increasing depth in raw coals, a trend that also applies to the inertinite-enriched samples despite the fluctuations. Like the vitrinite-enriched and inertinite-enriched samples originating from YH5, the vitrinite-enriched sample has higher NH4Cl-Na content than the inertinite-enriched sample, which may be caused by the fact that the higher O content in vitrinite means it has more carboxyl groups to associate with Na. In terms of the contents of HCl-Na, NH4-EDTA-Na, and insoluble Na, there are no obvious rules in either raw coals or maceral samples due to the observational difficulties created by the low ratios of these three Na occurrence modes. However, it is worth mentioning that the contents of NH4-EDTA-Na and HCl-Na in YH5V are higher than those in YH5I, but the reverse occurs for H2O-Na and insoluble Na. The most likely reason for this, as mentioned above, is that the higher O content in vitrinite allows for the presence of additional oxygen-containing groups and soluble minerals, which provides better formation conditions for NH4-EDTA-Na and HCl-Na; meanwhile, as it has more pores, inertinite provides more formation space for H2O-Na and insoluble Na.

4.3. Sodium Occurrence Characteristics in Macerals

4.3.1. Sodium Occurrence Characteristics of YH1 and YH1I

Although the contents of different Na species can be measured by sequential extraction, the observation of the specific occurrence status of sodium in Zhundong coal has always been challenging. In this study, all samples were first prepared as polished sections and successively subjected to SEM-EDS and microscopy tests for elementary analyses and maceral identification.
The observations of YH1 and YH1I are shown in Figure 5 and Figure 6, respectively. The Na-containing particle found in YH1 is a typical fusinite grain, one cell of which is filled by an irregularly shaped white crystal. Two spots on the crystal were selected and analyzed by EDS, and a certain amount of Na enrichment (3%–3.5%) was detected on both of them. The elementary composition shows that C and O are the major components, which should be attributed to the coal matrix beneath the surface, and the atomic ratio of Na, Al, and Si is approximately 1:1:3, which indicates that this crystal is mainly albite (NaAlSi3O8). Moreover, small quantities of K and Ca are also found, which appear as isomorphism hosting for Na and Al, respectively, in the lattice of albite. In addition to this albite crystal, which should be ascribed to insoluble Na, no other Na-containing particles were found in YH1. Typical H2O-Na was detected in a cell of a fusinite particle of YH1I. According to the Na, Cl, Ca, and Mg EDS mapping (Figure 6D) of the related area, a cluster of halite (NaCl) is embedded in the cell, and the two larger clusters exhibit a clear cubic structure. Spots 3 and 4 are separately sampled on these two cubes, and the EDS spectra demonstrate evident Na aggregation (over 15 wt%) in each cube. The elementary composition of the two spots is also similar, with basic C and O as basement, and Na and Cl holding the rest in a ratio of approximately 1:1. Due to the slightly higher atomic content of Na, there may be a tiny amount of Na2CO3. Furthermore, some dolomite (CaMg[CO3]2) may also exist as an associated mineral considering that Ca and Mg are also present at similar levels.

4.3.2. Sodium Occurrence Characteristics of YH2 and YH2I

YH2 has a lower total Na content than YH1, so only a modest amount of Na is found in YH2. As illustrated in Figure 7, Na is discovered in the cell-embedded clay of a particle composed mainly of fusinite and semifusinite. According to the elementary composition (Figure 7D,E) of spots 5 and 6 sampled on the cell-filling mineral, the atomic ratio of Al and Si is approximately 1:1 in both spots, which indicates that kaolinite (Al4[Si4O10]OH8) is the major filling clay. In spot 6, Na is presented in equal proportion with Cl and only accounts for a tiny amount (<0.2 wt%) in kaolinite. In this case, Na was possibly derived from soluble halite (H2O-Na), which was retained in this kaolinite-filling cell. The situation is more complicated in the case of spot 5. Na has a higher content than Cl, and some Ca is also mixed in. A possible explanation for this finding is that extra Na and Ca may appear separately as interlayer-adsorbed cations and isomorphisms of Al in kaolinite.
Compared with YH2, the trace of Na is considerably more obvious in YH2I. Sodium aggregations were detected in two cells of a large fusinite particle, which are presented in Figure 8 and Figure 9. As shown in Figure 8, no crystalloid was found in area 1, but the Na EDS mapping indicates several Na aggregations. In addition, the chlorine aggregations of Cl EDS mapping in the same region are in good agreement with the Na distribution. Spot 7 was sampled at the most notable Na gathering place, and the EDS spectrum reveals over 9 wt% sodium; this is primarily ascribed to halite (NaCl, H2O-Na), considering the similar atomic ratios of Na and Cl. Moreover, in addition to Na and Cl, there are other elements in minor quantities, such as K, Ca, Mg, and S. According to the approximate atom percentages and similar coverage areas in the EDS mappings of Ca and S, a tiny amount of anhydrite (CaSO4) might be associated with halite. K and Mg are present mainly as KCl and MgCl2, respectively, given the relatively higher Cl content. The other Na-containing cell in YH2I, labelled area 2, is located right below area 1, and the corresponding microscopic images and EDS mapping results are displayed in Figure 9. Some vague crystalline outlines can be observed in the image, and the Na and Cl EDS-mapping-covered areas basically match it. Spot 8 was sampled on the most evident crystalloid surface where maximum Na was enriched. The Na weight percentage of spot 8 was over 17%, indicating a highly enriched level of Na; moreover, the similar atomic ratio of Cl proves that halite (NaCl) is the major occurrence mode of Na in this case. Considering the relatively high content of Na and the existence of other elements, such as K, Ca, Mg, and S, and minor amounts of Na2CO3 and KCl, some commonly associated minerals of halite, such as calcite (CaCO3) and dolomite (CaMg[CO3]2), may also exist.

4.3.3. Sodium Occurrence Characteristics of YH3 and YH3I

Although the Na content of YH3 is lower than that of YH1 and YH2, the occurrence of Na in YH3 is more complicated. As displayed in Figure 10, the Na-containing particles found in YH3 are composed of fractions of telinite, fusinite, and semifusinite. The cell in which Na was detected is located approximately at the junction of telinite and fusinite, and it was hard to determine the exact category because the sample had been polished before the test. Some gray granular substances are distributed in the Na-containing area, and two spots were selected and tested by EDS. The EDS spectrum of spot 9 shows a minor quantity of Na enrichment (1.16 wt%), which should occur as halite (NaCl) given the similar atomic ratio of Na and Cl. In consideration of the high contents and approximate atom percentages of Ca and S, the major mineral at spot 9 should be anhydrite (CaSO4). In addition to these two minerals, some clays may also be present, given the existence of K, Mg, Al, and Si. The situation is simpler in YH3I, where the Na-containing particle is purely fusinite. A white crystalloid is clearly embedded in a cell of fusinite; spot 11 is sampled on its surface and the EDS spectrum is shown in Figure 11. Over 5% weight percentage of Na was enriched in spot 11, which forms halite (NaCl) with a similar level of Cl. Therefore, this Na-containing crystalloid was mainly composed of halite, and a minor amount of dolomite (CaMg[CO3]2) might also have been associated, considering the existence of Ca and Mg. In addition, due to the relatively high Na content compared to Cl, some Na2CO3 and Na2SO4 may also occur in this region.

4.3.4. Sodium Occurrence Characteristics of YH4 and YH4I

Only a small amount of Na was discovered in YH4, mixed in the cell-filling pyrite of a particle consisting of a large piece of semifusinite and fusinite fractions. As presented in Figure 12 cell-filling pyrite was distributed in the semifusinite, and Na was tested in two spots sampled on it. The EDS elemental composition of both spots shows a good 2:1 atomic ratio of S to Fe. However, the Na distribution in the two spots was different: Na2CO3 should be the major occurrence of Na in spot 12, while a tiny quantity of halite (NaCl) might be mixed in the pyrite in spot 13 given the perfect atom ratio of Na and Cl. The enrichment degree of Na is considerably higher in YH4I than in YH4. Na was detected in one cell of a typical fusinite grain. A cluster of crystal cubes and a crystalloid were embedded in the cell (Figure 13), and spots 14, 15, and 16 were sampled on two larger cubes and the crystalloid, respectively. According to the elementary composition results, not counting C and O, all spots have the atomic ratio of halite (NaCl) and a strong Na enrichment degree (18 wt%, 15.5 wt%, and 11.03 wt% in spots 14, 15, and 16, respectively). Moreover, KCl and typically associated minerals of halite, such as calcite (CaCO3), anhydrite (CaSO4), and dolomite (CaMg[CO3]2) should also occur given the existence of K, Ca, Mg, and S. In addition to the above analyses, there may exist a small amount of Na2CO3 in spots 15 and 16 due to the slightly higher content of Na compared with Cl. Additionally, it should be noted that the Na enrichment of the crystalloid is significantly lower than that of the two cubic crystals, despite it having the largest bulk, which may be caused by its incomplete crystal structure.

4.3.5. Sodium Occurrence Characteristics of YH5, YH5V, and YH5I

There are distinct differences in the distribution of Na in YH5, YH5V, and YH5I. In YH5, a small amount of Na was discovered in a cell of a semifusinite particle. Although some impurities were mixed in this cell, Na was found in the surrounding coal matrix. As shown in Figure 14, the elementary compositions of spots 16 and 17 are 4.09 wt% and 2.83 wt% Na, respectively, and the Na in these two spots occurs as halite (NaCl), with a similar atomic ratio to Cl. However, there are some slight differences between spots 16 and 17: there are extra Cl forms KCl, MgCl2, and CaCl2 in spot 16, whereas a tiny quantity of calcite (CaCO3) is associated with halite in spot 17, in addition to the above chlorine salts. It should be noted that no locally enriched Na was detected in the extracted vitrinite sample (YH5V) under SEM, and Na EDS mappings of all particles show a disordered uniform distribution. Two typical grains in YH5V are presented in Figure 15; one is composed of collodetrinite and scattered inertodetrinite, while the other is composed of collotelinite and micrinite. Although they comprise different macerals, they share a similar sodium distribution. It could be speculated that the Na in the vitrinite sample might be homogeneously associated with the organic coal matrix as NH4Cl-Na or NH4-EDTA-Na. The local Na enrichment is significant in YH5I, where Na was also detected in a cell of the particle that was mainly composed of fusinite and semifusinite, as in the above cases. However, the fusinite and semifusinite present in this particle are fragmentary, and some collodetrinite and macrinite are also mixed in the particle, possibly due to the pressure of overlying coal layers. As displayed in Figure 16, some substances hosting crystal-like structures are embedded in the cell of the particle, and, due to the depth of the cell, the color and specific details could not be observed. The EDS mapping of Na, Cl, Ca, and Mg in this region reveals two distinct halite crystals. Spots 19 and 20 are sampled on them, and the elementary spectra accord with the mapping results. Moreover, 12.18 wt% and 12.22 wt% Na is enriched separately in both crystals, and a similar amount of Cl is also aggregated. Additionally, the higher content of Na in both spots indicates the existence of Na2CO3, and a tiny amount of dolomite (CaMg[CO3]2) may also be associated with halite, considering the similar atomic ratios of Ca and Mg.

4.3.6. Sodium Occurrence Relationship with Maceral Groups

The vitrinite-enriched sample was only extracted in YH5 due to the high inertinite content of Zhundong coal. For a better exploration of the occurrence relationship between Na and maceral groups, the characteristics of the Na in raw coals was also considered. Raw coals generally have lower Na levels, ranging from 0.12% to 4.63%. Moreover, the Na detected in raw coals occurs as H2O-Na (halite NaCl) or insoluble Na (albite, or associated with clays/pyrite/anhydrite), and all were found in the cell of fusinite or semifusinite. This indicates that sodium tends to be associated with mineral matters in inertinite. The enrichment of Na in the inertinite-enriched sample is far more pronounced and uniform than in the raw coal. Except for YH3I, the inertinite-enriched samples all exhibit local H2O-Na (halite NaCl) enrichment of over 10%, rising to above 15% in YH1I, YH2I, and YH4I. This not only suggests a clear occurrence relationship between Na-associated minerals and inertinite but also implies a higher degree of Na enrichment in inertinite-enriched samples with homogeneous patterns. This occurrence affinity of Na is also quite remarkable in comparison to the vitrinite-enriched and inertinite-enriched samples extracted from YH5. As shown in Figure 15, the Na mappings of all particles show an even Na distribution on the surface in vitrinite, whereas a cluster of NaCl crystalloids with over 12% enrichment was detected in inertinite. In conclusion, there is a positive occurrence relationship between Na and inertinite in Zhundong coal, while no marked relationship between Na and vitrinite was observed. Moreover, in the inertinite group, semifusinite and particularly fusinite, are more easily associated with Na.
Different Na species also seem to have different occurrence tendencies with maceral groups in Zhundong coal. Most Na found in inertinite-enriched samples occurs as NaCl, which has been categorized as H2O-Na, while the rest is presented as insoluble Na associated with cell-filled minerals. This observation is consistent with the sequential extraction results: H2O-Na occupies the primary position in Zhundong coal, and the contents of H2O-Na and insoluble Na in the inertinite-enriched samples are clearly higher than those in vitrinite-enriched sample. Moreover, it should be noted that the elevated levels of H2O-Na and insoluble Na in inertinite can be largely attributed to the presence of fusinite and semifusinite, which have abundant pore structures that can be filled by soluble salts and insoluble minerals.

4.4. Triboelectrostatic Separation

Triboelectrostatic separation was performed on all five raw coals, and the separate data, total Na levels, and maceral composition of the products are shown in Table 4. A certain degree of separation was achieved in all five raw coals: concentrates with reduced ash content are collected at the negative end, while the tailings at the positive end have increased ash content. It is evident that vitrinite tends to be enriched in the concentrates, whereas the tailings contain more inertinite, suggesting that the maceral enrichment of Zhundong coal can be obtained after triboelectrostatic separation. The vitrinite content in all five concentrates generally ranges from 55% to 60%, whereas the inertinite enrichment in the tailings is in the range of 60% to 75%. In addition, while no significant difference in ash yield was found among the products, there are distinct variations in maceral enrichment. Compared to the raw coals, the content of vitrinite in concentrates increased by 25.0%, 28.0%, 11.0%, 21.5%, and 10.0% in YH1–YH5, respectively, whereas the inertinite in tailings increased by 6.0%, 4.0%, 11.5%, 7%, and 12.5%. The vitrinite enrichment of concentrates was considerably more obvious than the inertinite of tailings in YH1–YH4, whereas the reverse occurred in YH5. A possible reason for this is that the enrichment effect of macerals depends on the maceral composition of the feed. The maceral group that occupies a smaller portion of raw coal has a better enrichment effect, which explains the better enrichment effect of vitrinite in YH1–YH4.
In terms of Na removal, there is a positive correlation between the total Na content and inertinite level in products (the correlation coefficient r is 0.98 and 0.65 in concentrates and tailings, respectively). As a result, the total Na content of the concentrates is significantly lower than that of the tailings. The lowest total Na content reaches 2735 μg/g in the concentrate of YH4 with the highest vitrinite level (59.5%), while the maximum value of 3883 μg/g occurs in the tailing of YH1 with the highest inertinite level (74.5%). This suggests that Na displays a preferential enrichment depending on the maceral content of products. In addition, compared to an equal decrease in the vitrinite level of the concentrates, a rise in the inertinite level of the tailings tends to dramatically enhance the total Na content. Therefore, the maximum difference (~1000 μg/g) in total Na content between concentrate and tailing occurs in YHI. The mixing of minerals in the tailing may explain this: minerals tend to be negatively charged, like inertinite particles, which may raise the total Na level due to their potential association with Na.
In conclusion, both Na removal and maceral separation can be achieved after the triboelectrostatic separation of Zhundong coal. The high inertinite content of Zhundong coal also benefits the Na removal effect by considerably enhancing the total Na level in the tailings. Tailings with high inertinite content levels can also be supplied to industries that use coal-based materials and monomers of aromatic polymers [56].

5. Conclusions

Tests including sequential extraction, combined microscopy, and SEM-EDS observations were carried out to investigate the occurrence and separation relationship between Na and maceral groups in five Zhundong raw coals from different depths. The conclusions are as follows:
  • The total Na content of the five raw coals decreases with increasing depth, falling from 3613 μg/g in YH1 to 3296 μg/g in YH5. All raw coals and corresponding maceral-enriched samples share a similar content order of five Na occurrence modes: H2O-Na (~50%) and NH4Cl-Na (~20%) occupy the first and second positions, respectively, while NH4-EDTA-Na, HCl-Na, and insoluble Na share the rest, with no significant differences. The total Na levels in the inertinite-enriched samples are significantly higher than those in their corresponding raw coal and vitrinite-enriched sample. In addition, H2O-Na and insoluble Na show a clear enrichment trend in the inertinite-enriched samples, while NH4-EDTA-Na and HCl-Na are slightly more concentrated in the vitrinite-enriched sample.
  • Na was found in all the raw coal and inertinite-enriched samples except for the vitrinite-enriched one. Na was detected as H2O-Na (NaCl) and insoluble Na (associated with cell-filling minerals) in the raw coals with under 5 wt%, whereas the Na enrichments in inertinite-enriched samples mostly reached values of over 10 wt% (highest in YH4I with 18 wt%) with the uniform H2O-Na mode as NaCl. In all of the above cases, sodium was found in the cells of fusinite or semifusinite, indicating a clear positive occurrence correlation between Na-enriched minerals and the inertinite group, especially with fusinite.
  • Maceral separation and Na removal were simultaneously achieved after the triboelectrostatic separation of Zhundong raw coals. Inertinite tends to be enriched in the tailings, whereas vitrinite enrichment occurs in the concentrates. The total Na level of the products rises with the inertinite content, which results in an obvious difference in the Na contents between the tailings and concentrates (up to ~1000 μg/g in YH1). In addition, the possible mixing of minerals in the tailings also contributes to the rise in Na levels.

Author Contributions

Conceptualization, X.H.; Methodology, X.H.; Validation, W.W. and H.S.; Formal Analysis, X.H.; Investigation, X.H. and K.C.; Resources, J.P.; Data Curation, C.Z.; Writing—original draft preparation, X.H.; Writing—review and editing, W.W., C.Z. and J.P.; Visualization, K.C. and H.S.; Supervision, W.W.; Project Administration, W.W.; Funding Acquisition, X.H. and K.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research is supported by National Natural Science Foundation of China (No. U1903207, No. 42002184, No. 41972176, and No. 92062109), Natural Science Foundation of Jiangsu Province (No. BK20200638), Fundamental Research Funds for the Central Universities (No. 2020CXNL11), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Data Availability Statement

At the request of other researchers, the authors of the article can provide the original data.

Acknowledgments

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

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Location and representative stratigraphic column of Zhundong coalfield: (A) distribution of the main coalfields and locations of sampling sites in Xinjiang Provence (▲); (B) location of Junggar Basin (★); (C) representative stratigraphic column of Zhundong coalfield.
Figure 1. Location and representative stratigraphic column of Zhundong coalfield: (A) distribution of the main coalfields and locations of sampling sites in Xinjiang Provence (▲); (B) location of Junggar Basin (★); (C) representative stratigraphic column of Zhundong coalfield.
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Figure 2. Five-step sequential extraction procedure used for this study.
Figure 2. Five-step sequential extraction procedure used for this study.
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Figure 3. Schematic of rotary triboelectrostatic separation system.
Figure 3. Schematic of rotary triboelectrostatic separation system.
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Figure 4. Content distribution characteristics of sodium in samples: (A) contents of different sodium species and the total in samples; (B) percentage of different sodium species in raw coals; (C) percentage of different sodium species in maceral samples.
Figure 4. Content distribution characteristics of sodium in samples: (A) contents of different sodium species and the total in samples; (B) percentage of different sodium species in raw coals; (C) percentage of different sodium species in maceral samples.
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Figure 5. Microscopy and SEM-EDS identification of YH1: (A) a fusinite (f) particle; (B) SEM image of (A); (C) magnified SEM image of a cell, marked yellow in (B); (D,E) are EDS results of spots 1 and 2, respectively.
Figure 5. Microscopy and SEM-EDS identification of YH1: (A) a fusinite (f) particle; (B) SEM image of (A); (C) magnified SEM image of a cell, marked yellow in (B); (D,E) are EDS results of spots 1 and 2, respectively.
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Figure 6. Microscopy and SEM-EDS identification of YH1I: (A) a fusinite (f) particle; (B) SEM image of (A); (C) magnified SEM image of a cell, marked yellow in (B); (D) Na, Cl, Ca, and Mg EDS mapping of (C); (E,F) are EDS spectra of spot 3 and 4, respectively.
Figure 6. Microscopy and SEM-EDS identification of YH1I: (A) a fusinite (f) particle; (B) SEM image of (A); (C) magnified SEM image of a cell, marked yellow in (B); (D) Na, Cl, Ca, and Mg EDS mapping of (C); (E,F) are EDS spectra of spot 3 and 4, respectively.
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Figure 7. Microscopy and SEM-EDS identification of YH2: (A) a particle composed of fusinite (f) and semifusinite (sf); (B) SEM image of (A); (C) magnified SEM image of the area marked yellow in (B); (D,E) are EDS results of spots 5 and 6, respectively.
Figure 7. Microscopy and SEM-EDS identification of YH2: (A) a particle composed of fusinite (f) and semifusinite (sf); (B) SEM image of (A); (C) magnified SEM image of the area marked yellow in (B); (D,E) are EDS results of spots 5 and 6, respectively.
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Figure 8. Microscopy and SEM–EDS identification of YH2I: (A) SEM image of (B); (B) a fusinite (f) particle; (C) magnified SEM image of the area marked yellow in (B); (D) magnified SEM image of area 1, marked yellow in (C); (E) EDS spectrum of spot 7; (F) Na, Cl, Ca, Mg, and S EDS mapping of (D).
Figure 8. Microscopy and SEM–EDS identification of YH2I: (A) SEM image of (B); (B) a fusinite (f) particle; (C) magnified SEM image of the area marked yellow in (B); (D) magnified SEM image of area 1, marked yellow in (C); (E) EDS spectrum of spot 7; (F) Na, Cl, Ca, Mg, and S EDS mapping of (D).
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Figure 9. Microscopy and SEM–EDS identification of YH2I: (A) magnified SEM image of area 2, marked red in Figure 8C; (B) EDS spectrum of spot 8; (C) Na, Cl, Mg, and Ca EDS mapping of (A).
Figure 9. Microscopy and SEM–EDS identification of YH2I: (A) magnified SEM image of area 2, marked red in Figure 8C; (B) EDS spectrum of spot 8; (C) Na, Cl, Mg, and Ca EDS mapping of (A).
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Figure 10. Microscopy and SEM–EDS identification of YH3: (A) SEM image of (B); (B) a particle composed of fusinite (f), semifusinite (sf), and telinite (te); (C) magnified SEM image of a cell marked yellow in (B); (D,E) are EDS spectra of spots 9 and 10, respectively.
Figure 10. Microscopy and SEM–EDS identification of YH3: (A) SEM image of (B); (B) a particle composed of fusinite (f), semifusinite (sf), and telinite (te); (C) magnified SEM image of a cell marked yellow in (B); (D,E) are EDS spectra of spots 9 and 10, respectively.
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Figure 11. Microscopy and SEM-EDS identification of YH3I: (A) SEM image of (B); (B) a fusinite (f) particle; (C) magnified SEM image of the area marked yellow in (A); (D) magnified SEM image of a cell, marked yellow in (C); (E) EDS spectrum of spot 11.
Figure 11. Microscopy and SEM-EDS identification of YH3I: (A) SEM image of (B); (B) a fusinite (f) particle; (C) magnified SEM image of the area marked yellow in (A); (D) magnified SEM image of a cell, marked yellow in (C); (E) EDS spectrum of spot 11.
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Figure 12. Microscopy and SEM-EDS identification of YH4: (A) a particle composed of fusinite (f) and semifusinite (sf); (B) SEM image of (A); (C) magnified SEM image of the area marked yellow in (B); (D,E) are EDS spectra of spots 12 and 13, respectively.
Figure 12. Microscopy and SEM-EDS identification of YH4: (A) a particle composed of fusinite (f) and semifusinite (sf); (B) SEM image of (A); (C) magnified SEM image of the area marked yellow in (B); (D,E) are EDS spectra of spots 12 and 13, respectively.
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Minerals 13 01155 g013
Figure 13. Microscopy and SEM-EDS identification of YH4I: (A) a fusinite (f) particle; (B) SEM image of (A); (C) magnified SEM image of the area marked yellow in (B); (D) magnified SEM image of the area marked red in (C); (E–G) are EDS results of spots 14, 15, and 16, respectively.
Figure 13. Microscopy and SEM-EDS identification of YH4I: (A) a fusinite (f) particle; (B) SEM image of (A); (C) magnified SEM image of the area marked yellow in (B); (D) magnified SEM image of the area marked red in (C); (E–G) are EDS results of spots 14, 15, and 16, respectively.
Minerals 13 01155 g013
Minerals 13 01155 g014
Figure 14. Microscopy and SEM-EDS identification of YH5: (A) a semifusinite (sf) particle; (B) SEM image of (A); (C) magnified SEM image of the area marked yellow in (B); (D,E) are EDS results of spots 17 and 18, respectively.
Figure 14. Microscopy and SEM-EDS identification of YH5: (A) a semifusinite (sf) particle; (B) SEM image of (A); (C) magnified SEM image of the area marked yellow in (B); (D,E) are EDS results of spots 17 and 18, respectively.
Minerals 13 01155 g014
Minerals 13 01155 g015
Figure 15. Microscopy and SEM-EDS identification of YH5V: (A) a collodetrinite (cd) particle; (B) SEM image of (A); (C) Na EDS mapping of (B); (D) a particle mainly composed of collotelinite (ct) and some micrinite (mi); (E) SEM image of (D); (F) Na EDS mapping of (E).
Figure 15. Microscopy and SEM-EDS identification of YH5V: (A) a collodetrinite (cd) particle; (B) SEM image of (A); (C) Na EDS mapping of (B); (D) a particle mainly composed of collotelinite (ct) and some micrinite (mi); (E) SEM image of (D); (F) Na EDS mapping of (E).
Minerals 13 01155 g015
Minerals 13 01155 g016
Figure 16. Microscope and SEM-EDS identification of YH5I: (A) SEM image of (C); (B) magnified SEM image of the area marked yellow in (A); (C) a particle mainly composed of fusinite (f), semifusinite (sf), some collodetrinite (cd), and a little macrinite (ma); (D) Na, Cl, Mg, and Ca EDS mapping of (B); (E,F) are EDS spectra of spots 19 and 20.
Figure 16. Microscope and SEM-EDS identification of YH5I: (A) SEM image of (C); (B) magnified SEM image of the area marked yellow in (A); (C) a particle mainly composed of fusinite (f), semifusinite (sf), some collodetrinite (cd), and a little macrinite (ma); (D) Na, Cl, Mg, and Ca EDS mapping of (B); (E,F) are EDS spectra of spots 19 and 20.
Minerals 13 01155 g016
Table 1. Total sulfur, proximate analyses, and ultimate analyses results of samples.
Table 1. Total sulfur, proximate analyses, and ultimate analyses results of samples.
SamplesYH1YH1IYH2YH2IYH3YH3IYH4YH4IYH5YH5VYH5I
UA (wt %)
Odaf18.5717.9217.8617.3916.5316.6416.2715.7917.5120.2816.15
Cdaf77.2778.3077.8878.5778.9779.2179.5980.1176.5874.3078.90
Hdaf3.193.073.173.303.373.252.993.333.714.453.91
Ndaf0.770.630.650.490.680.540.650.480.720.700.60
St,d0.180.080.220.200.450.360.340.280.500.210.33
PA (wt %)
Mad12.3212.6411.1010.9011.9011.1411.1210.9413.3112.359.86
Ad4.303.163.463.013.643.264.382.753.392.483.04
Vdaf30.2330.5730.1330.7930.0929.5029.5128.0837.9543.0035.20
FCad66.7767.2467.4567.1367.3768.2067.4169.9459.9255.5462.82
UA, ultimate analyses; PA, proximate analyses; 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.
Table 2. Oxides and elements in raw coals (wt %, on air dried basis).
Table 2. Oxides and elements in raw coals (wt %, on air dried basis).
Oxides and ElementsYH1YH2YH3YH4YH5China aWorld b
Na2O0.410.340.420.460.270.16nd
K2O0.010.010.010.010.010.19nd
CaO2.361.901.862.321.741.23nd
MgO1.050.830.780.630.460.22nd
SiO20.490.260.230.220.178.47nd
Al2O30.330.290.290.270.435.98nd
Fe2O30.180.100.030.110.154.85nd
S0.300.740.830.870.72ndnd
Cl0.050.070.210.260.05ndnd
REY c3.2419.766.2213.406.17138.0068.40
Li9.262.297.262.243.9331.8012.00
Nb0.090.100.220.110.209.443.70
Ta0.010.040.030.050.020.620.28
Zr1.8518.128.3515.206.2989.5036.00
Ga0.130.240.250.260.246.555.80
In0.010.010.020.010.020.050.03
Ge0.020.100.120.140.042.782.29
Cr2.6327.853.3220.766.6515.4016.00
Co2.460.761.100.741.507.085.10
a average values in the common Chinese coals [53]; b average values in worldwide coals [54]; c, rare earth elements and Y; nd, no data.
Table 3. Maceral identification results of samples (vol. %; on a whole basis).
Table 3. Maceral identification results of samples (vol. %; on a whole basis).
SamplesYH1YH1IYH2YH2IYH3YH3IYH4YH4IYH5YH5VYH5I
Vitrinite
Telinite15.506.0012.006.519.59.011.58.02.52.03.0
Collotelinite1.000.505.001.01.01.04.02.52.511.50.5
Vitrodetrinite6.004.508.004.56.02.07.04.01.0 2.5
Collodetrinite8.005.005.004.017.04.58.52.043.076.512.0
Corpogelinite0.50 0.50 1.0 0.50.5
Gelinite0.500.50 1.01.5
Total31.5016.5030.5016.044.517.533.017.049.090.018.0
Inertinite
Fusinite 25.0041.506.5015.521.049.015.528.019.00.545.0
Semifusinite 26.0035.0037.5054.013.023.034.539.516.51.025.5
Macrinite 7.501.5011.504.03.02.54.54.05.00.51.5
Micrinite 0.50 1.0 1.04.50.5
Inertodetrinite 9.505.5012.5010.518.06.511.010.09.03.08.5
Total68.5083.5068.0084.055.082.065.581.550.59.581.0
Liptinite
Cutinite 0.50 0.5
Sporinite 0.50 0.5 1.00.50.5
Total 1.00 0.5 1.00.50.5
Minerals
Clay
Carbonate
Sulfide 0.50 0.50.51.0 1.0
VRC0.450.410.450.420.460.400.450.420.410.480.39
Table 4. Triboelectrostatic separation results of Zhundong raw coals.
Table 4. Triboelectrostatic separation results of Zhundong raw coals.
FeedConcentratesTailings
SamplesASYieldASVGIGTNaYieldASVGIGTNa
%%%%%μg/g%%%%μg/g
YH14.3040.541.9256.5042.50288759.465.9222.0074.503883
YH23.4638.331.7558.5040.50276861.674.5226.5072.003623
YH33.6439.401.9755.5044.00299160.604.7331.5066.503561
YH44.3841.862.0554.5044.00293158.146.0623.5072.503479
YH53.3938.151.5259.5040.00273561.854.5437.0063.003542
AS, ash yield; VG, vitrinite group content; IG, inertinite group content; TNa, total Na content.
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He, X.; Che, K.; Pan, J.; Sun, H.; Zhou, C.; Wang, W. Occurrence Mode of Sodium in Zhundong Coal, China: Relationship to Maceral Groups. Minerals 2023, 13, 1155. https://doi.org/10.3390/min13091155

AMA Style

He X, Che K, Pan J, Sun H, Zhou C, Wang W. Occurrence Mode of Sodium in Zhundong Coal, China: Relationship to Maceral Groups. Minerals. 2023; 13(9):1155. https://doi.org/10.3390/min13091155

Chicago/Turabian Style

He, Xin, Kexin Che, Jinhe Pan, Hao Sun, Changchun Zhou, and Wenfeng Wang. 2023. "Occurrence Mode of Sodium in Zhundong Coal, China: Relationship to Maceral Groups" Minerals 13, no. 9: 1155. https://doi.org/10.3390/min13091155

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