Mineralogy and Geochemistry of the M9 High-Sulfur Coal from the Renjiazhuang Mining District, China.

According to coal lithotypes, the bottom, parting, roof, and 15 coal samples were collected by finely partitioning the M9 seam from the Renjiazhuang Mining District, Ningxia, China. Conventional chemical analysis, optical microscopy, scanning electron microscopy equipped with energy-dispersive X-ray spectrometry, X-ray diffractometry, inductively coupled plasma atomic emission spectrometry, inductively coupled plasma mass spectrometry, and atomic absorption spectrophotometry techniques were used on these samples to research the vertical variation between geochemistry and mineralogy in the high-sulfur coal. The weighted average content of total sulfur calculated from 15 coal samples is 3.07%, which belongs to the high-sulfur coal. However, the contents of morphological sulfur of 15 piles are significantly different: the contents of pyritic and organic sulfur are observed to range from 0.02 to 1.55% and from 1.88 to 3.91%. The results show that these differences are mainly controlled by marine conditions and the contents of organic matter and kaolinite. The mineralogy of the M9 coal is dominated by kaolinite, followed by dolomite, and it also contains minor amounts of illite, feldspar, pyrite, siderite, hematite, chalcopyrite, calcite, and marcasite. Moreover, pyrite is the main sulfide in coal, and agglomerated chalcopyrite and granular galena are partially visible. The forms of pyrite include fine-grained, spherical, irregular block-shaped, and clumps. Trace elements are mainly carried by pyrite and ash so that physical coal cleaning can be applied to partially remove them, while thalassophile elements Na, Ca, and Mg are closely related to organic sulfur, indicating that the coal blending can be used to decrease their contents.

Introduction

The existence of sulfur in coal seams restricts coal mining and utilization. Apart from that, the acid gases such as sulfur dioxide produced by its combustion seriously pollute the environment.[1−6] The accumulation of sulfur is mainly affected by the sedimentary microfacies in coal, forming a syngenetic stage and an early diagenetic stage.[7−11] In China, coal is utilized as a predominant non-renewable resource. The total coal consumption in 2020 (almost 4.98 billion tons) was the highest globally. Furthermore, medium–high sulfur coal and high-sulfur coal are important coal types in China. Additionally, the mode of occurrence of trace elements is the major factor that determines the toxicity of the elements and the degree of release.[12−15] In order to accurately identify the potential environmental impact, it is essential to further study the migration of trace elements and sulfur in coal.

The Renjiazhuang Mining District, Ningxia territory is one of the larger underground coal mines in China, with an annual output of almost 0.6 million tons. The M3, M5, and M9 coal seams are the main producing seams. Wu et al. studied the geochemistry of harmful elements and sulfur in these seams and found that the significant fluctuation of sulfur content could represent different sedimentary environments, and the distributions and contents of harmful elements are closely associated with the different forms of sulfur in coals.[16] At present, the M9 seam is being mined; however, previous studies and geological data indicate that the M9 coal seam is severely affected by the sedimentary environment of multiple transgressions and regressions.[17] According to the sedimentological analysis, the marine environment is the main reason for the difference of morphological sulfur content.[18−20] Hence, there are some differences in mineralogical and elemental contents in these coal seams, which need to be further studied.

This paper describes the results of petrological and geochemical analyses of the vertical section of the M9 coal seam from the Renjiazhuang Underground Mine through 15 coal samples collected ply by ply, which is associated with the depositional environment, and their impact on the elemental content.

Geological Setting and Sample Collection

The Renjiazhuang Coal Mine is nearly 32 km from Lingwu city in the northeast region of Ningxia Province. The coal-bearing strata in the area are mainly composed of the Pennsylvanian Taiyuan Formation and the Lower Permian Shanxi Formation. The average thicknesses of the coal seams are 15 m and 10 m (Figure ). The depositional environment of the Taiyuan Formation mainly includes tidal deltas, barrier sand, and lagoons.[11,17,21] The M9 coal formed in a marine environment with weak oxidation and multiple fluctuating transgressions.[17] The thickness of the M9 seam ranges from 0.73 to 7.84 m in the whole mining area, with an average of 5.12 m. The mean maximum vitrinite reflectance varies from 0.67 to 0.75%, with an average of 0.71%.[21] The roof is a major siltstone, and the floor is silty mudstone. The sampling was carried out in accordance with Chinese standards (GB/T 482-2008). 18 samples were collected, including 15 coal samples, 2 silty mudstone samples, and 1 mudstone sample (Figure ).

Figure 1

Stratigraphic column of the coal-bearing strata in the Renjiazhuang Mining District and sampling location.

Analytical Methods

Conventional chemical analysis, including the determination of moisture, volatile matter, and ash yield of the samples, was performed in Jiangsu Mineral Resources and Geological Design and Research Institute, with reference to the ASTM Standard D3173M-17a (2017), D3174-12 (2012), and D3175-17 (2017).[22−24] Forms of sulfur and total sulfur were determined following the ASTM standards D3177-02 (2011) and D2492-02 (2012), respectively (see the data results in Table ).[25,26] Al, Ca, Ba, Sr, Fe, and Mg in coal were determined by inductively coupled atomic emission spectrometry (ICP-AES) (Icap7400). Furthermore, Na, K, Ti, and P were analyzed using atomic absorption spectrophotometry (AAS) (AA-6880), and spectral photometry was used for Si (SiO2). Additionally, Li, Be, Sc, Th, U, Co, Ni, Zn, Ga, Rb, Nb, Cs, In, W, Tl, Pb, Bi, V, and Cr were analyzed by inductively coupled plasma mass spectrometry. The content of 28 trace and major elements in the samples are presented in Table . Coal samples were ground to a particle size of less than 75 μm using an agate grinding pot, and then, mineralogical analysis was conducted by X-ray diffractometry (D8 DISCOVER). In addition, the mineralogical morphology was studied using a DMRXP-MPM600 optical microscope (white light and ultraviolet light reflectance) and a scanning electron microscope (SIGMA) equipped with an energy-dispersive X-ray analyzer (X-Max 20).

Table 1

Moisture, Volatile Matter, Sulfur and Ash Content, and Coal Lithotypes of 18 Samples (%)a

sample nos.	coal lithotypes	Mad	Vdaf	Ad	St,d	Sp,d	Ss,d	So,d	VR	IR	ER
roof	silty mudstone	nd	nd	nd	5.34	4.24	0.11	0.99	nd	nd	nd
RZ-1	dull	0.57	32.08	26.19	1.99	0.02	0.00	1.97	39.54	54.30	6.16
parting	mudstone	1.46	nd	nd	0.03	0.01	0.00	0.02	nd	nd	nd
RZ-2	semi-dull	0.94	32.93	11.12	2.66	0.10	0.00	2.56	31.13	62.67	6.20
RZ-3	semi-bright	1.03	37.70	6.81	3.39	0.03	0.00	3.36	71.93	25.16	2.91
RZ-4	semi-dull	0.92	42.24	14.98	3.27	0.37	0.00	2.90	70.84	24.00	5.17
RZ-5	bright	0.81	36.66	6.90	3.51	0.14	0.00	3.37	56.56	39.04	4.41
RZ-6	semi-dull	0.75	42.29	5.91	5.46	1.55	0.00	3.91	77.88	16.99	5.13
RZ-7	semi-bright	0.91	38.98	4.20	3.53	0.18	0.05	3.30	62.96	30.76	6.28
RZ-8	semi-dull	0.76	37.21	5.12	3.27	0.25	0.00	3.02	54.12	42.08	3.81
RZ-9	semi-bright	0.96	34.55	18.17	2.10	0.12	0.00	1.98	33.52	53.92	12.56
RZ-10	semi-dull	1.06	36.32	13.25	3.21	0.34	0.01	2.86	60.57	36.05	3.38
RZ-11	semi-dull	0.88	33.96	17.11	2.55	0.18	0.01	2.36	35.62	53.38	11.00
RZ-12	semi-bright	1.01	35.80	12.47	2.50	0.02	0.01	2.47	49.73	43.46	6.80
RZ-13	semi-dull	0.98	39.73	12.77	3.2	0.38	0.01	2.81	71.19	25.47	3.34
RZ-14	dull	0.9	36.10	22.19	2.97	1.08	0.01	1.88	48.12	44.37	7.51
RZ-15	semi-dull	1.03	41.92	18.11	2.79	0.57	0.01	2.21	78.58	16.17	5.25
bottom	silty mudstone	nd	nd	nd	5.28	4.52	0.10	0.66	nd	nd	nd
weighted average of M9 coal		0.90	36.99	13.21	3.07	0.36	0.01	2.70	nd	nd	nd

M, moisture; V, volatile matter; A, ash; St; total sulfur; Ss, sulfate sulfur; Sp, pyritic sulfur; So, organic sulfur; VR, vitrinite; IR, inertinite; ER, exinite; ad, air-basis; d, dry basis; daf, dry and ash-free basis; and nd, no date. All coal sample data are from Wu et al.[21]

Table 2

Elemental Abundances in the M9 High Sulfur Coal from the Renjiazhuang Coal Mine, Ningxia Province, China

element	roof	RZ-1	parting	RZ-2	RZ-3	RZ-4	RZ-5	RZ-6	RZ-7	RZ-8	RZ-9	RZ-10	RZ-11	RZ-12	RZ-13	RZ-14	RZ-15	bottom
K	0.43	0.03	0.12	0.07	0.10	0.03	0.07	0.06	0.08	0.07	0.06	0.06	0.06	0.06	0.03	0.05	0.17	3.42
Na	0.07	0.13	0.07	0.29	0.38	0.10	0.26	0.39	0.56	0.38	0.16	0.17	0.14	0.16	0.19	0.08	0.09	0.22
Si	8.29	23.41	23.87	22.16	22.32	10.24	18.93	9.97	19.44	17.65	21.27	22.02	22.72	22.29	16.23	21.83	22.42	22.78
Al	5.06	23.67	23.02	23.03	22.52	10.48	18.87	10.09	19.08	17.86	22.55	22.23	23.69	23.46	17.42	22.43	22.51	19.09
Fe	6.84	0.07	0.07	1.05	0.70	3.02	2.22	30.59	4.90	5.45	0.87	3.25	1.17	0.29	3.08	5.01	3.04	3.32
Ca	35.14	0.16	0.16	0.58	0.73	15.34	5.61	4.05	2.59	4.92	2.11	0.89	0.39	1.22	9.51	0.70	0.44	0.49
Mg	1.60	0.11	0.16	0.42	0.31	8.11	0.62	1.97	0.82	2.29	1.31	0.46	0.19	0.55	0.43	0.04	0.16	0.77
Ti	0.26	0.31	0.48	0.54	0.44	0.38	0.73	0.40	0.43	0.47	0.73	0.56	0.97	0.53	0.22	0.47	1.03	0.94
P	0.11	0.06	0.01	0.25	0.23	0.05	1.30	0.03	0.44	0.43	0.12	0.02	0.03	0.08	1.98	0.03	0.02	0.04
Li	19.70	296.00	232.00	112.00	50.50	53.30	34.60	7.70	13.80	17.30	164.00	77.00	176.00	119.00	68.50	150.00	75.20	176.00
Be	0.75	1.84	0.96	1.22	0.88	1.35	1.29	0.79	1.13	1.09	1.85	1.16	1.56	1.32	1.46	2.52	2.29	3.26
Sc	6.44	12.20	7.76	3.41	2.26	4.72	2.75	3.03	1.28	1.27	6.70	3.75	4.31	2.69	3.15	5.18	8.68	23.20
Th	7.47	23.00	10.10	6.33	2.52	5.09	6.70	1.19	2.02	2.02	12.00	7.18	9.93	5.51	2.66	15.50	9.56	21.20
U	6.96	3.48	4.22	1.09	1.16	2.68	2.70	1.57	0.67	0.39	3.05	2.73	2.14	1.50	1.99	3.19	2.06	10.30
Ta	0.5	0.37	3.28	0.47	0.20	0.37	0.33	0.07	0.09	0.12	0.77	0.38	0.75	0.42	0.12	0.63	1.31	1.98
Ni	71.80	5.33	5.32	1.24	0.93	1.41	1.15	0.85	0.78	0.80	1.90	2.44	1.54	0.92	1.18	1.83	1.25	42.80
Zn	84.10	5.52	21.50	2.75	3.39	3.46	3.76	3.10	3.83	3.40	4.51	2.82	3.89	2.36	3.17	6.88	5.29	109.00
Ga	7.45	8.84	39.90	2.85	5.18	9.61	6.02	19.40	5.50	5.14	7.90	17.50	12.10	9.55	13.80	10.00	15.50	29.70
Rb	19.30	1.39	7.86	0.57	0.47	0.69	0.40	0.34	0.31	0.32	1.31	0.73	1.18	0.54	0.44	1.62	2.16	154.00
Nb	8.35	3.96	37.90	2.41	2.92	5.76	3.53	1.16	0.66	0.58	4.34	6.06	6.58	4.71	2.81	7.85	8.51	24.70
Cs	2.48	0.47	1.74	0.16	0.11	0.24	0.13	0.10	0.09	0.09	0.41	0.21	0.40	0.13	0.13	0.50	0.34	15.20
In	0.039	0.210	0.026	0.033	0.025	0.034	0.028	0.022	0.021	0.019	0.071	0.039	0.082	0.022	0.018	0.078	0.047	0.160
W	0.40	0.45	4.93	0.35	0.64	0.77	0.39	1.08	0.60	0.41	0.65	0.29	1.13	0.34	0.91	0.61	1.34	3.85
Tl	1.05	0.02	0.06	0.05	0.02	0.04	0.04	0.27	0.03	0.03	0.03	0.03	0.03	0.01	0.01	0.03	0.02	0.79
Pb	14.67	80.79	28.04	11.39	6.16	8.61	9.03	4.13	3.71	3.72	18.99	18.62	26.34	7.93	7.86	34.52	11.66	41.30
Bi	0.26	0.70	0.90	0.28	0.17	0.20	0.31	0.08	0.12	0.11	0.32	0.41	0.71	0.17	0.10	0.51	0.22	0.65
V	65.90	33.20	16.20	7.27	6.76	9.35	11.70	3.79	2.73	2.41	16.40	15.70	13.80	7.65	4.90	14.80	14.80	131.00
Cr	54.40	10.50	5.43	3.37	1.54	2.58	2.39	1.40	1.48	1.12	6.34	4.06	5.10	2.37	1.59	5.55	4.72	90.90
Th/U	1.073	6.609	2.393	5.807	2.172	1.899	2.481	0.758	3.015	5.179	3.934	2.630	4.640	3.673	1.337	4.859	4.641	2.058
(V + Ni)/V	2.09	1.16	1.33	1.17	1.14	1.15	1.10	1.22	1.29	1.33	1.12	1.16	1.11	1.12	1.24	1.12	1.08	1.33
Cr/V	0.83	0.32	0.34	0.46	0.23	0.28	0.20	0.37	0.54	0.46	0.39	0.26	0.37	0.31	0.32	0.38	0.32	0.69

Results and Discussion

Vertical Distribution of Elements

The M9 coal is classified as low-ash (13.21%) and high-sulfur (3.07%) coal based on the Chinese Standard GB/T 15224.1-2018 and GB/T 214-2007, suggesting that coals with ash yields 10–20% and sulfur >3% are considered low-ash and high-sulfur coals, respectively (Table ). However, the total sulfur content of the 15 coal samples varies significantly. RZ-1 coal is medium-sulfur coal (<2.0 wt %), while RZ-2, RZ-9, RZ-11, RZ-12, RZ-14, and RZ-15 samples are medium–high sulfur coals, and the remaining samples are high-sulfur coals (Table ). The lithotypes of the M9 coal seam are mainly composed of semi-bright coal and semi-dull coal, which contain high organic sulfur, followed by bright coal and dull coal. Pyritic sulfur content is relatively higher in semi-dull coal and dull coal (Table ).

There are obvious differences in the content of morphological sulfur in the M9 coal seam (Table ). The contents of organic sulfur and pyritic sulfur in coal are observed to range from 1.88 to 3.91% and from 0.02 to 1.55%, respectively, and the content of sulfate sulfur varies from 0 to 0.05% (Table ), indicating that sulfur in coal is mainly enriched in organic sulfur. Previous studies indicated that the content of highly enriched organic sulfur in coal range between 4 and 11%, and these coals were deposited in a special sedimentary environment.[14,27−29] Meanwhile, the vertical content variation of organic sulfur is largely similar to that of thalassophile elements Na, Ca, and Mg (except P and Ti) and trace elements Li, Be, Sc, Th, In, Ga, Bi, and U. In addition, similar vertical distribution curves are also found between ash and In, Si, Al, Li, Be, Sc, Th, and Bi (Figure ), indicating that these trace elements are mainly associated with organic matter and inorganic minerals.[2,14−16]

Figure 2

Vertical content variations of sulfur and associated elements in the M9 coal seam against depth (So,d, Ad, Si, Al, Na, Ca, Mg, P, and Ti in percentage and Li, Be, Sc, Th, In, Ga, Bi, and U in parts per million).

The vertical variation of total sulfur is similar to that of pyritic sulfur, and similar vertical variations can be observed for pyrite sulfur and Fe, Pb, W, and Tl (Figure ), indicating that these elements are related to pyrite.[9,14−16,29] Sulfate sulfur is similar to K, Ni, Zn, Rb, Cs, V, Cr, Ta, and Nb, indicating that these trace elements are mainly associated with inorganic minerals. At the same time, the content of total sulfur in silty mudstone ranges from 5.28 to 5.34% and the content of total sulfur varies from 4.24 to 4.52% (Figure ), indicating that the sulfur in the roof and floor is mainly enriched in pyrite.

Figure 3

Vertical content variations of sulfur and associated elements in the M9 coal seam against depth (St,d, Ss,d, Sp,d, Fe, and K in percentage and Cs, Cr, Nb, Rb, Zn, W, Tl, Ni, Ta, V, and Pb in parts per million).

Comparing trace elements in the M9 seam and the bottom and roof, the contents of Li, Be, Pb, and Ga gradually increase from the roof to the coal seam and then to the bottom (Table ). That is to say, the contents of Li, Be, Pb, and Ga in coal are affected not only by hydrodynamic conditions, pH, and material sources[29−32] but also by epigenetic leaching.[16,33]

Cr, U, V, Zn, and Ni can indicate the redox conditions of the sedimentary environment.[34,35] At the same time, Th/U, (V + Ni)/V, and Cr/V can also be used for the identification of paleo-sedimentary redox conditions, with a lower value of each ratio [Th/U < 0.80, (V + Ni)/V < 1.18, and Cr/V < 0.24], representing relative reducing conditions, and higher values [Th/U > 1.33, (V + Ni)/V < 2.50, and Cr/V > 0.50], representing relative oxidation conditions.[34−37] As shown in Figure , the higher contents of pyritic sulfur and organic sulfur are produced in a relatively reducing environment, especially the RZ-6 sample. This line graph presents an obvious rise and decline trend, which is prominently displayed about organic sulfur and Th/U and (V + Ni)/V, indicating that the M9 seam is formed in the marine environment of multiple fluctuating transgressions.

Figure 4

Sulfur content in the vertical section in contrast with the element ratio on the redox condition.

Correlation Analysis

Based on data from all coal samples, it can be observed that organic sulfur and vitrinite contents are positively correlated, reflecting that the formation of organic sulfur is related to the type and content of organic matter (Table and Figure a). The content of ash is positively correlated with the content of lithium in coal, with a correlation coefficient of 0.76 (Figure b), indicating that lithium is present in inorganic minerals. This is also consistent with the result that the carrier of lithium in coal mainly occurs in clay minerals and part of it occurs in mica and tourmaline.[38,39] There is a positive correlation between pyrite sulfur and Fe, with a correlation coefficient of 0.72 (Figure c), indicating that sulfur is related to the content of Fe2+ because sulfur and Fe2+ form FeS in coal.[40] Al has an apparently positive association with Si at the 99% confidence level (Figures and 5d), and the theoretical line is strongly close to the regression equation of kaolinite (the dotted line in Figure d). It can be considered that the changes in Al and Si are mainly attributed to kaolinite, which is present in each coal pile.

Figure 5

Relations between (a) pyritic sulfur and organic sulfur, (b) Ad and Li, (c) Fe and pyritic sulfur, and (d) Al and Si.

Mineralogy

X-ray diffractometry was used to determine the mineralogy of three layer samples (RZ-1, RZ-8, and RZ-13) and three layer mixed samples (RZ-4–6, RZ-9–11, and RZ-14–15). Kaolinite is the dominant mineral (except ply RZ-8), which is consistent with the observation explained above, followed by dolomite. Additionally, minor amounts of illite, feldspar, pyrite, siderite, hematite, chalcopyrite, calcite, and marcasite can be observed in local coal seams (Figure ).

Figure 6

X-ray diffraction patterns of some ply samples.

It can be seen that clay minerals in the M9 coal seam mostly occupy cell lumens in fusinite and collodetrinite (Figure a). Scanning electron microscopy energy-dispersive X-ray spectrometry (SEM/EDX) was used to further analyze the mineralogy. This could be better expressed as the abundance of Si and Al being due to the dominance of kaolinite (Table and Figure a).

Figure 7

Modes of occurrence of mineralogy: (a) clay filling the fusinite and collodetrinite lumens, (b) calcite in cell lumens, (c) well-developed spheroid pyrite occurring in calcite, (d) fine-grained pyrite distributed in micrinite and collodetrinite, and (e,f) pyrite contained in and around the resinite and feldspar embedded in the collodetrinite, with (e) the same as the observation area of (f), respectively; Cy, caly; Fs, fusinite; Co, collodetrinite; Sf, semifusinite; Ca, calcite; Py, pyrite; Mi, macrinite; Fe, feldspar; and Re, resinite.

Figure 8

SEM–EDX images of mineralogy in coal: (a) lamellar kaolinite, (b) granular calcite filling in cell lumens, (c) massive dolomite and a more granular dolomite filling in the interstices of coal, (d) well-spherical and granular pyrite, (e) granular galena, and (f) agglomerated chalcopyrite. Ka, kaolinite; Ca, calcite; Do, dolomite; Py, pyrite; Ga, galena; and Ch, chalcopyrite.

The RZ-1 sample is dominated by kaolinite, and the cellular cavity is filled with laminated kaolinite (Figure a). RZ-8 contains dolomite, followed by kaolinite and calcite (Figure ). Meanwhile, the interstices of coal are filled with a massive dolomite and a more granular dolomite (Figure c). The cell lumens are filled with granular calcite, and the particle size of calcite varies from 5 to 20 μm (Figures b and 8b). The fissures of coal are filled with vein calcite, and spherical pyrite occurs in calcite (Figure c). To conclude, the pyrite and calcite are of syngenetic origin, which is consistent with the development of pyrite in an alkaline environment.[41]

Modes of Sulfide Occurrence

The formation of organic sulfur is mainly affected by SO42–, Fe2+, organic matter, and bacterial activities,[11,42−44] while pyrite sulfur and total sulfur are mainly controlled by the action of seawater and parent plant material.[8,12,16,43] There are exceptions. Turkish lignite was formed in a freshwater environment and covered by natural alkali and carbonates.[44,45] The possible sources of sulfur in coal with high organic sulfur are sulfide mineralization, the peat environment, and regional volcanic activity,[46] as well as the basin fluid and pyroclastic or clastic materials.[47]

The well-developed spheroidal pyrite has a particle size of 1.0–5.0 μm and the pyrite is framboidal (Figure c), suggesting a biogenic origin. Meanwhile, the interstices of the coal are filled with layered dolomite (Figure c), which also provides abundant materials on the genesis of pyrite from the impact of sulfate-reducing bacteria.[18,43−48] Fine-grained pyrite occurs in micrinite and collodetrinite (Figure d), which is consistent with Wang et al.’s view that pyrite is mostly found in vitrinite and cutinite.[29] In other words, the spherical pyrite is authigenically deposited.[43,49] Well-spherical and granular pyrites are hosted in coal with a particle size of 0.2–5.0 μm (Figure d). The granular galena aggregates are arranged in different directions, and the particle size is 0.5–4.0 μm (Figure e). The agglomerated chalcopyrite is embedded in coal, with 1.0–10.0 μm cellular cavity cavities visible around it. There appear to be two phases present, and the darker phase may be replacing the lighter phase (Figure f), indicating that chalcopyrite developed in syngenesis and replaced by epigenesis. This was affected by the marine environment and activities of sulfate-reducing bacteria, which rapidly reduces SO42– in seawater to produce H2S.[43,44] This further indicates that the local chalcopyrite development may be formed by anaerobic bacteria reduction of sulfate.[45]

Fine-grained pyrite is distributed around the ellipsoidal resinite, and a massive feldspar occurs in collodetrinite according to the observation of reflected light and UV light reflectance analyses (Figure e,f). Pyrite occurs in the resinite, with a granular aggregation morphology, and their development is from the edge to the center of resinite (Figure c–f), indicating that the resinite is replaced by the H2S-rich fluid[43,50] and the genesis of pyrite requires the combination of ferrous irons with sulfur.

Variation of Organic and Pyritic Sulfur

The vertical variation of pyrite sulfur is significantly different from that of organic sulfur and trace elements (except Ga, Tl, and W). The vertical changes of pyrite sulfur and trace elements are directly limited by medium alkaline conditions, the concentration of ferrous ions, and the availability of sulfur.[50,51] The vertical curve of pyrite sulfur content and Fe content in the 15 coal plies is given in Figure . The Fe content of RZ-6 is significantly higher than that of other layers, indicating that pyrite is the principal mineralogical host of iron (Figure d). Although some layers could be interpreted as extensive marine transgression,[17] the organic sulfur contained in the M9 coal seam is apparently higher than the pyrite sulfur (Table and Figure ). Due to the fact that the reaction of organic matter with H2S can form organic sulfur, the output of H2S is high and Fe2+ supplements are insufficient.[29,52,53] Only a few minerals occur in the M9 coal, and there is a lack of clastic material (Figure ). High-organic-sulfur coal is generally affected by seawater and accumulation of algal material, which is considered to have been formed in clastic deficient basins.[44]

Relatively high pyrite can be formed in RZ-4, RZ-6, RZ-13, and RZ-14 piles that contain low Al and Si contents, representing clay minerals (Tables and 2). In contrast, the samples RZ-2, RZ-3, and RZ-12 have a relatively high clay minerals content, while the pyrite content is extremely low. This is inconsistent with previous conclusions about the positive correlation between pyrite sulfur and clay mineral content.[11,48] These samples are developed with the weaker oxidative environments (Figure ). Apart from that, the overall environment is under weak reduction–oxidation conditions and is hardly or weakly affected by brackish water.[17] Compared with other samples, the RZ-7 layer has a higher sulfate sulfur content (sulfate sulfur 0.05%, pyrite sulfur 0.18%, and organic sulfur 3.30), indicating that the H2S is sufficient (Table ), which is beneficial to sulfate sulfur accumulation. Coupled with active anaerobic bacteria, a sufficient supply of SO42–, and thorough decomposition of organic matter, it will lead to the formation and accumulation of organic sulfur.[44,52−54]

As shown in Table , the M9 coal has a high ash content (>20%) and high organic sulfur and low pyrite sulfur, although individual samples (RZ-1 and RZ-9) have low pyrite sulfur and organic sulfur. A prominently high content of pyrite sulfur (1.55% pyrite sulfur and 3.91% organic sulfur) has been detected in sample of RZ-6, which is more than 5 times that of the others. At the same time, the content of clay minerals in this layer is relatively low. It is also possible that the upper region of RZ-5 contains kaolinite with high Al and Si contents and has low permeability, a slow dissolution rate, strong resistance to the acid–base, and compactness between pores and cracks,[43] which can act as a barrier, preventing sulfate penetration.

Conclusions

All coal samples affected by the marine microenvironment are dominated by organic sulfur, which occurs in semi-dark coal and semi-bright coal lithotypes. However, locally high pyritic sulfur occurs in semi-dark coal and dull coal lithotypes.

Trace elements are mainly related to pyrite and ash, and physical cleaning can be applied to partially remove them. However, thalassophile elements Na, Ca, and Mg contained in organic sulfur are difficult to remove through physical methods.

The mineral composition of the M9 coal seam is dominated by kaolinite, followed by dolomite, and it also includes minor amounts of illite, feldspar, pyrite, siderite, hematite, chalcopyrite, calcite, and marcasite. In addition, pyrite is the main sulfide in coal, and agglomerated chalcopyrite and granular galena are partially visible. The modes of occurrence of pyrite include fine-grained, spherical, irregular block-shaped, and clumps.