Geochemical characteristics of REY, Li, Ga trace elements in the No. 9 coal seam of the Daheng mine, Ningwu coalfield, Shanxi Province, China

Jin-xi Wng, Zhi-heng Fu, Y-fn Hu, Zhen Yng, Ji-ling M, Yu-zhung Sun

a Key Laboratory of Resource Exploration Research of Hebei Province, Hebei University of Engineering, Handan 056038, China

b School of Resources and Geosciences, China University of Mining and Technology, Xuzhou 221116, China

c Institute for Atmospheric and Environmental Sciences, Faculty of Geoscience and Geography, Goethe-University Frankfurt, Frankfurt am Main 60438, Germany

Keywords:Trace elements Rare earth elements Geochemistry Coal Taiyuan Formation Mineral exploration engineering Shanxi Province China

ABSTRACT To understand the geochemical characteristics of the No.9 coal in the Daheng Mine of the Ningwu coalfield, the trace element analysis was conducted through X-ray fluorescence spectroscopy (XRF) and inductively coupled plasma mass spectrometry (ICP-MS). The sedimentary environment was discussed according to the element geochemical parameters. The results show that Li, Ga, Hf, Zr, Nb, Th, and Ta are slightly enriched in the No. 9 coal of Daheng Mine. The average value of the rare earth elements and yttrium (∑REY) in coal here is 144.20 μg/g (excluding parting), which is higher than the average value of∑REY in the world’s coal and China’s coal. The light rare earth elements (LREY) are enriched. The content of Eu was 0.12‒2.10 μg/g with an average of 0.57 μg/g, and the Eu is obviously negatively abnormal. Most of the trace elements in the coal are positively correlated with the ash content, which shows that the occurrence of these trace elements is related to inorganic minerals. The results of sequential chemical extraction experiments show that rare earth elements mainly exist in coal in the form of aluminosilicate. The value of the Sr/Ba and the content of S reflect that the coal-forming environment was influenced by seawater. The values of V/Cr and Ni/Co reflect that the peat swamp is in an anaerobic environment and a strongly reducing environment during the coal-forming period.

1. Introduction

In recent years, the study of elemental geochemistry in coal has attracted increasing attention, which makes the value of beneficial elements in coal fully and effectively utilized,and at the same time, controls the damage of harmful trace elements in coal to the environment and human health. Many scholars have studied the content, occurrence state, and genetic environment of trace elements in coal in different regions and eras of China (Tang XY and Huang WH, 2004;Zhao CL and Sun YZ, 2008; Dai SF et al., 2012; Sun YZ et al., 2012; Zhuang XG et al., 2013; Yuan Y et al., 2015; Huang SQ et al., 2021). For example, significant enrichment of Ga,Rb, Cs, REE, Y, and Ba was found in Jurassic coal in China,and the enrichment factors were studied as well (Zhao CL et al., 2014, 2017). The content and enrichment mechanism of associated metal elements in China’s coal were studied, and the comprehensive utilization of certain associated metal elements in coal was discussed in recent years (Sun YZ et al.,2010, 2013a, 2013b, 2014, 2017; Qin SJ et al., 2015). Qin SJ et al. (2016) studied the geochemical characteristics of associated elements in Late Permian coal in southwest Guizhou Province. Both hazardous and valuable elements have been reported in various types of China’s coal (Wang WF et al., 2003; Wang JX et al., 2013, 2019; Liu BJ et al.,2020). However, due to the differences in local sedimentary environments, the geochemical characteristics of the elements vary. The Daheng Mine is located in the north part of the Ningwu Coalfield, Shanxi Province. There is a large amount of coal storage, but there are a few reports on the geochemical characteristics of elements in coal in this area. Taking the No.9 coal of Daheng Mine of the Ningwu coalfield as the research object, this paper discusses the elemental geochemical characteristics and the environmental characteristics of a coal formation by using the elemental geochemical parameters of the No. 9 coal seam.

2. Geological setting

The Daheng Mine is located in the southeast of Pinglu District, Shuozhou City, Shanxi Province, and belongs to the eastern part of Maguan River in the Pingshuo mining area,which is in the north of Carboniferous‒Permian the Ningwu coalfield in North China. There are typical North China strata in the Mine, with the missed Upper Ordovician and the Lower Carboniferous, and the total thickness of coal-bearing strata reaching up to 500 m. The Mine is generally a syncline structure and the axis of which is located in the southwest of the Mine. The axis direction is north-west, and the eastern wing of the syncline contains most of the Mine. The Mine also has several faults, and the direction is generally consistent with the direction of the syncline axis, and the geological conditions are more complicated.

The main coal-bearing strata of the Daheng Mine are the Taiyuan Formation of the Late Carboniferous (Pennsylvanian)to the Early Permian (Cisuralian). At the end of the Late Carboniferous, under the north-south compressive stress, the northern Yinshan Mountains uplifted and the seawater receded, and the sedimentary system became a coastal weak barrier lagoon tidal flat system. In this lagoon tidal flat system, the coal seam of the Taiyuan Formation formed in the Ningwu Basin. The thickness of the stratum of the Taiyuan Formation is 75‒113 m, the average thickness is 91.02 m, and the average total thickness of the coal seam contained is 32.89 m.The No. 9 and No. 11 coal seams are mineable in this mine(Fig. 1). The No. 9 coal is a stable and minable coal seam with a thickness of about 10‒20 m. The No. 9 coal has a resource of over 70×106t and a recoverable reserve of 30×106t.The coal quality belongs to bituminous coal with medium ash and sulfur, which is good coal for power and oil refining.

3. Sampling and methods

Following the Chinese standard (GB/T482-2008), a total of nine samples, including seven coal samples (DH901‒DH903,DH905‒DH908), and two partings (DH904 and DH909) of relevant layers were systematically collected from Daheng Coal Mine. During the experiment, in order to ensure the accuracy of the data, the samples were crushed repeatedly by the sample preparation machine. Finally, the coal samples with different size of 18‒40 mesh and 200 mesh were screened out with a sieve. Among them, the 18‒40 mesh samples were prepared as briquettes for optical microscope and SEM observation, the 200 mesh samples were used for proximate analysis, inductively coupled plasma mass spectrometry (ICP-MS), and X-ray fluorescence analysis(XRF) analysis. Plasma low-temperature ashing (LTA)apparatus was used to conduct low-temperature ashing of all samples, and the temperature was less than 200℃ during the process. The X-ray diffraction analysis (XRD) on LTAresidues and five-step sequential chemical extraction have been carried out to determine the types of minerals and the state of element in all samples, respectively.

Fig. 1. a‒Location of the Daheng Mine and the Ningwu coalfield in Shanxi Province; b‒sedimentary sequences in the Daheng mine and the section of the No. 9 coal seam.

4. Results and discussion

4.1. Trace elements content and distribution characteristics

Table 1 lists the content and average value of trace elements in coal samples from the Daheng Mine. The content of most trace elements in the No. 9 coal of Daheng Mine is lower than that of Chinese coals (Dai SF et al., 2012) and the average value of coal in the world. Meanwhile, the proposedNote: nd represents no data.CC (CC=Element Content in Coal/World Coal Content) value is used to characterize the dispersion and enrichment of trace elements in coal (Dai SF et al., 2014). The enrichment coefficient is classified into six levels: CC<0.5 indicates losses; 0.5CC2 indicates normal; 2100 indicates abnormally high enrichment (Fig. 2).

Table 1. Content characteristics and ratio parameters of trace elements in the coal from Daheng Mine.

According to the enrichment coefficient, the CC values of Li, Ga, Hf, Zr, Nb, Th, and Ta are all greater than 2 and less than 5, which are slightly enriched in the No. 9 coal of Daheng Mine. The CC value of Cr, Co, Ni, Cu, Ge, Rb, Cs,and Bi is less than 0.5, which indicates the loss of elements.This phenomenon may be caused by the presence of several faults, which allow fluids to pass around the coal seam and carry these elements to be leached, and the coal seam can also exchange materials with the surrounding rock, resulting in relatively low levels of these elements (Yang L et al., 2008).The other elements are in the normal range. As a whole, the overall level of trace elements in the No. 9 coal seam of Daheng Mine is low, which is similar to the characteristic of low trace elements content in Late Paleozoic coal of North China (Dai SF et al., 2003).

4.2. Content and distribution characteristics of rare earth elements

Rare earth elements and yttrium (REY) are stable in nature and their geochemical parameters can reflect the characteristics of rare earth elements. The authors standardize the samples with upper crustal elements (Taylor SR and Mclennan SM, 1985), and discuss the geochemical characteristics of rare earth elements in Daheng Mine by using LREY, MREY, HREY, ΣREY, (La/Lu)N, (La/Sm)N,(Gd/Lu)N,δEu,δCe and other geochemical parameters (Table 2).

(i) The average value of LREY in the No. 9 coal of Daheng Mine is 116.52 μg/g, the average value of MREY is 23.39 μg/g, and the average value of HREY is 4.28 μg/g. The average value of ΣREY is 144.20 μg/g (excluding parting),which is 2.1 times the world’s coal mean (68.41 μg/g) and is similar to the Chinese coal mean (135.89 μg/g). Probably due to the different depositional microenvironments, the ΣREY content varies greatly in different stratified samples of the No.9 coal, with the maximum value being about 2.7 times the minimum value, and the distribution of rare earth elements in the coal seam profile is uneven.

Fig. 2. Enrichment coefficient of trace elements of the No. 9 coal in Daheng Mine of Ningwu coalfield, Shanxi Province.

Table 2. Content and geochemical parameters of rare earth elements in the coal from Daheng Mine of Ningwu coalfield, Shanxi Province.

The ΣREY value of the parting DH904 is 21.39 μg/g,which is very low. It is speculated that the rare earth elements in the parting are carried out by water leaching and filtered into the underlying coal seam. While the ΣREY value of the parting DH909 is 599.45 μg/g and such a high content may be due to the accumulation of rare earth elements from water leaching at the bottom.

(ii) Seredin VV and Dai SF (2012) classify rare earth elements and yttrium in coal into light rare earth enriched [Ltype, (La/Lu)N>1], medium rare earth enrichment type [Mtype, (La/Sm)N<1, (Gd/Lu)N>1] and heavy rare earth enrichment type [H-type, (La/Lu)N<1]. The range of (La/Lu)Nin the No. 9 coal of Daheng Mine is 0.64‒2.28, with an average value of 1.47, which is greater than 1. All samples are light rare earth enriched except DH903 and DH904 which are medium and heavy rare earth enriched, and DH906 is heavy rare earth enriched. In general, the Daheng Mine is obviously rich in LREY, while MHRY and HREY are relatively deficient.

(iii) The distribution pattern of rare earth elements can visually reflect the geochemical characteristics of rare earth elements. The standardization value of parting DH909 is obviously high (Fig. 3), and DH904 is opposite to it. The distribution curves of coal samples DH903 and DH906 are inclined to the LREY. The distribution patterns of rare earth elements in the other samples are basically the same, slanted from left to right, with a broad and slow “V” curve as a whole and a small “V” valley at Eu. The mean value ofδEu is 0.78,the negative anomaly is obvious, and there’s no obvious anomaly inδCe. It is inferred that the main sources of rare earth elements are the same during coal formation in No. 9 coal seam of Daheng Mine, and the supply of terrestrial materials is relatively stable, and the distribution pattern of rare earth elements is similar to the upper crust of North China, reflecting that they may have consistent material sources and a common tectonic background.

4.3. Element occurrence state

The occurrence state of trace elements in coal is more complex, and they can exist in a single mineral or as adsorption or isomorphism states in coal. When the elements are combined with organic matter or minerals in coal, they can form an organic bound state and an inorganic bound state accordingly. The correlation analysis of trace elements and ash in coal can be used as a way to determine the affinity of elements (Ketris MP and Yudovich YE, 2009; Cui GL et al.,2004).

There is a positive correlation between ash and trace elements in coal, with ash highly positively correlated with Ga (0.95), Ta (0.93), W (0.93), Nb (0.92), Ni (0.88), Rb(0.84), Cs (0.84), Sc (0.75), Li (0.72), Hf (0.70), and positively correlated with Mo (0.62), Bi (0.58), Cu (0.47), Tl(0.44), Zn (0.42), Zr (0.41), Pb (0.38), U (0.38), V (0.37), Co(0.36), but only negatively correlated with Sr (−0.47),showing that most of the elements are related to minerals in coal and occur in inorganic minerals. The rest of the elements have no obvious correlation with ash, and they correlate with both minerals and organic matter.

Under normal circumstances, rare earth elements in coal are mainly combined with silicate minerals, and their content is controlled by terrigenous clast or hydrothermal fluids, and also affected by the content of inorganic components (Dai SF et al., 2006; Swaine DJ and Goodarzi F, 1995), while rare earth elements in coal also can be combined with organic matter (Seredin VV and Dai SF, 2012). In this paper, the occurrence state of rare earth elements in coal is analyzed by the method of sequential chemical extraction experiments(five-step sequential extraction) (Dreler GB and Finkelman RB, 1992).

According to Fig. 4, rare earth elements have a similar occurrence state in coal: The highest percentage of rare earth elements in the residue state, mostly above 90%; followed by the percentage in the organic state, between 4% and 9%; the content in the exchangeable state, carbonate binding state, and iron-manganese oxide bounded state are very small, with an average of 2.6%, which is one of the important manifestations of the chemical stability of rare earth elements. Among the light and medium rare earth elements, with the increasing atomic number (excluding Y), the content of residual slag state tends to gradually decrease, and the content of rare earth elements combined in organic state and iron-manganese oxide state tends to gradually increase, while the changing trend of heavy rare earth elements is the opposite. The content of medium and heavy rare earth in the organic state is higher than that in light rare earth, indicating that medium and heavy rare earth show stronger binding ability with organic matter,which is consistent with the conclusion reached by Eskenazy GM (1999). In the 5-step sequential chemical extraction experiments, the general residue state includes aluminosilicate state and sulfide state, and the total sulfur content in the No. 9 coal seam is low, with a mean value of 1.78%. Therefore, it is believed that the aluminosilicate state is absolutely dominant in the residual state, and it can be inferred that the rare earth elements mainly exist in the aluminosilicate state.

According to low-temperature ashing and X-ray diffraction analysis, optical microscope, and scanning electron microscope observation with energy spectrum, it is found that the average mineral content of coal samples is 13.28%, with the most abundant clay minerals mainly kaolinite, followed by a small amount of pyrite and quartz and a small amount of calcite (Fig. 5). Finally, it is inferred that rare earth elements are mainly in the kaolinite-dominated clay minerals and to a lesser extent in organic matter.

4.4. Environmental significance of trace elements

Fig. 4. Percentage content of rare earth elements in the coal from Daheng Mine of Ningwu coalfield, Shanxi Province. I‒ion exchangeable state; II‒carbonates associated state; III‒ferromanganese oxide associated state; IV‒organic associated state; V‒residual state.

Fig. 5. XRD spectrum of minerals in the DH906 of coal sample from Daheng Mine of Ningwu coalfield, Shanxi Province. K‒kaolinite; Q‒quartz; P‒pyrite.

The distribution characteristics of trace elements in coal are greatly affected by the depositional environment. In turn,it can be used the ratio of certain trace elements or the geochemical characteristics of certain trace elements to infer and evaluate the depositional environment at that time.Elements, such as S, Sr, Ba, B, Th, U, Ga, V, Ni, and Co, are generally believed to be more sensitive elements reflecting the environment (Liu DM et al., 2000; Sun YZ et al., 2017; Zhao QJ et al., 2020). These elements can provide an indicative role in discerning the redox conditions of the sedimentary environment and reflecting the paleo-salinity of the water medium, as well as in distinguishing between freshwater and seawater sediments.

4.4.1. Sulfur (S), Strontium (Sr), Barium (Ba)

Generally, coals formed in sedimentary environments affected by seawater has higher sulfur contents, and coals formed in inland freshwater environments have lower sulfur contents (Sun YZ et al., 2017), but in this case, there may also be high sulfur content, presumably due to the formation of pyrite after the intrusion of hydrothermal fluid into the coal seam (Dai SF et al., 2012).

Tang SH et al., (2006) considered that the total sulfur content increases sequentially from terrestrial sedimentary coal (<1.5%) to terrestrial and marine transition coal (average 2%‒5%) to sedimentary coal in shallow sea environment (up to 6%‒10%). Sr/Ba value is used to judge the salinity of the water. Sr/Ba value greater than 1 is generally considered to be modern marine sediment, and Sr/Ba value less than 1 is often considered to be modern and ancient continental sediment(Wang YY et al., 1979). The total sulfur content in the No. 9 coal seam of Daheng Mine is 0.64%‒2.84%, with an average value of 1.78%, and the value of Sr/Ba is generally greater than 1. Based on this, it is inferred that the No. 9 coal seam was formed in a coal-forming environment affected by seawater.

4.4.2. Nickel (Ni), Vanadium (V), Chromium (Cr), and Cobalt(Co)

The values of V/Cr and Ni/Co can be used as reliable indexes to judge the sedimentary environment (Wang JX et al., 2015; Yuan K et al., 2020). When V/Cr<2, it is an oxidizing condition, the value of V/Cr between 2‒4.25 is an oxygen-lean environment, and V/Cr>4.25 is a reducing environment (Jones B and Manning DAC, 1994). When Ni/Co<5, it is an oxidation environment, when the value of Ni/Co is between 5 and 7, it is an oxygen-lean environment,and when Ni/Co>7, it is a reducing environment (Adams JAS and Weaver CE, 1958). Finkelman RB, (1999) used V/(V+Ni)values to classify the sedimentary environment as the anaerobic environment with water stratification (0.84‒0.89),anaerobic environment with not strong water stratification(0.54‒0.82), and the anaerobic environment with weak water stratification (0.46‒0.60).

The results show that most of the V/Cr values of the No. 9 coal seam of Daheng Mine are between 2 and 4.25, and the average value is 3.45. The Ni/Co value varies greatly, with the maximum value of 24.32, the minimum value of 4.36, and the average value of 8.20. Only two values of V/(V+Ni) are less than 0.82, and the average value is 0.82, reflecting that the overall coal-forming environment is a strongly reducing environment, and the water body at the No. 9 coal seam level is an oxygen-poor-anaerobic environment with strong stratification.

4.4.3. δEu, δCe

δEu andδCe represent the anomalies of Eu and Ce.Normally, coal samples controlled by land sources have Eu negative anomalies, and Eu negative anomalies indicate acidic or reducing environments. TheδEu value in the No. 9 coal seam of the Mine is 0.78, and the negative Eu anomaly is obvious, indicating that the coal formation process is mainly controlled by continental facies and the sedimentary environment is mainly an acidic and reducing condition. In the seawater environment, Ce3+is easily transformed into Ce4+, thus Ce negative anomaly is an indicator of the marine facies environment. However, in the marginal sea, shallow sea, and land-closed sea, there is basically no anomaly inδCe.In the outer sea and open sea, there is an obvious anomaly(Cui XN et al., 2016). The average value ofδCe in the No. 9 coal seam is 0.97, with no obvious negative anomaly,indicating that the coal-forming environment in this area is less affected by seawater.

5. Conclusions

(i) According to the enrichment coefficient CC, Li, Ga,Hf, Zr, Nb, Th, Ta, and other lithophile elements are slightly enriched in the No. 9 coal of Daheng Mine. Cr, Co, Ni, Cu,Ge, Rb, Cs, and Bi are severely deficient, other elements are equivalent to the content of trace elements in Chinese coal,and the overall level of trace elements in the No. 9 coal seam is low.

(ii) Rare earth elements and yttrium are enriched in the No. 9 coal seam, and the content of rare earth elements is 89.23‒246.61 μg/g, with an average value of 144.2 μg/g. In general, light rare earth elements are significantly enriched while medium and heavy rare earth elements are relatively deficient in Daheng Mine.δEu mean value is 0.78, with an obviously negative anomaly. The mean value ofδCe is 0.97 with no anomaly. By sequential chemical extraction experiments, XRD, XRF, and correlation analysis, it is concluded that rare earth elements mainly present in clay minerals with a dominance of kaolinite, in addition, a small amount of them present in organic matter.

(iii) The content of S and Sr/Ba, V/(V+Ni), V/Cr, Ni/Co,δEu, andδCe values generally reflect that the coal-forming environment of the No. 9 coal of Daheng Mine is reducing terrestrial peat with the affected of seawater.

CRediT authorship contribution statement

Jin-xi Wang conceived of the presented idea. Jin-xi Wang,Zhi-heng Fu, Ya-fan Hu, Zhen Yang, and Jia-liang Ma wrote the manuscript in consultation. Yu-zhuang Sun supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflict of interest.

Acknowledgment

This research was financially supported by the National Natural Science Foundation of China (41807305, 41872173),and the Science Foundation of Hebei (E2020209074,D202040 2013).