Distribution and Mobility Potential of Trace Elements in the Main Seam of the Most Coal Basin

https://doi.org/10.1016/j.coal.2018.07.005Get rights and content

Highlights

  • Coal bearing sediments of the Holešice Member are an important natural source of V, Cr, Ni, As and Pb.

  • Ash yield content does not correspond to the increased element content.

  • The potential source of increased trace elements considers to be the weathered volcanic material.

  • All trace elements evaluated are readily mobile under oxidising conditions.

Abstract

The Most Basin represents a coal basin with a long mining history in the Czech Republic. The majority of palaeo-environmental and geochemical studies have been focused on the upper part of the coal-bearing Most Formation where the coal is generally enriched in As, Cr, Ni, Cu and Zn. This study evaluated the distribution of V, Cr, Ni, Cu, Zn, Pb, Se, As, and Mn from the lower part. Regarding to the transport of inorganic elements into environment, the evaluation of mobility potential was also conducted.

The total of 51 samples were collected from the Lower Bench in the Holešice Member. The chemical analyses were performed together with optical microscopy. The X-ray spectroscopy and ICP-MS were used to detect trace element contents in the study set of samples. 4 samples were subjected to the Sequential Extraction Procedure. The prepared leachates were analysed using the ICP-MS method.

In total, 13 elements were screened in samples within the study profile. The mode of occurrence is the key factor for identifying a potential source of the trace elements. All evaluated trace elements (V, Cr, Ni, Cu, Zn, Pb, Se, As and Mn) leached most in the oxidisable fraction. On the other hand, As, Mn, Ni, Zn and Cd leached in more mobile fractions, e.g. reducible and acid.

Introduction

The study of trace elements in coal provides not only useful data related to the sedimentological environment, but also reveals information required to minimise the environmental impact during the use of coal. Inorganic components in the coal can originate from several sources: 1) original organic matter, 2) formation during the stages of coalification, 3) carrying away by water or wind, 5) products of alteration of primary minerals. Trace elements, as a part of the inorganic components of coals, have deserved much attention and many studies have been done on trace element content and their distribution on sub-bituminous and bituminous coals (Adedosu et al., 2007; Finkelman, 1995; Lewinska-Preis et al., 2009; Ren et al., 1999; Suárez-Ruíz et al., 2006; Swaine, 1990; Zhuang et al., 2012, among many others). On the contrary, such conventional studies are scarce for low-rank coals (Gentzis et al., 1996). The modes of occurrence of trace elements vary greatly among coals. In low-rank coals, elements are usually organically bound, but with the progress of coalification, the elements are removed by expulsion of moisture and by changes in the chemical structure of the organic matter (Ward, 2002; Li et al., 2007; Finkelman et al., 2018). Elements bound with discrete minerals remain unchanged (Ward, 2002). The fate of trace elements during coal conversion processes has become a matter of concern due to the large amount of coal that is often used for energy production. The behaviour of trace elements during coal combustion depends on their concentration, mode of occurrence, and combustion parameters. The organically associated trace elements tend to be vaporised, either escaping into the atmosphere or adsorbed on the fine fly ash particles upon combustion in the furnace. The inorganically associated elements are generally non-volatile and tend to retain in the bottom ash and/or the fly ash particles upon combustion. On the other hand, the presence of trace elements in coal may help in understanding phenomena such as the geological history of coal-bearing sequences in sedimentary basins, the conditions ongoing due to coal seam formation, the depositional environment, and the influence of tectonics (Dai et al., 2012).

The mobility potential has been successfully evaluated in coal by the modified Sequential Extraction Procedure (SEP) (Cabon et al., 2007). The general principle of the method is to gradually leach the elements with a decreasing pH agent. The mobility potential of any trace element increases in the following order: oxidisable, reducible and acid and allows to predict its behaviour in environment under given conditions.

The Most Basin (Late Eocene-Early Miocene) represents an economically significant coal basin for the Czech Republic with only surface mining (Rajchl et al., 2009). The basin belongs to the group of Podkrušnohorská basins (Fig. 1), being a subject of detailed palaeo-environmental and geochemical studies for its unique sedimentary sequence (Havelcová et al., 2012, Havelcová et al., 2013, Havelcová et al., 2015; Mach et al., 2013; Matys Grygar and Mach, 2013; Rajchl et al., 2008; Teodoridis et al., 2011; Teodoridis and Sakala, 2008). The basin formation started during the late Eocene and Oligocene by the intensive volcanic activity. The sedimentation followed by the clastic and organic deposits from the end of the Oligocene to the early Miocene (Mach et al., 2014). During the late Eocene-Oligocene, an intense alkaline volcanism mainly of basaltic character brought the pyroclastic materials and lava thick-bodies into the sedimentary basin, establishing the volcanic Střezov Formation. The following Most Formation is a unique coal-bearing strata (Fig. 2) subdivided to the Duchcov, Holešice, Libkovice and Lom members. The lowermost Duchcov Member contains alluvial sediments, followed by the Holešice Member. The last-mentioned member represents the Main Seam gradually passing to the Libkovice Member. The fine-grained lacustrine clays and lacustrine clay sediments form the uppermost Lom Member (Pešek and Sivek, 2016).

According to the geochemistry and mineralogy, two main complexes were distinguished in the Most Basin. The Lower Bench, including Duchcov Member and lower part of the Holešice Member, contained a higher content of Al, Ti, Nb, Zr, Cr. While the high content of Si, Mg, K, Rb and Cs was typical for the Upper Bench, forming the upper part of the Holešice Member (Elznic et al., 1998). This division was further supported by Mach et al. (2014) with using the significant correlation proxies, e.g. Al, K, Ti.

Taking into account these previous findings, the present work evaluates the mobility potential and the final distribution of V, Cr, Ni, Cu, Zn, Pb, Se, As, Mn in the Lower Bench (Holešice Member) of the Main Seam in the Most Coal Basin.

Section snippets

Samples and methods

Fifty-one samples were collected from the Bílina Mine in the Most Basin. The evaluated set represents a continuous vertical profile of the Holešice Member in the Lower Bench (Fig. 2). The proximate (moisture content Wa, volatile matter content Vd and ash yield content Ad) and ultimate analyses (CHNS), together with the calorific value Qsd, were conducted in all samples according to ISO 17246, 17247 and ISO 1928. The element migration potential in environment was evaluated based on the

Ultimate and proximate analyses

The ultimate and proximate analyses illustrated the variability of a vertical profile (Table 1, Fig. 3). The ash yield (Ad) significantly fluctuated with depth, ranging from 17.4 to 87.7 wt% throughout and reflected the varying amount of the detritus at the time of the deposition. The samples with the ash yield content higher than 50% were according to the ISO 11760 (2005) classified as coaly claystones. The study set of coaly claystones showed the variable Vd content, varying from 11.6 to

Mode of occurrence of trace elements

The results of the PCA revealed that 62.85% of the dispersed variable could be assigned to two factors (Fig. 5). The high positive loadings for Ti (0.84), Cd (0.80), V (0.91), Cr (0.90), Ni (0.85), Cu (0.71) and Pb (0.88) displayed a high positive Factor 1. The elements were probably organically associated and/or originated from Ti-bearing minerals.

In contrast, a high positive loading for As (0.79) and Sd (0.52) have been found in the Factor 2. Therefore, the origin of As from sulphide minerals

Conclusions

Our results have provided a detailed distribution of trace elements within the Lower Bench of the Holešice Member (Most Formation). The dominant maceral group is huminite with mean random reflectance Rr = 0.32%. The mean of huminite reflectance does not show a significant trend throughout the profile of the Lower Bench. The study samples are classified as lignites and coaly claystones.

Ash yield (Ad), a general parameter of coal, cannot be used to estimate trace element content in the study

Acknowledgements

We would like to thank Erasmus+ HE-2015 for supporting the research at the National Institution of Coal in Oviedo. Our thanks also go to the Operational Program Prague – Competitiveness, more specifically, the Centre for Texture Analysis project (project ID: CZ.2.16/3.1.00/21538) and the long-term conceptual development of the research organization RVO: 67985891. This work was financially supported by the RECETOX Research Infrastructure (LM2015051 and CZ.02.1.01/0.0/0.0/16_013/0001761). It is

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