Studies of mercury content in selected coal seams of the Upper Silesian Coal Basin

The structure of fuel consumption in Poland has changed very little in recent years, and coal and lignite are still the dominant fuels in the energy sector. The presence of mercury in coal is a common phenomenon. Its content in coal samples from various deposits from around the world typically ranges from 0.03 to 0.3 mg/kg; however, the amount of mercury in each seam varies. This paper presents the results of mercury content studies of nearly 100 samples collected from the coal seams of the Upper Silesian Coal Basin. The seams selected for examination represent different lithostratigraphic members (Libi1¿, £aziska, Orzesze, Ruda, Siod3o, Porêba, and Jaklowiec horizons). Coal samples from a total of 35 seams in 17 mines were studied. The mercury content analysis indicated that the amount of this element varied in the examined material. The average mercury content in the sampled seams was comparable with the data provided in exiting documentation or was much lower. The total mercury content measured in all the coal samples ranged from 0.0029 to 0.3026 mg/kg, and averaged 0.0739 mg/kg. Frequently, in individual samples from genetically similar deposits, mercury content in coal varied by several orders of magnitude, which was evident primarily within the £aziska and Ruda horizons. There is no clear differentiation in mercury content between the eastern and the central-western mining regions. In the samples collected in all the mines, the average mercury content ranged from 0.0331 to 0.1589 mg/kg. There was no relationship between the average mercury content in coal from different mines and the lithostratigraphic origin of the mined seams. The total mercury content was provided in the air-dry (analytical) state of the samples. Due to the small number of samples collected from the mined seams and the wide range of variability discovered, the conducted studies do not allow for the relation of total mercury content in selected seams to their lithostratigraphic position. These results, however, have significantly extended the scope of knowledge about mercury content in coal from the USCB seams. Moreover, the study of coal content variability has a distinct cognitive aspect. 106


Introduction
The presence of mercury in coal is well known.Mercury is a natural contaminating element present in coal and in other fossil fuels.The concentration of mercury in coal may vary, and its content in black coal is generally lower than in lignite (e.g.Bojakowska, Soko³owska 2001; Chmielniak et al. 2012).Mercury occurs in coal in several forms, probably mainly as divalent mercury.Most mercury compounds in coal (60-70%) occur in combination with sulphur as additives to FeS 2 pyrite and as HgS mercury sulphide.Some of it can also be bound to coal maceral.Mercury as a component of the organic fraction of carbon constitutes the remaining 30-40% (e.g.Bojakowska, Soko³owska 2001;G³odek, Pacyna 2007).
In Poland, no regular studies have been conducted to investigate the total mercury content in coal, and the available data indicate a significant divergence in the obtained results.A comparison of the results presented in past documentation shows that in the existing coal mines in Poland, the average mercury content in coal ranged from 0.06 to 0.15 mg/kg, although content which significantly differs from the average has also been reported (Bojakowska, Soko³owska 2001;Bojarska 2006;Wojnar, Wisz 2006;Smoliñski 2007;Chmielniak et al. 2012;Okoñska et al. 2013).Mercury content in coal deposits around the world also varies significantly.The content of this element in coal samples from different deposits generally ranges from 0.03 to 0.3 mg/kg.Test results of coal samples from the United States indicate mercury content ranging from 0.03 to 0.20 mg/kg, from Russia between 0.07 and 0.12 mg/kg, from China between 0.06 and 0.08 mg/kg, from Australia between 0.05 and 0.1 mg/kg, and from the U.K. of 0.11 mg/kg (http://ec.europa.eu/, 2004;Olkuski 2007;Smoliñski 2007).
The aim of the research presented in this paper was to determine the mercury content in coal seams of the Upper Silesian Coal Basin (USCB), and to present the test results in the context of prior studies.The various Polish coal deposit samples analysed were mined in 2011 and provided by mining operators.

Analysed research area
The area subjected to the mercury content study included selected coal seams of the Upper Silesian Coal Basin (USCB), which is the main Polish coal basin.The mined areas 96 corresponding to individual mines where coal samples were collected are shown in Figure 1 and Table 1.
Existing coal resources in the Upper Silesian Coal Basin have been documented in 128 deposits, including 36 which have been abandoned (as of 31 Dec. 2012).In 2012, coal mining in Poland was estimated at 71.3 million tons, of which 64.7 million tons were from the Upper Silesian Coal Basin (Szuflicki et al., edited, 2013).
In the present study, the USCB area was divided into three parts -the western, central, and eastern.A schematic cross-section through the region is shown in Figure 2 (Kotas 1987).In the Upper Carboniferous productive series, sediments were distinguished which differed in lithological and facies formation and in the thickness of the seams and resources.

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The mined coal seams were mainly located in the members of the Krakow sandstone series, the mudstone series, and the Upper Silesian sandstone series (Fig. 2).Within these series, a number of horizons may be distinguished where coal seams are mined.The Libi¹¿ (group 100) and £aziska (group 200) horizons are the shallowest.Deeper horizons include Orzesze (group 300) and Ruda (group 400).The deepest horizons are entirely located in the western part of the Upper Silesian Coal Basin and include Siod³o (group 500), Porêba (group 600), and Jaklowiec (group 700).Different types of coal, from power coal to coking coal, exist in coal seams.Power coal occurs in the eastern and central areas, while in the west the degree of coalification increases.The thickness of the individual horizons also increases westwards (Kotas 1987;Osika 1970).

The adopted research methodology
The outcome of the study was the determination of total mercury content in selected Polish coal deposits mined in the Upper Silesian Coal Basin.The examined seams represented different lithostratigraphic members.A variability analysis of the total mercury content in the selected seams of the Krakow sandstone series was conducted in the Libi¹¿ and £aziska horizons; in the mudstone series the Orzesze horizons; and in the Upper Silesia sandstone series the Ruda and Siod³o horizons as well as the Porêba and Jaklowiec horizons.Nearly 100 coal samples were analysed for total mercury content.From 1 to 6 samples were collected from each seam.Coal samples from a total of 35 seams in 17 mines (Table 1 and 2) were studied.Coal samples were collected directly from the seams and did not undergo the enrichment process.Raw sample coals were reduced to laboratory samples and subjected to grinding and mixing during the preparation of test samples (particle size < 0.2 mm).
Total mercury was determined in all the samples.For each sample two measurements were made, and the value analysed was the arithmetic mean of the measurements.Employed was a modification of the classical AAS method, which allows measurement of threshold values (in nanograms) of mercury in solid and liquid samples of different origin.The advantage of the applied methodology is that is it possible to eliminate the sample pretreatment step.Sample mineralisation takes place within the analyser, immediately prior to 99 the detection process.This minimizes losses of mercury which always occur in conventional analytical methods.To conduct the study AMA-254, a dedicated atomic absorption spectrometer by Altec was used.The study method applied corresponding analytical techniques recommended in the work of A. Okoñska et al. (2013) to determine mercury content in coal samples.

Analysis of study results on mercury content in coal seams
Total mercury content measured in all the samples of coal ranged from 0.0029--0.3026mg/kg.In the range limited by the upper and lower quartiles (25 and 75%), mercury content ranged from 0.0316-0.0989mg/kg, with an average value of 0.0567 mg/kg.The obtained results were comparable with values reported in the work of A. Michalska and B. Bia³ecka (2012) (Table 3).
The measured mercury content in all the coal samples collected from the seams in the Upper Silesian Coal Basin is listed in Figure 3.The total mercury content was provided in the air-dry (analytical) state of the samples.The moisture content in the carbon samples in the analytical state was not large, generally ranging from 1.3 to 4.7%, and only in individual 100 In individual mined regions, the total mercury content in the samples from all the seams of a particular mine (Table 1) was averaged.Mercury content in 17 coal mines averaged 0.0852 mg/kg.The highest values were obtained for mine No. 3 (seam in £aziska horizons) and mine No. 7 (seam in Orzesze horizons), where the averaged total mercury content in the samples was 0.15-0.16mg/kg.The lowest average total mercury content of 0.03-0.04mg/kg was reported in coal samples from mine No. 10 (seams in Siod³o and Porêba horizons), mine No. 1 (seams in Orzesze, Ruda, and Siod³o horizons), and No. 17 (seams in £aziska and Orzesze horizons).No relationship has been observed between the average mercury content in coal from different mines and the lithostratigraphic location of the seams.
Taking into account the fact that the coal samples came from different horizons and seams (Table 2, Figure 4), the highest mercury content of up to 0.3 mg/kg was found in samples from the seams of group 200 (£aziska horizon) and group 400 (Ruda horizon).The lowest mercury content of only 0.003 mg/kg was measured in samples from the seams of group 200 (Table 2).After the measured values from all samples within a particular seam were averaged, it can be said that the total mercury content in the analysed seams ranged between 0.031-0.101mg/kg.The highest average values were observed in the group of seams from the Orzesze horizon, and the lowest in the group of seams from the Libi¹¿ horizons.It should be noted that in two samples, high point concentrations of mercury in excess of 2 mg/kg were reported.In the analysis of the results and the estimation of average values however, this value was omitted.It may be assumed that such high levels of mercury are not typical for the seam and may only be associated with localized carbon mineralisation.The most likely causes of a local increase in mercury content in coal are trace inclusions of mercury minerals (e.g.cinnabar) observed in the Polish coal deposits, especially in the Wa³brzych area (Bojakowska, Soko³owska 2001).An analysis of the results leads to the conclusion that varying total mercury content in coal may be observed in the Upper Silesian Coal Basin coal deposits.However, a limited number of observations in individual seams does not allow any conclusions about the relationship between the content of this element in the examined material and different lithostratigraphic members.Nevertheless, it does supplement the scope of knowledge concerning the content of mercury in coals from the USCB seams.A significant divergence of the values depends rather on the number of samples collected within an analysed horizon of a particular group (Table 2).Similar conclusions were widely described in the work of Z. Kokesz (2010) with regard to sulphur.
The average mercury content in the samples from the mined seams analysed in this study amounted to 0.0739 mg/kg.There was no clear differentiation in mercury content between the eastern and the central-western mining regions.In the eastern part (mining and sampling area -mines Nos.16 and 17), the measured total mercury content ranged from 0.0029 to 0.2987 mg/kg, with an average of 0.0512 mg/kg.In the central-western part (mining and sampling area -mines Nos. 1 to 15), the total mercury content ranged from 0.0076 to 0.3026 mg/kg, and averaged 0.0899 mg/kg.The average mercury content in the sampled seams was much lower than in the case of power coal described by A. Okoñska et al. (2013) and was at a level comparable to that of the data provided in the papers by other authors (Bojakowska, Soko³owska 2001;Chmielniak et al. 2012) listed in Table 3.The research methodology adopted in the present study was the same as that prescribed and used in the works of A. Okoñska et al. (2013), T. Chmielniak et al. (2012), andA. Michalska andB. Bia³ecka (2012).W ostatnich latach struktura zu¿ycia paliw w Polsce zmieni³a siê w niewielkim stopniu, a paliwa dominuj¹ce w gospodarce energetycznej to nadal wêgiel kamienny oraz wêgiel brunatny.Powszechnie znanym zjawiskiem jest obecnooeae rtêci w wêglach.Jej zawartooeae w próbkach wêgla kamiennego pochodz¹cych z ró¿nych z³ó¿ oewiatowych kszta³tuje siê najczêoeciej na poziomie od 0,03 do 0,3 mg/kg, przy czym zawartooeae rtêci w poszczególnych pok³adach jest zró¿nicowana.
Ze wzglêdu na ma³¹ liczbê próbek pobranych z eksploatowanych pok³adów oraz stwierdzony szeroki zakres zmiennooeci, przeprowadzone badania nie pozwalaj¹ na powi¹zanie zawartooeci rtêci ca³kowitej w wybranych pok³adach z ich pozycj¹ litostratygraficzn¹.Wyniki pracy pozwoli³y jednak na zdecydowane rozszerzenie wiedzy dotycz¹cej zawartooeci rtêci w wêglach z pok³adów GZW, a badanie jej zmiennooeci wykazuje wyraŸny aspekt poznawczy.The structure of fuel consumption in Poland has changed very little in recent years, and coal and lignite are still the dominant fuels in the energy sector.The presence of mercury in coal is a common phenomenon.Its content in coal samples from various deposits from around the world typically ranges from 0.03 to 0.3 mg/kg; however, the amount of mercury in each seam varies.

STUDIES OF MERCURY
This paper presents the results of mercury content studies of nearly 100 samples collected from the coal seams of the Upper Silesian Coal Basin.The seams selected for examination represent different lithostratigraphic members (Libi¹¿, £aziska, Orzesze, Ruda, Siod³o, Porêba, and Jaklowiec horizons).Coal samples from a total of 35 seams in 17 mines were studied.The mercury content analysis indicated that the amount of this element varied in the examined material.The average mercury content in the sampled seams was comparable with the data provided in exiting documentation or was much lower.
The total mercury content measured in all the coal samples ranged from 0.0029 to 0.3026 mg/kg, and averaged 0.0739 mg/kg.Frequently, in individual samples from genetically similar deposits, mercury content in coal varied by several orders of magnitude, which was evident primarily within the £aziska and Ruda horizons.There is no clear differentiation in mercury content between the eastern and the central-western mining regions.In the samples collected in all the mines, the average mercury content ranged from 0.0331 to 0.1589 mg/kg.There was no relationship between the average mercury content in coal from different mines and the lithostratigraphic origin of the mined seams.The total mercury content was provided in the air-dry (analytical) state of the samples.
Due to the small number of samples collected from the mined seams and the wide range of variability discovered, the conducted studies do not allow for the relation of total mercury content in selected seams to their lithostratigraphic position.These results, however, have significantly extended the scope of knowledge about mercury content in coal from the USCB seams.Moreover, the study of coal content variability has a distinct cognitive aspect.

Fig. 4 .
Fig. 4. The averaged results of mercury content measurements in available coal samples from individual groups of seams -own research of authors Rys. 4. Uoerednione wyniki pomiarów zawartooeci rtêci w dostarczonych próbkach wêgli dla poszczególnych grup pok³adów -badania w³asne autorów CONTENT IN SELECTED COAL SEAMS OF THE UPPER SILESIAN COAL BASIN K e y w o r d s Upper Silesian Coal Basin, coal, seams, total mercury A b s t r a c t

TABLE 2
The results of mercury content measurements in supplied coal samples from individual lithostratigraphic members of the eastern, central, and western part of the Upper Silesian Coal Basin -

TABLE 3
Averaged mercury content in coal, including the results of the conducted research

Coal from USCB (averaged according to research authors)
cases reaching 10%.Such hygroscopic moisture values have been found in other studies (e.g.Baic 2013).