Geochemical Characteristics of Trace Elements in the No. 6 Coal Seam from the Chuancaogedan Mine, Jungar Coalfield, Inner Mongolia, China

Fourteen samples of No. 6 coal seam were obtained from the Chuancaogedan Mine, Jungar Coalfield, Inner Mongolia, China. The samples were analyzed by optical microscopic observation, X-ray diffraction (XRD), scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS), inductively coupled plasma mass spectrometry (ICP-MS) and X-ray fluorescence spectrometry (XRF) methods. The minerals mainly consist of kaolinite, pyrite, quartz, and calcite. The results of XRF and ICP-MS analyses indicate that the No. 6 coals from Chuancaogedan Mine are higher in Al 2 Th, and U are higher than those of world hard coals. The results of cluster analyses show that the most probable carrier of strontium in the coal is gorceixite; Lithium mainly occurs in clay minerals; gallium mainly occurs in inorganic association, including the clay minerals and diaspore; cadmium mainly occurs in sphalerite; and lead in the No. 6 coal may be associated with pyrite. Potentially valuable elements (e.g., Al, Li, and Ga) might be recovered as byproducts from coal ash. Other harmful elements (e.g., P, Pb, and U) may cause environmental impact during coal processing.


Introduction
Coal is the main fossil fuel resource and energy source in China.China's energy consumption has grown and will continue to grow along with its economic growth [1].In the process of coal utilization, the recovery of valuable elements from fly ash, as well as the impact on environment from harmful trace elements, have become important research topics [2,3].In some cases, Ge, Ga, Li and U can be enriched to higher levels than usual economic grades [4], while As, Pb, Hg, and F are potentially toxic.Furthermore, the modes of occurrence of trace elements in coal control the stratus of their emission in coal combustion processes [5].Moreover, Sr, Ba, B, and V have great significance to the environment regarding coal formation [5].
Minerals 2016, 6, 28 2 of 14 Jungar Coalfield, such as the Heidaigou Opencut Mine, the Haerwusu Opencut Mine, and the Guanbanwusu Mine (Figure 1) [4,5,7,17,18].In this study, the No. 6 coal seam from the Chuancaogedan Mine was chosen because this mine is located on the southern edge of the Jungar Coalfield (Figure 1).Compared with the northern and central parts, Chuancangedan is further away from the provenance-the Yinshan Upland [4,17,18].In this paper, the concentrations and modes of occurrence of the trace elements of No. 6 coal from the Chuancaogedan Mine are reported.The results provide new data on trace element enrichment in coal.

Geological Setting
The Jungar coalfield is located in the southern Yinshan Oldland (Figure 1), and is one of several Late Palaeozoic coal-bearing basins in this region [19].The Jungar coalfield is ~64 km long (N-S) by ~26 km wide (W-E), with a total area of 1700 km 2 .This coalfield was sustained by dynamic tectonic activities, and the formation, sedimentation, and evolution of the Jungar coalfield was controlled by the tectonic processes of the Central Asian Orogenic Belt (CAOB) [20,21].The Permo-Carboniferous denudation processes of the basin development started from the Cambrian-Ordovician periods, then gradually progressed to the Middle and Late Palaeozoic [22].During the period from the end of the Early Permian to the Late Permian, intermediate-felsic lavas erupted in the CAOB and this lava sequence might have provided an important source of minerals to the coal formations in the region.
The Taiyuan Formation, with a total thickness of 21-95 m, is mainly composed of grey and greyish-white quartzose sandstone, mudstone, siltstone, and coal which is interbedded with dark-grey mudstone, siltstone, limestone, and thin-bedded quartzose sandstone.The Taiyuan Formation was formed in paralic delta and tidal flat-barrier complex environments (Figure 2).
The No. 6 coal seam, the main minable seam, is located in the second section of the Taiyuan Formation.The thickness of the No. 6 coal varies from 0.30 to 16.80 m (11.61 m on average), with 0 to 7 partings.The partings consist mainly of mudstone.The floor and the roof are mainly composed of mudstone, sandy mudstone, and siltstone.In this study, the No. 6 coal seam from the Chuancaogedan Mine was chosen because this mine is located on the southern edge of the Jungar Coalfield (Figure 1).Compared with the northern and central parts, Chuancangedan is further away from the provenance-the Yinshan Upland [4,17,18].In this paper, the concentrations and modes of occurrence of the trace elements of No. 6 coal from the Chuancaogedan Mine are reported.The results provide new data on trace element enrichment in coal.

Geological Setting
The Jungar coalfield is located in the southern Yinshan Oldland (Figure 1), and is one of several Late Palaeozoic coal-bearing basins in this region [19].The Jungar coalfield is ~64 km long (N-S) by ~26 km wide (W-E), with a total area of 1700 km 2 .This coalfield was sustained by dynamic tectonic activities, and the formation, sedimentation, and evolution of the Jungar coalfield was controlled by the tectonic processes of the Central Asian Orogenic Belt (CAOB) [20,21].The Permo-Carboniferous denudation processes of the basin development started from the Cambrian-Ordovician periods, then gradually progressed to the Middle and Late Palaeozoic [22].During the period from the end of the Early Permian to the Late Permian, intermediate-felsic lavas erupted in the CAOB and this lava sequence might have provided an important source of minerals to the coal formations in the region.
The Taiyuan Formation, with a total thickness of 21-95 m, is mainly composed of grey and greyish-white quartzose sandstone, mudstone, siltstone, and coal which is interbedded with dark-grey mudstone, siltstone, limestone, and thin-bedded quartzose sandstone.The Taiyuan Formation was formed in paralic delta and tidal flat-barrier complex environments (Figure 2).
The No. 6 coal seam, the main minable seam, is located in the second section of the Taiyuan Formation.The thickness of the No. 6 coal varies from 0.30 to 16.80 m (11.61 m on average), with 0 to 7 partings.The partings consist mainly of mudstone.The floor and the roof are mainly composed of mudstone, sandy mudstone, and siltstone.

Samples and Methods
Fourteen bench samples were taken from the workface at the Chuancaogedan Mine, following the Chinese Standard Method GB 482-2008 [23].Every coal bench sample was cut over a column that was 10-cm wide, 10-cm deep, and 50-cm thick.All the collected samples were immediately stored in plastic bags to minimize contamination and oxidation.From bottom to top, the 14 bench samples were identified as 6-1 (roof) to 6-14 (Figure 2) The mineralogical composition was determined on the raw coal samples by coal-petrography microscopy (Leica DM 4500P microscope (Leica Microsystems, Solms, Germany) (at a magnification of 500×) equipped with a Craic QDI 302™ spectrophotometer, CRAIC, San Dimas, CA, USA).Low-temperature ashing of coal was performed on an EMITECH K1050× plasma asher (Quorum, Ashford, UK).The temperature for low-temperature ashing was kept lower than 200 °C (75 W power). X-ray diffraction (XRD) analyses on the resultant low-temperature ashes and the parting samples were performed on a D/max-2500/PC powder diffractometer (Rigaku, Tokyo, Japan) with Ni-filtered Cu-Kα radiation and a scintillation detector.The XRD patterns were recorded over a 2θ interval of 2.6°-70°, with a step size of 0.01°.
A scanning electron microscope (HITACHI UHR FE-SEM, SU8220, HITACHI, Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer (SEM-EDS) was used to study the distribution characteristics of the minerals, and the distribution patterns of some elements of interest in the coal.

Samples and Methods
Fourteen bench samples were taken from the workface at the Chuancaogedan Mine, following the Chinese Standard Method GB 482-2008 [23].Every coal bench sample was cut over a column that was 10-cm wide, 10-cm deep, and 50-cm thick.All the collected samples were immediately stored in plastic bags to minimize contamination and oxidation.From bottom to top, the 14 bench samples were identified as 6-1 (roof) to 6-14 (Figure 2) The mineralogical composition was determined on the raw coal samples by coal-petrography microscopy (Leica DM 4500P microscope (Leica Microsystems, Solms, Germany) (at a magnification of 500ˆ) equipped with a Craic QDI 302™ spectrophotometer, CRAIC, San Dimas, CA, USA).Low-temperature ashing of coal was performed on an EMITECH K1050ˆplasma asher (Quorum, Ashford, UK).The temperature for low-temperature ashing was kept lower than 200 ˝C (75 W power). X-ray diffraction (XRD) analyses on the resultant low-temperature ashes and the parting samples were performed on a D/max-2500/PC powder diffractometer (Rigaku, Tokyo, Japan) with Ni-filtered Cu-Kα radiation and a scintillation detector.The XRD patterns were recorded over a 2θ interval of 2.6 ˝-70 ˝, with a step size of 0.01 A scanning electron microscope (HITACHI UHR FE-SEM, SU8220, HITACHI, Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer (SEM-EDS) was used to study the distribution characteristics of the minerals, and the distribution patterns of some elements of interest in the coal.
All of the samples were crushed and ground to pass 200 mesh (75 µm) for elemental analysis.X-ray fluorescence spectrometry (XRF) was used to determine the oxides of the major elements in the coal ash (815 ˝C), including Na 2 O, MgO, Al 2 O 3 , SiO 2 , P 2 O 5 , K 2 O, CaO, TiO 2 , MnO, and Fe 2 O 3 [24].
Inductively coupled plasma mass spectrometry (ICP-MS) was applied to determine the trace element contents in the coal samples.For the ICP-MS analysis, microwave digestion of an approximate 200-mg sample (H <40 µm) was weighed into PTFE (Poly Tetra Fluoro Ethylene) vessels; 2 mL of HF (50%) + 5 mL of HNO 3 (65%) + 2 mL of H 2 O 2 (30%) were added, and microwave digestion was performed for 1 h at a temperature of 210 ˝C.This solution was then transferred into 125-mL FEP (Fluorinated Ethylene Propylene) bottles that were filled with 100 g of deionized water [24].

Minerals in the Coal
The XRD results from the low temperature ashes, optical microscopic observations, and SEM-EDS data show that the minerals in the No. 6 coal from Chuancaogedan are mainly composed of clay (kaolinite) pyrite, quartz and calcite (Figures 3-6).
Inductively coupled plasma mass spectrometry (ICP-MS) was applied to determine the trace element contents in the coal samples.For the ICP-MS analysis, microwave digestion of an approximate 200-mg sample ( <40 μm) was weighed into PTFE (Poly Tetra Fluoro Ethylene) vessels; 2 mL of HF (50%) + 5 mL of HNO3 (65%) + 2 mL of H2O2 (30%) were added, and microwave digestion was performed for 1 h at a temperature of 210 °C.This solution was then transferred into 125-mL FEP (Fluorinated Ethylene Propylene) bottles that were filled with 100 g of deionized water [24].

Minerals in the Coal
The XRD results from the low temperature ashes, optical microscopic observations, and SEM-EDS data show that the minerals in the No. 6 coal from Chuancaogedan are mainly composed of clay (kaolinite) pyrite, quartz and calcite (Figures 3-6).Inductively coupled plasma mass spectrometry (ICP-MS) was applied to determine the trace element contents in the coal samples.For the ICP-MS analysis, microwave digestion of an approximate 200-mg sample ( <40 μm) was weighed into PTFE (Poly Tetra Fluoro Ethylene) vessels; 2 mL of HF (50%) + 5 mL of HNO3 (65%) + 2 mL of H2O2 (30%) were added, and microwave digestion was performed for 1 h at a temperature of 210 °C.This solution was then transferred into 125-mL FEP (Fluorinated Ethylene Propylene) bottles that were filled with 100 g of deionized water [24].

Minerals in the Coal
The XRD results from the low temperature ashes, optical microscopic observations, and SEM-EDS data show that the minerals in the No. 6 coal from Chuancaogedan are mainly composed of clay (kaolinite) pyrite, quartz and calcite (Figures 3-6).The clay minerals mainly occur as lumps and cell-fillings with microgranular surfaces (Figure 4A) in telinite and fusinite (Figure 4B).This is common in many other coals and closely associated strata, and may indicate formation by authigenic processes.The results from XRD (Figure 3) and SEM-EDS studies (Figure 4C,D) show that the clay minerals consist mainly of kaolinite.Moreover, the data also indicates that kaolinite is well crystallized.
Pyrite is one of the most common sulfides occurring in coal, especially in coals formed in marine influenced depositional environments.Pyrite in the No. 6 coal predominantly occurs in pyritized cells (Figure 5A) and as fracture-fillings (Figure 5B); euhedral pyrite crystals are also found in the coal (Figure 5C,D), showing that pyrite can be of both syngenetic and epigenetic origin.
Quartz is commonly distributed in the macerals as irregular particles (Figure 6A) in this coal, suggesting a terrigenous detrital origin.Calcite mainly occurs as fracture-infillings (Figure 6B) in the No. 6 coal, indicating an epigenetic origin.

Major Element Contents in the No. 6 Coal
The contents of major oxides (Na2O, MgO, Al2O3, SiO2, P2O5, K2O, CaO, TiO2, MnO, and Fe2O3, on dry coal basis) in the No. 6 coal from Chuancaogedan Mine, in comparison to the average values of coals from Guanbanwusu, Haerwusu, Heidaigou and averages for Chinese coals, are listed in Table 1.Although the oxides of major elements in the No. 6 coal from the Chuancaogedan Mine are dominated by SiO2 (8.09% on average) and Al2O3 (6.76% on average) (Table 1), the SiO2/Al2O3 ratio of the No. 6 coal (1.20 on average) is lower than the common values of Chinese coals.Because quartz is virtually absent, the clay minerals are the major carrier of Si in the coal [25].The clay minerals mainly occur as lumps and cell-fillings with microgranular surfaces (Figure 4A) in telinite and fusinite (Figure 4B).This is common in many other coals and closely associated strata, and may indicate formation by authigenic processes.The results from XRD (Figure 3) and SEM-EDS studies (Figure 4C,D) show that the clay minerals consist mainly of kaolinite.Moreover, the data also indicates that kaolinite is well crystallized.
Pyrite is one of the most common sulfides occurring in coal, especially in coals formed in marine influenced depositional environments.Pyrite in the No. 6 coal predominantly occurs in pyritized cells (Figure 5A) and as fracture-fillings (Figure 5B); euhedral pyrite crystals are also found in the coal (Figure 5C,D), showing that pyrite can be of both syngenetic and epigenetic origin.
Quartz is commonly distributed in the macerals as irregular particles (Figure 6A) in this coal, suggesting a terrigenous detrital origin.Calcite mainly occurs as fracture-infillings (Figure 6B) in the No. 6 coal, indicating an epigenetic origin.

Major Element Contents in the No. 6 Coal
The contents of major oxides (Na2O, MgO, Al2O3, SiO2, P2O5, K2O, CaO, TiO2, MnO, and Fe2O3, on dry coal basis) in the No. 6 coal from Chuancaogedan Mine, in comparison to the average values of coals from Guanbanwusu, Haerwusu, Heidaigou and averages for Chinese coals, are listed in Table 1.Although the oxides of major elements in the No. 6 coal from the Chuancaogedan Mine are dominated by SiO2 (8.09% on average) and Al2O3 (6.76% on average) (Table 1), the SiO2/Al2O3 ratio of the No. 6 coal (1.20 on average) is lower than the common values of Chinese coals.Because quartz is virtually absent, the clay minerals are the major carrier of Si in the coal [25].The clay minerals mainly occur as lumps and cell-fillings with microgranular surfaces (Figure 4A) in telinite and fusinite (Figure 4B).This is common in many other coals and closely associated strata, and may indicate formation by authigenic processes.The results from XRD (Figure 3) and SEM-EDS studies (Figure 4C,D) show that the clay minerals consist mainly of kaolinite.Moreover, the data also indicates that kaolinite is well crystallized.
Pyrite is one of the most common sulfides occurring in coal, especially in coals formed in marine influenced depositional environments.Pyrite in the No. 6 coal predominantly occurs in pyritized cells (Figure 5A) and as fracture-fillings (Figure 5B); euhedral pyrite crystals are also found in the coal (Figure 5C,D), showing that pyrite can be of both syngenetic and epigenetic origin.
Quartz is commonly distributed in the macerals as irregular particles (Figure 6A) in this coal, suggesting a terrigenous detrital origin.Calcite mainly occurs as fracture-infillings (Figure 6B) in the No. 6 coal, indicating an epigenetic origin.

Major Element Contents in the No. 6 Coal
The contents of major oxides (Na The average content of Al 2 O 3 is higher than that in other Chinese coals, because abundant kaolinite and boehmite are present in these coals.In addition, Chuancaogedan coal displays higher SiO 2 , P 2 O 5 , and Fe 2 O 3 contents than those of Guanbanwusu, Haerwusu, and Heidaigou coals; this is maybe due to higher quartz and pyrite contents in the former than in the latter coal.

Trace Elements in the No. 6 Coal
The contents of trace elements in the coal samples, in comparison to the average values for Guanbanwusu, Haerwusu, and Heidaigou coals, as well as other Chinese coals [26] and world hard coals [27], are listed in Table 2.The abundance of trace elements in the No. 6 coal from the Chuancaogedan Mine, in comparison to the average values for Chinese and world hard coals, is shown in Figure 7.
Compared to world hard coals (Figure 7A), the only trace element with a CC > 5 (the CC (concentration coefficient) which is the ratio of the element concentration in Chuancaogedan and world hard coals) in the coals is Sr.Elements with a weak enrichment (2 < CC < 5) include Li, Ga, Zn, Zr, Gd, Hf, Pb, Th and U. Co, Ni, Rb, and Cs, which have lower concentrations than those of world hard coals (CC < 0.5).Beryllium, Sc, V, Cr, Nb, Mo, Ba, Ta, W, and Bi (0.5 < CC < 2) are close to the levels of abundance found in average world hard coals.
Compared to Chinese coals (Figure 7B), only Sr has a CC > 5, and Zn shows a weak enrichment (2 < CC < 5).Caesium is lower compared to world hard coals (CC < 0.5).The concentrations of the remaining elements are close to those found in average world hard coals.
The coals from Chuancaogedan Mine contain more Zn, Sr, Cd, and Ba than Guanbanwusu, Haerwusu and Heidaigou coals.The remaining elements in the Chuancaogedan Mine samples display lower contents compared to the coals from Guanbanwusu, Haerwusu and Heidaigou Mines.Trace elements bound in the organic matter of coal volatilize more easily during combustion than these bound in the inorganic matter, which tend to remain in the ash.Therefore, ash yield and trace element contents in coal often display a close relationship [29][30][31].
Four groups (Groups 1 to 4) of elements have been identified in the No. 6 coal from the present based on their correlation coefficients with ash yield (Table 3).
Table 3. Element affinities between the concentration of each element in the coal and ash yield or selected elements.
Group 2 includes elements with a relatively high inorganic affinity.The elements in this group (Sc, V, Co, Ni, Cu, W, U, Na 2 O, Cs, Ga, Rb, Sr, Nb, Ba, and K 2 O) are strongly correlated with the ash yield, with correlation coefficients between 0.50 and 0.80.
Group 3 includes only Cr, which has a correlation coefficient with the ash yield of 0.35.Group 4 includes Fe 2 O 3 , MnO, CaO, Pb, Th, SO 3 , P 2 O 5 , MgO, Hf, Zr, Mo, Cd, and Be.These elements have correlation coefficients with the ash yield that range from ´0.30 to 0.30, indicating an intermediate affinity.

Cluster Analysis
The elemental associations in the Chuancaogedan coals were studied by cluster analysis.Four groups of elemental association were identified (Figure 8), referred to as Groups 1, 2, 3 and 4.
Group 1.This group includes Al 2 O 3 , SiO 2 , Li, Rb, TiO 2 , Cs, Nb, and Ta (Figure 8).All of the elements in this group have high positive correlation coefficients with the ash yield, ranging from 0.53 to Group 2. This group includes the elements V, Cr, Co, Ni, MnO, and Cu.All of the elements in this group, except MnO, have relatively high correlation coefficients with the ash yield, ranging from 0.34 to 0.67.With the exception of V-MnO (0.27), the correlation coefficients between the pairs of elements in this association are higher than 0.50.The elements in this group, except Cu, are lithophile elements that are probably associated with the clay minerals.
Group 3.This group consists of Sr, Ba, MgO, Be, Zr, Hf, Th, Sc, U, K2O, Bi, and Na2O.With the exception of K2O, Bi, and Na2O, they have low correlation coefficients with the ash yield (Table 3), and these elements are probably associated with unidentified traces of sulfide minerals.K2O, Bi, and Na2O have high correlation coefficients with SiO2 and Al2O3, and these elements probably occur in clay minerals and diaspore.
Group 4. This group includes CaO, Zn, P2O5, Cd, SO3, Fe2O3, Pb, and Mo.All of these elements have negative correlation coefficients with the ash yield, possibly because they occur in phosphate minerals (gorceixite and fluorapatite).

Strontium
The concentration of Sr in the samples varies considerably, from 31.3 to 1915 μg/g, with a weighted average of 1037 μg/g.This level is much higher than that in common Chinese coals (140 μg/g on average) [26] and world hard coals (114 μg/g on average) [27].The correlation coefficient between Sr and Ba is high at 0.90 (Table 3).The most probable carrier of Ba in the coal is gorceixite, so Sr may be also contained in this mineral [18].

Lithium
The arithmetic average lithium content reaches 56.6 μg/g in the No. 6 coal, and is much higher than that in common Chinese coals and world hard coals.The highest content of Li is 253 μg/g (Table 2).Sun et al. suggest that the cut-off grade of Li for economic recovery should be 120 μg/g [32].However, the Li content of the coal from the Chuancaogedan Mine does not reach this level.
The modes of occurrence of lithium in coal have not been fully studied.Li mainly occurs in granite pegmatite deposits, alkali feldspar granite deposits and salt lake deposits [23,25,32].Researchers thought that Li in coal generally occurred in clay minerals and partially in mica and tourmaline [33,34].The high correlation coefficient between Li and ash indicates that a large proportion of the Li occurs in the inorganic matter [18].Lithium is positively correlated with Al2O3, Group 2. This group includes the elements V, Cr, Co, Ni, MnO, and Cu.All of the elements in this group, except MnO, have relatively high correlation coefficients with the ash yield, ranging from 0.34 to 0.67.With the exception of V-MnO (0.27), the correlation coefficients between the pairs of elements in this association are higher than 0.50.The elements in this group, except Cu, are lithophile elements that are probably associated with the clay minerals.
Group 3.This group consists of Sr, Ba, MgO, Be, Zr, Hf, Th, Sc, U, K 2 O, Bi, and Na 2 O.With the exception of K 2 O, Bi, and Na 2 O, they have low correlation coefficients with the ash yield (Table 3), and these elements are probably associated with unidentified traces of sulfide minerals.K 2 O, Bi, and Na 2 O have high correlation coefficients with SiO 2 and Al 2 O 3 , and these elements probably occur in clay minerals and diaspore.
Group 4. This group includes CaO, Zn, P 2 O 5 , Cd, SO 3 , Fe 2 O 3 , Pb, and Mo.All of these elements have negative correlation coefficients with the ash yield, possibly because they occur in phosphate minerals (gorceixite and fluorapatite).

Strontium
The concentration of Sr in the samples varies considerably, from 31.3 to 1915 µg/g, with a weighted average of 1037 µg/g.This level is much higher than that in common Chinese coals (140 µg/g on average) [26] and world hard coals (114 µg/g on average) [27].The correlation coefficient between Sr and Ba is high at 0.90 (Table 3).The most probable carrier of Ba in the coal is gorceixite, so Sr may be also contained in this mineral [18].

Lithium
The arithmetic average lithium content reaches 56.6 µg/g in the No. 6 coal, and is much higher than that in common Chinese coals and world hard coals.The highest content of Li is 253 µg/g (Table 2).Sun et al. suggest that the cut-off grade of Li for economic recovery should be 120 µg/g [32].However, the Li content of the coal from the Chuancaogedan Mine does not reach this level.
The modes of occurrence of lithium in coal have not been fully studied.Li mainly occurs in granite pegmatite deposits, alkali feldspar granite deposits and salt lake deposits [23,25,32].Researchers thought that Li in coal generally occurred in clay minerals and partially in mica and tourmaline [33,34].The high correlation coefficient between Li and ash indicates that a large proportion of the Li occurs in the inorganic matter [18].Lithium is positively correlated with Al 2 O 3 , SiO 2 , and K 2 O, with correlation coefficients of 0.99, 0.99 and 0.77, respectively (Table 3), indicating that Li may also be associated with kaolinite, chlorite, and possibly with illite.
Lithium is also positively correlated with some lithophile elements, including Rb, Nb, Bi, Ga, Ta, and W (Table 3), with correlation coefficients of 0.95, 0.94, 0.77, 0.81, 0.97, and 0.93, respectively, and it is further confirmed that Li occurs in aluminosilicate minerals.The distribution of the Li content in the coal benches, except sample 6-1, is uniform.The high content of Li in sample 6-1 may be caused by a parting within the seam.
It can be deduced that the Ga mainly occurs in inorganic association, including the clay minerals and diaspore, based on the positive correlations of Ga-ash (r = 0.81), Ga-Al 2 O 3 (r = 0.82), and Ga-SiO 2 (r = 0.80).Gallium may replace Zn in sphalerite, but the low-sulfur content and the negative correlation coefficient of Ga-S (r = ´0.25)indicate that Ga is not related to sulfide in the Chuancaogedan coals.

Zirconium
The average concentration of Zr is 132 µg/g, which is higher than that of common Chinese (89.5 µg/g) [26] and world coals (36 µg/g) [27].The correlation coefficients of Zr-Al 2 O 3 and Zr-SiO 2 are relatively low, 0.24 and 0.25, respectively.Dai et al. found that the content of Zr in the coals from the northern and central Jungar Coalfield is higher than that of common Chinese coals, and pointed out that the major carrier of Zr is zircon [18].

Cadmium
Cadmium is one of the toxic trace elements in coal.The average Cd content of the No. 6 coal (0.4 µg/g on average) is higher than that of common Chinese coals and world hard coals.Cd is a chalcophile element.Cd is positively correlated with S and Zn (Table 3), with correlation coefficients of 0.5 and 0.9, respectively, indicating that the Cd in the No. 6 coal mainly occurs in sphalerite.
Pb in coal mainly occurs in galena or is associated with other sulfide minerals.The relationships between Pb and Al 2 O 3 and SiO 2 in the No. 6 coal are poor, with correlation coefficients of 0.05 and 0.10, respectively.The correlation coefficient with Fe 2 O 3 is high, suggesting that the occurrence of Pb in the No. 6 coal may be associated with pyrite.

Thorium
The average Th content of the No. 6 coal (9.3 µg/g) is higher than that of common Chinese coals and world hard coals.Thorium in the Chuancaogedan coals has high correlation coefficients with Sc (0.68), V (0.64), Zr (0.89), and Hf (0.87), probably indicating the same source for these elements.The relatively low correlation coefficients of Th-Al 2 O 3 (0.21), Th-SiO 2 (0.23), and Th-ash (0.20) indicate that Th may occur in accessory minerals in the coal and probably in the organic matter as well.

1.
The No. 6 coal from Chuancaogedan Mine is significantly enriched in Zn and Sr and is slightly enriched in Li, Ga, Zr, Gd, Hf, Pb, Th, and U compared with world hard coals.The major elements exhibit enrichment in Al 2 O 3 (6.76%)and P 2 O 5 (0.169%), but with a lower SiO 2 /Al 2 O 3 ratio (1.20), compared to Chinese hard coals.The contents of Zn, Sr, Li, Ga, Zr, Gd, Hf, Pb, Th, and U are higher than those of world hard coals.Aluminum, Li, and Ga could be recovered as the byproducts from coal ash, and P, Pb, and U may be harmful to the environment during coal processing.

2.
The elements in the No. 6 coal may be classified into four groups of association according to their modes of occurrence.Most of the elements in Group 1 and Group 2 are strongly correlated with the ash yield, but the elements of the remaining two associations have negative or weak correlation coefficients with the ash yield.

3.
The most probable carriers of Sr in the coal are barite and gorceixite.Lithium is mainly associated with kaolinite and possibly with illite.Gallium mainly occurs in inorganic association, including the clay minerals and diaspore, but is not related to sulfide.Zirconium occurs in association with sulfide minerals.Cadmium mainly occurs in sphalerite.Lead in the No. 6 coal may be associated with pyrite.Thorium may occur in accessory minerals in the coal and probably in the organic matter as well.

Figure 2 .
Figure 2. Lithostratigraphical column of the Jungar Coalfield and lithological column of the sampling profile.

Figure 3 .
Figure 3. Identification of minerals in the X-ray diffraction (XRD) pattern of the low temperature ash (LTA) of Sample 6-10.

Figure 3 .
Figure 3. Identification of minerals in the X-ray diffraction (XRD) pattern of the low temperature ash (LTA) of Sample 6-10.

Figure 3 .
Figure 3. Identification of minerals in the X-ray diffraction (XRD) pattern of the low temperature ash (LTA) of Sample 6-10.

Figure 7 .Figure 7 .
Figure 7. (A) Concentrations coefficients (CC) of elements in the Chuancaogedan coal vs. world coals; (B) CC of elements in the Chuancaogedan coals vs. Chinese coals.

Figure 8 .
Figure 8. Cluster analyses of analytical results on 14 samples.

Figure 8 .
Figure 8. Cluster analyses of analytical results on 14 samples.

Table 1 .
[25] MgO, Al 2 O 3 , SiO 2 , P 2 O 5 , K 2 O, CaO, TiO 2 , MnO, and Fe 2 O 3 , on dry coal basis) in the No. 6 coal from Chuancaogedan Mine, in comparison to the average values of coals from Guanbanwusu, Haerwusu, Heidaigou and averages for Chinese coals, are listed in Table 1.Although the oxides of major elements in the No. 6 coal from the Chuancaogedan Mine are dominated by SiO 2 (8.09% on average) and Al 2 O 3 (6.76% on average) (Table1), the SiO 2 /Al 2 O 3 ratio of the No. 6 coal (1.20 on average) is lower than the common values of Chinese coals.Because quartz is virtually absent, the clay minerals are the major carrier of Si in the coal[25].Contents of major elements (re-calculated as oxides; in %, on dry coal basis) and total sulfur in No. 6 coal from Chuancaogedan.

Table 2 .
Concentrations of trace elements in the No. 6 coal from Chuancaogedan (µg/g, on dry coal basis).