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Chemistry, mineralogical, and residence of arsenic in a typical high arsenic coal
International Journal of Mineral Processing 141 (2015) 61–67
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International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Chemistry, mineralogical, and residence of arsenic in a typical high arsenic coal Chong Tian a,b,⁎, Junying Zhang a,⁎, Rajender Gupta b, Yongchun Zhao a, Shuai Wang c a b c
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China Department of Chemical and Material Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada Department of Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, PR China
a r t i c l e
i n f o
Article history: Received 12 March 2014 Received in revised form 17 March 2015 Accepted 5 June 2015 Available online 8 June 2015 Keywords: Low temperature ash Float–sink Heavy minerals Arsenic occurrence
a b s t r a c t The arsenic chemistry, mineral association and distributions in coal were evaluated using density fraction and a low temperature ashing process. A high arsenic coal from Guizhou province in southwestern China was chosen in the study. The results show that light mineral (b 2.89 g/cm3) fraction and heavy mineral (N 2.89 g/cm3) fraction were successfully separated from Guizhou coal by means of the process. Phase-mineral composition in low temperature ash and light mineral fraction are almost the same, mostly quartz and kaolinite. In contrast, pyrite dominates in the heavy mineral fraction. Arsenic concentration in Guizhou coal is as high as 265 μg/g. An obvious enrichment of arsenic in heavy mineral fraction is observed, and the arsenic concentration is approaching to 650 μg/g. Modes of occurrence of arsenic exhibit as multiple forms. Both organic and inorganic associated arsenic in Guizhou coal have been identified. The organic associated arsenic accounts for 28.40% (wt.%) of the total arsenic. The inorganic associated arsenic mostly occurs in pyrite. Direct evidence by using the EDX analysis on 27 pyrite particles confirms that arsenic is occurring in pyrite. Each distinct pyrite particle contains a certain amount of arsenic with an average of 3.5% (wt.%), while the maximum is reaching to 6.7% (wt.%). Endemic arseniasis is highly suspected to be link to the frequent use of coal-burning stove for heating and/or food drying, inhalation of indoor air polluted by arsenic derived from coal combustion in houses, and the contaminated water in Guizhou province, southwestern China. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Topics about “arsenic in the environment” have been of a concern for a long time, which were preceded the discovery of arsenic in coal (Yudovich and Ketris, 2005). Some of the arsenic compounds are very toxic when they are dispersed in the environment as an air or water pollutant (Zielinski et al., 2007). It has been reported that arsenic exposure may not only affect and disable organs of bodies, but may also interfere with proper functioning of the immune system (Duker et al., 2005). Incidents on adverse health risks resulted from arsenic contamination in the environment in many countries of the world have been reported. Guizhou has been recognized as the most typical area of arsenic contamination in China as well as in the world (Kang et al., 2011). It has
Abbreviations: LTA, low-temperature ash; LTAS, low temperature ashes; LMF, light mineral fraction as a function of the total weight; HMF, heavy mineral fraction as a function of the total weight; HTA, high temperature ash; GZ, Guizhou; TEs, trace elements; XRD, X-ray diffraction; FE-SEM/EDX, Field Emission Scanning Electron Microscope equipped with Energy-Dispersive X-ray analysis; ICP-MS, inductively coupled-plasma mass spectrometry; XRF, X-ray fluorescence; RIM, reference intensity method; h, hour; hrs, hours. ⁎ Corresponding authors at: State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China. E-mail addresses: [email protected] (C. Tian), [email protected] (J. Zhang).
http://dx.doi.org/10.1016/j.minpro.2015.06.010 0301-7516/© 2015 Elsevier B.V. All rights reserved.
also been reported that more than 3000 cases of arseniasis in villages in Guizhou were caused by the use of locally mined high arsenic coal (Zhang et al., 2002). Direct arsenic exposure for the local residences may come from the domestic use of coal for heating or food drying. Risks for arseniasis would be much higher if the high arsenic coal is frequently used indoors (He et al., 2002). Arsenic concentrations in Chinese coals increase in districts from northern to southern, and the weighted mean arsenic concentration is 3.18 μg/g (Kang et al., 2011). Abnormally high arsenic concentrations were identified in coals in southwestern China because of many geological factors (Dai et al., 2012; Ren et al., 1999). Chen et.al reported that the concentration of arsenic in coals in southwestern China was 18.2 μg/g (Chen et al., 1985). Ding et al. found that some coals in southwestern China contained arsenic exceeding to 1000 μg/g and some anomalous samples were with arsenic concentration as high as 35,000 μg/g (Ding et al., 2001). The origins of the high arsenic coal of southwestern China were related to the three aspects, including gold mineralization, hydrothermal activity in the late stage of coalification, and active volcanic effects (Zheng et al., 1999). The concentrations, distributions and modes of occurrence of arsenic in the high arsenic coals in southwest China have been studied extensively (Dai et al., 2005, 2006; Zheng et al., 1999). Quantitative understanding the modes of occurrence of arsenic in Guizhou, China might aid in the prediction of its
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mobility behavior and minimizing the health risks and the environmental impacts during coal utilization locally. The modes of occurrence of arsenic in coal are rather complex. Majority of arsenic is associated with mineral matter, and part of the arsenic is associated with organic matter in coals. Three major modes of arsenic occurrence are pyritic, organic, and arsenate (Yudovich and Ketris, 2005). Many techniques and measuring methods, including Mossbauer spectroscopy analysis, X-ray absorption fine structure (XAFS) analysis, a scanning electron microscope with energy dispersive X-ray analysis (SEM–EDX), and petro-graphic microscopic identification and sequential leaching experiments, have been applied to study arsenic in Chinese coals (Ding et al., 1999; Kang et al., 2011). Although it has been reported that arsenic may be organically-bounded or as (AsO4)3−, it was sulfides that have been extensively acknowledged to be the major carrier of arsenic, specifically, pyrite, marcasite, or arsenopyrite (Rieder et al., 2007). However, evidences on modes of occurrence of arsenic in coal were mostly obtained indirectly or to a lesser extent directly (Liu et al., 2007; Zhang et al., 2002) due to the limitation in direct methods. For instance, indirect evidences depended on the positive correlation As-S or As-Pyrite, or with the help of float–sink method with coals for heavy density fraction separation, in which sulfides dominate indicating that arsenic was associated with pyrite (FeS2) (Finkelman, 1995; Yudovich and Ketris, 2005). Later, direct characterizations of arsenic residence were added based on the measurement on pyrite separated by handpicking from coals (Zhang et al., 2002). Microprobe investigations also confirmed arsenic occurrence in pyrite, and it has been reported that the detectable amount of arsenic in some U.S. coals reached to 4.7 wt.% (Yudovich and Ketris, 2005). Previous research also reported that arsenic concentration would increase as the particle size of pyrite increase in Soutern Appalachian coal (Akers and Raleigh, 1998). However, direct assessments on the arsenic occurrence in coal are still necessary for achieving a step forwards. In the present study, the arsenic chemistry, mineral association and distributions were evaluated using density fraction and a low temperature ashing process (Tian et al., 2014). The typical high arsenic coal from Guizhou (GZ) province, in southwestern China was studied. Quantitative assessments on mineralogical characteristics and occurrence of arsenic in the GZ coal were achieved. Direct evidence on arsenic occurrence and content in the pyrite particles was obtained by using the Field Emission Scanning Electron Microscope equipped with EnergyDispersive X-ray analysis (FE-SEM/EDX) measuring. The possible origins of the arsenic exposure and contamination in GZ area have also been analyzed. 2. Material and methods 2.1. Coals and heavy mineral separation methods The typical high arsenic coals were collected from Xingren County, Guizhou southwestern China. It has been reported that arsenic accumulation in the coal in this district was related to gold mineralization (Zheng et al., 1999). The collection of the coals was in accordance with the Chinese Standard for Sampling of Coal in seam GB482-2008. Large batch coals were preserved in barrels to avoid man-made contamination. In the present study, the coals were air dried, crushed and sieved to obtain a representative 45–75 μm fraction firstly. Afterwards, the powdered coals were ashed at both high and low temperatures. The high temperature ash (HTA) was used to determine the oxides of the major elements, including SiO2, Al2O3, Fe3O4, CaO, MgO, Na2O, SO3, K2O, and TiO2 content, which would be served as a reference when describing mineral compositions in low temperature ash (LTA) and its different density fractions. Low temperature (120–150 °C) oxygen plasma ashing was considered to be a useful and subsidiary procedure for isolating the mineral matter, which were intimately distributed in the organic matrix of coals, for subsequent analysis by various methods (Vassilev and
Vassileva, 1996; Winburn et al., 2000). The LTA was obtained by means of a K1050X low temperature oxygen plasma asher. 1 g of powdered coal in a crucible was put in the chamber of the low temperature asher, and then set the radio frequency power at 70 W. After each step of the ashing process was completed (typically around 2 hrs), the crucible was taken out and weighed, and the weight loss for the coal was determined. The sample in the crucible was then stirred to make a new surface, and the ashing process was repeated until the weight loss was negligible. The total ashing time for the samples was typically 8–12 hrs. In the procedure of low temperature ashing, the organic constituents in the coal were almost completely removed by oxidation and the inorganic constituents were obtained without major alteration. Since the content of heavy minerals in the coal is rather small (usually b1 wt.% of the LTA), at least 5 g of LTA was needed to provide sufficient amount of sample for the follow-up analysis. Around half of a month was needed to obtain sufficient sample of the GZ coal for the following study. Afterwards, the LTA was firstly washed and filtered by the deionized water and oven dried. And then the washed LTA were subjected to float–sink separation in bromoform (density = 2.89 g/cm3). The float– sink process was conducted with the help of a centrifuge. The LTA was divided into two density fractions, the light mineral fraction (LMF) (density b 2.89 g/cm3) was floating on the top in the centrifugal tube, while the heavy mineral fraction (HMF) (density N 2.89 g/cm3) sank to the bottom. The LMF and HMF were filtered and oven dried overnight. 2.2. Analytical technique The oxides of the major elements, including SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, SO3, TiO2, P2O5, and K2O in HTA were determined by X-ray fluorescence spectrometry (XRF). Phase-mineral composition and content of the GZ coal were determined by XRD. The XRD investigations were carried out on an X'Pert PRO diffractometer equipped with a graphite diffracted-beam monochromator. The accelerating voltage was 40 kV and the current was 40 mA. XRD patterns were recorded for the LTA and the different density fractions over a 2θ interval of 10–80° using Cu-Kα radiation with a step size of 0.017° per 10 s. Final semi-quantitative mineral composition was determined by using the Reference Intensity Method (RIM) (Chung, 1974; Ward and French, 2006; Winburn et al., 2000). The small amounts of HMF were analyzed with the FE-SEM/EDX for micro-morphology observation and rare mineral identification. The FESEM/EDX was also applied to measure the arsenic occurrence and content in HMF for obtaining a direct evidence of arsenic occurring in specific mineral species in HMF. Investigations of FE-SEM were carried out on a Sirion200 microscope equipped with a GENESIS EDX (energy dispersive X-ray spectroscopy). Arsenic concentrations in coal, LTA, LMF and HMF were determined by using the high resolution inductively coupled-plasma mass spectrometry (ICP-MS). The solid samples for ICP-MS were digested according to the DZ/T0223-2001, with 1 ml HF, 3 ml HNO3, and 1 ml HClO4 for each 0.05 g solid sample in the low pressure sealed vessel. 3. Results and discussion 3.1. General observations of mineral separation of the coal studied Ultimate and proximate analyses of the typical GZ Coal studied were summarized in Table 1. According to Chinese standard GB/T 15224.12004 and GB/T 15224.2-2004. The GZ coal studied is a high sulfur and high ash anthracite. The chemical compositions were summarized in Table 2. Concentrations of SO3 is as high as 21.5% (wt.%). It has been reported that sulfur in coal existed as organic and inorganic form, and the inorganic sulfur were usually sulfides and sulfates, with pyrite as the major sulfur containing mineral in most coals (Calkins, 1994). The
C. Tian et al. / International Journal of Mineral Processing 141 (2015) 61–67 Table 1 Ultimate and proximate analysis of the coal studied. Ultimate analysis (wt.%) Sample GZ
Cdaf 75.29
Hdaf 1.56
Table 3 General observations of results from the float–sink experiment. Proximate analysis (wt.%)
Ndaf 0.79
St 20.31
Odaf 3.03
63
Mad 1.05
Ad 45.94
Vdaf 12.17
Sample Low-temperature ash
FCdaf 40.84
M: moisture; V: volatile; A: ash; FC: fix carbon; St: total sulfur; GZ: Guizhou. daf: dry and ash-free basis; ad: air-dried; d: dry basis.
Float–sink
Coal (g) LTA (g) LTA/Coal (%) LMF (g) LMF (%) HMF (g) HMF (%) GZ
8.781
4.261
48.525
3.955
92.82
0.306
7.18
GZ: Guizhou; LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction.
high ash and high sulfur GZ coal is expected to have a high concentration of pyrite and arsenic. Table 3 summarized the LTA yields and the general observations of LMF and HMF separated from LTA by means of float–sink technique. Yields of the LTA from GZ coal is about 50% (wt. %) of the coal, which is close to the ash content in the coal, indicating a complete depletion of organic constituents. The general observations from float–sink experiment show that the separated LMF accounts for most part in the LTA of the GZ coal, and the proportion is more than 90% (wt.%) of the LTA. In contrast, HMF only accounts for a very small amount in the LTA.
3.3. Residence of arsenic in the coal studied Ren et al. (1999) have reported that the arsenic concentrations in Chinese coals were ranging from 0.03 to 478.4 μg/g, with an average
3.2. Mineralogical characteristics of the coal studied The XRD measurement was used to determine the phase-mineral composition and content in the GZ coal. XRD patterns of LTA, LMF and HMF were shown in Fig. 1. Diffraction peaks appearing in the XRD patterns of the LTA and LMF are very similar, indicating that the phase-mineral composition in LTA and LMF are almost the same. While obvious different diffraction peaks are observed in the XRD patterns of the HMF in comparison to that in LTA and LMF, hinting the different phases in the HMF. In other words, it indicates that separation of heavy minerals by means of float–sink technique followed by low temperature ashing of the GZ coal can be successfully achieved, and a sample of HMF can be obtained for further quantitative assessments. Phase-mineral composition and content of LTA, LMF and HMF were summarized in Table 4. Phases in LTA and LMF are majorly composed of quartz and kaolinite, a little muscovite is also found. And the relative content of each mineral species in LTA and LMF varies a little bit. Phases identified in HMF are predominantly composed of pyrite. Semiquantitative results show that the relative content of pyrite in the HMF is 62.91% (wt.%). Minor amount of other heavy minerals, e.g. anatase, is also identified in HMF. Secondary electron microscope images of the HMF from GZ coal were shown in Fig. 2. And the EDX results were in line with the XRD analysis. Most of the mineral particles identified in the secondary electron microscope images are pyrite. And all the particles exhibit as distinct shapes with different size ranging from 10 μm to 20 μm. Most of these particles are euhedral massive pyrite, and cracks can be observed on the surface of some pyrite. Characteristic, type, genesis, morphology, and distributions of pyrite in coal seams from different deposits have been described by previous researchers (López-Buendía et al., 2007; Querol et al., 1989; Turnera and Richardson, 2004). Morphologies and genesis of pyrite are various, and both micrometer-sized framboids and massive crystals and vein-fillings can be observed (Vassilev and Vassileva, 1996; Widodo et al., 2010). The morphology of pyrite is of a help in reconstructing the depositional environment when peat formation (Dai et al., 2008). Pyrite is considered to be the major host for many hazardous trace elements of great environmental concern (Luttrell et al., 2000). Thus it can be deduced that TEs should be enriched in the HMF of GZ coal. Table 2 Chemical compositions (wt.%) of the coal studied. Sample
Na2O
MgO
Al2O3
SiO2
Fe2O3
SO3
K2O
CaO
TiO2
GZ
0.6
0.7
36.8
24.9
9.8
21.5
2.3
0.6
2.6
GZ: Guizhou.
M-Muscovite
K-Kaolinite
Q-Quartz
An-Anatase
P-pyrite
Fig. 1. XRD patterns of the fractions separated from the coal studied. M—muscovite; K—kaolinite; Q—quartz; An—anatase; P—pyrite.
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Table 4 Phase-mineral composition and content (wt.%) of fractions separated from the coal studied.
LTA LMF HMF
Muscovite
Kaolinite
Quartz
Anatase
Pyrite
15.4 17.6 4.24
29.8 28.3 14.91
45.2 42.9 9.21
– – 4.38
4.3 4.1 62.91
LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction.
of 3.79 μg/g. And this conclusion was depending on the evaluation of a total of 3386 coal samples from China. Arithmetic means of the arsenic levels have been extensively reported in the assessment of arsenic in Chinese coals (Dai et al., 2004, 2010; Wang et al., 2006). Some studies indicated that the arsenic concentrations in Chinese coals varied greatly from 0 to 35,037 μg/g (Ding et al., 1999). Zhang et.al reported that arsenic concentration in the Triassic coals from southwestern Guizhou ranged from 1.22 to 238 ppm (Zhang et al., 2004). The GZ coal studied also has a relatively high concentrations of arsenic in comparison to some other Chinese coals, which is as high as 265 μg/g. The relative enrichment of arsenic in this area is probably due to the low-temperature hydrothermal activity during the local geological history. The histogram in Fig. 3 shows the arsenic concentration trends in the GZ coal, LTA, LMF and HMF. It can be found that arsenic concentrations vary in the coal, LTA, and the two density fractions. Arsenic concentrations in LTA (391 μg/g) and LMF (360 μg/g) are very close to each other. In contrast, arsenic is obviously enriched in the HMF (approaching to 650 μg/g). It suggests that the arsenic be likely exhibiting as multiple modes. It has been reported that modes of occurrence of arsenic are majorly controlled by Aspyr or Asorg. If an arsenic accumulation in partitioning of sink fraction from coal or high-ash coals is observed, it means that the Aspry dominates. If an enrichment of arsenic in medium-density fractions or low- and medium-ash coals is observed, Asorg is expected to dominate in the coal (Yudovich and Ketris, 2005). In the present study, the fact that high-ash content of the GZ coal and the obvious enrichment of arsenic in the HMF indicate that the modes of occurrence of arsenic are highly expected to be majorly controlled by Aspyr. However, other modes of occurrence of arsenic are also believed to present in the GZ coal. For instance, a noticeable arsenic loss from coal to the washed LTA can not be ignored, and this part of arsenic is highly possible to be organically associated. Since the organically-bounded arsenic is mostly associated with C, O and organic S, and oxygen plasma would implant in the chemical bounds, e.g. As-C and As-S et.al, during the low temperature ashing process, thus causing the As-C and/or As-S being broken
LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction;
Fig. 3. Arsenic concentrations in the coal studied and its separated fractions. LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction.
down. In this case, the organically associated arsenic would be isolated and easily washed away. Taking into account the arsenic concentration in GZ coal and its LTA, and the yields of LTA from the GZ coal, the loss of arsenic can be calculated by the Eq. (1): L ¼ 1−ðC1=C2Þ Y
ð1Þ
Where, L: Arsenic loss; C1: Arsenic concentrations in LTA; C2: Arsenic concentrations in coal; Y: Yields of LTA from coal. It can be quantified that the loss of arsenic from coal to LTA is 28.4% (wt.%) of the total arsenic in GZ coal, thus making it clear that organically associated arsenic is presenting in the GZ coal. In contrast, more than 70% (wt.%) of the total arsenic are inorganically associated in GZ coal. The inorganically associated arsenic may be further divided into two forms, including alumina-silicate associated and pyrite associated. To be specific, certain amount of arsenic in LMF indicates that a small part of arsenic may be alumina-silicate associated. However, the remaining pyrite in the LMF caused by the experiment error may also contribute for the arsenic content. The relative higher concentrations of arsenic in the HMF in comparison to LMF suggests that arsenic has a strong affinity to mineral species in HMF, particularly with pyrites. It is deduced that most of the arsenic in GZ coal studied is pyrite associated, which is in line with the previous findings that the inorganically associated arsenic is mostly associated with pyrite in coals (Finkelman, 1994; Hower et al., 1997). Fig. 4 shows both the arsenic and pyrite content trends in coal,
LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction;
Fig. 2. Typical secondary electron microscope image of heavy mineral fraction in the coal studied.
Fig. 4. Arsenic content in the fractions separated from the coal studied. LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction.
C. Tian et al. / International Journal of Mineral Processing 141 (2015) 61–67
LTA and the two density fractions in GZ coal studied. The similar arsenic and pyrite concentration trends in the partitioning from GZ coal also directly confirm the conclusion that the arsenic has a strong positive correlation with the pyrite in coals. 3.4. Arsenic occurrence in pyrite Previous findings indicate that pyrite and arsenic are obviously enriched in the HMF separated from GZ coal, and their concentrations are expected to in exceeding to the detectable limitation. Hence, FESEM/EDX is highly applicable to study the arsenic distributions as well as mineral associations in HMF. Quantitative assessments on the arsenic occurrence and content in pyrite in GZ coal by using the EDX measurement by particle analysis within the separated HMF were obtained. Typical secondary electron microscope images with corresponding EDX results were shown in Fig. 5. It was found that pyrite particles were totally isolated from the organic constituents and presented as distinct shapes and with size larger than 10 μm. Fig. 5-A presented an irregular
65
shapes of pyrite particle, and the arsenic concentration was reaching to 4.34% (wt.%). Euhedral pyrite presents as decahedron was also identified (Fig. 5-B), with arsenic concentration of 2.62% (wt.%). Fig. 5-C showed a pyrite with some cracks on the surface, and the arsenic concentration was 2.67% (wt.%). Totally 27 particles were analyzed and the results were listed in Table 5. The EDX analysis show that major elements in all the particles are Fe and S, and the atomic ratio of Fe to S is about 1:2 suggesting it to be FeS2–pyrites. However, some pyrite particles contain certain amount of O and a small quantity of Al and Si. The presence of oxygen may be partly from the oxidation of the pyrite exposing in the air. Presence of Al and Si probably came from few clay minerals combined with the pyrite particles. The results from EDX, combined with the results obtained from the XRD analysis, confirmed that the single particles indentified in the FE-SEM/EDX are predominantly pyrite with some impurities such as Si and Al attached on the particles. EDX measuring by particle analysis in the HMF also indicates that each of the pyrite contains a certain amount of arsenic. Concentrations of arsenic in the isolated pyrite vary widely, ranging from 1.06% to
Arsenic concentrations in the images of A,B,C are 4.34%, 2.62%,2.67%, respectively. (wt.%) Fig. 5. Secondary electron microscope images and energy spectrum analysis on typical particles in heavy mineral fractions in the coal studied. Arsenic concentrations in the images of A, B, and C are 4.34%, 2.62%, and 2.67%, respectively (wt.%).
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6.7% (wt.%) with an average at around 3.5% (wt. %). It directly confirms the assumption that most of the arsenic in the GZ coal studied is occurring in pyrite. And the pyrite is the major carrier for the arsenic in coals. The incorporation of arsenic in the structure of pyrite includes arsenic substitution for sulfur and/or arsenic substitution for iron. While the data available varies with coals in different coalfields all over the world (Rieder et al., 2007). In order to determine the presence of arsenic in the structure of pyrite in the GZ coal, a statistic analysis on the 27 single particles showing the arsenic concentration as a function of S and Fe content in HMF were presented in Fig. 6. It indicates that the higher Fe and/or S concentrations in the single mineral particle, the higher arsenic concentrations can be observed. Herein, it indicates that arsenic has a strong affinity to pyrite. Although the statistical correlation determination only quantifies a weak but existing correlation between arsenic and Fe/S in the HMF, it would be statistical significance. Fig. 6. Correlations between concentrations of arsenic and S (A)/Fe (B) in the mineral particles.
3.5. Arsenic contaminations in the local area Arsenic emissions in coal combustion are partly depending on the modes of arsenic occurrence in coal. It is reported that forms of arsenic exists as Asorg, Aspyr in coal appear to be evaporated more easily in combustion, while Asclay tends to remain in bottom ash (Yudovich and Ketris, 2005). In the present study, it is found that modes of occurrence of arsenic in GZ coal mostly present as Asorg and Aspyr, and it makes clear that the arsenic in GZ coal is more easily volatilized in combustion. The frequent use of high arsenic coal in coal-burning stove in house with poor vent would result in the accumulation of arsenic in the air indoors. Subsequently, the inhaled air polluted by arsenic derived from coal combustion in house becomes harmful. It has been reported that each local resident in the endemic areas takes up 6.67 mg arsenic in summer daily on average, and 7.88 mg arsenic in winter when coal-burning stove is more frequently used for heating (Yudovich and Ketris, 2005). It is also found out that chili peppers dried over the coal-burning stoves contain much higher arsenic in comparison to the fresh chili peppers (Wang et al., 2006). Herein, it can be deduced that the life-style of using the high arsenic coal for heating and food drying with poorly
4. Conclusions
Table 5 Element compositions of distinct particles in the heavy mineral fraction. No.
Elements (At. %) O
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
47.27 58.94 34.02 25.88 18.64 27.45 34.81 31.28 23.01 33.37
Al
6.17 7.66 5.89 4.75 6.37 2.69 4.55 4.59 9.74 5.00
Si
S
5.22 7.74 5.73 4.91 5.48 3.18 5.12 4.87 9.75 5.10
56.64 55.93 65.01 59.16 65.06 67.7 64.32 63.34 66.36 66.43 62.21 67.87 64.76 57.42 58.15 58.89 41.47 21.8 15.62 33.4 36.78 16.27 38.59 30.47 37.87 22.54 35.14
K
Ti
1.16 1.46 1.05 0.92 1.45 1.15 2.10
0.74
vented stove indoors is believed to be the major reason for the endemic arsenism with resultant of mass cases of disease and deaths in southwestern Chinese Guizhou province. Further, the coal used for power generation would cause mass emissions of toxic metals, especially the volatile element, e.g. arsenic. And the condensation of volatile arsenic species could be enriched on the surface of inhalable fine particles in the atmosphere. Detrimental health risks when the arsenic contained air or particle matter is inhaled and/or got deposit in lungs (Ruppert et al., 2005). Moreover, the disperse of the arsenic enriched fly ash generated from coal combustion on the ground surface would also contaminate the soil and the underground water with resultant of arsenic poisoning in the local area. What's more, arsenic-bearing pyrite is usually to be porous and with high surface area, and some extraneous atoms would appear in the pyrite crystal lattice, which makes the arsenic-bearing pyrite susceptible to oxidation and leaching (Huggins, 2002; Ruppert et al., 2005). In this case, the process of coal beneficiation would easily result in the contamination at water deposit site.
Fe
As (wt.%)
40.88 41.55 34.42 38.42 33.65 30.39 32.43 35.23 30.55 31.60 34.45 29.39 34.05 38.98 39.65 40.03 54.35 17.37 7.59 18.93 25.18 49.86 26.73 22.02 20.29 13.63 20.37
4.34 4.93 1.06 2.42 2.38 3.56 5.9 2.62 5.7 3.66 3.35 5.1 2.2 6.3 3.88 1.92 6.7 2.64 3.06 2.35 3.44 3.49 2.98 2.67 2.6 3.23 2.41
A combining method by using float–sink technique followed by low temperature ashing of a coal was applied for achieving quantitative assessments on mineralogical characteristics and distributions of arsenic in a coal. Quantitative assessments on mineralogical characteristic and occurrence of arsenic in GZ coal have been successfully achieved by means of the combining method. Heavy minerals, mostly pyrites, were successfully isolated from the organic constituents and separated from the low temperature ash residues of the GZ coal. Phase-mineral composition in LTA and LMF in GZ coal is very similar, which is predominantly composed of quartz and kaolinite, and with a little muscovite. While phases in the HMF are completely different from those in LTA and LMF. Pyrite dominates in the HMF. Arsenic content in the GZ coal is as high as 265 μg/g. Both organically associated arsenic and inorganically associated arsenic are identified in the GZ coal. The organically associated arsenic accounts for 28.4% of the total arsenic in GZ coal. Accumulation of arsenic in the HMF is as expected, and its concentration is as high as 650 μg/g. The inorganically associated arsenic is specified as pyrite associated. Direct evidence by using FE-SEM/EDX analysis is obtained, thus confirming that arsenic is occurring in pyrite. It is found that each pyrite particle contains a certain amount of arsenic, and the maximum concentration is as high as 6.7% (wt.%). Average of arsenic concentration in the pyrite particles is 3.5% (wt.%). Endemic arseniasis is suspected to be linked to the coal-burning stove for heating and/or dried food, inhalation of indoor air polluted by arsenic derived from coal combustion and the contaminated water in Guizhou province, southwestern China.
C. Tian et al. / International Journal of Mineral Processing 141 (2015) 61–67
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International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Chemistry, mineralogical, and residence of arsenic in a typical high arsenic coal Chong Tian a,b,⁎, Junying Zhang a,⁎, Rajender Gupta b, Yongchun Zhao a, Shuai Wang c a b c
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China Department of Chemical and Material Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada Department of Chemical Engineering, Huazhong University of Science and Technology, Wuhan, 430074, PR China
a r t i c l e
i n f o
Article history: Received 12 March 2014 Received in revised form 17 March 2015 Accepted 5 June 2015 Available online 8 June 2015 Keywords: Low temperature ash Float–sink Heavy minerals Arsenic occurrence
a b s t r a c t The arsenic chemistry, mineral association and distributions in coal were evaluated using density fraction and a low temperature ashing process. A high arsenic coal from Guizhou province in southwestern China was chosen in the study. The results show that light mineral (b 2.89 g/cm3) fraction and heavy mineral (N 2.89 g/cm3) fraction were successfully separated from Guizhou coal by means of the process. Phase-mineral composition in low temperature ash and light mineral fraction are almost the same, mostly quartz and kaolinite. In contrast, pyrite dominates in the heavy mineral fraction. Arsenic concentration in Guizhou coal is as high as 265 μg/g. An obvious enrichment of arsenic in heavy mineral fraction is observed, and the arsenic concentration is approaching to 650 μg/g. Modes of occurrence of arsenic exhibit as multiple forms. Both organic and inorganic associated arsenic in Guizhou coal have been identified. The organic associated arsenic accounts for 28.40% (wt.%) of the total arsenic. The inorganic associated arsenic mostly occurs in pyrite. Direct evidence by using the EDX analysis on 27 pyrite particles confirms that arsenic is occurring in pyrite. Each distinct pyrite particle contains a certain amount of arsenic with an average of 3.5% (wt.%), while the maximum is reaching to 6.7% (wt.%). Endemic arseniasis is highly suspected to be link to the frequent use of coal-burning stove for heating and/or food drying, inhalation of indoor air polluted by arsenic derived from coal combustion in houses, and the contaminated water in Guizhou province, southwestern China. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Topics about “arsenic in the environment” have been of a concern for a long time, which were preceded the discovery of arsenic in coal (Yudovich and Ketris, 2005). Some of the arsenic compounds are very toxic when they are dispersed in the environment as an air or water pollutant (Zielinski et al., 2007). It has been reported that arsenic exposure may not only affect and disable organs of bodies, but may also interfere with proper functioning of the immune system (Duker et al., 2005). Incidents on adverse health risks resulted from arsenic contamination in the environment in many countries of the world have been reported. Guizhou has been recognized as the most typical area of arsenic contamination in China as well as in the world (Kang et al., 2011). It has
Abbreviations: LTA, low-temperature ash; LTAS, low temperature ashes; LMF, light mineral fraction as a function of the total weight; HMF, heavy mineral fraction as a function of the total weight; HTA, high temperature ash; GZ, Guizhou; TEs, trace elements; XRD, X-ray diffraction; FE-SEM/EDX, Field Emission Scanning Electron Microscope equipped with Energy-Dispersive X-ray analysis; ICP-MS, inductively coupled-plasma mass spectrometry; XRF, X-ray fluorescence; RIM, reference intensity method; h, hour; hrs, hours. ⁎ Corresponding authors at: State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, PR China. E-mail addresses: [email protected] (C. Tian), [email protected] (J. Zhang).
http://dx.doi.org/10.1016/j.minpro.2015.06.010 0301-7516/© 2015 Elsevier B.V. All rights reserved.
also been reported that more than 3000 cases of arseniasis in villages in Guizhou were caused by the use of locally mined high arsenic coal (Zhang et al., 2002). Direct arsenic exposure for the local residences may come from the domestic use of coal for heating or food drying. Risks for arseniasis would be much higher if the high arsenic coal is frequently used indoors (He et al., 2002). Arsenic concentrations in Chinese coals increase in districts from northern to southern, and the weighted mean arsenic concentration is 3.18 μg/g (Kang et al., 2011). Abnormally high arsenic concentrations were identified in coals in southwestern China because of many geological factors (Dai et al., 2012; Ren et al., 1999). Chen et.al reported that the concentration of arsenic in coals in southwestern China was 18.2 μg/g (Chen et al., 1985). Ding et al. found that some coals in southwestern China contained arsenic exceeding to 1000 μg/g and some anomalous samples were with arsenic concentration as high as 35,000 μg/g (Ding et al., 2001). The origins of the high arsenic coal of southwestern China were related to the three aspects, including gold mineralization, hydrothermal activity in the late stage of coalification, and active volcanic effects (Zheng et al., 1999). The concentrations, distributions and modes of occurrence of arsenic in the high arsenic coals in southwest China have been studied extensively (Dai et al., 2005, 2006; Zheng et al., 1999). Quantitative understanding the modes of occurrence of arsenic in Guizhou, China might aid in the prediction of its
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mobility behavior and minimizing the health risks and the environmental impacts during coal utilization locally. The modes of occurrence of arsenic in coal are rather complex. Majority of arsenic is associated with mineral matter, and part of the arsenic is associated with organic matter in coals. Three major modes of arsenic occurrence are pyritic, organic, and arsenate (Yudovich and Ketris, 2005). Many techniques and measuring methods, including Mossbauer spectroscopy analysis, X-ray absorption fine structure (XAFS) analysis, a scanning electron microscope with energy dispersive X-ray analysis (SEM–EDX), and petro-graphic microscopic identification and sequential leaching experiments, have been applied to study arsenic in Chinese coals (Ding et al., 1999; Kang et al., 2011). Although it has been reported that arsenic may be organically-bounded or as (AsO4)3−, it was sulfides that have been extensively acknowledged to be the major carrier of arsenic, specifically, pyrite, marcasite, or arsenopyrite (Rieder et al., 2007). However, evidences on modes of occurrence of arsenic in coal were mostly obtained indirectly or to a lesser extent directly (Liu et al., 2007; Zhang et al., 2002) due to the limitation in direct methods. For instance, indirect evidences depended on the positive correlation As-S or As-Pyrite, or with the help of float–sink method with coals for heavy density fraction separation, in which sulfides dominate indicating that arsenic was associated with pyrite (FeS2) (Finkelman, 1995; Yudovich and Ketris, 2005). Later, direct characterizations of arsenic residence were added based on the measurement on pyrite separated by handpicking from coals (Zhang et al., 2002). Microprobe investigations also confirmed arsenic occurrence in pyrite, and it has been reported that the detectable amount of arsenic in some U.S. coals reached to 4.7 wt.% (Yudovich and Ketris, 2005). Previous research also reported that arsenic concentration would increase as the particle size of pyrite increase in Soutern Appalachian coal (Akers and Raleigh, 1998). However, direct assessments on the arsenic occurrence in coal are still necessary for achieving a step forwards. In the present study, the arsenic chemistry, mineral association and distributions were evaluated using density fraction and a low temperature ashing process (Tian et al., 2014). The typical high arsenic coal from Guizhou (GZ) province, in southwestern China was studied. Quantitative assessments on mineralogical characteristics and occurrence of arsenic in the GZ coal were achieved. Direct evidence on arsenic occurrence and content in the pyrite particles was obtained by using the Field Emission Scanning Electron Microscope equipped with EnergyDispersive X-ray analysis (FE-SEM/EDX) measuring. The possible origins of the arsenic exposure and contamination in GZ area have also been analyzed. 2. Material and methods 2.1. Coals and heavy mineral separation methods The typical high arsenic coals were collected from Xingren County, Guizhou southwestern China. It has been reported that arsenic accumulation in the coal in this district was related to gold mineralization (Zheng et al., 1999). The collection of the coals was in accordance with the Chinese Standard for Sampling of Coal in seam GB482-2008. Large batch coals were preserved in barrels to avoid man-made contamination. In the present study, the coals were air dried, crushed and sieved to obtain a representative 45–75 μm fraction firstly. Afterwards, the powdered coals were ashed at both high and low temperatures. The high temperature ash (HTA) was used to determine the oxides of the major elements, including SiO2, Al2O3, Fe3O4, CaO, MgO, Na2O, SO3, K2O, and TiO2 content, which would be served as a reference when describing mineral compositions in low temperature ash (LTA) and its different density fractions. Low temperature (120–150 °C) oxygen plasma ashing was considered to be a useful and subsidiary procedure for isolating the mineral matter, which were intimately distributed in the organic matrix of coals, for subsequent analysis by various methods (Vassilev and
Vassileva, 1996; Winburn et al., 2000). The LTA was obtained by means of a K1050X low temperature oxygen plasma asher. 1 g of powdered coal in a crucible was put in the chamber of the low temperature asher, and then set the radio frequency power at 70 W. After each step of the ashing process was completed (typically around 2 hrs), the crucible was taken out and weighed, and the weight loss for the coal was determined. The sample in the crucible was then stirred to make a new surface, and the ashing process was repeated until the weight loss was negligible. The total ashing time for the samples was typically 8–12 hrs. In the procedure of low temperature ashing, the organic constituents in the coal were almost completely removed by oxidation and the inorganic constituents were obtained without major alteration. Since the content of heavy minerals in the coal is rather small (usually b1 wt.% of the LTA), at least 5 g of LTA was needed to provide sufficient amount of sample for the follow-up analysis. Around half of a month was needed to obtain sufficient sample of the GZ coal for the following study. Afterwards, the LTA was firstly washed and filtered by the deionized water and oven dried. And then the washed LTA were subjected to float–sink separation in bromoform (density = 2.89 g/cm3). The float– sink process was conducted with the help of a centrifuge. The LTA was divided into two density fractions, the light mineral fraction (LMF) (density b 2.89 g/cm3) was floating on the top in the centrifugal tube, while the heavy mineral fraction (HMF) (density N 2.89 g/cm3) sank to the bottom. The LMF and HMF were filtered and oven dried overnight. 2.2. Analytical technique The oxides of the major elements, including SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, SO3, TiO2, P2O5, and K2O in HTA were determined by X-ray fluorescence spectrometry (XRF). Phase-mineral composition and content of the GZ coal were determined by XRD. The XRD investigations were carried out on an X'Pert PRO diffractometer equipped with a graphite diffracted-beam monochromator. The accelerating voltage was 40 kV and the current was 40 mA. XRD patterns were recorded for the LTA and the different density fractions over a 2θ interval of 10–80° using Cu-Kα radiation with a step size of 0.017° per 10 s. Final semi-quantitative mineral composition was determined by using the Reference Intensity Method (RIM) (Chung, 1974; Ward and French, 2006; Winburn et al., 2000). The small amounts of HMF were analyzed with the FE-SEM/EDX for micro-morphology observation and rare mineral identification. The FESEM/EDX was also applied to measure the arsenic occurrence and content in HMF for obtaining a direct evidence of arsenic occurring in specific mineral species in HMF. Investigations of FE-SEM were carried out on a Sirion200 microscope equipped with a GENESIS EDX (energy dispersive X-ray spectroscopy). Arsenic concentrations in coal, LTA, LMF and HMF were determined by using the high resolution inductively coupled-plasma mass spectrometry (ICP-MS). The solid samples for ICP-MS were digested according to the DZ/T0223-2001, with 1 ml HF, 3 ml HNO3, and 1 ml HClO4 for each 0.05 g solid sample in the low pressure sealed vessel. 3. Results and discussion 3.1. General observations of mineral separation of the coal studied Ultimate and proximate analyses of the typical GZ Coal studied were summarized in Table 1. According to Chinese standard GB/T 15224.12004 and GB/T 15224.2-2004. The GZ coal studied is a high sulfur and high ash anthracite. The chemical compositions were summarized in Table 2. Concentrations of SO3 is as high as 21.5% (wt.%). It has been reported that sulfur in coal existed as organic and inorganic form, and the inorganic sulfur were usually sulfides and sulfates, with pyrite as the major sulfur containing mineral in most coals (Calkins, 1994). The
C. Tian et al. / International Journal of Mineral Processing 141 (2015) 61–67 Table 1 Ultimate and proximate analysis of the coal studied. Ultimate analysis (wt.%) Sample GZ
Cdaf 75.29
Hdaf 1.56
Table 3 General observations of results from the float–sink experiment. Proximate analysis (wt.%)
Ndaf 0.79
St 20.31
Odaf 3.03
63
Mad 1.05
Ad 45.94
Vdaf 12.17
Sample Low-temperature ash
FCdaf 40.84
M: moisture; V: volatile; A: ash; FC: fix carbon; St: total sulfur; GZ: Guizhou. daf: dry and ash-free basis; ad: air-dried; d: dry basis.
Float–sink
Coal (g) LTA (g) LTA/Coal (%) LMF (g) LMF (%) HMF (g) HMF (%) GZ
8.781
4.261
48.525
3.955
92.82
0.306
7.18
GZ: Guizhou; LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction.
high ash and high sulfur GZ coal is expected to have a high concentration of pyrite and arsenic. Table 3 summarized the LTA yields and the general observations of LMF and HMF separated from LTA by means of float–sink technique. Yields of the LTA from GZ coal is about 50% (wt. %) of the coal, which is close to the ash content in the coal, indicating a complete depletion of organic constituents. The general observations from float–sink experiment show that the separated LMF accounts for most part in the LTA of the GZ coal, and the proportion is more than 90% (wt.%) of the LTA. In contrast, HMF only accounts for a very small amount in the LTA.
3.3. Residence of arsenic in the coal studied Ren et al. (1999) have reported that the arsenic concentrations in Chinese coals were ranging from 0.03 to 478.4 μg/g, with an average
3.2. Mineralogical characteristics of the coal studied The XRD measurement was used to determine the phase-mineral composition and content in the GZ coal. XRD patterns of LTA, LMF and HMF were shown in Fig. 1. Diffraction peaks appearing in the XRD patterns of the LTA and LMF are very similar, indicating that the phase-mineral composition in LTA and LMF are almost the same. While obvious different diffraction peaks are observed in the XRD patterns of the HMF in comparison to that in LTA and LMF, hinting the different phases in the HMF. In other words, it indicates that separation of heavy minerals by means of float–sink technique followed by low temperature ashing of the GZ coal can be successfully achieved, and a sample of HMF can be obtained for further quantitative assessments. Phase-mineral composition and content of LTA, LMF and HMF were summarized in Table 4. Phases in LTA and LMF are majorly composed of quartz and kaolinite, a little muscovite is also found. And the relative content of each mineral species in LTA and LMF varies a little bit. Phases identified in HMF are predominantly composed of pyrite. Semiquantitative results show that the relative content of pyrite in the HMF is 62.91% (wt.%). Minor amount of other heavy minerals, e.g. anatase, is also identified in HMF. Secondary electron microscope images of the HMF from GZ coal were shown in Fig. 2. And the EDX results were in line with the XRD analysis. Most of the mineral particles identified in the secondary electron microscope images are pyrite. And all the particles exhibit as distinct shapes with different size ranging from 10 μm to 20 μm. Most of these particles are euhedral massive pyrite, and cracks can be observed on the surface of some pyrite. Characteristic, type, genesis, morphology, and distributions of pyrite in coal seams from different deposits have been described by previous researchers (López-Buendía et al., 2007; Querol et al., 1989; Turnera and Richardson, 2004). Morphologies and genesis of pyrite are various, and both micrometer-sized framboids and massive crystals and vein-fillings can be observed (Vassilev and Vassileva, 1996; Widodo et al., 2010). The morphology of pyrite is of a help in reconstructing the depositional environment when peat formation (Dai et al., 2008). Pyrite is considered to be the major host for many hazardous trace elements of great environmental concern (Luttrell et al., 2000). Thus it can be deduced that TEs should be enriched in the HMF of GZ coal. Table 2 Chemical compositions (wt.%) of the coal studied. Sample
Na2O
MgO
Al2O3
SiO2
Fe2O3
SO3
K2O
CaO
TiO2
GZ
0.6
0.7
36.8
24.9
9.8
21.5
2.3
0.6
2.6
GZ: Guizhou.
M-Muscovite
K-Kaolinite
Q-Quartz
An-Anatase
P-pyrite
Fig. 1. XRD patterns of the fractions separated from the coal studied. M—muscovite; K—kaolinite; Q—quartz; An—anatase; P—pyrite.
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Table 4 Phase-mineral composition and content (wt.%) of fractions separated from the coal studied.
LTA LMF HMF
Muscovite
Kaolinite
Quartz
Anatase
Pyrite
15.4 17.6 4.24
29.8 28.3 14.91
45.2 42.9 9.21
– – 4.38
4.3 4.1 62.91
LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction.
of 3.79 μg/g. And this conclusion was depending on the evaluation of a total of 3386 coal samples from China. Arithmetic means of the arsenic levels have been extensively reported in the assessment of arsenic in Chinese coals (Dai et al., 2004, 2010; Wang et al., 2006). Some studies indicated that the arsenic concentrations in Chinese coals varied greatly from 0 to 35,037 μg/g (Ding et al., 1999). Zhang et.al reported that arsenic concentration in the Triassic coals from southwestern Guizhou ranged from 1.22 to 238 ppm (Zhang et al., 2004). The GZ coal studied also has a relatively high concentrations of arsenic in comparison to some other Chinese coals, which is as high as 265 μg/g. The relative enrichment of arsenic in this area is probably due to the low-temperature hydrothermal activity during the local geological history. The histogram in Fig. 3 shows the arsenic concentration trends in the GZ coal, LTA, LMF and HMF. It can be found that arsenic concentrations vary in the coal, LTA, and the two density fractions. Arsenic concentrations in LTA (391 μg/g) and LMF (360 μg/g) are very close to each other. In contrast, arsenic is obviously enriched in the HMF (approaching to 650 μg/g). It suggests that the arsenic be likely exhibiting as multiple modes. It has been reported that modes of occurrence of arsenic are majorly controlled by Aspyr or Asorg. If an arsenic accumulation in partitioning of sink fraction from coal or high-ash coals is observed, it means that the Aspry dominates. If an enrichment of arsenic in medium-density fractions or low- and medium-ash coals is observed, Asorg is expected to dominate in the coal (Yudovich and Ketris, 2005). In the present study, the fact that high-ash content of the GZ coal and the obvious enrichment of arsenic in the HMF indicate that the modes of occurrence of arsenic are highly expected to be majorly controlled by Aspyr. However, other modes of occurrence of arsenic are also believed to present in the GZ coal. For instance, a noticeable arsenic loss from coal to the washed LTA can not be ignored, and this part of arsenic is highly possible to be organically associated. Since the organically-bounded arsenic is mostly associated with C, O and organic S, and oxygen plasma would implant in the chemical bounds, e.g. As-C and As-S et.al, during the low temperature ashing process, thus causing the As-C and/or As-S being broken
LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction;
Fig. 3. Arsenic concentrations in the coal studied and its separated fractions. LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction.
down. In this case, the organically associated arsenic would be isolated and easily washed away. Taking into account the arsenic concentration in GZ coal and its LTA, and the yields of LTA from the GZ coal, the loss of arsenic can be calculated by the Eq. (1): L ¼ 1−ðC1=C2Þ Y
ð1Þ
Where, L: Arsenic loss; C1: Arsenic concentrations in LTA; C2: Arsenic concentrations in coal; Y: Yields of LTA from coal. It can be quantified that the loss of arsenic from coal to LTA is 28.4% (wt.%) of the total arsenic in GZ coal, thus making it clear that organically associated arsenic is presenting in the GZ coal. In contrast, more than 70% (wt.%) of the total arsenic are inorganically associated in GZ coal. The inorganically associated arsenic may be further divided into two forms, including alumina-silicate associated and pyrite associated. To be specific, certain amount of arsenic in LMF indicates that a small part of arsenic may be alumina-silicate associated. However, the remaining pyrite in the LMF caused by the experiment error may also contribute for the arsenic content. The relative higher concentrations of arsenic in the HMF in comparison to LMF suggests that arsenic has a strong affinity to mineral species in HMF, particularly with pyrites. It is deduced that most of the arsenic in GZ coal studied is pyrite associated, which is in line with the previous findings that the inorganically associated arsenic is mostly associated with pyrite in coals (Finkelman, 1994; Hower et al., 1997). Fig. 4 shows both the arsenic and pyrite content trends in coal,
LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction;
Fig. 2. Typical secondary electron microscope image of heavy mineral fraction in the coal studied.
Fig. 4. Arsenic content in the fractions separated from the coal studied. LTA: low temperature ash; LMF: light mineral fraction; HMF: heavy mineral fraction.
C. Tian et al. / International Journal of Mineral Processing 141 (2015) 61–67
LTA and the two density fractions in GZ coal studied. The similar arsenic and pyrite concentration trends in the partitioning from GZ coal also directly confirm the conclusion that the arsenic has a strong positive correlation with the pyrite in coals. 3.4. Arsenic occurrence in pyrite Previous findings indicate that pyrite and arsenic are obviously enriched in the HMF separated from GZ coal, and their concentrations are expected to in exceeding to the detectable limitation. Hence, FESEM/EDX is highly applicable to study the arsenic distributions as well as mineral associations in HMF. Quantitative assessments on the arsenic occurrence and content in pyrite in GZ coal by using the EDX measurement by particle analysis within the separated HMF were obtained. Typical secondary electron microscope images with corresponding EDX results were shown in Fig. 5. It was found that pyrite particles were totally isolated from the organic constituents and presented as distinct shapes and with size larger than 10 μm. Fig. 5-A presented an irregular
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shapes of pyrite particle, and the arsenic concentration was reaching to 4.34% (wt.%). Euhedral pyrite presents as decahedron was also identified (Fig. 5-B), with arsenic concentration of 2.62% (wt.%). Fig. 5-C showed a pyrite with some cracks on the surface, and the arsenic concentration was 2.67% (wt.%). Totally 27 particles were analyzed and the results were listed in Table 5. The EDX analysis show that major elements in all the particles are Fe and S, and the atomic ratio of Fe to S is about 1:2 suggesting it to be FeS2–pyrites. However, some pyrite particles contain certain amount of O and a small quantity of Al and Si. The presence of oxygen may be partly from the oxidation of the pyrite exposing in the air. Presence of Al and Si probably came from few clay minerals combined with the pyrite particles. The results from EDX, combined with the results obtained from the XRD analysis, confirmed that the single particles indentified in the FE-SEM/EDX are predominantly pyrite with some impurities such as Si and Al attached on the particles. EDX measuring by particle analysis in the HMF also indicates that each of the pyrite contains a certain amount of arsenic. Concentrations of arsenic in the isolated pyrite vary widely, ranging from 1.06% to
Arsenic concentrations in the images of A,B,C are 4.34%, 2.62%,2.67%, respectively. (wt.%) Fig. 5. Secondary electron microscope images and energy spectrum analysis on typical particles in heavy mineral fractions in the coal studied. Arsenic concentrations in the images of A, B, and C are 4.34%, 2.62%, and 2.67%, respectively (wt.%).
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C. Tian et al. / International Journal of Mineral Processing 141 (2015) 61–67
6.7% (wt.%) with an average at around 3.5% (wt. %). It directly confirms the assumption that most of the arsenic in the GZ coal studied is occurring in pyrite. And the pyrite is the major carrier for the arsenic in coals. The incorporation of arsenic in the structure of pyrite includes arsenic substitution for sulfur and/or arsenic substitution for iron. While the data available varies with coals in different coalfields all over the world (Rieder et al., 2007). In order to determine the presence of arsenic in the structure of pyrite in the GZ coal, a statistic analysis on the 27 single particles showing the arsenic concentration as a function of S and Fe content in HMF were presented in Fig. 6. It indicates that the higher Fe and/or S concentrations in the single mineral particle, the higher arsenic concentrations can be observed. Herein, it indicates that arsenic has a strong affinity to pyrite. Although the statistical correlation determination only quantifies a weak but existing correlation between arsenic and Fe/S in the HMF, it would be statistical significance. Fig. 6. Correlations between concentrations of arsenic and S (A)/Fe (B) in the mineral particles.
3.5. Arsenic contaminations in the local area Arsenic emissions in coal combustion are partly depending on the modes of arsenic occurrence in coal. It is reported that forms of arsenic exists as Asorg, Aspyr in coal appear to be evaporated more easily in combustion, while Asclay tends to remain in bottom ash (Yudovich and Ketris, 2005). In the present study, it is found that modes of occurrence of arsenic in GZ coal mostly present as Asorg and Aspyr, and it makes clear that the arsenic in GZ coal is more easily volatilized in combustion. The frequent use of high arsenic coal in coal-burning stove in house with poor vent would result in the accumulation of arsenic in the air indoors. Subsequently, the inhaled air polluted by arsenic derived from coal combustion in house becomes harmful. It has been reported that each local resident in the endemic areas takes up 6.67 mg arsenic in summer daily on average, and 7.88 mg arsenic in winter when coal-burning stove is more frequently used for heating (Yudovich and Ketris, 2005). It is also found out that chili peppers dried over the coal-burning stoves contain much higher arsenic in comparison to the fresh chili peppers (Wang et al., 2006). Herein, it can be deduced that the life-style of using the high arsenic coal for heating and food drying with poorly
4. Conclusions
Table 5 Element compositions of distinct particles in the heavy mineral fraction. No.
Elements (At. %) O
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27
47.27 58.94 34.02 25.88 18.64 27.45 34.81 31.28 23.01 33.37
Al
6.17 7.66 5.89 4.75 6.37 2.69 4.55 4.59 9.74 5.00
Si
S
5.22 7.74 5.73 4.91 5.48 3.18 5.12 4.87 9.75 5.10
56.64 55.93 65.01 59.16 65.06 67.7 64.32 63.34 66.36 66.43 62.21 67.87 64.76 57.42 58.15 58.89 41.47 21.8 15.62 33.4 36.78 16.27 38.59 30.47 37.87 22.54 35.14
K
Ti
1.16 1.46 1.05 0.92 1.45 1.15 2.10
0.74
vented stove indoors is believed to be the major reason for the endemic arsenism with resultant of mass cases of disease and deaths in southwestern Chinese Guizhou province. Further, the coal used for power generation would cause mass emissions of toxic metals, especially the volatile element, e.g. arsenic. And the condensation of volatile arsenic species could be enriched on the surface of inhalable fine particles in the atmosphere. Detrimental health risks when the arsenic contained air or particle matter is inhaled and/or got deposit in lungs (Ruppert et al., 2005). Moreover, the disperse of the arsenic enriched fly ash generated from coal combustion on the ground surface would also contaminate the soil and the underground water with resultant of arsenic poisoning in the local area. What's more, arsenic-bearing pyrite is usually to be porous and with high surface area, and some extraneous atoms would appear in the pyrite crystal lattice, which makes the arsenic-bearing pyrite susceptible to oxidation and leaching (Huggins, 2002; Ruppert et al., 2005). In this case, the process of coal beneficiation would easily result in the contamination at water deposit site.
Fe
As (wt.%)
40.88 41.55 34.42 38.42 33.65 30.39 32.43 35.23 30.55 31.60 34.45 29.39 34.05 38.98 39.65 40.03 54.35 17.37 7.59 18.93 25.18 49.86 26.73 22.02 20.29 13.63 20.37
4.34 4.93 1.06 2.42 2.38 3.56 5.9 2.62 5.7 3.66 3.35 5.1 2.2 6.3 3.88 1.92 6.7 2.64 3.06 2.35 3.44 3.49 2.98 2.67 2.6 3.23 2.41
A combining method by using float–sink technique followed by low temperature ashing of a coal was applied for achieving quantitative assessments on mineralogical characteristics and distributions of arsenic in a coal. Quantitative assessments on mineralogical characteristic and occurrence of arsenic in GZ coal have been successfully achieved by means of the combining method. Heavy minerals, mostly pyrites, were successfully isolated from the organic constituents and separated from the low temperature ash residues of the GZ coal. Phase-mineral composition in LTA and LMF in GZ coal is very similar, which is predominantly composed of quartz and kaolinite, and with a little muscovite. While phases in the HMF are completely different from those in LTA and LMF. Pyrite dominates in the HMF. Arsenic content in the GZ coal is as high as 265 μg/g. Both organically associated arsenic and inorganically associated arsenic are identified in the GZ coal. The organically associated arsenic accounts for 28.4% of the total arsenic in GZ coal. Accumulation of arsenic in the HMF is as expected, and its concentration is as high as 650 μg/g. The inorganically associated arsenic is specified as pyrite associated. Direct evidence by using FE-SEM/EDX analysis is obtained, thus confirming that arsenic is occurring in pyrite. It is found that each pyrite particle contains a certain amount of arsenic, and the maximum concentration is as high as 6.7% (wt.%). Average of arsenic concentration in the pyrite particles is 3.5% (wt.%). Endemic arseniasis is suspected to be linked to the coal-burning stove for heating and/or dried food, inhalation of indoor air polluted by arsenic derived from coal combustion and the contaminated water in Guizhou province, southwestern China.
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