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A cut-off grade for gold and gallium in coal
Fuel 147 (2015) 62–66
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A cut-off grade for gold and gallium in coal Wenfeng Wang a,b,⇑, Shuxun Sang a,b, Weiduo Hao a,b, Ran Wang a,b, Jiefang Zhang a,b, Piaopiao Duan a,b, Yong Qin a,b, Shaochun Xu c a
Department of Source and Earth Science, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, China Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, China University of Mining and Technology, Ministry of Education, Xuzhou 221008, Jiangsu Province, China c Department of Computer Science and Mathematics, Algoma University, 1520 Queen Street East, Sault Ste. Marie, Ontario P6A 2G4, Canada b
h i g h l i g h t s We studied the Au and Ga contents in coal and ash by INAA. The average contents of Au and Ga in Guizhou coal are 6.92 ng/g and10.65 lg/g. Gallium is enriched in the residues more easily than Au during the coal ashing process. The cut-off grades of Au and Ga in coal are considered to be 200 ng/g and 60 lg/g.
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Article history: Received 30 November 2014 Received in revised form 27 December 2014 Accepted 21 January 2015 Available online 31 January 2015 Keywords: Cut-off grade Gold Gallium Coal Ash
a b s t r a c t The contents of Au, Ga, As, Sb and other elements in two hundred and thirty-four samples of coal, partings, roof and floor collected from Guizhou Province, China were analyzed by Instrumental Neutron-Activation Analysis (INAA). In order to discuss the relationship between Au, Ga in coal and in coal ash, the contents of Au and Ga in 815 °C coal ash were also determined by INAA and partial Ga by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The analysis results showed that: (1) the average contents of Au and Ga in upper Permian coal from Guizhou are 6.92 ng/g and 10.65 lg/g, respectively, far higher than that in Chinese coal and world coal; the average contents of Au (59.73 ng/g) and Ga (19.16 lg/g) in partings, roof and floor are higher than that in the coal and the Clarke value of sedimentary rock (especially for Au); (2) during the coal ashing process, Ga is enriched in the residues more easily than Au, which is mainly because of the difference in the mode of occurrence of the two elements. Gallium mainly occurs as the substitute of Al in the form of isomorph, which makes it hard to volatize; while Au shows a high association with organic matter, sulfide, and carbonates, and some may be nano-gold, which is easily volatile during coal combustion. Only Au occurred in the aluminosilicate is easily left in the residues; (3) we collected data on the Au and Ga contents in coal and coal ash from China and then conducted correlation analysis by combining data from coals and coal ashes worldwide; the cut-off grades of Au and Ga in coal are considered to be 200 ng/g and 60 lg/g, respectively. This method could also be used to determine the cut-off grade of other valuable elements in coal. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction China is the largest coal production and consumption country in the world. By 2013, coal production in China has exceeded 3.6 billion tons per year. Generally, every 4 tons of coal combustion will produce 1 ton of coal ash [1], but the low rate of comprehensive ⇑ Corresponding author at: Department of Source and Earth Science, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, China. Tel.: +86 51683591011; fax: +86 51683590998. E-mail address: [email protected] (W. Wang). http://dx.doi.org/10.1016/j.fuel.2015.01.066 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
utilization of coal ash in China not only leads to great environmental problems but also wastes a large amount of mineral resources and industrial valuable elements in the ash. In the 1990s, joint efforts were taken by USA and Canada to study Ga in coal ash, and Ga was successfully extracted from coal ash [2]. In Ordos, China, a pilot plant with an annual processing capacity of approximately 150 t Ga was built at the beginning of 2011 [3]. Gold was also extracted from wastes of the Reftinsk power station, Russian [4]. Although there has been a lot of research work done on extraction of trace elements from coal ash [5], the industrial cut-off grade of metals in coal has rarely been reported.
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The precious metal minerals in coal basins can be extracted as by-products during secondary treatment of the coal or as the products of coal combustion [6]. Recently, more and more researchers have found valuable trace elements enriched in coal [5,7–10]. Thus, the minimum mining grade of trace elements is of vital importance to the combined exploration of trace elements and coal, which could reduce the exploration budget and promote the exploitation of metals in coal ash. Unfortunately, there are only a few related researches. Sun et al. [11] suggested that the minimum mining grades of Li, U, Th and REE should be 120, 40, 150 and 300 lg/g, respectively. Their work is foresighted to some extent, however, the conclusion was drawn based on the 17% average ash yield in Chinese coal. Moreover, they might not have considered the occurrence mode of trace elements in coal and the combustion mode, therefore, their conclusion may not be applied to other elements in coal. In order to explore the relationship between the Au and Ga contents in coal and in coal ash, and further determine their cut-off grade in coal, we conduct an analysis of the Au and Ga contents in 234 coal and 75 coal ash samples. 2. Sampling and analytical method Samples were collected from working coal mines in Guizhou Province, China, 234 pieces in total, including 44 partings, roof and floor samples, and 190 coal samples. All of the coal samples are mixed and ply samples of upper Permian coal seam and feed of 4 power plants. Proximate analysis (total moisture, volatile matter, and ash yield) was conducted using ISO recommendations (ISO-11722 [12], ISO-562 [13], and ISO-1171 [14]). The total sulfur and forms of sulfur were determined following ISO-351 [15] and ISO-157 [16], respectively. All samples were ashed in a muffle oven at 815 °C to constant value of weight (ISO-1171). X-ray fluorescence spectrometry was used to determine the oxides of major elements for these ash samples. Some trace elements such as As, Au, Ga and Sb in solid samples, and Au in 34 ash samples and Ga in 25 ash samples were determined using Instrumental Neutron Activation Analysis (INAA). Inductively Coupled Plasma Mass Spectrometry (ICP-MS), in a pulse counting mode (three points per peak), was used to determine Ga in the other 50 ash samples. 3. Results and analysis 3.1. The Au and Ga contents in Guizhou coal and coal ash The contents of Au and Ga in Guizhou coal range from 1.16 to 313 ng/g and from 2.54 to 30.7 lg/g, averaged at 9.53 ng/g and 11.25 lg/g, respectively, which are far higher than the average contents in Chinese coal and world coal (Table 1), implicating their significant enrichment in Guizhou coal. However, the average contents of Au (especially Au, 59.73 ng/g) and Ga (19.16 lg/g) in partings, roof and floor are significantly higher than that in coal and the Clarke value of sedimentary rock, respectively. Moreover, the average Au content in coal is even higher than the Clarke value
of sedimentary rock (6 ng/g), but the Au content in ash (15.52 ng/g) is lower than that in world coal ash (22 ng/g). On the contrary, the average Ga content in Guizhou coal (11.25 lg/g) is lower than the Clarke value in sedimentary rock (12 lg/g), but in Guizhou coal ash, the Ga content (35.31 lg/g) is higher than the average content in world coal ash (33 lg/g). The results showed that compared with world coal, Au in Guizhou coal is easily volatile during coal combustion and only partial Au is left in the residue while Ga shows opposite behavior. Table 2 shows the contents of Au and Ga in some coal and coal ash samples. As Table 2 shows, the Au content in 7 out of 34 samples is significantly lower than that in raw coal samples, and the content in some coal and ash is approximately the same. By contrast, Ga in ash samples is higher than that in raw coal sample, which could indicate that Ga is enriched in the residues more easily during coal combustion. The different behavior of the two elements reflects their different modes of occurrence in coal. 3.2. The occurrence mode of Au and Ga in Guizhou coal Generally, it is believed that Ga in coal mainly occurs as inorganic phase and replaces Al in aluminiferous minerals (mainly boehmite) in the form of isomorph [7]. Thus, Ga usually shows a positive correlation with ash yield, and this is also shown in Guizhou coal but with a weak relationship (Fig. 1a). Moreover, Ga in coal also shows a weak correlation with Al2O3 (Fig. 1b), while in coal ash, an obviously positive correlation is shown between Ga and Al2O3 (Fig. 1c). This phenomenon shows that although Ga mainly occurs in the inorganic matter of coal, not all Ga replace Al in aluminiferous minerals in the form of isomorph. According to the above correlation analysis, it seems that Ga in coal in the form of isomorph remains more in ash during coal combustion, while the other occurrences of Ga (organic phase) are easily volatile. The occurrence mode of Au in coal is considerably complex. Seredin et al. [19] thought that there are four kinds of occurrences for Au in coal: dispersed native gold, associated with sulfide, absorbed by clay minerals, and organically bonded. Our previous research on the sequential chemical extraction of four pieces coal samples from Guizhou demonstrated that the occurrence mode of Au in the studied coal samples is complex [20]: Au occurs in coal bound to organic matter, as a sulfide, within clay minerals and as a carbonate (Table 3). Yang [21] conducted research on Puan coal samples from Guizhou with sequential chemical extraction method and the result also showed that the Au occurs mainly as a sulfide affinity while also associating with organic matter, silicate and carbonate. Our correlation analysis demonstrates that Au in coal has a weakly positive correlation with ash yield (Fig. 2a) but it is in association with a more positive correlation with pyrite, Sb and As (Fig. 2b–d), which might indicate that Au in coal mainly exists in pyrite, especially Sb/As-bearing pyrite. Iron in coal mainly exists in pyrite, and could also occur in aluminosilicate. During the coal ashing process, some Fe maybe escape with the decomposition of pyrite. As shown in Fig. 2e, Au in coal has a certainly positive correlation with Fe2O3, but less than the relationship between Au in
Table 1 The Au and Ga contents in upper Permian coal from Guizhou Province. Elements Coals Range
Coals ashes Total number Average Range of samples
Au (ng/g) 1.16–313 190 Ga (lg/g) 2.54–30.7 106
6.92 10.65
Partings, roof and floor Total number Average Range of samples
2.4–190 34 12.2–71.45 75
15.74 35.88
Chinese coals World
Total number Average of samples
1.41–1261 44 2.72–47.6 28
59.73 19.16
Coals Coal ashes 3 6.55
3.7 5.8
22 33
Chinese coals from Tang and Huang [17] and Dai et al. [9], world coal and Clarke value in sedimentary rock from Ketris and Yudovich [18].
Clarke value of sedimentary rock
6 12
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Table 2 The Au and Ga contents in coal samples and coal ashes. Coalfield
Location + sample types
Ash (%)
Hezhang Nayong Zhijin
Gaoqiao coal Feed coal of Nayong power plant Jinma coal Shibanzhai coal Jingxuan coal Feed coal of Faer power plant Dahebian coal Wangjiazhai coal Yushe coal Xiaohegou coal Shanhe coal Zijin coal + partings 1 Zijin coal + partings 2 Zijin coal Guojiadi coal Zhenzhong coal Jixing coal Madaodi coal + partings Hengtai coal Dabatian coal Jinqiao coal Wazicong coal Xiongxin coal Guanglong coal Xingfa coal Longkang coal Qianjin coal Dayakou coal Xinlong coal 1 Xinlong coal 2 Sancun coal + partings Silianxiang coal Erwan coal Xiashanzhen coal
8.69 32.83 9.26 13.82 27.29 62.97 12.63 23.99 13.11 16.14 7.98 54.22 62.56 27.94 18.93 19.12 13.89 69.76 14 14.97 21.38 45.58 18.21 28.93 15.72 24.26 26.3 30.91 21.62 19.37 69.66 31.29 17.83 24.77
Au (lg/g)
Ga (ng/g) In coal
Suicheng
Zhenfeng
Puan
Panxian Bijie Xinyi Anlong
Xinren
16.26
7.12 18.5 4.17 3.28 24.64 13.3 2.54 5.2 6.52 3.41 8.23 7.07 3.25 2.72 5.64 4.08 2.55 8.87 4.65 8.83 11.95 10.2 7.71 12.6 5.82 4.45 8.49 4.12 4.12 7.78
In ash
In coal
In ash
57.9 46.8 47.6 46.8 31.8 41.3 31.1 16.6 52.9 35.6 28.1 12.2 13.1 15 22.8 39.1 38.5 47.6 43.8 28.7 13.9 18.7 22.9 29.5 41.7 40.7 29.6 35.9 30.6 35 29.6 29.4 29.4 30.7
32.4 6.13 39.9 26.8 3.03 17.6 6.95 2.34 4.94 61.8 1.6 4.28 1.97 3.24 1.34 1.63 2.43 1.41 2.58 1.81 2.22 2.35 2.21 2.76 4.34 2.62 2.41 3.05 1.93 1.21 132 2.77 1.87 1.8
9.81 4.6 20.2 23.9 10.2 3.71 3.34 8.32 18.6 14.6 10.7 4.3 2.89 3.98 8.79 5.97 10.5 13.9 41.8 6.83 2.4 5.01 11.4 3.25 32.9 10.5 9.04 10.2 13.7 3.93 190 5.74 3.38 6.75
80 60 40 20 0
a 0
10
20
30
40
Ga in coal (µg/g)
Ash yield (%)
100
40 35 30 25 20 15 10 5 0
Ga in ash (µg/g)
Ad, ash yield, dry basis; Italic values, lower Au content in ash than in coal.
b 0
5 10 15 20 25 30 35 40 45
Ga (µg/g)
Al 2 O3 (%)
80 70 60 50 40 30 20 10 0
c 0
20
40
60
Al2 O3 (%)
Fig. 1. The relationship between Ga in coal and ash yield, Al2O3, and between Ga in coal ash and Al2O3.
coal ash and Fe2O3 (Fig. 2f). This indicates that organic-, pyriterelated and nano-gold (fusing point is 327 °C) are easily volatile during coal combustion, while more Au occurring in aluminasilicate remains in the coal ash (Fig. 2f). 3.3. The industrial cut-off grade of Au and Ga in coal Despite the large estimated resources of Ga in bauxites (1 billion kg) and Zn deposits, only a small percentage (40%) is economically recoverable [2,22], so there is considerable interest in evaluating Ga recovery from other significant sources, such as phosphate ores and coal [2]. Although the latter typically contains up to 0.10% Ga, this element, along with other valuable elements, is concentrated in coal fly ash during coal combustion [23]. As one kind of fuel, coal is mainly used by combusting to supply energy. If valuable elements present in coal concentrate in the coal ash and reach the industrial cut-off grade, it is essential to consider
their beneficial recovery as a by-product from coal combustion residues. Based on a review of a number of methods for recovery of metals from power plant solid wastes, Meawad et al. [24] believed that the solid wastes are actually a secondary resource of metals, and by applying mineral processing technologies and hydrometallurgical and biohydrometallurgical processes, it is possible to recover metals such as Al, Ga, Ge and V, from power plant solid wastes. As for Au, it also has been shown that economically efficient Au recovery from the wastes of the Reftinsk power plant in the Urals is conducted with an average Au grade of 0.1–0.2 lg/g in coal combustion products [4]. Recently, Yao et al. [25] can use combinations of processes, such as predesilication of fly ash and a lime-soda sinter process. It is also better to simultaneously recover more valuable materials, such as Al, Si, Ga and Ti. However, most reported methods are still in the early stages of commercialization [25]. Further studies are needed to turn this research into commercial reality.
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W. Wang et al. / Fuel 147 (2015) 62–66 Table 3 Sequential chemical extraction results of four Guizhou coals (%) [20]. Sample Nos.
Water soluble
Ion-exchangeable
Carbonate
Humic acid bonded
Iron–Mn oxidate
Organic bonded
Residue
Coal Coal Coal Coal
bdl bdl 3.59 12.33
0.07 0.12 3.12 5.84
13.89 1.71 5.5 10.13
12.11 30.24 21.71 9.91
15.79 13.42 34.17 38.24
33.54 18.72 22.8 13.22
24.6 35.79 9.09 10.33
1 2 3 4
bdl, below detection limit.
100
1000
100 80 60 40
1 0.1
20 0
100
10
Sb (µg/g)
Sp,d (%)
Ash yield (%)
120
a 1
10
100
1000
0.01
10000
1
10
0.01 0.1
1000
10 1 0.1
1000
d 100
1000
1000 10000
100 10 1 0.1
100000
1000
Au in ash (ng/g)
Au in coal (ng/g)
100
10
c 10
Au (ng/g)
10000
1000
As (µg/g)
100
Au (ng/g)
10000
1
1 0.1
b
Au (ng/g)
0.01 0.1
10
e 1
10
100
100
10
1
f 1
10
Fe 2 O3 (%)
Au (ng/g)
100
Fe 2 O3 (%)
Fig. 2. The relationship between Au in coal and ash yield, As, Sb, Sp,d and Fe2O3, and Au in ash and Fe2O3.
200
10000
Ga in coal ash
Au in coal ash
100000
1000 100 10 1
a 1
100 200
10000
Au in coal
150 100 50 0
b 0
20
40
60
80
Ga in coal
Fig. 3. The relationship between Au (ng/g), Ga (lg/g) in coal and Au, Ga in coal ash.
The industrial cut-off grade of some elements in coal could be judged by the cut-off grade in coal ash. However, due to the difference of coal category, mode of occurrence, combustion condition, it might be hard to propose a unified cut-off grade for one element with a specific case study on a coal mine. Therefore, we collected data worldwide about the contents of Au [6,19,26,27] and Ga [7,23,28–30] in coal and coal ash, conducted statistic analysis in order to determine the cut-off grade of Au and Ga in coal (Fig. 3). It was recommended in China, 1–2 lg/g as the cut-off grade for Au in hard-rock [31]. Thus, if the Au content in coal ash reaches 1 lg/g, Au in coal must be higher than 200 ng/g according to Fig. 3a. Seredin et al. [6] believed that if all the Au transfers into coal ash, the coal containing about 50 ng/g Au could be considered as precious metal ores. But in fact, as mentioned above, some contents of Au volatize during coal combustion process. The recent discovery of drop-like Au particles on the surface of fly ash from the Wulantuga and Lincang power plants in China [32], might indi-
cate that Au condenses from the gas phase and deposits on the surface of fly ash in the cooler zone of the electrostatic precipitator and baghouse filter. 30 lg/g was recommended as the required grade for Ga in coal in China [31]. As shown in Fig. 3b, the Ga content in coal ash corresponding to 30 lg/g of the Ga content in coal is 50 lg/g. Dai et al. [33] also thought that the cut-off grade for Ga in coal ash could be set at >50 lg/g. But specifications for hard-rock gold exploration of China standard has recommended that the cut-off grade of Ga in some kinds of minerals is generally above 0.01% [31], for example, 0.01–0.002% Ga in bauxite, 0.02–0.03% in pyrite; 0.01–0.02% in blende, 0.1–0.8% in germanite. According to this standard, if the Ga content in coal ash reaches 100 lg/g, the Ga content in coal must be above 60 lg/g (Fig. 3b). Therefore, we think that the cut-off grades of Au and Ga in coal are 200 ng/g and 60 lg/g, respectively. The treatment of coal ash to recover Au and Ga represents an interesting alternative to disposal,
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but their market price must be considered. The present prices of gold and gallium ingot are about $ 1200 an ounce and $ 200– 300 kg 1, respectively. Although specific economical process has not been discussed in detail, the extraction of Au and Ga from the coal ash with a content more than 1 lg/g and 100 lg/g, respectively, is believed to be economically feasible according to the extraction methods by Font et al. [2], Gutierrez et al. [34] and Leonov et al. [4]. 4. Conclusion In this study, we collected data on the Au and Ga contents from 190 coal samples in Guizhou province and then conducted a correlation analysis with data from coals and coal ashes worldwide. The research results are as below: (1) The average contents of Au and Ga in upper Permian coal, Guizhou Province, China, are 6.92 ng/g and 10.65 lg/g, respectively, far higher than those in Chinese coal and world coal. The average contents of Au (especially Au, 59.73 ng/g) and Ga (19.16 lg/g) in partings, roof and floor are significantly higher than those in coal and the Clarke value of sedimentary rock, respectively. However, the Au content in coal ash (15.72 ng/g) is lower than that in world coal ash, while Ga in coal ash (35.31 lg/g) is higher than that in world coal ash. (2) Gallium in Guizhou coal occurs mainly in the inorganic form, but not all of Ga occurs in the form of isomorph in aluminiferous minerals, partial Ga existing in other forms (organic phase) which could easily volatile during the coal combustion process. By contrast, Ga occurring as isomorph in aluminiferous minerals is easily enriched in the coal combustion residues. (3) The occurrence mode of Au in Guizhou coal is complex. Gold shows an association with organic matter, sulfide, carbonate and clay minerals; some Au may occur as nano-gold. The Au occurring in organic matter, sulfide, carbonate, and the nano-gold in coal is easily volatile during the coal combustion, resulting in the Au content in some coal samples higher than that in coal ash. (4) The cut-off grades of Au and Ga in coal are considered to be 200 ng/g and 60 lg/g, respectively. This study method could also be used to determine the cut-off grade of other valuable elements in coal.
Acknowledgments This study was supported by National Key Basic Research and Development Program of China (Nos. 2014CB238905 and 2012CB214901), the National Natural Science Foundation of China (Nos. 41330638 and 41372168), the Fundamental Research Funds for the Central Universities (Nos. 2014ZDPY25, 2013XK06 and 2013ZCX006), Program for National Excellent Doctoral Dissertation Award (No. 201165), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. We are grateful to anonymous reviewers for their valuable comments on the manuscript. References [1] Ren Q, Guo YW, Fu ZQ, Chen Y, Zhang JQ. Evaluation on fly ash recycling application based on analytic hierarchy process. World Sci-Tech R&D 2012;34:80–3 [in Chinese].
[2] Font O, Querol X, Juan R, Casado R, Ruiz CR, Lopez-Soler A, et al. Recovery of gallium and vanadium from gasification fly ash. J Hazard Mater 2007;139:413–23. [3] Seredin VV. From coal science to metal production and environmental protection: a new story of success. Int J Coal Geol 2012;90–91:1–3. [4] Leonov SB, Fedotov KV, Senchenko AE. Economic gold recovery from cinder dumps of thermal power plants. Gorn Zhurn 1998;5:67–8 [in Russian]. [5] Seredin VV, Dai SF, Sun YZ, Chekryzhov IY. Coal deposits as promising sources of rare metals for alternative power and energy-efficient technologies. Appl Geochem 2013;31:1–11. [6] Seredin VV, Finkelman RB. Metalliferous coals: a review of the main genetic and geochemical types. Int J Coal Geol 2008;76:253–89. [7] Dai S, Ren D, Li S. Discovery of the superlarge gallium ore deposit in Jungar, Inner Mongolia, North China. Chin Sci Bull 2006;51:2243–52. [8] Dai S, Zhao L, Peng S, Chou C-L, Wang X, Zhang Y, et al. Abundances and distribution of minerals and elements in high-alumina coal fly ash from the Jungar Power Plant, Inner Mongolia, China. Int J Coal Geol 2010;81:320–32. [9] Dai SF, Ren DY, Chou CL, Finkelman RB, Seredin VV, Zhou YP. Geochemistry of trace elements in Chinese coals: a review of abundances, genetic types, impacts on human health, and industrial utilization. Int J Coal Geol 2012;94:3–21. [10] Sun YZ, Zhao CL, Li YH, Wang JX, Zhang JY, Jin Z, et al. Further information of the associated Li deposits in the No. 6 coal seam at Jungar Coalfield, Inner Mongolia, Northern China. Acta Geol Sinica 2013;87:801–12. [11] Sun YZ, Zhao CL, Li YH, Wang JX. Minimum mining grade of the selected trace elements in Chinese coal. J China Coal Soc 2014;39:744–8 [in Chinese]. [12] ISO-11722. Solid mineral fuels – hard coal – determination of total moisture in the general analysis test sample by drying in nitrogen; 1999. [13] ISO-562. Hard coal and coke – determination of volatile matter; 2010. [14] ISO-1171. Solid mineral fuels – determination of ash; 1997. [15] ISO-351. Solid mineral fuels – determination of total sulfur – high temperature combustion method; 1996. [16] ISO-157. Coal – determination of forms of sulfur; 1996. [17] Tang XY, Huang WH. Trace elements in Chinese coals. Beijing: The Commercial Press; 2004. p. 49–65 [in Chinese]. [18] Ketris MP, Yudovich YE. Estimations of Clarkes for carbonaceous biolithes: world average for trace element contents in black shales and coals. Int J Coal Geol 2009;78:135–48. [19] Seredin VV. Distribution and formation conditions of noble metal mineralization in coal-bearing basins. Geol Ore Deposits 2007;49:3–36. [20] Wang R. Study on occurrence and distribution of Au in the coals and its enrichment mechanism in Western Guizhou Province, China. Ph.D. thesis, China University of Mining and Technology, Xuzhou; 2011 [in Chinese]. [21] Yang JY. Modes of occurrence and origins of noble metals in the Late Permian coals from the Puan Coalfield, Guizhou, southwest China. Fuel 2006;85:1679–84. [22] Moskalyk RR. Gallium: the backbone of the electronics industry. Miner Eng 2003;16:921–9. [23] Querol X, Fernandez-Turiel J, Lopez-Soler A. Trace elements in coal and their behavior during combustion in a large power station. Fuel 1995;74:331–43. [24] Meawad AS, Bojinova DY, Pelovsk YG. An overview of metals recovery from thermal power plant solid wastes. Waste Manage 2010;30:2548–59. [25] Yao ZT, Ji XS, Sarker PK, Tang JH, Ge LQ, Xia MS, et al. A comprehensive review on the applications of coal fly ash. Earth Sci Rev 2015;141:105–21. [26] Gayer R, Rickard D. Colloform gold in coal from southern Wales. Geology 1994;22:35–8. [27] Arbuzov SI, Rikhvanova LP, Maslova SG, Arhipov VS, Belyaeva AM. Anomalous gold contents in brown coals and peat in the south-eastern region of the Western-Siberian platform. Int J Coal Geol 2006;68:127–34. [28] Yang SJ, Qian QF, Jiang Z, Wang C, Yuan JW. The distribution of elements in coal combustion residues in a power plant. Environ Chem 1983;2:32–7 [in Chinese]. [29] Vassilev SV, Vassileva CG, Karayigit AI, Bulut Y, Alastuey A, Querol X. Phasemineral and chemical composition of composite samples from feed coals, bottomashes and fly ashes at the Soma power station, Turkey. Int J Coal Geol 2005;61:35–63. [30] Dai S, Zhou Y, Zhang M, Wang X, Wang J, Song X, et al. A new type of Nb (Ta)– Zr(Hf)–REE–Ga polymetallic deposit in the late Permian coal-bearing strata, eastern Yunnan, southwestern China: possible economic significance and genetic implications. Int J Coal Geol 2010;83:55–63. [31] Committee Office of the National Mineral Reserves (CONMR). Reference manual for the requirements of industrial mineral ores. Beijing: Geological Press; 1987. p. 195–6 [in Chinese]. [32] Seredin VV, Dai SF. The occurrence of gold in fly ash derived from high-Ge coal. Miner Deposita 2014;49:1–6. [33] Dai S, Jiang Y, Ward CR, Gu L, Seredin VV, Liu H, et al. Mineralogical and geochemical compositions of the coal in the Guanbanwusu Mine, Inner Mongolia, China: further evidence for the existence of an Al (Ga and REE) ore deposit in the Jungar Coalfield. Int J Coal Geol 2012;98:10–40. [34] Gutierrez B, Pazos C, Coca J. Recovery of gallium from coal fly ash by a dual reactive extraction process. Waste Manage Res 1997;15:371–82.
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A cut-off grade for gold and gallium in coal Wenfeng Wang a,b,⇑, Shuxun Sang a,b, Weiduo Hao a,b, Ran Wang a,b, Jiefang Zhang a,b, Piaopiao Duan a,b, Yong Qin a,b, Shaochun Xu c a
Department of Source and Earth Science, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, China Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, China University of Mining and Technology, Ministry of Education, Xuzhou 221008, Jiangsu Province, China c Department of Computer Science and Mathematics, Algoma University, 1520 Queen Street East, Sault Ste. Marie, Ontario P6A 2G4, Canada b
h i g h l i g h t s We studied the Au and Ga contents in coal and ash by INAA. The average contents of Au and Ga in Guizhou coal are 6.92 ng/g and10.65 lg/g. Gallium is enriched in the residues more easily than Au during the coal ashing process. The cut-off grades of Au and Ga in coal are considered to be 200 ng/g and 60 lg/g.
a r t i c l e
i n f o
Article history: Received 30 November 2014 Received in revised form 27 December 2014 Accepted 21 January 2015 Available online 31 January 2015 Keywords: Cut-off grade Gold Gallium Coal Ash
a b s t r a c t The contents of Au, Ga, As, Sb and other elements in two hundred and thirty-four samples of coal, partings, roof and floor collected from Guizhou Province, China were analyzed by Instrumental Neutron-Activation Analysis (INAA). In order to discuss the relationship between Au, Ga in coal and in coal ash, the contents of Au and Ga in 815 °C coal ash were also determined by INAA and partial Ga by Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The analysis results showed that: (1) the average contents of Au and Ga in upper Permian coal from Guizhou are 6.92 ng/g and 10.65 lg/g, respectively, far higher than that in Chinese coal and world coal; the average contents of Au (59.73 ng/g) and Ga (19.16 lg/g) in partings, roof and floor are higher than that in the coal and the Clarke value of sedimentary rock (especially for Au); (2) during the coal ashing process, Ga is enriched in the residues more easily than Au, which is mainly because of the difference in the mode of occurrence of the two elements. Gallium mainly occurs as the substitute of Al in the form of isomorph, which makes it hard to volatize; while Au shows a high association with organic matter, sulfide, and carbonates, and some may be nano-gold, which is easily volatile during coal combustion. Only Au occurred in the aluminosilicate is easily left in the residues; (3) we collected data on the Au and Ga contents in coal and coal ash from China and then conducted correlation analysis by combining data from coals and coal ashes worldwide; the cut-off grades of Au and Ga in coal are considered to be 200 ng/g and 60 lg/g, respectively. This method could also be used to determine the cut-off grade of other valuable elements in coal. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction China is the largest coal production and consumption country in the world. By 2013, coal production in China has exceeded 3.6 billion tons per year. Generally, every 4 tons of coal combustion will produce 1 ton of coal ash [1], but the low rate of comprehensive ⇑ Corresponding author at: Department of Source and Earth Science, China University of Mining and Technology, Xuzhou 221008, Jiangsu Province, China. Tel.: +86 51683591011; fax: +86 51683590998. E-mail address: [email protected] (W. Wang). http://dx.doi.org/10.1016/j.fuel.2015.01.066 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
utilization of coal ash in China not only leads to great environmental problems but also wastes a large amount of mineral resources and industrial valuable elements in the ash. In the 1990s, joint efforts were taken by USA and Canada to study Ga in coal ash, and Ga was successfully extracted from coal ash [2]. In Ordos, China, a pilot plant with an annual processing capacity of approximately 150 t Ga was built at the beginning of 2011 [3]. Gold was also extracted from wastes of the Reftinsk power station, Russian [4]. Although there has been a lot of research work done on extraction of trace elements from coal ash [5], the industrial cut-off grade of metals in coal has rarely been reported.
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The precious metal minerals in coal basins can be extracted as by-products during secondary treatment of the coal or as the products of coal combustion [6]. Recently, more and more researchers have found valuable trace elements enriched in coal [5,7–10]. Thus, the minimum mining grade of trace elements is of vital importance to the combined exploration of trace elements and coal, which could reduce the exploration budget and promote the exploitation of metals in coal ash. Unfortunately, there are only a few related researches. Sun et al. [11] suggested that the minimum mining grades of Li, U, Th and REE should be 120, 40, 150 and 300 lg/g, respectively. Their work is foresighted to some extent, however, the conclusion was drawn based on the 17% average ash yield in Chinese coal. Moreover, they might not have considered the occurrence mode of trace elements in coal and the combustion mode, therefore, their conclusion may not be applied to other elements in coal. In order to explore the relationship between the Au and Ga contents in coal and in coal ash, and further determine their cut-off grade in coal, we conduct an analysis of the Au and Ga contents in 234 coal and 75 coal ash samples. 2. Sampling and analytical method Samples were collected from working coal mines in Guizhou Province, China, 234 pieces in total, including 44 partings, roof and floor samples, and 190 coal samples. All of the coal samples are mixed and ply samples of upper Permian coal seam and feed of 4 power plants. Proximate analysis (total moisture, volatile matter, and ash yield) was conducted using ISO recommendations (ISO-11722 [12], ISO-562 [13], and ISO-1171 [14]). The total sulfur and forms of sulfur were determined following ISO-351 [15] and ISO-157 [16], respectively. All samples were ashed in a muffle oven at 815 °C to constant value of weight (ISO-1171). X-ray fluorescence spectrometry was used to determine the oxides of major elements for these ash samples. Some trace elements such as As, Au, Ga and Sb in solid samples, and Au in 34 ash samples and Ga in 25 ash samples were determined using Instrumental Neutron Activation Analysis (INAA). Inductively Coupled Plasma Mass Spectrometry (ICP-MS), in a pulse counting mode (three points per peak), was used to determine Ga in the other 50 ash samples. 3. Results and analysis 3.1. The Au and Ga contents in Guizhou coal and coal ash The contents of Au and Ga in Guizhou coal range from 1.16 to 313 ng/g and from 2.54 to 30.7 lg/g, averaged at 9.53 ng/g and 11.25 lg/g, respectively, which are far higher than the average contents in Chinese coal and world coal (Table 1), implicating their significant enrichment in Guizhou coal. However, the average contents of Au (especially Au, 59.73 ng/g) and Ga (19.16 lg/g) in partings, roof and floor are significantly higher than that in coal and the Clarke value of sedimentary rock, respectively. Moreover, the average Au content in coal is even higher than the Clarke value
of sedimentary rock (6 ng/g), but the Au content in ash (15.52 ng/g) is lower than that in world coal ash (22 ng/g). On the contrary, the average Ga content in Guizhou coal (11.25 lg/g) is lower than the Clarke value in sedimentary rock (12 lg/g), but in Guizhou coal ash, the Ga content (35.31 lg/g) is higher than the average content in world coal ash (33 lg/g). The results showed that compared with world coal, Au in Guizhou coal is easily volatile during coal combustion and only partial Au is left in the residue while Ga shows opposite behavior. Table 2 shows the contents of Au and Ga in some coal and coal ash samples. As Table 2 shows, the Au content in 7 out of 34 samples is significantly lower than that in raw coal samples, and the content in some coal and ash is approximately the same. By contrast, Ga in ash samples is higher than that in raw coal sample, which could indicate that Ga is enriched in the residues more easily during coal combustion. The different behavior of the two elements reflects their different modes of occurrence in coal. 3.2. The occurrence mode of Au and Ga in Guizhou coal Generally, it is believed that Ga in coal mainly occurs as inorganic phase and replaces Al in aluminiferous minerals (mainly boehmite) in the form of isomorph [7]. Thus, Ga usually shows a positive correlation with ash yield, and this is also shown in Guizhou coal but with a weak relationship (Fig. 1a). Moreover, Ga in coal also shows a weak correlation with Al2O3 (Fig. 1b), while in coal ash, an obviously positive correlation is shown between Ga and Al2O3 (Fig. 1c). This phenomenon shows that although Ga mainly occurs in the inorganic matter of coal, not all Ga replace Al in aluminiferous minerals in the form of isomorph. According to the above correlation analysis, it seems that Ga in coal in the form of isomorph remains more in ash during coal combustion, while the other occurrences of Ga (organic phase) are easily volatile. The occurrence mode of Au in coal is considerably complex. Seredin et al. [19] thought that there are four kinds of occurrences for Au in coal: dispersed native gold, associated with sulfide, absorbed by clay minerals, and organically bonded. Our previous research on the sequential chemical extraction of four pieces coal samples from Guizhou demonstrated that the occurrence mode of Au in the studied coal samples is complex [20]: Au occurs in coal bound to organic matter, as a sulfide, within clay minerals and as a carbonate (Table 3). Yang [21] conducted research on Puan coal samples from Guizhou with sequential chemical extraction method and the result also showed that the Au occurs mainly as a sulfide affinity while also associating with organic matter, silicate and carbonate. Our correlation analysis demonstrates that Au in coal has a weakly positive correlation with ash yield (Fig. 2a) but it is in association with a more positive correlation with pyrite, Sb and As (Fig. 2b–d), which might indicate that Au in coal mainly exists in pyrite, especially Sb/As-bearing pyrite. Iron in coal mainly exists in pyrite, and could also occur in aluminosilicate. During the coal ashing process, some Fe maybe escape with the decomposition of pyrite. As shown in Fig. 2e, Au in coal has a certainly positive correlation with Fe2O3, but less than the relationship between Au in
Table 1 The Au and Ga contents in upper Permian coal from Guizhou Province. Elements Coals Range
Coals ashes Total number Average Range of samples
Au (ng/g) 1.16–313 190 Ga (lg/g) 2.54–30.7 106
6.92 10.65
Partings, roof and floor Total number Average Range of samples
2.4–190 34 12.2–71.45 75
15.74 35.88
Chinese coals World
Total number Average of samples
1.41–1261 44 2.72–47.6 28
59.73 19.16
Coals Coal ashes 3 6.55
3.7 5.8
22 33
Chinese coals from Tang and Huang [17] and Dai et al. [9], world coal and Clarke value in sedimentary rock from Ketris and Yudovich [18].
Clarke value of sedimentary rock
6 12
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Table 2 The Au and Ga contents in coal samples and coal ashes. Coalfield
Location + sample types
Ash (%)
Hezhang Nayong Zhijin
Gaoqiao coal Feed coal of Nayong power plant Jinma coal Shibanzhai coal Jingxuan coal Feed coal of Faer power plant Dahebian coal Wangjiazhai coal Yushe coal Xiaohegou coal Shanhe coal Zijin coal + partings 1 Zijin coal + partings 2 Zijin coal Guojiadi coal Zhenzhong coal Jixing coal Madaodi coal + partings Hengtai coal Dabatian coal Jinqiao coal Wazicong coal Xiongxin coal Guanglong coal Xingfa coal Longkang coal Qianjin coal Dayakou coal Xinlong coal 1 Xinlong coal 2 Sancun coal + partings Silianxiang coal Erwan coal Xiashanzhen coal
8.69 32.83 9.26 13.82 27.29 62.97 12.63 23.99 13.11 16.14 7.98 54.22 62.56 27.94 18.93 19.12 13.89 69.76 14 14.97 21.38 45.58 18.21 28.93 15.72 24.26 26.3 30.91 21.62 19.37 69.66 31.29 17.83 24.77
Au (lg/g)
Ga (ng/g) In coal
Suicheng
Zhenfeng
Puan
Panxian Bijie Xinyi Anlong
Xinren
16.26
7.12 18.5 4.17 3.28 24.64 13.3 2.54 5.2 6.52 3.41 8.23 7.07 3.25 2.72 5.64 4.08 2.55 8.87 4.65 8.83 11.95 10.2 7.71 12.6 5.82 4.45 8.49 4.12 4.12 7.78
In ash
In coal
In ash
57.9 46.8 47.6 46.8 31.8 41.3 31.1 16.6 52.9 35.6 28.1 12.2 13.1 15 22.8 39.1 38.5 47.6 43.8 28.7 13.9 18.7 22.9 29.5 41.7 40.7 29.6 35.9 30.6 35 29.6 29.4 29.4 30.7
32.4 6.13 39.9 26.8 3.03 17.6 6.95 2.34 4.94 61.8 1.6 4.28 1.97 3.24 1.34 1.63 2.43 1.41 2.58 1.81 2.22 2.35 2.21 2.76 4.34 2.62 2.41 3.05 1.93 1.21 132 2.77 1.87 1.8
9.81 4.6 20.2 23.9 10.2 3.71 3.34 8.32 18.6 14.6 10.7 4.3 2.89 3.98 8.79 5.97 10.5 13.9 41.8 6.83 2.4 5.01 11.4 3.25 32.9 10.5 9.04 10.2 13.7 3.93 190 5.74 3.38 6.75
80 60 40 20 0
a 0
10
20
30
40
Ga in coal (µg/g)
Ash yield (%)
100
40 35 30 25 20 15 10 5 0
Ga in ash (µg/g)
Ad, ash yield, dry basis; Italic values, lower Au content in ash than in coal.
b 0
5 10 15 20 25 30 35 40 45
Ga (µg/g)
Al 2 O3 (%)
80 70 60 50 40 30 20 10 0
c 0
20
40
60
Al2 O3 (%)
Fig. 1. The relationship between Ga in coal and ash yield, Al2O3, and between Ga in coal ash and Al2O3.
coal ash and Fe2O3 (Fig. 2f). This indicates that organic-, pyriterelated and nano-gold (fusing point is 327 °C) are easily volatile during coal combustion, while more Au occurring in aluminasilicate remains in the coal ash (Fig. 2f). 3.3. The industrial cut-off grade of Au and Ga in coal Despite the large estimated resources of Ga in bauxites (1 billion kg) and Zn deposits, only a small percentage (40%) is economically recoverable [2,22], so there is considerable interest in evaluating Ga recovery from other significant sources, such as phosphate ores and coal [2]. Although the latter typically contains up to 0.10% Ga, this element, along with other valuable elements, is concentrated in coal fly ash during coal combustion [23]. As one kind of fuel, coal is mainly used by combusting to supply energy. If valuable elements present in coal concentrate in the coal ash and reach the industrial cut-off grade, it is essential to consider
their beneficial recovery as a by-product from coal combustion residues. Based on a review of a number of methods for recovery of metals from power plant solid wastes, Meawad et al. [24] believed that the solid wastes are actually a secondary resource of metals, and by applying mineral processing technologies and hydrometallurgical and biohydrometallurgical processes, it is possible to recover metals such as Al, Ga, Ge and V, from power plant solid wastes. As for Au, it also has been shown that economically efficient Au recovery from the wastes of the Reftinsk power plant in the Urals is conducted with an average Au grade of 0.1–0.2 lg/g in coal combustion products [4]. Recently, Yao et al. [25] can use combinations of processes, such as predesilication of fly ash and a lime-soda sinter process. It is also better to simultaneously recover more valuable materials, such as Al, Si, Ga and Ti. However, most reported methods are still in the early stages of commercialization [25]. Further studies are needed to turn this research into commercial reality.
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W. Wang et al. / Fuel 147 (2015) 62–66 Table 3 Sequential chemical extraction results of four Guizhou coals (%) [20]. Sample Nos.
Water soluble
Ion-exchangeable
Carbonate
Humic acid bonded
Iron–Mn oxidate
Organic bonded
Residue
Coal Coal Coal Coal
bdl bdl 3.59 12.33
0.07 0.12 3.12 5.84
13.89 1.71 5.5 10.13
12.11 30.24 21.71 9.91
15.79 13.42 34.17 38.24
33.54 18.72 22.8 13.22
24.6 35.79 9.09 10.33
1 2 3 4
bdl, below detection limit.
100
1000
100 80 60 40
1 0.1
20 0
100
10
Sb (µg/g)
Sp,d (%)
Ash yield (%)
120
a 1
10
100
1000
0.01
10000
1
10
0.01 0.1
1000
10 1 0.1
1000
d 100
1000
1000 10000
100 10 1 0.1
100000
1000
Au in ash (ng/g)
Au in coal (ng/g)
100
10
c 10
Au (ng/g)
10000
1000
As (µg/g)
100
Au (ng/g)
10000
1
1 0.1
b
Au (ng/g)
0.01 0.1
10
e 1
10
100
100
10
1
f 1
10
Fe 2 O3 (%)
Au (ng/g)
100
Fe 2 O3 (%)
Fig. 2. The relationship between Au in coal and ash yield, As, Sb, Sp,d and Fe2O3, and Au in ash and Fe2O3.
200
10000
Ga in coal ash
Au in coal ash
100000
1000 100 10 1
a 1
100 200
10000
Au in coal
150 100 50 0
b 0
20
40
60
80
Ga in coal
Fig. 3. The relationship between Au (ng/g), Ga (lg/g) in coal and Au, Ga in coal ash.
The industrial cut-off grade of some elements in coal could be judged by the cut-off grade in coal ash. However, due to the difference of coal category, mode of occurrence, combustion condition, it might be hard to propose a unified cut-off grade for one element with a specific case study on a coal mine. Therefore, we collected data worldwide about the contents of Au [6,19,26,27] and Ga [7,23,28–30] in coal and coal ash, conducted statistic analysis in order to determine the cut-off grade of Au and Ga in coal (Fig. 3). It was recommended in China, 1–2 lg/g as the cut-off grade for Au in hard-rock [31]. Thus, if the Au content in coal ash reaches 1 lg/g, Au in coal must be higher than 200 ng/g according to Fig. 3a. Seredin et al. [6] believed that if all the Au transfers into coal ash, the coal containing about 50 ng/g Au could be considered as precious metal ores. But in fact, as mentioned above, some contents of Au volatize during coal combustion process. The recent discovery of drop-like Au particles on the surface of fly ash from the Wulantuga and Lincang power plants in China [32], might indi-
cate that Au condenses from the gas phase and deposits on the surface of fly ash in the cooler zone of the electrostatic precipitator and baghouse filter. 30 lg/g was recommended as the required grade for Ga in coal in China [31]. As shown in Fig. 3b, the Ga content in coal ash corresponding to 30 lg/g of the Ga content in coal is 50 lg/g. Dai et al. [33] also thought that the cut-off grade for Ga in coal ash could be set at >50 lg/g. But specifications for hard-rock gold exploration of China standard has recommended that the cut-off grade of Ga in some kinds of minerals is generally above 0.01% [31], for example, 0.01–0.002% Ga in bauxite, 0.02–0.03% in pyrite; 0.01–0.02% in blende, 0.1–0.8% in germanite. According to this standard, if the Ga content in coal ash reaches 100 lg/g, the Ga content in coal must be above 60 lg/g (Fig. 3b). Therefore, we think that the cut-off grades of Au and Ga in coal are 200 ng/g and 60 lg/g, respectively. The treatment of coal ash to recover Au and Ga represents an interesting alternative to disposal,
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but their market price must be considered. The present prices of gold and gallium ingot are about $ 1200 an ounce and $ 200– 300 kg 1, respectively. Although specific economical process has not been discussed in detail, the extraction of Au and Ga from the coal ash with a content more than 1 lg/g and 100 lg/g, respectively, is believed to be economically feasible according to the extraction methods by Font et al. [2], Gutierrez et al. [34] and Leonov et al. [4]. 4. Conclusion In this study, we collected data on the Au and Ga contents from 190 coal samples in Guizhou province and then conducted a correlation analysis with data from coals and coal ashes worldwide. The research results are as below: (1) The average contents of Au and Ga in upper Permian coal, Guizhou Province, China, are 6.92 ng/g and 10.65 lg/g, respectively, far higher than those in Chinese coal and world coal. The average contents of Au (especially Au, 59.73 ng/g) and Ga (19.16 lg/g) in partings, roof and floor are significantly higher than those in coal and the Clarke value of sedimentary rock, respectively. However, the Au content in coal ash (15.72 ng/g) is lower than that in world coal ash, while Ga in coal ash (35.31 lg/g) is higher than that in world coal ash. (2) Gallium in Guizhou coal occurs mainly in the inorganic form, but not all of Ga occurs in the form of isomorph in aluminiferous minerals, partial Ga existing in other forms (organic phase) which could easily volatile during the coal combustion process. By contrast, Ga occurring as isomorph in aluminiferous minerals is easily enriched in the coal combustion residues. (3) The occurrence mode of Au in Guizhou coal is complex. Gold shows an association with organic matter, sulfide, carbonate and clay minerals; some Au may occur as nano-gold. The Au occurring in organic matter, sulfide, carbonate, and the nano-gold in coal is easily volatile during the coal combustion, resulting in the Au content in some coal samples higher than that in coal ash. (4) The cut-off grades of Au and Ga in coal are considered to be 200 ng/g and 60 lg/g, respectively. This study method could also be used to determine the cut-off grade of other valuable elements in coal.
Acknowledgments This study was supported by National Key Basic Research and Development Program of China (Nos. 2014CB238905 and 2012CB214901), the National Natural Science Foundation of China (Nos. 41330638 and 41372168), the Fundamental Research Funds for the Central Universities (Nos. 2014ZDPY25, 2013XK06 and 2013ZCX006), Program for National Excellent Doctoral Dissertation Award (No. 201165), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. We are grateful to anonymous reviewers for their valuable comments on the manuscript. References [1] Ren Q, Guo YW, Fu ZQ, Chen Y, Zhang JQ. Evaluation on fly ash recycling application based on analytic hierarchy process. World Sci-Tech R&D 2012;34:80–3 [in Chinese].
[2] Font O, Querol X, Juan R, Casado R, Ruiz CR, Lopez-Soler A, et al. Recovery of gallium and vanadium from gasification fly ash. J Hazard Mater 2007;139:413–23. [3] Seredin VV. From coal science to metal production and environmental protection: a new story of success. Int J Coal Geol 2012;90–91:1–3. [4] Leonov SB, Fedotov KV, Senchenko AE. Economic gold recovery from cinder dumps of thermal power plants. Gorn Zhurn 1998;5:67–8 [in Russian]. [5] Seredin VV, Dai SF, Sun YZ, Chekryzhov IY. Coal deposits as promising sources of rare metals for alternative power and energy-efficient technologies. Appl Geochem 2013;31:1–11. [6] Seredin VV, Finkelman RB. Metalliferous coals: a review of the main genetic and geochemical types. Int J Coal Geol 2008;76:253–89. [7] Dai S, Ren D, Li S. Discovery of the superlarge gallium ore deposit in Jungar, Inner Mongolia, North China. Chin Sci Bull 2006;51:2243–52. [8] Dai S, Zhao L, Peng S, Chou C-L, Wang X, Zhang Y, et al. Abundances and distribution of minerals and elements in high-alumina coal fly ash from the Jungar Power Plant, Inner Mongolia, China. Int J Coal Geol 2010;81:320–32. [9] Dai SF, Ren DY, Chou CL, Finkelman RB, Seredin VV, Zhou YP. Geochemistry of trace elements in Chinese coals: a review of abundances, genetic types, impacts on human health, and industrial utilization. Int J Coal Geol 2012;94:3–21. [10] Sun YZ, Zhao CL, Li YH, Wang JX, Zhang JY, Jin Z, et al. Further information of the associated Li deposits in the No. 6 coal seam at Jungar Coalfield, Inner Mongolia, Northern China. Acta Geol Sinica 2013;87:801–12. [11] Sun YZ, Zhao CL, Li YH, Wang JX. Minimum mining grade of the selected trace elements in Chinese coal. J China Coal Soc 2014;39:744–8 [in Chinese]. [12] ISO-11722. Solid mineral fuels – hard coal – determination of total moisture in the general analysis test sample by drying in nitrogen; 1999. [13] ISO-562. Hard coal and coke – determination of volatile matter; 2010. [14] ISO-1171. Solid mineral fuels – determination of ash; 1997. [15] ISO-351. Solid mineral fuels – determination of total sulfur – high temperature combustion method; 1996. [16] ISO-157. Coal – determination of forms of sulfur; 1996. [17] Tang XY, Huang WH. Trace elements in Chinese coals. Beijing: The Commercial Press; 2004. p. 49–65 [in Chinese]. [18] Ketris MP, Yudovich YE. Estimations of Clarkes for carbonaceous biolithes: world average for trace element contents in black shales and coals. Int J Coal Geol 2009;78:135–48. [19] Seredin VV. Distribution and formation conditions of noble metal mineralization in coal-bearing basins. Geol Ore Deposits 2007;49:3–36. [20] Wang R. Study on occurrence and distribution of Au in the coals and its enrichment mechanism in Western Guizhou Province, China. Ph.D. thesis, China University of Mining and Technology, Xuzhou; 2011 [in Chinese]. [21] Yang JY. Modes of occurrence and origins of noble metals in the Late Permian coals from the Puan Coalfield, Guizhou, southwest China. Fuel 2006;85:1679–84. [22] Moskalyk RR. Gallium: the backbone of the electronics industry. Miner Eng 2003;16:921–9. [23] Querol X, Fernandez-Turiel J, Lopez-Soler A. Trace elements in coal and their behavior during combustion in a large power station. Fuel 1995;74:331–43. [24] Meawad AS, Bojinova DY, Pelovsk YG. An overview of metals recovery from thermal power plant solid wastes. Waste Manage 2010;30:2548–59. [25] Yao ZT, Ji XS, Sarker PK, Tang JH, Ge LQ, Xia MS, et al. A comprehensive review on the applications of coal fly ash. Earth Sci Rev 2015;141:105–21. [26] Gayer R, Rickard D. Colloform gold in coal from southern Wales. Geology 1994;22:35–8. [27] Arbuzov SI, Rikhvanova LP, Maslova SG, Arhipov VS, Belyaeva AM. Anomalous gold contents in brown coals and peat in the south-eastern region of the Western-Siberian platform. Int J Coal Geol 2006;68:127–34. [28] Yang SJ, Qian QF, Jiang Z, Wang C, Yuan JW. The distribution of elements in coal combustion residues in a power plant. Environ Chem 1983;2:32–7 [in Chinese]. [29] Vassilev SV, Vassileva CG, Karayigit AI, Bulut Y, Alastuey A, Querol X. Phasemineral and chemical composition of composite samples from feed coals, bottomashes and fly ashes at the Soma power station, Turkey. Int J Coal Geol 2005;61:35–63. [30] Dai S, Zhou Y, Zhang M, Wang X, Wang J, Song X, et al. A new type of Nb (Ta)– Zr(Hf)–REE–Ga polymetallic deposit in the late Permian coal-bearing strata, eastern Yunnan, southwestern China: possible economic significance and genetic implications. Int J Coal Geol 2010;83:55–63. [31] Committee Office of the National Mineral Reserves (CONMR). Reference manual for the requirements of industrial mineral ores. Beijing: Geological Press; 1987. p. 195–6 [in Chinese]. [32] Seredin VV, Dai SF. The occurrence of gold in fly ash derived from high-Ge coal. Miner Deposita 2014;49:1–6. [33] Dai S, Jiang Y, Ward CR, Gu L, Seredin VV, Liu H, et al. Mineralogical and geochemical compositions of the coal in the Guanbanwusu Mine, Inner Mongolia, China: further evidence for the existence of an Al (Ga and REE) ore deposit in the Jungar Coalfield. Int J Coal Geol 2012;98:10–40. [34] Gutierrez B, Pazos C, Coca J. Recovery of gallium from coal fly ash by a dual reactive extraction process. Waste Manage Res 1997;15:371–82.