Statistical analysis of the concentrations of trace elements in a wide diversity of coals and its implications for understanding elemental modes of occurrence

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Fuel 87 (2008) 2211–2222 www.fuelfirst.com

Statistical analysis of the concentrations of trace elements in a wide diversity of coals and its implications for understanding elemental modes of occurrence Jie Wang

b

a,*

, Osamu Yamada b,*, Tetsuya Nakazato b, Zhan-Guo Zhang b, Yoshizo Suzuki b, Kinya Sakanishi c

a East China University of Science and Technology, No. 130 Meilong Road, Shanghai 200237, PR China National Institute of Advanced Industrial Science and Technology (AIST), Onogawa 16-1, Tsukuba, Ibaraki 305-8569, Japan c AIST, Suehiro2-2-2, Hiro, Kure, Hiroshima 737-0197, Japan

Received 28 February 2007; received in revised form 16 October 2007; accepted 17 October 2007 Available online 13 November 2007

Abstract Seventeen trace elements in 24 coals from worldwide deposits of differing ranks and sulfur contents were determined with the use of inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), and flow injection (FI) ICP-MS. By examining multiple correlations between each trace element and three major elements, calcium, aluminum, and iron, we have found that thirteen trace elements (Li, Be, V, Cr, Mn, Ni, Cu, Zn, Ga, As, Se, Sr, and Ba) in the coals show significant correspondence. Elements correlating with aluminum are lithium, beryllium, vanadium, chromium, copper, gallium, and selenium; of these elements, vanadium, chromium, and copper also have a relationship with iron. Manganese, strontium and barium are correlated with calcium, while nickel, zinc, and arsenic are correlated with iron. In the geochemical and mineralogical senses, the significant correlation of a trace element with calcium reflects its common association with carbonate minerals for medium- to high-rank coals, while that with aluminum is implicative of the common association with aluminosilicate minerals and that with iron is characteristic of the association with sulfide minerals for high-sulfur coals, and with iron-bearing carbonate and clay minerals for low-sulfur coals. It is observed that most trace elements have more than one common association(s) in the 24 coals. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Trace elements; Mineral matter; Statistical analysis; Modes of occurrence; Geochemistry

1. Introduction Information on the modes of occurrence of trace elements in coal is fundamental to the understanding of thermal, chemical and environmental behavior of trace elements during coal conversion processes, as well as the removal of trace elements by coal cleaning. Various direct and indirect analytical methods have been applied to identify the modes of occurrence of trace elements in coal [1,2]. *

Corresponding authors. Tel./fax: +86 21 64252853. E-mail addresses: [email protected] (J. Wang), osamu.yamada @aist.go.jp (O. Yamada). 0016-2361/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2007.10.012

Finkelman [3] conducted a direct probe of trace elements in coal with the use of scanning electron microscopy combined with an energy dispersive X-ray analyzer (SEMEDXA). This technique can shed light on the distribution of certain heavy trace elements in different mineral phases. However, many trace elements are not detectable because of insufficient sensitivity. Diehl et al. [4] used a more sensitive spectrometer of laser ablation inductively coupled plasma mass (LA-ICP-MS) to survey trace elements in coal and pyrite. Huggins and his collaborators [5–9] performed a series of works on fingerprinting the chemical forms of several trace elements in coal, including titanium, vanadium, chromium, manganese, zinc, selenium, and others,

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by using X-ray absorption fine structure spectroscopy (XAFS). XAFS is a powerful instrumental analyzer, which is capable of detecting the chemical structures of a specific element at a concentration as low as 2 lg g1, but it has not become a routine analytical approach in most laboratories over the world. Indirect methods include various technical applications such as selective acid leaching [10–14], mineral grain separation [15–17], maceral separation [18], and organic solvent extraction [19,20], followed by the elemental quantification, for example, with instrumental neutron activation analysis (INAA), atomic absorption spectrometry (AAS), ICPOES, or ICP-MS. By physical separation, the distribution of trace elements in different float/sink fractions of coal can be obtained, but the information generally lacks quantitative significance. Physical separation cannot distinguish between chemically-bound elements and elements intermixed physically as a fine discrete mineral. Sequential acid leaching method is a chemical separation method, which is widely used to determine the modes of occurrence of trace elements in coal, based on selective dissolution of the major minerals into different acid solutions. In general, this method identifies trace elements in coal into ion-exchangeable, HCl-soluble (carbonate and mono-sulfide), HF-soluble (aluminosilicate and silicate), and HNO3-soluble (pyrite) forms. A shortcoming of this method is that the organically combined trace elements are estimated by difference. Lachas et al. [19] put forward the solvent extraction method for more reliable determination of the organically associated trace elements. Wang et al. [21] proposed a leaching method by comparison of the elemental leachability between raw coal and the corresponding ashes. This method was validated to determine some organic and pyritic trace elements. Statistical analysis is another indirect method for characterizing trace elements in coal. Spears et al. [22] found positive correlations between nickel, copper, zinc, arsenic, selenium, molybdenum, antimony and lead in a UK coal by multigravity separation, and this was ascribed to their associations with pyrite. Spears and Zheng [23] determined 46 major and trace elements in 24 UK coals with ranges of 4.6–37.6% volatile matter and 0.02–1.34% pyritic sulfur content, and they further investigated the statistical relationships between various pairs of major and trace elements. They found that the trace elements correlating with silicon and aluminum were vanadium, chromium, gallium, rubidium, yttrium, zirconium, niobium, lanthanum and tantalum; the trace elements correlating with iron were arsenic, selenium, molybdenum, antimony and thallium; no trace elements were linked to calcium. Mukherjee and Srivastava [24] observed that gallium in four Assam coals was correlated with alumina and silica whereas lead was correlated with pyritic sulfur. Recently, Shaver et al. [25] studied the relationships in concentration between various elements for Bon Air coals, and they inferred trace element geochemistry and their modes of occurrence in coal. They claimed that most titanium and chromium as well as some

manganese, nickel, and barium were hosted by clay; most arsenic as well as some vanadium, nickel, and copper were hosted by pyrite; most mercury and some manganese were hosted by calcite. The modes of trace elements vary greatly among coals, and it is always possible that there exist extraordinary forms in a specific coal. A question we wish to address is whether there are any significant relations between trace elements and major elements for a wide diversity of coals rather than for the specific coals as examined in the aforementioned works. In this paper, we have first determined seventeen trace elements in twenty four coals from globally distributed deposits by validated analytical method [26,27]. We have further implemented multiple linear regressions between trace elements and three major elements, calcium, aluminum, and iron, for all 24 coals, and also for the coals of different ranks and of different concentrations of pyrite. The regression results are evaluated to generalize about how a given trace element occurs in common associations in different coals. 2. Experimental 2.1. Coal samples Twenty four coals were used in this study. Among these coals, 21 coals were supplied by the Japanese Coal Bank, and these coals are denoted as SS followed by different numbers. The particle size of each SS coal was smaller than 0.147 mm. For each SS coal, some 1000 g of the sample were thoroughly mixed, and then equally divided into several parts for use. The other three coals were from the Argonne Coal Sample Bank. The production countries of both the SS coals and the Argonne coals and their basic properties are shown in Table 1. The coals are of quite different grades and ranks. 2.2. Experimental methods The coal samples were first subjected to microwave digestion in ultra-pure concentrated HNO3 using highpressure quartz vessels. The rigorous digestion (temperature up to 250 °C and pressure up to 7.5 MPa) allowed coal organic and mineral matrices to be extensively decomposed and dissolved in the HNO3 solution, forming a colorless and transparent solution after filtering out a small amount of quartz-like mineral residue through a polytetrafluoroethylene (PTFE) filter with a pore size of 0.45 lm. This solution was used for subsequent spectrometric analyses. Six major elements (Na, Mg, Al, K, Ca, and Fe) and three trace elements (Li, Sr, Ba) were determined by inductively coupled plasma optical emission spectrometry (ICP-OES, Optima 4300 DV, Perkin–Elmer). Beryllium, vanadium, chromium, manganese, cobalt, nickel, copper, zinc, and lead were determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500a, Agilent Technologies Inc.). Gallium, arsenic, selenium, and cadmium were

J. Wang et al. / Fuel 87 (2008) 2211–2222

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Table 1 Properties of coal samples (all on dry basis) Sample code

Production country

Proximate analysis, wt% Ash

Volatile

C

H

N

Total

Pyritic

SS20 SS22 SS23 SS25 SS30 SS33 SS37 SS38 SS40 SS56 SS59 SS62 SS63 SS64 SS66 SS69 SS80 SS82 SS90 SS95 SS100 IL WY BZ

South Africa South Africa USA USA Japan India China Colombia India China Australia China Australia China India India Australia Australia India China Australia USA USA USA

13.4 14.1 7.1 8.2 19.8 1.0 16.8 1.0 7.9 18.5 11.5 5.4 13.2 14.5 1.9 4.7 14.1 7.0 9.0 7.8 12.1 15.5 8.8 9.7

32.0 27.2 43.3 39.6 37.9 48.9 8.6 40.9 46.1 10.6 27.5 36.7 27.7 18.6 45.0 47.7 41.4 35.6 47.2 34.5 33.6 40.1 44.8 44.9

73.7 73.7 76.7 75.5 65.9 80.7 76.6 84.1 75.1 74.0 77.6 83.4 74.9 76.6 83.4 85.8 73.4 83.9 73.7 79.6 74.5 65.7 68.4 65.9

4.4 4.2 5.5 5.6 5.1 6.0 3.6 5.9 6.1 3.4 4.6 5.2 4.6 4.06 5.9 6.3 5.75 5.4 5.9 4.6 4.4 4.2 4.9 4.4

1.8 1.7 1.5 1.6 1.1 1.2 1.1 1.7 1.3 1.2 1.8 1.0 1.6 1.3 1.8 1.3 1.45 2.2 1.2 1.2 1.7 1.2 1.0 1.0

0.89 0.67 0.50 0.63 2.26 0.12 0.44 0.49 0.70 0.42 0.67 0.49 0.39 0.42 0.31 0.83 0.60 0.54 1.26 1.22 0.77 4.82 0.63 0.80

0.38 0.26 0.14 0.14 0.88 0.01 0.26 0.36 0.52 0.08 0.37 0.31 0.10 0.09 0.05 0.05 0.01 0.03 0.47 0.14 0.15 2.80 0.17 0.14

quantified by flow injection ICP-MS (FI-ICP-MS). FIICP-MS has a lower limit of quantification than more common ICP-MS, and this method is effective for determining ultra-trace elements in coal. For example, selenium and cadmium in coal are usually difficult to quantify with ICP-MS [19,27], whereas these elements could be determined with enhanced accuracy and precision by means of FI-ICP-MS [27]. The instrumentation and detailed procedures of analysis were described elsewhere [26,27]. The multiple linear regressions were performed using a calculation program compiled ourselves on a common Microsoft Office Excel software. 3. Results

Ultimate analysis, wt%

Sulfur analysis, wt%

The figures of zinc and lead exhibit larger variations. These variations were also observed in additional repetitive experiments [19,26–28]. A probable reason is that zinc and lead are readily susceptible to contamination during analysis, but the exact reason is still unknown. For reference, the concentration ranges and the average concentrations of some trace elements for the worldwide coals [29] are transcribed in Table 3. For some chalcophile elements such as arsenic and lead, their average concentrations in the studied coals are a few times lower than those in the worldwide coals. This may be because many of the SS coals, except for SS30 coal, that were imported from overseas to Japan had been subjected to beneficiation, leading to a low concentration of pyrite. Detailed information on coal cleaning is not available.

3.1. Concentrations of major and trace elements in coal 3.2. Correlations between major elements Table 2 shows the concentrations of the major elements in coals, as well as their ranges and arithmetic means calculated from all 24 coals. We can see that the major elements in the studied coals have wide ranges of concentration. The concentrations of trace elements in coal are shown in Table 3, where the data, except those for the Argonne coals (IL, WY and BZ coals), are the arithmetic means of the results obtained by analysis of two separately digested coal samples. The data for the Argonne coals were obtained by analysis of three separately digested coal samples. Unfortunately, the analytical variations were not recorded for SS82, IL, WY, and BZ coals. The data for all the trace elements except zinc and lead show very good reproducibility.

Fig. 1 shows the correlations of iron with total sulfur and pyritic sulfur, respectively. The correlation coefficients are 0.95 and 0.93, respectively, while no significant correlations are found between calcium or aluminum and total sulfur or pyritic sulfur (the correlation coefficients range 0.01–0.27). However, if four pyrite-rich coals, SS30 coal, SS40 coal, SS90 coal and IL coal, are omitted from the statistical calculation, the correlations of iron with both total sulfur and pyritic sulfur are insignificant (the coefficients are 0.17 for both total sulfur and pyritic sulfur). On the contrary, the correlation coefficients of iron with calcium and aluminum increase from 0.14 and 0.37 to 0.45 and

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J. Wang et al. / Fuel 87 (2008) 2211–2222

Table 2 Concentrations of some major metallic elements in coal samples (in mg g1, dry basis) Coal sample

Na

Mg

Al

K

Ca

Fe

SS20 SS22 SS23 SS25 SS30 SS33 SS37 SS38 SS40 SS56 SS59 SS62 SS63 SS64 SS66 SS69 SS80 SS82 SS90 SS95 SS100 IL WY BZ

0.35 0.33 0.57 1.21 2.35 0.29 0.74 0.07 0.19 1.13 0.33 0.97 0.28 1.15 0.16 0.23 1.12 0.34 0.16 0.22 0.06 1.09 1.28 4.51

2.08 2.00 6.95 0.57 1.61 0.24 1.01 0.06 0.28 0.73 0.73 0.36 0.65 0.75 0.36 0.40 0.76 0.18 0.22 0.67 0.67 0.70 2.70 3.80

21.0 20.5 4.44 10.4 17.6 1.17 27.2 0.96 12.7 33.6 16.3 1.98 23.9 25.8 2.18 3.08 18.5 13.4 15.5 9.62 13.8 11.2 7.26 3.90

0.91 0.90 0.74 0.37 0.99 0.05 1.16 0.03 0.39 0.89 0.66 0.17 1.03 0.62 0.20 0.03 0.63 1.08 0.34 0.62 0.18 1.56 0.32 0.38

7.08 10.6 4.62 1.73 12.3 1.20 8.71 0.15 1.03 4.72 2.91 15.7 1.21 4.21 0.51 1.01 0.87 1.09 1.88 9.49 13.6 9.24 13.6 14.8

5.00 6.28 3.17 3.48 16.1 1.34 5.49 0.72 3.23 3.71 6.88 4.10 4.38 3.34 1.62 0.69 1.93 1.74 7.37 6.11 5.04 27.9 4.21 4.43

Range Average

0.06–4.51 0.80

0.06–8.95 1.27

0.96–33.6 12.51

0.03–1.56 0.59

0.60, respectively, by excluding the four pyrite-rich coals. It is worthwhile mentioning the work by Spears and Zheng [23]. They found a positive correlation between iron and pyritic sulfur in 24 UK coals. In geochemistry, the abundance linkage between iron and sulfur for pyrite-rich coals was thought to be controlled by a seawater sedimentary origin. Under a marine-influenced diagenesis environment, the solution immersed in coal swamp generally contains high concentrations of sulfate and iron salts, which leads to the formation of pyrite and organic sulfur. However, iron occurs in many forms, such as iron disulfide (pyrite and marcasite), iron carbonate (siderite and ankerite, etc.), iron-bearing clay, iron sulfate (ferrous sulfate and jarosite etc.), and organically bound iron. For low-sulfur coals, iron occurs primarily as carbonate and clay rather than pyrite. This is responsible for the poor relationship between iron and sulfur but the improved relationships of iron with calcium and aluminum when the pyrite-rich coals are excluded from the statistical calculation. The calculated correlation coefficient between aluminum and potassium is 0.63, demonstrating some relationship between these two elements. Aluminum in coal is mainly due to the presence of clay minerals such as kaolinite and illite, while potassium is mainly due to the presence of potassium-bearing clay like illite and feldspar. Kaolinite contains little potassium. It is therefore reasonable to expect some relationship between these two elements, but such a relationship would not generally be strong. Spear and Zheng [23] found a positive relationship between sili-

0.15–15.7 5.93

0.69–27.9 5.34

con and aluminum with an atomic ratio of silicon to aluminum close to the stoichiometric ratio for kaolinite, but they found no significant relationship between aluminum and potassium in UK coals. Their observations were attributed to the enrichment of kaolinite in UK coals. 3.3. Multiple linear regressions for all coal samples It is recognized that trace elements in coal occur in both organic association and inorganic association. Inorganic trace elements are associated primarily with three major mineral groups in coal, carbonate, aluminosilicate, and sulfide [1]. Quartz, although ubiquitous in coals, is a very minor host towards trace elements [1]. Carbonate minerals in coal have several species. Calcite and siderite are the most important carbonate minerals. Calcium in mediumto high-rank coals can be an element implicative of carbonate, but an important occurrence form of this element in low-rank coals including lignite and subbituminous coal is often carboxyl-bound. We can reasonably assume that aluminum in coal is a marker for aluminosilicate in coal, despite different species of aluminosilicate. Kaolinite and illite are two typical mineral species in coal. In general, kaolinite scarcely accommodates trace elements but potassium-containing illite is enriched in trace elements, so trace elements in clay should also have a relationship with potassium. As described in Section 3.2, iron in high-sulfur coals is mainly implicative of sulfide, whereas iron in low-sulfur coals is mainly due to iron-bearing carbonate and/or clay.

Table 3 Concentrations of trace elements in coal samples (in lg g1, dry basis) SS020

SS022

SS023

SS025

SS030

SS033

SS037

SS038

Li Be V Cr Mn Co Ni Cu Zn Ga As Se Cd Sr Ba Pb Hg

16.4 ± 0.4 1.90 ± 0.08 22.3 ± 0.7 26.0 ± 1.1 81.5 ± 0.9 10.1 ± 0.3 20.5 ± 0.8 12.1 ± 0.9 16.8 ± 3.3 10.0 ± 0.4 3.15 ± 0.09 1.00 ± 0.05 0.09 ± 0.01 266.6 ± 1.5 169.7 ± 0.6 9.9 ± 0.6 0.41 ± 0.03

43.83 ± 0.03 1.84 ± 0.02 18.9 ± 0.3 23.9 ± 1.9 57.1 ± 0.6 3.91 ± 0.07 10.2 ± 0.08 10.8 ± 0.5 12.2 ± 1.6 7.84 ± 0.05 1.36 ± 0.13 1.13 ± 0.07 0.094 ± 0.003 458.2 ± 5.3 441.3 ± 0.3 6.47 ± 0.07 0.33 ± 0.01

5.7 ± 0.2 0.42 ± 0.03 7.1 ± 0.3 20.0 ± 2.8 12.3 ± 0.7 1.11 ± 0.07 8.2 ± 0.5 5.7 ± 0.2 39.7 ± 28 1.6 ± 0.1 0.547 ± 0.001 1.18 ± 0.02 1.1 ± 0.2 46.6 ± 2.6 37.5 ± 4.7 9.3 ± 0.6 0.185 ± 0.004

6.94 ± 0.04 0.34 ± 0.01 8.37 ± 0.09 7.14 ± 0.06 6.7 ± 0.8 1.21 ± 0.03 4.9 ± 0.3 13.5 ± 0.8 9.1 ± 0.5 3.29 ± 0.01 2.7 ± 0.4 1.02 ± 0.01 0.10 ± 0.01 279 ± 0.7 293 ± 3.4 4.1 ± 0.3 0.21 ± 0.02

47.0 ± 0.6 0.68 ± 0.06 33.0 ± 0.1 30.2 ± 4.7 65.5 ± 0.6 5.20 ± 0.04 10.6 ± 0.5 14.6 ± 1.8 52 ± 23 6.43 ± 0.01 1.61 ± 0.03 0.98 ± 0.02 0.63 ± 0.38 190.8 ± 2.0 101.9 ± 0.3 6.6 ± 2.9 0.1 ± 0.002

0.51 ± 0.06 0.25 ± 0.05 2.51 ± 0.01 3.6 ± 0.4 17.1 ± 0.4 4.47 ± 0.07 7.14 ± 0.02 5.7 ± 2.2 42 ± 19 0.33 ± 0.07 0.84 ± 0.03 0.16 ± 0.03 0.8 ± 0.1 381.2 ± 7.3 69.7 ± 2.0 32 ± 29 0.20 ± 0.04

49.54 ± 0.07 1.19 ± 0.07 25.5 ± 1.5 17.8 ± 0.9 31.8 ± 4.2 6.2 ± 0.2 11.8 ± 0.6 24.3 ± 0.7 10.7 ± 0.6 8.7 ± 0.4 1.38 ± 0.04 6.5 ± 0.2 0.089 ± 0.003 230.4 ± 1.8 222.8 ± 1.7 19.5 ± 0.9 0.55 ± 0.03

1.20 ± 0.04 0.27 ± 0.01 4.70 ± 0.05 3.91 ± 0.06 3.7 ± 0.1 0.52 ± 0.04 2.6 ± 0.1 4.2 ± 0.8 70 ± 21 0.70 ± 0.01 0.197 ± 0.002 3.653 ± 0.001 0.28 ± 0.06 9.1 ± 0.3 8.0 ± 0.1 2.39 ± 0.04 0.14 ± 0.01

SS040

SS056

SS059

SS062

SS063

SS064

SS066

SS069

16.1 ± 0.6 0.64 ± 0.01 30.8 ± 0.8 14.1 ± 3.9 21.4 ± 7.5 5.6 ± 0.2 16.4 ± 0.3 9.1 ± 1.1 29 ± 17 4.4 ± 0.2 2.0 ± 0.2 0.83 ± 0.04 0.18 ± 0.01 110.9 ± 0.6 66.3 ± 1.3 9.7 ± 0.8 0.24 ± 0.01

113.2 ± 0.3 1.65 ± 0.02 18.4 ± 0.9 12.9 ± 0.2 16.3 ± 0.7 3.0 ± 0.4 7.6 ± 0.1 17.7 ± 0.5 43 ± 13 11.3 ± 0.2 1.8 ± 0.5 5.2 ± 0.3 0.17 ± 0.01 280.4 ± 2.9 158 ± 11 30 ± 11 0.077 ± 0.002

25.8 ± 0.2 0.84 ± 0.01 20.54 ± 0.02 7.9 ± 0.3 46.7 ± 0.5 11.2 ± 0.1 15.5 ± 0.6 16.5 ± 0.9 8.9 ± 1.0 4.79 ± 0.02 0.87 ± 0.02 0.770 ± 0.005 0.05 ± 0.01 51.1 ± 1.0 27.0 ± 3.8 6.4 ± 1.3 0.066 ± 0.001

1.29 ± 0.03 0.36 ± 0.02 1.98 ± 0.09 3.90 ± 0.9 178.1 ± 4.5 2.66 ± 0.08 6.8 ± 0.2 6.8 ± 0.5 4.4 ± 0.6 0.65 ± 0.03 0.89 ± 0.01 0.26 ± 0.01 0.011 ± 0.003 217.2 ± 1.0 58.8 ± 8.4 4.7 ± 3.0 0.173 ± 0.001

104.3 ± 2.0 0.71 ± 0.05 25.6 ± 0.9 15.5 ± 2.9 21.1 ± 1.8 4.65 ± 0.14 8.44 ± 0.24 22.1 ± 0.4 30.6 ± 4.0 7.1 ± 0.3 1.244 ± 0.003 0.75 ± 0.05 1.1 ± 0.3 345.4 ± 6.6 63.1 ± 1.8 29 ± 26 0.275 ± 0.003

81.3 ± 0.3 1.48 ± 0.03 18.7 ± 0.4 10.0 ± 0.4 20.6 ± 0.5 3.51 ± 0.08 7.64 ± 0.02 15.3 ± 0.2 9.6 ± 1.8 8.46 ± 0.08 0.91 ± 0.04 4.7 ± 0.1 0.18 ± 0.03 241.1 ± 7.8 136.3 ± 1.9 17.1 ± 1.0 0.3 ± 0.2

2.67 ± 0.01 0.45 ± 0.04 5.12 ± 0.05 2.72 ± 0.04 3.7 ± 0.6 1.84 ± 0.03 4.29 ± 0.09 3.0 ± 0.4 33 ± 20 0.78 ± 0.04 2.34 ± 0.04 0.20 ± 0.03 0.6 ± 0.2 23.9 ± 2.6 27.2 ± 1.7 2.6 ± 0.2 0.095 ± 0.001

3.33 ± 0.03 0.62 ± 0.01 2.75 ± 0.03 0.98 ± 0.01 10.8 ± 0.09 2.16 ± 0.01 1.12 ± 0.02 2.4 ± 0.3 22.7 ± 4.4 0.967 ± 0.003 0.55 ± 0.01 0.365 ± 0.002 0.12 ± 0.02 28.7 ± 0.3 13.8 ± 0.1 1.83 ± 0.01 0.18 ± 0.03

Li Be V Cr Mn Co Ni Cu Zn Ga As Se Cd Sr Ba Pb Hg

SS82

SS090

SS95

SS100

IL

WY

BZ

Range

Average

11.4 ± 0.3 2.19 ± 0.02 54.8 ± 1.6 9.6 ± 0.7 8.5 ± 0.3 8.2 ± 0.3 7.3 ± 0.3 25.5 ± 0.9 19.8 ± 0.6

25.5 1.13 28.1 3.4 9.59 2.72 3.1 11.6 52

27.3 ± 0.4 0.73 ± 0.01 25.8 ± 2.0 8.4 ± 0.8 34.7 ± 1.4 6.72 ± 0.04 8.6 ± 0.2 8.8 ± 0.2 66 ± 33

13.1 ± 0.7 1.15 ± 0.04 13.1 ± 0.4 10.3 ± 0.1 178 ± 23 5.2 ± 0.2 9.67 ± 0.04 10.5 ± 0.6 53.7 ± 7.0

8.26 ± 0.03 0.64 ± 0.01 19.87 ± 0.05 25.7 ± 1.7 37.3 ± 1.1 19.3 ± 0.1 68.1 ± 1.1 16.1 ± 0.4 41 ± 11

26.3 0.87 74.1 43.0 134 8.0 50.9 28.5 405

8.68 0.3 14.4 6.11 8.68 1.72 5.4 13.3 20.1

12.1 0.29 16.7 1.2 126 1.2 8.0 19.5 33.2

0.45–114 0.2–2.1 1.8–61 1–61 3.1–162 0.5–18.5 0.9–65 2.0–28.5 4.0–405

26.5 0.85 19.6 13.5 38.6 4.9 12.2 12.9 46.5

Rangea

Averagea

0.1–15

2

0.5–60 5–300 0.5–30 0.5–50

20 70 5 20

(continued on next page)

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Li Be V Cr Mn Co Ni Cu Zn

SS080

J. Wang et al. / Fuel 87 (2008) 2211–2222

Element

J. Wang et al. / Fuel 87 (2008) 2211–2222

S concentration in coal (%)

40 0.1 2–80 0.02–1

0.5–80 0.2–10 0.1–3

10 1 0.5

5

4.6 1.6 1.7 0.3 183 137 10.9 0.21

Average

Rangea

Averagea

2216

4 3 2 1 0 0

10

20

0.3–11.3 0.2–4.6 0.1–6.4 0.01–1.0 6.9–552 3.9–702 1.7–34 0.07–0.55

Fig. 1. Correlations of iron with total sulfur (r) and pyritic sulfur (d).

1.47 2.9 0.64 0.02 550 702 33.9 nd

From the above statement, we assume that trace elements in coal are correlated with three major elements, calcium, aluminum, and iron. The multiple correlation formula is given as follows:

4.15 ± 0.01 2.47 ± 0.06 2.42 ± 0.08 0.206 ± 0.002 109.4 ± 0.6 65.4 ± 1.3 3.3 ± 0.4 0.40 ± 0.01

5.1 4.6 4.1 0.71 49.7 126 34.1 0.12

3.62 3.83 1.6 0.083 315 390 4.1 0.15

Y ¼ a þ bc X c þ b a X a þ b i X i

a

The data for world coals cited from reference [25].

3.8 ± 0.1 1.1 ± 0.1 0.13 ± 0.03 0.15 ± 0.07 586 ± 52 307 ± 37 7.1 ± 0.4 0.13 ± 0.05 5.5 ± 0.7 4.2 ± 0.1 0.708 ± 0.004 0.27 ± 0.09 37.1 ± 0.8 20.7 ± 1.2 4.3 ± 0.6 0.14 ± 0.06 4.64 0.31 0.93 0.22 57.45 39.1 5.7 0.12 7.46 ± 0.01 0.56 ± 0.01 0.69 ± 0.04 0.7 ± 0.3 78.4 ± 0.9 58.2 ± 1.3 9.5 ± 0.7 0.159 ± 0.004 Ga As Se Cd Sr Ba Pb Hg

SS080

Table 3 (continued)

SS82

SS090

SS95

SS100

IL

WY

BZ

Range

Fe concentration in coal

30

(mg.g-1)

ð1Þ

where Y is the concentration of trace element in coal (%), Xc is the concentration of calcium in coal (%), Xa is that of aluminum in coal (%), and Xi is that of iron in coal (%). In the equation, bcXc, baXa, and biXi refer to the fractions of trace element associated with minerals relevant to calcium, aluminum, and iron, respectively, and a refers to the concentration of trace element in the coals free of major minerals, roughly implying the organically associated fraction. For various coals, Xc, Xa, and Xi are independent variables, and Y is a dependent variable. The experimental data shown in Tables 2 and 3 give 24 points for each trace element (except Hg which has one less point) in the four dimensional space with the coordinates (Xcj, Xaj, Xij, Yj; j = 1, 2, . . . ,24, and for Hg, j = 1, 2, . . . ,23). Least squares multiple linear regression is then used to determine the parameters, a, bc, ba, and ba. The results are shown in Table 4, where F is the variance ratio; qyc, qya, and qyi are the partial correlation coefficients of Y versus Xc, Xa, and Xi, respectively. The correlation is assessed to be significant if the value of variance ratio (F) is larger than or close to the value of F distribution. In this case, the value of F distribution with (3, 24-3-1) degrees of freedom and 95% confidence level is 3.1. We can consequently see that all trace elements, with the exception of cobalt, cadmium, lead and mercury, show significant correspondence. Furthermore, the partial correlation coefficients indicate the correlations with the corresponding major element. Gallium is excellently correlated with aluminum, as is depicted in Fig. 2. Other aluminum-correlated elements are lithium, beryllium, copper, selenium, vanadium and chromium. Zinc, vanadium, chromium, nickel, copper, and arsenic are correlated with iron, whereas manganese, strontium, and barium are correlated with calcium.

J. Wang et al. / Fuel 87 (2008) 2211–2222

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Table 4 Multiple regression results for the correlation between concentrations of trace elements and those of major elements Element

Equation

F value

Partial correlation coefficient qyc

qya

qyi

Li Be V Cr Mn Co Ni Cu Zn Ga As Se Cd Sr Ba Pb Hg

Y = 9.59–0.361Xc + 2.88Xa + 0.187Xi Y = 0.313–0.003Xc + 0.046Xa  0.006Xi Y = 6.23–0.735Xc + 0.527Xa + 2.19Xi Y = 0.310 + 0.169Xc + 0.343Xa + 1.31Xi Y = 13.8 + 5.59Xc  1.07Xa + 2.68Xi Y = 1.94 + 0.101Xc + 0.132Xa + 0.136Xi Y = 1.66 + 0.592Xc + 0.083Xa + 1.20Xi Y = 4.19 + 0.115Xc + 0.422Xa + 0.493Xi Y = 28.6  3.79Xc  2.13Xa + 13.1Xi Y = 0.032 + 0.0174Xc + 0.334Xa + 0.033Xi Y = 1.04 + 0.0350Xc + 0.00365Xa + 0.105Xi Y = 0.0185 + 0.0148Xc + 0.101Xa + 0.0423Xi Y = 0.461–0.0302Xc  0.00542Xa + 0.0227Xi Y = 97.6 + 18.8Xc + 3.50Xa  9.17Xi Y = 39.9 + 19.6Xc + 1.44Xa  5.28Xi Y = 6.10–0.0654Xc + 0.265Xa + 0. 567Xi Y = 0.125–0.00866Xc + 0.00526Xa  0.00702Xi

15.8 7.8 11.9 15.6 7.3 1.3 3.2 8.1 25.0 106.8 2.9 3.1 2.1 3.1 3.5 1.1 2.5

0.10 0.04 0.34 0.13 0.59 0.12 0.22 0.14 0.45 0.11 0.16 0.05 0.44 0.54 0.58 0.03 0.36

0.83 0.73 0.43 0.46 0.25 0.30 0.06 0.62 0.47 0.97 0.03 0.52 0.16 0.22 0.10 0.22 0.42

0.06 0.08 0.75 0.75 0.35 0.18 0.45 0.47 0.89 0.21 0.47 0.15 0.38 0.32 0.20 0.27 0.34

Ga concentration (μg.g-1)

14 12 10 8 6 4 2 0

0

10

20

Al concentration

30

40

(mg.g-1)

Fig. 2. Correlation between gallium and aluminum.

The least squares multiple linear regressions have been conducted using the data of potassium instead of those of aluminum. The values of F and the partial correlation coefficients of gallium, lithium, beryllium, copper, selenium, chromium and vanadium are presented, respectively, as follows: 7.2 and 0.71 for gallium, 3.9 and 0.59 for lithium, 4.4 and 0.63 for beryllium, 6.4 and 0.55 for copper, 1.3 and 0.34 for selenium, 15.6 and 0.43 for chromium, and 12.0 and 0.43 for vanadium. By comparing these data with the corresponding data listed in Table 4, we are interested to see that the aluminum-correlated elements except selenium also show significant correlations with potassium; however, the values of F with potassium overall tend to be smaller than those with aluminum. 3.4. Regressions by considering coal-rank and pyrite content Of the 24 coals tabulated in Table 1, WY and BZ coals are the lowest-rank coals (BZ coal is a lignite; WY is a subbituminous coal). Previous studies [17,20] and our supple-

mentary study showed that the two coals contained high concentrations of organically associated calcium. To observe the effects of coal rank on the regression results, we have performed the least squares multiple linear regressions for each trace element using Eq. (1) by omitting data for WY and BZ coals. Similarly, the pyrite-rich coals, SS30, SS40, SS90, and IL coals, were omitted to investigate the effect of pyrite content on the regression results. The results obtained from these two cases are shown in Table 5. Comparison of Table 5 with Table 4 demonstrates that the total correlations of each trace element are not essentially changed by excluding WY and BZ coals; however, the partial correlation coefficients of chromium, manganese, and nickel with calcium become slightly larger, whereas those of strontium and barium with calcium become slightly smaller. Deletion of the pyrite-rich coals results in a substantial change in that the correlations of vanadium, chromium, nickel, copper, zinc, and selenium with iron become insignificant, whereas the correlations of manganese, strontium, and barium with iron become slightly enhanced. 4. Discussion A voluminous amount of work has been directed to identifying the modes of occurrence of trace elements in coal, which is comprehensively described in the literature [1,9,12,30]. To our knowledge, the present work has, for the first time, studied multiple correlations between trace elements and the major elements for a wide diversity of coals. Of the 17 trace elements examined in this study, 13 trace elements (Li, Be, V, Cr, Mn, Ni, Cu, Zn, Ga, As, Se, Sr, and Ba) show significant correspondence. Such correlations can imply some common elemental modes of occurrence in different coals. However, the implications may be different with coal rank and pyrite content because

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J. Wang et al. / Fuel 87 (2008) 2211–2222

Table 5 Correlations between concentrations of trace elements and those of major elements in medium- to high-rank coals (MHRC)a as well as in low-sulfur coals (LSC)b Element

Li Be V Cr Mn Co Ni Cu Zn Ga As Se Cd Sr Ba Pb Hg a b

MHRC

LSC

F value

Partial correlation coefficient

F value

qyc

qya

qyi

14.7 6.3 12.1 16.4 9.3 1.5 3.4 11.2 24.5 96.2 3.8 2.6 1.4 1.8 2.1 2.6 2.8

0.19 0.08 0.43 0.37 0.67 0.29 0.33 0.14 0.50 0.03 0.15 0.02 0.36 0.40 0.36 0.29 0.42

0.84 0.71 0.49 0.40 0.33 0.23 0.01 0.71 0.43 0.97 0.10 0.52 0.18 0.29 0.35 0.39 0.40

0.10 0.12 0.77 0.75 0.32 0.10 0.40 0.59 0.89 0.24 0.60 0.15 0.35 0.27 0.07 0.42 0.38

13.6 7.2 3.4 4.1 6.0 1.5 1.4 6.2 0.7 87.1 1.4 6.1 1.8 2.4 3.0 1.3 2.2

Partial correlation coefficient qyc

qya

qyi

0.00 0.01 0.00 0.08 0.39 0.02 0.25 0.16 0.20 0.20 0.36 0.33 0.37 0.27 0.40 0.18 0.39

0.80 0.70 0.56 0.40 0.36 0.13 0.10 0.63 0.26 0.96 0.04 0.73 0.09 0.03 0.00 0.42 0.43

0.17 0.10 0.08 0.27 0.37 0.28 0.11 0.05 0.05 0.12 0.04 0.50 0.01 0.22 0.13 0.27 0.15

By excluding WY and BZ coals. By excluding SS30, SS40, SS90 and IL coals.

the coals have undergone different geochemical processes during the coal formation. The relationship between trace element and aluminum is thought to be due to the association of trace element with aluminosilicate or clay. As described above, all aluminumrelated trace elements (Ga, Li, Be, Cu, Se, Cr, and V), with the exception of selenium, are also correlated positively with potassium. This is consistent with the well-known fact that potassium-rich clay like illite rather than potassiumlean kaolinite accommodates trace elements appreciably. The relationship between trace elements and calcium is characteristic of the association of trace element with carbonate in medium- to high-rank coals, but more commonly with the organic matter in low-rank coals. The relationship between trace element and iron can be implicative for the association of trace element with sulfide in pyrite-rich coals, but more so with carbonate and/or clay in low-pyrite coals. According to these implications, we thus use the statistical analysis results to infer the common modes of occurrence of each trace element, with reference to the relevant literature, as follows. 4.1. Lithium Maybe because lithium in coal is of less environmental concern and it is difficult to detect by direct X-ray and electron-scanning methods, its modes of occurrence in coal have rarely been reported. Our present study has shown that lithium correlates well with aluminum, indicating that this element has a common association with aluminosilicate. In contrast, lithium has poor relationships with calcium and iron. We conducted an accessory sequential

leaching experiment to examine the modes of occurrence of lithium in IL and BZ coals. Greater than 80% of lithium was selectively removed by HF leaching from both coals. We believe that lithium is associated predominantly with aluminosilicate. 4.2. Beryllium This element is also difficult to detect directly by instrumental methods, and the experimental information on the modes of occurrence of beryllium is not so much. Finkelman stated that beryllium was consistently concentrated in the organic matter, and the possibility of some beryllium being associated with clay cannot be excluded [30]. Wang et al. [31] reported that beryllium was partly associated with the organic matter, which was evident by leaching four raw coals and their corresponding ashes and chars. In the present study, beryllium shows a very positive correlation with aluminum. Similar to lithium, beryllium is poorly correlated with calcium and iron. Moreover, a large a value (0.31), relative to the average concentration of beryllium in the 24 coals (0.82), suggests that an appreciable part of beryllium may be associated with the organic matter. We thus believe that beryllium has two common modes of occurrence, aluminosilicate association and organic association. 4.3. Vanadium Its mode of occurrence has been investigated extensively [6,7]. It is well known that a major association of vanadium is with clay, particularly with illite [6]. This conforms to the

J. Wang et al. / Fuel 87 (2008) 2211–2222

correlations between vanadium and aluminum or potassium obtained by the present statistical analysis. Besides the clay-associated vanadium, a part of vanadium was reported to be associated tightly with the organic matrix, being substantially resistant to the acid leaching [7,21]. In the correlation equation, a large a value may be due to the fraction of the organically associated vanadium. How this lithophile element might be associated with the organic matrix is discussed in the literature [7]. In the present study, we are surprised to see that vanadium is correlated strongly with iron when all twenty four coals are counted in the statistical analysis, and no significant correlation is found when the four high-pyrite coals are excluded (see Table 5). It is generally regarded that vanadium does not exist as a solid solution element in pyrite. However, recently Shaver et al. [25] also presented an observation that vanadium was correlated with pyrite in the Bon Air coals. It is suggested that vanadium may be rich in pyrite-rich coals but may not be chemically combined with pyrite.

4.4. Chromium Chromium is a lithophile element analogous to vanadium. Previous studies showed that chromium has two major forms as an organic association and as an illite association [7,21]. Good correlations between chromium and aluminum or potassium, as revealed in this study, are consistent with the well-known association of chromium with clay, particularly with illite. Our accessory sequential leaching experiment showed that chromium in IL and BZ coals was largely leached with a HF solution, while only a small fraction of chromium was leached with a HNO3 solution. Moreover, a significant fraction of chromium in IL and BZ coals was not dissolved by successive HCl, HF, and HNO3 leaching. This part of acid-insoluble chromium was characterized as being mainly associated with the organic matter [21,31]. However, the organically associated part of chromium may be masked in the present statistical analysis. The correlations of chromium with iron are quite similar to those of vanadium. Such correlations are perplexing, because like vanadium, chromium is not commonly observed as a solid solution element in pyrite. Chromite (FeCrO4) has been found in coal; but it was reported to be restricted to an ultramafic deposit origin [7,32], and the enrichment of chromite should not have any relationship with pyrite in coal. It should be noted that the correlations of vanadium and chromium with aluminum are changed insignificantly by excluding the high-pyrite coals, in contrast to the results of these two elements with iron. This implies that the abundance of vanadium and chromium in coal may have different geochemical origins, probably one being related to aluminum abundance and another being related to iron abundance. The latter origin is probably an introduction process of vanadium and chromium ions from sea water into peat swamp together with iron salts. However, it is likely that these

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two lithophile elements form organic association and/or clay during coalification while iron forms pyrite. 4.5. Manganese The significant correlation between manganese and calcium is explicable because much evidence demonstrates that manganese occurs in coal principally as carbonate [6,30]. It is usually true that carbonate-rich coals have high concentrations of calcium; however, low-rank coals with high concentrations of calcium may dominantly contain carboxyl-bound calcium rather than carbonate. Therefore, the correlation between manganese and calcium can essentially represent the associations of manganese with carbonate for medium- to high-rank coals but not for low-rank coals. Of the 24 coals, WY and BZ coals are two low-rank coals with high concentrations of organically associated calcium, while the other coals are of medium to high rank. The present statistical analysis reveals that the correlation between manganese and calcium is somewhat improved by excluding WY and BZ coals (see Tables 4 and 5), implying that carbonate is actually a common occurrence form for manganese. In addition, manganese is somewhat correlated with iron, and the correlation between the two elements is neither significantly changed by excluding the low-rank coals nor by excluding the pyrite-rich coals (see Tables 4 and 5). This relation may be explained by the common presence of manganese in iron carbonate. Huggins and Huffman [6] pointed out that the forms of manganese in coal are very similar to calcium, with an ion-exchangeable form or a crystalline carbonate form [(Ca, Mn)CO3]. Finkelman [30] stated that manganese can be a trace constituent substituting for iron in carbonate. Although manganese is often assumed to be associated with clay [6,30], no significant correlation has been observed in the present study for deducing this association. 4.6. Nickel This chalcophile element has diverse modes of occurrence in coal, partly as acid-insoluble organic association [21] and partly as an ion-exchangeable form [30]. In the present study, nickel shows moderate relationships with iron and calcium. However, the correlation with iron is greatly weakened by excluding the pyrite-rich coals, whereas the correlation with calcium appears to be enhanced by excluding the pyrite-rich coals or the low-rank coals. This implies that nickel occurs probably in both the sulfide and carbonate forms. 4.7. Copper Copper is a chalcophile element and its primary mineral in coal is thought to be chalcopyrite (CuFeS2) [12,33]. The statistical analysis shows that copper is correlated with both aluminum and iron. The correlation with iron becomes insignificant by excluding the pyrite-rich coals,

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J. Wang et al. / Fuel 87 (2008) 2211–2222

whereas the correlations of copper with aluminum remain significant by excluding the low-rank coals or the pyriterich coals. This implies that sulfide is a common mode of occurrence of copper in pyrite-rich coals; aluminosilicate is another common mode of occurrence of copper in coal, irrespective of coal rank and pyrite content. No significant correlation is found between copper and calcium, suggesting that copper in coal may not occur as carbonate. 4.8. Zinc Zinc is also a chalcophile element and it occurs in coal probably as sphalerite (ZnS) and pyrite [12,33]. The present statistical analysis shows that zinc has a good correlation with iron when the pyrite-rich coals are included. Such a significant correlation between zinc and iron disappears by excluding the pyrite-rich coals. Since most zinc in coal dissolves in hydrochloric acid whereas pyrite does not, one may assume that there should not be any relationships between zinc and pyrite. However, one does not know whether the solid solution zinc in pyrite can be leached selectively by HCl or not, and this makes it difficult to elucidate whether zinc occurs in pyrite as a solid solution element or as a physically intermixed mineral. The correlation between zinc and iron may be because geological environments for the formation of high-pyrite coals is probably also compatible with the formation of zinc sulfide. No significant correlations are found between zinc and calcium or aluminum. Therefore, we believe that zinc has a common form as sulfide in high-sulfur coal but it may have no characteristic mineral form in low-sulfur coal. A positive a value suggests that a fraction of zinc is probably organically associated. 4.9. Gallium The modes of occurrence of gallium in coal are not widely documented. Mukeherrjee reported that gallium in four Assam coals was associated with clay [24]. In the present study, gallium exhibits an excellent correlation with aluminum. It can be certain that the principal phase of gallium in coal is aluminosilicate. In contrast to the results of chromium and vanadium, the correlation of gallium with potassium is clearly poorer than that with aluminum. This suggests that kaolinite may be an important host to this element. 4.10. Arsenic Although arsenic in coal is characterized to have various associations with organic matter, carbonate, clay and sulfide [30], the two common forms are sulfide and organic associations [8,12]. Previous acid leaching experiment demonstrated that part of arsenic in the SS20 coal (named EL coal in the references) behaved as a solid solution in pyrite [21,31]. Huggins et al. [5] early presented direct evidence for the solid-solution arsenic in pyrite by XAFS. The present

statistical study has also confirmed that arsenic is commonly associated with sulfide in pyrite-rich coals. Also, the partial correlation coefficient with calcium obtained by excluding the pyrite-rich coals is obviously larger than that obtained from all coals, whereas the partial correlation coefficient with calcium obtained by excluding the low-rank coals is smaller than that obtained from all coals (see Tables 4 and 5). Because calcium in the low-rank coals is mainly organically associated, the result suggests that some arsenic in low-rank coals may be organically associated. 4.11. Selenium Because selenium can substitute for both sulfur in sulfide minerals and organic sulfur, it is widely regarded that selenium in coal is associated with pyrite, galena and organic matter. Finkelman et al. [30] determined that most of selenium in a Powder River basin coal was associated with the organics, with only 5–10% selenium being associated with pyrite. In the present study, we are curious to note that selenium is poorly correlated with iron but significantly with aluminum, unlike other chalcophile elements such as nickel, zinc and arsenic. The relations between selenium and aluminum become better for the low-sulfur coals (see Tables 4 and 5). It appears that selenium may have an association with aluminosilicate, but the speciation of selenium with aluminum is not well known. As an alternative explanation, selenium in coal may have a geochemical origin related to the formation of aluminosilicate in coal but it is not eventually concentrated in aluminosilicate. Furthermore, selenium is poorly correlated with potassium, differing from all the rest of aluminum-related elements. The elements correlating with aluminum form from both detrital and diagenetic origins, but those with potassium are of a more detrital origin [23]. Therefore, selenium in coal may be mainly of diagenitic origin. Selenium in high-sulfur coals is probably associated with sulfide to some extent because the correlation of selenium with iron appreciably becomes worse by excluding the pyrite-rich coals. 4.12. Strontium and barium These two elements show very similar correlation characteristics. Davison stated that strontium commonly substitutes for calcium in calcite and barium may occur as witherite (BaCO3), so these two elements are generally enriched in calcium-rich coals [1]. This is consistent with their significant correlation with calcium observed in the present statistical analysis. However, the correlation with calcium is weakened by excluding the low-rank coals, unlike the result for manganese. The two alkaline-earth elements are likely to be less carbonate-associated but more carboxyl-bound than manganese. Previous study reported that WY coal had more organically associated strontium and barium than IL coal [20]. For barium, sulfate is believed to be a common mineral species, and this would worsen the relationship between this element and calcium.

J. Wang et al. / Fuel 87 (2008) 2211–2222

A positive a value implies that fractional part of these two elements may also occur in associations without three main elements, calcium, aluminum, and iron. In addition, strontium and barium are somewhat correlated with aluminum. It is reported in the literature [1,25] that feldspar and crandallite – group aluminum phosphate often contain barium and strontium. 4.13. Other elements (cobalt, cadmium, lead and mercury) Overall, these four trace elements are poorly correlated with the major elements. Nevertheless, the partial correlation coefficients of cadmium and lead with iron are relatively significant. For low-pyrite coals (see Table 5), less significant correlations are found. This indicates that cadmium and lead in pyrite-rich coals have a relatively common occurrence as sulfide, in agreement with other observations on this element [1,30]. In addition, the partial correlation coefficients of lead with aluminum are relatively significant, also consistent with the report on this element being associated with aluminosilicate [1]. Mercury is somewhat correlated with calcium and aluminum but not with iron. A significant part of mercury may also occur in an organic association. The modes of occurrence of mercury in coal appear to be diverse. Shaver et al. [25] pointed out that calcite possesses the ability of adsorbing mercury from an aqueous environment, precipitating mercury carbonate or other mercury species during coalification, so it might be expected to observe some relationship between mercury and calcium. Sulfide is generally regarded as the dominant speciation of mercury in coal, but this was not the case for Pennsylvania bituminous coals [34]. Tewalt et al. [34] reported that mercury correlated poorly with sulfur, arsenic, pyrite, and iron-bearing compounds in these coals. The present study also shows that at least mercury is not generally rich in pyrite-rich coals, differing from many other chalcophile elements. Cobalt has a very poor relationship with three major elements. The modes of occurrence of cobalt in coal appear to be extremely irregular. Shaver et al. [25] also reported this observation. 5. Conclusions Least squares multiple linear regressions have been performed to examine the correlations between trace elements and three major elements for widely differing coals. The present method shows the probability of a trace element occurring in common forms in a wide diversity of coals. This statistical method can be used as a supplement to the other determination methods which are usually limited to a few specific coals. We are interested to find that among the seventeen trace elements examined in this study, 13 trace elements have significant correlations with calcium, aluminum and iron. Manganese, strontium, and barium are correlated with calcium; lithium, beryllium, vanadium, chromium, and gallium are correlated with aluminum; nickel, copper, zinc, and arsenic are correlated with iron

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