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Mar 7, 2016 - Gaomi Campus, Qingdao University of Science and Technology, Weifang, 261500, China. ABSTRACT: The content and modes of occurrence ...
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Arsenic in Coal: Modes of Occurrence, Distribution in Different Fractions, and Partitioning Behavior during Coal Separation—A Case Study Changchun Zhou, Ningning Zhang, Changbin Peng, Longfei Cong, Changheng Ouyang, and Rui Han Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b02669 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 7, 2016

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Arsenic in Coal: Modes of Occurrence, Distribution in Different Fractions, and Partitioning Behavior during Coal Separation—a Case Study Chang-Chun Zhou*,a,b, Ning-Ning Zhanga, Chang-Bin Penga, Long-Fei Conga, Chang-Heng Ouyanga, Rui Hanc a) School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou 221116, China b) Key Laboratory of Coal Processing & Efficient Utilization,Ministry of Education, Xuzhou 221116, China c) Gaomi Campus, Qingdao University of Science and Technology, Weifang, 261500, China *Corresponding author. E-mail address: [email protected] (C. Zhou).

Fax number: +86 0516 83591066

Corresponding author at: Room A505, School of Chemical Engineering and Technology, South Lake Campus, China University of Mining & Technology, Xuzhou 221116, Jiangsu Province, P.R.CHINA

ABSTRACT The content and modes of occurrence of arsenic and its distribution in Yunnan coal of China as well as its partitioning behavior during coal separation process were investigated. The following apparatus like proximate analyzer, ultimate analyzer, sulfur analyzer, inductively coupled plasma mass spectrometer and the methods including sequential chemical extraction process, screening analysis, float-and-sink analysis, heavy liquid separation and progressive release flotation were frequently used during the research process. The coal sample has a high sulfur content of 8.21%, and its arsenic content is 15.1 µg/g, which is within the range of the mild enrichment level. Content relationship among the various modes of occurrence in order is: sulfide associated form (47.38%) > organically bounded form (18.09%) > silicate associated form (17.51%) > carbonate associated form (12.04%) > ion-exchangeable form (3.84%) > water soluble form (1.14%). Sulfide associated form is the dominant modes of occurrence of arsenic in the raw coal, which means arsenic has an affinity to sulfur. Arsenic in sulfide associated form is mainly occurred in the inorganic sulfide minerals (especially in pyrite). Besides, the arsenic content increases with the decrease of coal particle size, and arsenic is concentrated in high-density products. There is a good correlation between the removal rate of arsenic and clean coal ash in either gravity separation or flotation, and arsenic removal rate 57.96% and 70.77% could be obtained through gravity separation and flotation respectively. To while ensuring arsenic removal rate and clean coal yield, combined approach of physics and chemistry should be developed. Keywords: Arsenic in coal; Modes of occurrence; Distribution; Partitioning behavior; Coal separation 1

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1. Introduction Arsenic is one of the most hazardous elements to the environment as well as one of the most common carcinogenic elements to humans.1-2 And as one of the common toxic trace elements in coal, arsenic, without being removed efficiently during the utilization process, will enter the atmosphere with the burning of coal.2-5 Paying attention to arsenic in coal is of great significance for the development of clean coal technology and environmental protection. The modes of occurrence of arsenic and other trace elements in coal have been studied extensively by many scholars. However, the distribution of trace elements in different fractions has not been investigated independently, which is as a part of the research about the modes of occurrence of trace elements in coal. Although the direct characterization methods such as the utilization of X-ray absorption fine structure spectroscopy (XAFS)6-9 and scanning electron microscopy combined with an energy dispersive X-ray analyzer (SEM-EDX)10-14 have been reported, the modes of occurrence of trace elements in coal have been determined mostly indirectly since the limitations of direct characterization methods, such as the required test conditions are too harsh.15-16 Generally, there are three indirect methods applied in investigating the modes of occurrence of trace elements in coal, i.e. statistical analysis, float-and-sink experiments and sequential chemical extraction. For example, the method of statistical analysis has been widely reported to study the mode of occurrence of trace elements.12, 17-21 By using float-and-sink experiments, Tian et al. 13, Luo et al.22, Querol et al.23, Senior et al.24, and Zhuang et al.25 focused on the distribution of trace elements in density-fractionated samples and attempted to show the relationship between trace elements and minerals in different density fractions. It is also reported that the method of sequential chemical extraction was used to investigate the modes of occurrence of trace elements in coal in literatures.26-28 Partitioning behavior of trace elements during coal separation process has been widely studied. Finkelman29 analyzed the factors which have an influence on the washability of trace elements in coal.

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Garcia et al.30-31 removed the trace elements from coal using flotation and selective flocculation processes. Demir et al.32 examined the effect of froth flotation on the removal of the trace element(s) content from coal. Morán et al.33 found that some pro-sulfur elements, such as Ni, Cu, Zn, As, etc., can be partly removed by the biological desulfurization process. Helble34 investigated the effects of coal separation on the reduction of emissions of trace elements during coal combustion. Senior et al.24, Zhou and Ren35 evaluated the washability of trace elements in coal using the float-and-sink test. Huggins et al.8 prepared several density fractions of coal by physical separation and measured the content of trace elements in these fractions to gain a better insight into elemental partitioning between clean coal and tailings. Vassileva et al.36 described the partitioning behaviour of trace elements among feed coals, high-grade coals, low-grade coals, coal slimes and host rocks during coal preparation. Finkelman29 and Wang et al.37-38 indicated that physical coal cleaning not only effectively removes the ash and sulfur from coal, but also reduces the concentration of most hazardous trace elements. This paper aimed at the comprehensively study on the quality characteristics of typical high arsenic content coal, the modes of occurrence of arsenic in coal, the distribution of arsenic in different size fractions and different density fractions, and the partitioning behavior of arsenic during the coal separation process. The purpose of this study was to show a theoretical foundation for the removal of arsenic in subsequent coal cleaning processes.

2. Material and methods 2.1. Coal sample The coal sample was taken from a Coal Preparation Plant in Yanshan County, Yunnan province of China. It is the feed of the coal preparation plant. The coal-bearing strata in the Yanshan Coalfield includes the Changxing and Wujiaping Formations, both of which are of the late Permian age.39 The concentrations of trace element in local coal are affected by the intra-seam tonstein bands.40

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Sample was crushed to -3mm using SP100 × 100 jaw crusher. Then, one part of the crushed coals was split as the stored sample and the other part was forwarded to chemical test, screening test, float-and-sink test and separation process. Results of proximate analysis, ultimate analysis, total sulfur analysis and forms of sulfur analysis of raw coal sample are shown in Table 1. As can be seen, this coal studied as meagre coal with high organic sulfur and high ash content. Table 1

Coal chemistry analysis results of raw coal sample, wt.% Proximate analysis (wt.%)

Ultimate analysis (wt.%)

Sulfur analysis (wt.%)

Md

Ad

Vd

FCd

Cdaf

Hdaf

Ndaf

Odaf

St,d

Sp,d

Ss,d

So,d

2.04

42.89

10.95

46.16

78.66

4.02

0.78

2.16

8.21

1.11

0.42

6.68

M: moisture; A: ash; V: volatile; FC: fix carbon; St: total sulfur; Sp: pyritic sulfur; Ss: sulfate sulfur; So: organic sulfur; ad: air-dried; d: dry basis; daf: dry and ash-free basis.

2.2. Technology roadmap and analytical methods Technical route used in this study is shown in Fig.1.

Fig.1. Technology roadmaps.

Coal sample was pulverized to pass a 0.2 mm sieve firstly, and then the sample was forwarded to the

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5E-MAG6700 automatic proximate analyzer for the indication of moisture, ash, volatile matter, and fixed carbon. The content of carbon, hydrogen, oxygen and nitrogen were determined using a vario MACRO cube. The total sulfur content and forms of sulfur were determined following ASTM standard methods D3177-0241 and D2492-0242, respectively. Sample was crushed and then ground to pass through a 74 µm mesh for trace element analysis. After the digestion by microwave digestion system, arsenic concentration in coal sample was determined by using the inductively coupled-plasma mass spectrometry (ICP-MS) with high resolution. ICP-MS has good reproducibility and low detection limit for the determination of trace elements and its Relative Standard Deviation (RSD) is less than 4.0%. Both the microwave digestion procedures and the ICP-MS analysis were carried out according to the methods proposed by Dai et al.43. Sequential chemical extraction process with classes of density, presented by Dai44, was used to research the modes of occurrence of arsenic. Six associated forms of arsenic in coal were obtained, they are water soluble form, ion-exchangeable form, organically bounded form, carbonate associated form, silicate associated form and sulfide associated form. Then the contents of each form were measured through ICP-MS. Screening tests according to Chinese standard GB/T477-2008 were employed to investigate the distribution of arsenic in different size fraction of coal. And float-and-sink test, following Chinese standard GB/T478-2008, was employed to investigate the distribution of arsenic at different density fraction of coal. Heavy liquid of float-and-sink tests for coarse particles (+0.5mm) was configured by zinc chloride while for fine particles (-0.5mm) it was configured by carbon tetrachloride, bromoform and benzene. Gravity separation and flotation tests were carried out to explore the partitioning of arsenic during coal dressing. Gravity separation experiments were conducted using the heavy liquid separation by which the raw coal was sorted into different density fractions. The densities of heavy liquid configured with zinc chloride

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are 1.5 g/cm3, 1.6 g/cm3, 1.8 g/cm3 and 2.0 g/cm3, respectively. Flotation tests were conducted using the progressive release flotation method in a 1.5 L single slot flotation machine, which are in accordance with the Chinese coal industry standard MT/T144-1997. The process of flotation tests is shown in Fig. 2. 100g/L coal pulp, 1.5L Stirred for 2min, 0.2mL N-dodecane added Stirred for 1min, 0.0185mL methyl isobutyl carbinol added Stirred for 10s Roughing, 3min Stirred for 30s Cleaning, 3min Tailings (product 6)

Stirred for 30s Cleaning, 3min

Tailings (product 5)

Stirred for 30s Cleaning, 3min

Tailings (product 4)

Stirred for 30s Cleaning, 3min

Tailings (product 3)

Tailings (product 2)

Clean coal (product 1)

Fig.2. Flowchart of progressive release flotation test.

3. Results and discussion 3.1. Modes of occurrence of arsenic in coal In this study, the enrichment degree of trace elements in coal is divided into 6 levels depended on the concentration coefficient (CC). CC is the ratio of the trace elements content in detected coal samples with average content of trace elements in Chinese coal. These 6 grades are as follows: CC> 100 as abnormal enrichment, 10 ion-exchangeable form (3.84%) > water soluble form (1.14%). There is a significant positive correlation between arsenic content and coal ash. Arsenic tends to concentrate in small-size and high-density products. Organic sulfur content is much greater than the content of inorganic sulfur, and arsenic in sulfide associated form is mainly occurred in the inorganic sulfide minerals (especially in pyrite). There is a good correlation between the removal rate of arsenic and clean coal ash in either gravity separation or flotation. Arsenic removal rate 57.96% and 70.77% could be obtained through gravity separation and flotation respectively. However, the removal rate of arsenic is limited by using physical method solely when considering the clean coal yield. To obtain a higher removal rate of arsenic and ensure the yield clean coal,

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developing a new approach combined physical and chemical processing methods like leaching-flotation or other forms should be a research focus in the field of trace element removal in the future.

Acknowledgements This research was supported by the National Key Basic Research Program of China (No. 2014CB238905), the National Natural Science Foundation of China (No. 51174205), the Program for New Century Excellent Talents in University (NCET-12-0962) and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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