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Removal of mercury from fine coal based on combined coal processing approaches Cheng Liu, Changchun Zhou, Longfei Cong, Ningning Zhang, Jin-he Pan, Mengcheng Tang, and Shan Shan Cao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02133 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017
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Removal of mercury from fine coal based on combined coal processing approaches Cheng Liu, Changchun Zhou*, Longfei Cong, Ningning Zhang, Jinhe Pan, Mengcheng Tang, Shanshan Cao Key Laboratory of Coal Processing & Efficient Utilization of Ministry of Education, School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou, Jiangsu 221116, China *Corresponding author. E-mail:
[email protected] (C.C Zhou). Fax number: +86 0516 83591066
Abstract Hg is one of the most harmful trace elements in coal. Removal of Hg from fine coal by combined coal processing approaches was investigated in this paper. Two flotation feed coal samples collected from FGZ and TX coal preparation plants in China were used as the research objects. FGZ coal was identified as high-sulfur coal while TX coal was ultra-low sulfur coal. Sequential chemical extraction was used to determine the modes of occurrence of Hg in coal samples. Two combined coal processing
methods,
namely
flotation-gravity
separation
process
and
flotation-chemical leaching process respectively, were used to obtain a reduction of Hg content in clean coal. The results indicate that the dominant mode of occurrence of Hg in FGZ coal is sulfide-bound form, while that in TX coal is un-leachable form. Dosages of collector and frother influence the removal of Hg in the flotation process and the addition of calcium oxide as depressant contribute to the Hg removal. However, simply using conventional flotation has a limitation in reducing the Hg 1
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content in clean coal. Flotation-gravity separation process has a good applicability in the treatment of FGZ coal in which Hg mainly associated with inorganic minerals and distributed unevenly in different density fractions, and clean coal with Hg content of 48.98µg/kg can be produced when the gravity separation density is 1.4 kg/m3. Flotation-chemical leaching process is effective in the reducing Hg content both in treating FGZ coal and TX coal. Through the latter advanced coal processing approach, relative removal rates of Hg of FGZ coal and TX coal can reach up to 61% and 66%, respectively, and clean coals with Hg content of 59.87µg /kg and 93µg /kg respectively can be obtained.
Key words: fine coal; Hg removal; flotation; gravity separation; leaching 1. Introduction In recent years, trace elements in coal has been a widely studied subject in the world as much attention had been put on the environmental problems caused by coal combustion1. Mercury, as one of the trace elements in coal, has been put in a significant place, owing to its adverse impacts on human health resulting from its volatility, permanent poisonousness and enrichment through the food chain after its discharge during combustion2-4. Coal burning had been reported to be the uppermost source of anthropogenic emission of Hg5-6. It is reported that 38% of the Hg emission in China derived from coal combustion notwithstanding the concentration of Hg in Chinese coal is low, with an average value of about 0.2µg /kg7. For environmental protection, a lot of work has been done for the removal of trace elements from coal, such as Hg
8-9
. The methods for removing trace elements 2
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from coal mainly include pre-combustion removal, in-combustion removal and post-combustion removal. Comparatively speaking, pre-combustion removal method, such as coal cleaning, is a low-cost method. In traditional coal clean process, coal preparation technology plays an important role in reducing ash and sulfur content in coal for its advantage of high-efficiency and low-cost. In addition, it is generally recognized that pyrite or other ash minerals are the main carrier of Hg in coal10-12. As for the low-S coal, the dominant forms of Hg are mainly sulfide-bound form and organic-bound form13. Therefore, the coal preparation technology can theoretically be employed as an effective method for the removal of Hg before coal combustion. A great deal of explorations and applications concerning on trace elements removal through coal cleaning processes have been done by many researchers14-16. Davidson et al.17 reviewed the effect of clean coal methods on the removal of trace elements. Akers et al.
18
researched the factors that influence the removal effect of
trace elements from coal by laboratory tests, including the liberation degree of the carrier materials of trace elements, relative intensity of cleaning, modes of occurrence of trace elements as well as coal cleaning methods. Luttrell et al.
19
and Mohanty et
al.20 used a joint process of froth flotation combined with enhanced gravity separation to reduce the trace elements content of coal in the pilot test. The results showed that more than 80% of the Hg and Se could be removed when the combustibles recovery was as high as 80%. Pan et al.21 and Feng et al. 22 studied the modes of occurrence of Hg in coal and its migration regularity during the coal cleaning process and concluded that most of mineral-forming Hg is potential to be removed. Furthermore, they also 3
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confirmed that the size of Hg-carrier materials is also an important factor that affects the removal degree of Hg as Hg would be more likely to be enriched in the fine size fractions of coal samples. However, remove limitation always exists during single conventional coal cleaning processing, especially for the removal of Hg15,23. Similar results concerning removal limitation were also found in our previous studies focused on the removal arsenic and fluorine in coal 24-25. Froth flotation and gravity separation are conventional methods used in the coal cleaning process, and chemical leaching is an advanced method which also plays an important role in mineral processing and coal cleaning. Studies showed that chemical leaching is often able to achieve the desired coal cleanliness effect when there is difficulty with conventional coal preparation methods14,26. Therefore, the hypothesis can be put forward that a combination of these methods may be efficient in reducing the Hg content in fine coal, from which clean coal with lower Hg content can be obtained and the Hg emission amount during coal burning can be decreased. Based on the above assumptions, two fine coal samples were used to carry out a series of removal tests, including flotation tests, flotation-gravity separation tests, flotation-chemical leaching tests. In addition, before the above tests, modes of occurrence of Hg, which will significantly influence the reduction of trace elements during coal cleaning processes, were investigated firstly.
2. Materials and methods 2.1 Coal samples Coal samples used in this study are the feed coal of flotation process collected 4
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from Fangezhuang (FGZ) coal preparation plant located in Hebei province and TaiXi (TX) coal preparation plant located in Ningxia province of China, which are totally different coal types. The feed coal of FGZ coal preparation plant come from Fangezhuang coal mine, and the coal-bearing strata belong to Carboniferous-Permian, and they are fat coal27. The feed coal of TX coal preparation plant come from Rujigou mining district in which the coal seams in the Jurassic Yanan Formation strata and all the coal are anthracite formed because of regional igneous thermal metamorphism28. Samples were collected carefully to avoid the loss of volatile mercury and then stored in a refrigerator before further treatment. According to the XRD analysis (shown in Figure 1), the mainly mineral composition in FGZ coal sample is clay minerals (kaolinite, halloysite and montmorillonite), quartz, and some carbonate minerals (calcite). The mainly composition of TX coal sample is quartz, and some kaolinite. And the basic characteristics of coal samples are listed in Table 1. The sulfur content of TX coal is only 0.17%, which is far lower than that of FGZ coal (1.08%) and belong to ultra-low sulfur coal. The Hg contents of two samples are 162.35µg/kg and 253.81µg/kg, respectively, both of which are greatly higher than the average value of the word coal29-30 and at an enrichment level of “significantly enriched” (10
