Clean Coal Desulfurization Pretreatment ... - ACS Publications

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Clean Coal Desulfurization Pretreatment: Microwave Magnetic Separation, Response Surface, and Pyrite Magnetic Strengthen Bo Zhang,*,†,§ Zhijun Ma,*,‡ Guangqing Zhu,†,‡,§ Guanghui Yan,†,§ and Chenyang Zhou†,§ †

Key Laboratory of Coal Processing and Efficient Utilization of Ministry of Education, China University of Mining & Technology, Xuzhou, Jiangsu, 221116, China ‡ School of Mining, Liaoning Technology University, Fuxin, 123000, China § School of Chemical Engineering and Technology, China University of Mining & Technology, Xuzhou, Jiangsu, 221116, China ABSTRACT: The throughput of pyrite extraction from coal using the conventional magnetic separation method can be effectively improved by increasing the magnetic susceptibility of pyrite. The electromagnetic characteristics and absorption response of fine high-sulfur coal were studied to evaluate the strengthening effect of microwave treatment on the magnetic susceptibility of pyrite. In actual production, the microwave treated pyrite was affected by oxygen and water vapor in the air. Magnetism of fine coal stored under room temperature in air for long term will become weaker, due to the formation of pyrrhotite. Also, the pyrite is not stable in air during the microwave pretreatment process. Due to the porosity of fine coal, when it is placed in air, oxidation reaction occurs between a small amount of oxygen and coal pyrite. This is the reason for the decrease in the specific magnetic susceptibility and the weakening of the magnetic behavior of fine coal. Analysis of different sputtering times indicates the different depths from the surface. During the microwave treatment, pyrite Fe2(SO4)3 and ferrous sulfate FeSO4 are mainly present on the surface of pyrite, and surface oxidation takes place. The relative content of Fe2(SO4)3 in the treated coal is higher than that of FeSO4, which indicated that the study proposed a novel way for the coal desulfurization pretreatment.

1. INTRODUCTION Coal is the world’s second largest energy resource and the primary energy resource in China.1 Globally, wet coal preparation technology is mainly used to prepare the coal. However, due to recent regulations in China, the new coal mining areas have been transferred to cold and arid regions,2 and over two-thirds of the coal resources are now scattered all over the drought-hit region of western China.3 Therefore, it is urgent to study and develop efficient dry coal separation techniques. The existing dry separation technology mainly includes wind jigging machines,4−6 compound dry separation technology,7−9 and dry dense medium fluidized bed technology.10−12 At present, desulfurization and separation methods for fine coal have become an important research area in dry coal preparation technology. The complicated abrasion of magnetite particles and the formation of fine particles during these processes can be analyzed by studying the abrasion model and particle size distribution of the magnetite particles.13 Chemical and biological desulfurization technologies can remove inorganic sulfur and organic sulfur impurities in coal, but problems such as high cost, long production cycles, and pollution still remain to be addressed. While physical desulfurization methods can only remove inorganic sulfur, they are economical and practical in nature.14,15 High gradient magnetic separation technology is an effective method for physical desulfurization, which can be used to deal with fine particles of paramagnetic minerals and has been applied to the desulfurization of fine coal (mainly pyrite).16,17 The magnetic properties of pyrite can be improved by heat treatment.18 Compared with coal, pyrite can be heated more rapidly due to its higher dielectric constant.19 Microwave radiation is an © XXXX American Chemical Society

effective technique for heat treatment and has the advantages of rapid heating, selective heating, and uniform heat distribution.20 The magnetic properties of the pyrite in coal are enhanced after heat treatment, which would be more favorable for the magnetic separation technology.21−23 For low rank coal (lignite), microwave heat treatment can significantly reduce the water content, and increase its calorific value and fixed carbon content, hence improving the quality of the lignite energy.24,25 Additionally, microwave heat treatment can also be used as a follow-up treatment step to the frictional electrical selection operation.26,27 Therefore, a different coal sample was chosen to study the electromagnetic characteristics and absorption response of fine high-sulfur coal. The strengthening effect of microwave treatment on the magnetic susceptibility of pyrite was also evaluated. In addition, Fourier transform infrared spectroscopy (FTIR) was used to investigate the strengthening process of pyrite in the coal sample.

2. MATERIALS AND METHODS Microwave pretreatment of the fine coal studied in this paper was performed in a nitrogen/air processing system, as shown in Figure 1. The system consists of a nitrogen protective device, microwave treatment device, nitrogen controller, and nitrogen storage device. A microwave frequency of 2.45 GHz was set for the treatment, with a continuously adjustable power ranging from 0 to 1000 W. A CTP-II Gouy magnetic balance was used for the magnetic susceptibility determination of paramagnetic and diamagnetic mineral samples. The system mainly consists of a control system, electromagnet, magnetic Received: November 15, 2017 Revised: January 17, 2018 Published: January 23, 2018 A

DOI: 10.1021/acs.energyfuels.7b03561 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Microwave processing system schematic diagram. measuring device, Holzer sample tube, and electronic analytical balance. The system is divided into a magnetic control system, electronic balance, and electromagnetic testing system. The chemical structure of the sample was analyzed using a Fourier transform infrared spectrometer (FTIR, VERTEX 80 V, Bruker, Germany), with a high spectral resolution reaching 0.06 cm−1. The resolution is continuously adjustable to full-wave band. Table 1 shows the numerical analysis for

Table 1. Morphological Analysis of the Sulfur of the Coal Samples form of sulfur no.

row coal

St.d. %

Sp.d. %

Ss.d. %

So.d. %

1 2 3 4

Xin’an Xiaoyi Yiluo Mengdong

1.57 2.96 2.18 3.09

0.60 1.05 0.32 1.24

0.03 0.29 0.02 0.28

0.94 1.62 1.84 1.56

the sulfur in four kinds of the test samples. Four kinds of coal samples are different in coal type classification, sulfur content (coal classification), and pyrite sulfur content: Xin’an (lean coal, medium sulfur coal), Yiluo (medium sticky coal, medium high sulfur coal), Xiaoyi (weak sticky coal, medium high sulfur coal), and Mengdong (brown coal, high-sulfur coal). The pyrite sulfur content of Yiluo was 0.32%, which is low; the pyrite sulfur content of Xiaoyi was 1.05%, which is high.

3. RESULTS AND DISCUSSION 3.1. Microwave Strengthened Magnetic Behavior of Fine Coal. 3.1.1. Strengthening Effect of Microwave on High-Sulfur Coal Magnetism. The magnetic change tests for microwave energy processing of Xiaoyi coal and Mengdong coal (Mengdong lignite) were performed at different times using a Guoy magnetic balance analysis of −0.5 mm fine coal. The microwave processing conditions were microwave power of 1000 W, frequency of 2.45 GHz, and processing times of 1, 2, 3, 4, 5, and 6 min. Figure 2 shows the variation trend of the specific magnetic susceptibility of Xiaoyi fine coal with different magnetic field intensities. Similar to the behavior observed for the Weinan coal, the magnetization of the coal sample increased first and then decreased with the increase in

Figure 2. Magnetic susceptibility of Xiaoyi and Mengdong −0.5 mm coal strengthened by microwave.

processing time. The best result was achieved at 4 min (3.5− 4.5 min), which indicates that the microwave time required for the pyrite and pyrrhotite to reach dynamic equilibrium with troilite is about 4 min. The specific magnetic susceptibility of B


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Energy & Fuels pulverized coal is 1 order of magnitude higher than that of the original coal. The magnetic susceptibility of Xiaoyi fine coal is not in the same order of magnitude because of the different coal pyrite formation process, and a variety of mixtures containing pyrite and pyrrhotite. The magnetic susceptibility of coal pyrite pyrrhotite is much higher than that of pyrite. As shown in Figure 2b, the variation of specific magnetic susceptibility at different microwave treatment times for the Mengdong lignite is almost the same as the overall trend for the Xiaoyi coal. The magnetic susceptibility of Mengdong coal also shows an initial increase, followed by decrease, and it is higher than that of the Xiaoyi coal. The specific magnetic susceptibility of Mengdong lignite was in the order of 10−9, higher than that of Xiaoyi coal. The highest value of specific magnetic susceptibility increased by nearly 50% after the microwave treatment. The total sulfur lignite was 3.09%, and 1.24% of sulfur was present in the form of pyrite. The specific magnetic susceptibility of pyrite in coal increased by about 50 times, and this significant improvement in the magnetic susceptibility of pyrite can effectively improve the removal of inorganic sulfur. The specific magnetic susceptibility of high-sulfur coal is related to its pyrite content and magnetic susceptibility of pyrite in the coal sample. The magnetic susceptibility of the two kinds of high-sulfur coal follows the order of Mengdong lignite > Xiaoyi coal. The highest value of the specific magnetic susceptibility after microwave treatment is the same as that of the raw coal. After the most optimum microwave treatment time, the sensitivity of magnetic susceptibility to microwave treatment time was positively correlated with the content of pyrite in coal. 3.1.2. Strengthening Effect of Microwave on Medium High Sulfur Coal Magnetism. The strengthening effect of microwave was evaluated on two kinds of medium high sulfur coal samples, Xin’an and Yiluo coal. Magnetic properties of −0.5 mm fine coal powder were analyzed by a Guoy magnetic balance at different times. The microwave power was 1000 W, with a frequency of 2.45 GHz, and the processing times were 1, 2, 3, 4, 5, and 6 min. Figure 3 shows the variation in magnetic susceptibility of the Xin’an and Yiluo sulfur coal under different microwave processing times. With the increase in microwave treatment time, the specific magnetic susceptibility increased slowly and then decreased rapidly. At the same time, the increase range and the ratio were far less than those of the two kinds of high-sulfur coal, and only increased by about 20%. The optimum microwave treatment time for the Xin’an and Yiluo coal samples was 2 min, and lower than that for the previous four kinds of high-sulfur coal. The specific magnetic susceptibility values of Xin’an and Yiluo coal were in the order of 10−10, the total sulfur content was 1.57% and 2.18%, and the amount of sulfur present as pyrite was 0.60% and 0.32%, respectively. The magnetic susceptibility of Yiluo coal is higher than that of the Xin’an coal. When the Xin’an and Yiluo coal samples were heated for longer than the optimum microwave processing time, their magnetization rate decreased rapidly, and their magnetic susceptibility became far lower than that of raw coal. 3.2. Weak Magnetic Behavior of Fine Coal Particles in Air (Oxygen and Water Vapor). In actual production, the pyrite after microwave pretreatment was affected by oxygen and water vapor in the air, so it is necessary to consider the effects of various factors on the magnetic enhancement after microwave pretreatment. The variation of specific magnetic susceptibility of fine coal in air at room temperature was studied

Figure 3. Magnetic susceptibility of Yiluo and Xin’an −0.5 mm coal strengthened by microwave.

and the weakening magnetism of high-sulfur coal was investigated. The optimum microwave time selected for each fine coal sample was 3, 4, and 2 min. The specific magnetic susceptibility measurement time of fine coal was set to 6, 12, 24, 48, 72, 96, 120, and 144 h. The measured intensities were 300 and 350 mT. Figure 4 shows the variation of the magnetic susceptibility of Xiaoyi and Mengdong coal samples in the air (oxygen and water vapor) at room temperature at different times. The magnetic susceptibility values of Yiluo fine coal and Mengdong lignite were reduced by about 30% within the first 6 h. The magnetic susceptibility of Yiluo coal decreased faster than that of Mengdong coal. The results show that the magnetic properties of the fine coal particles become weaker when they are placed in air (oxygen and water vapor) at room temperature for a long time, which is due to the instability of pyrite. Due to the presence of small voids or pores in the fine coal, the coal pyrite gets oxidized due to the oxygen and water vapor in air, resulting in the formation of a small amount of pyrrhotite. This was the reason for the decrease in the specific magnetic susceptibility and the weakening of the magnetic behavior of fine coal. Figure 5 shows the variation of magnetic susceptibility in high-sulfur fine coal and Yiluo coal at room temperature in air C


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Figure 5. Variation of magnetic susceptibility of Yiluo and Xin’an fine coal with different times.

Figure 4. Variation of magnetic susceptibility of Xiaoyi and Mengdong fine coal with different times.

surface oxidation of pyrite. XPS ion beam sputter depth profiling technology was used for analysis of the elemental contents and surface chemical state after stripping. Moreover, the element distribution along the depth direction, the element chemical environment, and chemical binding energy were also determined. The smooth fresh surface of Mengdong pyrite was used as the depth analysis object. The microwave pretreatment time was 180 s, the microwave power was 1000 W, and the microwave frequency was 2.45 GHz. The area of depth profiling was 4.36 mm × 4.36 mm, ion sputtering intensity was 2000 eV, moderate intensity, and the sputtering time was 90 s. Different sputtering times were used to represent different surface depths. The mechanism of surface oxidation and the distribution of chemical elements during the microwave treatment of pyrite were thus obtained. Figures 6 and 7 show the broad spectrum XPS spectrum of the Mengdong pyrite after microwave treatment and the corresponding depth analysis of the XPS spectra of the narrow spectrum of S element, respectively. The different sputtering times in Figures 6 and 7 are 0, 90, 180, 270, 360, 450, and 540 s. With the increase in sputtering time, the peak of O 1s shifted to lower binding energy, while the C 1s peak decreased significantly. The peak intensity of the Fe 2p peak at 0 s sputtering time was low,

(oxygen and water vapor) at different times. The magnetic susceptibility values of Yiluo coal and Xin’an coal showed an overall decreasing trend with the increase in storage time. However, the decreasing speed was slowed down, and at around 72 h, the magnetic susceptibility was unchanged or slightly increased. However, compared to the other two kinds of coal, the magnetic susceptibility values of Yiluo coal and Xin’an coal decreased only a little, and the different stages were entered earlier. Due to the small content of two kinds of pyrite in coal, there is less conversion of the magnetic pyrrhotite in the microwave. The pyrite and pyrrhotite produced during the microwave pretreatment in the air (oxygen and water vapor) are not stable, due to some amount of porosity in fine coal. The decrease in specific magnetic susceptibility caused by the reaction between oxygen and pyrrhotite in air (oxygen and water vapor) is also related to the trend of magnetic enhancement in microwave pretreatment. The magnetic susceptibility of Yiluo coal which was placed in air for a long time decreased by 15% and that of Xin’an coal decreased by 10%. 3.3. Mechanism of Microwave Enhanced Surface Oxidation of Coal Pyrite. Microwave pretreatment was used to strengthen the magnetism of coal pyrite, leading to D


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Figure 6. Analysis of wide spectrum of XPS spectra of Mengdong pyrite after microwave treatment.

Figure 7. Analysis of narrow spectrum of XPS spectra of Mengdong pyrite after microwave treatment.

2p3/2 Scan D peak is 161.78 eV, from S2− (161.8 + 0.1 eV), and the compound is FeS. There may also be a small amount of pyrrhotite Fe2S (162.2 + 0.4 eV). After microwave treatment, mainly Fe2(SO4)3 and FeSO4 are present on the surface of coal pyrite (Figure 8) due to the surface oxidation in the microwave process. The characteristic peak intensity of Fe2(SO4)3 on the surface of coal pyrite is stronger than that of FeSO4 peak, which indicates that the relative content of Fe2(SO4)3 is higher than that of FeSO4. With the increase in ion sputtering time stripping, the relative contents of ferric sulfate Fe2(SO4)3 and ferrous sulfate FeSO4 were significantly reduced, with the relative content of the latter being higher than that of the former. When the sputtering depth increased, FeS2 and FeS appeared with similar contents. The peak intensity of FeS2 was lower than that of pyrite. 3.4. Infrared Spectrum Analysis of Coal Series Pyrite Strengthening Process. Fourier transform infrared spectroscopy (FTIR) was used to investigate the strengthening process of coal series pyrite using the KBr tablet method. The test conditions were 20 scans, resolution of 8 cm−1, and

while there were small changes in the other peaks. The S element had the most obvious changes in the peak position, so a narrow sweep depth analysis of the S element was performed, and the sputtering times of 0 and 450 s were chosen. Figure 8 shows the depth analysis of coal pyrite after microwave treatment, in which the 0 s spectrum of the S element is used to fit the XPS spectrum. According to Figure 8, we can get the fitting results of the two S 2p3/2 peaks. In the Scan A XPS spectrum, the binding energy of the S 2p3/2 for the S element is 167.28 eV, which is attributed to SO42− (167.4 + 0.2 eV), and the chemical state is FeSO4. In the Scan B spectrum, the S 2p3/2 peak is at 169.03 eV, which is from SO42− (169.1 + 0.1 eV), and the chemical state is Fe2(SO4)3. According to Figure 9, we can get the fitting results of the four S 2p3/2 peaks. The binding energy of S 2p3/2 Scan A XPS of the S element in the peak is 167.38 eV, from SO42− sulfate (167.4 + 0.1 eV), and the chemical state is FeSO4; the S 2p3/2 Scan B peak is at 169.18 eV, from Fe2(SO4)3 (169.1 + 0.1 eV) SO42+; the S 2p3/2 Scan C peak is at 162.98 eV FeS2, from pyrite (163.1 + 0.2 eV); and finally, the binding energy of the S E


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absorption peak. Especially after 60 s, the peak intensity decreased obviously, which indicates that, during the microwave treatment process, the content of FeS2 decreased with the increase in microwave time. When the microwave treatment is continued further than 60 s, the peak intensities decreased faster. By analyzing the infrared fingerprint area shown in Figure 10, it can be seen that the peaks for ferric oxide (Fe2O3) at 1086 and 1047 cm−1 are overlapping. The absorption peak at 796 cm−1 is characteristic of the ferric oxide (Fe2O3) and attributed to stretching bending vibration. The absorption peak was found to contain a certain amount of Fe2O3 by examining the characteristic peaks of 1086 and 796 cm−1 in the raw samples. With the increase in microwave time, the absorption peak intensities were increased to different extents, which shows that there is a certain content of Fe2O3 in the raw untreated samples. In the microwave treatment process, Fe2O3 was not generated before the microwave time of 60 s. Between the microwave times of 60−90 s, Fe2O3 began to be generated and the production of Fe2O3 was the highest in this region. In the time range of 90−180 s, Fe2O3 continued to be produced. The characteristic peaks of pyrrhotite are at 469 and 537 cm−1, where 469 cm−1 is the absorption peak of flexural vibration, and 537 cm−1 is the peak of flexural vibration absorption. These two characteristic peaks of pyrrhotite increased obviously at 90 s, indicating the formation of pyrrhotite from 60 to 90 s interval. The 537 cm−1 absorption band is wide, and is composed of the characteristic absorption peak of 565 cm−1 of magnetite and 540 cm−1 absorption peaks of pyrrhotite. The intensity of absorption peaks was significantly increased for the treatment times of 90 and 180 s, indicating a certain amount of magnetite formation during the reaction.

Figure 8. Analysis of narrow spectrum of peak fitting points XPS spectra of S after microwave treatment (0 s) (S 2p3/2 Scan A: FeSO4, Scan B: Fe2(SO4)3).

4. CONCLUSIONS 1. Magnetism of fine coal can become weaker when it is stored at room temperature under air for a long time. This is due to the formation of pyrrhotite as the pyrite is not stable in the air during the microwave pretreatment process. As fine coal has a certain degree of porosity, when placed in the air, oxidation reaction occurs between a small amount of oxygen and the coal pyrite to give pyrrhotite. This is the reason for the decrease in the specific magnetic susceptibility and the weakening of the magnetic behavior of fine coal. 2. Compared with the Xiaoyi and Mengdong coal samples, the magnetic susceptibility values of Yiluo coal and Xin’an coal decreased slightly, and entered different stages earlier. Due to the small amount of pyrite in these two kinds of coal, there was less conversion of pyrrhotite during the process of microwave enhanced magnetism. The specific magnetic susceptibility caused by the reaction of oxygen and pyrrhotite in the air was decreased, which was related to the trend of magnetic enhancement in microwave pretreatment. 3. Analysis of different sputtering times represents the different depths from the surface. During the microwave treatment, pyrite Fe2(SO4)3 and ferrous sulfate FeSO4 were mainly on the surface of pyrite and surface oxidation occurred. The relative content of Fe2(SO4)3 was higher than that of FeSO4. With the increase in ion stripping time, the relative content of ferrous sulfate Fe2(SO4)3 and ferrous sulfate FeSO4 decreased ob-

Figure 9. Analysis of narrow spectrum of peak fitting points XPS spectra of S after microwave treatment (450 s) (S 2p3/2 Scan A: FeSO4, Scan B: Fe2(SO4)3, Scan C: FeS2, Scan D: S2−and Fe2S).

measurement range from 4000 to 400 cm−1. The Mengdong coal pyrite was selected as the simulated mineral in fine coal. As the microwave energy absorption ability of pyrite is strong, it is not consistent with the microwave time that was used for the microwave magnetic testing of fine grain coal. Microwave times of 60, 90, and 180 s were used in this analysis, but the other microwave processing conditions were unchanged. The microwave power was 1000 W, the microwave frequency was 2.45 GHz, and the content of pyrite was 70%. The microwave reaction was performed as per the above conditions with air as the atmosphere, and FTIR was used to analyze the reaction process and response of the untreated and treated samples (at 60, 90, and 120 s). The most common oxide minerals show IR absorption bands between 4000 and 400 cm−1, while the sulfide absorption band is located in a long wavenumber region of 400−0 m−1. The infrared spectrum is divided into the feature area (4000−1400 cm−1) and the fingerprint region (1400−666 cm−1). Figure 10 shows the FTIR spectra after the microwave pretreatment of Mengdong coal pyrite. The overall absorption peak intensity showed a decreasing trend in the vicinity of 420 cm−1 (FeS2), which corresponds to the stretching vibration F


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Figure 10. FTIR spectra of Mengdong pyrite after microwave pretreatment. (Pyrite content: 70%, microwave conditions: 1000 W, 2.45 GHz, microwave time sequence of 0, 60, 90, and 180 s.)

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ACKNOWLEDGMENTS The authors acknowledge the financial support by the Fundamental Research Funds for Central Universities (Grant 2017XKZD04) and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

viously. The relative content of ferrous sulfate FeSO4 was greater than that of ferric sulfate Fe2(SO4)3. Pyrite FeS2 and ferrous sulfide FeS appeared with further increase in sputtering time. With the increase in the sputtering depth, the content of these two products remained unchanged. 4. Through the investigation of the strengthening process of coal pyrite by FTIR, it came to light that untreated pyrite contained a certain amount of Fe2O3, which was not formed during the first 60 s of microwave time. Between the microwave time of 60−90 s, Fe2O3 began to be generated, and the generated quantity was the largest. At the same time, in the 90−180 s region, Fe2O3 continued to be produced. There was an evident increase in the amount of pyrrhotite in 90 s, which shows that the formation time of pyrrhotite was between 60 and 90 s. The absorption peak intensity of the samples at 90 and 180 s increased obviously, which indicated that there was a certain amount of magnetite formation in the reaction process.

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AUTHOR INFORMATION

Corresponding Authors

*(B.Z.) Telephone: +86-516-83591102. E-mail: [email protected]. * ( Z . M . ) T e l e p h o n e : + 8 6 - 51 6 - 8 3 5 9 1 1 0 2 . E - m a i l : [email protected]. ORCID

Bo Zhang: 0000-0001-7963-4572 Notes

The authors declare no competing financial interest. G


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