Article pubs.acs.org/EF
Fine Coal Desulfurization by Magnetic Separation and the Behavior of Sulfur Component Response in Microwave Energy Pretreatment Bo Zhang,*,†,‡ Yuemin Zhao,*,†,‡ Chenyang Zhou,†,‡ Chenlong Duan,†,‡ and Liang Dong†,‡ †
National Research Center of Coal Preparation and Purification, and ‡School of Chemical Engineering and Technology, China University of Mining and Technology, Xuzhou, Jiangsu 221116, People’s Republic of China ABSTRACT: Pyrite in fine coal was heated selectively by microwave energy to strengthen its magnetism in this study. Then, the inorganic sulfur in fine coal (−0.5 mm) was removed using a 2 T strong magnetic roller magnetic separator. The migrating regulation of organic sulfur-containing groups and harmful elements was analyzed by employing a microwave energy-intensive process. As treatment time increased, the infrared absorption peaks of the sulfur compound were as follows: swing vibration absorption peak of C−S in the −CH2SH structure, sulfone-based (OSO) asymmetric stretching vibration absorption peak, vibration absorption peak of sulfoxide (including the SO bond), and CS key vibration absorption peak. The magnetic susceptibility of Weinan and Xiaoyi coal increased as time decreased. The magnetic susceptibility then reached its optimal value at 4 min, at which point the dynamic balance of the transformation between pyrite, pyrrhotite, and meteorite pyrite was achieved. The maximum magnetic susceptibility was improved by 1 order of magnitude. The coal desulfurization rate and magnetic susceptibility of Weinan and Xiaoyi coal showed the same variation trend, with their optimal values emerging at 4 min. a microwave radiation environment.26−28 In addition, magnetic susceptibility showed a growing tendency under 650 W of microwave radiation. Microwave radiation is largely used in the large-scale processing of ilmenite combined with magnetic separation in Norway.29,30 The use of microwave radiation to pretreat fine coal involves a very complex physical and chemical process.31 Given that microwave energy could be applied for selective heating coal through different dielectric constants, the pure heating effect of coal is significantly lower than that of water, pyrite, and other inorganic minerals. However, when heat is transferred at the edge of inorganic and pure coal, the coal undergoes several changes in its organic structural behavior. Fourier transform infrared spectroscopy (FTIR) is one of the most common analysis technologies that can provide information on functional groups. It can also be used to obtain coal oxidation change information because infrared spectroscopy is sensitive to the oxygen group, which can be used to qualitatively and semi-quantitatively test the aromatic and aliphatic variation of C−H.31 Many studies have emphasized the use of microwave energy in drying, grinding, and desulfurization. Thus, the present study explores the use of microwave energy to deal with the fine coal chemical and physical response of organic linkers and inorganic minerals. However, determining the structure of this change is very difficult because of the complex structure and heterogeneity of coal. Many factors are involved in this process, and these factors does not wholly depend upon the chemical composition of the volatile content, grade, size, water, etc.
1. INTRODUCTION Coal consumption accounted for 70.48% of the primary energy consumption.1 In 2011, China’s raw coal production was 35.2 million tons, which accounted for 77.35% of the primary energy production.2 Coal is the main energy supply in China, and this situation will remain unchanged for a long time. Thus, efficient and clean usage of coal resources is significant. Coal preparation technology is a simple and effective method in improving the quality of coal. This method is presently being used and developed worldwide. Wet coal preparation technology currently dominates, but as the global coal mining gradually transfers its focus to the cold region and the growing water shortage,3 an efficient dry separation technology is urgently needed. Many scholars all over the world have studied fine coal separation. Research in this area has made significant progress in recent years, given the development of air dense medium fluidized bed,4−8 vibration fluidized beds,9,10 magnetically stabilized fluidized beds (MSFBs),11−14 friction electric separation,15,16 shallow bed air dense medium fluidized beds,17 jigging separators,18−21 and pulse dense medium fluidized beds.22 Generally, either the real part or the imaginary part of the dielectric constant is very small. Thus, we often use the ratio of the vacuum dielectric constant by itself. This constant is often called the relative dielectric constant.23−25The dielectric constant of coal was measured experimentally and found to decrease as the coal rank decreased. However, moisture and mineral compositions may lead to opposite results. Low-rank coal that contains a large amount of water was proven to possess a high dielectric constant. The relative dielectric constant of coal is greater than that of most compounds, except pyrite minerals. Hence, with an adequate quantity, it could increase the dielectric constant of coal. No significant change in the dielectric constant was observed in the range of the variable frequency testing. A large number of minerals, such as chalcopyrite, hematite, and wolframite, were easily heated in © XXXX American Chemical Society
Received: September 8, 2014 Revised: December 30, 2014
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DOI: 10.1021/ef502003g Energy Fuels XXXX, XXX, XXX−XXX
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2. MAGNETISM STRENGTHENING THEORY The dielectric constant is a complex number composed of a real part and an imaginary part. The relationship between the two parts is a loss angle tangent called the out phase angle of material in the electric field. The sum of the two parts is the composite dielectric constant. The loss angle is the ratio of the electric field and the composite dielectric constant. The real dielectric constant is considered to be the same phase in alternating current and the virtual polarizations on behalf of the phase lag in Figure 1. The real part of the virtual dielectric
Table 1. Experimental Coal Sample Basic Indicators coal sample
name of index
index
Weinan coal (WN)
type of coal size (mm) ash (%) sulfur (%) type of coal size (mm) ash (%) sulfur (%)
non-coking coal −0.5 29.58 3.96 coking coal −0.5 23.75 2.95
Xiaoyi coal (XY)
samples taken from Weinan and Xiaoyi had ash contents of 29.58 and 23.75%, respectively. The sulfur content of the coal samples was analyzed using a CTS3000B sulfur analyzer. Meanwhile, the mercury levels were tested using a Hydra II C automatic mercury measurement instrument. The harmful trace elements in the coal samples were analyzed through inductively coupled plasma mass spectrometry (ICP−MS), which is a destructive analysis method. At present, ICP−MS is used widely because it is one of the best methods for measuring microelements in coal. Its accuracy can reach 10−9 (part per billion). The test and analysis of the response of sulfur were performed via Fourier transform infrared spectroscopy (FTIR, Bruker, Vertex 80v). This method is widely used in the study of the coal structure, and its good characterization facilitates the identification of all kinds of functional groups in macromolecular structures. Pulverized coal magnetism after microwave processing was measured using a Guoy magnetic balance. To evaluate the methods for the desulfurization effect in the process of magnetic separation desulfurization, the best desulfurization test conditions should be determined. However, a unified desulfurization effect evaluation criterion is lacking at both home and abroad. In this study, we select the most common desulfurization effect evaluation criterion, i.e., the desulfurization rate of Sd (%)
Figure 1. Behavior of material within a microwave field:24 (a) microwave transparency, (b) microwave reflectance, (c) microwave absorption, and (d) microwave absorption and reflectance.
Md =
constant is equal to the real virtual dielectric constant. The loss angle tangent can be used to show the loss of energy in each field phase.24 Given that the dielectric loss factor of pyrite is higher than that of pure coal (excluding minerals), pyrite in coal could be pyrolyzed under selectivity microwave heating circumvents. Then, two new phases, pyrrhotite (Fe1−xS) and meteorite pyrite (FeS), can be generated. The composition of coal is very complicated, and its structure is highly homogeneous. In the process of brief microwave irradiation, the decomposition of pyrite in coal is not very consistent. The result is the coexistence of the 4C ultra structure of central Asia pyrrhotite, the ferromagnetic phase of Fe7S8, and other antiferromagnetic structures. This condition leads to the improvement of coal desulfurization by microwave irradiation. In summary, when microwave irradiation time is properly controlled, it can maximize Fe1−xS generation and, consequently, achieve the best available magnetic separation desulfurization effect. Intensity magnetic separation desulfurization through microwave pretreatment uses microwaves to heat pyrite in coal. The process can accelerate the pyrolysis of pyrite into pyrrhotite and meteorite pyrite, which could enhance the magnetism of pyrite. It is then removed through magnetic separation.
M r − McY × 100% Mr
where Md is the desulfurization rate (%), Mr is the lack of sulfur content in raw coal (%), Mc is the sulfur content in the plant after sorting (%), and Y is the cleaned coal production rate.
4. RESULTS AND DISCUSSION 4.1. Response of the Sulfur Function to Microwave Energy. FTIR is one of the most common analysis technologies for analyzing organic molecular structures. It can provide information about functional groups. The fine coal samples (−0.5 mm) from Weinan were tested using microwave devices with 1000 W and 2450 MHz. The microwave pretreatment times for the samples were 1, 2, 3, 4, 5, 6, 7, and 9 min. The weight of each sample was 100 g. The response patterns of the sulfur compounds in the coal samples after microwave strengthening were analyzed by FTIR (Bruker, Vertex 80v). The experiment was conducted using a KBr tablet, scanned 20 times with a resolution of 8 cm−1 in the measurement range of 4000−400 cm−1. The FTIR spectra of the response of the sulfur organic groups in different microwave pretreatment times are shown in Figure 2. As shown in Figure 2, the following four absorption peaks weakened as pretreatment time increased: the wagging vibration absorption peak of the C−S bond in −CH2SH at 1432 cm−1, the asymmetric stretching vibration absorption peak of sulfuryl (OSO) at 1373 and 1324 cm−1, the absorption peak at 1090−1200 cm−1 that is probably the vibration absorption peak of sulfoxide (>SO), and the vibration absorption peak of the CS bond at 1086 cm−1.
3. TEST MATERIAL AND CALCULATION METHODS In this study, −0.5 mm coal of Weinan and Xiaoyi was used as the experimental material. As shown in Table 1, the coal from Weinan was not sticky and had a sulfur content of 3.96%. It is thus categorized as high-sulfur coal. The coal from Xiaoyi was coking coal and had a sulfur content of 2.95%. It is categorized as middle- and high-sulfur coal. The B
DOI: 10.1021/ef502003g Energy Fuels XXXX, XXX, XXX−XXX
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Figure 2. FTIR of the response of the sulfur organic groups in different microwave pretreatment times.
The wagging vibration absorption peak of the C−S bond in the CH2SH structure decreased under two pretreatment times, but it did not decrease as time increased. As pretreatment time increased, the wagging vibration absorption peak of the C−S bond in the −CH2SH structure was reduced. These findings show the reaction of the C−S bond in the −CH2SH structure to microwave energy. As microwave energy treatment time increased, the vibration absorption peak of the CS bond (1086 cm −1) and the asymmetric stretching vibration absorption peak of sulfuryl (OSO) (1373 and 1324 cm−1) showed a significant decreasing trend. Thus, microwave energy had an evident effect on the CS bond and sulfuryl. The absorption peak at 1090−1200 cm−1 may be the vibration absorption peak of sulfoxide (>SO) and showed a weak response to the change in the pretreatment time. 4.2. Harmful Element Cleaning in the Microwave Strengthening Process. ICP−MS was used in this study to analyze the harmful trace elements in coal. It is one of the most common analysis technologies for the analysis of trace elements in coal. The precision of ICP−MS can reach 10−9. In this study, the effect of the microwave pretreatment time on the harmful trace elements in Weinan coal was assessed. The microwave pretreatment time was changed from 1 to 5 min. ICP−MS was used to study the response of U, Se, As, Cd, Mo, Co, V, Pb, Ni, Mn, and Cu after microwave pretreatment. The content of Hg was analyzed using a Hydra II C mercury analyzer. The study focused on the 12 trace elements mentioned above. Figure 3 shows the response patterns of the harmful elements in coal after microwave pretreatment. V, Pb, Mn, Ni, Cu, and As are harmful trace elements with content greater
than 100 ppt (Figure 3a). The content of Mn and Ni showed no response to the changing microwave pretreatment time. In contrast, the content of V, Pb, and C decreased with the change in the pretreatment time. After 4 min of microwave pretreatment, the content of Pb and Cu significantly decreased and the removal rate reached 45 and 26%, respectively. Under the same condition, the removal rate for V and As could only reach 11.66 and 4.62%, respectively. However, after 8 min of microwave pretreatment, the removal rate of V and As reached 51.58 and 49.68%, respectively. Figure 3b shows Hg, U, Se, Mo, Cd, and Co contents at less than 100 ppt. The microwave pretreatment time showed no significant effect on U. The content of Hg, Se, Mo, Cd, and Co decreased as the microwave pretreatment time increased. The content of Hg and Cd slightly decreased after 2 min of microwave pretreatment. When the pretreatment time was extended to 4 min, the removal rate for Hg and Cd reached 86.63 and 70.21%, respectively. A certain amount of Se and Mo could be removed under a microwave pretreatment time of 2 or 4 min. The content of U decreased when the pretreatment time reached 8 min. The content of V, Pb, Cu, As, Hg, Se, Mo, Cd, and Co decreased after 4 min of microwave pretreatment. In particular, the content of Pb, Cu, Hg, and Cd showed a significant decrease. The content of Mn, Ni, and U did not show a clear response to the microwave pretreatment. 4.3. Enhanced Magnetism by Microwave Treatment. The coal samples from Weinan and Xiaoyi were pretreated in a microwave for 1−6 min. A Guoy magnetic balance was used to measure the magnetic change after microwave pretreatment. Figure 4 shows the relationship between the pretreatment time C
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microwave pretreatment. The best pretreatment time appeared to be 3.5−4.5 min. Figure 5 shows the relationship between the
Figure 5. Relationship between the pretreatment time and the specific susceptibility of Xiaoyi coal (−0.5 mm) in different magnetic field intensities.
pretreatment time and the specific susceptibility of Xiaoyi coal (−0.5 mm) under different magnetic field intensities. Figure 5 shows the same trend as that for Weinan coal. After 4 min of microwave pretreatment, the dynamic balance between the variation of pyrite, pyrrhotite, and meteorite pyrite was reached. The magnetism reached its maximum value. Microwave pretreatment enhanced the specific susceptibility of the coal samples by 1 order of magnitude. 4.4. Magnetic Desulfurization Experiment. The pretreated coal samples from Weinan and Xiaoyi were desulfurizated using RE Roll. The magnetic strength was 2 T; the microwave power was 1000 W; and the frequency was 2450 MHz. The pretreatment times were 2, 3, 3.5, 4, 4.5, 5, 7, and 9 min. Figure 6 shows that the fine coal from Xiaoyi had a sulfur
Figure 3. Response patterns of the harmful elements in coal after microwave pretreatment.
Figure 6. Desulfurization effect of −0.5 mm Xiaoyi coal samples using RE Roll in different microwave pretreatment times.
Figure 4. Relationship between the pretreatment time and the specific susceptibility of Weinan coal (−0.5 mm) in different magnetic field intensities.
content of 2.96% and is thus categorized as middle- and highsulfur coal. As microwave pretreatment time increased, the desulfurization rate first increased and then decreased. It reached the maximum of 39.52% under a pretreatment time of 4 min, during which the sulfur content was 2.02%.
and the specific susceptibility of Weinan coal (−0.5 mm) under different magnetic field intensities. As pretreatment time increased, the specific susceptibility of the Weinan coal samples first showed an increasing trend and then decreased. The specific susceptibility reached its maximum value at 4 min of D
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Figure 7 shows the RE Roll separation result of the Weinan coal samples subjected to different microwave pretreatment
Article
AUTHOR INFORMATION
Corresponding Authors
*Telephone: +86-516-83590092. E-mail: zhangbocumt@126. com. *Telephone: +86-516-83590092. E-mail: ymzhao_paper@126. com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research work involved in this paper received the financial support by the National Fundamental Research and Development Program (973 Program) (2012CB214904), the National Natural Science Foundation of China for Innovative Research Group (51221462), the Fundamental Research Funds for the Central Universities (2014XT05), the National Natural Science Foundation of China (51134022, 91434133, and 51304196), the Specialized Research Fund for the Doctoral Program of Higher Education (20120095130001), the Research and Innovation Project for College Graduates of Jiangsu Province, China (XYLX_1407), and a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
Figure 7. Desulfurization effect of −0.5 mm Weinan coal samples using RE Roll in different microwave pretreatment times.
times. The coal from Weinan had a sulfur content of 3.96% and is thus categorized as high-sulfur coal. The relationship between the desulfurization rate and microwave pretreatment time was the same as that described above. The best results were a desulfurization rate of 32.01% and a sulfur content of 2.92% in 4 min of microwave pretreatment time. After desulfurization, Weinan coal became a middle- and high-sulfur coal, while Xiaoyi coal became a middle-sulfur coal.
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5. CONCLUSION In this study, pyrite in fine coal was heated selectively by microwave energy to strengthen its magnetism. The inorganic sulfur in fine coal (−0.5 mm) was removed using a 2 T strong magnetic roller/magnetic separator. The migrating regulation of organic sulfur-containing groups and harmful elements was analyzed by employing a microwave energy-intensive process. As treatment time increased, the infrared absorption peaks of the sulfur compound were as follows: swing vibration absorption peak of C−S in the −CH2SH structure, sulfonebased (OSO) asymmetric stretching vibration absorption peak, vibration absorption peak of sulfoxide (including the S O bond), and CS key vibration absorption peak. The magnetic susceptibility of Weinan and Xiaoyi coal increased as time decreased. The magnetic susceptibility then reached its optimal value at 4 min, at which point the dynamic balance of the transformation between pyrite, pyrrhotite, and meteorite pyrite was achieved. The maximum magnetic susceptibility was improved by 1 order of magnitude. The coal desulfurization rate and magnetic susceptibility of Weinan and Xiaoyi coal showed the same variation trend, with their optimal values emerging at approximately 4 min. In the magnetic separation desulfurization experiment, the sulfur content of Weinan coal decreased from 3.95 to 2.92%. The desulfurization rate reached 32.01%. The sulfur content of Xiaoyi coal decreased from 2.96 to 2.02%. Thus, microwave-enhanced magnetic separation turned the fine coal from Weinan into middle- to high-sulfur coal and the fine coal from Xiaoyi into low- to middle-sulfur coal. E
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