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Enhanced desulfurizing flotation of different size fractions of high sulfur coal using sonoelectrochemical method
Fuel Processing Technology 97 (2012) 9–14
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Enhanced desulfurizing flotation of different size fractions of high sulfur coal using sonoelectrochemical method Hong-Xi Zhang a,⁎, Hong-Jin Bai a, Xian-Shu Dong b, Zhi-Zhong Wang c a b c
College of Life Science, Tarim University, Alaer 843300, Xinjiang, China College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China College of Chemistry & Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
a r t i c l e
i n f o
Article history: Received 6 June 2011 Received in revised form 5 December 2011 Accepted 8 January 2012 Available online 3 February 2012 Keywords: Sonoelectrochemistry Sonocavitation Enhanced flotation Desulfurization
a b s t r a c t Enhanced desulfurizing flotation of different size fractions of high sulfur coal was investigated using the sonoelectrochemical method. The supporting electrolyte used in this process was calcium hydroxide and the additive was anhydrous ethanol. The effects of treatment conditions on desulfurization were studied by a single-factor method. The conditions include anhydrous ethanol concentration, sonoelectrolytic time, current density, and ultrasound intensity. For the coal samples with different size fractions, the optimal experimental conditions for anhydrous ethanol concentration, sonoelectrolytic time, current density, and ultrasound intensity, respectively, are achieved. Under optimal conditions, the raw and treated coals were analyzed by infrared spectroscopy and a chemical method. Pyritic sulfur, organic sulfur, and ash are partially removed. Compared with different size fractions coal, desulfurizing flotation of high sulfur coal by sonoelectrochemistry is an effective technology. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction The increasing demand for coal has led to more extensive mining of coal seams, so that the mined coal has high sulfur content. Reducing sulfur content in coal prior to its use in many applications is essential. To this end, researchers have focused on searching for simple, economic, and efficient desulfurization methods as this has become an issue of worldwide concern. The process of enhanced desulfurizing flotation of coal using electrochemistry involves simple technology; this process has been reported in several published articles [1–5]. The method efficiently removes pyritic and organic sulfur, as well as ash. Some studies have reported a form of energy, ultrasound waves, that has been increasingly applied in desulfurizing flotation [6–9]. Ultrasound can enhance clean coal yield and promote the separation of pyrite in high sulfur coal, factors that are favorable for flotation desulfurization. The combination of electrochemical flotation and ultrasonic flotation methods presents promising prospects for effective desulfurization of coal. In recent years, the integration of electrochemistry and sonochemistry has given rise to an interdisciplinary method called sonoelectrochemistry. Research in this area has attracted considerable attention, and some of the advantages reported in literature include the improvement of electrochemical progress while obtaining results superior to those obtained by electrochemistry alone [10–12].
⁎ Corresponding author. Tel.: + 86 997 4681613; fax: + 86 997 4681612. E-mail address: [email protected] (H.-X. Zhang).
Studies on a variety of areas including electroanalysis, electroplating, electroorganic synthesis, electropolymerization, and pollutant degradation in emulsion have used sonoelectrochemical methods. However, few reports on enhanced desulfurizing flotation of coal by sonoelectrochemistry have been published. We believe that research on the feasibility of such application is necessary. The combination of ultrasound technology and electrochemistry has also been extensively applied in the industry, leading us to expect that desulfurizing flotation of high sulfur coal via sonoelectrochemistry will find potential industrial applications.
2. Experimental 2.1. Materials and reagents High sulfur coal from the Tunlan coal plant in Shanxi, China was used for the experiment. By screening, the coal was separated into samples of different size fractions. The date at which proximate analysis was conducted and the sulfur content of the different size fractions of coal are listed in Table 1. In the present study, first of all, coal with size fraction of 0.125–0.2 mm was chosen as the experimental coal sample. According to previous experimental experience, the concentration of slurry and supporting electrolyte were 96 g L − 1 and 2.0 g L − 1, respectively. Other reagents used in this experiment were calcium hydroxide (AR grade), anhydrous ethanol (AR grade), kerosene as the flotation reagent (Chemical Pure, collector), and sec-n-octyl alcohol (CP, frother).
0378-3820/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.01.005
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H.-X. Zhang et al. / Fuel Processing Technology 97 (2012) 9–14
Table 1 Proximate analysis and sulfur content (W%, ad) of Tunlan coal. Granulometry
− 0.076 mm 0.076-0.125 mm 0.125–0.2 mm 0.2–0.5 mm
Weight
Proximate analysis
(%)
Moisture
Ash
Volatile matter
Total
Sulfur content Pyritic
Sulfate
Organic
(value, kJ/kg)
17.69 22.87 16.86 42.58
1.56 1.35 1.46 1.32
22.35 19.93 18.97 18.66
18.16 20.85 19.08 16.87
6.12 5.26 4.14 3.03
3.53 3.19 1.89 1.36
1.08 0.75 0.50 0.33
1.51 1.32 1.75 1.34
25022 26510 28269 28179
y1 y2
2.2. Instruments Electrolytic power was provided by a DH1722 DC-regulated power supply. Graphite (97.4 cm 2) and stainless steel (180.3 cm 2) were used as the anode and cathode, respectively; the electrolytic trough was set at 300 mL cup. The experimental scheme is presented in Fig. 1, and the flotation process and conditions are shown in Fig. 2. Other equipment used in the study are an SK250HP ultrasonic cleaner (59 kHz, 250 W, ultrasonic irradiation area: 203.3 cm 2), XFGC-80 flotation cell, HXZ-S3 sulfur determinator, and Shimadzu FTIR8400S (KBr:coal = 200:1). 2.3. Experimental method Specific amounts of supporting electrolyte, additive, and coal sample were placed in the electrolytic trough, to which distilled water was added to obtain a volume of 250 mL. Under a certain current density, ultrasonic intensity, and agitation rate (300 rpm), the slurry was subjected to sonoelectrolysis for a preset duration at room temperature. Subsequently, the slurry was immediately transferred to the flotation column. The floating coal was washed, dried in vacuum at 80 °C for 4 h, and then subjected to sulfur determination. The calculations of sulfur and ash reduction are expressed as [13] sulfur reduction ðwt:%Þ ¼ 100½x1 −x2 ðm2 =m1 Þ=x1
ð1Þ
ash reduction ðwt:%Þ ¼ 100½y1 −y2 ðm2 =m1 Þ=y1
ð2Þ
Calorific
is the ash percentage in the original sample, and denotes the ash percentage in the coal obtained from leaching.
3. Results and discussion 3.1. Effect of sonoelectrolytic conditions 3.1.1. Effect of ethanol concentration on sulfur reduction The effects of sulfur reduction, clean coal yield and sulfur content plotted against ethanol concentration are presented in Fig. 3. With increasing ethanol concentration, sulfur content initially exhibits a gradual reduction to a minimum, and then gradually increases; the sulfur reduction rate rapidly increases to a maximum, and then rapidly declines. Before the ethanol concentration reaches 1.0 mol/L, the clean coal yield exhibits a gradual fluctuation. This phenomenon can be attributed to the presence of specific numbers of ethanol molecules, which could have a function of desulfurization, resulted in the reduction of sulfur content in the clean coal using enhanced desulfurizing flotation by sonoelectrochemistry. However, as ethanol concentration reaches 1.0 mol/L, the wettability of the coal surface is enhanced and the clean coal yield begins to rapidly increase. The hydrophilicity of pyrite be weakened and resulted in part of pyrite sulfur transfer to the clean coal by flotation. Therefore, the sulfur content begins to gradually increase; and the sulfur reduction rate begins to rapidly decline. Therefore, the optimal ethanol concentration is 1.0 mol/L. These results indicate that ethanol concentration has a significant effect on sulfur reduction, in which a specific amount of ethanol proves favorable for enhanced desulfurizing flotation by sonoelectrochemistry.
where m1 m2 x1 x2
is the weight of the original dried sample, is the weight of the original dried sample after leaching, denotes the sulfur percentage in the original sample, represents the sulfur percentage in the coal obtained from leaching,
Fig. 1. Test device of sonoelectrochemistry. 1—regular speed stirrer, 2—graphite tube (anode), 3—electrolytic bath(cathode), 4—SK250HP ultrasonic cleaner, 5—DH1722 DC-regulated power supply, 6—ultrasonic output power knob, 7—ultrasonic output time knob, 8—hollow stents.
3.1.2. Effect of sonoelectrolytic time on sulfur reduction Fig. 4 depicts the sulfur reduction, clean coal yield and sulfur content plotted against sonoelectrolytic time. With time progression, the sulfur reduction rate rapidly increases to a maximum, and then slowly declines; the sulfur content initially exhibits a rapid reduction to a minimum, and then gradually increases. Initially, several groups containing sulfur are observed on the coal surface. The sulfur on the coal surface is easy to remove by the electrochemical oxidation of the anode. When a sonoelectrolytic time of about 20 min elapses, part of the inorganic and organic sulfur may have been converted into elemental sulfur because of intense oxidation; the elemental
Fig. 2. Flotation experimental processing and conditions.
1.4
80
1.2
1.0
78
0.8 76
11
14
12
pH
82
Sulfur content (%)
Sulfur reduction or yield (%)
H.-X. Zhang et al. / Fuel Processing Technology 97 (2012) 9–14
10
8
0.6 74 0.0
0.5
1.0
0.4 2.0
1.5
6
0
10
C (ethanol) (mol/L)
20
30
40
50
Sonoelectrolytic time (min)
Fig. 3. Effects of ethanol concentration on sulfur reduction, clean coal yield and sulfur content. Sonoelectrolytic time: 20 min; current density: 10 × 10− 3 A/cm2; ultrasonic intensity: 1.2 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
Fig. 5. Effects of sonoelectrolytic time on pH. C (ethanol): 1.0 mol/L; current density: 10 × 10− 3 A/cm2; ultrasonic intensity: 1.2 W/cm2.
sulfur generated is difficult to remove by washing or flotation [14]. With continued oxidation, the sulfur content in the clean coal increases. Thus, the sulfur reduction rate again begins to slowly drop. The optimal sonoelectrolytic time is 20 min. When subjected to the common function of desulfurization and oxidation, the clean coal yield initially exhibits rapid increases, and then exhibits a slight fluctuation. The effect of pH plotted against sonoelectrolytic time under the same experimental conditions is presented in Fig. 5. With increasing duration of the process, the pH continuously declines, indicating that oxidation advances in the course of sonoelectrolysis.
RSH→R−S−S−R→ R−SðO2 Þ−SðO2 Þ−R → R−OH
ð3Þ
2−
16OH þ 4FeS2 þ 15O2 → 4FeðOHÞ3 þ 8SO4 þ 2H2 O
−
2−
8OH þ 2FeS2 þ 7O2 → 2FeðOHÞ2 þ 4SO4
ð4Þ
þ 2H2 O
ð5Þ
−
ð6Þ
85
2.0
80
1.8
75
1.6
70
1.4
65
1.2
60
0
10
20
30
40
Sulfur content (%)
Sulfur reduction or yield (%)
4OH → O2 þ H2 O þ 4e
1.0
Sonoelectrolytic time (min) Fig. 4. Effects of sonoelectrolytic time on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 1.0 mol/L; current density: 10 × 10− 3 A/cm2; ultrasonic intensity: 1.2 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
=
2−
=
OH =H2 O
þ
þ 4H
ð7Þ
ð8Þ
þ
At cathode : 2H þ 2e→H2 ↑
ð9Þ
In alkaline system, the OH − on anode surface is lose charge and generate highly reactive groups such as OH ·, H2O −, O − and O2−. The reactive groups can oxygenate pyrite sulfur and partically organic sulfur into soluble sulfate or sulfonic acid root. 3.1.3. Effect of current density on sulfur reduction Fig. 6 shows the sulfur reduction, clean coal yield and sulfur content plotted against current density. With an increase in current density, sulfur reduction rate initially gradually increases to its maximum, and then decreases slowly; sulfur content initially gradually declines, and then increases slowly. The mechanism behind this phenomenon might be postulated as follows. Initially, the increase in current density imposes a favorable effect on flotation desulfurization and facilitates an
80
1.4
75
1.3
70
1.2
65
1.1 0
5
10
15
20
Sulfur content (%)
þ
O2
þ R −OH þ 2SO4
Sulfur reduction or yield (%)
electrolyte
At anode : 2H2 O→O2 þ 4H þ 4e
O2
25
103 Current density (A/cm2) Fig. 6. Effects of current density on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 1.0 mol/L; sonoelectrolytic time: 20 min; ultrasonic intensity: 1.2 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
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H.-X. Zhang et al. / Fuel Processing Technology 97 (2012) 9–14
8
6
0
5
10
103
Current density
4000
20
15
541.4 476.7 420.8
10
a 910-1040
pH
12
1608
Transmittance (%)
14
b
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
(A/cm2)
Fig. 7. Effect of current density on pH. C (ethanol): 1.0 mol/L; sonoelectrolytic time: 20 min; ultrasonic intensity: 1.2 W/cm2.
Fig. 9. IR spectra of 0.2–0.5 mm coal before and after treatment by sonoelectrochemistryenhanced flotation. (a) raw coal; (b) after treatment ( c(ethanol)=0.7 mol L−1; sonoelectrolytic time: 20 min; current density: 12.5×10− 3 A cm−2; ultrasonic intensity: 1.2 Wcm− 2 ).
1.4
0.8 60 0.3
0.6
0.9
1.2
1.5
b 1600
0.0
0.6
a
1.8
Ultrasonic intensity (W/cm2) 4000
Fig. 8. Effects of ultrasonic intensity on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 1.0 mol/L; sonoelectrolytic time: 20 min; current density: 15 × 10-3 A/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
ideal clean coal yield. As current density reaches 5 × 10− 3 A/cm2, the clean coal yield begins to decline because of the oxidation on the coal surface; oxidation weakens hydrophobicity on the surface of coal particles. The optimal current density is 15 × 10− 3 A/cm 2 and pH acts against current density under the same conditions (Fig. 7). As current density increases, pH continues to decrease, indicating that electrochemical oxidation is strengthened with the progression of the reaction. 3.1.4. Effect of ultrasonic intensity on sulfur reduction Fig. 8 shows the sulfur reduction, clean coal yield and sulfur content plotted against ultrasonic intensity. With an increase in ultrasonic intensity, the rate of sulfur reduction initially exhibits a rapid reduction to a minimum, and then gradually increases; and the clean coal yield
3500
3000
2500
2000
Wavenumber
1500
538.2 474.9 420.5
1.0 70
910-1040
1.2
Transmittance (%)
80
Sulfur content (%)
Sulfur reducation or yield (%)
90
1000
500
(cm-1)
Fig. 10. IR spectra of 0.125–0.2 mm coal before and after treatment by sonoelectrochemistryenhanced flotation. (a) raw coal; (b) after treatment (c (ethanol)=1.0 mol L−1 ; sonoelectrolytic time: 20 min; current density: 15×10−3 A cm−2; ultrasonic intensity: 1.2 Wcm−2).
initially exhibits a rapid increment to a maximum, and then gradually declines. Compared with no irradiation, ultrasonic radiation at low intensities remarkably increases clean coal yield. As ultrasonic intensity approaches 0.5 W/cm 2, the rate of sulfur reduction and sulfur content in clean coal almost stabilizes. As ultrasonic intensity reaches 1.0 W/cm 2, the sulfur reduction rapidly increases. The mechanism for this phenomenon may be elucidated as follows. The surface cleaning effects of ultrasonic treatment in slurry are characterized by cavitation and accompanied by a local increase in pressure and temperature. Given that solid/liquid interactions are weaker than liquid cohesion forces,
Table 2 Comparison of optimally experimental conditions for different size fractions of high sulfur coal with enhanced flotation by sonoelectrochemistry. Size fractions
− 0.076 mm 0.076–0.125 mm 0.125–0.2 mm 0.2–0.5 mm
C (ethanol)
Ultrasonic intensity
time
Current density
Sulfur
Yield
Sulfur reduction
Ash reduction
Calorific value
Moisture
(mol L− 1)
(W cm− 2)
(min)
(103 A cm− 2)
(%)
(%)
(%)
(%)
(kJ kg− 1)
(%)
2.1 2.1 1.0 0.7
1.2 1.2 1.2 1.2
20 40 20 20
15 7.5 15 12.5
2.1 1.3 1.1 1.1
72.2 72.5 73.0 68.9
75.4 82.7 81.2 76.1
65.4 67.0 59.8 63.2
30942 32789 32625 33533
0.63 0.52 0.52 0.49
3.2. Comparison 3.2.1. Optimal experimental conditions The sulfur content, yield and sulfur reduction for different size fractions coal when subjected to enhanced flotation using sonoelectrochemical methods, under optimal conditions, are listed in Table 2. Under their optimal conditions, the optimal ultrasonic intensity are 1.2 W cm− 2 for different size fractions of high sulfur coal. and the optimal sonoelectrolytic time are mostly focus on 20 min. The highest rate of sulfur reduction is 82.7% for 0.076–0.125 mm coal. The lowest yield of clean coal is 68.9% for 0.2–0.5 mm coal. The highest rate of ash reduction is 67.0% for 0.076–0.125 mm coal. Furthermore, the method of enhanced flotation by sonoelectrochemistry is effective in calorific value increment and moisture reduction for different size fractions of high sulfur coal. 3.2.2. Infrared spectroscopy analysis With difference size fractions, the infrared spectra of raw and treated clean coal generated by sonoelectrochemistry-enhanced flotation are shown in Figs. 9–12. Comparing curves b and a, we can postulate that: (1) The weakening of the aromatic disulfide and hydrosulfonyl
a
3500
3000
2500
2000
1500
1000
477.5 415.2
b 910-1040
4000
13
1533
solid/liquid interfaces more easily lend themselves to cavitation formation. The unsettled conditions occurring at a solid/liquid interface can modify the surface properties of minerals, leading to changes in the adsorption of collectors on minerals, and accordingly, in their flotation responses. Ultrasonic cavitation is strengthened with the continuous increase in ultrasonic intensity. A high-speed liquid jet directed toward the surface is generated. The jet and associated shockwave impingement induces localized erosion [15], causing particle fragmentation [16]. The shockwave and turbulent flow from cavitation result in the interparticle collision of the coal particles with sufficient force to bring about changes on the surface [17]. The liquid–solid interface areas between coal particles and the solvent increases and improve mass transfer; this results in more favorable conditions for desulfurization. Moreover, ultrasonic cavitation [18,19] breaks the weak bonds of the organic sulfide on the interior of the coal particles and generates sulfide-free radicals[20]. These radicals combine with the organic solvent, and then dissolve into solution. Therefore, a proportion of total sulfur is removed by this process [21]. As ultrasonic intensity reaches 1.5 W/cm 2, the clean coal yield further decreases to 68.1%. This effect is negligible for flotation desulfurization because of the low clean coal yield. Therefore, the optimal ultrasonic intensity is 1.2 W/cm2.
Transmittance (%)
H.-X. Zhang et al. / Fuel Processing Technology 97 (2012) 9–14
500
Wavenumber (cm-1) Fig. 12. IR spectra of −0.076 mm coal before and after treatment by sonoelectrochemistryenhanced flotation. (a) Raw coal; (b) after treatment (c (ethanol)=2.1 mol/L; sonoelectrolytic time: 20 min; current density: 15×10− 3 A cm-2; ultrasonic intensity: 1.2 Wcm-2).
absorption band at about 537 cm-1 and 477 cm− 1 can be attributed to the removal of part of the organic sulfur by sonoelectrochemistryenhanced flotation. (2) The weakening of the pyrite absorption band at about 420 cm− 1 can be attributed to the partial removal of pyritic sulfur by sonoelectrochemistry-enhanced flotation. (3) The weakening of the broad band in the range of 910–1040 cm− 1 can be attributed to the removal of ash material, such as kaolinite. (4) The weakening of aromatic C C absorption band at about 1600 cm− 1 be attributed to oxidation during sonoelectrochemistry-enhanced flotation. Therefore, the method of enhanced flotation by sonoelectrochemistry can partially remove organic and pyritic sulfur as well as ash.
3.2.3. Chemical analysis Under their optimal conditions, the sulfur content analysis of different size fractions of clean coal by sonoelectrochemistry-enhanced flotation is shown in Table 3. By chemical analysis, under their optimal conditions, the sulfur content of total sulfur, pyritic sulfur, sulfate sulfur and organic sulfur, for different size fractions, have varying degrees of decline after sonoelectrochemistry-enhanced flotation.
418.58
a
4000
3500
3000
2500
2000
1500
1000
537.9 478.0
910-1040
1600
b 2350.9
Transmittance (%)
4. Conclusions
500
The study aimed to evaluate the feasibility of enhanced desulfurizing flotation of different size fractions of high-sulfur coal using the sonoelectrochemical method. For the different size fractions coal, the optimal experimental conditions for anhydrous ethanol, sonoelectrolytic time, current density, and ultrasound intensity, respectively, are achieved. Under their optimal conditions, the optimal ultrasonic intensity are 1.2 W cm− 2 for different size fractions of high sulfur coal. The highest rate of sulfur reduction is 82.7% for 0.076–0.125 mm coal. The highest rate of ash reduction is 67.0% for 0.076–0.125 mm coal. The lowest yield of clean coal is 68.9% for 0.2–0.5 mm coal. The method is effective in calorific value increment and moisture reduction for different size fractions of high sulfur coal. IR and chemical analyses indicate that parts of pyritic and organic sulfur as well as ash, are removed by sonoelectrochemistry-enhanced flotation.
Wavenumber (cm-1) Fig. 11. IR spectra of 0.076–0.125 mm coal before and after treatment by sonoelectrochemistry-enhanced flotation. (a) raw coal; (b) after treatment ( c (ethanol) =2.1 mol L− 1; sonoelectrolytic time: 40 min; current density: 7.5 × 10− 3 A cm2; ultrasonic intensity: 1.2 W cm− 2 ).
Acknowledgments This project was supported by the Tarim University President Foundation Master's Programs (TDZKSS09006).
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H.-X. Zhang et al. / Fuel Processing Technology 97 (2012) 9–14
Table 3 Sulfur content analysis of different size fractions of clean coal by sonoelectrochemistry-enhanced flotation with their optimal conditions (W%, ad). Size fractions
− 0.076 mm 0.076–0.125 mm 0.125–0.2 mm 0.2–0.5 mm
C (ethanol)
Ultrasonic intensity
Sonoelectrolytic
Current density
Sulfur content
(mol L− 1)
(W cm− 2)
time (min)
(103 A cm− 2)
Total
Pyritic
Sulfate
Organic
2.07 2.07 1.04 0.69
1.23 1.23 1.23 1.23
20 40 20 20
15 7.5 15 12.5
2.08 1.25 1.07 1.05
0.59 0.002 0.002 0.08
0.33 0.137 0.110 0.09
1.16 1.111 0.958 0.88
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Enhanced desulfurizing flotation of different size fractions of high sulfur coal using sonoelectrochemical method Hong-Xi Zhang a,⁎, Hong-Jin Bai a, Xian-Shu Dong b, Zhi-Zhong Wang c a b c
College of Life Science, Tarim University, Alaer 843300, Xinjiang, China College of Mining Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China College of Chemistry & Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
a r t i c l e
i n f o
Article history: Received 6 June 2011 Received in revised form 5 December 2011 Accepted 8 January 2012 Available online 3 February 2012 Keywords: Sonoelectrochemistry Sonocavitation Enhanced flotation Desulfurization
a b s t r a c t Enhanced desulfurizing flotation of different size fractions of high sulfur coal was investigated using the sonoelectrochemical method. The supporting electrolyte used in this process was calcium hydroxide and the additive was anhydrous ethanol. The effects of treatment conditions on desulfurization were studied by a single-factor method. The conditions include anhydrous ethanol concentration, sonoelectrolytic time, current density, and ultrasound intensity. For the coal samples with different size fractions, the optimal experimental conditions for anhydrous ethanol concentration, sonoelectrolytic time, current density, and ultrasound intensity, respectively, are achieved. Under optimal conditions, the raw and treated coals were analyzed by infrared spectroscopy and a chemical method. Pyritic sulfur, organic sulfur, and ash are partially removed. Compared with different size fractions coal, desulfurizing flotation of high sulfur coal by sonoelectrochemistry is an effective technology. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction The increasing demand for coal has led to more extensive mining of coal seams, so that the mined coal has high sulfur content. Reducing sulfur content in coal prior to its use in many applications is essential. To this end, researchers have focused on searching for simple, economic, and efficient desulfurization methods as this has become an issue of worldwide concern. The process of enhanced desulfurizing flotation of coal using electrochemistry involves simple technology; this process has been reported in several published articles [1–5]. The method efficiently removes pyritic and organic sulfur, as well as ash. Some studies have reported a form of energy, ultrasound waves, that has been increasingly applied in desulfurizing flotation [6–9]. Ultrasound can enhance clean coal yield and promote the separation of pyrite in high sulfur coal, factors that are favorable for flotation desulfurization. The combination of electrochemical flotation and ultrasonic flotation methods presents promising prospects for effective desulfurization of coal. In recent years, the integration of electrochemistry and sonochemistry has given rise to an interdisciplinary method called sonoelectrochemistry. Research in this area has attracted considerable attention, and some of the advantages reported in literature include the improvement of electrochemical progress while obtaining results superior to those obtained by electrochemistry alone [10–12].
⁎ Corresponding author. Tel.: + 86 997 4681613; fax: + 86 997 4681612. E-mail address: [email protected] (H.-X. Zhang).
Studies on a variety of areas including electroanalysis, electroplating, electroorganic synthesis, electropolymerization, and pollutant degradation in emulsion have used sonoelectrochemical methods. However, few reports on enhanced desulfurizing flotation of coal by sonoelectrochemistry have been published. We believe that research on the feasibility of such application is necessary. The combination of ultrasound technology and electrochemistry has also been extensively applied in the industry, leading us to expect that desulfurizing flotation of high sulfur coal via sonoelectrochemistry will find potential industrial applications.
2. Experimental 2.1. Materials and reagents High sulfur coal from the Tunlan coal plant in Shanxi, China was used for the experiment. By screening, the coal was separated into samples of different size fractions. The date at which proximate analysis was conducted and the sulfur content of the different size fractions of coal are listed in Table 1. In the present study, first of all, coal with size fraction of 0.125–0.2 mm was chosen as the experimental coal sample. According to previous experimental experience, the concentration of slurry and supporting electrolyte were 96 g L − 1 and 2.0 g L − 1, respectively. Other reagents used in this experiment were calcium hydroxide (AR grade), anhydrous ethanol (AR grade), kerosene as the flotation reagent (Chemical Pure, collector), and sec-n-octyl alcohol (CP, frother).
0378-3820/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.01.005
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Table 1 Proximate analysis and sulfur content (W%, ad) of Tunlan coal. Granulometry
− 0.076 mm 0.076-0.125 mm 0.125–0.2 mm 0.2–0.5 mm
Weight
Proximate analysis
(%)
Moisture
Ash
Volatile matter
Total
Sulfur content Pyritic
Sulfate
Organic
(value, kJ/kg)
17.69 22.87 16.86 42.58
1.56 1.35 1.46 1.32
22.35 19.93 18.97 18.66
18.16 20.85 19.08 16.87
6.12 5.26 4.14 3.03
3.53 3.19 1.89 1.36
1.08 0.75 0.50 0.33
1.51 1.32 1.75 1.34
25022 26510 28269 28179
y1 y2
2.2. Instruments Electrolytic power was provided by a DH1722 DC-regulated power supply. Graphite (97.4 cm 2) and stainless steel (180.3 cm 2) were used as the anode and cathode, respectively; the electrolytic trough was set at 300 mL cup. The experimental scheme is presented in Fig. 1, and the flotation process and conditions are shown in Fig. 2. Other equipment used in the study are an SK250HP ultrasonic cleaner (59 kHz, 250 W, ultrasonic irradiation area: 203.3 cm 2), XFGC-80 flotation cell, HXZ-S3 sulfur determinator, and Shimadzu FTIR8400S (KBr:coal = 200:1). 2.3. Experimental method Specific amounts of supporting electrolyte, additive, and coal sample were placed in the electrolytic trough, to which distilled water was added to obtain a volume of 250 mL. Under a certain current density, ultrasonic intensity, and agitation rate (300 rpm), the slurry was subjected to sonoelectrolysis for a preset duration at room temperature. Subsequently, the slurry was immediately transferred to the flotation column. The floating coal was washed, dried in vacuum at 80 °C for 4 h, and then subjected to sulfur determination. The calculations of sulfur and ash reduction are expressed as [13] sulfur reduction ðwt:%Þ ¼ 100½x1 −x2 ðm2 =m1 Þ=x1
ð1Þ
ash reduction ðwt:%Þ ¼ 100½y1 −y2 ðm2 =m1 Þ=y1
ð2Þ
Calorific
is the ash percentage in the original sample, and denotes the ash percentage in the coal obtained from leaching.
3. Results and discussion 3.1. Effect of sonoelectrolytic conditions 3.1.1. Effect of ethanol concentration on sulfur reduction The effects of sulfur reduction, clean coal yield and sulfur content plotted against ethanol concentration are presented in Fig. 3. With increasing ethanol concentration, sulfur content initially exhibits a gradual reduction to a minimum, and then gradually increases; the sulfur reduction rate rapidly increases to a maximum, and then rapidly declines. Before the ethanol concentration reaches 1.0 mol/L, the clean coal yield exhibits a gradual fluctuation. This phenomenon can be attributed to the presence of specific numbers of ethanol molecules, which could have a function of desulfurization, resulted in the reduction of sulfur content in the clean coal using enhanced desulfurizing flotation by sonoelectrochemistry. However, as ethanol concentration reaches 1.0 mol/L, the wettability of the coal surface is enhanced and the clean coal yield begins to rapidly increase. The hydrophilicity of pyrite be weakened and resulted in part of pyrite sulfur transfer to the clean coal by flotation. Therefore, the sulfur content begins to gradually increase; and the sulfur reduction rate begins to rapidly decline. Therefore, the optimal ethanol concentration is 1.0 mol/L. These results indicate that ethanol concentration has a significant effect on sulfur reduction, in which a specific amount of ethanol proves favorable for enhanced desulfurizing flotation by sonoelectrochemistry.
where m1 m2 x1 x2
is the weight of the original dried sample, is the weight of the original dried sample after leaching, denotes the sulfur percentage in the original sample, represents the sulfur percentage in the coal obtained from leaching,
Fig. 1. Test device of sonoelectrochemistry. 1—regular speed stirrer, 2—graphite tube (anode), 3—electrolytic bath(cathode), 4—SK250HP ultrasonic cleaner, 5—DH1722 DC-regulated power supply, 6—ultrasonic output power knob, 7—ultrasonic output time knob, 8—hollow stents.
3.1.2. Effect of sonoelectrolytic time on sulfur reduction Fig. 4 depicts the sulfur reduction, clean coal yield and sulfur content plotted against sonoelectrolytic time. With time progression, the sulfur reduction rate rapidly increases to a maximum, and then slowly declines; the sulfur content initially exhibits a rapid reduction to a minimum, and then gradually increases. Initially, several groups containing sulfur are observed on the coal surface. The sulfur on the coal surface is easy to remove by the electrochemical oxidation of the anode. When a sonoelectrolytic time of about 20 min elapses, part of the inorganic and organic sulfur may have been converted into elemental sulfur because of intense oxidation; the elemental
Fig. 2. Flotation experimental processing and conditions.
1.4
80
1.2
1.0
78
0.8 76
11
14
12
pH
82
Sulfur content (%)
Sulfur reduction or yield (%)
H.-X. Zhang et al. / Fuel Processing Technology 97 (2012) 9–14
10
8
0.6 74 0.0
0.5
1.0
0.4 2.0
1.5
6
0
10
C (ethanol) (mol/L)
20
30
40
50
Sonoelectrolytic time (min)
Fig. 3. Effects of ethanol concentration on sulfur reduction, clean coal yield and sulfur content. Sonoelectrolytic time: 20 min; current density: 10 × 10− 3 A/cm2; ultrasonic intensity: 1.2 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
Fig. 5. Effects of sonoelectrolytic time on pH. C (ethanol): 1.0 mol/L; current density: 10 × 10− 3 A/cm2; ultrasonic intensity: 1.2 W/cm2.
sulfur generated is difficult to remove by washing or flotation [14]. With continued oxidation, the sulfur content in the clean coal increases. Thus, the sulfur reduction rate again begins to slowly drop. The optimal sonoelectrolytic time is 20 min. When subjected to the common function of desulfurization and oxidation, the clean coal yield initially exhibits rapid increases, and then exhibits a slight fluctuation. The effect of pH plotted against sonoelectrolytic time under the same experimental conditions is presented in Fig. 5. With increasing duration of the process, the pH continuously declines, indicating that oxidation advances in the course of sonoelectrolysis.
RSH→R−S−S−R→ R−SðO2 Þ−SðO2 Þ−R → R−OH
ð3Þ
2−
16OH þ 4FeS2 þ 15O2 → 4FeðOHÞ3 þ 8SO4 þ 2H2 O
−
2−
8OH þ 2FeS2 þ 7O2 → 2FeðOHÞ2 þ 4SO4
ð4Þ
þ 2H2 O
ð5Þ
−
ð6Þ
85
2.0
80
1.8
75
1.6
70
1.4
65
1.2
60
0
10
20
30
40
Sulfur content (%)
Sulfur reduction or yield (%)
4OH → O2 þ H2 O þ 4e
1.0
Sonoelectrolytic time (min) Fig. 4. Effects of sonoelectrolytic time on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 1.0 mol/L; current density: 10 × 10− 3 A/cm2; ultrasonic intensity: 1.2 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
=
2−
=
OH =H2 O
þ
þ 4H
ð7Þ
ð8Þ
þ
At cathode : 2H þ 2e→H2 ↑
ð9Þ
In alkaline system, the OH − on anode surface is lose charge and generate highly reactive groups such as OH ·, H2O −, O − and O2−. The reactive groups can oxygenate pyrite sulfur and partically organic sulfur into soluble sulfate or sulfonic acid root. 3.1.3. Effect of current density on sulfur reduction Fig. 6 shows the sulfur reduction, clean coal yield and sulfur content plotted against current density. With an increase in current density, sulfur reduction rate initially gradually increases to its maximum, and then decreases slowly; sulfur content initially gradually declines, and then increases slowly. The mechanism behind this phenomenon might be postulated as follows. Initially, the increase in current density imposes a favorable effect on flotation desulfurization and facilitates an
80
1.4
75
1.3
70
1.2
65
1.1 0
5
10
15
20
Sulfur content (%)
þ
O2
þ R −OH þ 2SO4
Sulfur reduction or yield (%)
electrolyte
At anode : 2H2 O→O2 þ 4H þ 4e
O2
25
103 Current density (A/cm2) Fig. 6. Effects of current density on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 1.0 mol/L; sonoelectrolytic time: 20 min; ultrasonic intensity: 1.2 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
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H.-X. Zhang et al. / Fuel Processing Technology 97 (2012) 9–14
8
6
0
5
10
103
Current density
4000
20
15
541.4 476.7 420.8
10
a 910-1040
pH
12
1608
Transmittance (%)
14
b
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
(A/cm2)
Fig. 7. Effect of current density on pH. C (ethanol): 1.0 mol/L; sonoelectrolytic time: 20 min; ultrasonic intensity: 1.2 W/cm2.
Fig. 9. IR spectra of 0.2–0.5 mm coal before and after treatment by sonoelectrochemistryenhanced flotation. (a) raw coal; (b) after treatment ( c(ethanol)=0.7 mol L−1; sonoelectrolytic time: 20 min; current density: 12.5×10− 3 A cm−2; ultrasonic intensity: 1.2 Wcm− 2 ).
1.4
0.8 60 0.3
0.6
0.9
1.2
1.5
b 1600
0.0
0.6
a
1.8
Ultrasonic intensity (W/cm2) 4000
Fig. 8. Effects of ultrasonic intensity on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 1.0 mol/L; sonoelectrolytic time: 20 min; current density: 15 × 10-3 A/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
ideal clean coal yield. As current density reaches 5 × 10− 3 A/cm2, the clean coal yield begins to decline because of the oxidation on the coal surface; oxidation weakens hydrophobicity on the surface of coal particles. The optimal current density is 15 × 10− 3 A/cm 2 and pH acts against current density under the same conditions (Fig. 7). As current density increases, pH continues to decrease, indicating that electrochemical oxidation is strengthened with the progression of the reaction. 3.1.4. Effect of ultrasonic intensity on sulfur reduction Fig. 8 shows the sulfur reduction, clean coal yield and sulfur content plotted against ultrasonic intensity. With an increase in ultrasonic intensity, the rate of sulfur reduction initially exhibits a rapid reduction to a minimum, and then gradually increases; and the clean coal yield
3500
3000
2500
2000
Wavenumber
1500
538.2 474.9 420.5
1.0 70
910-1040
1.2
Transmittance (%)
80
Sulfur content (%)
Sulfur reducation or yield (%)
90
1000
500
(cm-1)
Fig. 10. IR spectra of 0.125–0.2 mm coal before and after treatment by sonoelectrochemistryenhanced flotation. (a) raw coal; (b) after treatment (c (ethanol)=1.0 mol L−1 ; sonoelectrolytic time: 20 min; current density: 15×10−3 A cm−2; ultrasonic intensity: 1.2 Wcm−2).
initially exhibits a rapid increment to a maximum, and then gradually declines. Compared with no irradiation, ultrasonic radiation at low intensities remarkably increases clean coal yield. As ultrasonic intensity approaches 0.5 W/cm 2, the rate of sulfur reduction and sulfur content in clean coal almost stabilizes. As ultrasonic intensity reaches 1.0 W/cm 2, the sulfur reduction rapidly increases. The mechanism for this phenomenon may be elucidated as follows. The surface cleaning effects of ultrasonic treatment in slurry are characterized by cavitation and accompanied by a local increase in pressure and temperature. Given that solid/liquid interactions are weaker than liquid cohesion forces,
Table 2 Comparison of optimally experimental conditions for different size fractions of high sulfur coal with enhanced flotation by sonoelectrochemistry. Size fractions
− 0.076 mm 0.076–0.125 mm 0.125–0.2 mm 0.2–0.5 mm
C (ethanol)
Ultrasonic intensity
time
Current density
Sulfur
Yield
Sulfur reduction
Ash reduction
Calorific value
Moisture
(mol L− 1)
(W cm− 2)
(min)
(103 A cm− 2)
(%)
(%)
(%)
(%)
(kJ kg− 1)
(%)
2.1 2.1 1.0 0.7
1.2 1.2 1.2 1.2
20 40 20 20
15 7.5 15 12.5
2.1 1.3 1.1 1.1
72.2 72.5 73.0 68.9
75.4 82.7 81.2 76.1
65.4 67.0 59.8 63.2
30942 32789 32625 33533
0.63 0.52 0.52 0.49
3.2. Comparison 3.2.1. Optimal experimental conditions The sulfur content, yield and sulfur reduction for different size fractions coal when subjected to enhanced flotation using sonoelectrochemical methods, under optimal conditions, are listed in Table 2. Under their optimal conditions, the optimal ultrasonic intensity are 1.2 W cm− 2 for different size fractions of high sulfur coal. and the optimal sonoelectrolytic time are mostly focus on 20 min. The highest rate of sulfur reduction is 82.7% for 0.076–0.125 mm coal. The lowest yield of clean coal is 68.9% for 0.2–0.5 mm coal. The highest rate of ash reduction is 67.0% for 0.076–0.125 mm coal. Furthermore, the method of enhanced flotation by sonoelectrochemistry is effective in calorific value increment and moisture reduction for different size fractions of high sulfur coal. 3.2.2. Infrared spectroscopy analysis With difference size fractions, the infrared spectra of raw and treated clean coal generated by sonoelectrochemistry-enhanced flotation are shown in Figs. 9–12. Comparing curves b and a, we can postulate that: (1) The weakening of the aromatic disulfide and hydrosulfonyl
a
3500
3000
2500
2000
1500
1000
477.5 415.2
b 910-1040
4000
13
1533
solid/liquid interfaces more easily lend themselves to cavitation formation. The unsettled conditions occurring at a solid/liquid interface can modify the surface properties of minerals, leading to changes in the adsorption of collectors on minerals, and accordingly, in their flotation responses. Ultrasonic cavitation is strengthened with the continuous increase in ultrasonic intensity. A high-speed liquid jet directed toward the surface is generated. The jet and associated shockwave impingement induces localized erosion [15], causing particle fragmentation [16]. The shockwave and turbulent flow from cavitation result in the interparticle collision of the coal particles with sufficient force to bring about changes on the surface [17]. The liquid–solid interface areas between coal particles and the solvent increases and improve mass transfer; this results in more favorable conditions for desulfurization. Moreover, ultrasonic cavitation [18,19] breaks the weak bonds of the organic sulfide on the interior of the coal particles and generates sulfide-free radicals[20]. These radicals combine with the organic solvent, and then dissolve into solution. Therefore, a proportion of total sulfur is removed by this process [21]. As ultrasonic intensity reaches 1.5 W/cm 2, the clean coal yield further decreases to 68.1%. This effect is negligible for flotation desulfurization because of the low clean coal yield. Therefore, the optimal ultrasonic intensity is 1.2 W/cm2.
Transmittance (%)
H.-X. Zhang et al. / Fuel Processing Technology 97 (2012) 9–14
500
Wavenumber (cm-1) Fig. 12. IR spectra of −0.076 mm coal before and after treatment by sonoelectrochemistryenhanced flotation. (a) Raw coal; (b) after treatment (c (ethanol)=2.1 mol/L; sonoelectrolytic time: 20 min; current density: 15×10− 3 A cm-2; ultrasonic intensity: 1.2 Wcm-2).
absorption band at about 537 cm-1 and 477 cm− 1 can be attributed to the removal of part of the organic sulfur by sonoelectrochemistryenhanced flotation. (2) The weakening of the pyrite absorption band at about 420 cm− 1 can be attributed to the partial removal of pyritic sulfur by sonoelectrochemistry-enhanced flotation. (3) The weakening of the broad band in the range of 910–1040 cm− 1 can be attributed to the removal of ash material, such as kaolinite. (4) The weakening of aromatic C C absorption band at about 1600 cm− 1 be attributed to oxidation during sonoelectrochemistry-enhanced flotation. Therefore, the method of enhanced flotation by sonoelectrochemistry can partially remove organic and pyritic sulfur as well as ash.
3.2.3. Chemical analysis Under their optimal conditions, the sulfur content analysis of different size fractions of clean coal by sonoelectrochemistry-enhanced flotation is shown in Table 3. By chemical analysis, under their optimal conditions, the sulfur content of total sulfur, pyritic sulfur, sulfate sulfur and organic sulfur, for different size fractions, have varying degrees of decline after sonoelectrochemistry-enhanced flotation.
418.58
a
4000
3500
3000
2500
2000
1500
1000
537.9 478.0
910-1040
1600
b 2350.9
Transmittance (%)
4. Conclusions
500
The study aimed to evaluate the feasibility of enhanced desulfurizing flotation of different size fractions of high-sulfur coal using the sonoelectrochemical method. For the different size fractions coal, the optimal experimental conditions for anhydrous ethanol, sonoelectrolytic time, current density, and ultrasound intensity, respectively, are achieved. Under their optimal conditions, the optimal ultrasonic intensity are 1.2 W cm− 2 for different size fractions of high sulfur coal. The highest rate of sulfur reduction is 82.7% for 0.076–0.125 mm coal. The highest rate of ash reduction is 67.0% for 0.076–0.125 mm coal. The lowest yield of clean coal is 68.9% for 0.2–0.5 mm coal. The method is effective in calorific value increment and moisture reduction for different size fractions of high sulfur coal. IR and chemical analyses indicate that parts of pyritic and organic sulfur as well as ash, are removed by sonoelectrochemistry-enhanced flotation.
Wavenumber (cm-1) Fig. 11. IR spectra of 0.076–0.125 mm coal before and after treatment by sonoelectrochemistry-enhanced flotation. (a) raw coal; (b) after treatment ( c (ethanol) =2.1 mol L− 1; sonoelectrolytic time: 40 min; current density: 7.5 × 10− 3 A cm2; ultrasonic intensity: 1.2 W cm− 2 ).
Acknowledgments This project was supported by the Tarim University President Foundation Master's Programs (TDZKSS09006).
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H.-X. Zhang et al. / Fuel Processing Technology 97 (2012) 9–14
Table 3 Sulfur content analysis of different size fractions of clean coal by sonoelectrochemistry-enhanced flotation with their optimal conditions (W%, ad). Size fractions
− 0.076 mm 0.076–0.125 mm 0.125–0.2 mm 0.2–0.5 mm
C (ethanol)
Ultrasonic intensity
Sonoelectrolytic
Current density
Sulfur content
(mol L− 1)
(W cm− 2)
time (min)
(103 A cm− 2)
Total
Pyritic
Sulfate
Organic
2.07 2.07 1.04 0.69
1.23 1.23 1.23 1.23
20 40 20 20
15 7.5 15 12.5
2.08 1.25 1.07 1.05
0.59 0.002 0.002 0.08
0.33 0.137 0.110 0.09
1.16 1.111 0.958 0.88
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