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Enhanced desulfurizing flotation of high sulfur coal by sonoelectrochemical method
Fuel Processing Technology 93 (2012) 13–17
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Enhanced desulfurizing flotation of high sulfur coal by sonoelectrochemical method Hong-Xi Zhang a,⁎, Xiao-Yan Ma a, Xian-Shu Dong b, Zhi-Zhong Wang c, Hong-Jin Bai a 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 20 December 2010 Received in revised form 23 July 2011 Accepted 4 September 2011 Available online 7 October 2011 Keywords: Sonoelectrochemistry Sonocavitation Enhanced flotation Desulfurization
a b s t r a c t Enhanced desulfurizing flotation 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 sample with a particle size of −0.076 mm, the optimal experimental conditions achieved for anhydrous ethanol, sonoelectrolytic time, current density, and ultrasound intensity are 2.1 mol/L, 20 min, 15× 10− 3 A/cm2, and 1.2 W/cm2, respectively. Optimal conditions cause a sulfur reduction of up to 75.4%. 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 enhanced flotation by ultrasound or electrochemistry, desulfurizing flotation of high sulfur coal by sonoelectrochemistry is an effective technology. © 2011 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. However, the technique still generates an un-ideal yield of clean coal. 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
⁎ Corresponding author. Tel.: + 86 997 4681613; fax: + 86 997 4681612. E-mail address: [email protected] (H.-X. Zhang). 0378-3820/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.09.007
obtained by electrochemistry alone [10–13]. 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 granulometry. For the coal sample with a particle size of −0.076 mm, the contents of sulfur and ash were much higher than those reflected in the granulometry of the other samples. In the present study, coal with a granulometry of −0.076 mm was chosen as the experimental coal sample. The date at which proximate analysis was conducted and the sulfur content of the coal are listed in Table 1. The concentrations of slurry and supporting electrolyte were 96 and 2.0 g/L, 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).
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H.-X. Zhang et al. / Fuel Processing Technology 93 (2012) 13–17
Table 1 Proximate analysis and sulfur content (W%, ad) of Tunlan coal. Proximate analysis
Sulfur content
Moisture
Ash
Volatile matter
Calorific value (kJ/kg)
Total
Pyritic
Sulfate
Organic
1.56
22.35
18.16
25,022
6.12
3.53
1.08
1.51
2.2. Instruments Fig. 2. Flotation experimental processing and conditions.
Electrolytic power was provided by a DH1722 DC-regulated power supply. Graphite (97.4 cm2) and stainless steel (180.3 cm2) 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 cm2), 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 [14] sulf ur reduction ðwt:%Þ ¼ 100½x1 −x2 ðm2 =m1 Þ=x1
ð1Þ
ash reduction ðwt:%Þ ¼ 100½y1 −y2 ðm2 =m1 Þ=y1
ð2Þ
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 rapidly increases. Before the ethanol concentration reaches 0.7 mol/L, the clean coal yield continues to rapidly increase. This phenomenon can be attributed to the presence of specific numbers of ethanol molecules, which can improve coal surface wettability and generate a favorable condition for clean coal recovery. However, as ethanol concentration reaches 0.7 mol/L, the hydrophilicity of the coal surface is enhanced and the clean coal yield begins to diminish. When ethanol concentration reaches 2.8 mol/L, the clean coal yield drops to 65.0%. This low yield is negligible for desulfurizing flotation. Therefore, the optimal ethanol concentration is 2.1 mol/L. These results indicate that ethanol concentration has a significant effect on clean coal yield, in which a specific amount of ethanol proves favorable for enhanced desulfurizing flotation by sonoelectrochemistry.
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, is the ash percentage in the original sample, and denotes the ash percentage in the coal obtained from leaching.
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. Initially, several groups containing sulfur are absorbed on the coal surface. The sulfur on the coal surface is easy to remove by the electrochemical oxidation of the anode. As time progresses, however, the reaction comes to the interior coal particles instead of on the surface. Therefore, sulfur removal becomes difficult at this point, and sulfur reduction proceeds more slowly. When a sonoelectrolytic time of about 20 min elapses, part of the inorganic and organic sulfur may have been converted into 3.0
Sulfur reduction or yield (%)
81 78
2.8
75 2.6 72 2.4 69 2.2
66 63 0.0
0.5
1.0
1.5
2.0
2.5
Sulfur content (%)
y1 y2
3.1. Effect of ethanol concentration on sulfur reduction
3.2. Effect of sonoelectrolytic time on sulfur reduction
where m1 m2 x1 x2
3. Results and discussion
2.0 3.0
c (ethanol) (mol/L) 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.
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: 1.23 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
H.-X. Zhang et al. / Fuel Processing Technology 93 (2012) 13–17
75
3.5
70 3.0 65 2.5
60
55
−
4.0
10
0
20
30
40
2−
8OH þ 2FeS2 þ 7O2 →2FeðOHÞ2 þ 4SO4
þ 2H 2 O
ð5Þ
−
4OH →O2 þ H2 O þ 4e
Sulfur content (%)
Sulfur reduction or yield (%)
80
15
O2
ð6Þ
O2
=
OH =H 2 O
RSH → R–S–S–R → R–SðO2 Þ–SðO2 Þ−R → R–OH =
2−
þ R –OH þ 2SO4
þ 4H
þ
ð7Þ
ð8Þ
2.0
50
Sonoelectrolytic time (min)
elemental sulfur because of intense oxidation; the elemental sulfur is difficult to remove by washing or flotation. 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 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 and the declining rate proceeds even more slowly, indicating that oxidation advances in the course of sonoelectrolysis. At the beginning of the process, the electrochemical oxidation is mostly restricted to the anode surface. This reaction proceeds easily, thereby causing pH to decline rapidly. As the sonoelectrolytic time is extended, the reaction occurs in the interior coal particles instead of on the surface. Thus, the rate of reaction slows down and the drop in pH increasingly decelerates. In addition, the effect of pH plotted against electrolytic time without ultrasound irradiation under the same experimental conditions is also presented in Fig. 5. Comparing electrochemistry without ultrasound irradiation with sonoelectrochemistry, the drop in pH using electrochemistry without ultrasound irradiation is slower than sonoelectrochemistry.
electrolyte
þ
At anode : 2H 2 O → O2 þ 4H þ 4e −
16OH þ 4FeS2 þ 15O2 →4FeðOHÞ3 þ 8SO4
ð3Þ 2−
þ 2H2 O
ð4Þ
3.3. 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 increases to its maximum, and then decreases rapidly. The mechanism behind this phenomenon may be postulated as follows. Initially, the increase in current density imposes a favorable effect on flotation desulfurization and facilitates an ideal clean coal yield. As current density reaches 10× 10− 3 A/cm2, the rate of clean coal yield begins to drop because of the oxidation on the coal surface; oxidation weakens hydrophobicity on the surface of coal particles. However, as current density approaches 15 × 10 − 3 A/cm 2, portions of inorganic and organic sulfur are converted into elemental sulfur because of intense oxidation. The elemental sulfur generated is difficult to remove by washing or flotation. Thereafter, the rate of sulfur reduction falls rapidly. The optimal current density is 15× 10− 3 A/cm2 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. 3.4. Effect of ultrasonic intensity on sulfur reduction Fig. 8 shows the sulfur reduction, clean coal yield and sulfur content plotted against ultrasonic intensity. In the beginning, the rate of sulfur
Sulfur reduction or yield (%)
pH
10
8
6
0
10
20
30
40
50
Electrolytic time (min) Fig. 5. Effects of electrolytic time on pH. C (ethanol): 2.07 mol/L; current density: 10 × 10− 3 A/cm2; (▼) electrochemical flotation without ultrasound irradiation; (△) flotation using sonoelectrochemistry (ultrasonic intensity: 1.23 W/cm2).
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 [15] and partially organic sulfur into soluble sulfate or sulfonic acid root.
14
12
electrolyte
þ
At cathode : 2H þ 2e → H 2 ↑
76
3.0
75
2.8
74
2.6
73
2.4
72
2.2
71
6
8
10
12
14
16
18
20
Sulfur content (%)
Fig. 4. Effects of sonoelectrolytic time on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 2.07 mol/L; current density: 10 × 10− 3 A/cm2; ultrasonic intensity: 1.23 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
2.0 22
103Current density (A/cm2) Fig. 6. Effects of current density on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 2.07 mol/L; sonoelectrolytic time: 20 min; ultrasonic intensity: 1.23 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
16
H.-X. Zhang et al. / Fuel Processing Technology 93 (2012) 13–17
reaches 1.5 W/cm 2, the clean coal yield further decreases to 68.0%. This effect is negligible for flotation desulfurization because of the low clean coal yield. Therefore, the optimal ultrasonic intensity is 1.2 W/cm2.
10
3.5. Infrared spectroscopy and chemical analysis
pH
12
8
5
10
15
20
25
103Current density (A/cm2) Fig. 7. Effect of current density on pH. C (ethanol): 2.07 mol/L; sonoelectrolytic time: 20 min; ultrasonic intensity: 1.23 W/cm2.
81
Table 2 lists the sulfur content and yield for different granulometry of clean coal when subjected to three different enhanced-flotation methods under optimal conditions. The sulfur content in coal that was cleaned by ultrasound-enhanced flotation is considerably higher than that in the other samples. Likewise, the clean coal yield obtained by electrochemistry-enhanced flotation is considerably lower than that obtained by other methods. The combination of high sulfur reduction, high yield, and low sulfur content observed in the newly developed method of enhanced flotation by sonoelectrochemistry makes it potentially feasible under certain conditions. Furthermore, the method of enhanced flotation by sonoelectrochemistry is effective in ash reduction, and partially effective in calorific value increase in special granulometry such as −0.076 mm and 0.076–0.125 mm under certain conditions. 4. Conclusions This study aimed to evaluate the feasibility of enhanced desulfurizing flotation of high sulfur coal via the sonoelectrochemical method. In the −0.076 mm coal, optimal conditions include an anhydrous ethanol concentration of 2.1 mol/L, a sonoelectrolytic time of 20 min, current
66 2.0
63 0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.9 1.6
Ultrasonic intensity (W/cm2) Fig. 8. Effect of ultrasonic intensity on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 2.07 mol/L; sonoelectrolytic time: 20 min; current density: 15 × 10− 3 A/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
4000
a
3500
3000
2500
2000
1500
415.2
477.5
b
910-1040
910-1040
1000
477.5
2.1
415.2
69
1373.2
2.2
1527.2
72
1373.2
2.3 75
1527.2
78
60 0.0
3.6. Comparison
2.4
Sulfur content (%)
Sulfur reducation or yield (%)
reduction decreases rapidly. Compared with no irradiation, ultrasonic radiation at low intensities remarkably increases clean coal yield. As ultrasonic intensity approaches 0.5 W/cm2, the rate of sulfur reduction and clean coal yield almost stabilizes. As ultrasonic intensity reaches 1.0 W/cm2, the sulfur reduction continuously 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, 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 [16], causing particle fragmentation [17]. 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 [18]. The liquid–solid interface areas between coal particles and the solvent increase and improve mass transfer; this results in more favorable conditions for desulfurization. Moreover, ultrasonic cavitation [19,20] breaks the weak bonds of the simple organic sulfide on the interior of the coal particles and generates sulfide-free radicals [21–23]. These radicals combine with the organic solvent, and then dissolve into solution. Therefore, a proportion of total sulfur is removed by this process [24,25]. As ultrasonic intensity
Transmittance (%)
6
The infrared spectra of raw and treated clean coal generated by sonoelectrochemistry-enhanced flotation are shown in Fig. 9. Comparing curves b and a, we can postulate that: 1) The weakening of the hydrosulfonyl absorption band at 477.5 cm− 1 can be attributed to the removal of part of the organic sulfur by sonoelectrochemistry-enhanced flotation. 2) The weakening of the pyrite absorption band at 415.2 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 910–1040 cm− 1 can be attributed to the removal of ash material, such as kaolinite. 4) The weakening of aromatic C_C and the methyl absorption band at about 1600 cm− 1 and 1373.2 cm− 1 can be attributed to oxidation during sonoelectrochemistry-enhanced flotation. Therefore, enhanced flotation by sonoelectrochemistry can partially remove organic and pyritic sulfur, as well as ash. Under the optimal conditions, the contents of total sulfur, organic sulfur, pyritic sulfur, and sulfate sulfur decrease to 2.1%, 1.2%, 0.6% and 0.3%, respectively, as determined by chemical analysis. These results indicate that enhanced flotation by sonoelectrochemistry can partially remove organic sulfur, pyritic sulfur, and sulfate sulfur.
500
Wavenumber (cm-1) Fig. 9. IR spectra of Tunlan coal before and after treatment by sonoelectrochemistryenhanced flotation. (a) Raw coal; (b) after treatment (c (ethanol) = 2.07 mol/L; sonoelectrolytic time: 20 min; current density: 15 × 10− 3 A/cm2; ultrasonic intensity: 1.23 W/cm2).
H.-X. Zhang et al. / Fuel Processing Technology 93 (2012) 13–17
17
Table 2 Comparison of sulfur content and yield for different granularities of cleaned coal subjected to three different methods. Method of enhanced flotation − 0.076 mm Ultrasound Electrochemistry Sonoelectrochemistry 0.076–0.125 mm Ultrasound Electrochemistry Sonoelectrochemistry 0.125–0.2 mm Ultrasound Electrochemistry Sonoelectrochemistry 0.2–0.5 mm Ultrasound Electrochemistry Sonoelectrochemistry
C (ethanol) (mol/L)
Time (min)
Ultrasonic intensity (W/cm2)
20 20 20
1.2
2.1 2.1
40 40 40
1.2
2.1 2.1
20 20 20
1.2
1.0 1.0
20 20 20
1.2
0.7 0.7
1.2
1.2
1.2
1.2
Current density (103 A/cm2)
Sulfur (%)
Yield (%)
Sulfur reduction (%)
Ash reduction (%)
Moisture (%)
Calorific value (kJ/kg)
15 15
3.4 2.0 2.1
73.3 61.1 72.2
59.2 79.9 75.4
51.8 63.4 65.4
0.56 0.59 0.63
27,661 28,121 30,942
1.9 1.2 1.3
74.3 66.0 72.5
73.9 85.1 82.7
59.4 68.3 67.0
0.45 0.57 0.52
27,898 32,074 32,789
15 15
1.4 1.0 1.1
64.0 59.8 73.0
78.2 86.1 81.2
65.5 59.3 59.8
0.44 0.56 0.52
29,089 33,595 32,625
12.5 12.5
1.3 1.0 1.1
63.7 57.2 68.9
72.9 80.9 76.1
64.9 62.0 63.2
0.36 0.53 0.49
30,074 34,648 33,533
7.5 7.5
density at 15 × 10− 3 A/cm2, and ultrasonic intensity at 1.2 W/cm2. The rate of sulfur reduction reaches 75.4%. IR and chemical analyses indicate that parts of pyritic and organic sulfur, as well as ash, are removed using this method. The combination of high sulfur reduction, high yield, and low sulfur content obtained in the newly developed method of enhanced flotation by sonoelectrochemistry makes it a potentially feasible approach under specific conditions. The method can be applied effectively in the desulfurization of coal under specific controlled conditions. Acknowledgments This project was supported by the Tarim University President Foundation Master's Programs (TDZKSS09006). References [1] A.M. Buswell, M.J. Nicol, Some aspects of the electrochemistry of the flotation of pyrrhotite, Journal of Applied Electrochemistry 32 (2002) 1321–1329. [2] S. Chander, Electrochemistry of sulfide flotation: growth characteristics of surface coatings and their properties, with special reference to chalcopyrite and pyrite, International Journal of Mineral Processing 33 (1991) 121–134. [3] W. Zhao, H. Zhu, Z.M. Zong, J.H. Xia, X.Y. Wei, Electrochemical reduction of pyrite in aqueous NaCl solution, Fuel 84 (2005) 235–238. [4] X.H. Wang, K.S.E. Forssberg, The solution electrochemistry of sulfide-xanthate-cyanide systems in sulfide mineral flotation, Minerals Engineering 9 (5) (1996) 527–546. [5] R. Woods, Electrochemical potential controlling flotation, International Journal of Mineral Processing 72 (2003) 151–162. [6] E.C. Cilek, S. Ozgen, Effect of ultrasound on separation selectivity and efficiency of flotation, Minerals Engineering 22 (14) (2009) 1209–1217. [7] Ş.G. Özkan, H.Z. Kuyumcu, Investigation of mechanism of ultrasound on coal flotation, International Journal of Mineral Processing 81 (3) (2006) 201–203. [8] Ş.G. Özkan, H.Z. Kuyumcu, Design of a flotation cell equipped with ultrasound transducers to enhance coal flotation, Ultrasonics Sonochemistry 14 (5) (2007) 639–645. [9] W.Z. Kang, H.X. Xun, J. Hu, Study of the effect of ultrasonic treatment on the surface composition and the flotation performance of high-sulfur coal, Fuel Processing Technology 89 (12) (2008) 1337–1344.
[10] B. Lakard, L. Ploux, K. Anselme, F. Lallemand, S. Lakard, M. Nardin, J.Y. Hihn, Effect of ultrasounds on the electrochemical synthesis of polypyrrole, application to the adhesion and growth of biological cells, Bioelectrochemistry 75 (2) (2009) 148–157. [11] A. Mandroyan, R. Viennet, Y. Bailly, M.L. Doche, J.Y. Hihn, Modification of the ultrasound induced activity by the presence of an electrode in a sonoreactor working at two low frequencies (20 and 40 kHz). Part I: active zone visualization by laser tomography, Ultrasonics Sonochemistry 16 (1) (2009) 88–96. [12] G.G. José, D.E. María, B. Pedro, V.H. Yolanda, G.G. Luís, S. Verónica, Current topics on sonoelectrochemistry, Ultrasonics 50 (2) (2010) 318–322. [13] H. Lei, Y.J. Tang, J.J. Wei, J. Li, X.B. Li, H.L. Shi, Synthesis of tungsten nanoparticles by sonoelectrochemistry, Ultrasonics Sonochemistry 14 (1) (2007) 81–83. [14] M. Abdollahi, A.Z. Moghaddam, K. Rami, Fuel 85 (2006) 1117–1124. [15] X.M. Zhu, J. Li, D.M. Bodily, M.E. Wadsworth, Transpassive oxidation of pyrite, Journal of the Electrochemical Society 140 (7) (1993) 1927–1935. [16] W. Lauterborn, W. Hentschel, Cavitation bubble dynamics studied by high speed photography and holography: part two, Ultrasonics 24 (1986) 59–65. [17] K.S. Suslick, D.J. Casadonte, M.L.H. Green, M.E. Thompson, Effects of high intensity ultrasound on inorganic solids, Ultrasonics 25 (1987) 56–59. [18] K.S. Suslick, S.J. Doktycz, The sonochemistry of zinc powder, Journal of the American Chemical Society 111 (6) (1989) 2342–2344. [19] A. Gedanken, Doping nanoparticles into polymers and ceramics using ultrasound radiation, Ultrasonics Sonochemistry 14 (2007) 418–430. [20] W. Lauterborn, T. Kurz, R. Geisler, D. Schanz, O. Lindau, Acoustic cavitation, bubble dynamics and sonoluminescence, Ultrasonics Sonochemistry 14 (4) (2007) 484–491. [21] S.A.H. Zaidi, Ultrasonically enhanced coal desulphurization, Fuel Processing Technology 33 (2) (1993) 95–100. [22] T.J. Mason, J.P. Lorimer, Sonochemistry: Theory, Applications and Uses of Ultrasound in Chemistry, Ellis Horwood, Chichester, 1988. [23] C. Petrier, J.L. Luche, Synthetic Organic Sonochemistry, in: J.L. Luche (Ed.), Plenum Press, New York, 1998, (Chapter 2). [24] G. Cravotto, P. Cintas, Power ultrasound in organic synthesis: moving cavitational chemistry from academia to innovative and large-scale applications, Chemical Society Reviews 35 (2006) 180–196. [25] J.L. Zhao, Y.Y. Zhang, H.W. Wang, Q.Y. Chen, Desulfurization of high organic sulfur coal by tetrachloroethylene extraction under ultrasonic field, Journal of Fuel Chemistry and Technology (Chinese) 30 (3) (2002) 234–237.
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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Enhanced desulfurizing flotation of high sulfur coal by sonoelectrochemical method Hong-Xi Zhang a,⁎, Xiao-Yan Ma a, Xian-Shu Dong b, Zhi-Zhong Wang c, Hong-Jin Bai a 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 20 December 2010 Received in revised form 23 July 2011 Accepted 4 September 2011 Available online 7 October 2011 Keywords: Sonoelectrochemistry Sonocavitation Enhanced flotation Desulfurization
a b s t r a c t Enhanced desulfurizing flotation 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 sample with a particle size of −0.076 mm, the optimal experimental conditions achieved for anhydrous ethanol, sonoelectrolytic time, current density, and ultrasound intensity are 2.1 mol/L, 20 min, 15× 10− 3 A/cm2, and 1.2 W/cm2, respectively. Optimal conditions cause a sulfur reduction of up to 75.4%. 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 enhanced flotation by ultrasound or electrochemistry, desulfurizing flotation of high sulfur coal by sonoelectrochemistry is an effective technology. © 2011 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. However, the technique still generates an un-ideal yield of clean coal. 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
⁎ Corresponding author. Tel.: + 86 997 4681613; fax: + 86 997 4681612. E-mail address: [email protected] (H.-X. Zhang). 0378-3820/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2011.09.007
obtained by electrochemistry alone [10–13]. 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 granulometry. For the coal sample with a particle size of −0.076 mm, the contents of sulfur and ash were much higher than those reflected in the granulometry of the other samples. In the present study, coal with a granulometry of −0.076 mm was chosen as the experimental coal sample. The date at which proximate analysis was conducted and the sulfur content of the coal are listed in Table 1. The concentrations of slurry and supporting electrolyte were 96 and 2.0 g/L, 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).
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Table 1 Proximate analysis and sulfur content (W%, ad) of Tunlan coal. Proximate analysis
Sulfur content
Moisture
Ash
Volatile matter
Calorific value (kJ/kg)
Total
Pyritic
Sulfate
Organic
1.56
22.35
18.16
25,022
6.12
3.53
1.08
1.51
2.2. Instruments Fig. 2. Flotation experimental processing and conditions.
Electrolytic power was provided by a DH1722 DC-regulated power supply. Graphite (97.4 cm2) and stainless steel (180.3 cm2) 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 cm2), 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 [14] sulf ur reduction ðwt:%Þ ¼ 100½x1 −x2 ðm2 =m1 Þ=x1
ð1Þ
ash reduction ðwt:%Þ ¼ 100½y1 −y2 ðm2 =m1 Þ=y1
ð2Þ
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 rapidly increases. Before the ethanol concentration reaches 0.7 mol/L, the clean coal yield continues to rapidly increase. This phenomenon can be attributed to the presence of specific numbers of ethanol molecules, which can improve coal surface wettability and generate a favorable condition for clean coal recovery. However, as ethanol concentration reaches 0.7 mol/L, the hydrophilicity of the coal surface is enhanced and the clean coal yield begins to diminish. When ethanol concentration reaches 2.8 mol/L, the clean coal yield drops to 65.0%. This low yield is negligible for desulfurizing flotation. Therefore, the optimal ethanol concentration is 2.1 mol/L. These results indicate that ethanol concentration has a significant effect on clean coal yield, in which a specific amount of ethanol proves favorable for enhanced desulfurizing flotation by sonoelectrochemistry.
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, is the ash percentage in the original sample, and denotes the ash percentage in the coal obtained from leaching.
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. Initially, several groups containing sulfur are absorbed on the coal surface. The sulfur on the coal surface is easy to remove by the electrochemical oxidation of the anode. As time progresses, however, the reaction comes to the interior coal particles instead of on the surface. Therefore, sulfur removal becomes difficult at this point, and sulfur reduction proceeds more slowly. When a sonoelectrolytic time of about 20 min elapses, part of the inorganic and organic sulfur may have been converted into 3.0
Sulfur reduction or yield (%)
81 78
2.8
75 2.6 72 2.4 69 2.2
66 63 0.0
0.5
1.0
1.5
2.0
2.5
Sulfur content (%)
y1 y2
3.1. Effect of ethanol concentration on sulfur reduction
3.2. Effect of sonoelectrolytic time on sulfur reduction
where m1 m2 x1 x2
3. Results and discussion
2.0 3.0
c (ethanol) (mol/L) 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.
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: 1.23 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
H.-X. Zhang et al. / Fuel Processing Technology 93 (2012) 13–17
75
3.5
70 3.0 65 2.5
60
55
−
4.0
10
0
20
30
40
2−
8OH þ 2FeS2 þ 7O2 →2FeðOHÞ2 þ 4SO4
þ 2H 2 O
ð5Þ
−
4OH →O2 þ H2 O þ 4e
Sulfur content (%)
Sulfur reduction or yield (%)
80
15
O2
ð6Þ
O2
=
OH =H 2 O
RSH → R–S–S–R → R–SðO2 Þ–SðO2 Þ−R → R–OH =
2−
þ R –OH þ 2SO4
þ 4H
þ
ð7Þ
ð8Þ
2.0
50
Sonoelectrolytic time (min)
elemental sulfur because of intense oxidation; the elemental sulfur is difficult to remove by washing or flotation. 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 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 and the declining rate proceeds even more slowly, indicating that oxidation advances in the course of sonoelectrolysis. At the beginning of the process, the electrochemical oxidation is mostly restricted to the anode surface. This reaction proceeds easily, thereby causing pH to decline rapidly. As the sonoelectrolytic time is extended, the reaction occurs in the interior coal particles instead of on the surface. Thus, the rate of reaction slows down and the drop in pH increasingly decelerates. In addition, the effect of pH plotted against electrolytic time without ultrasound irradiation under the same experimental conditions is also presented in Fig. 5. Comparing electrochemistry without ultrasound irradiation with sonoelectrochemistry, the drop in pH using electrochemistry without ultrasound irradiation is slower than sonoelectrochemistry.
electrolyte
þ
At anode : 2H 2 O → O2 þ 4H þ 4e −
16OH þ 4FeS2 þ 15O2 →4FeðOHÞ3 þ 8SO4
ð3Þ 2−
þ 2H2 O
ð4Þ
3.3. 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 increases to its maximum, and then decreases rapidly. The mechanism behind this phenomenon may be postulated as follows. Initially, the increase in current density imposes a favorable effect on flotation desulfurization and facilitates an ideal clean coal yield. As current density reaches 10× 10− 3 A/cm2, the rate of clean coal yield begins to drop because of the oxidation on the coal surface; oxidation weakens hydrophobicity on the surface of coal particles. However, as current density approaches 15 × 10 − 3 A/cm 2, portions of inorganic and organic sulfur are converted into elemental sulfur because of intense oxidation. The elemental sulfur generated is difficult to remove by washing or flotation. Thereafter, the rate of sulfur reduction falls rapidly. The optimal current density is 15× 10− 3 A/cm2 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. 3.4. Effect of ultrasonic intensity on sulfur reduction Fig. 8 shows the sulfur reduction, clean coal yield and sulfur content plotted against ultrasonic intensity. In the beginning, the rate of sulfur
Sulfur reduction or yield (%)
pH
10
8
6
0
10
20
30
40
50
Electrolytic time (min) Fig. 5. Effects of electrolytic time on pH. C (ethanol): 2.07 mol/L; current density: 10 × 10− 3 A/cm2; (▼) electrochemical flotation without ultrasound irradiation; (△) flotation using sonoelectrochemistry (ultrasonic intensity: 1.23 W/cm2).
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 [15] and partially organic sulfur into soluble sulfate or sulfonic acid root.
14
12
electrolyte
þ
At cathode : 2H þ 2e → H 2 ↑
76
3.0
75
2.8
74
2.6
73
2.4
72
2.2
71
6
8
10
12
14
16
18
20
Sulfur content (%)
Fig. 4. Effects of sonoelectrolytic time on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 2.07 mol/L; current density: 10 × 10− 3 A/cm2; ultrasonic intensity: 1.23 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
2.0 22
103Current density (A/cm2) Fig. 6. Effects of current density on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 2.07 mol/L; sonoelectrolytic time: 20 min; ultrasonic intensity: 1.23 W/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
16
H.-X. Zhang et al. / Fuel Processing Technology 93 (2012) 13–17
reaches 1.5 W/cm 2, the clean coal yield further decreases to 68.0%. This effect is negligible for flotation desulfurization because of the low clean coal yield. Therefore, the optimal ultrasonic intensity is 1.2 W/cm2.
10
3.5. Infrared spectroscopy and chemical analysis
pH
12
8
5
10
15
20
25
103Current density (A/cm2) Fig. 7. Effect of current density on pH. C (ethanol): 2.07 mol/L; sonoelectrolytic time: 20 min; ultrasonic intensity: 1.23 W/cm2.
81
Table 2 lists the sulfur content and yield for different granulometry of clean coal when subjected to three different enhanced-flotation methods under optimal conditions. The sulfur content in coal that was cleaned by ultrasound-enhanced flotation is considerably higher than that in the other samples. Likewise, the clean coal yield obtained by electrochemistry-enhanced flotation is considerably lower than that obtained by other methods. The combination of high sulfur reduction, high yield, and low sulfur content observed in the newly developed method of enhanced flotation by sonoelectrochemistry makes it potentially feasible under certain conditions. Furthermore, the method of enhanced flotation by sonoelectrochemistry is effective in ash reduction, and partially effective in calorific value increase in special granulometry such as −0.076 mm and 0.076–0.125 mm under certain conditions. 4. Conclusions This study aimed to evaluate the feasibility of enhanced desulfurizing flotation of high sulfur coal via the sonoelectrochemical method. In the −0.076 mm coal, optimal conditions include an anhydrous ethanol concentration of 2.1 mol/L, a sonoelectrolytic time of 20 min, current
66 2.0
63 0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.9 1.6
Ultrasonic intensity (W/cm2) Fig. 8. Effect of ultrasonic intensity on sulfur reduction, clean coal yield and sulfur content. C (ethanol): 2.07 mol/L; sonoelectrolytic time: 20 min; current density: 15 × 10− 3 A/cm2; (●) yield; (■) sulfur reduction; (★) sulfur content.
4000
a
3500
3000
2500
2000
1500
415.2
477.5
b
910-1040
910-1040
1000
477.5
2.1
415.2
69
1373.2
2.2
1527.2
72
1373.2
2.3 75
1527.2
78
60 0.0
3.6. Comparison
2.4
Sulfur content (%)
Sulfur reducation or yield (%)
reduction decreases rapidly. Compared with no irradiation, ultrasonic radiation at low intensities remarkably increases clean coal yield. As ultrasonic intensity approaches 0.5 W/cm2, the rate of sulfur reduction and clean coal yield almost stabilizes. As ultrasonic intensity reaches 1.0 W/cm2, the sulfur reduction continuously 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, 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 [16], causing particle fragmentation [17]. 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 [18]. The liquid–solid interface areas between coal particles and the solvent increase and improve mass transfer; this results in more favorable conditions for desulfurization. Moreover, ultrasonic cavitation [19,20] breaks the weak bonds of the simple organic sulfide on the interior of the coal particles and generates sulfide-free radicals [21–23]. These radicals combine with the organic solvent, and then dissolve into solution. Therefore, a proportion of total sulfur is removed by this process [24,25]. As ultrasonic intensity
Transmittance (%)
6
The infrared spectra of raw and treated clean coal generated by sonoelectrochemistry-enhanced flotation are shown in Fig. 9. Comparing curves b and a, we can postulate that: 1) The weakening of the hydrosulfonyl absorption band at 477.5 cm− 1 can be attributed to the removal of part of the organic sulfur by sonoelectrochemistry-enhanced flotation. 2) The weakening of the pyrite absorption band at 415.2 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 910–1040 cm− 1 can be attributed to the removal of ash material, such as kaolinite. 4) The weakening of aromatic C_C and the methyl absorption band at about 1600 cm− 1 and 1373.2 cm− 1 can be attributed to oxidation during sonoelectrochemistry-enhanced flotation. Therefore, enhanced flotation by sonoelectrochemistry can partially remove organic and pyritic sulfur, as well as ash. Under the optimal conditions, the contents of total sulfur, organic sulfur, pyritic sulfur, and sulfate sulfur decrease to 2.1%, 1.2%, 0.6% and 0.3%, respectively, as determined by chemical analysis. These results indicate that enhanced flotation by sonoelectrochemistry can partially remove organic sulfur, pyritic sulfur, and sulfate sulfur.
500
Wavenumber (cm-1) Fig. 9. IR spectra of Tunlan coal before and after treatment by sonoelectrochemistryenhanced flotation. (a) Raw coal; (b) after treatment (c (ethanol) = 2.07 mol/L; sonoelectrolytic time: 20 min; current density: 15 × 10− 3 A/cm2; ultrasonic intensity: 1.23 W/cm2).
H.-X. Zhang et al. / Fuel Processing Technology 93 (2012) 13–17
17
Table 2 Comparison of sulfur content and yield for different granularities of cleaned coal subjected to three different methods. Method of enhanced flotation − 0.076 mm Ultrasound Electrochemistry Sonoelectrochemistry 0.076–0.125 mm Ultrasound Electrochemistry Sonoelectrochemistry 0.125–0.2 mm Ultrasound Electrochemistry Sonoelectrochemistry 0.2–0.5 mm Ultrasound Electrochemistry Sonoelectrochemistry
C (ethanol) (mol/L)
Time (min)
Ultrasonic intensity (W/cm2)
20 20 20
1.2
2.1 2.1
40 40 40
1.2
2.1 2.1
20 20 20
1.2
1.0 1.0
20 20 20
1.2
0.7 0.7
1.2
1.2
1.2
1.2
Current density (103 A/cm2)
Sulfur (%)
Yield (%)
Sulfur reduction (%)
Ash reduction (%)
Moisture (%)
Calorific value (kJ/kg)
15 15
3.4 2.0 2.1
73.3 61.1 72.2
59.2 79.9 75.4
51.8 63.4 65.4
0.56 0.59 0.63
27,661 28,121 30,942
1.9 1.2 1.3
74.3 66.0 72.5
73.9 85.1 82.7
59.4 68.3 67.0
0.45 0.57 0.52
27,898 32,074 32,789
15 15
1.4 1.0 1.1
64.0 59.8 73.0
78.2 86.1 81.2
65.5 59.3 59.8
0.44 0.56 0.52
29,089 33,595 32,625
12.5 12.5
1.3 1.0 1.1
63.7 57.2 68.9
72.9 80.9 76.1
64.9 62.0 63.2
0.36 0.53 0.49
30,074 34,648 33,533
7.5 7.5
density at 15 × 10− 3 A/cm2, and ultrasonic intensity at 1.2 W/cm2. The rate of sulfur reduction reaches 75.4%. IR and chemical analyses indicate that parts of pyritic and organic sulfur, as well as ash, are removed using this method. The combination of high sulfur reduction, high yield, and low sulfur content obtained in the newly developed method of enhanced flotation by sonoelectrochemistry makes it a potentially feasible approach under specific conditions. The method can be applied effectively in the desulfurization of coal under specific controlled conditions. Acknowledgments This project was supported by the Tarim University President Foundation Master's Programs (TDZKSS09006). References [1] A.M. Buswell, M.J. Nicol, Some aspects of the electrochemistry of the flotation of pyrrhotite, Journal of Applied Electrochemistry 32 (2002) 1321–1329. [2] S. Chander, Electrochemistry of sulfide flotation: growth characteristics of surface coatings and their properties, with special reference to chalcopyrite and pyrite, International Journal of Mineral Processing 33 (1991) 121–134. [3] W. Zhao, H. Zhu, Z.M. Zong, J.H. Xia, X.Y. Wei, Electrochemical reduction of pyrite in aqueous NaCl solution, Fuel 84 (2005) 235–238. [4] X.H. Wang, K.S.E. Forssberg, The solution electrochemistry of sulfide-xanthate-cyanide systems in sulfide mineral flotation, Minerals Engineering 9 (5) (1996) 527–546. [5] R. Woods, Electrochemical potential controlling flotation, International Journal of Mineral Processing 72 (2003) 151–162. [6] E.C. Cilek, S. Ozgen, Effect of ultrasound on separation selectivity and efficiency of flotation, Minerals Engineering 22 (14) (2009) 1209–1217. [7] Ş.G. Özkan, H.Z. Kuyumcu, Investigation of mechanism of ultrasound on coal flotation, International Journal of Mineral Processing 81 (3) (2006) 201–203. [8] Ş.G. Özkan, H.Z. Kuyumcu, Design of a flotation cell equipped with ultrasound transducers to enhance coal flotation, Ultrasonics Sonochemistry 14 (5) (2007) 639–645. [9] W.Z. Kang, H.X. Xun, J. Hu, Study of the effect of ultrasonic treatment on the surface composition and the flotation performance of high-sulfur coal, Fuel Processing Technology 89 (12) (2008) 1337–1344.
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