Energy & Fuels 2005, 19, 1003-1007
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Desulfurization and Deashing of Hazro Coal via a Flotation Method Fatma Deniz Ayhan,* Halime Abakay, and Abdurrahman Saydut Department of Mining Engineering, Dicle University, 21280 Diyarbakır, Turkey Received October 5, 2004. Revised Manuscript Received February 9, 2005
In this study, desulfurization and deashing of Hazro coal via a flotation method were studied. For this purpose, experimental studies were conducted on the coal sample with ash, sulfur, volatile matter, and fixed carbon contents of 24.77%, 6.90%, 35.18%, and 38.06%, respectively, and two groups of flotation experiments were made. The effects of pH, collector amount, and various frothers (MIBC, AF 76, pine oil, DF 250) on the depressing and floating pyrite from Hazro coal were investigated in the first and second group flotation experiments, respectively. Flotation results indicated that, when compared to various frothers, the following order for the depressing of pyrite was obtained: MIBC > pine oil > AF 76 > DF 250. For the floating of pyrite, the following order was obtained: DF 250 > pine oil > AF 76 > MIBC. A concentrate containing 1.12% pyritic sulfur and 13.02% ash with a pyritic sulfur reduction of 66.86% was obtained from a feed that contained 4.95% pyritic sulfur and 24.77% ash. The flotation method removed most of the sulfate sulfur (>90%) and 66.86% of the pyritic sulfur from the coal sample.
Introduction Because of the energy situation and the gradual depletion of high-quality coal reserves of the world, the demineralization and/or desulfurization of low-grade high-ash and/or high-sulfur coals to obtain environmentally acceptable clean fuels has attracted greater attention.1-3 Sulfur and mineral matter contents of a coal are important, because direct utilization of highash and high-sulfur content coals effects serious technological and environmental problems. Various properties of these type coals may be improved by application of a convenient cleaning procedure. The subject of coal upgrading has received increased attention recently, and the subject is actively under study.4-6 Mineral matter and sulfur exhibit harmful effects on the utilization of coal for purposes of combustion, carbonization, gasification, liquefaction, etc. These impurities poison the catalyst that is used in coal conversion processes. SO2 produced during combustion leads to atmospheric pollution, acid rain, and the corrosion of boilers, pipelines, and other machinery. Therefore, it is necessary to remove mineral matter and sulfur from coal prior to utilization. The most common types of mineral matter in coal include various forms of silica (such as quartz, opal, cherts, etc.) and clay minerals. Coal also contains * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Kara, H.; Ceylan, R. Fuel 1988, 67, 170-172. (2) Sharma, D. K.; Gihar, S. Fuel 1991, 70, 663-665. (3) Mukherjee, S.; Borthakur, P. C. Fuel Process. Technol. 2003, 85, 157-164. (4) Karaca, H.; Ceylan, K. Fuel Process. Technol. 1997, 50, 19-33. (5) Karaca, S.; Akyu¨rek, M.; Bayrakc¸ eken, S. Fuel Process. Technol. 2003, 80, 1-8. (6) Mukherjee, S.; Borthakur, P. C. Fuel 2001, 80, 2037-2040.
various carbonates, sulfates, sulfides, oxides, etc. Both physical and chemical methods are adopted to remove the mineral matter and sulfur from coal.7,8 Coals are heterogeneous, complex, and noncrystalline macromolecules that contain both organic and inorganic materials. Inorganic materials of coal contain several inorganic constituents, especially sulfur, which has an important role in almost all coal utilization systems. Some techniques have been applied to coal to remove its inorganic materials from the organic component. One of the most suitable methods for the removal of pyritic sulfur from coal is froth flotation.9 Flotation is a process for separating a mixture of two species of finely ground solids, based on differences in the species surface properties.10,11 Primarily, the ashforming and sulfur-bearing minerals found in coal are hydrophilic and, therefore, should remain in the tailings from coal flotation.12,13 To remove sulfur and ash from Hazro coal, the flotation method was applied. The effect of different parameters on flotation was investigated, and the experimental results are presented here. (7) Mukherjee, S.; Borthakur, P. C. Fuel Process. Technol. 2003, 85, 93-101. (8) Mukherjee, S.; Borthakur, P. C. Fuel 2003, 82, 783-788. (9) Demirbas¸ , A. Energy Convers. Manage. 2002, 43, 885-895. (10) Banford, A. W.; Aktas¸ , Z.; and Woodburn, E. T. Powder Technol. 1998, 98, 61-73. (11) Vamvuka, D.; Agridiotis, V. Int. J. Miner. Process. 2001, 61, 209-224. (12) Hirt, W. C.; Aplan, F. F. In Processing and Utilization of High Sulfur Coal, IV; Dungan, P. R., Quigley, D. R., Attia, Y. A., Eds.; Coal Science and Technology, Vol. 18; Elsevier: Amsterdam, 1991; pp 339356. (13) Olson, T. J.; Aplan F. F. In Applied Mineralogy; Park, W. C, Hausen, D. M., Hagni, R. D., Eds.; Proceedings of the Second International Congress on Applied Mineralogy in the Minerals Industry, Los Angeles, CA, February 22-25, 1984; Metallurgical Society of AIME: Warrendale, PA, 1984; pp 367-393.
10.1021/ef049747r CCC: $30.25 © 2005 American Chemical Society Published on Web 03/10/2005
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Table 1. Proximate Analysis Results of the Coal Sample component
as-received
air-dried
moisture content (%) ash content (%) volatile matter content (%) fixed carbon content (%) upper heating value (kcal/kg) total sulfur content (%) pyritic sulfur content (%) sulfate sulfur content (%) organic sulfur content (%)
2.76 24.57 34.90 37.76 5890 6.90
1.99 24.77 35.18 38.06 5937 6.90 4.95 0.10 1.85
a
drieda 25.27 35.89 38.84 6058 7.00
Figure 1. Zeta potential values of the coal sample, with respect to solution pH.
The sample was dried to constant mass at 105 °C.
Table 2. Ultimate Analysis Results of the Coal Sample component
content (%, daf)
C H S (total) N O (diff.)
70.24 5.67 7.00 0.66 16.43
Table 3. Major Element Contents of the Coal Sample component
composition (%)
SiO2 Fe2O3 Al2O3 CaO MgO Na2O K2O SO3
42.0 16.5 33.9 0.9 0.5 0.2 0.9 0.8
Table 4. Size Analysis of the Coal Sample size fraction (mm)
amount (wt %, dry)
cumulative amount under size (wt %)
-0.106/+0.075 -0.075/+0.053 -0.053/+0.045 -0.045/+0.038 -0.038 total
12.70 17.80 14.90 18.40 36.20 100.00
100.00 87.30 69.50 54.60 36.20
Experimental Section 1. Material. The coal sample used in this work was obtained from Hazro, Turkey. Proximate analysis, ultimate analysis, and major element contents of the coal sample are given in Tables 1, 2, and 3, respectively. The proximate and ultimate analyses were performed using Turkish and ASTM standards.14 The coal sample was ground to a nominal top size of -0.1 mm in a ball mill (Denver type) for flotation tests. The screen analysis of the coal sample is given in Table 4. 2. Reagents Used in the Experiments. Kerosene and sodium isopropyl xanthate (Aero 343) were used as collectors in the first and second group experiments, respectively. Pine oil, MIBC, AF 76, and DF 250 were used as frother types in both groups. NaOH, HCl, and NaOH, H2SO4 were used as pH modifiers and Na2SiO3 and sodium diphosphate (Na4P2O7) were used as depressants for the first and second group flotation experiments, respectively. In the studies, all reagents were reagent-grade chemicals, and all reagents were prepared as solutions of 1 wt %. 3. Method. In this study, the removal of sulfur and ash from the Hazro coal was the intent. For this purpose, the study was performed in two groups. Flotation tests were performed in a cell with a capacity of 1 L, using a Denver laboratory flotation machine. The concentrate and the tailings were filtered, dried, weighed, and analyzed for ash, pyritic sulfur, and sulfate (14) Sevinc¸ , M. Yurt Madencilis¸ ini Gelis¸ tirme Vakfı, 1997.
sulfur. Ash content was determined according to the methodology defined in ASTM 3174. Pyritic sulfur and sulfate sulfur contents were determined according to the methodology defined in ASTM D2492-77.
Results and Discussion 1. Zeta Potential Measurements of Coal Samples. Zeta potential measurements of the coal samples were achieved using a “Zeta Meter System 3.0” apparatus. Zero point of charge (zpc) measurements were performed on the concentrate obtained by flotation. Results of the zpc measurements are given in Figure 1. As observed in Figure 1, the zpc value of coal is located at pH 7.0. The surface charges of the coal sample are positive at pH 7. The dissociation of surface functional groups (e.g., -COOH) increased as the hydroxide concentration [OH-] increased.15 Figure 1 also shows that the zeta potential of coal is negative over the pH range of 7-12, with the potential becoming negative as the pH increased. Functionality of the coal surface also is very important in the electrokinetic behavior of the coal particles. The results are consistent with those reported by Crawford.15 2. Flotation Experiments. 2.1. The First Group Flotation Experiments. Evaluation of the flotation results was made according to the pyritic sulfur, sulfate sulfur, and ash contents of the concentrates obtained from the first group experiments. The operating conditions of the first group experiments were as follows: impeller speed, 1000 rpm; solidliquid ratio, 10%; wetting conditioning time, 10 min; depressant conditioning time, 10 min; collector conditioning time, 10 min; frother conditioning time, 5 min; depressant (Na2SiO3) amount, 200 g/t; collector (kerosene) amount, 300 g/t; frother (pine oil) amount, 100 g/t; flotation time, 3 min; and particle size, 100% -0.1 mm. The operating conditions were determined according to pre-experiments. 2.1.1. Effect of pH. The effect of pH was established. Test results are given in Figures 2 and 3. As shown in Figure 2, the pyritic sulfur contents of the concentrates obtained at pH 5, 6, and 7 were 3.01%, 3.32%, and 3.43%, respectively, in which the pyritic sulfur reductions were lower than the other pH values because the floating of pyrite was high. Pyrite was usually depressed during the polymetallic ore treatment by increasing the pH to ∼11 via the use of lime.16 High conditioning pH values, long conditioning times, and extensive surface oxidation were usually required to (15) Crawford, R. J.; Mainwaring, D. E. Fuel 2001, 80, 313-320. (16) Benzaazoua, M.; Kongolo, M. Int. J. Miner. Process. 2003, 66, 221-234.
Desulfurization and Deashing of Hazro Coal
Figure 2. Effect of pH on the pyritic sulfur reduction of coal and the pyritic sulfur content of the concentrate.
Figure 3. Effect of pH on the ash content and combustible yield of concentrate.
depress pyrite flotation.17,18 The pyritic sulfur contents of the concentrates obtained at pH 8, 9, and 10 were 2.14%, 2.01%, and 2.19%, respectively, in which the pyrite was floated at minimum levels. As shown in Figure 3, the ash contents of the concentrates obtained at pH 2, 3, 4, and 5 were 21.27%, 20.78%, 20.31%, and 20.01%, respectively; the ash contents of these concentrates were higher than those at the other pH values. The Al2O3 content of the coal sample was 33.90%. The isoelectric points of Al2O3 were pH 6.94, pH 8.4, and pH 9.4.19 The floatability of Al2O3 was high in an acidic medium in which the surface charge of Al2O3 was positive. Therefore, the ash contents of the concentrates obtained in an acidic medium were high. The best pH value for desulfurization was 9. The pyritic sulfur reduction and pyritic sulfur content of the concentrate obtained at pH 9 were 59.39% and 2.01%, respectively. The pyritic sulfur content of the Hazro coal was reduced from 4.95% to 2.01% at pH 9. Ash content and combustible yield of the concentrate obtained at pH 9 were 19.10% and 82.80%, respectively. The reduction in ash content of the coal sample was 5.76%. 2.1.2. Effect of Kerosene Amount. The effect of kerosene amount in the experiments was established, and the test results are given in Figures 4 and 5. A kerosene amount of 250 g/t was established as being the best, because of the low pyritic sulfur content (1.98%) and high pyritic sulfur reduction (60%) of the concentrate obtained. (17) Kristal, Z.; Grano, S. R.; Reynolds, K.; Smart, R. St.; Ralston, J. In Proceedings of the Fifth Mill Operators’ Conference; AUSIMM: 1994, pp 171-179. (18) Shen, W. Z.; Fornasiero, D.; Ralston, J. Mineral. Eng. 1998, 11 (2), 145-158. (19) de Bruyn, P. L., Agar, G. E., Fuerstenau, D. W., Eds. Froth Flotation 50th Anniversary Volume; The American Institute of Mining, Metallurgical, and Petroleum Engineers: New York, 1962; pp 91-138.
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Figure 4. Effect of kerosene amount on the pyritic sulfur reduction of coal and the pyritic sulfur content of the concentrate.
Figure 5. Effect of kerosene amount on the ash content and combustible yield of the concentrate.
Figure 6. Effect of frother type on the pyritic sulfur reduction of coal and the pyritic sulfur content of concentrate (A, DF 250; B, AF 76; C, pine oil; and D, MIBC).
Figures 4 and 5 show that increasing the amount of kerosene decreased the pyritic sulfur reduction and increased the ash contents of the concentrates obtained, particularly in kerosene amounts of 300, 350, and 400 g/t, in which the ash contents of the concentrates were 19.10%, 19.69%, and 19.98%, respectively. 2.1.3. Effect of Frother Type. The effect of frother type in the experiments was established, and test results are given in Figures 6 and 7. Figure 6 shows that MIBC was the most effective of the investigated frothers, in regard to removing pyritic sulfur from the coal sample. The low pyritic sulfur content (1.83%) and high pyritic sulfur reduction (63.03%) of the concentrate were obtained while MIBC was being used. The following order for reduction in pyritic sulfur of the concentrates was obtained: MIBC > pine oil > AF 76 > DF 250. For pyritic sulfur content, the following order was obtained: MIBC < pine oil < AF 76 < DF 250.
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Figure 8. Effect of pH on the pyritic sulfur reduction of coal and the pyritic sulfur content of tailing.
Figure 7. Effect of frother type on the ash content and combustible yield of concentrate (A, DF 250; B, AF 76; C, pine oil; and D, MIBC). Table 5. Results of Cleaning Flotation of the First Group Flotation Experiments product
amount (wt %)
pyritic sulfur (%)
ash (%)
carbon yield (%)
concentrate middling 3 middling 2 middling 1 tailing total
43 9 11 13 24 100
1.50 1.71 2.13 2.75 14.83 4.95
14.16 17.26 20.45 22.31 49.91 24.77
49.06 9.90 11.63 13.43 15.98 100.00
Figure 7 shows that, when MIBC was used, the ash content was 16.83% and the combustible yield of the concentrate obtained was 84.02%. The reduction in ash content of the coal sample was 7.94%. The highest amount of ash and combustible yield of concentrate were achieved using DF 250: 93% and 98.50%, respectively. The following order for both ash content and combustible yield of the concentrates was obtained: DF 250 > AF 76 > pine oil > MIBC. The pyritic sulfur content of the coal was reduced from 4.95% to 1.83% when MIBC was used, according to the test results. The rougher concentrate, which was comprised of 1.83% pyritic sulfur, with 63.03% pyritic sulfur reduction and an ash content of 16.83%, obtained by the first group experiments, was subjected to three cleaning flotation processes. The results are given in Table 5. 2.2. Second Group Flotation Experiments. The evaluation of flotation results was made according to the pyritic sulfur, sulfate sulfur, and ash contents of the tailings obtained from the second group experiments. For the nonselective flotation of sulfide mineral, the most commonly used and most investigated reagents were the xanthate-based collectors, which were generally characterized by their ability to collect sulfides. They were very selective, because of the length of their radical chains.20-22 It was reported that coal-pyrite responded to flotation more differently than ore-pyrite does, and it was also reported that the pyrite from one coal source was different than that from another source.23,24 It was also reported that coal-source pyrite was much less floatable with the conventional sulfide collector, (20) Crozier, R. D. Flotation: Theory, Reagents and Ore Testing; Pergamon Press: 1992. (ISBN 0-08-041864-3.) (21) Sirkeci, A. A. Int. J. Miner. Process. 2000, 60, 263-276. (22) Zojic, J. E. Academic Press: New York, 1969; p 65. (23) Aplan, F. F. Industrial Practice of Fine Coal Processing, 1988, pp 99-111. (24) Aplan, F. F. Min. Eng. (Littleton, Colo.) 1993.
xanthate, than was ore-source pyrite.25,26 There was a wide variability in the flotation of coal-pyrite, depending on the source, and coal pyrites may require 10-1000 times more xanthate collector for their flotation than ore-pyrite does.27,28 Therefore, sodium isopropyl xanthate (Aero 343) was used for the floating of pyrite as a collector in the second group experiments. The operating conditions of the second group experiments were as follows: impeller speed, 1000 rpm; solidliquid ratio, 10%; wetting conditioning time, 5 min; depressant conditioning time, 5 min; collector conditioning time, 5 min; frother conditioning time, 2 min; depressant (sodium diphosphate) amount, 250 g/t; collector (Aero 343) amount, 50 g/t; frother (pine oil) amount, 50 g/t; flotation time, 30 s; and particle size, 100% -0.1 mm. The operating conditions were determined according to pre-experiments. 2.2.1. Effect of pH. The effect of pH was established, and test results are given in Figure 8. As this figure shows, the best pyritic sulfur reduction of the coal sample was achieved at pH 6, in which the pyritic sulfur content and pyritic sulfur reduction of the tailing obtained were 3.41% and 31.11%, respectively. Therefore, pH 6 was established as the best pH. In the flotation of complex sulfide ores, a high pH value was generally used to separate valuable sulfide minerals from pyrite or pyrrhotite with xanthate collectors.18,29 pH values of >11 were necessary to markedly depress pyrite flotation in the presence of xanthate collectors.30 The partitioning behavior of pyrite in aqueous biphase systems was strongly pH-dependent. Below pH 5, pyrite particles transferred into the polymer-rich (top) phase; under these relatively acidic conditions, the surface reacted to form a hydrophobic metal-depleted product (Fe1-xS2). Above pH ∼9, all particles transferred into the salt-rich (bottom) phase; under the prevailing alkaline conditions, the metal sulfide surface was oxidized to a hydrophobic iron oxide. These trends demonstrate that pH could be used to change the surface chemistry of pyrite (e.g., from FeS2 to Fe1-xS2 (25) Chernowsky, F. J.; Lyon, M. Trans. Soc. Min. Eng., AIME 1972, 252, 11-14. (26) Miller, K. J. Trans. Soc. Min. Eng., AIME 1975, 258, 30-33. (27) Aplan, F. F.; Arnold, B. J. Coal Preparation, 5th Edition; Society for Mining, Metallurgy and Exploration: Litttleton, CO, 1991; Section 3. (28) Chen, Y. H.; Aplan, F. F. Presented at the AIME Annual Meeting, Denver, CO, February 1991, Paper No. 24-8. (29) Ball, B.; Rickard, R. S. In Flotation: A. M. Gaudin Memorial Volume; Fuerstenau, M. C., Ed.; American Institute of Mining, Metallurgical, and Petroleum Engineers (AIME): New York, 1976; Vol. 1, pp 458-484. (30) Boulton, A.; Fornasiero, D.; Ralston, J. Int. J. Miner. Process. 2001, 61, 13-22.
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Energy & Fuels, Vol. 19, No. 3, 2005 1007
group experiments was subjected to three cleaning flotation processes. These three cleaning flotation processes were performed under the best conditions of the first group experiments. The results are given in Table 6. The sulfate sulfur content of the coal sample was reduced to pine oil > AF 76 > MIBC. For pyritic sulfur content, the following order was obtained: DF 250 < pine oil < AF 76 < MIBC. DF 250 was chosen as the best frother type. The rougher tailing of 3.38% pyritic sulfur content, and 31.72% pyritic sulfur reduction obtained by the second (31) Osseo-Asare, K.; Zeng, X. Int. J. Miner. Process. 2000, 58, 319330. (32) Zeng, X.; Osseo-Asare, K. Colloids Surf., A 2001, 177, 247254.
The results obtained from this study are as follows: (1) Two groups of flotation experiments were conducted on the desulfurization and deashing of Hazro coal. (2) The effect of pH, collector (kerosene) amount, and frother type (MIBC, AF 76, pine oil, DF 250) on the depressing pyrite from the Hazro coal was investigated in the first group experiments. The best flotation conditions were as follows: pH 9; kerosene amount, 250 g/t; and frother type, MIBC. (3) The effect of pH and frother type (MIBC, AF 76, pine oil, DF 250) on the floating pyrite from the Hazro coal was investigated in the second group experiments. The best flotation conditions were as follows: pH 6; and frother type, DF 250. (4) The flotation data indicated that pH and, particularly, frother type were the most effective parameters in the pyritic sulfur and ash removal. MIBC and DF 250 were determined to be very effective in the depressing and floating of pyrite in the first and second group experiments, respectively. (5) The rougher flotation concentrate obtained by the first group experiments was subjected to three cleaning flotation processes, and a clean coal that contained 1.50% pyritic sulfur and 14.16% ash, with 69.70% pyritic sulfur reduction, was obtained. (6) The rougher flotation tailing obtained by the second group experiments was subjected to three cleaning flotation processes, and a clean coal that contained 1.12% pyritic sulfur and 13.02% ash, with 66.86% pyritic sulfur reduction, was obtained. The flotation method removed most of the sulfate sulfur (>90%) from the Hazro coal. Acknowledgment. The authors are grateful to the referees (anonymous) of the manuscript for their valuable suggestions. EF049747R
