2052
Energy & Fuels 2006, 20, 2052-2055
Desulfurization and Deashing of Hazro Coal by Selective Oil Agglomeration in Various Water Mediums Halime Abakay Temel* and Fatma Deniz Ayhan Department of Mining Engineering, Dicle UniVersity, 21280 Diyarbakir, Turkey ReceiVed February 22, 2006. ReVised Manuscript ReceiVed June 22, 2006
The aim of this study was to study the effects of various water mediums on desulfurization and deashing of Hazro coal by the agglomeration method. For this purpose, three groups of agglomeration experiments were made. The effects of some parameters that markedly influence the effectiveness of selective oil agglomeration, such as solid concentration, bridging liquid concentration, and pH, on the agglomeration were investigated in the first group of experiments. The effects of different salts (NaCl, MgCl2, and FeCl3) on the agglomeration were investigated in the second group of experiments. The effects of lake water and sea water on the agglomeration were investigated in the third group of experiments. The influences of the Mediterranean Sea water and Aegean Sea water on the removal of ash and total sulfur were found to be important.
Introduction High-ash and high-sulfur coals are unsuitable for efficient use in carbonization, combustion, gasification, liquefaction, etc. purposes.1,2 Coal usually contains a significant quantity of different metallic and nonmetallic impurities that cause environmental or process problems in its usage cycle. Because of this fact, many studies have been carried out to reduce these polluting impurities in coal, including those of ash, sulfur, and silicates. The existence of sulfur compounds in coal limits its industrial application because of environmental as well as technical problems. Harmful effects on agricultural products, disruption of the natural equilibrium of the ozone layer, corrosion of metal structures, and respiratory problems of humans and animals are undesirable effects of sulfur and its compounds.3,4 The conventional coal beneficiation methods are inefficient in the cleaning of fine coal particles. Therefore, flotation, selective flocculation, and oil agglomeration methods have gained importance to clean fine particles.5,6 One of the more promising methods for cleaning coal involves suspending finely ground coal in water and selectively agglomerating the more hydrophobic and oleophilic components with oil or low-molecular-weight hydrocarbons such as pentane or heptane as the suspension is agitated vigorously.7-9 * To whom correspondence should be addressed. E-mail: habakay@ dicle.edu.tr. (1) Mukherjee, S.; Borthakur, P. C. Fuel 2003, 82, 783-788. (2) Mukherjee, S.; Borthakur, P. C. Fuel 2001, 80, 2037-2040. (3) Abdollayh, M.; Moghaddam, A. Z.; Rami, K. Fuel 2006, 85, 11171124. (4) Demirbas¸ , A. Energy ConVers. Manage. 2002, 43, 885-895. (5) Capes, C. E. Coal Preparation, 5th ed.; SME: Littleton, CO, 1991; Vol. 9 (part 4), p 1021. (6) Capes, C. E.; Coleman, R. D. Proceedings of 4th International Symposium on Agglomeration; Toronto, Canada, 1985, pp 857-866. (7) Wheelock, T. D.; Markuszewski, R. The Science and Technology of Coal and Coal Utilization; Cooper B. R., Ellingson W. A., Eds.; Plenum Press: New York, 1984; pp 47-123. (8) Steedman, W. G.; Krishnan, s. V. Fine Coal Processing; Mishra S. K., Klimpel R. R., Eds.; Noyes Publications: Park Ridge, NJ, 1987; pp 179-205. (9) Keller, D. V.; Burry, W. M., Jr. Coal Prep. 1990, 8, 1-17.
The organic macerals tend to be agglomerated in preference to the inorganic minerals. Dependent upon their relative size and density, the agglomerates can be recovered from the suspension by floating, skimming, or screening. It was observed that the effects of pH and ionic strength on coal recovery and separation efficiency depend upon the surface properties of the particles.10-13 An increase in the ionic strength was found to favor the recovery of hydrophobic, oleophilic coals and to disfavor the recovery of weakly oleophilic, hydrophilic coals and pyrite.13 It was further observed that the effect of ionic strength on the recovery of a hydrophobic coal depends upon the relative hydrophobicity of the material.14 Thus, a greater effect was observed with a strongly hydrophobic coal than with a weakly hydrophobic coal. The effect of increasing ionic strength on hydrophobic coals was believed because of the compression of the electrical double layer surrounding individual particles, while the effect on hyrophilic coals was thought because of the adsorption of hydrated cations.10,11,13 In this study, the effects of various water mediums on the agglomeration of Hazro coal were studied, and the experimental results are presented here. Experimental Section 1. Materials. The coal sample used in this work was obtained from Hazro, Turkey. Proximate analysis, ultimate analysis, and petrographic analysis of the coal sample and major element contents of the coal ash sample are given in Tables 1-4. Proximate and ultimate analysis were performed by using Turkish and ASTM standards.16 (10) Fan, C. W.; Markuszewski, R.; Wheelock, T. D. Fizykochem. Probl. Mineralurgii 1987, 19, 17-26. (11) Yang, G. C. C.; Markuszewski, R.; Wheelock, T. D. Coal Prep. 1988, 5, 133-146. (12) Sadowski, R.; Venkatadri, J. M.; Druding, R.; Markuszewski, R.; Wheelock, T. D. Coal Prep. 1988, 6, 17-34. (13) Fan, C. W.; Hu, Y. C.; Markuszewski, R.; Wheelock, T. D. Energy Fuels 1989, 3, 376-381. (14) Wheelock, T. D.; Markuszewski R.; Fan C. W.; Hu Y. C.; Tyson D. Fossil Energy Quarterly Report for April 1, Ames, IA, 1988, IS-4975.
10.1021/ef060079e CCC: $33.50 © 2006 American Chemical Society Published on Web 08/09/2006
Desulfurization and Deashing of Hazro Coal
Energy & Fuels, Vol. 20, No. 5, 2006 2053
Table 1. Proximate Analysis Results of the Coal Sample15 component
as received
air dried
drieda
moisture (%) ash (%) volatile matter (%) fixed carbon (%) upper heating value (kcal/kg) total sulfur (%) pyritic sulfur (%) sulfate sulfur (%) organic sulfur (%) organic sulfur (%)
2.76 24.57 34.90 37.76 5890
1.99 24.77 35.18 38.06 5937
25.27 35.89 38.84 6058
6.90
6.90 4.95 0.10 1.85
a
7.00
The sample was dried to constant mass at 105 °C. Table 2. Ultimate Analysis Results of the Coal Sample15 ultimate analysis (daf)
C H S (total)
70.24 5.67 7.00
ultimate analysis (daf) N O (diff.)
0.66 16.43
Table 3. Petrographic Analysis Results of the Coal Sample17 maceral group
percent by volume
huminite liptinite inertinite pyrite clay and silicate minerals
72 6 6 6 10
Table 4. Major Element Contents of the Coal Ash Sample15 components
composition (%)
components
composition (%)
SiO2 Fe2O3 Al2O3 CaO
42.0 16.5 33.9 0.9
MgO Na2O K 2O SO3
0.5 0.2 0.9 0.8
Figure 1. Effect of the solid concentration on agglomeration (bridging liquid concentration, 40%).
A total of 50% kerosene plus 50% fuel oil (number 4) was used as bridging liquid. The density of 50% kerosene plus 50% fuel oil (number 4) was determined as 0.84 g/cm3. Tap water was used as the water for the suspension (pH ∼ 7.5) in the first group of experiments. NaOH and H2SO4 were used as pH modifiers in the three group experiments. NaCl, MgCl2, and FeCl3 were used in the second group of experiments. The salts were of reagent-grade chemicals. The Mediterranean Sea water, Aegean Sea water, and soda lake water taken from the Mediterranean Sea (Mersin, Turkey), Aegean Sea (I˙ zmir, Turkey), and soda lake (Van, Turkey), respectively, and were used in the three group experiments. In the evaluation of experimental results, total sulfur and ash contents of products were considered.
Table 5. Size Analysis of the Coal Sample15 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
It was determinated that the rank of the coal sample was subbituminous coal according to petrographic analysis results.17 The coal sample was ground (dry) to a nominal top size of -0.1 mm in a ball mill (Denver type) for agglomeration tests. The screen analysis of the coal sample is given in Table 5. 2. General Method. The agglomeration experiments were performed in a 250 mL beaker with four baffles at the borders to create turbulence using a FRAMO-Geratetechnik, LR20-type mechanical stirrer. The agitation was provided by a centrally located flat blade turbine impeller (consisting of four blades, 50 mm in diameter and 10 mm in width) at a fixed distance from the bottom of the vessel. The coal-water suspension of a given solid content (wt %) was prepared. The suspension was conditioned at 1800 rpm for 35 min. Afterward, an appropriate amount of oil (mL of oil/g of coal) was added, and mixing was continued for another 10 min. After agglomeration, the suspension was transferred to a 0.106 mm (140 mesh, U.S. Standard) screen. The agglomerates were taken as overscreen products, and the tailings were taken as underscreen products. The agglomerates and tailings were filtered and, afterward, dried in an oven at 100-105 °C. Ash and total sulfur contents were determined for both the agglomerates and tailings. (15) Ayhan, F. D.; Abakay, H.; Saydut, A. Energy Fuels 2005, 19, 10031007. (16) Sevinc¸ , M. Yurt Madenciligˇini Gelis¸ tirme Vakfı, 1997. (17) Abakay, H. Master’s Thesis, Dicle University, Turkey, 2001.
Results and Discussion 1. The First Group of Agglomeration Experiments. The effects of the solid concentration, bridging liquid concentration, and pH on the agglomeration were investigated in the first group of experiments. The operating conditions of the agglomeration tests were as follows: stirring speed, 1800 rpm; bridging liquid, 50% kerosene plus 50% fuel oil; bridging liquid concentration, 40% (mL of oil/g of coal); pH 6. The operating conditions were determined according to preexperiments. 1.1. The Effect of the Solid Concentration. The effect of the solid concentration was established. Test results are given in Figure 1. As shown in Figure 1, the best solid concentration for desulfurization and deashing was found to be 10% solids. Ash content and combustible yield of the agglomerate obtained at 10% solids were 19.10 and 74.45%, respectively. The total sulfur content of the coal was reduced from 6.90 to 3.53% at 10% solids. 1.2. The Effect of the Bridging Liquid Concentration. The effect of bridging liquid was established. The bridging liquid concentration was varried between 10 and 50%. Test results are given in Figure 2. As shown in Figure 2, increasing the amount of 50% kerosene plus 50% fuel oil decreased both ash and total sulfur contents of the agglomerates. A bridging liquid concentration of 45% was established as being the best, because of the low ash content (18.45%) and high total sulfur reduction (53.33%) of the agglomerate obtained. It was clear that the combustible yield
2054 Energy & Fuels, Vol. 20, No. 5, 2006
Temel and Ayhan
Figure 4. Effect of NaCl on agglomeration. Figure 2. Effect of the bridging liquid concentration on agglomeration (solid concentration, 10% solids).
Figure 5. Effect of MgCl2 on agglomeration. Figure 3. Effect of pH on agglomeration (solid concentration, 10% solids; bridging liquid concentration, 45%).
increased with an increasing oil concentration. For S¸ ırnak asphaltite-containing high-ash content, similiar results were reported.18 1.3. The effect of pH. The effect of pH was established. Test results are given in Figure 3. As shown in Figure 3, the total sulfur contents of the agglomerates obtained at pH 6, 7, and 8 were 3.22, 3.09, and 2.91%, respectively, in which the sulfurbearing minerals were agglomerated at minimum levels. The total sulfur contents of the agglomerates obtained at pH 3, 4, and 5 were 4.03, 3.81, and 3.52%, respectively, in which the total sulfur reductions were higher than the other pH values because the agglomeration of sulfur-bearing minerals was high. In an aqueous suspension, the surface of pyrite can oxidize under acidic conditions to form both elemental sulfur and sulfate, whereas under basic conditions at low temperature, the surface can oxidize to produce a surface layer of iron oxides.19 When the oxidized pyrite was suspended in an acidic solution, it behaved like a hydrophobic material and agglomerated readily with heptane, but when it was suspended in a basic solution, it behaved like a hydrophilic material and did not agglomerate. The oxidized surface appeared to be coated with both a hydrophilic material such as basic ferric sulfate and a hydrophobic material such as elemental sulfur. In an acidic solution, the hydrophilic material was dissolved, leaving the hydrophobic material, while in a basic solution, the hydrophilic material became hydrated and dominated the surface characteristics.20 The practical application of pH control in suppressing the agglomeration of pyrite by oil was demonstrated by Leonard et (18) Abakay, H.; Ayhan, F. D.; Kahraman, F. Fuel 2004, 83, 20812086. (19) Hiskey, J. B.; Schlitt, W. J. Interfacing Technologies in Solution Mining; AIME: New York, 1982; pp 55-74. (20) Drzymala, J.; Wheelock, T. D. Coal Prep. 1992, 10, 189-201.
al.,21 who showed that the maximum reduction of the pyritic sulfur content of an Iowa bituminous coal was achieved by using a pH of 9-11 during agglomeration. The best pH value for desulfurization was 8. The ash contents of agglomerates obtained at pH 3 and 4 were 20.95 and 20.50%, respectively, in which the ash contents of the agglomerates were higher than those at the other pH values because ash-forming minerals were agglomerated at high ratios. Ayhan et al.15 found that the zero point of charge of Hazro coal is located at pH 7.0. The greatest combustible yield was realized at the isoelectric point for the coal, where the hydrophobicity of the coal surface should have been a maximum. Similarly, in this study, the response of Hazro coal to agglomeration at pH 7 was good depending upon the effect of the isoelectric point. Therefore, the best pH value for deashing was 7. The best agglomeration conditions were as follows: solid concentration, 10%; bridging liquid concentration, 45%; pH 7. It was found that, when Hazro coal was subjected to three cleaning flotation processes, a clean coal that contained 1.50% pyritic sulfur and 14.16% ash with 69.70% pyritic sulfur reduction was obtained.15 However, when Hazro coal was subjected to agglomeration at the best conditions, an agglomerate product that contained 3.09% total sulfur and 17.14% ash with 55.22% total sulfur reduction was obtained. It was clear that the agglomeration method did not discriminate well between coal- and sulfur-bearing mineral in coal. The results are consistent with those reported by Leonard et al.21 2. The Second Group of Agglomeration Experiments. The effects of NaCl, MgCl2, and FeCl3 on the agglomeration were established. The best agglomeration conditions were used in the second group of experiments. Test results are given in Figures 4-6. 2.1. The Effect of NaCl. As shown in Figure 4, the ash content of agglomerates decreased to 300 mg/L and then (21) Leonard, W. G.; Greer, R. T.; Markuszewski, R, Wheelock, T. D. Sep. Sci. Technol. 1981, 16, 1589-1609.
Desulfurization and Deashing of Hazro Coal
Figure 6. Effect of FeCl3 on agglomeration.
increased. Increasing the amount of NaCl decreased the total sulfur content of agglomerates. The best results for desulfurization and deashing were obtained at 450 and 300 mg/L, respectively. The total sulfur reduction of agglomerates obtained at 450 mg/L was 65.07%. At 300 mg/L, the ash content of agglomerates was 16.23%. 2.2. The Effect of MgCl2. As shown in Figure 5, increasing the amount of MgCl2 decreased the ash content of agglomerates and increased the total sulfur content of agglomerates. The best results for desulfurization and deashing were obtained at 200 and 450 mg/L, respectively. The total sulfur reduction and total sulfur content of the agglomerate obtained at 200 mg/L were 58.70 and 2.85%, respectively. The ash content and combustible yield of the agglomerate obtained at 450 mg/L were 17.10 and 73.47%, respectively. 2.3. The Effect of FeCl3. FeCl3 was shown to be an effective depressant for sulfurized pyrite.20 Also, FeCl3 was used by Baker et al.22 as a pyrite depressant for coal pyrite. As shown in Figure 6, the best results for desulfurization and deashing were obtained at 450 and 300 mg/L, respectively. The total sulfur reduction and total sulfur content of the agglomerate obtained at 450 mg/L were 60.43 and 2.73%, respectively. The ash content and combustible yield of the agglomerate obtained at 300 mg/L were 19.66 and 70.48%, respectively. 3. The Third Group of Agglomeration Experiments. The effects of the Mediterranean Sea water, Aegean Sea water, and soda lake water on agglomeration were established. The best agglomeration conditions were used in the third group of experiments. 3.1. The Effect of Soda Lake Water. For this experiment, the soda lake water was mixed with tap water at different ratios. The soda lake water ratio was varied between 1 and 100%. Test results are given in Figure 7. As shown in Figure 7, the ash content of agglomerates decreased to a 10% ratio and then increased. Increasing the ratio of soda lake water increased the total sulfur content of agglomerates. The best results for desulfurization and deashing were obtained at 1 and 10% ratios, respectively. The total sulfur reduction and total sulfur content of the agglomerate obtained at a 1% ratio were 61.74 and 2.64%, respectively. The ash content and combustible yield of the agglomerate obtained at a 10% ratio were 16.82 and 75.93%, respectively. 3.2. The Effects of the Mediterranean Sea Water and Aegean Sea Water. Test results are given in Table 6. The best agglomeration conditions were used in the experiments. Experimental conditions were as follows: stirring speed, 1800 rpm; (22) Baker, A. F.; Miller, K. J. U.S. Bureau of Mines Report of Investigations RI 7518, 1971.
Energy & Fuels, Vol. 20, No. 5, 2006 2055
Figure 7. Effect of the soda lake water agglomeration. Table 6. Effects of the Mediterranean Sea Water, Aegean Sea Water, and Tap Water on Agglomeration agglomerates
Mediterranean Sea water
Aegean Sea water
tap water
ash (%) total sulfur (%) total sulfur reduction (%) combustible yield (%)
14.24 2.19 68.26 81.87
15.71 2.23 67.68 78.26
17.14 3.09 55.22 78.83
bridging liquid, 50% kerosene plus 50% fuel oil; solid concentration, 10%; bridging liquid concentration, 45%; pH 7. As shown in Table 5, the total sulfur contents of agglomerates achieved with the Mediterranean Sea water and Aegean Sea water were 2.19 and 2.23%, respectively. The combustible yields obtained in the usage of the sea waters were higher than those obtained at the other mediums. Also, the combustible yields obtained in the agglomeration of S¸ ırnak asphaltite were found to be high when the Mediterranean Sea water and soda lake water were used in agglomeration medium.15 Conclusions The results obtained from this study are as follows: (1) Three group of agglomeration experiments were conducted on various water mediums. (2) The effects of the solid concentration, bridging liquid concentration, and pH on agglomeration were investigated in the first group of experiments. The best agglomeration conditions were as follows: pH 7; solid concentration, 10%; bridging liquid concentration, 45%. (3) The effects of three salts (NaCl, MgCl2, and FeCl3) on agglomeration were investigated in the second group of experiments. The usage of NaCl, MgCl2, and FeCl3 in the agglomeration medium had a positive effect on the reduction of ash and sulfur content of agglomerates. NaCl was the most effective of the investigated salts, in regard to removing total sulfur and ash from the coal sample. (4) The effects of the soda lake water, Mediterranean Sea water, and Aegean Sea water on agglomeration were investigated in the third group of experiments. (5) Agglomeration results indicated that, when compared to various water mediums, the following order for the ash content was obtained: Mediterranean Sea water < Aegean Sea water < NaCl < soda lake water < FeCl3 < MgCl2 < tap water, and the following order for the reduction of total sulfur was obtained: Mediterranean Sea water > Aegean Sea water > NaCl > soda lake water > FeCl3 > MgCl2 > tap water. When the Mediterranean Sea water was used as an agglomeration medium, an agglomerate product containing 2.19% total sulfur and 14.24% ash with a total sulfur reduction of 68.26% was obtained from a feed that contained 6.90% total sulfur and 24.77% ash. EF060079E
