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Coal recovery from bituminous coal by aggloflotation with petroleum oils
Fuel 85 (2006) 1138–1142 www.fuelfirst.com
Coal recovery from bituminous coal by aggloflotation with petroleum oils Nermin Gence * Vocational School of Bozu¨yu¨k, Anadolu University, Bozu¨yu¨k/Bilecik, Turkey Received 28 July 2004; received in revised form 4 October 2005; accepted 2 November 2005 Available online 28 November 2005
Abstract The objective of this work is to obtain high calorific value/low ash content products from fine bituminous coals by aggloflotation with petroleum oils such as hexane, heptane, toluene and pentane. The two methods used were agglomeration and aggloflotation for cleaning fine bituminous coal. Oil agglomeration experiments first were carried out followed by aggloflotation at the optimum conditions obtained from agglomeration. The effects of fundamental parameters such as oil type and dosage, pulp density, conditioning time; pH and Na2SiO3 amount were studied. The combustible recoveries and ash content of concentrates for optimal aggloflotation test conditions were with hexane: 92.17% combustible recovery (dry basis), 10.87% ash (dry basis) and lower calorific value (LHV) 5864 kcal/kg. q 2005 Elsevier Ltd. All rights reserved. Keywords: Bituminous coal; Aggloflotation; Petroleum oils
1. Introduction 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. Aggloflotation being a combination of agglomeration and flotation can be considered as an alternative to both processes for beneficiation of coal fines [1–5]. Coal recovery and separation efficiency, which are achieved by oil agglomeration depend on a number of factors including the surface properties of the solids, the nature of the agglomerates and certain properties of the aqueous medium. It has been observed that ionic strength of the aqueous medium affects the agglomeration performance. The presence of electrolytes also destabilises the wetting films on hydrophobic solids and helps to thin the film during the process of establishing a contact angle on coal particles. The positive effect of ionic strength on the agglomeration of the more oleophilic coals appears to be compression at the electrical double layer-surrounding individual particles, whereas the effect on the less oleophilic coals seems to be adsorption of the hydrated cation. By compressing the electrical double layer around the negatively charged coal particles and oil droplets, * Tel.: C90 228 314 11 95; fax: C90 228 314 11 96. E-mail address: [email protected]
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.11.001
the repulsive electrostatic forces between particles and droplets seem to be largely overcome [6–10]. Oil agglomeration is used mainly to remove mineral matter from coal. The differences between the surface properties of coal and those of its mineral impurities are exploited in the oil agglomeration process. During vigorous agitation of water slurry, hydrophobic coal particles are selectively coated with oil and cluster upon collision to form agglomerates. The hydrophilic mineral matter remains in the aqueous phase and is rejected. Most of liquid hydrocarbons, e.g. kerosene, diesel oil, other petroleum derivatives and vegetable oils, may be used as a bridging liquid. Unlike mineral oils, vegetable oils with negligible sulphur, nitrogen and metal content are renewable, available and non-polluting energy sources. Having these important characteristics and the present European agricultural situation of excess production result in these oils are being suitable candidates for alternative uses such as bio diesel production, the raw material in the chemical industry and as an agglomerating agent to clean coal fines [10–15]. The objective of this work is to obtain high calorific value/low ash content from fine bituminous coals by aggloflotation with petroleum oils such as hexane, heptane, toluene and pentane. 2. Sample and method 2.1. Properties of the coal sample The sample used in the experiments was collected from Zonguldak coal basin in Turkey. According to chemical
2.2. Experimental procedure The two methods used in the test wash were agglomeration and aggloflotation for cleaning fine bituminous coal. Oil agglomeration experiments were conducted in a commercial 10-speed Electro-mag M12 blender equipped with a 1000 ml glass vessel. For each agglomeration test, 400 ml of distilled water and 16 g of sample were placed in the blender. Prior to the oil addition, the coal slurry was preconditioned for specific periods of time at several agitation speeds. The oil was then added to the coal-water slurry, which was then conditioned for periods of time and agitation speed to agglomerate coal particles. The agglomerated product was separated from tailings by screening and ashed after own drying. The effect of various parameters such as oil type and dosage, preconditioning time and agitation speed, agglomeration time and agitation speed, pulp temperature, pH, and Na2SiO3 amount on the ash content and combustible yield were investigated in the agglomeration experiments. In the second phase of experimental studies, aggloflotation process was carried out at the optimum conditions obtained from the agglomeration test wash. In the aggloflotation test work, the pulp was transferred to a flotation cell. The agglomerated product was separated from the tailings as a froth product. The effects of pulp density, oil dosage, conditioning time, flotation time, pH, Na2SiO3 amount and pulp temperature on the ash content and combustible yield were investigated. In the experiments, petroleum oils such as hexane, heptane, toluene, pentane were used as a bridging and flotation reagents. 3. Results and discussion 3.1. Agglomeration experiments A number of variables such as oil type and dosage, pH, pulp density, preconditioning time, agglomeration time, pulp temperature and Na2SiO3 amount, considered important in determining the optimum conditions for agglomeration, were tested. In order to determine the effect of the oil type on agglomeration, a series of experiments were carried out by using hexane, heptane, toluene and pentane under the following conditions; Oil type Oil dosage Pulp pH Pulp density Preconditioning time and agitation speed Agglomeration time and agitation speed Pulp temperature Na2SiO3 amount
Hexane/heptane/toluene/pentane 1000 g tK1 7.0 10% by weight 5 min and 500 rev minK1 10 min and 500 rev minK1 20 8C 100 g tK1
1139 Combustible Recovery, %
Ash Content, %
100 80 60 40 20 0 Hexane
Heptane
Toluene
Pentane
Oil Type Fig. 1. The effect of oil type on agglomeration.
The best results were obtained with hexane. As it can be seen from the results, illustrated in Fig. 1, which ash content was decreasing from 49.49 to 29.56% giving an 82.15 combustible recovery. The hexane dosage was varied between 1000 and 2000 g tK1 to determine the effect of oil dosage on agglomeration. The experimental conditions were kept constant between tests and the optimum results were obtained a dosage of 1750 g tK1. Results show that a clean coal, containing 24.02% ash with 83.98% combustible recovery (Fig. 2). At lower oil dosages combustible recovery was low while that of ash content high. Combustible recovery of concentrate was increased while ash content was decreased with increasing oil dosage up to 1750 g tK1. At higher oil dosage, 2000 g tK1, ash content of concentrate was increased while combustible recovery was decreased. The pulp density of pulp was varied between 10 and 35% while other conditions kept constant, but oil dosage was set as 1750 g tK1. The best results obtained at 25% solids concentration. As it can be seen from the results that concentrate containing 19.55% ash with 85.11% combustible recovery (Fig. 3). It was observed that the selectivity was decreased at high solids content, causing a reduction in combustible recovery of concentrate while ash content was increasing. The solids were conditioned in tap water to determine the effects of conditioning time on agglomeration. Combustible Recovery/Ash Content, %
analysis, the sample contains 49.49% ash (dry basis), 6.87% moisture, 25.85% volatile matter, 16.87% fixed carbon and 0.92% total sulphur. Lower calorific value (LHV) is 2998 kcal/kg.
Combustible Recovery/Ash Content, %
N. Gence / Fuel 85 (2006) 1138–1142
Combustible Recovery, %
Ash Content, %
100 80 60 40 20 0 750
1250
Oil Dosage
1750
(g.t–1)
Fig. 2. The effect of oil dosage on agglomeration.
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1200
Agitation Speed (rev.min–1)
The preconditioning time was varied between 1 and 10 min to determine the effect of preconditioning time on agglomeration of coal. The experimental conditions were kept constant. Pulp density was set as 25%. Results showed that at lower preconditioning time ash content was high while that of combustible recovery low. The optimum results were obtained at 7 min. Results show that concentrate containing 16.75% ash with 85.55% combustible recovery (Fig. 4). The combustible recovery of concentrate was increased while ash content was decreased with increasing preconditioning time up to 7 min. At high preconditioning time, 10 min, the ash content of concentrate was continued to increase while combustible recovery was sharply decreased. During conditioning, there have obviously been changes in minerals surfaces as a result of chemical or physical interaction between minerals surfaces and reagents, involved. Conditioning should be long enough to allow the reactions to take place. It can easily be claimed that short conditioning times were not enough and selectivity was not achieved. It was observed that prolonged conditioning caused a decrease in combustible recovery while ash content was increased. The agitation speed was varied between 500 and 1200 rev minK1 in order to determine the effect of agitation speed on agglomeration. Experimental conditions were kept constant as before, and preconditioning time was set as 7 min. The results showed that optimum results obtained that at 1000 rev minK1. As it can be seen from the results that concentrate containing 16.00% ash with 86.01% combustible
recovery (Fig. 5). At lower speed, 500 rev minK1, and high speed, 1200 rev minK1, the combustible recovery of concentrate were decreased while ash content was increased. Agglomeration time was varied between 5 and 20 min. The experimental conditions were kept constant as before, and agitation speed was set as 1000 rev minK1. The best results were obtained at 15 min agglomeration time. Results show that concentrate containing 15.21% ash with 86.38% combustible recovery (Fig. 6). At short agglomeration time, 5 min, and long agglomeration time, 20 min, combustible recovery of concentrate was decreased while ash content was increased. Temperature of pulp was varied between 20 and 50 8C in order to determine the effect of pulp temperature on agglomeration. The conditions of agglomeration were taken as before but the agglomeration time was set as 15 min. Optimum results obtained at 35 8C. As it can be seen from the results that concentrate containing 13.96% ash with 86.98% combustible recovery (Fig. 7). At room temperature, 20 8C, and high temperature, 50 8C, combustible recovery of concentrate was decreased while ash content was increased. It was pointed out by many investigators that solubility of petroleum oils increases with increasing pulp temperature, causing an increase in combustible recovery which explains the poor results obtained at low temperature. This was also attributed to micelle formation: this class of reagents form aggregates (called micelles) when their solutions reach concentrations higher than the critical micelle concentration (CMC) whenever pulp temperature is above a certain minimum temperature [16–19].
Combustible Recovery, %
Ash Content, %
100 80 60 40 20 0 0
2
4
6
8
10
Preconditioning Time (min) Fig. 4. The effect of preconditioning time on agglomeration.
12
Combustible Recovery/Ash Content, %
Fig. 5. The effect of agitation speed on agglomeration.
Combustible Rcovery/Ash Content, %
Fig. 3. The effect of pulp density on agglomeration.
Combustible Recovery, %
Ash Content, %
100 80 60 40 20 0 0
5
10
15
20
Agglomeration Time (min) Fig. 6. The effect of agglomeration time on agglomeration.
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200
It was also explained in literature due to micelles formation: this class of reagents form aggregates (called micelles) when their solutions reach concentrations higher than a so-called critical micelle concentration (CMC) whenever pulp temperature is above a certain minimum temperature [7,16–19]. In this case, micelle formation was achieved around the 50 8C and as a result, a reduction in combustible recovery was obtained. The pulp pH was varied between 6 and 10 while other conditions kept constant. But pulp temperature was set as 35 8C. Pulp pH was adjusting by using Na2CO3. The best results obtained at pH 7.0 (natural pH). Results show that concentrate containing 13.96% ash with 86.98% combustible recovery (Fig. 8). The combustible recovery of concentrate was decreased while ash content was increased with increasing pH value up to optimum point, and afterwards combustible recovery was decreased significantly while ash content was increased. A series of agglomeration tests were carried out to determine the effect of Na2SiO3 dosage on agglomeration and Na2SiO3 dosage was varied between 100 and 1000 g tK1. The variables of agglomeration were kept constant as given above but pH was set as 7.0. The results showed that 400 g tK1 Na2SiO3 dosages produced the best results. As it can be seen from the results that concentrate containing 11.98% ash with 87.69% combustible recovery (Fig.9). At high Na2SiO3 dosage, 1000 g tK1, and low Na2SiO3 dosage, 100 g tK1, combustible recovery was decreased significantly while ash content was increased. Combustible Recovery/Ash Content, %
400
600
800
1000
1200
Na2SiO3 Amount (g.t–1)
Fig. 7. The effect of pulp temperature on agglomeration.
Combustible Recovery, %
Ash Content, %
100
Fig. 9. The effect of Na2SiO3 dosage on agglomeration.
The best results, obtained at following conditions, the combustible recovery and ash content of clean coal produced at the agglomeration tests are given Fig. 9, respectively. Oil type Oil dosage Pulp density Preconditioning time and agitation speed Agglomeration time and agitation speed Pulp temperature Pulp pH Na2SiO3 amount
100 80 60 40
15 min and 1000 rev minK1 35 8C Natural pH (7.0) 400 g tK1
First, the optimum conditions of oil agglomeration were determined by using Hexane afterwards these conditions were applied to Toluene. The results are given in Table 1. 3.2. Aggloflotation experiments In the second phase of experimental studies aggloflotation tests were carried out at the optimum conditions obtained from agglomeration studies, investigating the effect of oil type and dosage, pulp density, flotation time, conditioning time, pH, Na2SiO3 amount and pulp temperature on the combustible recoveries and ash content. In the aggloflotation experiments, the pulp was transferred to a flotation cell. The agglomerated product was separated from the pulp as a froth product. The optimum conditions of aggloflotation experiments are given below; Oil type Oil dosage Pulp pH Pulp density Na2SiO3 amount Conditioning time Flotation time Pulp temperature
Ash Content, %
Hexane 1750 g tK1 25% by weight 7 min and 1000 rev minK1.
Hexane 800 g tK1 Natural pH (7.0) 15% by weight 300 g tK1 7 min 4 min 25 8C
Table 1 The results of agglomeration tests
20 0 5
6
7
8
9
pH Fig. 8. The effect of pH on agglomeration.
10
Oil type
Ash (%)
Combustible recovery (%)
Lower calorific value (kcal/kg)
Hexane Toluene
11.98 10.81
87.69 83.48
5376 4982
11
Combustible Recovery/Ash Content, %
1142
N. Gence / Fuel 85 (2006) 1138–1142 Combustible Recovery, %
Ash Content, %
100 80
Oil Type Combustible Recovery Ash Content Low Calorific Value
Hexane 92.17% 10.87% 5864 kcal/kg
60
References
40 20 0 Hexane
Toluene
Heptane
Pentane
Oil Type
Fig. 10. The Results of Aggloflotation Tests.
The combustible recoveries and ash content of concentrates for optimal aggloflotation test conditions were with hexane: 92.17% combustible recovery, 10.87% ash and lower calorific value 5864 kcal/kg, toluene: 87.36% combustible recovery, 9.83% ash and lower calorific value 5672 kcal/kg. The combustible recovery and ash content of clean coal produced at the aggloflotation tests are given in Fig. 10. 4. Conclusions (1) In the agglomeration tests, low ash and high combustible recovery product was achieved by using high dosage of oil and, therefore, agglomeration is not a economically suitable method to clean the bituminous coal. On the other hand oil dosage was decreased in the aggloflotation experiments. Therefore, aggloflotation is a suitable technical process but also it is an economic in order to clean the bituminous coal. (2) The results obtained from the experiments indicated that was possible to enrich the bituminous coals by using aggloflotation. (3) Aggloflotation can be used to reduce ash content of fine bituminous coals with reasonable combustible recovery. The data, presented, clearly shows that the reduction of inorganic minerals content of bituminous coals was obtained and the optimum conditions of aggloflotation were determined as follows: Oil type Oil dosage Pulp pH Pulp density Na2SiO3 amount Conditioning time Flotation time Pulp temperature
Hexane 800 g tK1 Natural pH (7.0) 15% by weight 300 g tK1 7 min 4 min 25 8C
(4) The best results, obtained at optimum conditions in the aggloflotation experiments, are as follows:
[1] Cuhadaroglu D, Ozdag H. Flotation and oil agglomeration studies for beneficiation of K0.5 mm coal of Zonguldak Washery. In: Kemal L, Arslan V, Akar A, Canbazoglu M, editors. Changing scopes in mineral processing. Rotterdam: Balkema; 1996. p. 475–9. [2] Cuhadaroglu D, Ozdag H. Beneficiation of fine coal by selective aggloflotation. VIII Balkan Mineral Processing Conference, Belgrade, Yugoslavia; September 1999. p. 13–8. [3] Mishra SK, Klimpel RR. Fine coal processing. New Jersey: Noyes Publications; 1987 p. 78–135 [see also p. 179–204]. [4] Capes CE. Oil agglomeration process principles and commercial application for fine coal cleaning Coal preparation. 5th ed. Littleton: SME; 1991 p.1021–9 [Part: 4]. [5] Capes CE, Coleman RD. Oil agglomeration for fine coal processing. Proceedings of Fourth international symposium on agglomeration, Toronto, Canada; 1985. pp. 857–66. [6] Cebeci Y, Ulusoy U, Simsek S. Investigation of the effects of agglomeration time, ph and various salts on the cleaning of Zonguldak bituminous coal by oil agglomeration. Fuel 2002;81:1131–7. [7] Fuerstenau MC, Miller JD, Khun MC. Chemistry of flotation. New York: SME; 1985 p. 10 [see also pages 19–26, 35–43, 74]. [8] Cebeci Y. The investigation of effects some parameters on the performance on selective agglomeration of Lignites. Sixth international mineral processing symposium, Kusadasi, Turkey; 1996. p. 445–59. [9] Celik MS, Somasundaran P. Desulphrization of coal. In: Kural Orhan, editor. Coal; 1994, 1994. p. 253–69. [10] Mehrotra VP, Sastry KVS, Morey BW. Review of oil agglomeration techniques for processing of fine coals. Int Miner Process 1983;11: 99–175. [11] Alonso MI, Valde´s AF, Martı´nez-Tarazona RM, Garcia AB. Coal recovery from fines cleaning wastes by agglomeration with colza oil: a contribution to the environment and energy preservation. Fuel Process Technol 2002;75:85–95. [12] Alonso MI, Valde´s AF, Martı´nez-Tarazona RM, Garcia AB. Coal recovery from fines cleaning wastes by agglomeration with vegetable oils. Effects of oil type and concentration. Fuel 1999;78:753–9. [13] Zhou ZA, Xu Z, Masliyah JH, Czarnecki J. Coagulation of bitumen with fine silica in model system, colloids and surfaces. Physicochem Eng Aspects 1999;148:199–211. [14] Gryglewicz G, Grabas K, Larenc-Grabowska E. Preparation and characterization of spherical activated carbons from oil agglomerated bituminous coals for removing organic impurities from water. Carbon 2002;40:2403–11. [15] Couch GR. Advanced coal cleaning technology, IEACR/44. London: IEA Coal Research; 1991 p. 48–56. [16] Baruah MK, Kotoky P, Baruah J, Bora GC. Cleaning of Indian coals by agglomeration with xylene and hexane. Sep Purif Technol 2000;20: 235–41. [17] Steedman WG, Krishnan SV. In: Mishra SK, Klimpel RR, editors. Oil agglomeration process for the treatment of fine coal, fine coal processing. USA: Noyes Publications; 1987. p. 179–204. [18] Koca S, Ozdag H. Flotation of alunite from kaoline. Proceedings of 5th international mineral processing symposium, Cappadocia, Turkey; 1994. p. 135–40. [19] Leja J. Surface chemistry of froth flotation, New York; 1983. p. 43–4, see also pages 210–1, 265.
Coal recovery from bituminous coal by aggloflotation with petroleum oils Nermin Gence * Vocational School of Bozu¨yu¨k, Anadolu University, Bozu¨yu¨k/Bilecik, Turkey Received 28 July 2004; received in revised form 4 October 2005; accepted 2 November 2005 Available online 28 November 2005
Abstract The objective of this work is to obtain high calorific value/low ash content products from fine bituminous coals by aggloflotation with petroleum oils such as hexane, heptane, toluene and pentane. The two methods used were agglomeration and aggloflotation for cleaning fine bituminous coal. Oil agglomeration experiments first were carried out followed by aggloflotation at the optimum conditions obtained from agglomeration. The effects of fundamental parameters such as oil type and dosage, pulp density, conditioning time; pH and Na2SiO3 amount were studied. The combustible recoveries and ash content of concentrates for optimal aggloflotation test conditions were with hexane: 92.17% combustible recovery (dry basis), 10.87% ash (dry basis) and lower calorific value (LHV) 5864 kcal/kg. q 2005 Elsevier Ltd. All rights reserved. Keywords: Bituminous coal; Aggloflotation; Petroleum oils
1. Introduction 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. Aggloflotation being a combination of agglomeration and flotation can be considered as an alternative to both processes for beneficiation of coal fines [1–5]. Coal recovery and separation efficiency, which are achieved by oil agglomeration depend on a number of factors including the surface properties of the solids, the nature of the agglomerates and certain properties of the aqueous medium. It has been observed that ionic strength of the aqueous medium affects the agglomeration performance. The presence of electrolytes also destabilises the wetting films on hydrophobic solids and helps to thin the film during the process of establishing a contact angle on coal particles. The positive effect of ionic strength on the agglomeration of the more oleophilic coals appears to be compression at the electrical double layer-surrounding individual particles, whereas the effect on the less oleophilic coals seems to be adsorption of the hydrated cation. By compressing the electrical double layer around the negatively charged coal particles and oil droplets, * Tel.: C90 228 314 11 95; fax: C90 228 314 11 96. E-mail address: [email protected]
0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.11.001
the repulsive electrostatic forces between particles and droplets seem to be largely overcome [6–10]. Oil agglomeration is used mainly to remove mineral matter from coal. The differences between the surface properties of coal and those of its mineral impurities are exploited in the oil agglomeration process. During vigorous agitation of water slurry, hydrophobic coal particles are selectively coated with oil and cluster upon collision to form agglomerates. The hydrophilic mineral matter remains in the aqueous phase and is rejected. Most of liquid hydrocarbons, e.g. kerosene, diesel oil, other petroleum derivatives and vegetable oils, may be used as a bridging liquid. Unlike mineral oils, vegetable oils with negligible sulphur, nitrogen and metal content are renewable, available and non-polluting energy sources. Having these important characteristics and the present European agricultural situation of excess production result in these oils are being suitable candidates for alternative uses such as bio diesel production, the raw material in the chemical industry and as an agglomerating agent to clean coal fines [10–15]. The objective of this work is to obtain high calorific value/low ash content from fine bituminous coals by aggloflotation with petroleum oils such as hexane, heptane, toluene and pentane. 2. Sample and method 2.1. Properties of the coal sample The sample used in the experiments was collected from Zonguldak coal basin in Turkey. According to chemical
2.2. Experimental procedure The two methods used in the test wash were agglomeration and aggloflotation for cleaning fine bituminous coal. Oil agglomeration experiments were conducted in a commercial 10-speed Electro-mag M12 blender equipped with a 1000 ml glass vessel. For each agglomeration test, 400 ml of distilled water and 16 g of sample were placed in the blender. Prior to the oil addition, the coal slurry was preconditioned for specific periods of time at several agitation speeds. The oil was then added to the coal-water slurry, which was then conditioned for periods of time and agitation speed to agglomerate coal particles. The agglomerated product was separated from tailings by screening and ashed after own drying. The effect of various parameters such as oil type and dosage, preconditioning time and agitation speed, agglomeration time and agitation speed, pulp temperature, pH, and Na2SiO3 amount on the ash content and combustible yield were investigated in the agglomeration experiments. In the second phase of experimental studies, aggloflotation process was carried out at the optimum conditions obtained from the agglomeration test wash. In the aggloflotation test work, the pulp was transferred to a flotation cell. The agglomerated product was separated from the tailings as a froth product. The effects of pulp density, oil dosage, conditioning time, flotation time, pH, Na2SiO3 amount and pulp temperature on the ash content and combustible yield were investigated. In the experiments, petroleum oils such as hexane, heptane, toluene, pentane were used as a bridging and flotation reagents. 3. Results and discussion 3.1. Agglomeration experiments A number of variables such as oil type and dosage, pH, pulp density, preconditioning time, agglomeration time, pulp temperature and Na2SiO3 amount, considered important in determining the optimum conditions for agglomeration, were tested. In order to determine the effect of the oil type on agglomeration, a series of experiments were carried out by using hexane, heptane, toluene and pentane under the following conditions; Oil type Oil dosage Pulp pH Pulp density Preconditioning time and agitation speed Agglomeration time and agitation speed Pulp temperature Na2SiO3 amount
Hexane/heptane/toluene/pentane 1000 g tK1 7.0 10% by weight 5 min and 500 rev minK1 10 min and 500 rev minK1 20 8C 100 g tK1
1139 Combustible Recovery, %
Ash Content, %
100 80 60 40 20 0 Hexane
Heptane
Toluene
Pentane
Oil Type Fig. 1. The effect of oil type on agglomeration.
The best results were obtained with hexane. As it can be seen from the results, illustrated in Fig. 1, which ash content was decreasing from 49.49 to 29.56% giving an 82.15 combustible recovery. The hexane dosage was varied between 1000 and 2000 g tK1 to determine the effect of oil dosage on agglomeration. The experimental conditions were kept constant between tests and the optimum results were obtained a dosage of 1750 g tK1. Results show that a clean coal, containing 24.02% ash with 83.98% combustible recovery (Fig. 2). At lower oil dosages combustible recovery was low while that of ash content high. Combustible recovery of concentrate was increased while ash content was decreased with increasing oil dosage up to 1750 g tK1. At higher oil dosage, 2000 g tK1, ash content of concentrate was increased while combustible recovery was decreased. The pulp density of pulp was varied between 10 and 35% while other conditions kept constant, but oil dosage was set as 1750 g tK1. The best results obtained at 25% solids concentration. As it can be seen from the results that concentrate containing 19.55% ash with 85.11% combustible recovery (Fig. 3). It was observed that the selectivity was decreased at high solids content, causing a reduction in combustible recovery of concentrate while ash content was increasing. The solids were conditioned in tap water to determine the effects of conditioning time on agglomeration. Combustible Recovery/Ash Content, %
analysis, the sample contains 49.49% ash (dry basis), 6.87% moisture, 25.85% volatile matter, 16.87% fixed carbon and 0.92% total sulphur. Lower calorific value (LHV) is 2998 kcal/kg.
Combustible Recovery/Ash Content, %
N. Gence / Fuel 85 (2006) 1138–1142
Combustible Recovery, %
Ash Content, %
100 80 60 40 20 0 750
1250
Oil Dosage
1750
(g.t–1)
Fig. 2. The effect of oil dosage on agglomeration.
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N. Gence / Fuel 85 (2006) 1138–1142 Combustible Recovery, %
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35
40
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Combustible Recovery/Ash Content, %
1140
Combustible Recovery, %
Ash Content, %
100 80 60 40 20 0 400
600
Pulp Density, %
800
1000
1200
Agitation Speed (rev.min–1)
The preconditioning time was varied between 1 and 10 min to determine the effect of preconditioning time on agglomeration of coal. The experimental conditions were kept constant. Pulp density was set as 25%. Results showed that at lower preconditioning time ash content was high while that of combustible recovery low. The optimum results were obtained at 7 min. Results show that concentrate containing 16.75% ash with 85.55% combustible recovery (Fig. 4). The combustible recovery of concentrate was increased while ash content was decreased with increasing preconditioning time up to 7 min. At high preconditioning time, 10 min, the ash content of concentrate was continued to increase while combustible recovery was sharply decreased. During conditioning, there have obviously been changes in minerals surfaces as a result of chemical or physical interaction between minerals surfaces and reagents, involved. Conditioning should be long enough to allow the reactions to take place. It can easily be claimed that short conditioning times were not enough and selectivity was not achieved. It was observed that prolonged conditioning caused a decrease in combustible recovery while ash content was increased. The agitation speed was varied between 500 and 1200 rev minK1 in order to determine the effect of agitation speed on agglomeration. Experimental conditions were kept constant as before, and preconditioning time was set as 7 min. The results showed that optimum results obtained that at 1000 rev minK1. As it can be seen from the results that concentrate containing 16.00% ash with 86.01% combustible
recovery (Fig. 5). At lower speed, 500 rev minK1, and high speed, 1200 rev minK1, the combustible recovery of concentrate were decreased while ash content was increased. Agglomeration time was varied between 5 and 20 min. The experimental conditions were kept constant as before, and agitation speed was set as 1000 rev minK1. The best results were obtained at 15 min agglomeration time. Results show that concentrate containing 15.21% ash with 86.38% combustible recovery (Fig. 6). At short agglomeration time, 5 min, and long agglomeration time, 20 min, combustible recovery of concentrate was decreased while ash content was increased. Temperature of pulp was varied between 20 and 50 8C in order to determine the effect of pulp temperature on agglomeration. The conditions of agglomeration were taken as before but the agglomeration time was set as 15 min. Optimum results obtained at 35 8C. As it can be seen from the results that concentrate containing 13.96% ash with 86.98% combustible recovery (Fig. 7). At room temperature, 20 8C, and high temperature, 50 8C, combustible recovery of concentrate was decreased while ash content was increased. It was pointed out by many investigators that solubility of petroleum oils increases with increasing pulp temperature, causing an increase in combustible recovery which explains the poor results obtained at low temperature. This was also attributed to micelle formation: this class of reagents form aggregates (called micelles) when their solutions reach concentrations higher than the critical micelle concentration (CMC) whenever pulp temperature is above a certain minimum temperature [16–19].
Combustible Recovery, %
Ash Content, %
100 80 60 40 20 0 0
2
4
6
8
10
Preconditioning Time (min) Fig. 4. The effect of preconditioning time on agglomeration.
12
Combustible Recovery/Ash Content, %
Fig. 5. The effect of agitation speed on agglomeration.
Combustible Rcovery/Ash Content, %
Fig. 3. The effect of pulp density on agglomeration.
Combustible Recovery, %
Ash Content, %
100 80 60 40 20 0 0
5
10
15
20
Agglomeration Time (min) Fig. 6. The effect of agglomeration time on agglomeration.
25
Combustible Recovery, %
1141
\\attenborough\work\els\JFUE\Articles\189819\Graphics
Ash Content, %
100 80 60 40 20 0 10
20
30
40
50
60
Pulp Temperature (°C)
Combustible Recovery/Ash Content, %
Combustible Recovery/Ash Content, %
N. Gence / Fuel 85 (2006) 1138–1142
Combustible Recovery, %
80 60 40 20 0
0
200
It was also explained in literature due to micelles formation: this class of reagents form aggregates (called micelles) when their solutions reach concentrations higher than a so-called critical micelle concentration (CMC) whenever pulp temperature is above a certain minimum temperature [7,16–19]. In this case, micelle formation was achieved around the 50 8C and as a result, a reduction in combustible recovery was obtained. The pulp pH was varied between 6 and 10 while other conditions kept constant. But pulp temperature was set as 35 8C. Pulp pH was adjusting by using Na2CO3. The best results obtained at pH 7.0 (natural pH). Results show that concentrate containing 13.96% ash with 86.98% combustible recovery (Fig. 8). The combustible recovery of concentrate was decreased while ash content was increased with increasing pH value up to optimum point, and afterwards combustible recovery was decreased significantly while ash content was increased. A series of agglomeration tests were carried out to determine the effect of Na2SiO3 dosage on agglomeration and Na2SiO3 dosage was varied between 100 and 1000 g tK1. The variables of agglomeration were kept constant as given above but pH was set as 7.0. The results showed that 400 g tK1 Na2SiO3 dosages produced the best results. As it can be seen from the results that concentrate containing 11.98% ash with 87.69% combustible recovery (Fig.9). At high Na2SiO3 dosage, 1000 g tK1, and low Na2SiO3 dosage, 100 g tK1, combustible recovery was decreased significantly while ash content was increased. Combustible Recovery/Ash Content, %
400
600
800
1000
1200
Na2SiO3 Amount (g.t–1)
Fig. 7. The effect of pulp temperature on agglomeration.
Combustible Recovery, %
Ash Content, %
100
Fig. 9. The effect of Na2SiO3 dosage on agglomeration.
The best results, obtained at following conditions, the combustible recovery and ash content of clean coal produced at the agglomeration tests are given Fig. 9, respectively. Oil type Oil dosage Pulp density Preconditioning time and agitation speed Agglomeration time and agitation speed Pulp temperature Pulp pH Na2SiO3 amount
100 80 60 40
15 min and 1000 rev minK1 35 8C Natural pH (7.0) 400 g tK1
First, the optimum conditions of oil agglomeration were determined by using Hexane afterwards these conditions were applied to Toluene. The results are given in Table 1. 3.2. Aggloflotation experiments In the second phase of experimental studies aggloflotation tests were carried out at the optimum conditions obtained from agglomeration studies, investigating the effect of oil type and dosage, pulp density, flotation time, conditioning time, pH, Na2SiO3 amount and pulp temperature on the combustible recoveries and ash content. In the aggloflotation experiments, the pulp was transferred to a flotation cell. The agglomerated product was separated from the pulp as a froth product. The optimum conditions of aggloflotation experiments are given below; Oil type Oil dosage Pulp pH Pulp density Na2SiO3 amount Conditioning time Flotation time Pulp temperature
Ash Content, %
Hexane 1750 g tK1 25% by weight 7 min and 1000 rev minK1.
Hexane 800 g tK1 Natural pH (7.0) 15% by weight 300 g tK1 7 min 4 min 25 8C
Table 1 The results of agglomeration tests
20 0 5
6
7
8
9
pH Fig. 8. The effect of pH on agglomeration.
10
Oil type
Ash (%)
Combustible recovery (%)
Lower calorific value (kcal/kg)
Hexane Toluene
11.98 10.81
87.69 83.48
5376 4982
11
Combustible Recovery/Ash Content, %
1142
N. Gence / Fuel 85 (2006) 1138–1142 Combustible Recovery, %
Ash Content, %
100 80
Oil Type Combustible Recovery Ash Content Low Calorific Value
Hexane 92.17% 10.87% 5864 kcal/kg
60
References
40 20 0 Hexane
Toluene
Heptane
Pentane
Oil Type
Fig. 10. The Results of Aggloflotation Tests.
The combustible recoveries and ash content of concentrates for optimal aggloflotation test conditions were with hexane: 92.17% combustible recovery, 10.87% ash and lower calorific value 5864 kcal/kg, toluene: 87.36% combustible recovery, 9.83% ash and lower calorific value 5672 kcal/kg. The combustible recovery and ash content of clean coal produced at the aggloflotation tests are given in Fig. 10. 4. Conclusions (1) In the agglomeration tests, low ash and high combustible recovery product was achieved by using high dosage of oil and, therefore, agglomeration is not a economically suitable method to clean the bituminous coal. On the other hand oil dosage was decreased in the aggloflotation experiments. Therefore, aggloflotation is a suitable technical process but also it is an economic in order to clean the bituminous coal. (2) The results obtained from the experiments indicated that was possible to enrich the bituminous coals by using aggloflotation. (3) Aggloflotation can be used to reduce ash content of fine bituminous coals with reasonable combustible recovery. The data, presented, clearly shows that the reduction of inorganic minerals content of bituminous coals was obtained and the optimum conditions of aggloflotation were determined as follows: Oil type Oil dosage Pulp pH Pulp density Na2SiO3 amount Conditioning time Flotation time Pulp temperature
Hexane 800 g tK1 Natural pH (7.0) 15% by weight 300 g tK1 7 min 4 min 25 8C
(4) The best results, obtained at optimum conditions in the aggloflotation experiments, are as follows:
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