Coal Froth Flotation: Effects of Reagent Adsorption on the Froth

Apr 28, 2006 - The initial amounts of SDS markedly affected the performance data, froth structure, and stability (Figure 11), as the initial reagent l...
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Energy & Fuels 2006, 20, 1123-1130

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Coal Froth Flotation: Effects of Reagent Adsorption on the Froth Structure Meryem Ozmak and Zeki Aktas* Faculty of Engineering, Department of Chemical Engineering, Ankara UniVersity, Tandogan 06100 Ankara, Turkey ReceiVed October 11, 2005. ReVised Manuscript ReceiVed January 25, 2006

The amount and quality of concentrate obtained from froth flotation of a coal are very important to determine the efficiency of the separation process. The shape and size of the bubbles in the froth directly affect the amount and purity of the concentrate overflowed during the froth flotation of the coal. The froth structure is significantly dependent on parameters such as the size of the solid particles, the surface properties of the particles, the chemical structure of surface active agents, the reagents adsorbed onto solid particles, and the reagents remaining in water. This work was performed to determine the relationship between the reagents adsorbed on the solid particles, froth structure, and froth flotation performance. The -53 µm size fraction of a bituminous coal was used to perform froth flotation experiments. The froth flotation of the coal used was performed in the presence of two nonionic surfactants, Triton x-100 (poly(ethylene glycol) tert-octylphenyl ether) and MIBC (methyl isobutyl carbinol), and an anionic surfactant, SDS (sodium dodecyl sulfate). The results showed that the adsorption of a high amount of reagent on the particles decreased the ability of separation, thus a substantial amount of mineral particles overflowed along with the hydrophobic coal particles. The use of MIBC with Triton x-100 or SDS as mixture increased solid recovery, and it was concluded that MIBC selectively adsorbed on solids acting as collector as well as a frother. Reagent adsorption has a crucial effect on the froth structure, which is strongly related to flotation performance.

Introduction The environmental and operational problems caused by coal usage can be reduced by removing the inert noncombustible mineral matter from the carbonaceous materials of coal. Because of its low capital and space requirements, froth flotation is preferable to other coal beneficiation methods.1 Froth flotation is an important mineral separation technique, and it involves the application of surface chemistry. The principle of froth flotation is based on the differences in the physicochemical surface properties of the particles. Hydrophobic coal particles selectively attach to air bubbles depending on the degree of hydrophobicity. The bubble-particle aggregates rise through the pulp, and when they reach the pulp surface, threephase froths are formed. Hydrophobic coal particles are carried and collected with the overflowing froth, as the hydrophilic waste material returns to the pulp by drainage and remains in the flotation cell. This separation is related to the selective drainage of liquid and solid particles at the air-liquid interface. A continuous drainage for the beneficiation depends on the froth stability generated in the flotation cell. For this reason, froth structure and stability are very important in the froth flotation process. To obtain a sufficient separation in froth flotation, it is necessary to use additional reagents, which act as frothers, collectors, or both.2,3 The type and amount of reagent added * Corresponding author. Tel: +90-312-2126720/1300. Fax: +90-3122121546. E-mail: [email protected]. (1) Mishra, S. K.; Klimpel, R. R. Fine Coal Processing; Noyes Publications: Park Ridge, NJ, 1987; pp 78-79. (2) Woodburn, E. T.; Flynn, S. A.; Cressey, B. A.; Cressey, G. Powder Technol. 1984, 40, 167-177. (3) Subrahmanyam, T. V.; Forssberg, E. Int. J. Miner. Process. 1988, 23, 33-53.

are important parameters in the froth flotation of coal. Several studies have been carried out to determine the effects of reagents on froth flotation performance.4-8 In recent years, some shortchain volatile fatty acids and commercial fatty acids were used in the froth flotation of coal fines as an alternative to the conventional collectors.9,10 Attempts to enhance the froth flotation performance resulted in the use of different reagents together. But the studies on the effects of reagent mixtures on froth flotation performance are limited to the conventional reagent mixtures.11-13 Erol et al. studied the effects of reagent type and dosage on the recovery and ash rejection in froth flotation of coal. Selected reagents (Triton x-100, Brij-35, and SDS) were used with MIBC in various ratios. Relatively high grade values were obtained in addition to high recovery using reagent mixtures.8 Because of the naturally porous structure of coal, reagents can be adsorbed onto the coal particles. Adsorption of molecules on solids from solution is important in controlling interfacial (4) Rosenbaum, J. M.; Fuerstenau, D. W. AIChE Symp. Ser. 1982, 1928. (5) Jia, R.; Harris, H. G.; Fuerstenau, D. W. Int. J. Miner. Process. 2000, 58, 99-118. (6) Vamvuka, D.; Agridiotis, V. Int. J. Miner. Process. 2001, 61, 209224. (7) Sripriya, R.; Rao, P. V. T.; Choudhury, B. R. Int. J. Miner. Process. 2003, 68, 109-127. (8) Erol, M.; Colduroglu, C.; Aktas, Z. Int. J. Miner. Process. 2003, 71, 131-145. (9) Denby, B.; Elverson, C.; Hal, S. Fuel 2002, 81, 595-603. (10) Sis, H.; Ozbayoglu, G.; Sarikaya, M. Miner. Eng. 2003, 16, 399401. (11) Bustamante, H.; Warren, L. J. Int. J. Miner. Process. 1984, 13, 1328. (12) Bonner, C. M.; Aplan, F. F. Sep. Sci. Technol. 1993, 28, 747-764. (13) Ates¸ ok, G.; C¸ elik, M. S. Fuel 2001, 79, 1509-1513.

10.1021/ef0503358 CCC: $33.50 © 2006 American Chemical Society Published on Web 04/28/2006

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processes such as froth flotation. The adsorption of reagent (collector) molecules at the solid-liquid interface modifies the dispersion properties and increases the probability of bubbleparticle attachment.14,15 The reagent (frother) molecules adsorbed at the air-liquid interface prevent bubbles from coalescing by decreasing the surface tension.16 Pope and Sutton studied the collector adsorption in flotation in a wide range of collector concentrations. They reported that the amount of adsorbed collector on the particles collected as concentrates was more than the feed. The increase in the adsorption rate by the increase of reagent loading was explained with the molecular orientation.17 Perry and Aplan obtained similar results. They reported that the amount of polysaccharides adsorbed on pyrite was more than on the coal particles because of hydrogen bonding and electrostatic forces.18 Aktas and Woodburn studied the adsorption behavior of the reagents used in froth flotation; they declared the relationship between the reagent adsorption, froth structure, and flotation performance.19,20 The change in froth structure during flotation can be used to control the parameters that affect the flotation performance, such as reagent dosage.14,15,21,22 Image analysis is one of the most widely used techniques for the determination of bubble size and distribution in froth flotation.23,24 In the image analysis technique, the shape of bubbles is assumed to be spherical, and the edges of the froth regions are determined. The continuity of these edges is important to obtain minimum error. Banford et al. used image analysis to determine the effect of froth structure on the flotation performance of low-rank Bickershaw coal.25 The effects of reagent addition strategies on the froth structure and performance of coal flotation in a batch cell were studied using image analysis to characterize flotation froths.15 Citir et al.26 used a pixel tracing technique developed by Banford et al.25 for off-line image analysis and provided some improvements to the previous studies of Biland27 and those of Wiklund and Grankund.28 This work was a part of a research program carried out to determine the effects of the reagent and reagent mixtures on the froth flotation performance and froth structure. In the first attempt, the effects of pure reagents and reagent mixtures on the froth flotation performance were determined.8 The objective of this study was to explain the relationship between the froth structure and flotation performance depending on the effect of reagent adsorption. (14) Banford, A. W. Ph.D. Thesis, The Victoria University of Manchester, Manchester, U.K., 1996. (15) Banford, A. W.; Aktas, Z. Miner. Eng. 2004, 17, 745-760. (16) Gourram-Badri, F.; Conil, P.; Morizot, G. Int. J. Miner. Process. 1997, 51, 197-208. (17) Pope, M. I.; Sutton, D. I. Powder Technol. 1971, 5, 101-104. (18) Perry, R. W.; Aplan, F. F. Sep. Sci. Technol. 1988, 23, 2097-2112. (19) Aktas, Z.; Woodburn, E. T. Miner. Eng. 1994, 7, 1115-1126. (20) Aktas, Z.; Woodburn, E. T. Powder Technol. 1995, 83, 149-158. (21) Sadr-Kazemi, N.; Cilliers, J. J. Miner. Eng. 1997, 10, 1075-1083. (22) Holtham, P. N.; Nguyen, K. K. Int. J. Miner. Process. 2002, 64, 163-180. (23) Aldrich, C.; Feng, D. Miner. Eng. 2000, 13, 1049-1057. (24) Rodrigues, R. T.; Rubio, J. Miner. Eng. 2003, 16, 757-765. (25) Banford, A. W.; Aktas, Z.; Woodburn, E. T. Powder Technol. 1998, 98, 61-73. (26) Citir, C.; Aktas, Z.; Berber, R. Comput. Chem. Eng. 2004, 28, 625632. (27) Biland, H. P. In Proceedings of the Second International Workshop on Time-Varying Image Processing and MoVing Object Recognition; Florence, Italy, September 8-9 1986; AIIMB: Naples, Italy, 1986; pp 251258. (28) Wiklund, J.; Granlund, G. In Time-Varying Image Processing and MoVing Object Recognition; Cappellini, V., Ed.; Elsevier Science Publishers: New York, 1987; pp 241-250.

Ozmak and Aktas

Figure 1. Particle size distributions and some size parameters of the feed (d10, d50, and d90 are the diameters at 10, 50, and 90%, respectively).

Experimental Section Materials Used. Coal and Reagents. A Turkish bituminous coal, sampled from the Zonguldak colliery, was used in this study. Size reduction of the coal was carried out using a crusher and a ball mill. The coal was first crushed in the crusher to less than 5 mm in diameter. The crushed coal particles were then milled in the ball mill to liberate the mineral matter. The milled sample was sieved and the -53 µm size fraction of the coal was collected and stored in sealed plastic bags. The reagents chosen were Triton x-100 (Merck), which is a perfect dispersant and surface modifier for coals, MIBC (Merck), both of which are nonionic, and SDS (Merck), which is anionic. These reagents were used as aqueous solutions or as mixtures with MIBC at given ratios. The reagents (MIBC and SDS) are commonly used in the froth flotation of coal. Some physical properties of the reagents were previously reported by Erol et al. 8 Sample Characterization. The ash and moisture contents of the samples were determined according to ASTM standards (ASTM D3172-73 and ASTM D3176-74). The ash and moisture contents of the coal were 23.95 and 0.88%, respectively. The particle size distributions of the solid samples were determined using a Malvern Mastersizer 2000 (Hydro 2000 MU) particle size analyzer. The results of particle size analysis are given as distribution and cumulative plots in Figure 1 including some particle size parameters. The BET surface area and pore size distribution of the coal samples were determined using a Quantachrome NOVA 2200 series volumetric gas adsorption instrument. The pore size distributions of the coal samples were determined from the desorption isotherms of nitrogen according to the BJH method.29 The BET surface area and desorption pore diameter of the -53 µm size fraction of the feed were 6.88 m2/g and 21.73 Å, respectively. Denver Cell Flotation Tests. The flotation tests were performed in a Denver laboratory flotation machine, which has a 1 dm3 capacity. Experimental details of the flotation tests were previously reported by Erol et al.8 The flotation tests were carried out at natural pH conditions using 600 g of 5% (wt %) coal slurry with single-stage reagent addition. The slurry, containing a known amount of reagent, was conditioned for 2 min. After this conditioning period, the aeration was started. The aeration rate was controlled and kept constant at 2.1 L/min using a rotameter. Once aeration started, the fine bubbles dispersed in the pulp rose to the upper surface and formed a froth. The overflowed froths were collected at various time intervals. The froth concentrates were weighed and dried in the evaporating dishes. The tailings left in the cell were dried and stored in small glass jars for subsequent particle size and ash content analysis. Solid, water, and ash contents of the concentrates and tailings were calculated (29) Barret, E. P.; Joyner, L. G.; Halenda, P. H. J. Am. Chem. Soc. 1951, 73, 373.

Coal Froth Flotation separately to determine the flotation performance. The flotation performance of each test was defined as recovery and grade of the final product. Solids and water balances for each experiment were done and deviations were between 2 and 5% for the solids and 1 and 4% for water. The variables studied were the nature and initial loading of the reagents. The adsorption of the reagents and their effects on the separation performance and the mean bubble diameter of the froth were determined. Reagent Adsorption. The reagents used in flotation tests adsorbed onto the coal particles because of the porous structure of coal. After all flotation tests and at various time intervals during the flotation, 5-10 mL liquid samples were taken from the slurry using a glass syringe, and they were filtered by filter holder. Adsorption kinetic tests of each reagent were also performed to find out the adsorption rate and equilibrium time. These tests were carried out in a cell with 1 L capacity without aeration. The same operational conditions with froth flotation tests, such as volume and solid content of slurry, agitation rate, wetting time, and initial reagent concentration, were ensured in the cell. Liquid samples were taken from the slurry frequently over a 6 h time period. These liquid samples were then analyzed with UV or GC depending on the chemical structure of the reagents used to determine the concentration of reagent remained in the pulp. Triton x-100 and SDS were analyzed with a Shimadzu UV-1601 spectrophotometer at 279.8 and 224.2 nm, respectively. The samples containing MIBC were analyzed using a ThermoQuest Trace GC 2000 gas chromatograph. A capillary column (Zebron ZB-Wax), 30 m long and 0.25 mm in diameter, was used with a flame ionization detector (FID). All analyses were carried out under isothermal conditions. The amounts of reagent adsorbed were calculated from the difference between the initial and unadsorbed amounts of reagents in water. Image Analysis. Photographs of the overflowing froths were taken from the beginning of each flotation test until the overflow ceased. These photographs were analyzed by the off-line image analysis technique to determine the changing bubble structure of the froth during the froth flotation process. The image processing of the froth region involves mainly three stages: identifying local intensity minima, border thinning, and calculation of the mean bubble diameter. The procedure of this image analysis technique, which is basically based on Banford’s method,14,25 was reported by Citir et al.26 A computer program called RADIS 026 (developed by Citir et al.26) was used for image analysis of the froths. From the image processing, the average bubble size, based on the bubble distribution, was obtained. The mean bubble diameter data were used to describe the froth structure.

Results and Discussion Froth flotation of Zonguldak bituminous coal was carried out using two nonionic reagents (Triton x-100 and MIBC) and an anionic reagent (SDS). The effects of reagents and reagent mixtures in froth flotation were evaluated with the froth structure and flotation performance depending on the reagent adsorption. The performance of froth flotation was given in terms of cumulative combustible recovery and cumulative grade as

recovery ) Wc,daf/Wf,daf, grade ) 1 - (Ac/Af) where Ac and Af are the ash contents of the cumulative concentrate and original feed coal (wt %), respectively, and Wc,daf and Wf,daf are the weight of the dry ash-free cumulative concentrate and original feed coal (g), respectively. Reagent Adsorption during Froth Flotation. As reported in the Experimental Section, prior to the froth flotation experiments, adsorption kinetic tests of each reagent were performed. The initial concentrations chosen for the adsorption kinetic tests were 0.9 mg of Triton x-100/g of coal, 0.6 mg of MIBC/g of

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Figure 2. Variations of reagent adsorptions with initial reagent concentration at the end of froth flotation.

Figure 3. Variations of Triton x-100 and MIBC adsorbed with MIBC fraction in the reagent mixture (initial concentrations of Triton x-100 and MIBC: 0.9 mg/g of coal).

coal, and 0.6 mg of SDS/g of coal. Almost 65-70% of the initial loading of each reagent is adsorbed in the first 2 min. The variation of the reagent adsorption with the initial reagent concentrations is given in Figure 2. The amount of reagent adsorbed was determined after the flotation process ceased. The amount of reagents adsorbed increased linearly with the increase of the initial reagent loadings. For the same initial concentrations, the amounts of MIBC adsorbed are greater than those of Triton x-100 and SDS at the end of flotation tests. Mixtures of two reagents were also used for the froth flotation experiments. The variation of surface active agents adsorbed with the ratio of MIBC in the mixture of the two reagents was investigated. The reagents adsorbed for the Triton x-100-MIBC and SDS-MIBC mixtures are shown in Figures 3 and 4, respectively. At the beginning of the test, the total amount of Triton x-100 and MIBC mixture was kept constant as 0.9 mg/g of coal, however the fraction of each reagent in the mixtures was varied. While the MIBC fraction in the mixture increases, the amount of Triton x-100 adsorbed decreases as seen in Figure 3. It is useful to compare the amount adsorbed of each reagent in the mixture if the mixture is prepared using the same amounts of MIBC and Triton x-100 (that is 50% for MIBC and 50% for Triton x-100, Figure 3). Although the same amounts of MIBC and Triton x-100 are used at the beginning of the test, the amounts adsorbed for MIBC and Triton x-100 are considerably different (0.45 and ∼0.25 mg/g og coal, respectively). This means that MIBC is selectively adsorbed onto the solid particles ∼1.8 times more than Triton x-100. In other words, in terms of unit surface area using the BET surface area of the coal, the

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Figure 4. Variations of SDS and MIBC adsorbed with MIBC fraction in the reagent mixture (initial concentration of SDS and MIBC: 0.375 mg/g of coal).

Figure 6. Variation of reagent adsorption with time (initial concentration of the reagent mixture: 0.9 mg/g of coal, Triton x-100 (0.855 mg) and MIBC (0.045 mg)).

Figure 5. Adsorption of Triton x-100, MIBC, and SDS with time during the froth flotation test.

Figure 7. Variation of reagent adsorption with time (initial concentration of the reagent mixture: 0.375 mg/g of coal, SDS (0.175 mg) and MIBC (0.20 mg)).

amounts adsorbed onto the coal per unit area (1 m2) of the solids are 0.0654 mg/m2 for MIBC and ∼0.036 mg/m2 for Triton x-100. Total concentration at the start of test was 0.375 mg/g of coal for the SDS and MIBC mixtures. As shown for Triton x-100-MIBC mixtures in Figure 3, similar trends were also obtained for SDS-MIBC mixtures shown in Figure 4. The total amounts of the reagent mixture adsorbed practically did not change, as shown in the figures. To investigate the influence of reagent adsorption during the froth flotation on the performance and froth structure, the amounts of reagents adsorbed were determined during the test for different adsorption periods. Figure 5 shows the amounts of reagents adsorbed (Trion x-100, MIBC, and SDS) as a function of time during froth flotation of the coal. The adsorption of each reagent is very fast in the first 30 s, and the majority of the reagent was adsorbed in 60 s. Thereafter the amount of reagent adsorbed increased slightly with time. Although the initial loadings of MIBC and SDS are the same (0.6 mg/g coal), the amount of MIBC adsorbed onto the particles is higher than the amount of SDS adsorbed. The reason is the different chemical structures of the reagents; 71.5, 69.1, and 66.4% of initial loadings of the reagents, namely, Triton x-100, MIBC, and SDS, respectively, adsorbed onto the solid particles in the first two minutes, which is the conditioning time. As stated in the Experimental Section, binary mixtures of reagents were also used for the froth flotation experiments. The results obtained from the Triton x-100 and MIBC mixture, of which the initial total reagent loading was 0.9 mg/g of coal,

are shown in Figure 6. The percentage of MIBC in the mixture was 5% at the beginning of the test. However, no MIBC was detected in the water at the end of test. This means that all MIBC adsorbed onto the solid particles during the froth flotation test. In another experiment (Figure 7), a mixture of SDS and MIBC was used, but the fraction of MIBC in the mixture was 53.3% (initial reagent loading SDS + MIBC ) 0.375 mg/g of coal). Almost all of the MIBC was adsorbed onto the solid particles at the end of experiment, whereas the amount of SDS adsorbed was about 0.10 mg/g of coal for the same experiment. Both Figure 6 and Figure 7 show that MIBC is selectively adsorbed onto the solids, compared to Triton x-100 and SDS. The total amount of reagent adsorbed is about 0.8 mg/g of coal for Triton x-100 and MIBC (Figure 6) and about 0.3 mg/g of coal for the SDS-MIBC mixture. The solid particles adsorbed much more of the MIBC-Triton x-100 reagent mixture than of the SDSMIBC mixture. Effect of Reagents on Froth Structure. A stable froth should be formed in a flotation cell to get highly effective separation during froth flotation. Froth stability is directly related to sufficient amounts of the appropriate surface active agents.2,3,30 The images, which are given as examples, shown in Figure 8 were obtained using two nonionic surfactants, Triton x-100 and MIBC, and one anionic surfactant, SDS. Polyhedral stable bubbles formed in the presence of Triton x-100 during the flotation. As reported previously,20 polyhedral froth structure allows sufficient drainage resulting in a product (30) Gaudin, A. M. Flotation, 1st ed.; McGraw-Hill: New York, 1932.

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Figure 9. Variation of mean bubble diameter with time for different initial concentrations of Triton x-100.

Figure 8. Variation of the froth structure (images) with time during the froth flotation for the reagents: (a) 0.9 mg of Triton x-100/g of coal, (b) 0.6 mg MIBC/ g of coal, and (c) 0.6 mg of SDS/g of coal.

with high grade. However, polyhedral but unstable froth was observed with only MIBC. The reason may be the overloaded bubbles because overloading of the surface causes accumulation of solid particles on the bubble surfaces, as a result the rising of froth in the cell is restricted because of the collapsing of the froth. This means that the froth formed is unstable. Small bursting spherical bubbles formed with the anionic surface active agent, SDS. The lamellae curvature is almost the same as it is anywhere else in the spherical froth; therefore, the resulting pressure difference between various points in the lamellae (or between lamellae and the plateau border) will be too low. Consequently, a low grade product could be obtained because of the insufficient drainage.31,32 The images of froths taken from each experiment for known periods were analyzed by image processing to determine the mean bubble diameter. Variations of the mean bubble diameter of the froths with time are represented in Figures 9-11. The froth flotation tests were performed in the presence of Triton x-100, MIBC, and SDS solutions at various concentrations. The mean bubble diameter increases with time as a result of bubble coalescence. However, after a certain time, froth collapsing was observed depending on decrease in surface active agent and the solid content of pulp in the flotation cell. The mean bubble diameter increased with the increase in initial reagent loading of nonionic reagents, Triton x-100 and MIBC. However it was determined that the mean bubble diameters were smaller when the initial loading of SDS was increased from 0.45 to 0.90 mg/g of coal. The shapes of the bubbles obtained from these tests were spherical. The values of the grade and recovery are also shown in Figure 9. While initial reagent loadings increase, very small bubbles were generated at the beginning of each experiment. After 25 (31) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons: New York, 1989. (32) Aktas, Z. Ph.D. Thesis, The Victoria University of Manchester, Manchester, U.K., 1993.

Figure 10. Variation of mean bubble diameter with time during froth flotation with different initial concentrations of MIBC.

Figure 11. Variation of mean bubble diameter with time during froth flotation with different initial concentrations of SDS.

s, the mean bubble diameter sharply increased until 50 s, as can be seen in the figure. The recovery values were 0.71, 0.80, and 0.84 for 0.45 (23.7 ppm), 0.72 (37.9 ppm), and 0.90 (47.4 ppm) mg/g of coal initial concentrations of Triton x-100; in terms of the grade, the values decreased from 0.76 to 0.72 for the same initial loadings. The reagent concentrations remaining (residual reagent concentrations) in water were 5.07, 7.73, and 15.31 ppm. While unstable and bursting bubbles were created at lower residual Triton x-100 concentrations (5.07 ppm), at higher residual reagent concentrations (15.31 ppm), the froth was stable but a collapsed froth was observed toward the end of experiment. The froth structures of the two experiments performed in the presence of MIBC were considerably different, as can be seen

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Figure 13. Variation of the mean bubble diameter with time during froth flotation with a mixture of Triton x-100 and MIBC (total initial concentration: 0.9 mg/g of coal). Figure 12. Variation of the froth structure (images) with time during the froth flotation for the reagent mixtures: (a) 0.9 mg (Triton x-100MIBC)/g of coal (5% MIBC) and (b) 0.375 mg (SDS-MIBC)/g of coal (40% MIBC).

in Figure 10. The recovery values were almost the same for the two tests; however, the grade values were 0.66 and 0.63 for the tests. Initial reagent loading influenced the grade values slightly, but the mean bubble diameter was significantly affected. Interestingly there was no reagent remaining in water for the experiment performed with 0.6 mg/g of coal (31.6 ppm) initial loading. For the other experiment (1.2 mg/g of coal, 63.2 ppm), a fairly stable froth was obtained, and the residual concentration was 5.86 ppm. The initial amounts of SDS markedly affected the performance data, froth structure, and stability (Figure 11), as the initial reagent loadings decreased from 0.9 (47.4 ppm) to 0.45 (23.7 ppm) mg/g of coal, the recovery value also decreased sharply from 0.94 to 0.73, however the grade values increased from 0.55 to 0.69. Very small spherical bubbles (2.8-3.8 mm) were generated during the experiment with a 0.9 mg/g of coal initial loading. Larger bubbles were generated for the experiments conducted with 0.6 (31.6 ppm) and 0.45 (23.7 ppm) mg/g of coal initial loadings. Variation of the mean bubble diameter with time during the experiments steadily increased in contrast to the experiments performed in the presence of Triton x-100 and MIBC. The froth morphology was quite different with the use of SDS (ionic) compared to that with the other reagents (nonionic). The residual concentrations of SDS were 3.75, 6.50, and 11.80 ppm for the experiments conducted with 0.45, 0.6, and 0.9 mg/g of coal initial loadings. The froth was fairly stable as the residual SDS concentration was 6.50 ppm. Considerable changes of the froth structure were observed when MIBC was used as a mixture of reagents, Triton x-100 or SDS (Figure 12). Small bubbles were produced when the fraction of MIBC in the reagent mixture was increased (Figures 13 and 14). It is clear that MIBC helps the attachment probabilities of the particles on the bubble surfaces and in the reduction of the mean bubble diameter. Experiments were performed to investigate the effect of the amount of surface active agent adsorbed with time on the froth structure and flotation performance. The results of experiments also showed that there were variations in the froth structure. Images were taken at different flotation periods for the experiments performed with 0.9 mg of Triton x-100/g of coal and 0.6 mg of SDS/g of coal loadings, respectively. Image analysis results showed that the mean bubble diameters increased with time, Figure 15. It was observed that polyhedral structure and stable froth formed for the experiment performed with 0.9 mg

Figure 14. Variation of the mean bubble diameter with time during froth flotation with a mixture of SDS and MIBC (total initial concentration: 0.375 mg/g of coal).

Figure 15. Variation of the mean bubble diameter with time during flotation for the tests selected.

of Triton x-100/g of coal. In case of 0.6 mg of SDS/g of coal the bubbles formed were small and spherical. The spherical bubbles were slightly deformed when the mixture of MIBC and SDS was used. Effect of Reagents on Froth Flotation Performance. The effects of reagents and reagent mixtures on the flotation performance have been reported in the first study of this project.8 Briefly the flotation performance was changed significantly according to the addition of the reagent or reagent mixture. The highest-recovery and lower-grade values were obtained with the use of MIBC or SDS. The grade of final product was considerably improved by using a mixture of reagents. The effect of the type and amount of surface active agent on the recovery

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Figure 16. Variation of the cumulative grade of the froth concentrates with time during the flotation tests selected.

and grade was investigated in a number of studies and similar findings were reported in those studies.5, 6, 20, 33 The grade value of froth overflowed is evaluated by using either the ash content of each concentrate collected at different times during the froth flotation process or the ash content of the cumulative concentrate collected at the end of experiment. It was determined that the ash contents increased with time depending on the type of the reagent. The grade value was the highest for the first concentrate and was the lowest for the last concentrate. Cumulative grade values as a function of time are shown in Figure 16 for the reagents used in this study. Initial loadings of the reagents for the tests were 0.9 mg/g of coal for Triton x-100 and SDS and 1.2 mg/g of coal for MIBC. The total recoveries reported in previous paper8 were 0.84, 0.96, and 0.94 for the reagents, Triton x-100, MIBC, and SDS, respectively. Relationship between Reagent Adsorption, Froth Structure, and Flotation Performance. As stated in Figure 5, it was determined that the amount of reagent adsorbed increased with time. There is a relationship between the mean bubble diameter and reagent adsorbed onto the solid particles. The relationship between the amount of surface active agent adsorbed and the mean bubble diameter of the froth was mathematically determined from the fitted functions (Figures 5 and 15). The values of reagent adsorbed and the mean bubble diameters in corresponding times were determined and plotted against each other. For instance, the relationship between the amount of reagent adsorbed and the mean bubble diameter is shown in Figure 17 for the experiments performed with Triton x-100 (0.9 mg/g of coal) and SDS (0.6 mg/g of coal). While the amount of reagent adsorbed increases, the mean bubble diameter increases too. In similar way, variation of the cumulative grade as a function of the mean bubble diameter was also mathematically determined by the fitted functions as shown in Figure 18 for the experiments reported in the same figure. Decreases in the cumulative grade are seen as the mean bubble diameter increases for the experiments. This relationship, which was produced from the experimental data, clearly shows that froth flotation performance strongly depends on froth structure. Conclusions Separation efficiency in the froth flotation of the coal strongly depends on the amount of reagent adsorbed. Adsorption of a high amount of surface active agent on the particles decreases (33) Flynn, S. A.; Woodburn, E. T. Powder Technol. 1987, 49, 127142.

Figure 17. Variation of the mean bubble diameter with the reagent adsorbed for the tests selected.

Figure 18. Variation of cumulative grade with the mean bubble diameter.

the effectiveness of separation; consequently, considerable amounts of mineral particles overflow along with hydrophobic coal particles. The use of mixtures of MIBC with Triton x-100 or SDS increased the solid recovery, this obviously shows that MIBC selectively adsorbed on solid particles. Reagent adsorption has crucial effect on the froth structure, which is strongly related to the flotation performance. The majority of the reagent adsorbed in the first 2 min, which is typical of the conditioning time used in flotation. The adsorption rate is considerably high during the conditioning period. Spherical bubbles were created in the presence of SDS, and polyhedral bubbles were obtained with Triton x-100 and MIBC. When the initial loading of each reagent is increased, the grade of the concentrate sharply decreased for SDS and slightly decreased for Triton x-100 and MIBC, as reported in our previous paper.8 It may be suggested that the higher initial loading of SDS results in a less selective separation process. With Triton x-100, small bubbles that were observed at the beginning of the tests substantially improved the combustible recovery and slightly diminished the grade. Larger bubbles were observed toward the end of the tests for higher initial reagent loadings. Homogeneous bubble size distributions and small shiny spherical bubbles were obtained in the presence of SDS. The size of the spherical bubble strongly influences the performance values; as the size of the bubbles decrease, the grade values decrease dramatically as well, whereas the recovery values significantly improve. With Triton x-100 or SDS, the froth stability not only depends on the amount of the reagent adsorbed onto the solid particles

1130 Energy & Fuels, Vol. 20, No. 3, 2006

but also on the reagent concentration remaining (residual reagent concentration) in the liquid phase. It might be recommended that the residual reagent concentration should also be considered to generate the stable froth, as well as the amout of reagent adsorbed.

Ozmak and Aktas Acknowledgment. The authors gratefully acknowledge that this research was supported by the State Planning Organization (DPT), Turkey, Project 98K120740. EF0503358