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Energy & Fuels 1996, 10, 1202-1207
Surface Energy and Induction Time of Fine Coals Treated with Various Levels of Dispersed Collector and Their Correlation to Flotation Responses Felicia F. Peng* Department of Mineral Processing Engineering, College of Engineering and Mineral Resources, West Virginia University, Morgantown, West Virginia 26506-6070 Received April 24, 1995. Revised Manuscript Received June 28, 1996X
Separation of fine coal in froth flotation relies upon the wettability difference between the coal-rich and mineral-rich particles in the aqueous solution. Two methods were used to measure the wettability of six ranks of coal as well as coal samples treated with various levels of dispersed collector in aqueous solution. Wettability was determined by measuring the distribution of critical wetting surface tension, i.e., surface energy, using the film flotation technique and by measuring the induction time, i.e., bubble-particle attachment time, of the material. The wetting of coal particles is strongly dependent upon the coalification processes and can be affected by ash content. For anthracite coal with a high ash content, very high surface energy and intermediate induction time were measured, but intermediate floatability with a very good ash rejection was obtained. Sub-bituminous coal with low mineral inclusion was found to have a relatively low surface energy, and thus a high floatability, but very poor selectivity was observed, which was reflected in lengthened induction time. When dynamics of flotation behaviors are involved, flotation results can be better interpreted by induction time. For dispersed collector-treated coal samples, an increase in collector dispersion (i.e., a decrease in kerosene droplet size), by direct liquid collector mechanical agitation, ultrasonic energy emulsification, or atomization, caused decreases in surface energy and induction time and closely matched the increase of flotation recovery and selectivity. With similar particle density, mineral liberation conditions, and particle size, the induction time was found to be closely related to the mean critical surface tension for untreated coal samples and for HV-bituminous coal samples treated by various levels of dispersed collector.
Introduction The efficiencies of surface-property-based separation processes such as fine coal flotation, coal dust abatement, and coal-water slurry preparation rely upon the wettability characteristics (hydrophobic/hydrophilic) of fine coal particles.1-3 In froth flotation, coal particles are often subjected to a suitable hydrocarbon oil treatment to alter their hydrophobicity, enhance recovery, and/or improve selectivity of fine coal flotation. Conventionally, the oily collector is added directly to the coal-water slurry. This procedure may lead to inadequate collector dispersion and nonselective adsorption. To improve the separation efficiency of fine coal flotation, an appropriate collector dispersion technique must be used to generate small and well-dispersed collector (hydrocarbon oil) droplets in aqueous solution which selectively adhere to the carbonaceous surface of coal particles.4,5 Several methods have been developed to determine the wettability of coal particle surfaces. The most * E-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, August 15, 1996. (1) Chander, S.; Mohal, B. R.; Aplan, F. F. Colloids Surf. 1987, 26, 205-216. (2) Laskowski, J. S.; He, Y. B.; Zhan, Y. Preprint 92-125, SME Annual Meeting, Phoenix, AZ, Feb 1992. (3) Somasundaran, P.; Ramesh, R. Coal Prep. (Gordon & Breach) 1991, 9, 121-130. (4) Misra, M.; Anazia, I. Miner. Metall. Process. 1988, Nov, 233236. (5) Peng, F. F.; Zhang, X.; Li, L.; Cho, E. H. Preprint 92-235, SME Annual Meeting, Phoenix, AZ, Feb 1992.
widely used is the contact angle measurement involving an air bubble applied to a flat, polished, solid surface in aqueous solution. This contact angle measurement may not yield sensible corresponding information for coal in powder form, yet when used properly, the contact angles can provide valuable information.6 Walker et al.7 modified the immersion/sink time procedure to measure wetting ability of coal dust with various surfactant treatments. They used an electronic balance to measure the wetting rate and determine the minimum surfactant requirement in which the coal would sink instantaneously. Mohal and Chander8 developed an imbibition time method to assess coal powder wettability in nonionic surfactant solution. The imbibition time is from the instant when the particle first impacts the liquidgas interface to the instant when it detaches from the interface. Tampy et al.9 modified the Washburn technique by developing an analytical method to determine the surface energy changes associated with wetting based on the fundamental equations of fluid transport through a packed powder bed of solids. Darcovich et al.10 used the adhesion technique to study the surface (6) Gutierrez-Rodriguez, J. A.; Purcell, R. J., Jr.; Aplan, F. F. Colloids Surf. 1984, 12, 1-25. (7) Walker, P. L.; Peterson, E. E.; Wright, C. C. Ind. Eng. Chem. 1952, 44 (10), 2389-2392. (8) Mohal, B. R.; Chander, S. Colloids Surf. 1986, 21, 193-203. (9) Tampy, G. K.; Chen, W. J.; Prudich, M. E.; Savage, R. L. Energy Fuels 1988, 2 (6), 782-786. (10) Darcovich, K.; Capes, C. E.; Talbot, F. D. F. Energy Fuels 1989, 3, 64-70.
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Energy & Fuels, Vol. 10, No. 6, 1996 1203
Table 1. Coal Ranks, Seam Names, Proximate Analyses, and Mine Locations coal rank
coal seam
mine location
moisture (%)
fixed C (%)
volatile matter (%)
ash (%)
anthracite LV-bituminous MV-bituminous HV-bituminous sub-bituminous lignite
Mammoth Lower Kittanning Upper Freeport Pittsburgh No. 8 Wyodak Wilcox Formation
Schuylkill Co., Pennsylvania Cambria Co., Pennsylvania Indiana Co., Pennsylvania Belmont Co., Ohio Gillette Co., Wyoming Freestone Co., Texas
1.8 0.6 0.8 1.5 23.0 28.7
56.1 66.0 49.5 36.9 32.4 24.7
4.5 17.2 23.2 34.3 37.9 34.7
37.6 16.2 26.5 28.3 6.7 11.9
properties of oil agglomerates made from high-purity coal, using hexadecane as the bridging liquid. Fuerstenau et al.11 and Marmur et al.12 developed a film flotation technique to obtain the distribution of the critical wetting surface energy and the fractions of hydrophobicity for the fine particle populations from film flotation responses. Fuerstenau and Diao13 used this technique to further study the wettability behavior of oxidized fine coal. This technique was also applied by Sablik and Wierzchowski14 to evaluate the effect of polar and apolar reagents on the surface energy of fine coals which were prewetted outside the flotation environment. The reagents they used consisted of dodecane, a mixture of fuel oil with MIBC, and a mixture of fuel oil with a frothing agent (AC). Another useful measure of wettability is induction time, or bubble-particle attachment time. This is the minimum time required to disjoin a water film between colliding particle surfaces and a gas bubble surface in froth flotation.15-17 The induction time measurement was used by Yoon and Yordan to evaluate the effects of particle size, dodecylamminium hydrochloride concentration, temperature, and indifferent electrolyte concentration on the quartz-amine flotation system. Ye et al. utilized induction time to characterize the surface properties of the six ranks of coal and to study the particle size effect. Induction time was also used by Fan et al.18 to assess the effects of heptane and sodium chloride salt in water and saltwater on the oil agglomeration of graphite, mineral pyrite, and coal samples with varying degrees of hydrophobicity. In this study, surface energy distribution and induction time were determined for six ranks of coal particles as well as for dispersed collector-treated Pittsburgh No. 8 coal particles. The Pittsburgh No. 8 coal particles were treated with various levels of dispersed hydrocarbon oil collector, using mechanical agitation, ultrasonic energy emulsification, or atomization. These treatments were used to alter the degree of hydrophobicity of coal particles to enhance the flotation recovery and selectivity. Wetting characteristics of untreated and treated coal particles were correlated with floatability and flotation recovery. (11) Fuerstenau, D. W.; Diao, J.; Williams, M. C. Colloids Surf. 1991, 10, 1-17. (12) Marmur, A.; Chen, W.; Zografi, G. J. Colloid Interface Sci. 1986, 113 (1), 114-120. (13) Fuerstenau, D. W.; Diao, J. Coal Prep. (Gordon & Breach) 1992, 10, 1-17. (14) Sablik, J.; Wierzchowski, K. Coal Prep. (Gordon & Breach) 1994, 15, 25-34. (15) Eigeles, M. A.; Volova, M. L. Proceedings, 5th International Mineral Processing Congress; IMM: London, 1960; pp 271-284. (16) Ye, Y.; Khandrika, S. M.; Miller, J. D. Int. J. Miner. Process. 1989, 25, 221-240. (17) Yoon, R. H.; Yordan, J. L. J. Colloid Interface Sci. 1991, 141 (2), 374-383. (18) Fan, C.-W.; Hu, Y.-C.; Markuszewski, R.; Wheelock, T. D. Energy & Fuels 1989, 3, 376-381. (19) Zhang, X.; Peng, F. F. Preprint 92-234, SME Annual Meeting, Phoenix, AZ, Feb 1992.
Materials and Experimental Methods Materials and Dispersed Collector-Treated Coal Samples. The coal samples used in this study included six ranks and are listed in Table 1. Coal samples were crushed, ground, and sieved to a fraction of 105-152 µm (mean particle size of 128 µm) and -600 µm (No. 30 U.S. sieve) particle sizes. Distilled/demineralized water was used to prepare all coal slurry samples except for the stirred tank flotation tests, which used tap water. Three collector dispersion techniques were used to generate small and well-dispersed kerosene (collector) droplets in aqueous solution to alter the hydrophobicity of coal particle surfaces. They are (1) direct liquid collector mechanical agitation, during which a collector was dispersed in aqueous solution for 1.5 min at an impeller speed of 1500 rpm in a flotation machine, (2) ultrasonic energy collector emulsification, in which a Branson ultrasonic power supply unit and a horn produced the output frequency of 20 kHz and 5.6 kW was used to emulsify the collector in aqueous solution, and (3) collector atomization, for which an air supply line at an inlet pressure of 55.2 kPa (8 psi) and a capillary tubing providing the suction of collector were used to generate the atomized collector in aqueous solution. The oil agglomerate sample was prepared by blending 1 g of Pittsburgh No. 8 seam coal, 0.5 g of kerosene as a bridging liquid, and 600 mL of distilled water for 8 min with a high-speed blender. The mixture was then washed with water on a sieve to remove the hydrophilic (mineral-rich) particles from the hydrophobic (coal-rich) particles coated with kerosene. Film Flotation Experiments. Film flotation tests were carried out in a 110 mm i.d., 45 mm depth Teflon container. A series of methanol solutions ranging from 22.4 to 72.8 mN/m surface tension of the liquids at 23 °C was used for separation. The schematic diagram of the film flotation apparatus is presented in Figure 1a. A 0.4 g coal sample of 128 µm mean particle size was spread over the surface of the methanol solution to form a monolayer of particle film. The coal particles that floated on the surface of the solution were collected by vacuum into a flask.20 The film flotation response is presented as a cumulative weight percent of lyophobic (hydrophobic) particles versus surface tension of separation solution. The critical wetting surface tension of the hydrophobic particles is taken to be the surface tension of the liquid at which the particle just sinks or is imbibed into the wetting liquid. The mean critical wetting surface tension of the coal particles, γj c, is defined as the surface energy which corresponds to 50% of the cumulative lyophobic particles. This may be calculated by γj c ) ∫γcf(γc) dγc, where f(γc) is the frequency distribution function.13 The heterogeneity of the particle surfaces can be described by f(γc) or characterized by the dispersion of the frequency curve, ∆γc′ ) γcmax - γcmin, where γcmax is maximum surface energy and γcmin is minimum surface energy. Induction Time Measurement. Induction time was measured using an electronic induction timer Model MCT-100 (Mineral & Coal Technologies, Blacksburg, VA). A 2 mm diameter air bubble was generated in the water at the end of the capillary tube by a microsyringe, 0.1 mm above the particle bed consisting of 128 µm mean size particles as shown in Figure 1b. The bubble was brought into contact with the (20) Peng, F. F.; Zhang, X.; Inoue, J. Preprint 93-266, SME Annual Meeting, Reno, NV, Feb 1993.
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Figure 2. Wettability distributions of lyophobic particles for six ranks of coal.
Figure 1. (a) Film flotation cell and (b) induction timer. particles for a timed interval during which the number of coal particles that adhered to the bubble was counted. The percent of coal particles attached to the bubble versus various exposure times was plotted.20 Induction time was defined as the exposure time that resulted in a 50% particle attachment for a 10-run trial. Froth Flotation Experiments. The natural floatability of six ranks of coal was determined using Hallimond tube flotation with 150 mL of distilled water, 3 g of -600µm coal, and 100 mL/min nitrogen gas. No frother or collector was used in these flotation experiments; that is, this was designated a reagentless condition. The froth sample was collected for 5 min. Stirred tank flotation experiments were performed in a 2 L cell with an automated Denver flotation machine Model D-12 at 10% of pulp density, an air flow rate of 3.1 cm3 min-1 (L cell volume)-1, 1500 rpm impeller speed, and the natural pH.5,20 The froth product was collected for 90 s. The collector in liquid or dispersed form (0.5 kg/ton kerosene) was added to the coal slurry prior to MIBC frother addition (0.05 kg/ton, methylisobutylcarbinol).
Figure 3. Bubble to particle attachment time distributions for high-rank coals. Table 2. Floatabilities, Induction Times, and Surface Energies of Six Ranks of Coals
Results and Discussion
coal seam
Coal Ranks, Particle Surface Characteristics, and Floatability. The cumulative weight percentage of lyophobic particles is plotted as a function of the wetting surface tension of the separation solutions (methanol) for all six ranks of coal in Figure 2. The surface energies obtained from these film flotation responses are summarized in Table 2. The wide ranges of surface energy dispersion, ∆γ ′, displayed in Figure 2, illustrate the distributed nature of coal particle surfaces which result from the heterogeneity of coal particles. This heterogeneity of coal particles can be due to (1) random dissemination of mineral matter in the coal matrix and (2) inherent heterogeneity of the coal surface itself.3 The former is related to the liberation of mineral matters from coal particles by crushing and grinding. Thus, it is possible to use a size reduction process to produce more distinct coal-rich and mineral-rich particles (fewer middling particles) of the material. The latter may be caused by oxidation state, rank of coal, and various organic
Mammoth Lower Kittanning Upper Freeport Pittsburgh No. 8 Wyodak Wilcox Formation
floatability ash recovery rejection induction (%) (%) time (ms) 26.84 84.12 82.26 37.88 61.95 11.22
81.39 49.48 63.74 71.19 41.81 88.32
7.35 6.27 6.40 9.40 62.63 384.0
surface energy (mN/m) contact γj c angle (deg) 63.61 43.89 44.17 49.72 45.83 53.60
27.51 72.74 72.74 66.63 70.82 63.40
moieties of coal matrix and their distribution on the coal particle surfaces. Comparison of various coal samples with similar ash content shows that as the rank of coal increases, the surface energy decreases from about 54 to 44 mN/m and the wettability distribution curve becomes steeper (small ∆γ ′). For anthracite coal particles, which have the highest ash content, the surface energy is also higher, about 64 mN/m. The bubbleparticle attachment frequency as a function of contact time is shown in Figure 3 for four high-rank coal. The extremely high induction times of Wyodak and Wilcox seam coal samples are excluded from the figure, but the values for all six ranks of coal are included in Table 2. The distribution range of the bubble-particle attach-
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Figure 4. Correlation of mean critical wetting surface tension, induction time, and floatability for six ranks of coal: (1) Mammoth, (2) Lower Kittanning, (3) Upper Freeport, (4) Pittsburgh No. 8, (5) Wyodak, and (6) Wilcox seam coals.
ment frequency also represents the extent of the heterogeneity of coal particle surfaces in the particle population. The correlations of the coal ranks with mean critical wetting surface tension, floatability, and induction time are given in Figure 4. Induction time ranged from 384 ms for low-rank lignite to 6.3 ms for high-rank lowvolatile (LV)-bituminous coal. The floatability results presented in Figure 4 show a close correlation with the mean critical wetting surface tension and induction time for various ranks of coal, the Wyodak and Mammoth coal samples being the exceptions. LV- and MVbituminous coals such as Lower Kittanning and Upper Freeport coal samples with high carbon content and aromacity exhibit the lowest mean surface energy (4244 mN/m) and induction time (6.3 ms), resulting in the higher floatability (84%) than the other ranks of coal. Pittsburgh No. 8 seam coal, a high-volatile (HV)bituminous coal, has a higher mean critical wetting surface tension (50 mN/m) and induction time (9.4 ms) than Lower Kittanning and Upper Freeport coals, giving a moderate natural floatability of 38%. The anthracite coal from the Mammoth seam has the highest ash content of 37.6% and mean wetting surface tension of 63.6 mN/m among the six ranks of coal samples, resulting in a 26.8% natural floatability. With a 7.4 ms induction time, Mammoth seam coal is expected to float as well as HV-bituminous coal but should not float better than LV-bituminous or medium-volatile (MV)-bituminous coals. A lower induction time and a
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Energy & Fuels, Vol. 10, No. 6, 1996 1205
very good ash rejection of 81.4% for Mammoth coal indicate possibly low numbers of middling particles in the samples. A higher degree of mineral liberation of this sample by grinding to produce more distinct coalrich particles and mineral-rich particles may be responsible for its good ash rejection. This sample appears to have an intermediate amount of coal particles containing a higher percentage of hydrophobic sites. This is reflected in a relatively low induction time of 7.4 ms, a value closer to that of the Pittsburgh No. 8 coal sample (9.4 ms). Therefore, good selectivity can be expected for both coal samples in the flotation process with an appropriate collector treatment. Wyodak coal, a sub-bituminous class, is a weakly hydrophobic material. However, it has a mean wetting surface tension of 46 mN/m, which is close to that of Lower Kittanning and Upper Freeport coals. As expected, its natural floatability is relatively high (62%) due to very low mineral matter inclusion (6.7%) and the highest level of volatile matter (37.9%) among the bituminous coals. A higher proportion of carbonaceous surfaces in the Wyodak coal sample might be exposed during the film flotation measurement. This, along with much lighter particles, may cause a relatively low surface energy measurement. From ash analyses of the flotation products, there was almost no difference between the ash contents of the froth product and the tailings,19 corresponding to a lowest ash rejection (41.9%), which in turn showed a very high induction time (62.6 ms). It is clear that the coal particle density (ash content) and coalification processes have significant effects on the homogeneity and overall hydrophobicity of the coal particle surface. A direct correspondence between the mean critical wetting surface tension and the induction time is not observed for Wyodak and Mammoth seam coals due to much different particle densities. A long induction time indicates more oxidized particle surfaces (due to the coal rank in this case) and possibly low mineral liberation conditions. The lignite from the Wilcox Formation seam shows a very high mean surface energy of 53.6 mN/m and an extremely high induction time of 384.0 ms, indicating a very low percentage of hydrophobic sites on the particle surfaces. Correspondingly, a very low natural floatability of 11.2% was determined. Clearly, the Wilcox Formation particle surfaces have considerably higher concentrations of oxygen functional groups, such as carboxylic and phenolic groups, interacting with water molecules (hydrophilicity), or they have poor mineral liberation conditions. Thus, longer bubbleparticle attachment time and less selective attachment of the air bubble took place on the carbonaceous surfaces of these particles during induction time measurement. The correlation results show that both surface characterization methods may be used to distinguish the levels of heterogeneity among the six ranks of coal. Induction time, however, is capable of measuring the significant change of dynamic behavior of bubbleparticle attachment related to the surface chemistry of particles, the heterogeneity of coal particle populations, and flotation systems.21 For the coal samples investigated, the use of induction time is more likely to predict the separation efficiency or selectivity of flotation than using wetting surface tension. (21) Laskowski, J. S. Miner. Sci. Eng. 1974, 6 (4), 223.
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Table 3. Flotation Responses, Induction Times, and Surface Energies of Pittsburgh No. 8 Seam Coal (High-Volatile Bituminous) Treated with Various Dispersed Collectors flotation response collector dispersion techniques reagentless mechanical agitation atomizing ultrasonic emulsification oil agglomeration
recovery (%)
ash rejection (%)
(37.88) 78.26
(71.19) 76.23
86.82 83.29
75.40 75.29
92.13
65.36
Figure 5. Wettability distributions of lyophobic particles for Pittsburgh No. 8 coal particles untreated and treated with various levels of dispersed collector.
Levels of Collector Dispersion, Dispersed Collector-Treated Particle Surface Characteristics, and Recovery. The cumulative weight percent of lyophobic (hydrophobic) particles as a function of surface tension of separation solution is shown in Figure 5 for untreated and treated Pittsburgh No. 8 seam coal with various levels of dispersed collector and for coal-oil agglomerates. The surface critical wetting property data of dispersed collector-treated coal samples are summarized in Table 3. The coal sample agglomerated with kerosene is used to represent a high dosage of collector adhered to the coal particles. As the level of kerosene dispersion increases or the amount of collector increases, the surface energy and induction time decrease. There is a considerable reduction in the mean wetting surface tension from untreated (natural state, 49.7 mN/m) to treated coal samples, with dispersed collector generated by direct liquid collector mechanical agitation (2 mN/m difference), atomization (6 mN/m difference), and ultrasonic energy emulsification (8 mN/m difference) as well as to oil agglomeration (18 mN/m difference). Furthermore, the wettability distribution curve for coal-oil agglomerates is sharper, narrower, and more uniform (∆γ ′ ∼ 17 mN/m) than those for dispersed collector-treated coal samples (∆γ ′ ∼ 37-58 mN/m). Increasing the level of collector dispersion (decreasing collector droplet size) and increasing the amount of kerosene also cause a reduction (∆t) in the induction time from its natural state (9.4 ms). The sequence is direct liquid collector mechanical agitation (∆t ∼ 2.4 ms), atomization (2.8 ms), ultrasonic energy emulsification (2.9 ms), and oil agglomeration (>7 ms). The
surface energy (mN/m) γcmax - γcmin
γj c
contact angle (deg)
9.40 7.00
72.3-34.9 84.9-27.2
49.72 47.78
66.63 68.66
6.56 6.52
86.0-31.9 87.6-31.9
43.89 41.67
73.09 75.69
41.2-24.6
31.94
87.81
induction time (ms)
