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Langmuir 1996, 12, 4808-4813
Flotation of Inherently Hydrophobic Particles in Aqueous Solutions of Inorganic Electrolytes O. Paulson and R. J. Pugh* The Institute for Surface Chemistry, Box 5607, Stockholm, Sweden, S-11486 Received February 12, 1996. In Final Form: June 5, 1996X The flotation of graphite particles in aqueous solutions of inorganic electrolytes was shown to depend on both the nature of the cation/anion pair and the range of the bubble/particle electrostatic interaction. For several electrolytes, as the reduction in the Debye length of the solution approached the decay length of the hydrophobic attraction, then flotation began to occur. Also using earlier reported data, it was possible to relate the flotation to surface tension/electrolyte concentration gradients and bubble coalescence behavior of the different electrolyte solutions. Higher flotation recoveries were attributed to an increased collision probability between the graphite particles, a higher concentration of small noncoalescing bubbles, and an increased stability of the froth. Furthermore, it has also been shown from previous studies that increasing electrolyte concentration causes a decrease in gas solubility. In fact, gas solubility has been shown to be dependent on the hydration entropy of the cation. This phenomenon was explained in terms of competitive utilization of water molecules in the hydration of cations and a consequent loss or gain in gas solubility. Overall, it was shown that a reduction in the electrostatic interactions between particle and bubble assisted flotation. However, in addition, an increase in flotation performance resulted from the inhibition of coalescence of bubbles, which is also linked with dissolved gas concentration gradients (structural differences at the air/solution interfacial region relative to the the bulk electrolyte solution).
Introduction The ability of electrolyte solutions to control the stability of bubbles and the flotation of naturally hydrophobic particles was first documented during 1932-1934 in the USSR. This work was essentially related to the flotation of coals in saline waters. It was also firmly established that inorganic electrolytes do not cause flotation of minerals which were not inherently hydrophobic. Following these early studies, many other cases have been reported in the general area of mineral flotation. For example, there are several sulfide mineral systems, hydrophobized in xanthate collectors, where it has been clearly demonstrated that different types of inorganic electrolytes can have a profound influence on the flotation kinetics. This area has been reviewed by Wellam et al.,1 Yoon,2 and Yarar.3 A summary indicating the types of electrolytes tested and the characteristics of some of these systems is shown in Table 1. According to these reviews, several surface chemical mechanisms have been proposed to explain the flotation process. These range from the action of the electrolytes in (a) disruption of hydration layers surrounding the particles and enhancing bubble-particle capture, (b) reducing the electrostatic interactions, and (c) increasing the charge on the surface of the bubbles to prevent primary bubble coalescence. However, none of these appears to satisfactorily explain the experimental behavior, and even today, the precise mechanism still remains obscure. In addition, from these early studies, the following general observations were noted: (a) the greater the frothing capacity of the inorganic electrolyte, the greater the flotation recovery; (b) the cations play a more important role than the anions in the process efficiency. From more detailed batch flotation coal cleaning studies by Yoon4 using inorganic electrolyte, it was concluded that
Table 1. Examples of Salt Flotation Practice, From Reference 3 electrolyte solution seawater NaCl, CaCl2, Na2SO4 NaCl, KCl, CaCl2, saline pit water Al2(SO4), Na2SO4, NaCl, CaCl2 NaCl, Na2CO3, Na2PO4, Na2SO4, (NH4)2SO4, CuSO4, FeSO4, Fe2(SO4)3, Al2(SO4)3
system floated Zn concentrate from 3.4% to 47.7%a coals; recoveries, 12-83%; grades; 4.5-8.4% ashb,c coals; recoveries, ∼96%;d flotation response is rank-dependent coals; recoveries, up to ∼90%; response is Macerel-dependent coals; recoveries, 50-77%; ash, 11-21%
a This is a Pb-Zn xanthate flotation circuit which used seawater. 12 Russian references are of direct relevance. c Data are also presented on talc, sulfur, and graphite flotation. Recoveries range between 52 and 95%. d A number of his previous papers on the subject have been published in Polish or Russian.
b
* Corresponding author: e-mail address, bob.pugh@surfchem. kth.se. X Abstract published in Advance ACS Abstracts, July 15, 1996.
the flotation efficiency increased with increase in salt concentration, and divalent anions such as S2O32- and SO42- proved very effective. Flotation speeds and separation efficiency were found to be improved. These workers attributed the mechanism to a reduction in the double layer thickness around the hydrophobic particle enhancing bubble-particle adhesion. Also from wetting (contact angle) and heat of immersion data, they showed that the hydrophobicity of the surface was not increased by the salt concentration. However, they could not eliminate the theory that the hydration sheath was reduced by electrolytes, leading to a reduction in the disjoining pressure and enabling bubble-particle contact to occur. More recently Craig et al.5,6 assessed the inhibition of bubbles to coalescence in electrolytes by the application of a combining rule based on the nature of the cationic/ anionic pair. This rule enabled one to predicted whether or not the electrolyte would inhibit coalescence of gas bubbles in the electrolyte solutions. Surface tensions,
(1) Wellham, E. J.; Elber, L.; Yan, D. S. Miner. Eng. 1992, 5 (3-5), 381. (2) Yoon, R. H. Min. Congr. J. 1982, 68 (12), 76. (3) Yarar, B. In Froth Flotation; Castro, S. H., Alvarez, J., Eds.; Elsevier Publishing: Amsterdam, 1988; Chapter 3, p 41.
(4) Yoon R. H. Surf. Sci. Ser. 1990, 87. (5) Craig, V. S. J.; Ninham, B.; Pashley, R. M. J. Phys. Chem. 1993, 97, 10192. (6) Craig, V. S. J.; Ninham, B.; Pashley, R. M. Nature 1993, 364, 317.
S0743-7463(96)00128-X CCC: $12.00
© 1996 American Chemical Society
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viscosity and electrostatic repulsion were ruled out as possible explanations and it was suggested the a reduction in hydrophobic attraction between bubbles was occurring with certain ion pairs which caused a reduction in coalescence. Finally, Weisenborn and Pugh7,8 re-evaluated the coalescence behavior in terms of the surface tension concentration gradients and the solubility of gases in the electrolyte solutions. Alternative interpretations have also been put forward including Gibbs surface elasticity.9,10 In the present study, we have determined the flotation of model hydrophobic particles in a series of electrolytes, under well-defined conditions. These experiments enable current theories of flotation, bubble coalescence, and bubble-particle attachment to be evaluated. In addition to mineral processing, flotation in electrolytes is of particular interest to the deinking processes, where high concentrations of inorganic species are present in solution. In fact, Larsson et al.11 clearly demonstrated that the flotation efficiency of ink particles (hydrophobized in oleate solution) increased with increase in sodium chloride, within the normal concentration ranges found in deinking plants (10-50 mM). These studies are also relevant where bubbles are generated in an inorganic environment, such as an electrolytic flotation cell. Although salt solutions can have a detrimental corrosive effect on plant hardware, in some cases the occurrence of readily available underground water sources can outweigh this disadvantage, enabling them to be used in flotation. Experimental Section Water was purified by a Milli-Q plus 185 system. All glassware was cleaned in chromic acid and washed in Milli-Q water. Stock solution of a series of inorganic electrolytes were prepared in water. The electrolytes were of analytical grade and, if available, of purity greater than 99.9%. Amorphous hydrophobic graphite is a well-known naturally hydrophobic material which primary occurs as a product of thermal metamorphism of coal seams. Previous studies have reported contact angles on amorphous graphite between 77° and 81° in the pH range 2-9.12,13 For the present study, amorphous graphite powder supplied by Merck, Damstadt, Germany (>99.9% purity), was chosen as a model system. The average particle size of the material used in the present study (after initially dispersing the primary particles in water) was determined using a Malvern Master Sizer. Initially, the Malvern Master Sizer sample chamber was filled with deionized water which was recirculated by pumping, throughout the system and through the measuring cell. Background checks were made to ensure no air bubbles were present in the water. Then a graphite suspension (2 wt%) was prepared by gently dispersing the powder in deionized water and the suspension added by syringe to the mixing chamber, which was set to operate in the higher stirring rate mode (1600 rpm). Additions of the suspensions were continued until a obscuration reading of about 20% was recorded from the measuring cell. This enabled particle size measurements to be carried out. The size distribution of the sample was then determined under this relatively high state of dispersion. Following this measurement, further particles size distributions were obtained at reduced stirring rates (800 and 400 rpm). It was found that coagulation of the hydrophobic particles occurred in these lower shear rate regimes. This was revealed by an increase in particles size distribution caused by the formation of coagula. The zeta potential of a dilute suspension (7) Weissenborn, P.; Pugh, R. J. Langmuir 1995, 11, 1422. (8) Weissenborn, P.; Pugh, R. J. Submitted for publication in J. Colloid Interface Sci. (9) Christenson H. K.; Yaminsky V. V. J. Phys. Chem. 1995, 99, 10420. (10) Hofmeier, U.; Yaminsky, V. V.; Christenson, H. K. J. Colloid Interface Sci. 1995, 174, 199. (11) Larsson, A.; Stenius, P.; Odberg, L. Svensk papperstidning, No. 18, 1984 87 R-158-R164 (1984). (12) Wakamatsu, T.; Numata, T. Miner. Eng. 1991, 4 (7-11), 975. (13) Chander, S.; Wie, J. M.; Fuerstenau, D. W. AIChE Symp. Ser. 1975, 71 (150) 183.
Figure 1. Particle size distribution determined by Master Sizer (Malvern Instruments) of graphite particles dispersed in water under a range of shear conditions: (a) high shear rate (stirrer speed 1600 rpm); (b) medium shear rate (stirrer speed 800 rpm); (c) low shear (stirrer speed 400 rpm). of the graphite particles, after dispersing in electrolyte and allowing the larger particles to sediment, was determined using the Malvern Zeta Sizer following standard procedures. Measurements were made in a range of electrolyte (NaCl) concentrations. Flotation of graphite was carried out in a small glass cylindrical column cell (height 19 cm, diameter 5.5 cm) as previously described in ref 14. A magnetic stirrer was positioned in the base of the cell (stirring rate 500 rpm) to disperse the graphite particles. A suspension of graphite (2 wt %) in distilled water was used throughout the experiments. The volume of the suspension for each experiment was 440 mL. Dry air was bubbled through the cell after passing through a glass frit (size 5 pore, hole size 1.0-1.7 µm with a cross-sectional area of 9.8 × 10-3 m2) in the cell base. The flow rate of air was maintained at 48 cm3/ min during the flotation tests. The graphite was recovered by scaping-off the froth into the outer recover cylinder and collecting in a beaker. Flotation time was maintained at 15 min for each test, and graphite was removed from the froth every 30 s. The recovered graphite was filtered-off on a glass microfiber filter (Whatman GF/A). The filtrate was dried in a oven (80 °C for 14 h) and weighed. All tests were performed under conditions of neutral pH (5.5-6.5) except in NH4Cl (which had a pH of 4.9) and in strong acid and alkali systems.
Results and Discussion Dispersion of Particles in Electrolyte. In Figure 1, particle size distributions are shown for the dispersion of graphite in sodium chloride solution. The particle size distribution for the dispersed system at high shear (1600 rpm) is shown in Figure 1a and indicates an average particle diameter of 14 µm. However, at lower shear rates (800 and 400 rpm as indicated in parts a and b of Figure (14) Nishkov, I.; Pugh, R. J. Processing of Fine Particles; Pumpton, A. J., Ed.; Canadian Institute of Mining and Metallurgy and Petroleum: Montreal, 1988; p 141.
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Figure 2. Flotation recovery of graphite particles versus the electrolyte concentration for a series of different types of electrolytes: group A, high flotation performance; group B, intermediate flotation performance; group C, low flotation performance.
1, respectively) the data indicate an increase in size distribution. In the case of the lower shear rate, a cut-off at about 90 µm was observed in the higher particle size regime, as indicated by the asymmetric shape of the hystergram. This result can be explained by hydrophobic coagulation of the graphite particles, followed by settling of larger coagula to the bottom of the measuring cell, outside the path of the laser. The coagulation of hydrophobic particles (methylated silica) has been studied by Xu and Yoon.15 These workers showed that at low electrolyte concentration and under conditions of low shear, the electrostatic repulsive forces could be easily overcome by the a long range hydrophobic attraction leading to weak coagula structures, even in cases where the surface potential of the particles was over -50 mV. More recent studies by Zhou et al.16 showed the influence of gas nucei on hydrophobic coagulation. The presence of nuclei decreased the density and increased the size of coal aggregates formed from the coagulation of fine particles. From modeling studies it was shown that gas nuclei had a volumetric mean diameter between 6 and 17 µm. Flotation. The flotation of graphite as a function of the electrolyte concentration is shown in Figure 2. From this plot it can be seen that the recovery generally increases with concentration and varies according to the cationic/ anionic pair. In fact, it is possible to classify the electrolytes into three groups according to their flotation performance. Group A, salts with divalent and trivalent cations or anions, includes MgCl2, CaCl2, Na2SO4, MgSO4, and LaCl3 and gives a high flotation response. In this group, flotation begins at low electrolyte concentrations (at about 0.02 M) and reaches maximum recovery at about 0.06-0.1 M. Group B includes NaCl, LiCl, KCl, CsCl, and NH4Cl, which give medium flotation response with flotation beginning at about 0.05 to 0.1 M electrolyte.
Finally group C (NaAc, NaClO4, HClO4, HCl, H2SO4, LiClO4, H3PO4) gives a very low flotation response, even up to concentrations as high as 0.3 M electrolyte. Electrostatics. The mechanism of the interaction between a hydrophobic mineral surface and an air bubble is still incomplete. It has been generally accepted that in weak electrolytes, both the air bubble and particle are negatively charged and both interfaces may be considered to be hydrophobic (high interfacial energy). However, at higher concentrations, specific adsorption of cations may occur which will determine the hydration states of the surfaces and may change the charge. Several earlier workers have also stressed the importance of the role of electrostatic interaction between particle and bubble in controlling the attachment process. At high electrolyte concentration, the electrostatic interaction would be sufficiently low to enable the hydrophobic forces to dominate the interaction. This would enable attachment to occur causing a high flotation response. In Figure 3, the zeta potential of the graphite particles versus the electrolyte concentration in the neutral pH range (7-8) is shown. In this case NaCl was chosen, corresponding to a typical type B electrolyte. From this plot it can be seen that as the electrolyte concentration is increased, the original surface potential is reduced from about -60 mV to about -5 to -10 mV. These results are similar to previous reported electrokinetic studies of graphite in NaCl solution.12 In the case of the surface potential of bubbles in electrolyte solutions, a series of experiments have been carried out by Li and Somasundra17-19 where the zeta potential has been determined as a function of pH. Measurements were carried out in 1:1, 2:1, and 3:1 valent electrolytes. From these results, it was found that the increase in electrolyte
(15) Xu, Z.; Yoon, R. H. J. Colloid Interface Sci. 1990, 134, 427. (16) Zhou, Z. A.; Xu, Z.; Finch, J. A. J. Colloid Interface Sci. 1996, 179, 311.
(17) Li, C.; Somasundaran, P. Colloid Interface Sci. 1993, 81, 13. (18) Li, C.; Somasundaran, P. Colloid Interface Sci. 1992, 148, 587. (19) Li, C.; Somasundaran, P. Colloid Interface Sci. 1991, 146, 215.
Flotation of Hydrophobic Particles
Figure 3. Zeta potential of graphite particles (O) and bubbles as a function of sodium chloride concentration, neutral pH (78). The region where graphite flotation occurs and inhibition of gas bubbles is indicated.
concentration had the general effect of decreasing the surface potential for all electrolytes studied, the trivalent cation having the greatest effect, followed by the divalent and finally the univalent cation. In Figure 3, the zeta potential versus concentration data for air bubbles in NaCl at neutral pH is included using the data of Li and Somasundra.17 This enables a comparison to be made with the graphite particles. The potential of the air bubbles is higher than the particles, but is reduced to about -30 mV in 0.1 M solution. Also in Figure 3, the range of flotation recovery of graphite in sodium chloride solution is indicated. Flotation begins with the particles having a zeta potential of less than -10 mV and the bubble -30 mV, but these values are reduced to about -5 mV and about -20 mV, respectively (obtained by extrapolation), in the region corresponding to 100% flotation recovery. When considering the bubble-particle interaction in terms of the electrostatic forces, at low electrolyte concentrations (0.01 M) we would anticipate that there would be sufficient repulsion to give only weak heteroflocculation. However, at high electrolyte concentrations, the electrostatic repulsion would be almost completely eliminated. In the intermediate electrolyte concentration range, both the short range repulsive van der Waals interaction (combined negative Hamaker constant) and the long range attractive hydrophobic interactions must be also taken into consideration to balance the electrostatic forces. Since the hydrophobic force is much stronger than the van der Waals force, it may be expected that as the Debye length of the electrolyte solution approaches the decay length of the hydrophobic attraction, then according to the modified DLVO theory15 the potential energy primary maximum will be exceeded. This will correspond in the beginning of the heterocoagulation process. Hence, an important parameter controlling the flotation process would be the Debye length or the thickness of the electrostatic double layer (1/κ). In Figure 4, a plot is shown of 1/κ versus the flotation recovery. For the electrolytes in groups A and B, the points more or less lie on the same curve and show a correlation between the double layer length and the flotation performance. Flotation appears to begin at a value of 1/κ of about 1 nm and reach 100% recovery at 1/κ of about 0.5 nm. Since the latter value of the double layer thickness is rather low, the electrostatic repulsion based on Poisson-Boltzmann theory between bubble and particle must be almost zero in this region. However, it can also be seen from Figure 4 that in the case of the group C electrolytes, the flotation response appears to be independent of the double layer length. From these results
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it can be concluded that the electrostatic interaction plays an important role but cannot be solely responsible for the flotation. With regard to the hydrophobic particles, since the zeta potential in dilute electrolyte is lower than the bubbles, it can be anticipated that hydrophobic coagulation occurs fairly readily. Also, since amorphous graphite particles will probably entrap gas on the surface, particle bridging by two gas nucei will probably enhance this hydrophobic effect. Bubble Coalescence, Surface Tension, and Flotation Response. In addition to the interaction between the bubble and particle, the bubble-bubble interaction (bubble coalescence) must also be taken into consideration. For two approaching bubbles in water, the thin water film must be squeezed out before coalescence can occur. This is resisted by a hydrodynamic repulsive force and Craig et al.5,6 suggested that the hydrophobic attractive force could overcome this repulsion leading to coalescence. In earlier studies, the inhibition of bubble coalescence by electrolyte beyond a critical transition concentration has been carefully documented.5 A relationship between the value of the transition concentration and surface tension gradient of the electrolyte was also reported.7 Also a correlation was made between the decrease in gas solubility (as represented by the expontial decay coefficient of oxygen solubility) and the increase in electrolyte concentration. In Table 2, the flotation recovery of the graphite, together with the surface tension concentration gradient of the electrolytes (data taken from ref 7) is shown. In addition, the concentration of electrolyte, which was found to inhibit bubble coalescence, is indicated (taken from ref 5). This table clearly shows that high flotation corresponds to the nature of the ion pair, and certain combinations of ion pairs appear significantly more effective than others in inhibiting bubble coalescence. This is could be also related to high values of the surface tension concentration gradients. To summarize, the electrolytes which produce stable bubbles in solution appear to give higher flotation response, whereas the electrolytes which have no effect in inhibiting the coalescence of the bubbles give low flotation. It is also clear that the nature of the cation plays an important role. In fact, it has been earlier suggested that the inhibition of bubble coalescence in electrolyte solutions appears to be linked to the utilization of water molecules in the hydration of cations and a consequent reduction in water available for gas solubility.20 From these results it is rather surprising to see that NaAc and NaClO4 give a low flotation/foaming performance and a low surface tension/electrolyte concentration gradient, while NaCl gives reasonably high flotation/foaming and a high surface tension/electrolyte concentration gradient. A possible explanation for this behavior is that ion pairs are not fully dissociated in the aqueous environment for NaAc and NaClO4. Flotation and Bubble Size. For a flotation process, the fractional removal of particles (R) can be described by the following first-order equation
dR/dt ) k(1 - R)
(1)
where t is the time and k is the flotation rate constant. R is defined by
R ) (C0 - C)/C0
(2)
where C is the number of particles per unit volume (particle concentration) and C0 is the particle concentration at t ) (20) Weissenborn, P.; Pugh, R. J. Proceedings of the International Mineral Processing Conference, San Francisco, 1995; Chapter 21, p 125.
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Figure 4. Relationship between the Debye length (1/κ) and the flotation performance of graphite in different inorganic electrolytes. Table 2. Flotation Response and Surface Tension Concentration Gradients of Electrolytes
electrolyte
surface tension/ electrolyte concn gradient d(∆γ)/dc at 1.5 sa literaturea
transition concn for inhibition of bubble coalescenceb
Group A. High Flotation Response (Flotation Range 0.04-0.08 M Electrolyte Solution) MgCl2 4.06 ( 0.10 3.14 no data CaCl2 4.02 ( 0.08 3.22 0.037 M LaCl3 5.91 ( 0.30 4.73 no data Na2SO4 2.90 ( 0.08 2.96 no data MgSO4 2.44 ( 0.05 2.24 0.02 M Group B. Medium Flotation Response (Flotation Range 0.1-0.25 M electrolyte solution) LiCl 1.98 ( 0.09 1.63 no data NaCl 2.08 ( 0.08 1.55 0.078 M KCl 1.85 ( 0.05 1.60, 1.65 0.12 M CsCl 1.52 ( 0.07 1.56 no data NH4Cl 1.59 ( 0.09 1.34 0.10 M CH3COONa H3PO4 H2SO4 HNO3 HClO4 LiClO4 NaClO4 HCl a
Group C. Low Flotation Response 0.93 ( 0.03 0.54 no transition 0.85 ( 0.06 0.98 no data 0.44 ( 0.06 0.64 no transition -0.83 ( 0.10 -0.70 no transition -2.15 ( 0.08 -1.64 0.070 M 0.27 ( 0.06 not available no transition 0.22 ( 0.06 0.73 no transition -0.27 ( 0.04 -0.29 no transition
References 7 and 8.
b
Reference 5.
0. In addition, from ref 21 the rate constant (k) has been expressed as
k ) (3GfrEL)/(2DbVr)
(3)
where Gfr is the gas volumetric flow rate at a fixed pressure, L is the length of the bubble rise (which is approximately the same distance as the cell height), Vr is the cell volume, (21) Schmidt, D. C.; Bergs, J. C. Prog. Paper Recycl. 1996, Feb, 67.
Db is the diameter of the bubble, and E is the efficiency number which represents the probability that a particle within the path of a rising bubble will be removed by that bubble. E is dependent on the bubble/particle collision efficiency, the bubble/particle attachment efficiency, and the bubble/particle aggregate stability efficiency. The above equations illustrate the importance of the bubble size on the flotation rate. For flotation in the same cell, providing the gas flow and the efficiency number are constant, smaller bubbles will cause an increase in flotation rate. In fact, this effect has been well established in the field of flotation technology.22 In the presence study it was found difficult to determine the bubble size in the cell. However, an estimate of the bubble size could be made from the foam. In order to study the foam structure, a video recording was made during the flotation process with the camera positioned directly above the head of the cell. This enabled the characteristics of the froth to be recorded for different electrolyte systems. The results clearly indicate the differences in bubble size of the froth for the coalescing (NaClO4) and noncoalescing (NaCl) bubble systems (Figure 5). In fact, from these Figures, it appears that the graphite particles in NaCl increase the froth stability leading to a larger bubble size in the froth. It is obvious that the stability of the bubbles and froth is a primarily requirement for flotation. Hence, higher flotation recoveries in increasing concentration of type A and to some extent type B electrolytes, must be partly attributed to an increase in concentrations of smaller bubbles and an increase in froth stability. In addition, the secondary role of electrolyte must be taken into consideration; the concentration must be sufficiently high to cause a reduction in the electrostatic interactions enhancing bubbleparticle contact. (22) Yoon, R. H.; Luttrell, G. H. Mineral Processing and Extractive Metallurgy; Gorden Breach, Science Publishers: Reading, U.K., 1989; Vol. 5, p 101.
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Figure 5. Flotation cell seen from above. Prior to flotation the column is filled with 1% (w/w) of graphite in aqueous solutions of inorganic electrolytes, performance (a) in 0.2 M NaClO4, (b) in 0.2 M NaCl ,and (c) in 0.2 M NaCl; after 8 min of flotation, all graphite is floated and removed. In (a) the air bubbles coalesce together and the electrolyte is therefore nonfoaming, whereas in (b) the bubble coalescence is inhibited and the foam is more stable and the graphite particles collected also increase the stability of the foam.
Conclusion Graphite particles were floated in aqueous solutions of different inorganic electrolytes. Under low shear conditions, the fine particles appear to easily coagulate due to hydrophobic forces, leading to the build-up of coagula. According to their flotation recovery, the electrolytes could be classified into three distinct groups. Group A electrolytes (such as LaCl3, MgCl2, MgSO4, Na2SO4, etc.) were found to give high flotation while group B electrolytes (NaCl, LiCl, KCl, CsCl, NH4Cl) were found to give an intermediate flotation recovery. Finally group C electrolytes (NaAc, NaClO4, HClO4, HCl, H2SO4, LiClO4) produced only low flotation recovery. For group A and B electrolytes, a correlation was obtained between the flotation performance and the Debye length (1/κ). This suggests that the electrostatic interaction plays an important role in the flotation process. Using earlier reported data,7 it was also found possible to relate the flotation to surface tension/electrolyte concentration gradients and bubble coalescence behaviour of the different
electrolyte solutions. Also it has been shown that the gas solubility decreased with increasing electrolyte concentration and depended on the entropy of hydration of the cation and the Jones-Dole viscosity coefficient.7 This can be understood in terms of competitive utilization of water molecules in the hydration of cations and a consequent loss or gain in gas solubility. We propose that the increase flotation performance of the hydrophobic graphite in electrolytes is linked with the dissolved gas concentration gradients in the electrolyte solutions. Higher flotation recoveries were attributed to an increase in the collision probability with higher concentration of smaller noncoalescing bubbles and a reduction in the electrostatic interactions between particle and bubble. Acknowledgment. The authors thank the Bo Rydin Foundation for a research award to O.P. Thanks to Dr. P. Weissenborn for discussions and support. LA960128N
