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Electrolyte System
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
203, 254–259 (1998)
CS985478
Flotation of Heterocoagulated Particulates in Ulexite/SDS/Electrolyte System M. S. Celik,* ,1 E. Yasar,* H. El-Shall† *Mining Faculty, Istanbul Technical University, Ayazaga, Istanbul 80626, Turkey; and †Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-6135 Received May 12, 1997; accepted February 19, 1998
Salt-type minerals can be usually floated with either anionic or cationic collectors. In a number of systems, flotation has been reported to remarkably increase above the concentrations where precipitation of the collector salt is initiated. Some studies attribute this phenomenon to heterocoagulation of oppositely charged colloidal precipitate and mineral particles. In this study, ulexite, a semisoluble boron mineral, in the presence of various multivalent ions, i.e. Ba 2/ , Mg 2/ , Ca 2/ , and Al 3/ , was found to exhibit excellent flotation even when particles, colloidal precipitates, and bubbles acquire a similar charge, which indicates that attractive structural forces exceed the forces of electrostatic repulsion. q 1998 Academic Press
Key Words: coagulation; flotation; boron minerals; ulexite; precipitation; hydrophobic interaction; zeta potential.
INTRODUCTION
Non-sulfide minerals are usually floated with long-chain collectors such as sulfonates, carboxylates, and amines. These collectors may, however, undergo precipitation upon exceeding their solubility limits. The weak electrolyte-type surfactants such as fatty acids and amines, depending on pH, may precipitate because the solubility limit of free acid or free amine is much lower than the corresponding solubility of the ionized species. These molecules form colloidal species with a clear isoelectric point (1). However, strong electrolyte-type surfactants such as sodium dodecyl sulfate ionize into its respective ions, and if precipitation can be accomplished with polyvalent cations, the precipitate exhibits no iep in the entire pH region (2). Due to their relatively high solubilities, salt-type minerals release sufficient amounts of multivalent cations (e.g. Ca 2/ and Mg 2/ ) that react with oppositely charged collector anions to form insoluble colloidal precipitates. A number of studies, notably the one reported by Ananthapadmanabhan and Somasundaran (3), have demonstrated 1
To whom correspondence should be addressed. E-mail: mcelik@sariyer. cc.itu.edu.tr.
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Materials
Ultrapure ulexite crystals handpicked from the Bigadic boron deposit were used. The lump size crystals were crushed by a hammer and then ground in an agate mortar
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that flotation in a number of systems is initiated at the onset of precipitation of the collector salt. Above this critical point, in the region of bulk precipitation, the flotation rate usually increases almost exponentially. The major question posed here is whether this region of improved flotation is due to surface or bulk precipitation. As early as 1933, Halbich (4) proposed a model involving the formation of insoluble surface reaction products to explain the mechanism of collector adsorption in soluble-salt flotation systems. Laskowski et al. (5) reports that dodecylamine floats quartz in the region of colloidal precipitate formation. A similar mechanism has been proposed by Miller and co-workers for soluble-salt systems (6, 7), who showed that flotation of alkali halides in their saturated brines is greatly enhanced when bulk precipitation of the collector salt (amine hydrochloride) occurs. The possibility of heterocoagulation between oppositely charged colloidal precipitate and soluble-salt mineral particles has been verified both with colloidal precipitate and with monosize latex particles which served as model collector colloids (7). The aggregation between oppositely charged particles was explored on the basis of an attractive hydration force arising from hydrogen bonding. Our previous studies, on the other hand, have shown that flotation of borax in its saturated solutions using alkyl sulfonates, alkyl sulfates, and amines is possible in the absence of collector precipitation, i.e., without colloidal precipitate (8, 9). In this study, a semisoluble boron mineral, ulexite (NaCaB5O9r8H2O), in the presence of various multivalent ions is used to further examine whether the formation of collector colloid of opposite charge is a prerequisite for the flotation.
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mately 10 measurements. All measurements were made at ambient temperature and converted to 22 { 17C, at which flotation tests were conducted. RESULTS AND DISCUSSION
Electrokinetic Studies
Ulexite is a semisoluble boron mineral with relatively high solubility of 4.63 g/L (1.3 1 10 02 M) at 207C (11). This solubility corresponds to that obtained in saturated ulexite suspensions containing material less than 53 mm material with a 0.2 g/mL solids concentration. The high solubility is ascribed to the presence of Na / in its structure. Since ulexite also contains Ca 2/ in its lattice, it produces the following overall reaction upon reacting with CO2 in the atmosphere: NaCaB5O9r8H2O / 3CO2 / 6H2O B Na / / Ca 2/ / 5H3BO3 / 3HCO3 0 / 5H2O. FIG. 1. Variation of zeta potential of ulexite with pH.
to obtain a sample of 150 1 210 mm in size for microflotation studies. The fine fractions were further ground to obtain a sample less than 53 mm in size for zeta potential measurements. A high-purity sodium dodecyl sulfate (SDS) collector purchased from Fluka was used in flotation studies. The pH was adjusted by HCl and NaOH. The inorganic electrolytes, NaCl, CaCl2r2H2O, MgCl2r6H2O, BaCl2r2H2O, and Al(NO3 )3r2H2O, were all Fluka made certified grade chemicals. Distilled and deionized water was used in all experiments.
[1]
The electrokinetic studies reveal that ulexite exhibits a negative charge in the pH region of practical importance and thus is devoid of an iep (Fig. 1). The potential-determining / and OH 0 ions, ions for ulexite are Ca 2/ , B4O20 7 , and H 0 20 which control the ratio of HCO 3 /CO 3 (12). Microflotation Studies
The floatability of ulexite with SDS as a function of collector concentration is presented in Fig. 2 at the natural
Methods
Microflotation tests were carried out in a 150-mL column cell ( 25 1 220 mm) with a 15 mm fine frit and magnetic stirrer. The samples containing 1.5 g ulexite were conditioned in 150 mL of solution containing the desired chemicals for 10 min and then floated for 1 min using nitrogen gas at a flow rate of 50 cm3 /min. An automatically controlled apparatus was used to control nitrogen flow rate and flotation time. Details of the experimental setup are given elsewhere ( 10 ) . The electrokinetic measurements were made by a Zeta Meter 3.0 equipped with a microprocessor unit to directly calculate the zeta potential. One gram of mineral or appropriate proportions of chemicals in 100 mL of solution was conditioned for 10 min. Electrokinetic measurements were carried out in equilibriated solutions. No significant change of zeta potential was found for up to 30 min of conditioning time. The suspension was kept still for 5 min to let larger particles settle. Each data point is an average of approxi-
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FIG. 2. Floatability of ulexite against the concentration of sodium dodecyl sulfate.
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FIG. 3. Effect of different multivalent ion concentrations on the floatability of ulexite at 2 1 10 04 M SDS concentration.
pH of 9.3. It is evident that the flotation recovery with SDS attains only about 40% at concentrations as high as 5 1 10 04 M. As ulexite is negatively charged in the entire pH range, SDS might not be expected to be effective due to electrostatic repulsion of the surface. The corresponding zeta potential data, which yield almost a straight line as a function of SDS concentration, support the results given in Fig. 2 ( 11 ) . In our previous studies, multivalent ions with SDS were found to activate colemanite (Ca2B4O7r5H2O) in accordance with the solubility product of the salt in the order of Ba ú Ca ú Mg (13). A similar approach was adapted to see if multivalent ions also activate ulexite. Figure 3 illustrates the effect of multivalent ion concentration on the floatability of ulexite with SDS at pH 9.3. These ions appeared to either depress or to have little influence on ulexite flotation at this pH. However, the same ions resulted in enhanced floatability at higher pH values, as shown in Fig. 4. It is interesting to note that in the absence of any addition of multivalent cation the flotation recoveries remain low in the same pH region. Also, irrespective of the type of ion, the flotation recoveries exhibit approximately the same trend. Although aluminum does not form appreciable amounts of either Al 3/ or its hydroxy complexes such as Al(OH) 2/ above pH 9, the flotation recoveries remain as high as those for other multivalent ions, i.e. Ca 2/ , Mg 2/ , and Ba 2/ . This indicates that the system is governed by a similar mechanism in terms of the effectiveness of ions. In previous studies (13), the effect of multivalent ions (Ca 2/ , Mg 2/ , and Ba 2/ ) on the enhanced floatability of colemanite was explained on
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the basis of bulk precipitation of the polyvalent cation collector salt. However, the formation of aluminum dodecyl sulfonate, Al(DDS)3 , or any other collector/aluminum species has been reported to cease above pH 10 (14). While the precipitate is Al(DDS)3 below pH 3, above this pH up to pH 10, in addition to Al(DDS)3 , the precipitate is found to be in the form of hydroxy sulfonate precipitates such as AlOHR2 or Al(OH)2R (14). This finding is in conflict with those reported in Fig. 4, at least for aluminum ions. Since ulexite in the absence of the anionic collector was found to be marginally floatable in Al(NO3 )3 solutions in the pH range 9–12, only the release of calcium from ulexite that is as high as 10 02 M (11) may be able to explain the enhanced floatability. Figure 5 illustrates the electrokinetic behavior of various particulates in the presence of magnesium ion as a function of pH with different combinations. For all ions investigated in this study, three combinations were tested. While the addition of 10 03 M MgCl2 alone leads to precipitation of Mg(OH)2 at pH values above about 10, the formation of Mg(DDS)2 precipitate through the interaction of Magnesium with 2 1 10 04 M SDS prevails at all pH values. Similarly, since both ulexite and Mg(DDS)2 precipitate alone are negatively charged in most of the pH range, the precipitate coated ulexite also exhibits negative charges at pH values below approximately 10. Evidently, the precipitates or particulates produced by MgCl2 , MgCl2 /SDS, and MgCl2 / SDS/ ulexite systems, though with significant data scatter, can be represented by a normalized line. The charge reversal of magnesium precipitates occurs at about 10.3 for all the three systems studied. Our results with Mg(OH)2 are in agreement with those reported by Li and Somasundaran
FIG. 4. Flotability of ulexite in the presence of various multivalent ions (10 03 M) as a function of pH.
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FIG. 5. Zeta potential of various particulates in the presence of 10 03 M magnesium ion and 2 1 10 04 M SDS: ( s ) Mg(OH)2 precipitate; ( h ) magnesium dodeyl sulfate precipitate; ( n ) precipitate-coated ulexite particle.
FIG. 6. Zeta potential of various particulates in the presence of 10 03 M aluminum ion and 2 1 10 04 M SDS: ( s ) Al(OH)3 precipitate; ( h ) aluminum dodecyl sulfate precipitate; ( n ) precipitate-coated ulexite particle.
(15). Most interestingly, the bubble charge reported by the same authors in the presence of MgCl2 almost coincides with the zeta potential of Mg(OH)2 precipitate. Considering this independent study, it might be stated that bubbles, particles and precipitates in the same system acquire a similar charge and are possibly charged by the same mechanism. The above definitions for the precipitate applies to all other multivalent cations described below. Another supporting evidence comes from the aluminum ion, which behaves in a manner similar to the magnesium ion under the same conditions. Figure 6 shows the effect of aluminum ion on the zeta potential of Al(NO3 )3 , Al(NO3 )3 / SDS, and Al(NO3 )3 /SDS/ulexite systems. Again, similar to magnesium, all three systems exhibit a similar charge profile. In the pH range 4.5–8.5, the Al(OH)3 (s) precipitate remains positively charged whereas above this pH the precipitate undergoes a charge reversal; the iep of the precipitate is 8.5. The zeta potential measurements in aluminum containing solutions against pH reported by Li and Somasundaran (16) reveal a striking similarity with those of the precipitate presented in Fig. 6. Similar zeta potential results as a function of pH are illustrated in Figs. 7 and 8 for calcium and barium ions. Apparently, both calcium and barium precipitates undergo charge reversals and form peaks at positive zeta potentials corresponding to pH values of approximately 10.4 and 11.4, respectively. All the foregoing zeta potential results together with the bubble charge measurements quoted from the literature demonstrate that particles, bubbles, and precipitates all have the
same charge in the pH region of bulk precipitation. This, in turn, reveals that the charging mechanism with all these particulates are similar and the precipitate has the dominant influence over the particulates in the following manner. Air bubbles in water have been shown to exhibit negative charge in the entire pH region with the charge increasing as the pH
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FIG. 7. Zeta potential of various particulates in the presence of 10 03 M calcium ion and 2 1 10 04 M SDS: ( s ) Ca(OH)2 precipitate; ( h ) calcium dodecyl sulfate precipitate; ( n ) precipitate-coated ulexite particle.
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FIG. 8. Zeta potential of various particulates in the presence of 10 03 M barium ions and 2 1 10 04 M SDS: ( s ) Ba(OH)2 precipitate; ( h ) barium dodecyl sulfate precipitate; ( n ) precipitate-coated ulexite particle.
is increased (15). However, in the presence of multivalent ions the bubble acquires a charge in accord with the dominant metal (Me n / ) or hydroxylated (MeOH) species in the absence of precipitation. A similar charging mechanism occurs with the precipitate. For instance, in Fig. 6, the Al(OH)3 precipitate is positively charged at pH values from 4.5 to 8.5, due to the adsorption of positively charged AlOH 2/ and Al(OH)2/ species, and negatively charged above pH 8.5, owing to those of Al(OH) 40 species. The zeta potential and species distribution of precipitates and bubbles correlate well in this respect. Introduction of an anionic surfactant leads to the interaction of oppositely charged species such as positively charged multivalent or hydroxylated species. Solid particles, starting with surface precipitation and followed by bulk precipitation, undergo a similar charging mechanism thereby acquire a hydrophobic coating in the presence of surfactants. The colloidal precipitates composed of longchain surfactants have been reported to be generally hydrophobic (17, 7). The interaction between a particle and bubble during the attachment process depends on the electrical charge of the solid/liquid and liquid/gas interfaces (19) and also on their relative hydrophobicities. Laskowski et al. (19) showed that the electrical charge of bubbles in weak electrolyte-type surfactants results from the armoring of bubbles by the partially hydrophobic precipitate. The role of air bubble charge as a third phase has not been well delineated. Studies reported in saturated KCl brines using amine as collector reveal that the positively charged amine collector acts as a bridge between the negatively charged oil droplet and the negatively
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charged KCl particle. This is perhaps achieved by the addition of oil, which coats the bubble and acquires a compatible charge by which it coagulates with rest of the assembly. Li and Somasundaran (15, 16) have shown that the electrokinetic profile of air bubbles in water and in solutions of AlCl3 and MgCl2 as a function of pH are similar in the region of electrolyte precipitation. Similarly, Celik and Yoon (18) have found that amine-coated bubbles, amine precipitates, and amine adsorbed by hydrophobic coal particles have the same iep. It should be noted that the van der Waals forces are independent of the surface charge. These forces are usually repulsive for bubble/water/particle in the absence of surfactants (20) whereas in the presence of surfactants they often are attractive. Hydrophobic force, which is far stronger than the van der Waals attraction, is usually equivalent to structural forces in hydrophobic systems but structural forces in hydrophilic systems includes hydration repulsion. The long-range nature of the hydrophobic interaction has been shown to account for the rapid coagulation of hydrophobic particles in water (21). At separations below 10 nm the hydrophobic force appears to be insensitive to changes in the type and concentration of the electrolyte ions in solution. The absence of screening effect by ions suggests the nonelectrostatic origin of this interaction (22). Christenson and Claesson have shown that in dilute solutions or in solutions containing bivalent ions the hydrophobic force exceeds the van der Waals attraction out to separations of 80 nm (23). Despite these assertions, it is questionable whether the hydration or hydrophobic force be viewed as an ordinary type of solvation or structural force, simply reflecting the packing of the water molecules (22). The most crucial question that remains to be answered is whether all of these hydrophobic particulates—bubbles, particles, and precipitates—indeed coagulate due to hydrophobic interactions in the form of structural forces or coagulation is induced through electrostatic attraction of oppositely charged entities as advocated by Miller and his associates (6, 7). The flotation recoveries in Fig. 4 and zeta potential results in Figs. 5–8 do not support the possibility of the latter postulate. While flotation is gradually increased above pH 9 and reaches a maximum above pH 12, the precipitate, particle, and bubble take both positive and negative charges. For instance, although calcium and barium precipitates acquire negative and positive charges in a narrow pH range, no parallel change is observed with the corresponding flotation curves shown in Fig. 4. These results demonstrate that more convincing evidence, if available, is required to definitively distinguish between the role of electrostatic interactions in the system. CONCLUSIONS
1. Ulexite is not very floatable with an anionic surfactant (SDS) at or above natural pH of 9.3. However, it becomes
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remarkably floatable with the addition of multivalent ions, i.e. calcium, magnesium, barium, and aluminum. 2. It is proposed that bubbles, particles, and precipitates, which all become hydrophobic particulates in the region of collector–metal ion precipitation, coagulate due to hydrophobic interactions in the form of structural forces. The charge character of the particulate seems to be of no relevance to the coagulation process. ACKNOWLEDGMENT We thank Mr. R. Bulut for performing part of the flotation experiments reported in this work.
REFERENCES 1. Laskowski, J. S., in ‘‘Challenges in Mineral Processing’’ (C. Sastry and M. C. Fuerstenau, Eds.), p. 15. SME, Littleton, CO, 1989. 2. Yordan, J. L., and Yoon, R. H., J. Colloid Interface Sci. 113, 430 (1986). 3. Ananthapadmanabhan, K. P., and Somasundaran, P., Colloids Surf. 13, 151 (1985). 4. Halbich, W., Metall Erz. 30, 431 (1933). 5. Laskowski, J. S., Vurdela M., and Liu, Q., ‘‘Proceedings of XVI International Mineral Processing Congress’’ (E. Forssberg, Ed.), p. 703. Elsevier, Amsterdam, 1988.
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6. Yalamanchili, M. R., and Miller, J. D., Miner. Eng. 7, 305 (1994). 7. Yalamanchili, M. R., Kellar, J. J., and Miller, J. D., Int. J. Min. Process. 39, 137 (1993). 8. Celik, M. S., Atak, S., and Onal, G., Miner. Metallurg. Process. 10, 149 (1993). 9. Celik, M. S., and Hancer, M., Miner. Metallurg. Process. 11, 121 (1994). 10. Hancer, M., and Celik, M. S., Separation Sci. Technol. 28, 1703 (1993). 11. Celik, M. S., and Bulut, R., Separation Sci. Technol. 31, 1817 (1996). 12. Celik, M. S., and Yasar, E., J. Colloid Interface Sci. 173, 181 (1995). 13. Celik, M. S., Hancer, M., Atesok, G., and Emrullahoglu, O. F., in ‘‘Beneficiation of Phosphates: Theory and Practice’’ (H. El-Shall et al., Eds.), Chap. 6, p. 57. SME, New York, 1993. 14. Somasundaran, P., Ananthapadmanabhan, K. P., and Celik, M. S., Langmuir 4, 1061 (1988). 15. Li, C., and Somasundaran, P., J. Colloid Interface Sci. 146, 215 (1991). 16. Li, C., and Somasundaran, P., J. Colloid Interface Sci. 148, 587 (1992). 17. Roman, R. J., Fuerstenau, M. C., and Seidel, D., Trans. AIME 241, 56 (1968). 18. Celik, M. S., and Yoon, R. H., ‘‘Correlation of electrokinetic potential of ionic surfactants adsorbed on hydrophobic Particulates.’’ [In preparation] 19. Laskowski, J. S., Yordan, J. L., and Yoon, R. H., Langmuir 5, 373 (1989). 20. Usui, H., J. Colloid Interface Sci. 137, 281 (1990). 21. Xu, Z., and Yoon, R. H., J. Colloid Interface Sci. 134, 427 (1990). 22. Israelachvili, J., ‘‘Intermolecular and Surface Forces,’’ 2nd ed., p. 286. Academic Press, San Diego, 1995. 23. Christenson, H. K., and Claesson, P. M., Science 239, 390 (1988).
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CS985478
Flotation of Heterocoagulated Particulates in Ulexite/SDS/Electrolyte System M. S. Celik,* ,1 E. Yasar,* H. El-Shall† *Mining Faculty, Istanbul Technical University, Ayazaga, Istanbul 80626, Turkey; and †Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611-6135 Received May 12, 1997; accepted February 19, 1998
Salt-type minerals can be usually floated with either anionic or cationic collectors. In a number of systems, flotation has been reported to remarkably increase above the concentrations where precipitation of the collector salt is initiated. Some studies attribute this phenomenon to heterocoagulation of oppositely charged colloidal precipitate and mineral particles. In this study, ulexite, a semisoluble boron mineral, in the presence of various multivalent ions, i.e. Ba 2/ , Mg 2/ , Ca 2/ , and Al 3/ , was found to exhibit excellent flotation even when particles, colloidal precipitates, and bubbles acquire a similar charge, which indicates that attractive structural forces exceed the forces of electrostatic repulsion. q 1998 Academic Press
Key Words: coagulation; flotation; boron minerals; ulexite; precipitation; hydrophobic interaction; zeta potential.
INTRODUCTION
Non-sulfide minerals are usually floated with long-chain collectors such as sulfonates, carboxylates, and amines. These collectors may, however, undergo precipitation upon exceeding their solubility limits. The weak electrolyte-type surfactants such as fatty acids and amines, depending on pH, may precipitate because the solubility limit of free acid or free amine is much lower than the corresponding solubility of the ionized species. These molecules form colloidal species with a clear isoelectric point (1). However, strong electrolyte-type surfactants such as sodium dodecyl sulfate ionize into its respective ions, and if precipitation can be accomplished with polyvalent cations, the precipitate exhibits no iep in the entire pH region (2). Due to their relatively high solubilities, salt-type minerals release sufficient amounts of multivalent cations (e.g. Ca 2/ and Mg 2/ ) that react with oppositely charged collector anions to form insoluble colloidal precipitates. A number of studies, notably the one reported by Ananthapadmanabhan and Somasundaran (3), have demonstrated 1
To whom correspondence should be addressed. E-mail: mcelik@sariyer. cc.itu.edu.tr.
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Materials
Ultrapure ulexite crystals handpicked from the Bigadic boron deposit were used. The lump size crystals were crushed by a hammer and then ground in an agate mortar
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that flotation in a number of systems is initiated at the onset of precipitation of the collector salt. Above this critical point, in the region of bulk precipitation, the flotation rate usually increases almost exponentially. The major question posed here is whether this region of improved flotation is due to surface or bulk precipitation. As early as 1933, Halbich (4) proposed a model involving the formation of insoluble surface reaction products to explain the mechanism of collector adsorption in soluble-salt flotation systems. Laskowski et al. (5) reports that dodecylamine floats quartz in the region of colloidal precipitate formation. A similar mechanism has been proposed by Miller and co-workers for soluble-salt systems (6, 7), who showed that flotation of alkali halides in their saturated brines is greatly enhanced when bulk precipitation of the collector salt (amine hydrochloride) occurs. The possibility of heterocoagulation between oppositely charged colloidal precipitate and soluble-salt mineral particles has been verified both with colloidal precipitate and with monosize latex particles which served as model collector colloids (7). The aggregation between oppositely charged particles was explored on the basis of an attractive hydration force arising from hydrogen bonding. Our previous studies, on the other hand, have shown that flotation of borax in its saturated solutions using alkyl sulfonates, alkyl sulfates, and amines is possible in the absence of collector precipitation, i.e., without colloidal precipitate (8, 9). In this study, a semisoluble boron mineral, ulexite (NaCaB5O9r8H2O), in the presence of various multivalent ions is used to further examine whether the formation of collector colloid of opposite charge is a prerequisite for the flotation.
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mately 10 measurements. All measurements were made at ambient temperature and converted to 22 { 17C, at which flotation tests were conducted. RESULTS AND DISCUSSION
Electrokinetic Studies
Ulexite is a semisoluble boron mineral with relatively high solubility of 4.63 g/L (1.3 1 10 02 M) at 207C (11). This solubility corresponds to that obtained in saturated ulexite suspensions containing material less than 53 mm material with a 0.2 g/mL solids concentration. The high solubility is ascribed to the presence of Na / in its structure. Since ulexite also contains Ca 2/ in its lattice, it produces the following overall reaction upon reacting with CO2 in the atmosphere: NaCaB5O9r8H2O / 3CO2 / 6H2O B Na / / Ca 2/ / 5H3BO3 / 3HCO3 0 / 5H2O. FIG. 1. Variation of zeta potential of ulexite with pH.
to obtain a sample of 150 1 210 mm in size for microflotation studies. The fine fractions were further ground to obtain a sample less than 53 mm in size for zeta potential measurements. A high-purity sodium dodecyl sulfate (SDS) collector purchased from Fluka was used in flotation studies. The pH was adjusted by HCl and NaOH. The inorganic electrolytes, NaCl, CaCl2r2H2O, MgCl2r6H2O, BaCl2r2H2O, and Al(NO3 )3r2H2O, were all Fluka made certified grade chemicals. Distilled and deionized water was used in all experiments.
[1]
The electrokinetic studies reveal that ulexite exhibits a negative charge in the pH region of practical importance and thus is devoid of an iep (Fig. 1). The potential-determining / and OH 0 ions, ions for ulexite are Ca 2/ , B4O20 7 , and H 0 20 which control the ratio of HCO 3 /CO 3 (12). Microflotation Studies
The floatability of ulexite with SDS as a function of collector concentration is presented in Fig. 2 at the natural
Methods
Microflotation tests were carried out in a 150-mL column cell ( 25 1 220 mm) with a 15 mm fine frit and magnetic stirrer. The samples containing 1.5 g ulexite were conditioned in 150 mL of solution containing the desired chemicals for 10 min and then floated for 1 min using nitrogen gas at a flow rate of 50 cm3 /min. An automatically controlled apparatus was used to control nitrogen flow rate and flotation time. Details of the experimental setup are given elsewhere ( 10 ) . The electrokinetic measurements were made by a Zeta Meter 3.0 equipped with a microprocessor unit to directly calculate the zeta potential. One gram of mineral or appropriate proportions of chemicals in 100 mL of solution was conditioned for 10 min. Electrokinetic measurements were carried out in equilibriated solutions. No significant change of zeta potential was found for up to 30 min of conditioning time. The suspension was kept still for 5 min to let larger particles settle. Each data point is an average of approxi-
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FIG. 2. Floatability of ulexite against the concentration of sodium dodecyl sulfate.
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FIG. 3. Effect of different multivalent ion concentrations on the floatability of ulexite at 2 1 10 04 M SDS concentration.
pH of 9.3. It is evident that the flotation recovery with SDS attains only about 40% at concentrations as high as 5 1 10 04 M. As ulexite is negatively charged in the entire pH range, SDS might not be expected to be effective due to electrostatic repulsion of the surface. The corresponding zeta potential data, which yield almost a straight line as a function of SDS concentration, support the results given in Fig. 2 ( 11 ) . In our previous studies, multivalent ions with SDS were found to activate colemanite (Ca2B4O7r5H2O) in accordance with the solubility product of the salt in the order of Ba ú Ca ú Mg (13). A similar approach was adapted to see if multivalent ions also activate ulexite. Figure 3 illustrates the effect of multivalent ion concentration on the floatability of ulexite with SDS at pH 9.3. These ions appeared to either depress or to have little influence on ulexite flotation at this pH. However, the same ions resulted in enhanced floatability at higher pH values, as shown in Fig. 4. It is interesting to note that in the absence of any addition of multivalent cation the flotation recoveries remain low in the same pH region. Also, irrespective of the type of ion, the flotation recoveries exhibit approximately the same trend. Although aluminum does not form appreciable amounts of either Al 3/ or its hydroxy complexes such as Al(OH) 2/ above pH 9, the flotation recoveries remain as high as those for other multivalent ions, i.e. Ca 2/ , Mg 2/ , and Ba 2/ . This indicates that the system is governed by a similar mechanism in terms of the effectiveness of ions. In previous studies (13), the effect of multivalent ions (Ca 2/ , Mg 2/ , and Ba 2/ ) on the enhanced floatability of colemanite was explained on
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the basis of bulk precipitation of the polyvalent cation collector salt. However, the formation of aluminum dodecyl sulfonate, Al(DDS)3 , or any other collector/aluminum species has been reported to cease above pH 10 (14). While the precipitate is Al(DDS)3 below pH 3, above this pH up to pH 10, in addition to Al(DDS)3 , the precipitate is found to be in the form of hydroxy sulfonate precipitates such as AlOHR2 or Al(OH)2R (14). This finding is in conflict with those reported in Fig. 4, at least for aluminum ions. Since ulexite in the absence of the anionic collector was found to be marginally floatable in Al(NO3 )3 solutions in the pH range 9–12, only the release of calcium from ulexite that is as high as 10 02 M (11) may be able to explain the enhanced floatability. Figure 5 illustrates the electrokinetic behavior of various particulates in the presence of magnesium ion as a function of pH with different combinations. For all ions investigated in this study, three combinations were tested. While the addition of 10 03 M MgCl2 alone leads to precipitation of Mg(OH)2 at pH values above about 10, the formation of Mg(DDS)2 precipitate through the interaction of Magnesium with 2 1 10 04 M SDS prevails at all pH values. Similarly, since both ulexite and Mg(DDS)2 precipitate alone are negatively charged in most of the pH range, the precipitate coated ulexite also exhibits negative charges at pH values below approximately 10. Evidently, the precipitates or particulates produced by MgCl2 , MgCl2 /SDS, and MgCl2 / SDS/ ulexite systems, though with significant data scatter, can be represented by a normalized line. The charge reversal of magnesium precipitates occurs at about 10.3 for all the three systems studied. Our results with Mg(OH)2 are in agreement with those reported by Li and Somasundaran
FIG. 4. Flotability of ulexite in the presence of various multivalent ions (10 03 M) as a function of pH.
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FIG. 5. Zeta potential of various particulates in the presence of 10 03 M magnesium ion and 2 1 10 04 M SDS: ( s ) Mg(OH)2 precipitate; ( h ) magnesium dodeyl sulfate precipitate; ( n ) precipitate-coated ulexite particle.
FIG. 6. Zeta potential of various particulates in the presence of 10 03 M aluminum ion and 2 1 10 04 M SDS: ( s ) Al(OH)3 precipitate; ( h ) aluminum dodecyl sulfate precipitate; ( n ) precipitate-coated ulexite particle.
(15). Most interestingly, the bubble charge reported by the same authors in the presence of MgCl2 almost coincides with the zeta potential of Mg(OH)2 precipitate. Considering this independent study, it might be stated that bubbles, particles and precipitates in the same system acquire a similar charge and are possibly charged by the same mechanism. The above definitions for the precipitate applies to all other multivalent cations described below. Another supporting evidence comes from the aluminum ion, which behaves in a manner similar to the magnesium ion under the same conditions. Figure 6 shows the effect of aluminum ion on the zeta potential of Al(NO3 )3 , Al(NO3 )3 / SDS, and Al(NO3 )3 /SDS/ulexite systems. Again, similar to magnesium, all three systems exhibit a similar charge profile. In the pH range 4.5–8.5, the Al(OH)3 (s) precipitate remains positively charged whereas above this pH the precipitate undergoes a charge reversal; the iep of the precipitate is 8.5. The zeta potential measurements in aluminum containing solutions against pH reported by Li and Somasundaran (16) reveal a striking similarity with those of the precipitate presented in Fig. 6. Similar zeta potential results as a function of pH are illustrated in Figs. 7 and 8 for calcium and barium ions. Apparently, both calcium and barium precipitates undergo charge reversals and form peaks at positive zeta potentials corresponding to pH values of approximately 10.4 and 11.4, respectively. All the foregoing zeta potential results together with the bubble charge measurements quoted from the literature demonstrate that particles, bubbles, and precipitates all have the
same charge in the pH region of bulk precipitation. This, in turn, reveals that the charging mechanism with all these particulates are similar and the precipitate has the dominant influence over the particulates in the following manner. Air bubbles in water have been shown to exhibit negative charge in the entire pH region with the charge increasing as the pH
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FIG. 7. Zeta potential of various particulates in the presence of 10 03 M calcium ion and 2 1 10 04 M SDS: ( s ) Ca(OH)2 precipitate; ( h ) calcium dodecyl sulfate precipitate; ( n ) precipitate-coated ulexite particle.
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FIG. 8. Zeta potential of various particulates in the presence of 10 03 M barium ions and 2 1 10 04 M SDS: ( s ) Ba(OH)2 precipitate; ( h ) barium dodecyl sulfate precipitate; ( n ) precipitate-coated ulexite particle.
is increased (15). However, in the presence of multivalent ions the bubble acquires a charge in accord with the dominant metal (Me n / ) or hydroxylated (MeOH) species in the absence of precipitation. A similar charging mechanism occurs with the precipitate. For instance, in Fig. 6, the Al(OH)3 precipitate is positively charged at pH values from 4.5 to 8.5, due to the adsorption of positively charged AlOH 2/ and Al(OH)2/ species, and negatively charged above pH 8.5, owing to those of Al(OH) 40 species. The zeta potential and species distribution of precipitates and bubbles correlate well in this respect. Introduction of an anionic surfactant leads to the interaction of oppositely charged species such as positively charged multivalent or hydroxylated species. Solid particles, starting with surface precipitation and followed by bulk precipitation, undergo a similar charging mechanism thereby acquire a hydrophobic coating in the presence of surfactants. The colloidal precipitates composed of longchain surfactants have been reported to be generally hydrophobic (17, 7). The interaction between a particle and bubble during the attachment process depends on the electrical charge of the solid/liquid and liquid/gas interfaces (19) and also on their relative hydrophobicities. Laskowski et al. (19) showed that the electrical charge of bubbles in weak electrolyte-type surfactants results from the armoring of bubbles by the partially hydrophobic precipitate. The role of air bubble charge as a third phase has not been well delineated. Studies reported in saturated KCl brines using amine as collector reveal that the positively charged amine collector acts as a bridge between the negatively charged oil droplet and the negatively
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charged KCl particle. This is perhaps achieved by the addition of oil, which coats the bubble and acquires a compatible charge by which it coagulates with rest of the assembly. Li and Somasundaran (15, 16) have shown that the electrokinetic profile of air bubbles in water and in solutions of AlCl3 and MgCl2 as a function of pH are similar in the region of electrolyte precipitation. Similarly, Celik and Yoon (18) have found that amine-coated bubbles, amine precipitates, and amine adsorbed by hydrophobic coal particles have the same iep. It should be noted that the van der Waals forces are independent of the surface charge. These forces are usually repulsive for bubble/water/particle in the absence of surfactants (20) whereas in the presence of surfactants they often are attractive. Hydrophobic force, which is far stronger than the van der Waals attraction, is usually equivalent to structural forces in hydrophobic systems but structural forces in hydrophilic systems includes hydration repulsion. The long-range nature of the hydrophobic interaction has been shown to account for the rapid coagulation of hydrophobic particles in water (21). At separations below 10 nm the hydrophobic force appears to be insensitive to changes in the type and concentration of the electrolyte ions in solution. The absence of screening effect by ions suggests the nonelectrostatic origin of this interaction (22). Christenson and Claesson have shown that in dilute solutions or in solutions containing bivalent ions the hydrophobic force exceeds the van der Waals attraction out to separations of 80 nm (23). Despite these assertions, it is questionable whether the hydration or hydrophobic force be viewed as an ordinary type of solvation or structural force, simply reflecting the packing of the water molecules (22). The most crucial question that remains to be answered is whether all of these hydrophobic particulates—bubbles, particles, and precipitates—indeed coagulate due to hydrophobic interactions in the form of structural forces or coagulation is induced through electrostatic attraction of oppositely charged entities as advocated by Miller and his associates (6, 7). The flotation recoveries in Fig. 4 and zeta potential results in Figs. 5–8 do not support the possibility of the latter postulate. While flotation is gradually increased above pH 9 and reaches a maximum above pH 12, the precipitate, particle, and bubble take both positive and negative charges. For instance, although calcium and barium precipitates acquire negative and positive charges in a narrow pH range, no parallel change is observed with the corresponding flotation curves shown in Fig. 4. These results demonstrate that more convincing evidence, if available, is required to definitively distinguish between the role of electrostatic interactions in the system. CONCLUSIONS
1. Ulexite is not very floatable with an anionic surfactant (SDS) at or above natural pH of 9.3. However, it becomes
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remarkably floatable with the addition of multivalent ions, i.e. calcium, magnesium, barium, and aluminum. 2. It is proposed that bubbles, particles, and precipitates, which all become hydrophobic particulates in the region of collector–metal ion precipitation, coagulate due to hydrophobic interactions in the form of structural forces. The charge character of the particulate seems to be of no relevance to the coagulation process. ACKNOWLEDGMENT We thank Mr. R. Bulut for performing part of the flotation experiments reported in this work.
REFERENCES 1. Laskowski, J. S., in ‘‘Challenges in Mineral Processing’’ (C. Sastry and M. C. Fuerstenau, Eds.), p. 15. SME, Littleton, CO, 1989. 2. Yordan, J. L., and Yoon, R. H., J. Colloid Interface Sci. 113, 430 (1986). 3. Ananthapadmanabhan, K. P., and Somasundaran, P., Colloids Surf. 13, 151 (1985). 4. Halbich, W., Metall Erz. 30, 431 (1933). 5. Laskowski, J. S., Vurdela M., and Liu, Q., ‘‘Proceedings of XVI International Mineral Processing Congress’’ (E. Forssberg, Ed.), p. 703. Elsevier, Amsterdam, 1988.
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6. Yalamanchili, M. R., and Miller, J. D., Miner. Eng. 7, 305 (1994). 7. Yalamanchili, M. R., Kellar, J. J., and Miller, J. D., Int. J. Min. Process. 39, 137 (1993). 8. Celik, M. S., Atak, S., and Onal, G., Miner. Metallurg. Process. 10, 149 (1993). 9. Celik, M. S., and Hancer, M., Miner. Metallurg. Process. 11, 121 (1994). 10. Hancer, M., and Celik, M. S., Separation Sci. Technol. 28, 1703 (1993). 11. Celik, M. S., and Bulut, R., Separation Sci. Technol. 31, 1817 (1996). 12. Celik, M. S., and Yasar, E., J. Colloid Interface Sci. 173, 181 (1995). 13. Celik, M. S., Hancer, M., Atesok, G., and Emrullahoglu, O. F., in ‘‘Beneficiation of Phosphates: Theory and Practice’’ (H. El-Shall et al., Eds.), Chap. 6, p. 57. SME, New York, 1993. 14. Somasundaran, P., Ananthapadmanabhan, K. P., and Celik, M. S., Langmuir 4, 1061 (1988). 15. Li, C., and Somasundaran, P., J. Colloid Interface Sci. 146, 215 (1991). 16. Li, C., and Somasundaran, P., J. Colloid Interface Sci. 148, 587 (1992). 17. Roman, R. J., Fuerstenau, M. C., and Seidel, D., Trans. AIME 241, 56 (1968). 18. Celik, M. S., and Yoon, R. H., ‘‘Correlation of electrokinetic potential of ionic surfactants adsorbed on hydrophobic Particulates.’’ [In preparation] 19. Laskowski, J. S., Yordan, J. L., and Yoon, R. H., Langmuir 5, 373 (1989). 20. Usui, H., J. Colloid Interface Sci. 137, 281 (1990). 21. Xu, Z., and Yoon, R. H., J. Colloid Interface Sci. 134, 427 (1990). 22. Israelachvili, J., ‘‘Intermolecular and Surface Forces,’’ 2nd ed., p. 286. Academic Press, San Diego, 1995. 23. Christenson, H. K., and Claesson, P. M., Science 239, 390 (1988).
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