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Effect of hydrolysed metal cations on the liquid–liquid extraction of silica fines with cetyltrimethylammonium chloride
Colloids and Surfaces A: Physicochemical and Engineering Aspects 139 (1998) 155–162
Effect of hydrolysed metal cations on the liquid–liquid extraction of silica fines with cetyltrimethylammonium chloride E. Kusaka *, Y. Kamata 1, Y. Fukunaka, Y. Nakahiro 2 Department of Energy Science and Technology, Faculty of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan Received 11 August 1997; accepted 12 January 1998
Abstract Silica fines of <5 mm were liquid–liquid extracted from aqueous suspension containing cationic surfactant cetyltrimethylammonium chloride (CTAC ) to the isooctane–water interface region. The silica depression on addition of hydrolysable metal cations Fe(III ), Al(III ) and La(III ) was investigated to clarify the role of such cations in the hydrophile–lipophile transition of the silica fines together with CTAC. At an addition of only 5 mM CTAC, the silica fines were completely recovered in the dense emulsion phase from metal salt-free suspension in the entire pH range investigated. In contrast, addition of 1-mM total metal cations caused silica depression in a certain pH range, specific for the metal salt used. Speciation distribution diagrams for the respective metal–H O systems indicate that decreases 2 in percentage recoveries of the silica in the lower pH range are in line with increases in concentrations of the metal hydroxo complexes. Furthermore, the silica depression disappears when the pH approaches the point-of-zero charge of the hydroxide precipitate. In these pH ranges, the neutral to negative metal precipitates are also extracted into the oil–water interface during the liquid–liquid extraction. It seems that the lipophilic-to-hydrophobic transition, that is, depression by metal-salt addition of silica fines in cationic liquid–liquid extraction is controlled by the presence of metal-hydroxide species and their charges, attributable to the hydroxide coating on the silica surface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Cetyltrimethylammonium chloride; Fine particle processing; Hydrolysed metal cation; Liquid–liquid extraction; Silica
1. Introduction The development of successive fine particle processing, which includes innovative separation techniques such as novel flotation and oil-assisted separation, in mineral processing and recycling, is * Corresponding author. Fax: +81 75 753 5428; e-mail: [email protected] 1 Present address: Kubota Co., 64 Ishizu-kitamachi, Sakai 590-0823, Japan. 2 Present address: Fukui University of Technology, 3-6-1 Gakuen, Fukui 910-0028, Japan. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 8 ) 0 0 24 9 - 0
expected to save, as well as to prolong, the use of natural resources. Liquid–liquid extraction for separating mineral fines of several microns or less is one such technique. However, the separation mechanisms involving the interaction between the oil droplets and the mineral particles are not well understood due to the complexity of the oil–water–solid three-phase system. To perceive such complex phenomena in the three-phase system, one needs to expedite the development of the oilassisted method in fine particle processing. The hydrolysable metal cations adsorb specifi-
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cally at the mineral–water interfaces to control a number of industrial as well as natural processes [1–5]. These phenomena are affected mainly by the aqueous pH of the suspension system. As a rule, the adsorption/desorption criteria of such metal species are associated with abrupt changes in electrokinetic properties; these also involve changes in the performance of the physicochemical separations and in the colloidal phenomena. Presently, attention has focused on depression through the addition of hydrolysable metal cations in the liquid–liquid extraction of silica fines. Silica particles are negatively charged in a wide pH range. Hence, silica adsorbs cationic surfactant collectors in this pH range by electrostatic attraction [6,7]. Nevertheless, in the presence of another cation that silica can adsorb, silica cannot adsorb cationic surfactants in sufficient amounts. This study describes the depressing effect of trivalent metal cations Fe( III ), Al(III ) and La(III ) on the liquid–liquid extraction recovery of silica fines using a cationic collector cetyltrimethylammonium chloride (CTAC ). Distribution diagrams, electrokinetic measurements, and residual metal concentrations are presented. 2. Experimental 2.1. Materials Guaranteed reagent grade precipitated silicon(IV ) oxide, SiO , of approximately 99% was 2 selected as a silica sample for the present investigation. An X-ray diffraction study confirmed that the SiO was crystalline quartz. The powder SiO 2 2 was ground by a vibratory ball mill using a steel cell and balls, leached with successive washes of hot aqua regia and hot concentrated hydrochloric acid to remove impurities arising from the steel cell and balls, and rinsed with distilled water. The fine fraction <5 mm in Stokes diameter was then collected by decantation. A particle-size distribution analysis using a centrifugal particle analyzer, Particle Analyzer PA-101 ( Union Giken Co., Japan), confirmed that the SiO sample has a 2 50 wt% passing size of 2.0 mm. Isooctane (2,2,4-trimethylpentane), of >99.0% was used as the oil phase without further purification.
CTAC of >95% ( Wako Pure Chemicals Industries Ltd, Japan) was used as a cationic collector, and reagent grade sodium chloride, NaCl, of >99.5% as a supporting electrolyte. These two reagents were also used without further purification. The metal salts used in this study included iron(III ) nitrate nonahydrate, Fe(III ) (NO ) · 33 9H O, aluminum(III ) nitrate nonahydrate, 2 Al(III ) (NO ) · 9H O, and lanthanum(III ) nitrate 33 2 hexahydrate, La(III ) (NO ) · 6H O. In the present 33 2 work, nitrates were used to maintain the aqueous chlorine level at very nearly 0.01 M. These metal nitrates were all reagent grades and used as received. All aqueous solutions were prepared from distilled water, and an aqueous pH was adjusted by the addition of small amounts of hydrochloric acid, HCl, or sodium hydroxide, NaOH. Nacalai Tesque Inc., Japan, supplied all the reagents used in this study, unless otherwise mentioned. 2.2. Methods The liquid–liquid extraction test was conducted with a 100-ml Pyrex separating funnel. First, 80 ml of aqueous phase containing the silica fines, given amounts of the metal salt and the supporting electrolyte and the pH regulator were put into the funnel and conditioned for 10 min using a shaking apparatus at a constant rotational speed of 340 rpm. Then, 5 ml of the stock solution of CTAC were added, and the suspension was again conditioned for another 10 min. After the aqueous phase conditioning, 15 ml of isooctane were added to make a total liquid mixture of 100 ml. This solid–water–oil mixture was then shaken for 1 h, within which time the layer of oil gradually broke down into fine droplets, forming an O/W emulsion. Then, the system was allowed to stand for 20 min to separate a dense O/W emulsion phase, or an oil phase, which rose to the upper part of the funnel from an aqueous phase, after which only the aqueous phase was drained, filtered, and dried. The equilibrium pH value was taken to be that of the aqueous phase after the draining process. Then, the solids were weighed, and the weight percentage
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of the silica fines recovered in the dense emulsion phase, or the oil phase, was determined. The aqueous solution after the liquid–liquid extraction was subject to chemical analysis to determine the residual concentration of metal species in water. The metal concentration was measured by an ICP spectrophotometer, ICAP-575II (Nippon Jarrell Ash Co., Japan). Zeta potentials of silica, isooctane and hydroxide precipitate were measured using Zetamaster (Malvern Instruments Ltd, UK ). The hydroxide precipitate was prepared by neutralizing the metalsalt solution with NaOH before rinsing the precipitate with distilled water.
3. Results and discussion 3.1. Liquid–liquid extraction of silica fines in metal salt-free solution of 5 mm CTAC Fig. 1 shows the effect of pH on the liquid–liquid extraction recovery of the silica fines in the metal salt-free solution containing 5 mM CTAC as a cationic collector and 0.01 M NaCl as a supporting electrolyte. As can be seen from the figure, without CTAC, the recovery of the silica fines is impractical. Similar results have been obtained in previous studies [4,8,9], in which the supporting electrolyte and its concentration varied. However, with the
Fig. 1. Liquid–liquid extraction recovery of silica fines as a function of pH with and without 5 mM cetyltrimethylammonium chloride at 0.01 M NaCl.
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addition of only 5 mM CTAC, almost all the silica fines are recovered in the dense emulsion phase in the entire pH range investigated, that is, the silica is rendered lipophilic by the addition of 5 mM CTAC. Since the electrokinetic properties of both the silica particles and the isooctane droplets are important in the understanding of the mechanism of collecting the solid at the isooctane–water interfaces in the liquid–liquid extraction process, the zeta potential measurements were conducted in conditions similar to those in the extraction tests. Figs. 2 and 3 reveal the pH variation in zeta potential of the silica particles and that of the isooctane droplets, respectively, in the metal saltfree solutions with and without 5 mM CTAC. The electrokinetic behavior of quartz in aqueous solution with only indifferent electrolytes such as NaCl has been widely investigated and is now well understood; bare quartz surfaces carry negative charges at all pH values above about 2 (e.g. [10,11]). The silica, as in Fig. 2, exhibits a negative zeta potential at the pH values above around 2, which is supported by the reported values in the literature. The isooctane droplets, as presented in Fig. 3, also carry negative charges in the pH range studied in the metal salt- and CTAC-free solution. The electrokinetic behavior of the pure alkane droplets containing C to C hydrocarbon chains and that 6 16
Fig. 2. Zeta potential of silica as a function of pH with and without 5 mM cetyltrimethylammonium chloride at 0.01 M NaCl.
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Fig. 3. Zeta potential of isooctane as a function of pH with and without 5 mM cetyltrimethylammonium chloride at 0.01 M NaCl.
of the industrial oils has been extensively investigated [12–15]. As a result, the ionic strength of the solution tested, particularly the concentration of chloride ions Cl−, significantly affects the electrophoretic mobility of hydrocarbons in the solutions even without any other specifically adsorbing ions, since in the neutral-to-week acidic pH range, the Cl− ions preferentially adsorb at the oil–water interfaces. Hence, the maximum and minimum in the oil zeta potential curve occurs in this pH range. If the Cl− concentration is sufficiently high and carefully regulated, the hydrogen and hydroxyl ions appear to act as potential determining ions of the oil droplet without such a maximum and minimum; the pH-zeta potential profile obtained in the present study seems to correspond with this case. With the addition of only 5 mM CTAC, as can be seen from Figs. 2 and 3, positive shifts in zeta potentials of the silica particle and the isooctane droplet were observed. This provides evidence of strong adsorption of cetyltrimethylammonium ions (CTA+) at both the silica- and the isooctane–water interfaces at all pHs. Also, it is noteworthy that at 5 mM CTAC, the silica particles do not have high zeta potentials throughout the investigated pH range; the silica zeta potential ranges from +4 mV at the acidic pH to −16 mV at the alkaline pH. When the results of the liquid–liquid extraction
of silica fines (Fig. 1) are compared with those of the electrokinetic measurements ( Figs. 2 and 3), the silica appears to concentrate at the isooctane–water interface, or otherwise into the oil phase, with the reduction in the zeta potentials of silica and/or isooctane due to the adsorption of CTA+ ions. Consequently, the attraction, such as the interaction due to the van der Waals forces and the interaction between the hydrophobic groups of the adsorbed CTA+ ions and oil droplets, overcomes the electrostatic repulsion to enhance rapid coagulation between the silica particles and the isooctane droplets to such an extent that the silica can be completely recovered in the dense emulsion phase. However, without CTAC, the isooctane phase cannot be divided into fine droplets under the investigated condition; this may be attributed to the low percentage recovery of silica in the range around pH 2 at which both silica and isooctane possess low zeta potentials. 3.2. Effect of metal-salt addition on liquid–liquid extraction of silica fines with 5 mm CTAC Figs. 4–6 show the effect of pH on the liquid–liquid extraction recovery of the silica fines in the presence of 1 mM Fe(III ), Al(III ) and La(III ), respectively, using 5 mM CTAC as a cationic collector and 0.01 M of NaCl as a supporting electrolyte.
Fig. 4. Effect of pH on the liquid–liquid extraction recovery of silica at 5 mM cetyltrimethylammonium chloride with and without 1 mM Fe(III ) at 0.01 M NaCl.
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Fig. 5. Effect of pH on the liquid–liquid extraction recovery of silica at 5 mM cetyltrimethylammonium chloride with and without 1 mM Al(III ) at 0.01 M NaCl. Fig. 7. Speciation distribution diagram for 1 mM Fe(III ).
Fig. 6. Effect of pH on the liquid–liquid extraction recovery of silica at 5 mM cetyltrimethylammonium chloride with and without 1 mM La(III ) at 0.01 M NaCl. Fig. 8. Speciation distribution diagram for 1 mM Al(III ).
For comparison, the recovery without the metal salts is also indicated. The silica fines remain in the aqueous phase in the presence of metal salts in a certain pH range, depending upon the particular metal salt, whereas without the metal salt, the silica is almost completely recovered in the entire pH range studied. Less than 90% recovery is graphically observed at pH<6.1 for Fe(III ), pH 4.1–7.5 for Al(III ), and pH 7.3–9.1 for La(III ). To identify the metal species that adsorb on to the silica surface, calculations of solution equilibria
were conducted as a function of pH using the available equilibrium constants for the Fe( III )-, Al(III )-, and La( III )–H O system [16 ]. The calcu2 lations are presented in Figs. 7–9, corresponding to the 1 mM of total metal salts. These distribution diagrams indicate that the precipitation pH values (pH ) of Fe(III ), Al(III ) and La(III ) are, PPT respectively, 1.8, 4.1 and 7.6, which are arrowed in Figs. 4–6. The hydroxide precipitates are predominant above pH , with only Al(OH ) (s) disPPT 3 solving above pH 11.2 because of the formation
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Fig. 9. Speciation distribution diagram for 1 mM La(III ).
of soluble Al(OH )−. Detailed comparison of these 4 pH values with the corresponding recovery PPT curves suggests that the minimum pHs at which the silica begins to be depressed are lower than the pH values for Fe(III ) and La(III ) and in PPT agreement with the pH for Al(III ). Moreover, PPT it can be noted that the depressing effect disappears around pH 7–8 for Fe(III ), pH 9 for Al( III ) and pH 9–10 for La(III ). The zeta potential of the respective metal hydroxide precipitate in 0.01 M NaCl solution is presented in Fig. 10. This reveals that the precipi-
Fig. 10. Zeta potential of Fe(OH ) (s), Al(OH ) (s) and 3 3 La(OH ) (s) precipitate as a function of pH at 0.01 M NaCl. 3
tate is rendered zero-charged at pH 8.0 for Fe(OH ) (s), pH 9.5 for Al(OH ) (s) and pH 10.3 3 3 for La(OH ) (s), which are comparable to the 3 values reported by Parks [10]. These PZCs are also arrowed in Figs. 4–6. Each of these pH values agrees well with the pH at which the depressing effect disappears. These findings suggest that the depression of the silica fines under the suspension condition tested in this study is affected by the presence of metal hydroxide precipitate and its charge, which determines the hydroxide coating in the silica surface. This fact is further confirmed by the following figures. Figs. 11–13 show the residual concentration of the metal species in the aqueous phase after liquid–liquid extraction as a function of pH. The corresponding recovery curves are again shown in these figures for comparison. These figures apparently reveal that the concentration of metal species in the aqueous phase is decreased as the pH approaches the PZC of the precipitate; only Al(III ) species remain in water when the pH increases in the alkaline region of more than about 9 due to the formation of more hydrophilic Al(III ) (OH )−. 4 3.3. Depression mechanism by metal-salt addition in liquid–liquid extraction of silica fines with 5 mM CTAC The depressing effect observed with the silica fines in the presence of hydrolysable metal ions
Fig. 11. Effect of pH on the residual concentration of aqueous Fe(III ) species in the corresponding liquid–liquid extraction test.
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Fig. 12. Effect of pH on the residual concentration of aqueous Al(III ) species in the corresponding liquid–liquid extraction test.
Fig. 13. Effect of pH on the residual concentration of aqueous La(III ) species in the corresponding liquid–liquid extraction test.
and the cationic collector may be summarized as follows. In the low pH region where the free trivalent metal ions M3+ are the predominant species, since sufficient amounts of metal hydroxide precipitates are not generated, negatively charged silica particles are electrostatically attracted only to surfactant cations. Therefore, in this pH range, a higher recovery is obtained, whereas positively charged metal species are not extracted. At pHs where cationic hydroxo complexes,
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MOH2+ and M(OH )+ , are present in a proper 2 quantity, an abrupt decrease in the recovery of silica can be seen. In other words, depression occurs at a pH below pH . Metal hydroxide PPT species may precipitate preferentially at the silica– water interface in this pH range [2]. Accordingly, metal hydroxide precipitates may compete with surfactant cations to adsorb on the silica surface, and hence depression may occur. Oil droplets are positively charged in the presence of cationic surfactant as in Fig. 3. In this pH range, hydroxide species cannot precipitate on the isooctane droplets that are now coated with the cationic surfactant layer, and therefore remain in aqueous phase without being extracted in the dense O/W emulsion phase. In the pH range from pH values to the PZC PPT of the metal hydroxide, the metal species can be hydrolysed to precipitate on not only the silica surface but also in the bulk solution, since the solid species are predominant as shown in the distribution diagrams described above. Also, in this pH range, both the silica and the precipitate are positive. Conclusively, depression occurs due to the competitive adsorption of the precipitates; metal species remain in water due to electrostatic repulsion with the oil droplets. Above the PZC of M(OH ) (s), the precipitates 3 are negatively charged. Therefore, the negatively charged silica surface cannot be coated with the precipitates, and the depressing effect disappears completely. Additionally, at this pH, electrostatic attraction between negatively charged precipitates and positively charged oil droplets would make the precipitates themselves concentrate at the isooctane–water interface, and almost zero residual metal concentration is observed. Fig. 14 shows the effect of La(III ) concentration on the depressing effect in the liquid–liquid extraction of the silica fines as a function of pH. The decrease in the recovery of the silica approximately at a pH between 7 and 9 is less pronounced at a lower La(III ) concentration. Possibly, these results reflect the reduction in the percentage of hydroxide coating due to the lower concentration of La(III ) species relative to available surface area of the silica fines.
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charged oil droplets and extracted at the isooctane–water interface due to electrostatic attraction to oil droplets.
Acknowledgment This research was financially supported by the Ministry of Education, Science, Sports and Culture, Japan, under allotment Grand-in-Aid for Scientific Research (A), No. 08555257.
Fig. 14. Effect of La(III ) concentration on the liquid–liquid extraction recovery of silica at 5 mM cetyltrimethylammonium chloride and 0.01 M NaCl.
4. Conclusions In this study, the depression of silica fines by trivalent metal salts was examined by liquid–liquid extraction test using CTAC as a cationic surfactant. In the absence of metal cations, complete recovery is obtained at the pHs investigated, whereas in the presence of metal cations, recovery was reduced in a certain pH range specific for the metal salt added. Less than 90% recovery was observed at pH<6.1 for Fe(III ), pH 4.1–7.5 for Al(III ) and pH 7.3–9.1 for La(III ). This depression was attributed to the coating of metal hydroxide species, complexes or precipitates, on to the silica surface competing with surfactant cations. Also, at a pH below pH [1.8 for Fe(III ), 4.1 for Al(III ) and PPT 7.6 for La(III )], metal hydroxide species may precipitate preferentially at the silica–water interface, and depression would occur. The depressing effect disappears around the precipitate PZC since the precipitates are negatively charged above the PZC. The depressing effect is less pronounced when lower concentrations of metal ions are added. Additionally, above the PZC, metal hydroxide precipitates would be attracted to positively
References [1] R.O. James, T.W. Healy, J. Colloid Interface Sci. 40 (1972) 42. [2] R.O. James, T.W. Healy, J. Colloid Interface Sci. 40 (1972) 53. [3] A.P. Oliveira, M.L. Torem, Colloid Surf. A 110 (1996) 75. [4] E. Kusaka, Y. Nakahiro, T. Wakamatsu, Miner. Eng. 8 (1995) 817. [5] R. Herrera-Urbina, D.W. Fuerstenau, Colloids Surf. A 98 (1995) 25. [6 ] P. Wangnerud, G. Olofsson, J. Colloid Interface Sci. 153 (1992) 392. [7] P. Wangnerud, D. Bering, G. Olofsson, J. Colloid Interface Sci. 169 (1994) 365. [8] E. Kusaka, Y. Nakahiro, T. Wakamatsu, Int. J. Miner. Process. 41 (1994) 257. [9] E. Kusaka, N. Amano, Y. Nakahiro, T. Wakamatsu, in: Min. Materials Process. Inst. Japan and Australasian Inst. Min. Metall., New Horizons in Resources Handling and Geo-engineering, Proc. 1994 mMIJ/AusIMM Joint Symp., Ube, 1–5 October 1994, Min. Materials Process. Inst. Japan, Tokyo, 1994, p. 233. [10] G.A. Parks, Chem. Rev. 65 (1965) 177. [11] J. Lyklema, Fundamentals of Interface and Colloid Science, Academic Press, London, 1995, Volume II, p. A3.2. [12] J. Stachurski, M. Michalek, Colloids Surf. 15 (1985) 255. [13] J. Stachurski, M. Michalek, J. Colloids Interface Sci. 184 (1996) 433. [14] W.W. Wen, S.C. Sun, Sep. Sci. Tech. 16 (1981) 1494. [15] J.M.W. Mackenzie, Trans. Soc. Min. Eng. AIME 244 (1969) 393. [16 ] J. Kragten, Atlas of Metal–Ligand Equilibria in Aqueous Solutions, Wiley, New York, 1978.
Effect of hydrolysed metal cations on the liquid–liquid extraction of silica fines with cetyltrimethylammonium chloride E. Kusaka *, Y. Kamata 1, Y. Fukunaka, Y. Nakahiro 2 Department of Energy Science and Technology, Faculty of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan Received 11 August 1997; accepted 12 January 1998
Abstract Silica fines of <5 mm were liquid–liquid extracted from aqueous suspension containing cationic surfactant cetyltrimethylammonium chloride (CTAC ) to the isooctane–water interface region. The silica depression on addition of hydrolysable metal cations Fe(III ), Al(III ) and La(III ) was investigated to clarify the role of such cations in the hydrophile–lipophile transition of the silica fines together with CTAC. At an addition of only 5 mM CTAC, the silica fines were completely recovered in the dense emulsion phase from metal salt-free suspension in the entire pH range investigated. In contrast, addition of 1-mM total metal cations caused silica depression in a certain pH range, specific for the metal salt used. Speciation distribution diagrams for the respective metal–H O systems indicate that decreases 2 in percentage recoveries of the silica in the lower pH range are in line with increases in concentrations of the metal hydroxo complexes. Furthermore, the silica depression disappears when the pH approaches the point-of-zero charge of the hydroxide precipitate. In these pH ranges, the neutral to negative metal precipitates are also extracted into the oil–water interface during the liquid–liquid extraction. It seems that the lipophilic-to-hydrophobic transition, that is, depression by metal-salt addition of silica fines in cationic liquid–liquid extraction is controlled by the presence of metal-hydroxide species and their charges, attributable to the hydroxide coating on the silica surface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Cetyltrimethylammonium chloride; Fine particle processing; Hydrolysed metal cation; Liquid–liquid extraction; Silica
1. Introduction The development of successive fine particle processing, which includes innovative separation techniques such as novel flotation and oil-assisted separation, in mineral processing and recycling, is * Corresponding author. Fax: +81 75 753 5428; e-mail: [email protected] 1 Present address: Kubota Co., 64 Ishizu-kitamachi, Sakai 590-0823, Japan. 2 Present address: Fukui University of Technology, 3-6-1 Gakuen, Fukui 910-0028, Japan. 0927-7757/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 9 2 7 -7 7 5 7 ( 9 8 ) 0 0 24 9 - 0
expected to save, as well as to prolong, the use of natural resources. Liquid–liquid extraction for separating mineral fines of several microns or less is one such technique. However, the separation mechanisms involving the interaction between the oil droplets and the mineral particles are not well understood due to the complexity of the oil–water–solid three-phase system. To perceive such complex phenomena in the three-phase system, one needs to expedite the development of the oilassisted method in fine particle processing. The hydrolysable metal cations adsorb specifi-
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cally at the mineral–water interfaces to control a number of industrial as well as natural processes [1–5]. These phenomena are affected mainly by the aqueous pH of the suspension system. As a rule, the adsorption/desorption criteria of such metal species are associated with abrupt changes in electrokinetic properties; these also involve changes in the performance of the physicochemical separations and in the colloidal phenomena. Presently, attention has focused on depression through the addition of hydrolysable metal cations in the liquid–liquid extraction of silica fines. Silica particles are negatively charged in a wide pH range. Hence, silica adsorbs cationic surfactant collectors in this pH range by electrostatic attraction [6,7]. Nevertheless, in the presence of another cation that silica can adsorb, silica cannot adsorb cationic surfactants in sufficient amounts. This study describes the depressing effect of trivalent metal cations Fe( III ), Al(III ) and La(III ) on the liquid–liquid extraction recovery of silica fines using a cationic collector cetyltrimethylammonium chloride (CTAC ). Distribution diagrams, electrokinetic measurements, and residual metal concentrations are presented. 2. Experimental 2.1. Materials Guaranteed reagent grade precipitated silicon(IV ) oxide, SiO , of approximately 99% was 2 selected as a silica sample for the present investigation. An X-ray diffraction study confirmed that the SiO was crystalline quartz. The powder SiO 2 2 was ground by a vibratory ball mill using a steel cell and balls, leached with successive washes of hot aqua regia and hot concentrated hydrochloric acid to remove impurities arising from the steel cell and balls, and rinsed with distilled water. The fine fraction <5 mm in Stokes diameter was then collected by decantation. A particle-size distribution analysis using a centrifugal particle analyzer, Particle Analyzer PA-101 ( Union Giken Co., Japan), confirmed that the SiO sample has a 2 50 wt% passing size of 2.0 mm. Isooctane (2,2,4-trimethylpentane), of >99.0% was used as the oil phase without further purification.
CTAC of >95% ( Wako Pure Chemicals Industries Ltd, Japan) was used as a cationic collector, and reagent grade sodium chloride, NaCl, of >99.5% as a supporting electrolyte. These two reagents were also used without further purification. The metal salts used in this study included iron(III ) nitrate nonahydrate, Fe(III ) (NO ) · 33 9H O, aluminum(III ) nitrate nonahydrate, 2 Al(III ) (NO ) · 9H O, and lanthanum(III ) nitrate 33 2 hexahydrate, La(III ) (NO ) · 6H O. In the present 33 2 work, nitrates were used to maintain the aqueous chlorine level at very nearly 0.01 M. These metal nitrates were all reagent grades and used as received. All aqueous solutions were prepared from distilled water, and an aqueous pH was adjusted by the addition of small amounts of hydrochloric acid, HCl, or sodium hydroxide, NaOH. Nacalai Tesque Inc., Japan, supplied all the reagents used in this study, unless otherwise mentioned. 2.2. Methods The liquid–liquid extraction test was conducted with a 100-ml Pyrex separating funnel. First, 80 ml of aqueous phase containing the silica fines, given amounts of the metal salt and the supporting electrolyte and the pH regulator were put into the funnel and conditioned for 10 min using a shaking apparatus at a constant rotational speed of 340 rpm. Then, 5 ml of the stock solution of CTAC were added, and the suspension was again conditioned for another 10 min. After the aqueous phase conditioning, 15 ml of isooctane were added to make a total liquid mixture of 100 ml. This solid–water–oil mixture was then shaken for 1 h, within which time the layer of oil gradually broke down into fine droplets, forming an O/W emulsion. Then, the system was allowed to stand for 20 min to separate a dense O/W emulsion phase, or an oil phase, which rose to the upper part of the funnel from an aqueous phase, after which only the aqueous phase was drained, filtered, and dried. The equilibrium pH value was taken to be that of the aqueous phase after the draining process. Then, the solids were weighed, and the weight percentage
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of the silica fines recovered in the dense emulsion phase, or the oil phase, was determined. The aqueous solution after the liquid–liquid extraction was subject to chemical analysis to determine the residual concentration of metal species in water. The metal concentration was measured by an ICP spectrophotometer, ICAP-575II (Nippon Jarrell Ash Co., Japan). Zeta potentials of silica, isooctane and hydroxide precipitate were measured using Zetamaster (Malvern Instruments Ltd, UK ). The hydroxide precipitate was prepared by neutralizing the metalsalt solution with NaOH before rinsing the precipitate with distilled water.
3. Results and discussion 3.1. Liquid–liquid extraction of silica fines in metal salt-free solution of 5 mm CTAC Fig. 1 shows the effect of pH on the liquid–liquid extraction recovery of the silica fines in the metal salt-free solution containing 5 mM CTAC as a cationic collector and 0.01 M NaCl as a supporting electrolyte. As can be seen from the figure, without CTAC, the recovery of the silica fines is impractical. Similar results have been obtained in previous studies [4,8,9], in which the supporting electrolyte and its concentration varied. However, with the
Fig. 1. Liquid–liquid extraction recovery of silica fines as a function of pH with and without 5 mM cetyltrimethylammonium chloride at 0.01 M NaCl.
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addition of only 5 mM CTAC, almost all the silica fines are recovered in the dense emulsion phase in the entire pH range investigated, that is, the silica is rendered lipophilic by the addition of 5 mM CTAC. Since the electrokinetic properties of both the silica particles and the isooctane droplets are important in the understanding of the mechanism of collecting the solid at the isooctane–water interfaces in the liquid–liquid extraction process, the zeta potential measurements were conducted in conditions similar to those in the extraction tests. Figs. 2 and 3 reveal the pH variation in zeta potential of the silica particles and that of the isooctane droplets, respectively, in the metal saltfree solutions with and without 5 mM CTAC. The electrokinetic behavior of quartz in aqueous solution with only indifferent electrolytes such as NaCl has been widely investigated and is now well understood; bare quartz surfaces carry negative charges at all pH values above about 2 (e.g. [10,11]). The silica, as in Fig. 2, exhibits a negative zeta potential at the pH values above around 2, which is supported by the reported values in the literature. The isooctane droplets, as presented in Fig. 3, also carry negative charges in the pH range studied in the metal salt- and CTAC-free solution. The electrokinetic behavior of the pure alkane droplets containing C to C hydrocarbon chains and that 6 16
Fig. 2. Zeta potential of silica as a function of pH with and without 5 mM cetyltrimethylammonium chloride at 0.01 M NaCl.
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Fig. 3. Zeta potential of isooctane as a function of pH with and without 5 mM cetyltrimethylammonium chloride at 0.01 M NaCl.
of the industrial oils has been extensively investigated [12–15]. As a result, the ionic strength of the solution tested, particularly the concentration of chloride ions Cl−, significantly affects the electrophoretic mobility of hydrocarbons in the solutions even without any other specifically adsorbing ions, since in the neutral-to-week acidic pH range, the Cl− ions preferentially adsorb at the oil–water interfaces. Hence, the maximum and minimum in the oil zeta potential curve occurs in this pH range. If the Cl− concentration is sufficiently high and carefully regulated, the hydrogen and hydroxyl ions appear to act as potential determining ions of the oil droplet without such a maximum and minimum; the pH-zeta potential profile obtained in the present study seems to correspond with this case. With the addition of only 5 mM CTAC, as can be seen from Figs. 2 and 3, positive shifts in zeta potentials of the silica particle and the isooctane droplet were observed. This provides evidence of strong adsorption of cetyltrimethylammonium ions (CTA+) at both the silica- and the isooctane–water interfaces at all pHs. Also, it is noteworthy that at 5 mM CTAC, the silica particles do not have high zeta potentials throughout the investigated pH range; the silica zeta potential ranges from +4 mV at the acidic pH to −16 mV at the alkaline pH. When the results of the liquid–liquid extraction
of silica fines (Fig. 1) are compared with those of the electrokinetic measurements ( Figs. 2 and 3), the silica appears to concentrate at the isooctane–water interface, or otherwise into the oil phase, with the reduction in the zeta potentials of silica and/or isooctane due to the adsorption of CTA+ ions. Consequently, the attraction, such as the interaction due to the van der Waals forces and the interaction between the hydrophobic groups of the adsorbed CTA+ ions and oil droplets, overcomes the electrostatic repulsion to enhance rapid coagulation between the silica particles and the isooctane droplets to such an extent that the silica can be completely recovered in the dense emulsion phase. However, without CTAC, the isooctane phase cannot be divided into fine droplets under the investigated condition; this may be attributed to the low percentage recovery of silica in the range around pH 2 at which both silica and isooctane possess low zeta potentials. 3.2. Effect of metal-salt addition on liquid–liquid extraction of silica fines with 5 mm CTAC Figs. 4–6 show the effect of pH on the liquid–liquid extraction recovery of the silica fines in the presence of 1 mM Fe(III ), Al(III ) and La(III ), respectively, using 5 mM CTAC as a cationic collector and 0.01 M of NaCl as a supporting electrolyte.
Fig. 4. Effect of pH on the liquid–liquid extraction recovery of silica at 5 mM cetyltrimethylammonium chloride with and without 1 mM Fe(III ) at 0.01 M NaCl.
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Fig. 5. Effect of pH on the liquid–liquid extraction recovery of silica at 5 mM cetyltrimethylammonium chloride with and without 1 mM Al(III ) at 0.01 M NaCl. Fig. 7. Speciation distribution diagram for 1 mM Fe(III ).
Fig. 6. Effect of pH on the liquid–liquid extraction recovery of silica at 5 mM cetyltrimethylammonium chloride with and without 1 mM La(III ) at 0.01 M NaCl. Fig. 8. Speciation distribution diagram for 1 mM Al(III ).
For comparison, the recovery without the metal salts is also indicated. The silica fines remain in the aqueous phase in the presence of metal salts in a certain pH range, depending upon the particular metal salt, whereas without the metal salt, the silica is almost completely recovered in the entire pH range studied. Less than 90% recovery is graphically observed at pH<6.1 for Fe(III ), pH 4.1–7.5 for Al(III ), and pH 7.3–9.1 for La(III ). To identify the metal species that adsorb on to the silica surface, calculations of solution equilibria
were conducted as a function of pH using the available equilibrium constants for the Fe( III )-, Al(III )-, and La( III )–H O system [16 ]. The calcu2 lations are presented in Figs. 7–9, corresponding to the 1 mM of total metal salts. These distribution diagrams indicate that the precipitation pH values (pH ) of Fe(III ), Al(III ) and La(III ) are, PPT respectively, 1.8, 4.1 and 7.6, which are arrowed in Figs. 4–6. The hydroxide precipitates are predominant above pH , with only Al(OH ) (s) disPPT 3 solving above pH 11.2 because of the formation
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Fig. 9. Speciation distribution diagram for 1 mM La(III ).
of soluble Al(OH )−. Detailed comparison of these 4 pH values with the corresponding recovery PPT curves suggests that the minimum pHs at which the silica begins to be depressed are lower than the pH values for Fe(III ) and La(III ) and in PPT agreement with the pH for Al(III ). Moreover, PPT it can be noted that the depressing effect disappears around pH 7–8 for Fe(III ), pH 9 for Al( III ) and pH 9–10 for La(III ). The zeta potential of the respective metal hydroxide precipitate in 0.01 M NaCl solution is presented in Fig. 10. This reveals that the precipi-
Fig. 10. Zeta potential of Fe(OH ) (s), Al(OH ) (s) and 3 3 La(OH ) (s) precipitate as a function of pH at 0.01 M NaCl. 3
tate is rendered zero-charged at pH 8.0 for Fe(OH ) (s), pH 9.5 for Al(OH ) (s) and pH 10.3 3 3 for La(OH ) (s), which are comparable to the 3 values reported by Parks [10]. These PZCs are also arrowed in Figs. 4–6. Each of these pH values agrees well with the pH at which the depressing effect disappears. These findings suggest that the depression of the silica fines under the suspension condition tested in this study is affected by the presence of metal hydroxide precipitate and its charge, which determines the hydroxide coating in the silica surface. This fact is further confirmed by the following figures. Figs. 11–13 show the residual concentration of the metal species in the aqueous phase after liquid–liquid extraction as a function of pH. The corresponding recovery curves are again shown in these figures for comparison. These figures apparently reveal that the concentration of metal species in the aqueous phase is decreased as the pH approaches the PZC of the precipitate; only Al(III ) species remain in water when the pH increases in the alkaline region of more than about 9 due to the formation of more hydrophilic Al(III ) (OH )−. 4 3.3. Depression mechanism by metal-salt addition in liquid–liquid extraction of silica fines with 5 mM CTAC The depressing effect observed with the silica fines in the presence of hydrolysable metal ions
Fig. 11. Effect of pH on the residual concentration of aqueous Fe(III ) species in the corresponding liquid–liquid extraction test.
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Fig. 12. Effect of pH on the residual concentration of aqueous Al(III ) species in the corresponding liquid–liquid extraction test.
Fig. 13. Effect of pH on the residual concentration of aqueous La(III ) species in the corresponding liquid–liquid extraction test.
and the cationic collector may be summarized as follows. In the low pH region where the free trivalent metal ions M3+ are the predominant species, since sufficient amounts of metal hydroxide precipitates are not generated, negatively charged silica particles are electrostatically attracted only to surfactant cations. Therefore, in this pH range, a higher recovery is obtained, whereas positively charged metal species are not extracted. At pHs where cationic hydroxo complexes,
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MOH2+ and M(OH )+ , are present in a proper 2 quantity, an abrupt decrease in the recovery of silica can be seen. In other words, depression occurs at a pH below pH . Metal hydroxide PPT species may precipitate preferentially at the silica– water interface in this pH range [2]. Accordingly, metal hydroxide precipitates may compete with surfactant cations to adsorb on the silica surface, and hence depression may occur. Oil droplets are positively charged in the presence of cationic surfactant as in Fig. 3. In this pH range, hydroxide species cannot precipitate on the isooctane droplets that are now coated with the cationic surfactant layer, and therefore remain in aqueous phase without being extracted in the dense O/W emulsion phase. In the pH range from pH values to the PZC PPT of the metal hydroxide, the metal species can be hydrolysed to precipitate on not only the silica surface but also in the bulk solution, since the solid species are predominant as shown in the distribution diagrams described above. Also, in this pH range, both the silica and the precipitate are positive. Conclusively, depression occurs due to the competitive adsorption of the precipitates; metal species remain in water due to electrostatic repulsion with the oil droplets. Above the PZC of M(OH ) (s), the precipitates 3 are negatively charged. Therefore, the negatively charged silica surface cannot be coated with the precipitates, and the depressing effect disappears completely. Additionally, at this pH, electrostatic attraction between negatively charged precipitates and positively charged oil droplets would make the precipitates themselves concentrate at the isooctane–water interface, and almost zero residual metal concentration is observed. Fig. 14 shows the effect of La(III ) concentration on the depressing effect in the liquid–liquid extraction of the silica fines as a function of pH. The decrease in the recovery of the silica approximately at a pH between 7 and 9 is less pronounced at a lower La(III ) concentration. Possibly, these results reflect the reduction in the percentage of hydroxide coating due to the lower concentration of La(III ) species relative to available surface area of the silica fines.
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charged oil droplets and extracted at the isooctane–water interface due to electrostatic attraction to oil droplets.
Acknowledgment This research was financially supported by the Ministry of Education, Science, Sports and Culture, Japan, under allotment Grand-in-Aid for Scientific Research (A), No. 08555257.
Fig. 14. Effect of La(III ) concentration on the liquid–liquid extraction recovery of silica at 5 mM cetyltrimethylammonium chloride and 0.01 M NaCl.
4. Conclusions In this study, the depression of silica fines by trivalent metal salts was examined by liquid–liquid extraction test using CTAC as a cationic surfactant. In the absence of metal cations, complete recovery is obtained at the pHs investigated, whereas in the presence of metal cations, recovery was reduced in a certain pH range specific for the metal salt added. Less than 90% recovery was observed at pH<6.1 for Fe(III ), pH 4.1–7.5 for Al(III ) and pH 7.3–9.1 for La(III ). This depression was attributed to the coating of metal hydroxide species, complexes or precipitates, on to the silica surface competing with surfactant cations. Also, at a pH below pH [1.8 for Fe(III ), 4.1 for Al(III ) and PPT 7.6 for La(III )], metal hydroxide species may precipitate preferentially at the silica–water interface, and depression would occur. The depressing effect disappears around the precipitate PZC since the precipitates are negatively charged above the PZC. The depressing effect is less pronounced when lower concentrations of metal ions are added. Additionally, above the PZC, metal hydroxide precipitates would be attracted to positively
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