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The role of zeta potentials of oil droplets and quartz particles during collectorless liquid-liquid extraction
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Int. J. Miner. Process. 41 (1994) 257-269
The role of zeta potentials of oil droplets and quartz particles during collectorless liquid-liquid extraction E. Kusaka, Y. Nakahiro, T. Wakamatsu Department ofMineral Scienceand Technology, FacultyofEngineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-01, Japan (Received 24 May 1993; accepted after revision 26 November 1993)
Abstract As a fundamental study on the recovery of fine panicles of valuable minerals by liquidliquid extraction, the concentration of fine quartz particles (0.24 #m average diameter) at the oil-water interface in the absence of a surfactant collector has been investigated. Isooctane and water were used as the two liquid phases. The recovery percentage of the quartz fines was determined as a function of the concentration of an added electrolyte (sodium, calcium and lanthanum chloride) and the pH value of the aqueous suspension. Based upon electrokinetic measurements, the liquid-liquid extraction results are interpreted in terms of colloid stability theory. Calculations of the total potential energy of interaction between isooctane and quartz indicate that height of the primary potential energy peak, which acts as an energy barrier in the quartz-isooctane coagulation process, correlates closely with the collectodess liquid-liquid extraction recovery of the quartz fines.
1. Introduction In mineral processing, the amount of finely divided particles that require processing has increased significantly in recent years. There is considerable interest in developing a process that could successfully handle such fine particles. Froth flotation is a well known such technique, but requires a narrow particle-size distribution because otherwise the finer particles have less chance of attaching themselves to the air bubbles (e.g. Derjaguin and Dukhin, 1960-61,1981 ). A wide variety of techniques, as summarized by Fuerstenau (1980) and Sivamohan (1990), have been tried to recover or remove fine mineral particles. The technique of liquid-liquid extraction is one example of such attempts, and has been investigated for the recovery of - 44/an hematite (Shergold and Mellgren, 0301-7516/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved SSD10301-7516 ( 94 )00002-H
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E. Kusaka et al. / Ira. J. Miner. Process. 41 (1994) 25 7-269
1969 ), ultrafine alumina (Lai and Fuerstenau, 1968 ), - 10 #m cassiterite (Zambrana et al., 1974; Marinakis and Shergold, 1988 ), fine tungsten ores (Marinakis and Kelsall, 1987) and fine beach sand minerals (Kusaka et al., 1988, 1989, 1991 ). This technique is also expected to be suitable for removing hydrophilic mineral matter from finely divided coal (Nakahiro et al., 1990) and from coalderived liquid product in the coal-liquefaction process (Ku et al., 1985 ). There are, however, very few studies on the mechanism of adhesion between oil droplets and solid particles. Discussions on the mechanism of this adhesion have been mainly in terms of the oil-wettability of the particles based on their surface hydrophobicity, that is, the variation in contact angle in the mineral-oilwater system, associated with the collector adsorption (e.g. Lai and Fuerstenau, 1968; Shergold and Mellgren, 1970; Raghavan and Fuerstenau, 1975 ). In aqueous solution, both oil-water and solid-water interfaces will generally carry a charge, due partly to interactions with chemical species in the solution. These surface charges may play a very important role in oil-solid coagulation, even in aqueous solutions which contain no surface-active agent as a collector. This investigation involved the testing of the liquid-liquid extraction method in the absence of a surfactant collector. Hydrophilic quartz fines having an average diameter of 0.24/zm were used as the solid phase and isooctane as the oil phase. In addition, electrokinetic measurements were also conducted on the isooctane droplets and the quartz particles under similar conditions to those for the extraction tests, in order to discuss the mechanism of the coagulation between the oil droplets and the hydrophilic quartz particles in terms of colloid-stability theory.
2. Experimental 2.1. Materials
Natural quartz from Fukushima Prefecture, Japan, was used as the test material for the solid phase. Pieces of high grade material were selected and ground for several hours in an alumina vibratory ball mill. The fine fraction was then collected by means of sedimentation. Fig. 1 shows the particle-size distribution for the quartz sample as determined using a centrifugal particle analyzer. The sample was dispersed by ultrasonic vibration before the analysis. It can be seen that the quartz sample has an average diameter (50%passing size) of 0.24/an on a weight basis. The purity of the quartz sample was also confirmed by X-ray diffraction studies. Reagent grade 2,2,4-trimethylpentane (isooctane) of + 99% purity was used as the oil phase throughout this investigation. Other reagents included HC1 and NaOH as pH regulators, and the electrol~es NaCl, CaC12 and LaCI3.
E. Kusaka et al. ~Int. J. Miner. Process. 41 (1994) 257-269 --
A
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Fig. 1. Particle-size distribution of the quartz sample used in this investigation.
2.2. Liquid-liquid extraction tests An aqueous suspension (25 c m 3 ) containing 0.1% of the finely ground quartz, an aqueous solution (25 em 3) containing the desired amounts of the pH regulator and the electrolyte, and isooctane (5 cm 3) were placed in a 100 cm 3 Pyrex separation funnel with a ground-glass stopper. The funnel was then shaken for 30 rain in a mechanical shaker. During this operation, some of the particles were observed to be concentrated at the isooctane-water interfaces, while others remained in the water phase. The mixture was then allowed to stand for 5 rain, during which time a dense emulsion phase containing the quartz fines separated from the aqueous phase. The two phases were drained offseparately, fdtered (after the aqueous pH had been measured), and dried. The dried products were weighed to determine the recovery percentage concentrated into the dense emulsion phase. 2.3. Electrokinetic measurements The electrophoretic mobilities of the isooctane droplets and the quartz particles were measured using a Zeta Meter (Organo Co., Ltd., Japan). The values of the zeta potentials were obtained using the Smoluehowski equation.
3. Results and discussion
3.1. Effect of adding various concentrations of an electrolytes at pH 5.5 The effect of adding various concentrations of NaC1, C a C I 2 and LaCI3 on the zeta potential of the isooctane droplets at pH 5.5 is shown in Fig. 2. A significant decrease in the magnitude of the zeta potential is observed within the electrolyte
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concentration range from 1 × 1 0 - 4 to 2 × 10 -2 mol dm -3 for NaCl, from 2 × 1 0 - 4 to 1×10 -2 tool dm -3 for CaCI2, and from 2X10 -s to 1×10 -3 mol dm -3 for LaC13. As the cation valency of the added electrolyte increases, the concentration at which the magnitude of the zeta potential begins to decrease is lowered. The variation in the zeta potential of the quartz particles at pH 5.5, as shown in Fig. 3, shows a similar trend to that for the isooctane droplets. The effect of varying the concentration of add~l NaCI, CaCI2 and LaC13 on the recovery percentage ofthe quartz fines concentrated into the dense emulsion phase
E. Kusaka et al. ~Int. J. Miner. Process. 41 (1994) 257-269
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10.8 10.6 10.4 10.2 100 ELECTROLYTE CONCENTRATION (tool dm"3) Fig. 4. Effect of concentration of added NaC1, CaCI2 and LaC13 on the recovery of the quartz fines by collectorless liquid-liquid extraction at pH 5.5.
in the collectorless liquid-liquid extraction tests at pH 5.5, is shown in Fig. 4. As can be seen, the percentage of quartz recovered in the NaC1 solution increases rapidly as the NaC1 concentration increases above 1 X 10-2 mol d m - 3 and reaches 100% at 5 × 10-~ tool dm -3. The addition of CaCI2 at concentrations above 1 × 10 -4 tool dm -3 results in a remarkable increase in the percentage of quartz recovered. Using LaCI3, the recovery increases as the electrolyte concentration rises above 1 × 10-5 mol din-3. Here again, the electrolyte concentration at which the quartz recovery begins to increase is progressively lowered as the cation valency of the added electrolyte increases. A comparison between the results of the liquid-liquid extraction tests and those of the electrokinetic measurements indicates that the increase in the recovery percentage seems to correlate with the decreases in the zeta potentials of both the oil droplets and the quartz particles and, therefore, the decrease in the electrostatic repulsion between them. Since in a preparatory test of collectorless liquidliquid extraction at an oil-volume concentration of 50%, no quartz particles have been observed to be transferred from water into isooctane and to be dispersed in the isooctane phase, it is considered that the quartz fines are concentrated in the isooctane-water interface region. This is attributed to the hydrophilic nature of the quartz surfaces. Consequently, the quartz-isooctane coagulation during collectorless liquid-liquid extraction is regarded as a heterocoagulation process, in which the total potential energy of the interaction can be quantified in terms of colloid stability theory (Verwey and Overbeek, 1948). According to this theory, the total potential energy of the interaction, VT, is expressed as the sum of two long-range forces; the potential energy of interaction due to the overlap of electrical double layers, VE, and that due to van der Waals' forces, VA:
VT=Ve+VA
(1)
262
E. Kusaka et al. ~Int..L Mmer. Process. 41 (1994)257-26~
Assuming that the average radius of the quartz fines (0.12/lm ) is much smaller than that of the isooctane droplets (which are visible in the funnel), VE can be approximated by an expression for the interaction between a sphere and an infinite fiat plate with opposite electrical charges. This expression was originally formulated for dissimilar spheres by Hogg et al., ( 1966 ) and later used to describe the interaction between an oppositely-charged sphere and an infinite fiat plate by Hull and Kichener (1969), Imamura and Tokiwa ( 1972 ) and Imamura ( 1975 ). In S.I. units it takes the form (Tokiwa and Imamura, 1984): VE=na~o~r[2~l~21n l + e x p ( - x H ) ~- ( ~ + ~ v ~ ) l n { 1 - e x p ( - 2 x H ) ) ] 1 - exp ( - xH) -
(2)
where a is the radius of the quartz sphere, eo the dielectric constant of free space, ~r the dielectric coefficient of water, ~vl and q/2 the surface potentials of quartz and isooctane, respectively, H the distance of separation and x the Debye-Hiickel reciprocal length parameter. Also, in Eq. (2), the upper positive sign is for the condition of constant surface potentials, and the lower negative sign for the condition of constant charge density. In this investigation, the condition of constant surface potentials was assumed, and the zeta potentials were used as substitutes for the surface potentials. When e is the elementary electrical charge, NA Avogadro's number, Z+ and Z_ the cation and the anion valencies of the added electrolyte, C+ and C_ the cation and anion concentrations, k Boltzmann's constant, and T the absolute temperature in degrees Kelvin, the relation among them is given by: (3)
I ¢ 2 = e 2 N A ( C + Z 2 Jr" C _ Z 2_ ) / ~ o ~ r k T
Disregarding the effect of retardation, the van der Waals' force, VA, between the sphere and the infinite fiat plate is given by (Hull and Kichener, 1969):
VA= (-Ai2/3/6) [ 2 a ( n + a ) / n ( H + 2 a ) - l n {
(n+2a)/H} ]
(4)
where A ~2/3 is the overall Hamaker constant for isooctane-water-quartz. If A 11/ 3 and A22/3 are the Hamaker constants for quartz-water-quartz and for isooctanewater-isooctane, respectively, A 1z~3 is given by the following relation:
(A12/3)2=All/3.A22/3
(5)
In calculating the values for Vx, the crystalline quartz-water-crystalline quartz Hamaker constant of 1.7 × 10-zo j was used for A I~/3 and the Cs alkane-waterCs alkane Hamaker constant of 4.1 × 10- 21 j for A2z/3 ( Hough and White, 1980 ). In Fig. 5, the results of calculations of the total potential energy of interaction between the quartz sphere and the isooctane plate for the system containing NaC1, CaCI2 and LaC13, are presented as a function of distance of separation at various electrolyte concentrations, which corresponds approximately to the extraction tests shown in Fig. 4. The primary peak height, VM, which behaves as an energy barrier to the interaction, reaches 84kT at 5× 10 -3 mol dm -3 NaC1, 71kT at 1 × 10 -4 mol dm -3 CaCI2, and 28kTat 1 × 10 -s tool d m -3 LaCl3. In all systems,
E. Kusaka et al. ~Int. J. Miner. Process. 41 (1994) 257-269
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SEPARATION DISTANCE (nm) Fig. 5. Calculated total potential energies of interaction between the quartz sphere and the isooctane plate as a function of distance of separation in the aqueous solution of the electrolyte: (A) NaCl, (a) 5× 10 -3 tool dm -3, (b) 1 × 10 -2 tool dm -3, (c) 2X 10 -2 tool dm-3; (B) CaCl2, (a) 1 × 10 -4 mol dm -3, (b) 1×10 -3 tool dm -3, (c) 6 × 1 0 -3 mol din-3; (C) LaC13, (a) 1×10 -5 tool dm -3, (b) 1 X 10 -4 mol dm -3, (c) 1 × 10 -3 tool dm -3.
VM tends to decrease as the electrolyte concentration increases. It decreases to
4kT at 2 X 10- 2 tool din- 3NaC1 and completely disappears at 6 × 10- 3 tool d m - 3 CaC12 and 1 × 10-3 tool dm-3 LaCI3. A comparison between the recoveries and the VT curves indicates that the increase in the recovery is in good agreement with the decrease in the VM value. The theoretical data show that at low electrolyte concentrations no coagulation between quartz and isooctane should occur because of high VM values. On the other hand, higher recoveries are obtained at higher electrolyte concentrations, at which the VM values are less than a few kT units or even zero. Under these conditions, the quartz particles surmount the energy barrier so that they can be rapidly concentrated at the isooctane-water interface, even without a collector.
3.2. Effect of pH in the isooctane-water-quartz system in the presence of NaCl Hydrogen and hydroxyl ions act as potential-determining ions in the interface region between water and an oxide mineral such as quartz, and the surface potential of the oxide mineral-water interface varies with the pH of the aqueous solution. In addition, the oil droplets in an aqueous solution containing no surfaceactive reagent are supposed to be charged due to the preferential adsorption of the hydroxyl ions. Their zeta potential therefore also varies with the pH (e.g.
E. Kusaka et al. ,' Int. A Miner. Process. 41 (1994) 25 "7-209
264
Roberts, 1936, 1937; Dickinson, 1941; Stachurski and Michalek, 1985 ). An interesting question is how the hydrogen and hydroxyl ions affect the charges of the oil-water and particle-water interfaces and the concentration of fine particles at the oil-water interface in collectorless liquid-liquid extraction. The influence of the zeta potentials of the isooctane droplets and the quartz particles on the concentration of the quartz fines at the isooctane-water interface was examined as a function of the aqueous pH, particularly in the system containing NaC1. Fig. 6 shows the effect of pH on the zeta potential of the isooctane droplets in the NaC1 solution. The hydrogen and hydroxyl ions appear to act as potentialdetermining ions in the isooctane-water interface region. Sodium and chloride ions act as indifferent ions that compress the thickness of the electrical double layer. No reversal of zeta potential was observed within the investigated range of pH values. The isoelectric point (IEP) of isooctane may, however, be around pH 2.5, i.e. outside the pH range investigated. In Fig. 7, the variation in zeta potential of the quartz particles as a function of pH in the presence of NaC1 is presented. As reported in many publications dealing with the electrokinetic property of the quartz-water interface, the hydrogen and hydroxyl ions appear to act as potential-determining ions in the quartz-water interface region. The figure also shows no reversal of the zeta potential of the quartz sample within the investigated range of pH values, but its IEP might be below pH 2. The IEP of the Fukushima quartz seems to be somewhat low; this may be due to the impurities present which are mainly feldspars. Fig. 8 presents the influence of pH on the collectorless liquid-liquid extraction recovery of the quartz fines at three different NaC1 concentrations. The recovery percentage tends to increase as the pH decreases below 4; this effect becomes more pronounced at higher NaCI concentrations. A comparison of the results of the liquid-liquid extraction tests and the electrokinetic measurements, suggests that the increase in the recovery percentage corresponds to a decrease in the negative zeta potential of isooctane rather than of -100
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quartz. Rapid heterocoagulation between two dissimilar spheres as proposed by Hogg et al. (1966) should occur within the pH range between their IEP's. This seems to explain the pH-dependence of the recovery. Theoretical data interpreting the recovery is provided by Fig. 9, which shows the calculated primary peak height, VM,of the interaction between the quartz sphere and the isooctane plate as a function of pH at various NaC1 concentrations. As can be seen in Fig. 9, the VMis reduced with decreasing pH, and rapidly falls when the pH approaches the IEP of isooctane. This tendency becomes increasingly pronounced at higher electrolyte concentrations.
266
E. Kusaka et al. / Int. J. Mmer. Process. 41 (1994) 2 5 7 - 2 6 9
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3.3. Correlation between the primary peak height and the collectorless fiquidliquid extraction recovery The recovery percentage of the quartz fines in the collectofless liquid-liquid extraction tests is closely related to the decrease in the primary peak height, VM, of the total potential energy of interaction between isooctane and quartz. It is of great importance to identify the criterion in terms of the I'M for the attainment of high recoveries in collectorless liquid-liquid extraction. Therefore, the relationship between the recovery percentage and the I'M value was examined. Fig. 10 presents the re-plot of the recovery percentage of the quartz fines in the coUectorless liquid-liquid extraction tests shown in Fig. 4 as a function of I'M. In calculating the VM values, the experimental data of the zeta potentials of isooctane and quartz corresponding to the conditions of the extraction tests were substituted for ~1 and ¢2 in Eq. (2). A VM value greater than 70kTgives a recovery of less than 20%, whereas a I'M of zero gives a 40 to 100% recovery. The critical I'M value to attain a 50% recovery ranges from 1.2kTfor NaCI to 5.TkTfor CaCI:, obtained from the respective fitted VM-recovery curves. (A recovery percentage value of 50% was chosen to indicate the critical condition, taking the broad size distribution of the quartz sample used in this investigation into account.) According to colloid stability theory, if the VMvalue is high, the particles should be unable to surmount the energy barrier. These results suggest that the zeta potentials of the oil droplets and mineral particles determine the contribution of the electrostatic force to the interaction between them when the interaction due to the van der Waals forces is not responsible for oil-particle coagulation, and that
E. Kusaka et al. ~Int. J. Miner. Process. 41 (1994) 257-269 100:
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in such a system the VM value becomes very important in estimating the recovery of fine particles by collectorless liquid-liquid extraction.
4. Conclusions
The recovery of fine quartz particles of an average diameter of 0.24/an by means of liquid-liquid extraction without the use of a collector has been studied. The results are discussed in terms of colloid stability theory, on the basis of the results of electrokinetic measurements. The following conclusions can be drawn from this investigation. ( 1 ) Electrokinetic results at pH 5.5 show that the magnitudes of the zeta potentials of the isooctane droplets and the quartz particles in an aqueous electrolyte solution decrease, as the electrolyte concentration is increased. Furthermore, as the cation valency of the electrolyte increases, the concentration at which the magnitudes of the zeta potentials begin to decrease is lowered. (2) In liquid-liquid extraction using quartz fines at pH 5.5, the recovery of the quartz particles is possible even without a collector when the concentration of the added electrolyte is increased. (3) The extraction behavior is well explained in terms of colloid stability theory. Calculations of the total potential energy of interaction between quartz and isooctane due to electrostatic and van tier Waals' forces indicate that the recovery percentage should increase under conditions where the height of the primary peak of the potential energy of interaction is low. (4) Electrokinetic measurements show that the IEP of isooctane is near pH 2.5, while that ofthe Fukushima quartz is below pH 2. (5) The liquid-liquid extraction results at various pH values in the system
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E. Kusaka et al. /Int. 3+.Miner. Process. 41 (1994,) 257-269
containing NaCI, show that the recovery percentage tends to increase as the pH approaches the IEP of the isooctane droplet. Colloid stability theory suggests that the heterocoagulation between isooctane and quartz may be accelerated near IEP. On the other hand, the recovery of the quartz fines is impossible in the neutral to basic pH range, due to high electrostatic repulsion. (6) The criterion in terms of the primary peak height of the total potential energy of the interaction between quartz and isooctane for attaining a 50% liquid-liquid extraction recovery of the quartz fines ranges from 1.2kT to 5.7kT, depending upon the electrolyte added into the liquid-liquid system.
Acknowledgement This research was supported by the Japanese Ministry of Education, Science and Culture, under allotment Grant-in-Aid for Scientific Research in 1992 (Grantin-Aid for Encouragement of Young Scientists, No. 04750597). References Derjaguin, B.V. and Dukhin, S.S., 1960-61. Theory of flotation of small and medium-size particles. Trans. Inst. Min. Metall., 70: 221-246. Derjaguin, B.V. and Dukhin, S.S., 1981. Kinetic theory of the flotation of fine particles (in English, with French, German and Russian abstracts). In: J. Laskowski (Editor), Mineral Processing, Proceedings of the 13th International Mineral Processing Congress, Warsaw. Elsevier, Amsterdam, Part A, pp. 21-62. Dickinson, W., 1941. The effect of pH upon the electrophoretic mobility of emulsions of certain hydrocarbons and aliphatic halide. Trans. Faraday Soc., 37:140-148. Fuerstenau, D.W., 1980. Fine particle flotation. In: P. Somasundaran (Editor), Fine Particles Processing-- Proceedings of the International Symposium on Fine Particles Processing, Vol. 1. Soc. Min. Eng. AIME, New York, pp. 669-705. Hogg, R., Healy, T.W. and Fuerstenau, D.W., 1966. Mutual coagulation of colloidal dispersion. Trans. Faraday Soc., 62: 1638-1651. Hough, D.B. and White, L.R., 1980. The calculation of Hamaker constants from Lffshitz theory with applications to wetting phenomena. Adv. Colloid Interface Sci., 14:3-41. Hull, M. and Kichener, J.A., 1969. Interaction of spherical colloidal particles with planar surface. Trans. Faraday Soc., 65: 3093-3104. Imamura, T., 1975. The role of interracial electrical conditions in detergency (in Japanese, with English abstract). Nippon Kagakukwai-Shi, (6): 943-947. Imamura, T. and Tokiwa, F., 1972. Relation between deposition of ferric oxide onto various fabrics and their potential energies in aqueous solutions of sodium tripolyphosphate and sodium chloride (in Japanese, with English abstract ). Nippon Ka~a~kwai-Shi, ( 11 ): 2177-2183. Ku, C., Henry, J.D., Jr., Siriwardane, R. and Roberts, L., 1985. Particle transfer from a continuous oil to a dispersed water phase: model particle study. J. Colloid Interface Sci., 106: 377-387. Kusaka, E., NakAhiro, Y., Wakamatsu, T. and Sri Murdiati, 1988. Recovery of free ilmenite particles from beach sand by liquid-liquid extraction (in Japanese, with English abstract). J. Min. Metall. Inst. Jpn. (Nippon Kogyokwai-Shi), 104: 795-801. Kusaka, E., Nakahiro, Y. and Wakamatsu, T., 1989. Recovery of fine monazite particles by liquidliquid extraction (in Japanese, with English abstract). J. Min. Mater. Process. Inst. Jpp_ (Shigento-Sozai), 105: 553-557.
E. Kusaka et al. ~Int. J. Miner. Process. 41 (1994)257-269
269
Kusaka, E., Nakahiro, Y. and Wakamatsu, T., 1991. Recovery of fine xenotime particles by liquidliquid extraction using alkylarnine salts as surfactant. In: Proc. XVII Int. Miner. Process. Congr., Vol. III, Polygraphischer Bereich/Bergakademie Freiberg/Sa., Freiberg, pp. 363-371. Lai, R.W.M. and Fuerstenau, D.W., 1968. Liquid-liquid extraction of ultrafme particles. Trans. AIME, 241: 549-556. Marinakis, K.I. and Kelsall, G.H., 1987. Effect of lattice cations, silicate ions and fluorosilicate ions on the two-liquid flotation of tungsten minerals. Trans. Inst. Min. Metall., 96, Sect. C: 63-68. Marinakis, K.I. and Shergold, H.L., 1988. The treatment of tin-bearing slimes by two-liquid flotation. In: K.S.E. Forssberg (Editor), Proc. XVI Int. Miner. Process. Congr., Part A, Elsevier, Amsterdam, pp. 487-498. Nakahiro, Y., Wakamatsu, T., Matsumoto, E. and Kusaka, E., 1990. Removal of mineral matter from fine coal by liquid-liquid extraction. In: Proc. 1 lth Int. Coal Preparation Congr. Mining and Materials Processing Institute of Japan, Tokyo, pp. 163-167. Raghavan, S. and Fuerstenau, D.W., 1975. On the wettability and flotation concentration of submicron hematite particles with octhylhydroxmate as collector. In: P. Somasundaran and R.B. Grieves (Editors), Advances in interfacial phenomena of particulate/solution/gas systems; applications to flotation research, MChE Symposium Series, Voi. 71. AIChE, New York, pp. 59-67. Roberts, A.L., 1936. The electrophoretic mobility of purified tristearin. Part I. The acid region, pH 2 to 7. Trans. Faraday Soc., 32: 1705-1717. Roberts, A.L., 1937. The electrophoretic mobility of purified tristearin. Part II. The alkaline region. Trans. Faraday Soc., 33: 643-650. Shergold, H.L. and Mellgren, O., 1969. Concentration of minerals at the oil-water interface: hematite-isooctane-water system in the presence of sodium dodecyl sulphate. Trans. Inst. Min. Metall., Sect. C, 78: 121-132. Shergold, H.L. and Mellgren, O., 1970. Concentration of minerals at the oil/water interface. Trans. AIME, 247: 149-159. Sivamohan, R., 1990. The problems of recovering very fine particles in mineral processing - - A review. Int. J. Miner. Process., 28: 247-288. Stachurski, J. and Michalek, M., 1985. The zeta potential of emulsion droplets of the alphatic hydrocarbons in aqueous solutions. Colloids Surf., 15: 255-259. Tokiwa, F. and Imamura, T., 1984. Detergency. In: A. Kitahara and A. Watanabe (Editors), Electrical Phenomena at Interfaces - - Fundamental, Measurements, and Applications. Marcel Dekker, New York, pp. 269-283. Verwey, E.J.W. and Overheek, J .Th.G. (Editors), 1948. Theory of the Stability of Lyophobic Colloids - - The Interaction of Sol Particles Having an Electric Double Layer. Elsevier, Amsterdam. Zambrana, G.Z., Medina, R.T., Gutierrez, G.B. and Vargas, R.R., 1974. Recovery of minus ten micron cassiterite by liquid-liquid extraction. Int. J. Miner. Process., 1: 335-345.
ELSEVIER
Int. J. Miner. Process. 41 (1994) 257-269
The role of zeta potentials of oil droplets and quartz particles during collectorless liquid-liquid extraction E. Kusaka, Y. Nakahiro, T. Wakamatsu Department ofMineral Scienceand Technology, FacultyofEngineering, Kyoto University, Yoshida-honmachi, Sakyo-ku, Kyoto 606-01, Japan (Received 24 May 1993; accepted after revision 26 November 1993)
Abstract As a fundamental study on the recovery of fine panicles of valuable minerals by liquidliquid extraction, the concentration of fine quartz particles (0.24 #m average diameter) at the oil-water interface in the absence of a surfactant collector has been investigated. Isooctane and water were used as the two liquid phases. The recovery percentage of the quartz fines was determined as a function of the concentration of an added electrolyte (sodium, calcium and lanthanum chloride) and the pH value of the aqueous suspension. Based upon electrokinetic measurements, the liquid-liquid extraction results are interpreted in terms of colloid stability theory. Calculations of the total potential energy of interaction between isooctane and quartz indicate that height of the primary potential energy peak, which acts as an energy barrier in the quartz-isooctane coagulation process, correlates closely with the collectodess liquid-liquid extraction recovery of the quartz fines.
1. Introduction In mineral processing, the amount of finely divided particles that require processing has increased significantly in recent years. There is considerable interest in developing a process that could successfully handle such fine particles. Froth flotation is a well known such technique, but requires a narrow particle-size distribution because otherwise the finer particles have less chance of attaching themselves to the air bubbles (e.g. Derjaguin and Dukhin, 1960-61,1981 ). A wide variety of techniques, as summarized by Fuerstenau (1980) and Sivamohan (1990), have been tried to recover or remove fine mineral particles. The technique of liquid-liquid extraction is one example of such attempts, and has been investigated for the recovery of - 44/an hematite (Shergold and Mellgren, 0301-7516/94/$07.00 © 1994 Elsevier Science B.V. All fights reserved SSD10301-7516 ( 94 )00002-H
258
E. Kusaka et al. / Ira. J. Miner. Process. 41 (1994) 25 7-269
1969 ), ultrafine alumina (Lai and Fuerstenau, 1968 ), - 10 #m cassiterite (Zambrana et al., 1974; Marinakis and Shergold, 1988 ), fine tungsten ores (Marinakis and Kelsall, 1987) and fine beach sand minerals (Kusaka et al., 1988, 1989, 1991 ). This technique is also expected to be suitable for removing hydrophilic mineral matter from finely divided coal (Nakahiro et al., 1990) and from coalderived liquid product in the coal-liquefaction process (Ku et al., 1985 ). There are, however, very few studies on the mechanism of adhesion between oil droplets and solid particles. Discussions on the mechanism of this adhesion have been mainly in terms of the oil-wettability of the particles based on their surface hydrophobicity, that is, the variation in contact angle in the mineral-oilwater system, associated with the collector adsorption (e.g. Lai and Fuerstenau, 1968; Shergold and Mellgren, 1970; Raghavan and Fuerstenau, 1975 ). In aqueous solution, both oil-water and solid-water interfaces will generally carry a charge, due partly to interactions with chemical species in the solution. These surface charges may play a very important role in oil-solid coagulation, even in aqueous solutions which contain no surface-active agent as a collector. This investigation involved the testing of the liquid-liquid extraction method in the absence of a surfactant collector. Hydrophilic quartz fines having an average diameter of 0.24/zm were used as the solid phase and isooctane as the oil phase. In addition, electrokinetic measurements were also conducted on the isooctane droplets and the quartz particles under similar conditions to those for the extraction tests, in order to discuss the mechanism of the coagulation between the oil droplets and the hydrophilic quartz particles in terms of colloid-stability theory.
2. Experimental 2.1. Materials
Natural quartz from Fukushima Prefecture, Japan, was used as the test material for the solid phase. Pieces of high grade material were selected and ground for several hours in an alumina vibratory ball mill. The fine fraction was then collected by means of sedimentation. Fig. 1 shows the particle-size distribution for the quartz sample as determined using a centrifugal particle analyzer. The sample was dispersed by ultrasonic vibration before the analysis. It can be seen that the quartz sample has an average diameter (50%passing size) of 0.24/an on a weight basis. The purity of the quartz sample was also confirmed by X-ray diffraction studies. Reagent grade 2,2,4-trimethylpentane (isooctane) of + 99% purity was used as the oil phase throughout this investigation. Other reagents included HC1 and NaOH as pH regulators, and the electrol~es NaCl, CaC12 and LaCI3.
E. Kusaka et al. ~Int. J. Miner. Process. 41 (1994) 257-269 --
A
259
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PARTICLE DIAMETER (~tm)
Fig. 1. Particle-size distribution of the quartz sample used in this investigation.
2.2. Liquid-liquid extraction tests An aqueous suspension (25 c m 3 ) containing 0.1% of the finely ground quartz, an aqueous solution (25 em 3) containing the desired amounts of the pH regulator and the electrolyte, and isooctane (5 cm 3) were placed in a 100 cm 3 Pyrex separation funnel with a ground-glass stopper. The funnel was then shaken for 30 rain in a mechanical shaker. During this operation, some of the particles were observed to be concentrated at the isooctane-water interfaces, while others remained in the water phase. The mixture was then allowed to stand for 5 rain, during which time a dense emulsion phase containing the quartz fines separated from the aqueous phase. The two phases were drained offseparately, fdtered (after the aqueous pH had been measured), and dried. The dried products were weighed to determine the recovery percentage concentrated into the dense emulsion phase. 2.3. Electrokinetic measurements The electrophoretic mobilities of the isooctane droplets and the quartz particles were measured using a Zeta Meter (Organo Co., Ltd., Japan). The values of the zeta potentials were obtained using the Smoluehowski equation.
3. Results and discussion
3.1. Effect of adding various concentrations of an electrolytes at pH 5.5 The effect of adding various concentrations of NaC1, C a C I 2 and LaCI3 on the zeta potential of the isooctane droplets at pH 5.5 is shown in Fig. 2. A significant decrease in the magnitude of the zeta potential is observed within the electrolyte
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concentration range from 1 × 1 0 - 4 to 2 × 10 -2 mol dm -3 for NaCl, from 2 × 1 0 - 4 to 1×10 -2 tool dm -3 for CaCI2, and from 2X10 -s to 1×10 -3 mol dm -3 for LaC13. As the cation valency of the added electrolyte increases, the concentration at which the magnitude of the zeta potential begins to decrease is lowered. The variation in the zeta potential of the quartz particles at pH 5.5, as shown in Fig. 3, shows a similar trend to that for the isooctane droplets. The effect of varying the concentration of add~l NaCI, CaCI2 and LaC13 on the recovery percentage ofthe quartz fines concentrated into the dense emulsion phase
E. Kusaka et al. ~Int. J. Miner. Process. 41 (1994) 257-269
261
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10.8 10.6 10.4 10.2 100 ELECTROLYTE CONCENTRATION (tool dm"3) Fig. 4. Effect of concentration of added NaC1, CaCI2 and LaC13 on the recovery of the quartz fines by collectorless liquid-liquid extraction at pH 5.5.
in the collectorless liquid-liquid extraction tests at pH 5.5, is shown in Fig. 4. As can be seen, the percentage of quartz recovered in the NaC1 solution increases rapidly as the NaC1 concentration increases above 1 X 10-2 mol d m - 3 and reaches 100% at 5 × 10-~ tool dm -3. The addition of CaCI2 at concentrations above 1 × 10 -4 tool dm -3 results in a remarkable increase in the percentage of quartz recovered. Using LaCI3, the recovery increases as the electrolyte concentration rises above 1 × 10-5 mol din-3. Here again, the electrolyte concentration at which the quartz recovery begins to increase is progressively lowered as the cation valency of the added electrolyte increases. A comparison between the results of the liquid-liquid extraction tests and those of the electrokinetic measurements indicates that the increase in the recovery percentage seems to correlate with the decreases in the zeta potentials of both the oil droplets and the quartz particles and, therefore, the decrease in the electrostatic repulsion between them. Since in a preparatory test of collectorless liquidliquid extraction at an oil-volume concentration of 50%, no quartz particles have been observed to be transferred from water into isooctane and to be dispersed in the isooctane phase, it is considered that the quartz fines are concentrated in the isooctane-water interface region. This is attributed to the hydrophilic nature of the quartz surfaces. Consequently, the quartz-isooctane coagulation during collectorless liquid-liquid extraction is regarded as a heterocoagulation process, in which the total potential energy of the interaction can be quantified in terms of colloid stability theory (Verwey and Overbeek, 1948). According to this theory, the total potential energy of the interaction, VT, is expressed as the sum of two long-range forces; the potential energy of interaction due to the overlap of electrical double layers, VE, and that due to van der Waals' forces, VA:
VT=Ve+VA
(1)
262
E. Kusaka et al. ~Int..L Mmer. Process. 41 (1994)257-26~
Assuming that the average radius of the quartz fines (0.12/lm ) is much smaller than that of the isooctane droplets (which are visible in the funnel), VE can be approximated by an expression for the interaction between a sphere and an infinite fiat plate with opposite electrical charges. This expression was originally formulated for dissimilar spheres by Hogg et al., ( 1966 ) and later used to describe the interaction between an oppositely-charged sphere and an infinite fiat plate by Hull and Kichener (1969), Imamura and Tokiwa ( 1972 ) and Imamura ( 1975 ). In S.I. units it takes the form (Tokiwa and Imamura, 1984): VE=na~o~r[2~l~21n l + e x p ( - x H ) ~- ( ~ + ~ v ~ ) l n { 1 - e x p ( - 2 x H ) ) ] 1 - exp ( - xH) -
(2)
where a is the radius of the quartz sphere, eo the dielectric constant of free space, ~r the dielectric coefficient of water, ~vl and q/2 the surface potentials of quartz and isooctane, respectively, H the distance of separation and x the Debye-Hiickel reciprocal length parameter. Also, in Eq. (2), the upper positive sign is for the condition of constant surface potentials, and the lower negative sign for the condition of constant charge density. In this investigation, the condition of constant surface potentials was assumed, and the zeta potentials were used as substitutes for the surface potentials. When e is the elementary electrical charge, NA Avogadro's number, Z+ and Z_ the cation and the anion valencies of the added electrolyte, C+ and C_ the cation and anion concentrations, k Boltzmann's constant, and T the absolute temperature in degrees Kelvin, the relation among them is given by: (3)
I ¢ 2 = e 2 N A ( C + Z 2 Jr" C _ Z 2_ ) / ~ o ~ r k T
Disregarding the effect of retardation, the van der Waals' force, VA, between the sphere and the infinite fiat plate is given by (Hull and Kichener, 1969):
VA= (-Ai2/3/6) [ 2 a ( n + a ) / n ( H + 2 a ) - l n {
(n+2a)/H} ]
(4)
where A ~2/3 is the overall Hamaker constant for isooctane-water-quartz. If A 11/ 3 and A22/3 are the Hamaker constants for quartz-water-quartz and for isooctanewater-isooctane, respectively, A 1z~3 is given by the following relation:
(A12/3)2=All/3.A22/3
(5)
In calculating the values for Vx, the crystalline quartz-water-crystalline quartz Hamaker constant of 1.7 × 10-zo j was used for A I~/3 and the Cs alkane-waterCs alkane Hamaker constant of 4.1 × 10- 21 j for A2z/3 ( Hough and White, 1980 ). In Fig. 5, the results of calculations of the total potential energy of interaction between the quartz sphere and the isooctane plate for the system containing NaC1, CaCI2 and LaC13, are presented as a function of distance of separation at various electrolyte concentrations, which corresponds approximately to the extraction tests shown in Fig. 4. The primary peak height, VM, which behaves as an energy barrier to the interaction, reaches 84kT at 5× 10 -3 mol dm -3 NaC1, 71kT at 1 × 10 -4 mol dm -3 CaCI2, and 28kTat 1 × 10 -s tool d m -3 LaCl3. In all systems,
E. Kusaka et al. ~Int. J. Miner. Process. 41 (1994) 257-269
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SEPARATION DISTANCE (nm) Fig. 5. Calculated total potential energies of interaction between the quartz sphere and the isooctane plate as a function of distance of separation in the aqueous solution of the electrolyte: (A) NaCl, (a) 5× 10 -3 tool dm -3, (b) 1 × 10 -2 tool dm -3, (c) 2X 10 -2 tool dm-3; (B) CaCl2, (a) 1 × 10 -4 mol dm -3, (b) 1×10 -3 tool dm -3, (c) 6 × 1 0 -3 mol din-3; (C) LaC13, (a) 1×10 -5 tool dm -3, (b) 1 X 10 -4 mol dm -3, (c) 1 × 10 -3 tool dm -3.
VM tends to decrease as the electrolyte concentration increases. It decreases to
4kT at 2 X 10- 2 tool din- 3NaC1 and completely disappears at 6 × 10- 3 tool d m - 3 CaC12 and 1 × 10-3 tool dm-3 LaCI3. A comparison between the recoveries and the VT curves indicates that the increase in the recovery is in good agreement with the decrease in the VM value. The theoretical data show that at low electrolyte concentrations no coagulation between quartz and isooctane should occur because of high VM values. On the other hand, higher recoveries are obtained at higher electrolyte concentrations, at which the VM values are less than a few kT units or even zero. Under these conditions, the quartz particles surmount the energy barrier so that they can be rapidly concentrated at the isooctane-water interface, even without a collector.
3.2. Effect of pH in the isooctane-water-quartz system in the presence of NaCl Hydrogen and hydroxyl ions act as potential-determining ions in the interface region between water and an oxide mineral such as quartz, and the surface potential of the oxide mineral-water interface varies with the pH of the aqueous solution. In addition, the oil droplets in an aqueous solution containing no surfaceactive reagent are supposed to be charged due to the preferential adsorption of the hydroxyl ions. Their zeta potential therefore also varies with the pH (e.g.
E. Kusaka et al. ,' Int. A Miner. Process. 41 (1994) 25 "7-209
264
Roberts, 1936, 1937; Dickinson, 1941; Stachurski and Michalek, 1985 ). An interesting question is how the hydrogen and hydroxyl ions affect the charges of the oil-water and particle-water interfaces and the concentration of fine particles at the oil-water interface in collectorless liquid-liquid extraction. The influence of the zeta potentials of the isooctane droplets and the quartz particles on the concentration of the quartz fines at the isooctane-water interface was examined as a function of the aqueous pH, particularly in the system containing NaC1. Fig. 6 shows the effect of pH on the zeta potential of the isooctane droplets in the NaC1 solution. The hydrogen and hydroxyl ions appear to act as potentialdetermining ions in the isooctane-water interface region. Sodium and chloride ions act as indifferent ions that compress the thickness of the electrical double layer. No reversal of zeta potential was observed within the investigated range of pH values. The isoelectric point (IEP) of isooctane may, however, be around pH 2.5, i.e. outside the pH range investigated. In Fig. 7, the variation in zeta potential of the quartz particles as a function of pH in the presence of NaC1 is presented. As reported in many publications dealing with the electrokinetic property of the quartz-water interface, the hydrogen and hydroxyl ions appear to act as potential-determining ions in the quartz-water interface region. The figure also shows no reversal of the zeta potential of the quartz sample within the investigated range of pH values, but its IEP might be below pH 2. The IEP of the Fukushima quartz seems to be somewhat low; this may be due to the impurities present which are mainly feldspars. Fig. 8 presents the influence of pH on the collectorless liquid-liquid extraction recovery of the quartz fines at three different NaC1 concentrations. The recovery percentage tends to increase as the pH decreases below 4; this effect becomes more pronounced at higher NaCI concentrations. A comparison of the results of the liquid-liquid extraction tests and the electrokinetic measurements, suggests that the increase in the recovery percentage corresponds to a decrease in the negative zeta potential of isooctane rather than of -100
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E. Kusaka et al. /Int. J. Miner. Process. 41 (1994) 257-269 -50
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quartz. Rapid heterocoagulation between two dissimilar spheres as proposed by Hogg et al. (1966) should occur within the pH range between their IEP's. This seems to explain the pH-dependence of the recovery. Theoretical data interpreting the recovery is provided by Fig. 9, which shows the calculated primary peak height, VM,of the interaction between the quartz sphere and the isooctane plate as a function of pH at various NaC1 concentrations. As can be seen in Fig. 9, the VMis reduced with decreasing pH, and rapidly falls when the pH approaches the IEP of isooctane. This tendency becomes increasingly pronounced at higher electrolyte concentrations.
266
E. Kusaka et al. / Int. J. Mmer. Process. 41 (1994) 2 5 7 - 2 6 9
103 ~-- ~----V-~--7--.--, /---r ---T-~--~ ~- Quartz sphere NaCI, tool dm"3 "-" >~ ~ - Isooctane plate / 10-4 !
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3.0
4.0
5.0
6.0
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pH Fig. 9. Calculated primary maximum height, VM, of the total potential energy of interaction between the quartz sphere and the isooctane plate as a function of pH at different NaCl concentrations. For calculating VM, the electrokinetic data for isooctane and quartz were graphically obtained from Figs. 6 and 7, respectively.
3.3. Correlation between the primary peak height and the collectorless fiquidliquid extraction recovery The recovery percentage of the quartz fines in the collectofless liquid-liquid extraction tests is closely related to the decrease in the primary peak height, VM, of the total potential energy of interaction between isooctane and quartz. It is of great importance to identify the criterion in terms of the I'M for the attainment of high recoveries in collectorless liquid-liquid extraction. Therefore, the relationship between the recovery percentage and the I'M value was examined. Fig. 10 presents the re-plot of the recovery percentage of the quartz fines in the coUectorless liquid-liquid extraction tests shown in Fig. 4 as a function of I'M. In calculating the VM values, the experimental data of the zeta potentials of isooctane and quartz corresponding to the conditions of the extraction tests were substituted for ~1 and ¢2 in Eq. (2). A VM value greater than 70kTgives a recovery of less than 20%, whereas a I'M of zero gives a 40 to 100% recovery. The critical I'M value to attain a 50% recovery ranges from 1.2kTfor NaCI to 5.TkTfor CaCI:, obtained from the respective fitted VM-recovery curves. (A recovery percentage value of 50% was chosen to indicate the critical condition, taking the broad size distribution of the quartz sample used in this investigation into account.) According to colloid stability theory, if the VMvalue is high, the particles should be unable to surmount the energy barrier. These results suggest that the zeta potentials of the oil droplets and mineral particles determine the contribution of the electrostatic force to the interaction between them when the interaction due to the van der Waals forces is not responsible for oil-particle coagulation, and that
E. Kusaka et al. ~Int. J. Miner. Process. 41 (1994) 257-269 100:
I
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0 20 40 60 80 100 PRIMARY MAXIMUMH~OHT (kT units) Fig. 10. Relationship between the primary maximum height, VM,and the recovery of the quartz fines by collectodess liquid-liquidextraction in the aqueous solution containingthe electrolytes NaC1, CaCI~ and LaCI3.
in such a system the VM value becomes very important in estimating the recovery of fine particles by collectorless liquid-liquid extraction.
4. Conclusions
The recovery of fine quartz particles of an average diameter of 0.24/an by means of liquid-liquid extraction without the use of a collector has been studied. The results are discussed in terms of colloid stability theory, on the basis of the results of electrokinetic measurements. The following conclusions can be drawn from this investigation. ( 1 ) Electrokinetic results at pH 5.5 show that the magnitudes of the zeta potentials of the isooctane droplets and the quartz particles in an aqueous electrolyte solution decrease, as the electrolyte concentration is increased. Furthermore, as the cation valency of the electrolyte increases, the concentration at which the magnitudes of the zeta potentials begin to decrease is lowered. (2) In liquid-liquid extraction using quartz fines at pH 5.5, the recovery of the quartz particles is possible even without a collector when the concentration of the added electrolyte is increased. (3) The extraction behavior is well explained in terms of colloid stability theory. Calculations of the total potential energy of interaction between quartz and isooctane due to electrostatic and van tier Waals' forces indicate that the recovery percentage should increase under conditions where the height of the primary peak of the potential energy of interaction is low. (4) Electrokinetic measurements show that the IEP of isooctane is near pH 2.5, while that ofthe Fukushima quartz is below pH 2. (5) The liquid-liquid extraction results at various pH values in the system
268
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containing NaCI, show that the recovery percentage tends to increase as the pH approaches the IEP of the isooctane droplet. Colloid stability theory suggests that the heterocoagulation between isooctane and quartz may be accelerated near IEP. On the other hand, the recovery of the quartz fines is impossible in the neutral to basic pH range, due to high electrostatic repulsion. (6) The criterion in terms of the primary peak height of the total potential energy of the interaction between quartz and isooctane for attaining a 50% liquid-liquid extraction recovery of the quartz fines ranges from 1.2kT to 5.7kT, depending upon the electrolyte added into the liquid-liquid system.
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