Photon correlation spectroscopy applied to hydrometallurgical solvent extraction systems

Colloids and Surfaces, 46 (1990) 45-61 Elsevier Science Publishers B.V., Amsterdam -

45 Printed in The Netherlands

Photon Correlation Spectroscopy Applied to Hydrometallurgical Solvent Extraction Systems RONALD D. NEUMAN*, MATTHEW A. JONES and NAI-FU ZHOU Department of Chemical Engineering, Auburn University, Auburn, AL 36849 (U.S.A.) (Received 19 July 1989; accepted 16 August 1989)

ABSTRACT Photon correlation spectroscopy (PCS) was applied to hydrometallurgical solvent extraction systems to shed insight into the physicochemical nature of the very small (d>,2 nm) metal-extractant aggregates which form in the organic phase. For the three extraction systems di (2-ethylhexyl)phosphoric acid (HDEHP)/n-hexane/metal salt solutions of Ni*+, Zn2+ and Ca2+, it was found that the mean effective hydrodynamic radius and polydispersity index of the metal-extractant aggregatesbecome larger and smaller, respectively, with an increase in the aqueous phase pH and the extractant concentration [ HDEHP 1. Histograms show the parallel effects of pH and [HDEHP] on the size distribution of the micellar aggregates. In addition, PCS measurements were performed on several reversed micellar systems including di (2-ethylhexyl ) sulfosuccinate (AOT)/isooctane/H,O and NaDEHP/benzene/H,O for comparison. It seems the model originally proposed by Eicke and co-workers [Ber. Bunsenges. Phys. Chem., 79 (1975) 6671 for the aggregation of surfactants in nonpolar solvents is applicable to these solvent extraction systems.

INTRODUCTION

The separation process of solvent extraction, whereby two immiscible liquid phases are contacted in order to transfer a solute from one phase to the other, has been widely used in industrial processes. For example, in hydrometallurgy, a metal-bearing aqueous feed solution is contacted with an immiscible extractant-containing organic phase so that a metal-extra&ant complex results in the organic solvent. The organic phase is then scrubbed to remove impurities or unwanted metals and is subsequently stripped, thereby resulting in a relatively concentrated aqueous metal solution. This is followed by precipitation, crystallization or electrowinning to recover the metal in solid form. Some of the most commonly used extractants are derivatives of organophosphorus acids. In particular, di (2-ethylhexyl)phosphoric acid (HDEHP) has received wide usage, being originally used for the extraction of uranium, but more recently for cobalt, nickel, and zinc [ 11. Although HDEHP has been *To whom correspondence should be addressed.

0166-6622/90/$03.50

0 1990 Elsevier Science Publishers B.V.

studied extensively, one of the unresolved questions is the physicochemical nature of the metal-extractant species formed when the organic phase is highly loaded. HDEHP is also surface active, so it is not too surprising that interfacial tension studies of solvent extraction systems composed of HDEHP/n-hexane/ aqueous metal chloride (including cobalt, nickel and zinc) indicate aggregates, possibly reversed micelles, can form in these systems [ 2-51. Although the formation of reversed micelles by HDEHP in solvent extraction systems is not a generally accepted phenomenon, it is known that the sodium salt of HDEHP ( NaDEHP) forms reversed micelles [ 6-91. Furthermore, it has been suggested that strontium [lo]and nickel [ 111 are extracted into NaDEHP reversed micelles. Among the various parameters employed in characterizing extractant (surfactant) solutions, particle size and, in particular, particle size distribution would be important and give additional information concerning the metalextractant aggregates which, in turn, can lend support to an aggregation model and aid in understanding the mechanism of solvent extraction. Photon correlation spectroscopy (PCS) is a light-scattering technique in which the intensity fluctuations of the scattered light are measured to obtain information on the Brownian motion of particles in solution, namely, the diffusion coefficient, which can be related to the particle size and size distribution for simple assumed shapes. In order to provide size information on solvent extraction systems of practical interest, PCS measurements were attempted on HDEHP extraction systems of nickel, cobalt, and zinc. The separation of nickel and cobalt is important from a strategic viewpoint, while the extraction of zinc by HDEHP is a recent industrial process. The extraction of calcium by HDEHP involves a simpler chemistry and has often been studied, thus this metal was also included in this preliminary study. To our knowledge, this investigation is the first to report PCS measurements on hydrometallurgical solvent extraction systems. When PCS is applied to very small micellar aggregates such as those found in hydrometallurgical systems, additional attention must be given to experimental details, as discussed in this communication, in order to obtain reproducible measurements. EXPERIMENTAL

The measurement of very small micellar aggregates is not a trivial problem. Therefore, prior to PCS measurements on solvent extraction systems, the experimental approach involved not only employing polystyrene latex standards but also di (2-ethylhexyl ) sulfosuccinate ( AOT ) as an “external” reversed micellar standard for comparison. In addition, two model solvent extraction systems were studied, namely, the sodium and calcium salts of HDEHP in nonpolar solvents.

Apparatus Dynamic light-scattering experiments were performed using a Brookhaven BI-BOOSM multiangle goniometer in conjunction with a Malvern RR51 photomultiplier tube (PMT) and a Lexel Model 95-4 argon ion laser equipped with an etalon. The incident laser beam (488 nm) was vertically polarized with, typically, a power of 200-300 mW. A Brookhaven 128channel BI-2030 digital correlator/computer was used to process the output PMT signal and to obtain particle size information. The software available can fit the intensity autocorrelation function to a single exponential and perform a cumulant analysis, while the size distribution was obtained using software based on the method of exponential sampling [ 121. Proper alignment of the light-scattering apparatus was verified using additional (Isin 0) software.

Materials Since the aggregation behavior of surfactants can be greatly affected by impurities, stringent cleaning and purification practices were employed. As such, all glassware was cleaned in a mixture of Nochromix (Godax Laboratories) and concentrated sulfuric acid and then copiously rinsed with doubly distilled water purified as described elsewhere [ 21. AOT (Fisher) was purified by dissolving in methanol, stirring with activated charcoal, filtering with 0.7 p filter paper and drying under high vacuum prior to use. This AOT sample was further purified using a method described by Martin and Magid [ 131: a benzene solution of AOT was treated with activated charcoal again; after filtration, the solution was washed with water twice; the benzene was evaporated and the solid was dissolved in a 1: 3 (vol) mixed solvent of methanol and water prior to washing with ligroine; and then the aqueous methanol solution was evaporated and dried as above. All solvents employed in the above purification scheme were redistilled. The organic phase diluent isooctane (HPLC grade, Aldrich) was dried by distilling with sodium metal and stored over Linde 4A molecular sieves. Solutions (0.025 M) of AOT in isooctane were prepared, ultrasonicated, and equilibrated at 298.15 K for at least 24 h in a shaker water bath. After PCS measurements, the water content was analyzed by volumetric Karl Fisher titration (KFT). Sodium and calcium salts of HDEHP were prepared as follows: First, HDEHP (96%, Morton Thiokol) was purified by the copper salt method following the procedures of Partridge and Jensen [ 141. In addition, the solvents acetone and diethyl ether (Burdick & Jackson) were distilled prior to use in the purification of HDEHP. In the preparation of NaDEHP a purified sample of HDEHP was neutralized with a standardized solution of sodium hydroxide in redistilled absolute ethanol. The ethanol was evaporated, and the solid was dissolved in distilled diethyl ether, filtering if necessary. The solvent was re-

48

moved with a rotating evaporator after which the solid was dried under high vacuum prior to use. An elemental analysis yielded 55.89% C, 9.71% H, 7.68% P, 7.13% Na and 1.16% HzO. In the preparation of Ca ( DEHP)2, purified HDEHP was added to distilled diethyl ether to give a 200 g 1-l solution. This was contacted with an aqueous solution of (50% excess) 0.5 A4 calcium hydroxide which reacts to form Ca(DEHP), in the organic phase. After discarding the aqueous phase, the organic solution was washed repeatedly with doubly distilled water to remove any unreacted calcium hydroxide, and the organic solvent was evaporated. Finally, the solid Ca(DEHP), was dried under high vacuum prior to use. An elemental analysis yielded 54.65% C, 10.03% H, 8.55% P, 6.26% Ca and 1.95% HzO. In the model solvent extraction system studies, the solvents benzene (99 mol%, Fisher) and n-hexane (99 mol%, Phillips) were used as organic phase diluents for NaDEHP and Ca (DEHP ) 2, respectively. The n-hexane and benzene were purified as described earlier [2,15] and, in addition, were dried by distilling with sodium metal and stored over Linde 4A molecular sieves. Stock solutions [21.5 g 1-l NaDEHP and 21.2 g 1-l for Ca(DEHP),] of the HDEHP metal salts were made, ultrasonicated and equilibrated in a shaker water bath at 293.15 K. Samples of various water contents were made by adding doubly distilled water by weight to clean glass vials, pipetting a known amount of stock solution into each vial and equilibrating for at least 24 h. After PCS measurements, as before, the water content was analyzed by KFT. In solvent extraction experiments two phases are equilibrated: an HDEHPcontaining organic phase and an aqueous metal salt solution. The solvent nhexane was extracted with doubly distilled water to remove any surface-active material remaining after purification. The metal chloride salts (99.99 + % ) of nickel, zinc, and calcium were obtained from Spex Industries while high-purity cobalt sulfate (99.999% ) was obtained from Johnson Matthey. The metal salt solutions were extracted with purified n-hexane to remove any extraneous surface-active compounds. In preparing solvent extraction samples for PCS experiments, equal volumes (3 ml) of organic and aqueous phases were equilibrated at 293.15 K. The pH was measured and adjusted to a specific value by adding sodium hydroxide (99.996%, Spex Industries) or hydrochloric acid (Ultrex, J.T. Baker). After equilibration, PCS measurements were performed on the.organic phase. Methods One of the difficulties encountered in light-scattering experiments is the presence of dust. For this reason, great care was taken to eliminate its effects by filtering the solution inside a low-dust hood directly into the sample cell and centrifuging it at 2000 rpm for 15 min. The organic samples were filtered

49

with an organic resistant 0.2 p Acrodisc (Gelman), while the aqueous samples were filtered with a 1 pm Millipore filter unit. The samples were then allowed to re-equilibrate at the desired temperature to within rt 0.02 K in the specimen cell assembly before commencing PCS measurements. During a typical experiment both angular (15-60”) and replicated (6-10) measurements were made. The criteria for accepting or rejecting PCS data can be grouped into four categories: count rate, appearance of autocorrelation function, percent difference between calculated and measured baselines and goodness of fit. In these experiments both criteria for the count rate ( 104-5*106 counts s-l) and the /I parameter (0.2-0.8)) which is proportional to the signal height above the baseline [see Eqn ( 1) 1, were satisfied by selecting the diameter of the second pinhole of the main detector optics to be 100 pm. If the sample time is chosen properly, the curve will decay smoothly to the baseline. The appearance of the autocorrelation function is a qualitative check that the best sample time was chosen. A better indication, however, is the percent difference between the calculated and measured baselines. It was possible for the two baselines to agree to within 0.01% for clean samples with no dust or flare light. The experimental data were fitted to two functional forms: a single exponential and a cumulant form. The goodness of fit of the data to either functional form is represented by a reduced chi square, which is a measure of both random deviations and differences in the assumed form. Typical acceptable values were 10B6 and less. Analysis The measured homodyne intensity autocorrelation function C (?) is related to the first-order (or field) autocorrelation function g (‘) (7) by C(z)=A(l+/?]g”‘(z)

12)

(I)

where A is the experimentally determined baseline and /.Iis an optical constant [ 16-181. For a monodisperse suspension of particles, g”‘(r)

--exp

(-Z?)

(2)

where the linewidth r (or decay rate) is related to the particle diffusion coefficient D by l-=Dq2.

(3)

The amplitude of the scattering wave vector q is defined by q= [4xn/l]sin(8/2)

(4)

where n is the index of refraction, A is the wavelength of the incident light in vacua, and 8 is the scattering angle. A least-squares technique permits deter-

50

mination of the two parameters p and r from experimental data. Then, using Eqns (3) and (4)) the particle diffusion coefficient of a monodisperse suspension can be obtained. Finally, the diffusion coefficient is related to the radius (r ) for spherical particles by the Stokes-Einstein relationship D = kT/Gmp

(5)

where k is the Boltzmann constant, T is the absolute temperature and r] is the viscosity of the medium in which the particles are diffusing. The resulting radius is often termed the effective hydrodynamic radius. For a polydisperse suspension, each size contributes an exponential similar to Eqn (2)) and the first-order autocorrelation function is written as a distribution of exponentials m

g(l)(z) = jG(T)exp(

-R)cir

(6)

0

with 03

I

G(Z-)dl-=

1

(7)

0

where G(r) represents the normalized linewidth distribution. In the method of cumulants [ 191, Eqn (6) is expanded about a mean linewidth r, and a leastsquares fit gives the three parameters, j3, r and ~1where p and p are the first and second cumulants of the linewidth distribution, respectively. The mean diffusion coefficient D can be obtained from T&q2

(8)

which can be related to the mean radius (i;) through the Stokes-Einstein relationship as before. In this case, however, a z-average radius is obtained. The second quantity obtained from the cumulant analysis is the polydispersity index Q which is an indication of the width of the size distribution and is defined as

Q=PP

(9)

A lower limit on Q is about 0.02 for monodisperse samples, while a higher value indicates polydispersity. The exponential sampling method which was employed to obtain the size distribution of a polydisperse sample is described elsewhere by Ostrowsky et al. [12].

51 RESULTS AND DISCUSSION

Polystyreti latex standards The light-scattering apparatus employed does not require calibration, nevertheless it is appropriate to check its performance. For this reason, PCS measurements were first performed on two standard polystyrene latex particles. One was a 0.3 pm nominal diameter sample from the National Bureau of Standards (NBS), while the other was a specially prepared 0.021 pm sample from Duke Scientific. The PCS results are summarized in Table 1. The diameter obtained for the NBS sample is in excellent agreement with the stated diameter. Furthermore, this sample can be considered to be monodisperse as evidenced by the low polydispersity index Q and a size distribution observed to be very narrow. The diameter obtained for the Duke Scientific sample also agrees very well with the given diameter. The sample, however, was not monodisperse as shown by the higher Q value of 0.20 and an observed bimodal size distribution. Later measurements on another 0.051 pm diameter standard gave a Q value of 0.05 with a narrower size distribution, but a different diameter (0.061 pm). These results clearly support the caution implied by Weiner and Tscharnuter [20] with regard to the use of presumed reference particles. Reversed micellur solution Although the polystyrene spheres are in the submicron size range, they are still much larger than the expected size of the metal-extra&ant aggregates (d 2 2 nm). Since the aggregates could be reversed micelles [2-51, it is appropriate that PCS be performed on a known reversed micellar system whose particle size is similar to that of the aggregates formed in hydrometallurgical systems. TABLE 1 PCS measurements of polystyrene latex particles Standard diameter

Measured diameteP

(Pm)

(Pm)

0.269 k 0.007 0.021 f 0.005

0.273 +0.004b 0.0218 + 0.0001”

Q 0.02 0.20

“The uncertainty is the standard deviation of repeat measurements at a scattering angle of 90”. ‘The measured diameter was obtained by a single exponential fit which is valid for monodisperse suspensions. The measured diameter was determined using the cumulant analysis since the had a certain degree of polydispersity with the expected result that the cumulant fit was better than the single exponential fit (as judged by a consistently lower chi square value).

sampie

52 TABLE 2 PCS measurements of AOT in isooctane at 298.15 K F (nm)

Q

WO

1.36f0.01 1.48 + 0.02 1.64 k 0.03

0.17+0.01 0.18 f 0.02 0.28 f0.01

0.18 0.15 0.11

The surfactant AOT forms reversed micelles in nonpolar solvents. Since PCS has been applied to AOT in the past by Day et al. [21], Zulauf and Eicke [ 221, Bedwell and Gulari [ 231, as well as Nicholson and co-workers [ 24,251, PCS measurements were carried out on AOT in isooctane to serve as an additional check of the light-scattering apparatus and experimental techniques. The results of the PCS measurements on three freshly prepared samples are given in Table 2. Here, F and Q are the mean effective hydrodynamic radius and polydispersity index, respectively, from a cumulant fit. The uncertainties in Fand Q are indicated by the standard deviations of the repeat measurements. The hydrodynamic radius of AOT in isooctane, without added water, was determined in previous PCS investigations to be 1.5 nm [ 22,231. Thus, the measured and literature results agree very well. Another finding was that the lower limit of &=0.02 for a monodisperse system was not achieved. Instead, the size distribution is relatively broad with Q tending to decrease with increasing molar ratio of water to surfactant ( IV,). Measurements were also undertaken as a function of scattering angle (15 60”)) and no angular dependence was observed. These results were not entirely unexpected since polydispersity can be anticipated assuming a stepwise association process as discussed later. In addition, at the very low AOT volume fraction ($-0.01) used in this work, the fractional differences which occur upon the addition or subtraction of one AOT molecule (or solubilized water molecule) will be greater for smaller particles than for larger particles [ 241. As also shown in Table 2, IV, in the AOT reversed micellar system, even without added water, is found to be about one water molecule per ten surfactant molecules as recently reported by Ueda and Schelly [ 261. The most important conclusion obtained from this phase of the study was that reproducible results could be achieved in general agreement with previous investigations. This is crucial since reliable characterization of solvent extraction systems requires accurate measurements by the PCS technique, especially so for the very small aggregates existing in these systems. Model solvent extraction systems The metal salt of the extractant is formed as the metal is extracted into the organic phase. If this loading process were to continue indefinitely, all of the

53

extractant would be converted into its metal salt. In practice, however, the degree of loading is optimized so that in general total conversion does not occur. On the other hand, sodium and calcium salts of HDEHP can be formed in the laboratory and represent the logical conclusion of a fully loaded organic phase. As such, they can be used for studies modelling solvent extraction systems. Two model solvent extraction systems were examined: NaDEHP in benzene and Ca (DEHP ) 2 in n-hexane. The sodium salt of HDEHP in benzene is known to form reversed micelles [ 6-91 which grow in size with increasing water content [ 27 J. The calcium salt of HDEHP probably behaves in a similar manner as NaDEHP. Therefore, PCS was performed on NaDEHP in benzene and Ca ( DEHP)2 in n-hexane as a function of water content to compare the results with those of previous reversed micellar studies and those obtained using solvent extraction systems, respectively. The PCS results for the reversed micellar system of NaDEHP in benzene are shown in Fig. 1. The mean effective hydrodynamic radius obtained from a cumulant fit is plotted versus IV,. It was found that added water (IV,= 1.2) was necessary for dissolution of the surfactant and that phase separation occurred at W,=6. These findings are in agreement with those of Faure et al. [8]. Below a water content of W,=2.5, however, the light-scattering measurements were angularly dependent and gave larger sizes with values of ?as large

Fig. 1. Mean effective hydrodynamic radius versus W,,. (O), NaDEHP in benzene at 293.15 K with W,= [H,O]/[NaDEHP]; (0), Ca(DEHP) 2 in n-hexane at 293.15 K with W,= [H,O]/ [Ca(DEHP),].

54

as 6 nm which may be due to the formation of liquid crystalline microstructures. As a result, PCS measurements were restricted to water contents ranging from IV, = 2.5 to W,,= 5.5, where f increased from 2.2 to 4.8 nm as shown in Fig. 1. Another finding was that the aggregate size distribution became narrower with increasing water content, for example, compare Q= 0.20 at W, = 2.6 with Q=O.OS at IV,= 5.4. PCS measurements were also performed at various angles, and no angular dependence was observed. The results of PCS measurements on Ca( DEHP)2 in n-hexane are also shown in Fig. 1 where F varies from 4.2 to 6.2 nm at water contents ranging from W,= 1.0 to W,= 3.8. Since the calcium salt of HDEHP is very hygroscopic, water contents lower than W,,= 1.0 could not be achieved. Furthermore, phase separation occurred at WO= 3.8. These results support a previous investigation which indicates an increase in average molecular weight for the calcium salt of HDEHP in n-hexane upon the addition of aqueous CaCl, solution [5]. Another finding was that the aggregate size distribution also became narrower with increasing water content, for example, Q=O.17 at W,= 1.0 and Q=O.O9 at W, = 3.7. In addition, no angular dependence was observed.

Solvent extraction systems PCS measurements were performed on the organic phase of HDEHP solvent extraction systems of nickel, cobalt, zinc and calcium. In the case of cobalt, as might be expected, the laser light was strongly absorbed by the organic solution. At high HDEHP concentrations, microbubbles formed with the concomitant scattering of light, thereby making size determinations impossible. On the other hand, for the remaining metals, size information was obtainable. It should be noted that exploratory light-scattering experiments, in the case of calcium and zinc, found the intensity of scattered light was too small under the same extraction conditions employed in earlier interfacial tension measurements by Gaonkar and Neuman [4,5]. Therefore, as a means to enhance the scattering, it was necessary to alter the extraction conditions so that size determinations were possible. For example, the metal concentration was increased from 0.01 A4 to 0.5 M and from 0.1 M to 0.3 M for calcium and zinc, respectively. In addition, the pH was raised from 3.5 to 4.5 for calcium. In the case of nickel, however, it was possible to employ the same extraction conditions in the PCS measurements as were used in the previous interfacial tension experiments. Table 3 summarizes the results of our first series of PCS measurements performed on the three solvent extraction systems [ 281. The concentration of HDEHP is expressed in terms of formality (F) because the stoichiometry of the extractant species in the organic phase varies in a complex manner. It appears that the minimum f observed for the metal-extractant aggregates is about 1 nm. This is to be compared with the radius of gyration for HDEHP

55 TABLE 3 PCS measurements of organic phase of solvent extraction system HDEHP/n-hexane/metal ride solution at 293.15 K pH

F (nm)

Q

0.10 0.30

4.5 4.5

1.06 1.11

0.25 0.22

Zn*+ (0.3OM)

0.10 0.30

2.5 2.5

1.32 1.39

0.21 0.21

Ni*+ (0.05 M)

0.10 0.30

5.0 5.0

1.61 1.59”

0.16 0.14

Metal ion (Cone)

HDEHP

Ca2+ (0.50 M)

(F)

chlo-

*Later runs by N.F.Z. and R.D.N. gave a higher value (1.8 nm).

which is calculated to be approximately 0.8 nm. Increasing the HDEHP concentration, while maintaining the other variables of metal ion concentration and pH constant, resulted in only a small increase in r; In a similar manner, increasing the metal ion concentration over the range examined in this study did not appear to have a major effect on the observed aggregate size. It is believed that an increase in the HDEHP (or metal ion) concentration increases primarily the number rather than the size of the individual aggregates. On the other hand, changes in the hydrogen ion concentration can have a pronounced effect on the aggregate size as will be shown later. In addition, it should be remarked that no angular dependence was observed over the range of angles examined in this study. In more recent work two of the authors (N.F.Z. and R.D.N.) investigated the effects of aqueous phase pH and extractant concentration in greater detail. In general, the higher the pH, the larger the metal-extra&ant aggregates. The pH effect is very evident in the case of nickel extraction, especially when the pH of the aqueous phase is higher than pH 5. For example, at pH 6, r of the Ni-HDEHP aggregates becomes as large as- 6 nm. On the other hand, as pointed out earlier, the effect of [HDEHP] is not so dramatic. In the same nickel extraction system, but at pH 5, a ten-fold increase in [ HDEHP ] from 0.03 to 0.3 F leads to only a small increase in Ffrom about 1.5 to 1.8 nm. Histograms of the size distribution of the metal-extra&ant aggregates formed in the three solvent extraction systems are shown in Figs 2-4. The size distributions are plotted with the scattered light intensity as a function of particle diameter and appear similar for the three solvent extraction systems. At lower pH values, the size distributions are very broad and generally appear somewhat bimodal. For example, consider the nickel extraction system shown in Fig. 4, although f is 1.2 nm at pH 4.5, the particle radii range from a few tenths of a nanometer to 5 nm. It should be recalled, however, that the intensity of the scattered light is proportional to the sixth power of the particle diameter, thus

56

loopH 5.7 7 = 2.7

0.5

5

50 DIAMETER,

nm

500 nm

Fig. 2. Effect of pH on size distribution of metal-extra&ant aggregates formed in the system HDEHP/n-hexane/O.S mol dmm3 CaCl, at [HDEHP] =O.l F and 293.15 K.

the larger particles can scatter light strongly even if their number is very small. At higher pH, the size distribution shifts to larger particle sizes and the bimodal nature more or less disappears. This is particularly true for the nickel extraction system at pH 6 where the histogram (see Fig. 4) reveals a narrower size distribution (Q-0.1) with the formation of large aggregates in the size range of microemulsion droplets. Size distributions were also obtained on several related systems. In the absence of metal ions, the histogram still shows a bimodal nature for the system HDEHP/n-hexane (or n-heptane) /HZ0 even when [HDEHP] =0.5 F with % 1.2 nm and Q - 0.3. These findings are to be contrasted with the results of Bhattacharyya and Ganguly [291who reported that for HDEHP (0.5 F) in nheptane the diameter of the observed aggregates (via electron microscopy of evaporated solutions) in the presence of water was 60-170 nm. It should also be noted that we found the size distributions to be somewhat bimodal for

57

‘ooL-J-L pH 2.0

z z J f

T = 1.0 nm

jjl,;i

0.5

100

5

50

500

pH 2.5

:: z 6

T=l.4nm

f 0 0.5

5

50

DIAMETER,

500 nm

Fig. 3. Effect of pH on size distribution of metal-extra&ant aggregates formed in the system HDEHP/n-hexane/O.J mol dmm3 i&Cl2 at [HDEHP] =O.l Fand 293.15 K.

NaDEHP in a mixed isooctyl alcohol/kerosene solvent [ 30 ] * as well as for the reversed micellar system AOT/isooctane/H,O**. It should be mentioned that the PCS data were analyzed assuming negligible interactions among the metal-extra&ant aggregates. In concentrated systems, however, D (and hence F) depends on the concentration (or volume fraction) of the dispersed phase. Dilution techniques, which are usually employed to reduce particle-particle interactions, cannot be applied to swollen reversed micellar or microemulsion systems since difficulties can arise from possible variation of particle size with volume fraction. Another complication in applying PCS to solvent extraction systems involves the question of the proper value of the viscosity for calculation of fusing the Stokes-Einstein equation. In this work the viscosity of the pure solvent was used neglecting the complex equilib*PCS measurements indicated that the size distribution of the microemulsion droplets formed by NaDEHP in the mixed solvent of isooctyl alcohol and kerosene (where isooctyl alcohol:kerosene=20:80 by volume, [NaDEHP] =0.44 F, and [H,O]/ [NaDEHP] ~3.3) covers a wide range (Q = 0.27) from 50 to 1000 nm with F= 120 nm. **A preliminary study of AOT reversed micelles in isooctane showed that their stability can be affected by aging and centrifugation. Generally speaking, the larger the micellar particles (e.g., W, N 20) the more evident the effects. After aging, the micellar size decreased and the bimodal nature of the size distribution disappeared more or less. Similar phenomena upon centrifugation were also observed. However, no aging or centrifugation effects on the aggregates formed in solvent extraction systems were observed.

58 100 pH 4.5 F-1.2

0 L 0.5

5

50

“Ill

500

100 pH 5.0 L

0 0.5

i-l.5

5

50

nm

500

1

L-l!!_ pH 6.0

r-s.9

0 0.5

5

50

DIAMETER,

“In

500

“m

Fig. 4. Effect of pH on size distribution of metal-extractant aggregates formed in the system HDEHP/n-hexane/0.05 mol dmT3 NiCl, at [HDEHP] =O.l F and 293.15 K.

ria involving HDEHP in the organic phase. For some W/O microemulsions at low water volume fractions (& < 0.1)) provided there is a small degree of polydispersity, the effects of interparticle interactions tend to cancel such that D is reasonably close to the diffusion coefficient extrapolated to zero concentration, i.e., D, [ 311. In this study both the HDEHP volume fraction $* and & were ;5 0.1. Thus, r computed from Eqn (5) can be taken as a first approximation to the true mean hydrodynamic radius r;, in the absence of interparticle interactions. Alternatively, one can attempt to relate D to D, by D=D,[l+

(K,+K,)$].

(IO)

The calculations of the coefficients Kt and Kh consider direct potential interactions and hydrodynamic interactions, respectively [32]. Our limited PCS results indicate a weak concentration dependence of the effective hydrodynamic radius, and the increase in F (or decrease in D) can be interpreted as being a consequence of attractive interactions. In any event, it is believed that our assumption of negligible interactions does not seriously affect the conclusions of this preliminary study. Future investigations, nevertheless, need to

59

investigate the nature of any particle interactions in these systems in greater detail. It is also worthwhile to note that further studies of the size distribution should consider the multiangle analysis reported recently by Cummins and Staples [ 331. As indicated earlier, it was expected that PCS measurements would give insight into the nature of the aggregates in hydrometallurgical solvent extraction systems which could lend support to an aggregation model. In general, three models have been commonly employed to describe the aggregation behavior of surfactants in nonpolar solvents: the phase separation model [ 341, the mass action (single equilibrium) model [34,35] and the stepwise aggregation (multiple equilibrium) model [ 34,361. In addition, another model [ 3739] has been proposed for the aggregation of surfactants whereby groups of three (in the case of AOT) and four (in the case of NaDEHP) monomers act as nuclei from which linear aggregates form in a stepwise fashion for the association process. This model defines an “operational c.m.c.” as the concentration range over which the linear aggregates reorganize into cyclic or reversed micelles which, in turn, subsequently grow in size. Since the model has been used to describe the association behavior of NaDEHP, it might be useful in explaining the aggregation of HDEHP in solvent extraction systems. This latter model implies a certain degree of polydispersity which, at higher surfactant concentrations, should decrease with increasing surfactant concentration. For solvent extraction systems, this also appears to be the situation. At fixed [ HDEHP] , Q is lower and the size distribution is narrower at higher pH values. In addition, although not too obvious, Q does appear to be lower and the size distribution narrower with increasing HDEHP concentration at constant pH. The importance of the pH dependence of the size distribution lies in exploring the mechanism of the aggregation process since smaller aggregates disappear and grow into larger aggregates. From a thermodynamic point of view, however, the larger aggregates are more stable when they exist as reversed micelles with the associated water in the core as “water pools”*. Accordingly, it appears that, as the loading increases in solvent extraction systems, aggregate growth occurs initially by stepwise “polymerization” with the subsequent formation of cyclic aggregates or reversed micelles which, under suitable conditions, grow into microemulsion droplets. On the other hand, it cannot be stated with certainty that there are reversed micelles or microemulsion droplets based solely on the present PCS measurements. The experimental results presented herein are necessary but not sufficient proof of their existence. For this reason, additional experiments are * ‘H-NMR [ 401 and fluorescence polarization [unpublished] results obtained in our laboratory with several HDEHP-based extraction systems have shown the existence of “bulk” water in the organic phase, thereby lending strong evidence supporting the formation of reversed micelles.

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underway to confirm whether the proposed aggregation mechanism cable to hydrometallurgical solvent extraction systems.

is appli-

CONCLUSIONS

Preliminary results reported herein indicate that PCS is capable of providing valuable size information on the metal-extractant aggregates of hydrometallurgical solvent extraction systems. The general model originally proposed by Eicke and co-workers for the aggregation of surfactants in nonpolar solvents appears to be applicable to HDEHP-based extraction systems since the hydrodynamic radius, polydispersity index, and size distribution results are consistent with the proposed mechanism. ACKNOWLEDGEMENTS

Doctors W.J. McDowell and B.A. Moyer are gratefully acknowledged for their enlightening discussions on solvent extraction as well as the critical comments of the reviewers. In addition, the authors would like to thank Dr A.G. Gaonkar for laboratory assistance. This research was supported by the Office of Basic Energy Sciences, Division of Chemical Sciences, Department of Energy under Grant No DE-FG05435ER13357.

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