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Preparation of Rare-Earth-Metal Oxalate Spherical Particles in Emulsion Liquid Membrane System Using Alkylphosphinic Acid as Cation Carrier Takayuki Hirai,* Norihiko Okamoto, and Isao Komasawa Department of Chemical Science and Engineering, Graduate School of Engineering Science, Osaka University, Machikaneyama-cho 1-3, Toyonaka, Osaka 560-8531, Japan Received April 6, 1998
Oxalate particles of rare-earth-metal elements were prepared by using an emulsion liquid membrane (ELM, water-in-oil-in-water (W/O/W) emulsion) system, consisting of Span 83 (sorbitan sesquioleate) as a surfactant and bis(1,1,3,3-tetramethylbutyl)phosphinic acid as an extractant (cation carrier). Rareearth-metal ions were extracted from the external water phase and stripped into the internal water phase to make oxalate particles. Well-defined spherical oxalate particles, smaller than 0.5 µm in diameter, were obtained for lighter and heavier rare-earth-metals such as Pr, Sm, Eu, Gd, Dy, Y, and Yb, following the formation of primary particles of about 20 nm in size. The present result differs from that obtained using EHPNA (2-ethylhexylphosphonic acid mono-2-ethylhexyl ester) as an extractant, where no submicrometersized spherical particles are obtained for the heavier rare-earth metals such as Dy, Y, and Yb. The smaller extracting ability of alkylphosphinic acid eases the stripping of rare-earth-metal ions by oxalic acid into the internal phase. Effects of the internal water droplet size, the rare-earth-metal ion concentration in the external water phase, and the volume ratio of organic membrane phase to internal water phase of the W/O emulsion (O/A ratio), on the size of spherical particles, were investigated. The control of the particle size was found to be feasible by control of the feed rare-earth-metal concentration and the size of the internal water droplets.
Introduction An emulsion liquid membrane (ELM, water-in-oil-inwater (W/O/W) emulsion) system has been studied for the selective separation of metals, where the metal ions in the external water phase are extracted into the organic membrane phase and then stripped and concentrated into the internal water phase. Recently, the internal water droplets of the ELM system were found to be capable of being used to prepare size-controlled and morphologycontrolled fine particles such as precious-metal particles,1 copper oxalate particles,2,3 rare-earth-metal oxalate particles,4,5 calcium carbonate particles,6,7 and composite strontium-lead oxalate particles.8 Submicrometer-sized particles were obtained, and especially in the case of calcium carbonate, spherical vaterite particles having hollow structure were obtained in the ELM system by freeze-thaw demulsification.7 This indicates that the crystal structure can also be controlled in the internal water droplets, since only calcite rhombs are obtained in homogeneous aqueous solutions. * To whom correspondence should be addressed. Telephone: +816-850-6272. Fax: +81-6-850-6273. E-mail:
[email protected]. (1) Majima, H.; Hirato, T.; Awakura, Y.; Hibi, T. Metall. Trans. B 1991, 22B, 397-404. (2) Yang, M.; Davies, G. A.; Garside, J. Powder Technol. 1991, 65, 235-242. (3) Hirai, T.; Nagaoka, K.; Okamoto, N.; Komasawa, I. J. Chem. Eng. Jpn. 1996, 29, 842-850. (4) Hirai, T.; Okamoto, N.; Komasawa, I. AIChE J. 1998, 44, 197206. (5) Hirai, T.; Okamoto, N.; Komasawa, I. J. Chem. Eng. Jpn. 1998, 31, 474-477. (6) Davey, R. J.; Hirai, T. J. Cryst. Growth 1997, 171, 318-320. (7) Hirai, T.; Hariguchi, S.; Komasawa, I.; Davey, R. J. Langmuir 1997, 13, 6650-6653. (8) Hirai, T.; Kobayashi, J.; Komasawa, I. J. Chem. Eng. Jpn. 1998, 31, in press.
Oxalate particles of rare-earth-metal elements are important materials in such industrial applications as the precursors of rare-earth-metal oxides, since oxalates give oxides readily by calcination. As for yttrium, Doyle et al. reported on oxalate precipitation from Y-loaded organic solution.9-13 Yttrium oxalate particles were obtained from an organic kerosene solution containing Y-loaded D2EHPA (bis(2-ethylhexyl)phosphoric acid) or neodecanoic acid by contact with an aqueous solution containing dimethyloxalate or oxalic acid. Span 20 (sorbitan monolaurate), Span 60 (sorbitan monostearate), or Tween 80 (polyoxyethylene-20-sorbitan monooleate) were used as surfactants to form a water-in-oil (W/O) or oil-in-water (O/W) emulsion. This was done by mechanical stirring and ultrasonic agitation, where one is contacting the organic phase with the aqueous phase and aiming to control the particle size. However, the control of size and morphology of the particles was found to be fairly difficult, probably because precipitation started before formation of stable emulsion and water droplets. Thus, in most cases, (9) Yoon, J. H.; Doyle, F. M. Precipitation of Rare-Earth Powders from Aqueous Solutions and Emulsions. In Innovations in Materials Processing Using Aqueous, Colloid, and Surface Chemistry; Doyle, F. M., Raghavan, S., Somasundaran, P., Eds.; TMS: Warrendale, PA, 1988; pp 195-211. (10) Yoon, J. H.; Doyle, F. M. Preparation of Lanthanide Oxalate Powders Using Carboxylate-Based Emulsions. In Light Metals 1990; Bickert, C. M., Ed.; TMS: Warrendale, PA, 1990; pp 991-997. (11) Lee, J.-C.; Doyle, F. M. Precipitation of Yttrium Oxalate from Di-2-ethylhexyl Phosphoric Acid Solution. In Rare Earths: Resources, Science, Technology and Applications; Bautista, R. G., Jackson, N., Eds.; TMS, Warrendale, PA, 1991; pp 139-150. (12) Doyle, F. M. Hydrometallurgy 1992, 29, 527-545. (13) Doyle, F. M.; Choi, H.; Antico, E.; Valiente, M.; Lee, J.-C. Influence of Organic Phase Speciation on the Characteristics of Yttria Precursor Powders Precipitated from Yttrium-Loaded D2EHPA Solutions. In Processing Materials for Properties; Henein H., Oki, T., Eds.; TMS, Warrendale, PA, 1993; pp 545-548.
10.1021/la9803732 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/22/1998
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the particles formed had a characteristic lath or elongated shape and developed to several micrometers in size. Submicrometer-sized (about 0.2 µm in size), almost spherical yttrium oxalate particles were obtained only in the case using Span 60 to make the W/O emulsion.9,10 More precise control of the particle size is expected by using the ELM system, where the metal ions are transported into the considerably stable internal water droplets to make a precipitate. In the present study, preparation of size-controlled and morphology-controlled oxalate particles of rare-earth-metals was investigated using ELM system with alkylphosphinic acid as cation carrier. Experimental Section Bis(1,1,3,3-tetramethylbutyl)phosphinic acid (DTMBPA, hereafter), supplied by Nippon Chemical Industrial Co. Ltd., Tokyo, was used as the extractant, and sorbitan sesquioleate (Span 83, supplied by Tokyo Kasei Kogyo Co., Ltd., Tokyo) was used as the surfactant. EHPNA (2-ethylhexylphosphonic acid mono-2-ethylhexyl ester) and D2EHPA (bis(2-ethylhexyl)phosphoric acid) were supplied as PC-88A and DP-8R, respectively, by Daihachi Chemical Industry Co. Ltd., Osaka. VA-10, tertiary aliphatic monocarboxylic acid (2-methyl-2-ethyl heptanoic acid) was supplied by Shell Chemical Co. Rare-earth-metal nitrates (Ln(NO3)3), oxalic acid, and naphthenic acid (NA, hereafter) were supplied by Wako Pure Chemical Industries, Ltd. The internal water phase for the emulsion (0.1 mol/L oxalic acid) and the organic membrane phase (kerosene containing 0.5 mol/L extractant and 5 wt % Span 83) were mixed at a volume ratio of 1:1 or 2:1 (O/A ratio ) 1 or 0.5) and were emulsified mechanically by use of a homogenizer (12 000 rpm). The resulting W/O emulsion (10 mL) was added to an external water phase (50 mL of 4 mmol/L Ln(NO3)3 aqueous solution) and was stirred vigorously by a magnetic stirrer to form a W/O/W emulsion. CH3COONa (0.02 mol/L) was added to the external solution for the cases using VA-10 and NA to maintain the external phase pH higher than 4. The size of the emulsion drops dispersed in the external water phase was less than 2 mm. The W/O emulsion was then separated from the external solution and demulsified by adding about 50 mL of ethylene glycol or acetone. The particles, formed in the water droplets, were separated by centrifuge and were washed with acetone. The size of the internal water droplets and the particles obtained were measured by a laser scattering particle-size distribution analyzer (Horiba LA910W). The particles were also characterized with a scanning electron microscope (SEM, Hitachi S-5000) and an X-ray diffractometer (XRD, Philips PW-3050). To determine the Ln concentration in each phase, the separated W/O emulsion was demulsified electrically and the organic membrane phase was then stripped with 0.1 mol/L HNO3. The Ln concentration of the resulting aqueous solution and of the external aqueous solution was determined with an inductively coupled argon plasma atomic emission spectrometer (ICP-AES, Nippon Jarrell-Ash ICAP-575 Mark II). The Ln concentration in the internal phase was then estimated by mass balance.
Results and Discussion Extractability of Rare-Earth Metals. Figure 1 shows the relationship between aqueous phase equilibrium pH and distribution ratio D of the rare-earth-metal elements, defined as the ratio of metal concentration in the organic phase and that in the aqueous phase, and determined using the conventional organic/aqueous twophase systems, where extractant concentration is 0.5 mol/L and feed rare-earth-metal concentration is 5 mmol/L. The plots give straight lines with slope ) 2 for DTMBPA, while slope ) 3 is obtained for D2EHPA and EHPNA. This suggests that the positive charges of Ln3+ are compensated by two anionic species of DTMBPA and one NO3- anion, to form the extractable complex. A straight line of slope 2 is obtained when D [H+]2 is plotted vs [(RH)2] (upper
Figure 1. Relationship between aqueous phase equilibrium pH and distribution ratio of rare-earth-metal elements, D, measured in organic/aqueous two-phase systems where the extractant concentration is 0.5 mol/L and the feed rare-earthmetal concentration is 5 mmol/L.
line denotes a species in the organic phase), indicating that the extraction equilibrium formulation is shown as
Ln3+ + 2(RH)2 + NO3- f Ln(NO3)R2(RH)2 + 2H+ (1) where RH denotes the extractant molecule, while that with D2EHPA, EHPNA, and another alkylphosphinic extractant, PIA-226 (bis(2-ethylhexyl)phosphinic acid),14 is shown as
Ln3+ + 3(RH)2 f LnR3(RH)3 + 3H+
(2)
This relationship has been reported previously.15 Size and Morphology of Particles. Following the extraction of the Ln ions from the external water phase into the internal water phase, the following internal water phase reaction occurs and rare-earth-metal oxalate particles are precipitated.
2Ln3+ + 3H2C2O4 + nH2O f Ln2(C2O4)3‚nH2O + 6H+ (3) SEM images of typical oxalate particles for Pr, Sm, Eu, Gd, Dy, Y, and Yb obtained at an O/A ratio ) 1 and initial rare-earth-metal concentration in the external phase [Ln]0 ) 4 mmol/L are shown in Figure 2. Well-defined spherical particles smaller than 0.5 µm in diameter (mean diameter for yttrium oxalate: 0.40 µm) were obtained for these rareearth-metals. In homogeneous aqueous solutions under similar concentration conditions, however, oxalate particles obtained were not spherical but were tabular and elongated, and the particle size was greater than 1 µm, as shown in the previous work.4 The ELM system is therefore effective in controlling both the particle size and the morphology. The obtained particles showed almost no characteristic XRD peaks. In the ELM system using EHPNA as an extractant,4 submicrometer-sized spherical particles were obtained for Ce, Pr, Nd, Sm, and Gd; however, tabular or elongated (not spherical) micrometer-sized oxalate particles were instead formed for the heavier rare-earth metals such as Dy, Y, and Yb. More obviously, in the case using D2EHPA, (14) Hino, A.; Hirai, T.; Komasawa, I. J. Chem. Eng. Jpn. 1996, 29, 1041-1044. (15) Yuan, C.; Ye, W.; Ma, G.; Wang, G.; Long, H.; Xie, J.; Qin, X.; Zhou, Y. Sci. Sin. B 1982, 25, 7-20.
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Figure 2. Scanning electron micrographs of oxalate particles for (a) Pr, (b) Sm, (c) Eu, (d) Gd, (e) Dy, (f) Y, and (g) Yb prepared in the ELM system containing DTMBPA, with an O/A ratio (volume ratio of organic membrane phase to internal water phase) ) 1, [Ln]0 ) 4 mmol/L, and t ) 2 h.
spherical particles were obtained for Ce, Pr, and Nd, but nonspherical particles were found for Sm. These results can be explained with the extractability of these extrac-
tants for rare-earth-metal elements. The order of extractability of the extractants is D2EHPA (phosphoric acid) > EHPNA (phosphonic acid) > DTMBPA (phosphinic
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Figure 4. Time-course variation for the mole fractions of Y ion in the external water phase, organic membrane phase, and internal water phase in the ELM system containing DTMBPA. [Y]0 ) 4 mmol/L and O/A ratio ) 1. The mole fraction in the initial external water phase is set as 100%.
Figure 3. Scanning electron micrographs for yttrium oxide particles obtained by calcination of yttrium oxalate particles for 6 h at (a) 823 K and (b) 1073 K.
acid), and the heavier rare-earth metals are more extractable than the lighter rare-earth metals for all extractants, as shown in Figure 1. Thus, the heavier elements are not stripped effectively from the organic membrane phase with oxalic acid, when D2EHPA or EHPNA is employed. The particles obtained for Sm with D2EHPA and for Dy and Y with EHPNA were likely to be formed in the external water phase following possible breakage of the emulsion drop during the long stirring time, and their shape became tabular or elongated. In the present case using DTMBPA, the lower extractability of this extractant for the elements compared with D2EHPA and EHPNA enables the effective stripping of heavier rareearth-metal ions with oxalic acid and thus the formation of spherical particles in the internal water droplets. Thus, spherical oxalate particles of almost all rare-earth-metals are feasible in the ELM systems employing an appropriate extractant, EHPNA or DTMBPA. The spherical yttrium oxalate particles obtained in the present ELM system were calcined and then characterized by SEM and XRD. Calcination at 823 K brings about some contraction of the particles, as shown in Figure 3a, probably caused by CO2 elimination. At 1073 K, the surface of particles become rough, keeping their size smaller than 0.3 µm as shown in Figure 3b. Formation of Y2O3 was observed in XRD analysis by these calcination, though the peak intensity was low. Mechanism of Particle Formation. Figure 4 shows the time-course variation in the mole fractions of Y ions in the external water phase, organic membrane phase, and internal water phase in the ELM system. The transport of Y ions into the internal phase is completed in 30 min, while about 20% of Y ions are retained in the organic phase. The transport rate in the present ELM
system is smaller than that in the system using EHPNA,4 and it is comparable to that in the system using VA-10.5 Figure 5 shows the SEM images for the yttrium oxalate particles, obtained at different reaction stirring times, t. At t ) 15 min, mainly smaller particles of about 20 nm in size are observed, and are considered to be formed in the internal phase immediately after the transport of Y ions. These primary particles then decrease in number, with increasing reaction time, and almost disappear following 2 h of stirring, as shown in parts b (t ) 30 min) and c of Figure 5 (t ) 1 h), and Figure 2f (t ) 2 h). This shows that the primary particles slowly aggregate (or possibly via Ostwald ripening) together to form final submicrometer-sized spherical particles in each water droplets. The shape of the internal water droplets is likely to influence the morphology of the formed particles. The size of the spherical particles is, however, smaller than the size of the water droplets (mean diameter: about 2.30 µm at O/A ratio ) 1 and 3.91 µm at O/A ratio ) 0.5). Control of Particle Size. In the previous study using EHPNA,4 the size of rare-earth-metal oxalate particles obtained was almost independent of the O/A ratio of W/O emulsion at about [Ln]0 > 2 mmol/L. Under this condition, the quantity of oxalate formed in an internal water droplet is governed by the quantity of oxalic acid fed into the droplet, since all of the oxalic acid in a droplet located near the W/O emulsion/external phase interface is consumed for oxalate precipitation. Similar result is expected in the present case where [Y]0 ) 4 mmol/L. Reflecting a slight difference in the droplet size between the cases at O/A ratios ) 1 and 0.5, fairly greater particles (mean diameter: 0.51 µm) are obtained at O/A ratio ) 0.5, as shown in Figures 6a and 7a. The particle-size distribution can be expected from the water droplet size distribution, as in the previous study.4 That is, the histogram of dropletsize distribution gives the quantity of oxalic acid and thus the possible quantity of yttrium oxalate in a water droplet for each diameter range, which enables calculation of the particle size by using the density of the solid yttrium oxalate. Some difference between the estimated and observed particle-size distributions is seen, as shown by Figure 7a. Smaller particles formed without consuming all of the oxalic acid in the water droplets may affect the measurement of particle-size distribution. Change in the quantity of oxalic acid in a water droplet is expected to bring about a change in the particle size. Thus, the quantity of oxalic acid in each water droplet was increased by increasing the internal water droplet size to 6.54 µm (mean diameter), by decreasing the Span
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Figure 5. Scanning electron micrographs of yttrium oxalate particles prepared in the ELM system containing DTMBPA with an O/A ratio ) 1, [Y]0 ) 4 mmol/L, and different reaction times: t ) (a) 15 min, (b) 30 min, and (c) 1 h.
83 concentration to 2 wt % and the agitation speed of the homogenizer to about 2,000 rpm (O/A ratio ) 1). Particles of larger size, shown in Figures 6b and 7b (mean diameter: 1.07 µm), were formed as expected. Another approach for controlling particle size is to reduce [Y]0 to 1 mmol/L. Under such a condition, the particle size is governed by quantity of Y ions distributed into each water droplet. Size distribution of the particles formed at [Y]0 ) 1 mmol/L is thus shifted toward the smaller diameter region, as shown in Figures 6c and 7c, since the quantity of Y ions supplied in one droplet is decreased, compared to the case of [Y]0 ) 4 mmol/L. Thus, the control of particle size is feasible in the present ELM system, in the similar way proposed in the previous study using EHPNA.4 Effect of Extractant on Particle Morphology. In the case using VA-10 as extractant,5 smaller oxalate particles of 20-60 nm in size were mainly obtained for Y
Hirai et al.
Figure 6. Scanning electron micrographs of yttrium oxalate particles prepared in the ELM systems containing DTMBPA (t ) 2 h) with (a) [Span 83] ) 5 wt %, O/A ratio ) 0.5, and [Y]0 ) 4 mmol/L, (b) [Span 83] ) 2 wt %, O/A ratio ) 1, and [Y]0 ) 4 mmol/L, and (c) [Span 83] ) 5 wt %, O/A ratio ) 1, and [Y]0 ) 1 mmol/L.
and Pr, and the aggregation of these particles seemed not to proceed effectively.5 Similar result was obtained with another carboxylic acid extractant, NA. Although the extractability of rare-earth metals with these carboxylic acid extractants is much smaller than that of alkyl phosphorus extractants as shown in Figure 1, no significant difference in the extraction rate for Y was observed between the cases using DTMBPA (Figure 4) and VA-10. Thus, the extraction rate is not a main factor controlling the aggregation of the primary particles. TG-DTA analyses for the oxalate particles were carried out. The particles formed in the ELM system showed a smaller decrease in TG, corresponding to dehydration at around 370 K, than the particles prepared in a homogeneous aqueous solution. Thus, the particles formed in the ELM system contained oxalate of lower hydration number. However, after washing with water, they showed
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boxylic acid extractants are employed. In the case using VA-10,5 the amount of spherical aggregates increased with decreasing Span 83 concentration. This suggests that some interaction between the primary particles and carboxylic acid/Span 83 at the organic membrane phase/ internal water phase interface prevented the primary particles from aggregation in the internal water droplets. A fairly weaker interaction between alkylphosphonic/ phosphinic acid and primary particles probably allows the small particles to aggregate. Thus, the particle morphology can be controlled by using appropriate extractant/surfactant. In the case using Span 20 instead of Span 83, similar particles were obtained. In the case where Span 85 was employed, a stable W/O/W emulsion was not obtained. Another possible interpretation for the different morphology is the difference in ion concentration in the internal phase. Since carboxylic acid is a weaker extractant, the stripping of Ln3+ from organic phase proceeds more easily, until pHeq approaches 4 as observed from Figure 1, than in the case using phosphorus extractants. Proton concentration and, thus, oxalate anion concentration in the internal droplets are, therefore, much reduced in this case. The reduced ion concentration may suppress the coagulation of primary particles in the internal water droplets. The effect of ion concentration should be investigated further, but in the present system, addition of other salts brought about change in chemical component of particles or decrease in stability of W/O/W emulsion.
Figure 7. Size distributions for internal water droplets and yttrium oxalate particles prepared in the ELM systems containing DTMBPA (t ) 2 h) under different conditions: (a) O/A ratio ) 1 and 0.5, (b) [Span 83] ) 5 and 2 wt %, and (c) [Y]0 ) 4 and 1 mmol/L. Comparison of estimated data with observed data. [Span 83] ) 5 wt % for parts a and c; [Y]0 ) 4 mmol/L for parts a and b.
a TG curve similar to that obtained for oxalate formed in a homogeneous aqueous solution, and they aggregated to give lath-shaped particles, which showed characteristic XRD peaks of Y2(C2O4)3‚10H2O and Pr2(C2O4)3‚10H2O. Thus, a possible interpretation for the formation of spherical oxalate particles in the ELM system is that they contain crystalline or amorphous oxalate of lower hydration number, which tend not to form lath-shaped crystals. However, this interpretation does not explain why the spherical oxalate particles are not obtained when car-
Conclusion Rare-earth-metal oxalate particles were prepared by using an emulsion liquid membrane (ELM, W/O/W emulsion) system, consisting of Span 83 (sorbitan sesquioleate) as a surfactant and bis(1,1,3,3-tetramethylbutyl)phosphinic acid (DTMBPA) as a cation carrier (extractant). Well-defined spherical oxalate particles of smaller than 0.5 µm in diameter were obtained, for lighter and heavier elements such as Pr, Sm, Eu, Gd, Dy, Y, and Yb, via the formation of primary particles of about 20 nm in size, followed by aggregation. Submicrometer-sized Y2O3 particles were obtained by calcination of yttrium oxalate. The present result differs from that obtained using EHPNA (2-ethylhexylphosphonic acid mono-2-ethylhexyl ester) as an extractant, where no submicrometer-sized spherical particles are obtained for the heavier rare-earth metals such as Dy, Y, and Yb. The smaller extracting ability of alkylphosphinic acid eases the stripping of rare-earthmetal ions by oxalic acid in the internal phase. The control of the particle size is shown to be feasible by control of the feed rare-earth-metal concentration and the size of the internal water droplets. Acknowledgment. The authors are grateful to the Division of Chemical Engineering, Department of Chemical Science and Engineering, Osaka University, for scientific support with respect to the gas-hydrate analyzing system (GHAS) constructed by use of a supplementary budget of 1995 and for the financial support through Grant-in-Aid for Scientific Research (No. 09650829) from the Ministry of Education, Science, Sports, and Culture, Japan. LA9803732
