ANALYTICAL SCIENCES FEBRUARY 2011, VOL. 27 171

Introduction

Liquid–liquid extraction (solvent extraction) is important in analytical chemistry as a method for collecting a target component in an aqueous solution by using an organic solvent immiscible with water. This method has the advantages of large extraction capacity, high processing speed, and high selectivity. These merits are favorable not only for chemical analysis but also for recovering valuable resources. Recently, the availability of liquid–liquid extraction is re-acknowledged and has moved into the limelight with the recovery and recycling of rare metals from industrial wastewaters and processing solutions.1–5 As liquid–liquid extraction apparatuses of industrial scale, mixer-settler,6–12 centrifugal extractor,13–16 spray extraction column,17–20 and pulsed or reciprocating-plate plug-flow extraction column21–32 are known.

In order to achieve high performance liquid–liquid extraction, it is favorable to mix aqueous and organic phases into an emulsion continually. Actually, only an apparatus that enables the two liquid phases to keep an emulsified state for a long time by using a continual mechanical force (stirring, shaking, centrifuging) can make the liquid–liquid system reach a condition of distribution equilibrium. The most popular liquid–liquid extraction apparatus that actualizes such an equilibrium condition is a mixer-settler. The two liquid phases are stirred and mixed by using an impeller in a mixer part, and then separated by gravitational settling in a settler part. Meanwhile, a centrifugal extractor that utilizes centrifugal force for the phase separation does not need to wait for the gravity settling, which means more rapid processing than mixer-settler

with a downsized apparatus. However, the centrifugal extractor has a disadvantage of being complicated in its structure, and thus, it needs high cost compared with a mixer-settler that has a relatively simple structure.

On the other hand, a spray extraction column and a pulsed or reciprocating-plate plug-flow extraction column are not intended to reach an equilibrium condition in the distribution of a target component between aqueous and organic phases. Thus, these apparatuses do not make the two liquid phases into an emulsion, or rather, they avoid the emulsion formation which makes it difficult to coalesce out. Even though the extractability of a target component with such apparatuses that cannot actualize an equilibrium condition is usually insufficient, such apparatuses are chosen in the case of needing to make a quick extraction treatment, e.g., in nuclear fuel reprocessing where long-time radiation from radioactive nuclei on solvent and extractant is unfavorable.

A new apparatus for liquid–liquid extraction, named “emulsion flow” extractor, is introduced in the present study. This apparatus has two prominent features. One is high emulsification ability comparable to that available by stirring with an impeller. In the emulsion flow extractor, a flow of a white cloudy state fluid (emulsion), referred to as “emulsion flow”, is generated by spraying micrometer-sized droplets of an aqueous phase from a head that is set inside a column part into an organic phase loaded in the apparatus. The emulsion condition obtained in the emulsion flow extractor is equivalent to that achieved in a mixer-settler. That is to say, the emulsion flow extractor has a mechanism to perform efficient liquid–liquid extraction comparable to that of a mixer-settler by only solution sending without continually adding mechanical forces (stirring, shaking, centrifuging). This manner is similar to a method using a column filled with an ion-exchange resin or an adsorbent. Also, no high-pressure pump is necessary for solution sending in the

2011 © The Japan Society for Analytical Chemistry

† To whom correspondence should be addressed.E-mail: yanase.nobuyuki@jaea.go.jp

New Apparatus for Liquid–Liquid Extraction, “Emulsion Flow” Extractor

Nobuyuki YANASE,*† Hirochika NAGANAWA,* Tetsushi NAGANO,* and Junji NORO**

*NuclearScienceandEngineeringDirectorate,JapanAtomicEnergyAgency,Tokai,Ibaraki319–1195,Japan**ResearchDepartment,NISSANARC,LTD.,Yokosuka,Kanagawa237–0061,Japan

A simple and low-cost apparatus for continuous and efficient liquid–liquid extraction, which does not need continual mechanical forces (stirring, shaking, etc.) other than solution sending, has newly been developed. This apparatus, named “emulsion flow” extractor, is composed of a column part where an emulsified state fluid flow (emulsion flow) is generated by spraying micrometer-sized droplets of an aqueous phase into an organic phase and a phase-separating part where the emulsion flow is destabilized by means of a sudden decrease in its vertical liner velocity due to a drastic increase in cross-section area of the emulsion flow passing through. In the present study, the performance of a desktop emulsion flow extractor in the extraction of Yb(III) and U(VI) from aqueous HNO3 solutions into isooctane containing bis(2-ethylhexyl) phosphoric acid (D2EHPA) was evaluated. The mixing efficiency of the emulsion flow extractor was found to be comparable with that of a popular liquid–liquid extractor, mixer-settler. Moreover, the emulsion flow extractor proved to have an overwhelming advantage in terms of phase-separating ability.

(Received November 11, 2010; Accepted December 13, 2010; Published February 10, 2011)

Original Papers

172 ANALYTICAL SCIENCES FEBRUARY 2011, VOL. 27

emulsion flow extractor like spray extraction column. This means markedly low fabrication and running costs of the emulsion flow extractor due to its simplicity compared with conventional extractors that use continual mechanical forces for the two liquid-phase mixing. The other feature is the high performance of phase separation dramatically superior to that of a mixer-settler, like centrifugal extractor that uses centrifugal force for the phase separation. The flow of emulsified state fluid (emulsion flow) generated in a column part disappears promptly by itself when it reaches a phase-separating part. In the phase-separating part, the emulsion flow is destabilized by means of a sudden decrease in its vertical linear velocity due to a drastic increase in cross-section area of the emulsion flow passing through. Consequently, the emulsified state is rapidly separated into the two liquid phases as before. That is, the emulsion is forced to disappear instantly by such a mechanism in the phase-separating part without waiting for gravity settling. This means that the liquid–liquid extraction procedure with the emulsion flow extractor is performed more rapidly than with a mixer-settler. In this connection, the size of the emulsion flow extractor, which is equivalent to a mixer-settler in performance (processing capacity), is much smaller than that of the mixer-settler. In the present study, the extraction of Yb(III) and U(VI) from aqueous HNO3 solutions into isooctane containing bis(2-ethylhexyl) phosphoric acid (D2EHPA) as an extractant was carried out with the emulsion flow extractor to evaluate its performance.

Experimental

Outlineof“emulsion flow”continuous liquid–liquidextractionapparatusofsinglecurrenttype

Figure 1 shows the outline of an “emulsion flow” continuous liquid–liquid extraction apparatus of a single current type.33 A desktop apparatus filled with aqueous and organic phases, i.e., in a standby condition, is also shown in Fig. 1. This apparatus

is composed of two parts; a column part and a phase-separating one. In the column part, micrometer-sized droplets of an aqueous phase are generated in an organic phase to create a flow of an emulsified state fluid (referred to as “emulsion flow”) that is a fine mixture of the aqueous and organic phases as is seen in Fig. 2; the aqueous phase is sent into the organic phase by a pump through a head having a number of micrometer-sized holes. Meanwhile, in the phase-separating part, the emulsion flow is promptly extinguished (see Fig. 2). The vertical linear velocity of the flow of the micrometer-sized water droplets in the organic phase (water-in-oil emulsion) generated in the column part is suddenly slowed down at the entrance to the phase-separating part by a drastic increase in cross-section area of the emulsion flow passing through. The sudden decrease in the vertical linear velocity causes collisions among the micrometer-sized droplets that make them coalesce out (see Fig. 2). Thus, the emulsification of the two liquid phases and their phase-separation occur continuously in the same vessel of the apparatus, which enables a rapid and efficient liquid–liquid extraction.

Structureofthesinglecurrent“emulsionflow”extractorA desktop emulsion flow extractor of a single current type in

Fig. 1 was originally fabricated. The structure of the apparatus is described below. The column part is a circular cylinder made of poly(vinyl chloride) whose diameter and height are 30 and 290 mm, respectively. A head inside the column part is a hollow cylinder made of polypropylene having 10 mm diameter and 50 mm in height with its one end being closed. The sidewall of the head is provided with 24 holes of 2 mm diameter and is covered with a teflon sheet having mesh with 70 μm pitch. The phase-separating part is like a box whose upper side coupled to the bottom of the column part is open, i.e., there is nothing between the column part and the phase-separating part. The height, width and depth of the phase-separating part are 90, 100 and 100 mm, respectively. The total volume inside the apparatus is about 1.1 L.

Fig. 1　Outline of “emulsion flow” continuous liquid–liquid extraction apparatus of a single current type and a desktop apparatus filled with aqueous and organic phases, i.e., in a standby condition.

ANALYTICAL SCIENCES FEBRUARY 2011, VOL. 27 173

An organic phase of 0.05 L is set in the column part. An aqueous phase of processing object is continuously introduced into the column part through the head by using a pump (gear pump, EFNIC Co., Ltd., GPE-PUA-2L) whose solution sending speed can be controlled from 0 to 2 L min–1. The phase-separating part is wholly filled with the aqueous phase after processing and has a drain outlet at the bottom.

Liquid–liquidextractionexperimentsA liquid–liquid extraction system with bis(2-ethylhexyl)

phosphoric acid (D2EHPA), which is a popular extractant for rare-earth metals and uranium,34–44 was chosen so as to evaluate the performance of the desktop emulsion flow extractor described above. The extraction of Yb(III) and U(VI) from aqueous HNO3 solutions into isooctane containing D2EHPA was carried out at room temperature (298 K). All reagents used in the extraction experiments were of commercially available analytical grade. The extractant D2EHPA (purity more than 95%) was purchased from Wako Pure Chemical Industries, Ltd. and used without further purification. The aqueous phase is an aqueous HNO3 solution adjusted at pH 2 containing 6 × 10–6 M Yb(III) or 4 × 10–8 M U(VI) as a metal ion. The organic phase is an isooctane solution containing 0.01 M D2EHPA. The volume of the organic phase confined in the column part of the apparatus without circulation was 0.05 L. The feed rate of the aqueous phase (processing object) was 0.50 or 0.22 L min–1. The total volume of the aqueous phase treated with the apparatus in a single experiment was 10 L. A portion of the aqueous phase drained after processing was sampled every 2 L. The concentration of Yb(III) or U(VI) in the drainage was measured by inductively-coupled plasma mass spectrometry (ICP-MS, Hewlett Packard HP4500). On the basis of the data thus obtained, the extractability of these metal ions was calculated by the following equation:

Extractability (%) = ([M]init – [M]aq)/[M]init × 100

where [M]init and [M]aq denote the concentrations of Yb(III) and

U(VI) initially added and those remaining in the aqueous phase (drainage), respectively.

The experimental results obtained by using the emulsion flow extractor were compared with those obtained by using a commercially available mixer-settler. The mixer-settler equipped with a pump (metering pump, IWAKI Co., Ltd., Hicera Pump V-05), whose maximum solution sending speed is 0.051 L min–1, was purchased from Tokyo Rikakikai Co., Ltd. (EYELA). The height, width and depth of this box-shaped apparatus are 70, 225 and 70 mm, respectively. The total volume inside the apparatus is about 1.1 L.

Results and Discussion

Generationof“emulsionflow”anditsdisappearanceAs is seen in Fig. 2, “emulsion flow” is generated in the

column part of the desktop emulsion flow extractor during the liquid–liquid extraction experiment described above. A fine mixture of aqueous and organic phases as an emulsion is produced by sending the aqueous phase into the organic phase, which means an equally efficient mixing ability to that of stirring or shaking. In addition, it is seen that the emulsion is completely extinguished at the entrance of the phase-separating part. The letters of “EXTRACTION” affixed in the backside of the phase-separating part can clearly be seen through the unclouded aqueous phase. Thus, the emulsion is rapidly separated into the two liquid phases without waiting for gravitational settling. The formation of an emulsion (a fine mixture of the two liquid phases) in the column part and successively occurring rapid phase separation in the phase-separating part in a single vessel of the apparatus having no partition give assurance a speedy and efficient liquid–liquid extraction.

Liquid–liquidextractionofYb(III)andU(VI)The extraction experiments were carried out with the single

current emulsion flow extractor shown in Fig. 1. The feed rate

Fig. 2　Appearance and disappearance of emulsion flow with an illustration showing emulsion flow generation and phase separation mechanisms.

174 ANALYTICAL SCIENCES FEBRUARY 2011, VOL. 27

of the aqueous phase was fixed at 0.50 L min–1. The total volume of the aqueous phase processed by the apparatus was 10 L. Figure 3A shows the extractability of Yb(III) or U(VI) from an aqueous nitric acid solution (pH 2) into isooctane containing 0.01 M D2EHPA obtained by measuring the metal concentration in the drainage sampled every 2 L as a function of the feed volume of the aqueous phase that has been totally processed. About 90% extractability was constantly obtained in the extraction of both Yb(III) and U(VI). In relation to this, batch tests with shaking the two liquid phases of the same volume in a stoppered test tube for 5 min by a vortex-mixer showed quantitative extraction of both Yb(III) and U(VI).

EffectoffeedrateThe effect of feed rate of the aqueous phase on the extractability

of Yb(III) was also examined. Although the feed rate decreased by half (from 0.50 to 0.22 L min–1), the extractability of about 90% was kept constant, as is seen in Fig. 3B. Thus, the performance of the emulsion flow extractor is quite stable against such a feed rate change.

In addition, time taken for restarting the emulsion flow extractor was also measured; once the apparatus was completely stopped and then restarted. It took about 5 s to reach a steady state of its operation forming an emulsion.

Comparisonwithmixer-settlerThe performance of the emulsion flow extractor was compared

with that of a commercially available mixer-settler. Figure 4A shows the mixer-settler used in the comparative experiments that is filled with aqueous and organic phases (standby condition). The compositions of the aqueous and organic phases in the experiments with the mixer-settler were the same as those with the emulsion flow extractor.

The mixer-settler is composed of a mixer part and a settler part. An aqueous phase and an organic phase are mixed with an impeller in the mixer part, and then separated by gravitational settling in the settler part. In the extraction experiments, both

Fig. 3　Extraction performance of emulsion flow extractor: (A) the extractability (%) of Yb(III) and U(VI) at 0.50 L min–1 feed rate and (B) the extractability (%) of Yb(III) at 0.50 and 0.22 L min–1 feed rates as functions of feed volume of the total aqueous phase that has been sent.

Fig. 4　Comparison in mixing and phase-separating abilities between emulsion flow extractor and mixer-settler: (A) mixer-settler in standby condition, (B) mixer-settler in operation at 0.04 L min–1 feed rate and 1200 rpm impeller rotation speed, and (C) emulsion flow extractor in operation at 0.50 L min–1 feed rate.

ANALYTICAL SCIENCES FEBRUARY 2011, VOL. 27 175

the aqueous and the organic phases were sent to the bottom beneath the impeller with the same feed rate. The aqueous phase was continuously introduced into the mixer part, while the organic phase was circulated between the mixer part and the settler part in order to keep the volume balance of the two liquid phases in these parts. The feed rate of the aqueous and organic phases was fixed to be 0.04 L min–1, about 80% of the maximum pump speed. The mixing and phase separation conditions were  checked at various impeller rotation speeds from 400 to 1200 rpm (revolutions per minute). For instance, at 600 rpm, the mixing state of the two liquid phases in the mixer part seems insufficient, while the aqueous phase in the settler part appears not so cloudy. In contrast, at 1200 rpm (about 80% of the maximum rotation speed) the two liquid-phase mixing in the mixer part seems sufficient, as is seen in Fig. 4B. The mixing state in the mixer-settler at 1200 rpm is similar to that in the emulsion flow extractor (see Figs. 4B and 4C). However, at 1200 rpm, the aqueous phase in the settler part is too cloudy to drain, which makes the liquid–liquid extraction actually impossible; the phase-separating ability of the emulsion flow extractor is far superior to that of the mixer-settler, as is clearly seen by comparing Figs. 4B and 4C. Therefore, the extraction experiment with the mixer-settler was performed at 600 rpm impeller rotation speed with 0.04 L per minute feed rate. The total volume of the aqueous phase processed by the mixer-settler was 10 L.

Figure 5 shows the extractability of Yb(III) as a function of the feed volume of the aqueous phase, comparing the emulsion flow extractor with the mixer-settler. The size (total volume) of the emulsion flow extractor is almost the same as that of the mixer-settler, i.e., about 1.1 L for both. The extraction performance of the emulsion flow extractor is always higher than that of the mixer-settler whose impeller rotation speed is 600 rpm, while the processing speed of the emulsion flow extractor (0.50 L min–1) is about 13 times higher than that of the mixer-settler (0.04 L min–1). Moreover, with the mixer-settler, even at 600 rpm impeller rotation speed, an oil film appears on the surface of the drained aqueous phase after settling for a long time, e.g., 10 h. Some droplets produced near an impeller may be too small to disappear in a short time, and thus, kept floating

in the aqueous phase for a long time. In contrast, no oil film appears on the surface of the drainage processed with the emulsion flow extractor after settling for 10 h. That may be because the too small droplets such as those produced in the mixer settler do not form in the emulsion flow mechanism.

PluggingofheadholeswithparticulatecomponentsA single current type emulsion flow extractor has a fatal

defect for practical use. When an aqueous solution (processing target) contains some particulate components, a head of the apparatus is easily clogged. A liquid–liquid extraction experiment under the same conditions as those described above except containing a lot of Al2O3 particles of 20 – 25 μm diameters was made with using the apparatus. As the result, the head was immediately clogged and the apparatus became inoperative. Therefore, it is necessary for the practical use of the apparatus to remove such particles in advance by using a filter or membrane, or to discover some mechanism to trap them within the apparatus.

Conclusions

A single current “emulsion flow” extractor that performs continuous and efficient liquid–liquid extraction by only solution sending was introduced in the present study. The performance of a desktop emulsion flow extractor originally fabricated was evaluated in the extraction of Yb(III) and U(VI) from aqueous HNO3 solutions (pH 2) into isooctane containing 0.01 M D2EHPA and compared with that of a commercially available mixer-settler. The extractability of these metal ions with the emulsion flow extractor was constantly about 90%. The extraction performance was not much affected by a change in the feed rate of the aqueous phase (processing target). The efficiency of mixing two liquid phases in the column part of the emulsion flow extractor was found to be comparable with that in the mixer part of the mixer-settler. That is, the emulsion flow extractor performs efficient liquid–liquid extraction comparable to the mixer-settler by only solution sending without any continual mechanical forces such as stirring or shaking. In other words, the initial and running costs of the emulsion flow extractor are markedly low due to its simplicity in structure compared with those of conventional extractors using a continual mechanical force for the two liquid-phase mixing. Moreover, the ability of separating two liquid phases in the phase-separating part of the emulsion flow extractor proved to be far superior to that in the gravitational settler part of the mixer-settler. It was confirmed that the emulsion flow extractor performed much speedier processing than the mixer settler. On the other hand, a  fatal defect of the single current emulsion flow extractor for practical use was pointed out. The head holes (meshes) in the apparatus were easily plugged with particulate components contained in an aqueous phase of the processing object.

Acknowledgements

This work is partially supported by Grants-in-Aid for Scientific Research (Nos. 17510054 and 20560750) from Japan Society for the Promotion of Science (JSPS).

References

1. M. Tanaka, Y. Huang, T. Yahagi, M. K. Hossain, Y. Sato,

Fig. 5　Comparison in extraction performance and processing speed between emulsion flow extractor and mixer-settler of the same size, based on the extractability (%) of Yb(III) at 0.50 L min–1 feed rate in emulsion flow extractor and 0.04 L min–1 feed rate and 600 rpm impeller rotation speed in mixer-settler as functions of feed volume of the total aqueous phase that has been sent.

176 ANALYTICAL SCIENCES FEBRUARY 2011, VOL. 27

and H. Narita, Sep.Purif.Technol., 2008, 62, 97. 2. Y. Huang and M. Tanaka, J. Hazard. Mater., 2009, 164,

1228. 3. T. Nakamura, S. Nishihama, and K. Yoshizuka, Solvent

Extr.Res.Dev.,Jpn., 2007, 14, 105. 4. D. D. Pereira, S. D. F. Rocha, and M. B. Mansur, Sep.

Purif.Technol., 2007, 53, 89. 5. T. Komulainen, P. Pekkala, A. Rantala, and S.-L.

Jamsa-Jounela, Hydrometallurgy, 2006, 81, 52. 6. G. A. Pinto, E. F. Gomes, F. O. Durao, C. M. N. Madureira,

M. M. B. L. Guimaraes, and S. Morais, Hydrometallurgy, 2009, 98, 224.

7. S. A. Ansari, D. R. Prabhu, R. B. Gujar, A. S. Kanekar, B. Rajeswari, M. J. Kulkarni, M. S. Murali, Y. Babu, V. Natarajan, S. Rajeswari, A. Suresh, R. Manivannan, M. P. Antony, T. G. Srinivasan, and V. K. Manchanda, Sep.Purif.Technol., 2009, 66, 118.

8. A. Suresh, N. L. Sreenivasan, R. Selvan, M. P. Antony, T. G. Srinivasan, S. B. Koganti, and P. R. V. Rao, Nucl.Technol., 2009, 167, 333.

9. C. M. Moreno, J. R. Perez-Correa, and A. Otero, Miner.Eng., 2009, 22, 1350.

10. J. C. B. S. Amaral and C. A. Morais, Miner.Eng., 2010, 23, 498.

11. A. Khakpay and H. Abolghasemi, Can. J. Chem. Eng., 2010, 88, 329.

12. M. Napeida, A. H. Asl, J. Safdari, and M. Torab-Mostaedi, Chem.Eng.Res.Des., 2010, 88, 703.

13. D. Serrano-Purroy, P. Baron, B. Christiansen, J.-P. Glatz, C. Madic, R. Malmbeck, and G. Modolo, Sep.Purif.Technol., 2005, 45, 157.

14. J.-Q. Zhu, J. Chen, C.-Y. Li, and W.-Y. Fei, Sep. Purif.Technol., 2007, 56, 237.

15. S. S. Deshmukh, M. J. Sathe, and J. B. Joshi, Ind. Eng.Chem.Res., 2009, 48, 37.

16. B. D. Kadam, J. B. Joshi, S. B. Koganti, and R. N. Patil, Chem.Eng.Res.Des., 2009, 87, 1379.

17. B.-S. Chun, H.-G. Lee, J.-K. Cheon, and G. Wilkinson, KoreanJ.Chem.Eng., 1996, 13, 234.

18. S. Mohanty, Rev.Chem.Eng., 2000, 16, 199.19. R. Egashira and J. Saito, J.Jpn.Pet.Inst., 2007, 50, 218.20. D. Bonam, G. Bhattacharyya, A. Bhowal, and S. Datta, Ind.

Eng.Chem.Res., 2009, 48, 7687.21. N. P. Ghildyal, M. Ramakrishna, B. K. Lonsane, and N. G.

Karanth, ProcessBiochem., 1991, 26, 235.22. K. Gottliebsen, B. Grinbaum, D. Chen, and G. W. Stevens,

Hydrometallurgy, 2000, 58, 203.23. N. Bardin-Monnier, P. Guiraud, and C. Gourdon, Chem.

Eng.Process., 2003, 42, 503.24. C.-H. He, Y.-H. Gao, S.-H. Yang, and D. W. Edwards, J.

LossPrev.ProcessInd., 2004, 17, 195.25. J. M. Bujalski, W. Yang, J. Nikolov, C. B. Solnordal, and

M. P. Schwarz, Chem.Eng.Sci., 2006, 61, 2930.26. A. Stella and H. R. C. Pratt, Ind. Eng. Chem. Res., 2006,

45, 6555.27. B. Grinbaum, SolventExtr.IonExch., 2006, 24, 795.28. S. Retieb, P. Guiraud, G. Angelov, and C. Gourdon, Chem.

Eng.Sci., 2007, 62, 4558.29. M. L. van Delden, G. S. Vos, N. J. M. Kuipers, and A. B. de

Haan, SolventExtr.IonExch., 2007, 25, 639.30. A. B. Jahya, G. W. Stevens, and H. R. C. Pratt, SolventExtr.

IonExch., 2009, 27, 63.31. S. S. S. Ebrahimi, H. Bahmanyar, and J. Safdari, J.Chem.

Eng.Jpn., 2009, 42, 10.32. A. E. Ferreira, S. Agarwal, R. M. Machado, M. L. F.

Gameiro, S. M. C. Santos, M. T. A. Reis, M. R. C. Ismael, M. J. N. Correia, and J. M. R. Carvalho, Hydrometallurgy, 2010, 104, 66.

33. H. Naganawa, N. Yanase, and T. Nagano, Japanese Patent Application, 2007, 136496.

34. K. Ohto, S. Yoshida, K. Yoshizuka, K. Inoue, M. Ohtsuka, M. Goto, and F. Nakashio, Anal.Sci., 1995, 11, 637.

35. J. M. Sanchez, M. Hidalgo, and V. Salvado, Solvent Extr.IonExch., 1999, 17, 455.

36. A. Geist, W. Nitsch, and J. Kim, Chem.Eng.Sci., 1999, 54, 1903.

37. J. Wichterlova and V. Rod, Chem. Eng. Sci., 1999, 54, 4041.

38. L. M. Dai, D. V. Minh, and P. V. Hai, J. Alloys Compd., 2000, 311, 46.

39. E. Kamio and K. Kondo, J.Chem.Eng.Jpn., 2002, 35, 574.40. S. Girgin, N. Acarkan, and A. A. Sirkeci, J. Radioanal.

Nucl.Chem., 2002, 251, 263.41. C. A. Morais and V. S. T. Ciminelli, Hydrometallurgy,

2004, 73, 237.42. H. Naganawa, K. Shimojo, H. Mitamura, Y. Sugo, J. Noro,

and M. Goto, SolventExtr.Res.Dev.,Jpn., 2007, 14, 151.43. A. Kadous, M. A. Didi, and D. Villemin, J. Radioanal.

Nucl.Chem., 2009, 280, 157.44. G. S. Lee, M. Uchikoshi, K. Mimura, and M. Isshiki, Sep.

Purif.Technol., 2009, 67, 79.