Extraction of lanthanides by polysulfone microcapsules containing EHPNA. I. Piercing method

JOURNAL OF RARE EARTHS, Vol. 33, No. 6, Jun. 2015, P. 655

Extraction of lanthanides by polysulfone microcapsules containing EHPNA. I. Piercing method JING Yu (靖 宇)1, WANG Yue (王 月)1, HOU Hailong (侯海龙)1,2, XU Jianhong (徐建鸿)1, WANG Yundong (王运东)1,* (1. The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China; 2. School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, China) Received 9 October 2014; revised 6 April 2015

Abstract: Since the conventional liquid-liquid extraction method suffered from a series of problems such as inefficiency of one stage extraction, vast device occupation and severe emulsification, we adopted microcapsule (MC) technique to change the former liquid-liquid extraction to liquid-solid extraction. Firstly, the piercing method was performed to prepare the empty polysulfone (PSF) microcapsules, which was easy to implement and control. Secondly, the ultrasonic approach was utilized to prepare the functional microcapsules containing 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA). We focused on a key factor of the molar ratio of PSF over 1-Methyl-2-pyrrolidinone (NMP), attaining a loading ratio as high as 7.21 g-EHPNA/g-PSF. Thirdly, we examined the kinetics and thermodynamics of extraction. Kinetic results demonstrated that equilibrium was reached within two hours, with an extraction rate of Sm3+≈Er3+>La3+; Thermodynamic results showed that the extraction of lanthanides complied with the Langmuir law, with an extraction capacity of 0.25–0.30 mmol/g-microcapsule. Fourthly, stripping experiment indicated that three hours were required to accomplish equilibrium for La3+ and Sm3+ while longer hours for Er3+. Finally, seven extraction-stripping cyclic experiments were performed for three mixed elements, the results of which revealed that Sm3+ and Er3+ maintained constantly high extraction amount whilst La3+ leveled off at approximately 50%. This proposed polysulfone microcapsule containing EHPNA is suitable to be applied to extraction and concentration of rare earth metals. Keywords: extraction; microcapsule; rare earths; EHPNA

The separation of lanthanides is extremely difficult to perform due to the similarity in physical and chemical properties[1]. The widely used method to extract rare earths (REs) in large scale by saponated 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA)[2,3] suffers from some drawbacks, such as the extractant loss and high energy consumption. As the height equivalent to a theoretical plate (HETP) is relatively high during the extraction process of REs, the conventional mixer-settler device usually occupies large space[4,5]. If EHPNA can be impregnated into the microcapsules[6], the former liquid-liquid extraction can be transformed to a liquid-solid system which better addresses the abovementioned issues, especially for dilute RE solutions[7]. Moreover, functionalized microcapsule is convenient to be applied in the extraction or ion exchange column devices[8–11], which can expand mass transfer area as well as improve mass transfer coefficient. Hence, more theoretical stages and higher efficiency can be achieved without the repetitive input of energy. Currently, the preparation of microcapsules can be generally categorized into three paradigms[12], namely

chemical method[13,14], physical method (e.g., solvent evaporation[15–17], solvent extraction[18,19], spray drying) and polymerization method (e.g., in situ polymerization, interfacial polymerization). One of the most influential reports of the third methodology was divinylbenzene homopolymeric microcapsules with highly porous membranes by in situ polymerization by Yoshizawa et al.[20]. Using these microcapsules, hydrochloric acid could be sufficiently extracted from aqueous solution. Subsequently, the polymerization method was extensively reported in literatures, however, only a few groups have reported on lanthanide separation. The extraction of lanthanides such as Sm3+, Er3+, Pr3+, Nd3+ and Y3+ was studied by adopting microcapsules containing bis(2-ethylhexyl) phosphoric acid[21]. The separation of La3+ and Ce3+ by using microcapsules was explored via in situ microencapsulation process[22]. The large size microcapsules containing tri-n-octylamine by in situ polymerization in combination of a gel inclusion method were prepared for acetic acid extraction[23]. Recently, the metal extraction dynamics into microcapsules have been investigated[24–26].

Foundation item: Project supported by the National Key Basic Research Program of China (2012CBA01203), the Specialized Research Fund for Doctoral Program of Higher Education of Ministry of Education of China (20130002110018) * Corresponding author: WANG Yundong (E-mail: [email protected]; Tel.: +86-10-62770304) DOI: 10.1016/S1002-0721(14)60467-1

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Inherent drawbacks also exist in the polymerization method. For instance, some polymerization reactions required a few hours with low reaction rate. Additionally, reaction apparatuses were used with high costs and an enormous amount of extractant was lost during the synthetic process. Thus it is reasonable to present an easier and handier method for industrial applications. Solvent evaporation method is easy to control while the surface of microcapsules tends to be usually compact which is adverse to mass transfer. Likewise, the solvent extraction method is also easy to control and can produce porous microcapsules. Polysulfone enjoys the advantages of strong strength, anti-wear, non-toxic, anti-acid or antialkali and anti-heat with the available temperature of –100–150 ºC, which makes it fit for wall material of microcapsules. In previous works[18,19,27], we succeeded in preparing for the polysulfone microcapsules containing 1-octanol for caprolactam extraction by adopting one-step and two-step methods. The former referred to production of functionalized microcapsules in one step by mixing the wall material and the extractant; the latter represented the preparation of empty microcapsules in the first step and the encapsulation with extractant afterwards. Nonetheless, the extraction process was complicated when EHPNA reacts with lanthanides, quite different from the physical dissolution of 1-octanol and caprolactam. Accordingly, the research into the lanthanides extraction by microcapsules is of great significance both academically and practically. The present paper is organized as follows. The experimental section elaborated on the preparation of empty polysulfone microcapsules by employing piercing method. The section 2.1, 2.2 and 2.3 described the detailed the crucial properties of microcapsules. The section 2.4 and 2.5 dealt with kinetic and thermodynamic extraction performance of light (La3+), medium (Sm3+), heavy (Er3+) RE and their mixture, respectively. Cyclic experiments of the extraction-stripping for the functionalized microcapsules were conducted to evaluate the stability of microcapsules in the section 2.6.

1 Experimental

JOURNAL OF RARE EARTHS, Vol. 33, No. 6, Jun. 2015

further purification. 1.2 Preparation of microcapsules containing EHPNA The preparation methods of microcapsules can be divided into two categories: one-step and two-step method. In this paper, microcapsules were prepared by the twostep process: (1) prepare empty microcapsules and (2) load extractant (EHPNA). For the first step, the experimental scheme is illustrated in Fig. 1. Polymer solution was firstly prepared by dissolving the desired amount of PSF in NMP. Then the polymer solution was pumped through a nozzle (SNA-22G-50) by a syringe pump (LSP01-1BH, Longer, China) with a flow rate of 200 μL/min. The outer and inner diameter of the nozzle was 0.73 and 0.41 mm. The polymer drops were solidified in a solidification solution (500 mL ethanol and 1500 mL deionized water). In order to keep a good shape of microcapsules, the addition of ethanol is necessary to reduce the density and surface tension of the solidification solution. The formed microcapsules were kept in the solidification solution for more than one hour, and then filtrated and washed five times by using deionized water. Finally, the empty microcapsules were obtained after being dried under 120 ºC for six hours. For the second step, EHPNA was introduced into the empty microcapsules by using the ultrasound method. In addition, we further performed one-step preparation for microcapsules. The main procedure was as follows: (1) to dissolve the certain amount of EHPNA (extractant) and PSF (wall material) in NMP to obtain the polymer solution; (2) to solidify the microcapsules by utilizing a nozzle (SNA-22G-50). Nevertheless, one-step method failed in the preparation, which will be discussed below. 1.3 Characterization of microcapsules The outer diameter of microcapsules could be obtained through the camera (Cannon, 600D). The concrete procedure is as follows. Microcapsules were initially placed onto a ruler to be photographed. Then the pixel coordinates of the diameter could be measured by aiding software. Finally, the diameter of the microcapsules

1.1 Materials Polysulfone (PSF) (average MW=35,000) was purchased from Aldrich Co., Ltd. N-Methyl-2-pyrrolidone (NMP) (>99.5 %), ethanol (>99.7 %) were purchased from Sinopharm Chemical Regent Beijing Co., Ltd. The extractant, 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA) (>95%) was purchased from Luoyang Aoda Chemical Co., Ltd. LaCl3·7H2O, SmCl3·6H2O and ErCl3·6H2O (>99.99%) were purchased from the Biam Alloys Co., Ltd., Beijing Institute of Aeronautical Materials of AVIC. All the reagents were used without

Fig. 1 Experimental scheme of microcapsule preparation using the piercing method

JING Yu et al., Extraction of lanthanides by polysulfone microcapsules containing EHPNA. I. Piercing method

could be attained by calculating the ratio of the coordinates. The surface and internal structure could be scrutinized by a scanning electric-microscope (SEM) (Hitachi, TM3000). It has to be noted that the microcapsules should be sliced in liquid nitrogen when being watched the internal structure. Weighing method (Mettler, XS205) was adopted to calculate the EHPNA loading ratio. It was necessary to dry the extractant on surface of the microcapsules by oil-absorbing papers and then the microcapsules should be laid in the moving air for total evaporation of surface EHPNA after the loading process. The pore volume and mean pore diameter (based on pore volume) of the polysulfone microcapsules could be gained by using the mercury intrusion porosimetry (Micromeritics, AutoPore IV 9500). The samples had to be dried over 5 h under 110 ºC in order to drive away the water and CO2. 1.4 Extraction and stripping of lanthanides The single LaCl3, SmCl3, ErCl3 solutions and their mixture with the concentration of 345.5, 389.2, 403.4 and 306.5 ppm (Separately, La: 102.3 ppm; Sm: 112 ppm; Er: 92.2 ppm) were obtained by dissolving the LaCl3·7H2O, SmCl3·6H2O and ErCl3·6H2O in hydrochloric acid solutions. Although the stoichiometry of EHPNA and REs is 6:1, the molar ratio of non-saponified EHPNA and REs at 10:1 can contribute to a higher efficiency because of the non-ideality of chemical reaction and the effect of macroscopic mass transfer. Based on previous extensive investigations[28–31], the suitable pH value range of most RE extractions by unsaponified EHPNA is 3.5–4.0. In order to keep consistence of LaCl3, SmCl3 and ErCl3 and take account of the overall extraction performance, we finally adjusted the initial pH value of single light, medium, heavy REs and their mixture solutions to 3.85. Therefore, the ratio of EHPNA:RE ion was set 10 to 1. The pH value of initial aqueous phase was adjusted to 3.85. The pH value was determined by a Metler FiveEasy pH meter with a Sanxin electrode. Extraction kinetics, thermodynamic equilibrium, stripping and extraction-stripping cycles were conducted by the following procedure. Fetch 0.2 g microcapsules containing EHPNA and 20 mL RE solutions (or 2.0 mol/L stripping hydrochloric acid solutions) and place them into a 20 mL isotope bottle. Then perform the extraction (or stripping) in a shaker with 200 r/min revolution at 25 ºC (± 2 ºC). Analyze the samples in different time intervals. The concentrations of La3+, Sm3+ and Er3+ in the aqueous solution were determined by ICP-OES (IRIS Intrepid II XSP). In thermodynamic equilibrium and extraction-stripping cyclic experiment, the microcapsules contacted with the aqueous phase for over 20 h to attain

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equilibrium. It had to be noted that ionic strength should be consistent in the kinetic study. Consequently, the ionic strength was regulated via NaCl. The 0.1 mol/L (H+Na)Cl solution should be prepared. The formula of extraction and stripping amount could be expressed as below. qe=(cin–ce)·V/m (1) where cin and ce indicated the initial and equilibrium concentration of rare earth metal, respectively. V stood for the volume of aqueous phase and m represented the mass of microcapsules.

2 Results and discussion 2.1 Determination of preparing microcapsules The first experiment needs to be implemented to confirm the merits and demerits between one-step and twostep method. Despite the easy process for the former, functionalized microcapsules could not be obtained. The main reason was that when EHPNA and PSF dissolved in NMP, the polymer solution was hydrophobic, while solidification solution composed of water and ethanol was hydrophilic. When the polymer solution got in the solidification solution in small drops, it could not be built whilst microcapsules gradually sat together by stirring. Regarding the latter, the sole PSF was easy to disperse in the solidification solution owing to the suitable density. Thus two-step method was fit for producing the microcapsules containing EHPNA. 2.2 Structural properties of microcapsules The structural characteristics of the empty microcapsules are decisive for the subsequent import of extractant and the performance of extraction. The ratio of PSF in polymer solution plays a vital role in the structure of microcapsules. When the flowing speed of polymer solution remains a certain level, the less wall material of PSF appears in polymer solution, the larger the pore volume of the microcapsules will be, thereby resulting in more EHPNA loading and better extraction performance. The structural properties of different microcapsules and their compositions are listed in Table 1. As can be seen from the table, the pore volume shrunk from 8.36 to 7.01 mL/g with the PSF content rising from 6 wt.% to 10 wt.%, and the maximum EHPNA loading ratio could be larger compared with the previous highest report of 7.39 mL/g[18]. However, the PSF content could not be too low which could give rise to collapse owing to the limited walls (below 4 wt.%). In addition, we could discern from Table 1 that the mean outer diameter of microcapsules was almost consistent with the different contents of wall materials. A typical morphology of empty microcapsule can be seen in Fig. 2. The B2 microcapsule generated by two-

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Table 1 Preparation conditions and structural properties of microcapsules PSF/

NMP/

PSF content in polymer

Microcapsule mean

Pore volume/

Inner mean

Loading ratio/

g

g

solution/wt.%

diameter/mm

(mL/g)

diameter/nm

(g-EHPNA/g-PSF)

B1

1.00

24.00

4

–

–

–

–

B2

1.00

15.67

6

1.99

8.36

3255.9

7.21

B3

1.00

11.50

8

2.00

7.40

1188.4

6.53

B4

1.00

9.00

10

2.00

7.01

649.1

5.67

No.

Fig. 2 Representative morphology of microcapsule B2 (a) Appearance; (b) Cross section; (c) Quarter; (d) Surface

step method enjoyed uniformity, the inside of which was very empty with compact margins. Also the fissures could be seen in the surface. The B2 microcapsule enjoyed better mass transfer compared with other counterparts with small pores on surface. It could also be learned from Table 1 that the volume-based mean pore diameter decreased with the increase of PSF content, from 3256 to 649 nm. The representative pore size distribution of B2 microcapsule could be seen in Fig. 3. It could be discerned that the distribution was relatively extensive from hundreds to thousands of nanometers. There were two peaks, of which the right one corresponded to the big hole indicating the center of the microcapsule whilst the left one implied the margins of the microcapsules. To sum up, the pore volume and volume-based mean diameter of B2 microcapsule were the largest, which demonstrated that the extraction amount and mass transfer could be enhanced with more extractant and larger contact area between EHPNA and RE metals.

2.3 EHPNA loading ratio The import of the EHPNA could be achieved through automatic impregnation and ultrasound, of which the latter required less time and larger loading amount of extractant[18]. So the ultrasound method was adopted to import the EHPNA. Fig. 4 curves the loading ratio of

Fig. 3 Representative pore size distribution of microcapsule B2

JING Yu et al., Extraction of lanthanides by polysulfone microcapsules containing EHPNA. I. Piercing method

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Fig. 4 Loading ratio as the impregnating time with the assistance of ultrasound

EHPNA versus time. It could be seen that the loading ratio peaked after three hours, of which the B2 microcapsule reached 7.2 g-EHPNA/g-PSF, far outperforming the previous literatures[18,22]. Also it could be calculated theoretically that the volume of the B2 microcapsule was 8.36 mL/g, with the EHPNA density of 0.9480 g/mL under 20 ºC. Accordingly, the pore of the microcapsules was almost filled with EHPNA after three hours’ load. 2.4 Extraction performance In order to confirm the extraction time, we need first accomplish the extraction kinetics. Then the thermodynamics should be conducted to guide the design of the microcapsules into the extraction column. 2.4.1 Extraction kinetics Fig. 5(a) illustrates the amount of the extraction versus the time for La3+, Sm3+ and Er3+. It could be observed that the extraction rate of Sm3+ and Er3+ is higher than that of La3+. This was mainly due to the stronger affinity of medium, heavy RE metals and EHPNA. After two hours’ extraction, equilibrium was reached with the extraction efficiencies of Sm3+ and Er3+ reaching 100% while 90% for La3+. Fig. 5(b) is the kinetic results for the microcapsule extraction of three elements. Also we noted that the extraction rate of Sm3+ and Er3+ was higher than that of La3+. After 80 min extraction, equilibrium was reached for Sm3+ and Er3+ while it took 200 min for La3+. Competitive extraction existed between the three elements, where Sm3+ and Er3+ firstly reacted with EHPNA to evolve into complex formation and then diffused towards the inside of the microcapsule. Due to the occupation of Sm3+ and Er3+, there was less chance for La3+ to make contact with EHPNA, leading to a lower extraction rate. Selectivity is a very important factor for evaluating the separation of rare earths. The corresponding selectivities are shown in Fig. 6. As there were almost no REs in aqueous phase when a thorough extraction was reached, we plotted their selectivities for 0–40 min. Due to the sequence of extraction capacity and rate with Er3+≥Sm3+ >La3+, the selectivities of Er/La and Sm/La were higher

Fig. 5 Kinetic extraction of single element (a) and mixed elements (b) of La3+, Sm3+ and Er3+ into microcapsule

Fig. 6 Selectivity of Sm/La, Er/La and Sm/Er in the extraction process

than that of Sm/Er. 2.4.2 Extraction thermodynamics From the results of extraction kinetics, we allowed enough time (10 h) to reach equilibrium for extraction in the thermodynamic experiment. According to common reported practice[25,26] based on thermodynamics, we used mol instead of mg to describe the extraction data. Fig. 7 presentes the experimental results. The equilibrium extraction increased with the rise in the aqueous equilibrium concentration of lanthanides. This was due to larger mass transfer area and stronger driving force of the concentration. When the aqueous equilibrium concentration reached 0.7 mol/m3, the extraction amount leveled off. This could be attributed to the extreme capacity of the extraction. The maximum extraction capacities for

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whilst that of La3+ was low. It could also be seen that this microcapsule was suitable for extraction and concentration of RE metals when the aqueous concentration reached 0.3–1.0 mol/m3. Additionally, the thermodynamic experiments of three mixed elements were completed, the corresponding data of which can be seen in Table 3. The trend was quite different from the single element. This was mainly due to the competition between Sm3+, Er3+and La3+. Even when the aqueous concentration was at an extremely low level, Sm3+ and Er3+ had already reached the maximum amount with small extraction amount for La3+. Fig. 7 Extraction isotherm of single element of La3+, Sm3+ and Er3+ into microcapsule and theoretical line based on Langmuir model

2.5 Stripping performance Stripping is decisive to the yield of RE metals and the regenerability of microcapsules in the extraction process. The experimental results for the single element could be seen from Fig. 8(a). It could be discerned that the stripping rate for La3+was the highest, followed by Sm3+. The stripping rate for Er3+was the slowest. This was mainly because the strong affinity between EHPNA and Er3+. Because of the high concentration of single element (~300 ppm per element), it usually took as long as three hours for the stripping process and the stripping efficiencies for La3+ and Sm3+ could reach 80% whilst 50% for Er3+. Our previous work[30] reported that stripping needed to be performed twice to reach the thorough stripping of Er3+. The stripping results for the three mixed elements could be seen in Fig. 8(b). It could be noted that the stripping rate was boosted a lot due to the low concentration (~100 ppm per element). Hence, the stripping rates for La3+ and Sm3+ were higher with the stripping effi-

Sm3+, Er3+, and La3+ were 3.0×10–4, 2.8×10–4 and 2.7× 10–4 mol/g, respectively. In order to study into the mechanism of the thermodynamics, we use Langmuir isotherm equation to correlate (Eq. 2). qe =

qmax Kce

(2)

1 + Kce

where qmax denoted the maximum amount of metal extracted. K was the Langmuir constant. ce represented the aqueous equilibrium concentration of lanthanides. It could also be expressed as: ce qe

=

ce qmax

+

1

(3)

Kqmax

We could see that there existed a linear relationship c between e and ce. Also qmax and K could be derived qe

Table 2 Parameters calculated based on Langmuir isotherm model by thermodynamic experiment

through slope and intercept. The regression parameters could be found in Table 2. The theoretical thermodynamic curves are plotted in Fig. 7, from which we could see that the experimental results corresponded to the theory well. qmax indicated the maximum extraction amount and K stood for the extraction affinity. We could discern that the extraction capacities of Sm3+ and Er3+ were high

Element

qmax /(10–4 mol/g) This paper

ef. [25]

K/(m3/mol) This paper

Ref. [25]

3+

2.85 (pH=3.85) 1.37 (pH=3.00) 11.6 (pH=3.85) 27.0 (pH=3.00)

3+

Sm

3.05 (pH=3.85) 2.08 (pH=2.20) 33.4 (pH=3.85) 17.9 (pH=2.20)

Er3+

2.86 (pH=3.85) 3.22 (pH=1.75) 36.1 (pH=3.85) 12.0 (pH=1.75)

La

Table 3 Thermodynamic extraction data of mixed elements of La3+, Sm3+ and Er3+ into microcapsule La3+

Mass of MC/

Sm3+

mg

cin/(mg/L)

ce/(mg/L)

110.38

102.32

11.52

8.23

112

98.49

102.32

4.88

9.89

112

89.72

102.32

4.52

10.90

112

78.86

102.32

6.92

12.10

71.48

102.32

7.00

13.33

66.02

102.32

29.12

56.60

102.32

49.26

102.32

qe/(mg/g)

cin/(mg/L)

ce/(mg/L)

Er3+ qe/(mg/g)

cin/(mg/L)

ce/(mg/L)

qe/(mg/g)

0

10.15

92.24

2.32

8.15

0

11.30

92.24

1.40

9.22

0

12.48

92.24

1.44

10.12

112

0

14.20

92.24

1.44

11.51

112

0

15.67

92.24

1.04

12.76

11.09

112

0

16.96

92.24

0.96

13.83

38.48

11.28

112

0

19.79

92.24

1.12

16.10

39.20

12.81

112

0

22.65

92.24

1.80

18.36

Key: The mass of microcapsules was weighed by a micro balance (Mettler, XP6)

JING Yu et al., Extraction of lanthanides by polysulfone microcapsules containing EHPNA. I. Piercing method

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2.6 Cyclic extraction and stripping

Fig. 8 Stripping performance of single element (a) and mixed elements (b) of La3+, Sm3+ and Er3+ from microcapsule

ciency 83% within three hours whilst 67% for Er3+. Moreover, the stripping efficiency could reach >90% with 20 h. Consequently, only one-time stripping was required, which was easy to implement, thereby probably realizing the separation of different RE metals according to different stripping rates. From Fig. 9, the selectivities in the stripping process can be observed. The selectivities of piercing method were in a relatively modest rang of 1–3. This result was mainly attributed to the high resistance of mass transfer in the center of microcapsules. In order to shrink the resistance of mass transfer and improve the separation coefficient, the way of decreasing the size of the microcapsules may be likely a useful idea. The detailed selectivities in stripping process and better results by the microfluidic method will be presented in our next paper.

Fig. 9 Selectivity of La/Sm, La/Er and Sm/Er in the stripping process

The extraction-stripping cyclic experiments could best describe the stability and durability of the microcapsules. Thus, we performed extraction-stripping cyclic experiments with the same microcapsules in three mixed elements for seven times. The experimental results are shown in Fig. 10. In each run, the first set of data indicated the extraction amount of the lanthanides in microcapsules and the second one represented the remaining amount of lanthanides in the same microcapsules after stripping. It could be seen that the extraction amount of Sm3+ and Er3+ enjoyed good stability. These experiments were performed in the turbulent shaken for 20 h, thus we concluded that there was little loss for the EHPNA from the microcapsules. Therefore, this EHPNA-functionalized microcapsule is practical in extraction column where shake and impact will be much smaller. The extraction amount for La3+ remained stable at 50% after the fourth cyclic experiment. This was probably attributed to the low affinity between extractant and La3+. After the competitive extraction of Sm3+ and Er3+, their complex accumulated in the inside of the microcapsules, which even blocked the hole. Thus, the resistance of inner diffusion was enlarged. The more cycles were performed, the more remainders of lanthanides would be left in microcapsule. This demonstrated that a certain amount of complex accumulated in the internal of the microcapsules, which made it hard to strip due to the smaller contact area. In all, the extraction-stripping enjoyed good stability and renewability, which particularly demonstrated its potential for the extraction and concentration for medium and heavy RE metals. As known to us, the alkali saponified extractant is able to significantly improve the extraction kinetics and thermodynamics, whereas that is not an easy way to put into practice in microcapsule system. The pore blockage and the insufficient saponification restrict our thinking. However, this idea is still very interesting and worth further researching to improve extraction performance in microcapsule system in future.

Fig. 10 Extraction-stripping cycling of mixed elements of La3+, Sm3+ and Er3+ with microcapsule

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The residence time of extraction in microcapsule system is usually limited in practical applications in extraction or ion exchange column. In order to further enhance the extraction rate and boost the renewability of the microcapsules, we plan to minimize the size of the microcapsules. This is because the extraction rate would be enhanced with smaller diameter and larger specific surface area of the microcapsules. Also, the mass transfer area would be expanded to better improve the efficiency of mass transfer. Hence, we have prepared the microcapsules through the microfluidic techniques, which is shown in our next paper of this study. The extraction and stripping results described here are also used in our next paper to gain insights into the design of microcapsules with faster extraction rate.

3 Conclusions In order to solve the problems of the conventional liquid-liquid extraction, we prepared the microcapsules by adopting the solvent extraction method and piercing techniques. We optimized the content of the PSF in polymer solution and succeeded in preparation of the microcapsules containing EHPNA. We also applied the prepared microcapsules into the extraction of light, medium and heavy RE metals. Extensive kinetic and thermodynamic experiments were performed to evaluate the microcapsules, which could provide a good guidance for the design and application in extraction or ion exchange column. Besides, cyclic experiments were performed, which demonstrated that the microcapsules enjoyed both stability and renewability, which was particularly fit for the extraction and concentration of medium and heavy RE metals. Acknowledgments: This research was carried out under the State Key Laboratory of Chemical Engineering Key Program (SKL-ChE-14T01) in the State Key Laboratory of Chemical Engineering of Tsinghua University, Beijing, P. R. China. The authors gratefully acknowledge the grant.

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