Solvent extraction performance of Ce(III) from chloride acidic solution with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA) by using membrane dispersion micro-extractor

JOURNAL OF RARE EARTHS, Vol. 33, No. 10, Oct. 2015, P. 1114

Solvent extraction performance of Ce(III) from chloride acidic solution with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA) by using membrane dispersion micro-extractor HOU Hailong (侯海龙)1, JING Yu (靖 宇)2, WANG Yue (王 月)2, WANG Yundong (王运东)2,*, XU Jianhong (徐建鸿)2, CHEN Jinnan (陈晋南)1,* (1. School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, China; 2. The State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China) Received 22 January 2015; revised 18 May 2015

Abstract: By using membrane dispersion micro-extractor, Ce(III) solvent extraction experiments were conducted. Cerium chloride solution with certain acidity was used as aqueous phase and 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA) kerosene solution as organic phase. The effects of system physicochemical properties and operational conditions, such as initial EHEHPA concentration, initial aqueous acidity, total flow rate and continuous phase flow rate, etc., on the extraction efficiency and the overall volume mass transfer coefficient were evaluated. As the total flow rate increased from 20 to 160 mL/min, the overall volume mass transfer coefficient was enhanced from 0.1 to 0.54 s–1. Under the optimal conditions, the Ce(III) extraction efficiency could reach 99.92% in 2.98 s. A mathematical model was set up to predict the overall volume mass transfer coefficient, and the calculation results agreed well with the experimental results, most relative error was within ±10%. Keywords: rare earths; solvent extraction; membrane dispersion; overall volume mass transfer coefficient

Rare earth elements including yttrium and the lanthanides have extensive application in many spheres, such as superconductor[1], photonic device[2,3], permanent magnetic material[4,5], solar cell[6], and agriculture area[7], etc. Because of similar physicochemical properties, it is difficult to separate them completely. Due to large throughput, fast reaction, and easy phase separation, the major method used in the rare earth element separation industry is solvent extraction with mixer settler. However, this traditional equipment has many drawbacks, such as: large equipment area occupation, high energy consumption, and large solvent holdup[8]. To solve the problems, many types of methods and equipment are devised, such as hollow fiber contactor[9], impregnated polymer beads and resin[10–13], ionic liquid[14–17], magnetic absorption[18], and polymer sorbent[19], etc. However, an effective method is still in shortage. Membrane dispersion process was firstly developed for emulsification[20]. Under pressure, the dispersed phase passes through membrane dispersion medium to form many droplets in the continuous phase with size ranging from several to hundreds micrometers. By reasons of large mass transfer area, simple structure, low energy consumption, the membrane dispersion technology has

great potential applications in petroleum industry, food and pharmacy, and chemical processes. Abrahamse et al.[21] analyzed the droplets formation process in membrane emulsification process, and found that the number of active pores of the membrane increased linearly with the transmembrane pressure. Chen et al.[22] studied the effects of transmembrane pressure, continuous phase flow rate, and the geometric parameters of inner elements on the mass transfer performance of 30% TBP (in kerosene)/nitric acid/water system in a ceramic membrane dispersion extraction process. In the same year, Chen and his coworkers[23] prepared barium sulfate nanoparticles with average size in range of 20–200 nm by using membrane dispersion setup. Xu et al.[24] investigated the membrane dispersion theory by studying the forces analysis of droplets. The calculated results of mathematical model agree well with the experimental. In the same year, Xu et al.[25] investigated mass transfer performance of six systems with different interfacial tension forces. Under optimal conditions, the phosphoric acid extraction efficiency by 30% TBP in kerosene can reach 100% in 0.2 s, and the throughput can reach 500 m3/(m2·h). Tan et al.[26] studied the H2O2 extraction in a gas agitated membrane dispersion extraction setup. A

Foundation item: Project supported by the National Basic Research Program of China (2012CBA01203) and the National Natural Science Foundation of China (90210034, 20221603) * Corresponding authors: WANG Yundong, CHEN Jinnan (E-mail: [email protected], [email protected]; Tel.: +86-10-62794448, +86-10-68918603) DOI: 10.1016/S1002-0721(14)60534-2

HOU Hailong et al., Solvent extraction performance of Ce(III) from chloride acidic solution with 2-ethylhexyl …

large phase ratio of 50:1 extraction can be realized and the overall volume mass transfer coefficient was enhanced for 30 times comparing the membrane dispersion without gas agitation. The application of membrane dispersion on extraction of rare earth from dilute solution with non-precipitation has great attraction[27]. Due to good selectivity of the rare earths elements and low acidity of the stripping flow, 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (EHEHPA) has been extensively used as extractant in rare earth solvent extraction process. Fig. 1 shows the structure of EHEHPA. In this paper, EHEHPA was used as extractant. In order to improve the extraction capacity, EHEHPA was saponified by strong ammonium water. The sulfated kerosene was used as dilute. Our previous investigation[8] studied the effects of physicochemical properties and operation conditions on the mass transfer performance of La(III) in chloric acidic medium by using membrane dispersion setup. However the mass transfer coefficient was not analyzed. In this paper, we focused on the mass transfer performance and the overall volume mass transfer coefficient of the Ce(III) in membrane dispersion extraction process by considering physicochemical properties and operation conditions. A mathematical model was correlated.

1 Experimental 1.1 Reagents and apparatus CeCl3·7H2O (purity>99.99% (weight fraction)) was supplied by Beijing Biam Alloys Co., Ltd. The diluent, sulphonated kerosene (supplied by Hubei Prosperity Galaxy Chemical Co., Ltd.) and the acidic extractant, EHEHPA (purity>95 wt.%, purchased from Luoyang Aoda Chemical Co., Ltd.), were used without further purification. All of the other reagents were of analytical grade. In all experiments, 40 mol.% of the EHEHPA was saponified by 28 wt.% ammonium water. The organic phase was obtained by dissolving a certain amount of EHEHPA in sulphonated kerosene. The mixture was stirred until a transparent phase was formed. The aqueous phase was prepared by dissolving cerium chloride heptahydrate in hydrochloric acid solution.

Fig. 1 Chemical structure of EHEHPA

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1.2 Extraction procedure The equipment setup is illustrated in Fig. 2. Fig. 2(a) shows the membrane dispersion micro-extractor setup. In all experiments, the organic phase was used as dispersed phase, and the aqueous phase was used as continuous phase. Both phases were immersed in water bath tanks with magnetic stirring to adjust the temperature (purchased from Zhengzhou Great Wall Scientific Industrial and Trade Co., Ltd.). The two solutions were pumped into membrane micro-dispersive module by metering pumps (purchased from Beijing Satellite Manufacturing Plant). The dispersed phase was pressed through a dispersion medium, a stainless steel sintered membrane with 5 μm and 0.3 mm thickness (shown in Fig. 2(b)), and many small droplets formed in the continuous phase. The extraction time was controlled by the outlet capillary tube length with an inner diameter of 2 mm, and the tube was also immersed in a water bath to control the reaction temperature. The final two-phase mixture was separated by a separating funnel, and the reffinate samples were collected for Ce(III) concentration analysis. 1.3 Analysis The Ce(III) concentration in aqueous phase was titrated three times by standardized EDTA solution. The Ce(III) concentration in organic phase was determined by mass balance. The pH was obtained by Metler easy five pH meter with a Sanxin electrode. The extraction efficiency is defined as follows: E=

( Ci − C e ) Ci

× 100%

(1)

where E represents the extraction efficiency; Ci refers to

Fig. 2 Equipment setup (a) Membrane dispersion micro-extractor setup (1−Continuous phase, 2−Dispersed phase, 3−Metering pumps, 4−Membrane micro-dispersive module, 5−Capillary tube, 6−Phase separating funnel, 7−Organic extract, 8−Aqueous raffinate); (b) SEM picture of the stainless steel sintered membrane surface

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the initial Ce(III) concentration in the aqueous phase, mol/L, and Ce is the Ce(III) concentration in the aqueous reffinate phase after extraction, mol/L, respectively. The residence time of the mixture in the capillary tube is calculated by t=

Vtube

( Qa + Qo )

(2)

× 60

where t represents the residence time in the tube, s; Vtube refers to the volume of the capillary tube, mL; Qa and Qo are the aqueous phase and organic phase flow rates respectively, mL/min. In order to evaluate the performance of membrane dispersion micro-extractor, the overall volume mass transfer coefficient is analyzed by N (3) KO = ΔC where Ko is the overall volume mass transfer coefficient, s–1; N is the volume mass transfer rate, mol/(L·s); ΔC is the average concentration difference, mol/L, which is calculated by

(C − C ) − (C − C ) ΔC = ln ( C − C ) − ln ( C − C ) 1

* 1

1

* 1

* 2

2

2

* 2

(4)

where C1, C2 are Ce(III) concentration in the organic phase respectively before and after extraction, mol/L; C1* , C 2* are the equilibrium concentration of Ce(III) in organic phase, respectively before and after extraction process, mol/L.

Fig. 3 Effect of EHEHPA concentration on the extraction efficiency of Ce(III) (a), and the overall volume mass transfer coefficient (b) (Ci=2.66×10–3 mol/L; Qo/Qa=40/ 40; initial pH=4.23; 298 K) (1) CEHEHPA=0.01 mol/L; (2) CEHEHPA=0.015 mol/L; (3) CEHEHPA=0.02 mol/L; (4) CEHEHPA=0.03 mol/L; (5) CEHEHPA=0.035 mol/L; (6) CEHEHPA=0.04 mol/L

2 Results and discussion 2.1 Effect of EHEHPA concentration on the extraction Capillary tubes with lengths of 38, 154, 253, 466, and 631 cm were used to control residence time. The effect of EHEHPA concentration on the extraction efficiency is shown in Fig. 3(a). It can be found that the extraction efficiency increases with the growth of the extractant concentration from 0.01 to 0.04 mol/L. When the EHEHPA concentration is less than 0.02 mol/L, the extraction efficiency is less than 60%. After that the extraction efficiency increases abruptly. When EHEHPA concentration is more than 0.035 mol/L, the extraction efficiency reaches almost 100% as residence time above 6 s. Fig. 3(b) shows the effect of the EHEHPA concentration on the overall volume mass transfer coefficient. The overall volume mass transfer coefficient increases with the EHEHPA concentration. It is known that saponified EHEHPA is a kind of surfactant. The increment of the surfactant concentration can reduce the interfacial tension. According to Xu’s dispersion theory[24], the reduction of interfacial tension can lead to smaller droplet diameter, i.e. larger interfacial area, which is included in the overall volume mass transfer coefficient. However, excessive saponified EHEHPA can cause the cerium ion

hydrolysis easily. In addition, when the extractant concentration grows, the clarification time increases. For example, for the cases with 0.04 mol/L, it takes almost 2 min to obtain 50 mL clear two phases. In the following experiments, 0.035 mol/L extractant is used. 2.2 Effect of initial aqueous phase acidity on the extraction The effect of initial aqueous phase pH on the extraction is shown in Fig. 4(a). In Fig. 4(a), when the pH is below 2.63, the extraction efficiency is less than 60%. When pH is above 3.14 and residence time above 11 s, the extraction efficiency is more than 90%. Increasing the aqueous pH can lead to a higher extraction efficiency. However, when the pH is above 3.63, a decline of extraction was found at pH 4.12. The reason may be the hydrolysis of the cerium ion. The effect of initial aqueous phase acidity on overall volume mass transfer coefficient is shown in Fig. 4(b). The overall volume mass transfer coefficient increases with the pH, reaches the maximum value 0.27 s–1 at pH 3.63, after that decreases to 0.17 s–1. The extraction reaction by using saponified EHEHPA can be expressed as:

HOU Hailong et al., Solvent extraction performance of Ce(III) from chloride acidic solution with 2-ethylhexyl …

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unsaponified EHEHEPA. The cerium ion extraction process with EHEHPA is a cation exchange process: (7) Ce3+ + 3H2L2 ↔ Ce(HL)3L3 +3H+ Higher acidity can restrain the extraction process. There is an abrupt reduction of overall volume mass transfer coefficient as initial aqueous pH increases from 3.60 to 4.12. The hydrolysis should be the cause for this variation. In the following experiments, the initial aqueous pH is limited from 3.14 to 3.63. 2.3 Effect of initial Ce(III) concentration on the extraction

Fig. 4 Effect of initial aqueous phase acidity on the extraction efficiency of Ce(III) (a), and the overall volume mass transfer coefficient (b) (Ci=2.66×10–3 mol/L; Qo/Qa=40/ 40; CEHEHPA=0.035 mol/L; 298 K)

2Ce3+ + 6NH4 L + 3H 2 L 2 ↔ 2Ce(HL)3 L 3 + 6NH4 +

(5)

where NH4L is the saponified EHEHPA, H2L2 represents EHEHPA dimmer, and Ce(HL)3L3 is the extracted complex. Increasing the initial aqueous acidity can reduce the saponified EHEHPA concentration and increase the unsaponified EHEHPA concentration: 2NH 4 L + 2H + ↔ H 2 L 2 + 2NH 4 +

(6)

Saponified EHEHPA is a surfactant. Reduction of the saponified EHEHPA concentration can increase the interfacial tension between the two phases. Table 1 shows the interfacial tension between aqueous phase of different acidity and EHEHPA of 0.035 mol/L. From Table 1, there is an abrupt reduction of interfacial tension as initial aqueous pH increases from 1.98 to 2.50. Correspondingly, there is also a significant increase in both extraction efficiency and overall volume mass transfer coefficient. According to Xu’s droplet dispersion theory[24], the increase of the interfacial tension can produce larger diameter droplets. The growth of the droplet diameter will reduce the volume mass transfer rate as well as the overall mass transfer coefficient. What’s more, the increase of the unsaponified EHEHPA concentration can result in many cerium ions reacting with Table 1 Interfacial tension between different acidic aqueous solutions and 0.035 mol/L EHEHPA 2.50

3.01

3.50

4.02

4.49

5.08

γ/(mN/m) 23.97 23.78 22.65 8.71

pH

0.99

1.52

1.98

7.85

7.24

6.76

6.67

6.90

The influence of initial cerium ion concentration in the feed phase is studied at the concentration range from 1.27×10–3 to 2.66×10–3 mol/L (shown in Fig. 5). It is known that the mass transfer driving force and volume mass transfer rate will increase with increasing initial cerium ion concentration. This is the reason for the increase of the cerium concentration in the organic phase with the initial cerium concentration (shown in Fig. 5(b)). However, the extraction efficiency decreases with the increasing of cerium ion concentration and the time of reaching the equilibrium state increases with the growth of initial Ce(III) concentration. In less than 12 s, the organic phases with 0.035 mol/L EHEHPA can extract all cerium for cases with a concentration of less than 2.66 mol/L, which shows a fast volume mass transfer rate. The effect of initial cerium ion concentration on overall volume mass transfer coefficient is shown in Fig. 5(c). The overall volume mass transfer coefficient increases from 0.15 to 0.28 s–1 with initial cerium ion concentration. Increasing initial cerium ion in the aqueous can enhance the interaction between molecules in two phases, which can increase the interfacial tension between the two phases and the droplet diameter. If this is a crucial factor, the overall mass transfer coefficient will decrease. The reason is probably that the increasing initial cerium ion concentration can enhance the reaction probability around the interfacial areas where reaction occurred. And this enhancement is reflected in the reduction of the reaction resistance which is also a component of the overall mass transfer resistance. From this point, it can be concluded that the mass transfer process is determined by both the chemical reaction and the mass transfer process in the two phases. 2.4 Effect of temperature on the extraction During investigating traditional extraction equipment, such as mixer settler and the extraction column, the reaction resistance is always neglected due to relative larger mass transfer resistances in both the organic and aqueous phases. Fig. 6(a) shows the effect of temperature on the extraction. It can be found that the higher temperature can lead to faster volume mass transfer rate, which means that the reaction resistance cannot be neglected in

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Fig. 5 Effect of initial Ce(III) concentration on the extraction efficiency of Ce(III) (a), the cerium concentration in organic phase (b), and the overall volume mass transfer coefficient (c) (pH=3.64; Qo/Qa= 40/40; CEHEHPA=0.035 mol/L; 298 K)

Fig. 6 Effect of temperature on the extraction efficiency of Ce(III) (a), and the overall volume mass transfer coefficient (b) (Ci= 2.66×10–3 mol/L; pH=3.64; Qo/Qa=40/40; CEHEHPA=0.035 mol/L)

membrane dispersion extraction process. Membrane dispersion process can not only increase the interfacial area of the two phases, but also reduce the transfer distance in the organic droplets. Mass transfer resistances in two phases reduce to the level of reaction resistance, so the temperature has great influence on the extraction efficiency. Fig. 6(b) shows the influence of the temperature on the overall volume mass transfer coefficient. The overall volume mass transfer coefficient increases from 0.30 to 0.47 s−1 with the temperature, which confirms that the reaction resistance in membrane dispersion extraction process cannot be neglected. 2.5 Effect of total flow rate on the extraction The effect of total flow rate on the extraction efficiency is shown in Fig. 7. In this part, the flow ratio was fixed at 1:1. The total flow rate is the sum of the both phases flow rates ranging from 10 to 80 mL/min. Capillary tubes of different lengths were used to control residence time. It can be found that the extraction efficiency reaches 97.6%, 96.8%, 99.68%, 98.88%, and 96.4% in 14.6 s (Fig. 7(a)), 11.9 s (Fig. 7(b)), 5.9 s (Fig. 6(a)), 3.9 s (Fig. 7(c)), and 1.8 s (Fig. 7(d)), respectively. A growth trend of volume mass transfer rate is shown. With organic to aqueous flow ratio of 80:80 (mL·min−1/mL·min−1), the extraction efficiency can reach 99.92% in 2.98 s. According to Xu’s theoretical research[24], due to an enhancement of crossflow drag force,

the increase of continuous phase flow rate can reduce the diameters of the dispersion droplets and increase the interfacial area between the two phases. It is known that the extraction process is an interfacial reaction. Enlargement of interfacial area can improve the volume mass transfer rate greatly. What’s more, the increase of the dispersed phase can cause inner circulation in the droplets, which can reduce the inner mass transfer resistance greatly. It is interested to find that the extraction efficiency of second data of all four cases (capillary tube length=154 cm) firstly reduces from 97.6% to 78.9% while total flow rate increases from 20 to 120 mL/min. After that, the extraction efficiency increases sharply to 96.4% as total flow rate increases to 160 mL/min. When total flow rate is low, the extraction efficiency is controlled by the residence time. Enough residence time results in high extraction efficiency. As the total flow rate increases, the residence time reduces, which leads to reduction of the extraction efficiency. As the total flow rate increases, the dispersed droplet diameter reduces and the interfacial area between two phases increases, which enhances the extraction and becomes dominant factor for the extraction process. The influence of the total flow rate on the overall volume mass transfer coefficient is shown in Fig. 8. When the total flow rate is in the range from 10 to 20 mL/min, there is no significance of the overall volume mass transfer coefficient. As the total flow rate increases

HOU Hailong et al., Solvent extraction performance of Ce(III) from chloride acidic solution with 2-ethylhexyl …

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Fig. 7 Effect of total flow rate on the extraction efficiency of Ce(III) (Ci=2.66×10–3 mol/L; pH=3.64; CEHEHPA=0.035 mol/L; 298 K) (a) Qo/Qa=10/10 mL·min−1/mL·min−1; (b) Qo/Qa=20/20 mL·min−1/mL·min−1; (c) Qo/Qa=60/60 mL·min−1/mL·min−1; (d) Qo/Qa=80/80 mL·min−1/mL·min−1

quently, the overall mass transfer coefficient increases. When the total flow rate exceeds 160 mL/min, the droplets diameter reduces insignificantly. The continuous phase boundary layer thickness becomes stable. The droplet turns like a rigid ball. The inner fluid becomes stagnant. As a result, the growth of the overall mass transfer coefficient becomes insignificant. 2.6 Effect of continuous phase flow rate on the extraction

Fig. 8 Effect of total flow rate on the overall volume mass transfer coefficient (Ci=2.66×10–3 mol/L; pH=3.64; CEHEHPA=0.035 mol/L; 298 K)

from 20 to 120 mL/min, the overall volume mass transfer coefficient increases sharply from 0.10 to 0.54 s–1. After that, the overall volume mass transfer coefficient increases slightly. Except increment of interfacial area, the increment of continuous phase flow rate can also reduce the thickness of the continuous phase boundary layer which is also the main resistance for the mass transfer. The growth of the dispersed phase flow rate can also cause circulation in the droplets, which helps to reduce the thickness of dispersed phase boundary layer and the mass transfer resistance in the dispersed phase. Conse-

The effect of continuous phase flow rate on extraction efficiency is shown in Fig. 9. As described above, the growth of continuous phase flow rate can reduce the diameter of the droplet and increases the interfacial area between the two phases. The mass transfer process is improved. However, in our experiments, the aqueous phase is continuous phase. Increasing continuous phase flow rate can supply more Ce(III). Fig. 9(a) shows that as continuous phase flow rate is 20 mL/min and residence time exceeds 11.9 s, the extraction efficiency is above 96.8%. However, when the continuous phase flow rate goes up to 40 mL/min and residence time exceeds 14.6 s (shown in Fig. 9(b)), the extraction efficiency is around 50%. However, as continuous phase flow rate increases from 20 to 40 mL/min, the overall volume mass transfer coefficient increases from 0.10 to 0.24 s–1. The reason is the increment of the interfacial area. Another reason may

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Fig. 10 Comparison of Ko between experimental data and calculated values by Eq. (11)

Fig. 10 shows the comparison of Ko between the calculation and the experimental data. The calculation values show good coincidence with experimental data. And the most relative error is within ±10%.

3 Conclusions Fig. 9 Effect of the continuous phase flow rate on the extraction efficiency (Ci=2.66×10–3 mol/L; pH=3.64; CEHEHPA= 0.035 mol/L; 298 K)

be the reduction of the continuous phase boundary layer thickness. 2.7 Mathematic model The correlation of overall volume mass transfer coefficient can be used to predict mass transfer process in the membrane dispersion micro-extractor. By considering the interfacial tension, two phase ratio, droplet diameter, and the shear force during dispersion, the dimensionless numbers of We and Φ are used to correlate the overall volume mass transfer coefficient. Two dimensionless numbers are defined as: (8) We = ρ uc2 d γ where We is the Weber number; ρ is the density of the continuous phase, kg/m3; uc is the superficial velocity of the continuous phase, m/s; d is the inner diameter of the flowing tube, m; γ is the interfacial tension of two phases, N/m; QD (9) Φ= QD + QC where Φ is the dispersion phase fraction; QD is the dispersed phase flow rate, mL/min; QC is the continuous phase flow rate, mL/min. Ko can be determined by the relationship: (10) Ko = aWe bΦ c The correlation equation is determined as K o = 0.036We 0.8Φ 2.1

(11)

Ce(III) solvent extraction process with membrane dispersion micro-extractor was investigated. The saponified EHEHPA in kerosene was used as organic phase, and the cerium chloride solution as the aqueous phase. Effects of initial extractant concentration, aqueous phase acidity, initial Ce(III) concentration in aqueous phase, temperature, total flow rate and continuous phase flow rate on the extraction efficiency and overall volume mass transfer coefficient were determined. Under the optimal conditions, the extraction efficiency could reach 99.92% in 2.98 seconds. According to the effect of the temperature on the overall volume mass transfer coefficient, the reaction resistance in the membrane dispersion extraction process could not be neglected. The overall mass transfer coefficient could be enhanced from 0.10 to the 0.54 as the total flow rate increased from 20 to 160 mL/min. A mathematic model was set up to predict the overall volume mass transfer coefficient in the membrane dispersion extraction process, and the relative error was within ±10%.

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