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Microfluidic solvent extraction of La(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (P507) by a microreactor
Chemical Engineering and Processing 91 (2015) 1–6
Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Microfluidic solvent extraction of La(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (P507) by a microreactor Shaohua Yin a,b,c,d,e, Libo Zhang a,b,c,d,e , Jinhui Peng a,b,c,d,e, Shiwei Li a,b,c, d, * , Shaohua Ju a,b,c,d,e , Lihua Zhang a,b,c,d,e a State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan 650093, China b Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China c Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming, Yunnan 650093, China d National Local Joint Laboratory of Engineering Application of Microwave Energy and Equipment Technology, Kunming, Yunnan 650093, China e Yunnan Provincial Key Laboratory of Intensification Metallurgy, Kunming, Yunnan 650093, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 December 2014 Received in revised form 2 March 2015 Accepted 4 March 2015 Available online 5 March 2015
A new solvent extraction system of extracting La(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (P507) has been investigated to intensify the extraction process in microreactor, and solve the problems like long mixing time, low processing capacity, large factory area occupation, and high energy consumption in the conventional rare earth solvent extraction equipment. In this work, extraction equilibrium studies show that the initial aqueous pH value 4.00 and saponification rate 40% are the optimal operation condition. The effects of volumetric flow rate on extraction efficiency are analyzed, and the results indicate that increasing flow ratio could improve the extraction efficiency greatly, up to almost 100%, and the two phases keep parallel flow while keeping an aqueous-organic interface in the microchannel. The mass transfer rate is proportional to the initial pH and P507 concentration, and approaches almost a constant value at high pH and extractant concentration, and the transfer process between the two phases accompanied with an interface chemical reaction is confirmed to proceed satisfactorily in a short time (residence time = 0.37 s). The features of the microreactors, i.e., large specific surface area and short diffusion distance are effective for the efficient extraction of La(III). ã 2015 Elsevier B.V. All rights reserved.
Keywords: La(III) Solvent extraction P507 Microreactor
1. Introduction Rare earth elements are abundant in China, and are extensively applied in metallurgy, functional ceramics and glass, rare earth electrode, fluorescent, permanent magnet material and so on [1]. Some important bulk applications of rare earth element lanthanum (La) are used in the nickel–lanthanum alloy, catalysts, special ceramics and organic synthesis nickel [2,3]. For example, fluid catalytic cracking (FCC) catalysts which are extensively used in the petrochemical industry contain about 3.5 wt.% rare earth oxides, mainly lanthanum, and smaller amounts of cerium, praseodymium and neodymium [4]. With the increasing demand for them, the
* Corresponding author at: Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China. Tel.: +86 871 65174949. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.cep.2015.03.003 0255-2701/ ã 2015 Elsevier B.V. All rights reserved.
extraction and separation of them has gained considerable importance. In the extraction process, the mixer-settler is one of the most common extraction equipment in the conventional extraction industry process. But it has an important shortcoming, more extraction stages are needed to obtain high purity rare earth products with respect to low separation coefficient, especially, the following problems emerge, such as long mixing time, low processing capacity, large factory area occupation, high energy consumption and so on. Currently, some researchers have proposed many types of extraction equipment and methods, e. g., the hollow fiber membrane extractor [5,6], the extraction column [7], the ion exchange resin [8], the ionic liquid [9,10], complexing extraction [11–13], synergistic extraction [14,15], etc. Although the above extraction technologies have some advantages, they could not change the longer mass transfer distance and mixing time, especially, a large amount of organic solvent is needed to dissolve both the extractant and the extracted species. Therefore, it is necessary to look for a new extraction technology of
2
S. Yin et al. / Chemical Engineering and Processing 91 (2015) 1–6
“green, efficient, safety and energy conservation” to overcome the above problems. Along with the progress in microfabrication, microstructured devices have recently attracted much attention, especially in the fields of analytical chemistry, extraction, and chemical engineering, and nanoparticles synthesis [16–18]. Microstructured devices are reactors with channel structures that have internal diameters in the range of 10–1000 mm [19]. Such microreactors have various advantages like a large special surface area of the liquid–liquid interface, a short diffusion distance for the chemical species and confident scale-up from laboratory to factory due to numbering-up approach when compared with macro scale reactors [20,21], in addition, using reactors in micro size has many other advantages such as safety and process intensification beside process cost reduction [22]. Though there are some limitations such as inability to deal with a solution containing solids to its further utilization, the advantages outnumber the limitations making microchannels very attractive for carrying out liquid–liquid extraction. Also, an efficient extraction process, in a short transfer time and by a small amount of solvent, is feasible when such a microreactor can be utilized as the extractors. Kubota et al. performed the extraction of rare earth ions such as Y, Eu and La with PC-88A using a Y type microreactor, and it was observed that two phases successfully
flowed in a micro-channel at volumetric flow rates from 5.6 1010 to 2.8 109 m3/s in a short residence time of 0.7 s [23]. Nishihama and Yoshizuka studied a micro solvent extraction system for the separation of lanthanides, and confirmed the exchange reaction between a lighter lanthanide in the organic phase and a heavier lanthanide in the aqueous phase [24]. Hou et al. investigated the La(III) extraction process in the HCl–EHEHPA kerosene system by membrane dispersion micro-extractor, and found that the extraction efficiency was higher when the organic solution was the dispersion phase, also, the recovery efficiency was larger than 82% [25]. The above studies show that microreactor has potential application in the rare earth extraction and separation process. Currently, there are still relatively few reports concerning the process of rare earth extraction on a micro-chip with solvent extraction. In the present work, we use a microfluidic solvent extraction system to study the extraction performance of rare earth element La(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (P507) as the extractant dissolved in sulphonated kerosene. The micro solvent extraction chip is fabricated on a PMMA (polymethyl methacrylate) plate. In addition, the experiments of extraction equilibrium, the effect of flow rate on extraction efficiency, the residence time of the aqueous phase in the microchannel, and
Fig. 1. Schematic illustration of the experimental apparatus ((a) microreactor system; (b) two phases microchannel schematic; (c) cross-section geometry).
S. Yin et al. / Chemical Engineering and Processing 91 (2015) 1–6
mass transfer process are studied. The aim of this study is to apply the microreactor to solvent extraction for the development of an efficient extraction process for the rare earths. 2. Experimental work 2.1. Materials LaCl37H2O (purity >99.9%) is purchased from Ganzhou City Yuan Jiang Mining Co., Ltd. The aqueous phase is obtained by dissolving the LaCl37H2O in deionized water. The concentration of La(III) in the aqueous solution is determined by titration using EDTA standard solution with xylenol orange as an indicator in hexamethylenetetramine buffer solutions. The acidic extractant, P507 supplied by Luoyang Aoda Chemical Co., Ltd., is used without further purification and dissolved in sulfonated kerosene. The organic phase is prepared by mixing the P507 in sulphonated kerosene with certain volume of ammonia water (3 mol/L) to obtain different saponification rates of organic phase. The mixture is stirred until a single phase formed, and the saponification degree is determined by HCl titration. NaCl is used in all extraction experiments to keep the ionic strength constant at m = 1 mol/L. All of the other reagents are analytical grade. A model pHs-3C pH meter (Leici, Shanghai, China) is employed to measure pH values of the aqueous phase. 2.2. Microfluidic device Microfluidic extractions are carried out in PyrexTM microchips (Institute of Microchemical Technology, Japan). The experimental apparatus employed is schematically illustrated in Fig. 1a. Two microchannels converge at a Y-junction to form a single microchannel (120 mm 160 mm 40 mm) which is divided into two sections by a guide structure which maintains the co-flow of the two liquids. This is where extraction takes place. The two phases are separated at a second Y-junction. The details of the geometry for the Y–Y type channel are shown in Fig. 1b. The cross-section of the extraction channel includes a guide structure which helps to stabilize the water/oil interface (Fig. 1c). 2.3. Extraction procedure The organic and aqueous feed solutions (O:A = 1:1) are fed into the micro flow channel via two inlets at the same flow rate using a programmable syringe pump (Harvard, PHD 2000-M). The two
Fig. 2. Effects of initial aqueous pH value and saponification rate on the extraction efficiency ([LaCl3] = 0.2 mol/L, [P507] = 1.5 mol/L).
3
phases pass through the microreactor, and the resulting aqueous and organic solutions are separated and fed out from the two outlets. The organic samples collected are stripped with an equal volume of 6 mol/L HCl, and the resulting aqueous samples are analyzed using the usual EDTA complexometric titration. Each extraction experiment is repetitively operated three times, and all the concentrations in the aqueous phase are obtained from three replicate measurements. Flow stability in the microchip is monitored using optical microscopy (Olympus, BH2-UMA). The extraction efficiency E [%] is defined by Eq. (1): E¼
Co Ci 100% Co
(1)
where Co and Ci are the La(III) concentrations (mol/L) in the aqueous phase before and after extraction, respectively. 3. Results and discussion 3.1. Extraction equilibrium In order to determine the optimal operational condition in the microreactor, the extraction equilibrium experiments are carried out. The effects of initial aqueous pH value and saponification rate on the extraction efficiency are shown in Fig. 2. As can be seen, extraction efficiency is enhanced significantly in the pH range of 1.50–4.50 when saponification rates are 0%, 20%, and 40%, respectively. Especially, extraction efficiency in these cases with saponification rates of 40% and 60% are almost 100% at pH values of 4.00 and 3.00 respectively. However, as pH rises above 3.0 in this situation with saponification rates of 60%, extraction efficiency starts to decrease. This may be explained that, the strong basicity of the saponified P507 extractant promotes to drive such hydrolysis reaction of La(III), reducing the extraction efficiency greatly. In view of the above reasons, the system with initial aqueous pH 4.00 and saponification rate 40% of P507 is used in the following experiments. 3.2. Effect of volumetric flow rate on extraction efficiency In this section, flow rates of the two phases are equal in all the experiments. Fig. 3 shows the effect of volumetric flow rate (naq) in the aqueous phase on the extraction efficiency and the corresponding relationship between residence time and extraction efficiency at pH 4 and saponification rate 40% of P507, The
Fig. 3. Relationship between extraction efficiency E and volumetric flow rate naq.
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extraction efficiency decreases with an increase in naq from 8.33 1010 to 4.17 108 m3/s, the longer the residence time and the higher extraction efficiency, and the highest extraction efficiency is almost 100% (actual value of 99.5%), that is because the longer contact time of both phases at lower volumetric flow rate naq. Hereafter, the volumetric flow rate naq for the later experiments is set at 8.33 1010 m3/s. In addition, two distinct photos are observed in the optical microscopy during the whole extraction process from the inlets to outlets (shown in Fig. 4). It is observed that the organic phase and aqueous phase successfully parallel flow without mixing while keeping an aqueous-organic interfacial in a micro-channel at all the flow rates from 8.33 1010 to 4.17 108 m3/s, if beyond the above experimental condition, segmented flow forms or emulsification appears. So, using microreactor to extract La(III) can avoid the phenomenon of emulsion occurred in the traditional mixing settler. 3.3. Extraction of La(III) in the microreactor
t¼
naq
m2 þðR 5Þ2 ¼ R2
(3)
ð80 mÞ2 þðR 40Þ2 ¼ R2
(4)
So m and R are calculated as 23.9 mm and 59.6 mm, respectively, and then n = 19.6 mm. The cross-section area of the aqueous channel (Saq) and the volume of aqueous phase (Vaq) are estimated by Eqs. (5) and (6) respectively. Z 80 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Saq ¼ ½ 59:62 ðx 23:9Þ2 19:6dx (5) 0
V aq ¼ Saq L
In this study, the residence time of the aqueous phase in the micro-channel, t (s), is calculated by the following equation: V aq
s. According to the cross-sectional geometry in the micro-channel in Fig. 5 [26], the actual size of the cross-sectional geometry is calculated, according to the Eqs. (3) and (4), as well as the known size.
(2)
where t represents the residence time, s; Vaq is the volume of the aqueous phase, m3; naq is the aqueous phase volume flow rate, m3/
(6)
In Eq. (6),L of 120 mm is the total length of the micro-channel. So the volume of aqueous phase Vaq is calculated as 3.08 1010 m3, and the residence time of the aqueous phase t value is about 0.37 s at the optimal volumetric flow rate naq 8.33 1010 m3/s. The result indicates that the extractability could be measured for a short contact time of both phases in the microreactor, while the extraction equilibrium is achieved at least 5 min in the conventional extraction equipment (not shown). From the results, a conclusion can be obtained that using the micro-reactor to extract metal ion can shorten the chemical reaction time under the same standard. 3.4. Mass transfer process As we known, microreactor has a large mass transfer rate through the interface of two phases to enhance the rate of chemical reaction because of the high interface area to volume ratio. The mass transfer rate of La(III), J (mol/m2 s) for the extraction, are based on the change in the metal concentration of the aqueous phase, and is defined as follows: J¼
naq ðC o C i Þ A
(7)
where the aqueous-organic interfacial area in the microchannel, A (m2), is calculated by Eq. (8) according to the size of cross-sectional geometry shown in Fig. 1c, and the value is 4.2 106 m2.
Fig. 4. Microscopic image magnified 100 mm of two phase flows from inlets to outlets.
Fig. 5. The actual size (mm) of the cross-sectional geometry in the microchannel.
S. Yin et al. / Chemical Engineering and Processing 91 (2015) 1–6
5
Fig. 6. Effect of initial pH on the mass transfer rate J ([P507] = 1.5 mol/L, saponification rate 40%).
Fig. 7. Effect of P507 concentrations on the mass transfer rate J (pH = 4.0, saponification rate 40% of P507).
A ¼ ðd hg Þ L
4. Conclusion
(8)
Fig. 6 shows the effect of initial pH in the aqueous phase on the mass transfer rate, with an increase in initial pH the mass transfer rate improves until to a flat curve, with a maximum of 3.9 102 mol/m2 s. The relationship between J and the initial concentration of the dimeric extractant P507 ([P507]) in the organic phase is shown in Fig. 7, as can be seen that, mass transfer rate follows an increasing trend until gently as extractant concentration increases. In general, an extraction process is described as follows: the metal ion in the aqueous phase diffuses from the bulk to the interface at first and then reacts with the extractant at the aqueous-organic interface to form the metal complexes, lastly the complexes transfer into the bulk organic phase. From the Figs. 6 and 7, it is considered that transfer process is controlled by the rate of La3+ diffusion in the aqueous phase due to the independent relationship between mass transfer rate J and the concentration of chemical species at high pH and high extractant concentration; however, mass transfer rate J is believed to be controlled by the interfacial chemical reaction or the diffusion rate of the metal complex in the organic phase at low pH and low [P507] because the reactivity of the complex formation is influenced by the pH and extractant concentration. 3.5. Separation process In general, the phase separation of the two phases after extraction is one of the key factors in the extractive separation process. At one time, some researches carried out a modification of the chip surface with the hydrophobic group to enhance the phase separation [27], but the employed combined hydrophobic group is easy to remove after long term operation. In the present work, therefore, the alternative method, involving the use of a complicated cross section for the phase separation is developed. The microchannel design consists of a Y-junction, two parallel channels separated by a guide structure, followed by a second Y-junction. The immiscible streams of liquid arrive at a Y-junction where they form a liquid–liquid interface that is maintained between the concurrent flows for the length of the main channel, before branching to recover the two phases. The latter is considered as a “phase separation” for the purposes of this paper. The phase separation of the aqueous and organic phases after extraction can be carried out by changing the cross section of the micro flow channel. Currently, in our group, we are study the separation performance of La/Ce, Ce/Pr, Nd/Sm, and Ho/Er/Y using this Y–Y type microchannel.
The extraction behavior of La(III) with saponified P507 has been investigated using a Y–Y shaped microreactor, and the following conclusions are drawn as follows: (1) The extraction equilibrium shows that the initial aqueous pH
value 4.00 and saponification rate 40% are the optimal operation condition. (2) Extraction efficiency decreases with an increase of the volumetric flow rate, and the highest extraction efficiency is almost 100%, and the two phases keep parallel flow without mixing in the microreactor. (3) The residence time of the aqueous phase in the microchannel is only 0.37 s at the optimal condition, which shortens the extraction time greatly. (4) Mass transfer rate increases with an increase in initial pH and [P507] converging to a flat curve, with a maximum of 3.9 102 mol/m2 s, accompanying with the interfacial chemical reaction at low pH and low extractant concentration while with diffusion process at high pH and high [P507]. In a word, the microreactor could be applicable to a system for the extraction of rare earths. Acknowledgments Financial aid from the following programs is gratefully acknowledged: the National Natural Science Foundation of China (U1302271), the National Program on Key Basic Research Project of China, (973 Program, 2014CB643404), the Young and Middle-aged Academic Technology Leader Backup Talent Cultivation Program in Yunnan Province, China (2012HB008), the Yunnan Provincial Science and Technology Innovation Talents scheme – Technological Leading Talent (2013HA002), and Kunming University of Science and Technology Personnel Training Fund (KKSY201452088). References [1] D.S. Song, S.J. Park, H.W. Kang, S.B. Park, J.I. Han, Recovery of lithium(I) strontium(II), and lanthanum(III) using Ca-alginate beads, J. Chem. Eng. Data 58 (2013) 2455–2464. [2] T. Saito, H. Sato, T. Motegi, Extraction of rare earth from La–Ni alloys by the glass slag method, J. Mater. Res. 18 (2003) 2814–2819. [3] Abhilash, S. Sinha, M.K. Sinha, B.D. Pandey, Extraction of lanthanum and cerium from Indian red mud, Int. J. Miner. Process. 127 (2014) 70–73. [4] K. Binnemans, P.T. Jones, B. Blanpain, T. Van Gerven, Y.X. Yang, A. Walton, M. Buchert, Recycling of rare earths: a critical review, J. Cleaner Prod. 51 (2013) 1– 22.
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Contents lists available at ScienceDirect
Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep
Microfluidic solvent extraction of La(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (P507) by a microreactor Shaohua Yin a,b,c,d,e, Libo Zhang a,b,c,d,e , Jinhui Peng a,b,c,d,e, Shiwei Li a,b,c, d, * , Shaohua Ju a,b,c,d,e , Lihua Zhang a,b,c,d,e a State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, Yunnan 650093, China b Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China c Key Laboratory of Unconventional Metallurgy, Ministry of Education, Kunming, Yunnan 650093, China d National Local Joint Laboratory of Engineering Application of Microwave Energy and Equipment Technology, Kunming, Yunnan 650093, China e Yunnan Provincial Key Laboratory of Intensification Metallurgy, Kunming, Yunnan 650093, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 December 2014 Received in revised form 2 March 2015 Accepted 4 March 2015 Available online 5 March 2015
A new solvent extraction system of extracting La(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (P507) has been investigated to intensify the extraction process in microreactor, and solve the problems like long mixing time, low processing capacity, large factory area occupation, and high energy consumption in the conventional rare earth solvent extraction equipment. In this work, extraction equilibrium studies show that the initial aqueous pH value 4.00 and saponification rate 40% are the optimal operation condition. The effects of volumetric flow rate on extraction efficiency are analyzed, and the results indicate that increasing flow ratio could improve the extraction efficiency greatly, up to almost 100%, and the two phases keep parallel flow while keeping an aqueous-organic interface in the microchannel. The mass transfer rate is proportional to the initial pH and P507 concentration, and approaches almost a constant value at high pH and extractant concentration, and the transfer process between the two phases accompanied with an interface chemical reaction is confirmed to proceed satisfactorily in a short time (residence time = 0.37 s). The features of the microreactors, i.e., large specific surface area and short diffusion distance are effective for the efficient extraction of La(III). ã 2015 Elsevier B.V. All rights reserved.
Keywords: La(III) Solvent extraction P507 Microreactor
1. Introduction Rare earth elements are abundant in China, and are extensively applied in metallurgy, functional ceramics and glass, rare earth electrode, fluorescent, permanent magnet material and so on [1]. Some important bulk applications of rare earth element lanthanum (La) are used in the nickel–lanthanum alloy, catalysts, special ceramics and organic synthesis nickel [2,3]. For example, fluid catalytic cracking (FCC) catalysts which are extensively used in the petrochemical industry contain about 3.5 wt.% rare earth oxides, mainly lanthanum, and smaller amounts of cerium, praseodymium and neodymium [4]. With the increasing demand for them, the
* Corresponding author at: Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan 650093, China. Tel.: +86 871 65174949. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.cep.2015.03.003 0255-2701/ ã 2015 Elsevier B.V. All rights reserved.
extraction and separation of them has gained considerable importance. In the extraction process, the mixer-settler is one of the most common extraction equipment in the conventional extraction industry process. But it has an important shortcoming, more extraction stages are needed to obtain high purity rare earth products with respect to low separation coefficient, especially, the following problems emerge, such as long mixing time, low processing capacity, large factory area occupation, high energy consumption and so on. Currently, some researchers have proposed many types of extraction equipment and methods, e. g., the hollow fiber membrane extractor [5,6], the extraction column [7], the ion exchange resin [8], the ionic liquid [9,10], complexing extraction [11–13], synergistic extraction [14,15], etc. Although the above extraction technologies have some advantages, they could not change the longer mass transfer distance and mixing time, especially, a large amount of organic solvent is needed to dissolve both the extractant and the extracted species. Therefore, it is necessary to look for a new extraction technology of
2
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“green, efficient, safety and energy conservation” to overcome the above problems. Along with the progress in microfabrication, microstructured devices have recently attracted much attention, especially in the fields of analytical chemistry, extraction, and chemical engineering, and nanoparticles synthesis [16–18]. Microstructured devices are reactors with channel structures that have internal diameters in the range of 10–1000 mm [19]. Such microreactors have various advantages like a large special surface area of the liquid–liquid interface, a short diffusion distance for the chemical species and confident scale-up from laboratory to factory due to numbering-up approach when compared with macro scale reactors [20,21], in addition, using reactors in micro size has many other advantages such as safety and process intensification beside process cost reduction [22]. Though there are some limitations such as inability to deal with a solution containing solids to its further utilization, the advantages outnumber the limitations making microchannels very attractive for carrying out liquid–liquid extraction. Also, an efficient extraction process, in a short transfer time and by a small amount of solvent, is feasible when such a microreactor can be utilized as the extractors. Kubota et al. performed the extraction of rare earth ions such as Y, Eu and La with PC-88A using a Y type microreactor, and it was observed that two phases successfully
flowed in a micro-channel at volumetric flow rates from 5.6 1010 to 2.8 109 m3/s in a short residence time of 0.7 s [23]. Nishihama and Yoshizuka studied a micro solvent extraction system for the separation of lanthanides, and confirmed the exchange reaction between a lighter lanthanide in the organic phase and a heavier lanthanide in the aqueous phase [24]. Hou et al. investigated the La(III) extraction process in the HCl–EHEHPA kerosene system by membrane dispersion micro-extractor, and found that the extraction efficiency was higher when the organic solution was the dispersion phase, also, the recovery efficiency was larger than 82% [25]. The above studies show that microreactor has potential application in the rare earth extraction and separation process. Currently, there are still relatively few reports concerning the process of rare earth extraction on a micro-chip with solvent extraction. In the present work, we use a microfluidic solvent extraction system to study the extraction performance of rare earth element La(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (P507) as the extractant dissolved in sulphonated kerosene. The micro solvent extraction chip is fabricated on a PMMA (polymethyl methacrylate) plate. In addition, the experiments of extraction equilibrium, the effect of flow rate on extraction efficiency, the residence time of the aqueous phase in the microchannel, and
Fig. 1. Schematic illustration of the experimental apparatus ((a) microreactor system; (b) two phases microchannel schematic; (c) cross-section geometry).
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mass transfer process are studied. The aim of this study is to apply the microreactor to solvent extraction for the development of an efficient extraction process for the rare earths. 2. Experimental work 2.1. Materials LaCl37H2O (purity >99.9%) is purchased from Ganzhou City Yuan Jiang Mining Co., Ltd. The aqueous phase is obtained by dissolving the LaCl37H2O in deionized water. The concentration of La(III) in the aqueous solution is determined by titration using EDTA standard solution with xylenol orange as an indicator in hexamethylenetetramine buffer solutions. The acidic extractant, P507 supplied by Luoyang Aoda Chemical Co., Ltd., is used without further purification and dissolved in sulfonated kerosene. The organic phase is prepared by mixing the P507 in sulphonated kerosene with certain volume of ammonia water (3 mol/L) to obtain different saponification rates of organic phase. The mixture is stirred until a single phase formed, and the saponification degree is determined by HCl titration. NaCl is used in all extraction experiments to keep the ionic strength constant at m = 1 mol/L. All of the other reagents are analytical grade. A model pHs-3C pH meter (Leici, Shanghai, China) is employed to measure pH values of the aqueous phase. 2.2. Microfluidic device Microfluidic extractions are carried out in PyrexTM microchips (Institute of Microchemical Technology, Japan). The experimental apparatus employed is schematically illustrated in Fig. 1a. Two microchannels converge at a Y-junction to form a single microchannel (120 mm 160 mm 40 mm) which is divided into two sections by a guide structure which maintains the co-flow of the two liquids. This is where extraction takes place. The two phases are separated at a second Y-junction. The details of the geometry for the Y–Y type channel are shown in Fig. 1b. The cross-section of the extraction channel includes a guide structure which helps to stabilize the water/oil interface (Fig. 1c). 2.3. Extraction procedure The organic and aqueous feed solutions (O:A = 1:1) are fed into the micro flow channel via two inlets at the same flow rate using a programmable syringe pump (Harvard, PHD 2000-M). The two
Fig. 2. Effects of initial aqueous pH value and saponification rate on the extraction efficiency ([LaCl3] = 0.2 mol/L, [P507] = 1.5 mol/L).
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phases pass through the microreactor, and the resulting aqueous and organic solutions are separated and fed out from the two outlets. The organic samples collected are stripped with an equal volume of 6 mol/L HCl, and the resulting aqueous samples are analyzed using the usual EDTA complexometric titration. Each extraction experiment is repetitively operated three times, and all the concentrations in the aqueous phase are obtained from three replicate measurements. Flow stability in the microchip is monitored using optical microscopy (Olympus, BH2-UMA). The extraction efficiency E [%] is defined by Eq. (1): E¼
Co Ci 100% Co
(1)
where Co and Ci are the La(III) concentrations (mol/L) in the aqueous phase before and after extraction, respectively. 3. Results and discussion 3.1. Extraction equilibrium In order to determine the optimal operational condition in the microreactor, the extraction equilibrium experiments are carried out. The effects of initial aqueous pH value and saponification rate on the extraction efficiency are shown in Fig. 2. As can be seen, extraction efficiency is enhanced significantly in the pH range of 1.50–4.50 when saponification rates are 0%, 20%, and 40%, respectively. Especially, extraction efficiency in these cases with saponification rates of 40% and 60% are almost 100% at pH values of 4.00 and 3.00 respectively. However, as pH rises above 3.0 in this situation with saponification rates of 60%, extraction efficiency starts to decrease. This may be explained that, the strong basicity of the saponified P507 extractant promotes to drive such hydrolysis reaction of La(III), reducing the extraction efficiency greatly. In view of the above reasons, the system with initial aqueous pH 4.00 and saponification rate 40% of P507 is used in the following experiments. 3.2. Effect of volumetric flow rate on extraction efficiency In this section, flow rates of the two phases are equal in all the experiments. Fig. 3 shows the effect of volumetric flow rate (naq) in the aqueous phase on the extraction efficiency and the corresponding relationship between residence time and extraction efficiency at pH 4 and saponification rate 40% of P507, The
Fig. 3. Relationship between extraction efficiency E and volumetric flow rate naq.
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extraction efficiency decreases with an increase in naq from 8.33 1010 to 4.17 108 m3/s, the longer the residence time and the higher extraction efficiency, and the highest extraction efficiency is almost 100% (actual value of 99.5%), that is because the longer contact time of both phases at lower volumetric flow rate naq. Hereafter, the volumetric flow rate naq for the later experiments is set at 8.33 1010 m3/s. In addition, two distinct photos are observed in the optical microscopy during the whole extraction process from the inlets to outlets (shown in Fig. 4). It is observed that the organic phase and aqueous phase successfully parallel flow without mixing while keeping an aqueous-organic interfacial in a micro-channel at all the flow rates from 8.33 1010 to 4.17 108 m3/s, if beyond the above experimental condition, segmented flow forms or emulsification appears. So, using microreactor to extract La(III) can avoid the phenomenon of emulsion occurred in the traditional mixing settler. 3.3. Extraction of La(III) in the microreactor
t¼
naq
m2 þðR 5Þ2 ¼ R2
(3)
ð80 mÞ2 þðR 40Þ2 ¼ R2
(4)
So m and R are calculated as 23.9 mm and 59.6 mm, respectively, and then n = 19.6 mm. The cross-section area of the aqueous channel (Saq) and the volume of aqueous phase (Vaq) are estimated by Eqs. (5) and (6) respectively. Z 80 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Saq ¼ ½ 59:62 ðx 23:9Þ2 19:6dx (5) 0
V aq ¼ Saq L
In this study, the residence time of the aqueous phase in the micro-channel, t (s), is calculated by the following equation: V aq
s. According to the cross-sectional geometry in the micro-channel in Fig. 5 [26], the actual size of the cross-sectional geometry is calculated, according to the Eqs. (3) and (4), as well as the known size.
(2)
where t represents the residence time, s; Vaq is the volume of the aqueous phase, m3; naq is the aqueous phase volume flow rate, m3/
(6)
In Eq. (6),L of 120 mm is the total length of the micro-channel. So the volume of aqueous phase Vaq is calculated as 3.08 1010 m3, and the residence time of the aqueous phase t value is about 0.37 s at the optimal volumetric flow rate naq 8.33 1010 m3/s. The result indicates that the extractability could be measured for a short contact time of both phases in the microreactor, while the extraction equilibrium is achieved at least 5 min in the conventional extraction equipment (not shown). From the results, a conclusion can be obtained that using the micro-reactor to extract metal ion can shorten the chemical reaction time under the same standard. 3.4. Mass transfer process As we known, microreactor has a large mass transfer rate through the interface of two phases to enhance the rate of chemical reaction because of the high interface area to volume ratio. The mass transfer rate of La(III), J (mol/m2 s) for the extraction, are based on the change in the metal concentration of the aqueous phase, and is defined as follows: J¼
naq ðC o C i Þ A
(7)
where the aqueous-organic interfacial area in the microchannel, A (m2), is calculated by Eq. (8) according to the size of cross-sectional geometry shown in Fig. 1c, and the value is 4.2 106 m2.
Fig. 4. Microscopic image magnified 100 mm of two phase flows from inlets to outlets.
Fig. 5. The actual size (mm) of the cross-sectional geometry in the microchannel.
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Fig. 6. Effect of initial pH on the mass transfer rate J ([P507] = 1.5 mol/L, saponification rate 40%).
Fig. 7. Effect of P507 concentrations on the mass transfer rate J (pH = 4.0, saponification rate 40% of P507).
A ¼ ðd hg Þ L
4. Conclusion
(8)
Fig. 6 shows the effect of initial pH in the aqueous phase on the mass transfer rate, with an increase in initial pH the mass transfer rate improves until to a flat curve, with a maximum of 3.9 102 mol/m2 s. The relationship between J and the initial concentration of the dimeric extractant P507 ([P507]) in the organic phase is shown in Fig. 7, as can be seen that, mass transfer rate follows an increasing trend until gently as extractant concentration increases. In general, an extraction process is described as follows: the metal ion in the aqueous phase diffuses from the bulk to the interface at first and then reacts with the extractant at the aqueous-organic interface to form the metal complexes, lastly the complexes transfer into the bulk organic phase. From the Figs. 6 and 7, it is considered that transfer process is controlled by the rate of La3+ diffusion in the aqueous phase due to the independent relationship between mass transfer rate J and the concentration of chemical species at high pH and high extractant concentration; however, mass transfer rate J is believed to be controlled by the interfacial chemical reaction or the diffusion rate of the metal complex in the organic phase at low pH and low [P507] because the reactivity of the complex formation is influenced by the pH and extractant concentration. 3.5. Separation process In general, the phase separation of the two phases after extraction is one of the key factors in the extractive separation process. At one time, some researches carried out a modification of the chip surface with the hydrophobic group to enhance the phase separation [27], but the employed combined hydrophobic group is easy to remove after long term operation. In the present work, therefore, the alternative method, involving the use of a complicated cross section for the phase separation is developed. The microchannel design consists of a Y-junction, two parallel channels separated by a guide structure, followed by a second Y-junction. The immiscible streams of liquid arrive at a Y-junction where they form a liquid–liquid interface that is maintained between the concurrent flows for the length of the main channel, before branching to recover the two phases. The latter is considered as a “phase separation” for the purposes of this paper. The phase separation of the aqueous and organic phases after extraction can be carried out by changing the cross section of the micro flow channel. Currently, in our group, we are study the separation performance of La/Ce, Ce/Pr, Nd/Sm, and Ho/Er/Y using this Y–Y type microchannel.
The extraction behavior of La(III) with saponified P507 has been investigated using a Y–Y shaped microreactor, and the following conclusions are drawn as follows: (1) The extraction equilibrium shows that the initial aqueous pH
value 4.00 and saponification rate 40% are the optimal operation condition. (2) Extraction efficiency decreases with an increase of the volumetric flow rate, and the highest extraction efficiency is almost 100%, and the two phases keep parallel flow without mixing in the microreactor. (3) The residence time of the aqueous phase in the microchannel is only 0.37 s at the optimal condition, which shortens the extraction time greatly. (4) Mass transfer rate increases with an increase in initial pH and [P507] converging to a flat curve, with a maximum of 3.9 102 mol/m2 s, accompanying with the interfacial chemical reaction at low pH and low extractant concentration while with diffusion process at high pH and high [P507]. In a word, the microreactor could be applicable to a system for the extraction of rare earths. Acknowledgments Financial aid from the following programs is gratefully acknowledged: the National Natural Science Foundation of China (U1302271), the National Program on Key Basic Research Project of China, (973 Program, 2014CB643404), the Young and Middle-aged Academic Technology Leader Backup Talent Cultivation Program in Yunnan Province, China (2012HB008), the Yunnan Provincial Science and Technology Innovation Talents scheme – Technological Leading Talent (2013HA002), and Kunming University of Science and Technology Personnel Training Fund (KKSY201452088). References [1] D.S. Song, S.J. Park, H.W. Kang, S.B. Park, J.I. Han, Recovery of lithium(I) strontium(II), and lanthanum(III) using Ca-alginate beads, J. Chem. Eng. Data 58 (2013) 2455–2464. [2] T. Saito, H. Sato, T. Motegi, Extraction of rare earth from La–Ni alloys by the glass slag method, J. Mater. Res. 18 (2003) 2814–2819. [3] Abhilash, S. Sinha, M.K. Sinha, B.D. Pandey, Extraction of lanthanum and cerium from Indian red mud, Int. J. Miner. Process. 127 (2014) 70–73. [4] K. Binnemans, P.T. Jones, B. Blanpain, T. Van Gerven, Y.X. Yang, A. Walton, M. Buchert, Recycling of rare earths: a critical review, J. Cleaner Prod. 51 (2013) 1– 22.
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