Efficient and Sustainable Regeneration of Bifunctional Ionic Liquid for

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Efficient and Sustainable Regeneration of Bifunctional Ionic Liquid for Rare Earth Separation Chao Huang, Bin Huang, Yamin Dong, Jinqing Chen, Yanliang Wang, and Xiao-Qi Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b00159 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 7, 2017

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ACS Sustainable Chemistry & Engineering

Efficient and Sustainable Regeneration of Bifunctional Ionic Liquid for Rare Earth Separation

Chao Huang1,2, Bin Huang1,2, Yamin Dong1, Jinqing Chen2, Yanliang Wang1, Xiaoqi Sun1*

1. Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences, No.1300 Jimei Road, Jimei District, Xiamen, Fujian, 361021, P.R. China. 2. School of Metallurgy and Chemical Engineering, Jiangxi University of Science & Technology, No.156, Kejia Avenue, Ganzhou, Jiangxi, 341000, P.R. China

*Author for Correspondence: Xiaoqi Sun E-mail: [email protected] Tel.: +86 592 6376370; Fax: +86 592 6376370;

ABSTRACT: An effective and sustainable strategy of regeneration is important for the industrial application of functionalized ionic liquid in hydrometallurgy. The transformations of [tri-n-octylmethylammonium][bis-sec-octylphenoxyl acetate] ([N1888][SOPAA]) and [trihexyl (tetradecyl) phosphonium][sec-octylphenoxy acetate] ([P6,6,6,14][SOPAA])

ACS Paragon Plus Environment


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under acidic and alkaline conditions were investigated. This paper developed a novel approach for sustainable stripping and efficient regeneration of acid-base coupling bifunctional ionic liquid (ABC-BIL) for rare earth element (REE) separation. After the stripping using deionized water, both of NaOH and Na2C2O4 were indicated to be effective for the regeneration of organic phase containing [N1888][SOPAA]. In order to recycle REE, the precipitates of REE(OH)3 and REE2(C2O4)3 were analysed, and sedimentation-calcination

strategy

was

proposed.

The

investigation

of

sedimentation-calcination also offered a promising method for the preparation of REE nanomaterial.

KEYWORDS：：Ionic liquid; Extraction; Stripping; Regeneration; Nanomaterial

INTRODUCTION Ionic liquids (ILs) are molten salts, which are composed of bulky organic cations in combination with organic or inorganic anions.1 The interesting properties of ILs, i.e., non-flammable, non-volatile, wide liquid range and adjustable functional group, gained masses of interests. It shows a great deal of promise in using ILs as diluents and/or extractants. Because of their excellent extractabilities, many functional ILs were studied in solvent extraction for metal ions. Functionalized ionic liquids (FILs) incorporate functional groups in their cations and/or anions, they can behave as both the organic phases and extracting agents.2,3 It is possible to design some FILs with


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specific chemical and physical properties that meet the need of metal ions separation. Recently, the acid-base coupling bifunctional ionic liquid (ABC-BIL) has been widely studied because of their interesting extractabilities and selectivities for metal ions. The inner synergistic effect of ABC-BIL for solvent extraction was reported using [tricaprylmethylammonium][di-2-ethylhexylphosphinate] as an extractant for Eu (III).4 [N1888][SOPAA] was proved to be efficient for extracting and separating heavy REEs in the chloride medium.5 The application of [trihexyl (tetradecyl) phosphonium][bis

(2-ethylhexyl)

amine]

and

[trihexyl

(tetradecyl)

phosphonium][NO3] to Eu (III) extraction and separation was studied.6 Stripping and regeneration of IL are important because the unimpeded pathway of

extraction-stripping-regeneration

is

an

essential

presuppose

for

the

commercialization of IL based extraction. To our knowledge, the step of stripping-regeneration for ABC-BIL based extraction is still not studied extensively. The stripping and regeneration strategy depends on the extraction mechanism and properties of IL. In previous investigations, part of ILs could be regenerated by electro-deposition, benefiting from high conductivity, low viscosity and low water uptake.7, 8 Unfortunately, the resistances and viscosities of most ILs had difficulties in the processes of electro-deposition. It was also decided that electro-deposition with functionalized IL [tetraoctylammonium][oleate] was not possible.9 Chemical stripping and regeneration of extractant by high concentration of acidity was widely used in conventional hydrometallurgy.10 As for ABC-BIL based extraction, high acidity was



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also effective for the stripping. For example, [trihexyl (tetradecyl) phosphonium][bis (2,4,4-trimethylpentyl) phosphinate] was used to separate REEs and nickel by solvent extraction, and the REEs were stripped with a 2 mol/L HNO3 solution.11 However, the high stripping acidity was not sustainable. Also, it was quite likely that the extracted acid associated with functionalized ionic liquid leading to the formation of its precursors.12 Benefiting from the adjustable functional groups of ABC-BILs, some ABC-BIL based extraction systems can be stripped more easily so as to reduce/remove the consumption of acid or/and base. As can be seen in Table 1, the stripping properties of some ABC-BILs are compared. [tricaprylmethylammonium][secoctylphenoxy acetate] and [tricaprylmethylammonium][secnonylphenoxy acetate] were studied to extract and separate La(III) from chloride medium, the La(III) could be stripped more than 95% from the loaded organic phase when the acidity was higher than 0.03 mol/L.13 Very recently, the strategy of metals stripped with water was put forward, a countercurrent extraction processing by [trihexyl (tetradecyl) phosphonium][sec-octylphenoxy acetate] was developed to separate Y3+ and Lu3+.14 Also, [N1888][SOPAA] was used to extract Y (III) from Yttrium-enriched REEs by countercurrent extraction.15 The clean extraction-washing-stripping-regeneration process is desirable to improve separation efficiency and sustainability of IL-based extraction. After stripping for 5 times with equal volume of water, the total stripping ratio of REE3+ in the organic phase containing [N1888][SOPAA] arrived at 79.38%.16 However, the organic phase of


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[N1888][SOPAA] tended to be emulsification after 5 times of stripping with water, and the residuary REE was an obstacle of high-purity REE preparation. Based on our recent investigation, a novel strategy is further investigated for the stripping and regeneration of ABC-BIL in this article. Table 1. The stripping properties of ABC-BIL based extraction systems Stripping IL

Metal ion

Recycling Reagents

Time

Ratio

0.5 mol/L Cyphos IL 10120

Fe (III)

3

99.5%

1

98%/93.6%

/

H2SO4

0.02 mol/L Cyphos IL 10422

Gd (III)

No negative effect and the organic

HCl/Na2S2O3

[N1888][CA12],

phase could be easily recycled.

0.03 mol/L La (III)

[N1888][CA100] 13

1

>95%

4

100%

Scrubbed with NaOH and 96.5%

0.5 mol/L [A336][DGA] 21

Nd (III) HNO3

[N1888][SOPAA]18

REE (III)

[A336][P507],

water

extraction efficiency was obtained.

4

79.38%

1

>95%

0.3 mol/L REE (III)

[A336][P204] 16 19

Without loss of extraction efficiency.

HCl

HNO3

/

The loss of the extraction efficiency was almost negligible.



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[P66614][R2POO]10

Ni (II),

2 mol/L

Scrubbed with NH3 and without a 2

11

REEs

HNO3

[P66614][SOPAA]13

Y (III),

0.008 mol/L

14

Lu (III)

HCl

REEs,

1.5 mol/L

[Hbet][Tf2N] 15 Co (II)

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100% significant loss in the extraction power.

1

100%

1

>99%

/

Scrubbed with Na2SO4 to prevent the loss of IL

H2C2O4 1 mol/L HCl,

Y (III), [Hbet][Tf2N] 16

1.5 mol/L

1

100%

/

Eu (III) H2C2O4

MATERIALS AND METHODS Reagents and Materials [N1888][SOPAA] and [P6,6,6,14][SOPAA] was prepared using the combination of ion-exchange and neutralizing reactions (As described in the Supporting Information).17 (2-sec-octylphenoxy) acetic acid (HSOPAA, CA-12) was purchased from Luoyang Aoda Chemical Co. Ltd (China) without any purification. The 260# kerosene used as diluent was provided by Shanghai Rare-earth Chemical Co., Ltd. (China). Methyltrioctylammonium chloride ([N1888]Cl) was supplied by Xiamen Pioneer Technology Co. Ltd (China). Yttrium-enriched rare earth feed was provided by Xiamen Tungsten Corporation, Ltd., China, and its ingredients are shown in Table 2. Sodium hydroxide (NaOH, 99 wt% purity), sodium oxalate (Na2C2O4, 99 wt% purity), disodium ethylenediaminetetraacetate dihydrate (C10H14N2Na2O8•2H20, 99 wt%


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purity) were supplied by Sinopharm Chemical Reagent Co., Ltd. The total concentrations of mixed REEs and free H+ were determined by volumetric titration with standard solution of EDTA and NaOH, respectively. All the other chemicals used in this study were of analytical grade. OCH2COO

CH3 N C8H17

C8H17 C8H17 C8H17

[N1888][SOPAA]

OCH2COO

C14H29 P C6H13

C6H13 C6H13 C8H17

[P6,6,6,14][SOPAA]

Figure 1. Structure of [N1888][SOPAA] and [P6,6,6,14][SOPAA] Table. 2 Rare earth composition of the feed solution, REE3+ = 0.22mol/L, pH = 6. REEO

Ho2O3

Er2O3

Tm2O3

Yb2O3

Lu2O3

Y2O3

Wt(%)

3.60

4.73

1.35

2.7

0.30

87.3

Apparatus and Measurements A model PHSJ-4F pH meter（Leici shanghai, China）was used to measure the pH value of aqueous phase. NMR spectra were recorded using an AVANCEIII 500 MHz spectrometer (Bruker). Infrared spectra were obtained on a Nicolet IS 50 infrared spectrometer in the range of 3500-750 cm-1. Thermos scientific iCAP 6500 series inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used for the ingredient analysis of REE. The differential and thermogracimetric analysis



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(TGA-DSC) of dried precursor was performed on a TGA-DSC analyzer (Mettler Toledo, TGA/DSC1) with a heating rate of 10℃/min in a static air atmosphere and calcined at 900℃ for half an hour. Scanning electron microscopy (FESEM, S4800, Hitachi, Japan) was used to observe the morphologies of obtained materials. Regeneration and Recycling Experiments The regeneration experiments were performed by contacting equal volumes (except specifically defined) of organic phase with REE solution for 30 min in a thermostatic air bath oscillator at 25℃. After the aqueous phase was separated, the equilibrium concentration of REE in aqueous phase ([M]aq) was determined by titration method or ICP-OES determination. The amount of REE in organic phase ([M]org) was calculated by mass balance. The regeneration experiment was conducted by contacting 5ml of organic phase containing [N1888][SOPAA] with 5ml of NaOH or Na2C2O4 with different concentrations for 30min in a thermostatic air bath oscillator at 25 ℃ . After settling until organic phase and aqueous phase separated, the concentration of REE in aqueous phase was determined, and the concentration of REE in organic phase was calculated by mass balance. Because of the formed REE2(C2O4)3 and REE(OH)3 precipitates were dispersed in the organic phase, centrifugal and membrane filtration were used to separate the precipitates from the organic

phase.

The

precipitates

for

analyzing

were

washed

with

ethanol/trichloromethane for several times to remove the residual organic phase. After drying in a vacuum drying oven at 80℃ for 5 hours, the product was obtained by


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calcining the precipitates at 900℃ for 3 hours. In order to evaluate the effectiveness of stripping and regeneration, removing ratio was set up to evaluate the removed metals. The extraction efficiency (E), stripping ratio (S) and removing ratio (R) are defined as follows: E% =

[ M ]org ×100% [ M ]org + [ M ]aq

(1)

S% =

[ M ]aq,a × 100% [ M ]org,t

(2)

[ M ]org ,b ×100% [ M ]org ,a

(3)

R% =

[M]aq and [M]org are the equilibrium concentration of REE in aqueous and organic phase, respectively. [M]aq,a is the equilibrium concentration of REE in aqueous and [M]org,t is the initial concentration of REE in loaded organic phase. [M]org,b is the concentration of REE before regeneration and [M]org,a is the concentration of REE after regeneration. All the concentration values of REEs were measured in duplicate with the uncertainty within 5%.

RESULTS AND DISCUSSION Transformation of ABC-BIL NMR is an effective technology to characterize the chemical environment of extractant before and after extraction.23 To further gain insight into the status of [N1888][SOPAA] and [P6,6,6,14][SOPAA] under acidic and alkaline conditions, the 1H NMR spectra of [N1888][SOPAA] and [P6,6,6,14][SOPAA] equilibrated with HCl were



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analyzed. Also, the acid-equilibrated [N1888][SOPAA] and [P6,6,6,14][SOPAA] were re-equilibrated with NaOH. As shown in Figure 1(a)-1, there is a pronounced chemical shift of hydrogen atom in COOH from HSOPAA at 13.043 ppm. The chemical shift of hydrogen atom is also quite pronounced for the mixture of [N1888]Cl and HSOPAA (Figure 1(a)-2). When the [N1888]Cl and HSOPAA were prepared to be [N1888][SOPAA] (Figure 1(a)-3) by neutralizing reaction, the chemical shift of hydrogen atom in COOH at 13.043 ppm disappears. As can be seen in Figure 1(a)-4, the chemical shift of hydrogen atom in COOH at 13.043 ppm appears when the [N1888][SOPAA] was equilibrated with 1.2 mol/L HCl. The emerged chemical shift illustrates the transformation of [N1888][SOPAA] to HSOPAA under highly acidic conditions. Interestingly, the chemical shift of hydrogen atom from COOH disappears when the decomposed [N1888][SOPAA] (Figure 1(a)-4) was re-equilibrated with 1.2 mol/L NaOH (Figure 1(a)-5). The phenomenon reveals the regeneration of decomposed [N1888][SOPAA] under the alkaline condition. In addition, we compared the IR spectra of [N1888][SOPAA] under acidic and alkaline conditions, which was corresponding with the NMR results. As shown in Figure 1 (b), it can be observed that the equilibrated [N1888][SOPAA] with 1.2mol/L HCl (Figure 1(b)-4) and mixture of [N1888]Cl and HSOPAA (Figure 1(b)-2) are exactly the same. The comparison also indicates the conversion of [N1888][SOPAA] to its precursors i.e., [N1888]Cl and HSOPAA under the condition of higher acidity. Moreover, the IR spectrum of regenerated [N1888][SOPAA] (Figure 1(b)-5) is almost the same as that of the initial


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[N1888][SOPAA] (Figure 1(b)-3). The comparison of spectra of (1), (2), (3), (4) and (5) from 1H NMR and IR mentioned above confirms that there exists a transformation between [N1888][SOPAA] and the mixture of [N1888]Cl, HSOPAA at the higher acidity and alkaline. Interestingly, the similar transformation can be also observed using 1H NMR (Figure 1(c)) and IR (Figure 1(d)) for [P6,6,6,14][SOPAA]. Accordingly, the transformations of [N1888][SOPAA] and [P6,6,6,14][SOPAA] under acidic and alkaline conditions can be represented by the following equation (4) and (5): (4) (5) The comparison confirms the dissociation of ABC-BIL under high acidity.9,24 By comparing the 1H NMR spectra in Figure 1 (a) and Figure 1 (c), it can be inferred that the dissociation acidity of [P6,6,6,14][SOPAA] (1.2 mol/L) is easier than that of [N1888][CA12] (1.2 mol/L). While the additional alkali contributes to the regeneration of ABC-BIL, the application of acid and alkali reveals negative effect for the sustainability of ABC-BIL based extraction.



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Figure 2. 1H NMR spectra and IR transmittance spectra of extracting agent in different

extracting

systems.

(1)

HSOPAA,

(2)

[N1888]Cl+[HSOPAA],

(3)

[N1888][SOPAA], (4) [N1888][SOPAA]-HCl, (5) [N1888][SOPAA]-HCl-NaOH, (6) [P6,6,6,14]Cl+HSOPAA,

(7)

[P6,6,6,14][SOPAA],

(8)

[P6,6,6,14][SOPAA]-HCl,

(9)

[P6,6,6,14][SOPAA]-HCl-NaOH. Different Chemicals for ABC-BIL Regeneration In order to strip REE3+ fast and effectively, high concentration of HCl was commonly used in conventional extraction system. However, the vase consumption of acid is not sustainable, reducing the dosage of acid during the stripping is an important trend.25 As mentioned above, the ABC-BIL will be transformed to its precursors under higher acidity. In this study, the [N1888][SOPAA] loaded with REEs was stripped by water firstly, and the total stripping ratio of REE3+ in organic phase of


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[N1888][SOPAA] arrived at about 80% without emulsification, the residual REE(III) was removed by Na2C2O4, NaOH, Disodium EDTA, respectively. Because of the different solubilities of Na2C2O4, NaOH, Disodium EDTA in water,26 appropriate concentrations of the chemicals were used in the regeneration experiments. As shown in Figure 2, the stripping rates of REEs increase with the increasing concentrations of Na2C2O4 and Disodium EDTA. Unlike Na2C2O4 and Disodium EDTA, the removing ratio of REE3+ by NaOH increases firstly and then decreases. Because of the stronger removing ratios under lower concentrations, Na2C2O4 and NaOH were further studied for REE removal. Concentration of NaOH, mol/L 0

2

4

6

8

60%

Removing ratio, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a)

45%

Disodium EDTA Na2C2O4

30%

(b)

NaOH (c)

15%

0% 0.00

0.05

0.10

0.15

0.20

Concentration of Na2C2O4 and Disodium EDTA , mol/L

Figure 3. Removing ratios of REE with different chemicals；(a) Na2C2O4, (b) NaOH, (c) C10H14N2Na2O8.

Effects of Duration, Centrifugation and Membrane After mixing NaOH or Na2C2O4 with organic phase containing [N1888][SOPAA] loaded with REEs, precipitates could be always observed. The precipitates of REE(OH)3 or REE2(C2O4)3 were indicated to be effective to remove the loaded REEs



from organic phase containing [N1888][SOPAA]. As show in Figure 3, both the removing ratio of REE3+ by 0.5mol/L NaOH and 0.2mol/L Na2C2O4 increase as time grown. Their highest removing ratios of REE3+ from organic phases arrive at 89.06% and 98.86%, respectively. The removing ratios of REE3+ by 0.2 mol/L Na2C2O4 are higher than those by 0.5 mol/L NaOH. This difference may be attributed to the sizes of different precipitates i.e., REE(OH)3 and REE2(C2O4)3. In the previous study, RE2(C2O4)3 was indicated to be the crystalline precipitate, but REE(OH)3 was the amorphous sediment.27 As a result, the settling velocity of REE2(C2O4)3 was faster than that of REE(OH)3.

100%

80%

Na2C2O4 NaOH

Removing rate, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60%

40%

20%

0% 1

2

3

4

Time， days

5

6

7

Figure 4. Effect of duration on the removing ratio Centrifuge was widely used in electronics, metallurgy, chemical and other industries.28 In search of a more efficient way, centrifuge method was carried out to wipe off the precipitates in this study. As show in Figure 4, the removing ratio of REE3+ increases as the centrifugal speed was raised. Clearly, the removing ratio of REE3+ from extracting phase regenerated by 0.2 mol/L Na2C2O4 is superior to that


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regenerated by 0.5 mol/L NaOH. The removing ratio of REE3+ is 85.1% under the conditions of 0.2 mol/L Na2C2O4 and 4000 r/min centrifugation. However, the removing ratio of REE3+ is still 48.4% when 0.5 mol/L NaOH was used as the regenerator.

100%

Na2C2O4 80%

Removing ratio (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NaOH

60%

40% 20% 0% 500

1000

1500

2000

2500

3000

3500

4000

Revolving speed (r/min)

Figure 5. Effect of centrifugal speed on the removing ratio To evaluate the possibility of membrane separation, the microfiltration membranes with different pore sizes were conducted to remove the crystalline precipitate of REE2(C2O4)3 and amorphous sediment of REE(OH)3. The results revealed that the removing ratios increased as the pore sizes of microfiltration membranes were decreased. As can be seen in Figure 5, the complete removing of REE(OH)3 can be achieved by 0.22 um pore microfiltration membrane. The removing ratio of REE(OH)3 decreases sharply when the pore size of microfiltration membrane is bigger than 2 um. The obvious decrease can be attributed to the size of REE(OH)3 is close to be 2 um. As for the REE2(C2O4)3, the increase of microfiltration membrane pore size also leads to a clearly decrease of REE3+ removing ratio.



It's worth noting that the removing ratio of 0.5 mol/L NaOH was better when the pore size of microfiltration membrane was less than 1 um. The concentrations of REEs in organic phase filtrated by microfiltration membrane of 0.22 um pore were studied. The removing ratio of REE3+ is 99.2% and concentrations of REEs in organic phase are below 3 mg/L when 0.5 mol/L NaOH was used as a regenerate agent.

100%

Na2C2O4 NaOH

80%

Removing rate, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

60%

40% 20% 0% 0.22

0.45

1

2

5

Pore size, um

Figure 6. Effect of pore size in microfiltration membrane on the removing ratio To develop a better strategy of removing REE3+, organic phase was centrifuged within 4000 r/min firstly. Then the precipitates of REE(OH)3 and REE2(C2O4)3 were filtrated by microfiltration membrane with pore size of 0.22 um. The removing ratios of REE3+ arrived at 96.8% and 99.4% when 0.2 mol/L Na2C2O4 and 0.5 mol/L NaOH were used as the regenerators, respectively. In combination, the removing effects of combined centrifugation and filtration method were superior to any of individual centrifugation or filtration. Investigation of Precipitate


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(a) 1430 555

(b)

Transmittance, %

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3415

(c)

1375 1321

1620

1430 558

(d) 3410

4000

3500

1505 1375

3000

2500

2000

1500

-1 Wavenumber, cm

1000

500

Figure 7. IR spectra of precipitates and calcined precipitates: (a) calcined precipitate of REE2(C2O4)3; (b) precipitate of REE2(C2O4)3; (c) calcined precipitate of REE(OH)3; (d) precipitate of REE(OH)3. The precipitates of REE(OH)3 and REE2(C2O4)3 were gathered from regenerated organic phases after centrifugation, serial experiments were carried out to recycle the REEs. As shown in Figure 6, the IR spectra of precipitates and calcined precipitates at 800℃ was compared. The bending modes at 1620, 1375, 1321 cm-1 in Figure 6 (a) can be assigned to the C=O in Na2C2O4 and [N1888][SOPAA], C-N in [N1888][SOPAA], C-H in [N1888][SOPAA], respectively. As for Figure 6 (c), the bending modes appear at 1505, 1375 cm-1 can be assigned to C=O in [N1888][SOPAA] and C-N in [N1888][SOPAA]. In addition, the bending modes that appear at 3415 cm-1 and 3410 cm-1 in Figure 6 (b) and Figure 6 (d) can be attributed to H2O, the bending modes at 555 and 558 cm-1 are C-O-REE, and the bending modes at 1430 cm-1 may be attributed to -NO2.29 Similar to the previous investigation30, 31, the band of -NO2 can be contributed to residuary organic phase containing extractant [N1888][SOPAA]. The



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comparison of Figure 6 (a), (c) and Figure 6 (b), (d) confirms that the precipitates can be turned into REE oxide by calcining.

Figure 8. SEM of precipitates and calcined precipitates: (a) calcined precipitate of REE2(C2O4)3; (b) precipitate of REE2(C2O4)3; (c) calcined precipitate of REE(OH)3; (d) precipitate of REE(OH)3. Further structural characterization of the precipitate powders using scanning electron microscopy (SEM) reveals that there was no obvious difference between REE(OH)3 and calcined REE(OH)3. However, the powders of REE2(C2O4)3 turned into graininess from schistose after calcining. Figure 7 (a) and Figure 7 (c) reveal that the sizes of REE2(C2O4)3 particles are bigger than those of REE(OH)3 particles. The superior crystallization property of REE2(C2O4)3 made a significant contribution to its centrifuging and filtering. Figure 7 (a) shows the adhesion of REE2(C2O4)3 is not evident, but the dispersity of calcined REE2(C2O4)3 is not well (Figure 7 (b)). While,


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the adhesion of REE(OH)3 and calcined REE(OH)3 are obvious, and the interparticle spacing of powders are widened after the calcining process. There might be certain heredity of structural characterization between the powders of REE oxide and precipitate of REE(OH)3 (Figure 7 (c) and Figure 7 (d)). The calcined REE(OH)3 crystallites in Figure 7 (d) with diameters of 50 nm to 70 nm can be called nanoparticles or ultrafine particles.

100% (a) (b)

80% Weight loss, %

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REE2(C2O4)3 weight loss 82.50% REE(OH)3 weight loss 69.86%

60% 40%

(b)

20% (a)

0% 0

200

400 Temperature,

600

800

Figure 9. TGA curves of REE(OH)3 and REE2(C2O4)3 precipitates After being collected, the precipitates were analyzed by TGA to determine the calcination temperature of REE(OH)3 and REE2(C2O4)3 (Figure 8). For both of the samples, there are three major stages of mass losses. Figure 8 (a) shows that the first stage of REE(OH)3 is at Troom ≤ T ≤ 200 ℃ due to water and kerosene evaporation, the second stage is at 200℃ ≤ T ≤ 500℃ caused by decomposition reaction of REE(OH)3. Above 500℃, the slight slope can be attributed to the release of gas inside the sample. With regard to REE2(C2O4)3 (Figure 8 (b)), the first stage is also at Troom ≤ T ≤ 200 ℃ , but the second stage from REE2(C2O4)3



decomposition is at 200℃ ≤ T ≤ 310℃. When the temperature exceeds 310℃, the mass loss of REE2(C2O4)3 slows down and levels off, the slight slope can be attributed to the release of gas inside the sample. For these reasons, 500℃ and 310℃ were used as the calcination temperatures for the precipitates of REE2(C2O4)3 and REE(OH)3, respectively. Recycling Experiment

Na2C2O4 60%

NaOH

40%

E%

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20%

0% fresh IL

1st

2nd

3rd

4th

Number of extraction-strpping cycle

Figure 10. Extraction efficiency of REE3+ with regenerated [N1888][SOPAA], [N1888][SOPAA]= 0.4 mol/L, ∑REEs = 0.12 mol/L, NaCl = 2.5 mol/L, pH = 6, O/A = 1:1 To verify the recyclability of organic phase regenerated by NaOH or Na2C2O4, a series of extraction-stripping-regenerating experiments were performed. The fresh organic phase containing [N1888][SOPAA] was equilibrated with an equal volume of REE feed solution. After phase separation, the organic phase loaded with REE was stripped for three times by an equal volume of water. The regenerated organic phase by NaOH or Na2C2O4 was then reused for a new extraction under similar experimental conditions. As revealed in Figure 9, it is clearly that the regenerated


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organic phase possessed similar extraction behaviors as the fresh organic phase, the comparison indicates that the developed ABC-BIL regeneration strategy in this study is effective for REEs industrial feed. CONCLUSIONS In summary, [N1888][SOPAA] was used for extracting and separating Y3+ from Yttrium-enrichments. The transformation of [N1888][SOPAA] under acidic and alkaline conditions was investigated by NMR and IR. To develop an sustainable regeneration strategy for [N1888][SOPAA], the organic phase loaded with REEs was stripped by water firstly to remove about 80% loaded REEs. After regenerated by 0.2 mol/L Na2C2O4 or 0.5 mol/L NaOH, the precipitates were wiped off by centrifugal and filtration. A series of extraction-stripping-regenerating cycles showed that the organic phase containing [N1888][SOPAA] could be recycled without performance loss. As for the recovery of REE3+ in precipitate, the calcination temperatures for precipitates of REE(OH)3 and REE2(C2O4)3 were identified to be 500℃ and 310℃, respectively. In the efficient and sustainable strategy, water is the simplest, cheapest and cleanest stripping reagent. After stripping with water, the organic phase loaded with residual REEs was regenerated by a lower consumption of chemicals, i.e. NaOH and Na2C2O4. Either of NaOH and Na2C2O4 can be used to remove the REE3+. However, C2O42+ is harmful to the subsequent extraction of REE due to the insoluble REE2(C2O4)3. Because of the high removing ratio for REE3+, harmlessness for extraction process, potentiality for REE nano-material preparation, it can be



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concluded that NaOH is an optimizing agent for the regeneration of bifunctional ionic liquid [N1888][SOPAA] for rare earth separation. In this study, an efficient and sustainable regeneration strategy of bifunctional IL for rare earth separation was proposed, further development on the IL based extraction is underway in this lab. ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (21571179), ‘Hundred Talents Program’ of the Chinese Academy of Sciences and Science and Technology Major Projects of Fujian Province (2015HZ0001-3). The authors wish to thank Xiamen Tungsten Corporation, Ltd. for the supply of heavy REE enrichments from ion-adsorbed mineral. REFERENCES (1) Wilkes, J. S.; A short history of ionic liquids-from molten salts to neoteric solvents.

Green Chem. 2002, 4, 73-80. (2) Sun, X. Q.; Luo, H. M.; Dai, S. Ionic liquids-based extraction: a promising strategy for the advanced nuclear fuel cycle. Chem. Rev. 2012, 112, 2100−2128. (3) Wellens, S.; Goovaerts, R.; Möller, C.; Luyten, J.; Thijsb, B.; Binnemans, K. A continuous ionic liquid extraction process for the separation of cobalt from nickel.

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Efficient and Sustainable Regeneration of Bifunctional Ionic Liquid for Rare Earth Separation

Chao Huang1,2, Bin Huang1,2, Yamin Dong1, Jinqing Chen2, Yanliang Wang1, Xiaoqi Sun1*

The

stripping-precipitation-regeneration-preparation

strategy

is

efficient

and

sustainable for regeneration of bifunctional ionic liquid and preparation of RE nanomaterial.

Graphical abstract for manuscript (For Table of Contents Use Only)


Efficient and Sustainable Regeneration of Bifunctional Ionic Liquid for

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