Solvent extraction kinetics of Sm(III), Eu(III) and Gd(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester

    Solvent extraction kinetics of Sm(III), Eu(III) and Gd(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester Zhuo Chen, Yundong Wang PII: DOI: Reference:

S1004-9541(17)30206-9 doi:10.1016/j.cjche.2017.06.022 CJCHE 865

To appear in: Received date: Revised date: Accepted date:

17 February 2017 5 June 2017 14 June 2017

Please cite this article as: Zhuo Chen, Yundong Wang, Solvent extraction kinetics of Sm(III), Eu(III) and Gd(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ester, (2017), doi:10.1016/j.cjche.2017.06.022

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ACCEPTED MANUSCRIPT Catalysis, Kinetics and Reaction Engineering

Solvent extraction kinetics of Sm(III), Eu(III) and

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Gd(III) with 2-ethylhexyl phosphoric acid-2-ethylhexyl ☆

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ester

Zhuo Chen1, Yundong Wang1,* 1

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State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua

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University, Beijing 100084, China.

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Abstract: Solvent extraction kinetics of Sm (III), Eu (III) and Gd (III) from hydrochloric acid have

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been focused on using 2-ethylhexyl phosphoric acid-2-ethylhexyl ester (P507) with Anordning for

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Kontinuerlig Undersokning av Fordelningsfaktore vid Vatske Extraction (AKUFVE). Compared with the conventional set-up, some advantages emerge obviously, for example, fast phase separation, easy operation and convenience of kinetic data acquisition. First of all, the extraction mechanism was

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discussed based on the dimeric model of P507. Secondly, the effects of stirring speed was investigated and 420 r·min-1 was determined of the following experiments. The effects of pH, concentration of rare earth elements (REEs) and P507 on the extraction rate were analyzed. The results indicated that the extraction mechanism changed with the increasing concentration of P507. Then, the experiments with different temperature were carried out. It turned out that the values of apparent activation energy (Ea) for Sm(III), Eu(III) and Gd(III) extracted by P507 were 26.80 kJ/mol, 13.40 kJ ·mol-1 and 11.10 kJ ·mol-1 respectively, the resistance of the entire process was limited by diffusion or both of diffusion and ☆

Supported by the National Natural Science Foundation of China (21476121, 21322604), NSAF (U1530107), the Natural Science Foundation of Beijing (2162020) and Tsinghua University Initiative Scientific Research Program (2014z21026) *

Corresponding author: [email protected] (Y.D. Wang)

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ACCEPTED MANUSCRIPT reaction. Finally, the correlation equations were obtained, and the theoretical results fit with the experimental data well, most relative error was within ±30%.

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Keywords: rare earth elements (REEs); extraction; kinetics; model

1. Introduction

Rare earth elements (REEs) play an important part in industry, and have been widely used in

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metallurgy, ceramics, glass, petrochemical industry, nuclear, electronics, agriculture and medicine [1-4].

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With the increasing demand of high purity REEs, differences in extraction kinetics provide a possibility for the quantitative separation of REEs that cannot be separated in equilibrium state [5]. Kovalancik [6]

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et al. investigated the extraction separation of Ce and Y by benzyldibutylamine (BDBA) in the

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presence of ethylenediaminetemlacetic acid (EDTA) as complexing agent. The concentration of Y after

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extraction in the organic phase was below the detection limit of the analytical method and the separation factor, αCe/Y, ranged from 200 to 400. Chen [7] et al. developed a selective separation

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strategy for Sc (III) from yttrium and lanthanides in aqueous solution by using ionic liquids. It was shown that the extraction efficiency of Sc (III) was as high as 99.5% at pH of 4.2. Sc (III) could be extracted selectively from yttrium and lanthanides by simple tuning of pH value of the aqueous phase. Sc (III) was preferentially extracted over yttrium and lanthanides with the ionic liquid extractants and the separation factor was 103. Some researchers discussed the extraction of lanthanum from the mixture solution of lanthanum nitrate and lanthanum chloride with Tri-Butyl-Phosphate (TBP), and concluded that the extraction rate increased with the increase of atomic number [8]. However, there exist some basic problems such as the obscure kinetic mechanism during the mass transfer from aqueous phase to organic phase. Therefore, it is necessary to investigate the extraction kinetics of REEs for further

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ACCEPTED MANUSCRIPT development of system and acquisition of data. The conventional set-up of extraction kinetics includes constant interfacial area cell [9], constant

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interfacial area cell with laminar flow [10], single drop technique [11-13], centrifugal partition

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chromatography (CPC) [14-15], stopped flow [16], growing drop cell [17] and so on. However, the following problems emerge, such as slow phase separation and difficult data acquisition. AKUFVE, integrating the reaction and separation, is used to solve the above problem effectively. No further

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centrifugal or settling phase separation are required, so the rapid and continuous determination and

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acquisition of data can be achieved. To the best of our knowledge, there has been no in-depth investigation on extraction kinetics of rare earth elements by AKUFVE until now.

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In this work, the extraction mechanism was discussed on the basis of dimeric model of P507. Then

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we used AKUFVE to study the extraction kinetics of medium rare earth elements Sm (III), Eu (III) and

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Gd (III). As for light and heavy rare earth elements, Kao [18] and Wang [19] et al. have done some researches. Here we mainly focused on the investigation of medium rare earth elements. The factors

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affecting the extraction rate such as the stirring speed, pH, concentration of REEs and P507 as well as temperature were all analyzed and the obtained experimental results of Sm (III), Eu (III) and Gd (III ) were compared with the theoretical model. 2. Theoretical kinetic model P507 (HA) usually form dimers in the organic solvent, abbreviated as H 2A2. And H2A2 can react with REEs in two different mechanisms: one is the monomeric model and the other one is dimeric. For example, the mechanisms of VO2+ reacting with P204 [20] and lanthanum reacting with Cyanex 272 [21] comply with the monomeric and dimeric model respectively. And the reaction order with respect to the concentration of H2A2 is 0.5 and 1.0. The following experimental results indicate that the 3

ACCEPTED MANUSCRIPT extraction kinetics of Sm, Eu and Gd are more consistent with the dimeric model. The total reaction between the REEs and extractant is shown in Eq. (1).

M 3  3H 2 A2  MA3 ( HA)3  3H 

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(1)

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where M3+, H2A2 and MA3(HA)3 represent the REEs, extractant P507 and the extracted complex respectively.

According to the theory proposed by Danesi and Chiarizia [22], the following equations are

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considered:

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k1 H 2 A2   2 HA

(2)

where k1 refers to the dissociation constant between the dimeric and monomeric form. 

(3)

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HA HA   HA(i )

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where αHA is the distribution constant of the extractant between the interface and organic phase, HA(i)

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represents the extractant P507 of interface. (4)

K2 MA(2i)  HA(i )   MA2( i )  H 

(5)

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K1 M 3  HA(i )  MA(2i)  H 

K3 MA2( i )  HA(i )   MA3(i )  H 

(6)

K4 MA3(i )  3HA(i )   MA3 ( HA)3

(7)

where K1, K2, K3, K4 represent equilibrium constants. The definitions of the above factors can be written as the following Eq. (8) to (12):

 HA  [ HA(i ) ] / [ HA]

(8)

2 k1  [HA]2 / [H2 A2 ]  [ HA(i ) ]2 / (HA [ H2 A2 ])

(9)

K1  [MA(2i) ][ H  ] / ([M 3 ][ HA(i ) ])

(10)

K3  [MA3(i ) ][H  ] / ([MA2( i ) ][ HA(i ) ])

(11)

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ACCEPTED MANUSCRIPT K4  [MA3 (HA)3 ] / ([MA3(i ) ][HA(i ) ]3 )

(12)

Considering Eq. (5) as the rate-controlling step [22-23], so the rate of extraction can be obtained

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as followed:

The definition of k is as the following Eq. (15):

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2 k  k1 K1 K 2 HA

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2 Rf  k1K1K2 HA [M 3 ][H2 A2 ][H  ]1

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Rf  K2[MA(2i) ][HA(i ) ]

(13) (14)

(15)

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Above all, Eq. (14) can be transformed into an alternative form:

Rf  k[M 3 ][H2 A2 ][H  ]1

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3.1 Reagents and materials

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3. Experimental

(16)

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The extractant P507 (supplied by Luoyang Aoda Chemical Co., Ltd.,) was used without further purification. And sulphonated kerosene (purchased from Hubei Prosperity Galaxy Chemical Co., Ltd.)

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was used as the diluent. The SmCl3·6H2O, EuCl3·6H2O and GdCl3·6H2O (purity>99.9%) were purchased from the Biam Alloys Co., Ltd., Beijing Institute of Aeronautical Materials of AVIC. All the other reagents were analytical grade. Initial concentration of Sm(III), Eu(III), and Gd(III) was maintained at 10 g·L-1 except researching the effect of REEs concentration on extraction rate. The concentration of REEs in aqueous phase before and after extraction were all determined by titration with standard disodium ethylene diamine tetraacetic acid (EDTA-2Na) using xylenol orange as indicator, and the amount of REEs extracted to organic phase was obtained by mass balance. The concentration of hydrogen ion in aqueous phase was determined using a FiveEasy pH meter with an electrode.

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ACCEPTED MANUSCRIPT 3.2 Apparatus and measurements Kinetics experiments were carried out with AKUFVE [24-25], which was imported from

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5

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METALLEXTRAKTION AB of Sweden. The picture of AKUFVE is shown in Fig. 1.

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Fig. 1 Picture and structure of AKUFVE

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1-Injecting hole; 2-Mixing chamber; 3-Centrifugal chamber; 4-Flowmeter; 5-Condenser

Equal volume of the organic phase and aqueous phase were added into the mixing chamber from

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injecting holes with funnel. At the same time, stirring, centrifuging and timing started at once. The volumes of both aqueous and organic phase were 150 ml. The stirring speed and centrifugal speed were regulated with the rotary knob which was on the front plate and the concrete values were displayed on the LED display. Compared with the other set-up, by using AKUFVE the data obtained and the operation are more efficient and reliable in reproducibility and stability. Once the stirring started, the organic phase and the aqueous phase contacted with each other and the mass transfer process occurred. At each 30 s interval, 2 ml of the aqueous phase was sampled for titrating analysis and 2 ml of the organic phase was also taken out to ensure a constant phase ratio. The kinetics experiments were carried out at ambient environment pressure and (25±0.1) ℃ except

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ACCEPTED MANUSCRIPT researching the effect of temperature on extraction rate. 3.3 Data analysis

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The experimental data were calculated as follows. Assuming that the mass transfer process could

forward extraction rate of extraction can be expressed as:

M (3aq )  M (o)

M (3aq )

(17)

(18)

and M ( o ) represent the REEs of aqueous phase and organic phase, respectively; V is

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where

V d[M (o) ] 1 d[M (o) ]    A dt Q dt

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Rf 

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be treated as a pseudo-first-order reversible reaction with respect to the metal cation [26], the initial

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the volume of either the aqueous or organic phase and A is the phase boundary area, Q refers to the

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specific interfacial area; t represents the reaction time. When data were processed, the relationship between concentration of M(o) in organic phase and t

rate Rf.

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was plotted for each experiment, and the slopes of the plots were used to calculate the initial extraction

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4. Results and discussion

4.1 Effect of stirring speed on extraction rate The effect of stirring speed on the extraction rate Rf was investigated for P507-kerosene system and the results are shown in Fig. 2. Actually diffusion and chemical reaction both have a close relationship with extraction rate. When chemical reaction is relatively slow, the resistance of the entire process is concentrated on reaction; In contrast, the extraction rate is controlled by diffusion; otherwise it is co-controlled by both chemical reaction and diffusion. With AKUFVE the relationship between the extraction rate and stirring speed is almost into a straight line with the stirring speed below 420 rpm. This probably results from the reduction of the boundary thickness and faster mass transfer as 7

ACCEPTED MANUSCRIPT increasing the stirring speed, and the rate-determining step is diffusion or both of diffusion and reaction which limits the extraction rate. Above a certain stirring speed (420 r·min-1), the emulsification

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phenomenon occurs slightly in P507 system. Therefore in order to guarantee the same experimental

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condition all the other kinetics experiments were carried out at the stirring speed of 420 r·min-1.

-1

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Fig. 2 Dependence of lgRf on stirring speed with P507-kerosene system (experimental conditions: [M3+] = 10 g·L , [P507] = -1

0.35 mol·L , T = 298 K, pH = 3.00).

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4.2 Effect of pH on extraction rate

The relationship of extraction rate has been obtained based on the theoretical model (Eq. (16)).

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Logarithmic operations are performed on both sides of Eq. (16), so the following equation is obtained.

lg Rf   lg[ H  ]  lg k  lg[ H2 A2 ]  lg[M 3 ]

(19)

The effect of pH on the experiments was investigated from the range of 1.00 to 4.00 at fixed concentration of rare earth elements and extractant as shown in Fig. 3. The slope of log-log relationship was the reaction order of hydrogen ion concentration. The experimental results showed that the reaction order of hydrogen ion concentration for Sm(III), Eu(III), and Gd(III) is approximate to -1, which validated the validity of theoretical model. Besides the rate constant k can be calculated according to the intercept. It was also concluded that the extraction rate raised as the pH increased from 1.00 to 3.00, for example, the logarithm of Sm (III) ranges from -4.70 to -3.05 in P507 system, after 8

ACCEPTED MANUSCRIPT that the extraction rate decreases. Therefore, the highest extraction rate can be reached at the pH of 3.00. Actually, the extractant cannot be fully dissociated in strong acidic solution, so the extraction rate is not

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the maximum. However, in weak acidic solution hydrolysis of REEs usually occurs and REEs become

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M(OH)3 precipitation. The hydrolysis product cannot be extracted, hence hydrolysis reduces the rate of extraction sharply.

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Slope:-0.82

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Slope:-0.84

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Slope:-0.82

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Fig. 3 Dependence of lgRf on lg[H+] with P507-kerosene system (experimental conditions: [M3+] = 10 g·L , [P507] = 0.35 -1

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mol·L , T = 298 K, Stirring speed = 420

r·min-1).

4.3 Effect of P507 concentration in organic phase on extraction rate

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The effect of the organic phase concentration was investigated in P507-kerosene system at fixed concentration of rare earth elements and hydrogen ion as shown in Fig. 4. The reaction order of the extractant for Sm(III), Eu(III), and Gd(III) was validated and the rate constant k was also obtained according to the similar method as described in section 4.2. In Fig. 4, it can be seen that with increasing extractant concentration, the extraction rate also linearly increase and the slopes of lgRf versus lg[P507] plot are approximately equal to 1.0 in the lower extractant concentration ranges. By contrast, in the higher extractant concentration ranges, the rate does not increase correspondingly and the slopes trend to zero. The reasonable explanation may be this fact: the extraction rate depends on the extractant interfacial concentration rather bulk concentration [27]. 9

ACCEPTED MANUSCRIPT The extractant in the interface increases with the increase of bulk concentration in lower P507 concentration ranges, and the extraction rate also increases. Otherwise P507 concentration in the

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interface is greatly excessive after dissociation and distribution in higher extractant concentration

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ranges. Therefore, the reaction order does not change with increasing extractant concentrations.

Slope:0.80

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Slope:0.97

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Slope:0.81

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Fig. 4 Dependence of lgRf on lg[P507] (experimental conditions: [M3+] = 10 g·L , Stirring speed = 420 r·min , T = 298 K,

pH = 3.00).

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4.4 Effect of REEs concentration in aqueous phase on extraction rate The effect of REEs concentration was also investigated. As shown in Fig. 5, the slopes of the plots

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for these three REEs are close to 1. The results indicate that the mass transfer process can be treated as a pseudo-first-order reversible reaction with respect to the metal cation.

Slope: 0.94

Slope: 1.18

Slope: 0.97

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Fig. 5 Dependence of lgRf on lg[M3+] with P507-kerosene system (experimental conditions: pH = 3.00, [P507] = 0.35 mol·L , T

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= 298 K, Stirring speed = 420 r·min ).

4.5 Effect of temperature on extraction rate

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The impact of temperature on extraction rate was investigated from 20 ℃ to 50 ℃. The results

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are shown in Fig. 6. Arrhenius equation describes the relationship between temperature and rate constant.

k  A exp(Ea / RT )

(20)

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activation energy and temperature respectively.

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Where k is rate constant, A is pre-exponential factor, R refers to gas constant, Ea and T represent

We perform logarithmic operations on both sides of Eq. (20), so the following equation is

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obtained. Then Ea can be calculated by the slope of the fitting line.

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lg k  lg A  ( Ea  lg e / R)  (1/ T )

(21) -1

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-1

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The activation energy for Sm(III), Eu(III), and Gd(III) are 26.80 kJ ·mol , 13.40 kJ ·mol and -1

11.10 kJ ·mol , respectively. It is generally believed that when Ea is greater than 42 kJ ·mol , -1

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extraction process is usually determined by chemical reaction [28]. When Ea is lower than 20 kJ ·mol , the main resistance is diffusion. Otherwise the extraction rate is controlled by both diffusion and chemical reaction. The activation energy results indicate that the extraction is determined by diffusion of Eu and Gd in P507 - kerosene system and Sm is controlled by chemical reaction and diffusion. However, Danesi and Vandegrift [26] reported that Ea was not sufficient for the determination of the kinetics process in the multi-component chemical reaction system. Therefore, several comprehensive analysis methods are required for the determination of the control procedure in further research.

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Slope:-0.70

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Slope:-0.58

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Slope:-1.40

-1

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Fig. 6 Dependence of lgk on temperature with P507-kerosene system (experimental conditions: [M3+] = 10 g·L , [P507] = 0.35 -1

-1

mol·L , pH = 3.00, Stirring speed = 420 r·min )

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Above all, the correlation equations are determined as:

(22)

Rf  9.09 105[ Eu3 ][ P507][ H  ]1

(23)

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Rf  7.32 105[Sm3 ][ P507][ H  ]1

Rf  11.50 105[Gd 3 ][P507][H  ]1

(24)

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As shown in Fig. 7, the correlation as Eq. (22) to (24) show good coincidence with the experimental values, most relative error is within ±30%. Wu [29] et al. proposed that the monomeric

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and dimeric models would be suitable when the reaction order with respect to [H2A2] is 0.5 and 1.0 respectively, so the experimental results obtained in the present study for Sm (III), Eu (III) and Gd (III) are expected to be best fitted by the dimeric model of P507.

+30%

-30%

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ACCEPTED MANUSCRIPT (a)

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-30%

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+30%

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(b)

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+30%

(c)

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-30%

Fig. 7 (a)-(c) Comparison of calculated and experiment values of Sm(III), Eu(III) and Gd(III).

5. Conclusions

The extraction kinetics of Sm(III), Eu(III) and Gd(III) with P507 by AKUFVE were studied. Firstly, -1

the extraction mechanism was obtained according to the dimeric model of P507. Then 420 r·min was considered as the optimal stirring speed of the following experiments. Next the factors that affect the extraction rate such as pH, concentration of REEs and P507 were also analyzed. The results showed that the extraction mechanism changed with the increasing concentration of P507. The experiments with different temperature were also carried out, it turned out that the values of Ea for Sm(III), Eu(III), -1

-1

-1

and Gd(III) extracted by P507 were 26.80 kJ ·mol , 13.40 kJ ·mol and 11.10 kJ ·mol respectively, the resistance of the entire process was concentrated on diffusion or both of diffusion and reaction. Finally, the correlation equations were obtained on the basis of the above experiments, and the theoretical results were consistent with the experimental data, most relative error was within ±30%.

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