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Recovery of Th(IV) from leaching solutions of rare earth residues using a synergistic solvent extraction system consisting of Cyanex 572 and n-octyl diphenyl phosphate (ODP)
Hydrometallurgy 183 (2019) 186–192
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Recovery of Th(IV) from leaching solutions of rare earth residues using a synergistic solvent extraction system consisting of Cyanex 572 and n-octyl diphenyl phosphate (ODP) Haiyue Zhoua,c, Yamin Donga, Yabin Wanga, Zeyuan Zhaoa, Yu Xiaoa, Xiaoqi Suna,b,
T
⁎
a
CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China b Ganzhou Rare Earth Group Co., Ltd., China Southern Rare Earth, Ganzhou 341000, PR China c Fujian Normal University, Fuzhou, Fujian 35007, PR China
ARTICLE INFO
ABSTRACT
Keywords: Synergistic extraction Cyanex 572 n-octyl diphenyl phosphate Rare earth Thorium (Th)
The synergistic effect of mixed Cyanex 572 and n-octyl diphenyl phosphate for Th(IV) extraction from nitric acid solution has been investigated in this paper. The effect of acidity together with mole fraction of extractant on the synergistic effect was studied. Based on the obtained results, the maximal synergistic enhancement coefficient reaches 12.40 at a ratio of 1:2 of Cyanex 572 and n-octyl diphenyl phosphate. Under the experimental conditions, the phase separation performance, stripping property and selectivity of Th and REE in the mixed extraction system are better than those from individual Cyanex 572 or n-octyl diphenyl phosphate system. The synergistic 4+ + [(HA)2 ]org + extraction mechanism for Th(IV) in nitric acid medium is proposed as Thaq K ex
2.5Borg + 3(NO3 )aq [Th(NO3 )3 HA2B2.5 ]org + H+aq . The mixed extraction system is successfully used for the separation of Th(IV) and REE(III) from the industrial feed solution leached from ion-absorbed type rare earth mineral waste residue. Lower stripping acid, stronger extraction ability, better phase separation performance and selectivity provide the synergistic extraction system with good application prospects.
1. Introduction As an important natural radioactive element, Thorium (Th) is widely distributed in the earth's crust (Lin et al., 2010). Th is indicated to be the most potential alternative of uranium to produce nuclear fuel (Arnold et al., 2014; Ashley et al., 2012). Also, Th is applied in some other fields, i.e., catalyst, functional material, agriculture, optics and aerospace (Lin et al., 2010; Cheng et al., 2011). Although widely used, the radioactivity and toxicity of Th, even at trace level, still poses a serious threat to the ecosystem and human health (Yuan et al., 2014). In addition, thorium and rare earth elements (REEs) are always occurred together in the minerals such as monazite, bastnasite and ion-adsorption type rare earth minerals, which greatly limits their respective applications (Lu et al., 2013). Accordingly, the separation of Th and REEs from rare earth minerals or waste residues is significant from the point of economic value and ecological effect (Deng et al., 2013; Wang et al., 2017). The recovery and separation of Th from REEs is an important task in
hydrometallurgy, which leads to larger amount of investigation on Th separation in the past few years (Nasab et al., 2011; Zhang et al., 2012; Zuo et al., 2008). Attracted by its simplicity, rapidity and ease of operation, solvent extraction has turned to be an efficient and promising technique in the separation and purity of Th (Paiva and Malik, 2004; Tan et al., 2015; Shrivastav et al., 2001). Common extractants including organophosphorus compounds (Yaftian et al., 2003; Sato, 1966), amines (Bismondo et al., 1999), carboxylic acids (Jain et al., 2005), amides (Pathak et al., 2001) and crown ethers (El-Dessouky and Borai, 2006). Cyanex 572 was a new commercial extractant, it has been studied for extracting REEs and Th from waste residues including ion-absorbed type minerals or fluorescent lamp (Tunsu et al., 2016). Recently, more and more attentions have been paid for developing new and efficient extraction systems for Th extraction. Synergistic extraction is an important research field in solvent extraction. Owing to its unique advantages like better extraction efficiency, selectivity, phase separation and stripping properties (Ma et al., 2017; Fan et al., 2011), synergistic extraction causes increasing interests. Several synergistic
⁎ Corresponding author at: CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China. E-mail address: [email protected] (X. Sun).
https://doi.org/10.1016/j.hydromet.2018.12.008 Received 11 July 2018; Received in revised form 25 November 2018; Accepted 7 December 2018 Available online 12 December 2018 0304-386X/ © 2018 Published by Elsevier B.V.
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extraction systems like Cyanex 572-Cyanex 923 (Ma et al., 2018), Nnoctylaniline-Trioctylamine (Patkar et al., 2009), HPMPP-TBP/CMPO/ TOPO (Meera et al., 2004), HFBPI-DC18C6/DB18C6/B15C5 (Reddy and Meera, 2001), PMBP–TOPO (Mohapatra et al., 1999), HTTA-TBP (Patil et al., 1979) and Cyanex 923–P204/P507/Cyanex 272 (Tong et al., 2013) have been studied to further optimize the extraction process of Th. Patkar et al. investigated the extraction of Th from aqueous sulphuric acid medium with a synergistic mixture of N-n-octylaniline and TOA (Patkar et al., 2009). Meera et al. researched the synergistic extraction behaviors of HPMPP and TBP/CMPO/TOPO for Th(IV) and U (VI), respectively (Meera et al., 2004). Very recently, n-octyl diphenyl phosphate (ODP) and Cyanex 572 were reported for the separation of Th and REEs in this lab (Dong et al., 2017; Wang et al., 2017). In this paper, attracted by the excellent performance of synergistic extraction, a new combination of Cyanex 572 and n-octyl diphenyl phosphate was investigated in the case of the synergistic extraction for Th(IV) from nitric acid solutions.
(2)
R=
Dmix Da + Db
(3)
=
[M]aq,a × Vaq [M]org,t × Vorg
× 100
D1 D2
(4)
(5)
where, [M]t and [M]a denote the concentrations of metal ions in aqueous phase before and after extraction, respectively. Da is the distribution ratio of one extractant for a metal; Db is the distribution ratio of the other extractant for a metal; Dmix is the distribution ratio of their mixture for a metal. [M]aq,a is equilibrium concentration of a metal in stripping aqueous phase. [M]org,t is the concentration of a metal loaded in the organic phase for stripping. Vaq and Vorg are the volumes of the aqueous phase and organic phase, respectively. D1 and D2 are the distribution ratios of metal 1 and 2, respectively. All the concentration values of metal were measured in duplicate with the uncertainty within 5%. Th(IV) complexes for IR spectra were prepared by equilibrating equal volume of aqueous phase containing 1 × 10−3 mol/L Th(IV) with 0.1 mol/L mixed Cyanex 572 and n-octyl diphenyl phosphate (XODP:XCyanex 572 = 2:1) in n-heptane, respectively. After the phase separation, the n-heptane was evaporated and the samples of Th(IV) complexes were obtained. To confirm the stability of Cyanex 572 in HNO3 media, stability test was prepared by equilibrating 4 mL of 0.25 mol/L HNO3 with 0.6 g Cyanex 572 for 1 h. After the phase separation, the equilibrated Cyanex 572 were obtained for IR spectra.
2.1. Reagents and apparatus Cyanex 572 were obtained from Cytec Industries Inc. (USA), which was used without further purification and saponification (Wang et al., 2017). n-octyl diphenyl phosphate was synthesized and purified according to our previous work (Dong et al., 2017), diphenyl chlorophosphate, 1-bromooctane and magnesium were purchased from Xiya Reagen (China). Heptane of analytical reagent was purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used as the diluent. The organic phases were prepared by dissolving the extractants in nheptane. Th(NO)4 stock solution was prepared by dissolving Th(NO3)4 with deionized water. The feed solution for Th, Fe and RE separation was leached from the ion-absorbed rare earth waste residue. All the other chemicals in the work were used without further purification.
3. Results and discussion
2.2. Instrumentation
3.1. The concentration effect of HNO3 on Th(IV) extraction
Thermo scientific iCAP 6500 series inductively coupled plasmaatomic emission spectroscopy (ICP-OES) instrument was used to determine the concentration of metal ions in the aqueous phase. IR spectra were recorded with a Nicolet iS50 spectrometer. The pH values of aqueous solution were determined by PHSJ-4F made by INESA scientific instrument CO., Ltd. or acid-base titration.
The effect of aqueous acidity on Th(IV) extraction efficiency was obtained by the extraction experiments of Cyanex 572, ODP and their mixture. It is observed from Fig. 1(a), the extraction efficiencies of individual Cyanex 572 and mixed Cyanex 572 and n-octyl diphenyl phosphate on the Th(IV) decrease with increasing the concentration of HNO3, the similar tendency can be attributed the competitive extraction between Th(IV) and proton. As for individual n-octyl diphenyl phosphate, its extraction efficiency increases firstly and then decreases with increasing the nitric acid concentration, which is consistent with previously reported in this lab (Dong et al., 2017). One possible explanation for the tendency is that the salting-out effect of HNO3 results in the initial increase of Th(IV) extraction efficiency (Pathak et al., 1999). With the further increase of HNO3 concentration, a high fraction of n-octyl diphenyl phosphate is complexed by HNO3, and decreases the concentration of free n-octyl diphenyl phosphate for Th(IV) extraction (Li et al., 2016). In addition, it is obvious that the extraction ability of mixed Cyanex 572 and n-octyl diphenyl phosphate is stronger than those of individual Cyanex 572 and individual n-octyl diphenyl phosphate. In order to show the synergistic effect directly, the synergistic enhancement coefficients are calculated and shown in Fig. 1(b), it is observed that the mixture of n-octyl diphenyl phosphate and Cyanex 572 exhibits obvious synergistic effect on Th(IV) extraction (R > 1). The values of R vary with the different concentration of HNO3 and the largest synergistic enhancement coefficient of 5.43 is obtained at 0.25 mol/L HNO3. Accordingly, 0.25 mol/L HNO3 is used for the following investigation.
2.3. Extraction and stripping experiments All the extraction experiments were performed at 298 K by equilibrating equal volumes (5 mL) of organic and aqueous phases in a thermostatic air bath oscillator for 40 min. After centrifugation at 2500 rpm for 5 min, the aqueous phase was separated, and the concentration of Th(IV) in the aqueous phase was determined using ICPOES. The concentration of Th(IV) extracted to the organic phase was calculated by mass balance. The stripping experiments were conducted by equilibrating 5 mL organic phase loaded with Th(IV) with 5 mL of different stripping reagents for 40 min in a thermostatic air bath oscillator. After centrifugation at 2500 rpm for 5 min, the aqueous phase was separated, and the concentration of Th(IV) in stripping acid was determined using ICP-OES. The concentration of Th(IV) in the organic phase was calculated by mass balance. The extraction efficiency (E), distribution ratio (D), synergy coefficient (R), stripping rate (S) and separation factor (β) are defined as follows:
[M]t [M]a × 100 [M]t
Vaq ([M]t [M]a ) × [M]a Vorg
S% =
2. Experimental
E% =
D=
(1) 187
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ODP (mol/L) 0.005
0.004
0.003
0.002
0.001
0.000
ODP Cyanex 572 ODP + Cyanex 572
6
6
4
2
2
0
0
D
D
4
0.000
0.001
0.002
0.003
0.004
0.005
Cyanex 572 (mol/L) Fig. 2. Synergistic extraction of Th(IV) with the mixture of n-octyl diphenyl phosphate and Cyanex 572. [Th(IV)] = 5 × 10−4 mol/L, [HNO3] = 0.25 mol/ L, [Cyanex 572 + ODP]o = 5.5 × 10−3 mol/L. Table 1 Synergistic enhancement coefficients of Th(IV) with mixed Cyanex 572 and noctyl diphenyl phosphate. [Th(IV)] = 5 × 10−4 mol/L, [HNO3] = 0.25 mol/L, [Cyanex 572 + ODP]o = 5.5 × 10−3 mol/L. CODP (×10−3) CCyanex 572 (×10−3) R
4.95 0.55
4.4 1.1
3.93 1.57
3.67 1.83
3.3 2.2
2.75 2.75
1.83 3.67
1.1 4.4
0.55 4.95
2.62
6.81
10.83
12.40
6.67
5.20
3.35
1.88
1.39
572 and n-octyl diphenyl phosphate can be observed under the experimental conditions (R > 1). Furthermore, it is worthy to mention that the R varies with the different mole fractions of Cyanex 572 and reaches a maximum of 12.40 at the mole fraction XODP:XCyanex 572 = 2:1. The following extraction experiments are measured at a ratio of n-octyl diphenyl phosphate to Cyanex 572 of 2 to 1 in order to maintain the synergistic effect. The extraction phenomena of Th(IV) by individual Cyanex 572, individual n-octyl diphenyl phosphate and the mixture of Cyanex 572 and n-octyl diphenyl phosphate are shown in Fig. 3. Slight emulsion can be observed in individual n-octyl diphenyl phosphate extraction system, whereas the extraction interface phenomenon can be obviously improved in the mixture of Cyanex 572 and n-octyl diphenyl phosphate extraction system.
Fig. 1. Extraction efficiency and synergistic enhancement coefficient of Th(IV) extracted by Cyanex 572, ODP, mixed Cyanex 572 and ODP. [Th (IV)] = 5 × 10−4 mol/L, [Cyanex 572] = [ODP] = 2.75 × 10−3 mol/L. [HNO3] = 0.1–0.7 mol/L.
3.2. The extraction of Th(IV) using Cyanex 572, n-octyl diphenyl phosphate and their mixture For investigating the synergistic effect of Th(IV) in the mixture system, the extraction behaviors of Th(IV) using Cyanex 572, n-octyl diphenyl phosphate and their mixture in n-heptane are studied. The total concentration of Cyanex 572 and n-octyl diphenyl phosphate is fixed at 5.5 × 10−3 mol/L, but their mole fractions are varied. As shown in Fig. 2, the extraction distribution ratios of Th(IV) increase with increasing mole fractions of either individual Cyanex 572 or individual n-octyl diphenyl phosphate, respectively. This indicates that both Cyanex 572 and n-octyl diphenyl phosphate show extraction ability for Th(IV). Moreover, it is evident that the distribution ratios of Th(IV) with the mixture of Cyanex 572 and n-octyl diphenyl phosphate are much higher and the distribution ratios increase firstly and then decrease with increasing the mole fraction of Cyanex 572 and decreasing the mole fraction of n-octyl diphenyl phosphate. This indicates that the extractability of the mixture of Cyanex 572 and n-octyl diphenyl phosphate are much stronger and the synergistic extraction effect is greatly influenced by the composition of organic phase. In order to assess the synergistic extraction effect of mixed Cyanex 572 and n-octyl diphenyl phosphate for Th(IV), synergistic enhancement coefficients (R) are calculated and listed in Table 1. It is obvious that an evident synergistic effect for Th(IV) with mixtures of Cyanex
Fig. 3. Extraction phenomena of Th(IV) by (a) n-octyl diphenyl phosphate, (b) Cyanex 572 and (c) mixed Cyanex 572 and n-octyl diphenyl phosphate. 188
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3.3. Extraction mechanism As mentioned in our previous studies, the extraction reactions of Th (IV) by single Cyanex 572 and n-octyl diphenyl phosphate can be expressed by the Eqs. (6) and (7). 4+ Thaq + 4[(HA) 2 ]org = [Th(HA2)4 ]org + 4H+aq
(6)
4+ Thaq + 4(NO3 )aq + 2ODP = [Th(NO3 )4 2ODP]org
(7)
where (HA)2 represent the dimeric species of Cyanex 572. If the synergistic extraction equation of Th(IV) in nitric acid medium with the mixture of Cyanex 572 and n-octyl diphenyl phosphate is expressed by the Eq. (8). 4+ Thaq + x[(HA) 2 ]org + yBorg + z(NO3 )aq
+
K ex
[Th(NO3 )z H2x
m A2x B y ]org
mH+aq
(8) Fig. 5. Effect of ODP concentration on the extraction of Th(IV). [Cyanex 572] = 1.8 × 10−3 mol/L, [Th(IV)] = 5.8 × 10−4 mol/L, [HNO3] = 0.25 mol/ L.
where x, y, z, m represent the stoichiometry of Cyanex 572, ODP, NO3− and H+, respectively. The distribution ratios (D) and the equilibrium constant (Kex) of the synergistic extraction system should be written as Eqs. (9) and (10)
D=
[Th(NO3 )z H2x
K ex =
m A2x B y ]org
(9)
[Th4 +]aq
[Th(NO3 )z H2x m A2x B y ]org [H+]m aq x y z [Th4 +]aq [(HA) 2 ]org [B]org [NO3 ]aq
(10)
Eq. (10) can be modified as Eq. (11)
K ex =
D [H+]m aq x y [(HA)2 ]org [B]org [NO3 ]zaq
(11)
Taking logarithms of Eq. (11):
logD = logK ex
mlog[H+]aq + xlog[(HA)2 ]org + ylog[B]org (12)
+ zlog[NO3 ]aq
The conventional slope analysis method was used to determine the stoichiometry including x, y and m in the synergistic extraction system. Fig. 4 shows the relationship of logD versus logCCyanex 572 with the slope of 1.13, indicating 1 mole Cyanex 572 can coordinate with 1 mole Th (IV), thus, the stoichiometry of Cyanex 572 was about 1. As shown in Fig. 5, the slope of logD versus logCODP is closed to be 2.5, suggesting the stoichiometry of n-octyl diphenyl phosphate is 2.5. Moreover, Fig. 6 shows the plot of logD versus log [H+]free with a slope of −1.12, which
0.6
Fig. 6. The effect of proton concentration on the extraction of Th(IV) with mixed Cyanex 572 and ODP. [Th(IV)] = 5.8 × 10−4 mol/L, [Cyanex 572 + ODP]o = 5.5 × 10−3 mol/L.
can be explained that one mole H+ is released by 1 mol Cyanex 572 during the synergistic extraction process. The concentrations of protons in Fig. 6 are obtained by acid-base titration. Considering the charge and atomic mass, the stoichiometry of NO3− should be 3. In other words, x = 1, y = 2.5, z = 3, m = 1. Therefore, the extraction equilibrium of Th(IV) in nitric acid medium with the mixture of Cyanex 572 and noctyl diphenyl phosphate can be expressed by the Eq. (13).
y = 1.13x + 3.23 2
R = 0.98 4+ Thaq + [(HA) 2 ]org + 2.5Borg + 3(NO3 )aq
0.4
LogD
+
H+aq
K ex
[Th(NO3 )3 HA2 B2.5 ]org (13)
3.4. IR spectra
0.2
IR spectra was studied to further confirm the extraction mechanism of Th(IV) in synergistic extraction system. The result was shown in Fig. 7, the bands at 1173 cm−1 in Fig. 7 (a), 1173 cm−1 in the Fig. 7 (b), 1173 cm−1in Fig. 7 (c), 1173 cm−1 in Fig. 7(d) and 1193 cm−1 in the Fig. 7(e) can be assigned to the P]O stretching vibrations. In this paper, most of the extraction experiments are conducted under the acidity of 0.25 mol/L HNO3. As revealed in Fig. 7, the IR spectra of (a) and (b) are almost the same, which demonstrates the stability of Cyanex 572 in nitric acid medium. In addition, the characteristic peak of P]O increased from 1173 cm−1 in (d) to 1193 cm−1 in (e) indicates that P]
0.0
-2.85
-2.70
-2.55
-2.40
LogCCyanex 572 Fig. 4. Effect of Cyanex 572 concentration on the extraction of Th(IV). [ODP] = 1.8 × 10−3 mol/L, [Th(IV)] = 5.8 × 10−4 mol/L, [HNO3] = 0.25 mol/L.
189
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100
S%
80
(a)
60
ODP Cyanex 572 ODP + Cyanex 572
40
0.0
0.5
1.0
1.5
2.0
2.5
3.0
HCl (mol/L) 100 Fig. 7. IR spectra of (a) Cyanex 572, (b) equilibrated Cyanex 572 with 0.25 mol/L HNO3, (c) ODP, (d) mixed Cyanex 572 and ODP (XCyanex 572:XODP = 1:2), (e) mixed Cyanex 572 and ODP loaded with Th(IV).
80
O groups in Cyanex 572 and ODP are involved in the extraction reaction. Furthermore, the new peak appeared at 1091 cm−1 in Fig. 7(e) can be attributed to the NO3− vibration, which implies that NO3− is also involved in the extraction process (Lu et al., 2016).
S%
60
(b) 40
3.5. Stripping properties
ODP Cyanex 572 ODP + Cyanex 572
20
For investigating the stripping properties of above mentioned extraction system, the organic phases loaded with Th(IV) were prepared by equilibrating 5 mL of organic phases containing 5 × 10−3 mol/L of Cyanex 572, n-octyl diphenyl phosphate, mixed Cyanex 572 and n-octyl diphenyl phosphate (XCyanex 572:XODP = 1:2) with 5 mL of aqueous phases containing 5 × 10−5 mol/L of Th(IV). In this study, hydrochloric acid and nitric acid used as stripping agent are investigated. As shown in Fig. 8(a) and (b), almost all the stripping rates increase with increasing the concentration of HCl or HNO3. The maximum stripping rates reach 78%, 88% and 98% when the concentration of HCl is 0.5 mol/L, while the maximum stripping rates of 80%, 84% and 95% are obtained at 5 mol/L HNO3. It is evident that the stripping ability of HCl was better than HNO3. The difference may be attributed that HCl is more effective to decompose the extracting complexes. In addition, it is worthy to mention that stripping behavior of synergistic extraction system is better than those of individual n-octyl diphenyl phosphate and individual Cyanex 572 extraction system under low acidity conditions when HCl is used as a stripping agent. It indicates that the synergistic extraction system shows better stripping performance than the individual extraction systems of n-octyl diphenyl phosphate and Cyanex 572.
0 1
2
3
4
5
6
HNO3 (mol/L) Fig. 8. Stripping rates of Th(IV) with (a) HCl and (b) HNO3 from the loaded organic phase, respectively. (a) [HCl] = 0.1–3 mol/L. (b) [HNO3] = 0.5–6 mol/ L.
3.6. The effect of temperature on synergistic extraction The effect of temperature on the synergistic extraction of Th(IV) is studied by varying the temperature (298 K – 328 K) while keeping the other conditions constant. The plot of Log D versus 1000/T is shown in Fig. 9, the change of enthalpy (∆H) can be calculated from the slope of 1.03 according to Van't hoff equation in the form (14):
log D =
H 1 +C 2.303R T
Fig. 9. Effect of experimental temperature on the extraction of Th(IV) with mixed Cyanex 572 and ODP. Th(IV) = 5.67 × 10−4 mol/L, [Cyanex 572 + ODP]o = 5.5 × 10−3 mol/L, XCyanex 572:XODP = 1:2, [HNO3] = 0.25 mol/L.
(14)
where R is gas constant and C is a constant. The change of Gibbs free energy (∆G) and the change of entropy (∆S) in 298 K can be obtained according to the Eqs. (15) and (16), respectively.
G=
RTlnK
G= H
T S
(16)
The value of log K in 298 K is calculated according to Eq. (12). Thermodynamic parameters including log K, ∆H, ∆G and ∆S are given
(15) 190
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hydrophobicity and stability of extracted species in the organic phase. The maximal synergistic enhancement coefficient reaches 12.40 at a ratio of 1:2 of Cyanex 572 and n-octyl diphenyl phosphate. In addition, the addition of Cyanex 572 to n-octyl diphenyl phosphate can avoid the emulsification in individual n-octyl diphenyl phosphate extraction system. By slopes analysis method and IR spectra analysis, the extraction mechanim of Th(IV) from nitric acid medium with the mixed noctyl diphenyl phosphate and Cyanex 572 system is investigated. The synergistic extracted complexes are confirmed to be Th(NO3)3HA2B. Thermodynamic study indicates that the synergistic extraction reaction is spontaneous. Lower stripping acid are required according to the stripping experiment. The extraction and separation of REE and Th from the feed solution leached from the ion-absorbed type rare earth mineral waste residue with the mixture of n-octyl diphenyl phosphate and Cyanex 572 are studied, which indicates that the mixed extraction system shows better separation performance for REE and Th. Hence, the proposed synergistic extraction system is a promising system in the separation of Th and REE, that is, efficient extraction ability, selectivity, phase separation performance and stripping property.
Table 2 Thermodynamic parameters in the synergistic extraction of Th(IV) at 298 K. Log K
ΔH (KJ/mol)
ΔG (KJ/mol)
ΔS (J·mol−1·K−1)
10.702
−19.72
−61.05
138.7
Acknowledgement This work was supported by The National Key Technology R&D Program (2017YFE0106900), ‘Hundreds Talents Program’ and Service Network Initiative from Chinese Academy of Sciences and Science and Technology Major Project of Ganzhou (2018). Fig. 10. The separation of Th and REE from industrial feed solution. [ODP + Cyanex 572]o = 1.1 × 10−3 mol/L, [HNO3] = 0.25 mol/L, [REE (III)] = 2.5 × 10−2 mmol/L, [Fe] = 0.936 mmol/L, [Th(IV)] = 1.41 × 10−3 mmol/L.
References Arnold, J., Gianetti, T.L., Kashtan, Y., 2014. Thorium lends a fiery hand. Nat. Chem. 6, 554. Ashley, S.F., Parks, G.T., Nuttall, W.J., Boxall, C., Grimes, R.W., 2012. Nuclear energy: thorium fuel has risks. Nature 492, 31. Bismondo, A., Comuzzi, C., Bernardo, P.D., Zanonato, P.L., 1999. Complexation of thorium(IV) by tris((2,3-dihydroxybenzylamino)ethyl)amine-a new strong chelating agent. Inorg. Chim. Acta 286, 103–107. Cheng, Z., Wang, H., Wang, Y., He, F., Zhang, H., Yang, S., 2011. Synthesis and characterization of an ion-imprinted polymer for selective solid phase extraction of thorium(iv). Microchim. Acta 173, 423–431. Deng, Q., Jin, Y., Wang, Q., Zhao, R., Pan, N., Zhai, F., Luo, M., Xia, C., 2013. New cyclen derivative ligand for thorium(IV) separation by solvent extraction. J. Radioanal. Nucl. Ch. 295, 125–133. Dong, Y., Li, S., Wang, Y., Shen, Y., Sun, X., 2017. Separation of thorium from rare earths with high-performance diphenyl phosphate extractant. Hydrometallurgy 171, 387–393. El-Dessouky, S.I., Borai, E.H., 2006. Extraction chromatography of thorium ion by solid phase impregnated resins containing bi-functional organic extractants. J. Radioanal. Nucl. Ch. 268, 247–254. Fan, S., Tian, M., Song, N., Jia, Q., Liao, W., 2011. Synergistic extraction and separation study of rare earth elements from nitrate medium by mixtures of sec -nonylphenoxy acetic acid and 2,2′-bipyridyl. J. Chem. Technol. Biotechnol. 86, 719–723. Jain, V.K., Pillai, S.G., Pandya, R.A., Agrawal, Y.K., Shrivastav, P.S., 2005. Selective extraction, preconcentration and transport studies of thorium(IV) using octa-functionalized calix[4]resorcinarene-hydroxamic acid. Anal. Sci. 21, 129–135. Li, S.M., Tan, M.L., Ding, S.D., Li, F., Li, Q.N., Zhang, L., Liu, C.X., 2016. Extraction of U (VI) and Th(IV) with three neutral extractants containing phosphorus. Chem. Res. Appl. 28, 307–315. Lin, C., Wang, H., Wang, Y., Cheng, Z., 2010. Selective solid-phase extraction of trace thorium(IV) using surface-grafted Th(IV)-imprinted polymers with pyrazole derivative. Talanta 81, 30–36. Lu, Y., Bi, Y., Bai, Y., Liao, W., 2013. Extraction and separation of thorium and rare earths from nitrate medium with p-phosphorylated calixarene. J. Chem. Technol. Biotechnol. 88, 1836–1840. Lu, Y., Wei, H., Zhang, Z., Li, Y., Wu, G., Liao, W., 2016. Selective extraction and separation of thorium from rare earths by a phosphorodiamidate extractant. Hydrometallurgy 163, 192–197. Ma, L., Zhao, Z., Dong, Y., Sun, X., 2017. A synergistic extraction strategy by [N 1888] [SOPAA] and Cyphos IL 104 for heavy rare earth elements separation. Sep. Purif. Technol. 174, 474–481. Ma, L., Zhao, Z., Dong, Y., Sun, X., 2018. A synergistic extraction strategy by Cyanex 572 and Cyanex923 for Th (IV) separation. Sep. Purif. Technol. 191, 307–313. Meera, R., Varma, R.L., Reddy, M.L.P., 2004. Enhanced extraction of thorium (IV) and uranium (VI) with 1-phenyl-3-methyl-4-pivaloyl-5-pyrazolone in the presence of various neutral organophosphorus extractants. Radiochim. Acta 92, 17–24. Mohapatra, P., Veeraraghavan, R., Manchanda, V., 1999. Extraction of thorium (IV) by a mixture of 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone and tri-n-octyl phosphine oxide. J. Radioanal. Nucl. Ch. 240, 31–38.
in Table 2, it can be found that the sign of ∆H and ∆G is minus, which indicates the synergistic extraction system is exothermic and spontaneous. In addition, ∆S > 0 indicates the mixture system is more disorder, further demonstrating its easily implement. 3.7. Extraction and separation of Th(IV), REE(III) from industrial feed solution Industrial feed solution leached from the ion-absorbed type rare earth mineral waste residue is applied to test selectivety of the mixed Cyanex 572 and n-octyl diphenyl phosphate. As shown in Fig. 10, the extraction effiencies of Th(IV) are better than REE and Fe in all the extraction systems. Compared with the individual Cyanex 572 or noctyl diphenyl phosphate extraction system, the mixed Cyanex 572 and n-octyl diphenyl phosphate extraction system reaveals evident synergistic extraction effect of Th(IV) (R = 2.3). In addition, separation factors of Th to REEs are calculated to be 4.7, 1.1 and 7.6 in individual n-octyl diphenyl phosphate, individual Cyanex 572 and mixed Cyanex 572 and n-octyl diphenyl phosphate extraction systems. Based on the results, the synergistic system shows higher extraction efficiency for Th (IV) and better separation performance of Th(IV) and REEs than the individual extraction systems. Hence, the synergistic extractant has revealed a promising application potential for the extraction and separation of Th. 4. Conclusion In this paper, a novel synergistic extraction system of mixed n-octyl diphenyl phosphate and Cyanex 572 was investigated for the Th(IV) extraction. Comparing the extraction of Th(IV) with individual n-octyl diphenyl phosphate, individual Cyanex 572 and mixed n-octyl diphenyl phosphate and Cyanex 572, it is found that the mixed extraction system presents significant synergistic effect on Th(IV) and better phase separation performance. This can be probably attributed to the greater 191
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H. Zhou et al. Nasab, M.E., Sam, A., Milani, S.A., 2011. Determination of optimum process conditions for the separation of thorium and rare earth elements by solvent extraction. Hydrometallurgy 106, 141–147. Paiva, A.P., Malik, P., 2004. Recent advances on the chemistry of solvent extraction applied to the reprocessing of spent nuclear fuels and radioactive wastes. J. Radioanal. Nucl. Ch. 261, 485–496. Pathak, P.N., Veeraraghavan, R., Manchanda, V.K., 1999. Separation of uranium and thorium using tris(2-ethylhexyl) phosphate as extractant. J. Radioanal. Nucl. Ch. 240, 15–18. Pathak, P.N., Kumbhare, L.B., Manchanda, V.K., 2001. Effect of structure of N, N dialkyl amides on the extraction of U (VI) and Th (IV): a thermodynamic study. Radiochim. Acta 89, 447–452. Patil, S.K., Ramakrishna, V.V., Sajun, M.S., Thiagarajan, R., 1979. Synergistic extraction of Th(IV) by mixtures of HTTA and TBP in benzene. Sep. Sci. Technol. 14, 859–868. Patkar, S.N., Burungale, A.S., Patil, R.J., 2009. Separation and liquid liquid extraction of thorium (IV) as sulphate complex with synergistic mixture of N-octylaniline and trioctylamine as an extractant. Rasayan J. Chem. 2, 4825–4832. Reddy, M.L.P., Meera, R., 2001. Crown ether as size selective synergist in the solvent extraction of thorium (IV) and uranium (VI) with 3-phenyl-4-(4-fluorobenzoyl)-5isoxazolone. Radiochim. Acta 89, 453–460. Sato, T., 1966. Extraction of uranium (VI) and thorium from hydrochloric acid solutions by tri-n-butyl phosphate. J. Chem. Technol. Biotechnol. 16, 53–57. Shrivastav, P., Menon, S.K., Agrawal, Y.K., 2001. Selective extraction and inductively coupled plasma atomic emission spectrophotometric determination of thorium using a chromogenic crown ether. J. Radioanal. Nucl. Ch. 250, 459–464.
Tan, M., Huang, C., Ding, S., Li, F., Li, Q., Zhang, L., Liu, C., Li, S., 2015. Highly efficient extraction separation of uranium (VI) and thorium (IV) from nitric acid solution with di (1-methyl-heptyl) methyl phosphonate. Sep. Purif. Technol. 146, 192–198. Tong, H., Wang, Y., Liao, W., Li, D., 2013. Synergistic extraction of Ce (IV) and Th (IV) with mixtures of Cyanex 923 and organophosphorus acids in sulfuric acid media. Sep. Purif. Technol. 118, 487–491. Tunsu, C., Lapp, J.B., Ekberg, C., Retegan, T., 2016. Selective separation of yttrium and europium using Cyanex 572 for applications in fluorescent lamp waste processing. Hydrometallurgy 166, 98–106. Wang, Y., Huang, C., Li, F., Dong, Y., Sun, X., 2017. Process for the separation of thorium and rare earth elements from radioactive waste residues using Cyanex® 572 as a new extractant. Hydrometallurgy 169, 158–164. Yaftian, M.R., Hassanzadeh, L., Eshraghi, M.E., Matt, D., 2003. Solvent extraction of thorium (IV) and europium (III) ions by diphenyl-N, N-dimethylcarbamoylmethylphosphine oxide from aqueous nitrate media. Sep. Purif. Technol. 31, 261–268. Yuan, L., Bai, Z., Zhao, R., Liu, Y., Li, Z., Chu, S., Zheng, L., Zhang, L., Zhao, Y., Chai, Z., Shi, W., 2014. Introduction of bifunctional groups into mesoporous silica for enhancing uptake of thorium(IV) from aqueous solution. ACS Appl. Mater. Interfaces 6, 4786–4796. Zhang, Y., Xu, Y., Huang, X., Long, Z., Cui, D., Hu, F., 2012. Study on thorium recovery from bastnaesite treatment process. J. Rare Earth 30, 374–377. Zuo, Y., Chen, J., Li, D., 2008. Reversed micellar solubilization extraction and separation of thorium (IV) from rare earth (III) by primary amine N1923 in ionic liquid. Sep. Purif. Technol. 63, 684–690.
192
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Recovery of Th(IV) from leaching solutions of rare earth residues using a synergistic solvent extraction system consisting of Cyanex 572 and n-octyl diphenyl phosphate (ODP) Haiyue Zhoua,c, Yamin Donga, Yabin Wanga, Zeyuan Zhaoa, Yu Xiaoa, Xiaoqi Suna,b,
T
⁎
a
CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China b Ganzhou Rare Earth Group Co., Ltd., China Southern Rare Earth, Ganzhou 341000, PR China c Fujian Normal University, Fuzhou, Fujian 35007, PR China
ARTICLE INFO
ABSTRACT
Keywords: Synergistic extraction Cyanex 572 n-octyl diphenyl phosphate Rare earth Thorium (Th)
The synergistic effect of mixed Cyanex 572 and n-octyl diphenyl phosphate for Th(IV) extraction from nitric acid solution has been investigated in this paper. The effect of acidity together with mole fraction of extractant on the synergistic effect was studied. Based on the obtained results, the maximal synergistic enhancement coefficient reaches 12.40 at a ratio of 1:2 of Cyanex 572 and n-octyl diphenyl phosphate. Under the experimental conditions, the phase separation performance, stripping property and selectivity of Th and REE in the mixed extraction system are better than those from individual Cyanex 572 or n-octyl diphenyl phosphate system. The synergistic 4+ + [(HA)2 ]org + extraction mechanism for Th(IV) in nitric acid medium is proposed as Thaq K ex
2.5Borg + 3(NO3 )aq [Th(NO3 )3 HA2B2.5 ]org + H+aq . The mixed extraction system is successfully used for the separation of Th(IV) and REE(III) from the industrial feed solution leached from ion-absorbed type rare earth mineral waste residue. Lower stripping acid, stronger extraction ability, better phase separation performance and selectivity provide the synergistic extraction system with good application prospects.
1. Introduction As an important natural radioactive element, Thorium (Th) is widely distributed in the earth's crust (Lin et al., 2010). Th is indicated to be the most potential alternative of uranium to produce nuclear fuel (Arnold et al., 2014; Ashley et al., 2012). Also, Th is applied in some other fields, i.e., catalyst, functional material, agriculture, optics and aerospace (Lin et al., 2010; Cheng et al., 2011). Although widely used, the radioactivity and toxicity of Th, even at trace level, still poses a serious threat to the ecosystem and human health (Yuan et al., 2014). In addition, thorium and rare earth elements (REEs) are always occurred together in the minerals such as monazite, bastnasite and ion-adsorption type rare earth minerals, which greatly limits their respective applications (Lu et al., 2013). Accordingly, the separation of Th and REEs from rare earth minerals or waste residues is significant from the point of economic value and ecological effect (Deng et al., 2013; Wang et al., 2017). The recovery and separation of Th from REEs is an important task in
hydrometallurgy, which leads to larger amount of investigation on Th separation in the past few years (Nasab et al., 2011; Zhang et al., 2012; Zuo et al., 2008). Attracted by its simplicity, rapidity and ease of operation, solvent extraction has turned to be an efficient and promising technique in the separation and purity of Th (Paiva and Malik, 2004; Tan et al., 2015; Shrivastav et al., 2001). Common extractants including organophosphorus compounds (Yaftian et al., 2003; Sato, 1966), amines (Bismondo et al., 1999), carboxylic acids (Jain et al., 2005), amides (Pathak et al., 2001) and crown ethers (El-Dessouky and Borai, 2006). Cyanex 572 was a new commercial extractant, it has been studied for extracting REEs and Th from waste residues including ion-absorbed type minerals or fluorescent lamp (Tunsu et al., 2016). Recently, more and more attentions have been paid for developing new and efficient extraction systems for Th extraction. Synergistic extraction is an important research field in solvent extraction. Owing to its unique advantages like better extraction efficiency, selectivity, phase separation and stripping properties (Ma et al., 2017; Fan et al., 2011), synergistic extraction causes increasing interests. Several synergistic
⁎ Corresponding author at: CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, PR China. E-mail address: [email protected] (X. Sun).
https://doi.org/10.1016/j.hydromet.2018.12.008 Received 11 July 2018; Received in revised form 25 November 2018; Accepted 7 December 2018 Available online 12 December 2018 0304-386X/ © 2018 Published by Elsevier B.V.
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extraction systems like Cyanex 572-Cyanex 923 (Ma et al., 2018), Nnoctylaniline-Trioctylamine (Patkar et al., 2009), HPMPP-TBP/CMPO/ TOPO (Meera et al., 2004), HFBPI-DC18C6/DB18C6/B15C5 (Reddy and Meera, 2001), PMBP–TOPO (Mohapatra et al., 1999), HTTA-TBP (Patil et al., 1979) and Cyanex 923–P204/P507/Cyanex 272 (Tong et al., 2013) have been studied to further optimize the extraction process of Th. Patkar et al. investigated the extraction of Th from aqueous sulphuric acid medium with a synergistic mixture of N-n-octylaniline and TOA (Patkar et al., 2009). Meera et al. researched the synergistic extraction behaviors of HPMPP and TBP/CMPO/TOPO for Th(IV) and U (VI), respectively (Meera et al., 2004). Very recently, n-octyl diphenyl phosphate (ODP) and Cyanex 572 were reported for the separation of Th and REEs in this lab (Dong et al., 2017; Wang et al., 2017). In this paper, attracted by the excellent performance of synergistic extraction, a new combination of Cyanex 572 and n-octyl diphenyl phosphate was investigated in the case of the synergistic extraction for Th(IV) from nitric acid solutions.
(2)
R=
Dmix Da + Db
(3)
=
[M]aq,a × Vaq [M]org,t × Vorg
× 100
D1 D2
(4)
(5)
where, [M]t and [M]a denote the concentrations of metal ions in aqueous phase before and after extraction, respectively. Da is the distribution ratio of one extractant for a metal; Db is the distribution ratio of the other extractant for a metal; Dmix is the distribution ratio of their mixture for a metal. [M]aq,a is equilibrium concentration of a metal in stripping aqueous phase. [M]org,t is the concentration of a metal loaded in the organic phase for stripping. Vaq and Vorg are the volumes of the aqueous phase and organic phase, respectively. D1 and D2 are the distribution ratios of metal 1 and 2, respectively. All the concentration values of metal were measured in duplicate with the uncertainty within 5%. Th(IV) complexes for IR spectra were prepared by equilibrating equal volume of aqueous phase containing 1 × 10−3 mol/L Th(IV) with 0.1 mol/L mixed Cyanex 572 and n-octyl diphenyl phosphate (XODP:XCyanex 572 = 2:1) in n-heptane, respectively. After the phase separation, the n-heptane was evaporated and the samples of Th(IV) complexes were obtained. To confirm the stability of Cyanex 572 in HNO3 media, stability test was prepared by equilibrating 4 mL of 0.25 mol/L HNO3 with 0.6 g Cyanex 572 for 1 h. After the phase separation, the equilibrated Cyanex 572 were obtained for IR spectra.
2.1. Reagents and apparatus Cyanex 572 were obtained from Cytec Industries Inc. (USA), which was used without further purification and saponification (Wang et al., 2017). n-octyl diphenyl phosphate was synthesized and purified according to our previous work (Dong et al., 2017), diphenyl chlorophosphate, 1-bromooctane and magnesium were purchased from Xiya Reagen (China). Heptane of analytical reagent was purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used as the diluent. The organic phases were prepared by dissolving the extractants in nheptane. Th(NO)4 stock solution was prepared by dissolving Th(NO3)4 with deionized water. The feed solution for Th, Fe and RE separation was leached from the ion-absorbed rare earth waste residue. All the other chemicals in the work were used without further purification.
3. Results and discussion
2.2. Instrumentation
3.1. The concentration effect of HNO3 on Th(IV) extraction
Thermo scientific iCAP 6500 series inductively coupled plasmaatomic emission spectroscopy (ICP-OES) instrument was used to determine the concentration of metal ions in the aqueous phase. IR spectra were recorded with a Nicolet iS50 spectrometer. The pH values of aqueous solution were determined by PHSJ-4F made by INESA scientific instrument CO., Ltd. or acid-base titration.
The effect of aqueous acidity on Th(IV) extraction efficiency was obtained by the extraction experiments of Cyanex 572, ODP and their mixture. It is observed from Fig. 1(a), the extraction efficiencies of individual Cyanex 572 and mixed Cyanex 572 and n-octyl diphenyl phosphate on the Th(IV) decrease with increasing the concentration of HNO3, the similar tendency can be attributed the competitive extraction between Th(IV) and proton. As for individual n-octyl diphenyl phosphate, its extraction efficiency increases firstly and then decreases with increasing the nitric acid concentration, which is consistent with previously reported in this lab (Dong et al., 2017). One possible explanation for the tendency is that the salting-out effect of HNO3 results in the initial increase of Th(IV) extraction efficiency (Pathak et al., 1999). With the further increase of HNO3 concentration, a high fraction of n-octyl diphenyl phosphate is complexed by HNO3, and decreases the concentration of free n-octyl diphenyl phosphate for Th(IV) extraction (Li et al., 2016). In addition, it is obvious that the extraction ability of mixed Cyanex 572 and n-octyl diphenyl phosphate is stronger than those of individual Cyanex 572 and individual n-octyl diphenyl phosphate. In order to show the synergistic effect directly, the synergistic enhancement coefficients are calculated and shown in Fig. 1(b), it is observed that the mixture of n-octyl diphenyl phosphate and Cyanex 572 exhibits obvious synergistic effect on Th(IV) extraction (R > 1). The values of R vary with the different concentration of HNO3 and the largest synergistic enhancement coefficient of 5.43 is obtained at 0.25 mol/L HNO3. Accordingly, 0.25 mol/L HNO3 is used for the following investigation.
2.3. Extraction and stripping experiments All the extraction experiments were performed at 298 K by equilibrating equal volumes (5 mL) of organic and aqueous phases in a thermostatic air bath oscillator for 40 min. After centrifugation at 2500 rpm for 5 min, the aqueous phase was separated, and the concentration of Th(IV) in the aqueous phase was determined using ICPOES. The concentration of Th(IV) extracted to the organic phase was calculated by mass balance. The stripping experiments were conducted by equilibrating 5 mL organic phase loaded with Th(IV) with 5 mL of different stripping reagents for 40 min in a thermostatic air bath oscillator. After centrifugation at 2500 rpm for 5 min, the aqueous phase was separated, and the concentration of Th(IV) in stripping acid was determined using ICP-OES. The concentration of Th(IV) in the organic phase was calculated by mass balance. The extraction efficiency (E), distribution ratio (D), synergy coefficient (R), stripping rate (S) and separation factor (β) are defined as follows:
[M]t [M]a × 100 [M]t
Vaq ([M]t [M]a ) × [M]a Vorg
S% =
2. Experimental
E% =
D=
(1) 187
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ODP (mol/L) 0.005
0.004
0.003
0.002
0.001
0.000
ODP Cyanex 572 ODP + Cyanex 572
6
6
4
2
2
0
0
D
D
4
0.000
0.001
0.002
0.003
0.004
0.005
Cyanex 572 (mol/L) Fig. 2. Synergistic extraction of Th(IV) with the mixture of n-octyl diphenyl phosphate and Cyanex 572. [Th(IV)] = 5 × 10−4 mol/L, [HNO3] = 0.25 mol/ L, [Cyanex 572 + ODP]o = 5.5 × 10−3 mol/L. Table 1 Synergistic enhancement coefficients of Th(IV) with mixed Cyanex 572 and noctyl diphenyl phosphate. [Th(IV)] = 5 × 10−4 mol/L, [HNO3] = 0.25 mol/L, [Cyanex 572 + ODP]o = 5.5 × 10−3 mol/L. CODP (×10−3) CCyanex 572 (×10−3) R
4.95 0.55
4.4 1.1
3.93 1.57
3.67 1.83
3.3 2.2
2.75 2.75
1.83 3.67
1.1 4.4
0.55 4.95
2.62
6.81
10.83
12.40
6.67
5.20
3.35
1.88
1.39
572 and n-octyl diphenyl phosphate can be observed under the experimental conditions (R > 1). Furthermore, it is worthy to mention that the R varies with the different mole fractions of Cyanex 572 and reaches a maximum of 12.40 at the mole fraction XODP:XCyanex 572 = 2:1. The following extraction experiments are measured at a ratio of n-octyl diphenyl phosphate to Cyanex 572 of 2 to 1 in order to maintain the synergistic effect. The extraction phenomena of Th(IV) by individual Cyanex 572, individual n-octyl diphenyl phosphate and the mixture of Cyanex 572 and n-octyl diphenyl phosphate are shown in Fig. 3. Slight emulsion can be observed in individual n-octyl diphenyl phosphate extraction system, whereas the extraction interface phenomenon can be obviously improved in the mixture of Cyanex 572 and n-octyl diphenyl phosphate extraction system.
Fig. 1. Extraction efficiency and synergistic enhancement coefficient of Th(IV) extracted by Cyanex 572, ODP, mixed Cyanex 572 and ODP. [Th (IV)] = 5 × 10−4 mol/L, [Cyanex 572] = [ODP] = 2.75 × 10−3 mol/L. [HNO3] = 0.1–0.7 mol/L.
3.2. The extraction of Th(IV) using Cyanex 572, n-octyl diphenyl phosphate and their mixture For investigating the synergistic effect of Th(IV) in the mixture system, the extraction behaviors of Th(IV) using Cyanex 572, n-octyl diphenyl phosphate and their mixture in n-heptane are studied. The total concentration of Cyanex 572 and n-octyl diphenyl phosphate is fixed at 5.5 × 10−3 mol/L, but their mole fractions are varied. As shown in Fig. 2, the extraction distribution ratios of Th(IV) increase with increasing mole fractions of either individual Cyanex 572 or individual n-octyl diphenyl phosphate, respectively. This indicates that both Cyanex 572 and n-octyl diphenyl phosphate show extraction ability for Th(IV). Moreover, it is evident that the distribution ratios of Th(IV) with the mixture of Cyanex 572 and n-octyl diphenyl phosphate are much higher and the distribution ratios increase firstly and then decrease with increasing the mole fraction of Cyanex 572 and decreasing the mole fraction of n-octyl diphenyl phosphate. This indicates that the extractability of the mixture of Cyanex 572 and n-octyl diphenyl phosphate are much stronger and the synergistic extraction effect is greatly influenced by the composition of organic phase. In order to assess the synergistic extraction effect of mixed Cyanex 572 and n-octyl diphenyl phosphate for Th(IV), synergistic enhancement coefficients (R) are calculated and listed in Table 1. It is obvious that an evident synergistic effect for Th(IV) with mixtures of Cyanex
Fig. 3. Extraction phenomena of Th(IV) by (a) n-octyl diphenyl phosphate, (b) Cyanex 572 and (c) mixed Cyanex 572 and n-octyl diphenyl phosphate. 188
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3.3. Extraction mechanism As mentioned in our previous studies, the extraction reactions of Th (IV) by single Cyanex 572 and n-octyl diphenyl phosphate can be expressed by the Eqs. (6) and (7). 4+ Thaq + 4[(HA) 2 ]org = [Th(HA2)4 ]org + 4H+aq
(6)
4+ Thaq + 4(NO3 )aq + 2ODP = [Th(NO3 )4 2ODP]org
(7)
where (HA)2 represent the dimeric species of Cyanex 572. If the synergistic extraction equation of Th(IV) in nitric acid medium with the mixture of Cyanex 572 and n-octyl diphenyl phosphate is expressed by the Eq. (8). 4+ Thaq + x[(HA) 2 ]org + yBorg + z(NO3 )aq
+
K ex
[Th(NO3 )z H2x
m A2x B y ]org
mH+aq
(8) Fig. 5. Effect of ODP concentration on the extraction of Th(IV). [Cyanex 572] = 1.8 × 10−3 mol/L, [Th(IV)] = 5.8 × 10−4 mol/L, [HNO3] = 0.25 mol/ L.
where x, y, z, m represent the stoichiometry of Cyanex 572, ODP, NO3− and H+, respectively. The distribution ratios (D) and the equilibrium constant (Kex) of the synergistic extraction system should be written as Eqs. (9) and (10)
D=
[Th(NO3 )z H2x
K ex =
m A2x B y ]org
(9)
[Th4 +]aq
[Th(NO3 )z H2x m A2x B y ]org [H+]m aq x y z [Th4 +]aq [(HA) 2 ]org [B]org [NO3 ]aq
(10)
Eq. (10) can be modified as Eq. (11)
K ex =
D [H+]m aq x y [(HA)2 ]org [B]org [NO3 ]zaq
(11)
Taking logarithms of Eq. (11):
logD = logK ex
mlog[H+]aq + xlog[(HA)2 ]org + ylog[B]org (12)
+ zlog[NO3 ]aq
The conventional slope analysis method was used to determine the stoichiometry including x, y and m in the synergistic extraction system. Fig. 4 shows the relationship of logD versus logCCyanex 572 with the slope of 1.13, indicating 1 mole Cyanex 572 can coordinate with 1 mole Th (IV), thus, the stoichiometry of Cyanex 572 was about 1. As shown in Fig. 5, the slope of logD versus logCODP is closed to be 2.5, suggesting the stoichiometry of n-octyl diphenyl phosphate is 2.5. Moreover, Fig. 6 shows the plot of logD versus log [H+]free with a slope of −1.12, which
0.6
Fig. 6. The effect of proton concentration on the extraction of Th(IV) with mixed Cyanex 572 and ODP. [Th(IV)] = 5.8 × 10−4 mol/L, [Cyanex 572 + ODP]o = 5.5 × 10−3 mol/L.
can be explained that one mole H+ is released by 1 mol Cyanex 572 during the synergistic extraction process. The concentrations of protons in Fig. 6 are obtained by acid-base titration. Considering the charge and atomic mass, the stoichiometry of NO3− should be 3. In other words, x = 1, y = 2.5, z = 3, m = 1. Therefore, the extraction equilibrium of Th(IV) in nitric acid medium with the mixture of Cyanex 572 and noctyl diphenyl phosphate can be expressed by the Eq. (13).
y = 1.13x + 3.23 2
R = 0.98 4+ Thaq + [(HA) 2 ]org + 2.5Borg + 3(NO3 )aq
0.4
LogD
+
H+aq
K ex
[Th(NO3 )3 HA2 B2.5 ]org (13)
3.4. IR spectra
0.2
IR spectra was studied to further confirm the extraction mechanism of Th(IV) in synergistic extraction system. The result was shown in Fig. 7, the bands at 1173 cm−1 in Fig. 7 (a), 1173 cm−1 in the Fig. 7 (b), 1173 cm−1in Fig. 7 (c), 1173 cm−1 in Fig. 7(d) and 1193 cm−1 in the Fig. 7(e) can be assigned to the P]O stretching vibrations. In this paper, most of the extraction experiments are conducted under the acidity of 0.25 mol/L HNO3. As revealed in Fig. 7, the IR spectra of (a) and (b) are almost the same, which demonstrates the stability of Cyanex 572 in nitric acid medium. In addition, the characteristic peak of P]O increased from 1173 cm−1 in (d) to 1193 cm−1 in (e) indicates that P]
0.0
-2.85
-2.70
-2.55
-2.40
LogCCyanex 572 Fig. 4. Effect of Cyanex 572 concentration on the extraction of Th(IV). [ODP] = 1.8 × 10−3 mol/L, [Th(IV)] = 5.8 × 10−4 mol/L, [HNO3] = 0.25 mol/L.
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100
S%
80
(a)
60
ODP Cyanex 572 ODP + Cyanex 572
40
0.0
0.5
1.0
1.5
2.0
2.5
3.0
HCl (mol/L) 100 Fig. 7. IR spectra of (a) Cyanex 572, (b) equilibrated Cyanex 572 with 0.25 mol/L HNO3, (c) ODP, (d) mixed Cyanex 572 and ODP (XCyanex 572:XODP = 1:2), (e) mixed Cyanex 572 and ODP loaded with Th(IV).
80
O groups in Cyanex 572 and ODP are involved in the extraction reaction. Furthermore, the new peak appeared at 1091 cm−1 in Fig. 7(e) can be attributed to the NO3− vibration, which implies that NO3− is also involved in the extraction process (Lu et al., 2016).
S%
60
(b) 40
3.5. Stripping properties
ODP Cyanex 572 ODP + Cyanex 572
20
For investigating the stripping properties of above mentioned extraction system, the organic phases loaded with Th(IV) were prepared by equilibrating 5 mL of organic phases containing 5 × 10−3 mol/L of Cyanex 572, n-octyl diphenyl phosphate, mixed Cyanex 572 and n-octyl diphenyl phosphate (XCyanex 572:XODP = 1:2) with 5 mL of aqueous phases containing 5 × 10−5 mol/L of Th(IV). In this study, hydrochloric acid and nitric acid used as stripping agent are investigated. As shown in Fig. 8(a) and (b), almost all the stripping rates increase with increasing the concentration of HCl or HNO3. The maximum stripping rates reach 78%, 88% and 98% when the concentration of HCl is 0.5 mol/L, while the maximum stripping rates of 80%, 84% and 95% are obtained at 5 mol/L HNO3. It is evident that the stripping ability of HCl was better than HNO3. The difference may be attributed that HCl is more effective to decompose the extracting complexes. In addition, it is worthy to mention that stripping behavior of synergistic extraction system is better than those of individual n-octyl diphenyl phosphate and individual Cyanex 572 extraction system under low acidity conditions when HCl is used as a stripping agent. It indicates that the synergistic extraction system shows better stripping performance than the individual extraction systems of n-octyl diphenyl phosphate and Cyanex 572.
0 1
2
3
4
5
6
HNO3 (mol/L) Fig. 8. Stripping rates of Th(IV) with (a) HCl and (b) HNO3 from the loaded organic phase, respectively. (a) [HCl] = 0.1–3 mol/L. (b) [HNO3] = 0.5–6 mol/ L.
3.6. The effect of temperature on synergistic extraction The effect of temperature on the synergistic extraction of Th(IV) is studied by varying the temperature (298 K – 328 K) while keeping the other conditions constant. The plot of Log D versus 1000/T is shown in Fig. 9, the change of enthalpy (∆H) can be calculated from the slope of 1.03 according to Van't hoff equation in the form (14):
log D =
H 1 +C 2.303R T
Fig. 9. Effect of experimental temperature on the extraction of Th(IV) with mixed Cyanex 572 and ODP. Th(IV) = 5.67 × 10−4 mol/L, [Cyanex 572 + ODP]o = 5.5 × 10−3 mol/L, XCyanex 572:XODP = 1:2, [HNO3] = 0.25 mol/L.
(14)
where R is gas constant and C is a constant. The change of Gibbs free energy (∆G) and the change of entropy (∆S) in 298 K can be obtained according to the Eqs. (15) and (16), respectively.
G=
RTlnK
G= H
T S
(16)
The value of log K in 298 K is calculated according to Eq. (12). Thermodynamic parameters including log K, ∆H, ∆G and ∆S are given
(15) 190
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hydrophobicity and stability of extracted species in the organic phase. The maximal synergistic enhancement coefficient reaches 12.40 at a ratio of 1:2 of Cyanex 572 and n-octyl diphenyl phosphate. In addition, the addition of Cyanex 572 to n-octyl diphenyl phosphate can avoid the emulsification in individual n-octyl diphenyl phosphate extraction system. By slopes analysis method and IR spectra analysis, the extraction mechanim of Th(IV) from nitric acid medium with the mixed noctyl diphenyl phosphate and Cyanex 572 system is investigated. The synergistic extracted complexes are confirmed to be Th(NO3)3HA2B. Thermodynamic study indicates that the synergistic extraction reaction is spontaneous. Lower stripping acid are required according to the stripping experiment. The extraction and separation of REE and Th from the feed solution leached from the ion-absorbed type rare earth mineral waste residue with the mixture of n-octyl diphenyl phosphate and Cyanex 572 are studied, which indicates that the mixed extraction system shows better separation performance for REE and Th. Hence, the proposed synergistic extraction system is a promising system in the separation of Th and REE, that is, efficient extraction ability, selectivity, phase separation performance and stripping property.
Table 2 Thermodynamic parameters in the synergistic extraction of Th(IV) at 298 K. Log K
ΔH (KJ/mol)
ΔG (KJ/mol)
ΔS (J·mol−1·K−1)
10.702
−19.72
−61.05
138.7
Acknowledgement This work was supported by The National Key Technology R&D Program (2017YFE0106900), ‘Hundreds Talents Program’ and Service Network Initiative from Chinese Academy of Sciences and Science and Technology Major Project of Ganzhou (2018). Fig. 10. The separation of Th and REE from industrial feed solution. [ODP + Cyanex 572]o = 1.1 × 10−3 mol/L, [HNO3] = 0.25 mol/L, [REE (III)] = 2.5 × 10−2 mmol/L, [Fe] = 0.936 mmol/L, [Th(IV)] = 1.41 × 10−3 mmol/L.
References Arnold, J., Gianetti, T.L., Kashtan, Y., 2014. Thorium lends a fiery hand. Nat. Chem. 6, 554. Ashley, S.F., Parks, G.T., Nuttall, W.J., Boxall, C., Grimes, R.W., 2012. Nuclear energy: thorium fuel has risks. Nature 492, 31. Bismondo, A., Comuzzi, C., Bernardo, P.D., Zanonato, P.L., 1999. Complexation of thorium(IV) by tris((2,3-dihydroxybenzylamino)ethyl)amine-a new strong chelating agent. Inorg. Chim. Acta 286, 103–107. Cheng, Z., Wang, H., Wang, Y., He, F., Zhang, H., Yang, S., 2011. Synthesis and characterization of an ion-imprinted polymer for selective solid phase extraction of thorium(iv). Microchim. Acta 173, 423–431. Deng, Q., Jin, Y., Wang, Q., Zhao, R., Pan, N., Zhai, F., Luo, M., Xia, C., 2013. New cyclen derivative ligand for thorium(IV) separation by solvent extraction. J. Radioanal. Nucl. Ch. 295, 125–133. Dong, Y., Li, S., Wang, Y., Shen, Y., Sun, X., 2017. Separation of thorium from rare earths with high-performance diphenyl phosphate extractant. Hydrometallurgy 171, 387–393. El-Dessouky, S.I., Borai, E.H., 2006. Extraction chromatography of thorium ion by solid phase impregnated resins containing bi-functional organic extractants. J. Radioanal. Nucl. Ch. 268, 247–254. Fan, S., Tian, M., Song, N., Jia, Q., Liao, W., 2011. Synergistic extraction and separation study of rare earth elements from nitrate medium by mixtures of sec -nonylphenoxy acetic acid and 2,2′-bipyridyl. J. Chem. Technol. Biotechnol. 86, 719–723. Jain, V.K., Pillai, S.G., Pandya, R.A., Agrawal, Y.K., Shrivastav, P.S., 2005. Selective extraction, preconcentration and transport studies of thorium(IV) using octa-functionalized calix[4]resorcinarene-hydroxamic acid. Anal. Sci. 21, 129–135. Li, S.M., Tan, M.L., Ding, S.D., Li, F., Li, Q.N., Zhang, L., Liu, C.X., 2016. Extraction of U (VI) and Th(IV) with three neutral extractants containing phosphorus. Chem. Res. Appl. 28, 307–315. Lin, C., Wang, H., Wang, Y., Cheng, Z., 2010. Selective solid-phase extraction of trace thorium(IV) using surface-grafted Th(IV)-imprinted polymers with pyrazole derivative. Talanta 81, 30–36. Lu, Y., Bi, Y., Bai, Y., Liao, W., 2013. Extraction and separation of thorium and rare earths from nitrate medium with p-phosphorylated calixarene. J. Chem. Technol. Biotechnol. 88, 1836–1840. Lu, Y., Wei, H., Zhang, Z., Li, Y., Wu, G., Liao, W., 2016. Selective extraction and separation of thorium from rare earths by a phosphorodiamidate extractant. Hydrometallurgy 163, 192–197. Ma, L., Zhao, Z., Dong, Y., Sun, X., 2017. A synergistic extraction strategy by [N 1888] [SOPAA] and Cyphos IL 104 for heavy rare earth elements separation. Sep. Purif. Technol. 174, 474–481. Ma, L., Zhao, Z., Dong, Y., Sun, X., 2018. A synergistic extraction strategy by Cyanex 572 and Cyanex923 for Th (IV) separation. Sep. Purif. Technol. 191, 307–313. Meera, R., Varma, R.L., Reddy, M.L.P., 2004. Enhanced extraction of thorium (IV) and uranium (VI) with 1-phenyl-3-methyl-4-pivaloyl-5-pyrazolone in the presence of various neutral organophosphorus extractants. Radiochim. Acta 92, 17–24. Mohapatra, P., Veeraraghavan, R., Manchanda, V., 1999. Extraction of thorium (IV) by a mixture of 1-phenyl-3-methyl-4-benzoyl-5-pyrazolone and tri-n-octyl phosphine oxide. J. Radioanal. Nucl. Ch. 240, 31–38.
in Table 2, it can be found that the sign of ∆H and ∆G is minus, which indicates the synergistic extraction system is exothermic and spontaneous. In addition, ∆S > 0 indicates the mixture system is more disorder, further demonstrating its easily implement. 3.7. Extraction and separation of Th(IV), REE(III) from industrial feed solution Industrial feed solution leached from the ion-absorbed type rare earth mineral waste residue is applied to test selectivety of the mixed Cyanex 572 and n-octyl diphenyl phosphate. As shown in Fig. 10, the extraction effiencies of Th(IV) are better than REE and Fe in all the extraction systems. Compared with the individual Cyanex 572 or noctyl diphenyl phosphate extraction system, the mixed Cyanex 572 and n-octyl diphenyl phosphate extraction system reaveals evident synergistic extraction effect of Th(IV) (R = 2.3). In addition, separation factors of Th to REEs are calculated to be 4.7, 1.1 and 7.6 in individual n-octyl diphenyl phosphate, individual Cyanex 572 and mixed Cyanex 572 and n-octyl diphenyl phosphate extraction systems. Based on the results, the synergistic system shows higher extraction efficiency for Th (IV) and better separation performance of Th(IV) and REEs than the individual extraction systems. Hence, the synergistic extractant has revealed a promising application potential for the extraction and separation of Th. 4. Conclusion In this paper, a novel synergistic extraction system of mixed n-octyl diphenyl phosphate and Cyanex 572 was investigated for the Th(IV) extraction. Comparing the extraction of Th(IV) with individual n-octyl diphenyl phosphate, individual Cyanex 572 and mixed n-octyl diphenyl phosphate and Cyanex 572, it is found that the mixed extraction system presents significant synergistic effect on Th(IV) and better phase separation performance. This can be probably attributed to the greater 191
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H. Zhou et al. Nasab, M.E., Sam, A., Milani, S.A., 2011. Determination of optimum process conditions for the separation of thorium and rare earth elements by solvent extraction. Hydrometallurgy 106, 141–147. Paiva, A.P., Malik, P., 2004. Recent advances on the chemistry of solvent extraction applied to the reprocessing of spent nuclear fuels and radioactive wastes. J. Radioanal. Nucl. Ch. 261, 485–496. Pathak, P.N., Veeraraghavan, R., Manchanda, V.K., 1999. Separation of uranium and thorium using tris(2-ethylhexyl) phosphate as extractant. J. Radioanal. Nucl. Ch. 240, 15–18. Pathak, P.N., Kumbhare, L.B., Manchanda, V.K., 2001. Effect of structure of N, N dialkyl amides on the extraction of U (VI) and Th (IV): a thermodynamic study. Radiochim. Acta 89, 447–452. Patil, S.K., Ramakrishna, V.V., Sajun, M.S., Thiagarajan, R., 1979. Synergistic extraction of Th(IV) by mixtures of HTTA and TBP in benzene. Sep. Sci. Technol. 14, 859–868. Patkar, S.N., Burungale, A.S., Patil, R.J., 2009. Separation and liquid liquid extraction of thorium (IV) as sulphate complex with synergistic mixture of N-octylaniline and trioctylamine as an extractant. Rasayan J. Chem. 2, 4825–4832. Reddy, M.L.P., Meera, R., 2001. Crown ether as size selective synergist in the solvent extraction of thorium (IV) and uranium (VI) with 3-phenyl-4-(4-fluorobenzoyl)-5isoxazolone. Radiochim. Acta 89, 453–460. Sato, T., 1966. Extraction of uranium (VI) and thorium from hydrochloric acid solutions by tri-n-butyl phosphate. J. Chem. Technol. Biotechnol. 16, 53–57. Shrivastav, P., Menon, S.K., Agrawal, Y.K., 2001. Selective extraction and inductively coupled plasma atomic emission spectrophotometric determination of thorium using a chromogenic crown ether. J. Radioanal. Nucl. Ch. 250, 459–464.
Tan, M., Huang, C., Ding, S., Li, F., Li, Q., Zhang, L., Liu, C., Li, S., 2015. Highly efficient extraction separation of uranium (VI) and thorium (IV) from nitric acid solution with di (1-methyl-heptyl) methyl phosphonate. Sep. Purif. Technol. 146, 192–198. Tong, H., Wang, Y., Liao, W., Li, D., 2013. Synergistic extraction of Ce (IV) and Th (IV) with mixtures of Cyanex 923 and organophosphorus acids in sulfuric acid media. Sep. Purif. Technol. 118, 487–491. Tunsu, C., Lapp, J.B., Ekberg, C., Retegan, T., 2016. Selective separation of yttrium and europium using Cyanex 572 for applications in fluorescent lamp waste processing. Hydrometallurgy 166, 98–106. Wang, Y., Huang, C., Li, F., Dong, Y., Sun, X., 2017. Process for the separation of thorium and rare earth elements from radioactive waste residues using Cyanex® 572 as a new extractant. Hydrometallurgy 169, 158–164. Yaftian, M.R., Hassanzadeh, L., Eshraghi, M.E., Matt, D., 2003. Solvent extraction of thorium (IV) and europium (III) ions by diphenyl-N, N-dimethylcarbamoylmethylphosphine oxide from aqueous nitrate media. Sep. Purif. Technol. 31, 261–268. Yuan, L., Bai, Z., Zhao, R., Liu, Y., Li, Z., Chu, S., Zheng, L., Zhang, L., Zhao, Y., Chai, Z., Shi, W., 2014. Introduction of bifunctional groups into mesoporous silica for enhancing uptake of thorium(IV) from aqueous solution. ACS Appl. Mater. Interfaces 6, 4786–4796. Zhang, Y., Xu, Y., Huang, X., Long, Z., Cui, D., Hu, F., 2012. Study on thorium recovery from bastnaesite treatment process. J. Rare Earth 30, 374–377. Zuo, Y., Chen, J., Li, D., 2008. Reversed micellar solubilization extraction and separation of thorium (IV) from rare earth (III) by primary amine N1923 in ionic liquid. Sep. Purif. Technol. 63, 684–690.
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