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Preparation of high-purity neodymium chloride by solvent extraction in the mixer-settlers: A pilot-scale investigation
Accepted Manuscript Preparation of high-purity neodymium chloride by solvent extraction in the mixersettlers: a pilot-scale investigation Fujian Li, Qiaoyan He, Yanliang Wang, Hong Zhou, Shuibo Zhang, Xiaoqi Sun PII:
S1002-0721(17)30038-8
DOI:
10.1016/j.jre.2017.06.002
Reference:
JRE 29
To appear in:
Journal of Rare Earths
Received Date: 24 January 2017 Revised Date:
8 June 2017
Accepted Date: 8 June 2017
Please cite this article as: Li F, He Q, Wang Y, Zhou H, Zhang S, Sun X, Preparation of high-purity neodymium chloride by solvent extraction in the mixer-settlers: a pilot-scale investigation, Journal of Rare Earths (2017), doi: 10.1016/j.jre.2017.06.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Preparation of high-purity neodymium chloride by solvent extraction in the mixer-settlers: a pilot-scale investigation
(
) 1,2,3,4, HE Qiaoyan (
)4, ZHANG Shuibo (
)4, WANG Yanliang (
)4, SUN Xiaoqi (
)1,2,3*
)1,2,3, ZHOU Hong
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LI Fujian (
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(1. Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China; 2. Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences, Xiamen 361021, China; 3. University of Chinese Academy of Sciences, Beijing, 100039, P. R. China; 4. Ganzhou Rare Earth Group Co., Ltd., China Southern Rare Earth, Ganzhou 341000, China)
Foundation item: This work was supported by ‘Hundreds Talents Program’ from Chinese Academy of Sciences, Science and Technology Major Project of Fujian Province (2015HZ0001-3), National Natural Science Foundation of China (21571179) and Science and Technology Major Project of Ganzhou (2017-8). Corresponding author: SUN Xiaoqi (E-mail: [email protected], Tel.:0592-6376370) DOI:
ACCEPTED MANUSCRIPT Abstract: A novel purification process based on mixer-settlers for high-purity NdCl3 was developed. Acidic solution and pure NdCl3 solution were compared to scrub the less-extractable rare earths (REs) (La, Ce, and Pr) from loaded organic phase. The extractant with low-degree saponification was evaluated to remove the more-extractable element (Sm). Then the RE impurities (La, Ce, Pr, and Sm) in Nd were purified by an
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integrated process. Furthermore, effect of smuggling behavior on the purification of NdCl3 in mixer-settlers was studied. Based on the investigation mentioned above, the pilot-scale purification process for NdCl3 with a purity of 99.999% was developed, and the total recovery was about 99%.
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Keywords: Solvent extraction, Purification, Neodymium, High-Purity, Mixer-settlers
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Highlights
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Graphic abstract
A novel process was developed in the mixer-settlers to purify NdCl3 with a purity of 99.999%. Impurities of La, Ce, and Pr can be removed using pure NdCl3 solution as scrubbing reagent. Impurities of Sm can be separated using low-degree saponification extractant. Smuggling reveals negative impact on the REE purification process. Optimized technical parameters for industrial application were developed.
ACCEPTED MANUSCRIPT Neodymium (Nd) is an important element in astronavigation, clean energy and electronic industries [1]. As applications in these specialized areas increasing, high-purity Nd has been commonly required. For example, the product of Nd2O3 applied in batteries and semiconductor laser requires that the content of RE (rare earth)
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impurities should be lower than 1×10-4 µmol/mol [2]. In general, the impurities in Nd2O3 product are light REs (La, Ce, Pr, and Sm) and transition elements. The separation of light REs is particularly difficult because of their similar physical and chemical properties
[3]
. Among the common methods for REs separation, solvent
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extraction is one of the most popular and versatile techniques [4]. Over the past years, many extraction processes
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were developed for REs separation, especially for the separation of Pr/Nd [5], Tm/Yb/Lu [6], Ho/Er/Y [7], RE/Th [8]
, etc. However, most of them were focused on the purifications of REs from 98% to 99.9%. Recently, the
preparation of high-purity REs has drawn much attention. Using the extraction chromatography, Li et al. prepared Nd with an impurity of Pr at 9.5 ug/g [9], and Tm with a purity of 99.995%
using electricity
[12]
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contactor was also used to prepare high-purity Y (>99.99%) and ultrasonic
[13]
[10]
. Annular centrifugal
[11]
. Moreover, optimized extraction processes
were developed to produce Gd and Nd with high purities (>99.99%).
Based on high-purity RE oxides, some ultra-purity RE metals were purified by vacuum distillation
[14]
and
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not been mentioned.
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external getter [15]. To the best of our knowledge, the pilot-scale preparation of high-purity NdCl3 (99.999%) has
Solvent extraction based on mixer-settlers is widely used in the industry of REs separation. In the process of solvent extraction, a noticeable smuggling of aqueous phase by organic phase at a volumetric ratio of above 0.1% occurs normally. It is possible in an extreme situation that the volumetric ratio of smuggling arrived at 10% [16]
. The smuggling resulted in negative impacts on the separation efficiencies for REs. The backmixing of
aqueous phase aroused by smuggling was studied since 1960s [17] and the accurate results could be obtained by computer simulation [18]. The parameters such as RE concentration, organic to aqueous phase ratio and initial
ACCEPTED MANUSCRIPT pH of RE solution were optimized, and a membrane phase separator was adapted to decrease the smuggling of aqueous phase [19]. A microfluidic chip was also employed to improve the extraction efficiency [20]. Being the main component in the formation of stabilizing interfacial film, surfactant was indicated to be the most [21]
. In the extraction process, a composition of metal ions and
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predominant contributor to emulsion stability
extractant behaves similarly as surfactant, which has a significant influence on extraction efficiency
[22]
.
Generally, both extraction efficiency and economic burden should be considered together to improve the RE
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purity. Therefore, the systematical investigation of smuggling together with corresponding control methods is
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important in both of academic research and industrial application for high-purity REs preparation. In this paper, an integrated Nd purification process was developed to discriminatively treat the less-extractable REs (La, Ce, and Pr) and more-extractable RE (Sm) on bench and pilot-plant scales. Also, the controlling method of smuggling behavior on the extraction process was investigated. Based on these studies, a novel and efficient
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pilot-scale solvent extraction process for high-purity Nd (99.999%) was developed. The related work has been applied for Chinese patent protection [23].
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1. Experimental
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1.1 Reagents and apparatus
2-Ethylhexyl phosphoric acid mono-2-ethylhexyl ester (P507) and sulfonated kerosene were purchased from Luoyang Aoda Chemical Co., Ltd., (China), and used without further purification. Structural formulas of P507 and RE(HA2)3 are shown as Fig. 1. Neodymium oxide (Nd2O3, 99.9%) used for feed solution was kindly provided by Ganzhou Rare Earth Group Co., Ltd., (China). Neodymium oxide (Nd2O3, 99.999%) used as scrubbing reagent was purchased from Alfa Aesar (A Johnson Matthey Company). NdCl3 feed solution (99.9%)
ACCEPTED MANUSCRIPT and pure NdCl3 scrubbing solution (99.999%) were prepared by dissolving the corresponding neodymium oxides with hydrochloric acid (HCl) and diluting with deionized water [7].
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(Insert Fig. 1)
Inductively coupled plasma atomic emission spectroscopy (Horiba ULTIMA, ICP-AES) and inductively
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coupled plasma mass spectrometry (ICP-MS, 300Q, PE) was used to determine the concentrations of REs in
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aqueous phase. The concentrations of acidic extractants were titrated by standard solution of sodium hydroxide. The required pH value of aqueous phase was adjusted with HCl or NaOH solution. The concentration of water in the organic phase was determined by the Karl-fischer titration method using an automatic titrator (KLS-411;
1.2 Solvent extraction process
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Shanghai INESA). All the chemicals used in the bench scale study were of analytical grade.
Bench scale solvent extraction studies were carried out by mixing different volumes of feed solution (6.5
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mL) and organic phase (50 mL) with the help of a mechanical stirrer for 10 min. After equilibration, the phases
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were settled and separated for 30 min. Stripping of metal ions from the loaded organic phase was carried out using 6 mol/L HCl solution. All the experiments were conducted at room temperature, which was about 25. Based on the experiments mentioned above, an integrated Nd separation investigation was carried out as follows: Adjusting the pH value around to be 4, NdCl3 feed solution was mixed to organic phases with the same saponification degree for 10 minutes by a mechanical stirrer. Then the loaded organic phase was scrubbed using pure NdCl3 scrubbing solution at a scrubbing ratio of 20%, the scrubbing procedure was repeated for ten times. Also, the loaded organic phase was stripped by 6 mol/L HCl solution, the stripped aqueous phase mentioned
ACCEPTED MANUSCRIPT above was mixed in sequence with a saponification degree of 0.09 mol/L (nNaOH/Vorg) organic phase (25ml) for eight times. Finally, the aqueous raffinate was collected for further analysis. In the pilot-scale study, the organic phase was formed with a mixture of P507 and kerosene (1:1 by
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volume). Stock solutions of RECl3 were prepared by dissolving ion-adsorbed RE deposits in HCl solution. NdCl3 solution (99.9%) was prepared from leaching solution of the deposits by traditional RE separation process
[24]
. The organic phase was saponified by NaOH solution and stripped by HCl solution. All the
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chemicals used in the pilot scale study were of commercial grade. RE compositions of the feed solution were
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analyzed by inductively the ICP-AES (shown in Table 1). NdCl3 solution (>99.9%) was analyzed by the ICP-MS.
As shown in Fig. 2, a separation process containing three-stage countercurrent solvent extraction, forty-stage RE scrubbing, thirteen-stage stripping, and two-stage HCl scrubbing by water were developed. The
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product of NdCl3 solution (99.999%) was led at the stage forty-fifth. The volume of mixing chamber in extraction tank was 60 L, and the ratio of mixing chamber/clarification chamber was 1:3. The design capacity of
(Insert Fig. 2)
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process was 1000 kilograms Nd2O3 (99.999%) every month.
In both bench and pilot scale experiments, the extraction efficiency (E), scrubbing ratio (S), saponification degree (SD), are defined as the following Eq. (1) to Eq. (3): % =
×
100% 1
% = × 100% 2
=
!"#
3
ACCEPTED MANUSCRIPT where [M]at and [M]a represent the initial and final concentration of REs in aqueous phase, nHCl and nore represent the molar mass of HCl solution and REs in the loaded organic phase. Scrubbing ratio (S%) represents the molar ratio of scrubbing reagent and loaded REs in the organic phase initially. SD% denotes the
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saponification degree. nNaOH denotes the molar mass of NaOH solution. Vorg denotes the volume of organic phase. All the concentration values of REs were measured in duplicate with the uncertainty within 5%. In the extraction process, the impurities of La3+, Ce3+, and Pr3+ in the loaded organic phase could be
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replaced by Nd3+ in a suitable acidity, the extraction reaction was represented as the following Eq. (4) [25]:
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%&' + )*+,# -./ = %&-./ + ) ' 0123 4
where (HA)2 denotes the dimeric form of acidic cationic extractant (P507) and subscript org represents the organic phase. RE3+imle denotes the impurities of less-extractable REs (La3+, Ce3+, and Pr3+).
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1.3 Smuggling of aqueous phase by organic phase in mixer-settlers
As shown in following Eq. (5), AISR represents the adjacent stage impurity ratio. AISR is defined as the
scrubbing segment [16]. 67,9
.5) = 6
= ' "> 5 7 ' ">
≈ ?/AB
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7,9:;
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ratio of an impurity’s concentrations in the aqueous phase (or the organic phase) of two adjacent stages in the
where βab represents the separation factor. Xb,n represents the neat molar flow rates of the less-extractable component b in nth stage in the aqueous phase, while Xb,n+1 represents that in the (n+1)th stage. rx represents the backmixing ratio of aqueous phase, which is defined as rx = V'x /Vx. rs represents the volumetric ratio of aqueous phase smuggled by organic phase, which is defined by means of volumetric flow rate ratio as rs = V'x/Vy. Vx and Vy denote the neat volumetric flow rates of aqueous and
ACCEPTED MANUSCRIPT organic phase between adjacent two stages, V'x represents the flow rate of aqueous phase smuggled by organic phase. If S = Ya + Yb and W = Xa + Xb are defined as the total RE flow rate (by molar) of the organic and aqueous
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phases, respectively. S ≈ Ya = Vy × [M]ot and W ≈ Xa = Vx × [M]at can be deduced in the scrubbing segments; [M]ot and [M]at denote the concentrations of REs in the organic phase and aqueous phase at equilibrium state, respectively. Then ASIR can be stated as the following Eq. (6): 1 + DE F / G F H!I /J K G H I + DE F /
G
=
G / F + DE
H!I /H I 1/J K + DE
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.5) =
6
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In the scrubbing segment, D denotes the distribution ratio which is defined as D= [M]ot / [M]at. It is fixed when the saponification rate and the concentration of the stripping acid are predetermined. Then ASIR can be stated as the following Eq. (7): .5) ≈
)M/ − × 1/J K )M/ + DE = 1 + 7 1 × 1/J K + DE × N O + DE J K
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where RA/O denotes the phase flow rate (by volume) of aqueous phase and organic phase. And the estimated stage number N for scrubbing segment can be expressed as the following Eq. (8): ,R S /S
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T ; % = ,R M?SU 8
where I0 denotes the RE impurities in NdCl3 feed solution, I1 denotes the RE impurities in the target
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product. In some extraction process, the stage number N of the scrubbing segment is fixed while the rs is variable via the factors mentioned above. To evaluate the effect of rs on I2, Eq. (9) is given as follows: 5/ = 5W / .5)
T
≈ 5W / X
UY/Z ' "[
; O ' "[ ]7
\ × N
^
T
9
where N0 denotes the estimated stage number N for scrubbing segment when rs is limited to be zero. 2. Results and discussion
2.1 Removal of La, Ce, and Pr from Nd feed solution
ACCEPTED MANUSCRIPT (1) Effect of extraction on RE impurities removal
In the NdCl3 feed solution, La3+, Ce3+ and Pr3+ could be regarded as less-extractable components because Nd3+ was a more-extractable component in P507 extraction process [26, 27]. Extraction ratio (E%) represents the
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molar ratio of RE in the loaded organic phase and feed solution at an equilibrium state. As E% decreasing, the concentration of Nd3+ in raffinate phase was increased. The impurities of La3+, Ce3+, and Pr3+ in loaded organic
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phase were replaced in a suitable acidity, and the extraction reaction could be represented as Eq. (4). The effect of extraction ratio on RE impurity removal was varied from 66.67% to 100%. As presented in Fig. 3, various
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volumes of NdCl3 feed solutions were mixed with organic phases with the same saponification degree. The loaded organic phase was stripped by HCl (6 mol/L) and the stripping phase was collected for further analysis. With the increase of E%, a slight decrease of the impurities of La3+, Ce3+, and Pr3+ in aqueous phase was observed. The phenomenon indicates the RE impurities in the organic phase were not absolutely scrubbed by
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Nd3+ in the raffinate phase under the experimental conditions. Because the impurities amount of La3+, Ce3+, and Pr3+ were slight in the loaded organic phase and there was almost the same amount in the raffinate phase. It
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indicates clearly that the purity of NdCl3 product was difficult to be improved either by increasing the REs concentration in raffinate or by adding the extraction stages. As previously mentioned, it is better to extract the
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metal ions together and then to separate them by scrubbing [28].
(Insert Fig. 3)
(2) Effect of scrubbing on RE impurities removal
ACCEPTED MANUSCRIPT To get the RE products with purities from 99.5% to 99.9%, acidic solution was widely used as scrubbing reagent to purify the loaded organic phase in RE industry [25, 29]. Using 2.5965 mol/L HCl, the loaded organic phase obtained from the extraction experiments was subjected to different scrubbing ratios from 0 to 40%. Each
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loaded organic phase was scrubbed twice with the same ratio. As presented in Fig. 4, the impurity of La3+ was scrubbed effectively at the scrubbing ratio of 10%, then decreased rapidly at the scrubbing ratio of 20%. However, there was slight change due to the scrubbing ratio from 20% to 40%. It worthwhile to mention that the
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impurities of Ce3+ and Pr3+ were nearly not reduced in our experimental range. The effect can be attributed to the
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lower separation factors of Ce/Nd and Pr/Nd, which indicates the REs cannot be scrubbed in a single stage, especially when the absolute content is below 200 ug/g. Because it is not a good choice to purify the microscale RE impurities using HCl solution, pure NdCl3 solution was further studied as scrubbing reagent in the further
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experiments.
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(Insert Fig. 4)
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(3) RE impurities removal by pure NdCl3 solution
For the purification of loaded organic phase by scrubbing, there are two essential approaches: one is the use of acidic aqueous solution at certain pH to remove the unwanted metal and keep the metal of interest in the loaded organic phase. The other approach is the use of metal solution of interest as a scrubbing reagent to replace the co-extracted unwanted metal. To evaluate the selective scrubbing of La3+, Ce3+, and Pr3+ from loaded organic phase, additional scrubbing experiments were performed by varying the concentration of pure NdCl3 solutions as a scrubbing reagent (at pH-4.0) and obtained results are represented in Fig. 5. The impurities of La3+,
ACCEPTED MANUSCRIPT Ce3+ and Pr3+ were reduced at the concentration of 0.2 mol/L pure NdCl3 solution. Tiny change could be observed by increasing the concentration of scrubbing reagent (Fig. 5). The varied scrubbing ratios from 0 to 40% were investigated at the concentration of 1.0898 mol/L pure
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NdCl3 solutions. Each loaded organic phase was scrubbed twice with the same ratio. The impurities of La3+ and Pr3+ were rapidly reduced as the scrubbing ratio increasing from 0 to 20%. It implied that La3+and Pr3+ were efficiently scrubbed by the Nd3+ at the scrubbing ratios of 20%.
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Afterwards, a slight decrease in impurities was observed at the scrubbing ratios of 30% and 40%. However,
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Ce3+ revealed a low amount in the feed solution and remained unchanged at the experiments range (Fig. 6). The decrease indicates that the neighboring REs in front of Nd in the periodic table could be purified by pure NdCl3 solution to some degree. The pure metal solution was also used as scrubbing reagent for preparation of Nd with a purity of 99% [5]. Nevertheless, it was difficult to improve the purity by increasing the scrubbing ratios in a
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single stage. Therefore, the pilot scale experiment was carried out by designing more scrubbing stages rather than extraction stages. In the pilot-scale experiments, the stripped liquor containing high-purity NdCl3 solution from loaded organic phase was partly diverted into the scrubbing section, which is called as the technique of
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sharing the scrubbing reagents with stripping reagents. On the one hand, the technique is efficient to prepare the
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RE product with required purity by pure metal scrubbing. On the other hand, it contributes to reducing the amounts of scrubbing acid together with alkali in the saponification section, which reveals economic and environmental advantages.
(Insert Fig. 5 and Fig. 6)
Exchange mechanism:
ACCEPTED MANUSCRIPT The extraction equilibrium of less-extractable REs (La3+, Ce3+, and Pr3+) can be expressed as the following Eq. (10) [30]: ) ' *+,# + 3-/ ./ = )*+,# -./ + 3- ' △ a= = −)bcd= 10
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The extraction equilibrium of Nd3+ can be expressed as the following Eq. (11): %&' + 3-/ ./ = %&-./ + 3- ' △ a/ = −)bcd/ 11
The exchange mechanism between the less-extractable REs and Nd3+ may be expressed as the following
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Eq. (12):
%&' + )*+,# -./ = %&-./ + ) ' *+,# △ a = −)bcd 12
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bℎf, g. 12 = g. 11 − g. 10
-fif,△ a = △ a/ − △ a= = −)bc N
d/ jk: O = −)bc N O = −)bcJ 13 d= l"k:
In the scrubbing section, [Ln()](a) was approximate equal to the total concentration multiplied by each
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composition of scrubbing solution, while the [Ln()](o) was approximate equal to the total concentration multiply by each composition of feed solution. Then ∆G3 of scrubbing equilibrium of La3+, Ce3+, and Pr3+ by Nd3+ were -23.16, -23.34, and -14.32 kJ/mol, respectively. As shown in table 1, the Gibbs free energies were all
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less than zero, which means that they were spontaneous processes at the room temperature and pressure.
(Insert Table 1)
The separation factors of βNd/Pr, βNd/Ce and βNd/La were always greater than 1 in traditional extraction processes using organophosphoric extractants [26] [31]. Then ∆G3 can also be obtained as Eq. (13), which indicates they are all less than zero in the experiment condition.
ACCEPTED MANUSCRIPT Furthermore, the complex structures of RE(HA2)3 can be given in Fig.1. Compared with the original hydrogen-oxygen bond, the lanthanum-oxygen bond is stronger because the ionic radius of lanthanum is smaller than that of hydrogen. The size effect contributes to forming stable extracting complex formed by
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lanthanum with the P507 molecule [27]. As the well-known lanthanide contraction, the ionic radius of Nd3+ is smaller than those of La3+, Ce3+, and Pr3+. Accordingly, neodymium-oxygen bond is the strongest among the
between the less-extractable REs and Nd3+ is easily occurred.
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2.2 Removal of Sm from Nd feed solution
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four RE-oxygen bonds due to the biggest electron affinity. As shown in Eq (12), the exchange mechanism
As mentioned above, the impurity of Sm can be scrubbed by neither acidic solution nor pure NdCl3 solution. Sm is the first extracted RE by the organic phase and the last scrubbed RE by the acid in NdCl3 feed [26]
. In this paper, additional experiments were carried out using 1.5 mol/L P507 with verified
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solution
saponification degree from 0 to 0.09 mol/L (nNaOH/Vorg) to quantitatively extract 81 µmol/mol Sm from NdCl3 feed solution. The saponified organic phase was contacted with 0.9763 mol/L NdCl3 feed solution (99.9%) at an
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O/A ratio of 2.5, and the aqueous raffinate was collected for further analysis. As shown in Fig. 7, the impurity of
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Sm was removed efficiently with the saponification degree increasing. As more organic phase was saponified, more REs were extracted and less Sm could stay in aqueous phase. Because of the microscale of Sm in the feed, the target product Nd was extracted together with Sm. Thus, the increased saponification degree decreased the yield of Nd. An optimized saponification degree was 0.03 mol/L in this study. To acquire Nd product with high yield, more stripping stages should be developed.
(Insert Fig. 7)
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2.3 Bench-scale purification
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Based on the investigation mentioned above, an integrated Nd purification process was developed to discriminatively treat the less-extractable REs (La, Ce, and Pr) and more-extractable RE (Sm) on the bench scale. The experimental procedures were presented as mentioned above. Table 2 shows that the purities of feed
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solution and obtained NdCl3 solution are 99.9% and 99.999%, respectively. NdCl3 with a purity of 99.9% is the
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main product in the RE industry of China [29]. It should be noted that the less-extractable REs (La, Ce, and Pr) in NdCl3 feed solution were removed efficiently and the residual amount was 0.1, 0.1 and 0.8 µmol/mol, respectively. It was lower than or approached the level of the scrubbing Nd reagent purchased from Alfa Aesar. The data in Table 2 show that P507 is an efficient extractant for the removal of La, Ce, Pr, Sm and some other
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RE purities from NdCl3 feed solution on the bench-scale experiment.
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(Insert Table 2)
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2.4 Effect of backmixing on RE impurities removal in mixer-settlers
Solvent extraction based on mixer-settlers is the most popular technology for REs industrial separation[19]. In the mixer-settlers extraction process, the backmixing of aqueous phase cannot be avoided due to the insufficient settling time in normal circumstances. Wu et al.
[16]
stated that the backmixing of aqueous phase
might have serious impact on the design and application of REs separation, especially for the scrubbing process. In this study, the backmixing effect in scrubbing section was presented, and appropriate controlling method was
ACCEPTED MANUSCRIPT developed. Base on the essential assumptions of the article [16], a stable state in both the flow and backmixing of two phases was supposed. ASIR in the scrubbing segment could be estimated as Eq. (7), and I2 could be estimated as Eq. (9). In a certain extraction process, the influence factors of AISR and I2 can be attributed to (a)
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separation factor βab; (b) the volumetric ratio of the aqueous phase smuggled by the organic phase rs; (c) the flow rate ratio of aqueous phase and organic phase RA/O. In some process, the separation factor βab was fixed. However, RA/O could be designed and rs should be controlled to improve the separation efficiency.
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In this study, the effects of rs on impurities and ASIR were investigated. As mentioned above, rs in the
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extraction process in the funnels are showed in Fig. 8. With the concentrations of HCl were ranged from 0 to 6.8086 mol/L in pure HCl solution and RECl3 solution, the volumetric ratios of aqueous phase smuggled by the organic phase were all around 15 ml/L, excepting for that rs was 49.54 ml/L and 7.21 ml/L when the concentration of pure HCl was 0 and 6.8086 mol/L, respectively. That indicates the value of rs is about 15 ml/L
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in the experimental conditions. While in industry, the value of rs is much higher than that in funnel, which is
(Insert Fig. 8)
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about 82.13 ml/L (Sample provided by Ganzhou Rare Earth Group Co., Ltd).
To illustrate the influence of the backmixing effect on the RE separation process more clearly, our results were compared with those in some other article [16]. As shown in Table 3, the relevant parameters were given. The critical separation factor was the same between the two papers, while the other parameters were of a minor difference. As the rs increasing, ASIR decreased with the same tendency (see Fig. 9). However, ASIR in this paper was the largest among the three groups of rs, and even larger than
Pr/Nd
(value,1.5[5]). It should be
noted that the ratio of total RE flow rate (by molar) of organic and aqueous phases (W:S) in this paper and that in
ACCEPTED MANUSCRIPT Wu’s article were 1.3 and 0.8, respectively. The tendencies of ASIR and W:S in the scrubbing segment and extraction segment in Wu’s article was similar [16]. In some process, the saponification degree and concentration of the stripping HCl solution were predetermined. The RE concentrations in both phase were fixed. Hence, the
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value of W:S was determined by the phase ratio RA/O. As shown in Eq. (7), enlarging RA/O could promote the value of ASIR and improve the purity of the product in the scrubbing segment.
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(Insert Table 3 and Fig. 9)
Therefore, the effect of backmixing on the impurities of RE product was calculated as Eq. (9) and the result was shown in Fig. 9. With the increase of rs from 0 to 15%, ASIR was decreased from 2.0 to 1.31. Meanwhile, the impurities in the RE product increased sharply from 1.0 to 56.3 µmol/mol (Table 4). Therefore, backmixing
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of the aqueous phase has serious impact on the design and application of RE separation, especially for the
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high-purity RE products in the scrubbing section.
(Insert Table 4)
To control the smuggling, the viscosity of organic phase was reduced by optimizing the extractant concentration and saponification degree. Furthermore, the settling time of two phases was extended as much as possible. A suitable phase ratio (RA/O) is the most important for the extraction process.
2.5 Industrial application
ACCEPTED MANUSCRIPT P507 has been widely used in Chinese RE separation industry [32]. Several extraction processes based on P507 were developed to purify the ion-adsorbed RE deposit leaching [7]. Based on our results mentioned above, P507 was used for the separation of La, Ce, Pr, Sm and Nd from 1.1025mol/L NdCl3 feed solutions (purity,
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(Insert Table 5)
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99.9%). As the feed solution of the pilot experiment, composition of NdCl3 solutions was presented in Table 5.
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Fig. 10 presents the overall equilibrium distributions of REs in the pilot-scale process. Tables 5 shows that the purity of obtained NaCl3 product was over 99.999%. With a yield of 99% in the extraction process, P507 was indicated to be an efficient extractant for the separation and purification of NaCl3 from ion-adsorbed deposit leaching solution. So far, the novel separation processing has been run for six months, and more than
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4100 kilograms of Nd2O3 products with purity of 99.999% have been produced in Ganzhou Rare Earth Group
(Insert Fig. 10)
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Co., Ltd.
3 Conclusions
In this paper, the effects of HCl solution and pure NdCl3 solution on the scrubbing of La, Ce, and Pr from the loaded organic phase were comparatively studied. Sm was selectively extracted from the NdCl3 feed solution. The RE impurities in Nd (La, Ce, Pr, and Sm) could be removed by an integrated process containing pure Nd scrubbing and low-degree saponification extraction in a bench-scale experiment. Then the effect of smuggling behavior on the purities of NdCl3 was systematically investigated in the mixer-settlers. Backmixing
ACCEPTED MANUSCRIPT of the aqueous phase revealed serious impact on the purity of Nd product, especially for the Nd product with high-purity grade (>99.99%). Increasing the phase ratios (A/O) and decreasing the feed concentration were efficient to control the smuggling in a reasonable range. Based on these investigations, a pilot-scale purification
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process for NdCl3 with 99.999% purity was developed. The total recovery of Nd was indicated to be about 99%. Because the separation of Pr/Nd is the most difficult in the P507 separation processing, the developed strategy reveals importance to the preparation of some other REs from ion-adsorbed deposit with the purities of
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corresponding measures for the high-purity REs preparation.
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99.999%. More investigation should be carried out to understand the smuggling mechanism and develop
Acknowledgments
This work was supported by ‘Hundreds Talents Program’ from Chinese Academy of Sciences,
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Science and Technology Major Project of Fujian Province
(2015HZ0001-3),
National Natural Science Foundation of China (21571179) and Science and Technology Major Project of
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Ganzhou (2017-8).
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References:
[1] Guyonnet D, Planchon M, Rollat A, Escalon V, Tuduri J, Charles N. Material flow analysis applied to rare earth elements in Europe. J. Clean. Prod., 2015, 107: 215. [2] Zhu J X, Zhou D F, Guo S R, Ye J F, Hao X F, Cao X Q, Meng J. Grain boundary conductivity of high purity neodymium-doped ceria nanosystem with and without the doping of molybdenum oxide. J. Power. Sources., 2007,174: 114.
ACCEPTED MANUSCRIPT [3] Lee G S, Chikoshi M U, Mimura K, Isshiki M. Separation of major impurities Ce, Pr, Nd, Sm, Al, Ca, Fe, and Zn from La using bis(2-ethylhexyl)phosphoric acid (D2EHPA)-impregnated resin in a hydrochloric acid medium. Sep. Purif. Technol., 2010,71: 186.
solutions. Miner. Eng., 2014, 56: 10.
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[4] Xie F, Zhang T A, Dreisinger D, Doyle F. A critical review on solvent extraction of rare earths from aqueous
[5] Banda R, Jeon H S, Lee M S. Separation of Nd from mixed chloride solutions with Pr by extraction with
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saponified PC 88A and scrubbing. J. Ind. Eng. Chem., 2015, 21: 436.
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[6] Chen L, Chen J, Jing Y, Li D Q. Comprehensive appraisal and application of novel extraction system for heavy rare earth separation on the basis of coordination equilibrium effect. Hydrometallurgy, 2016, 165: 351. [7] Wang Y L, Liao W P, Li D Q. A solvent extraction process with mixture of CA12 and Cyanex272 for the preparation of high purity yttrium oxide from rare earth ores. Sep. Purif. Technol., 2011, 82: 197.
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[8] Lu Y C, Zhang Z F, Li Y L, Liao W P. Extraction and recovery of cerium(IV) and thorium(IV) from sulphate medium by an α-aminophosphonate extractant. J. Rare Earths, 2017, 35: 34. [9] Li Y J, Wang Y P, Wang G Y, Cheng H R, Ni Y M. Study on purification of highly pure neodymium oxide
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by extraction chromatography with CL-P-507 HCl system. Chemical Reagents (in Chinese), 1999, 2: 65.
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[10] Li Y J, Wang Y P, Zhang R H, Wang J X, Ni Y M. Study on separation and preparation of high purity thulium oxide. Inorganic Chemicals Industry (in Chinese), 2000, 32: 4. [11] Zhou J Z, Duan W H, Zhou X Z, Zhang C Q. Application of annular centrifugal contactors in the extraction flowsheet for producing high purity yttrium. Hydrometallurgy, 2007, 85: 154. [12] Pan Q P, Zhu W C, Cai Z S, Chen J G, Gao P. Chinese Patent, CN 101824537A. [13] Xu W L, Wang Y Q, Wei X T, Zhu J Y, Wei W, Zhao H K. Chinese Patent, CN 101024505A. [14] Isshiki M. Purification of rare earth metals. Vacuum, 1996, 47: 885.
ACCEPTED MANUSCRIPT [15] Li G L, Li L, Miao R Y, Tian W H, Yan S H, Li X G. Research on the removal of impurity elements during ultra-high purification process of terbium. Vacuum, 2016, 125: 21. [16] Wu S, Cheng F X, Liao C S, Yan C H. Impact of backmixing of the aqueous phase on two-component rare
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earth separation process. J. Rare Earths, 2013, 31: 517. [17] Mecklenburgh J C, Hartland S. Two phase countercurrent extraction with high backmixing. Chem. Eng. Sci., 1968, 23: 1421.
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[18] Yeow Y L, Doan H A N, Doan P A N. A simplified method of obtaining the backmixing coefficient of a
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solvent extraction column from tracer pulse data. Chem. Eng. Sci., 2005, 60: 2871.
[19] Leblebici M E, Kuhn S, Stefanidis G D, Gerven T V. Milli-channel mixer and phase separator for solvent extraction of rare earth elements. Chem. Eng. J., 2016, 293: 280.
[20] Kolar E, Catthoor R P R, Kriel F H, Sedev R, Middlemas S, Klier E, Hatch G, Priest C. Microfluidic
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solvent extraction of rare earth elements from a mixed oxide concentrate leach solution using Cyanex® 572.
[21] Zolfaghari R, Fakhru’l-Razi A, Abdullah L C, Elnashaie S S, Pendashteh A. Demulsification techniques of
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water-in-oil and oil-in-water emulsions in petroleum industry. Sep. Purif. Technol., 2016, 170: 377.
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[22] Lindell E, Jääskeläinen E, Paatero E, Nyman B. Effect of reversed micelles in Co/Ni separation by Cyanex 272. Hydrometallurgy, 2000, 56: 337. [23] Zhang S B, Sun X Q, Li F J, Wang Y L, Zhou H. Chinese Patent, CN105883889A. [24] Lee M S, Lee J Y, Kim J S, Lee G S. Solvent extraction of neodymium ions from hydrochloric acid solution using PC88A and saponified PC88A. Sep. Purif. Technol., 2005, 46: 72. [25] Kao H C, Yen P S, Juang R S. Solvent extraction of La(III) and Nd(III) from nitrate solutions with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester. Chem. Eng. J., 2006, 119: 167.
ACCEPTED MANUSCRIPT [26] Lee G S, Uchikoshi M, Mimura K, Isshiki M. Distribution coefficients of La, Ce, Pr, Nd, and Sm on Cyanex 923-, D2EHPA-, and PC88A-impregnated resins. Sep. Purif. Technol., 2009, 67: 79. [27] Kosinski F E, Bostian H. Lanthanum solvent extraction mechanisms using di-(2-ethylhexyl) phosphoric
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acid. J. Inorg. Nucl. Chem., 1969, 31: 3623. [28] Liu Y, Jeon H S, Lee M S. Solvent extraction of Pr and Nd from chloride solutions using ternary extractant system of Cyanex 272, Alamine 336 and TBP. J. Ind. Eng. Chem., 2015, 31: 74.
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earth resources in China. J. Rare Earths, 2013, 31: 331.
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[29] Liao C S, Wu S, Cheng F X, Wang S L, Liu Y, Zhang B, Yan C H. Clean separation technologies of rare
[30] Li D Q, Zhang J, Xu M. Studies of extraction mechanism of rare earth compounds with mono (2-ethyl hexyl) 2-ethyl hexyl phosphonate (HEH[EHP]) I. The distribution of rare earth compounds in the system HNO3–H2O–HEH[EHP] and the effect of temperatures and solvents (in Chinese). Chin. J. Appl. Chem., 1985, 2:
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17.
[31] Zhang Y G, Li J N, Huang X W, Wang C M, Zhu Z W, Zhang G C. Synergistic extraction of rare earths by mixture of HDEHP and HEH/EHP in sulfuric acid medium. J. Rare Earths, 2008, 26: 688.
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11: 241.
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[32] Yan C H, Jia J T, Liao C S, Wu S, Xu G X. Rare Earth Separation in China. Tsinghua Sci. Technol., 2006,
ACCEPTED MANUSCRIPT Table 1 Calculated gibbs free energy of La3+, Ce3+, and Pr3+ scrubbed by Nd3+ La3+
Ce3+
Pr3+
Nd3+
[Ln()] (a) (10-6mol/L)
0.65
0.11
0.76
1089789.10
[Ln()] (o) (10-6mol/L)
69.32
27.34
199.17
975323.70
2.5
5.5
15
lnKɵ
-8.29
-7.51
-6.50
-6.27
△G1ɵ or △G2ɵ (kJ/mol)
20.55
18.60
16.11
15.52
△G3ɵ (kJ/mol)
-5.02
-3.07
-0.59
-
△G3 (kJ/mol)
-16.85
19
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K ɵ [30] (10-4)
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Ln()
-17.03
-14.65
-
Table 2 The composition of feed solution and NdCl3 product (µmol/mol) CeO2
Feed
71
28
NdCl3
0.1
0.1
Alfa Aesar*
0.6
Pr6O11
Sm2O3
Eu2O3
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La2O3
0.1
Ho2O3
Y2O3
Er2O3
Nd2O3/TREO
81
0.2
27
20
89
17
99.9%
0.8
0.3
0.2
0.6
0.2
0.8
0.4
99.999%
0.3
0.2
0.3
0.05
0.07
0.1
99.999%
0.7
Note: * The data was provided by Alfa Aesar.
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Gd2O3
204
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REO
ACCEPTED MANUSCRIPT Table 3 The parameters in the calculation of ASIR nPr/nNd in feed solution
nPr/nNd in product
βPr/Nd
[M]ot
[M]at
Vy
Vx
W:S
This paper
0.1 / 99.9
0.0001 / 99.9999
1.5
0.12
1.2
600
80
1.3
Scrb. seg. [16]
25 / 75
0.0025 / 99.9975
1.5
0.18
1.5
10
0.96
0.8
Extr. seg. [16]
25 / 75
99.9925/0.0075
1.5
0.18
1.5
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Notes: [M]ot , [M]at : mol/L ; Vy, Vx : L/h
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Parameters
10
1.28
1.1
ACCEPTED MANUSCRIPT Table 4 The influence of rs in the scrubbing segment 0%
0.10%
0.50%
1%
2%
3%
4%
5%
10%
13.33%
V'x (L/h)
0
0.6
3
6
12
18
24
30
60
79.98
rx
0.0%
0.8%
3.8%
7.5%
15.0%
22.5%
30.0%
37.5%
75.0%
100.0%
I2 (µmol/mol)
1.0
1.1
1.4
1.9
3.3
5.3
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rs
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7.8
10.9
34.6
56.3
ACCEPTED MANUSCRIPT Table 5 Composition of feed solution and NdCl3 product (µmol/mol) La2O3
CeO2
Pr6O11
Sm2O3
Eu2O3
Gd2O3
Ho2O3
Y2O3
Er2O3
Nd2O3/TREO
Feed
71
28
204
81
0.2
27
20
89
17
99.9%
NdCl3
0.5
0.1
1.8
0.9
0.2
0.5
1
1
0.2
99.999%
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REO
ACCEPTED MANUSCRIPT Captions of Illustrations Fig. 1 Structural formulas of P507 and RE(HA2)3. Fig. 2 The Nd-P507-Cl extraction processing. Note: If RE concentration was beyond 0.001 mol/L, fractions of
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the raffinate (R1) were metered into stage 5th. Fig. 3 Effect of extraction ratio on the impurity removal. Aqueous phase: 0.9763mol/L NdCl3 feed solution, i.e., 6.8, 7.2, 7.9, 8.9, 10.2 ml. Organic phase: 1.5 mol/L P507 in sulfonated kerosene, 50.0 ml. Saponification degree:
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0.4 mol /L (nNaOH/Vorg).
Fig. 4 Effect of scrubbing ratio on RE impurities removal by HCl. Organic phase: 1.5 mol/L P507 in sulfonated
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kerosene, 50 ml. Saponification degree: 0.4 mol/L. Aqueous phase: 0.9763mol/L NdCl3 feed solution, 6.5 ml. HCl scrubbing solution: 2.5965 mol/L, first scrubbing, i.e., (1) 0.73, (2) 1.47, (3) 2.20, (4) 2.93 ml, second scrubbing, i.e., (1) 0.66, (2) 1.20, (3) 1.54, (4) 1.76 ml. Scrubbing ratio of 0 represents the feed solution with no
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scrubbing.
Fig. 5 Effect of pure NdCl3 concentrations on RE impurities removal. Organic phase: 1.5 mol/L P507 in sulfonated kerosene, 50 ml. Saponification degree: 0.4 mol/L. Aqueous phase: 0.9763 mol/L NdCl3 feed
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solution, 6.83 ml. Pure NdCl3 scrubbing solution: 1.0898 mol/L, first scrubbing, 1.16 ml with adding water, i.e.,
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(1) 0.29, (2) 0.77, (3) 1.74, (4) 4.64 ml, second scrubbing, 0.93 ml with adding water, i.e., (1) 0.23, (2) 0.62, (3) 1.40, (4) 3.72 ml. Concentration of 0 mol/L represents the feed solution with pure water scrubbing. Fig. 6 Effect of pure NdCl3 concentrations on RE impurities removal. Organic phase: 1.5 mol/L P507 in sulfonated kerosene, 50 ml. Saponification degree: 0.4 mol/L. Aqueous phase: 0.9763mol/L NdCl3 feed solution, 6.5 ml. pure NdCl3 scrubbing solution: 1.0898 mol/L, first scrubbing, i.e., (1) 0.58, (2) 1.16, (3) 1.75, (4) 2.33 ml, second scrubbing, (1) 0.52, (2) 0.93, (3) 1.22, (4) 1.40 ml. Scrubbing ratio of 0 represents the feed solution with no scrubbing.
ACCEPTED MANUSCRIPT Fig. 7 Effect of saponification rate on RE impurities removal. Organic phase: 1.5 mol/L P507 in sulfonated kerosene, 25 ml. Saponification NaOH solution: 0.0526 mol/L, i.e., 4.8, 14.3, 28.5, 42.8 ml. Aqueous phase: 0.9763 mol/L NdCl3 feed solution, 10 ml. saponification of 0 represents the feed solution with no extraction.
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Fig. 8 Effect of concentration on rs in the extraction process using funnels. Fig. 9 Effect of concentration on rs in the extraction process using funnels. Notes: * the data cited from the article [16]
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Fig. 10 Equilibrium distribution of REs in the pilot aqueous solutions.
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Illustrations
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a
(b)
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(a) P507; (b) RE(HA2)3 Fig. 1
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Fig. 2
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La Ce Pr Sm
180
120
60
0 80
90
Extraction ratio (E%)
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Fig. 3
100
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70
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Impurity‘s contents (umol/mol)
240
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La Ce Pr Sm
180
120
60
0 10
20
Scrubbing ratio (%)
40
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Fig. 4
30
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0
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Impurity‘s contents (umol/mol)
240
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La Ce Pr Sm
180
120
60
0 0.0
0.2
0.4
0.6
0.8
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Fig. 5
1.0
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Pure NdCl3 scrubbing solution (mol/L)
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Impurity‘s contents (umol/mol)
240
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Impurity‘s contents (umol/mol)
240
La Ce Pr Sm
180
120
0
10
20
30
Scrubbing ratio (%)
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Fig. 6
40
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0
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60
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La Ce Pr Sm
180
120
60
0
0.00
0.02
0.04
0.06
0.08
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Fig. 7
0.10
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Saponification degree (mol/L)
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Impurity‘s contents (umol/mol)
240
ACCEPTED MANUSCRIPT 60 49.54ml/L
pure HCl HCl with RECl3 solution
50
rs (ml/L)
40
30
20
10
-1
0
1
2
3
4
5
Concentration of HCl (mol/L)
7
8
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Fig. 8
6
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0
7.21ml/L
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14.31ml/L
ACCEPTED MANUSCRIPT 2.4
ASIR in Scrb.Seg.--rs ASIR in Scrb.Seg.--rs* ASIR in Extr.Seg.--rs*
1.6
1.2
3
6
9
rs (%)
15
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Fig. 9
12
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0
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ASIR
2.0
ACCEPTED MANUSCRIPT 600 Pr Sm
Nd
1.6
0.8 300 0.0 150 -0.8
0
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Impurity (umol/mol)
450
Concentration (mol/L)
La Ce
-1.6
0
10
20
30
40
Extraction stages
60
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Fig. 10
50
S1002-0721(17)30038-8
DOI:
10.1016/j.jre.2017.06.002
Reference:
JRE 29
To appear in:
Journal of Rare Earths
Received Date: 24 January 2017 Revised Date:
8 June 2017
Accepted Date: 8 June 2017
Please cite this article as: Li F, He Q, Wang Y, Zhou H, Zhang S, Sun X, Preparation of high-purity neodymium chloride by solvent extraction in the mixer-settlers: a pilot-scale investigation, Journal of Rare Earths (2017), doi: 10.1016/j.jre.2017.06.002. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Preparation of high-purity neodymium chloride by solvent extraction in the mixer-settlers: a pilot-scale investigation
(
) 1,2,3,4, HE Qiaoyan (
)4, ZHANG Shuibo (
)4, WANG Yanliang (
)4, SUN Xiaoqi (
)1,2,3*
)1,2,3, ZHOU Hong
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LI Fujian (
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(1. Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China; 2. Xiamen Institute of Rare Earth Materials, Haixi Institute, Chinese Academy of Sciences, Xiamen 361021, China; 3. University of Chinese Academy of Sciences, Beijing, 100039, P. R. China; 4. Ganzhou Rare Earth Group Co., Ltd., China Southern Rare Earth, Ganzhou 341000, China)
Foundation item: This work was supported by ‘Hundreds Talents Program’ from Chinese Academy of Sciences, Science and Technology Major Project of Fujian Province (2015HZ0001-3), National Natural Science Foundation of China (21571179) and Science and Technology Major Project of Ganzhou (2017-8). Corresponding author: SUN Xiaoqi (E-mail: [email protected], Tel.:0592-6376370) DOI:
ACCEPTED MANUSCRIPT Abstract: A novel purification process based on mixer-settlers for high-purity NdCl3 was developed. Acidic solution and pure NdCl3 solution were compared to scrub the less-extractable rare earths (REs) (La, Ce, and Pr) from loaded organic phase. The extractant with low-degree saponification was evaluated to remove the more-extractable element (Sm). Then the RE impurities (La, Ce, Pr, and Sm) in Nd were purified by an
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integrated process. Furthermore, effect of smuggling behavior on the purification of NdCl3 in mixer-settlers was studied. Based on the investigation mentioned above, the pilot-scale purification process for NdCl3 with a purity of 99.999% was developed, and the total recovery was about 99%.
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Keywords: Solvent extraction, Purification, Neodymium, High-Purity, Mixer-settlers
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Highlights
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Graphic abstract
A novel process was developed in the mixer-settlers to purify NdCl3 with a purity of 99.999%. Impurities of La, Ce, and Pr can be removed using pure NdCl3 solution as scrubbing reagent. Impurities of Sm can be separated using low-degree saponification extractant. Smuggling reveals negative impact on the REE purification process. Optimized technical parameters for industrial application were developed.
ACCEPTED MANUSCRIPT Neodymium (Nd) is an important element in astronavigation, clean energy and electronic industries [1]. As applications in these specialized areas increasing, high-purity Nd has been commonly required. For example, the product of Nd2O3 applied in batteries and semiconductor laser requires that the content of RE (rare earth)
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impurities should be lower than 1×10-4 µmol/mol [2]. In general, the impurities in Nd2O3 product are light REs (La, Ce, Pr, and Sm) and transition elements. The separation of light REs is particularly difficult because of their similar physical and chemical properties
[3]
. Among the common methods for REs separation, solvent
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extraction is one of the most popular and versatile techniques [4]. Over the past years, many extraction processes
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were developed for REs separation, especially for the separation of Pr/Nd [5], Tm/Yb/Lu [6], Ho/Er/Y [7], RE/Th [8]
, etc. However, most of them were focused on the purifications of REs from 98% to 99.9%. Recently, the
preparation of high-purity REs has drawn much attention. Using the extraction chromatography, Li et al. prepared Nd with an impurity of Pr at 9.5 ug/g [9], and Tm with a purity of 99.995%
using electricity
[12]
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contactor was also used to prepare high-purity Y (>99.99%) and ultrasonic
[13]
[10]
. Annular centrifugal
[11]
. Moreover, optimized extraction processes
were developed to produce Gd and Nd with high purities (>99.99%).
Based on high-purity RE oxides, some ultra-purity RE metals were purified by vacuum distillation
[14]
and
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not been mentioned.
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external getter [15]. To the best of our knowledge, the pilot-scale preparation of high-purity NdCl3 (99.999%) has
Solvent extraction based on mixer-settlers is widely used in the industry of REs separation. In the process of solvent extraction, a noticeable smuggling of aqueous phase by organic phase at a volumetric ratio of above 0.1% occurs normally. It is possible in an extreme situation that the volumetric ratio of smuggling arrived at 10% [16]
. The smuggling resulted in negative impacts on the separation efficiencies for REs. The backmixing of
aqueous phase aroused by smuggling was studied since 1960s [17] and the accurate results could be obtained by computer simulation [18]. The parameters such as RE concentration, organic to aqueous phase ratio and initial
ACCEPTED MANUSCRIPT pH of RE solution were optimized, and a membrane phase separator was adapted to decrease the smuggling of aqueous phase [19]. A microfluidic chip was also employed to improve the extraction efficiency [20]. Being the main component in the formation of stabilizing interfacial film, surfactant was indicated to be the most [21]
. In the extraction process, a composition of metal ions and
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predominant contributor to emulsion stability
extractant behaves similarly as surfactant, which has a significant influence on extraction efficiency
[22]
.
Generally, both extraction efficiency and economic burden should be considered together to improve the RE
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purity. Therefore, the systematical investigation of smuggling together with corresponding control methods is
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important in both of academic research and industrial application for high-purity REs preparation. In this paper, an integrated Nd purification process was developed to discriminatively treat the less-extractable REs (La, Ce, and Pr) and more-extractable RE (Sm) on bench and pilot-plant scales. Also, the controlling method of smuggling behavior on the extraction process was investigated. Based on these studies, a novel and efficient
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pilot-scale solvent extraction process for high-purity Nd (99.999%) was developed. The related work has been applied for Chinese patent protection [23].
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1. Experimental
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1.1 Reagents and apparatus
2-Ethylhexyl phosphoric acid mono-2-ethylhexyl ester (P507) and sulfonated kerosene were purchased from Luoyang Aoda Chemical Co., Ltd., (China), and used without further purification. Structural formulas of P507 and RE(HA2)3 are shown as Fig. 1. Neodymium oxide (Nd2O3, 99.9%) used for feed solution was kindly provided by Ganzhou Rare Earth Group Co., Ltd., (China). Neodymium oxide (Nd2O3, 99.999%) used as scrubbing reagent was purchased from Alfa Aesar (A Johnson Matthey Company). NdCl3 feed solution (99.9%)
ACCEPTED MANUSCRIPT and pure NdCl3 scrubbing solution (99.999%) were prepared by dissolving the corresponding neodymium oxides with hydrochloric acid (HCl) and diluting with deionized water [7].
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(Insert Fig. 1)
Inductively coupled plasma atomic emission spectroscopy (Horiba ULTIMA, ICP-AES) and inductively
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coupled plasma mass spectrometry (ICP-MS, 300Q, PE) was used to determine the concentrations of REs in
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aqueous phase. The concentrations of acidic extractants were titrated by standard solution of sodium hydroxide. The required pH value of aqueous phase was adjusted with HCl or NaOH solution. The concentration of water in the organic phase was determined by the Karl-fischer titration method using an automatic titrator (KLS-411;
1.2 Solvent extraction process
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Shanghai INESA). All the chemicals used in the bench scale study were of analytical grade.
Bench scale solvent extraction studies were carried out by mixing different volumes of feed solution (6.5
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mL) and organic phase (50 mL) with the help of a mechanical stirrer for 10 min. After equilibration, the phases
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were settled and separated for 30 min. Stripping of metal ions from the loaded organic phase was carried out using 6 mol/L HCl solution. All the experiments were conducted at room temperature, which was about 25. Based on the experiments mentioned above, an integrated Nd separation investigation was carried out as follows: Adjusting the pH value around to be 4, NdCl3 feed solution was mixed to organic phases with the same saponification degree for 10 minutes by a mechanical stirrer. Then the loaded organic phase was scrubbed using pure NdCl3 scrubbing solution at a scrubbing ratio of 20%, the scrubbing procedure was repeated for ten times. Also, the loaded organic phase was stripped by 6 mol/L HCl solution, the stripped aqueous phase mentioned
ACCEPTED MANUSCRIPT above was mixed in sequence with a saponification degree of 0.09 mol/L (nNaOH/Vorg) organic phase (25ml) for eight times. Finally, the aqueous raffinate was collected for further analysis. In the pilot-scale study, the organic phase was formed with a mixture of P507 and kerosene (1:1 by
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volume). Stock solutions of RECl3 were prepared by dissolving ion-adsorbed RE deposits in HCl solution. NdCl3 solution (99.9%) was prepared from leaching solution of the deposits by traditional RE separation process
[24]
. The organic phase was saponified by NaOH solution and stripped by HCl solution. All the
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chemicals used in the pilot scale study were of commercial grade. RE compositions of the feed solution were
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analyzed by inductively the ICP-AES (shown in Table 1). NdCl3 solution (>99.9%) was analyzed by the ICP-MS.
As shown in Fig. 2, a separation process containing three-stage countercurrent solvent extraction, forty-stage RE scrubbing, thirteen-stage stripping, and two-stage HCl scrubbing by water were developed. The
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product of NdCl3 solution (99.999%) was led at the stage forty-fifth. The volume of mixing chamber in extraction tank was 60 L, and the ratio of mixing chamber/clarification chamber was 1:3. The design capacity of
(Insert Fig. 2)
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process was 1000 kilograms Nd2O3 (99.999%) every month.
In both bench and pilot scale experiments, the extraction efficiency (E), scrubbing ratio (S), saponification degree (SD), are defined as the following Eq. (1) to Eq. (3): % =
×
100% 1
% = × 100% 2
=
!"#
3
ACCEPTED MANUSCRIPT where [M]at and [M]a represent the initial and final concentration of REs in aqueous phase, nHCl and nore represent the molar mass of HCl solution and REs in the loaded organic phase. Scrubbing ratio (S%) represents the molar ratio of scrubbing reagent and loaded REs in the organic phase initially. SD% denotes the
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saponification degree. nNaOH denotes the molar mass of NaOH solution. Vorg denotes the volume of organic phase. All the concentration values of REs were measured in duplicate with the uncertainty within 5%. In the extraction process, the impurities of La3+, Ce3+, and Pr3+ in the loaded organic phase could be
SC
replaced by Nd3+ in a suitable acidity, the extraction reaction was represented as the following Eq. (4) [25]:
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%&' + )*+,# -./ = %&-./ + ) ' 0123 4
where (HA)2 denotes the dimeric form of acidic cationic extractant (P507) and subscript org represents the organic phase. RE3+imle denotes the impurities of less-extractable REs (La3+, Ce3+, and Pr3+).
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1.3 Smuggling of aqueous phase by organic phase in mixer-settlers
As shown in following Eq. (5), AISR represents the adjacent stage impurity ratio. AISR is defined as the
scrubbing segment [16]. 67,9
.5) = 6
= ' "> 5 7 ' ">
≈ ?/AB
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7,9:;
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ratio of an impurity’s concentrations in the aqueous phase (or the organic phase) of two adjacent stages in the
where βab represents the separation factor. Xb,n represents the neat molar flow rates of the less-extractable component b in nth stage in the aqueous phase, while Xb,n+1 represents that in the (n+1)th stage. rx represents the backmixing ratio of aqueous phase, which is defined as rx = V'x /Vx. rs represents the volumetric ratio of aqueous phase smuggled by organic phase, which is defined by means of volumetric flow rate ratio as rs = V'x/Vy. Vx and Vy denote the neat volumetric flow rates of aqueous and
ACCEPTED MANUSCRIPT organic phase between adjacent two stages, V'x represents the flow rate of aqueous phase smuggled by organic phase. If S = Ya + Yb and W = Xa + Xb are defined as the total RE flow rate (by molar) of the organic and aqueous
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phases, respectively. S ≈ Ya = Vy × [M]ot and W ≈ Xa = Vx × [M]at can be deduced in the scrubbing segments; [M]ot and [M]at denote the concentrations of REs in the organic phase and aqueous phase at equilibrium state, respectively. Then ASIR can be stated as the following Eq. (6): 1 + DE F / G F H!I /J K G H I + DE F /
G
=
G / F + DE
H!I /H I 1/J K + DE
SC
.5) =
6
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In the scrubbing segment, D denotes the distribution ratio which is defined as D= [M]ot / [M]at. It is fixed when the saponification rate and the concentration of the stripping acid are predetermined. Then ASIR can be stated as the following Eq. (7): .5) ≈
)M/ − × 1/J K )M/ + DE = 1 + 7 1 × 1/J K + DE × N O + DE J K
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where RA/O denotes the phase flow rate (by volume) of aqueous phase and organic phase. And the estimated stage number N for scrubbing segment can be expressed as the following Eq. (8): ,R S /S
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T ; % = ,R M?SU 8
where I0 denotes the RE impurities in NdCl3 feed solution, I1 denotes the RE impurities in the target
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product. In some extraction process, the stage number N of the scrubbing segment is fixed while the rs is variable via the factors mentioned above. To evaluate the effect of rs on I2, Eq. (9) is given as follows: 5/ = 5W / .5)
T
≈ 5W / X
UY/Z ' "[
; O ' "[ ]7
\ × N
^
T
9
where N0 denotes the estimated stage number N for scrubbing segment when rs is limited to be zero. 2. Results and discussion
2.1 Removal of La, Ce, and Pr from Nd feed solution
ACCEPTED MANUSCRIPT (1) Effect of extraction on RE impurities removal
In the NdCl3 feed solution, La3+, Ce3+ and Pr3+ could be regarded as less-extractable components because Nd3+ was a more-extractable component in P507 extraction process [26, 27]. Extraction ratio (E%) represents the
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molar ratio of RE in the loaded organic phase and feed solution at an equilibrium state. As E% decreasing, the concentration of Nd3+ in raffinate phase was increased. The impurities of La3+, Ce3+, and Pr3+ in loaded organic
SC
phase were replaced in a suitable acidity, and the extraction reaction could be represented as Eq. (4). The effect of extraction ratio on RE impurity removal was varied from 66.67% to 100%. As presented in Fig. 3, various
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volumes of NdCl3 feed solutions were mixed with organic phases with the same saponification degree. The loaded organic phase was stripped by HCl (6 mol/L) and the stripping phase was collected for further analysis. With the increase of E%, a slight decrease of the impurities of La3+, Ce3+, and Pr3+ in aqueous phase was observed. The phenomenon indicates the RE impurities in the organic phase were not absolutely scrubbed by
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Nd3+ in the raffinate phase under the experimental conditions. Because the impurities amount of La3+, Ce3+, and Pr3+ were slight in the loaded organic phase and there was almost the same amount in the raffinate phase. It
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indicates clearly that the purity of NdCl3 product was difficult to be improved either by increasing the REs concentration in raffinate or by adding the extraction stages. As previously mentioned, it is better to extract the
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metal ions together and then to separate them by scrubbing [28].
(Insert Fig. 3)
(2) Effect of scrubbing on RE impurities removal
ACCEPTED MANUSCRIPT To get the RE products with purities from 99.5% to 99.9%, acidic solution was widely used as scrubbing reagent to purify the loaded organic phase in RE industry [25, 29]. Using 2.5965 mol/L HCl, the loaded organic phase obtained from the extraction experiments was subjected to different scrubbing ratios from 0 to 40%. Each
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loaded organic phase was scrubbed twice with the same ratio. As presented in Fig. 4, the impurity of La3+ was scrubbed effectively at the scrubbing ratio of 10%, then decreased rapidly at the scrubbing ratio of 20%. However, there was slight change due to the scrubbing ratio from 20% to 40%. It worthwhile to mention that the
SC
impurities of Ce3+ and Pr3+ were nearly not reduced in our experimental range. The effect can be attributed to the
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lower separation factors of Ce/Nd and Pr/Nd, which indicates the REs cannot be scrubbed in a single stage, especially when the absolute content is below 200 ug/g. Because it is not a good choice to purify the microscale RE impurities using HCl solution, pure NdCl3 solution was further studied as scrubbing reagent in the further
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experiments.
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(Insert Fig. 4)
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(3) RE impurities removal by pure NdCl3 solution
For the purification of loaded organic phase by scrubbing, there are two essential approaches: one is the use of acidic aqueous solution at certain pH to remove the unwanted metal and keep the metal of interest in the loaded organic phase. The other approach is the use of metal solution of interest as a scrubbing reagent to replace the co-extracted unwanted metal. To evaluate the selective scrubbing of La3+, Ce3+, and Pr3+ from loaded organic phase, additional scrubbing experiments were performed by varying the concentration of pure NdCl3 solutions as a scrubbing reagent (at pH-4.0) and obtained results are represented in Fig. 5. The impurities of La3+,
ACCEPTED MANUSCRIPT Ce3+ and Pr3+ were reduced at the concentration of 0.2 mol/L pure NdCl3 solution. Tiny change could be observed by increasing the concentration of scrubbing reagent (Fig. 5). The varied scrubbing ratios from 0 to 40% were investigated at the concentration of 1.0898 mol/L pure
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NdCl3 solutions. Each loaded organic phase was scrubbed twice with the same ratio. The impurities of La3+ and Pr3+ were rapidly reduced as the scrubbing ratio increasing from 0 to 20%. It implied that La3+and Pr3+ were efficiently scrubbed by the Nd3+ at the scrubbing ratios of 20%.
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Afterwards, a slight decrease in impurities was observed at the scrubbing ratios of 30% and 40%. However,
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Ce3+ revealed a low amount in the feed solution and remained unchanged at the experiments range (Fig. 6). The decrease indicates that the neighboring REs in front of Nd in the periodic table could be purified by pure NdCl3 solution to some degree. The pure metal solution was also used as scrubbing reagent for preparation of Nd with a purity of 99% [5]. Nevertheless, it was difficult to improve the purity by increasing the scrubbing ratios in a
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single stage. Therefore, the pilot scale experiment was carried out by designing more scrubbing stages rather than extraction stages. In the pilot-scale experiments, the stripped liquor containing high-purity NdCl3 solution from loaded organic phase was partly diverted into the scrubbing section, which is called as the technique of
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sharing the scrubbing reagents with stripping reagents. On the one hand, the technique is efficient to prepare the
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RE product with required purity by pure metal scrubbing. On the other hand, it contributes to reducing the amounts of scrubbing acid together with alkali in the saponification section, which reveals economic and environmental advantages.
(Insert Fig. 5 and Fig. 6)
Exchange mechanism:
ACCEPTED MANUSCRIPT The extraction equilibrium of less-extractable REs (La3+, Ce3+, and Pr3+) can be expressed as the following Eq. (10) [30]: ) ' *+,# + 3-/ ./ = )*+,# -./ + 3- ' △ a= = −)bcd= 10
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The extraction equilibrium of Nd3+ can be expressed as the following Eq. (11): %&' + 3-/ ./ = %&-./ + 3- ' △ a/ = −)bcd/ 11
The exchange mechanism between the less-extractable REs and Nd3+ may be expressed as the following
SC
Eq. (12):
%&' + )*+,# -./ = %&-./ + ) ' *+,# △ a = −)bcd 12
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bℎf, g. 12 = g. 11 − g. 10
-fif,△ a = △ a/ − △ a= = −)bc N
d/ jk: O = −)bc N O = −)bcJ 13 d= l"k:
In the scrubbing section, [Ln()](a) was approximate equal to the total concentration multiplied by each
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composition of scrubbing solution, while the [Ln()](o) was approximate equal to the total concentration multiply by each composition of feed solution. Then ∆G3 of scrubbing equilibrium of La3+, Ce3+, and Pr3+ by Nd3+ were -23.16, -23.34, and -14.32 kJ/mol, respectively. As shown in table 1, the Gibbs free energies were all
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less than zero, which means that they were spontaneous processes at the room temperature and pressure.
(Insert Table 1)
The separation factors of βNd/Pr, βNd/Ce and βNd/La were always greater than 1 in traditional extraction processes using organophosphoric extractants [26] [31]. Then ∆G3 can also be obtained as Eq. (13), which indicates they are all less than zero in the experiment condition.
ACCEPTED MANUSCRIPT Furthermore, the complex structures of RE(HA2)3 can be given in Fig.1. Compared with the original hydrogen-oxygen bond, the lanthanum-oxygen bond is stronger because the ionic radius of lanthanum is smaller than that of hydrogen. The size effect contributes to forming stable extracting complex formed by
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lanthanum with the P507 molecule [27]. As the well-known lanthanide contraction, the ionic radius of Nd3+ is smaller than those of La3+, Ce3+, and Pr3+. Accordingly, neodymium-oxygen bond is the strongest among the
between the less-extractable REs and Nd3+ is easily occurred.
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2.2 Removal of Sm from Nd feed solution
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four RE-oxygen bonds due to the biggest electron affinity. As shown in Eq (12), the exchange mechanism
As mentioned above, the impurity of Sm can be scrubbed by neither acidic solution nor pure NdCl3 solution. Sm is the first extracted RE by the organic phase and the last scrubbed RE by the acid in NdCl3 feed [26]
. In this paper, additional experiments were carried out using 1.5 mol/L P507 with verified
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solution
saponification degree from 0 to 0.09 mol/L (nNaOH/Vorg) to quantitatively extract 81 µmol/mol Sm from NdCl3 feed solution. The saponified organic phase was contacted with 0.9763 mol/L NdCl3 feed solution (99.9%) at an
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O/A ratio of 2.5, and the aqueous raffinate was collected for further analysis. As shown in Fig. 7, the impurity of
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Sm was removed efficiently with the saponification degree increasing. As more organic phase was saponified, more REs were extracted and less Sm could stay in aqueous phase. Because of the microscale of Sm in the feed, the target product Nd was extracted together with Sm. Thus, the increased saponification degree decreased the yield of Nd. An optimized saponification degree was 0.03 mol/L in this study. To acquire Nd product with high yield, more stripping stages should be developed.
(Insert Fig. 7)
ACCEPTED MANUSCRIPT
2.3 Bench-scale purification
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Based on the investigation mentioned above, an integrated Nd purification process was developed to discriminatively treat the less-extractable REs (La, Ce, and Pr) and more-extractable RE (Sm) on the bench scale. The experimental procedures were presented as mentioned above. Table 2 shows that the purities of feed
SC
solution and obtained NdCl3 solution are 99.9% and 99.999%, respectively. NdCl3 with a purity of 99.9% is the
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main product in the RE industry of China [29]. It should be noted that the less-extractable REs (La, Ce, and Pr) in NdCl3 feed solution were removed efficiently and the residual amount was 0.1, 0.1 and 0.8 µmol/mol, respectively. It was lower than or approached the level of the scrubbing Nd reagent purchased from Alfa Aesar. The data in Table 2 show that P507 is an efficient extractant for the removal of La, Ce, Pr, Sm and some other
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RE purities from NdCl3 feed solution on the bench-scale experiment.
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(Insert Table 2)
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2.4 Effect of backmixing on RE impurities removal in mixer-settlers
Solvent extraction based on mixer-settlers is the most popular technology for REs industrial separation[19]. In the mixer-settlers extraction process, the backmixing of aqueous phase cannot be avoided due to the insufficient settling time in normal circumstances. Wu et al.
[16]
stated that the backmixing of aqueous phase
might have serious impact on the design and application of REs separation, especially for the scrubbing process. In this study, the backmixing effect in scrubbing section was presented, and appropriate controlling method was
ACCEPTED MANUSCRIPT developed. Base on the essential assumptions of the article [16], a stable state in both the flow and backmixing of two phases was supposed. ASIR in the scrubbing segment could be estimated as Eq. (7), and I2 could be estimated as Eq. (9). In a certain extraction process, the influence factors of AISR and I2 can be attributed to (a)
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separation factor βab; (b) the volumetric ratio of the aqueous phase smuggled by the organic phase rs; (c) the flow rate ratio of aqueous phase and organic phase RA/O. In some process, the separation factor βab was fixed. However, RA/O could be designed and rs should be controlled to improve the separation efficiency.
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In this study, the effects of rs on impurities and ASIR were investigated. As mentioned above, rs in the
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extraction process in the funnels are showed in Fig. 8. With the concentrations of HCl were ranged from 0 to 6.8086 mol/L in pure HCl solution and RECl3 solution, the volumetric ratios of aqueous phase smuggled by the organic phase were all around 15 ml/L, excepting for that rs was 49.54 ml/L and 7.21 ml/L when the concentration of pure HCl was 0 and 6.8086 mol/L, respectively. That indicates the value of rs is about 15 ml/L
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in the experimental conditions. While in industry, the value of rs is much higher than that in funnel, which is
(Insert Fig. 8)
AC C
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about 82.13 ml/L (Sample provided by Ganzhou Rare Earth Group Co., Ltd).
To illustrate the influence of the backmixing effect on the RE separation process more clearly, our results were compared with those in some other article [16]. As shown in Table 3, the relevant parameters were given. The critical separation factor was the same between the two papers, while the other parameters were of a minor difference. As the rs increasing, ASIR decreased with the same tendency (see Fig. 9). However, ASIR in this paper was the largest among the three groups of rs, and even larger than
Pr/Nd
(value,1.5[5]). It should be
noted that the ratio of total RE flow rate (by molar) of organic and aqueous phases (W:S) in this paper and that in
ACCEPTED MANUSCRIPT Wu’s article were 1.3 and 0.8, respectively. The tendencies of ASIR and W:S in the scrubbing segment and extraction segment in Wu’s article was similar [16]. In some process, the saponification degree and concentration of the stripping HCl solution were predetermined. The RE concentrations in both phase were fixed. Hence, the
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value of W:S was determined by the phase ratio RA/O. As shown in Eq. (7), enlarging RA/O could promote the value of ASIR and improve the purity of the product in the scrubbing segment.
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SC
(Insert Table 3 and Fig. 9)
Therefore, the effect of backmixing on the impurities of RE product was calculated as Eq. (9) and the result was shown in Fig. 9. With the increase of rs from 0 to 15%, ASIR was decreased from 2.0 to 1.31. Meanwhile, the impurities in the RE product increased sharply from 1.0 to 56.3 µmol/mol (Table 4). Therefore, backmixing
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of the aqueous phase has serious impact on the design and application of RE separation, especially for the
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EP
high-purity RE products in the scrubbing section.
(Insert Table 4)
To control the smuggling, the viscosity of organic phase was reduced by optimizing the extractant concentration and saponification degree. Furthermore, the settling time of two phases was extended as much as possible. A suitable phase ratio (RA/O) is the most important for the extraction process.
2.5 Industrial application
ACCEPTED MANUSCRIPT P507 has been widely used in Chinese RE separation industry [32]. Several extraction processes based on P507 were developed to purify the ion-adsorbed RE deposit leaching [7]. Based on our results mentioned above, P507 was used for the separation of La, Ce, Pr, Sm and Nd from 1.1025mol/L NdCl3 feed solutions (purity,
SC
(Insert Table 5)
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99.9%). As the feed solution of the pilot experiment, composition of NdCl3 solutions was presented in Table 5.
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Fig. 10 presents the overall equilibrium distributions of REs in the pilot-scale process. Tables 5 shows that the purity of obtained NaCl3 product was over 99.999%. With a yield of 99% in the extraction process, P507 was indicated to be an efficient extractant for the separation and purification of NaCl3 from ion-adsorbed deposit leaching solution. So far, the novel separation processing has been run for six months, and more than
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4100 kilograms of Nd2O3 products with purity of 99.999% have been produced in Ganzhou Rare Earth Group
(Insert Fig. 10)
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Co., Ltd.
3 Conclusions
In this paper, the effects of HCl solution and pure NdCl3 solution on the scrubbing of La, Ce, and Pr from the loaded organic phase were comparatively studied. Sm was selectively extracted from the NdCl3 feed solution. The RE impurities in Nd (La, Ce, Pr, and Sm) could be removed by an integrated process containing pure Nd scrubbing and low-degree saponification extraction in a bench-scale experiment. Then the effect of smuggling behavior on the purities of NdCl3 was systematically investigated in the mixer-settlers. Backmixing
ACCEPTED MANUSCRIPT of the aqueous phase revealed serious impact on the purity of Nd product, especially for the Nd product with high-purity grade (>99.99%). Increasing the phase ratios (A/O) and decreasing the feed concentration were efficient to control the smuggling in a reasonable range. Based on these investigations, a pilot-scale purification
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process for NdCl3 with 99.999% purity was developed. The total recovery of Nd was indicated to be about 99%. Because the separation of Pr/Nd is the most difficult in the P507 separation processing, the developed strategy reveals importance to the preparation of some other REs from ion-adsorbed deposit with the purities of
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corresponding measures for the high-purity REs preparation.
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99.999%. More investigation should be carried out to understand the smuggling mechanism and develop
Acknowledgments
This work was supported by ‘Hundreds Talents Program’ from Chinese Academy of Sciences,
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Science and Technology Major Project of Fujian Province
(2015HZ0001-3),
National Natural Science Foundation of China (21571179) and Science and Technology Major Project of
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Ganzhou (2017-8).
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References:
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ACCEPTED MANUSCRIPT [3] Lee G S, Chikoshi M U, Mimura K, Isshiki M. Separation of major impurities Ce, Pr, Nd, Sm, Al, Ca, Fe, and Zn from La using bis(2-ethylhexyl)phosphoric acid (D2EHPA)-impregnated resin in a hydrochloric acid medium. Sep. Purif. Technol., 2010,71: 186.
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[8] Lu Y C, Zhang Z F, Li Y L, Liao W P. Extraction and recovery of cerium(IV) and thorium(IV) from sulphate medium by an α-aminophosphonate extractant. J. Rare Earths, 2017, 35: 34. [9] Li Y J, Wang Y P, Wang G Y, Cheng H R, Ni Y M. Study on purification of highly pure neodymium oxide
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solvent extraction of rare earth elements from a mixed oxide concentrate leach solution using Cyanex® 572.
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[32] Yan C H, Jia J T, Liao C S, Wu S, Xu G X. Rare Earth Separation in China. Tsinghua Sci. Technol., 2006,
ACCEPTED MANUSCRIPT Table 1 Calculated gibbs free energy of La3+, Ce3+, and Pr3+ scrubbed by Nd3+ La3+
Ce3+
Pr3+
Nd3+
[Ln()] (a) (10-6mol/L)
0.65
0.11
0.76
1089789.10
[Ln()] (o) (10-6mol/L)
69.32
27.34
199.17
975323.70
2.5
5.5
15
lnKɵ
-8.29
-7.51
-6.50
-6.27
△G1ɵ or △G2ɵ (kJ/mol)
20.55
18.60
16.11
15.52
△G3ɵ (kJ/mol)
-5.02
-3.07
-0.59
-
△G3 (kJ/mol)
-16.85
19
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K ɵ [30] (10-4)
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Ln()
-17.03
-14.65
-
Table 2 The composition of feed solution and NdCl3 product (µmol/mol) CeO2
Feed
71
28
NdCl3
0.1
0.1
Alfa Aesar*
0.6
Pr6O11
Sm2O3
Eu2O3
TE D
La2O3
0.1
Ho2O3
Y2O3
Er2O3
Nd2O3/TREO
81
0.2
27
20
89
17
99.9%
0.8
0.3
0.2
0.6
0.2
0.8
0.4
99.999%
0.3
0.2
0.3
0.05
0.07
0.1
99.999%
0.7
Note: * The data was provided by Alfa Aesar.
AC C
Gd2O3
204
EP
REO
ACCEPTED MANUSCRIPT Table 3 The parameters in the calculation of ASIR nPr/nNd in feed solution
nPr/nNd in product
βPr/Nd
[M]ot
[M]at
Vy
Vx
W:S
This paper
0.1 / 99.9
0.0001 / 99.9999
1.5
0.12
1.2
600
80
1.3
Scrb. seg. [16]
25 / 75
0.0025 / 99.9975
1.5
0.18
1.5
10
0.96
0.8
Extr. seg. [16]
25 / 75
99.9925/0.0075
1.5
0.18
1.5
AC C
EP
TE D
M AN U
SC
Notes: [M]ot , [M]at : mol/L ; Vy, Vx : L/h
RI PT
Parameters
10
1.28
1.1
ACCEPTED MANUSCRIPT Table 4 The influence of rs in the scrubbing segment 0%
0.10%
0.50%
1%
2%
3%
4%
5%
10%
13.33%
V'x (L/h)
0
0.6
3
6
12
18
24
30
60
79.98
rx
0.0%
0.8%
3.8%
7.5%
15.0%
22.5%
30.0%
37.5%
75.0%
100.0%
I2 (µmol/mol)
1.0
1.1
1.4
1.9
3.3
5.3
RI PT
rs
AC C
EP
TE D
M AN U
SC
7.8
10.9
34.6
56.3
ACCEPTED MANUSCRIPT Table 5 Composition of feed solution and NdCl3 product (µmol/mol) La2O3
CeO2
Pr6O11
Sm2O3
Eu2O3
Gd2O3
Ho2O3
Y2O3
Er2O3
Nd2O3/TREO
Feed
71
28
204
81
0.2
27
20
89
17
99.9%
NdCl3
0.5
0.1
1.8
0.9
0.2
0.5
1
1
0.2
99.999%
AC C
EP
TE D
M AN U
SC
RI PT
REO
ACCEPTED MANUSCRIPT Captions of Illustrations Fig. 1 Structural formulas of P507 and RE(HA2)3. Fig. 2 The Nd-P507-Cl extraction processing. Note: If RE concentration was beyond 0.001 mol/L, fractions of
RI PT
the raffinate (R1) were metered into stage 5th. Fig. 3 Effect of extraction ratio on the impurity removal. Aqueous phase: 0.9763mol/L NdCl3 feed solution, i.e., 6.8, 7.2, 7.9, 8.9, 10.2 ml. Organic phase: 1.5 mol/L P507 in sulfonated kerosene, 50.0 ml. Saponification degree:
SC
0.4 mol /L (nNaOH/Vorg).
Fig. 4 Effect of scrubbing ratio on RE impurities removal by HCl. Organic phase: 1.5 mol/L P507 in sulfonated
M AN U
kerosene, 50 ml. Saponification degree: 0.4 mol/L. Aqueous phase: 0.9763mol/L NdCl3 feed solution, 6.5 ml. HCl scrubbing solution: 2.5965 mol/L, first scrubbing, i.e., (1) 0.73, (2) 1.47, (3) 2.20, (4) 2.93 ml, second scrubbing, i.e., (1) 0.66, (2) 1.20, (3) 1.54, (4) 1.76 ml. Scrubbing ratio of 0 represents the feed solution with no
TE D
scrubbing.
Fig. 5 Effect of pure NdCl3 concentrations on RE impurities removal. Organic phase: 1.5 mol/L P507 in sulfonated kerosene, 50 ml. Saponification degree: 0.4 mol/L. Aqueous phase: 0.9763 mol/L NdCl3 feed
EP
solution, 6.83 ml. Pure NdCl3 scrubbing solution: 1.0898 mol/L, first scrubbing, 1.16 ml with adding water, i.e.,
AC C
(1) 0.29, (2) 0.77, (3) 1.74, (4) 4.64 ml, second scrubbing, 0.93 ml with adding water, i.e., (1) 0.23, (2) 0.62, (3) 1.40, (4) 3.72 ml. Concentration of 0 mol/L represents the feed solution with pure water scrubbing. Fig. 6 Effect of pure NdCl3 concentrations on RE impurities removal. Organic phase: 1.5 mol/L P507 in sulfonated kerosene, 50 ml. Saponification degree: 0.4 mol/L. Aqueous phase: 0.9763mol/L NdCl3 feed solution, 6.5 ml. pure NdCl3 scrubbing solution: 1.0898 mol/L, first scrubbing, i.e., (1) 0.58, (2) 1.16, (3) 1.75, (4) 2.33 ml, second scrubbing, (1) 0.52, (2) 0.93, (3) 1.22, (4) 1.40 ml. Scrubbing ratio of 0 represents the feed solution with no scrubbing.
ACCEPTED MANUSCRIPT Fig. 7 Effect of saponification rate on RE impurities removal. Organic phase: 1.5 mol/L P507 in sulfonated kerosene, 25 ml. Saponification NaOH solution: 0.0526 mol/L, i.e., 4.8, 14.3, 28.5, 42.8 ml. Aqueous phase: 0.9763 mol/L NdCl3 feed solution, 10 ml. saponification of 0 represents the feed solution with no extraction.
RI PT
Fig. 8 Effect of concentration on rs in the extraction process using funnels. Fig. 9 Effect of concentration on rs in the extraction process using funnels. Notes: * the data cited from the article [16]
AC C
EP
TE D
M AN U
SC
Fig. 10 Equilibrium distribution of REs in the pilot aqueous solutions.
ACCEPTED MANUSCRIPT
RI PT
Illustrations
TE D
M AN U
SC
a
(b)
AC C
EP
(a) P507; (b) RE(HA2)3 Fig. 1
AC C
EP
TE D
SC
M AN U
Fig. 2
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
La Ce Pr Sm
180
120
60
0 80
90
Extraction ratio (E%)
AC C
EP
TE D
M AN U
Fig. 3
100
SC
70
RI PT
Impurity‘s contents (umol/mol)
240
ACCEPTED MANUSCRIPT
La Ce Pr Sm
180
120
60
0 10
20
Scrubbing ratio (%)
40
AC C
EP
TE D
M AN U
Fig. 4
30
SC
0
RI PT
Impurity‘s contents (umol/mol)
240
ACCEPTED MANUSCRIPT
La Ce Pr Sm
180
120
60
0 0.0
0.2
0.4
0.6
0.8
AC C
EP
TE D
M AN U
Fig. 5
1.0
SC
Pure NdCl3 scrubbing solution (mol/L)
RI PT
Impurity‘s contents (umol/mol)
240
ACCEPTED MANUSCRIPT
Impurity‘s contents (umol/mol)
240
La Ce Pr Sm
180
120
0
10
20
30
Scrubbing ratio (%)
AC C
EP
TE D
M AN U
Fig. 6
40
SC
0
RI PT
60
ACCEPTED MANUSCRIPT
La Ce Pr Sm
180
120
60
0
0.00
0.02
0.04
0.06
0.08
AC C
EP
TE D
M AN U
Fig. 7
0.10
SC
Saponification degree (mol/L)
RI PT
Impurity‘s contents (umol/mol)
240
ACCEPTED MANUSCRIPT 60 49.54ml/L
pure HCl HCl with RECl3 solution
50
rs (ml/L)
40
30
20
10
-1
0
1
2
3
4
5
Concentration of HCl (mol/L)
7
8
AC C
EP
TE D
M AN U
Fig. 8
6
SC
0
7.21ml/L
RI PT
14.31ml/L
ACCEPTED MANUSCRIPT 2.4
ASIR in Scrb.Seg.--rs ASIR in Scrb.Seg.--rs* ASIR in Extr.Seg.--rs*
1.6
1.2
3
6
9
rs (%)
15
AC C
EP
TE D
M AN U
Fig. 9
12
SC
0
RI PT
ASIR
2.0
ACCEPTED MANUSCRIPT 600 Pr Sm
Nd
1.6
0.8 300 0.0 150 -0.8
0
RI PT
Impurity (umol/mol)
450
Concentration (mol/L)
La Ce
-1.6
0
10
20
30
40
Extraction stages
60
AC C
EP
TE D
M AN U
SC
Fig. 10
50