Separation of thorium and uranium from xenotime leach solutions by solvent extraction using primary and tertiary amines

Journal Pre-proof Separation of thorium and uranium from xenotime leach solutions by solvent extraction using primary and tertiary amines

Nguyen Trong Hung, Le Ba Thuan, Tran Chi Thanh, Masayuki Watanabe, Do Van Khoai, Nguyen Thanh Thuy, Hoang Nhuan, Pham Quang Minh, Tran Hoang Mai, Nguyen Van Tung, Doan Thi Thu Tra, Manish Kumar Jha, Jin-Young Lee, Rajesh Kumar Jyothi PII:

S0304-386X(20)30878-1

DOI:

https://doi.org/10.1016/j.hydromet.2020.105506

Reference:

HYDROM 105506

To appear in:

Hydrometallurgy

Received date:

15 January 2020

Revised date:

7 October 2020

Accepted date:

19 October 2020

Please cite this article as: N.T. Hung, L.B. Thuan, T.C. Thanh, et al., Separation of thorium and uranium from xenotime leach solutions by solvent extraction using primary and tertiary amines, Hydrometallurgy (2020), https://doi.org/10.1016/j.hydromet.2020.105506

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© 2020 Published by Elsevier.

Journal Pre-proof

Separation of thorium and uranium from xenotime leach solutions by solvent extraction using primary and tertiary amines

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Nguyen Trong Hunga,*, Le Ba Thuana, Tran Chi Thanhb, Masayuki Watanabec, Do Van khoaic, Nguyen Thanh Thuya, Hoang Nhuana, Pham Quang Minhb, Tran Hoang Maia, Nguyen Van Tunga, Doan Thi Thu Trad, Manish Kumar Jhae, Jin-Young Leef, Rajesh Kumar Jyothif,* [email protected] a Institute for Technology of Radioactive and Rare Elements (ITRRE)-VINATOM-MOST, 48 Lang Ha, Dong Da, Hanoi, Vietnam b Vietnam Atomic Energy Institute (VINATOM)-Ministry of Science and Technology (MOST), 59 Ly Thuong Kiet, Hoan Kiem, Hanoi, Vietnam c Japan Atomic Energy Agency (JAEA), 2-4 Tokaimura, Nakagun, Ibaraki, 319-1195, Japan d Institute of Geological Sciences (IGS)-Vietnam Academy of Science and Technology (VAST), 84 Chua Lang, Dong Da, Hanoi, Vietnam e MEF Division, CSIR-National Metallurgical Laboratory (NML), Jamshedpur 831 007, India f Convergence Research Center for Development of Mineral Resources (DMR),Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132, Korea

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*Corresponding Authors at: N. T. Hung, Institute for Technology of Radioactive and Rare Elements (ITRRE)-VINATOM-MOST, 48 Lang Ha, Dong Da, Hanoi, Vietnam R. K. Jyothi, Convergence Research Center for Development of Mineral Resources (DMR),Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 34132, Korea

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E-mail addresses: [email protected] (N. T. Hung), [email protected] (R. K. Jyothi)

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Abstract

This study addressed the development of a continuous countercurrent extraction-scrubbing-

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stripping technique using a mixture of primary and tertiary amines as an effective extractant for

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both thorium (Th) and uranium (U) to simultaneously separate them from the leach solutions of xenotime concentrate from the Yen Phu mine (Vietnam). Systematic studies determined the optimum parameters of the separation, including the optimum concentrations of the mixture of the primary amine (N1923) and tertiary amine (tri-n-octyl amine), the optimum acidity (pH) of the feed liquor (Yen Phu xenotime leachate), the optimum contact time between phases for extraction, scrubbing and stripping processes, and the most suitable stripping reagent mixture. Using the optimum parameters, the optimum stage number and phase volumetric ratio for extraction, scrubbing, and stripping processes were calculated using the calculus method based on the law of matter conservation. The flow rates of both phases for extraction, scrubbing, and stripping were determined from the results of these studies and calculations. To optimize the separation of uranium

Journal Pre-proof and thorium from the Yen Phu xenotime leachate, counter-current simulations of extraction, scrubbing and stripping were done in a series of mixer-settler units. The results indicated that the selective separation of Th and U with almost no loss of rare earths (REs) and a minimal contamination of iron were obtained. The continuous countercurrent extraction-scrubbing-stripping technique shows potential applications in the commercial separation of Th and U from RE leachate after tests using a sequence of mixer-settler units on a pilot scale.

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Keywords: Rare Earths; Thorium, Uranium; Continuous countercurrent extraction, Scrubbing, Stripping, Amine extractants

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1. Introduction

Based on our previous studies (Hung et al., 2020; Noboru et al., 2020), Yen Phu mine in

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Vietnam is considered as one of the most important rare earth mining sites. However, considerable

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contents of thorium (Th) and uranium (U) in xenotime concentrate from Yen Phu mine are

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drawbacks. Therefore, studies on the separation of Th and U from Yen Phu xenotime leachate are important.

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Several technologies exist to separate Th and U from REs, such as precipitation and solvent

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extraction or ion-exchange. Generally, the precipitation method is too complicated and generates much waste; therefore, it is disadvantageous to apply on a large scale (Jha et al., 2016; Kumari et

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al., 2015; Zhu et al., 2015; Judge et al., 2020; Amer et al., 2017; Gomes da Silva et al., 2018; Lapidus et al., 2015; Sadri et al., 2017; Vijayalakshmi et al., 2001). Industrial separation is often conducted using solvent extraction or ion-exchange (Jha et al., 2016; Kumari et al., 2015; Jyothi et al., 2011; Zhu et al., 2015; Judge et al., 2020). The former process, also known as liquid-liquid extraction, has been popular and widely applied to recover and separate metal ions from their sources. The solvent extraction process is easy to manage, less expensive to set up, and can be set up to generate zero waste (Jha et al., 2016; Jyothi et al., 2010; 2011; Kumari et al., 2015; Lee et al., 2009; Qi, 2018; Xie et al., 2014; Zhu et al., 2015; Judge et al., 2020; Gupta et al., 2005; Ted et al., 1983; Phillip, 1997; Kislik, 2012).

Journal Pre-proof As reviewed (Amer et al., 2017 ; Amaral et al., 2018, 2010 ; Lapidus et al., 2015 ; Li et al., 2004 ; Nasab et al., 2010 ; Sadri et al., 2017 ; Singh et al., 2000 ; Zhu et al., 2015; Judge et al., 2020; Zuo et al., 2008) in sulfate solutions, tertiary amine has a significantly high selectivity in extracting U(VI) over Th(IV) and RE(III); in contrast, it was shown a high selectivity in extracting Th(IV) over U(VI) and RE(III) when using primary amines. The most used commercial primary and tertiary amines are N1923 and tri-n-octyl amine (TOA), respectively (Amaral et al., 2018; 2010; El-Yamani et al., 1985; Lapidus et al., 2015; Li et al., 2004; Liu et al., 2008; 2007; Morais et

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al., 2005; Nasab et al., 2011; Yang et al., 1997; Yu et al., 1989; Zhu et al., 2015; Judge et al., 2020;

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Zuo et al., 2008). Several investigations were conducted and several new processes were worked on extraction with tertiary and primary amines to simultaneously separate Th and U from REs in acidic

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sulfate leach solutions (Amaral et al., 2018; 2010; Zhu et al., 2015; Judge et al., 2020).

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A full flow sheet of the solvent extraction technique (Fig. 9) consists of three processes,

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namely extraction, scrubbing, and stripping (Jha et al., 2016; Jyothi et al., 2010; 2011; Kumari et al., 2015; Lee et al., 2009; Qi, 2018; Xie et al., 2014; Ted et al., 1983; Phillip, 1997). In the

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extraction process with amines, an anion exchange mechanism is proposed and it depends on the

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availability of the metal in anionic or neutral species in the aqueous phase; therefore, the extractant has a key (Jha et al., 2016; Jyothi et al., 2010; 2011; Kumari et al., 2015; Lee et al., 2009; Amaral et

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al., 2018; 2010; Zhu et al., 2015; Judge et al., 2020; Li et al., 2004; Lu et al., 2016). The extraction mechanism for Th and U using amines in a sulfate medium can be expressed using the following equations proposed by Amaral et al. (2018, 2010), Zhu et al. (2015), Judge et al. (2020), Li et al. (2004), Lu et al. (2016), and Yang et al. (2020): Th(SO4)2(aq)+ 2(RpNH2)H2SO4(org) ⇌ (RpNH3)2Th(SO4)3(org)

(1)

UO2(SO4)(aq) + 2(RtN)H2SO4(org) ⇌ (RtNH)2UO2(SO4)2(org)

(2)

where (aq) is the aqueous and (org) is the organic phase; Rp and Rt are CH(C9H19 ~ C11H23)2 and (C8H17)3, respectively. (RpNH3)2Th(SO4)3 and (RtNH)2UO2(SO4)2 are not meant to represent the

Journal Pre-proof actual Th and U complex species in the organic phase and are used here only to simplify the presentation. The next process is scrubbing. Generally, REs and iron (Fe) are co-extracted during the extraction process; therefore, most of the REs and Fe must be removed from the LO phase before stripping. Here, a dilute sulfuric acid scrubbing liquor is used generally in the scrubbing stage (Zhu et al., 2015; Judge et al., 2020; Li et al., 2004). The final process in the solvent extraction technique is stripping. In conventional stripping,

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the metal loaded in the organic phase is transferred to the aqueous phase using a specific stripping

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solution. The most commonly used stripping reagents of Th and U are (i) sodium, ammonium carbonate, or hydroxide (carbonate or hydroxide stripping process), (ii) sodium or ammonium

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nitrate salt and nitric acid (nitrate stripping process), and (iii) sodium or ammonium chloride salt

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and hydrochloric acid (chloride stripping process) ) (Jha et al., 2016; Jyothi et al., 2010; 2011;

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Kumari et al., 2015; Lee et al., 2009; Li et al., 2004; Amaral et al., 2018; 2010; Zhu et al., 2015; Judge et al., 2020; Morais et al., 2005; Nasab et al., 2011; Wei et al., 2016; Xie et al., 2014; Li et

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al., 2004). Nitrate solutions are more efficient stripping reagents than chloride solutions at the same

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concentrations. However, the nitrate-stripping process has several disadvantages: (i) the regeneration of nitrate-free solvents is very difficult, and even the presence of small amounts of

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nitrates will cause a severe decrease in extraction efficiency; (ii) the oxidation characteristics of nitrates decrease the stability of amine extractants during the process (Amaral et al., 2018; 2010; Morais et al., 2005; Nasab et al., 2011; Zhu et al., 2015; Judge et al., 2020). The stripping using carbonates or hydroxides provides a simple process flow pattern since the organic phase may be stripped in a single-stage operation and Th and U precipitates are obtained as a direct result of the stripping stage; simultaneously, the amine sulfate salts are converted into free amines, which can be directly recycled to the extraction system (Amaral et al., 2018; 2010; Morais et al., 2005; Nasab et al., 2011; Zhu et al., 2015; Judge et al., 2020). However, the physical characteristics of the system still were not appealing since the slimy Th and U precipitates did not

Journal Pre-proof settle in the aqueous phase but floated in the organic phase and collected at the interface. Although these precipitates settle in the organic phase and can be easily recovered through filtration or centrifugation. However, a considerable amount of the organic phase is occluded or adsorbed by the precipitate such that the cost associated with these organic losses is rather excessive (Amaral et al., 2018; 2010; Morais et al., 2005; Nasab et al., 2011; Zhu et al., 2015; Judge et al., 2020). The chloride stripping process is employed as the effective stripping and low-cost reagent for Th and U; it is the preferred process owing to low base requirements to precipitate Th and U from a

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pregnant stripping liquor. Additionally, the chloride stripping process overcomes the disadvantages

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of the nitrate and base stripping processes (Amaral et al., 2018; 2010; Morais et al., 2005; Nasab et

may be expressed as equations 3 and 4:

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al., 2011; Zhu et al., 2015; Judge et al., 2020). Simplified expressions for the stripping reactions

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(RpNH3)2[Th(SO4)3](org) + 2Cl-(aq) ⇌ 2RpNH3Cl(org) + Th4+(aq) + 3SO42-(aq)

(4)

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(RtNH)2UO2(SO4)2(org) + 2Cl-(aq) ⇌ 2RtNHCl(org) + UO22+(aq) + 2SO42-(aq)

(3)

As highlighted in the above discussion and complying to develop an extraction-scrubbing-

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stripping technique applied to separate Th and U from the Yen Phu xenotime leachate, the

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parameters required for this process must be optimized to accurately describe the process and potentially apply the technique on a commercial scale. In addition, the recovery of Th and U increases the commercial value of the Yen Phu xenotime concentrate. The Vietnam Atomic Energy Institute (VINATOM) and MOST have an interest in REs (especially REs: Tb, Dy and Nd) and, Th and U for the application of them in socio-economic branches. 2. Experiments 2.1. Feed liquor The feed liquor (Yen Phu xenotime leachate) was prepared through the sulfuric acid process of the concentrate on a 50kg batch pilot scale using a rotary furnace test plant (Rotary kiln simulator, Japan). The process was conducted at the optimum temperature of 593±15 K and the acid-to-

Journal Pre-proof concentrate-mass ratio of 1.3 for 4 h (Hung et al., 2020). The concentrate composition is indicated in our previous study (Hung et al., 2020). Subsequently, the processed concentrate was dissolved with a water-to-solid mass ratio of 8:1 for 2 h to recover the RE sulfate solution by filtering and washing. The pH of the solution was then adjusted from less than 1 to 1.5 using a neutralizing reagent of MgO milk and filtered to obtain the feed liquor. The purpose of the neutralizing was to significantly remove the impurities from the solution, particularly Fe to maintain the lost REs at a minimum (Chen et al., 2020; Li et al., 2004). Table 1 shows the concentrations of Th, U, REs, and

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Fe in the Yen Phu xenotime leachate.

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ThO2 reagent 99.9% was extracted from Vietnamese monazite at the Institute for Technology

dissolving the ThO2 in sulfuric acid (Merck).

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of Radioactive and Rare Elements (ITRRE), and a control liquor of Th(SO4)2 was prepared by

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A UO3 reagent of nuclear grade was purified using the solvent extraction method of tri-butyl

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phosphate from yellowcake that was extracted from Palua-Parong (Vietnam) uranium sandstone ores at ITRRE (Lien et al, 2020; Hung et al, 2016a; 2016b; 2017; 2018); and a control liquor of

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2.2. Reagents

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UO2SO4 was prepared by dissolving the UO3 in sulfuric acid (Merck).

The primary and tertiary amines used in this study were N1923 (China) and TOA (Merck),

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respectively. n-decanol (Merck) was used as a modifier and added to the solvent with a 5% (v/v) fixed amount to prevent third phase formation, which often occurs when insoluble salts form in the organic phase during extraction due to amine an mineral acid contact (Jha et al., 2016; Jyothi et al., 2011; Kumari et al., 2015; Lee et al., 2009). The diluent reagent used in this study was iso-paraffin IP-2028 (Japan). Table 2 shows the characteristics of the solvents. The organic-phase N1923 or TOA or a mixture of both was prepared at a given concentration by dissolving N1923 or TOA or a mixture of both in IP-2028; the amine group in the solvent was pre-converted to the hydrosulfate form by shaking three times with a sulfuric acid solution before extraction.

Journal Pre-proof In addition, all reagents (MgO, H2SO4, HCl and NaCl, Na2CO3, etc.) for this research are of analytical grade and used as they are dissolved in deionized water. 2.3. Experimental procedure The experiments to study on Th or U extraction isotherms at the different concentrations of Th(SO4)2 or UO2SO4 liquor were conducted by dipping a 100-mL glass beaker in a water bath of a thermostat (Grant GD 100, UK) at a constant temperature of 2980.2 K. 30 mL of Th(SO4)2 or UO2SO4 liquor (pH = 1.5 and 0.5–9 g·L-1 Th or pH = 1.5 and 0.1–0.75 g·L-1 U) followed by 30 mL

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of organic N1923 (0.1 mol·L-1) or TOA (0.015 mol·L-1) were carefully added to the beaker, and

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they were mixed for the desired time using a lab stirrer. For the majority of experiments, unless specified, the stirring speed was kept at 200rpm. The organic and aqueous phases brought into

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contact for ten minutes to ensure the extraction attained equilibrium. After the extraction

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equilibrium at a given concentration was complete, the two phases were allowed to settle and were

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separated using a separation funnel; a sample of the aqueous phase in the extraction equilibrium was analyzed using an inductively coupled plasma mass spectrometry (ICP-MS) (Agilent 7500,

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USA) for its remaining metal concentration to calculate the extracted Th or U. The concentrations

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of Th or U in the organic phase in the extraction equilibrium at each given concentration were

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calculated using the mass balance (equation 5): (5)

where Ci, Caq, and Corg (in g·L-1) are the concentrations of Th or U in the initial liquor and aqueous, and organic phase in the extraction equilibrium, respectively. A satisfactory mass balance was obtained for each extraction experiment; five replications were performed for each experiment on the Th or U extraction at different concentrations (Ci). The Corg and Caq at each given Ci were employed to plot the isotherms of Th or U extraction. The experiments to study on the thermodynamics of Th and U extractions at a given temperature were conducted by dipping the 250-mL glass beaker in a water bath of the thermostat. 95 mL of Th(SO4)2 or UO2SO4 liquor (pH = 1.5 and 3 g·L-1 Th or pH = 1.5 and 0.5 g·L-1 U)

Journal Pre-proof followed by 95 mL of N1923 organic (0.1 mol·L-1) or TOA (0.015 mol·L-1) were added carefully to the beaker and were mixed for the desired time using the lab stirrer. To ensure equilibrium is attained in extraction, the aqueous and organic phase were in contact for ten minutes. The extraction experiments were performed at different temperatures ranging from 298 to 330 K. When the extraction equilibrium at a given temperature was complete, the two phases were allowed to settle and 0.1ml of the aqueous phase was collected for further analysis ensuring that volume was kept constant. The extraction equilibrium concentration of Th or U in the aqueous phase was analyzed

(6)

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each given temperature were calculated using equation 6:

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using ICP-MS. The distribution coefficients (D) of Th or U extraction using the relevant amines at

in the extraction equilibrium, respectively.

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where Ci and Caq (in g·L-1) are the concentrations of Th or U in the initial liquor and aqueous phases

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A satisfactory mass balance was obtained for each extraction experiment; five replications were

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conducted for each experiment of Th or U extraction at different temperatures ranging from 298 to 330 K. The D at each given temperature was employed to calculate the thermodynamics of Th or U

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extraction.

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The experimental procedure to study on the stripping of Th and U from the primary amine N1923 and tertiary amine TOA was as follows: The 0.1-mol·L-1 N1923 and 0.015-mol·L-1 TOA solvents used in the stripping experiments were loaded to saturation of Th and U, respectively. The relevant stripping factors, namely the molarity of the HCl and NaCl and contact time, were studied. The stripping experiments were conducted at O/A=1 and a temperature of 2980.2 K. 10.0 mL of organic and aqueous phases, respectively, were equilibrated in a mechanical shaker for ten minutes to ensure the stripping equilibrium was obtained. Subsequently, a separation funnel was used to detach the organic and aqueous phases. Different concentrations of hydrochloric acid (HCl) and sodium chloride (NaCl) were used for the

Journal Pre-proof stripping experiments. The aqueous phases in the stripping equilibrium were analyzed using ICPMS to determine the stripping efficiency of Th or U as follows (equation 7): (7) where

(in %) is the stripping efficiency of Th or U, Caq (in g·L-1) is the concentration of Th or U

in the aqueous phase after equilibrium, and Ci is the saturation concentration of Th or U in the initial organic phase (Ci = 3.10.2 g·L-1 of Th and 0.740.03 g·L-1 of U). When the optimum stripping liquor concentration was determined, the effect of stripping time on

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the efficiency of Th and U was studied for different intervals ranging from one to ten minutes. The

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aqueous phases in the stripping equilibrium for the different intervals were analyzed using ICP-MS

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to define the required time to attain stripping equilibrium.

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Five replications for each experiment of Th or U stripping were performed. The experimental procedure to study on the extraction of Th and U from the Yen Phu

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xenotime leachate using the mixture of N1923 and TOA was as follows:

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The concentration effect of N1923 and TOA on the extraction of Th, U, REs, and Fe from the Yen Phu xenotime leachate (the feed liquor) was investigated using each extractant separately. The

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liquor.

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relevant factors in the extraction were the molarity of the amines, contact time, and pH of the feed

The extraction experiments were conducted at an aqueous-to-organic volumetric ratio of 10 and a temperature of 2980.2 K. 10.0 mL of the organic phase and 100 mL of the feed liquor were contacted in a mechanical shaker for ten minutes to ensure the extraction equilibrium. Different concentrations of N1923 and TOA were used for each extraction experiment. The aqueous phase in the extraction equilibrium was analyzed using ICP-MS to obtain the extraction efficiencies of Th, U, REs, and Fe as follows (equation 8): (8) where Ci and Caq (in g·L-1) are the concentrations of Th, U, REs, or Fe in the feed liquor and aqueous phases in the extraction equilibrium, respectively.

Journal Pre-proof When the optimum solvent composition was determined, the effect of contact time and pH in the feed liquor on the extraction efficiencies of Th, U, REs, and Fe using the optimum N1923 and TOA mixture was studied. The extraction experiments were conducted at an aqueous-to-organic volumetric ratio of 10 and a temperature of 2980.2 K. The aqueous phase in the extraction equilibrium was analyzed using ICP-MS to obtain the extraction efficiencies of Th, U, REs, and Fe (equation 8). When the optimum contact time of the extraction was determined, the equilibrium pH effect in the

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feed liquor on the extraction efficiencies of Th, U, REs, and Fe using the optimum N1923 and TOA

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mixture for the optimum contact time was studied. The aqueous phase in the extraction equilibrium was analyzed using ICP-MS to obtain the extraction efficiencies of Th, U, REs, and Fe (equation 8).

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Five replications were performed for each experiment on the extraction of Th, U, REs, and Fe from

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the Yen Phu xenotime leachate using a mixture of N1923 and TOA.

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Continuous countercurrent simulative extraction experiments using a separation funnel system were implemented to determine the parameters of the loaded and scrubbed organic phases and

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values of D of Th, U, REs, and Fe for extraction, scrubbing, and stripping processes. The principle

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of the continuous countercurrent extraction process was described in detail by Jyothi et al. (2010), Lee et al. (2009), Qi (2018), Teh et al. (1983), Xie et al. (2015), Phillip (1997), and Kislik (2012),

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and can be summarized as follows: the aqueous and organic phases in the (n-1)th funnel were separated into two phases. The aqueous phase was filled in the nth funnel and the organic phase was introduced in the (n-2)th funnel (Fig. 8). As Fig. 9 shows, the feed, scrubbing, and stripping liquor (aqueous phase) were fed into the first funnel of the extraction, scrubbing, and stripping processes, respectively. The aqueous phase in the last funnel of the extraction and stripping processes were raffinate and pregnant stripping liquor, respectively. The aqueous phase in the last funnel of the scrubbing process (pregnant scrubbing liquor) was introduced in the first funnel of the extraction process. The organic phase was fed into the last funnel of the extraction stage and came out from

Journal Pre-proof the first funnel of the stripping stage. The concentrations of Th, U, REs, and Fe in aqueous and organic phases in the extraction, scrubbing, and stripping equilibriums were obtained by ICP-MS. The continuous countercurrent experiments were conducted in a series of mixer-settler units. The experimental setup comprised three processes: extraction of six stages, scrubbing of four stages, and stripping of six stages (Fig. 9). A mixture of 0.1-mol·L-1 N1923 and 0.015-mol·L-1 TOA was used as the solvent and the Yen Phu xenotime leachate as a feed liquor. The flow rates of the organic phase and feed liquor in the extraction process were calculated using equation 9 and

(9) (10)

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equation 10:

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where

f(org) and f(feed) (in mL·min-1) are flow rates of the organic phase and feed liquor, respectively;

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is the volumetric ratio of the feed liquor to organic phase;

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VM (in mL) is the volume of the mixer unit; t (in min) is the contact time of the phases.

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equation 12:

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The flow rates of the scrubbing and stripping liquors were calculated using equation 11 and

(11) (12)

where f(scrub) and f(strip) (in mL·min-1) are the flow rates of the scrubbing and stripping liquors, respectively; R is the volumetric ratio of the organic phase to scrubbing or stripping liquor; The concentrations of Th, U, REs, and Fe in the aqueous and organic phases for every process were analyzed using ICP-MS for a period of 2 h. 2.4. Solvent regeneration procedure

Journal Pre-proof At the end of the stripping operation, the amine extractants are in the form of chloride salts. Regenerate them into free amine with a base, e.g., solutions of NH3, NaOH, and Na2CO3 is necessary because (i) the introduction of chloride into the extraction system causes a decrease in the overall extraction efficiency owing to the interference of chloride ions, and (ii) small amounts of Th and U that are still in the stripped organic phase will impair the extraction efficiency. The simplified expressions for the regeneration reactions may be expressed as equations 13 and 14: (13)

2RtNHCl(org) + Na2CO3(aq) = 2RtN(org) + 2NaCl(aq) + H2O + CO2

(14)

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2RpNH3Cl(org) + Na2CO3(aq) = 2RpNH2(org) + 2NaCl(aq) + H2O + CO2

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For the complete regeneration of the extractants, while maintaining the carbonate

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requirements at a low level, a multistage operation is used. In our experiments on the regeneration of the N1923 and TOA extractants, the results indicated that the process was operated continuously,

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counter-currently in an arrangement of mixer-settler units of four steps, using a sodium carbonate

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solution of 1.0 mol·L-1 as the regenerant and an organic-to-aqueous volumetric ratio of 4. The flow

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rate of the regenerating liquor (f(reg)) can be calculated using equation 15: (15)

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3. Theory

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The regenerated extractants can be recycled in the extraction process.

The main parameters that must be determined in the liquid-liquid extraction technique are the stage numbers and phase volumetric ratios in the extraction, scrubbing, and stripping processes. Owing to low accuracy in the graphical representation of the extraction isotherm plot, the determination of the theoretical extraction stages and the phase volumetric ratios by constructing the McCabe-Thiele diagram often results in certain errors. These parameters can be defined using the calculus method to make the theoretical stage and phase volumetric ratio calculations more accurate. The theory of the calculus method to define the theoretical stages and the phase volumetric ratios is based on the law of matter conservation. Which states that during any physical or chemical

Journal Pre-proof contact, the total mass of the products and the total mass of the reactants remains constant. A flow diagram of the extraction stages is shown in Fig. 8 (Jyothi et al., 2010; Lee et al., 2009; Qi, 2018; Teh et al., 1983; Xie et al., 2015; Phillip, 1997; Kislik, 2012). The organic-to-aqueous volumetric ratio R is determined by the equation of matter conservation (equations 16 and 17). XoVa = XnVa + Y1Vo

(16)

or

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(17)

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where, as Fig. 8 shows,

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X0 and Xn are the concentrations of thorium (and uranium) in the feed and raffinate liquor, respectively;

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Y1 is the concentration of Th (and U) in the loaded organic phase (the first extraction stage in Fig.

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9);

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Vo and Va are volumes of the organic phase and feed liquor, respectively. As Fig. 8 depicts, the equation of matter conservation employed for the first extraction stage is (18)

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X0Va = XnVa + VoY1

X0 = Xn + RY1

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Dividing both sides by Va results in

(19)

Similarly, the equation of matter conservation employed for the second extraction stage is X1 = Xn + RY2

(20)

and the equation of matter conservation employed for the third extraction stage is X2 = Xn + RY3

(21)

Assuming that distribution coefficients in all of the extraction stages remain unchanged, (22) Substituting

into equation 19 results in

Journal Pre-proof (23) Substituting

and equation 23 into equation 20 results in (24)

Similarly, substituting

and equation 24 into equation 21 results in (25)

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Finally, the equation of matter conservation employed for the nth extraction stage is

+

+ is a sum (S) of a geometric progression with a

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. Thus,

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common ratio (

(27)

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The expression *

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*

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Dividing both sides by Xn yields

(26)

(28)

)

(29)

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(

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Substituting equation 28 into equation 27 and solving it yields

Performing the common logarithm of both sides results in *

(

)+

(30) where n is the number of stages. 4. Results and discussions 4.1. Study on the extraction of thorium and uranium from control sulfate liquors using the primary amine N1923 and the tertiary amine TOA 4.1.1. Extraction isotherms of thorium and uranium

Journal Pre-proof The extractions of Th and U from control sulfate liquors using primary and tertiary amines, respectively, were studied. 0.1 mol·L-1 of N1923 (primary amine) and 0.015 mol·L-1 of TOA (tertiary amine) were used to extract Th and U, respectively. The pH of the sulfate liquors was 1.5, and the aqueous-to-organic volumetric ratio was 1. Th and U extraction experiments to plot their extraction isotherm curves were conducted at a fixed temperature of 298±0.2 K. Th and U extraction isotherms were schemed from the experimental data, and McCabe-Thiele diagrams were used to obtain the theoretical stages and the phase volumetric ratio for effective

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extraction and to be used in continuous countercurrent extraction experiments. The extraction

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isotherms were drawn with reference to the successive contact technique to deplete the liquor and load the organic phase. Fig. 1(A) shows the Th extraction isotherm, and Fig. 1(B) shows the U

-p

isotherm. The McCabe-Thiele diagrams obtained for the Th and U extractions indicated that six

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extraction stages were necessary to load the organic phase with the Th saturation capacity of 3.1

lP

g·L-1 and U saturation capacity of 0.7 g·L-1 at an aqueous-to-organic volumetric ratio of 18. The ideal number of stages for the extraction of Th and U required to be defined by continuous

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countercurrent extraction experiments.

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4.1.2. Thermodynamics of the thorium and uranium extractions Th and U extraction experiments in sulfate medium using 0.1-mol·L-1 N1923 and 0.015-

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mol·L-1 TOA, respectively, were performed while temperature was varied from 2980.2 to 3300.2 K and using Th and U control sulfate liquors of pH = 1.5 containing 3 g·L-1 of thorium and 0.5 g·L-1 of uranium. The studies on the effect of temperature on Th and U extraction processes using each extractant separately are depicted in Fig. 2. As Fig. 2 shows, when temperature increased from 298 to 330 K, the D of Th and U decreased from 1.95 to 0.81 and from 2.86 to 1.21, respectively. The thermodynamic calculations give a better insight of the effect of temperature in the extraction process. The enthalpy of the extraction process (ΔH) is derived from equation 31, which is also known as the Van’t Hoff equation (Lu et al., 2016; Yang et al., 2020): (31)

Journal Pre-proof where R is the gas constant and C is an integration constant including the equilibrium constant for the extraction reaction and the activity coefficients for other components, which are assumed to be fixed under determined experimental conditions. The Van’t Hoff plots of logD of Th and U extraction processes of the amines versus is shown in Fig. 2. By using equation 31, the enthalpy can be determined using the least square method at different extraction temperatures (Fig. 2). According to the Van’t Hoff equation,

is the slope of the straight lines in Fig. 2. Hence, the

H of Th and U extraction processes using the primary and tertiary amines, respectively, can be

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calculated. The ΔH values of Th and U extraction processes using N1923 and TOA, respectively, at

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a liquor pH of 1.5 were –21.2 and –24.1 kJ·mol-1, respectively.

-p

When ΔH is negative that the reaction is exothermic; hence, an increase in temperature decrease

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the amount of Th (and U) in the solvent. Furthermore, the Gibbs free energy (ΔG) and the entropy (S) are determined using the following equations:

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G = –RTlnD = –2.303RTlogD

(33)

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and

(32)

To determine the thermodynamic data listed in Table 3 equation 32 and equation 33 were

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used. The negative Gibbs free energy values indicated that the Th and U extractions from the control sulfate liquors using N1923 and TOA, respectively, were thermodynamically favorable. They were similar to those in the studies on Th and U extractions using amines, wherein Th was extracted using the primary amines Primene JM-T and N1923, and U was extracted using the tertiary amine Alamin 336 in sulfate media (Lu et al. 2016; Yang et al., 2020; Liu et al., 2008; 2007; Yang et al., 1997; Singh et al., 2000; El-Yamani et al., 1985). Thus, the thermodynamic parameters implied that Th and U extraction processes using amines should be implemented at room temperature.

Journal Pre-proof 4.2. Study on the stripping of thorium and uranium from the primary amine N1923 and the tertiary amine TOA 4.2.1. Chloride stripping of thorium and uranium Preliminary stripping experiments of Th and U using chloride reagents from 0.1-mol·L-1 N1923 and 0.015-mol·L-1 TOA, respectively, were performed. The N1923 and TOA solvents used in the stripping experiments were loaded to saturation of the Th and U, respectively. Table 4 shows the characterization of these loaded solvents obtained through successive discontinuous contacts

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with the liquors.

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The stripping efficiencies of HCl and a NaCl-HCl mixture were investigated using an aqueous-to-organic volumetric ratio of 1 at 2980.2 K. The stripping time was ten minutes and the

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phase disengagement time was lower than five minutes. The HCl concentration effect was studied

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by 0.5 to 3.5 mol·L-1 on Th and U stripping. Fig. 3(A) shows the plot of the results of the stripping

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efficiencies. As Fig. 3(A) shows, the Th stripping percentage increased from 511% to 851%. For the HCl concentration levels above 2.0 mol·L-1, practically no increase was observed in the Th

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stripping; at these HCl concentration levels, the U stripping was practically constant at 921%.

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Thus, the optimum HCl concentration for Th and U stripping was 2.0 mol·L-1.

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However, the pregnant stripping liquor obtained had high acidity; this caused difficulties for the further precipitation of Th and U using an oxalic acid or a base. Therefore, mixtures of NaCl and HCl acid were used to study the Th and U stripping. The results of the study on the effect of the molarity ratios of NaCl to HCl on Th and U stripping are shown in Fig. 3(B). As Fig. 3(B) shows, by increasing the NaCl molarity to 1.5 mol·L-1 and decreasing the HCl molarity to 1.0 mol·L-1, the Th and U stripping attained the maximum efficiencies of 851% and 921%, respectively. Additionally, no enhancement was observed in the stripping efficiency over the NaCl-to-HCl molar ratio of 1.5:1.0. Thus, the NaCl and HCl mixture was an effective stripping and low-cost reagent for Th and U. Moreover, the pregnant stripping liquor had low acidity; hence, this is advantageous for further Th and U treatments using oxalate precipitation or a base.

Journal Pre-proof 4.2.2. Effect of reaction time on thorium and uranium stripping The studies on the effect of stripping time on the Th and U stripping efficiencies aimed to determine data required to design a relevant continuously countercurrent mixer-settler unit of the stripping process (Qi, 2018). The 0.1-mol·L-1 N1923 and 0.015-mol·L-1 TOA solvents used in these testing were loaded to the saturation of Th and U, respectively; and the stripping liquor was a chloride mixture of 1-mol·L-1 HCl and 1.5-mol·L-1 NaCl. These studies were performed at 2980.2 K and an aqueous-to-organic volumetric ratio of 1. The reaction time between the aqueous and

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organic phases for the stripping process was studied from one to ten minutes to determine the

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required time to attain the stripping equilibrium. The effects of the contact time are graphically depicted in Fig. 4. As the figure shows, Th and U stripping efficiencies rapidly increased to attain

-p

values of 2.70.1 and 0.650.05 g·L-1 for Th and U in six and five minutes, respectively; no

re

improvement was observed in Th and U stripping efficiencies for more than six and five minutes,

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respectively. Therefore, the contact time of six minutes was considered the most efficient time for both Th and U stripping.

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4.3. Study on thorium and uranium extraction from the Yen Phu xenotime leachate using a mixture

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of the primary amine N1923 and tertiary amine TOA

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4.3.1. Effect of the concentrations of N1923 and TOA on thorium and uranium extraction This study investigated the selective extraction of Th and U from an RE liquor in sulfuric acid media using a mixture of N1923 and TOA. The solvent composition was studied and the variation in the concentration effect of N1923 and TOA on the extraction of Th, U, and REs was first investigated using each extractant separately. The feed liquor was a Yen Phu xenotime leachate containing Th, U, and REs compositions (Table 1). These studies were performed at 2980.2 K and an aqueous-to-organic volumetric ratio of 10. The effect of N1923 concentrations on the extraction of Th, U, REs, and Fe from the Yen Phu xenotime leachate were evaluated by varying the concentration from 0.05 to 0.2 mol·L-1. Fig. 5(A) shows the results obtained in the experiment. As anticipated, the Th distribution coefficient

Journal Pre-proof increased when the concentration of the extractant increased. The increasing percentages of the U and Fe extractions required observation. The U and Fe extractions become significant as the N1923 concentration exceeded 0.1 mol·L-1, being approximately 20% when extractant concentration reached of 0.2 mol·L-1. However, in this scenario, RE extraction was unremarkable as the RE extraction percentage was lower than 2%. The results led to the conclusion that the optimum concentration of N1923 should be lower than 0.1 mol·L-1 (Fig. 5(A)). Additionally, the TOA concentration effect on the extraction of Th, U, REs, and Fe from the

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Yen Phu xenotime leachate was investigated in the range from 0.01 to 0.03 mol·L-1. Fig. 5(B)

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shows the obtained results. In the investigated interval, the efficiency of U extraction increased from 681% to 921%. Above the TOA concentration of 0.015 mol·L-1, the U extraction reached a

-p

plateau. On the other hand, Th extraction increased, which was negligible (4%) at 0.015 mol·L-1

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and as high as 18% at a TOA concentration of 0.03 mol·L-1. In the studied interval of extractant

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concentrations, the extraction percentages of REs and Fe were below 1%, suggesting that extractions of REs and Fe extractions are not a restricting factor in determining the concentration of

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TOA to be used in the process. The results led to the conclusion that TOA concentration should be

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kept below 0.015 mol·L-1 (Fig. 5(B)).

Thus, the best solvent composition to extract Th and U from the Yen Phu xenotime leachate is

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a mixture of 0.1-mol·L-1 N1923 and 0.015-mol·L-1 TOA. 4.3.2. Rate of thorium and uranium extraction The studies on the effect of reaction time on the efficiencies of Th and U extraction aimed to determine the equilibrium time at which maximum Th and U recovery occurred with the least amount extracted from REs and flow rates of the organic phase and feed liquor that are fed into a continuous countercurrent mixer-settler extractor of the extraction process. A mixture of 0.1-mol·L1

N1923 and 0.015-mol·L-1 TOA was used as an organic phase in these experiments; the feed

liquor was Yen Phu xenotime leachate containing Th, U, REs, and Fe compositions as shown in Table 1. These studies were performed at 2980.2 K and an aqueous-to-organic volumetric ratio of

Journal Pre-proof 10. The contact time between both phases for the extraction was studied from one to eight minutes to determine the required time to attain the extraction equilibrium. The results of the contact time effect are graphically represented in Fig. 6. It was determined that the extraction efficiencies of Th and U rapidly increased to 831% and 861% in four minutes, respectively; Th and U extraction remained unchanged in a contact time of more than four minutes. Therefore, four minutes was considered the most efficient contact time for the extraction of thorium and uranium to ensure complete extraction.

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By prolonging the contact time from one to four minutes, the extraction of REs was enhanced from

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0% to higher than 2% and then decreased to lower than 1%. In the first minutes of the extraction process, the N1923 and TOA extractants did not attain the saturation of Th and U; hence, REs were

-p

extracted by the extractants. Additionally, because the extractability (distribution coefficients) of Th

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and U using the amines were greater than those of REs, the extraction competition between Th and

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U with REs forced the REs to keep in the aqueous phase while Th and U transferred to the organic phase. As a result, the extraction of REs decreased when contact time increased.

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4.3.3. Effect of the feed liquor pH on thorium and uranium extraction

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An important parameter in the extraction of metals is the feed liquor’s pH. To study the effect

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of the liquor’s pH on the extraction efficiency of Th, U, REs, and Fe, several experiments with a pH range of the feed liquor from 0.5 to 2.5 were conducted at 2980.2 K and an aqueous-to-organic volumetric ratio of 10 using a mixture of 0.1-mol·L-1 N1923 and 0.015-mol·L-1 TOA as the organic phase and Yen Phu xenotime leachate containing Th, U, REs, and Fe compositions (Table 1) as the feed liquor. To modify the initial liquor pH (pH 1.5) a sulfuric acid solution (acid) or an MgO milk solution (alkaline) were used. The extraction results for Th, U, and REs, and Fe are graphically depicted in Fig. 7. As Fig. 7 shows, Th and U extractions dramatically increased when the pH increased from 0.5 to 2.0. Approximately 90% of Th and U were extracted in pH ranges from 1.5 to 2.0; the extraction of Th and U was not affected by the feed liquor pH, remaining at approximately 90% after pH 2 was reached. Similarly, the extraction of REs remained at approximately 1.5%.

Journal Pre-proof However, a significant increase in the Fe extraction was observed with the increase in the pH of the liquor (from 1 to 14%). The effect on the Fe extraction can be explained by a synergistic effect favored by the increase in the pH. Here, the increase in the Fe extraction was related to the predominance of the species

and

in a sulfate medium (Alguacil et al., 1986;

Schrötterová et al., 1999; Wei et al., 2016; Yu et al., 1989). Thus, the feed-liquor pH of 1.5 was the preferable value that achieved the optimum extraction for Th and U; the REs and Fe remained in the effluent liquor.

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4.4. Determining the parameters of the continuous countercurrent extraction, scrubbing, and

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stripping processes

Initially, several experiments must be conducted to load the organic phase in the saturation

-p

capacities of Th, U, REs, and Fe. A mixture of 0.1-mol·L-1 N1923 and 0.015-mol·L-1 TOA as the

re

organic phase and Yen Phu xenotime leachate containing Th, U, REs, and Fe compositions (Table

lP

1) as the feed liquor were employed in these experiments at 2980.2 K and an aqueous-to-organic volumetric ratio of 10. The solvent loaded to the saturation of Th, U, REs, and Fe after four contacts

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under the abovementioned operating conditions. Table 5 shows the saturation of the loaded organic

ur

after a series of contact with the liquors.

4.4.1. Determining the parameters of the extraction

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The concentrations of thorium and uranium in feed liquor X0, as shown in Table 1, were 0.17 and 0.04 g·L-1, respectively; the concentration of Th and U in the loaded organic (Y1), as shown in Table 5, were 2.9 and 0.68 g·L-1, respectively. Assuming that 99.9% of both Th and U was separated from the Yen Phu xenotime leachate, this meant that the amounts of Th and U in raffinate liquor Xn were at approximately 0.001 g·L-1. Thus, R in the extraction process of Th and U by the mixture of 0.1-mol·L-1 N1923 and 0.015-mol·L-1 TOA calculated from equation 17 was

.

As indicated from our continuous countercurrent extraction testing on the separation funnel system, at saturation concentration in the organic phase of Th from 2 to 3 g·L-1 and U from 0.5 to 0.7 g·L-1, the D coefficients of Th and U were in the ranges from 30 to 50 and from 20 to 40,

Journal Pre-proof respectively. Therefore, the mean D coefficients of Th and U were approximately 40 and 30, respectively. As calculated from equation 30, the number of theoretical extraction stages of Th and U extraction process was approximately 5.6, which can be rounded off to 6 stages. The numbers of theoretical extraction stages determined using the calculus method and McCabe-Thiele diagram (Fig. 1) were the same.

4.4.2. Determining the parameters of the scrubbing process

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As Table 5 shows, the REs and Fe were loaded into Y1. Most of the REs and Fe from the Y1

ro

must be removed before the stripping operation. The scrubbing operation aims to transfer REs and Fe from the organic into the aqueous phase and maintain Th and U in the organic phase. Here, a

-p

dilute sulfuric acid scrubbing liquor of pH = 1.5 is used generally for the scrubbing process (Jyothi

re

et al., 2010; Lee et al., 2009).

lP

Similar to equation 17, the organic-to-aqueous volumetric ratio R of the scrubbing process is

na

determined using the equation of matter conservation:

ur

where

(34)

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X1 is the concentration of Fe (and REs) in the pregnant scrubbing liquor; Y0 and Yn are concentrations of Fe (and REs) in the loaded and scrubbed organic phases, respectively (Fig. 9);

Vo and Va are volumes of the organic phase and scrubbing liquor, respectively; Similar to equation 30, the number of stages of the scrubbing process is determined by equation 35: *

(

)+

(35) where n is the number of scrubbing stages. As indicated in Tables 1 and 4, the D coefficients of Fe and REs in the scrubbing process were approximately 0.25 and 0.1, respectively. Assuming that 99% of Fe was scrubbed from the

Journal Pre-proof loaded organic phase, the concentrations of Fe organic phase after scrubbing Yn were approximately 0.004 g·L-1. As calculated from equation 34 and equation 35, the optimum R and n of the scrubbing process were 1.5 (or 2.0) and 4 (or 6), respectively. Moreover, as calculated from equation 35, 99.99% of REs were scrubbed from the loaded organic phase. 4.4.3. Determining the parameters of the stripping process Similar to equation 34, the R of the stripping process is determined by the equation of matter conservation:

of

(36)

ro

where

-p

X1 is the concentration of Th (and U) in the pregnant stripping liquor; Y0 and Yn are concentrations of Th (and U) in the scrubbed and stripped organic phases,

re

respectively;

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Vo and Va are volumes of the organic phase and stripping liquor, respectively;

(

)+

(37)

ur

*

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Similar to equation 35, the number of stages of the stripping process is determined by equation 37:

where n is the number of stripping stages.

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As indicated from our continuous countercurrent scrubbing testing on the separation funnel system, amounts of Th and U in the scrubbed organic phase Y0 were 2.60.1 and 0.620.04 g·L-1, respectively. The D coefficients of Th and U in the stripping process were approximately 0.12 and 0.15, respectively, which were determined in the stripping experiments using a mixture of 1-mol·L-1 HCl and 1.5-mol·L-1 NaCl as a stripping liquor. Assuming that 99.9% of Th and U were stripped from the organic phase, the concentrations of Th and U in the stripped organic phase Yn were approximately 0.001 g·L-1. As calculated from equation 36 and equation 37, the optimum R and n of the stripping process were 2.4 and 6, respectively.

Journal Pre-proof 4.5. Study on continuous countercurrent extraction, scrubbing, and stripping processes The purpose of these experiments was the optimizing the separation of Th and U from the Yen Phu xenotime leachate. They were conducted using a mixture of 0.1-mol·L-1 N1923 and 0.015mol·L-1 TOA as the extractants, in an array of mixer-settler units. The experimental setup comprised three processes: extraction, scrubbing, and stripping (Fig. 9). Table 6 summaries the optimum parameters of the continuous countercurrent extraction, scrubbing, and stripping processes that resulted from studies discussed in Sections 4.1, 4.2, 4.3, and 4.4.

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The volumes of the mixer-settler units for the extraction, scrubbing, and stripping processes

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were 500 mL mixers and 2000 mL settlers, and 60 mL mixers and 240 mL settlers, respectively. The flow rate of the organic phase in the extraction, scrubbing, and stripping processes was

-p

calculated as 6.50.5 mL·min-1 (equation 9); the flow rate of the feed liquor in the extraction

re

process, the scrubbing liquor in the scrubbing process, and the stripping liquor in the stripping

lP

process were calculated as 118.52.5 mL·min-1 (equation 10), 4.50.5 mL·min-1 (equation 11), and 2.70.3 mL·min-1 (equation 12), respectively.

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The efficiencies of the continuous countercurrent extraction, scrubbing, and stripping

ur

processes and the profiles of Th, U, Fe, and RE amounts in both phases are depicted in Figs. 10 and

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11. As Fig. 10(A) shows, in the extraction process, practically all Th and U were extracted from the feed liquor; Th and U concentrations in the raffinate liquor were lower than 0.001 g·L-1. The Th, U, Fe, and RE concentrations in the loaded organic phase were 2.90.1, 0.680.02, 0.40.1, and 1.60.1 g·L-1, respectively. In the scrubbing process, 99% of Fe and 99.9% of REs were scrubbed from the loaded organic phase (Figs. 11); the pregnant scrubbing liquor could be fed into the extraction process as a feed liquor. Th and U concentrations in the loaded organic phase decreased by 2.60.1 and 0.620.01 g·L-1, respectively. In the stripping process, Th and U concentrations in the pregnant stripping liquor were 6.20.4 and 1.50.1 g·L-1, respectively. Fig. 10 indicates that 99.9% of both Th and U were stripped from the scrubbed organic phase.

Journal Pre-proof Table 7 summarizes the results of the continuous countercurrent extraction, scrubbing, and stripping processes. The results indicated that practically all Th and U were separated from the Yen Phu xenotime leachate and the loss of REs was insignificant (less than 0.1%). Thus, the optimum parameters of the process were determined as shown in Table 6. The optimum parameters were tested at a pilot scale using a array of mixer-settler units, with 4-L mixers and 16-L settlers for the extraction process and 0.5-L mixers and 2.0-L settlers for the scrubbing and stripping processes to get insight of the industrial application of the process. The

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satisfactory results demonstrated the potential of the continuous countercurrent extraction-

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scrubbing-stripping technique to be applied in commercially separating Th and U from RE leachate. Conclusions

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A continuous countercurrent extraction-scrubbing-stripping technique to separating Th and U

re

from Yen Phu xenotime leachate is proposed in this paper; a mixture of the primary amine N1923

lP

and the tertiary amine TOA was employed as an effective extractant for both Th and U. The steps of the process can be concluded as follows:

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① In the extraction process, the optimum pH of the feed liquor was 1.5 and the optimum t, n, and

Jo

feed liquor.

, respectively; 99.9% of both Th and U were extracted from the

ur

R were 4 min, 6 stages, and

② In the scrubbing process, the scrubbing liquor was dilute sulfuric acid of pH = 1.5 and the optimum t, n, and R were 5.5 min, 4 (or 6) stages, and 1.5 (or 2.0), respectively; 99.9% of REs and 99% of Fe were scrubbed from the loaded organic phase, and loss of REs was at a minimum. ③ In the stripping process, the stripping liquor was a mixture of 1-mol·L-1 HCl and 1.5-mol·L-1 NaCl, and the optimum t, n, and R were 6 min, 6 stages, and 2.4, respectively; 99.9% of both Th and U were stripped from the loaded organic phase. The separation of Th and U from Yen Phu xenotime leachate is an important strategy to entirely remove radioactive nuclides from the further RE products and subsequently recover Th and U.

Journal Pre-proof Acknowledgments Dr. Nguyen Trong Hung expresses his sincere gratitude and thanks to the authorities of VINATOM and MOST. This research was financially supported by the Ministry projects 20202021 (code DTCB.10/20/VCNXH) and by the National project 2018-2020 (code KC.02.11/16-20). Dr. Nguyen Trong Hung and Dr. Masayuki Watanabe express their sincere gratitude and thanks to the authorities of VAST and, Japan Society for the Promotion of Science (JSPS) and JAEA. This research was supported by the VAST-JSPS Bilateral Joint Research Projects: “Analysis

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of rare-earths by femto-second molecular spectroscopy”.

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Dr. Jin-Young Lee and Dr. Rajesh Kumar Jyothi express their sincere gratitude and thanks to the authorities of KIGAM funded by the Ministry of Science, ICT, and Future Planning of Korea.

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This research was supported by the Convergence Research Project (CRC-15-06-KIGAM) funded

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by the National Research Council of Science and Technology (NST).

References

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Amaral Janúbia C.B.S., Michelle L. C. G. Sá, Carlos A. Morais, 2018. Recovery of uranium,

ur

thorium and rare earth from industrial residues. Hydrometallurgy 181, 148-155. Amaral Janúbia C.B.S., Carlos A. Morais, 2010. Thorium and uranium extraction from rare earth

498-503.

Jo

elements in monazite sulfuric acid liquor through solvent extraction. Minerals Engineering 23,

Amer T. E., El-Sheikh E. M., M. A. Gado, Abu-Khoziem H. A. & Zaki S. A., 2017. Selective recovery of lanthanides, uranium and thorium from Rosetta monazite mineral concentrate. Separation Science and Technology 53 (10), 1522-1530. Alguacil F. J. and Amer S., 1986. The extraction of iron(III) from aqueous sulphate solutions by primene 81R sulphate. Hydrornetallurgy 15, 337-350.

Journal Pre-proof Chen Shiliang, Huang Xiaowei, Feng Zongyu, Xu Yang, Wang Meng, Xia Chao, Zhao Longsheng, 2020. Behavior of rare earth, iron, and phosphorus during purification of rare earth sulfate leach solution using magnesium oxide. Hydrometallurgy 196, 105377 Danesi P.R., Renato Chiarizia and Charles F. Coleman, 1980. The Kinetics of Metal Solvent Extraction. CRC Critical Reviews in Analytical Chemistry, November 1980. El-Yamani I. S., Shabana E. I., 1985. Studies on extraction of thorium(IV) from sulfate media with mixtures of long-chain primary amines and tributyl phosphate. Journal of Radioanalytical and

of

Nuclear Chemistry 88/2, 273-280.

ro

Gomes da Silva R., Carlos Antonio de Morais, Leandro Viana Teixeira, Éder Domingos de Oliveira, 2018. Selective removal of impurities from rare earth sulphuric liquor using

-p

different reagents. Minerals Engineering 127, 238-246.

re

Gupta C.K., Krishnamurthy N., 2005. Extractive Metallurgy of Rare Earths. CRC Press.

lP

Hung N.T., Thuan L.B., Thanh T.C., Watanabe M., Nhuan H., Khoai D.V., Thuy N.T., Tung N.V., Aoyagi N., Tra D.T.T., Minh N.T., Jha M.K., Lee J.Y. and Jyothi R.K., 2020. Optimization of

na

sulfuric acid leaching of a Vietnamese rare earth concentrate. Hydrometallurgy 191, 105195.

ur

Hung N.T., Thuan L.B., Thanh T.C., Nhuan H., Khoai D.V., Tung N.V., Lee J.Y., Jyothi R.K., 2018. Modeling the UO2 ex-AUC pellet process and predicting the fuel rod temperature

197.

Jo

distribution under steady-state operating condition. Journal of Nuclear Materials 504, 191–

Hung N.T., Thuan L.B., Tung N.V, Thuy N.T., Lee J.Y., Jyothi R.K., 2017. The UO 2 ex-ADU powder preparation and pellet sintering for optimum efficiency: experimental and modeling studies. Journal of Nuclear Materials 496, 177–181. Hung N.T., Thuan L.B., Khoai D.V., Lee J.Y., Kumar J.R., 2016. Modeling conversion of ammonium diuranate (ADU) into uranium dioxide (UO2) powder. Journal of Materials 479, 483-488.

Nuclear

Journal Pre-proof Hung N.T., Thuan L.B., Khoai D.V., Lee J.Y., Kumar J.R., 2016. Brandon mathematical model describing the effect of calcination and reduction parameters on specific surface area of UO 2 powders. Journal of Nuclear Materials 474, 150-154. Jha Manis Kumar, Archana Kumar, Rekha Panda, Jyothi Rajesh Kumar, Kyoungkeun Yoo, Jin Young Lee, 2016. Review on hydrometallurgical recovery of rare earth metals. Hydrometallurgy 165, 2-26. Judge W.D., Azimi G., 2020. Recent progress in impurity removal during rare earth element

of

processing: A review. Hydrometallurgy 196, 105435.

ro

Jyothi Rajesh Kumar, Joon-Soo Kim, Jin-Young Lee, and Ho-Sung Yoon, 2010. Solvent extraction of uranium(VI) and separation of vanadium(V) from sulfate solutions using Alamine 336.

-p

Journal of Radioanalytical and Nuclear Chemistry 285, 301-308.

re

Jyothi Rajesh Kumar, Joon-Soo Kim, Jin-Young Lee, and Ho-Sung Yoon, 2011. A brief review on

lP

solvent extraction of uranium from acidic solutions. Separation & Purification Reviews 40, 77-125.

na

Kislik V. S., 2012. Solvent Extraction: Classical and Novel Approaches. Elsevier.

ur

Kumari Archana, Rekha Panda, Manis Kumar Jha, Rajesh Kumar Jyothi, Jin Young Lee, 2015. Process development to recover rare earth metals from monazite mineral: A review. Minerals

Jo

Engineering 79, 102-115.

Lapidus G.T., Doyle F.M., 2015. Selective thorium and uranium extraction from monazite: II. Approaches to enhance the removal of radioactive contaminants. Hydrometallurgy 155, 161167. Lee J. Y. and Kumar J. R., 2009. Liquid-liquid extraction general principles-a review. J. Kor. Inst. Res. Recycl.18, 3-9. Li Deqian, Yong Zuo, Shulan Meng, 2004. Separation of thorium(IV) and extracting Rare Earths from sulfuric and phosphoric acid solutions by solvent extraction method. Journal of Alloys and Compounds, 431-433.

Journal Pre-proof Lien T.V., Dinh T.T., Dung N.T.K., 2020. Study on leaching systems and recovery for PALUA– PARONG low grade uranium sandstone ores. Hydrometallurgy 191, 105164. Liu Jianjun, Yanliang Wang, and Deqian Li, 2008. Extraction kinetics of thorium(IV) with primary amine N1923 in sulfate media using a constant interfacial cell with laminar flow. Separation Science and Technology 43, 431-445. Liu Jianjun, WeiWei Wang, Deqian Li, 2007. Interfacial behavior of primary amine N1923 and the kinetics of thorium(IV) extraction in sulfate media. Colloids and Surfaces A: Physicochem.

of

Eng. Aspects 311, 124-130.

ro

Lu, Y., Wei, H., Zhang, Z., Li, Y., Wu, G. and Liao, W., 2016. Selective extraction and separation of thorium from rare earths by a phosphorodiamidate extractant. Hydrometallurgy 163, 192-

-p

197.

re

Morais C.A., Gomiero L.A., Scassiotti Filho W., Rangel Jr H., 2005. Uranium stripping from

Engineering 18, 1331-1333.

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tertiary amine by sulfuric acid solution and its precipitation as uranium peroxide. Minerals

na

Nasab M. E., Sam A., Milani S.A., 2011. Determination of optimum process conditions for the

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separation of thorium and rare earth elements by solvent extraction. Hydrometallurgy 106, 141-147.

Jo

Noboru Aoyagi, Thuy T. Nguyen, Yuta Kumagai, Tung V. Nguyen, egawa, Hung T. Nguyen, and Thuan

a

asami Na ada,

e, 2 2 . pectroscopic tudies of

u ari ssbauer,

Infrared, and Laser-Induced Luminescence for Classifying Rare-Earth Minerals Enriched in Iron Rich Deposits. ACS Omega 5, 7096-7105. Philip A. Schweitzer, 1997. Handbook of Separation techniques for chemical engineering. 3 rd ed. McGraw-Hill Press. Qi Dezhi, 2018. Hydrometallurgy of Rare Earths: Extraction and Separation. Elsevier.

Journal Pre-proof Sadri Farzaneh, Fereshteh Rashchi, Ahmad Amini, 2017. Hydrometallurgical digestion and leaching of Iranian monazite concentrate containing rare earth elements Th, Ce, La and Nd. International Journal of Mineral Processing 159, 7-15. chr tterová D. and Ne ovář P., 1999. Extraction of Iron (III) from Aqueous Sulfate Solutions by Primene JMT. Chem. Papers 53 (6), 412-416. Singh H. and Gupta C. K., 2000. Solvent extraction and processing of uranium and thorium in production uranium. Mineral Processing and Extractive Metallurgy Review: An International

of

Journal 21, 307-349.

ro

Teh C. Lo, Malcolm H. I. Baird and Carl Hanson, 1983. Handbook of Solvent extraction. John Wiley & Sons.

-p

Vijayalakshmi R., Mishra S.L., Singh H., Gupta C.K., 2001. Processing of xenotime concentrate by

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sulphuric acid digestion and selective thorium precipitation for separation of Rare Earths.

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Hydrometallurgy 61, 75-80.

Wei Qifeng, Ren Xiulian, Guo jingjing, Chen Yongxing, 2016. Recovery and separation of sulfuric

na

acid and iron from dilute acidic sulfate effluent and waste sulfuric acid by solvent extraction

ur

and stripping. Journal of Hazardous Materials 304, 1-9. Xie Feng, Ting An Zhang, David Dreisinger, Fiona Doyle, 2014. A critical review on solvent

Jo

extraction of Rare Earths from aqueous solutions. Minerals Engineering 56, 10-28. Yang Xiaojing, Zhang Zhifeng, Kuang Shengting, Wei Haiqin, Li Yanling, Wu Guolong, Geng Aifang, Li Yunhui, Liao Wuping, 2020. Removal of thorium and uranium from leach solutions of ion-adsorption rare earth ores by solvent extraction with Cextrant 230. Hydrometallurgy 194 (2020) 105343. Yang Yonghui, Sun Sixiu, Xue Shuyun, Yang Zhikun, Wang Youshao, Bao Borong, 1997. Extraction of uranium(VI) through reversed micelle by primary amine N1923. Journal of Radioanalytical and Nuclear Chemistry 222 (1-2), 239-241.

Journal Pre-proof Yu Shuqiu and Jiayong Chen, 1989. Synergistic Extraction of Ferric Iron in Sulfate Solutions by Tertiary

Amine

and

2-Ethylhexyl

2-Ethylhexylphosphonic

Acid

(HEHEHP)

or

Dialkylphosphonic Acid. Hydrometallurgy 22, 183-192. Zhu Zhaowu, Yoko Pranolo, Chu Yong Cheng, 2015. Separation of uranium and thorium from Rare Earths for rare earth production-A review. Minerals Engineering 77, 185-196. Zuo Yong, Ji Chen, Deqian Li, 2008. Reversed micellar solubilization extraction and separation of thorium(IV) from rare earth(III) by primary amine N1923 in ionic liquid. Separation and

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Purification Technology 63, 684-690.

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Fig. 1. Thorium (A) and Uranium (B) extraction isotherms.

, in K-1 of U(VI) and Th(IV) extraction.

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Fig. 2. Van't Hoff plots of log D against

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Fig. 3. Influence of the HCl concentration (A) and molar ratio of NaCl to HCl (B) on Th and U stripping efficiencies.

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Fig. 4. Effect of contact time on Th and U stripping efficiencies.

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Fig. 5. Effect of N1923 (A) and TOA (B) concentrations on the extractability of Th and U, and REs (Fe) from Yen Phu xenotime leachate.

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Fig. 6. Effect of contact time on the extractability of Th and U, and REs from Yen Phu xenotime leachate.

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Fig. 7. Effect of pH on the extractability of Th and U, and REs (Fe) from Yen Phu xenotime leachate.

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Fig. 8. Flow diagram of the extraction stages.

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Fig. 9. Proposed flow diagram illustrating the successive extraction, scrubbing and stripping stages.

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Fig. 10. Profiles of Th (A) and U (B) concentrations in continuous countercurrent extraction, scrubbing and stripping experiments.

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Fig. 11. Profiles of Fe (A) and REs (B) concentrations in continuous countercurrent scrubbing experiments.

Journal Pre-proof Table 1 Chemical composition of Yen Phu xenotime leachate (pH=1.5). Component

Concentration, in g·L-1

Th

0.170.06

U

0.040.01

REs

18.41.2

Fe

1.60.1

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324

Table 2

Journal Pre-proof Characteristics of the solvents. Solvents

Formula

M, g·mol-1

d, g·cm-3

Purity

354

0.820

99%

310

0.809

98%

158

0.822

99%

170-226

0.789

99%

tri-n-octyl amine (TOA) N1923 n- decanol

C10H21OH CnH2n+2

IP-2028

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(n=12-16)

Table 3 Thermodynamic parameters for Th(IV) and U(VI) extraction.

Journal Pre-proof G, in kJ·mol-1

S, in J·mol-1·K-1

H, in kJ·mol-1

298

-1.65

-71.22

-21.2

306

-0.82

-69.37

314

-0.34

-67.60

322

-0.03

-65.92

330

0.58

-64.32

298

-2.60

-80.90

306

-2.18

-78.78

314

-1.19

-76.78

322

-0.62

-74.87

330

-0.52

-73.06

Temp., in K Th extraction

U extraction

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Table 4 Chemical characterization of the loaded solvents.

Journal Pre-proof Content of Th, in g·L-1 Content of U, in g·L-1

Primary amine N1923 0.1 mol·L-1

3.10.2

0

Tertiary amine TOA 0.015 mol·L-1

0

0.740.03

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Solvent

Table 5 Chemical characterization of the loaded solvents. Items

Th

U

REs

Fe

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2.90.1

0.680.02 1.60.1

Noted in the extraction step

Y1 (of Th)

Y1 (of U)

-

-

-

-

Y0 (of REs) Y0 (of Fe)

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Noted in the scrubbing step

0.40.1

Table 6 The optimum extraction and scrubbing-stripping parameters. Parameters

Extraction

Scrubbing

Acidity of liquor

pH=1.5

Dilute sulfuric acid Mixture of HCl 1 mol·L-1 and solution; pH=1.5

Stripping

NaCl 1.5 mol·L-1

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5.5 minutes

6 minutes

Organic to aqueous volumetric ratio

1 to 18

1.5 to 1 (or 2.0 to 1)

2.4 to 1

Number of stages

6

4 (or 6)

6

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Contact time

Table 7 Concentration of Th, U, REs and Fe in aqueous and organic phases. Concentration, in g·L-1

Items Th

U

REs

Fe

Raffinate liquor

0.001

0.001

18.41.2

1.60.1

Loaded organic

2.90.1

0.680.02

1.60.1

0.40.1

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0.06

2.580.06

0.610.02

Scrubbed organic

2.60.1

0.620.01

0.001

0.004

Pregnant stripping liquor

6.20.4

1.50.1

< 0.001

< 0.004

Stripped organic

0.001

0.001

< 0.001

< 0.004

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Pregnant scrubbing liquor

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CRediT author statement Name of the author

Role

Nguyen Trong Hung

Conceptualization; Methodology; Writing & Editing manuscript Conceptualization; Methodology

Tran Chi Thanh

Methodology

Masayuki Watanabe

Methodology

Do Van khoai

Methodology; Writing - Original Draft

Nguyen Thanh Thuy

Validation; Investigation

Hoang Nhuan

Validation

Pham Quang Minh

Validation

Tran Hoang Mai

Data Curation; Formal analysis

Nguyen Van Tung

Investigation

Doan Thi Thu Tra

Formal analysis

Manish Kumar Jha

Collaborative author

Jin-Young Lee

Collaborative author

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Collaborative author and edited the

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Rajesh Kumar Jyothi

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Le Ba Thuan

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manuscript

Journal Pre-proof Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Graphical Abstract Separation of thorium and uranium from xenotime leach solutions by solvent extraction using primary and tertiary amines Nguyen Trong Hunga*, Le Ba Thuana, Tran Chi Thanhb, Masayuki Watanabec, Do Van khoaic, Nguyen Thanh Thuya, Hoang Nhuana, Pham Quang Minhb, Tran Hoang Maia, Nguyen Van Tunga, Doan Thi Thu Trad, Manish Kumar Jhae, Jin-Young Leef, Rajesh Kumar Jyothif* a

Institute for Technology of Radioactive and Rare Elements (ITRRE)-VINATOM-MOST, 48 Lang

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Ha, Dong Da, Hanoi, Vietnam; Vietnam Atomic Energy Institute (VINATOM)-Ministry of Science and Technology (MOST), 59 Ly

d

Institute of Geological Sciences (IGS)-Vietnam Academy of Science and Technology (VAST), 84

Chua Lang, Dong Da, Hanoi, Vietnam;

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MEF Division, CSIR-National Metallurgical Laboratory (NML), Jamshedpur 831-007, India.

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Japan Atomic Energy Agency (JAEA), 2-4 Tokaimura, Nakagun, Ibaraki, 319-1195, Japan;

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Thuong Kiet, Hoan Kiem, Hanoi, Vietnam;

Convergence Research Center for Development of Mineral Resources (DMR),Korea Institute of

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Geoscience and Mineral Resources (KIGAM), Daejeon 34132,Korea;

Flow diagram illustrating the continuous countercurrent extraction-scrubbing-stripping technique

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Highlights  Separation of Th and U from xenotime leach liquor by liquid-liquid (solvent) extraction  Primary and tertiary amines used in the continuous countercurrent separation of Th and U  The optimum parameters for extraction, scrubbing and stripping were determined  99.9% of both, Th and U, are extracted from feed liquor in the extraction stage  99.9% of REs and 99% of Fe are scrubbed from the loaded organic phase in scrubbing  The method could be used commercially to separate Th and U from xenotime

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 Further recovery of REs (Tb, Dy, Nd, etc.) is proposed for commercial applications