Solvent Extraction of Didymium by TBP, Aliquat 336 and HDEHP in the Presence of Ca(NO3)2

applied sciences

Article Solvent Extraction of Didymium by TBP, Aliquat 336 and HDEHP in The Presence of Ca(NO3)2

Petr I. Matveev * and Vladimir G. Petrov Department of Chemistry, Lomonosov Moscow State University, Leninskie gory 1, bld. 3, Moscow 119991, Russia; [email protected] * Correspondence: [email protected]

Received: 3 February 2020; Accepted: 14 March 2020; Published: 17 March 2020

Abstract: Three commercially available extractants (tri-butyl-phosphate (TBP), di-(2-ethylhexyl) phosphoric acid (HDEHP) and Aliquat 336 (a mixture of quaternary ammonium bases)) were tested for separation of Pr(III) and Nd(III) in both static and dynamic modes. In the case of HDEHP, phase stability and influence of nitric acid were considered. Extraction isotherms were constructed, and influence of water-soluble complexing agents on the separation factor was investigated for Aliquat 336. In the case of TBP, influence of calcium nitrate in aqueous phase on the extraction efficiency was investigated. Model countercurrent experiments were conducted for TBP and Aliquat 336. It was shown that TBP is the best choice due to its high capacity and cation extraction order (Nd > Pr).

Keywords: rare earth elements; solvent extraction; separation; neodymium; praseodymium; TBP; HDEHP; Aliquat 336

1. Introduction Rare earth elements (REEs) are a group consisting of lanthanides as well as yttrium and scandium. Most part of these elements demonstrate unique physical properties that are used in a wide range of high-tech industries. However, in many cases (e.g., production of luminescent and magnet materials, additives in nuclear fuels, etc.), implementation of an individual pure REE is required. From a chemical point of view, all of these elements possess very close chemical properties: the same oxidation state (+3), the same character of electronic shells ([Xe]4fn5d16s2 for lanthanides) and close values of ionic radii. Separation of rare earth elements is a complex chemical and technological task. One of the preferable ways to separate REEs and to isolate individual elements is with liquid–liquid extraction. This method allows the organization of a continuous separation process and the possibility to deal with high concentration of metals (>100 g L 1)[1]. · − Separation of a REE is complicated by the fact that isolation of a certain individual rare earth element requires its separation from two neighboring elements. For this reason, the processing of a total REE concentrate is a multi-stage procedure where all REEs are divided into groups (e.g., “light” and “heavy” REEs). Then, components of these groups are separated with isolation of pairs of elements. The last step is a separation of an individual element. The whole separation scheme is determined by the content of certain groups of elements in the initial ore, the media used to dissolve the REE ore [2], the availability of suitable equipment and the technological and economic aspects of the REE production [3]. The highest content in the natural REE mixtures have elements of the “light” group La-Nd [2]. Among La-Nd, neodymium has the greatest value as a component of permanent magnets (Nd-Fe-B) [4–6] and lasers [7,8]. Praseodymium based materials have a prospective use in catalysis [9–11]. A group of La-Nd can be separated from a whole mixture of REEs relatively easily due to the absence of promethium in natural REEs. Thus, a gap between Nd and Sm allows

Appl. Sci. 2020, 10, 2032; doi:10.3390/app10062032 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 2032 2 of 12 their easier separation than a pair of neighboring elements. Mutual separation of Ce and Nd can be simplified by oxidation of cerium from +3 to +4 oxidation state [12], so the separation of the neodymium–praseodymium pair is the most difficult part in isolation of pure Nd and pure Pr (for a long time, this pair was considered as one element called didymium [13]). The basis of any extraction system is an extractant, which defines the extraction efficiency (can be estimated as a distribution ratio, D, see Section 2.2.1) and the selectivity (can be estimated as a separation factor, SF, see Section 2.2.1). In the case of metal cations with close chemical properties (close values of D and small values of SF), technological realization of their separation requires a multi-stage countercurrent process. The distribution ratios and separation factors affect the main parameters of the separation process—number of stages, volume ratios of organic and aqueous phases, product yields, etc. [14]. One of the acute research topics is the search for new extractants with high selectivity towards a certain REE. The main drawback of this approach is a high cost of new extractants and their unavailability in amounts sufficient for industrial scale applications. On the other hand, there are many classes of commercially available extractants that are already used in industry. Organophosphorus extractants and quaternary ammonium bases (QABs) are some of the most studied extractants in hydrometallurgy [15]. In particular, in the treatment and separation of f-elements tri-n-butyl-phosphate (TBP) [16–18], soluble in organic phases phosphoric acids [19–22], and mixtures of QABs [23–25] are widely used. Beside the extractant, other important factors are the medium REEs are extracted from and the presence of a salting-out agent. It is a non-extractable salt containing the same anion as the extractable components, the addition of which increases the total content of anions and reduces the activity coefficient of water. This results in a shift of the equilibrium to the formation of the organic soluble complex, i.e., enhances the extraction. Nitrate media is more preferable during ore processing due to high solubility of REE nitrates in aqueous solutions and lower corrosion activity towards materials of the separation equipment (e.g., in comparison with chlorides). The choice of a salting-out agent is largely determined by its availability. Previously, we have shown that calcium nitrate can be used as an effective salting-out agent [18] for the REE separation in the system “100% TBP–nitric acid solutions of REE(NO3)3”. Despite the large volume of experimental results existing at the moment, very often there are no data in literature on specific conditions of separation (concentrations of extractants and other components of extraction systems, temperature, stream flows, etc.), especially in the case of macro-quantities (grams per liter) of REEs. The existing data are insufficient to calculate parameters of the process for separating a pair of elements with a certain composition. In particular, such an object can be a mixture of praseodymium and neodymium with a Pr/Nd ratio = 70:30, which can be formed in technological schemes during the isolation of neodymium [26]. The main systems for the separation of a Pr-Nd pair described in literature are Aliquat 336 in the presence of NH4NO3 as a salting-out agent [27,28]; organophosphorus acids (di-(2-ethylhexyl) phosphoric acid, HDEHP, and 2-ethyl-hexylphosphonic acid mono-2-ethylhexyl ester, EHEHPA) in chloride media [29] or synergetic systems of EHEHPA + 8-hydroxyquinoline [30]. TBP was also used to separate of the “light” group of REEs in the presence of nitric acid as a salting-out agent [31] and to model the extraction of Nd(NO3)3 and Pr(NO3)3 in the presence of NH4NO3 as a salting-out agent [32]. There are also some examples of the application of phosphine oxides for extraction of REEs from nitric acid media [33]. Nevertheless, even with using these systems, separation of the Pr-Nd pair is still a difficult task, and the growing consumption of these metals requires the development of new effective and cheap extraction systems. Therefore, the aim of this work was to apply existing commercially available extractants as a basis for new extraction systems for Pr and Nd separation (in both static and dynamic conditions). We have chosen the following extractants: tri-butyl-phosphate (TBP), di-(2-ethylhexyl) phosphoric acid (HDEHP) and Aliquat 336 (a mixture of quaternary ammonium bases). The choice of these three Appl. Sci. 2020, 10, 2032 3 of 12 extractants is based on their availability and their fundamentally different extraction mechanisms: solvation (TBP), cationic exchange (HDEHP) and anionic exchange (Aliquat 336) [15]. As already mentioned, a large amount of experimental data have been obtained for extraction systems based on these extractants, but in this work, it was important to examine the extraction of a specific mixture with a Pr/Nd ratio of 70:30 and assess how the main affecting factors can influence extraction and separation performance. As the main factors, we have chosen concentration of Ca(NO3)2 as a salting-out agent in the case of TBP and Aliquat 336, concentration of REEs and nitric acid in the case of HDEHP and presence of a water-soluble complexing agents in the case of Aliquat 336.

2. Materials and Methods

2.1. Reagents, Chemicals and Analytical Tools Tri-butyl-phosphate (TBP) (99% purity, Acros, Geel, Belgium), di-(2-ethylhexyl)phosphoric acid (HDEHP) (99% purity, Acros, Geel, Belgium) and a mixture of quaternary ammonium bases (Aliquat 336) (99% purity, Acros, Geel, Belgium) were used as extractants. TBP was used without a solvent, and HDEHP was dissolved in dodecane (99%, Himmed company, Moscow, Russia). A relatively stable emulsion (stratification time was about 30 minutes without application of centrifugation) formed when dodecane was used as a solvent for Aliquat 336. The addition of 10 vol. % 1-octanol (99% purity, Acros, Geel, Belgium) suppressed the formation of emulsion [34]. The formation of the stable emulsion was not observed in the case of use of toluene as a solvent. However, due to the high volatility of toluene, a mixture of dodecane and 1-octanol was used in all the experiments. The commercially available Aliquat 336 is a mixture of quaternary ammonium bases with chloride as a counter-ion. Its use in a nitrate medium requires conversion it to a nitrate form. For this, 0.5 mol L 1 · − solution of Aliquat 336 was contacted with a solution of 1 mol L 1 of calcium nitrate twice. According · − to argentometry [35], the degree of conversion was 99.7% after two contacts. Aqueous solutions of REE(III) were prepared by dissolving nitrates of praseodymium and neodymium with the declared purity of 99.9% in deionized MiliQ water. Calcium nitrate (chemical grade purity, Himmed company, Moscow, Russia) was used as the salting-out agent. Acidity of aqueous solutions was adjusted by nitric acid (chemical grade purity, Himmed company, Moscow, Russia). All reagents were used as received without additional purification. The mixed aqueous solutions of neodymium and praseodymium nitrates with total content of 80–590 g L 1 (as a REE(NO ) ) were prepared by dissolution of rare earth element nitrates in volumetric · − 3 3 flasks with adding of a required amount of a standardized nitric acid solution and a certain mass of calcium nitrate. The final concentration of HNO was 0.3 mol L 1, and the concentration of Ca(NO ) 3 · − 3 2 varied from 0 to 300 g L 1. · − Solutions were analyzed by ICP-OES technique (Agilent ICP-OES 720, Santa Clara, CA, US), but preliminarily were checked by UV–Vis analysis (Shimadzu UV 1800, Kyoto, Japan) for rough estimation of a metal concentration range.

2.2. Solvent Extraction Experiments

2.2.1. Batch Extraction Experiments The organic phase was preliminarily equilibrated with an aqueous phase. The pre-equilibration was performed by contact (three times) with the aqueous phase of the same composition (concentration of nitric acid and calcium nitrate) as the aqueous phase in a subsequent batch or countercurrent experiment, except that the aqueous phase for pre-equilibration did not contain praseodymium and neodymium nitrates. The organic-to-aqueous phase (O/A) volume ratio was 1:1. This was necessary to prevent the extraction of water further in the experiment, which could lead to a change in the volume of the organic phase and the extraction of nitric acid, which could lead to the precipitation of REE hydroxides in the aqueous phase during the experiment due to a decrease of nitric acid concentration. Appl. Sci. 2020, 10, 2032 4 of 12

In the batch extraction experiments, the pre-equilibrated organic phase was contacted three times with a praseodymium and neodymium aqueous solution every time a fresh portion of the aqueous phase was used. Composition of the aqueous phase after the third contact was identical to the initial solution. The O/A volume ratio was 15:15 mL. Then, rare earth elements were back-extracted three times by a pure 0.3 mol L 1 HNO aqueous solution in the case of TBP and Aliquat 336 or by a 3 mol L 1 · − 3 · − HNO3 aqueous solution in the case of HDEHP. Tests with radioactive tracers have shown that such a procedure results in complete back-extraction of REEs. All back-extracts from certain experiment were mixed together. All experiments were carried out in separatory funnels. The distribution ratio, D, for each element was calculated as a ratio of its content in a back-extract (accounting the dilution factor “3”) to its content in the aqueous phase after phase separation (Equation (1)): 3 C(Me) D = · be (1) C(Me)aq where C(Me)be is concentration of a metal cation in the ﬁnal back-extract; C(Me)aq is concentration of a metal cation in the aqueous phase after extraction and “3” is dilution factor. Separation factor was calculated according to Equation (2):

D(Me ) SF = 1 (2) D(Me2) where D(Me1) and D(Me2) are distribution ratios for metal (1) and metal (2). All the experiments were performed three times. Corresponding D values were averaged, and the resulting D value was plotted in graphs along with relative errors.

2.2.2. Countercurrent Extraction Mode Modeling of constant flow countercurrent extraction was performed according to Craig’s method [36] with a ratio of 1:2, i.e., one funnel was equal to two theoretical stages of a cascade process. The experimental scheme is presented in Figure1. Funnels were filled with organic and aqueous solutions and then were shaken. These starting funnels are indicated with superscript “1” on the scheme. After phase separation, an aqueous phase from the 5th funnel (51) was removed (a raffinate, R1) and an aqueous phase from the 4th funnel (41) was transferred to the 5th funnel (52), as well as from the 3rd (31) to the 4th (42), from the 2nd (21) to the 3rd (32) and from the 1st (11) to the 2nd (22). Then, a fresh aqueous solution (F, a feeding solution) was added to the first funnel. All funnels were shaken again. After phase separation, in this second step an organic phase from the 1st funnel (12) was removed (an extract, E1), and an organic phase from the 2nd funnel (22) was transferred to the 1st funnel (13), as well as from the 3rd (32) to the 2nd (23), from the 4th (42) to the 3rd (33) and from the 5th (52) to the 4th (43). Then, a fresh organic phase (S, a solvent) was added to the fifth funnel (53). These operations were repeated until a steady state had been reached, so each aqueous (or organic) phase in one funnel contacted two times with another organic (or aqueous) phase every time contact was with a new phase. Thus, five funnels realized a ten-stage countercurrent cascade. The O/A volume ratio was 15:15 mL. Achievement of the steady state was controlled by analysis of the raffinates (Ri, aqueous outputs) coming from the 5th funnel. It was found for all of the investigated extraction systems that the steady state was reached after passing three cascade volumes; five funnels contained 5 organic and 5 aqueous phases, and at the steady state, we obtained 15 raffinates and 15 extracts. After achievement of the steady state, every odd step of repetition of the “shaking procedure” corresponded to stage numbers 1, 3, 5, 7, 9 and every even step to stage numbers 2, 4, 6, 8, 10. For all countercurrent extraction experiments, we analyzed stage numbers 1, 3, 5, 7, 9. Calculated mass balance was not less than 96%. Appl. Sci. 2020, 10, 2032 5 of 12 Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 12

Figure 1. A schematic diagram of simulating a 10-stage countercurrent cascade with 5 separatory Figure 1. A schematic diagram of simulating a 10-stage countercurrent cascade with 5 separatory funnels: F: feed aqueous solution of the REEs, S: fresh organic phase, E: extract and R: raﬃnate. funnels: F: feed aqueous solution of the REEs, S: fresh organic phase, E: extract and R: raffinate. 3. Results 3. Results 3.1. Batch Extraction 3.1. Batch Extraction 3.1.1. Di-(2-ethylhexyl)phosphoric Acid (HDEHP) 3.1.1. Di-(2-ethylhexyl)phosphoric Acid (HDEHP) (a) Phase(a) StabilityPhase Stability DuringDuring solvent solvent extraction, extraction it, isit is necessary necessary that that onlyonly two liquid liquid phases phases remain remain in the in thesystem. system. It is It is knownknown that inthat the in casethe case of aof cationic a cationic exchanger exchanger classclass of extractants extractants,, there there is isa possibility a possibility of the of third the third phase formation [37,38]. Table 1 presents data on the stability of heterogeneous systems containing 1 phase formation [37,38]. Table1 presents data on the stability of heterogeneous systems containing mol∙L−1 solutions of HDEHP in dodecane and aqueous phases containing different amounts of nitric 1 mol L 1 solutions of HDEHP in dodecane and aqueous phases containing diﬀerent amounts of acid· − and neodymium and praseodymium nitrates. In general, due to the nature of HDEHP as a nitriccationic acid and exchanger neodymium and a and weak praseodymium acid, the phase nitrates.stability of In such general, systems due incr toeases the nature with a ofdecrease HDEHP in as a cationicthe exchangerREE content and in the a weaksolution acid, and the with phase an increase stability in acidity of such that systems resultsincreases in lower concentration with a decrease of in the REEREE content–HDEHP in complexes the solution. The and instability with an of increase such systems in acidity is associated that results either in with lower the concentrationformation of of REE–HDEHPthe third phase complexes. (see Figure The S1 instability), or with the of suchformation systems of a ishomogeneous associated gel. either Gel with formation the formation usually of the thirdoccurred phase at (see high Figure REE levels S1), or in withthe solution. the formation The third of phase a homogeneous is a precipitate gel. of Gel REE formation organic salts usually occurredinsoluble at high in the REE organic levels phase. in the solution. The third phase is a precipitate of REE organic salts insoluble in the organic phase. Table 1. Third phase formation in systems containing nitric acid and didymium. Organic phase—1 mol∙L−1 of di-(2-ethylhexyl)phosphoric acid (HDEHP) in dodecane. Table 1. Third phase formation in systems containing nitric acid and didymium. Organic phase—1 mol L 1 of di-(2-ethylhexyl)phosphoricC(Pr + Nd), g∙L−1 acid (HDEHP) in dodecane. · − 10 25 50 75 С(HNO3), mol∙L−1 C(Pr + Nd), g L 1 · 0.005− 10stable Third 25 phase Gel 50Gel 75 1 C(HNO3), mol L0.03 stable stable Third phase Gel · − 0.07 stable stable stable Gel 0.005 stable Third phase Gel Gel 0.1 stable stable stable Gel 0.03 stable stable Third phase Gel 0.07 stable stable stable Gel 0.1 stable stable stable Gel Appl. Sci. 2020, 10, 2032 6 of 12

Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 12 Based on the data on phase stability, we selected the extraction system with the maximum content of REE—50 g L 1 and 0.1 mol L 1 nitric acid—for further research. Based on· − the data on phase· − stability, we selected the extraction system with the maximum (b)content Inﬂuence of REE of Nitric—50 g∙L Acid−1 and Concentration 0.1 mol∙L−1 nitric acid—for further research. (b) Influence of Nitric Acid Concentration In the following experiments, initial total content of Nd and Pr was 50 g L 1, and nitric acid In the following experiments, initial total content of Nd and Pr was 50 · g∙L− −1, and nitric acid concentration was varied from 0.1 to 1 mol L 1. As expected (due to cationic nature of extractant), with concentration was varied from 0.1 to 1 mol∙L· − −1. As expected (due to cationic nature of extractant), an increase in the content of nitric acid in the aqueous phase, both the extraction eﬃciency and the with an increase in the content of nitric acid in the aqueous phase, both the extraction efficiency and selectivity of this extractant towards Nd(III) in the presence of Pr(III) decreases (Figure2). The maximal the selectivity of this extractant towards Nd(III) in the presence of Pr(III) decreases (Figure 2). The SF-value in a concentration range of 0.1–1 mol L 1 of nitric acid is 1.3. The organic phase capacity was maximal SF-value in a concentration range ·of− 0.1–1 mol∙L−1 of nitric acid is 1.3. The organic phase 20 g L 1 (0.1 mol L 1 of nitric acid) and 5 g L 1 for 1 mol L 1 of nitric acid. capacity· − was 20 ·g∙L− −1 (0.1 mol∙L−1 of nitric ·acid)− and 5 g∙L· −1− for 1 mol∙L−1 of nitric acid.

1.4

1.2

1.0

SF(Nd/Pr) 0.8 D(Nd)

D,SF D(Pr) 0.6

0.4

0.2

0.0 0.0 0.2 0.4 0.6 0.8 1.0 + [H ], mol/L Figure 2. Distribution ratios and separation factors for Nd(III) and Pr(III) in the system Figure 2.1 Distribution ratios and separation1 factors for Nd(III) and Pr(III) in the system “1 “1 mol L− HDEHP in dodecane—50 g L− REE(NO3)3 + HNO3”. mol∙L·−1HDEHP in dodecane—50 g∙L−1· REE(NO3)3 + HNO3”. Thus, this extraction system is not appropriate for the separation of Nd(III) and Pr(III) due to the low selectivityThus, this and extraction high sensitivity system is tonot the appropriate acidity of thefor aqueousthe separation phase. of Nd(III) and Pr(III) due to the low selectivity and high sensitivity to the acidity of the aqueous phase. 3.1.2. Aliquat 336 3.1.2. Aliquat 336 Earlier, it was shown that Aliquat 336 has the highest separation factor (the highest selectivity) for Pr/NdEarlier pair among, it was other shown extractants that Aliquat (SF(Pr 336/Nd) has the2) [27highest]. However, separation it should factor be (the mentioned highest thatselectivity) QABs ≈ preferentiallyfor Pr/Nd pair extract among lighter other elements, extractants which (SF(Pr/Nd) makes them≈ 2) [27] diff. erentHowever, compared it should to organophosphorus be mentioned that extractantsQABs preferentially (TBP and HDEHP). extract On lighter the other elements hand,, manywhich aqueousmakes soluble them polyaminocarboxylatesdifferent compared to suchorganophosphorus as diethylenetriaminepentaacetic extractants (TBP acid and(DTPA) HDEHP). and On ethylenediaminetetraacetic the other hand, many acid aqueous (EDTA) soluble bind “heavy”polyaminocarboxylates REEs (Tb-Lu) more such efficiently as than diethylenetriaminepentaacetic “light” REEs (La-Gd) [39], so addition acid of such(DTPA) acids as and a complexingethylenediaminetetraacetic agents can increase acid the (EDTA)SF(Pr/ Nd) bind values. “heavy” REEs (Tb-Lu) more efficiently than “light” REEs (La-Gd) [39], so addition of such acids as a complexing agents can increase the SF(Pr/Nd) (a) Extraction Isotherms for REEs by Aliquat 336 in The Presence of Ca(NO ) values. 3 2 1 (a) AnExtraction extraction Isotherms isotherm for was REEs constructed by Aliquat for 336 the in system The P “0.5resence mol Lof− Ca(NOAliquat3)2 336 in dodecane/10% 1 · −1 vol. 1-octanol—Pr(NOAn extraction isotherm3)3 and was Nd(NO constructed3)3 + Ca(NO for the3)2 system+ 0.3 mol “0.5L mol∙L− HNO Aliquat3 ” (Figure 3363 in). Itdodecane/10% is seen that · 1 thevol. capacity 1-octanol of— thePr(NO organic3)3 and phase Nd(NO is relatively3)3 + Ca(NO low and3)2 + is0.3 7 mol∙L10 g −1L HNO, which3 ” (Figure is consistent 3). It is with seen the that data the − · − obtainedcapacity inof thethe systemorganic containing phase is relatively ammonium low nitrate and is as 7 a−1 salting-out0 g∙L−1, which agent is [consistent27]. With anwith increase the data in theobtained content in of the the system salting-out containin agentg inammonium the aqueous nitrate phase, as thea salting shape-out of the agent isotherms [27]. With becomes an increase steeper; in thethe saturation content of ofthe the salting organic-out phase agent isin achievedthe aqueous with phase, smaller the amounts shape of ofthe REEs isotherms in the becomes aqueous steeper phase.; Alongthe saturation with this of trend, the organic the separation phase factoris achieved of neodymium with smaller and amounts praseodymium of REEs falls in down.the aqueous The highest phase. valueAlong of with the separation this trend, factor the was separation found atfactor a calcium of neodymium nitrate concentration and praseodymium of 1 mol L 1 fallsand has down. a value The · − highest value of the separation factor was found at a calcium nitrate concentration of 1 mol∙L−1 and has a value of 2.0 (in the case of 2,5 mol·L−1 Ca(NO3)2 SF(Pr/Nd) ≈ 1.2), which is the same as the Appl. Sci. 2020, 10, 2032 7 of 12 of 2.0Appl. (in theSci. 20 case20, 10 of, x 2,5FORmol PEERL REVIEW1 Ca(NO ) SF(Pr/Nd) 1.2), which is the same as the maximum7 of 12 value · − 3 2 ≈ of SF(Pr/Nd) published in literature [27]. Further decrease of the salting-out agent content leads to a maximum value of SF(Pr/Nd) published in literature [27]. Further decrease of the salting-out agent dramatic decrease in the efficiency of REE extraction (D values are less than 0.1). content leads to a dramatic decrease in the efficiency of REE extraction (D values are less than 0.1).

1 FigureFigure 3. Extraction 3. Extraction isotherms isotherms for for the system “0.5 “0.5 mol∙L mol−1L Aliquat− Aliquat 336 in 336 dodecane/10% in dodecane vol./10% 1- vol. · 1 1-octanol—Pr(NO ) and Nd(NO ) + Ca(NO ) + 0.3 mol−1 L HNO ”. octanol—Pr(NO3 33)3 and Nd(NO33)33 + Ca(NO3)23 + 20.3 mol∙L HNO· − 3”. 3

(b) Eﬀ(b)ect ofEffect Complexing of Complexing Agents A ongents The on Extraction The Extraction of REEs of byREE Aliquats by Aliquat 336 in 336 The in Presence The Presence of Ca(NO of 3)2 and NaNOCa(NO3 3)2 and NaNO3 As indicated above, in order to achieve higher SF(Pr/Nd)s, additional experiments were As indicated above, in order to achieve higher SF(Pr/Nd)s, additional experiments were performed performed to determine the effect of the water-soluble complexing agents (EDTA and DTPA). Initial to determineconcentration the eﬀ ofect the of theREE water-soluble mixture was complexing 0.01 mol∙L−1, agentsand initial (EDTA pH and of the DTPA). solution Initial was concentration 2.0–2.5. of the REE mixture was 0.01 mol L 1, and initial pH of the solution was 2.0–2.5. Addition of calcium Addition of calcium nitrate· − as a salting-out agent resulted in precipitation of calcium nitratepolyaminocarboxylates as a salting-out agent. Thus, resulted calcium in precipitation nitrate was replaced of calcium with polyaminocarboxylates. sodium nitrate. The obtained Thus, data calcium nitrateare was presented replaced in Table with sodium2. nitrate. The obtained data are presented in Table2.

Table 2. Distribution ratios and separation factors for the system “0.5 mol∙L−1 Aliquat1 336 in Table 2. Distribution ratios and separation factors for the system “0.5 mol L− Aliquat 336 in dodecane/10% vol. 1-octanol—Pr(NO3)3, Nd(NO3)3 + 1 mol∙L−1 1NaNO3 + polyaminocarboxylate”.· dodecane/10% vol. 1-octanol—Pr(NO3)3, Nd(NO3)3 + 1 mol L− NaNO3 + polyaminocarboxylate”. C(REE) = 0.01 mol∙L1 −1, initial pH = 2.0–2.5. “Na2EDTA or Na2·DTPA”, %: percentage relative to REE C(REE) = 0.01 mol L− , initial pH = 2.0–2.5. “Na2EDTA or Na2DTPA”, %: percentage relative to concentration. · REE concentration. Na2EDTA, % 20 40 60 80 100 Na2DEDTA,(Nd) %0.07 20 ± 0.01 400.06 ± 0.005 600.03 ± 0.002 0.02 80 ± 0.002 0.01 100 ± 0.001 DD(Nd)(Pr) 0.070.15 ±0.01 0.02 0.06 0.140.005 ± 0.005 0.03 0.080.002 ± 0.005 0.020.040.002 ± 0.002 0.010.016 0.001± 0.002 SF(Pr/Nd) 1.8± ± 0.3 ± 2.3 ± 0.2 ± 2.6 ± 0.2 ±2.1 ± 0.2 1.6± ± 0.2 D(Pr) 0.15 0.02 0.14 0.005 0.08 0.005 0.04 0.002 0.016 0.002 SFmax ± ± 2.6± ± 0.2 ± ± NaSF2DTPA,(Pr/Nd) % 1.8 200.3 2.3 0.240 2.6 0.260 2.1 0.280 1.6 1000.2 D(Nd) 0.08± ± 0.01 ±0.05 ± 0.005 ±0.02 ± 0.002 0.014± ± 0.003 0.011± ± 0.002 SFmax 2.6 0.2 D(Pr) 0.16 ± 0.01 0.14 ± 0.01 ±0.07 ± 0.005 0.040 ± 0.006 0.019 ± 0.005 NaSF2DTPA,(Pr/Nd) %2.1 20 ± 0.2 402.7 ± 0.2 603.6 ± 0.3 2.9 80 ± 0.3 1.8 100 ± 0.2 DSF(Nd)max 0.08 0.01 0.05 0.005 0.023.6 ±0.002 0.3 0.014 0.003 0.011 0.002 From the data in Table± 2, it can be ±stated that the presence± of water-soluble± complexing± agents D(Pr) 0.16 0.01 0.14 0.01 0.07 0.005 0.040 0.006 0.019 0.005 can actually increase the± separation factor± for the Pr-Nd± pair from 2 to ±2.6 (EDTA) and ±up to 3.6 (DTPA).SF(Pr/ Nd)However, under 2.1 0.2 experimental 2.7 conditions,0.2 the 3.6 maximum0.3 values 2.9 of0.3 the separation 1.8 0.2factors ± ± ± ± ± SFmax 3.6 0.3 ± Appl. Sci. 2020, 10, 2032 8 of 12

From the data in Table2, it can be stated that the presence of water-soluble complexing agents can actually increase the separation factor for the Pr-Nd pair from 2 to 2.6 (EDTA) and up to 3.6 (DTPA). However,Appl. Sci. 20 under20, 10, x experimental FOR PEER REVIEW conditions, the maximum values of the separation factors were reached8 of 12 at low values of distribution ratios (<0.1), which will require the use of high (>10) O/A volume ratios, were reached at low values of distribution ratios (<0.1), which will require the use of high (>10) O/A which will reduce the overall eﬃciency of the separation industrial process. volume ratios, which will reduce the overall efficiency of the separation industrial process. 3.1.3. Tri-N-Butyl-Phosphate (TBP) 3.1.3. Tri-N-Butyl-Phosphate (TBP) 1 It is well known that tri-n-butyl phosphate has a high capacity (over 100 g L− ) towards REEs. · −1 Use ofIt TBP is well without known the diluentthat tri- cann-butyl achieve phosphate the maximum has a possiblehigh capacity capacity. (over The 100 distribution g∙L ) towards ratios wereREEs. Use of TBP without the diluent can achieve the maximum possible capacity. The distribution ratios1 determined for the system “Pure TBP—neodymium + praseodymium + calcium nitrates + 0.3 mol L− were determined for the system “Pure TBP1 —neodymium + praseodymium + calcium nitrates ·+ 0.3 HNO3”. The total REE content was 300 g L− . An increase in the concentration of the salting-out agent −1 · −1 inmol∙L the aqueous HNO3 phase”. The leads total toREE an increasecontent inwas both 300 the g∙L distribution. An increase ratios in and the separationconcentration factors. of the The salting most - out agent in the aqueous phase leads to an increase in both the distribution1 ratios and separation intensive increase of these parameters is observed in a range of 0–300 g L− in the concentration of factors. The most intensive increase of these parameters is observed in ·a range of 0–300 g∙L−1 in the the salting-out agent (SF(Nd/Pr) = 1.4–1.5) with a slow growth at its higher concentration (Figure4). concentration of the salting-out agent (SF(Nd/Pr) = 1.4–1.5) with a slow growth at its higher There was also an increase in the viscosity of aqueous solutions, which decreased the eﬃciency of the concentration (Figure 4). There was also an increase in the viscosity of aqueous solutions, which whole process. Therefore, further extraction experiments were carried out with the content of calcium decreased the efficiency1 of the whole process. Therefore, further extraction experiments were carried nitrate of 300 g L− . out with the content· of calcium nitrate of 300 g∙L−1.

1.6

1.4

1.2

1.0 SF(Nd/Pr) D(Pr) D(Nd)

0.8 D,SF

0.6

0.4

0.2

0.0 0 100 200 300 400 500 c(Ca(NO ) ), g/L 3 2 Figure 4. Dependence of distributions ratios and separation factors for Nd and Pr in the system: “Pure Figure 4. Dependence of distributions ratios and separation factors1 for Nd and Pr in the system: “Pure TBP—neodymium + praseodymium + calcium nitrate + 0.3 mol L− HNO3”. TBP—neodymium + praseodymium + calcium nitrate + 0.3 mol∙L· −1 HNO3”. 3.2. Countercurrent Experiments 3.2. Countercurrent Experiments Since the process of the separation of REEs is performed in a multistage countercurrent process, the distributionSince the ratios process should of bethe established separation inof twoREE modes—batchs is performed extractionin a multistage and countercurrent.countercurrent Forprocess the , modelingthe distribution of a countercurrent ratios should mode,be established we choose in two two modes extraction—batch systems: extraction “Pure and TBP countercurrent - neodymium. For+ the modeling of a countercurrent mode, we choose1 two extraction systems:1 “Pure TBP - neodymium praseodymium + calcium nitrates + 0.3 mol L− HNO3” and ”0.5 mol L− Aliquat 336 in dodecane/10% + praseodymium + calcium nitrates +· 0.3 mol∙L−1 HNO3” and· ”0.5 mol∙L1 −1 Aliquat 336 in vol. 1-octanol—neodymium + praseodymium + calcium nitrates + 0.3 mol L− HNO3”. dodecane/10% vol. 1-octanol—neodymium + praseodymium + calcium nitrates· + 0.3 mol∙L−1 HNO3”. 3.2.1. Countercurrent Experiment with Aliquat 336 3.2.1. Countercurrent Experiment with Aliquat 336 The simulated process consisted of 10 stages. Due to the low capacity of this extractant, the experimentThe simulated was performed process at consisted an O/A volume of 10 stages. ratio ofDue 2:1. to The the totallow capacity content of of REEs this extractant, in the initial the solutionexperiment was was 35 g Lperformed1 with a ratioat an of O/A 70:30 volume (Pr/Nd), ratio and of the 2:1. content The total of calcium content nitrate of REE wass in 1 the mol initialL 1. · − · − Thesolution distribution was 35ratios g∙L−1 with were a the ratio same of for70:30 all (Pr/Nd), stages in and the system:the contentD(Pr) of =calcium0.24 and nitrateD(Nd) was= 0.12.1 mol∙L In a−1. The distribution ratios were the same for all stages in the system: D(Pr) = 0.24 and D(Nd) = 0.12. In a steady state of the dynamic mode (a stationary regime), the raffinate contained 60:40 (Pr/Nd) with a total content of 24 g∙L−1 REE(NO3)3, and the extract contained 5 g∙L−1 (82% Pr and 18% Nd).

Appl. Sci. 2020, 10, 2032 9 of 12 steady state of the dynamic mode (a stationary regime), the raﬃnate contained 60:40 (Pr/Nd) with a total content of 24 g L 1 REE(NO ) , and the extract contained 5 g L 1 (82% Pr and 18% Nd). · − 3 3 · − 3.2.2. CountercurrentAppl. Sci. 2020, 10, x FOR Experiment PEER REVIEW with TBP 9 of 12 3.2.2. Countercurrent Experiment with TBP Appl.The Sci. experiment 2020, 10, x FOR was PEER performed REVIEW in a countercurrent mode on a ten-stage cascade. As a9 result,of 12 the total content of didymium in the raﬃnate was 90 g L 1 (initial solution: 300 g L 1 70% Pr and 30% Nd) The experiment was performed in a countercurrent· − mode on a ten-stage cascade.· − As a result, the with3.2.2. praseodymiumtotal Countercurrent content of didymium content Experiment of in 90%. the with raffinate Figure TBP 5was represents 90 g∙L−1 (initial compositions solution: 300 of g∙L the−1 aqueous 70% Pr and phases 30% Nd in) the cascadewith inThe aprase steadyexperimentodymium state. was cont performedent of 90%. in aFigure countercurrent 5 represents mode compositions on a ten-stage of the cascade. aqueous As aphases result, in the the totalcascade content in aof steady didymium state .in the raffinate was 90 g∙L−1 (initial solution: 300 g∙L−1 70% Pr and 30% Nd) with praseodymium cont220ent of 90%. Figure 5 represents compositions of the aqueous phases in the cascade in a steady state.200 220180

200160 180140 Pr 160120 Nd 100 140 Pr 120g/L C(REE), 80 Nd 100 60

C(REE), g/L C(REE), 80 40

60 20

40 0 2 4 6 8 10 20 Stage number 0 2 4 6 8 10 FigureFigure 5. Concentration 5. Concentration profiles profiles for for the the aqueous aqueous phases phases inin aa steadysteady state. O/A O/A volume volume ratio ratio = 1:1=. 1:1. Stage number OrganicOrganic phase—100% phase—100% TBP. TBP. Figure 5. Concentration profiles for the aqueous phases in a steady state. O/A volume ratio = 1:1. In orderOrganicIn order to phase determine to —determine100% theTBP .the possibility possibility to to improve improve the the separationseparation of of Pr/Nd Pr/Nd by by increasing increasing the theO/A O /A volumevolume ratio, countercurrentratio, countercurrent experiments experiments were also were performed also performed at ratios at of ratios1.5 and of 1.8. 1.5 The and concentration 1.8. The concentration profiles of the stages obtained in this case are shown in Figure 6. One can see that at profiles ofIn the order stages to determine obtained the in thispossibility case are to shownimprove in the Figure separation6. One of can Pr/Nd see thatby increasing at the ratio the ofO/A 1.5, the volumethe ratio ratio, of 1.5, countercurrent the total content experiments of didymium were in alsothe raffinate performed significantly at ratios decreases of 1.5 and (from1 1.8. 90 Theto 75 total content of didymium in the raffinate significantly decreases (from 90 to 75 g L− ). In this case, concentrationg∙L−1). In this profiles case, there of theis no stages significant obtained change in thi ins the case ratio are of shown neodymium in Figure and 6 .praseodymium. One can· see that When at there is no significant change in the ratio of neodymium and praseodymium. When the volume ratio is thethe ratio volume of 1.5, ratio the istotal even content higher of (1.8) didymium, the metals in theare completelyraffinate significantly extracted into decreases the organic (from phase 90 to— 75the even higher (1.8), the metals are completely extracted into the organic phase—the aqueous phases of g∙Laqueous−1). In this phases case, thereof stages is no 1 significant–3 contain changeREEs at in concentrations the ratio of neodymium close to zero and. The praseodymium. total REEs content When in stagesthethe 1–3 volume aqueous contain ratio phase REEs is even of at stage concentrationshigher 4 is(1.8) 2.5, g∙Lthe−1 closemetals with topraseodymium are zero. completely The total contentextracted REEs of content into 95%, the which inorganic the is aqueousslightly phase— higher phasethe of 1 stageaqueous 4than is 2.5 for phases g theL− volume withof stagespraseodymium ratio 1– of3 contain1.5, but REEsthis content separation at concentrations of 95%, still whichcannot close isbe toslightly assumed zero. The higher as total sufficient. thanREEs for content the volumein 200 · 200 ratiothe of 1.5,aqueous but thisphase separation of stage 4 is still 2.5 cannot g∙L−1 with be praseodymium assumed as su contentfficient. of 95%, which is slightly higher 180 180 than for the volume ratio of 1.5, but this separation still cannot be assumed as sufficient. 160 160 200 200 140 140 180 180 Pr 120 120 Pr 160 Nd 160 Nd 100 100 140 140

80 Pr 80 Pr C(REE), g/L C(REE), 120 120g/L C(REE), Nd 60 60 Nd 100 100 40 40

80 80

C(REE), g/L C(REE), C(REE), g/L C(REE), 20 20 60 60 0 0 40 40 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 20 Stage number 20 Stage number 0 0 1 2 3 4 5(a) 6 7 8 9 10 1 2 3 4 5(b) 6 7 8 9 10 Stage number Stage number Figure 6. Concentration profiles for the aqueous phases in a steady state. O/A volume ratios = 1.5:1; (a) 1.8:1, (b) organic(a phase) —100% TBP. (b) FigureFigure 6. Concentration 6. Concentration proﬁles profiles for for the the aqueousaqueous phases phases in in a asteady steady state. state. O/A O volume/A volume ratios ratios = 1.5:=1; 1.5:1; 4. Discussion (a) 1.8:1,(a) 1.8:1 (b), organic(b) organic phase—100% phase—100% TBP. TBP. Considering the extraction properties of the three studied extractants, we can conclude that 4. HDEHPDiscussion has the lowest selectivity, while Aliquat 336 has the highest value of the separation factor Considering the extraction properties of the three studied extractants, we can conclude that HDEHP has the lowest selectivity, while Aliquat 336 has the highest value of the separation factor Appl. Sci. 2020, 10, 2032 10 of 12

4. Discussion Considering the extraction properties of the three studied extractants, we can conclude that HDEHP has the lowest selectivity, while Aliquat 336 has the highest value of the separation factor for a single (static) mode of extraction, but in a countercurrent mode, an eﬃcient use of Aliquat 336 is possible only at high (5:1) volume ratios of organic and aqueous phases. Tri-n-butyl phosphate appears to be the most promising among these extractants as a basis of the new extraction system containing calcium nitrate. This new system combines moderate selectivity with a high capacity of the organic phase towards rare earth elements. An extraction order, i.e., an order of metals with the highest to the lowest values of distribution ratios, is also important in the case of TBP. Neodymium, the content of which is much smaller, is extracted better than praseodymium making it possible to obtain praseodymium as a product in raﬃnate. This process has a smaller number of extraction stages and has no need for the scrubbing stage, which is not the case for Aliquat 336.

Supplementary Materials: The following are available online at http://www.mdpi.com/2076-3417/10/6/2032/s1. Figure S1, XRF spectra for third phase obtained after extraction of 50 g/L (Pr + Nd) from 0.005 mol/L nitric acid. Author Contributions: Extraction of experiments and original draft preparation, P.I.M.; conceptualization, review and editing, V.G.P. All authors have read and agreed to the published version of the manuscript. Funding: The reported study was funded by RFBR according to the research project, grant number № 18-33-0616/мoл_a. Acknowledgments: This work was supported in part by M.V. Lomonosov Moscow State University Program of Development. Conﬂicts of Interest: The authors declare no conﬂicts of interest.

References

1. Gupta, C.K.K.; Krishnamurthy, N. Extractive Metallurgy of Rare Earths; CRC Press: Boca Raton, FL, USA, 2005; ISBN 0-415-33340-7. 2. Jha, M.K.; Kumari, A.; Panda, R.; Rajesh Kumar, J.; Yoo, K.; Lee, J.Y. Review on hydrometallurgical recovery of rare earth metals. Hydrometallurgy 2016, 165, 2–26. [CrossRef] 3. McLellan, B.C.; Corder, G.D.; Ali, S.H. Sustainability of rare earths—An overview of the state of knowledge. Minerals 2013, 3, 304–317. [CrossRef] 4. Herbst, J.F.; Croat, J.J.; Pinkerton, F.E.; Yelon, W.B. Relationships between crystal structure and magnetic

properties in Nd2Fe14B. Phys. Rev. B 1984, 29, 4176. [CrossRef] 5. Gergoric, M.; Ravaux, C.; Steenari, B.M.; Espegren, F.; Retegan, T. Leaching and recovery of rare-earth elements from neodymium magnet waste using organic acids. Metals 2018, 8, 721. [CrossRef] 6. Marx, J.; Schreiber, A.; Zapp, P.; Walachowicz, F. Comparative life cycle assessment of NdFeB permanent magnet production from different rare earth deposits. ACS Sustain. Chem. Eng. 2018, 6, 5858–5867. [CrossRef] 7. Mohammed, M.K.; Umer, U.; Abdulhameed, O.; Alkhalefah, H. Effects of laser fluence and pulse overlap on machining of microchannels in alumina ceramics using an Nd: YAG laser. Appl. Sci. 2019, 9, 3962. [CrossRef] 8. Gui, K.; Zhang, Z.; Xing, Y.; Zhang, H.; Zhao, C. Frequency difference thermally and electrically tunable

dual-frequency Nd:YAG/LiTaO3 microchip laser. Appl. Sci. 2019, 9, 1969. [CrossRef] 9. Höcker, J.; Krisponeit, J.O.; Cambeis, J.; Zakharov, A.; Niu, Y.; Wei, G.; Colombi Ciacchi, L.; Falta, J.; Schaefer, A.; Flege, J.I. Growth and structure of ultrathin praseodymium oxide layers on ruthenium(0001). Phys. Chem. Chem. Phys. 2017, 19, 3480–3485. [CrossRef][PubMed] 10. Vaiano, V.; Matarangolo, M.; Sacco, O.; Sannino, D. Photocatalytic treatment of aqueous solutions at high dye concentration using praseodymium-doped ZnO catalysts. Appl. Catal. B Environ. 2017, 209, 621–630. [CrossRef] 11. Zinatloo-Ajabshir, S.; Salavati-Niasari, M.; Zinatloo-Ajabshir, Z. Facile size-controlled preparation of highly photocatalytically active praseodymium zirconate nanostructures for degradation and removal of organic pollutants. Sep. Purif. Technol. 2017, 177, 110–120. [CrossRef] 12. Rabie, K.A. A group separation and puriﬁcation of Sm, Eu and Gd from Egyptian beach monazite mineral using solvent extraction. Hydrometallurgy 2007, 85, 81–86. [CrossRef] Appl. Sci. 2020, 10, 2032 11 of 12

13. Dingle, A. Praseodymium unpaired. Nat. Chem. 2018, 10, 576. [CrossRef][PubMed] 14. Xie, F.; Zhang, T.A.; Dreisinger, D.; Doyle, F. A critical review on solvent extraction of rare earths from aqueous solutions. Miner. Eng. 2014, 56, 10–28. [CrossRef] 15. Wilson, A.M.; Bailey, P.J.; Tasker, P.A.; Turkington, J.R.; Grant, R.A.; Love, J.B. Solvent extraction: The coordination chemistry behind extractive metallurgy. Chem. Soc. Rev. 2014, 43, 123–134. [CrossRef][PubMed] 16. Jorjani, E.; Shahbazi, M. The production of rare earth elements group via tributyl phosphate extraction and precipitation stripping using oxalic acid. Arab. J. Chem. 2016, 9, S1532–S1539. [CrossRef] 17. Schulz, W.W.; Navratil, J.D.; Bess, T. Science and Technology of Tributyl Phosphate; CRC Press: Boca Raton, FL, USA, 1987. 18. Matveev, P.I.; Petrov, V.G.; Egorova, B.V.; Senik, N.N.; Semenkova, A.S.; Dubovaya, O.V.; Valkov, A.V.; Kalmykov, S.N. Solvent extraction of rare earth elements by tri-n-butyl phosphate and tri-iso-amyl phosphate

in the presence of Ca(NO3)2. Hydrometallurgy 2018, 175, 218–223. [CrossRef] 19. Scal, M.L.W.; Seruﬀ, L.A.; Vera, Y.M. Study of the separation of didymium from lanthanum using liquid-liquid extraction: Comparison between saponiﬁcation of the extractant and use of lactic acid. Miner. Eng. 2020, 148, 106200. [CrossRef] 20. Santos, G.; Vera, Y.M. Estudio de la extracción de tierras raras ligeras a partir de la extracción líquido – líquido utilizando ácidos organofosforados y ácido ascórbico. Rev. Metal. 2019, 55, 142. [CrossRef] 21. Basualto, C.; Valenzuela, F.; Molina, L.; Muñoz, J.P.; Sapag, J. Study of the solvent extraction of the lighter lanthanide metal ions by means of organophosphorus extractants. J. Chil. Chem. Soc. 2013, 2, 1785–1789. [CrossRef] 22. Panda, N.; Devi, N.; Mishra, S. Solvent extraction of neodymium(III) from acidic nitrate medium using Cyanex 921 in kerosene. J. Rare Earths 2012, 30, 794–797. [CrossRef] 23. Li, Z.; Li, X.; Raiguel, S.; Binnemans, K. Separation of transition metals from rare earths by non-aqueous solvent extraction from ethylene glycol solutions using Aliquat 336. Sep. Purif. Technol. 2018, 201, 318–326. [CrossRef] 24. Padhan, E.; Sarangi, K. Recovery of Nd and Pr from NdFeB magnet leachates with bi-functional ionic liquids based on Aliquat 336 and Cyanex 272. Hydrometallurgy 2017, 167, 134–140. [CrossRef] 25. Abreu, R.D.; Morais, C.A. Study on separation of heavy rare earth elements by solvent extraction with organophosphorus acids and amine reagents. Miner. Eng. 2014, 61, 82–87. [CrossRef]

26. Igumnov, S.N.; Valkov, A.V. Separation of rare earth elements in the tributyl phosphate–Ln(NO3)3–Ca(NO3)2 system in the counter current process. Mosc. Univ. Chem. Bull. 2017, 72, 115–119. [CrossRef] 27. Preston, J.S. The recovery of rare earth oxides from a phosphoric acid byproduct. Part 4. The preparation of magnet-grade neodymium oxide from the light rare earth fraction. Hydrometallurgy 1996, 42, 151–167. [CrossRef] 28. Lu, D.; Horng, J.S.; Hoh, Y.C. The separation of neodymium by quaternary amine from didymium nitrate solution. J. Less-Common Metals 1989, 149, 219–224. [CrossRef] 29. Reddy, M.L.P.; Prasada Rao, T.; Damodaran, A.D. Liquid-liquid extraction processes for the separation and puriﬁcation of rare earths. Miner. Process. Extr. Metall. Rev. 1993, 12, 91–113. [CrossRef] 30. Wu, D.; Zhang, Q.; Bao, B. Synergistic eﬀects in extraction and separation of praseodymium(III) and neodymium(III) with 8-hydroxyquinoline in the presence of 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester. Ind. Eng. Chem. Res. 2007, 46, 6320–6325. [CrossRef] 31. Thakur, N.V. Separation of rare earths by solvent extraction. Miner. Process. Extr. Metall. Rev. 2000, 21, 277–306. [CrossRef] 32. Mokili, B.; Poitrenaud, C. Modelling of the extraction of neodymium and praseodymium nitrates from aqueous solutions containing a salting-out agent or nitric acid by tri-n-butylphosphate. Solvent Extr. Ion Exch. 1996, 14, 617–634. [CrossRef] 33. Panda, N.; Mishra, S. Binary mixture of Cyanex 921 and Cyanex 923 as extractant for praseodymium (III) and neodymium (III). J. Chem. Technol. Metall. 2017, 52, 113–125. 34. de Fábrega, F.M.; Mansur, M.B. Liquid-liquid extraction of mercury (II) from hydrochloric acid solutions by Aliquat 336. Hydrometallurgy 2007, 87, 83–90. [CrossRef] 35. Bae, H.S.; Hwang, H.; Eom, I.Y. Argentometric titration apparatus with a light emitting diode-based nephelometric detection system. Bull. Korean Chem. Soc. 2015, 36, 2725–2729. [CrossRef] Appl. Sci. 2020, 10, 2032 12 of 12

36. Craig, L.C.; Hausmann, W.; Ahrens, E.H.; Harfenist, E.J. Automatic countercurrent distribution equipment. Anal. Chem. 1951, 23, 1236–1244. [CrossRef] 37. Melnik, M.I.; Spiryakov, V.I.; Filimonov, V.T.; Karelin, E.A. Preparation of lanthanide (III) neutral compound

ligands in the Ln(CH3COO)3–HDEHP–decane system and study of their solubility in HDEHP–decane solutions. J. Alloys Compd. 1998, 277, 863–867. [CrossRef] 38. Peppard, F.; Mason, G.W.; Maier, J.L. Fractional extraction of the lanthanides as their di-alkyl orthophosphates. J. Nucl. Inorg. Chem. 1957, 4, 334–343. [CrossRef] 39. Nash, K.L. The chemistry of TALSPEAK: A review of the science. Solvent Extr. Ion Exch. 2015, 33, 37–41. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).