phosphoric Acid, Water, Nitric Acid - American Chemical Society

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Liquid−Liquid Equilibria in Multicomponent Systems Containing o‑Xylene, Di-(2-ethylhexyl)phosphoric Acid, Water, Nitric Acid, and Europium (Gadolinium, Dysprosium) Nitrate at 298.15 K Svetlana V. Kurdakova,*,†,‡ Nikita A. Kovalenko,‡ Vladimir G. Petrov,‡ and Irina A. Uspenskaya‡ †

Department of Materials Science, and ‡Department of Chemistry, Lomonosov Moscow State University, 119991, Leninskie gory, 1 building 3, Moscow, Russia

ABSTRACT: The extraction of europium(III), dysprosium(III), and gadolinium(III) nitrates with di-(2-ethylhexyl) phosphoric acid (D2EHPA) in o-xylene was investigated at 298.15 K. Spectrophotometry and ICP-AES techniques were used for rare earth elements (REE) content determination. The content of nitric acid was measured by acid−base titration. The density of the equilibrium phases was determined. In the case of high content of rare earth element nitrates in the initial aqueous solutions, the third solid phase formation was observed during extraction experiments.

1. INTRODUCTION Liquid−liquid extraction is one of the most popular methods for separation and purification of rare earth elements (REEs). A list of the most widely used extractants for transferring REEs from aqueous solutions to the organic phase is given in the review by Xie et al.1 The organic derivatives of phosphoric acid are highly demanded reagents for intra- and intergroup separation of REEs; di-(2-ethylhexyl)phosphoric acid (D2EHPA, C16H35PO4, HA) is one of the most promising of these compounds.2−5 Because of its high viscosity, D2EHPA is used in practice by mixing with an organic diluent. Zikovsky studied the effect of diluents on REEs extraction with D2EHPA and reported that the recovery was enhanced according to the following order of diluents: heptane, cyclohexane, butyl ether, carbon tetrachloride, toluene, xylene, nitrobenzene, bromoform, chlorobenzene, benzene, hexane, dichloromethane, and chloroform.6 REEs extraction by D2EHPA was investigated in the presence of various mineral acids in such organic diluents as n-heptane,7−9 kerosene,10−16 benzene,17−19 n-dodecane,20−22 Amsco,23−26 hexane,10 and toluene.9,27−29 The mechanism of complexation REEs with D2EHPA depends on different conditions, one of them is the acidity (pH) of solution. REEs are extracted into the organic phase at low acidity (pH∼1-3) in the form of soluble lanthanide di-(2ethylhexyl) phosphate complexes (LnA3).22 As was reported by Harada et al., at some extraction process conditions the third © 2017 American Chemical Society

phase formation was possible as a result of the precipitation of LnA3 complexes.30 This phenomenon is highly undesirable for technological processes. The basis of separation and purification by solvent extraction technique is the distribution of a solute between two immiscible or poorly miscible solvents. The distribution coefficient (D) is an essential parameter for technologists unlike for scientists who consider D as a possible but rather uninformative way to present the phase equilibrium condition of the investigated system. To calculate equilibrium composition of coexisting phases in a wide range of variables a thermodynamic model of the system should be constructed, so thermodynamic modeling of liquid−liquid equilibrium (LLE) is a first step of mathematical models creation that are necessary for the design of equipment. The availability of a thermodynamic model allows optimization of the extraction parameters and improves the efficiency of separation technology. The quality of thermodynamic models largely depends on the reliability and completeness of experimental data used to determine the model parameters. Only the equilibrium content of REEs in two phases (or the value of the distribution coefficient D) and the acidity of the Received: July 29, 2017 Accepted: October 13, 2017 Published: November 9, 2017 4337

DOI: 10.1021/acs.jced.7b00696 J. Chem. Eng. Data 2017, 62, 4337−4343

Journal of Chemical & Engineering Data

Article

aqueous phase at a fixed temperature are usually presented in many works on extraction.6,10,19,31 At the same time there are no data on the density of the solutions, which makes impossible the conversion from the molar concentration to the mole fraction scale which is used in thermodynamic models. Most of the authors tried to identify certain correlations between the REE distribution coefficients and conditions of the extraction process, but they did not report full information about the composition of the studied systems.6,8,18,22 With the thermodynamic characteristics of REE complexation and/or extraction, authors usually determined concentration equilibrium constants instead of thermodynamic constants. In this case equilibrium is studied in the presence of a buffer for used to retain a constant pH value or ionic strength.7,12,32 The aforementioned data can be used to validate existing models of the extraction processes. However, this is not enough for calculating the interaction parameters of the components and developing new thermodynamic models, which are necessary for the optimization of the present methods and developing the alternative techniques for the recovery and separation of REEs. The aim of this study was to obtain experimental data on the phase equilibria in the water−nitric acid−REE nitrate (Eu, Gd, Dy)−o-xylene - D2EHPA systems under conditions close to the industrial separation for REEs. The experimental and thermodynamic data on the extraction equilibrium of individual REEs in nitric acid medium and mixed organic solvent D2EHPA−o-xylene are missing. Preston and du Preez33 reported the results of an experimental study of the REE extraction using o-xylene as diluent. All the data refer to the mixture of lanthanides, and therefore they are useless for the construction of thermodynamic models. We have found only one work,6 containing the distribution coefficients of europium and thulium nitrates between aqueous and organic (D2EHPA + o-xylene) phases and the concentration equilibrium constants of the complexation process. It is impossible to estimate true (thermodynamic) equilibrium constants from these data, because sodium nitrate was added to the investigated solutions to maintain constant ionic strength. In the experimental part of this work special attention was paid to the following points: (i) determination of the content of all the components that are present in the coexisting phases, if their amounts exceed the detection limit, (ii) estimation of the accuracy of the experimentally determined values; (iii) detection of factors that have the greatest impact on the value of the distribution coefficients. The data obtained in this work will be used in the future for the thermodynamic modeling of multicomponent systems water−nitric acid−REE nitrate (light, medium and heavy group)−o-xylene−D2EHPA.

Table 1. Sources and Purities of Chemicals chemical name Dy2O3, dysprosium oxide Eu2O3, europium oxide Gd2O3, gadolinium oxide HNO3, nitric acid o-C8H10, o-xylene C16H35PO4, D2EHPA

CASRN

source

1308-87-8

Ganzhou Jin Chengyuan New Material Co., Ltd. (China)

puritya/ weight percent

analysis methodb

99.95

XRD, EDX

1308-96-9

99.99

12064-62-9

99.99

XRD, EDX XRD, EDX ICPAES GC

7697-37-2 95-47-6

Komponent-Reactiv (Russia) Khimmed (Russia)

298-07-7

Sigma-Aldrich (USA)

≥70% 99.8 97.9

1

H and 31 P NMR

a As specified in the certificates of analysis provided by the chemical suppliers. bXRD, X-ray diffraction analysis; EDX, energy dispersive Xray; NMR, nuclear magnetic resonance; GC, gas chromatography; ICP-AES, inductively coupled plasma atomic emission spectroscopy.

identification. In this study we used D2EHPA without further purification for LLE experiments. Diffraction patterns of REE oxides were collected with STOE STADI P automatic powder diffractometer (Cu Kα1 radiation) at room temperature (2θ 5−80°, step 0.01°). Obtained patterns were compared with the Joint Committee on Powder Diffraction Standards Powder Diffraction File-2 (JCPDS PDF-2) databank. The STOE WinXPOW program package was applied to the X-ray diffraction analysis. X-ray microanalysis and elemental analysis of Dy2O3, Eu2O3, and Gd2O3 was carried out using Leo Supra 50 VP scanning electron microscopy (LEO Carl Zeiss SMT Ltd., Germany) with an INCA Energy dispersive X-ray spectrometer with Oxford 350-Max 80 silicon drift detector. The purity of o-xylene was determined by a Kristall 2000 M gas chromatograph with Supelcowax-10 column (30 m/0.32 mm/1.0 mkm) and a flame ionization detector (FID). Potentiometric (acid−base) titration, and 1H, 31P nuclear magnetic resonance spectroscopy were used for identification of impurities in D2EHPA. 1H and 31P spectra were obtained on a Bruker Avance 400 MHz NMR spectrometer. The water content in the samples was determined by Karl Fisher technique. The absence of significant impurities in nitric acid was determined by ICP-AES method. 2.2. Extraction Procedure. The extraction experiments were carried out in 100 mL separating funnels by mixing the initial aqueous {H2O + HNO3 + Ln(NO3)3} and organic {HA + o-C8H10} solutions. The mixtures were stirred on a shaker for 1 h, and then allowed to stand at 298.15 ± 0.50 K in an air thermostat for 2 days; equilibrated phases were separated after this procedure. The time of keeping at fixed temperature was chosen according to the results of Danesi et al. and Geist et al.,32,35 where kinetics of the REEs extraction processes were studied. 2.3. Analysis of Solutions. The acidity of the aqueous solution before and after extraction was determined by acid− base potentiometric titration on a TitroMatic 1S automatic titrator with glass electrode (Crison). Two different methods were used for the determination of the REEs in aqueous solutions before and after extraction: spectrophotometry and ICP-AES. Spectrophotometric analysis was performed, according to the technique from Uhrovchik et

2. EXPERIMENTAL SECTION 2.1. Reagents and Their Characterization. In this work, we used o-xylene and D2EHPA, nitric acid, europium, gadolinium, and dysprosium oxides, the characteristics of which are given in Table 1. The absence of significant impurities was confirmed by different analysis methods (see Table 1). All of the reagents were used without further purification. REE nitrates for extraction were prepared by dissolution of a certain amount of Eu2O3, Gd2O3, and Dy2O3 in nitric acid and distilled water. Oxides of REEs (Ln2O3) were preheated at 1223.15 K for 36 h to achieve their weight forms according to Eyring; 34 X-ray and EDX analyses were applied for 4338



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al.36 using Arsenazo-III (analytically pure, 99%). Photoelectric photometer (spectrophotometer) KFK-3 was employed for the absorbance measurements. The determination of europium, gadolinium, and dysprosium was carried out at pH 2.6−3.0, according to Rohwer et al.37 ICP-AES elemental analysis was carried out on iCAP 6300 ICP spectrometer (Thermo Scientific). Standard single-element solutions of REE in 2 wt % HNO3 (high-Purity Standards, USA) were used for preparing calibration solutions for the spectral analysis (spectrophotometry, ICP-AES). Maximum relative uncertainty of REE content measurements was estimated within 3%. The concentrations of REE in the organic phase were determined by mass balance calculations. 2.4. Density Measurements. The densities of solutions were measured by VIP-2MP density meter (Termex) at a constant temperature 298.15 ± 0.05 K. The limits of permissible absolute uncertainty of density measurements were 2 × 10−4 g·cm−3. The density was calculated from the results of measurements of the resonant vibration period of a cell with a sample:

second, mono-(2-ethylhexyl)phosphoric acid can form complexes with the REEs.38 To estimate the influence of the first factor, the vapor pressure of pure and crude D2EHPA solutions in o-xylene were compared. The measuring procedure was described earlier in detail.39 The results of experiments at three temperatures are shown in Figure 1; measurement uncertainties are comparable to the size of the symbols. As can be seen, the difference between the vapor pressure of the solutions with pure and crude D2EHPA belongs to the range of vapor pressure measurement errors (0.5% at 298.15 K, 1% at 308.15 K). So it is possible to conclude that the activity of mixed solvent does not change essentially due to the presence of a small quantity of some species in the organic phase. The amount of impurities of mono-(2-ethylhexyl)phosphoric acid in the crude D2EHPA is few and it can be neglected, because its influence on the LLE equilibrium is insignificant as compared with the accuracy of analytical techniques. This result can be considered as an indirect indication of a negligible impact of listed-above D2EHPA impurities on liquid−liquid equilibria. Thus, we used an extractant without further purification. 3.2. Data Analysis. According to the literature data different extraction mechanisms are presented for systems under investigation.17 The basic uncertainty concerns the structure of the complex (REE di-(2-ethylhexyl)phosphate), but for processing obtained primary data at low acidity (pH∼1−3) we took into account the following reaction scheme:

ρ = Aτ 2 + B

where ρ is the density of the solution, g·cm−3; τ is the oscillation period of the measuring cell, ms; A, B are coefficients, determined from calibration by known densities and oscillation periods of ambient air, distilled water, and standard materials (listed in Table 2) produced and certified by D.I. Mendeleyev Institute for Metrology (VNIIM).

3+ + Ln(aq) + 3HA (org) = LnA3(org) + 3H(aq)

Table 2. Density (ρ) of VNIIM Standard Materials at 298.15 Ka

a

tested sample

ρ/g·cm−3

tested sample

ρ/g·cm−3

REP-2 REP-5

0.72657 0.99704

REP-7 REP-12

1.31556 1.09032 −5

The combined expanded uncertainties are Uc = 5 × 10

g·cm

(1a)

or in equivalent form that takes into account the existence of a hydrogen-bonded dimer HA in the nonpolar organic solvents 3+ + 2Ln(aq) + 3H 2A 2(org) = 2LnA3(org) + 6H(aq)

−3

(1b)

3+

where Ln , HA, and LnA3 are REE cation, D2EHPA, and REE di-(2-ethylhexyl)phosphate, respectively. Subscripts “aq” and “org” denote aqueous and organic phases. The use of eq 1 allows us consider mass transfer between phases regardless of the complex structure. Because of the extremely low content of o-xylene, D2EHPA and LnA3 in the aqueous equilibrium phase and water, nitric acid, and REE nitrates in the organic equilibrium phase, we neglect the transfer of these components between phases. Thus, the only equation describing the transfer of substances between phases is eq 1. Partially the statements described above were proven by us experimentally using additional test experiments. The absence of nitrate-ion in the organic phase after extraction was proven by a qualitative reaction with diphenylamine in sulfuric acid (limit of NO3− detection is 18 mg·dm−3).40 The amount of water in the organic phase after extraction of europium nitrate was determined by Karl Fischer titration. The amount of water in the initial organic reagents (0.02−0.09 mol %) and in the organic phase after extraction was almost the same (it did not exceed 0.06 mol %). On the basis of these results, it was concluded that there is no enrichment of the organic phase with water during the extraction of europium (and other middle group REEs) by D2EHPA. The processing of primary data was based on calculation of equilibrium phase composition, and the calculated results must be in agreement with all the experimental data obtained within the limits of their determination. During the calculation the total mass of the initial aqueous solution (minaq/g), initial masses

3. RESULTS AND DISCUSSION Primary experimental data are reported in Table 3. The presented data are given with an excess number of significant figures to avoid the loss of accuracy due to subsequent algebraic operations. It should be noted that in some cases (see experiments number 14 and 22 in Table 3) the formation of the third solid phase was observed, which is a polymer complex di(2ethylhexyl)phosphate lanthanide (LnA3).30 A third-phase formation GdA3 was observed at an initial concentration of gadolinium nitrate solutions ≥0.1 mol·kg−1, whereas the thirdphase DyA3 precipitated at concentrations of dysprosium nitrate ≥0.05 mol·kg−1. 3.1. The Impact of Impurities. According to 1H, 31P NMR, and potentiometric (acid−base) titration used in this work D2EHPA contained some amount of mono-(2ethylhexyl)phosphoric acid (1.4 wt %), phosphoric acid (0.3 wt %), and inert impurities (0.4 wt %). Many research studies require high purity reagents, while purification of reagents in industrial processes is carried out just in some cases. In this regard, one should try to quantify the effect of impurities on the results of phase equilibria investigation in extraction systems. Two aspects should be kept in mind when discussing the impurity impact on the result of an extraction experiment. At first, the impurities can change the activity of a mixed solvent (D2EHPA + o-xylene) and 4339



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Table 3. Primary Experimental Liquid−Liquid Equilibrium Data for the o-Xylene−Di-(2-ethylhexyl)phosphoric Acid−Water− Nitric Acid−REE Nitrate Systems at Temperature T = 298.15 K and Pressure p = 0.1 MPaa initial solutions before mixing aqueous solution m N

b

g

equilibrium phases organic solution

ρ

c(Ln(NO3)3)

c(HNO3)

g·cm−3

mol·kg−1

mol·kg−1

c

c

aqueous phase

organic phase

m(D2EHPA)

m(o-xylene)

ρ

c(Ln(NO3)3)

c(HNO3)

ρ

g

g

g·cm−3

mol·kg−1

mol·kg−1

g·cm−3

10−1d 10−1e 10−1d 10−1e 10−2d 10−2e 10−2d 10−2e 10−2d 10−2e 10−2d 10−2e 10−2d 10−2e 10−2d 10−2e 10−3d 10−3e 10−3d 10−3e 10−5d 10−5e 10−7e

0.1565

0.8955

0.1561

0.8955

0.1302

0.8956

0.1301

0.8957

0.1268

0.8956

0.1271

0.8957

0.1127

0.8948

0.1126

0.8948

0.0795

0.8936

0.0793

0.8935

0.0410

0.8914

0.9977

2.434 2.488 2.434 2.474 6.690 6.735 6.655 6.777 6.666 6.771 6.863 6.794 1.979 2.006 1.975 1.988 2.012 2.031 2.041 2.075 4.358 4.408 1.120

0.0179

0.8904

0.9976

5.210 × 10−7e

0.0176

0.8940

o-Xylene + Di-(2-ethylhexyl)phosphoric Acid + Water + Nitric Acid + Europium(III) Nitrate 1 54.769 1.0851 2.649 × 10−1d 0.0639 6.825 37.521 2.704 × 10−1e 2 27.642 1.0851 2.649 × 10−1d 0.0639 3.443 18.993 2.704 × 10−1e 3 51.206 1.0272 9.898 × 10−2d 0.0298 6.814 37.502 9.858 × 10−2e 4 51.206 1.0272 9.898 × 10−2d 0.0298 6.822 37.506 9.858 × 10−2e 5 51.799 1.0272 9.898 × 10−2d 0.0298 6.819 37.510 9.858 × 10−2e 6 51.806 1.0272 9.898 × 10−2d 0.0298 6.819 37.506 9.858 × 10−2e 7 51.026 1.0117 4.922 × 10−2d 0.0198 6.812 37.514 5.022 × 10−2e 8 51.013 1.0117 4.922 × 10−2d 0.0197 6.818f 37.520 5.022 × 10−2e 9 50.597 1.0045 2.423 × 10−2d 0.0096 6.805 37.506 2.408 × 10−2e 10 50.592 1.0045 2.423 × 10−2d 0.0096 6.814 37.525 2.408 × 10−2e 11 50.477 0.9999 9.701 × 10−3d 0.0099 6.820 37.497 9.721 × 10−3e 12 50.328 0.9984 4.815 × 10−3d 0.0009 6.822 37.510 4.808 × 10−3e 13 50.332 0.9984 4.815 × 10−3d 0.0009 6.818 37.595 4.808 × 10−3e o-Xylene + Di-(2-ethylhexyl)phosphoric Acid + Water + Nitric Acid + Gadolinium(III) Nitrate 14 50.580 1.0271 9.882 × 10−2d 0.00974 6.750 37.210 9.917 × 10−2e 15 49.860 1.0124 5.170 × 10−2d 0.00988 6.750 37.210 5.123 × 10−2e 16 49.860 1.0124 5.170 × 10−2d 0.00988 6.750 37.210 5.123 × 10−2e 17 49.510 1.0049 2.672 × 10−2d 0.00995 6.750 37.200 2.640 × 10−2e 18 49.510 1.0049 2.672 × 10−2d 0.00995 6.750 37.200 2.640 × 10−2e 19 49.260 1.0004 1.063 × 10−2d 0.01000 6.750 37.200 1.070 × 10−2e 20 49.260 1.0004 1.063 × 10−2d 0.01000 6.750 37.200 1.070 × 10−2e 21 49.240 0.9989 5.420 × 10−3d 0.01001 6.760 37.210 5.410 × 10−3e o-Xylene + Di-(2-ethylhexyl)phosphoric Acid + Water + Nitric Acid + Dysprosium(III) Nitrate 22 52.708 1.0117 5.152 × 10−2d 0.01977 6.823 37.588 23 50.491 1.005 2.572 × 10−2d 0.01443 6.823 37.588 2.594 × 10−2e 24 50.491 1.005 2.572 × 10−2d 0.01443 6.823 37.588 2.594 × 10−2e 25 50.875 0.9998 1.064 × 10−2d 0.01080 6.879 37.858 1.064 × 10−2e

1.0797 1.0797 1.0214 1.0213 1.0214 1.0213 1.0065 1.0065 1.0002 0.9994 0.9981

c

× × × × × × × × × × × × × × × × × × × × × × ×

c

third phase formation 1.0058 1.0058 1.0004 1.0004 0.9985 0.9985 0.9980

0.9996 0.9996 0.9985

10−2d 10−2e 10−2d 10−2e 10−3d 10−3e 10−3d 10−3e 10−5d 10−5e 10−5d 10−5e 10−6d 10−6e

1.751 1.736 1.751 1.731 1.755 1.742 1.749 1.751 3.030 3.040 3.040 3.020 1.900 1.200

× × × × × × × × × × × × × ×

2.159 2.160 2.171 2.164 2.243 2.262

third phase formation × 10−4d 0.0929 × 10−4e × 10−4d 0.0929 × 10−4e × 10−6d 0.0447 × 10−6e

0.1166

0.8934

0.1166

0.8934

0.0843

0.8910

0.0843

0.8910

0.0427

0.8900

0.0427

0.8900

0.0282

0.8893

0.8943 0.8943 0.8915

The standard uncertainties are u(T) = 0.5 K, u(p) = 10 kPa, u(m) = 2 × 10−3 g, u(ρ) = 2 × 10−4 g·cm−3, u(c) = 1 × 10−3 mol·kg−1 for nitric acid concentration. For REE nitrate relative standard uncertainty is ur(c) = 0.03 when c(REE) > 1 × 10−5 mol·kg−1, standard uncertainty u(c) = 3 × 10−7 mol·kg−1 when c(REE) ≤ 1 × 10−5 mol·kg−1. bTotal mass of aqueous solution before mixing. cMoles of electrolyte per kilogram of solution. d Spectrophotometry. eICP-AES method a

4340



Article eq in eq nLn,aq = nLn,aq − nLn,org

(2)

eq in eq nH,aq = nH,aq + 3nLn,org

(3)

For REEs and nitric acid the concentration was calculated as moles of electrolyte per kilogram of solution. The concentration of REE (cinLn,aq/mol·kg−1) and nitric acid (cinH,aq/mol· kg−1) in the initial aqueous solution and the concentration of −1 −1 eq REE (ceq Ln,aq/mol·kg ) and nitric acid (cH,aq/mol·kg ) in the equilibrium aqueous phase may be determined as follows in in in c Ln,aq = nLn,aq × 103/maq

(4)

in in in c H,aq = nH,aq × 103/maq

(5)

eq c Ln,aq =

Figure 1. Influence of impurities on the vapor pressure in the D2EHPA−o-xylene system. Small symbols, solutions with purified D2EHPA (99.0%):39 crosses, 308.15 K; squares, 303.15 K; diamonds, 298.15 K. Large symbols (circles), solutions with crude D2EHPA (this work). Measurement uncertainties are comparable to the size of the symbols.

(minD2EHPA/g)

eq nLn,aq × 103 eq eq n Heq2O,aq M H2O + nLn,aq MLn(NO3)3 + nH,aq MHNO3

(6) eq c H,aq =

n Heq2O,aq M H2O +

eq nH,aq × 103 eq nLn,aq MLn(NO3)3

eq MHNO3 + nH,aq

(7)

where M/(g·mol−1) is the molar mass; the amount of water (nHeq2O,aq/mol) may be calculated as

and o-xylene (mino‑xylene/g) were used amounts of REEs (ninLn,aq/mol) and

of D2EHPA as fixed parameters. The nitric acid (ninH,aq/mol) in the initial aqueous solution as well as REEs in the equilibrium organic phase (neq Ln,org/mol) were chosen as variable parameters. The amounts of REE (neq Ln,aq/ mol) and nitric acid (neq /mol) in the equilibrium aqueous H,aq phase may be calculated according to eqs 2 and 3, respectively:

n Heq2O,aq = n Hin2O,aq =

in in in (maq − nLn,aq MLn(NO3)3 − nH,aq MHNO3)

M H 2O

(8)

Table 4. Recommended Liquid−Liquid Equilibrium Data for the o-Xylene−Di-(2-ethylhexyl)phosphoric Acid−Water−Nitric Acid−REE Nitrate Systems at Temperature T = 298.15 K and Pressure p = 0.1 MPa feeda N

x(H2O)

x(HNO3)

x(Ln(NO3)3)

aqueous phaseb x(oxylene)

x(H2O)

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3

0.1125 0.1128 0.1131 0.1131 0.1120 0.1120 0.1119 0.1119 0.1118 0.1119 0.1116 0.1117 0.1119

0.9921 0.9922 0.9964 0.9964 0.9964 0.9964 0.9976 0.9976 0.9985 0.9985 0.9993 0.9997 0.9997

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−4 10−4 10−4

4.77 4.76 1.24 1.24 1.24 1.26 3.61 3.59 3.64 3.70 7.91 2.02 9.40

× × × × × × × × × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3 10−4 10−4 10−5 10−5 10−7 10−9 10−9

4.52 4.48 4.59 4.58 4.48 4.50 4.22 4.21 3.07 3.06 1.33 6.47 6.46

× × × × × × × × × × × × ×

10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−4 10−4

4.35 4.34 4.32 4.33 4.35 4.35 4.42 4.43 4.75 4.76 5.27 5.46 5.45

10−3 10−3 10−3 10−3 10−3

0.1135 0.1134 0.1132 0.1132 0.1131

third phase formation 0.9976 2.12 × 10−3 0.9984 1.53 × 10−3 0.9992 7.66 × 10−4 0.9995 4.93 × 10−4

3.15 3.17 5.47 2.57

× × × ×

10−4 10−5 10−7 10−8

4.78 3.32 1.42 7.27

× × × ×

10−3 10−3 10−3 10−4

4.26 4.68 5.23 5.43

10−3 10−3 10−3

0.1106 0.1123 0.1118

third phase formation 0.9983 1.68 × 10−3 0.9992 7.95 × 10−4

3.91 × 10−6 4.07 × 10−8

x(D2EHPA)

H2O + HNO3 + Eu(NO3)3 + D2EHPA + o-Xylene + EuA3 1 0.8749 1.11 × 10−3 4.74 × 10−3 6.74 × 4.73 × 10−3 6.73 × 2 0.8746 1.11 × 10−3 1.63 × 10−3 6.77 × 3 0.8780 4.89 × 10−4 1.63 × 10−3 6.78 × 4 0.8780 4.89 × 10−4 1.62 × 10−3 6.70 × 5 0.8792 4.89 × 10−4 1.64 × 10−3 6.70 × 6 0.8792 4.90 × 10−4 8.14 × 10−4 6.69 × 7 0.8803 3.23 × 10−4 8 0.8803 3.22 × 10−4 8.12 × 10−4 6.70 × 3.93 × 10−4 6.68 × 9 0.8809 1.69 × 10−4 3.93 × 10−4 6.69 × 10 0.8809 1.69 × 10−4 1.58 × 10−4 6.68 × 11 0.8814 1.69 × 10−4 7.65 × 10−5· 6.69 × 12 0.8815 3.50 × 10−5 7.65 × 10−5 6.68 × 13 0.8813 3.26 × 10−5 H2O + HNO3 + Gd(NO3)3 + D2EHPA + o-Xylene + GdA3 14 0.8779 1.60 × 10−4 1.62 × 10−3 6.78 × 8.47 × 10−4 6.77 × 15, 16 0.8788 1.59 × 10−4 4.23 × 10−4 6.76 × 17, 18 0.8794 1.59 × 10−4 1.71 × 10−4 6.76 × 19, 20 0.8797 1.59 × 10−4 8.79 × 10−5 6.77 × 21 0.8799 1.60 × 10−4 H2O + HNO3 + Dy(NO3)3 + D2EHPA + o-Xylene + DyA3 22 0.8808 3.25 × 10−4 1.65 × 10−3 6.61 × 4.19 × 10−4 6.71 × 23, 24 0.8803 2.34 × 10−4 1.73 × 10−4 6.69 × 25 0.8812 1.83 × 10−4

organic phasec

x(HNO3) 3.08 3.07 2.41 2.41 2.35 2.35 2.05 2.05 1.42 1.42 7.26 3.00 2.97

× × × × × × × × × × × × ×

x(Ln(NO3)3)

x(LnA3)

3.51 × 10−3 1.46 × 10−3

x(D2EHPA)

x(oxylene)

× × × × × × × × × × × × ×

10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

0.9520 0.9521 0.9522 0.9521 0.9520 0.9520 0.9515 0.9515 0.9494 0.9494 0.9460 0.9447 0.9449

× × × ×

10−2 10−2 10−2 10−2

0.9527 0.9499 0.9463 0.9449

4.63 × 10−2 5.22 × 10−2

0.9502 0.9463

a

LnA3 is absence in the feed mixture. bIt was assumed that the content of LnA3, D2EHPA and o-xylene in the aqueous phase is insignificant. cIt was assumed that the content of H2O, HNO3 and Ln(NO3)3 in the organic phase is insignificant. 4341



Article

as well as with reducing the concentration of lanthanide in the initial solutions.

The estimation of variable parameters was performed by minimization of objective function N

F=

N

4. CONCLUSIONS A set of experimental data on the composition and bulk properties of aqueous and organic phases in the system water− nitric acid−REE nitrate (Eu, Gd, Dy)−o-xylene−D2EHPA at 298.15 K was obtained. Unlike most similar studies, the obtained data can be directly used to define the parameters of the thermodynamic model, and then these results may be used for calculation of extraction and separation processes of lanthanides (Eu, Gd, Dy) with D2EHPA in o-xylene.

in,exp in,calc 2 in,exp in,calc 2 ∑ w1(c Ln,aq − c Ln,aq ) + ∑ w2(c H,aq − c H,aq ) i

i N

+

N

eq,exp eq,calc 2 eq,exp eq,calc 2 − c Ln,aq ) + ∑ w4(c H,aq − c H,aq ) ∑ w3(c Ln,aq i

i

(9)

where the subscripts “calc” and “exp” indicate the calculated value corresponding to each measured experimental value; N was equal 1 or 2 depending on the number of repeat experiments; w is a statistical weight of the experimental point, it was assumed to be equal to the squared inverse values of the standard uncertainties of the experimental values. Calculated compositions of phases are presented in Table 4. The difference between experimental and calculated values does not exceed an error of the experimental determination. The only exception is a concentration of europium nitrate in the initial aqueous solution (experiment numbers 9 and 10 in Table 3) determined by the ICP method. The difference is about 3.4%, which is somewhat greater than the experimental error. In general, the agreement between the experimental and calculated results indicates the correct choice of the extraction mechanism and also the validity of the assumptions made in the calculations. On the basis of the data given in Table 4 REE distribution coefficients (D) according to eq 10 were calculated. eq eq D = x LnA /x Ln(NO 3,org 3)3 ,aq

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Svetlana V. Kurdakova: 0000-0002-6329-921X Funding

The investigations were financially supported by the URALCHEM OJSC. Notes

The authors declare no competing financial interest.

■

REFERENCES

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(10)

The dependence of europium distribution coefficient on the initial europium content and acidity in aqueous solution is shown in Figure 2. It is impossible to compare these results

Figure 2. Distribution coefficient of europium (lgD) vs initial concentration of europium nitrate and nitric acid (x(Eu(NO3)3), x(HNO3)) at constant content of the other components of the investigated system (see Table 4); experimental points and surface trend.

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phosphoric Acid, Water, Nitric Acid - American Chemical Society

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