­­Solvent extraction equilibrium of dysprosium(Ⅲ) from nitric acid solutions

with 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester

HUANG Ying^{1, 2}, TANAKA Mikiya¹

1. Research Institute for Environmental Management Technology,

National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba,

Ibaraki 305-8569, Japan;

2. School of Energy and Environment, Southeast University, Nanjing 210096, China

Received 12 December 2008; accepted 6 May 2009

Abstract: In view of the importance of solvent extraction of rare earth metals with the acidic organophosphorous reagent, the development of a chemically based model applicable to high concentration is desired. In the present study, the equilibrium distribution of dysprosium(Ⅲ) between an aqueous nitric acid solution and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) was measured in the range of an initial aqueous dysprosium(Ⅲ) concentration from 1.0×10^-3 to 1.0×10^-1 kmol/m³ and PC88A concentration from 0.16 to 0.65 kmol/m³ in Shellsol D70 as the diluent. The obtained data were analyzed using the chemically based model in order to correlate the equilibrium distribution ratios. In this model, dysprosium(Ⅲ) was assumed to be extracted with the PC88A dimer as a 1:3 complex, the activities were considered for the aqueous species, and the effective concentration of the PC88A dimer was calculated using Alstad’s empirical equation. As a result, the apparent extraction equilibrium constant was determined to be 253 (kmol?m^-3)^-2 with an excellent correlation between the experiment and calculation results in the wide range of the logarithm of the distribution ratio from -2 to 3.5. In conclusion, the methodology in this model would be effective for quantitative description of solvent extraction behavior of general rare earth elements as well as dysprosium.

Key words: solvent extraction; dysprosium; 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A); nitric acid; equilibrium

1 Introduction

Solvent extraction using acidic organophosphorus extractants has shown its effectiveness to separate and recover rare earth (RE) metals because of the high extraction and separation efficiencies[1-6]. In order to make the process more efficient, a chemically-based mathematical model for the numerical simulation applicable in the range from low (~10^-5 kmol/m³ RE) to high (＞10^-2 kmol/m³ RE) metal concentrations should be developed. As the initial step, it is desired to establish an extraction equilibrium model for RE. Up to now, the fundamental extraction equilibrium studies of RE using acidic organophosphorus extractants were carried out mainly under the ideal conditions: low metal concentration with a constant ionic strength and activity coefficients of chemical species[2, 7-12]. There are only few studies[13-16] on the extraction equilibrium of RE at high concentrations.

In this work, by considering the recent increasing importance of dysprosium due to the use as the dopant to the neodymium magnet and the limited reserve in nature, the distribution of dysprosium(Ⅲ) between aqueous nitric acid solution and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (PC88A) is measured in the range of an initial aqueous dysprosium(Ⅲ) concentration from 1.0×10^-3 to 1.0×10^-1 kmol/m³ and PC88A concentration from 0.16 to 0.65 kmol/m³ in Shellsol D70 as the diluent. The data obtained are analyzed in order to develop a chemically based extraction equilibrium model employing the activities for the aqueous species and the effective concentration of the extractant in the organic phase.

2 Experimental

The aqueous working solutions were prepared by diluting dysprosium(Ⅲ) nitrate hexahydrate (Dy(NO₃)₃? 6H₂O, Wako, 99.5% purity) with nitric acid at different concentrations. The original concentrations of Dy(NO₃)₃ and HNO₃ in the aqueous phase solutions were from 0.001 to 0.1 kmol/m³ and 0 to 0.8 kmol/m³, respectively. The organic phase contained PC88A (Daihachi) and Shellsol D70 (C11-C14 paraffins and naphthenes, Shell Chemicals), which were used as extractant and diluent, respectively. The concentration of PC88A in the organic phase was varied from 0.16 to 0.65 kmol/m³ as monomer. The concentration of the extractant was determined by the nonaqueous neutralization titration.

Equal volume (20 cm³) of the aqueous and organic working solutions was continuously shaken for 4 h in a water bath kept at (298±0.1) K to attain equilibrium. Then the two phases were separated centrifugally for   5 min. The pH values of the aqueous phases were measured using an HM-60G pH meter (TOA-DKK). The concentrations of Dy in the aqueous phases were determined by EDTA titration and by ICP-AES (ULTIMA 2, Horiba) when they were relatively high and low, respectively. The concentrations of Dy in the organic phases were measured by the same method after completely stripping Dy from the organic phases into   2 kmol/m³ HNO₃. The distribution　ratio,　D,　was　calculated as the ratio of the total Dy molar concentration in the organic phase to that in aqueous phase, i.e.,

where c, the subscript T, and over bar denote the molar concentration, the total concentration, and the organic phase, respectively.

Duplicated experiment under 18 different conditions indicated that the lg D was reproduced with the standard deviation of 0.040.

3 Results and discussion

The dependencies of lg D on the measured pH value, pH_a, at different initial concentrations of PC88A and Dy(NO₃)₃ (i.e.  and c(Dy³⁺)_in) are shown in Figs.1 and 2, respectively. In Fig.1, the distribution ratios increase with increasing pH and  When  is constant and c(Dy³⁺)_in≤0.01 kmol/m³, lg D changes little with increasing c(Dy³⁺)_in and increases linearly with increasing pH_a as shown in Fig.2. When c(Dy³⁺)_in=0.1 kmol/m³, however, lg D approaches the constant value with increasing pH_a. This is because the initial Dy concentration exceeds the extraction capacity of the extractant.

Fig.1 lgD vs pH_a for extraction of Dy with PC88A (c(Dy³⁺)_in= 0.001 kmol/m³)

Fig.2 lgD vs pH_a for extraction of Dy with PC88A (=0.16 kmol/m³)

It is accepted that Dy³⁺ is extracted by PC88A through the following cation exchange reaction at the low acid concentration[7, 14, 17], where (HR)₂ is a dimer of PC88A:

The apparent extraction equilibrium constant, K_ex, is defined as

Here a is the activity related to c by

a=yc                                        (4)

where y is the activity coefficient. When  is the only extracted species in the organic phase, Eqs.(3) and (4) lead to

where pH=-lg a(H⁺).

The experimental values of  with different extraction conditions are plotted versus pH according to Eq.(5) in Fig.3, where the straight line with the slope of 3 is fitted by the least squares method. The K_ex value of 117 (kmol?m^-3)^-2 has been obtained as the antilogarithm of the intercept of the solid line. In Eq.(5), y values are calculated using an OLI Analyzer 2.0, a comprehensive analysis tool for complex aqueous and multisolvent electrolyte solution chemistry [18]. Here, the material balance of the extractant and the charge balance, respectively, shown as Eqs.(6) and (7) were considered. However, the linearity of the analysis results of the experimental data is not good as shown in Fig.3.

The experimental values of lg D-3pH-lg y(Dy³⁺)_T with different initial HNO₃ concentrations (i.e., c(H⁺)) when c(Dy³⁺)_in is 0.001 kmol/m³ are plotted versus  in Fig.4. The slope value of the fitted straight line is 2.3 that is less than the theoretical value of 3 shown in Eq.(5). Due to this fact together with the unsatisfied linearity of the experimental data in Fig.3, it is concluded that the model based on Eq.(5) cannot describe the extraction equilibria successfully.

Fig.3 Analysis of extraction equilibrium data of Dy with PC88A based on Eq.(5)

Fig.4 Analysis of extraction equilibrium data of Dy with PC88A at c(Dy³⁺)_in of 0.001 kmol/m³ based on Eq.(5)

Several researchers pointed out for the acidic organophosphorous extractants that when aliphatic solvents were used as diluents, the slope values of the lgD vs logarithm of extractant dimer concentration plot were close to 2.5 in the extractant concentration range of 10^-2-1 kmol/m³[19-22]. The negative deviation from the third-order dependence can be attributed to the nonideality of  caused by weak solute-solute interactions[22], where the interacting solute species is the PC88A dimer. This nonideality can be corrected by the effective concentration of the PC88A dimer, , which is defined as

where  is the coefficient for the effective concentration of the extractant. The value of  is assumed to vary with  on the basis of the empirical equation by ALSTAD et al[23] as

where A is a constant. Thus, K_ex is redefined as

Eq.(5) is rewritten as

When the K_ex and A values are given, the value of the right hand side of Eq.(11) can be obtained from the experimental data. Therefore, the least squares method minimizing the difference between both sides of Eq.(11) was applied in order to determine K_ex and A. As a result, the values of K_ex=253 (kmol?m^-3)^-2 and A=0.473 (kmol?m^-3)^-0.5 were obtained, where the standard deviation in terms of lg D of Eq.(11) was 0.021.

Figs.5 and 6 show the analysis results using Eq.(11) in which the nonideality of the extractant was considered. It is observed that the linearity of Fig.5 becomes better as compared with Fig.3 and the slope of the fitting line in Fig.6 is increased to 3.0 which is 2.3 in Fig.4.

Fig.5 Analysis of extraction equilibrium data of Dy with PC88A based on Eq.(11)

Fig.6 Analysis of extraction equilibrium data of Dy with PC88A at c(Dy)_in of 0.001 kmol/m³ based on Eq.(11)

Fig.7 is the graphical expression for the result of the least squares analysis of Eq.(11) showing that the data points lie on the straight line of the unit slope with the y-intercept of lg K_ex in which the coefficient of determi- nation, R², is close to 1.

Fig.7 Relationship between experimental data of lg D and

The value of A in this research is in the same order as that reported by ALSTAD et al[23] for the extraction of rare earth metals by di(2-ethylhexyl)phosphoric acid diluted in Shellsol T (0.83 (kmol?m^-3)^-0.5).

From the good analysis results shown in Figs.5-7, it is concluded that Eq.(11) is successful to describe the extraction equilibrium of Dy with PC88A. Because of the similarity of the extraction behavior, the methodology in this model would be effective also for the quantitative description of the extraction equilibria of other rare earth metals with PC88A.

4 Conclusions

The equilibrium distribution of Dy(Ⅲ) between the aqueous nitric acid solution and the organic phase containing PC88A as the extractant and Shellsol D70 as the diluent was measured over wide concentration ranges of dysprosium, hydrogen ion and PC88A. A chemical equilibrium model considering the activities of the aqueous species and the effective concentration of the extractant was developed by analyzing the distribution data. The Alstad’s empirical equation was found to be applicable for calculating the effective concentration of the PC88A dimer. As a result, excellent correlation between the model and the experiment was obtained in the wide log D range from -2 to 3.5 with the apparent extraction equilibrium constant of 253 (kmol?m^-3)^-2. Consequently, the methodology in this model would be effective for quantitative description of solvent extraction behavior of general RE elements as well as dysprosium.

Acknowledgment

The authors are grateful to the Japan Society for the Promotion of Science for supporting this fellowship work (No. 07616).

Appendices

a       Activity

A       Constant in the Alstad’s empirical equation ((kmol?m^-3)^-0.5)

c       Concentration (kmol?m^-3)

c*     Effective concentration of the extractant (kmol?m^-3)

D      Distribution ratio

HR   Extractant monomer

K_ex      Apparent extraction equilibrium constant ((kmol?m^-3)^-2)

R²        Coefficient of determination

y       Activity coefficient

y*     Coefficient for the effective concentration of the extractant

Superscript (—)  Organic phase

Subscript T    Total

References

[1] RITCEY G M, ASHBROOK A W. Solvent extraction, principles and applications to process metallurgy (Part II) [M]. Amsterdam: Elsevier, 1979: 386-420.

[2] LI D Q, WAN X, LIN D Z, XIE Y F, LIN S X, WANG Z H, LI H, JI E Y. Extraction separation of rare earth elements, scandium and thorium with mono (2-ethyl hexyl) 2-ethyl hexyl-phosphate (HEH(EHP)) [C]// International Solvent Extraction Conference, Belgium: Liege, 1980: 1-10.

[3] ZHANG B Z, LU K Y, KING K C, WEI W C, WANG W C. Rare earth industry in China [J]. Hydrometallurgy, 1982, 9: 205-210.

[4] SHERRINGTON L. Commercial processes for rare earth and thorium [M]// Handbook of Solvent Extraction edited by Lo T C, Baird M H I, Hanson C. New York: John Wiley & Sons, 1983: 717- 723.

[5] SATO T. Liquid-liquid extraction of rare-earth elements from aqueous acid solutions by acid organophosphorus compounds [J]. Hydrometallurgy, 1989, 22: 121-140.

[6] PRESTON J S, PREEZ A C D. Solvent-extraction processes for the separation of the rare earth metals [C]// Solvent Extraction 1990, Part A, Amsterdam: Elsevier, 1992: 883-894.

[7] PEPPARD D F, MASON G W, DRISCOLL W J, SIRONEN R J. Acidic esters of orthophosphoric acid as selective extractants for. metallic cations-tracer studies [J]. Journal of Inorganic and Nuclear Chemistry, 1958, 7: 276-285.

[8] INOUE K, FUJINO O, ARITA H. Solvent extraction equilibria of neodymium and praseodymium with acidic organophosphorus compounds [J]. Journal of MMIJ, 1986, 102: 491-494. (in Japanese)

[9] MORI Y, OHYA H, ONO H, EGUCHI W. Extraction equilibrium of Ce(Ⅲ), Pr(Ⅲ) and Nd(Ⅲ) with acidic organophosphorus extractants [J]. Journal of Chemical Engineering of Japan, 1988, 21: 86-91.

[10] SANTHI P B, REDDY M L P, RAMAMOHAN T R, DAMODARAN A D. Liquid-liquid extraction of yttrium (Ⅲ) with mixtures of organophosphorus extractants: Theoretical analysis of extraction behaviour [J]. Hydrometallurgy, 1991, 27: 169-177.

[11] AKIBA K, YAMADA K, NAKAMURA S. Solvent extraction of lanthanides by acidic organophosphorus compounds [J]. Nihon Kagaku Kaishi, 1993: 555-560. (in Japanese)

[12] KUBOTA F, GOTO M, NAKASHIO F. Extraction of rare earth metals with 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester in the presence of diethylenetriaminepentaacetic acid in aqueous phase [J]. Solvent Extraction and Ion Exchange, 1993, 11: 437-453.

[13] HIRASHIMA Y, MUGITA M, SHIOKAWA J. Distribution equilibria of SmCl₃-EuCl₃-HCl and EuCl₃-GdCl₃-HCl systems with 1M di(2-ethylhexyl) phosphoric acid in n-heptane [J]. Journal of Inorganic and Nuclear Chemistry, 1975, 37: 1249-1253.

[14] HAN K S, TOZAWA K. Thermodynamic prediction of distribution coefficients for the solvent extraction of the rare earth metals [C]// Rare Earth Extraction, Preparation and Applications. Warrendale: TMS, 1988: 115-128.

[15] HINO A, NISHIHAMA S, HIRAI T, KOMASAWA I. Practical study of liquid-liquid extraction process for separation of rare earth elements with bis(2-ethylhexyl) phosphinic acid [J]. Journal of Chemical Engineering of Japan, 1997, 30: 1040-1046.

[16] FU N, TANAKA M. Modeling of the equilibria of yttrium(Ⅲ) and europium(Ⅲ) solvent extraction from nitric acid with PC88A [J]. Materials Transactions, 2006, 47: 136-142.

[17] MISHRA S L, SINGH H, GUPTA C K. Simultaneous purification of dysprosium and terbium from dysprosium concentrate using 2-ethyl hexyl phosphonic acid mono-2-ethyl hexyl ester as an extractant [J]. Hydrometallurgy, 2000, 56: 33-40.

[18] CASAS J M, PAPANGELAKIS V G, LIU H. Performance of three chemical models on the high-temperature aqueous Al₂(SO₄)₃- MgSO₄-H₂SO₄-H₂O system [J]. Industrial and Engineering Chemistry Research, 2005, 44: 2931-2941.

[19] MARCUS Y, KERTES A S. Ion exchange and solvent extraction of metal complexes [M]. London: Wiley, 1969: 499-574.

[20] MASON G W, SCHOFER N L, PEPPARD D F. The extraction of U(VI) and selected M(Ⅲ) cations by bis neo-octyl phosphoric acid in two different hydrocarbon diluents [J]. Journal of Inorganic and Nuclear Chemistry, 1970, 32: 3911-3922.

[21] MASON G W, METTA D N, PEPPARD D F. The extraction of selected M(Ⅲ) metals by bis 2-ethylhexyl phosphoric acid in n-heptane [J]. Journal of Inorganic and Nuclear Chemistry, 1976, 38: 2077-2079.

[22] DANESI P R, VANDEGRIFT G F. Activity coefficients of bis(2-ethylhexyl) phosphoric acid in n-dodecane [J]. Inorganic and Nuclear Chemistry Letters, 1981, 17: 109-115.

[23] ALSTAD J, AUGUSTON J H, DANIELSSEN T, FARBU L. A comparative study of the rare earth elements in extraction by HDEHP/SHELL SOL T from nitric and sulfuric acid solutions [C]// International Solvent Extraction Conference 1974, Lyon, France, 1974, 2: 1083-1102.

Corresponding author: TANAKA Mikiya; Tel: +81-29-861-8486; Fax: +81-29-861-8481; E-mail: mky-tanaka@aist.go.jp

DOI: 10.1016/S1003-6326(09)60202-8

(Edited by LI Xiang-qun)