Selective extraction of praseodymium ions from neodymium solution using a non-equilibrium extraction method

Journal of Alloys and Compounds, 192 (1993) 126-128 JALCOM 2147

126

Selective extraction of praseodymium ions from neodymium solution using a non-equilibrium extraction method Yukinori Minagawa Research Center, Mitsubishi Kasei Corporation, 1000 Kamoshida-cho, Midori-ku, Yokohama 227 (Japan)

Abstract A n advanced method for the separation of praseodymium ions from a mixed solution of neodymium and praseodymium ions has been investigated, using a non-equilibrium extraction with diethylenetriaminepentaacetic acid ( D T P A ) and a bis-(2-ethylhexyl)phosphoric acid (B2EHPA)-kerosine system in which the largest value of the separation factor /3vr-Nd (=D~/DNa), i.e. 5.0, was obtained for the first time. It was found that the experimental extraction rate was represented by the reversible pseudo-first-order reaction as V = k~[H + ]2[Ln3 + ]T. (a) - k"_ 1[H + ] [Ln 3÷ ](o) where subscript (a) and (0) represent aqueous and organic phases, respectively, and k"1 a n a~ k"-1 are kinetic constants for the forward and backward reactions respectively.

1. Introduction

Previously, we reported [1] an advanced purification method for removing yttrium ions from the other rare earth metal ions with the least extraction stages ever published. This was by using a non-equilibrium extraction system composed of diethylenetriaminepentaacetic acid (DTPA) and bis-(2-ethylhexyl)phosphoric acid (B2EHPA)-kerosine. Recently, pure neodymium oxide has been in demand as a source of the permanent magnetic alloy Fe-Nd-B. However, the separation of neodymium from praseodymium by solvent extraction has been extremely difficult. Therefore, it is necessary to develop an advanced extraction process with a larger separation factor. The non-equilibrium system [1] has been applied to separate these two ions because a larger separation factor can be expected than that achieved with the conventional extraction. In this paper, the author describes the extraction conditions to obtain a separation factor of 3 or more, since Omori et al. [2] reported a remarkably large factor of 2.5. Discussions on the extraction mechanism are also presented.

B2EHPA-kerosine solution in a separatory funnel of volume 300 c m 3 with a phase ratio of 50/50. Then, vigorous shaking was started by the shaker at room temperature (about 24 °C). After the shaking was stopped at a designated time, the aqueous phase was separated quickly from the organic phase. The concentrations of praseodymium and neodymium were determined spectrophotometrically (Hitachi 323) at 443 and at 574 nm for Pr 3+ and Nd 3+ respectively.

3. Results and discussion

The changes in the extraction fractions E(%) of praseodymium and neodymium are plotted as a function of the shaking time t in Fig. 1. The plots show that PrS.+ reached the extraction equilibrium faster than N d 3+ did under all the given extraction conditions. As the pH of the aqueous phase became lower, Pr 3+ was found to reach equilibrium faster than N d 3+. Then, the relationships between the separation factor flPr- Nd = DPr/DNd

D Ln= [Ln]T. (o)/[Ln]T. O) 2. Experimental details

The extraction experiments were carried out as follows: the mixed solution of praseodymium and neodymium chlorides and DTPA was added to a 0925-8388/93/$6.00

where the subscript T represents the total concentration of each ion, and (o) and (a) represent organic and aqueous phases, respectively, and the shaking time t are shown in Fig. 2, in which maximum values of the separation factor larger than 3 can be observed under all the experimental conditions applied in Fig. 1.

© 1 9 9 3 - Elsevier Sequoia. All rights reserved

Y. Minagawea Extraction of Pr ions from Nd solution 100

100

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time/min

where a is the initial total concentration of Ln 3+ in the aqueous phase, x is the concentration of Ln 3÷ extracted in the organic phase at time t, x~ is the concentration of Ln 3 ÷ extracted into the organic phase at equilibrium, k' =k'l +k'_l, and xe=k'la/(k~ +k'_ 1) is obtained from dx/dt=0. The following equation is easily derived from eqn.

(2): k' =k'l +k'_, = (l/t) ln{xe/(X e - x ) }

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and plots of In{xe/(xe-x)} vs. time for both ions gave straight lines passing around the origin. This means that the rate law must be a reversible pseudo-firstorder reaction as mentioned above. Then, the k' value obtained from the slope of line was separated into k] and k'_ ,. Relationships between the k'l and k'_, values for both ions and the hydrogen ion concentration at equilibrium are shown in Fig. 3. The experimental rate equation can thus be given V= k{ [H + ]2[Ln3 + IT. ( a )

5

(2)

[

Fig. 1. Extraction rates of Pr 3+ (©) and N d 3+ ( e ) ions: experiments 1-3 have the extraction conditions [DTPA]T/ ([pr3+]T+[Nd3+]T)=2, [ B 2 E H P A ] = I . 0 M as a monomer and p H values in the aqueous phase at equilibrium are 3.70 (experiment 1), 2.98 ( e x p e r i m e n t 2) a n d 2.49 (experiment 3); experiment 4 has the conditions [DTPA]T/([Pr3+]T+[Nd3+]T)=I.0 , [B2EHPA] = 1.5 M as a monomer and the pH is 2.98.

5

(1)

where the subscripts T, (a) and (o) represent the total concentration, and the aqueous phase and organic phase respectively, and k] and k'-a represent the forward and backward rate constants respectively. Now, we have

J'.<-~l

20

The largest value of 5.0 was obtained in experiment 4. The maximum values in experiments 1-3 become larger with decreasing pH values. The values of the separation factor near equilibrium, illustrated by an asterisk (*) in Fig. 2, are about 2.0, which is close to 2.4; i.e. the value calculated theoretically by the equation reported previously [1]. The extraction data from experiments 1-3 were analysed using eqns. (1) and (2) based on the rate law of reversible pseudo-first-order reaction, as predicted in the previous paper [1]. V= -d[Ln3+]T,(a)/dt=k'l[Ln3+]T,(a)-k'l[Ln3+](o)

I

100

127

4

i0

time/r~in

Fig. 2. Relationships between the separation factor /3Pr-Nd and shaking time: experiments 1--4 show the same extraction conditions as described in Fig. 1. Asterisks ( * ) represent values of the separation factor at equilibrium.

-- k t,_,[H

+ ][Ln 3+ ]T. (o)

(4)

where kj and k"_a represent the kinetic constants for the forward and backward reactions respectively. Figure 3 shows that the value of k'l for praseodymium is larger than that for neodymium in the given range of hydrogen ion concentration. This explains why praseodymium reaches equilibrium faster than neodymium does, as shown in Fig. 1. Figure 3 also shows that the value of k'_, for praseodymium is almost equal to that for neodymium in the hydrogen ion concentration. The reason was discussed by using SN2 mechanism in the author's previous paper [3]. However, it is not easy to elucidate why the forward and backward rates are proportional to the second and the first powers of the hydrogen ion concentration

Y. Minagawa / Extraction of Pr ions from Nd solution

128 i

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KH, 2

t

L n H Z - + H ÷ = LnHEZ

(7)

2.0

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1.0

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kl'

where KH, 1 and KH, 2 represent complex formation constants. The extraction reaction would be the reaction of LnH2Z with H X 2 - dissociated from the solvent of HzX2 at an interracial phase between the organic and aqueous phases such that

k ~'

LnH/Z + 3(HX2-)0) = Ln(HXE)aO) + H2Z3-

o o 0.0

K'x

(8)

Where the subscript i represents the interfacial phase. According to the SN1 mechanism, the following equations can be obtained from eqn. (8):

-1.0

A -2.0

I

I

I

t

I

-4

I

I

-3

z

f

LnHEZ

I

Ln 3+ + 3(HX2-)o) = Ln(HX2)30)

Fig. 3. Relationships between forward and backward rate constants k~ and k'-z of Pr 3+ (O) and Nd 3+ (A) ions, and the hydrogen ion concentration at equilibrium by using the kinetic data of experiments 1-3.

respectively. To confirm eqn. (4), the logarithmic values of the distribution ratio (D) at equilibrium for both ions were plotted against the logarithmic values of the hydrogen ion concentration in which the relations for both ions were given by D = [Ln 3+ ](o)/[Ln 3+ ]T. (a) = k"[H + ]1.o where k" is a proportional constant. This equation shows that eqn. (4) is valid, since the relation given by V=0 in eqn. (4) accounts for the above equation. Therefore, the following equation can be proposed as the overall reaction, by using a material balance of hydrogen ion and supposing that a predominant species in the aqueous phase is LnZ 2- formed by the general reaction of Ln 3+ with Z 5-, i.e. a species dissociated from DTPA. LnZ e- + H + + 3(H2X2)(o) = Ln(HX2)3(o) + n 4 z -

> Ln 3÷ +H2Z 3-

(9)

Kgx

-2

log ( Ill" ]/~d)

(10)

Supposing fast equilibria are established in eqns. (6), (7) and (10), the following theoretical rate equation can be given. V= - d [LnH2Z]/dt = d [Ln(HX2)3 ]0)/dt =ki[LnZ2-][H+] 2

(11)

where k'~ =kIKH, 1KH,2- Equation (11) is consistent with the terms of the forward rate in the experimental rate equation (eqn. (4)). The backward reaction rate was discussed by using the SN2 mechanism in which hydrogen ions might play a role as a catalyst in the complexforming reaction [3].

Acknowledgments The author is indebted to Prof. T. Yotsuyanagi of Tohoku University, and Drs. T. Onoda and K. Wada, and T. Okano, M. Tanaka and S. Kasuya of Mitsubishi Kasei Research Center for their encouragement and helpful discussions.

(5)

where H2X2 represents B2EHPA as a dimer. Equation (5) is acceptable, since the principal species of DTPA in the pH range 2.5--4.0 are considered to be H4Zand H3Z2- from the acid dissociation constants of DTPA reported by Moeller and Thompson [4]. Subsequently, an extracted species was supposed to be LnH2Z, which is formed by KH, 1

LnZ 2- + H + = L n H Z -

kz

(6)

References 1 Y. Minagawa and F. Yajima, Bull. Chem. Soc. Jpn., 65 (1992) 29. 2 H. Omori, H. Shibata, M. Sano and O. Nishimura, Nippon Kinzoku-Gakkaishi, 51 (1987) 645. 3 Y. Minagawa, DoctoralDissertation, TohokuUniversity, Japan, 1991. 4 T. Moeller and L. C. Thompson, J. Inorg. Nucl. Chem., 24 (1962) 499.