Hydrometallurgy44 (1997) 321-330
Extraction equilibrium and kinetics of neodymium with diisodecylphosphoric acid Kazuo Kondo *, Ling Xi Tao, Michiaki Matsumoto Department of Chemical Engineering and Materials Science, Doshisha University, Tanabe, Kyoto 610-03, Japan
Received 7 March 1996; accepted 27 June 1996
Abstract The extraction equilibrium of neodymium (Nd) with diisodecylphosphoric acid (DIDPA, HR) was measured at 303 K. The complex NdR 3 • 3HR was found to have formed and the extraction equilibrium constants were determined for n-heptane, toluene and benzene diluents. The effect of temperature on the extraction equilibrium was examined to elucidate the thermodynamics of the extraction reaction. This extraction process was found to be exothermic. The extraction rate of Nd with DIDPA was also measured using a stirred transfer cell. The dependencies of the extraction reaction on pH and extractant concentration were different for the various diluents used. The extraction rates were found to be limited by the diffusion processes of neodymium in the aqueous phase and the complex in the organic phase. The extractant, DIDPA, was found to be suitable for the extraction of lanthanides from strong acidic media from both equilibrium and kinetic aspects.
1. Introduction The solvent extraction technique has been applied to the treatment of high level liquid waste (HLW) from nuclear fuel reprocessing. Since H L W is generated as a nitric acid solution of approximately 2 m o l / d m 3, it is necessary that the extractant employed has an excellent extractability for lanthanides and actinides from the concentrated acidic solution. In the T R U E X solvent extraction process, a bifunctional neutral extractant, such as carbamoylmethylphosphoryl, is used . However, for the extraction and purification of lanthanides from mineral sources, acidic organophosphorus extractants, such as di(2-ethylhexyl) phosphoric acid (D2EI-IPA) and 2-ethylhexyl-phosphonic acid
* Corresponding author. Phone/Fax: + 81 774 65 6656. 0304-386X/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0304-3 86X(96)0005 4-0
K. Kondo et al. / Hydrometallurgy 44 (1997) 321-330
mono-2-ethylhexyl ester (EHPNA) have been used [2,3]. It is desirable to develop acidic organophosphorus extractants with a strong extractability for economic reasons. Ishida et al.  synthesized a series of alkyl cyclohexyl hydrogen phosphates to evaluate the effect of the structure of the extractants on the extraction behavior of rare earth metals. They clarified that the position of substituents on the cyclohexyl ring in the phosphate is closely related to the extractabilities and separation factors. Ohto et al. [5,6] pointed out that the extractability of acidic organophosphorus extractants depends on their acid dissociation constants. They found that the phenyl group adjacent to the phosphorus atom increased the acid dissociation constants of the extractants. In previous work [7,8], based on the above studies, we anticipated that dialkylphosphoric acids with bulky alkyl groups would show excellent extractability. From this viewpoint, we examined diisostearylphosphoric acid (DISPA), which has 18 carbon atoms and is commercially available, as an extractant. It was found that the complex formed (MR 3) using DISPA as extractant was different from that (MR 3 - 3HR) formed using D2EHPA or EHPNA as extractant, because of the steric hindrance of the bulky alkyl chain, and that the extractability of DISPA was not improved. This may be due to the low acid dissociation constant of DISPA . Tachimori et al.  studied the qualitative extraction of Nd(III) with diisodecylphosphoric acid (DIDPA) and found the potential ability of DIDPA for the extraction of lanthanides from a strong acidic solution. Few studies on extraction using DIDPA have been reported. Nakamura and Akiba  studied the transport of Eu(III) through a supported liquid membrane containing DIDPA as the carrier. A quantitative analysis of the extraction using DIDPA as the extractant has not been conducted at all so far. In this study, we investigated the equilibrium and kinetics of neodymium extraction with DIDPA. As the metal ion having the highest concentration, on average, of all the lanthanides present in the HLW , neodymium was chosen as a representative of the lanthanides. First, the extraction equilibrium was examined, and the diluent and temperature effects on the extraction equilibrium were clarified. Then the extraction kinetics of neodymium with DIDPA were examined using a stirred transfer cell.
2.1. Extraction equilibrium The extractant, DIDPA, was kindly supplied by Daihachi Chemical Co., Japan. The purity of DIDPA was determined by non-aqueous titration with ethanolic sodium hydroxide. Other organic and inorganic chemicals used were G.R. grade. The extraction equilibrium of neodymium with DIDPA was measured at various temperatures. The organic phase was prepared by dissolving the extractant in the diluent. The diluents used were benzene, toluene and n-heptane. The aqueous solution was prepared by dissolving neodymium chloride in 100 m o l / m 3 hydrochloric acid-sodium acetate solution whose pH was adjusted. Equal volumes of the aqueous and organic solutions were shaken in a thermostatically controlled bath for 24 h to attain equilibrium. After phase separation, the concentration of neodymium and the pH of the aqueous
K. Kondo et a l . / Hydrometallurgy 44 (1997) 321-330
solution were measured. The concentration of neodymium was determined spectrophotometrically by the Arsenazo III method. Experimental conditions were as follows; pH 0.4-3,5; CNo,0 = 0.1 m o l / m 3, C(HR)2,0 = 5--35 m o l / m 3, temperature 293-308 K. 2.2. Extraction rate
The stirred transfer cell shown in Fig. 1 was used to measure the rate of neodymium extraction by DIDPA at 303 K. This cell is used to elucidate the extraction mechanism of metal due to its definite interfacial area [7,8]. The cell consists of two compartments of equal volume (about 140 cm3), an upper compartment for the organic solution and a lower one for the aqueous solution. The interfacial area between both solutions is 1.3 × 10 - 3 m 2. The solutions in the cell were stirred in opposite directions by two flat-blade stirrers at 200 rpm, in order not to disturb the interface. The concentration change of neodymium in the aqueous solution with time was measured under various experimental conditions. The initial extraction rate of neodymium, R 0, was obtained from the concentration change of neodymium in the aqueous solution with time in the initial period. Experimental conditions were as follows; pH 0.8-3; CNd,0 = 0.02-1.5 mol/m3; C~HRI2,0 = 0.5--60 m o l / m 3.
A,B Upperand lower flange C
D,E Impeller F
Fig. 1. Experimental apparatus for measurement of extraction rate (stirred transfer cell).
K. Kondo et al./ Hydrometallurgy 44 (1997) 321-330
3. Results and discussion
3.1. Extraction equilibrium For the D2EHPA and EHPNA systems, it is known that the rare earth metal ion is extracted as a complex of composition, MR 3 • xHR into the organic solution with the dimer of the extractant . Therefore, the extraction equilibrium of neodymium with DIDPA is assumed to be:
(3 + x) Nd 3 ÷ + ~ ( H R ) 2 ~ N d R
where (HR) 2 represents the dimer of the extractant, and x and Kex are the solvation number and the extraction equilibrium constant, respectively. The distribution ratio of Nd, D, is given by: D = Cud •org/CNd ,aq = ~ "" ¢Xf'(3+x)/2/K~3 ~(HR)2 /~H
Eq. (2) can be rewritten as follows: l°g(DC3)
(3+x) 2 IogC(HR)2+ log Kex
The experimental results for the various organic solvents employed were plotted according to Eq. (3) and are shown in Fig. 2. It was found that the slopes of the straight lines are all 3, that is, x = 3. Therefore, the equation for the extraction of neodymium with DIDPA can be expressed as: N d 3+
+ 3(HR)2 ~ NdR 3 • 3HR + 3H +
O~ 64 ~ p e = 3 o 2
]D n-Heptane1.15] [O Toluene 1.95[ 0.5
log C(HR) 2
Fig. 2. Equilibrium plot of N d - D I D P A extraction system for various organic diluents.
K. Kondo et a l . / Hydrometallurgy 44 (1997) 321-330
Table 1 Extraction equilibrium constants Extractant
DIDPA DIDPA DIDPA EHPN A EHPNA EHPNA EHPNA D2EHPA D2EHPA
n-Heptane Toluene Benzene n-Heptane Toluene n-Dodecane Dispersol n-Heptane Toluene
1.9 × 10 s 1.1 × 10 ~ 8.7 8.2 X 10- 2 3.1 × 10 -4 6.9 x 10- 3 3.0X 10 -2 1.5 X 101 4.4 × 10- 2
This work This work This work     [ 12] [ 13]
The values of Kex obtained from the intercepts in Fig. 2 are listed in Table 1, along with those for EHPNA and D2EHPA as extractants [12-15]. From Table 1, it was found that the extractability of DIDPA is 10 2 and l 0 4 times as high as that of D2EHPA and EHPNA, respectively. Aliphatic diluents gave higher values of Kex compared with those for aromatic ones, which was the same result as seen in other extractant systems . DIDPA was found to be suitable for the extraction of rare earth metals from strong acidic media. The effect of temperature on the extraction equilibrium was also examined. Fig. 3 shows the temperature dependencies of the Kex values. The effect of temperature is relatively small and the extraction at low temperatures is somewhat better than that at high temperatures. This indicates that the extraction process is exothermic. This behavior was similar to that in E u - D 2 E H P A system .
 n-Heptane 0
101 100 3.2
~ i J 3.3 3.4 103(1/T) [K -1]
Fig. 3. Effect of temperature on extraction equilibrium constant of Nd with DIDPA.
K. Kondo et al./ Hydrometallurgy 44 (1997) 321-330
O Toluene &
 FIC~ ~ _ D_O6.a-O--- ./
./o ~ A
ONa,o = 0.5 m o l / m 3
C(.m~,0 = 25 m o l / m 3
pH Fig. 4. Effect of pH on initial extraction rate.
3.2. Extraction rate
Fig. 4 shows the relation between the logarithm of the initial extraction rate of neodymium, R 0, and pH. In the range of low pH, the slope of this relation is 1.0 for toluene and benzene, and 0.5 for n-heptane, but in the high pH range, the slopes approach zero. Fig. 5 shows the relation between R 0 and the initial concentration of neodymium, CNd.o, on a log scale. R 0 is proportional to CNd,0 for all the diluents. Fig. 6 shows the relation between R 0 and the initial concentration of dimer of the extractant,
O Toluene &
O / O /
pH = 2.2 C(HR)2,0 =
mol/m 3 i
CNd,0 [ m o l / m 3] Fig. 5. Effect of CNd,Oon initial extraction rate.
K. Kondo et a l . / Hydrometallurgy 44 (1997) 321-330
 n-Heptane O Toluene u}
1 0 -5
D  _ r-17 O--A~). O
CNd,0 = 0.5 mol/m 3 10"7 0.1
. . . . . . . .
C(HR)2,0 [mol/m 3] Fig. 6. Effect of C(HR)2, 0 on initial extraction rate.
C(~R)2.0, on a log scale. In the range of low C(HR)2,0, the slope of this relation is 1.5 for toluene and benzene, and 0.5 for n-heptane, but in the high C(HR)2.0, the slopes approach zero. It was found that the apparent reaction order for the overall extraction reaction was dependent on the solvents used. Each solid curve in Figs. 4 - 6 was calculated by a method shown later. In previous papers [7,17], the extraction rates of metals with D2EHPA, EHPNA and DISPA were reported to be limited by diffusion processes. In the present system, the influence of diffusion on the extraction process was examined. The diffusion equations of neodymium and the complex between the metal and the extractant should be considered as Eq. (5) and Eq. (6), respectively. It is not necessary to consider the diffusion equations for the extractant and hydrogen ion because the extractant is present in a large excess compared with neodymium and the buffer solution is used. R = kNo(CNd -- CNd,i )
where k i is the mass transfer coefficient of the species j and subscript i represents the zone adjacent to the interface. In the initial period of the extraction, Eq. (5) and Eq. (6) are converted to the following equations: R o = kNd( end,0 -- CNd,i )
n0 = kcCc,
Furthermore, the overall mass transfer coefficient of neodymium, K, is defined as: R o = KCNo, o
The value of K is obtained from each experimental result using Eq. (9).
K, Kondo et al. / Hydrometallurgy 44 (1997) 321-330 10 6
j/I O A
o A o ,--,
Yz 1 0 5
I  n-Heptane O Toluene [
10 4 ........ I ........ L ........ , ....... 1 0 "1 10 0 101 10 2 10 3
(CH,0/C(HR)2,0) 3 ['] Fig. 7. Relationship between 1/ K - l / kNd and (C H,o / C(HR)2,O)3"
In the interfacial zone, the species are assumed to be in an equilibrium state if the extraction reaction proceeds very rapidly: Kox =
3 3 cc,C.o/(CN ,q..,2.0)
From Eqs. (7)-(10), Eq. (11) is derived: 1
C H.0 3
k c Kex C?HR)~.o
Considering the above equations, in the low pH range in Fig. 4 and in the low C(HR):,0 range in Fig. 6, the extraction rate is anticipated to be limited by the diffusion process of the complex. On the other hand, in the high pH range in Fig. 4 and in the high C(HR):,0 range in Fig. 6, the extraction rate is anticipated to be limited by the diffusion process of neodymium. The results in Fig. 5 also suggest diffusion control of neodymium. The mass transfer coefficient of neodymium, kNd, was evaluated from Fig. 5 as 1.2 × 10 -5 m / s . This value is similar to those obtained in the samarium and europium extractions with DISPA using the identical experimental apparatus [7,8]. The mass transfer coefficient of the complex, k c, is obtained by rearranging the data in the diffusion control regime of the complex according to Eq. (11). The plot is shown in Fig. 7. The values of k c obtained were (1.5 _+ 1.0) × 10 -6, (2.6 _+ 2.0) × 10 .6 and (4.6 _+ 2.7) × 10 .6 m / s for n-heptane, toluene and benzene diluents, respectively. The solid lines in Figs. 4 - 6 are the calculated ones using the above values. Fairly good agreement was obtained. The behavior of the extraction kinetics in the diffusion control regime of the complex for the n-heptane system is very different from those for the toluene and benzene systems. This is caused by the large difference in the Kex values.
K. Kondo et aL / Hydrometallurgy 44 (1997) 321-330
4. Conclusions The extraction equilibrium of neodymium (Nd) with diisodecylphosphoric acid (DIDPA, HR) was measured at 303 K. The complex NdR 3 - 3HR was found to be formed and the extraction equilibrium constants were determined for n-heptane, toluene and benzene diluents. The extractability of DIDPA is much stronger than that of di(2-ethylhexyl)phosphoric acid and 2-ethylhexyl-phosphonic acid mono-2-ethylhexyl ester. The effect of temperature on the extraction equilibrium was examined. It was found that the extraction process is exothermic. The extractability remains approximately the same even at higher temperatures since the effect of temperature on the extraction is relatively small. The extraction rate of neodymium with DIDPA was measured using a stirred transfer cell. Dependencies of the initial extraction rate on the chemical species in a diffusion control regime of the complex were different for the various diluents used. n-Heptane diluent gave the highest extraction rate among the diluents investigated. The extraction rates were found to be limited by the diffusion processes of neodymium in the aqueous phase and by the complex in the organic phase. From the equilibrium and kinetic studies, it was found that DIDPA diluted by n-heptane was suitable for the extraction of neodymium from a concentrated acidic solution.
concentration of species j ( j = Nd, HR, H) distribution ratio of neodymium extraction equilibrium constant mass transfer coefficient of complex mass transfer coefficient of neodymium extraction rate solvation number
subscripts i 0
zone adjacent to interface initial state
m/s m/s mol/(m2s)
Acknowledgements The authors thank Daihachi Chemical Co. for supplying samples of diisodecylphosphoric acid.
References [l] Danesi, P.R., Chiarizia, R., Rickert P. and Horwitz, E.P., Separation of actinides and lanthanides from acidic nuclear wastes by a supported liquid membrane. Solvent Extr. Ion Exch., 3 (1985): 1 l 1-147.
K. Kondo et al. / Hydrometallurgy 44 (1997)321-330
 Koch, D.F.A., Rare earth extraction and separation. Mater. Austral., May (1987): 12-15.  Pierce, T.B. and Peck, P.F., The extraction of the lanthanide elements from perchloric acid by di-(2-ethylhexyl)hydrogen phosphate. Analyst (London), 88 ( 1963): 217-220.  Ishida, K., Takahashi, T., Nakamura, M. and Sato, T., Structural effect of phosphoric acid ester having cyclohexyl groups on the extraction of rare earths. Bunseki Kagaku, 42 (1983): 655-658.  Ohto, K., lnoue, K., Goto, M., Nakashio, F., Nagasaki, T., Shinkai, S. and Kago, T., Solvent extraction of trivalent yttrium, holmium, and erbium by novel types of acidic organophosphonates. Bull. Chem. Soc. Jpn., 66 (1993): 2528-2535.  Ohto, K., Yoshida, S., Inoue, K., Ohtsuka, M., Goto, M. and Nakashio, F., Solvent extraction equilibria of rare earth metals by acidic organophosphorus extractants with bulky substituents. Anal. Sci., I I (1995): 637-641.  Kondo, K., Hashimoto, T., Sumi, H. and Matsumoto, M., Mechanisms of samarium extraction with diisostearylphosphoric acid and its permeation through a supported liquid membrane. J. Chem. Eng. Jpn., 28 (1995): 511-516.  Kondo, K. and Matsumoto, M., Solvent extraction of europium with diisostearylphosphoric acid and its application to an emulsion liquid membrane technique. Separ. Sci. Technol., 31 (1996): 557-567.  Inoue, K., Yoshizuka, K. and Ohto, K., Solvent extraction characteristics for mutual separation of rare earths of Cyanex 272 and TR-83. Kagaku Kogaku Ronbunshu, 21 (1995): 603-607.  Tachimori, S., Sato, A. and Nakamura, H., Extraction of lanthanides(llI) with isodecyl phosphoric acid from nitric acid solution. J. Nucl. Sci. Technol., 15 (1978): 421-425.  Nakamura, S. and Akiba, K., Transport of europium(Ill) through a supported liquid membrane containing diisodecylphosphoric acid. Separ. Sci. Technol., 24 (1989): 673-683.  Kubota, F., Goto, M. and Nakashio, F., Extraction of rare earth metals with 2-ethylhexyl phosphinic acid mono-2-ethylhexyl ester in the presence of diethylenetriaminepentaacetic acid in the aqueous phase. Solvent Extr. Ion Exch., 11 (1993): 437-453.  Mori, Y., Ohya, H., Ono, H. and Eguchi, W., Extraction equilibrium of Ce(lll), Pr(IlI) and Nd(lll) with acidic organophosphorus extractants. J. Chem. Eng. Jpn., 21 (1988): 86-91. [t4] Ma, E., Yan, X., Wang, S, Long, H. and Yuan, C., Solvent extraction of lanthanides by 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester. Proc. Int. Solvent Extr. Conf. '80. No. 80-147 (1980).  Teramoto, M., Sakuramoto, T., Koyama, T., Matsuyama, H. and Miyake, Y., Extraction of lanthanides by a liquid surfactant membrane. Separ. Sci. Technol., 21 (1986): 229-250.  Danesi, P.R., Chiarizia, R., Raieh, M.A. and Scibona, G., Enthalpy and entropy variations in the liquid cation exchange of some lanthanide ions by dinonylnaphthalenesulfonic acid and bis(2-ethylhexyl)phosphoric acid. J. Inorg. Nucl. Chem., 37 (1975): 1489-1493.  Miyake, Y., Matsuyama, H., Nishida, M., Nakai, M., Nagase, N. and Teramoto, M., Kinetics and mechanism of metal extraction with acidic organophosphorus extractants (I): Extraction rate limited by diffusion process. Hydrometallurgy, 24 (1990): 19-35.