Modeling of the Extraction of Nitric Acid and ...

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Modeling of the Extraction of Nitric Acid and Neodymium Nitrate from Aqueous Solutions over a Wide Range of Activities by CMPO a

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S. Belair , A. Labet , C. Mariet & P. Dannus

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CEA‐SACLAY/INSTN/UECCC, Gif Sur Yvette Cedex, France Published online: 18 Aug 2006.

To cite this article: S. Belair , A. Labet , C. Mariet & P. Dannus (2005) Modeling of the Extraction of Nitric Acid and Neodymium Nitrate from Aqueous Solutions over a Wide Range of Activities by CMPO, Solvent Extraction and Ion Exchange, 23:4, 481-499, DOI: 10.1081/SEI-200068435 To link to this article: http://dx.doi.org/10.1081/SEI-200068435

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Solvent Extraction and Ion Exchange, 23: 481–499, 2005 Copyright # Taylor & Francis, Inc. ISSN 0736-6299 print/1532-2262 online DOI: 10.1081/SEI-200068435

Modeling of the Extraction of Nitric Acid and Neodymium Nitrate from Aqueous Solutions over a Wide Range of Activities by CMPO S. Belair, A. Labet, C. Mariet, and P. Dannus CEA-SACLAY/INSTN/UECCC, Gif Sur Yvette Cedex, France

Abstract: A thermodynamic model that allows one to determine the number and the stoichiometry of the complexes formed between nitric acid, neodymium nitrate (Nd(NO3)3), and octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) diluted in nitrophenylhexyl ether (NPHE) also called 1-(hexyloxy)-2nitrobenzene, is presented in this work. The Mikulin-Sergievskii-Dannus’ model was used to model the extraction at 258C of the HNO3-H2O-NPHE, HNO3-H2O-CMPO 0.2 mol kg21-NPHE, Nd(NO3)3-H2O-CMPO 0.2 mol kg21-NPHE and Nd(NO3)3HNO3-H2O-CMPO 0.2 mol kg21-NPHE systems. The stoichiometric mean activity coefficients of components in the binary aqueous phases were determined by interpolation from the experimental data published in the literature. For mixtures, they were calculated from experimental data using the Mikulin’s equation, whereas the activity coefficients of the species in the organic phase were calculated from the Sergievskii-Dannus’ equation. A satisfactory description of the distribution of neodymium nitrate, nitric acid, and water was then obtained over a wide range of neodymium and nitric acid concentrations in the aqueous phase by taking into account the formation of the following complexes: ðHNO3 ÞðNPHEÞ, ðHNO3 ÞðH2 OÞðNPHEÞ, ðHNO3 ÞðH2 OÞðCMPOÞ2 , ðHNO3 ÞðCMPOÞ, ðHNO3 Þ2 ðCMPOÞ, ðHNO3 Þ3 ðCMPOÞ, ðNdðNO3 Þ3 ÞðCMPOÞ3 (1 : 3), ðNdðNO3 Þ3 ÞðCMPOÞ2 (1 : 2), ðNdðNO3 Þ3 ÞðCMPOÞ (1 : 1), ðNdðNO3 Þ3 ÞðHNO3 ÞðCMPOÞ3 (1 : 1 : 3), ðNdðNO3 Þ3 ÞðHNO3 ÞðCMPOÞ2 (1 : 1 : 2) and ðNdðNO3 Þ3 ÞðHNO3 ÞðCMPOÞ (1 : 1 : 1). Keywords: Neodymium nitrate, nitric acid, extraction systems, extraction isotherms, CMPO, NPHE, thermodynamic model, activity coefficient

Received 10 December 2004, Accepted 7 April 2005 Address correspondence to S. Belair, CEA-SACLAY/INSTN/UECCC, 91191 Gif Sur Yvette Cedex, France. Fax: 33 1 69 08 77 82; E-mail: [email protected] 481

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INTRODUCTION Octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) is a well-known extractant for the extraction of lanthanide(III) or actinide(III) ions from diluted or concentrated salt aqueous solutions. The extraction is often carried out in the presence of a salting-out agent, mostly nitric acid. And although a lot of studies were described in many papers,[1 – 17] few studies were devoted to physico chemical modeling of these extraction systems. The number of CMPO molecules per metal varies from one study to another between 1 ðLnðNO3 Þ3 ÞðCMPOÞ (1 : 1)[22] and 4 ðLnðNO3 Þ3 ÞðCMPOÞ4 (1 : 4).[11] The polarity of the diluent[18] and the steric hindrance of the substituents bonded to the phosphoryl group[19] influence the coordination of the complex. The experimental conditions (CMPO/Ln concentration ratio, nitric acid content) also show an effect on the coordination. Moreover, the experimental conditions used are always limited to the predominance of CMPO relative to the lanthanide concentration. These conditions favor the formation of CMPO-rich species. Without nitric acid, authors indicate the 1 : 4,[11] 1 : 3,[12,14,20] and 1 : 2[15] stoichiometries using the slope-analysis technique. With nitric acid, the authors suggest the same stoichiometries that were mentioned previously.[1,2,6,19,21] Garcia-Carrera suggests a 1 : 1 complex for the Eu(NO3)3 or Ce(NO3)3/ HNO3/CMPO/nitrophenylhexyl ether (NPHE) systems.[22] Actually, the presence of the nitric acid increases lanthanide activity leading to the formation of lanthanide-rich species and especially the 1 : 1 complex. But, at the same time, the main drawback is probably the formation of mixed species, comprising lanthanide, CMPO, but also HNO3, that is to say ðLnðNO3 Þ3 Þx ðHNO3 Þy ðCMPOÞz species. Indeed, many authors [3,8,9,16] suggested the formation of mixed complexes ðLnðNO3 Þ3 ÞðHNO3 Þ13 ðCMPOÞ3 in the presence of concentrated nitric acid ([HNO3] . 1 mol L21). The present study was carried out on the Nd(NO3)3-HNO3-H2O-CMPO 0.2 mol kg21-NPHE system. The influence of the high polarity of the diluent nitrophenylalkyl ether on the ion-pair dissociation was previously discussed.[17] On the basis of this discussion, the ion-paired dissociation was considered as a minor phenomenon. In this goal, the subsystems (HNO3-H2O/NPHE, HNO3-H2O/CMPO-NPHE, Nd(NO3)3-H2O/CMPONPHE) were studied individually to differentiate the simple and the mixed complexes formed in the complete system Nd(NO3)3-HNO3-H2O-CMPO 0.2 mol kg21-NPHE. A previous thermodynamic study[17] has already been carried out in order to identify all species able to form in the Nd(NO3)3H2O-CMPO 0.2 mol kg21-NPHE system. The experimental extraction isotherms of acid and water from binary aqueous solutions (HNO3-H2O) and of neodymium, acid, and water from ternary aqueous solutions (Nd(NO3)3HNO3-H2O) were determined. The Mikulin-Sergievskii-Dannus’ model was applied to determine the number and the stoichiometry of the complexes in the organic phase as well as their associated extraction constants.

Modeling of the Extraction of Nitric Acid and Neodymium Nitrate

483

The results of this modeling were compared to those obtained with a spectrometric method. Namely, electrospray ionization/mass spectroscopy (ESI/ MS) was used, which allows the detection of complexes formed in the organic phase.


METHOD OF MODELING The method of modeling was described in detail by Mokili for the H2O-NH4NO3TBP-dodecane, HNO3-H2O-NH4NO3-TBP-dodecane,[23] and Ln(NO3)3-H2OTBP-dodecane, Ln(NO3)3-NH4NO3-H2O-TBP-dodecane, Ln(NO3)3-HNO3H2O-TBP-dodecane systems.[24] Software was developed to model as completely as possible the extraction system corresponding to a set of general extraction equilibria of the form,[25,26] iE þ jH þ þ kM þ þ ð j þ kÞNO 3 þ hH2 O þ dDil O ðEÞi ðHÞj ðMÞk ðNO3 Þ jþk ðH2 OÞh ðDilÞd

ð1Þ

where the over bar indicates the organic phase species. Depending on the value of the stoichiometric coefficients (i, j, k, h, d), it is possible to take into account various types of equilibria: . . . .

Polymerization of the extractant (E) and/or diluent molecule (Dil); Hydration of the extractant and/or diluent molecule; Distribution of the acid or salt between the aqueous and diluent phases; Extraction of the acid and or salt, with or without hydration, by the extractant or diluent.

The water molecules of hydration given in the extraction equilibrium show strong interactions with the organic complex. However, many studies[25,27] have shown that water interacting weakly with the organic-phase species (hydrated or not) took part in the general phenomenon of hydration of the organic phases of these systems. Thus, we refer to these weakly interacting water molecules as those that are solubilized by the organic species to distinguish them from the hydration water molecules without ambiguity. Those physical interactions involved with the organic-phase species can be described therefore by using the Sergievskii’s equation[28] improved by Dannus[25] in the way already detailed by Mokili.[23] For an organic species i this relation is given by: gi ¼ g i exp½si ð1 aH2 O Þ

ð2Þ

where g i is the activity coefficient of species i in the same organic phase (i.e, same diluent and same concentration), but in equilibrium with pure water (aH2O ¼ 1), s i is the number of water molecules solubilized by species i, for aH2O ¼ 1.

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For the specific case of an extraction equilibrium that corresponds to, iCMPO þ kNd 3þ þ jH þ þ ð j þ 3kÞNO 3 þ hH2 O þ dNPHE O ðCMPOÞi ðHÞj ðNdÞk ðNO3 Þð jþ3kÞ ðH2 OÞh ðNPHEÞd ð3Þ The associated thermodynamic extraction constant can be written, ½ijkhd


K0 ¼

d

i

g ijkhd i

ajH akNd ahH2 O ½CMPO ½NPHE g CMPO gNPHE h i exp ðs ijkhd i:sCMPO d:sNPHE Þ ð1 aH2 O Þ

d

ð4Þ

where ½ijkhd is the concentration of the complex in the organic phase, ½CMPO and ½NPHE are the concentrations of the free extractant and the free diluent in the organic phase. At this stage, only non-ideality induced by physical interactions between the organic species and the diluent are expressed in the terms g . Assuming for a constant extractant/diluent ratio, that the ratio between activity coefficients g is constant along an extraction isotherm, it is then possible to write an “effective” extraction constant defined for a given extractant/diluent proportion, i

0

K¼K

g CMPO gNPHE

d

g ijkhd

¼

½ijkhd ajH

akNd

ahH2 O

i

½CMPO ½NPHE h i exp ðs i:s d:s Þ ð1 a Þ H O 2 ijkhd CMPO NPHE

d

ð5Þ

where aNd ¼ aNd(NO3)3 ¼ gNd(NO3)34 . [Nd(III)] . [NO3]3T, and aH ¼ aHNO3 ¼ gHNO32 . [H]T . [NO3]T with gNd(NO3)3 and gHNO3, the stoichiometric mean activity coefficient of Nd(NO3)3 and HNO3 respectively, with [Nd(III)], [NO3]T, and [H]T, the total concentration of neodymium, nitrate and acid constituents respectively. In the case of a binary solution, the stoichiometric mean activity coefficient of the component and the activity of water are determined by interpolation from the experimental data published in the literature.[29 – 34] In the case of a mixture of N electrolytes, the stoichiometric mean activity coefficients are calculated by Mikulin’s relationship:[35,36]

gðMXÞ1 ¼

bi nðMXÞ1 mbi ðMXÞ1 gðMXÞ1 N P nðMXÞi mðMXÞi

ð6Þ

i¼1

where g(MX)1 is the activity coefficient for the electrolyte 1 in the mixture, g(MX)1bi is the activity coefficient for the electrolyte 1 in binary solution of the same water activity as the mixture, n(MX)1 is the number of ions


485

resulting from the total dissociation of the electrolyte 1, n(MX)i is the number of i ions resulting from the total dissociation of the electrolyte i ð1 ! NÞ, m(MX)i i bi is the molality of electrolyte i ð1 ! NÞ in the mixture, m(MX)1 is the molality of the electrolyte 1 in the binary solution of the same water activity as the mixture. The establishment of this formula is valid if the mixture obeys Zdanovskii’s rule,[37,38] which is expressed mathematically by the relationship:


N X mi ¼ 1 at aH2 O ¼ constant mbi i¼1 i

ð7Þ

where mi is the concentration of the electrolyte i in the mixture, mibi is the concentration of the electrolyte i in the binary solution of the same water activity as that of the mixture. Its validity has been demonstrated for a great number of systems by Zdanovskii himself and other workers.[25,37,39 – 43] The stoichiometric mean activity coefficients of the electrolytes in the mixture and the associated water activity were calculated by a computer program established in our laboratory on the basis of Equations (6) and (7) using physicochemical data (concentration, stoichiometric mean activity coefficient, water activity) of the binary solutions published in the literature constituting the mixture. In our study, the thermodynamic modeling consists of testing the validity of a set of organic species with respect to the experimental extraction isotherms. If this set constitutes a convincing representation of the organic phase, a good agreement between the calculated and experimental values for salt, acid, and water extraction isotherms should be observed. This step of the calculation consists in an adjustment of parameters (K, s) with respect to each of the selected species.

EXPERIMENTAL DATA The nitric acid binary aqueous solutions, the neodymium nitrate binary solutions, and the nitric acid—neodymium nitrate ternary solutions were equilibrated by shaking at 25 + 18C for 1 hour with equal volumes of CMPO diluted at 0.2 mol kg21 with NPHE. The phases were separated by 15 minutes of centrifugation at 5600 rpm. After separation, the density value for each phase was measured at 25 8C with a DMA 602 Anton Paar densimeter and the water content of the organic phase was determined by coulometry using the Karl-Fischer method.[44] The concentration of nitric acid in the aqueous and organic phases was determined by potentiometry. The concentration of neodymium in the aqueous phase was determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) (Perkin Elmer OPTIMA 2000). The neodymium content of the organic solution was determined according the following equation: mNdðNO3 Þ3eq ¼ mNdðNO3 Þ3i mNdðNO3 Þ3eq

ð8Þ


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S. Belair et al.

where mNdðNO3 Þ3eq is the lanthanide concentration in the organic phase at equilibrium, mNd(NO3)3i is the initial lanthanide concentration in the aqueous phase, mNd(NO3)3eq is the lanthanide concentration in the aqueous phase at equilibrium. The concentrations in the aqueous solutions are expressed in mol kg21 of water. The concentrations mH2 O ; mH , and mNd in the organic phase are expressed in mol kg21 of NPHE. The organic solutions were analyzed by electrospray ionization/mass spectroscopy (ESI/MS) (Quattro II Micromass). The mass spectrometer was operated by applying a voltage of 3.5 kV to the capillary. The electrospray source was used in the following typical conditions. The source temperature was maintained at 808C and nitrogen was used as bath and nebulization gas. A cone voltage value of 40 V was applied, and the mobile phase was pure methanol. An aliquot of the organic phase was diluted 200 times with methanol. RESULTS AND DISCUSSION Extraction System: Nitric Acid-Water-NPHE Preceding studies[22] have shown that the NPHE diluent could extract nitric acid, but no stoichiometry has been proposed. Figure 1a shows the extraction experimental isotherm for HNO3 along with the water isotherm. It can be seen that the shape of the isotherms shows an exponential increase of the acid and water concentrations extracted for an acid concentration in the aqueous phase higher than 4 mol kg21. Figure 1b shows the acid and water contents of the organic phase as a function of nitric acid activity. In this representation, the variation is linear indicating that there is certainly only one stoichiometry binding NPHE and nitric acid. Finally, the acid concentrations extracted by NPHE are weak, and 91% of NPHE molecules remain free. A good agreement between calculated and experimental isotherms (Fig. 1a) taking into account both species ðHNO3 ÞðNPHEÞ and ðHNO3 ÞðH2 OÞðNPHEÞ was obtained. The optimized parameters of these complexes are presented in Table 1. Extraction System: Nitric acid-Water-CMPO 0.2 mol kg21-NPHE Figure 2 shows the extraction isotherms of nitric acid and water by CMPO diluted at 0.2 mol kg21 in NPHE. To specify the CMPO component with respect to acid extraction, the acid and water contents extracted by the NPHE have been calculated with the parameters (K,s) previously adjusted and deduced to the experimental data. Then, the final result corresponds to the acid and water concentrations extracted by the CMPO only (Fig. 3). The acid concentration in the organic phase is higher than the total CMPO concentration for an acid content in the aqueous phase higher than

487



Figure 1. Experimental and calculated extraction isotherms as a function of nitric acid concentration (a) or activity (b) in the aqueous phase. Extraction system: HNO3-H2O-NPHE at 258C.

2.5 mol kg21. In the end of the isotherm, there are more than two acid molecules for one CMPO molecule. The modeling will thus require at least three successive complexes ðHNO3 Þn ðCMPOÞ with increasing stoichiometry in nitric acid (1 : 1, 2 : 1, and 3 : 1 complexes). Finally, the weak increase of

488

S. Belair et al. Table 1. Optimized parameters of the complexes present in the organic phase for the extraction system HNO3-H2O-NPHE (molality scale) at 258C Species


ðHNO3 ÞðNPHEÞ ðHNO3 ÞðH2 OÞðNPHEÞ

Keff.

s

3.1 1025 1.7 1024

0.2 0.0

water concentration for the higher acid concentrations seems to show a water solubilization rather as a hydration of acid-rich complexes. The parameters for each species concerning the NPHE have been already determined (Table 1). The adjustment is only realized on the complexes with CMPO, acid and water. A good agreement between calculated and experimental isotherms (Fig. 2) taking into account the species ðHNO3 ÞðH2 OÞðCMPOÞ2 , ðHNO3 ÞðCMPOÞ, ðHNO3 Þ2 ðCMPOÞ, and ðHNO3 Þ3 ðCMPOÞ was obtained. The parameters of each new complex are represented in Table 2. The complex containing two CMPO molecules is the only one to be hydrated and the 1 : 3 complex solubilizes a lot of water. These stoichiometries have been already suggested in the literature[22,45,46] but information about their hydration was not previously reported.

Figure 2. Experimental and calculated extraction isotherms as a function of nitric acid concentration in the aqueous phase. Extraction system: HNO3-H2O-CMPO 0.2 mol kg21-NPHE at 258C.



489

Figure 3. Experimental extraction isotherms as a function of nitric acid concentration in the aqueous phase. The y-axis labels mHNO3,org,CMPO and mH2O,org,CMPO indicate that the acid and water concentrations are extracted by the CMPO only. Extraction system: HNO3-H2O-CMPO 0.2 mol kg21 at 258C.

The species predominance diagram for nitric acid is given in Fig. 4. The proportion of 1 : 1 complex is higher than that for the other complexes for an acid concentration lower than 6 mol kg21 in the aqueous phase.

Extraction System: Neodymium Nitrate-Water-CMPO 0.2 mol kg21-NPHE The study of the extraction system Nd(NO3)3-H2O-CMPO 0.2 mol kg21NPHE was described in detail in a previous article.[17] The Table 3 recounts Table 2. Optimized parameters of the complexes, between HNO3 and CMPO, present in the organic phase for the extraction system HNO3-H2O-CMPO 0.2 mol kg21-NPHE (molality scale) at 258C Species ðHNO3 ÞðH2 OÞðCMPOÞ2 ðHNO3 ÞðCMPOÞ ðHNO3 Þ2 ðCMPOÞ ðHNO3 Þ3 ðCMPOÞ

Keff.

s

3.0 101 3.7 100 6.8 1022 9.2 1025

0.0 0.3 0.7 1.7


490

S. Belair et al.

Figure 4. Speciation diagram for nitric acid in the organic phase. Extraction system: HNO3-H2O-CMPO 0.2 mol kg21-NPHE at 258C.

the main results obtained by the thermodynamic modeling. The 1 : 1 complex is only present for high neodymium activities. These activities can be reached only in presence of a salting-out agent, such as NH4NO3. Extraction System: Neodymium Nitrate-Nitric Acid-Water-CMPO 0.2 mol kg21-NPHE To complete this study, extraction experiments of neodymium nitrate have been performed in the presence of nitric acid to reveal or not the formation of mixed species, that is to say ðNdðNO3 Þ3 Þx ðHNO3 Þy ðCMPOÞz species. Extraction experTable 3. Optimized parameters of the complexes present in the organic phase for the extraction system Nd(NO3)3-H2O-CMPO 0.2 mol kg21-NPHE (molality scale) at 258C[17] Species ðNdðNO3 Þ3 ÞðCMPOÞ3 ðNdðNO3 Þ3 ÞðCMPOÞ2 ðNdðNO3 Þ3 ÞðCMPOÞ

Keff.

s

1.8 106 5.6 104 2.0 101

0.0 0.0 0.0



491

iments were performed with increasing neodymium concentration and a nitric acid concentration about 3 mol kg21 at equilibrium. The neodymium extraction isotherm is compared with the isotherm calculated without nitric acid (Fig. 5). The isotherm obtained in the presence of HNO3 is higher than the one without it, and this isotherm exceeds the concentration limit of mNd ¼ 0:1 mol kg1 expected for a Nd:CMPO stoichiometry of 1 : 2, indicating that a complex with one molecule of neodymium for one molecule of CMPO is present. In Fig. 6, the neodymium concentration in the organic phase is presented as a function of the thermodynamic activities of neodymium in the aqueous phases at equilibrium. The experiments performed without nitric acid (0 , aNd , 0.13) have shown that the proportion of the ðNdðNO3 Þ3 ÞðCMPOÞ complex is always very weak.[17] With nitric acid, the neodymium activities (1023 , aNd , 1) reach values that favor formation of a complex with one molecule of neodymium for one molecule of CMPO in important proportion. The isotherm’s representation as a function of thermodynamic activity of neodymium (Fig. 6) also illustrates the competitive extraction phenomenon of the acid. At equilibrium, for even neodymium activities, the neodymium concentration in the organic phase is lowest in the presence of acid. Indeed, a part of the CMPO is consumed by the acid extraction. In these experiments, when the neodymium activity decreases,

Figure 5. Extraction isotherms as a function of neodymium nitrate concentration in the aqueous phase with (circle) and without (line) nitric acid. Extraction systems: Nd(NO3)3-H2O-CMPO 0.2 mol kg21-NPHE (line) and Nd(NO3)3-HNO3 3 mol kg21 -H2O-CMPO 0.2 mol kg21-NPHE (circle) at 258C.


492

S. Belair et al.

Figure 6. Experimental extraction isotherms as a function of neodymium nitrate activity in the aqueous phase with (square) and without (triangle) nitric acid. Extraction systems: Nd(NO3)3-H2O-CMPO 0.2 mol kg21-NPHE (triangle) and Nd(NO3)3-HNO3 3 mol kg21-H2O-CMPO 0.2 mol kg21-NPHE (square) at 258C.

the ratio aHNO3/aNd increases. Thus, the competitive extraction of the nitric acid is increasingly important. Fig. 6 also presents the calculated extraction isotherm of neodymium taking into account all the complexes adjusted until now, that is to say the “simple” complexes, from the subsystems (HNO3H2O/NPHE, HNO3-H2O/CMPO-NPHE, Nd(NO3)3- H2O/CMPO-NPHE), 1. 2. 3.

complexes with NPHE and acid: ðHNO3 ÞðNPHEÞ and ðHNO3 ÞðH2 OÞ ðNPHEÞ ðNPHEÞ; complexes with CMPO and acid: ðHNO3 ÞðH2 OÞðCMPOÞ2 , ðHNO3 Þ ðCMPOÞ ðCMPOÞ, ðHNO3 Þ2 ðCMPOÞ, and ðHNO3 Þ3 ðCMPOÞ, complexes with CMPO and neodymium: ðNdðNO3 Þ3 ÞðCMPOÞ3 , ðNdðNO3 Þ3 ÞðCMPOÞ2 , and ðNdðNO3 Þ3 ÞðCMPOÞ.[17]

The calculated amount of neodymium in the organic phase is much lower than the amount experimentally obtained. So, the simple complexes adjusted until now are not sufficient to predict the extracted concentration of neodymium in the organic phase. Over the range of neodymium activities examined, the experimental isotherm is above the isotherm calculated with only the simple complexes. It means that mixed complexes are formed over the entire range of concentrations. Good agreement between calculated and experimental isotherms of neodymium, acid, and water (Fig. 7) was obtained



493

Figure 7. Experimental and calculated extraction isotherms based on the model given in Tables 1, 2, 3, and 4. Extraction system: Nd(NO3)3-HNO3 3 mol kg21-H2OCMPO 0.2 mol kg21-NPHE at 258C.

taking into account the three mixed complexes ðNdðNO3 Þ3 ÞðHNO3 ÞðCMPOÞ3 , ðNdðNO3 Þ3 ÞðHNO3 ÞðCMPOÞ2 , and ðNdðNO3 Þ3 ÞðHNO3 ÞðCMPOÞ. Twelve complexes were necessary to model the extraction of the Nd(NO3)3-HNO3H2O-CMPO 0.2 mol kg21-NPHE system, and their optimized parameters are given in Tables 1, 2, 3, and 4. In order to understand the influence of the nitric acid over the neodymium extraction by CMPO, Fig. 8 shows the species predominance diagram for neodymium in the organic phase as a function of its aqueous concentration at equilibrium for different nitric acid concentrations. The proportion of simple complexes becomes very weak when the acid concentration increases. Thus, with nitric acid, we have to take into account the mixed Table 4. Optimized parameters of the mixed complexes present in the organic phase for the extraction system Nd(NO3)3-HNO3-H2O-CMPO 0.2 mol kg21-NPHE (molality scale) at 258C Species ðNdðNO3 Þ3 ÞðHNO3 ÞðCMPOÞ3 ðNdðNO3 Þ3 ÞðHNO3 ÞðCMPOÞ2 ðNdðNO3 Þ3 ÞðHNO3 ÞðCMPOÞ

Keff.

s

9.0 107 2.0 105 2.5 101

0.0 0.2 0.8

S. Belair et al.


494

Figure 8. Influence of the nitric acid concentration on the speciation of neodymium nitrate in the organic phase. Extraction system: Nd(NO3)3-HNO3-H2O-CMPO 0.2 mol kg21-NPHE at 258C. (a) [HNO3] ¼ 0 mol kg21; (b) [HNO3] ¼ 1 mol kg21; (c) [HNO3] ¼ 3 mol kg21.



495

complexes. Finally, with nitric acid, the thermodynamic modeling reveals the formation of a mixed complex containing one neodymium molecule for one CMPO molecule in important proportion (20% for [HNO3]eq ¼ 3 mol kg21 and [Nd(NO3)3]eq ¼ 0.4 mol kg21). Without nitric acid, the simple complex ðNdðNO3 Þ3 ÞðCMPOÞ is never formed to any significant extent whatever the neodymium concentration (Figure 8a). Each organic phase, from the Nd(NO3)3-HNO3 3 mol kg21-H2O-CMPO 0.2 mol kg21-NPHE system, was analyzed by ESI/MS (Fig. 9). The electronebulization causes the desolvation of one or more nitrate molecules but also the desolvatation of the acid molecules if they would be present in solution. Indeed the desolvation of the acid molecules is a usual phenomenon observed in ESI/MS.[47] The bonds between the extractant and the acid are weak; they easily break during the ionization. Three complexes were detected relative to the stoichiometries 1 : 3, 1 : 2, and 1 : 1. Under these acidity conditions ([HNO3]eq ¼ 3 mol kg21) and according to the results of modeling, the complexes can be only mixed complexes.

CONCLUSION The Mikulin-Sergievskii-Dannus’ model used in this work permits a complete modeling of the extraction isotherms of the Nd(NO3)3-HNO3-H2O-CMPO

Figure 9. ESI/MS spectrum. Extraction of neodymium nitrate in presence of nitric acid by CMPO. Extraction system: Nd(NO3)3-HNO3 3 mol kg21-H2O-CMPO 0.2 mol kg21-NPHE at 258C.

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0.2 mol kg21-NPHE system. Twelve complexes were indicated to be present in the organic phase, and their extraction constants were determined. Information about the hydration of the ðHNO3 Þx ðCMPOÞy complexes has been obtained to complete the results already reported in the literature. The study conducted over a very wide range of lanthanide and acid activities allowed us to observe that the predominance of one complex over the others depends on the thermodynamic activity of neodymium and acid. The thermodynamic approach consisting in a step-by-step study of the different subsystems allowed us to differentiate and quantify the competitive extraction of nitric acid and the formation of mixed complexes.

ACKNOWLEDGMENTS We are grateful to M. Tabarant from CEA-SACLAY/DEN/DPC/SECR/LSRI for the assistance in the ICP-AES measurements and to C. Lamouroux from CEA-SACLAY/DEN/DPC/SECR/LSRM for the assistance in the ESI/MS spectrum.

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