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Solvent extraction of rare earth metals from nitrate solutions with di(2,4,4-trimethylpentyl)phosphinate of methyltrioctylammonium
Journal of Molecular Liquids 172 (2012) 144–146
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Short Communication
Solvent extraction of rare earth metals from nitrate solutions with di(2,4,4-trimethylpentyl)phosphinate of methyltrioctylammonium V.V. Belova ⁎, A.A. Voshkin, N.S. Egorova, A.I. Kholkin Institute of General and Inorganic Chemistry of Russian Academy of Sciences, 31 Leninskii pr., Moscow, 119991, Russia
a r t i c l e
i n f o
Article history: Received 16 December 2011 Received in revised form 16 March 2012 Accepted 25 April 2012 Available online 19 May 2012 Keywords: Extraction Rare earth metals Cyanex 272 Binary extractant
a b s t r a c t The mechanisms of the extraction of rare earth metals from nitrate media with the binary extractant— methyltrioctylammonium di(2,4,4-trimethylpentyl)phosphinate with the formation of extracted species of different composition in the organic phase have been suggested. Compositions of the extracted species which distribution is satisfactorily described by the calculated curves have been determined. The calculated concentration values obtained by using the proposed equations were found to be in a good agreement with the experimentally obtained values for lanthanum and ytterbium extraction with the binary extractant from nitrate solutions. © 2012 Elsevier B.V. All rights reserved.
Dialkylphosphinic acids are efficient extractants for the recovery and separation of rare earth metals (REM) [1–3]. A study on binary extractants based on dialkylphosphinic acids is of considerable interest because, for example, in addition to the properties of initial cation-exchange extractants, new opportunities appear, such as an increase in the efficiency of separation, and stripping was facilitated in some cases [4–6]. Earlier [7,8], we have studied the extraction of lanthanide nitrates with the binary extractants such as salts of methyltrioctylammonium and dialkylphosphinic acids (Cyanex 272, Cyanex 301 and Cyanex 302). It was found that a binary extractant based on Cyanex 272 is characterized by the highest extraction power. Data for the extraction isotherms showed that under conditions of the loaded organic phase a ratio of REM concentration in the organic phase to the initial concentration of methyltrioctylammonium dialkylphosphinate was close to 1. The extraction of different rare earth metal complexes has been suggested. In this study, a description of distribution of rare earth metals from nitrate solutions taking into account formation of the extracted species of different composition in the organic phase by the example of the extraction of lanthanum and ytterbium nitrates with a solution of methyltrioctylammonium di(2,4,4-trimethylpentyl)phosphinate in toluene has been accomplished.
followed by evaporation of HNO3 to values of pH ~ 6–7 in solutions. The initial aqueous solutions were diluted to the desired metal concentrations. Constant concentration of nitrate ion in solutions was supported by adding NaNO3. A solution of methyltrioctylammonium di(2,4,4-trimethylpentyl) phosphinate in toluene was used as a binary extractant. The binary extractant (R4NA) was prepared by dissolving equimolar amounts of methyltrioctylammonium chloride (R4NCl) and di(2,4,4trimethylpentyl)phosphinic acid (HA) in toluene, followed by shaking the organic solution with an equal volume of 1 M NaOH solution for 10 min and followed by washing the organic phase with water. Concentrations of lanthanides in the initial solutions and aqueous phases after extraction were analyzed by titration with a standard solution of EDTA at pH 5.5 using xylenol orange as an indicator. The concentrations of lanthanides in the organic phase were determined by the differences between the concentrations in the initial solutions and the aqueous phase after extraction. The concentrations of metals in the organic phase were also determined after their stripping with 1–2 M solutions of mineral acids. All chemicals were of reagent grade. Extraction was performed at 20 C in test tubes with ground-in stoppers with equal volumes of the aqueous and organic phases. The duration of phase mixing was 15 min which was sufficient to establish the permanent values of the distribution coefficients of REM [7].
2. Experimental
3. Results and discussion
Stock solutions of lanthanide nitrates were prepared by dissolving the corresponding metal oxide in a 6 M nitric acid solution by heating
Fig. 1 shows the extraction isotherms of lanthanum and yttrium nitrates from 2 M solutions of NaNO3 with a 0.05 М solution of trioctylmethylammonium bis(2,4,4-trimethylpentyl)phosphinate in toluene. The analysis of the extraction isotherms of La and Yb nitrates (Fig. 1) indicated that they exhibited two characteristic plateaus at
1. Introduction
⁎ Corresponding author. Tel./fax: + 7 495 955 4834. E-mail address: [email protected] (VV. Belova). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2012.04.012
VV. Belova et al. / Journal of Molecular Liquids 172 (2012) 144–146
145
Table 1 Constants of binary extraction of lanthanum and ytterbium from nitrate solutions at CLa(o): СR4NA(init.) = 1:2 (P = 0.95, n = 6). Extracted species
K
log K
La(NO3)A2 Yb(NO3)A2 (R4N)2[La(NO3)3A2] (R4N)2[Yb(NO3)3A2]
0.48 ± 0.09 4.35 ± 0.25 (3.36 ± 0.31)·104 (9.49 ± 0.75)·104
− 0.82 ± 0.45 0.48 ± 0.09 3.97 ± 0.35 4.70 ± 0.48
with the effective extraction constant:
Fig. 1. Isotherms of the extraction of La and Yb nitrates from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene.
1:2 and 1:1 ratios between metal concentrations in the organic phase and the initial concentration of the binary extractant. This obviously indicates the formation of two extracted species in the organic phase, in contrast to the extraction of REM nitrates with tetraoctylammonium dialkylphosphate [6]. It was suggested that in the binary extraction of lanthanide nitrates with methyltrioctylammonium di(2,4,4-trimethylpentyl)phosphinate the extracted species, which are formed in the initial system involving organic acid such as LnA3 − x(NO3)x, where x values may vary in the range of 0–2 can be also formed in the organic phase. From the literature it is known [9] that in the extraction of lanthanides from nitrate solutions with nitrates of quaternary ammonium bases the extracted species involving metal-containing anions can be formed. Taking into account these data we can suggest formation of complexes (R4N)yLn(NO3)3 + y, as well as compounds involving metal-containing anions with mixed ligands—(R4N)zLnAz(NO3)3 in the organic phase in the binary extraction of Ln(NO3)3. According to this we can suggest several options for the interphase distribution of REM nitrates in the system involving the binary extractant: −
3þ
LnðaÞ þ 3NO3ðaÞ þ 2R4 NAðoÞ ↔LnðNO3 ÞA2ðoÞ þ 2R4 NNO3ðoÞ
ð1Þ
(where “a” and “o” represent the aqueous and the organic phases respectively) with the effective extraction constant:
K LnðNO3 ÞA2 ¼
C LnðoÞ C 2R4 NNO3 ðoÞ C LnðaÞ C 3NO− ðaÞ C 2R4 NAðoÞ γ 4ðaÞ
ð2Þ
3
3þ − LnðaÞ þ 3NO3ðaÞ þ 2R4 NAðoÞ ↔ðR4 NÞ2 LnðNO3 Þ3 A2 ðoÞ
ð3Þ
with the effective extraction constant:
K ðR
4 N Þ2
С LnðoÞ ½LnðNO3 Þ3 A2 ¼ С 3 2 4 С − LnðaÞ NO ðaÞ С R4 NAðoÞ γ ðaÞ
ð4Þ
3
3þ
−
LnðaÞ þ 3NO3ðaÞ þ R4 NAðoÞ ↔LnðNO3 Þ2 AðoÞ þ R4 NNO3ðoÞ
С LnðoÞ С R4 NNO3ðoÞ С LnðaÞ С 3NO− ðaÞ С R4 NAðoÞ γ4ðaÞ
When two extracted species are formed in the organic phase, the general expression would be as follows: 3þ − 2LnðaÞ þ 6NO3ðaÞ þ 2R4 NAðoÞ ↔R4 N LnðNO3 Þ3 A ðoÞ þ LnðNO3 Þ2 AðoÞ
ð9Þ
þR4 NNO3ðoÞ 2 С LnðoÞ =2 С R4 NNO3ðoÞ
K R N½LnðNO Þ AþLnðNO Þ A ¼ 2 4 3 3 3 2 С LnðвÞ С 6NO− ðaÞ С 2R4 NAðoÞ γ8ðaÞ
ð10Þ
3
Also, the formation of complexes of a different composition is possible, for example: 3þ − 2LnðaÞ þ 6NO3ðaÞ þ 2R4 NAðoÞ ↔ðR4 N Þ2 LnðNO3 Þ5 ðoÞ þ LnðNO3 ÞA2ðoÞ ð11Þ
K ðR
4 N Þ2
2 С LnðoÞ =2
½LnðNO3 Þ5 þLnðNO3 ÞA2 ¼ С 2 С 6 − С 2 8 LnðaÞ NO ðaÞ R4 NAðoÞ γ ðaÞ
ð12Þ
3
According to Eqs. (1) and (3) the ratio of lanthanum and binary extractant in the organic phase is equal to 1:2. For this ratio, we calculated the effective extraction constants of lanthanum and ytterbium nitrates, in accordance with Eqs. (2) and (4), which are listed in Table 1. To calculate the extraction constants, we used the experimental data for the extraction isotherms (Fig. 1). In this case, we assumed that in Eq. (2) СR4NNO3(o) = 2CLn(o), for Eqs. (2) and (4) СR4NA(o) = СR4NA(init.) − 2СLn(o). The activity coefficients of nitrate ion were taken from reference data [10]. According to Eqs. (6), (8), (10) and (12) the constants of the corresponding processes for lanthanum and ytterbium extraction were also calculated (Table 2). It was assumed that the extracted species of REM are in an equal ratio in the organic phase (Eqs. (9) and (11)). Using the data in Table 1 we plotted the calculated isotherms of the extraction of lanthanum and ytterbium nitrates from 2 M NaNO3 solution with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene (Figs. 2 and 3). To estimate the proximity of the calculated values of lanthanide concentrations in the organic phase to the experimental data, the least square regression method was chosen as a criterion. For the system “methyltrioctylammonium dialkylphosphinate—lanthanum nitrate” the values of the least square
Table 2 Constants of binary extraction of lanthanum and ytterbium from nitrate solutions at CLa(o): СR4NA(init.) = 1:1 (P = 0.95, n = 7).
ð6Þ
3
3þ − LnðaÞ þ 3NO3ðaÞ þ R4 NAðoÞ ↔R4 N LnðNO3 Þ3 A ðoÞ
ð8Þ
3
ð5Þ
with the effective extraction constant: K LnðNO3 Þ2 A ¼
С LnðoÞ K R N½LnðNO Þ A ¼ 4 3 3 С LnðaÞ С 3NO− ðaÞ С R4 NAðoÞ γ 4ðaÞ
ð7Þ
Extracted species
K La
K Yb
Ln(NO3)2A R4N[Ln(NO3)3A] Ln(NO3)2A + R4N[Ln(NO3)3A] (R4N)2[Ln(NO3)5] + Ln(NO3)A2
1.61 ± 0.34 (1.23 ± 0.12)·102 9.32 ± 0.64 (5.27 ± 0.53)·102
3.61 ± 0.12 (5.70 ± 0.62)·102 9.68 ± 1.11 (2.40 ± 0.25)·104
146
VV. Belova et al. / Journal of Molecular Liquids 172 (2012) 144–146
0.05
0.02 experimental data calculated curve calculated curve
La(NO3)A2 (R4N)2[La(NO3)3A2]
0.01
CYb(o), mol/L
CLa(o), mol/L
0.03
0 0
0.02
0.04
0.06
0.08
CLa(a), mol/L
0.04 0.03 0.02
experimental data calculated curve
0.01
Fig. 2. Calculated isotherms of extraction of lanthanum nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene.
R4N[Yb(NO3)3A]+Yb(NO3)2A
0
regression were 0.014 and 0.012 for the complexes of La(NO3)A2 and (R4N)2[La(NO3)3A2] respectively. For the system “methyltrioctylammonium dialkylphosphinate—ytterbium nitrate” the values of the least square regression were 0.009 and 0.012 for the complexes of Yb(NO3)A2 and (R4N)2[Yb(NO3)3A2] respectively. It can be concluded that the proposed equations reliably describe the extraction of REM from nitrate solutions with the binary extractant in the studied range of initial concentrations of metals, while in the organic phase the formation of both complexes is possible. The mathematical model is useful for predicting the concentrations of lanthanum and ytterbium in the organic and aqueous phases at any initial concentrations of these metals in the binary extraction system. To evaluate the assumptions, which were made about the possible occurrence of the binary extraction of lanthanide nitrates in accordance with Eqs. (5), (7), (9) and (11), the theoretical isotherms of the extraction of lanthanum and ytterbium nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene were also calculated using the experimental data of the extraction constants (Table 2).
cYb(o), mol/L
0.03 0.02 experimental data
Yb(NO3)A2
0.01
(R4N)2Yb(NO3)3A2
calculated curve calculated curve
0
0.1
0.2
0.3
0.4
0.5
CYb(a), mol/L Fig. 5. Calculated isotherms of extraction of ytterbium nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene in the case of forming Yb(NO3)2A and R4N[Yb(NO3)3A].
For the system “methyltrioctylammonium dialkylphosphinate”— lanthanum nitrate the values of the least square regression sum were 0.021, 0.025, 0.009 and 0.009 for complexes 1, 2, 3 and 4 respectively (Table 2). For the system “methyltrioctylammonium dialkylphosphinate”—ytterbium nitrate the values of the least square regression sum were 0.020, 0.033, 0.006 and 0.010 for complexes 1, 2, 3 and 4 respectively (Table 2). Figs. 4 and 5 show the calculated isotherms with the least square regression sum from which it follows that a more reliable extraction of REM from nitrate solutions is described by Eq. (9), taking into account the formation in the organic phase several extracted species such as Ln(NO3)2A and R4N[Ln(NO3)3A]. Thus, the calculated concentration values obtained by using Eq. (9) were found to be in a good agreement with the experimentally obtained values. The analysis of the proposed extraction equations shows that increasing the concentration of NO3− anions in the aqueous phase leads to an increase in the distribution coefficients of lanthanides independently on the composition of extracted species. Effect of extractant concentration is similar, but the degree of influence depends on the stoichiometric coefficients.
0 0
0.01
0.02
0.03
Acknowledgment
0.04
cYb(a), mol/L Fig. 3. Calculated isotherms of extraction of ytterbium nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene.
The authors gratefully acknowledge the Russian Foundation for Basic Research (project no. 10-03-00188) for the financial support. References [1] [2] [3] [4]
0.05
cLa(o), mol/L
0.04
[5]
0.03
[6] [7]
0.02
experimental data
[8]
calculated curve
0.01
R4N[La(NO3)3A]+La(NO3)2A
[9] [10]
0 0
0.05
0.1
0.15
0.2
cLa(a), mol/L Fig. 4. Calculated isotherms of extraction of lanthanum nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene in the case of forming La(NO3)2A and R4N[La(NO3)3A].
D. Wu, C. Niu, D. Li, Y. Bai, Journal of Alloys and Compounds 374 (2004) 442–446. Z.H. Wang, G.X. Ma, J. Lu, W.P. Liao, D.Q. Li, Hydrometallurgy 66 (2002) 95–99. M.I. Saleh, F. Bari, B. Saad, Hydrometallurgy 63 (2002) 75–84. A.I. Kholkin, V.V. Belova, G.L. Pashkov, I.Yu. Fleitlikh, V.V. Sergeev, Journal of Molecular Liquids 82 (1999) 131–146. V.V. Belova, A.I. Kholkin, Solvent Extraction and Ion Exchange 16 (1998) 1233–1255. S.N. Kalyakin, V.I. Kuzmin, Mendeleev Communications 10 (2000) 151–152. V.V. Belova, A.A. Voshkin, A.I. Kholkin, A.K. Pyartman, Hydrometallurgy 97 (2009) 198–203. N.S. Egorova, V.V. Belova, A.A. Voshkin, A.K. Pyartman, V.I. Zhilov, N.A. Minaeva, V.A. Keskinov, A.I. Kholkin, Russian Journal Khimicheskaya Tekhnologiya 7 (2006) 33–37. A.K. Pyartman, S.V. Kovalev, V.A. Keskinov, N.V. Khokhlova, Russian Journal of Inorganic Chemistry 43 (1998) 1626–1628. Yu.Yu. Lurye, The Analytical Chemistry Handbook (in Russian), Khimiya, Moscow, 1971.
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Short Communication
Solvent extraction of rare earth metals from nitrate solutions with di(2,4,4-trimethylpentyl)phosphinate of methyltrioctylammonium V.V. Belova ⁎, A.A. Voshkin, N.S. Egorova, A.I. Kholkin Institute of General and Inorganic Chemistry of Russian Academy of Sciences, 31 Leninskii pr., Moscow, 119991, Russia
a r t i c l e
i n f o
Article history: Received 16 December 2011 Received in revised form 16 March 2012 Accepted 25 April 2012 Available online 19 May 2012 Keywords: Extraction Rare earth metals Cyanex 272 Binary extractant
a b s t r a c t The mechanisms of the extraction of rare earth metals from nitrate media with the binary extractant— methyltrioctylammonium di(2,4,4-trimethylpentyl)phosphinate with the formation of extracted species of different composition in the organic phase have been suggested. Compositions of the extracted species which distribution is satisfactorily described by the calculated curves have been determined. The calculated concentration values obtained by using the proposed equations were found to be in a good agreement with the experimentally obtained values for lanthanum and ytterbium extraction with the binary extractant from nitrate solutions. © 2012 Elsevier B.V. All rights reserved.
Dialkylphosphinic acids are efficient extractants for the recovery and separation of rare earth metals (REM) [1–3]. A study on binary extractants based on dialkylphosphinic acids is of considerable interest because, for example, in addition to the properties of initial cation-exchange extractants, new opportunities appear, such as an increase in the efficiency of separation, and stripping was facilitated in some cases [4–6]. Earlier [7,8], we have studied the extraction of lanthanide nitrates with the binary extractants such as salts of methyltrioctylammonium and dialkylphosphinic acids (Cyanex 272, Cyanex 301 and Cyanex 302). It was found that a binary extractant based on Cyanex 272 is characterized by the highest extraction power. Data for the extraction isotherms showed that under conditions of the loaded organic phase a ratio of REM concentration in the organic phase to the initial concentration of methyltrioctylammonium dialkylphosphinate was close to 1. The extraction of different rare earth metal complexes has been suggested. In this study, a description of distribution of rare earth metals from nitrate solutions taking into account formation of the extracted species of different composition in the organic phase by the example of the extraction of lanthanum and ytterbium nitrates with a solution of methyltrioctylammonium di(2,4,4-trimethylpentyl)phosphinate in toluene has been accomplished.
followed by evaporation of HNO3 to values of pH ~ 6–7 in solutions. The initial aqueous solutions were diluted to the desired metal concentrations. Constant concentration of nitrate ion in solutions was supported by adding NaNO3. A solution of methyltrioctylammonium di(2,4,4-trimethylpentyl) phosphinate in toluene was used as a binary extractant. The binary extractant (R4NA) was prepared by dissolving equimolar amounts of methyltrioctylammonium chloride (R4NCl) and di(2,4,4trimethylpentyl)phosphinic acid (HA) in toluene, followed by shaking the organic solution with an equal volume of 1 M NaOH solution for 10 min and followed by washing the organic phase with water. Concentrations of lanthanides in the initial solutions and aqueous phases after extraction were analyzed by titration with a standard solution of EDTA at pH 5.5 using xylenol orange as an indicator. The concentrations of lanthanides in the organic phase were determined by the differences between the concentrations in the initial solutions and the aqueous phase after extraction. The concentrations of metals in the organic phase were also determined after their stripping with 1–2 M solutions of mineral acids. All chemicals were of reagent grade. Extraction was performed at 20 C in test tubes with ground-in stoppers with equal volumes of the aqueous and organic phases. The duration of phase mixing was 15 min which was sufficient to establish the permanent values of the distribution coefficients of REM [7].
2. Experimental
3. Results and discussion
Stock solutions of lanthanide nitrates were prepared by dissolving the corresponding metal oxide in a 6 M nitric acid solution by heating
Fig. 1 shows the extraction isotherms of lanthanum and yttrium nitrates from 2 M solutions of NaNO3 with a 0.05 М solution of trioctylmethylammonium bis(2,4,4-trimethylpentyl)phosphinate in toluene. The analysis of the extraction isotherms of La and Yb nitrates (Fig. 1) indicated that they exhibited two characteristic plateaus at
1. Introduction
⁎ Corresponding author. Tel./fax: + 7 495 955 4834. E-mail address: [email protected] (VV. Belova). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2012.04.012
VV. Belova et al. / Journal of Molecular Liquids 172 (2012) 144–146
145
Table 1 Constants of binary extraction of lanthanum and ytterbium from nitrate solutions at CLa(o): СR4NA(init.) = 1:2 (P = 0.95, n = 6). Extracted species
K
log K
La(NO3)A2 Yb(NO3)A2 (R4N)2[La(NO3)3A2] (R4N)2[Yb(NO3)3A2]
0.48 ± 0.09 4.35 ± 0.25 (3.36 ± 0.31)·104 (9.49 ± 0.75)·104
− 0.82 ± 0.45 0.48 ± 0.09 3.97 ± 0.35 4.70 ± 0.48
with the effective extraction constant:
Fig. 1. Isotherms of the extraction of La and Yb nitrates from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene.
1:2 and 1:1 ratios between metal concentrations in the organic phase and the initial concentration of the binary extractant. This obviously indicates the formation of two extracted species in the organic phase, in contrast to the extraction of REM nitrates with tetraoctylammonium dialkylphosphate [6]. It was suggested that in the binary extraction of lanthanide nitrates with methyltrioctylammonium di(2,4,4-trimethylpentyl)phosphinate the extracted species, which are formed in the initial system involving organic acid such as LnA3 − x(NO3)x, where x values may vary in the range of 0–2 can be also formed in the organic phase. From the literature it is known [9] that in the extraction of lanthanides from nitrate solutions with nitrates of quaternary ammonium bases the extracted species involving metal-containing anions can be formed. Taking into account these data we can suggest formation of complexes (R4N)yLn(NO3)3 + y, as well as compounds involving metal-containing anions with mixed ligands—(R4N)zLnAz(NO3)3 in the organic phase in the binary extraction of Ln(NO3)3. According to this we can suggest several options for the interphase distribution of REM nitrates in the system involving the binary extractant: −
3þ
LnðaÞ þ 3NO3ðaÞ þ 2R4 NAðoÞ ↔LnðNO3 ÞA2ðoÞ þ 2R4 NNO3ðoÞ
ð1Þ
(where “a” and “o” represent the aqueous and the organic phases respectively) with the effective extraction constant:
K LnðNO3 ÞA2 ¼
C LnðoÞ C 2R4 NNO3 ðoÞ C LnðaÞ C 3NO− ðaÞ C 2R4 NAðoÞ γ 4ðaÞ
ð2Þ
3
3þ − LnðaÞ þ 3NO3ðaÞ þ 2R4 NAðoÞ ↔ðR4 NÞ2 LnðNO3 Þ3 A2 ðoÞ
ð3Þ
with the effective extraction constant:
K ðR
4 N Þ2
С LnðoÞ ½LnðNO3 Þ3 A2 ¼ С 3 2 4 С − LnðaÞ NO ðaÞ С R4 NAðoÞ γ ðaÞ
ð4Þ
3
3þ
−
LnðaÞ þ 3NO3ðaÞ þ R4 NAðoÞ ↔LnðNO3 Þ2 AðoÞ þ R4 NNO3ðoÞ
С LnðoÞ С R4 NNO3ðoÞ С LnðaÞ С 3NO− ðaÞ С R4 NAðoÞ γ4ðaÞ
When two extracted species are formed in the organic phase, the general expression would be as follows: 3þ − 2LnðaÞ þ 6NO3ðaÞ þ 2R4 NAðoÞ ↔R4 N LnðNO3 Þ3 A ðoÞ þ LnðNO3 Þ2 AðoÞ
ð9Þ
þR4 NNO3ðoÞ 2 С LnðoÞ =2 С R4 NNO3ðoÞ
K R N½LnðNO Þ AþLnðNO Þ A ¼ 2 4 3 3 3 2 С LnðвÞ С 6NO− ðaÞ С 2R4 NAðoÞ γ8ðaÞ
ð10Þ
3
Also, the formation of complexes of a different composition is possible, for example: 3þ − 2LnðaÞ þ 6NO3ðaÞ þ 2R4 NAðoÞ ↔ðR4 N Þ2 LnðNO3 Þ5 ðoÞ þ LnðNO3 ÞA2ðoÞ ð11Þ
K ðR
4 N Þ2
2 С LnðoÞ =2
½LnðNO3 Þ5 þLnðNO3 ÞA2 ¼ С 2 С 6 − С 2 8 LnðaÞ NO ðaÞ R4 NAðoÞ γ ðaÞ
ð12Þ
3
According to Eqs. (1) and (3) the ratio of lanthanum and binary extractant in the organic phase is equal to 1:2. For this ratio, we calculated the effective extraction constants of lanthanum and ytterbium nitrates, in accordance with Eqs. (2) and (4), which are listed in Table 1. To calculate the extraction constants, we used the experimental data for the extraction isotherms (Fig. 1). In this case, we assumed that in Eq. (2) СR4NNO3(o) = 2CLn(o), for Eqs. (2) and (4) СR4NA(o) = СR4NA(init.) − 2СLn(o). The activity coefficients of nitrate ion were taken from reference data [10]. According to Eqs. (6), (8), (10) and (12) the constants of the corresponding processes for lanthanum and ytterbium extraction were also calculated (Table 2). It was assumed that the extracted species of REM are in an equal ratio in the organic phase (Eqs. (9) and (11)). Using the data in Table 1 we plotted the calculated isotherms of the extraction of lanthanum and ytterbium nitrates from 2 M NaNO3 solution with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene (Figs. 2 and 3). To estimate the proximity of the calculated values of lanthanide concentrations in the organic phase to the experimental data, the least square regression method was chosen as a criterion. For the system “methyltrioctylammonium dialkylphosphinate—lanthanum nitrate” the values of the least square
Table 2 Constants of binary extraction of lanthanum and ytterbium from nitrate solutions at CLa(o): СR4NA(init.) = 1:1 (P = 0.95, n = 7).
ð6Þ
3
3þ − LnðaÞ þ 3NO3ðaÞ þ R4 NAðoÞ ↔R4 N LnðNO3 Þ3 A ðoÞ
ð8Þ
3
ð5Þ
with the effective extraction constant: K LnðNO3 Þ2 A ¼
С LnðoÞ K R N½LnðNO Þ A ¼ 4 3 3 С LnðaÞ С 3NO− ðaÞ С R4 NAðoÞ γ 4ðaÞ
ð7Þ
Extracted species
K La
K Yb
Ln(NO3)2A R4N[Ln(NO3)3A] Ln(NO3)2A + R4N[Ln(NO3)3A] (R4N)2[Ln(NO3)5] + Ln(NO3)A2
1.61 ± 0.34 (1.23 ± 0.12)·102 9.32 ± 0.64 (5.27 ± 0.53)·102
3.61 ± 0.12 (5.70 ± 0.62)·102 9.68 ± 1.11 (2.40 ± 0.25)·104
146
VV. Belova et al. / Journal of Molecular Liquids 172 (2012) 144–146
0.05
0.02 experimental data calculated curve calculated curve
La(NO3)A2 (R4N)2[La(NO3)3A2]
0.01
CYb(o), mol/L
CLa(o), mol/L
0.03
0 0
0.02
0.04
0.06
0.08
CLa(a), mol/L
0.04 0.03 0.02
experimental data calculated curve
0.01
Fig. 2. Calculated isotherms of extraction of lanthanum nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene.
R4N[Yb(NO3)3A]+Yb(NO3)2A
0
regression were 0.014 and 0.012 for the complexes of La(NO3)A2 and (R4N)2[La(NO3)3A2] respectively. For the system “methyltrioctylammonium dialkylphosphinate—ytterbium nitrate” the values of the least square regression were 0.009 and 0.012 for the complexes of Yb(NO3)A2 and (R4N)2[Yb(NO3)3A2] respectively. It can be concluded that the proposed equations reliably describe the extraction of REM from nitrate solutions with the binary extractant in the studied range of initial concentrations of metals, while in the organic phase the formation of both complexes is possible. The mathematical model is useful for predicting the concentrations of lanthanum and ytterbium in the organic and aqueous phases at any initial concentrations of these metals in the binary extraction system. To evaluate the assumptions, which were made about the possible occurrence of the binary extraction of lanthanide nitrates in accordance with Eqs. (5), (7), (9) and (11), the theoretical isotherms of the extraction of lanthanum and ytterbium nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene were also calculated using the experimental data of the extraction constants (Table 2).
cYb(o), mol/L
0.03 0.02 experimental data
Yb(NO3)A2
0.01
(R4N)2Yb(NO3)3A2
calculated curve calculated curve
0
0.1
0.2
0.3
0.4
0.5
CYb(a), mol/L Fig. 5. Calculated isotherms of extraction of ytterbium nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene in the case of forming Yb(NO3)2A and R4N[Yb(NO3)3A].
For the system “methyltrioctylammonium dialkylphosphinate”— lanthanum nitrate the values of the least square regression sum were 0.021, 0.025, 0.009 and 0.009 for complexes 1, 2, 3 and 4 respectively (Table 2). For the system “methyltrioctylammonium dialkylphosphinate”—ytterbium nitrate the values of the least square regression sum were 0.020, 0.033, 0.006 and 0.010 for complexes 1, 2, 3 and 4 respectively (Table 2). Figs. 4 and 5 show the calculated isotherms with the least square regression sum from which it follows that a more reliable extraction of REM from nitrate solutions is described by Eq. (9), taking into account the formation in the organic phase several extracted species such as Ln(NO3)2A and R4N[Ln(NO3)3A]. Thus, the calculated concentration values obtained by using Eq. (9) were found to be in a good agreement with the experimentally obtained values. The analysis of the proposed extraction equations shows that increasing the concentration of NO3− anions in the aqueous phase leads to an increase in the distribution coefficients of lanthanides independently on the composition of extracted species. Effect of extractant concentration is similar, but the degree of influence depends on the stoichiometric coefficients.
0 0
0.01
0.02
0.03
Acknowledgment
0.04
cYb(a), mol/L Fig. 3. Calculated isotherms of extraction of ytterbium nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene.
The authors gratefully acknowledge the Russian Foundation for Basic Research (project no. 10-03-00188) for the financial support. References [1] [2] [3] [4]
0.05
cLa(o), mol/L
0.04
[5]
0.03
[6] [7]
0.02
experimental data
[8]
calculated curve
0.01
R4N[La(NO3)3A]+La(NO3)2A
[9] [10]
0 0
0.05
0.1
0.15
0.2
cLa(a), mol/L Fig. 4. Calculated isotherms of extraction of lanthanum nitrate from 2 M NaNO3 solutions with 0.05 M solution of methyltrioctylammonium dialkylphosphinate in toluene in the case of forming La(NO3)2A and R4N[La(NO3)3A].
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