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Separation of samarium and gadolinium solutions by solvent extraction
ELSEVIER
Journal of Alloys and Compounds 249 (1997) 116-l I8
Separation of, samarium and gadolinium solutions by solvent extraction P. Miranda Jr. , L. B. Zinner* Institute
de Quirnica-Universidade
de Scio Patio,
C.P. 26.077,
CEP 05599-970-
SL?O Pnulo,
SP, Brazil
Abstract Interest in the production of individual rare earth (RE) elements has increased in the last years, due to the advances in technological applications of RE. Samarium and gadolinium are used in several applications: nuclear industry, lasers, supermagnets among others. This paper presents a technological contribution to the study of RE separation, in order to obtain Sm and Gd concentrated solutions. Experiments were carried out on a laboratory scale using solvent extraction techniques. Separation of Sm and Gd is studied in a continuous system in a micropilot unit of mixer-settlers. The extractant used is 2-ethylhexyl phosphonic acid, mono-2-ethylhexyl ester,
dilutedin isododecane (1 M). The aqueousphaseprocessed wasa chloridesolutionof mediumandheavy RE producedfrom monazite leaching.The RE determinations in aqueoussolutionswereperformedusingan atomicemissionspectrometer with inductively coupled plasmaAton-Comp, serial Ash-800. The analytical results are presentedas RE concentrationsprofiles. Two continuous-circuit configurations are describedof extraction-scrubbing-stripping for the productionof SmandGd concentrated solutionswith purity higher than 90% and 94%, respectively. Keywords:
Samarium;
Gadolinium;
Separation
of RE; Solvent
extraction
1. Introduction The application of solvent extraction techniques to rare earth (RE) fractioning has been studied on a laboratory scale since the 40’s, mainly as a result of research works developed for nuclear fuel production. Liquid-liquid extraction is a separationtechnique based on the distribution of a solute between two phases: aqueous and organic, practically immiscible with each other. The organic phase generally contains a diluent and an extractant capable of combining differently with RE elements,then forming compoundsthat are more soluble, i.e., show greater affinity with the organic phase. According to Marcus and Kertes [I] and Ritcey and Ashbrook [2], solvent extraction processescan be classified basedeither on the chemical reaction involved or on the type of the extractant used. Generally, they are classified as extraction by solvation, extraction by compound formation, and extraction by ion-pair formation. The most used extractants for RE separation are the organophosphorusacids that belong to a compound formation class, with prominence of di-2-ethylhexyl phosphoric acid (D2EHPA), which has been widely applied since the pioneering work of Peppard et al. [3]. Some works of Preston and Du Preez [4], Benedetto et al. [5], Desphande “Corresponding
author.
0925~8388/97/$17.00 PII SO925-8388(96)02754-5
0
1997 Elsevier
Science
S.A. All rights reserved
et al. [6], Delmas et al. [7] and others have discussedthe use of 2-ethylhexylphosphonic acid mono-Zethylhexyl ester (HEH(EHP)) in RE separation. This extractant was chosen because generally it presents higher separation factors and allows easier stripping with relation to D2EHPA. In the extraction process, a cation-exchange reaction occurs, in which hydrogen ions of the extractant are exchangedfor RE ions from the aqueousphase,according to the equation: @+)a
+ 3(IHX),),~(RE(HX?),),
Where a: ;lLIX,2: K:
+ 3(H+),
(1)
aqueousphase organic phase extractant in dimeric form equilibrium constant
2. Experimental 2.1. Reagents The RE chloride solution studied contains medium and heavy RE elements; it originated from the Brazilian monazite processing, carried out by Nuclemon Mineroquimica-SP. This solution has an acidity of 1.18 M
P. Miranda,
L. B. Zimer
I Journal
of Alloys
and about 159 g 1-l of RE elements, having as main constituents: Sm (34.55%), Gd (23.X5%), Tb (2.31%), Dy (6.82%), and Y (24.45%). The RE solutions used in these tests were prepared by adjusting acidity and RE concentration by the addition of distilled water and ammonium hydroxide. The organic phase used was 1 M in the extractant. Diluent- was isododecane with low content of aromatic compounds commercialized with the name Isoparafina-17 /21 by Unipar Quimica. The extractant HEH(EHP), is commercialized with the trade name Ionquestby Albright and Wilson Americas. It was used without further purification and has the structural formula:
and
Compounds
249
(1997)
117
116-116
settlers of 360 cm3 capacities. Two configurations were tested for the fractioning of RECl, solution (Table 1). Configuration A was used for the fractioning of the initial solution in two groups: medium and heavy RE; and the other, configuration B, for the fractioning of the medium RE solution obtained with configuration A, in order to produce as final products Sm and Gd concentrated solutions. The following diagram shows the continuous-circuit used in both configurations for the fractioning of the solution being studied:
CHzCH3 I 1
CH#2HZCHzCH&HCHz0 \
/Ho
2.4. Analytical CH3CHZCH+ZH&HCHz
CHzCHs
2.2. Batch tests Batch tests were carried out in order to study the main parameters extraction kinetics, aqueous phase acidity, RE concentration in aqueous phase, organic/aqueous (0 /A) phase ratio, extractant concentration in organic phase and RE stripping from the loaded organic phase. From batch tests results it was possible to propose solvent extraction configurations to carry out tests in continuous-circuits to reach the fractioning of the RE solution. 2.3. Continuous
tests
Continuous tests were carried out using a micro-pilot unit of mixers-settlers, with mixers of 160 cm3 and Table 1 Characteristics Configuration
Individual RE determinations in aqueous phase were carried out using an atomic emission spectrometer with inductively coupled plasma, Aton-Comp, serial Ash-800 and total RE determinations were performed by EDTA complexometric titrations using xylenol orange as indicator. Acidity analysis of aqueous solutions were carried out by acid-base titration with sodium hydroxide, using as indicator an ethanolic solution of 0.1% methylene blue and methyl red.
3. Results and discussion With the results obtained from batch tests, it was possible to confirm the behavior of the RE-extractant system used with the ones mentioned in different articles in the literature and to propose an initial configuration of continuous-circuit for the fractioning of the solution being studied. In these tests, it was observed that:
of configurations A
Extraction
Scrubbing
Stages
Aqueous
12
50 g 1-l RI? pH=lS
Configuration
control
‘P\OH
feed
Stripping
OiA
Stages
HCl
O/A
Stages
HCI
O/A
4
18
1N
8
5
5N
4
B
Extraction
Scrubbing
Stages
Aqueous
feed
4
16.5 g 1-l RE pH= 1.5
Stripping
O/A
Stages
HCI
O/A
Stages
HCI
O/A
1.2
6
0.5 N
2
4
2N
3
118
P. Miranda,
L. B. Zirmer
I Jmrnnl
of Aliofs
Yttrium is the element showing the highest extraction efficiency. Y (Z =39) has a behavior, with organophosphorus acids, of an element between Ho (z=67) and Er (z=68). Extraction efficiency increases with the atomic number of the lanthanides, i.e., with the decrease of crystalline radius. Extraction is most efficient when: higher phase ratios O/A, higher concentrations of organic phase, higher contact times between organic and aqueous phases, and aqueous phases with low acidity and RE concentrations are used. Stripping is most effective when higher concentration acids are used; for lower concentrations, the stripping is more efficient for the lighter RE. The stripping behavior is opposite to the extraction, where the equilibrium of expression 1 is displaced to the left. From expresssion 1, extraction is dependent on aqueous phase acidity. A pH increase favors extraction, but there is a limit for the pH range, as in high pH solutions the formation of RE precipitates occurs, which is undesirable for the process. The equilibrium in extraction is attained fairly rapidly: in about 3 or 4 minutes practically all RE were extracted, showing quick kinetics as mentioned in other works, which is a characteristic of acid extractants. For the first fractioning process, with relation to batch tests (concentration ea. 50 g 1-l RE, pH = 1-2; O/A =4) the separation factor for Dy/Gd p is 4.8. For the second fractioning (concentration 16 g 1-l; pH= 1-1.5; O/A= I2) for Gd/Sm /3 is 4.1. Balint [S], Preston and Du Preez [4] and Benedetto [5] reported the separation factor values of 3.51, 2.52 and 3.70, respectively, for the Gd/Sm pair. Different configurations were tested, but only two configurations are presented in this paper, the ones that showed better results for the desired fractioning: configurations A and B. Using configuration A, it was possible to fraction the initial RE chloride solution into two groups: medium RE (Sm, Gd, Tb) and heavy RE (Tb, Dy, Y), producing concentrated solutions with near 60% Sm element for the former and 70% Y element for the latter. Configuration B corresponds to the fractioning of medium RE solution obtained previously with configuration A, producing another two fractions: one with a Sm element content higher than 90%, and the other with Gd element content higher than 94%. Fig. 1 shows the concentration profiles of Sm and Gd in aqueous and organic phases in the extraction (E) and scrubbing (S) stages of configuration B. Analyzing the profiles, a higher affinity of Gd for the organic phase can be observed. Gd presents a low concentration in the
and Compounds
249 (1997)
II&
118
Fig. 1. Profile of Sm and Gd concentrations in configuration B, for organic and aqueous phases of extraction (E) and scrubbing (S) stages.
raffinate (stage 1). Sm is back washed from the organic phases during scrubbing, thus producing a strip liquor rich in Gd. The system (HEH(EHP))-RE-HCI was adequate to achieve the separation proposed. Sm concentration was increased from 34% to 90% and Gd from 24% to 94%, with recoveries in the range of 99% for Sm and 95% for Gd. Mixers-settlers devices were applied successfully in this work. Formation of emulsions were not observed, nor RE precipitation from the loaded organic phases during the operation of the continuous-circuit.
Acknowledgments The authors are grateful for the use of analytical support by the chemists Luis Carlos Reino and SebastiZo Alder Pereira (IPEN-CNEN/SP), CTMP-SP, for the laboratories where the experiments were carried out, and for the financial support of CAPES and PADCT.
References [ 1] Y. Marcus and A.S. Kertes, Ion exchange and solvent extraction of metal complex, Wiley-Interscience, New York, 1969. [2] GM. Ritcey and A.W. Ashbrook, Solvent Extraction Principles and Applications to Process Metallurgy, Elsevier, New York, 1984. [3] D.F. Peppard, G. Mason, J. Laier and W.J. Driscoll, J. lnorg. Nucl. Chrm., 4 (1957) 334. [4] J. Preston and A.C. Du Preez, Proc. Inf. Solve,~ Extrncfiot~ Conf., Parr A, 1990, p. 883. [5] J.S. Benedetto, V.S.T. Ciminelli and J. Duarte Neto, XV Enconfm National de Trutcunento de MinCrios e Hidrorneic2lwgin, Pnrte 5, Tee. Min., I (I 992) 192. [6] SM. Desphande, S.L. Mishra, R.B. Gajankush, N.V. Thakur and K.S. Koppiker, Miu. Proc. Extr. Metoll. Rev., IO (1992) 267. [7] F. Delmas, C. Nogueira, F. Rodrigues and J. Duarte Neto, Proc. 1~. Solvent Exrraction Cot@., I (1993) 279. [8] B.J. Balint, Separation Factors between Adjacent Rare Earth Extracted from a Mixed RE Chloride Solution Using Ionquest 801, Proc. 2nd Int. Conj on Rare Earth Deueloprnent and Applications, Vol. 1, International Academic Publications, Beijing, 1991, p.386.
Journal of Alloys and Compounds 249 (1997) 116-l I8
Separation of, samarium and gadolinium solutions by solvent extraction P. Miranda Jr. , L. B. Zinner* Institute
de Quirnica-Universidade
de Scio Patio,
C.P. 26.077,
CEP 05599-970-
SL?O Pnulo,
SP, Brazil
Abstract Interest in the production of individual rare earth (RE) elements has increased in the last years, due to the advances in technological applications of RE. Samarium and gadolinium are used in several applications: nuclear industry, lasers, supermagnets among others. This paper presents a technological contribution to the study of RE separation, in order to obtain Sm and Gd concentrated solutions. Experiments were carried out on a laboratory scale using solvent extraction techniques. Separation of Sm and Gd is studied in a continuous system in a micropilot unit of mixer-settlers. The extractant used is 2-ethylhexyl phosphonic acid, mono-2-ethylhexyl ester,
dilutedin isododecane (1 M). The aqueousphaseprocessed wasa chloridesolutionof mediumandheavy RE producedfrom monazite leaching.The RE determinations in aqueoussolutionswereperformedusingan atomicemissionspectrometer with inductively coupled plasmaAton-Comp, serial Ash-800. The analytical results are presentedas RE concentrationsprofiles. Two continuous-circuit configurations are describedof extraction-scrubbing-stripping for the productionof SmandGd concentrated solutionswith purity higher than 90% and 94%, respectively. Keywords:
Samarium;
Gadolinium;
Separation
of RE; Solvent
extraction
1. Introduction The application of solvent extraction techniques to rare earth (RE) fractioning has been studied on a laboratory scale since the 40’s, mainly as a result of research works developed for nuclear fuel production. Liquid-liquid extraction is a separationtechnique based on the distribution of a solute between two phases: aqueous and organic, practically immiscible with each other. The organic phase generally contains a diluent and an extractant capable of combining differently with RE elements,then forming compoundsthat are more soluble, i.e., show greater affinity with the organic phase. According to Marcus and Kertes [I] and Ritcey and Ashbrook [2], solvent extraction processescan be classified basedeither on the chemical reaction involved or on the type of the extractant used. Generally, they are classified as extraction by solvation, extraction by compound formation, and extraction by ion-pair formation. The most used extractants for RE separation are the organophosphorusacids that belong to a compound formation class, with prominence of di-2-ethylhexyl phosphoric acid (D2EHPA), which has been widely applied since the pioneering work of Peppard et al. [3]. Some works of Preston and Du Preez [4], Benedetto et al. [5], Desphande “Corresponding
author.
0925~8388/97/$17.00 PII SO925-8388(96)02754-5
0
1997 Elsevier
Science
S.A. All rights reserved
et al. [6], Delmas et al. [7] and others have discussedthe use of 2-ethylhexylphosphonic acid mono-Zethylhexyl ester (HEH(EHP)) in RE separation. This extractant was chosen because generally it presents higher separation factors and allows easier stripping with relation to D2EHPA. In the extraction process, a cation-exchange reaction occurs, in which hydrogen ions of the extractant are exchangedfor RE ions from the aqueousphase,according to the equation: @+)a
+ 3(IHX),),~(RE(HX?),),
Where a: ;lLIX,2: K:
+ 3(H+),
(1)
aqueousphase organic phase extractant in dimeric form equilibrium constant
2. Experimental 2.1. Reagents The RE chloride solution studied contains medium and heavy RE elements; it originated from the Brazilian monazite processing, carried out by Nuclemon Mineroquimica-SP. This solution has an acidity of 1.18 M
P. Miranda,
L. B. Zimer
I Journal
of Alloys
and about 159 g 1-l of RE elements, having as main constituents: Sm (34.55%), Gd (23.X5%), Tb (2.31%), Dy (6.82%), and Y (24.45%). The RE solutions used in these tests were prepared by adjusting acidity and RE concentration by the addition of distilled water and ammonium hydroxide. The organic phase used was 1 M in the extractant. Diluent- was isododecane with low content of aromatic compounds commercialized with the name Isoparafina-17 /21 by Unipar Quimica. The extractant HEH(EHP), is commercialized with the trade name Ionquestby Albright and Wilson Americas. It was used without further purification and has the structural formula:
and
Compounds
249
(1997)
117
116-116
settlers of 360 cm3 capacities. Two configurations were tested for the fractioning of RECl, solution (Table 1). Configuration A was used for the fractioning of the initial solution in two groups: medium and heavy RE; and the other, configuration B, for the fractioning of the medium RE solution obtained with configuration A, in order to produce as final products Sm and Gd concentrated solutions. The following diagram shows the continuous-circuit used in both configurations for the fractioning of the solution being studied:
CHzCH3 I 1
CH#2HZCHzCH&HCHz0 \
/Ho
2.4. Analytical CH3CHZCH+ZH&HCHz
CHzCHs
2.2. Batch tests Batch tests were carried out in order to study the main parameters extraction kinetics, aqueous phase acidity, RE concentration in aqueous phase, organic/aqueous (0 /A) phase ratio, extractant concentration in organic phase and RE stripping from the loaded organic phase. From batch tests results it was possible to propose solvent extraction configurations to carry out tests in continuous-circuits to reach the fractioning of the RE solution. 2.3. Continuous
tests
Continuous tests were carried out using a micro-pilot unit of mixers-settlers, with mixers of 160 cm3 and Table 1 Characteristics Configuration
Individual RE determinations in aqueous phase were carried out using an atomic emission spectrometer with inductively coupled plasma, Aton-Comp, serial Ash-800 and total RE determinations were performed by EDTA complexometric titrations using xylenol orange as indicator. Acidity analysis of aqueous solutions were carried out by acid-base titration with sodium hydroxide, using as indicator an ethanolic solution of 0.1% methylene blue and methyl red.
3. Results and discussion With the results obtained from batch tests, it was possible to confirm the behavior of the RE-extractant system used with the ones mentioned in different articles in the literature and to propose an initial configuration of continuous-circuit for the fractioning of the solution being studied. In these tests, it was observed that:
of configurations A
Extraction
Scrubbing
Stages
Aqueous
12
50 g 1-l RI? pH=lS
Configuration
control
‘P\OH
feed
Stripping
OiA
Stages
HCl
O/A
Stages
HCI
O/A
4
18
1N
8
5
5N
4
B
Extraction
Scrubbing
Stages
Aqueous
feed
4
16.5 g 1-l RE pH= 1.5
Stripping
O/A
Stages
HCI
O/A
Stages
HCI
O/A
1.2
6
0.5 N
2
4
2N
3
118
P. Miranda,
L. B. Zirmer
I Jmrnnl
of Aliofs
Yttrium is the element showing the highest extraction efficiency. Y (Z =39) has a behavior, with organophosphorus acids, of an element between Ho (z=67) and Er (z=68). Extraction efficiency increases with the atomic number of the lanthanides, i.e., with the decrease of crystalline radius. Extraction is most efficient when: higher phase ratios O/A, higher concentrations of organic phase, higher contact times between organic and aqueous phases, and aqueous phases with low acidity and RE concentrations are used. Stripping is most effective when higher concentration acids are used; for lower concentrations, the stripping is more efficient for the lighter RE. The stripping behavior is opposite to the extraction, where the equilibrium of expression 1 is displaced to the left. From expresssion 1, extraction is dependent on aqueous phase acidity. A pH increase favors extraction, but there is a limit for the pH range, as in high pH solutions the formation of RE precipitates occurs, which is undesirable for the process. The equilibrium in extraction is attained fairly rapidly: in about 3 or 4 minutes practically all RE were extracted, showing quick kinetics as mentioned in other works, which is a characteristic of acid extractants. For the first fractioning process, with relation to batch tests (concentration ea. 50 g 1-l RE, pH = 1-2; O/A =4) the separation factor for Dy/Gd p is 4.8. For the second fractioning (concentration 16 g 1-l; pH= 1-1.5; O/A= I2) for Gd/Sm /3 is 4.1. Balint [S], Preston and Du Preez [4] and Benedetto [5] reported the separation factor values of 3.51, 2.52 and 3.70, respectively, for the Gd/Sm pair. Different configurations were tested, but only two configurations are presented in this paper, the ones that showed better results for the desired fractioning: configurations A and B. Using configuration A, it was possible to fraction the initial RE chloride solution into two groups: medium RE (Sm, Gd, Tb) and heavy RE (Tb, Dy, Y), producing concentrated solutions with near 60% Sm element for the former and 70% Y element for the latter. Configuration B corresponds to the fractioning of medium RE solution obtained previously with configuration A, producing another two fractions: one with a Sm element content higher than 90%, and the other with Gd element content higher than 94%. Fig. 1 shows the concentration profiles of Sm and Gd in aqueous and organic phases in the extraction (E) and scrubbing (S) stages of configuration B. Analyzing the profiles, a higher affinity of Gd for the organic phase can be observed. Gd presents a low concentration in the
and Compounds
249 (1997)
II&
118
Fig. 1. Profile of Sm and Gd concentrations in configuration B, for organic and aqueous phases of extraction (E) and scrubbing (S) stages.
raffinate (stage 1). Sm is back washed from the organic phases during scrubbing, thus producing a strip liquor rich in Gd. The system (HEH(EHP))-RE-HCI was adequate to achieve the separation proposed. Sm concentration was increased from 34% to 90% and Gd from 24% to 94%, with recoveries in the range of 99% for Sm and 95% for Gd. Mixers-settlers devices were applied successfully in this work. Formation of emulsions were not observed, nor RE precipitation from the loaded organic phases during the operation of the continuous-circuit.
Acknowledgments The authors are grateful for the use of analytical support by the chemists Luis Carlos Reino and SebastiZo Alder Pereira (IPEN-CNEN/SP), CTMP-SP, for the laboratories where the experiments were carried out, and for the financial support of CAPES and PADCT.
References [ 1] Y. Marcus and A.S. Kertes, Ion exchange and solvent extraction of metal complex, Wiley-Interscience, New York, 1969. [2] GM. Ritcey and A.W. Ashbrook, Solvent Extraction Principles and Applications to Process Metallurgy, Elsevier, New York, 1984. [3] D.F. Peppard, G. Mason, J. Laier and W.J. Driscoll, J. lnorg. Nucl. Chrm., 4 (1957) 334. [4] J. Preston and A.C. Du Preez, Proc. Inf. Solve,~ Extrncfiot~ Conf., Parr A, 1990, p. 883. [5] J.S. Benedetto, V.S.T. Ciminelli and J. Duarte Neto, XV Enconfm National de Trutcunento de MinCrios e Hidrorneic2lwgin, Pnrte 5, Tee. Min., I (I 992) 192. [6] SM. Desphande, S.L. Mishra, R.B. Gajankush, N.V. Thakur and K.S. Koppiker, Miu. Proc. Extr. Metoll. Rev., IO (1992) 267. [7] F. Delmas, C. Nogueira, F. Rodrigues and J. Duarte Neto, Proc. 1~. Solvent Exrraction Cot@., I (1993) 279. [8] B.J. Balint, Separation Factors between Adjacent Rare Earth Extracted from a Mixed RE Chloride Solution Using Ionquest 801, Proc. 2nd Int. Conj on Rare Earth Deueloprnent and Applications, Vol. 1, International Academic Publications, Beijing, 1991, p.386.