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1424−1430. Ion Flotation of ... Large amounts of REMs are used in the form of a mixture of oxides in ... collecting agent forms in solution surface-active ions.

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2011, Vol. 84, No. 9, pp. 1476−1482. © Pleiades Publishing, Ltd., 2011. Original Russian Text © D.E. Chirkst, O.L. Lobacheva, N.V. Dzhevaga, 2011, published in Zhurnal Prikladnoi Khimii, 2011, Vol. 84, No. 9, pp. 1424−1430.

INORGANIC SYNTHESIS AND INDUSTRIAL INORGANIC CHEMISTRY

Ion Flotation of Rare-Earth Metals with Sodium Dodecyl Sulfate D. E. Chirkst, O. L. Lobacheva, and N. V. Dzhevaga St. Petersburg State Mining Institute, St. Petersburg, Russia Received October 27, 2010

Abstract—Ion flotation of rare-earth metals with sodium dodecyl sulfate and the effect of chloride ions on this process were studied. Values of the distribution coefficients were obtained. DOI: 10.1134/S1070427211090035

Rare-earth metals (REMs) and their compounds become increasingly important for modern technology. Large amounts of REMs are used in the form of a mixture of oxides in metallurgical, glass-making, and ceramic industries. Cerium, neodymium, samarium, europium, and gadolinium are most widely used as compounds of individual REMs. Cerium oxide is applied in glass and mirror polishing. The misch metal containing 50% cerium, 30% lanthanum, 15% neodymium, and 5% praseodymium has found wide use. This alloy serves to purify steel by removal of free oxygen and sulfur and admixtures of lead and antimony. Individual REMs are in the highest demand in the world market. In Russia, the main source of rare-metal raw materials are loparite ores from the Lovozero deposit [1, 2]. Therefore, it is necessary to develop a technology for production of a wide assortment of individual rare-earth elements. A promising way is to use the flotation technique, which can produce a concentrate with 60–70% REM oxides. The ion flotation is a process in which ions present in solution are recovered with ionogenic surfactants serving as collecting agents [3]. To perform ion flotation, gas bubbles and a collecting agent are introduced into the starting solution. The collecting agent forms in solution surface-active ions whose charge is opposite in sign to that of the ion being recovered. A compound of surface-active and recovered ions is concentrated on the surface of gas bubbles and is carried by these bubbles to the foam.

It has been found previously [4] that, in extraction of REMs with tributyl phosphate (TBP) from nitrate solutions, the distribution coefficients vary within the range 1.1–1.4. No large distribution coefficients were observed in ion flotation of REMs, either [5]. It was noted in [6] that addition of chlorides in concentrations of 0.1–0.15 M makes lower the distribution coefficient in extraction with naphthenic acid due to formation of unextractable chloro complexes. Because of the different stabilities of the chloro complexes, this lowering varies and, consequently, the Ce/Y separation coefficient increases from 1.5 to 14. A method has been developed for raising the separation coefficient of REMs by changing the anionic composition of the aqueous phase [7]. Therefore, it is of interest to study the effect of chloride ions on the ion flotation process. We performed flotation in a 137 V-FL laboratory flotation machine with a 1-dm3 cell in the course of 5 min. As model solutions served 0.001 M solutions of cerium, samarium, europium, yttrium, or ytterbium nitrates. The solution volume was 200 ml. Dry sodium dodecyl sulfate (NaDS) was added as a surfactant to a concentration of 0.003 M. We also added sodium chloride to the starting solution in amounts corresponding to concentrations of 0.01 and 0.05 M. At a NaCl concentration of 0.1 M, the flotation of lanthanides is nearly fully suppressed. The resulting foam and chamber products were separated and analyzed. The foam was disintegrated with 1 M sulfuric acid. The REM concentration was determined

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ION FLOTATION OF RARE-EARTH METALS

photometrically with arsenazo III [8]; the concentration of chloride ions, by mercurimetric titration with a mixed indicator (ethanolic solution of 0.5 wt % diphenyl carbazide and 0.05 wt % Bromophenol Blue); and the concentration of dodecyl sulfate ions, by potentiometric titration with a 0.002 M solution of cetyltrimethylammonium chloride with an ion-selective electrode constituted by an EVL-1MZ silver chloride electrode placed in a solution of NaDS and NaCl and a membrane selective to the DS ion. The membrane was fabricated at the laboratory of ionometry, chair of physical chemistry, St. Petersburg State University [10]. The distribution coefficients of metal cations among the foam and chamber products were calculated from the

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ratio between [M3+] (Ce3+, Sm3+, Eu3+, Er3+, Y3+, Yb3+) in the foam product and the concentration [M3+] in the chamber residue by the formula [11]: D = corg/caq.

(1)

The distribution coefficients were found by the formula KM1/M2 = DM1/DM2.

(2)

The results we obtained are listed in Tables 1–6. Figures 1a–1c show as an example dependences of the distribution coefficients on pH at various sodium

(b)

(a)

11 pH (c)

11 pH Fig. 1. Dependence of the distribution coefficient of (a) cerium(III), (b) samarium(III), and (c) europium(III) on pH at various concentrations of chloride ions. Initial metal concentration 0.001 M. cNaCl (M): (1) 0, (2) 0.01, (3) 0.05, and (4) 0.1. RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 9 2011

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CHIRKST et al.

Table 1. Distribution coefficients of cerium(III) in ion flotation in NaCl-containing aqueous solutions сNaCl = 0

сNaCl = 0.01 M

сNaCl = 0.05 M

сNaCl = 0.1 M

рН

D

рН

D

рН

D

рН

D

4.47 4.89 5.30 5.80 6.05 6.70 7.00 7.43 7.85 8.50 8.90

6.07 5.22 5.82 7.62 10.12 14.82 60.92 83.69 107.43 102.33 75.05

4.00 4.56 4.98 5.64 6.12 6.28 6.50 7.00

4.10 5.25 4.13 4.82 5.69 49.2 38.0 22.1

6.03 7.15 7.65 7.95 8.10 8.56 9.20

1.72 2.69 3.02 6.14 29.0 36.7 35.4

6.30 7.16 7.50 7.72 8.18 8.61 9.35

5.30 5.40 4.94 7.01 13.56 8.59 3.66

Table 2. Distribution coefficients of samarium(III) in ion flotation in NaCl-containing aqueous solutions сNaCl = 0

сNaCl = 0.01 M

сNaCl = 0.05 M

Table 3. Distribution coefficients of europium(III) in ion flotation in NaCl-containing aqueous solutions сNaCl = 0

сNaCl = 0.01 M

сNaCl = 0.05 M

рН

D

рН

D

рН

D

рН

D

рН

D

рН

D

1.83

3.01

5.80

9.59

7.40

4.57

0.98

0.23

5.87

5.22

7.44

4.20

3.00

8.60

6.00

7.74

8.20

10.92

2.21

0.45

6.15

58.64

8.20

3.90

6.60

6.35

151.35

8.40

3.94

3.48

13.44

6.30

10.36

8.42

15.51

3.00

4.00

14.09

6.62

99.66

8.70

21.32

3.57

7.89

6.70

385.21

8.75

8.69

4.89

11.55

7.00

166.55

8.95

10.29

4.14

5.02

7.10

916.68

8.90

5.34

5.38

12.14

7.52

358.15

9.40

7.38

4.57

10.26

7.32

166.44

9.45

4.43

6.04

15.08

7.80

57.54

5.05

10.21

7.83

68.57

6.68

81.87

8.50

11.22

5.55

9.85

8.54

16.29

6.90

81.52

6.20

16.64

7.53

81.52

6.53

204.10

7.94

81.64

7.00

208.89

92.90

7.96

182.65

9.05

170.34

10.48

chloride concentrations for cerium, samarium, and europium cations. To find the reason why the flotation is suppressed when the concentration of chlorides is raised and the pH is shifted to larger values [12], it is necessary to determine the form of the complexes being floated. Having compared the pH values of complexation, i.e., the pH at which MOH2+ or M(OH)+2 complexes start to be formed, and the pH of hydrate formation with the pH at which the degree of recovery is the largest (Table 7), we came to the following conclusions. In flotation of

cerium(III), hydroxides Ce(OH)3 are recovered into the foam. For samarium and europium cations, the recoveryonset pH exceeds the complexation pH, but is lower than the hydrate formation pH, and, therefore, hydroxo salts of composition MOH(DS)2 are the first to be recovered to the foam and then this occurs with hydroxides M(OH)3. For yttrium, ytterbium, and erbium, there are two complexation stages, MOH2+ and M(OH)+2, respectively. Therefore, a mixture of the following composition is recovered into the foam in flotation of cations of these

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 9 2011

ION FLOTATION OF RARE-EARTH METALS Table 4. Distribution coefficients of erbium(III) in ion flotation in NaCl-containing aqueous solutions сNaCl = 0

сNaCl = 0.01 M

сNaCl = 0.05 M

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Table 6. Distribution coefficients of ytterbium(III) in ion flotation in NaCl-containing aqueous solutions сNaCl = 0

сNaCl = 0.01 M

сNaCl = 0.05 M

рН

D

рН

D

рН

D

рН

D

рН

D

рН

D

1.76

4.73

5.68

3.71

6.00

4.01

5.03

9.51

5.86

3.10

6.02

1.88

3.06

4.38

6.00

5.01

6.28

4.01

5.50

9.22

6.30

7.31

6.33

2.84

3.49

3.97

6.31

198.00

6.58

3.95

6.00

13.77

6.85

20.76

6.71

12.44

4.24

3.63

6.70

257.14

6.81

11.20

6.40

95.42

7.36

48.35

7.47

1.36

5.20

7.88

7.12

114.71

7.13

3.46

7.06

281.39

7.85

7.63

7.83

0.52

5.35

6.67

7.40

6.62

7.42

1.69

7.38

365.37

8.57

4.00

8.20

0.53

6.19

11.94

7.87

3.45

7.78

2.12

8.30

403.74

8.40

0.61

6.43

566.67

8.54

0.58

8.15

1.55

9.10

318.41

8.85

0.70

6.52

606.25

8.42

0.98

7.30

630.00

8.68

1.25

8.40

693.75

9.27

700.00

Table 5. Distribution coefficients of yttrium(III) in ion flotation in NaCl-containing aqueous solutions сNaCl = 0

сNaCl = 0.01 M

сNaCl = 0.05 M

With increasing sodium chloride concentration, the recovery-onset pH and the maximum-recovery pH grow for cations of the rare-earth metals under study. Ytterbium and cerium are exceptions. In flotation of ytterbium, the recovery-onset and maximum-recovery pH are shifted to smaller values as the concentration of chloride ions becomes higher. In the case of cerium, the recovery-onset and maximum-recovery pH values decrease at a NaCl concentration of 0.01 M and increase at 0.05 M. We determined the effect of chloride ions on the distribution coefficient by comparing the instability constants of chloro and hydroxo complexes (Table 8). The former were calculated by the formula

рН

D

рН

D

рН

D

2.99

8.96

3.92

3.41

5.17

3.08

3.65

8.41

4.50

4.58

6.40

3.10

4.10

9.41

5.13

4.28

7.03

9.42

4.59

11.17

6.03

3.81

7.40

7.23

Λf G°298 {MCl2+} = Λf G°{M3+} Λf G{Cl–}

5.10

12.57

6.72

3.87

7.87

4.47

+ RTln Kn.

6.12

30.46

7.41

279.34

8.53

1.13

6.67

264.48

7.80

379.49

6.95

855.64

8.11

50.60

7.99

736.18

8.50

8.39

8.97

3.12

metals: MOH(DS)2, M(OH)2DS, and M(OH)3. The only exception is yttrium, for which pHhydr is higher than the maximum-recovery pH, and, therefore, no hydroxides are recovered.

(3)

Having expressed the instability constant by means of Eq. (3), we obtained Kn = e

ΛcomplG°298 – ––––––––– RT

.

(4)

The Gibbs energies of formation of hydroxo complexes and lanthanide cations in an aqueous solution were taken in accordance with the database [14]. According to the data in Table 8, the instability constants of hydroxo complexes with Ce3+, Sm3+, Eu3+, Er3+, and Yb3+ are approximately equal. Y(OH)2+ has

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CHIRKST et al.

Table 7. REM distribution coefficients in relation to the recovery pH and sodium chloride concentration Parameter

Y

Ce

Sm

Eu

Er

Yb

рНcompl МОН2+

6.30

5.90

5.81

5.80

5.72

5.79

рНcompl М(ОН)2+

6.97







6.21

6.30

pHhydr

7.20

6.40

6.49

6.52

6.44

6.56

сNaCl = 0 Distribution coefficient

855.64

107.43

81.87

208.89

606.25

403.74

Maximum-recovery pH

6.95

7.85

6.68

7.00

6.52

8.30

Recovery-onset pH

6.12

6.70

6.04

6.20

6.19

6.00

сNaCl = 0.01 M Distribution coefficient

379.49

49.19

358.15

916.68

257.14

48.35

Maximum-recovery pH

7.80

6.28

7.52

7.10

6.70

7.36

Recovery-onset pH

6.72

6.12

6.30

5.87

6.00

5.86

сNaCl = 0.05 M Distribution coefficient

9.42

36.65

21.32

8.69

11.20

12.44

Maximum-recovery pH

7.03

8.56

8.70

8.75

6.81

6.71

Recovery-onset pH

6.40

7.65

7.40

8.40

6.58

6.02

Table 8. Instability constants of hydroxo complexes and chloro complexes with REMs Compound

Kn

ΔcomplG°298, kJ mol–1

Compound

Kn

ΔcomplG°298, kJ mol–1

YCl2+

0.054

–7.22

Y(OH)2+

1.56 × 10–8

–44.56

CeCl2+

0.092

–5.91

Ce(OH)2+

6.80 × 10–9

–46.62

SmCl2+

0.072

–6.52

Sm(OH)2+

5.04 × 10–9

–47.36

EuCl2+

0.076

–6.37

Eu(OH)2+

4.92 × 10–9

–47.42

ErCl2+

0.083

–6.16

Er(OH)2+

4.09 × 10–9

–47.88

YbCl2+

0.110

–5.46

Yb(OH)2+

4.99 × 10–9

–47.39

a lower stability. YCl2+ is the most stable chloro complex, and YbCl2+, the least stable. This fact accounts for the substantial decrease in the distribution coefficients in ion flotation of yttrium because its cations are bound into a stable unfloatable chloro complex, and for the decrease in the recovery-onset and maximum-recovery pH values for ytterbium with increasing concentration of chlorides. Using the distribution coefficients for various REM pairs, we calculated the separation coefficients at certain pH values in relation to the concentration of chloride

ions. The results we obtained are listed in Table 9. Noteworthy is the strongest effect of chloride ions on the increase in the separation coefficients. At a NaCl concentration of 0.01 M, KEu/Ce increased from 3.43 in a nitrate medium to 41.52 at pH 7.00; KSm/Y, from 0.31 to 25.73 at pH 6.62; KEu/Er, from 0.33 to 25.16 at pH 7.32; KEu/Y, from 0.77 to 99.45 at pH 6.70; KSm/Er, from 0.13 to 54.13 at pH 7.52; KY/Er, from 1.06 to 110.03 at pH 7.87; and KY/Yb, from 1.82 to 49.73 at pH 7.85. At a NaCl concentration of 0.05 M, KCe/Er increased from 0.15 in a nitrate medium to 37.25 at pH 8.56; KCe/Yb,

RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 9 2011

ION FLOTATION OF RARE-EARTH METALS

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Table 9. Maximum separation coefficients of lanthanides сNaCl = 0

сNaCl = 0.01 M

Separation coefficient

Kmax

рН

KSm/Ce

5.53

6.70

KEu/Ce

13.78

KEr/Ce

сNaCl = 0.05 M рН

Kmax

рН

7.54

7.00

1.51

7.60

6.70

41.52

7.00

1.39

7.65

40.92

6.70

6.77

6.50

2.32

6.00

KCe/Yb

0.74

6.00

6.73

6.30

60.15

8.56

KCe/Y

0.54

4.50

9.80

6.50

32.41

8.56

KEu/Sm

1.10

6.00

14.60

6.30

0.92

7.40

KSm/Er

3.89

4.00

54.13

7.52

17.08

8.70

KSm/Yb

1.32

5.40

7.54

7.80

25.45

8.40

KSm/Y

1.60

3.50

25.73

6.60

13.72

8.40

KEu/Er

1.48

5.55

25.16

7.30

6.96

8.75

KEu/Yb

1.21

6.10

20.69

6.35

7.57

8.90

KEu/Y

0.92

4.60

99.45

6.70

3.49

8.40

KEr/Yb

5.94

6.40

27.07

6.30

4.09

7.80

KY/Er

2.55

6.20

110.03

7.80

4.28

7.40

KY/Yb

3.04

7.00

49.73

7.80

8.62

7.80

Kmax

from 0.25 to 60.15 at pH 8.56; KCe/Y, from 0.15 to 32.41 at pH 8.56; and KSm/Yb, from 0.20 to 25.45 at pH 8.42. In flotation of rare-earth metals in a nitrate-chloride medium, the separation coefficients become 3–30 times larger, depending on the pH and concentration of chloride ions. In the nitrate medium, the separation coefficients are small (Table 9). The exceptions are the following REM pairs: Eu/Ce and Er/Ce. The separation coefficients are the largest at chloride concentrations of 0.01 M, with the exception of the Er/ Ce pair. At a sodium chloride concentration of 0.05 M, the separation coefficients decrease to a varied extent, with the exception of Ce/Yb, Ce/Y, and Sm/Yb. The maximum separation coefficients of certain pairs of rare-earth metals show a tendency toward a shift of the pH to higher values upon an increase in the concentration of chloride ions. The opposite tendency was observed for the pair Er/Ce. For the pairs Er/Yb, Y/Er, and Y/YB, the pH shifted to higher values at a chloride concentration of 0.01 M, and to lower values, compared with the pH in the absence of chloride ions, at a concentration of 0.05 M.

CONCLUSIONS (1) A tendency toward a decrease in the distribution coefficients and a shift of the maximum recovery to the range of higher pH values is observed in ion flotation of lanthanides upon addition of chloride ions. (2) The distribution coefficients reach the maximum value at a chloride concentration of 0.01 M. ACKNOWLEDGMENTS The study was performed under the Analytical departmental target program of the RF Ministry of Education and Science “Development of the scientific potential of the higher school (2009–2010)” (project no. 2.1.1./973) and RF Presidential program “Leading scientific schools” (grant no. NSh-6291.2010.3). REFERENCES 1. Naumov, A.V., Izv. Vyssh. Uchebn. Zaved., Tsv. Metall., 2008, no. 1, pp. 22–31.

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2. Samonov, A.E., Perspektivy razvitiya proizvodstva i potrebleniya redkozemel’noi produktsii v Rossii: Materialy Vserossiskoi nauchnoi konferentsii (Prospects for Development of Manufacture and Consumption of Rare-Earth Products in Russia: Proc. All–Russia Sci. Conf.), Moscow: GGM Ross. Akad. Nauk, 2008, pp. 134–138.

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11.

3. Sebba, F., Ion Flotation, Elsevier Publishing Company, 1962. 4. Chekmarev, A.M., Chizhevskaya, S.V., et al., Trudy Mosk. Khim.-Tekhnol. Inst. im. D.I. Mendeleeva, 1986, no. 143, pp. 3–7. 5. Chirkst, D.E., Lobacheva, O.L., and Berlinskii, I.V., Zh. Prikl. Khim., 2009, vol. 82, no. 8, pp. 1273–1276. 6. Chirkst, D.E., Litvinova, T.E., Starshinova, V.S., and Lutskii, D.S., Zap. Gorn. Inst., 2006, vol. 169, pp. 204– 208. 7. RF Patent 2320738. 8. Savin, S.B., Arsenazo III (Arsenazo III), Moscow: Atomizdat, 1966. 9. Kreshkov, A.P., Osnovy analiticheskoi Teoreticheskie osnovy: Kolichestvennyi

khimii: analiz

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(Fundamentals of Analytical Chemistry: Theoretical Foundations: Quantitative Analysis), Moscow: Khimiya, 1976. Timofeev, S.V., Materova, V.A., and Arkhangel’skii, L.K., Vestn. Len. Gos. Univ., Fiz. Khim., 1978, no. 16, issue 3, pp. 139–141. Osnovy analiticheskoi khimii. V 2 knigakh (Fundamentals of Analytical Chemistry in 2 books), Zolotov, Yu.A., Ed., Kniga 1. Obshchie voprosy: Metody razdeleniya (Book 1: General Problems: Separation Techniques), Moscow: Vysshaya Shkola, 2004. Chirkst, D.E., Lobacheva, O.L., Berlinskii, I.V., and Dzhevaga, N.V., Zh. Prikl. Khim., 2011, vol. 84, no. 2, pp. 345–348. Chirkst, D.E., Lobacheva, O.L., and Berlinskii, I.V., Zh. Fiz. Khim., 2010, vol. 84, no. 12, pp. 2241–2244. www.chem.msu.su. Khimicheskaya informatsionnaya set’: Elektronnaya biblioteka uchebnykh materialov po khimii: Uchebnye bazy dannykh: Baza dannykh “Termicheskie konstanty veshchestv.” (Chemical Information Network: Electronic Library of Educational Materials on Chemistry: Educational Databases: Database “Thermal Constants of Substances”).

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