Solvent Extraction of PtCl_{4} from Hydrochloric Acid Solution with ...

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Nov 25, 2008 - Solvent Extraction of PtCl4 from Hydrochloric Acid Solution with Alamine336. Man Seung Lee1;*, Jin Young Lee2, J. Rajesh Kumar2, Joon Soo ...
Materials Transactions, Vol. 49, No. 12 (2008) pp. 2823 to 2828 #2008 The Japan Institute of Metals

Solvent Extraction of PtCl4 from Hydrochloric Acid Solution with Alamine336 Man Seung Lee1; * , Jin Young Lee2 , J. Rajesh Kumar2 , Joon Soo Kim2 and Jeong Soo Sohn2 1

Department of Advanced Materials Science & Engineering, Mokpo National University, Chonnam 534-729, Korea Metals Recovery Group, Minerals & Materials Processing Division, Korea Institute of Geoscience & Mineral Resources, Daejeon 305-350, Korea 2

We have conducted solvent extraction of Pt(IV) from HCl solution with Alamine336. Solvent extraction reaction in our system was determined from the experimental results by graphical method. The equilibrium constant of the solvent extraction reaction was estimated from our experimental results by considering the activity coefficients of chemical species present in the aqueous phase with Bromley equation. Bromley interaction parameter between hydrogen ion and PtCl2 6 was evaluated from the solvent extraction data reported in the literature. Solvent extraction of PtCl4 by Alamine336 and the corresponding equilibrium constant in our experimental range can be represented by  3 PtCl2 6 þ R3 NH2 Cl2,org ¼ PtCl6 R3 NH2,org þ 2Cl , Kex ¼ 1:9  10 . The measured distribution coefficients of Pt(IV) agreed well with those calculated in this study. [doi:10.2320/matertrans.MRA2008305] (Received September 1, 2008; Accepted October 2, 2008; Published November 25, 2008) Keywords: PtCl4 , solvent extraction, Alamine336, HCl

1.

Introduction

Platinum group metals are very expensive and their specific physical and chemical properties have made them important materials for the automobile, chemical, and electronics industry. Pt, Pd and Rh are used in automobile catalysts to reduce the emission levels from the exhaust gases and the automobile industry is the major consumer of platinum group metals.1) Since platinum has aesthetic qualities and a permanent luster, it is also used in the manufacture of jewelry.1) Substitution of platinum metal in catalysts by other metals is difficult. Since the resources of the platinum metal are limited, platinum should be recovered not only from its natural ores but also from the industrial wastes. Development of an effective separation process to recover platinum from these resources, especially spent automotive catalysts, is very important. Separation and purification of platinum group metals is one of the most difficult processes of metal separation, because of the similarity in their chemical behavior in chloride media. The most prominent feature of the aqueous chemistry of platinum group metals in chloride solution is the strong tendency of the metals to form anion complexes with chloride ion.2) Since the concentration of platinum group metals in industrial wastes is very low, ion exchange and solvent extraction have been widely employed to separate and recover them. Solvent extraction of Pt(IV) with various extractants has been reported.3–7) Alamine336 (Tertiary amine, R3 N, R ¼ CH3 (CH2 )7 ), a sort of anionic extractants, is also used extensively in extracting various metal ions.8–11) In this study, solvent extraction of Pt(IV) from chloride solution by Alamine336 has been performed in the low concentration range of Pt(IV) at moderate HCl concentration. Solvent extraction reaction in our experimental range was identified from the experimental data. Interaction parameter between Hþ and PtCl6 2 , which is necessary to calculate the activity coefficient of PtCl6 2 , was evaluated from the solvent extraction data reported in the literature. The equilibrium constant for the solvent extraction *Corresponding

author, E-mail: [email protected]

of Pt(IV) by Alamine336 was estimated from our experimental data by considering the activity coefficients of solutes in the aqueous phase. 2.

Experimental

Stock solution of platinum(IV) was prepared by dissolving PtCl4 (Aldrich, 98%) and HCl in distilled water. Alamine336 was purchased from Henkel Corporation and used without further purification. Alamine336 was diluted with toluene. Equal volume (30 cm3 ) of aqueous and organic phase was placed in a 100 cm3 separatory funnel and shaken for 15 min with a mechanical shaker. The aqueous phase was separated after settling the mixture for 1 h. All the experiments were conducted at a temperature of 25  1 C. The concentration of Pt(IV) in the aqueous phase was measured with ICP-OES (Spectroflame EOP). The concentration of Pt in the organic phase was calculated from the mass balance. 3.

Results and Discussion

3.1 Solvent extraction of PtCl4 with Alamine336 Figure 1 shows the effect of shaking time on the distribution coefficients of Pt(IV) at the extraction condition of Pt(IV) 5:0  104 and HCl 1 kmol/m3 with 0.002 and 0.005 kmol/m3 Alamine336 (R3 N), respectively. In both concentrations of Alamine336, shaking time of 10 min was found to be sufficient to obtain equilibrium state. Therefore, the two phases were shaken for 15 min in further experiments. It has been reported that Alamine336 reacts with HCl to form Alamine336 salt (R3 NHCl) in HCl solution and that this Alamine336 salt takes part in the solvent extraction reaction of metals.10,11) The following reaction represents the formation of Alamine336 salt in HCl solution. R3 Norg þ HCl ¼ R3 NHClorg

ð1Þ

In the above equation, subscript org represents organic phase. Many studies have shown that most of the Pt(IV) in HCl solution exists as PtCl2 6 . Table 1 lists the stepwise

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M. S. Lee, J. Y. Lee, J. R. Kumar, J. S. Kim and J. S. Sohn 0

0.5

PtCl6

2-

0.4

-1

log (mole fraction)

0.3

log D

0.2

[R3N]total (kmol/m3) 0.002 0.005

0.1

0.0

-2

PtCl5

-3

-

-4

-0.1

PtCl4o

-5 -0.2

0

10

20

30

40

50

-6

60

0

1

Mixing time (min)

2

3

4

5 3

HCl concentration (kmol/m )

Fig. 1 Effect of shaking time on the distribution coefficients of Pt(IV) at the initial extraction condition of 5  104 kmol/m3 Pt(IV) and 1 kmol/ m3 HCl.

Fig. 2 Distribution of Pt(IV) containing species with HCl concentration at the initial Pt concentration of 1:0  103 kmol/m3 .

Table 1 Stepwise stability constant for the formation of Pt(IV)-chloro complexes at 25 C. logKI

PtCl4 þ Cl ¼ PtCl5 

2.016

PtCl5  þ Cl ¼ PtCl6 2

2.010

2 12) stability constants for the formation of PtCl 5 and PtCl6 . Distribution of Pt(IV) containing species with HCl concentration was obtained by considering the complex formation reaction shown in Table 1 together with the mass balance equations for Pt and Cl. The calculated mole fractions of Pt(IV) containing species are shown in Fig. 2 as a function of HCl concentration at the Pt(IV) concentration of 1:0  103 kmol/m3 . Figure 2 reveals that PtCl2 6 is the predominant species in our experimental range. Therefore, the following reaction may be responsible for the solvent extraction reaction of Pt(IV) from HCl solution by Alamine336.13)  PtCl2 ð2Þ 6 þ 2R3 NHClorg ¼ PtCl6 (R3 NH)2,org þ 2Cl

Figure 3 shows the effect of HCl concentration on the distribution coefficients of Pt(IV) when the initial concentrations of Pt(IV) and Alamine336 were 4:3  104 and 0.002 kmol/m3 , respectively. The distribution coefficients of Pt(IV) decreased with the increase of HCl concentration. Figures 4 and 5 show the effect of Alamine336 concentration on the distribution coefficients of Pt(IV) from HCl solution of 1.0 and 3.0 kmol/m3 , respectively. In Fig. 4, the initial concentration of Pt(IV) was 4:3  104 kmol/m3 , while the initial concentration of Pt(IV) in Fig. 5 was 9:1  104 kmol/m3 . In both figures, the concentration range of Alamine336 was from 0.002 to 0.007 kmol/m3 . Figures 4 and 5 show that log D increases linearly with the increase

1.2

1.0

log D

Reaction

1.4

0.8

0.6

0.4

0.2 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

log [HCl] Fig. 3 Effect of HCl concentration on the extraction of Pt(IV) at the initial Alamine336 concentration of 0.002 kmol/m3 . ([PtCl4 ]total ¼ 4:3  104 kmol/m3 )

of log [R3 N] in the experimental range and the slope is close to unity. Figure 6 shows the effect of Pt(IV) concentration when Alamine336 and HCl concentrations were fixed at 0.002 and 1.0 kmol/m3 , respectively. The logarithm of Pt(IV) concentration in organic phase is proportional to that of Pt(IV) in the aqueous phase and the slope of this plot is unity, indicating

Solvent Extraction of PtCl4 from Hydrochloric Acid Solution with Alamine336

2825

-3.35

0.8 -3.40

0.6

log [Pt]org

log D

-3.45

0.4

-3.50

-3.55

0.2

[HCl]total

slope

1.0 kmol/m3 3.0 kmol/m3

1.05 1.24 -3.60 -4.20

0.0 -3.0

-2.8

-2.6

-2.4

-2.0

-2.2

-4.10

-4.05

-4.00

-3.95

log [Pt(IV)]aq

log [R3N]total Fig. 4 Effect of Alamine336 concentration on the extraction of Pt(IV) at the HCl concentration of 1.0 and 3.0 kmol/m3 . ([PtCl4 ]total ¼ 4:3  104 kmol/m3 )

-4.15

Fig. 6 Effect of Pt(IV) concentration on the extraction of Pt at 1.0 kmol/m3 HCl and 0.002 kmol/m3 Alamine336 concentrations.

when the amount of acid is in excess to amine.14,15) Therefore, Alamine336 can form double salt with excess HCl, which is represented by

1.0

R3 NHClorg þ HCl ¼ R3 NH2 Cl2,org

The dependence of log D on log [R3 N] in Figs. 4 and 5 suggests the following reaction to occur in our experimental ranges rather than eq. (2).

0.8

log D

ð3Þ

 PtCl2 6 þ R3 NH2 Cl2,org ¼ PtCl6 R3 NH2,org þ 2Cl

0.6

ð4Þ

Evaluation of the interaction parameter between Hþ and PtCl6 2 We used Bromley equation to calculate the activity coefficients of chemical species present in the aqueous phase. The following equation represents Bromley equation for cation M at 25 C.16) 3.2

0.4

[HCl]total

0.2

1.0 kmol/m3 3.0 kmol/m3

slope 1.35 1.35

0.0 -2.7

-2.6

-2.5

-2.4

-2.3

-2.2

-2.1

0:5108ðzM Þ2 I0:5 þ FM ¼ AðzM Þ2 þ FM 1 þ I0:5 2 3 X ð0:06 þ 0:6BMX Þ  jzM zX j FM ¼ þ BMX 7 2  6 1:5 5 X 4 jI 1þ jzM zX

log M ¼  -2.0

log [R3N]total Fig. 5 Effect of Alamine336 concentration on the extraction of Pt(IV) at the HCl concentration of 1.0 and 3.0 kmol/m3 . ([PtCl4 ]total ¼ 9:1  104 kmol/m3 )

that the extracted Pt(IV) species exists as a monomer in the organic phase. Equation (2) suggests that the slope of the straight line in a plot of log D against log [R3 N] should be about two. However, the slope of the straight lines in Fig. 4 was close to unity, indicating that one mole of Alamine336 reacts with one mole of PtCl2 6 in our experimental range. Some studies have reported that amine extractant can form double salt

ð5Þ

ðjzM j þ jzX jÞ2 ½X ð6Þ 4 In the above equations, z is the ionic charge and I ionic strength of a solution and BMX the interaction parameter between cation M and anion X. In calculating the activity coefficients of solutes present in our system by Bromley equation, the activity coefficients of þ chloride ion and PtCl2 6 depend on the concentration of H and the interaction parameter between these two ions and hydrogen ion. The value of BHCl was obtained from the data reported by Bromley,16) while there is no Bromley interac

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M. S. Lee, J. Y. Lee, J. R. Kumar, J. S. Kim and J. S. Sohn

tion parameter between PtCl2 and Hþ . Lokhande et al. 6 reported the data on the solvent extraction of Pt(IV) from HCl solution by N-n-octylaniline (RR0 NH, R ¼ C6 H5 , R0 ¼ CH2 (CH2 )6 CH3 ).2) We evaluated Bromley interaction parameter between PtCl2 and Hþ from these data. They 6 supposed the following reaction to represent the solvent extraction of Pt(IV) from HCl solution by N-n-octylaniline. PtCl2 6

7

Y value of Eq. (12)

RR0 NHorg þ HCl ¼ RR0 NH2 Clorg

8

ð7Þ

0

0



þ 2RR NH2 Clorg ¼ PtCl6 (RR NH2 )2,org þ 2Cl

ð8Þ

The equilibrium constant of the above equation can be represented as follows14) o Kex ¼

[PtCl6 (RR0 NH2 )2 ][Cl ]2 0 [PtCl2 6 ][RR NH2 Cl]

I ¼ Kex 

2



ðCl Þ2 PtCl6 (RR0 NH2 )2  PtCl2 ðRR0 NH2 Cl Þ2 6

ðCl Þ2 PtCl6 (RR0 NH2 )2  PtCl2 ðRR0 NH2 Cl Þ2 6 o

[RR'NH] slope 0.0224 M 0.96 0.0448 M 0.84

ð9Þ 3

I

0 I log Kex ¼ log Kex þ 2 log Cl PtCl6 (RR0 NH2 )2  log PtCl2 þ log 6 ðRR0 NH2 Cl Þ2

2 0

1

2

3

4

5 3

Hydrogen ion concentration (kmol/m ) Fig. 7 Evaluation of the interaction parameter between PtCl6 2 and Hþ from the extraction data reported in the literature by using eq. (12).

ð10Þ

Expression for the activity coefficient of PtCl2 by 6 Bromley equation becomes 2 3 2ð0:06 þ 0:6BHþ ,PtCl2 Þ 6 log PtCl2 ¼ 4A þ 6 þ BHþ ,PtCl2   6 6 7 1:5 2 4 5 I 1þ 2 32 þ [H ] 4

5

4

In the above equation, Kex and Kex represent the equilibrium constant of the solvent extraction reaction at ionic strength of zero and I, respectively. Taking logarithms on both sides of the above equation leads to



6

ð11Þ

Substitution of the above equation into eq. (10) results in I log Kex þ 2 log Cl þ 4A 2 3 2ð0:06 þ 0:6BHþ ,PtCl2 Þ 0 6 ¼ log Kex þ 6 þ BHþ ,PtCl2   7 6 1:5 2 4 5 I 1þ 2 9 þ PtCl6 (RR0 NH2 )2  [H ]  log ð12Þ 4 ðRR0 NH2 Cl Þ2 I was obtained from the experimental The value of Kex results reported by Lokhande2) and the activity coefficient of chloride ion was calculated by Bromley equation. In Lokhandes’s experimental condition, the volume ratio of aqueous to organic phase was kept at 2.5. Therefore, the following mass balance equations were used in calculating I Kex from their experimental results.   Vaq [PtCl6 (RR0 NH2 )2 ]org ¼ ð13Þ ð[Pt]total  [Pt]aq Þ Vorg

[RR0 NH2 Cl] ¼ [RR0 NH2 Cl]total  2[PtCl6 (RR0 NH2 )2 ] ð14Þ [Cl ] ¼ [HCl]total þ 4[PtCl4 ]total  6[PtCl2 6 ]   Vorg  ð6[PtCl6 (RR0 NH2 )2 ] þ [RR0 NH2 Cl]Þ ð15Þ Vaq

[Hþ ] ¼ [HCl]total 

Vorg [RR0 NH]total Vaq

ð16Þ

In the above equation, subscript total represents the initial total concentration. Figure 7 shows the plot of the value of the left-hand side of eq. (12) against the concentration of hydrogen ion. The slope of the straight line in this figure is related to the interaction parameter between Hþ and PtCl2 6 . The value of Bromley interaction parameter between Hþ and PtCl2 6 was evaluated from the slope as follows BHþ ,PtCl2 ¼ 0:28 6 3.3

ð17Þ

Prediction of Pt(IV) distribution coefficients from initial extraction conditions In order to estimate the equilibrium constant of solvent extraction reaction, eq. (4), the equilibrium concentrations and activity coefficients of chemical species present in both phases are required. In calculating the equilibrium concentrations of chemical species present in the aqueous phase of our system, it was assumed that all of the Pt(IV) in the aqueous phase exists as PtCl2 6 . The concentration of HCl employed in this study was from 0.1 to 3.0 kmol/m3 . The concentration of hydroxide ion can be ignored at this HCl concentration range. On this assumption, the following mass and charge balance equations were obtained when the volume ratio of aqueous to organic was unity. [PtCl4 ]total ¼ [PtCl2 ð18Þ 6 ] þ [PtCl6 R3 NH2 ] ð19Þ [R3 N]total ¼ [R3 NH2 Cl2 ] þ [PtCl6 R3 NH2 ] [Cl]total ¼ 4[PtCl4 ]total þ [HCl]total ¼ [Cl ] þ 6[PtCl2 6 ] þ 2[R3 NH2 Cl2 ] þ 6[PtCl6 R3 NH2 ] ð20Þ

Solvent Extraction of PtCl4 from Hydrochloric Acid Solution with Alamine336 Table 2 Initial and equilibrium conditions together with calculated results. (Unit: kmol/m3 )

2.4

[HCl]t

[R3 N]t

logDexpt

LogDcal

1

4:3  10

0.1

0.002

1.34

2.37

2 3

4:3  104 4:3  104

0.2 0.3

0.002 0.002

1.05 0.94

1.76 1.44

2.0

4

4:3  104

0.5

0.002

0.75

1.06

1.6

5

4

4:3  10

0.7

0.002

0.70

0.83

6

4:3  104

0.9

0.002

0.61

0.67

7

4

4:3  10

1.0

0.002

0.52

0.61

8

4:3  104

1.0

0.001

0.39

0.25

9

4

4:3  10

1.0

0.0013

0.47

0.39

10 11

4:3  104 4:3  104

1.0 1.0

0.0015 0.0017

0.54 0.63

0.46 0.53

0.4

12

4:3  104

1.0

0.002

0.70

0.61

0.0

13

4

4:3  10

3.0

0.002

0.11

0.18

14

4:3  104

3.0

0.003

0.34

0.37

15

4

4:3  10

3.0

0.004

0.57

0.51

16

4:3  104

3.0

0.005

0.64

0.61

17

4

4:3  10

3.0

0.006

0.72

0.69

18 19

4:3  104 9:1  104

3.0 1.0

0.007 0.002

0.76 0.19

0.76 0.51

20

9:1  104

1.0

0.003

0.60

0.74

21

9:1  104

1.0

0.004

0.79

0.90

22

9:1  104

1.0

0.005

0.84

1.02

23

9:1  104

1.0

0.006

0.92

1.11

24

9:1  104

1.0

0.007

0.96

1.19

25

9:1  104

3.0

0.002

0.18

0.11

26 27

9:1  104 9:1  104

3.0 3.0

0.003 0.004

0.43 0.53

0.32 0.46

28

9:1  104

3.0

0.005

0.74

0.57

29

4

3.0

0.006

0.80

0.66

30

4

9:1  10

3.0

0.007

0.96

0.74

31

4:0  104

1.0

0.002

0.70

0.62

32

4

4:5  10

1.0

0.002

0.71

0.61

33

5:0  104

1.0

0.002

0.71

0.60

34 35

5:5  104 6:0  104

1.0 1.0

0.002 0.002

0.71 0.71

0.59 0.58

36

6:5  104

1.0

0.002

0.72

0.56

9:1  10

logDexpt : measured distribution coefficients of Pt(IV) LogDcal : calculated distribution coefficients of Pt(IV)

[Hþ ] ¼ [HCl]total  2[R3 N]total

ð21Þ

The number of chemical species present in both phases after extraction is 7 (Cl , Hþ , PtCl6 2 , R3 NH2 Cl2 , PtCl6 R3 NH2 , H2 O, toluene). In order to calculate the concentrations of these 5 species excluding H2 O and toluene, 5 independent equations are required. These 5 equations were obtained from solvent extraction reaction, three mass balance equations, and charge balance. The solution of these 5 nonlinear equations was obtained by using Newton-Raphson method. Since few equations are available to calculate the activity coefficients of chemical species present in the organic phase, the activity coefficients of Alamine336 double salt and the extracted species were assumed to be unity. In order to estimate the above equilibrium constant from the extraction data, an evaluation function was defined as follows:

log Dcalc

[PtCl4 ]t

2.8

4

N

2827

1.2

0.8

-0.4 -0.4

standard deviation = 0.06 0.0

0.4

0.8

1.2

1.6

2.0

2.4

2.8

log Dexpt Fig. 8 Comparison of the distribution coefficients of Pt(IV) between measured and calculated in this study.

1X ðDcalc  Dmeas Þ2 ð22Þ N where N denotes the number of experimental data and Dcalc and Dmeas represent the distribution coefficient of Pt(IV) calculated in this study and measured, respectively. The following equilibrium constant was obtained by minimizing the Err function. Err ¼

Kex ¼

[PtCl6 R3 NH2 ][Cl ]2 ðCl Þ2 ¼ 1:9  103 2 [PtCl2 ][R NH Cl ] 3 2 2 PtCl 6 6

ð23Þ

Table 2 gives the experimental conditions along with the results of extraction experiment. Figure 8 shows the measured distribution coefficients of Pt(IV) and the calculated values. The distribution coefficients of Pt(IV) calculated from the initial extraction conditions by using the above equilibrium constant are also shown in Table 2. It is seen in Table 2 and Fig. 8 that the difference between measured and calculated distribution coefficients of Pt(IV) is wide when HCl concentration is lower than 0.5 kmol/m3 . When HCl concentration is low, Alamine336 may exist as an Alamine336 salt (R3 NHCl) instead of a double salt (R3 NH2 Cl2 ). This difference in the form of Alamine336 salt at low and high HCl concentration may be the reason of the wide difference between the measured and calculated distribution coefficients of Pt(IV). Standard deviation between the measured and calculated distribution coefficients of Pt(IV) was 0.06. Therefore, it can be safely said that the proposed solvent extraction reaction and equilibrium constant estimated in this study might well represent the reaction occurring in the experimental ranges where HCl concentration is larger than 0.5 kmol/m3 . 4.

Conclusions

Chemical speciation of PtCl4 -HCl-H2 O system indicates that most of the Pt(IV) in the solution exists as PtCl2 6 in our experimental range. Applying slope analysis method to the

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M. S. Lee, J. Y. Lee, J. R. Kumar, J. S. Kim and J. S. Sohn

experimental results revealed that one mole of Alamine336 reacts with one mole of PtCl2 6 . Solvent extraction reaction was supposed to explain this dependence of Pt(IV) extraction on Alamine336 concentration. The equilibrium constant for this reaction was estimated from the experimental results by considering the activity coefficients of chemical species present in the aqueous phase with Bromley equation. The interaction parameter between Hþ and PtCl2 6 was evaluated from the data reported in the literature by using Bromley equation. The followings represent solvent extraction reaction supposed by us and the corresponding equilibrium constant estimated in this study.  PtCl2 6 þ R3 NH2 Cl2,org ¼ PtCl6 R3 NH2,org þ 2Cl ;

Kex ¼ 1:9  103 The measured distribution coefficients of Pt(IV) agreed well with those predicted by using the above equilibrium constant from the initial extraction conditions. Acknowledgments This research was supported by Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Knowledge and Economy of Korea (MKE).

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