Chemical Model on the Synergistic Solvent Extraction of Manganese(II

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Chemical Model on the Synergistic Solvent Extraction of Manganese(II) from Chloride Solutions by a Mixture of Cyanex 272 and Cyanex 301 B. Nagaphani Kumar,† Chong Ho Sonu,‡ and Man Seung Lee*,† †

Department of Advanced Materials Science & Engineering, Institute of Rare Metal, Mokpo National University, Chonnam 534-729, Korea ‡ Metallic Resources Technology Laboratory, LS-Nikko Copper Inc., Kyunggi, 463-400, Korea ABSTRACT: Synergistic solvent extraction of manganese(II) using a mixture of Cyanex 272 and Cyanex 301 from chloride solutions has been studied in the view of developing a chemical model. Slope analysis method was used for the determination of extraction stoichiometry. The equilibrium constant for the synergistic solvent extraction reaction was calculated by taking into account of the complex formation reactions between Mn(II) and chloride ions. The activity coefficients of species in the aqueous phase were estimated by applying the Bromley equation. The calculated distribution ratios of Mn(II) agreed well with the experimentally measured values.

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INTRODUCTION Liquid−liquid extraction is the most suitable method for the recovery of manganese.1−8 The solvent extraction equilibrium of manganese(II) using Cyanex 302 (bis(2,4,4trimethylpentyl)monothiophosphinic acid) from sulfate solutions was studied by Devi and Mishra.9 The authors proposed the extraction mechanism and calculated the extraction equilibrium constant based on slope analysis and distribution ratio data. Lee and Filiz calculated the extraction equilibrium constant for the solvent extraction reaction of Mn(II) from hydrochloric acid solutions with Alamine 336. In this estimation of equilibrium constant, the activity coefficients of species in the aqueous phase are calculated by applying the Bromley equation and the complex formation reactions between manganese and chloride ions.10 Sousa Junior et al. reported the equilibrium constant for the solvent extraction of manganese from sulfate solutions with D2EHPA in isoparaffin. The modified Davies equation was applied for the calculation of activity coefficients of all solutes in the aqueous phase.11 In our previous studies, we have reported the synergistic solvent extraction of Mn(II) from chloride solutions using mixture of Cyanex 272 and Cyanex 301 dissolved in kerosene.12 The extracted species in the organic phase was proposed on the basis of slope analysis.12 To the best of our knowledge, there are no reports on the development of a chemical model for the extraction of Mn(II) using mixture of extractants. The objective of the present work is to obtain the equilibrium constant for the synergistic solvent extraction equilibrium of Mn(II) from chloride solutions with a mixture of Cyanex 272 and Cyanex 301 by calculating the activity coefficients of chemical species in © XXXX American Chemical Society

the aqueous phase using the Bromley equation. The obtained equilibrium constant was verified by comparing the distribution ratios of experimentally measured and calculated values.

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EXPERIMENTAL SECTION Reagents and Apparatus. Cyanex 272 (C272, 85 %) and Cyanex 301 (C301, 70 %) were purchased from Cytec, Canada and used without purification. Kerosene was obtained from Dae Jung Chemicals, Korea. Solutions of Mn(II) were prepared by dissolving MnCl2·4H2O(Dae Jung, Korea) in deionized water. An inductively coupled plasma-optical emission spectrometer (ICP-OES, Spectro, Arcos) was used for the determination of concentration of Mn(II) in the aqueous solutions. A Thermo Orion star A211 pH meter was used to measure the pH of aqueous solutions. Procedure. Equal volumes (0.01 dm3) of aqueous and organic phases were shaken for 30 min (sufficient to attain equilibrium) at 298 K with a wrist action shaker. After equilibrium, the two phases were separated, and the equilibrium pH of the aqueous phase was measured. Metal ion concentrations in the aqueous phase before and after extraction were determined using ICP-OES after suitable dilutions. The concentration of metal in the organic phase was obtained by the difference. The distribution ratio, D, was calculated as the ratio of the concentration of metal present in the organic phase to that in the aqueous phase at equilibrium. Received: August 1, 2013 Accepted: September 18, 2013

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RESULTS AND DISCUSSION Solvent Extraction Reaction of Mn(II) with a Mixture of Cyanex 272 and Cyanex 301. The solvent extraction of Mn(II) with single Cyanex 272 and Cyanex 301 was welldocumented.4,6,12−14 According to these literature reports, the solvent extraction reactions of Mn(II) with single Cyanex 272 and Cyanex 301 can be represented by Mn+2(aq) + (HA)2(org) = MnA 2(org) + 2H+(aq)

(1)

Mn+2(aq) + (HB)2(org) = MnB2(org) + 2H+(aq)

(2)

where (HA)2 and (HB)2 represent the dimeric form of Cyanex 272 and Cyanex 301, respectively. In our previous studies, we found that there is a significant synergistic effect in the extraction of Mn(II) with a mixture of Cyanex 272 and Cyanex 301, keeping the mole fraction of Cyanex 272 at 0.6.12 The observed synergism may be explained as follows. First, Mn(II) was extracted by Cyanex 301 forming a complex,and then, Cyanex 272 will form as an adduct, increasing the hydrophobicity of the complex. The synergistic solvent extraction reaction of Mn(II) can be represented as

Figure 1. Effect of concentration of the total mixture of extractants on the distribution ratio of Mn(II) at three initial pH values, 4 (square, slope = 1.02), 3 (circle, slope = 0.99), and 2 (triangle, slope = 1.08). Conditions: [Mn(II)], initial = 0.00183 mol·dm−3, T = 298 K, volume phase ratio = 1, equilibration time = 30 min, C272 = Cyanex 272, C301 = Cyanex 301. The total concentration of extractants mixture is varied from 0.1 mol·dm−3 to 0.7 mol·dm−3 by keeping the mole fraction of Cyanex 272 at 0.6.

Mn+2(aq) + (HA)2(org) + (HB)2(org) = MnH 2A 2B2(org) + 2H+(aq)

(3)

Inserting the definition of the distribution ratio into the equilibrium constant (Kex) of eq 3 and rearrangement of the resulting equation leads to log Dmix = log Kex + 2 pH + log(HA)2 + log(HB)2

(4)

where Dmix is the distribution ratio of Mn(II) with mixture of Cyanex 272 and Cyanex 301. From eq 4, it can be seen that a plot of log D vs {log (HA)2 + log (HB)2} should be a straight line with a slope of unity, keeping all other parameters fixed. Figure 1 demonstrates the effect of concentration of mixture of extractants on the distribution ratios of Mn(II). The total concentration of the mixture was varied from 0.1 mol·dm−3 to 0.7 mol·dm−3 by keeping the mole fraction of Cyanex 272 at 0.6. The initial concentration of Mn(II) was fixed at 0.00183 mol·dm−3, and the initial pH of the solutions was fixed at 2, 3, and 4. The slopes of the plots were found to be unity. Similarly, Figures 2 and 3 represent the effect of concentration of mixture of extractants on the distribution ratios of Mn(II) where the initial concentration of Mn(II) was 0.00872 mol·dm−3 and 0.0183 mol·dm−3. The slope of the plots, log D vs {log[C272] + log[C301]} varies in between 0.53 and 0.81, therefore considered as unity. Therefore, our present experimental results agreed well with the reported solvent extraction reaction. Estimation of the Equilibrium Constant. In the extraction of metal by cationic extractants, the concentration of metal in the aqueous phase and the value of solution pH were measured before and after extraction. To estimate the equilibrium constant for the solvent extraction reaction, the activity of the chemical species which take part in the extraction reaction should be obtained from these extraction data. Since activity is a product of the concentration and activity coefficient of a species, the equilibrium concentration of the chemical species needs to be calculated from the extraction data. The formation of any metal complexes in aqueous solution affects the distribution of the chemical species containing the metal. In

Figure 2. Effect of concentration of the total mixture of extractants on the distribution ratio of Mn(II) at three initial pH values, 4 (square, slope = 0.65), 3 (circle, slope = 0.67) and 2 (triangle, slope = 0.81). Conditions: [Mn(II)], initial = 0.00872 mol·dm−3, T = 298 K, volume phase ratio = 1, equilibration time = 30 min, C272 = Cyanex 272, C301 = Cyanex 301. The total concentration of extractants mixture is varied from 0.1 mol·dm−3 to 0.7 mol·dm−3 by keeping the mole fraction of Cyanex 272 at 0.6.

chloride solution, Mn(II) can react with chloride and hydroxide ions to form complexes. Therefore, it is necessary to investigate the extent of the formation of Mn(II) complexes with hydroxide and chloride ion in our experimental range. According to the Eh−pH diagram of Mn(II) at 298 K, the relation between the concentration of Mn(II) and the precipitation pH15 is given by B

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Mn is 0.0183 mol·dm −3 . According to Table 2, the predominant species of Mn(II) is Mn2+ and only a small amount of MnCl+ exists in our experimental range. Therefore, Mn 2+ and MnCl + were considered in calculating the equilibrium concentration of free Mn ion. The concentration of hydrogen ion can be obtained from the value of equilibrium pH. From the equilibrium data, the equilibrium concentration of species which take part in the solvent extraction reaction was calculated by using mass balance and chemical equilibria as follows

Figure 3. Effect of concentration of total mixture of extractants on the distribution ratio of Mn(II) at three initial pH values, 4 (square, slope = 0.53), 3 (circle, slope = 0.57), and 2 (triangle, slope = 0.69). Conditions: [Mn(II)], initial = 0.0183 mol·dm−3, T = 298 K, volume phase ratio = 1, equilibration time = 30 min, C272 = Cyanex 272, C301 = Cyanex 301. The total concentration of extractants mixture is varied from 0.1 mol·dm−3 to 0.7 mol·dm−3 by keeping the mole fraction of Cyanex 272 at 0.6.

log[Mn ] = 15.31 − 2 pH

reaction

log K

reference

−0.21 −0.35 −0.57

10 16 16

(9)

FM =

−4

3.805·10 3.809·10−4 3.822·10−4 3.861·10−4 3.988·10−4 4.399·10−4

1 + I 0.5

(10)

+ FM = −A(z M)2 + FM

⎡ (0.06 + 0.6B ) ·|z z | MX M X ⎣ (1 + (1.5/|z Mz X|)I )2

(|z M| + |z X|)2 [X] 4

⎤ + BMX ⎥ · ⎦ (12)

In the above equations, z is ionic charge, I ionic strength of solution, and BMX the interaction parameter between cation M and anion X. Since general equations to calculate the activity coefficients of the solutes in the organic are not available at present, the activity coefficients of species present in the organic phase were assumed to be unity. To estimate the equilibrium constant from the extraction data, an evaluation function was defined as follows: Err =

1 ∑ (Dcal − Dexpt)2 N

(13)

where N denotes the number of experimental data and Dcal and Dexpt represent the distribution coefficient of Mn(II) calculated in this study and measured, respectively. The equilibrium constant was obtained by minimizing the Err function. By applying the above method, the equilibrium constant for the solvent extraction of Mn by mixture of Cyanex 272 and Cyanex 301 in which the mole fraction of Cyanex 272 kept at 0.6 was estimated to be 6.1·10−2. Prediction of the Distribution Coefficient of Mn from the Initial Extraction Conditions. The number of chemical species present in both phases after extraction is 7 (Cl−, H+, Mn2+, MnCl+, Cyanex 272, Cyanex 301, MnH2A2B2), excluding the solvent of the aqueous and organic phases. Therefore, seven independent equations are needed to calculate the distribution coefficients of Mn from the initial extraction conditions. These

mole fraction of species MnCl2

0.5108(z M)2 I 0.5

∑⎢ X

Table 2. Distribution of Mn(II) Species Containing Chloride at 25 °C (Concentration of Mn(II) Is 0.0183 mol·dm−3)

0.00935 0.00935 0.00936 0.00941 0.00955 0.00998

[(HB)2 ]initial = [(HB)2 ] + [MnH 2A 2B2]

(11)

distribution diagram of Mn(II) in our experimental range was obtained by using the equilibrium constants for the formation of complexes represented in Table 1. Table 2 shows the distribution of Mn(II) thus obtained when the concentration of

0.9903 0.9903 0.9903 0.9902 0.9901 0.9896

(8)

(5)

Mn2+ + Cl− = MnCl+ MnCl+ + Cl− = MnCl2,aq MnCl2,aq + Cl− = MnCl3−

−5 −4.5 −4 −3.5 −3 −2.5

[(HA)2 ]initial = [(HA)2 ] + [MnH 2A 2B2]

To consider the effect of ionic strength, the activity coefficients of solutes were calculated by using the Bromley equation. The following equation represents the Bromley equation17 for the activity coefficient of the cation, γM, at 298 K.

Table 1. Equilibrium Constants for the Formation of Mn(II) Complexes with a Chloride Ion

MnCl+

(7)

= [Cl−] + [MnCl+]

Equation 5 tells that Mn(II) hydroxide begins to precipitate at pH 8.5 when the concentration of Mn(II) is 0.0183 mol· dm−3. Therefore, the concentration of Mn(II) complexes containing hydroxide ion can be neglected without much error in our experimental range. The equilibrium constant for the formation of Mn(II) complexes with the chloride ion is listed in Table 1. A

Mn

[Mn]organic,total = [MnH 2A 2B2]

log γM = −

2+

log(HCl)

(6)

[Cl]initial = [HCl]initial + 2[MnCl 2]initial

Mn 2 + + H 2O = MnO + 2H+

2+

[Mn]aqueous,total = [Mn 2 +] + [MnCl+]

MnCl3− 6.702·10−6 6.713·10−6 6.747·10−6 6.858·10−6 7.217·10−6 8.424·10−6 C

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Table 3. Measured (Experimental) and Calculated Valuesa of the Distribution Ratio of Mn(II) at an Initial Concentration of 0.00183 mol·dm−3 [C272b]

[C301c]

sample no.

initial pH

equilibrium pH

mol·dm−3

mol·dm−3

log Dexpt

log Dcal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

4 4 4 4 4 4 3 3 3 3 3 3 2 2 2 2 2 2 2

2.46 2.41 2.4 2.41 2.41 2.41 2.44 2.37 2.34 2.34 2.33 2.33 1.98 1.98 1.95 1.9 1.9 1.88 1.87

0.06 0.12 0.18 0.24 0.3 0.36 0.06 0.12 0.18 0.24 0.3 0.36 0.06 0.12 0.18 0.24 0.3 0.36 0.42

0.04 0.08 0.12 0.16 0.2 0.24 0.04 0.08 0.12 0.16 0.2 0.24 0.04 0.08 0.12 0.16 0.2 0.24 0.28

0.245 0.839 1.225 1.501 2.604 2.604 0.096 0.693 1.069 1.364 1.594 1.594 −0.774 −0.111 0.288 0.541 0.755 0.934 1.056

0.519 1.002 1.323 1.562 1.751 1.907 0.35 0.825 1.14 1.375 1.563 1.718 −0.62 −0.091 0.216 0.44 0.618 0.766 0.893

a

Conditions: T = 298 K, volume phase ratio = 1, equilibration time = 30 min, specific gravity of Cyanex 272 = 0.92, specific gravity of Cyanex 301 = 0.95. Dexpt = experimentally measured distribution ratio, Dcal = calculated distribution ratio. bC272 = Cyanex 272. cC301 = Cyanex 301.

Table 4. Measured (Experimental) and Calculated Valuesa of the Distribution Ratio of Mn(II) at an Initial Concentration of 0.00872 mol·dm−3 [C272b] sample no.

initial pH

equilibrium pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

4 4 4 4 4 4 4 3 3 3 3 3 3 3 2 2 2 2 2 2 2

2.23 2.02 1.89 1.82 1.77 1.72 1.69 2.16 1.96 1.83 1.77 1.72 1.68 1.66 1.91 1.8 1.72 1.67 1.61 1.58 1.54

mol·dm 0.06 0.12 0.18 0.24 0.3 0.36 0.42 0.06 0.12 0.18 0.24 0.3 0.36 0.42 0.06 0.12 0.18 0.24 0.3 0.36 0.42

−3

[C301c] mol·dm−3

log Dexpt

log Dcal

0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.04 0.08 0.12 0.16 0.2 0.24 0.28

−0.446 −0.086 0.143 0.325 0.456 0.58 0.703 −0.516 −0.125 0.108 0.269 0.406 0.531 0.66 −0.959 −0.45 −0.155 0.045 0.188 0.311 0.4

−0.271 0.029 0.231 0.39 0.524 0.641 0.746 −0.323 −0.013 0.191 0.35 0.484 0.601 0.705 −0.818 −0.392 −0.153 0.02 0.158 0.274 0.376

a

Conditions: T = 298 K, volume phase ratio = 1, equilibration time = 30 min, specific gravity of Cyanex 272 = 0.92, specific gravity of Cyanex 301 = 0.95. Dexpt = experimentally measured distribution ratio, Dcal = calculated distribution ratio. bC272 = Cyanex 272. cC301 = Cyanex 301.

independent equations were obtained from one chemical equilibria, a solvent extraction reaction, four mass balance equations, and a charge balance equation. When the volume ratio of aqueous to organic is unity, the mass balance equation of Mn is given by [Mn]total = [Mn 2 +] + [MnCl+] + [MnH 2A 2B2]

Equation 15 represents charge balance. [H+] + 2[Mn 2 +] + [MnCl+] = [Cl−]

(15)

Tables 3, 4, and 5 give the experimental conditions together with the extraction results. For each experimental data shown in these tables, the distribution coefficients of Mn predicted from the initial extraction conditions are also shown. Figure 4 shows

(14) D

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Table 5. Measured (Experimental) and Calculated Valuesa of the Distribution Ratio of Mn(II) at an Initial Concentration of 0.0183 mol·dm−3 [C272b]

[C301c]

sample no.

initial pH

equilibrium pH

mol·dm−3

mol·dm−3

log Dexpt

log Dcal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

4 4 4 4 4 4 4 3 3 3 3 3 3 3 2 2 2 2 2 2 2

2.16 1.88 1.78 1.72 1.66 1.61 1.58 2.12 1.91 1.77 1.7 1.61 1.55 1.53 1.92 1.74 1.7 1.59 1.58 1.5 1.48

0.06 0.12 0.18 0.24 0.3 0.36 0.42 0.06 0.12 0.18 0.24 0.3 0.36 0.42 0.06 0.12 0.18 0.24 0.3 0.36 0.42

0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.04 0.08 0.12 0.16 0.2 0.24 0.28

−0.691 −0.401 −0.218 −0.073 0.04 0.147 0.237 −0.748 −0.418 −0.208 −0.077 0.046 0.15 0.247 −1.096 −0.623 −0.411 −0.237 −0.115 −0.007 0.084

−0.57 −0.307 −0.141 −0.014 0.091 0.182 0.263 −0.605 −0.333 −0.163 −0.035 0.071 0.162 0.243 −0.969 −0.59 −0.381 −0.234 −0.117 −0.02 0.065

a Conditions: T = 298 K, volume phase ratio = 1, equilibration time = 30 min, specific gravity of Cyanex 272 = 0.92, specific gravity of Cyanex 301 = 0.95. Dexpt = experimentally measured distribution ratio, Dcal = calculated distribution ratio. bC272 = Cyanex 272. cC301 = Cyanex 301.

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CONCLUSIONS The synergistic solvent extraction equilibria of Mn(II) from chloride solutions using a mixture of Cyanex 272 and Cyanex 301 was studied. The extraction stoichiometry was identified based on the results of experiments on effect of extractant concentration on the distribution of Mn(II). The extraction equilibrium constant was estimated using chemical equilibria and mass balance equations, and it was 6.1·10−2. The Bromley equation was applied for the calculation of activity coefficients of the solutes in the aqueous phase. A standard deviation value of 0.03 indicates the good agreement between experimentally measured distribution ratios and calculated distribution ratios of Mn(II).

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +82 61 450 2492; fax: +82 61 450 2498. E-mail address: [email protected]. Funding

This work was supported by a grant operated by LS-Nikko of Korea. The authors would like to thank them for the financial support.

Figure 4. Comparison of distribution ratios of Mn(II) between measured and calculated. [Mn(II)]: 0.00183 mol·dm−3 (square), 0.00872 mol·dm−3 (circle), and 0.0183 mol·dm−3 (triangle).

Notes

The authors declare no competing financial interest.

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the distribution coefficients of Mn(II) obtained experimentally and the calculated values. It indicates that the distribution coefficients of Mn(II) obtained experimentally agreed very well with those predicted in this study, which reflects in the standard deviation value (0.03) between the measured and the calculated distribution ratios of Mn(II). Considering that few works have been reported on the chemical model on the extraction of metal by a mixture of extractants, our method can be applied to predict the distribution coefficients of metals in a synergistic extraction system.

ACKNOWLEDGMENTS We gratefully thank the Gwangju branch of the Korea Basic Science (KBSI) for ICP data. REFERENCES

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