ARTICLE pubs.acs.org/IECR
Selective Extraction of Palladium and Platinum from Hydrochloric Acid Solutions by Trioctylammonium-Based Mixed Ionic Liquids Shoichi Katsuta,*,† Yuki Yoshimoto,† Miho Okai,† Yasuyuki Takeda,† and Kotaro Bessho‡ † ‡
Department of Chemistry, Graduate School of Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522, Japan Radiation Science Center, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba 305-0801, Japan ABSTRACT: Mixtures of the protic ionic liquids trioctylammonium bis(trifluoromethanesulfonyl)amide ([TOAH][NTf2]) and trioctylammonium nitrate ([TOAH][NO3]) were investigated as extractants for platinum-group elements. The mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2] has low viscosity (274.2 mPa s) and low aqueous solubility (2.6 105 mol dm3 as the cation). With the ionic liquid mixture, Pd(II) and Pt(IV) were extracted almost quantitatively from 0.10 mol dm3 hydrochloric acid. Under the same conditions, Na(I), Mg(II), K(I), Ca(II), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Ru(III), Rh(III), and Cd(II) were only slightly extracted. It was found that the extraction of Pd and Pt increases with increasing content of [TOAH][NO3] in the mixture. The metals could be back-extracted from the ionic liquid mixture with nitric acid solution, and selective stripping of Pt was possible by controlling the concentration of nitric acid. The metal extractability of the ionic liquid mixture after the back-extraction was equivalent to that of the fresh mixture, indicating that the species composition of the mixture returned to the original state through the back-extraction process. This extraction system provides an efficient separation method for Pd and Pt in acidic chloride media using ionic liquid mixtures, which are recyclable, easy to handle, safe, and environmentally friendly.
’ INTRODUCTION Platinum-group elements such as Pd and Pt are important metals that are used in jewelry, electrical contacts, dentistry, and antipollution devices in automobiles, among other applications. In wet refining or recycling processes, the platinum-group elements in ores are generally leached with hydrochloric acid containing chlorine or with aqua regia, in which the metals dissolve as anionic chloro complexes such as PdCl42 and PtCl62. Therefore, great efforts have been made to find efficient, cost-effective, and environmentally friendly methods for the separation of these metals from acidic chloride media. At present, solvent extraction is the most popular method used for this purpose.1 For platinum-group elements, some organic ligands such as dialkyl sulfides, β-hydroxyoximes, and tributyl phosphate are widely used as extracting agents.1,2 These compounds are generally used by dilution in proper organic solvents. Most of the diluents, as well as the extracting agents, are molecular compounds, and therefore, they are volatile, flammable, and harmful for the environment and human health. Thus, a key issue in the field of solvent extraction is to develop safer and more environmentally friendly diluents or extracting agents. Ionic liquids, which are now defined as salts having a melting point below 100 °C, have recently attracted considerable attention as potential alternatives to conventional organic solvents in various fields of chemistry. This is because they are recognized as green solvents owing to their nonvolatility, which assures nonflammability and a low impact on the environment and human health.3 Hydrophobic ionic liquids, which are almost immiscible with water, involve a hydrophobic organic cation such as alkylimidazolium, alkylpyridinium, or alkylammonium and an anions such as hexafluorophosphate or bis(trifluoromethanesulfonyl)amide. These r 2011 American Chemical Society
ionic liquids are, however, generally low in the ability to extract metal ions; therefore, they have been studied exclusively as diluents to dissolve conventional extracting agents.4,5 The salts known as liquid ion exchangers, such as alkylammonium chlorides and alkylphosphonium chlorides, can also be used for the extraction of Pd and Pt.68 Although these salts are ionic liquids, they are used diluted in proper solvents because of their high viscosities (e.g., 2088 mPa s for a common liquid ion exchanger, methyltrioctylammonium chloride).9 To overcome the safety and environmental problems of the solvent extraction method, it is preferable not to use any organic molecular compounds as diluents and extracting agents. Several functionalized ionic liquids for metal extraction have been synthesized as “task-specific ionic liquids”.911 However, in general, the high cost of ionic liquids limits their practical application. The purpose of this study was to find an ionic liquid for the selective extraction of Pd and Pt from hydrochloric acid solutions. It is desired that the ionic liquid be highly hydrophobic, sufficiently low in viscosity, and cost-effective. We propose the use of a mixture of different ionic liquids, namely, a hydrophobic low-viscosity ionic liquid as a diluent and a liquid ion exchanger as an extracting agent. The mixture should satisfy the needs of nonflammability, environment friendliness, high hydrophobicity, and low viscosity, as well as high extractability. The use of an ionic liquid mixture also enables control of the extractability by varying the content of the liquid ion exchanger. Recently, Zhao and coworkers12,13 reported a novel microextraction method, termed Received: June 20, 2011 Accepted: October 2, 2011 Revised: September 21, 2011 Published: October 05, 2011 12735
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Industrial & Engineering Chemistry Research ionic liquid/ionic liquid dispersive liquidliquid microextraction, for rapid enrichment of organic compounds, where mixtures of a hydrophobic ionic liquid (extraction solvent) and a hydrophilic ionic liquid (disperser solvent) were used. The targets of extraction and the purpose of using mixed ionic liquids in their study are different from those in our study. In this study, mixtures of protic ionic liquids, trioctylammonium bis(trifluoromethanesulfonyl)amide ([TOAH][NTf2]) and trioctylammonium nitrate ([TOAH][NO3]), were mainly used. The trioctylammonium salts can be readily prepared by neutralization of trioctylamine (TOA) which is available at a reasonable cost. For comparison with [TOAH][NTf2], an ionic liquid having a similar structure, methyltrioctylammonium bis(trifluoromethanesulfonyl)amide ([MTOA][NTf2]), and a common ionic liquid, 1-butyl-3methylimidazolium bis(trifluoromethanesulfonyl)amide ([BMIm][NTf2]), were also used. Moreover, we examined the possibility of stripping the metals from the ionic liquid mixture and reusing the ionic liquid mixture as an extractant.
’ EXPERIMENTAL DETAILS Reagents. TOA of 98% purity and methyltrioctylammonium chloride ([MTOA]Cl) of 97.0% purity were purchased from Sigma-Aldrich. 1,1,1-Trifluoro-N-(trifluoromethylsulfonyl)methanesulfonamide (HNTf2) of 99.0% purity, lithium bis(trifluoromethanesulfonyl)amide (Li[NTf2]) of 99.7% purity, and mineral acids (HCl and HNO3) of ultrapure grade or equivalent were purchased from Kanto Chemical. 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([BMIm][NTf2]) was the same as used previously.14 The above reagents were all used as received. Dichloromethane (Kanto Chemical, guaranteed reagent grade) was purified by distillation. Standard aqueous solutions of Na(I) and K(I) were prepared from NaCl and KCl (Merck, suprapure grade), which were dried at 250 °C under a vacuum. Standard aqueous solutions of Mg(II), Ca(II), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Ru(III), Rh(III), Pd(II), Cd(II), and Pt(IV) were purchased as 1000 ppm atomic absorption standards from Kanto Chemical or Wako Pure Chemicals and were used after appropriate dilution. Water was distilled and further deionized with a Milli-Q Lab system (Millipore). Preparation of Ionic Liquids. To prepare [TOAH][NTf2], 37.7 g (0.134 mol) of HNTf2 was dissolved slowly in 39.6 g (0.112 mol) of TOA; the mixture was washed about 10 times with deionized water until the pH of the aqueous phase became constant (ca. 3.8) and then dried under a vacuum at 5060 °C for 24 h. The product was obtained in quantitative yield as a pale yellow liquid. 1H NMR [400 MHz, CDCl3, tetramethylsilane (TMS)]: δ = 7.397 (br s, 1H, NH), 3.0293.072 (m, 6H, CH2), 1.6321.690 (m, 6H, CH2), 1.2811.334 (m, 30H, CH2), 0.8710.905 (t, 9H, CH3). MS (ESI, acetonitrile, m/z): positive, 354.4079, [TOA]+; negative, 279.9179, [NTf2]. The water content determined by Karl Fischer titrations was 0.2 wt %. [TOAH][NO3] was prepared by shaking a dichloromethane solution (200 cm3) of 0.10 mol dm3 TOA with an aqueous solution (200 cm3) of 1.0 mol dm3 nitric acid. After centrifugation, the dichloromethane phase was transferred to a boiling flask, rotary evaporated to dryness, and further dried on P2O5 under a vacuum for 24 h to afford the product as a pale yellow solid (7.2 g, 91% yield). 1H NMR (400 MHz, CDCl3, TMS): δ = 10.976 (br s, 1H, NH), 2.9903.044 (m, 6H, CH2), 1.6881.718 (m, 6H, CH2), 1.2651.318 (m, 30H, CH2), 0.8650.900 (t, 9H, CH3). The water content was 0.1 wt %.
ARTICLE
[TOAH]Cl was also obtained as a white solid (7.7 g, 91% yield) in a similar manner from a dichloromethane solution of TOA and an aqueous solution of HCl. 1H NMR (400 MHz, CDCl3, TMS): δ = 12.030 (br s, 1H, NH), 2.9232.978 (m, 6H, CH2), 1.7701.811 (m, 6H, CH2), 1.2641.333 (m, 30H, CH2), 0.8650.900 (t, 9H, CH3). The water content was 0.3 wt %. [MTOA][NTf2] was prepared by mixing 10 g (0.025 mol) of [MTOA]Cl and an aqueous solution (20 cm3) of 1.5 mol dm3 Li[NTf2]; the ionic liquid phase separated from the aqueous phase and was washed 10 times with deionized water and dried under a vacuum at 5060 °C for 24 h. The product was obtained in quantitative yield as a pale yellow liquid. 1H NMR (400 MHz, CDCl3, TMS): δ = 3.1633.206 (m, 6H, CH2), 3.015 (s, 3H, CH3), 1.6111.669 (m, 6H, CH2), 1.2781.354 (m, 30H, CH2), 0.8680.902 (t, 9H, CH3). MS (ESI, acetonitrile, m/z): positive, 368.4238, [MTOA]+; negative, 279.9182, [NTf2]. The contents of Li+ and Cl were checked by atomic absorption spectrophotometry and ion-selective electrode potentiometry, respectively (Li < 3 105 wt %, Cl < 1 104 wt %). The water content was 0.02 wt %. Mixtures of two ionic liquids were prepared by weight. Watersaturated ionic liquid mixtures were obtained by washing the mixtures of dry ionic liquids with the least amount of deionized water at 25 ( 0.2 °C. The water-saturated mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2] was also prepared by another method, namely, by washing the mixture of TOA (0.809 g) and [TOAH][NTf2] (8.41 g) with an aqueous nitric acid solution (1.0 mol dm3, 15 cm3). It was confirmed that the ionic liquid mixtures prepared by the two different methods were identical in terms of density and the extractability for Pd and Pt. Measurements of Fundamental Properties of Ionic Liquids. Melting points were measured by the rising melting point method using a temperature-controlled water bath (Thermo NESLAB, RTE-7) and a primary standard thermometer (Nihon Keiryoki Kogyo, No. 1, uncertainty = (0.03 K). Densities were measured at 25 ( 0.2 °C with an oscillating U-tube density meter (Anton Paar, DMA35n) calibrated with pure water. Kinematic viscosities were measured using an Ubbelohde-type viscometer (Kusano, No. 2, viscometer constant = 0.4718 mm2 s2) in a thermostatted water bath (25 ( 0.05 °C). Dynamic viscosity was calculated as the product of the kinematic viscosity and the density. Mutual solubility measurements for an ionic liquid and water were conducted for the two-phase mixture equilibrated at 25 ( 0.05 °C. The concentration of water in the ionic liquid was measured with a Karl Fischer coulometric titrator (Hiranuma, AQ-7). The concentration of the ionic liquid in water was determined as the cation concentration measured by the extraction and spectrophotometric method using bis[2-(5-bromo-2-pyridylazo)-5-(N-propyl-Nsulfopropylamino)phenolato]cobaltate(III).15 Forward Extraction of Metal Ions. Aqueous solutions containing HCl (0.104.0 mol dm3) and metal ions were prepared and stored for more than 24 h at room temperature; the metal concentrations were as follows: Pd, 9.4 105 mol dm3; Rh, 9.9 105 mol dm3; Pt, 5.1 104 mol dm3; Cd, 5.4 104 mol dm3; Na, K, and Ca, 6.5 104 mol dm3; Mg, 6.6 104 mol dm3; Ru, 7.8 104 mol dm3; Zn, 9.3 104 mol dm3; Cu, 9.5 104 mol dm3; Co and Ni, 1.0 103 mol dm3; Mn and Fe, 1.1 103 mol dm3. The aqueous solution of a metal ion was placed in a stoppered test tube, together with a single ionic liquid or a mixture of different ionic liquids. Here, the volume of the viscous ionic liquid phase was accurately evaluated from the mass by using the density listed in Table 1, and the 12736
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Table 1. Fundamental Properties of Ionic Liquids ionic liquid [TOAH][NTf2]
mp (°C)
Fa (g cm3)
3.7 ( 0.2
1.1043 ( 0.0003
200.4 ( 0.2
1.1018 ( 0.0002
179.4 ( 0.2
1.0834 ( 0.0002
274.2 ( 0.7
1.0814 ( 0.0003c
235.7 ( 0.1c
81.42
1.1113 ( 0.0001
608 ( 1
3e
1.1100 ( 0.0001 1.438e
485.4 ( 0.4 50.1e
c
mixture of 10 wt % [TOAH][NO3]
3.4 ( 0.9
solubility of watera
solubility in watera,b
(wt %)
(mol dm3)
ηa (mPa s)
1.02 ( 0.03
(6.3 ( 0.5) 106
0.60 ( 0.03
(2.6 ( 0.1) 105
0.287 ( 0.009
(2.56 ( 0.07) 105
1.4f
(1.643 ( 0.008) 102
c
in [TOAH][NTf2] d
[MTOA][NTf2]
c
[BMIm][NTf2]
c
c,f
1.3919 a
At 25 °C. b Concentration of the cation. c For the water-saturated state. d Glass transition temperature.16 e Reference 17. f Reference 18.
volume ratio of the ionic liquid phase to the aqueous phase was adjusted to 1:2. The tube was mechanically shaken for 1 h at 250 strokes per min in a thermostatted chamber (25 ( 0.2 °C). In the preliminary experiments, no shaking-time dependence of the extraction was observed for Pd(II) and Pt(IV) in the range from 15 min to 4 h. After the phases had been separated by centrifugation at 3000 rpm, the metals in the aqueous phase was determined with a Hitachi Z-5000 polarized Zeeman atomic absorption spectrophotometer in the flame or graphite furnace mode. The extraction percentage (E) and the distribution ratio (D) were calculated from the equations E ð%Þ ¼ 100
ðC°aq Caq Þ C°aq
° Vaq ðCaq Caq Þ D¼ VIL Caq
Table 2. Distribution Ratios of Pd(II) and Pt(IV) in Extractions with Different Ionic Liquids from 0.10 mol dm3 HCl Solutions at 25 °C D ionic liquid
VB, aq CB, aq VB, IL C°B, IL
ð2Þ
ð3Þ
where C°B,IL and CB,aq denote the metal concentrations in the ionic liquid phase before back-extraction and in the aqueous phase after back-extraction, respectively, and VB,aq and VB,IL are the volumes of the aqueous phase and the ionic liquid phase in the back-extraction, respectively.
’ RESULTS AND DISCUSSION Fundamental Properties of Ionic Liquids. Melting points, densities, dynamic viscosities, and mutual solubilities with water
16.4
6.32
[MTOA][NTf2]
2.53
3.26
[BMIm][NTf2]
0.05
2.10
Mixture of Ionic Liquids
C°aq
Eback ð%Þ ¼ 100
Pt(IV)
Pure Ionic Liquid [TOAH][NTf2]
ð1Þ
where and Caq denote the metal concentrations in the aqueous phase before and after extraction, respectively, and Vaq and VIL are the volumes of the aqueous phase and the ionic liquid phase, respectively. Back-Extraction of Metal Ions. An aliquot of the ionic liquid phase after the forward extraction was transferred into another stoppered test tube, and an aqueous nitric acid solution (0.108.0 mol dm3) was added into the tube. Here, the volume of the nitric acid solution was adjusted to twice that of the ionic liquid phase. The tube was mechanically shaken for 1 h at 25 ( 0.2 °C. After phase separation by centrifugation, the aqueous phase was subjected to atomic absorption spectrophotometry. The back-extraction percentage (Eback) was calculated as
Pd(II)
5 wt % [TOAH][NO3] in [TOAH][NTf2]
552
10 wt % [TOAH][NO3] in [TOAH][NTf2]
803
15 wt % [TOAH][NO3] in [TOAH][NTf2] 20 wt % [TOAH][NO3] in [TOAH][NTf2]
1.27 103 1.82 103
10 wt % [TOAH]Cl in [TOAH][NTf2]
3.52 103
10 wt % [TOAH][NO3] in [BMIm][NTf2]
2.02
22.7 24.7 36.4 51.0 187 13.8
were measured for [TOAH][NTf2], [MTOA][NTf2], [BMIm][NTf2], and a mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2]. The results are summarized in Table 1, where some values are cited from the literature.1618 At 25 °C, the [NTf2]based salts are liquids, whereas [TOAH][NO3] is a solid (mp = 30.7 ( 0.1 °C). [TOA][NTf2] and [MTOA][NTf2] are very similar in density. The viscosity of [TOAH][NTf2] is considerably lower than that of [MTOA][NTf2] and even lower than those of general liquid ion exchangers and task-specific ionic liquids.9,10 The solubility in water of [TOAH][NTf2] is very low and lower than those of [MTOA][NTf2] and [BMIm][NTf2]. The solubilities of water are not very different in the three [NTf2]-based ionic liquids. The properties of the mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2] are similar to those of pure [TOAH][NTf2]. Therefore, the ionic liquid mixture is low in viscosity and highly hydrophobic, meeting the requirements for the use in solvent extraction. We also examined the influence of water saturation on the density and viscosity of the ionic liquids, and the results are reported in Table 1. It was found that each of the ionic liquids had almost the same density when water-saturated as when dry, whereas the viscosity decreased with increasing water saturation. Extractabilities of Different Ionic Liquids for Palladium and Platinum. The extraction of Pd(II) and Pt(IV) from 0.10 mol dm3 12737
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Table 3. Distribution Ratios and Extraction Percentages of Metals with a Mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2] from HCl Solutions of Different Concentrations at 25 °Ca Na(I) 3
Mg(II)
Ca(II)
Mn(II)
concentration of HCl (mol dm )
D
E (%)
D
E (%)
D
E (%)
D
E (%)
D
E (%)
0.10
0.01
0.6
0.00
0.0
0.03
1.4
0.00
0.0
0.00
0.0
1.0
0.02
0.8
0.00
0.0
0.04
2.2
0.00
0.0
0.02
0.9
2.0 4.0
0.01 0.02
0.7 0.8
0.00 0.00
0.0 0.1
0.04 0.04
2.0 1.9
0.00 0.01
0.0 0.5
0.00 0.00
0.0 0.0
Fe(III) 3
Co(II)
Ni(II)
Cu(II)
Zn(II)
concentration of HCl (mol dm )
D
E (%)
D
E (%)
D
E (%)
D
E (%)
0.10
0.00
0.0
0.04
1.8
0.00
0.0
0.23
10.5
0.00
0.0
1.0
0.00
0.0
0.05
2.4
0.00
0.0
0.18
8.1
1.05
34.5
2.0
0.07
3.5
0.04
1.9
0.00
0.0
0.18
8.3
4.39
68.7
4.0
4.99
71.4
0.03
1.7
0.00
0.0
0.21
9.3
5.45
73.2
Ru(III)
a
K(I)
Rh(III)
Pd(II)
D
Cd(II)
E (%)
Pt(IV)
concentration of HCl (mol dm3)
D
E (%)
D
E (%)
D
E (%)
D
E (%)
D
E (%)
0.10 1.0
0.10 0.05
4.9 2.4
0.10 0.08
4.6 4.0
803 59.3
99.8 96.7
0.01 2.36
0.5 54.2
24.7 25.4
92.5 92.7
2.0
0.11
5.4
0.10
4.6
24.9
92.6
4.19
67.7
13.0
86.7
4.0
0.08
3.8
0.03
1.3
81.9
4.35
68.5
9.06
7.54
79.0
Volume ratio of ionic liquid phase/aqueous phase = 1:2.
hydrochloric acid was investigated with various ionic liquid phases. The data for D are summarized in Table 2. It was confirmed that the D value was almost independent of the initial aqueous concentration of the metals in the ranges of (1.99.4) 105 mol dm3 for Pd and (1.05.1) 104 mol dm3 for Pt when using the mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2]. When a single ionic liquid was used for the extraction, Pd was more extracted than Pt with [TOAH][NTf2], whereas the opposite was true with [MTOA][NTf2] or [BMIm][NTf2]. The extractability toward Pd varied in the order [TOAH][NTf2] > [MTOA][NTf2] > [BMIm][NTf2]. The extractability toward Pt varied in the same order but not significantly. The extraction percentages of Pd and Pt with [TOAH][NTf2] were 89.1% and 76.0%, respectively, when the volume ratio of the ionic liquid phase to the aqueous liquid phase is 1:2. Quantitative extraction of these metals is impossible by the single use of these ionic liquids. Upon addition of a small amount of [TOAH][NO3] to [TOAH][NTf2], the extractability was greatly enhanced. For example, the distribution ratios of Pd and Pt were enhanced by factors of 49 and 3.9, respectively, in the presence of 10 wt % [TOAH][NO3]. The extractability increased with increasing content of [TOAH][NO3] in the mixture. Consequently, both of the metals could be quantitatively extracted into the ionic liquid phase with the aid of [TOAH][NO3]. The enhancement of extractability can be attributed to the function of [TOAH][NO3] as an liquid ion exchanger. Extractability of [BMIm][NTf2] was also enhanced by the addition of [TOAH][NO3]; however, the extractability of the mixture of 10 wt % [TOAH][NO3] in [BMIm][NTf2] was lower than that of the same ratio mixture of [TOAH][NO3] in [TOAH][NTf2].
Addition of [TOAH]Cl is more effective than that of [TOAH][NO3]; the distribution ratios of Pd and Pt were 4.4 and 7.6 times larger, respectively, for the mixture of 10 wt % [TOAH]Cl in [TOAH][NTf2] than for the mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2]. This indicates that [TOAH]Cl has a stronger anion-exchange ability than [TOAH][NO3]. However, we used [TOAH][NO3] in the subsequent experiments, considering regeneration of the ionic liquid phase in the back-extraction process with aqueous nitric acid solutions as described later. Extraction of Various Metals from Hydrochloric Acid Solutions. The extraction of various metals with a mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2] from aqueous HCl solutions was investigated as a function of the HCl concentration. The data on D and E are summarized in Table 3. Over the HCl concentration range from 0.10 to 4.0 mol dm3, Na(I), Mg(II), Ca(II), Mn(II), and Ni(II) were not extracted (E < 1%). The extractabilities of K(I) (12%), Co(II) (∼2%), Cu(II) (810%), Ru(III) (25%), and Rh(III) (15%) were also low and nearly independent of the HCl concentration. Fe(III), Zn(II), and Cd(II) were extracted up to 70% from 4.0 mol dm3 HCl solution, but were not extracted from 0.10 mol dm3 HCl solution. The extractabilities of Pd(II) and Pt(IV) were high (80% or more) over the whole HCl concentration range, and those from 0.10 mol dm3 HCl were highest and nearly quantitative. Therefore, selective extraction of Pd and Pt is possible from dilute HCl solution. Back-Extraction with Nitric Acid Solutions. For the above ionic liquid phase after extraction of Pd(II) and Pt(IV) from 0.10 mol dm3 HCl solution, back-extraction of the metals with aqueous HNO3 solutions was examined. In Figure 1, the backextraction percentages, Eback, of the metals are shown as a function of the HNO3 concentration. The Eback value for each 12738
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Industrial & Engineering Chemistry Research
Figure 1. Back-extraction percentages of Pd (open circles) and Pt (solid circles) from the ionic liquid phase (mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2]) with aqueous HNO3 solutions. Volume ratio of ionic liquid phase/aqueous phase = 1:2.
metal increased with increasing concentration of HNO3 in the range from 0.10 to 8.0 mol dm3. It was found that 47% of the Pd and 86% of the Pt were back-extracted from the ionic liquid phase with 8.0 mol dm3 HNO3. When the back-extraction with 8.0 mol dm3 nitric acid was repeated twice, the total recoveries from the ionic liquid phase reached 57% for Pd and 91% for Pt. Because Pt is more effectively back-extracted than Pd, it is also possible to selectively strip Pt from the ionic liquid phase. For example, 60% of the Pt is stripped by a single backextraction with 1.0 mol dm3 HNO3, by which only 3.1% of Pd is stripped. Reusability of Ionic Liquid Phase for Metal Extraction. Pd(II) and Pt(IV) in HCl solutions are expected to be extracted as anionic chloro complexes such as PdCl42 and PtCl62 by anion exchange with hydrophilic nitrate ions in mixtures of [TOAH][NTf2] and [TOAH][NO3]. In the forward extraction process, the amount of nitrate ions in the ionic liquid phase would decrease, and in their place, the amounts of chloro complexes and chloride ions would increase. However, in the back-extraction process with nitric acid solution, nitrate ions can be supplied to the ionic liquid phase by exchange with the chloro complexes and chloride ions. If this idea is valid, the ionic liquid phase after the backextraction should be able to be reused for the extraction of these metals from HCl solutions. To verify this idea, we conducted the following experiments: A mixture of 10 wt % [TOAH][NO3] in [TOAH][NTf2] was shaken with a 0.10 mol dm3 HCl solution in a 1:2 volume ratio for 1 h at 25 °C. Then, the ionic liquid phase was shaken twice with 1.0 mol dm3 HNO3 solution in a 1:2 volume ratio for 1 h at 25 °C. This sequence of extraction operations was then repeated. The ionic liquid phases after the first and second sequences were used for the extraction of Pd(II) and Pt(IV) from 0.10 mol dm3 HCl solution, and the extractabilities were measured. The extraction percentages of Pd and Pt were 99.7% and 93.6%, respectively, for the ionic liquid phase after the first sequence and 99.7% and 94.0%, respectively, for that after the second sequence. These extraction percentages are nearly equal to those for the fresh ionic liquid mixture, namely, 99.8% (Pd) and 92.5% (Pt). Therefore, it was confirmed that the ionic liquid mixture is regenerated in the back-extraction process with HNO3 solution and can be reused for metal extraction.
ARTICLE
’ CONCLUSIONS Mixtures composed of a hydrophobic and low-viscosity ionic liquid and a liquid ion exchanger have some merits as metal extractants: (1) They are more hydrophobic and lower in viscosity than pure liquid ion exchangers. (2) They have higher extractabilities than general hydrophobic ionic liquids, and their extractabilities can be controlled through composition. (3) They are nonflammable and environmentally friendly because they contain no volatile compounds. The protic ionic liquids used in this work, namely, [TOAH][NTf2] and [TOAH][NO3], are easy to prepare and low in cost. The mixture of [TOAH][NTf2] and [TOAH][NO3] exhibits high extractability and selectivity for Pd(II) and Pt(IV) in acidic chloride media. Most of the metals extracted in the ionic liquid phase can be stripped through back-extraction with aqueous nitric acid solution, although the back-extraction efficiency still needs to be increased further. A rough separation between Pd and Pt is also possible in the backextraction process by controlling the nitric acid concentration. The ionic liquid mixture after the back-extraction can be used again for metal extraction. This extraction system has the possibility to be applied to industrial separation or recovery processes of platinum-group elements. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +81-43-2902781. Fax: +81-43-2902874. E-mail: katsuta@ faculty.chiba-u.jp.
’ ACKNOWLEDGMENT The authors thank Professor Emeritus Dr. Koichi Oguma, Chiba University, for his support concerning atomic absorption spectroscopy. This work was financially supported by a Grant-inAid for Scientific Research (No. 22550070) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and a Grant for Environmental Research Projects (No. 093155) from the Sumitomo Foundation. ’ REFERENCES (1) Bernardis, F. L.; Grant, R. A.; Sherrington, D. C. A Review of Methods of Separation of the Platinum-Group Metals through Their Chloro-Complexes. React. Funct. Polym. 2005, 65, 205–217. (2) Cox, M. Solvent Extraction in Hydrometallurgy. In Solvent Extraction Principles and Practice, 2nd ed.; Rydberg, J., Cox, M., Musikas, C., Choppin, G. R., Eds.; CRC/Taylor & Francis: Boca Raton, FL, 2004; pp 454503. (3) Rogers, R. D., Seddon, K. R., Eds. Ionic Liquids as Green Solvents. Progress and Prospects; ACS Symposium Series 856; American Chemical Society: Washington, DC, 2003. (4) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Willauer, H. D.; Huddleston, J. G.; Rogers, R. D. Room Temperature Ionic Liquids as Replacements for Traditional Organic Solvents and Their Applications towards “Green Chemistry” in Separation Processes. In Green Industrial Applications of Ionic Liquids; Rogers, R. D., Seddon, K. R., Volkov, S., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2003; pp 137156. (5) Diets, M. L. Ionic Liquids as Extraction Solvents: Where do We Stand ? Sep. Sci. Technol. 2006, 41, 2047–2063. (6) Giridhar, P.; Venkatesan, K. A.; Srinivasan, T. G.; Vasudeva; Rao, P. R. Extraction of Fission Palladium by Aliquat 336 and Electrochemical Studies on Direct Recovery from Ionic Liquid Phase. Hydrometallurgy 2006, 81, 30–39. 12739
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