One-Step Separation of Platinum, Palladium, and Rhodium: A Three

ARTICLE pubs.acs.org/IECR

One-Step Separation of Platinum, Palladium, and Rhodium: A Three-Liquid-Phase Extraction Approach Pinhua Yu,†,‡,§ Kun Huang,*,† Chao Zhang,†,‡,§ Keng Xie,†,‡,§ Xiuqiong He,†,‡,§ and Huizhou Liu*,† †

State Key Laboratory of Biochemical Engineering, Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P.R. China ‡ National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing 100190, China § Graduate University of the Chinese Academy of Sciences, Beijing 100049, P.R. China

bS Supporting Information ABSTRACT: Three-liquid-phase extraction and one-step separation of platinum, palladium, and rhodium in the system composed of diisoamyl sulphide (S201), polyethylene oxide-polypropylene oxide random block copolymer (EOPO), Na2SO4, and H2O were investigated. Experimental results indicated that phase-forming salt type, salt concentrations, coexisting H+ and Cl
concentrations in equilibrious Na2SO4 aqueous solutions have significant influences on the three-liquid-phase partitioning behaviors of Pt(IV), Pd(II), and Rh(III). Under the optimized operation parameters, over 99 wt % of Pd(II), about 90 wt % of Pt(IV), and 85 wt % of Rh(III) in initial feed solutions were respectively concentrated into S201 top phase, EOPO middle phase, and Na2SO4 bottom aqueous phase. The present work explores a possibility to develop a three-liquid-phase extraction approach for one-step separation of platinum metal ions in highly concentrated acidic chloride media obtained by hydrometallurgical processes.

’ INTRODUCTION Recently there is a growing demand for platinum group metals (PGMs), especially for platinum (Pt), palladium (Pd), and rhodium (Rh), due to their widespread use as catalysts in automobile exhaust emission control.1
3 Extensive efforts have been made for the development of new methods to recover and separate Pt, Pd, and Rh from various waste catalysts.4
7 Solvent extraction (SX) is one of the most effective techniques for the mutual separation between platinum metals since the classical precipitation and electrochemical methods appear to be rather complex and the percent recoveries are relatively low.8
10 However, the similar chemical and physical properties of platinum metals create difficulties in conventional SX processes. Therefore, solvent extraction and separation of PGMs, especially those in the same periodic column such as Pt and Pd, suffer from multisteps of pretreatment procedures or repeating adjustment of acidity in feed solutions for selective extraction of different metals. The difficulty in traditional solvent extraction increases with the presence of various complicated chloro-complexes of PGMs and differences in mass transfer kinetics for individual extractable species. Three-liquid-phase extraction (TLPE) is a promising technique for the treatment of a multicomponent complicated system due to the unique separation selectivity of three coexisting liquid phases with different physicochemical properties.11
13 The increase in phase numbers provides extended separation capacity over traditional liquid
liquid two-phase systems. It is possible for three-liquid-phase system (TLPS) to extract and synchronously concentrate different target components respectively into three different liquid phases by one-step operation. Mojski et al. proposed a TLPS composed of acetonitrile solution of pyridine or amine derivatives, hexane solution of carboxylic acid and aqueous solution of NaCl, KCl, or (NH4)2SO4, for one-step extraction and separation of Cr
Cu
Fe, V
Cu
Fe, or r 2011 American Chemical Society

V
Ni
Co ternary metal mixtures.14 However, the phase behaviors of that TLPS and the interphase mass transfer of target metals are hard to control. The TLPS consisting of an organic phase and a polymer-based aqueous biphasic system (ABS) is a new separation medium developing in our group.15 That system has exhibited potential applications in multiphase separation of various organic components such as glycyrrhizic acid and liquiritin,16 penicillin V and phenoxyacetic acid,17 phenol and p-nitrophenol,18 p-nitrophenol and o-nitrophenol.19 Recently, the TLPS of trialkylphosphine oxide(TRPO)-polyethylene glycol (PEG)-(NH4)2SO4 was applied to extraction and one-step separation of Ti(IV), Fe(III), and Mg(II) with the addition of water-soluble ethylene diamine tetraacetic acid (EDTA) into aqueous feed-in solutions.20 Owing to the unique advantages of easy to control phase-forming behaviors, the interphase mass transfer and simultaneous partition of different metals in three different liquid phases were achieved. The results indicated that Mg(II) was concentrated into a (NH4)2SO4-rich aqueous bottom phase, whereas Ti(IV) and Fe(III) were respectively transferred into trialkylphosphine oxide (TRPO)-rich organic top phase and PEG-rich aqueous middle phase. However, the extraction of platinum metal chlorocomplex anions is quite different from that of metal cations. The extractable species such as PtCl62
, PdCl42
, and RhCl63
exist only in concentrated hydrochloric acid solutions (>1 mol/L HCl). The three-liquid-phase systems reported in literature are not suitable for one-step separation of platinum metals.

Received: April 23, 2011 Accepted: June 26, 2011 Revised: June 25, 2011 Published: June 26, 2011 9368

dx.doi.org/10.1021/ie200883u | Ind. Eng. Chem. Res. 2011, 50, 9368–9376

Industrial & Engineering Chemistry Research The aim of this work is to develop the suitable TLPS for onestep separation of Pt(IV), Pd(II), and Rh(III) from highly concentrated acidic chloride media. Various operation parameters including phase-forming salt type, salt concentration, and H+ and Cl
concentrations were optimized. The possible extraction mechanism of Pt(IV), Pd(II), and Rh(III) by the suggested TLPS was also discussed.

’ EXPERIMENTAL SECTION Chemicals. Diisoamyl sulfide (S201, >98.5%), a palladium selective extractant,21 was provided by Beijing Ruile Kang Separation Technology Corporation, Ltd. Poly(ethylene oxide) (PEO) and polypropylene oxide (PPO) random block copolymer (EOPO, >98%, ca. 2500 g mol
1) was purchased from Sigma-Aldrich Corp. Ltd. Stock EOPO aqueous solution was prepared by dissolving weighed amount of EOPO in deionized water (typically EOPO concentration is 50 wt %). Analytical grade of PdCl2 (99%) and H2PtCl6 3 6H2O (99%) were provided by China Pharmaceutical Corp. RhCl3 3 3H2O (99.5%) was purchased from TianJin Jinbolan Fine Chemical Corp. Ltd. The metal stock solutions of Pt(IV), Pd(II), or Rh(III), respectively, containing 100 mg/L were prepared by dissolving their corresponding salts into 1 mol/L hydrochloric acid. Anhydrous sodium sulfate (Na2SO4, 99.9%) and other chemicals were purchased from Beijing Chemical Factory. Three-Liquid-Phase Extraction of PGMs. In TLPE experiments on the influence of Na2SO4 concentrations, an EOPO
based aqueous biphasic system (ABS) was first prepared according to the following procedures: (1) A 3 mL portion of each Pt(IV), Pd(II), and Rh(III) stock solutions, respectively, were thoroughly mixed with a certain amount of solid Na2SO4 and 4.50 g of 50 wt % EOPO stock solution for 5 min. (2) The mixtures were centrifugated for 10 min at 4000 rpm for clear phase separation. After twophase separation, 2 mL of S201 dissolved in n-nonane (S201 with volume fraction of 10% v/v) was added to mix with the above ABS for 5 min. The mixtures were then centrifugated for 10 min at 4000 rpm to finally obtain a three-layered system consisting of S201 organic top phase, EOPO
rich middle phase, and Na2SO4-rich aqueous bottom phase. In TLPE experiments on the influence of H+ and Cl
concentrations, 3 mL of each Pt(IV), Pd(II), and Rh(III) stock solutions were, respectively, first mixed together. Then, the mixtures were boiled dry. The obtained residues after boiling were dissolved into 9 mL of aqueous solution containing corresponding amounts of Cl
and H+. The concentrated H2SO4 (98 wt %) or solid NaCl were used for the adjustment of H+ or Cl
concentrations in above metal-mixed aqueous solutions. Then, 3.00 g of solid Na2SO4, 4.50 g of 50 wt % EOPO aqueous solution, and 2 mL of 10% (v/v) S201/n-nonane organic solution were added in turn. The system was mixed for 5 min followed by centrifugation at 4000 rpm for 10 min to obtain a stable three-liquid-phases coexisting system. All the above experiments were performed at room temperature (25 °C). Determination of PGMs. Determinations of palladium, platinum, and rhodium, respectively, in middle EOPO
rich aqueous phase and bottom Na2SO4-rich aqueous phase were conducted with an Optima 2  00/7000DV inductive coupled plasma optical emission spectrometer (ICP-OES), Perkin-Elmer. Standard platinum, palladium, and rhodium solution (The contents of Pt, Pd, and Rh were all 1000 μg/mL) were purchased from the Analytical Center of National Nonferrous Metal and Electronic Materials.

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External standard calibration was used throughout the experiment. A 0.2 mL portion of EOPO
rich phase or salt-rich phase was diluted 50 times into 10 mL of 1 mol/L HCl solution. To subtract the influence of matrix on the determined value, standard PGMs solution was also diluted into 1 mol/L HCl for references. Pre-experiments demonstrated that a 50 fold dilution of samples before ICP determination can effectively avoid influence from the background of concentrated salt aqueous solution or EOPO polymers. Errors on the determined concentrations of palladium, platinum, and rhodium could be neglected (usually 0.3 mol/L Pd(II) could not be reduced and almost all of it was extracted by S201 into the organic phase. In the Pt(IV)
Pd(II) system, interesting partition behavior of Pt(IV) in the EOPO phase can be observed in that the mass fraction of Pt(IV) in the EOPO phase first decreased when [Cl
] was below 0.3 mol/L, then increased continuously with the increase in Cl
concentration. The initial decrease in Pt distribution is probably due to the adsorption of PtCl62
by the reduced Pd(II) fine particles. In the Pt(IV)
Rh(III) system, the mass fraction of Pt(IV) in the EOPO phase kept climbing with the increase in Cl
concentration. The mass fractions of Rh(III) in separated EOPO phase both kept decreasing in the Pd(II)
Rh(III) and Pt(IV)
Rh(III) systems. Figure 9 shows the distribution of Pt(IV), Pd(II), and Rh(III) in TLPS of S201/nonane
EOPO
Na2SO4 for the extraction of ternary metal mixtures. This was similar to binary metal systems in that when the [Cl
] < 0.3 mol/L, Pd(II) was reduced to

Figure 9. Effect of Cl
concentrations in an equilibrious salt aqueous phase on the partition behavior of platinum, palladium, and rhodium in a TLPS composed of 10% (v/v) S201/nonane
EOPO
Na2SO4 (ternary metal mixtures of platinum, palladium, and rhodium were used. [H+] = 1 mol/L; [Na2SO4] = 2.14 mol/L. The Cl
concentrations were adjusted by the addition of NaCl.).

black solids. The mass fractions of Pt(IV) and Rh(III) both decreased sharply due to the coadsorption of PtCl62
and RhClx(H2O)6
x
(x
3) onto the surfaces of black solids. When [Cl
] > 0.3 mol/L, the mass fraction of Pd(II) in the organic phase remained at 100 wt %, while that of Pt(IV) and Rh(III) kept increasing and decreasing, respectively. The influence of Cl
concentration in an equilibrious salt aqueous phase on three-liquid-phase partitioning of PGM ions is similar to the influence of Na2SO4 concentration. The increase in Cl
concentration promotes the salting-out dehydration of EOPO molecules. The volumes of the EOPO middle phase keep decreasing with the increase in Cl
concentration. Moreover, the salting-out effect leads to the increase in dehydrated oxygen atoms in the PEO segments, and therefore favors the transfer of PtCl62
to the EOPO phase. The salting-out induced decrease in solvent polarity of the EOPO phase accounts for the decrease in partitioning of hydrophilic Rh(III) species into the separated EOPO phase. However, the salting-out strength of SO42
was far stronger than that of Cl
. More recently, a thermodynamic 9373

dx.doi.org/10.1021/ie200883u |Ind. Eng. Chem. Res. 2011, 50, 9368–9376

Industrial & Engineering Chemistry Research

Figure 10. The possible scheme for Pt(IV), Pd(II), and Rh(III) extraction mechanism in TLPS of S201/nonane
EOPO
Na2SO4.

approach utilizing Gibbs free energy of hydration (ΔGhyd) quantified the relative strength of the salting-out ability of different ions.43 Typically ΔGhyd of SO42
(
1145 kJ/mol) was far smaller than that of Cl
(
270 kJ/mol). Therefore, the salting out ability of Na2SO4 was much stronger than that of NaCl. No wonder the phase volume change with the variation of Na2SO4 concentration was far more remarkable than that with the change of NaCl concentration. In addition, the Cl
concentration can also affect the stability of PGMs chloro-anions and change the partition behavior of them. However, it is difficult for the hydrated Rh(III) to transform into chloro-anions by simply elevating Cl
concentrations. The high stability constant of PtCl62
enables most of the added Cl
ions to associate with Pt(IV), and thereafter the increased Cl
concentrations have no obvious influences on the association of Pt(IV). Consequently, the salting-out effect of NaCl dominates the partition process of Pt(IV) and Rh(III). On the contrary, the Pd(II) chloro-anions are easily influenced by Cl
concentrations owing to the low stability constant of PdCl42
. The reduction of Pd(II) occurs when the Cl
concentration is not sufficiently high to stabilize PdCl42-. The Possible Extraction Mechanism of TLPS for Pt(IV), Pd(II), and Rh(III). On the basis of the above discussion, a preliminary assumption of possible separation mechanism of Pt(IV), Pd(II), and Rh(III) in TLPS can be obtained, though further evidence still need to be provided. Figure 10 illustrates a schematic diagram of three-liquid-phase distribution of Pt(IV), Pd(II), and Rh(III) in TLPS. During the three-liquid-phase extraction process, the chloro-anion species of PGMs, for example, PtCl62
, PdCl62
, and RhCl63
, were concentrated in the vicinity of dehydrated EOPO molecules. However, the interaction strengths between PGMs chloro-anions and EOPO molecules are different. The complexing Cl atoms in PdCl42
can be substituted by S201, while PtCl62
with strong thermodynamic stability and dynamic inertia cannot be. As a result, most of the Pd(II) ions are extracted into the organic top phase leaving Pt(IV) ions in the EOPO phase. The strong hydration nature of Rh(III) enables RhCl63
to transform into hydrophilic species such as RhCl3(H2O)3, RhCl2(H2O)4+, RhCl(H2O)52+, and Rh(H2O)63+, which

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cannot combine with the pseudopolycations in EOPO molecules. Therefore, most of the Rh(III) species return back into the salt aqueous phase. Pd(II), Pt(IV), and Rh(III) are respectively isolated into S201 organic top phase, EOPO aqueous middle phase, and Na2SO4 aqueous bottom phase. Metal Recovery and Material Regeneration. The backextraction and recovery of metal ions from each phase of the TLPS can be achieved by different approaches. It is well accepted that Pd(II) can form more stable complexes with NH3 than with diisoamyl sulfide.44 In our experiment, equal volumes of Pd(II) loaded S201/nonane organic phase and 1 mol/L NH3 3 H2O aqueous solution were contacted for Pd(II) back-extraction. The results indicated that 73% Pd(II) was transferred back into aqueous solution. Two continuous strippings gave over 90% Pd(II) recovery. Electrodeposition can be directly applied to reduce Pd(II) from its ammonia aqueous solutions. The S201/ nonane organic phase can be regenerated for subsequent reuse by washing with 1 mol/L HCl aqueous solution. The Pt(IV) loaded EOPO phases in the TLPS contain a large amount of water and salt ions. Our previous work has demonstrated their excellent electrical conductivity.45 Therefore, electrodeposition can be used to reclaim Pt(IV). However, this work still deserves further research, and the results will be focused in another paper. After the stripping of Pt(IV), the EOPO copolymer can be refreshed or regenerated by solvent extraction with CHCl3 or CH2Cl246 or by properly elevating the ambient temperature to induce phase separation, leaving water, salt, and impurities in the separated aqueous phase.22 The remaining Rh(III) in the bottom aqueous phase after TLPE can be recovered by solvent extraction47 or traditional chemical reduction processes. Na2SO4 in the aqueous phase can be partly removed by crystallization through properly cooling the aqueous phase. In addition, the Na2SO4 aqueous solution after Rh(III) removal can be reused for subsequent TLPE.

’ CONCLUSIONS We have successfully employed a three-liquid-phase system composed of diisoamyl sulphide (S201) organic phase, polyethylene oxide-polypropylene oxide random block copolymer (EOPO), and Na2SO4 aqueous two phases for extraction and one-step separation of platinum, palladium, and rhodium from their hydrochloric acid leaching solutions. Experimental results indicated that Na2SO4 was the best phase-forming salt to construct a stable TLPS. Almost all of Pd was enriched into S201 organic top phase of TLPS. Increasing Na2SO4 concentration could induce the variation of selective partitioning of Pt(IV) and Rh(III) in the EOPO
Na2SO4 aqueous biphasic system. Most of Pt(IV) transferred into the EOPO aqueous middle phase, while Rh(III) went into the Na2SO4 aqueous bottom phase. Increase in Cl
concentration resulted in the variation of complexing-species of PGMs in aqueous solution and therefore affected the extractable species in the salting-out induced three-liquid-phase system. H+ concentration contributed to the coextraction of Pt(IV) and Rh(III) into the EOPO phase when H+ concentration was increased from 0.05 to 0.5 mol/L. The TLPE approach developed in the present work provides a suitable separation strategy for multiphase simultaneous extraction and a one-step separation of PGMs from their hydrochloric acid leaching liquors, and the application of a three-liquid-phase system has been extended to high acid, high chloride operation conditions. 9374

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’ ASSOCIATED CONTENT

bS Supporting Information. Additional figures as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. ’ AUTHOR INFORMATION Corresponding Author

*Email: [email protected]. Tel: 86 10 82544912. E-mail: [email protected]. Tel/Fax: 86 10 62554264.

’ ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (No. 51074150, No.21027004), Key Project of Chinese National Programs for Fundamental Research and Development (973 Programs No. 2007CB613507, No.2007CB714300) and Innovative Research Group Science Fund (No.20221603) ’ REFERENCES (1) Kaspar, J.; Fornasiero, P.; Hickey, N. Automotive catalytic converters: current status and some perspectives. Catal. Today 2003, 77, 419–449. (2) Platinum Series; Johnson Matthey: London, UK, published annually since 1985. (3) Buchanan, D. L. Platinum-Group Element Exploration; Elsevier Science Publishers: Amsterdam, The Netherlands, 1988. (4) Barakat, M. A.; Mahmoud, M. H. H. Recovery of platinum from spent catalyst. Hydrometallurgy 2004, 72, 179–184. (5) Chen, J.; Huang, K. A new technique for extraction of platinum group metals by pressure cyanidation. Hydrometallurgy 2006, 82, 164–171. (6) Kim, C. H.; Woo, S. I.; Jeon, S. H. Recovery of platinum-group metals from recycled automotive catalytic converters by carbochlorination. Ind. Eng. Chem. Res. 2000, 39, 1185–1192. (7) Lee, J. Y.; Raju, B.; Kumar, B. N.; Kumar, J. R.; Park, H. K.; Reddy, B. R. Solvent extraction separation and recovery of palladium and platinum from chloride leach liquors of spent automobile catalyst. Sep. Purif. Technol. 2010, 73, 213–218. (8) Schreier, G.; Edtmaier, C. Separation of Ir, Pd, and Rh from secondary Pt scrap by precipitation and calcinations. Hydrometallurgy 2003, 68, 69–75. (9) Kondo, Y.; Kunota, M. Precipitation behavior of platinum groupmetals from simulated high-level liquid waste in sequential denitration process. J. Nucl. Sci. Technol. 1992, 29, 140–148. (10) Chen, J. P.; Lim, L. L. Recovery of precious metals by an electrochemical deposition. Chemosphere 2005, 60, 1384–1392. (11) Tan, X. D.; Ji, Q. R.; Chang, Z. D.; Liu, H. Z. Partition behavior of penicillin in three-liquid-phase extraction system. Chin. J. Process Eng. 2006, 6, 363–368. (12) Gu, G. H.; Wu, Y. S.; Fu, X.; Hu, X. P.; Yu, W. R.; Xing, Y. H. Comparison between three-phase extraction system of TBP kerosene/ HClO4
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