Separation and Concentration of Pd, Pt, and Rh from

Stock solutions of palladium, platinum, and rhodium (500 mg L-1, pH = 1) were prepared by dissolving weighed amounts of palladium(II) chloride (Merck)...
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Ind. Eng. Chem. Res. 2002, 41, 1616-1620

Separation and Concentration of Pd, Pt, and Rh from Automotive Catalytic Converters by Combining Two Hollow-Fiber Liquid Membrane Systems Cla` udia Fonta` s, Victo` ria Salvado´ , and Manuela Hidalgo* Department de Quı´mica, Universitat de Girona, 17071 Girona, Spain

A new system combining two hollow-fiber-supported liquid membranes (HFSLMs) for the separation and concentration of platinum, palladium, and rhodium is presented. The best results were obtained when the feed solution was recirculated through a first module containing Cyanex 471 as the solvating extractant and then the depleted feed solution was recirculated through a second module containing the anion exchanger Aliquat 336 as the carrier. Under these conditions, palladium was selectively recovered in the stripping solution of module I, while platinum was recovered in the receiving phase of module II, and rhodium remained in the feed solution. It was observed that low concentrations of thiocyanate in the feed solution favored the permeability of platinum. This separation scheme was applied to the recovery of platinum-group metals from the leaching solutions of spent automotive pellet-type catalysts. Palladium and platinum were effectively separated and enriched, and only small amounts of aluminum were cotransported through the liquid membrane systems. Introduction The active components of automotive catalytic converters, which have been widely used since 1975,1-3 are platinum-group metals (PGMs) and especially platinum, palladium, and rhodium, which are finely dispersed on inert supports made of alumina. The concentrations of platinum-group metals in catalysts vary widely depending on the manufacturer.4 Platinum, which is generally present in larger amounts than palladium and rhodium, ranges in concentration from 300 to 1000 µg g-1; in the case of palladium, the concentrations vary from 200 to 800 µg g-1; and for rhodium, they vary from 50 to 100 µg g-1. These amounts are quite small, but even still, the concentrations are often richer than those found in mined ores.5 Given that the PGMs in catalytic converters are not destroyed, the accumulated quantities of these metals found in vehicles represent a significant above-ground resource that could potentially be exploited. In order that this be possible, effective processes for the recovery of PGMs from these converters must be developed. In addition to retaining PGMs, autocatalysts also expel PGMs within fine particulate matter, or dust, which is produced as a result of the abrasion and deterioration of the bulk catalysts.4,6-8 The toxicity of PGMs has led to an interest in the development of preconcentration methods for the ultratrace analysis of Pd and Pt in environmental matrixes. Many of the available methods require preconcentration steps for accurate determinations to be made and for the analyte to be separated from the sample matrix and interfering elements. Several methods have been described for the preconcentration of PGMs from geological materials, such as fire assays, chlorination, or acid dissolution.9-11 Some of these techniques have also been proposed for the recovery of PGMs from automotive catalysts.12 Ionexchange methods using anion-exchange resins13 have also been investigated for this purpose. * Corresponding author. Phone: (+34) 972418190. Fax: (+34) 972418150. E-mail: [email protected].

Supported liquid membranes (SLMs) containing a selective carrier14,15 are well-suited for the selective separation of metals. Moreover, enrichment and separation can be simultaneously achieved when the volume of the stripping solution is much smaller than that of the source solution.16,17 The results obtained with SLMs in a laminar configuration show that palladium can be effectively separated from platinum in the presence of small amounts of thiocyanate using triisobutylphosphine sulfide (Cyanex 471) as a carrier.18,19 The transport of platinum was accomplished using tricaprylylmethylammonium chloride (Aliquat 336) as the extractant and a NaClO4 solution as the receiving phase.20 It has also been found that no transport of rhodium is expected under these chemical conditions.21-23 The aim of this study is to design a scheme for the separation and enrichment of Pd, Pt, and Rh through the combination of two hollow-fiber-supported liquid membrane systems and to select the most suitable chemical composition of the source solution and the mode of operation. We also tested the efficiency of the proposed separation scheme using feed solutions that had been leached from used automotive pellet-type catalysts. Experimental Section Reagents and Solutions. Stock solutions of palladium, platinum, and rhodium (500 mg L-1, pH ) 1) were prepared by dissolving weighed amounts of palladium(II) chloride (Merck), platinum (IV) chloride (Johnson Matthey), and rhodium(III) chloride (Johnson Matthey) in concentrated HCl. The aqueous feed solutions containing different amounts of the metals and 0.5 M NaCl were obtained by diluting the stock metal solutions and adding the necessary amount of sodium chloride (analytical reagent grade, Panreac). Different volumes of a sodium thiocyanate solution (analytical reagent grade, Panreac), standardized by titration with silver nitrate, were added to the feed solutions. The standard reference material SRM-2556 (used auto catalyst pellets) (NIST, National Institute of

10.1021/ie010468q CCC: $22.00 © 2002 American Chemical Society Published on Web 02/09/2002

Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1617 Table 1. Compositions of the Solutions in the Two HFSLM Modules Used in the Experiments organic phase stripping solution

Figure 1. Scheme of the hollow-fiber-supported liquid membrane module used in the experiments.

Standards and Technology, Gaithersburg, MD) was also used as a source of PGMs. Cyanex 471, kindly supplied by Cyanamid Iberica and previously recrystalized from ethanol-water, was used as the carrier in module I. The organic phase was prepared by dissolving the extractant in Decaline (decahydronaphthalene, cis and trans) (Sigma-Aldrich) with 20% (v/v) cumene (isopropylbenzene) (Merck). The stripping solution consisted of 0.1 M NaSCN at pH 2 and 0.5 M ionic strength (adjusted with NaCl). Module II contained 0.15 M Aliquat 336 (Fluka Chemie) dissolved in dodecane (analytical reagent grade, Merck) modified with 4% dodecanol as the liquid membrane phase. A 0.5 M NaClO4 solution at pH 2 was used as the stripping solution. Hydrophobic polypropylene hollow fibers (inner diameter ) 0.3 mm, outer diameter ) 0.5 mm, pore size ) 0.2 µm, porosity ) 75%, length ) 57 cm; supplied by Azko, Enka, E.G.) were used as supports for the liquid membrane. Apparatus. The experiments were carried out using a HF module with one coiled hollow fiber as shown in Figure 1. A Gilson peristaltic pump (Pacisa, Spain) was used to continuously recirculate the aqueous phases. Palladium, platinum, and rhodium concentrations in the samples were determined with a coupled plasma atomic emission spectrometer (Varian Liberty series II ICP). Microwave-assisted digestion of the samples was performed in a CEM microwave oven (model MDS 81 D) equipped with closed vessels. Sample Dissolution Procedure. Portions (2.5-g) of previously dried SRM-2556 were accurately weighed and placed in a 125-mL pressure-resistant PFTE advanced composite vessel to which 20 mL of aqua regia was added. The vessel was then placed into a microwave oven. The magnetron was controlled by entering a power setting and an operating time. The power setting was 60% when six samples were processed and 40% when only three vessels were used. Maximum efficiency for the lixiviation process was obtained with an operating time of 20 min. Afterward, the vessel was left to cool, and the solution was filtered using a filter paper (Whatman 542). Finally, distilled water was added to the filtrate until a volume of 25 mL was reached. The resulting solutions were used as stock samples. Hollow-Fiber Liquid Membrane Experiments. The HFSLMs were prepared by slow impregnation of the tubular microporous fiber, which was achieved by passing the organic solution through the lumen of the hollow fiber.16 The feed solution was continuously

module I

module II

40 mM Cyanex 471 in Decaline + 20% cumene 0.1 M NaSCN, pH 2, I ) 0.5

0.15 M Aliquat 336 in dodecane + 4% dodecanol 0.5 M NaClO4, pH 2, I ) 0.5

recirculated through the fiber while the stripping solution was recirculated around the shell side. The flow rate of the aqueous solutions was usually kept at 0.6 mL min-1. The volume of aqueous solution and the length of the experiment varied depending on the type of experiment. All of the experiments involved the use of two HFSLM systems, and the best mode of operation when working with both modules was established by using synthetic mixtures containing platinum, palladium, and rhodium as the feed solutions. Once the mode of operation was optimized, the separation system was applied to the lixiviates of the automotive catalysts. Table 1 shows the composition of the organic phases and receiving solutions of the hollow-fiber liquid membrane system used in the experiments. Mode of Operation of the HFSLM Modules. Different combinations of the two SLM systems were evaluated (Figure 2). Scheme a. The feed solution was continuously recirculated through the lumen of the fiber of module I, while the stripping solution was recirculated on the shell side. Then, the depleted feed solution was introduced into module II and was recirculated in the same fashion (Figure 2a). Scheme b. In this case, the feed solution was continuously recirculated through both modules sequentially, i.e., the feed solution flowed inside the fiber of the first module at the same time as the stripping solution was recirculated, and it was then introduced into the fiber of the second module, which also had the stripping solution recirculating around the shell side. Obviously, the order in which modules I and II were placed was an important factor in this study (Figure 2b). Results and Discussion The optimum conditions for separating palladium, platinum, and rhodium from solutions containing mixtures of these metals were established by investigating the different parameters affecting the efficiency of the membrane system. Effect of the Thiocyanate Concentration in the Feed Solution. The presence of small amounts of thiocyanate is necessary to achieve the transport of Pd(II) when Cyanex 471 is used as the carrier, and the palladium permeability is heavily dependent on the SCN- concentration present in the feed solution.18,19 Moreover, it has been found that the transport of platinum using the anion exchanger Aliquat 336 as the carrier is reduced when the concentration of thiocyanate is high.22 Given that the solutions tested contain platinum, palladium, and rhodium, it is necessary to optimize the concentration of thiocyanate in the feed solution to enhance the extraction of palladium by Cyanex 471 without disturbing the platinum transport by Aliquat 336. The effect of the thiocyanate concentration in the feed solution on the separation of palladium and platinum

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Figure 2. Schematic view of the combination of the two HFSLM modules for the separation and recovery of Rh, Pd and Pt. Scheme a is run in batch mode; scheme b is run in recycling mode. 1, hollow-fiber contactor corresponding to module I; 2, hollow-fiber contactor corresponding to module II; 3, peristaltic pump; 4, stripping solution of module I; 5, feed solution; 5′, depleted feed solution; 6, stripping solution of module II.

was investigated using the two modules following scheme a (see Figure 2), where 250 mL of synthetic feed solution containing 10 mg L-1 of each metal was continuously recirculated inside the fiber of module I and 100 mL of the stripping solution was recirculated around the shell side. The flow rate of both solutions was 0.35 mL min-1. After 24 h, the depleted feed solution was recirculated through module II under the same conditions. The results obtained from these experiments are presented in Figure 3. Palladium is the only metal present in the stripping solution of module I, whereas platinum is transported through the membrane in module II, and rhodium remains in the feed solution. These results are explained by the selectivity of the solvating extractant Cyanex 471 for palladium19 and the fact that the use of the anion exchanger Aliquat 336 as the carrier in module II allowed for preferential platinum permeation under the chemical conditions used in these experiments.22 Low concentrations of thiocyanate are more favorable in terms of platinum permeability, whereas for palladium, no significant differences in the percentage of metal recovered in the stripping phase are observed over the range of SCN- concentrations studied (Figure 3). Influence of the Mode of Operation. As the two modules used in this study can be positioned in two different ways (module I followed by module II or the opposite), we investigated the influence of position on

Figure 3. Percentage of metal recovered in the stripping phase of modules I and II as a function of the thiocyanate concentration present in the feed solution using the two HFSLM modules following scheme a. The feed solution contained about 10 mg L-1 of each rhodium, palladium, and platinum.

the separation process, taking scheme b as the operational mode (see Figure 2). In both cases, 200 mL of synthetic feed solution containing about 10 mg L-1 of each metal and 10-4 M SCN- was continuously recirculated through the lumen of the fibers, while 20 mL of the corresponding stripping solution flowed around the shell side. The flow rate of both solutions was 0.6 mL min-1, and the experiments were run for 22 h.

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Figure 4. Effect of the thiocyanate concentration in the feed solution (obtained from a lixiviate of spent automotive catalyst) on the percentage recoveries of palladium and platinum in the stripping phases using the two HFSLM modules following scheme a. The feed solution contained about 10,000 mg L-1 of Al, 160 mg L-1 of Pb, 10 mg L-1 of Pd, and 20 mg L-1 of Pt. Table 2. Metal Recovered in the Stripping Phase (%) Scheme b with Module I followed by Module II module I (SCN-) module II (ClO4-) Pd Pt Rh

19 0 0

8 66 0

Scheme b with Module II followed by Module I module II (ClO4-) module I (SCN-) Pd Pt Rh

8.2 39.4 0

5.1 0 0

Table 2 shows the percentages of metal recovered in the stripping phase with respect to the concentrations in the initial feed solution. These results show that it is not possible to achieve complete separation of the metals using either of the two scheme b setups. With both setups, a significant amount of palladium was recovered in the stripping solution of module II, which contains Aliquat 336. However, the recovery of platinum is considerably lower in the second scheme b setup, as palladium and platinum compete for the anion exchanger extractant molecules. Separation of PGMs Contained in Automotive Catalysts. To evaluate the performance of the proposed separation system with real samples, a prepowered catalyst standard (NIST SRM-2556) was used. Lixiviation was performed with microwave-assisted leaching using aqua regia, which has been widely used for the dissolution of samples containing PGMs.24,25 After 20 min of digestion, about 75% of the palladium and 65% of the platinum had been dissolved, whereas the rhodium had not dissolved at all. In addition to platinum and palladium, large amounts of aluminum were present in the resulting solution (>0.5 M, 40% of the total metal content), as was lead, which was present as a result of the use of tetraethyl lead in gasoline. An increase in the time of digestion did not improve dissolution of the PGMs. Feed solutions were prepared by the filtration of the lixiviate solutions and the subsequent addition of distilled water until the desired volume was reached. To enrich palladium and platinum during their separation, we used volumes of stripping solutions that were much smaller than the source solution volumes.

Table 3. Concentration Factors for Palladium and Platinum at Different Thiocyanate Concentrations in the Feed Solution [SCN-] in the feed solution (M) palladium platinum

6 × 10-5

1.2 × 10-4

5 × 10-3

2.7 2.6

2.4 3.3

1.2 0

As has been discussed, the concentration of thiocyanate in the feed solution is a critical parameter in the extraction of palladium with Cyanex 471. To optimize the concentration of thiocyanate, we conducted a series of experiments involving the addition of different amounts of this anion to feed solutions prepared from 10 mL of the leaching solution and distilled water to make up a volume of 25 mL. The resulting solution contained about 10,000 mg L-1 of aluminum, 160 mg L-1 of lead, 10 mg L-1 of palladium, and 20 mg L-1 of platinum. The volume of each stripping solution was 3 mL, and all aqueous solutions were recirculated at a flow rate of 0.6 mL min-1 for 5 h. In these experiments, the two HFSLM modules operated following scheme a. The results are presented in Figure 4 as percentages of palladium and platinum recovery (with respect to the initial metal contents present in the feed solution). As can be seen, even under these unfavorable conditions (large amounts of aluminum, high acidity), the separation of the two platinum-group metals was accomplished: palladium was recovered in the stripping solution of module I, and platinum was found in module II. Only small amounts of aluminum (approximately 60 mg L-1) were found in the stripping solution of the first module, and no lead permeated through either of the HFSLM modules. The best results in terms of metal recovery, especially in the case of platinum, were obtained when low SCN- concentrations were used, as has been found in previous studies.22 The concentration factors for both metals are presented in Table 3. This parameter is defined as F(t)) Cst(t)/Cso, where Cst(t) is the metal concentration in the stripping solution at time t and Cso is the metal concentration in the source solution. After optimization of the concentration of thiocyanate, we conducted new separation experiments using 25 mL of source solution containing 6 × 10-5 M thiocyanate

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Table 4. Results Obtained When Two HFSLM Modules are Combined Following Scheme a for the Separation of Palladium and Platinum Obtained from Leaching Solutions of Spent Catalystsa

Al Pb Pd Pt a

initial feed

stripping module I

feed after module I

stripping module II

final feed

11 646 185 8.4 18.1

43.5 0 30.5 0

11 323 185 0 16.4

135 0 0 85.7

11 237 185 0 2.6

Metal concentrations reported in mg L-1.

and 3 mL of stripping solution. The feed solution was recirculated through the liquid membrane (LM) of module I for 20 h and then through the LM of module II for an additional 20-h period. The obtained results are summarized in Table 4. As can be seen, both palladium and platinum are effectively enriched, with concentrations factors of 3.6 and 5.6, respectively, while the percentage of recoveries of the metals in relation to the initial metal concentrations in the source solution are 43% for palladium and 57% for platinum. Aluminum is the only nonprecious metal that is transported, but taking into account its high presence in the source solution, the concentration of this metal found in the stripping solutions is insignificant. Conclusions The combination of two hollow-fiber-supported liquid membrane systems was successful in the simultaneous separation and concentration of Pd(II) and Pt(IV) from acidic samples obtained from the leaching of spent automotive catalytic converters. A selective separation of these metals was achieved when the feed solution (to which an small amount of thiocyanate had been added) was first introduced into the HFLM system containing Cyanex 471 as the extractant and then passed through the second liquid membrane system consisting of Aliquat 336 as the carrier. Only an insignificant amount of aluminum was cotransported. Furthermore, the possibility of obtaining enriched stripping solutions makes scheme a an attractive system for the separation and preconcentration of PGMs from environmental samples containing small amounts of these metals. Acknowledgment The present work was supported by the CICYT (Spanish Commission for Research and Development), Project QUI 1999-0749-C03-03. Literature Cited (1) Kallmann, S.; Blumberg, P. Analysis of automobile-exhaust emission-control catalysts. Talanta 1980, 27, 827. (2) Borisov, O. V.; Coleman, D. M.; Oudsema, K. A.; Carter, R. O. Determination of platinum, palladium, rhodium and titanium in automotive catalytic converters using inductively coupled plasma mass spectrometry with liquid nebulization. J. Anal. At. Spectrom. 1997, 12, 239. (3) Wayne, D. M. Direct determination of trace noble metals (palladium, platinum and rhodium) in automobile catalyst by gloss discharge mass spectrometry. J. Anal. At. Spectrom. 1997, 12, 1195. (4) Barefoot, R. R. Determination of platinum at trace levels in environmental and biological materials. Environ. Sci. Technol. 1997, 31 (2), 309.

(5) Malhotra, S. C. Future opportunities in the reclamation of precious metals from major sources of obsolete scrap. Precious Metals: mining, extraction and processing. In Proceedings of the AIME Annual Meeting; The Metallurgical Society of AIME: Los Angeles, CA, 1984. (6) Barefoot, R. R. Distribution and speciation of platinum group elements in environmental matrices. Trends Anal. Chem. 1999, 18 (11), 702. (7) Schierl, R.; Fruhmann, G. Airborne platinum concentrations in Munich city buses. Sci. Total Environ. 1996, 182, 21. (8) Go´mez, M.; Go´mez, B.; Palacios, M. A. Control of interference in the determination of Pt, Pd, and Rh in airborne particulate matter by inductively coupled plasma mass spectrometry. Anal. Chim. Acta 2000, 404, 285. (9) Perry, B. J.; Barefoot, R. R.; Van Loon, J. C. Inductively coupled mass spectrometry for the determination of the platinum group metals and gold. Trends Anal. Chem. 1995, 14 (8), 388. (10) Barefoot, R. R.; Van Loon, J. C. Recent advances in the determination of the platinum group elements and gold. Talanta 1999, 49, 1. (11) Reddi, G. S.; Rao, C. R. M. Analytical techniques for the determination of precious metals in geological and related materials. Analyst 1999, 124, 1531. (12) Hoffmann, J. E. Recovering platinum-group metals from auto catalysts. J. Met. 1988, June, 40. (13) Gaita, R.; Al-Bazi, S. J. An ion-exchange method for the selective separation of palladium, platinum and rhodium from solutions obtained by leaching automotive catalytic converters. Talanta 1995, 42 (2), 249. (14) Danesi, P. R. Separation of metal species by supported liquid membranes. Sep. Sci. Technol. 1984, 19, 857. (15) Noble, R. D.; Way, D. J. Liquid membrane technology: An overview. In Liquid Membranes: Theory and Applications; ACS Symposium Series 347; Noble, R. D., Way D. J., Eds.; American Chemical Society: Washington, D.C., 1987. (16) Danesi, P. R.; Rickert, P. G. Some observations on the performance of hollow-fiber supported liquid membrane for CoNi separations. Solvent Extr. Ion Exch. 1996, 4 (1), 149. (17) Parthasarathy, N.; Pelletier, M.; Buffle, J. Hollow fiber based supported liquid membrane: A novel analytical system for trace metals analysis. Anal. Chim. Acta 1997, 350, 183. (18) Hidalgo, M.; Masana, A.; Salvado´, V.; Freiser, H.; Al-Bazi, S. J.; Valiente, M. Accelerated mass transfer of palladium(II) through a selective solid-supported liquid membrane containing Cyanex 471. Anal. Chim. Acta 1991, 251, 233. (19) Fonta`s, C.; Salvado´, V.; Hidalgo, M. A liquid membrane system based on Cyanex 471 as carrier for the selective separation and concentration of palladium(II) from spent automotive catalysts. Anal. Chim. Acta, manuscript submitted. (20) Fonta`s, C.; Salvado´, V.; Hidalgo, M. Solvent extraction of Pt(IV) by Aliquat 336 and its application to a solid supported liquid membrane system. Solvent Extr. Ion Exch. 1999, 17 (1), 149. (21) Berengel, E.; Demopoulos, G. P.; Harris, G. B. Speciation and separation of rhodium(III) from chloride solutions: A critical review. Hydrometallurgy 1996, 40, 135. (22) Fonta`s, C.; Antico´, E.; Salvado´, V.; Valiente, M.; Hidalgo, M. Chemical pumping of rhodium by a supported liquid membrane containing Aliquat 336 as carrier. Anal. Chim. Acta 1997, 346, 199. (23) Fonta`s, C.; Palet, C.; Salvado´, V.; Hidalgo, M. A hollow fiber supported liquid membrane based on Aliquat 336 as a carrier for rhodium(III) transport and preconcentration. J. Membr. Sci. 2000, 178, 131. (24) Gowing, C. J. B.; Potts, P. J. Evaluation of a rapid technique for the determination of precious metals in geological samples based on a selective aqua regia leach. Analyst 1991, 116, 773. (25) Hinds, M. W.; Littau, S.; Moulinie´, P. Determination of trace metals in platinum by electrothermal atomic absorption spectrometry following a closed-vessel microwave dissolution procedure. Analyst 1992, 117, 1473.

Received for review May 29, 2001 Revised manuscript received November 12, 2001 Accepted November 15, 2001 IE010468Q