Extraction Properties of Palladium(II) in HCl Solution with Sulfide

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Extraction Properties of Palladium(II) in HCl Solution with SulfideContaining Monoamide Compounds Hirokazu Narita,†,* Kazuko Morisaku,† Ken Tamura,† Mikiya Tanaka,† Hideaki Shiwaku,‡ Yoshihiro Okamoto,‡ Shinichi Suzuki,‡ and Tsuyoshi Yaita‡ †

Research Institute for Environmental Management Technology, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan ‡ Quantum Beam Science Directorate, Japan Atomic Energy Agency (JAEA), 1-1-1 Koto, Sayo, Hyogo 679-5148, Japan S Supporting Information *

ABSTRACT: The extraction properties of palladium(II) in an HCl solution with four sulfide-containing monoamide (S-MA) compounds (N-methyl-N-n-octyl-4-thiapentanamide (MO4), N-methyl-N-n-octyl-3-thiapentanamide (MO3), N-methyl-N-noctyl-phenyl-3-thiapentanamide (MOPh), and N,N-di-n-octyl-3-thiapentanamide (DO3)) were investigated by solvent extraction, FT-IR, and EXAFS spectroscopies. All the S-MA compounds extract Pd(II) much faster than the conventional extractant, di-n-hexyl sulfide. The order of the extractabilities for Pd(II) is as follows: MO4 > DO3 ≈ MO3 > MOPh. The dominant extracted complex can be [PdCl2(S-MA)2] for each S-MA system. The structural studies suggest that the inner coordination sphere of Pd(II) in the extracted complexes consists of 2 chloride ions and 2 sulfur atoms from a pair of S-MA molecules for all the systems. This means that the amide oxygen atom does not take part in the direct coordination to Pd(II), although the amide group can promote the Pd(II) extraction rate.

1. INTRODUCTION Palladium is currently one of the most important metals in the industrial fields (especially, autocatalysts); this partly corresponds to the high demand and high price of palladium in recent years.1,2 Hence, improvement of the palladium recovery from wastes containing palladium, along with the palladium refining from ores, has become very important. The separation and purification of platinum group metals (PGM) in hydrometallurgical processes is frequently carried out by solvent extraction from an acidic chloride solution.3,4 In particular, dialkyl sulfide compounds (DAS) (e.g., di-n-octyl sulfide (DOS) and di-n-hexyl sulfide (DHS)) have been extensively introduced for practical use, which are also used in the INCO process.5 DAS can extract Pd(II) over Pt(IV) from an HCl solution; however, the slow extraction of Pd(II) is a serious problem.4 We have found that amide compounds containing a sulfide group, i.e., thiodiglycolamide (diamide)6,7 and 3-thiapentanamide (monoamide),8 have a good Pd(II) extractability. They show a rapid extraction of Pd(II) with a high selectivity. As for the thiodiglycolamide compounds, the N-substituents affect the apparent extraction rate; N,N′-dimethyl-N,N′-di-n-octyl-thiodiglycolamide extracts Pd(II) from an HCl solution faster than N,N,N′,N′-tetra-n-octyl-thiodiglycolamide.7 On the other hand, the effects of the N-substituents of the 3-thiapentanamide compounds have not yet been studied. Understanding the relationship between the metal extractabilities and the structure of the extractants is required to optimize the extractant structure for practical use. In addition to functional groups having coordinating atoms which are normally selected on the basis of the HSAB rule, side chains/ substituents have a significant effect on the metal extraction behavior.9 In the case of the Pd(II) extraction, dialkyl(aryl) © 2014 American Chemical Society

sulfide-type and pyridyl-type extractants have been systematically studied. Yuan et al. showed that the sulfide extractants containing phenyl groups are inferior to those containing alkyl (except sec-alkyl) groups in terms of the extraction percentage and extraction rate of Pd(II); the lower extractabilities are attributed to the lower electron density of the sulfur atom by the electron withdrawing effect of the phenyl group and higher steric effect by the bulky phenyl group. 10 In the pyridinecarboxamide―Pd(II)―HCl system, the extractabilities of Pd(II) are highly dependent on the basicity of the pyridine nitrogen atom that coordinates to Pd(II) and steric hindrance around the pyridine nitrogen atom that are varied with the position of the carboxyamide in the pyridine ring.11 For both of them, the surroundings of the coordinating atom can govern the extraction properties of Pd(II). The objective of this study is to clarify the structural effect of the sulfide-containing monoamide (S-MA) compounds on the Pd(II) extraction from hydrochloric acid. We synthesized four S-MA compounds: N-methyl-N-n-octyl-4-thiapentanamide (MO4), N-methyl-N-n-octyl-3-thiapentanamide (MO3), Nmethyl-N-n-octyl-phenyl-3-thiapentanamide (MOPh), and N,N-di-n-octyl-3-thiapentanamide (DO3) (Figure 1). This study investigated the dependencies of the Pd(II) extraction with S-MA compounds on the extraction time, the concentration of S-MA, H+, and Cl−, and the coordination properties of the extracted complexes using FT-IR and EXAFS methods. Received: August 6, 2013 Accepted: February 13, 2014 Published: February 13, 2014 3636

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organic phase were calculated on the basis of the mass balance of the metal ions before and after the extraction. 2.4. FT-IR Measurements. The samples for the extracted complexes were prepared by solvent extraction (organic phase 0.1 M S-MA in chloroform, aqueous phase 3 M HCl (Pd(II) absent) or 0.05 M Pd(II) in 3 M HCl, extraction time 120 min). The Pd(II) was quantitatively extracted into the organic phase, then the concentration ratio of Pd(II):S-MA was about 1:2 for all the S-MA systems. The organic solutions were deposited onto a ST-IR Card TYPE 1 (Thermo Electron Co.). All the IR spectra were collected using a Nicolet Magna 750. 2.5. XAFS Measurements and Analyses. For the XAFS measurements, an aqueous solution (0.05 M Pd(II) in 3 M HCl) and the Pd(II) complex extracted with DO3 were prepared. The extracted complex was obtained by solvent extraction under the condition of 0.2 M DO3 in 2ethylhexanol−0.05 M Pd(II) in 3 M HCl, in which the Pd(II) was quantitatively extracted after the 120-min extraction. The XAFS measurements were carried out at the BL11XU of SPring-8. The measurement conditions and analytical method are described in detail in the Supporting Information.

Figure 1. Structures of the S-MA compounds: (A) MO4, (B) MO3, (C) MOPh, and (D) DO3.

2. EXPERIMENTAL SECTION 2.1. Reagent. The stock solutions of Pd(II)/Pt(IV) were prepared by dissolving the metal chlorides (PdCl2 and H2PtCl6: Soekawa Chemical Co., Ltd.) into HCl solutions. The S-MA compounds were synthesized using a procedure similar to thiodiglycolamide;6 the detailed methods are described in the next section. All other chemicals used in this study were of reagent grade. 2.2. Synthesis of Extractants. Carboxylic acid (3(methylthio)propionic acid for MO4, 2-(ethylthio)acetic acid for MO3, and DO3, 2-(benzylthio)acetic acid for MOPh) was converted to carboxylic chloride by refluxing a solution of the carboxylic acid dissolved in thionyl chloride with a small amount of N,N-dimethylformamide for 1 h at 50 °C. After removing the thionyl chloride, the crude carboxylic chloride was dropwise added to a mixed solution of triethylamine and a secondary amine (N-methyloctylamine for MO4, MO3, and MOPh, N,N-di-n-octylamine for DO3) in chloroform at 10 °C. The resulting solution was then refluxed with stirring for 2−3 h at 60 °C. After cooling to room temperature, the organic solution was successively washed with water, 1 M HCl, and a 5% aqueous sodium carbonate solution. The organic phase was next dried over anhydrous sodium sulfate and then concentrated in vacuo. The residue was purified by column chromatography (elution with n-hexane:ethyl acetate 19:1 for MO3 and DO3, 49:1 for MO4 and 97:3 for MOPh). Their purities were over 99% on the basis of 1H NMR and gas chromatography. 2.3. Solvent Extraction. Chloroform was used as a diluent. The diluted extractants were pre-equilibrated with the same volume of an aqueous solution in the absence of the metal ions. One milliliter of the pre-equilibrated organic phase and the same volume of the aqueous solution containing the mixed metal ions (Pd(II) and Pt(IV)) or only a Pd(II) ion (5 × 10−5 M each) were vertically shaken in a 10-mL glass tube at an amplitude of 100 mm and a frequency of 200 spm, then centrifuged. A portion of the organic phase and the same volume of a 28% ammonia solution were shaken and the metal ions in the organic phase were back-extracted to the aqueous phase. The volume ratio of the organic phase/aqueous phase (O/A) was 1 during all the experiments. All the extractions were carried out at room temperature (23 ± 2 °C). The concentrations of the metal ions in the aqueous phase were measured by ICP-AES (Horiba ULTIMA2); those in the

3. RESULTS AND DISCUSSION 3.1. Effect of Extraction Time. Figure 2 shows the dependence of the extraction percentages (E%) of Pd(II) from

Figure 2. E% of Pd(II) as a function of extraction time: ( organic phase) 0.01 M S-MA or 0.1 M DHS; (aqueous phase) 3 M HCl.

3 M HCl with 0.01 M S-MA on the extraction (shaking) time. For comparison, the extraction data in the 0.1 M DHS in the chloroform−3 M HCl system is included in Figure 2. The plots of Pt(IV) were omitted, since Pt(IV) was hardly extracted under this condition. Obviously, all the S-MA compounds extract Pd(II) much faster than DHS. This is consistent with a previous extraction result using DO3 and DHS in 80 vol% ndodecane−20 vol% 2-ethylhexanol.8 The apparent extraction rate follows the sequence of MO4 > MO3 > MOPh > DO3 ≫ DHS. The same order is observed using the lower concentration (0.001 M) of S-MA (Supporting Information, Figure S1). The back E% of Pd(II) is over 80% for all the S-MA systems. 3.2. Effect of S-MA Concentration. The dependence of the distribution ratio (D) of Pd(II) from 3 M HCl on the S-MA concentration is shown in Figure 3. The slope of the straight lines calculated using the least-squares method in Figure 3 nearly corresponds to the number of extractants in the dominant extracted complex. The obtained slope values are around 2, indicating that the ratios of Pd(II) to S-MA in the 3637

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Figure 3. Dependence of D of Pd(II) on the S-MA concentration: (aqueous phase) 3 M HCl; (extraction time) 10 h.

Figure 5. Dependence of D of Pd(II) on the Cl− concentration: (organic phase) 0.0005 M for MO4, 0.001 M for MO3, MPh, and DO3; (aqueous phase) 3 M H (Cl, ClO4); (extraction time) 10 h.

extracted complex are probably 1:2 in all the systems. This result is verified by the Pd loading capacity (Supporting Information, Figure S2). The magnitude of the D of Pd(II) is as follows: MO4 > DO3 ≈ MO3 > MOPh. The characteristics of the extracted complexes will be discussed in a later section. 3.3. Effect of H+ and Cl− Concentration. The effect of the H+ concentration on the D of Pd(II) with 0.001 M S-MA was investigated (Figure 4). The Cl− concentration was fixed at

3.4. Coordination Property of the Extracted Complex. The structural analyses of the extracted complexes were performed using FT-IR and XAFS methods. We focused on the presence or absence of the amide oxygen atom in the inner coordination sphere of Pd(II) in the extracted complexes; the carbonyl stretching frequency, ν(C−O), of the S-MA compounds and the Pd−O correlation in the extracted complex were investigated by FT-IR and EXAFS spectroscopies, respectively. Figure 6 shows the FT-IR spectra of the four S-MA compounds before and after the equilibrium with 3 M HCl or

Figure 4. Dependence of D of Pd(II) on the H+ concentration: (organic phase) 0.001 M S-MA; (aqueous phase) 3 M (H, Li) Cl; (extraction time) 10 h.

3 M; the volume ratios of the 3 M HCl and 3 M LiCl solutions were varied. In all the systems, the D of Pd(II) is independent of the H+ concentration, suggesting that the participation of the protonated S-MA in the Pd(II) extraction is negligible. Figure 5 shows the dependence of the D of Pd(II) with S-MA (0.0005 M for MO4, 0.001 M for MO3, MPh and DO3) on the Cl− concentration. The H+ concentration was fixed at 3 M by using a mixed solution using 3 M HCl and 3 M HClO4. When the Cl− concentration is enhanced, the D values decrease with a slope of about −2. In the 3 M HCl solution, Pd(II) dominantly exists as PdCl42−.12 From the results of the dependencies on the concentration of S-MA, H+, and Cl−, the extraction equilibrium can be represented as

Figure 6. FT-IR spectra of the S-MA compounds. Raw S-MA (solid line), S-MA equilibrated with 3 M HCl (dashed line), and Pd(II) extracted complex with S-MA (dotted line).

Pd(II) in 3 M HCl. The raw S-MA compounds (i.e., before the equilibrium) have an intense peak at around 1645 cm−1 which corresponds to the ν(C−O). The shape and position of this peak has not been changed by equilibrium with the HCl solution, suggesting that the S-MA compounds hardly extract HCl under this condition. As for the Pd(II) extracted complexes, similarly, there is no obvious peak shift. In general, the peak assigned to ν(C−O) is shifted to lower wavenumber when the amide oxygen atom of a monoamide molecule directly coordinates to a metal ion; large shifts to lower wavenumber (Δν(C−O): 70−102 cm−1) are observed for lanthanide(III), thorium(IV), and plutonium(IV) nitrate complexes with the monoamide compounds (N,N-di-2-ethylhexyl-butyramide, N,N-di-2-ethylhexyl-3,3-dimethyl-butyramide, and N,N-di-2-ethylhexyl-iso-butyramide).13 Since each

[PdCl4]2 −(aq) + 2S‐MA (org) ↔ [PdCl 2(S‐MA)2 ](org) + 2Cl−(aq)

(1)

−

Such an extraction equilibrium (two Cl are replaced by two extractants) has been reported for the DAS systems.10 3638

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amide oxygen atom does not bind to the Pd(II) in the extracted complex. 3.5. Difference of Pd(II) Extraction Properties among the S-MA Compounds. In order to explain the different Pd(II) extractabilities among the four S-MA compounds, the donor ability of the sulfur atom should be considered. As for MOPh, the phenyl group adjacent to the sulfur atom, which is a typical electron-withdrawing group, probably weakens the donor ability of the sulfur atom. Moreover, steric hindrance can occur between the bulky phenyl group and the sulfur atom. These are attributable to the lowest Pd(II) extractability in the MOPh system. In contrast, MO4 shows a higher Pd(II) extractability than the other compounds. Such a high extractability in the MO4 system seems to be due to low effect of both the electron-withdrawing and steric hindrance, because the sulfur atom of MO4 is located at a long distance from the amide group which is also an electron-withdrawing one, compared to the other S-MA compounds. These tendencies were also seen in the DAS systems.10 The faster extraction of Pd(II) with the S-MA compounds than that with DHS suggests that the amide group plays an important role in the promotion of the extraction rate taking into account the structural difference between them. The structural studies, however, show that the amide oxygen atom would not bind to the Pd(II) ion. Therefore, the rapid Pd(II) extraction with the S-MA compounds is probably caused by factors other than the coordination of the amide oxygen atom to Pd(II). Baba et al. mentioned that the extraction mechanism of Pd(II) from an HCl solution is significantly affected by the solubility and the interfacial activity of the extractant;15−17 N,Ndioctylglycine, which has a high interfacial activity together with a very high aqueous solubility, shows a higher extraction rate than DHS.17 The compounds containing an N,N-dialkylamide group have a relatively high interfacial activity;18,19 this possibly provides the difference in the apparent extraction rate between the S-MA compounds and DHS. Regarding the difference among the S-MA compounds, DO3 shows the slowest Pd(II) extraction in spite of the same coordination site (3thiapentane) of DO3 as that of MO3. Since DO3 is more hydrophobic than MO3, the faster Pd(II) extraction with MO3 is consistent with the explanation in ref 17. However, we need a further study of the hydrophobicity and interfacial tension of the S-MA compounds in the future to understand the order of the apparent extraction rate.

S-MA−Pd(II) sample contains 0.1 M S-MA and 0.05 M Pd(II), few free S-MA molecules are present in the Pd(II) extracted samples due to the formation of 1:2 Pd(II):S-MA in the extracted complex as already mentioned. Accordingly, the amide oxygen atom of the S-MA molecule would not take part in the direct coordination to the Pd(II). The raw Pd K-edge k3-weighted EXAFS data and the corresponding Fourier transforms for the Pd(II) in 3 M HCl solution (Pd-Aq) and the Pd(II) complex extracted with DO3 (Pd-DO3) are shown in Figure 7. The analytical method and

Figure 7. (A) Pd K-edge k3-weighted EXAFS spectra and (B) the corresponding FTs for Pd(II) in 3 M HCl (Pd-Aq) and the Pd(II) complex extracted with DO3 (Pd-DO3). The phase shifts are not corrected. Experimental data (solid line), theoretical fit (dashed line).

the obtained structural parameters are described in detail in the Supporting Information. For Pd-Aq, the fitting results show that the Pd(II) has four Cl− ions at 2.304(5) Å. The EXAFS spectrum for Pd-DO3 is analogous to that for Pd-Aq as seen in Figure 7; however, the extraction equilibrium (eq 1) is not ionpair, but coordinative. In addition, Pd(II) complexes in solution normally form a square-planar configuration.14 These mean that the inner coordination sphere of Pd(II), which corresponds to the intense peak around 1.9 Å, consists of two Cl− ions and two S/O atoms of the S-MA molecules. Hence, curve fits for this intense peak were performed using three types of combinations of the correlations, (Pd−O and Pd−Cl), (Pd−O, Pd−S and Pd−Cl), and (Pd−S and Pd−Cl) in which the number of the Pd−Cl correlation was fixed at 2, then the obtained structural parameters (especially, the number of the correlation, N) were compared. As a result, the reasonable N value was obtained for (Pd−S and Pd−Cl) N = 1.8 for Pd−S; meanwhile, (Pd−O, Pd−S, and Pd−Cl) N = 0 for Pd−O and N = 1.8 for Pd−S, and (Pd−O and Pd−Cl) N = 4.6 for Pd−O. The other structural parameters for Pd−O and Pd−Cl (Table S1 in the Supporting Information) are also plausible. These verify the FT-IR results suggesting that the

4. CONCLUSIONS In this study, the relationship between the structure of the SMA compounds and their extraction properties of Pd(II) from an HCl solution was investigated by solvent extraction (dependencies of the E% or D of Pd(II) on the extraction time and the concentration of S-MA, H+, and Cl−) and structural analyses with FT-IR and EXAFS spectroscopies. As for the extraction time, all the S-MA compounds rapidly extract Pd(II) compared to the conventional extractant, DHS. The apparent extraction rate of Pd(II) follows the order of MO4 > MO3 > MOPh > DO3 ≫ DHS. A similar dependency of the D of Pd(II) on the concentration of S-MA, H+, and Cl− is seen among the S-MA compounds; the D increases with an increase in the S-MA concentration (slope ≈2); on the other hand, the D decreases with an increase in the Cl− concentration (slope ≈ −2); D does not depend on the H+ concentration. These dependencies of the D indicate that the [PdCl2(S-MA)2] complex is dominant in the organic phase. The magnitude of 3639

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the Pd(II) extraction is in the sequence of MO4 > DO3 ≈ MO3 > MOPh. The FT-IR study shows that the peak assigned to the ν(C−O) is hardly shifted between the raw S-MA compounds and the Pd(II) complex extracted with the S-MA compounds, suggesting that the amide oxygen atom does not participate in the direct coordination to Pd(II). This result was supported by the EXAFS analysis in which the inner coordination sphere of Pd(II) in the extracted complex with DO3 would consist of two chloride ions and two sulfur atoms. From these extraction and structural properties, the order of the extractability of MO4 > DO3 ≈ MO3 > MOPh could be explained by the donor ability of the sulfur atom and steric hindrance around it. Regarding the apparent extraction rate, however, we need a further study to clarify the order.

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(9) Rozen, A. M.; Krupnov, B. V. Dependence of the extraction ability of organic compounds on their structure. Russ. Chem. Rev. 1996, 65, 973. (10) Yuan, C.; Ma, H.; Cao, J.; Zhou, L.; Luo, R. Studies on the structural effect of dialkyl(aryl) sulfides in gold and palladium extraction. Solvent Extr. Ion Exch. 1988, 6, 739. (11) Szczepańska, I.; Borowiak-Resterna, A.; Wiśniewski, M. New pyridinecarboxyamides for rapid extraction of palladium from acidic chloride media. Hydrometallurgy 2003, 68, 159. (12) Colombo, C.; Oates, C. J.; Monhemius, A. J.; Plant, J. A. Complexation of platinum, palladium and rhodium with inorganic ligands in the environment. Geochem.-Explor. Environ. Anal. 2008, 8, 91. (13) Berthon, C.; Chachaty, C. NMR and IR spectrometric studies of monoamide complexes with plutonium(IV) and lanthanide(III) nitrates. Solvent Extr. Ion Exch. 1995, 13, 781. (14) Hellquist, B.; Bengtsson, L. A.; Holmberg, B.; Hedman, B.; Persson, I.; Elding, L. I. Structures of solvated cations of palladium(II) and platinum(II) in dimethyl sulfoxide, acetonitrile and aqueous solution studied by EXAFS and LAXS. Acta Chem. Scand. 1991, 45, 449. (15) Baba, Y.; Inoue, K. The kinetics of solvent extraction of palladium(II) from acidic chloride media with sulfur-containing extractants. Ind. Eng. Chem. Res. 1988, 27, 1613. (16) Baba, Y.; Inoue, K.; Yoshizuka, K.; Furusawa, T. Solvent extraction of palladium(II) with nonchelating oximes with different alkyl chain length. Ind. Eng. Chem. Res. 1990, 29, 2111. (17) Baba, Y.; Iwasaki, M.; Yoshizuka, K.; Inoue, K. Kinetics of palladium(II) extraction with N,N-dioctylglycine. Hydrometallurgy 1993, 33, 83. (18) Prochaska, K.; Cierpiszewski, R.; Jakubiak, A.; BorowiakResterna, A. Co-adsorption and rate of extraction in a copper chloride system containing decanol and hydrophobic pyridine acid derivatives. Solvent Extr. Ion Exch. 2000, 18, 479. (19) Vidyalakshmi, V.; Subramanian, M. S.; Rajeswari, S.; Srinivasan, T. G.; Rao, P. R. V. Interfacial tension studies of N,N-dialkyl amides. Solvent Extr. Ion Exch. 2003, 21, 399.

ASSOCIATED CONTENT

S Supporting Information *

Effect of extraction time using 0.001 M S-MA, Pd loading capacity, and details about the Pd K-edge XAFS study. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-29-861-7895. Fax: +81-29-861-8481. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank Ms. Kaeko Takada for technical assistance. The XAFS measurements have been performed with the approval of the Common-Use Facility Program of Japan Atomic Energy Agency (2010A-E01, 2010B-E01). A part of this study was financially supported by the Industrial Technology Research Grant Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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REFERENCES

(1) Butler, J. Platinum 2012; Johnson Matthey: Hertfordshire, England, 2012. (2) Platinum Today, Johnson Matthey. http://www.platinum. matthey.com/ (accessed Jul 2013). (3) Edward, R. I.; te Riele, W. A. M. Commercial processes for precious metals. In Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hanson, C., Eds.; John Wiley & Sons: New York, 1983; pp 725− 732. (4) Cox, M. Solvent extraction in hydrometallurgy. In Principles and Practices of Solvent Extraction; Rydberg, J., Cox, M., Musikas, C., Choppin, G. R., Eds.; Marcel Dekker, Inc.: New York, 2004, pp 455− 505. (5) Barnes, J. E.; Edwards, J. D. Solvent extraction at Inco’s Acton precious metal refinery. Chem. Ind. 1982, 6, 151. (6) Narita, H.; Tanaka, M.; Morisaku, K.; Abe, T. Rapid separation of palladium(II) from platinum(IV) in hydrochloric acid solution with thiodiglycolamide. Chem. Lett. 2004, 33, 1144. (7) Narita, H.; Tanaka, M.; Morisaku, K. Palladium extraction with N,N,N′,N′-tetra-n-octyl-thiodiglycolamide. Miner. Eng. 2008, 21, 483. (8) Narita, H.; Tanaka, M.; Morisaku, K. Separation of palladium in hydrochloric acid solution with sulfide-containing amide compounds. In Proceedings of ISEC 2008, International Solvent Extraction Conference, Moyer, B. A., Ed.; The Canadian Institute of Mining, Metallurgy and Petroleum: Montreal, 2008, Vol. II; pp 1445−1450. 3640

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