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Extraction and separation of gold (I) cyanide in polyethylene glycol-based aqueous biphasic systems
Hydrometallurgy 62 Ž2001. 41–46 www.elsevier.comrlocaterhydromet
Extraction and separation of gold žI/ cyanide in polyethylene glycol-based aqueous biphasic systems Tianxi Zhang a,b, Wenjun Li a,b, Weijin Zhou a,b, Hongcheng Gao a,b, Jinguang Wu a,b,) , Guangxian Xu a,b, Jing Chen c , Huizhou Liu d , Jiayong Chen d a
State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking UniÕersity, Beijing 100871, China b The UniÕersity of Hong Kong Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking UniÕersity, Beijing 100871, China c Institute of Precious Metals, Kunming 650221, China d Institute of Chemical Metallurgy, Chinese Academy of Sciences, Beijing 100080, China Received 29 August 2000; received in revised form 3 May 2001; accepted 18 May 2001
Abstract The extraction of gold ŽI. cyanide in polyethylene glycol-based aqueous biphasic systems has been investigated. Almost all of gold ŽI. cyanide Ž) 96%. was transferred from salt-rich phase into the polyethylene glycol ŽPEG.-rich phase without addition of any extractant. No significant influence of solution pH on gold ŽI. cyanide extraction was observed. The aqueous biphasic systems have high capability of extraction. The gold ŽI. cyanide in the PEG-rich phase can be easily reduced by addition of zinc. The PEG-rich phase could be reused for extraction of gold ŽI. cyanide without decrease of extraction capability. The PEGrsalt aqueous biphasic systems may provide a potential new technique for gold separation from cyanide solutions because all components of systems are virtually nontoxic and nonflammable. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Extraction; Gold ŽI. cyanide; Aqueous biphasic system; Polyethylene glycol; Partition
1. Introduction Gold is usually separated from alkaline cyanide solution by carbon adsorption or ion exchange. Traditional solvent extraction has been proven to be a useful technology for selective removal and recovery of metal ions from aqueous solutions with organic ) Corresponding author. State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. Tel.: q86-10-6275-7951; fax: q86-10-6275-1708. E-mail address: [email protected] ŽJ. Wu..
solvent and aqueous solution as two immiscible phases. Solvent extraction has not yet known any practical applications from such alkaline solutions although it has the potential to offer improvements over the well-established gold recovery processes. During last few years, there has been a renewed interest in the application of solvent extraction to the recovery of gold using various extractants ŽMooiman and Miller, 1983, 1991; Alguacil and Caravaca, 1996; Alguacil et al., 1994, 1997; Ma et al., 1999.. The aqueous biphasic systems have been extensively studied to separate biomolecules such as protein, nucleic acid, etc. ŽAlbertsson, 1986; Walter et
0304-386Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 1 . 0 0 1 7 9 - 7
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T. Zhang et al.r Hydrometallurgy 62 (2001) 41–46
al., 1985; Zaslavsky, 1995.. Many workers have studied polymer phase systems only, such as polyethylene glycol ŽPEG.rdextran. PEGrdextran systems appear to involve certain economical and operational limitations, which may be overcome by use of PEGrsalt phase systems. PEGrsalt systems also can offer a wide range of different hydrophobicity between two phases of these systems. PEG-based aqueous biphasic systems have all practical advantages of conventional liquid–liquid extraction and also have a number of unique advantages such as nontoxic, nonflammable and inexpensive components of systems. It seems that little attention has been paid to the partition of inorganic compounds in aqueous biphasic systems ŽRogers et al., 1993; Rogers and Zhang, 1997.. This work has attempted to study the application of PEGrsalt aqueous biphasic systems to the partition of gold ŽI. cyanide. Various parameters such as concentrations of gold ŽI., salt and PEG, solution pH on the partition of gold ŽI. were studied. 2. Materials and methods Polyethylene glycol ŽPEG2000. was purchased from Huamei, China. Beijing Chemical Reagent supplied Na 2 SO4 ŽAR., K 2 CO 3 ŽAR., Na 3 PO4 ŽAR.. KAuŽCN. 2 Ž) 99%. was prepared according to the literature ŽFischer et al., 1994.. Other chemicals were all commercially available reagents of analytical grade. 2.1. Partition of KAu(CN)2 PEGrsalt Žtypically Na 2 SO4 . aqueous biphasic systems were utilized in the partition of KAuŽCN. 2 . Stock solutions are typically prepared on a weight percentage for PEG 33.3% Žwrw. and Na 2 SO4 16.7% Žwrw.. Solution pH was adjusted to about 10.5 Žtypically.. Au ŽI. partition was carried out by shaking equal volumes of each stock solution for 10 min. The mixtures were then centrifuged to reach a clear separation of two phases. 2.2. Determination of Au (I) The Au ŽI. concentration both in top and bottom phases has been determined with 198AuŽCN.y 2 tracer. The salt stock solutions were prepared by addition of
very little 198AuŽCN.y 2 as tracer into certain KAu ŽCN. 2 concentration solutions wAu ŽI. s 2.0 grL, typicallyx. After separation of two phases, equal aliquots of each phase Žtypically 0.4 mL. are submitted for !-radiometric measurements. The recovery of Au ŽI. is calculated with salt stock solutions as standard sample. The radiometric distribution ratio Ž D . is defined as the count per minute Žcpm. in the upper Žusually PEG-rich. phase, divided by the cpm in the lower salt-rich phase. As given in the experimental method, this is equivalent to the ratio of Au ŽI. concentrations in the upper and lower phases Ždistribution ratio.. Experiments were carried out in triplicate with the experimental errors of less than 5.0%. 3. Results and discussion Spivakov et al. suggested Žfor the first time. the possible application for metal ion partitioning in aqueous biphasic systems ŽZvarova et al., 1984; Shkinev et al., 1985; Molochnikova et al., 1992.. The aqueous biphasic systems utilized for metal ion partitioning have been classified into three categories ŽRogers et al., 1993; Rogers and Zhang, 1997.. In the first category, the metal ion is transferred directly into the PEG-rich phase from the salt-rich phase without any added extractant. The most important discovery thus far in this category is the partitioning of the pertechnetate anions ŽTcO4y. ŽRogers et al., 1995, 1996.. A second category produces a metal complex that partitions to the PEG-rich phase from the salt-rich phase with the addition of an inorganic anion such as halide anions or SCNy ŽRogers and Zhang, 1997; Rogers et al., 1996.. The third category utilizes water-soluble extractants that coordinates the metal ion ŽAguinaga-Diaz and Guzman, 1996; Sun and Luo, 1990.. In this work, the partitioning of AuŽCN.y has been investigated in the PEGrsalt 2 aqueous biphasic systems without any added extractant. 3.1. Effect of Au(CN)2y concentration on the distribution ratio of Au(CN)2y Fig. 1 shows the effect of AuŽCN.y 2 concentration in Na 2 SO4 stock solution on distribution ratio of Ž.Ž . AuŽCN.y 2 . The distribution ratio of Au I ; 45 has no significantly change with an increase of Au ŽI.
T. Zhang et al.r Hydrometallurgy 62 (2001) 41–46
43
3.2. Effect of pH of Na2 SO4 stock solution on distribution ratio of Au(CN)2y
Fig. 1. Effect of AuŽCN.y 2 concentration in Na 2 SO4 stock soluŽ . tion on distribution ratio of AuŽCN.y 2 ; PEG 33.3% wrw r Na 2 SO4 16.7% Žwrw., pH f10.5.
The cyanide solution in leaching of Au ŽI. must be adjusted to alkali ŽpH ) 10, typically. to prevent escape of the poison HCN due to the presence of free cyanide ions in the solution Mooiman and Miller, 1983. Fig. 2 shows the effect of pH of Na 2 SO4 stock solution on the distribution ratio of AuŽCN.y 2 . The distribution ratio of Au ŽI. does not change very much with the change of solution pH from 3.7 to 12.9. Almost all of Au ŽI. Ž; 97.7%. has been extracted into PEG-rich phase from salt-rich phase. This demonstrated that the PEGrNa 2 SO4 aqueous biphasic systems can be used in the extraction of Au ŽI. from the alkaline cyanide solution. 3.3. Effect of PEG concentration in stock solution on distribution ratio of Au(CN)2y
concentration from 2.0 to 57.7 grL. Almost all of Au ŽI. has been directly transferred into the PEG-rich phase from the salt-rich phase without any added extractant. No significant trend for decrease of distribution ratio of Au ŽI. was observed when Au ŽI. concentration reached 57.7 grL, which indicated that PEGrNa 2 SO4 system had the high capability of Au ŽI. extraction. Most metal ions tend to stay in the salt-rich phase with distribution ratio of much less than 1.0 without any added extractant since they are highly hydrated and interact strongly with water ŽRogers and Zhang, 1997.. Rogers et al. ŽRogers et al., 1993, 1995, 1996, 1997; Rogers and Zhang, 1997. reported that the pertechnetate anion partitions quantitatively to the PEG-rich phase in a variety of PEGrsalt systems without any added extractant. This is an important discovery of metal ion partitioning in aqueous biphasic systems. Metal ion partitioning appears to be governed by the interactions of the ions with water ŽRogers and Zhang, 1997.. Thermodynamic parameters such as ion’s Gibbs free energy of hydration Ž DG hyd . were utilized to explain mechanism of the partition of pertechnetate anion ŽRogers and Zhang, 1997.. In this work, AuŽCN.y has a fairly high 2 affinity for the PEG-rich phase, which could be another discovery of metal ion partitioning in aqueous biphasic systems. The mechanism of the AuŽCN.y 2 partitioning needs further investigation.
The increase of incompatibility between the PEG-rich and salt-rich phases could enhance the affinity of a solute for a particular phase ŽRogers and Zhang, 1997; Rogers et al., 1995.. The phase incompatibility can be manipulated in several ways such as PEG concentration, salt concentration and types of salt. As shown in Fig. 3, the distribution ratio of Au ŽI. is influenced by the PEG concentration. The distribution of Au ŽI. increases significantly with the increase of PEG concentration. The phase incompatibility increases with increasing of PEG or salt concentration, which results to the increase of the difference of
Fig. 2. Effect of pH of Na 2 SO4 stock solution on distribution ratio Ž. Ž . of AuŽCN.y 2 ; Au I s1.4 mgrL, PEG 33.3% wrw rNa 2 SO4 16.7% Žwrw..
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T. Zhang et al.r Hydrometallurgy 62 (2001) 41–46
Fig. 3. Effect of PEG concentration in stock solution on distribuŽ. tion ratio of AuŽCN.y 2 ; Au I s1.2 mgrL, PEGrNa 2 SO4 16.7% Žwrw., pH f10.5.
PEG or salt concentration in each phase. Since Au ŽI. prefers the PEG-rich phase, the distribution ratio of Au ŽI. increases as the phase incompatibility increases. 3.4. Effect of Na2 SO4 concentration in stock solution on distribution ratio of Au(CN)2y Fig. 4 shows the effect of Na 2 SO4 concentration in stock solution on aqueous biphasic phase distribution ratio of AuŽCN.y 2 . The distribution ratio of Au ŽI. increases significantly with the increase of Na 2 SO4 concentration from 13.1% to 23.1% Žwrw.. Similar to the effect of PEG concentration, the distribution ratio of Au ŽI. increases as the phase incompatibility increases.
Fig. 4. Effect of Na 2 SO4 concentration in stock solution on Ž. distribution ratio of AuŽCN.y 2 ; Au I s1.2 mgrL, PEG 33.3% Žwrw.rNa 2 SO4 , pH f10.5.
Fig. 5. Effect of the type of two-phase forming salts on distribuŽ. Ž . tion ratio of AuŽCN.y 2 ; Au I s 2.0 grL, PEG 33.3% wrw r Na 2 SO4 or K 2 CO 3 or Na 3 PO4 , pH f10.5.
3.5. Effect of the type of two-phase forming salts on distribution ratio of Au(CN)2y The effect of three different salts forming two phases ŽNa 2 SO4 , K 2 CO 3 , Na 3 PO4 . on distribution ratio of AuŽCN.y 2 was shown in Fig. 5. The distribution ratio of Au ŽI. increases with the increase of salt concentration in the three salt systems. From the distribution ratio of Au ŽI., the relative salting out ability followed the order: Na 2 SO4 ) Na 3 PO4 ) K 2 CO 3 . The percentage of Au ŽI. extraction was in the range of 93.4–99.9% for the different concentrations of two-phase forming salts studied. This demonstrated that almost all of Au ŽI. has been extracted into PEG-rich phase with no significant influence of phase forming salt tested.
Fig. 6. Effect of average molecular weight of PEG on distribution Ž. Ž . ratio of AuŽCN.y 2 ; Au I s 2.0 grL, PEG 33.3% wrw rNa 2 SO4 16.7% Žwrw., pH f10.5.
T. Zhang et al.r Hydrometallurgy 62 (2001) 41–46
3.6. Effect of PEG aÕerage molecular weight on distribution ratio of Au(CN)2y The molecular weight of PEG used could also influence the observed distribution ratio. Fig. 6 shows the data on the partitioning of Au ŽI. in PEGrsalt system. For a given weight percent of PEG Ž33.3%., the number of PEG molecules present decrease with increasing of average molecular weight; however, the number of ethylene oxide units in solution remains approximately the same. As is now expected, there is also a general trend that the distribution ratio increases with the increase of molecular weight of PEG. This follows from phase diagram data that for a given phase forming salt, it requires less salt to salting-out from higher molecular weight PEGs. Thus, phase incompatibility is higher for higher molecular weight PEGs, the difference of PEG concentrations between the phases is greater, and therefore, the distribution ratio is higher.
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Table 2 Reuse of PEG-rich phase after the reduction of AuŽCN.y 2 Extraction time
Distribution ratio
Au ŽI. extraction Ž%.
1 2a
39.4 43.2
97.6 97.8
a The PEG-rich phase has been reused after the reduction of AuŽCN.y 2.
the reuse of the PEG-rich phase for AuŽCN.y 2 extraction. There is practically no decrease of the amount Ž .y of AuŽCN.y 2 extracted after Au CN 2 in PEG-rich phase reduced with zinc. This demonstrated that the PEG-rich phase could be reused for extraction of AuŽCN.y 2.
4. Conclusions 3.7. The reduction of Au(CN)2y in the PEG-rich phase The AuŽCN.y 2 in aqueous phase can be reduced by a reductant. Table 1 shows the reduction of AuŽCN.y 2 using zinc with the effect of the presence of PEG also studied. There is not much difference of AuŽCN.y 2 reduction with the presence of PEG. AlŽ; 97%. can be reduced to elemost all AuŽCN.y 2 mental gold. This indicates that metal gold can be easily isolated from the PEG-rich phase. 3.8. The reuse of PEG-rich phase for Au(CN)2y extraction
Polyethylene glycol-based aqueous biphasic systems can be used to partition AuŽCN.y 2 in alkaline Ž; 96%. transfers solution. Almost all of AuŽCN.y 2 to the PEG-rich phase from salt-rich phase. The distribution ratio of AuŽCN.y is about 40. The 2 AuŽCN.y 2 of PEG-rich phase can be easily reduced or isolated by addition of zinc. The PEG-rich phase could also be reused for the extraction of AuŽCN.y 2, which could lead to the continuous operation for the extraction of AuŽCN.y 2. The PEGrsalt aqueous biphasic systems may provide a potential technique for gold separation from alkaline cyanide solution with cleaner and safer advantages.
For continuous operation, the PEGrsalt aqueous biphasic systems should be reused. Table 2 shows Acknowledgements Table 1 Reduction of AuŽCN.y 2 using zinc comparing the presence of PEG or not Sample
PEG Ž%.
Au ŽI. reduction Ž%.
1 2
0 33.3
98.1 96.5
We acknowledge the financial support of the Collaboration Project between Yunnan Province and Peking University ŽD9808K., the National Natural Science Foundation of China Ž39730160. and the State Key Project for Fundamental Research ŽG1998061307..
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References Aguinaga-Diaz, P.A., Guzman, R.Z., 1996. Affinity partitioning of metal ions in aqueous polyethylene glycolrsalt two-phase systems with PEG-modified chelators. Sep. Sci. Technol. 31, 1483–1499. ˚ 1986. Partition of Cell Particles and MacroAlbertsson, P.A., molecules. 3rd edn. Wiley, New York. Alguacil, F.J., Caravaca, C., 1996. Synergistic extraction of gold ŽI. cyanide with the primary amine Primene JMT and the phosphine oxide Cyanex 921. Hydrometallurgy 42, 197–203. Alguacil, F.J., Caravaca, C., Martinez, S., Cobo, A., 1994. The phosphine oxides Cyanex 923 and Cyanex 925 as extractants for gold ŽI. cyanide aqueous solutions. Hydrometallurgy 36, 369–384. Alguacil, F.J., Caravaca, C., Mochon, J., Sastre, A., 1997. Solvent extraction of AuŽCN.y 2 with mixtures of the amine Primene JMT and the phosphine oxide Cyanex 923. Hydrometallurgy 44, 359–362. Fischer, D., Hones, G., Kreuzbicher, I., Neu-Becher, U., Schwa¨ ger, B., 1994. Gold. In: Keim, R., Schwager, B. ŽEds.., 1994. Gmelin Handbook of Inorganic and Organometallic Chemistry, vol. 62. Springer-Verlag, Berlin, pp. 290–291 ŽSuppl. B2.. Ma, G., Yan, W.F., Hu, T.D., Chen, J., Yan, C.H., Gao, H.C., Wu, J.G., Xu, G.X., 1999. FTIR and EXAFS investigations of microstructures of gold solvent extraction: hydrogen bonding between modifier and AuŽCN.y 2 . Phys. Chem. Chem. Phys. 1, 5215–5221. Molochnikova, N.P., Shkinev, V.M., Myasoedov, B.F., 1992. Two-phase aqueous systems based on polyŽethylene glycol. for extraction separation of actinides in various media. Solvent Extr. Ion Exch. 10, 697–712. Mooiman, M.B., Miller, J.D., 1983. The solvent extraction of gold from aurocyanide solutions. Proc. Int. Solvent Extr. Conf. ISEC ’83. AIChE, New York, pp. 530–531. Mooiman, M.B., Miller, J.D., 1991. The chemistry of gold solvent extraction from cyanide solution by solvating extractants. Hydrometallurgy 27, 29–33. Rogers, R.D., Zhang, J.H., 1997. New technologies for metal ion
separations polyethylene glycol-based aqueous biphasic systems and aqueous biphasic extraction chromatography. In: Marinsky, J., Marcus, Y. ŽEds.., Ion Exchange and Solvent Extraction, vol. 13. Marcel Dekker, New York, pp. 141–193. Rogers, R.D., Bond, A.H., Bauer, C.B., 1993. Metal ion separations in polyethylene glycol-based aqueous biphasic systems. Sep. Sci. Technol. 28, 1091–1126. Rogers, R.D., Zhang, J.H., Bond, A.H., Bauer, C.B., Jezl, M.L., Roden, D.M., 1995. Selective and quantitative partitioning of pertechnetate in polyethylene glycol-based aqueous biphasic systems. Solvent Extr. Ion Exch. 13, 665–688. Rogers, R.D., Bond, A.H., Bauer, C.B., Zhang, J.H., Griffin, S.T., 1996. Metal ion separations in polyethylene glycol-based aqueous biphasic systems: correlation of partitioning behavior with available thermodynamic hydration data. J. Chromatogr. 680, 221–229. Rogers, R.D., Bond, A.H., Zhang, J.H., Horwitz, E.P., 1997. New technetium-99m generator technologies utilizing polyethylene glycol-based aqueous biphasic systems. Sep. Sci. Technol. 32, 867–882. Shkinev, V.M., Molochnikova, N.P., Zvarova, T.I., Spivakov, B.Y., Myasoedov, B.F., Zolotov, Y.A., 1985. Extraction of complexes of lanthanides and actinides with arsenazo III in an ammonium sulfate–polyŽethylene glycol. –water two-phase system. J. Radioanal. Nucl. Chem. 88, 115–120. Sun, X.M., Luo, J.Q., 1990. Liquid–liquid extraction behavior of metals with chrome azurol S as extractant in PEG– ŽNH 4 . 2 SO4 –H 2 O system. Huaxue Shijie 12, 367–369 Žin Chinese.. Walter, H., Brooks, D.F., Fisher, D., 1985. Partitioning in Aqueous Two-Phase Systems: Theory, Methods, Uses and Applications to Biotechnology. Academic Press, New York. Zaslavsky, B.Y., 1995. Aqueous Two-Phase Partitioning: Physical Chemistry and Bioanalytical Applications. Marcel Dekker, New York. Zvarova, T.I., Shkinev, V.M., Vorob’eva, G.A., Spivakov, B.Y., Zolotov, Y.A., 1984. Liquid–liquid extraction in the absence of usual organic solvents: application of two-phase aqueous systems based on a water-soluble polymer. Mikrochim. Acta III, 449–458.
Extraction and separation of gold žI/ cyanide in polyethylene glycol-based aqueous biphasic systems Tianxi Zhang a,b, Wenjun Li a,b, Weijin Zhou a,b, Hongcheng Gao a,b, Jinguang Wu a,b,) , Guangxian Xu a,b, Jing Chen c , Huizhou Liu d , Jiayong Chen d a
State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking UniÕersity, Beijing 100871, China b The UniÕersity of Hong Kong Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking UniÕersity, Beijing 100871, China c Institute of Precious Metals, Kunming 650221, China d Institute of Chemical Metallurgy, Chinese Academy of Sciences, Beijing 100080, China Received 29 August 2000; received in revised form 3 May 2001; accepted 18 May 2001
Abstract The extraction of gold ŽI. cyanide in polyethylene glycol-based aqueous biphasic systems has been investigated. Almost all of gold ŽI. cyanide Ž) 96%. was transferred from salt-rich phase into the polyethylene glycol ŽPEG.-rich phase without addition of any extractant. No significant influence of solution pH on gold ŽI. cyanide extraction was observed. The aqueous biphasic systems have high capability of extraction. The gold ŽI. cyanide in the PEG-rich phase can be easily reduced by addition of zinc. The PEG-rich phase could be reused for extraction of gold ŽI. cyanide without decrease of extraction capability. The PEGrsalt aqueous biphasic systems may provide a potential new technique for gold separation from cyanide solutions because all components of systems are virtually nontoxic and nonflammable. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Extraction; Gold ŽI. cyanide; Aqueous biphasic system; Polyethylene glycol; Partition
1. Introduction Gold is usually separated from alkaline cyanide solution by carbon adsorption or ion exchange. Traditional solvent extraction has been proven to be a useful technology for selective removal and recovery of metal ions from aqueous solutions with organic ) Corresponding author. State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. Tel.: q86-10-6275-7951; fax: q86-10-6275-1708. E-mail address: [email protected] ŽJ. Wu..
solvent and aqueous solution as two immiscible phases. Solvent extraction has not yet known any practical applications from such alkaline solutions although it has the potential to offer improvements over the well-established gold recovery processes. During last few years, there has been a renewed interest in the application of solvent extraction to the recovery of gold using various extractants ŽMooiman and Miller, 1983, 1991; Alguacil and Caravaca, 1996; Alguacil et al., 1994, 1997; Ma et al., 1999.. The aqueous biphasic systems have been extensively studied to separate biomolecules such as protein, nucleic acid, etc. ŽAlbertsson, 1986; Walter et
0304-386Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 3 8 6 X Ž 0 1 . 0 0 1 7 9 - 7
42
T. Zhang et al.r Hydrometallurgy 62 (2001) 41–46
al., 1985; Zaslavsky, 1995.. Many workers have studied polymer phase systems only, such as polyethylene glycol ŽPEG.rdextran. PEGrdextran systems appear to involve certain economical and operational limitations, which may be overcome by use of PEGrsalt phase systems. PEGrsalt systems also can offer a wide range of different hydrophobicity between two phases of these systems. PEG-based aqueous biphasic systems have all practical advantages of conventional liquid–liquid extraction and also have a number of unique advantages such as nontoxic, nonflammable and inexpensive components of systems. It seems that little attention has been paid to the partition of inorganic compounds in aqueous biphasic systems ŽRogers et al., 1993; Rogers and Zhang, 1997.. This work has attempted to study the application of PEGrsalt aqueous biphasic systems to the partition of gold ŽI. cyanide. Various parameters such as concentrations of gold ŽI., salt and PEG, solution pH on the partition of gold ŽI. were studied. 2. Materials and methods Polyethylene glycol ŽPEG2000. was purchased from Huamei, China. Beijing Chemical Reagent supplied Na 2 SO4 ŽAR., K 2 CO 3 ŽAR., Na 3 PO4 ŽAR.. KAuŽCN. 2 Ž) 99%. was prepared according to the literature ŽFischer et al., 1994.. Other chemicals were all commercially available reagents of analytical grade. 2.1. Partition of KAu(CN)2 PEGrsalt Žtypically Na 2 SO4 . aqueous biphasic systems were utilized in the partition of KAuŽCN. 2 . Stock solutions are typically prepared on a weight percentage for PEG 33.3% Žwrw. and Na 2 SO4 16.7% Žwrw.. Solution pH was adjusted to about 10.5 Žtypically.. Au ŽI. partition was carried out by shaking equal volumes of each stock solution for 10 min. The mixtures were then centrifuged to reach a clear separation of two phases. 2.2. Determination of Au (I) The Au ŽI. concentration both in top and bottom phases has been determined with 198AuŽCN.y 2 tracer. The salt stock solutions were prepared by addition of
very little 198AuŽCN.y 2 as tracer into certain KAu ŽCN. 2 concentration solutions wAu ŽI. s 2.0 grL, typicallyx. After separation of two phases, equal aliquots of each phase Žtypically 0.4 mL. are submitted for !-radiometric measurements. The recovery of Au ŽI. is calculated with salt stock solutions as standard sample. The radiometric distribution ratio Ž D . is defined as the count per minute Žcpm. in the upper Žusually PEG-rich. phase, divided by the cpm in the lower salt-rich phase. As given in the experimental method, this is equivalent to the ratio of Au ŽI. concentrations in the upper and lower phases Ždistribution ratio.. Experiments were carried out in triplicate with the experimental errors of less than 5.0%. 3. Results and discussion Spivakov et al. suggested Žfor the first time. the possible application for metal ion partitioning in aqueous biphasic systems ŽZvarova et al., 1984; Shkinev et al., 1985; Molochnikova et al., 1992.. The aqueous biphasic systems utilized for metal ion partitioning have been classified into three categories ŽRogers et al., 1993; Rogers and Zhang, 1997.. In the first category, the metal ion is transferred directly into the PEG-rich phase from the salt-rich phase without any added extractant. The most important discovery thus far in this category is the partitioning of the pertechnetate anions ŽTcO4y. ŽRogers et al., 1995, 1996.. A second category produces a metal complex that partitions to the PEG-rich phase from the salt-rich phase with the addition of an inorganic anion such as halide anions or SCNy ŽRogers and Zhang, 1997; Rogers et al., 1996.. The third category utilizes water-soluble extractants that coordinates the metal ion ŽAguinaga-Diaz and Guzman, 1996; Sun and Luo, 1990.. In this work, the partitioning of AuŽCN.y has been investigated in the PEGrsalt 2 aqueous biphasic systems without any added extractant. 3.1. Effect of Au(CN)2y concentration on the distribution ratio of Au(CN)2y Fig. 1 shows the effect of AuŽCN.y 2 concentration in Na 2 SO4 stock solution on distribution ratio of Ž.Ž . AuŽCN.y 2 . The distribution ratio of Au I ; 45 has no significantly change with an increase of Au ŽI.
T. Zhang et al.r Hydrometallurgy 62 (2001) 41–46
43
3.2. Effect of pH of Na2 SO4 stock solution on distribution ratio of Au(CN)2y
Fig. 1. Effect of AuŽCN.y 2 concentration in Na 2 SO4 stock soluŽ . tion on distribution ratio of AuŽCN.y 2 ; PEG 33.3% wrw r Na 2 SO4 16.7% Žwrw., pH f10.5.
The cyanide solution in leaching of Au ŽI. must be adjusted to alkali ŽpH ) 10, typically. to prevent escape of the poison HCN due to the presence of free cyanide ions in the solution Mooiman and Miller, 1983. Fig. 2 shows the effect of pH of Na 2 SO4 stock solution on the distribution ratio of AuŽCN.y 2 . The distribution ratio of Au ŽI. does not change very much with the change of solution pH from 3.7 to 12.9. Almost all of Au ŽI. Ž; 97.7%. has been extracted into PEG-rich phase from salt-rich phase. This demonstrated that the PEGrNa 2 SO4 aqueous biphasic systems can be used in the extraction of Au ŽI. from the alkaline cyanide solution. 3.3. Effect of PEG concentration in stock solution on distribution ratio of Au(CN)2y
concentration from 2.0 to 57.7 grL. Almost all of Au ŽI. has been directly transferred into the PEG-rich phase from the salt-rich phase without any added extractant. No significant trend for decrease of distribution ratio of Au ŽI. was observed when Au ŽI. concentration reached 57.7 grL, which indicated that PEGrNa 2 SO4 system had the high capability of Au ŽI. extraction. Most metal ions tend to stay in the salt-rich phase with distribution ratio of much less than 1.0 without any added extractant since they are highly hydrated and interact strongly with water ŽRogers and Zhang, 1997.. Rogers et al. ŽRogers et al., 1993, 1995, 1996, 1997; Rogers and Zhang, 1997. reported that the pertechnetate anion partitions quantitatively to the PEG-rich phase in a variety of PEGrsalt systems without any added extractant. This is an important discovery of metal ion partitioning in aqueous biphasic systems. Metal ion partitioning appears to be governed by the interactions of the ions with water ŽRogers and Zhang, 1997.. Thermodynamic parameters such as ion’s Gibbs free energy of hydration Ž DG hyd . were utilized to explain mechanism of the partition of pertechnetate anion ŽRogers and Zhang, 1997.. In this work, AuŽCN.y has a fairly high 2 affinity for the PEG-rich phase, which could be another discovery of metal ion partitioning in aqueous biphasic systems. The mechanism of the AuŽCN.y 2 partitioning needs further investigation.
The increase of incompatibility between the PEG-rich and salt-rich phases could enhance the affinity of a solute for a particular phase ŽRogers and Zhang, 1997; Rogers et al., 1995.. The phase incompatibility can be manipulated in several ways such as PEG concentration, salt concentration and types of salt. As shown in Fig. 3, the distribution ratio of Au ŽI. is influenced by the PEG concentration. The distribution of Au ŽI. increases significantly with the increase of PEG concentration. The phase incompatibility increases with increasing of PEG or salt concentration, which results to the increase of the difference of
Fig. 2. Effect of pH of Na 2 SO4 stock solution on distribution ratio Ž. Ž . of AuŽCN.y 2 ; Au I s1.4 mgrL, PEG 33.3% wrw rNa 2 SO4 16.7% Žwrw..
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T. Zhang et al.r Hydrometallurgy 62 (2001) 41–46
Fig. 3. Effect of PEG concentration in stock solution on distribuŽ. tion ratio of AuŽCN.y 2 ; Au I s1.2 mgrL, PEGrNa 2 SO4 16.7% Žwrw., pH f10.5.
PEG or salt concentration in each phase. Since Au ŽI. prefers the PEG-rich phase, the distribution ratio of Au ŽI. increases as the phase incompatibility increases. 3.4. Effect of Na2 SO4 concentration in stock solution on distribution ratio of Au(CN)2y Fig. 4 shows the effect of Na 2 SO4 concentration in stock solution on aqueous biphasic phase distribution ratio of AuŽCN.y 2 . The distribution ratio of Au ŽI. increases significantly with the increase of Na 2 SO4 concentration from 13.1% to 23.1% Žwrw.. Similar to the effect of PEG concentration, the distribution ratio of Au ŽI. increases as the phase incompatibility increases.
Fig. 4. Effect of Na 2 SO4 concentration in stock solution on Ž. distribution ratio of AuŽCN.y 2 ; Au I s1.2 mgrL, PEG 33.3% Žwrw.rNa 2 SO4 , pH f10.5.
Fig. 5. Effect of the type of two-phase forming salts on distribuŽ. Ž . tion ratio of AuŽCN.y 2 ; Au I s 2.0 grL, PEG 33.3% wrw r Na 2 SO4 or K 2 CO 3 or Na 3 PO4 , pH f10.5.
3.5. Effect of the type of two-phase forming salts on distribution ratio of Au(CN)2y The effect of three different salts forming two phases ŽNa 2 SO4 , K 2 CO 3 , Na 3 PO4 . on distribution ratio of AuŽCN.y 2 was shown in Fig. 5. The distribution ratio of Au ŽI. increases with the increase of salt concentration in the three salt systems. From the distribution ratio of Au ŽI., the relative salting out ability followed the order: Na 2 SO4 ) Na 3 PO4 ) K 2 CO 3 . The percentage of Au ŽI. extraction was in the range of 93.4–99.9% for the different concentrations of two-phase forming salts studied. This demonstrated that almost all of Au ŽI. has been extracted into PEG-rich phase with no significant influence of phase forming salt tested.
Fig. 6. Effect of average molecular weight of PEG on distribution Ž. Ž . ratio of AuŽCN.y 2 ; Au I s 2.0 grL, PEG 33.3% wrw rNa 2 SO4 16.7% Žwrw., pH f10.5.
T. Zhang et al.r Hydrometallurgy 62 (2001) 41–46
3.6. Effect of PEG aÕerage molecular weight on distribution ratio of Au(CN)2y The molecular weight of PEG used could also influence the observed distribution ratio. Fig. 6 shows the data on the partitioning of Au ŽI. in PEGrsalt system. For a given weight percent of PEG Ž33.3%., the number of PEG molecules present decrease with increasing of average molecular weight; however, the number of ethylene oxide units in solution remains approximately the same. As is now expected, there is also a general trend that the distribution ratio increases with the increase of molecular weight of PEG. This follows from phase diagram data that for a given phase forming salt, it requires less salt to salting-out from higher molecular weight PEGs. Thus, phase incompatibility is higher for higher molecular weight PEGs, the difference of PEG concentrations between the phases is greater, and therefore, the distribution ratio is higher.
45
Table 2 Reuse of PEG-rich phase after the reduction of AuŽCN.y 2 Extraction time
Distribution ratio
Au ŽI. extraction Ž%.
1 2a
39.4 43.2
97.6 97.8
a The PEG-rich phase has been reused after the reduction of AuŽCN.y 2.
the reuse of the PEG-rich phase for AuŽCN.y 2 extraction. There is practically no decrease of the amount Ž .y of AuŽCN.y 2 extracted after Au CN 2 in PEG-rich phase reduced with zinc. This demonstrated that the PEG-rich phase could be reused for extraction of AuŽCN.y 2.
4. Conclusions 3.7. The reduction of Au(CN)2y in the PEG-rich phase The AuŽCN.y 2 in aqueous phase can be reduced by a reductant. Table 1 shows the reduction of AuŽCN.y 2 using zinc with the effect of the presence of PEG also studied. There is not much difference of AuŽCN.y 2 reduction with the presence of PEG. AlŽ; 97%. can be reduced to elemost all AuŽCN.y 2 mental gold. This indicates that metal gold can be easily isolated from the PEG-rich phase. 3.8. The reuse of PEG-rich phase for Au(CN)2y extraction
Polyethylene glycol-based aqueous biphasic systems can be used to partition AuŽCN.y 2 in alkaline Ž; 96%. transfers solution. Almost all of AuŽCN.y 2 to the PEG-rich phase from salt-rich phase. The distribution ratio of AuŽCN.y is about 40. The 2 AuŽCN.y 2 of PEG-rich phase can be easily reduced or isolated by addition of zinc. The PEG-rich phase could also be reused for the extraction of AuŽCN.y 2, which could lead to the continuous operation for the extraction of AuŽCN.y 2. The PEGrsalt aqueous biphasic systems may provide a potential technique for gold separation from alkaline cyanide solution with cleaner and safer advantages.
For continuous operation, the PEGrsalt aqueous biphasic systems should be reused. Table 2 shows Acknowledgements Table 1 Reduction of AuŽCN.y 2 using zinc comparing the presence of PEG or not Sample
PEG Ž%.
Au ŽI. reduction Ž%.
1 2
0 33.3
98.1 96.5
We acknowledge the financial support of the Collaboration Project between Yunnan Province and Peking University ŽD9808K., the National Natural Science Foundation of China Ž39730160. and the State Key Project for Fundamental Research ŽG1998061307..
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References Aguinaga-Diaz, P.A., Guzman, R.Z., 1996. Affinity partitioning of metal ions in aqueous polyethylene glycolrsalt two-phase systems with PEG-modified chelators. Sep. Sci. Technol. 31, 1483–1499. ˚ 1986. Partition of Cell Particles and MacroAlbertsson, P.A., molecules. 3rd edn. Wiley, New York. Alguacil, F.J., Caravaca, C., 1996. Synergistic extraction of gold ŽI. cyanide with the primary amine Primene JMT and the phosphine oxide Cyanex 921. Hydrometallurgy 42, 197–203. Alguacil, F.J., Caravaca, C., Martinez, S., Cobo, A., 1994. The phosphine oxides Cyanex 923 and Cyanex 925 as extractants for gold ŽI. cyanide aqueous solutions. Hydrometallurgy 36, 369–384. Alguacil, F.J., Caravaca, C., Mochon, J., Sastre, A., 1997. Solvent extraction of AuŽCN.y 2 with mixtures of the amine Primene JMT and the phosphine oxide Cyanex 923. Hydrometallurgy 44, 359–362. Fischer, D., Hones, G., Kreuzbicher, I., Neu-Becher, U., Schwa¨ ger, B., 1994. Gold. In: Keim, R., Schwager, B. ŽEds.., 1994. Gmelin Handbook of Inorganic and Organometallic Chemistry, vol. 62. Springer-Verlag, Berlin, pp. 290–291 ŽSuppl. B2.. Ma, G., Yan, W.F., Hu, T.D., Chen, J., Yan, C.H., Gao, H.C., Wu, J.G., Xu, G.X., 1999. FTIR and EXAFS investigations of microstructures of gold solvent extraction: hydrogen bonding between modifier and AuŽCN.y 2 . Phys. Chem. Chem. Phys. 1, 5215–5221. Molochnikova, N.P., Shkinev, V.M., Myasoedov, B.F., 1992. Two-phase aqueous systems based on polyŽethylene glycol. for extraction separation of actinides in various media. Solvent Extr. Ion Exch. 10, 697–712. Mooiman, M.B., Miller, J.D., 1983. The solvent extraction of gold from aurocyanide solutions. Proc. Int. Solvent Extr. Conf. ISEC ’83. AIChE, New York, pp. 530–531. Mooiman, M.B., Miller, J.D., 1991. The chemistry of gold solvent extraction from cyanide solution by solvating extractants. Hydrometallurgy 27, 29–33. Rogers, R.D., Zhang, J.H., 1997. New technologies for metal ion
separations polyethylene glycol-based aqueous biphasic systems and aqueous biphasic extraction chromatography. In: Marinsky, J., Marcus, Y. ŽEds.., Ion Exchange and Solvent Extraction, vol. 13. Marcel Dekker, New York, pp. 141–193. Rogers, R.D., Bond, A.H., Bauer, C.B., 1993. Metal ion separations in polyethylene glycol-based aqueous biphasic systems. Sep. Sci. Technol. 28, 1091–1126. Rogers, R.D., Zhang, J.H., Bond, A.H., Bauer, C.B., Jezl, M.L., Roden, D.M., 1995. Selective and quantitative partitioning of pertechnetate in polyethylene glycol-based aqueous biphasic systems. Solvent Extr. Ion Exch. 13, 665–688. Rogers, R.D., Bond, A.H., Bauer, C.B., Zhang, J.H., Griffin, S.T., 1996. Metal ion separations in polyethylene glycol-based aqueous biphasic systems: correlation of partitioning behavior with available thermodynamic hydration data. J. Chromatogr. 680, 221–229. Rogers, R.D., Bond, A.H., Zhang, J.H., Horwitz, E.P., 1997. New technetium-99m generator technologies utilizing polyethylene glycol-based aqueous biphasic systems. Sep. Sci. Technol. 32, 867–882. Shkinev, V.M., Molochnikova, N.P., Zvarova, T.I., Spivakov, B.Y., Myasoedov, B.F., Zolotov, Y.A., 1985. Extraction of complexes of lanthanides and actinides with arsenazo III in an ammonium sulfate–polyŽethylene glycol. –water two-phase system. J. Radioanal. Nucl. Chem. 88, 115–120. Sun, X.M., Luo, J.Q., 1990. Liquid–liquid extraction behavior of metals with chrome azurol S as extractant in PEG– ŽNH 4 . 2 SO4 –H 2 O system. Huaxue Shijie 12, 367–369 Žin Chinese.. Walter, H., Brooks, D.F., Fisher, D., 1985. Partitioning in Aqueous Two-Phase Systems: Theory, Methods, Uses and Applications to Biotechnology. Academic Press, New York. Zaslavsky, B.Y., 1995. Aqueous Two-Phase Partitioning: Physical Chemistry and Bioanalytical Applications. Marcel Dekker, New York. Zvarova, T.I., Shkinev, V.M., Vorob’eva, G.A., Spivakov, B.Y., Zolotov, Y.A., 1984. Liquid–liquid extraction in the absence of usual organic solvents: application of two-phase aqueous systems based on a water-soluble polymer. Mikrochim. Acta III, 449–458.