Solvent Extraction Research and Development, Japan, Vol. 19, 63 68 (212) Extraction Behavior of Gold from Hydrochloric Acid Solutions with Ionic Liquids as Extractants Takahiko KAKOI 1 *, Mayumi YOSHIYAMA 1, Hiroyuki ARIYOSHI 1, Masako KOMAI 1 and Masahiro GOTO 2 1 Department of Biochemistry and Applied Chemistry, Kurume National College of Technology, Kurume 83-8555, Japan 2 Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka 819-395, Japan (Received December 16, 211; Accepted January 17, 212) The extraction mechanism of gold(iii) with ionic liquids as extractants has been investigated in a liquid-liquid extraction system. Ionic liquids were found to possess a high extraction potential for gold. The composition of the complex between gold and an ionic liquid was elucidated by slope analysis and Job's method. The gold exists as a tetrachloroaurate anion complex in hydrochloric acid solutions, which reacted with an ionic liquid molecule in chloroform and was extracted into the organic solutions. Furthermore, gold could be separated from copper in an acidic chloride medium by using the ionic liquid as the extractant. 1. Introduction Precious metals are very expensive and have some specific physical and chemical properties and are very important materials as industrial catalysts, electrical and corrosion-resistant materials. However, the metallic resources are limited, therefore the development of efficient recycling techniques from secondary resources such as industrial and nuclear wastes is desirable from the viewpoint of energy conservation. Several practical separation techniques have been developed for the recovery of such precious metals. The solvent extraction process is already used commercially for the recovery of metal ions, and much fundamental information has been accumulated. Although many extensive studies on the extraction of metals have been conducted, few studies concerning the extraction behavior of rare metals with ionic liquids have been carried out [1-3]. - 63 -
In recent years, ionic liquids have been employing as reaction media for many chemical reactions. Ionic liquids are low-melting-point salts that have characteristic properties such as absence of flammability, lack of measurable vapor pressure, good ability to dissolve organic compounds, and catalytic effects [4]. In a previous study, we reported the usefulness of ionic liquids as solvents for the polymer synthesis, and the function of ionic liquids was clarified [5]. Gold is well known to be very useful as an electrical material as well as jewelry, and is extracted commercially using dibutylcarbitol from an acidic chloride medium [6]. Gold (III) exists as a very stable tetrachloroaurate anion complex in aqueous hydrochloric acid solution and is extracted into organic solutions as an ion pair with a neutral or basic extractant. In the present work, the separation of gold and copper from a hydrochloric acid solution has been performed by solvent extraction using an ionic liquid, 1-octyl-3-methyl imidazolium hexafluorophosphate (OMI), which has a quaternary ammonium functional group in its molecular structure. Quaternary ammonium extractants are well known to have a high extractability for precious metals such as platinum and palladium [7]. So, if an ionic liquid itself acts as an extractant, a successful solvent extraction process for metal ions will be established in a long-term operation because it is non-volatile and immiscible with water. Our aim is to assess how such an ionic liquid used normally as an organic solvent in the solvent extraction process affects the extraction behavior of gold. In this study, extraction equilibrium experiments were performed by changing several operating parameters such as gold and OMI concentrations with a view to determine the reaction stoichiometry between gold and the ionic liquid. 2. Experimental 2.1. Materials Figure 1 shows the molecular structure of the ionic liquids, 1-butyl-3-methyl imidazolium hexafluorophosphate (BMI), 1-hexyl-3-methyl imidazolium hexafluorophosphate (HMI), and 1-octyl-3- methyl imidazolium hexafluorophosphate (OMI), which were kindly supplied by The Nippon Synthetic Chemical Industry Co., Ltd.. Analytical grade chloroform was used as the organic solvent. All other reagents used were of guaranteed reagent grade and were used as received. Figure 1. Molecular structure of the ionic liquids used in this study. - 64 -
2.2. Extraction equilibria of metals Aqueous solutions containing gold and copper were prepared using a commercially available standard solution containing 1 ppm Au (III) and / or 1 ppm Cu (II). The hydrogen ion concentration was adjusted with hydrochloric acid. Organic solutions were prepared by dissolving the ionic liquids in chloroform. Equal volumes of the aqueous and organic solutions were shaken in a glass vial immersed in a thermostated water bath (298 K), and allowed to reach equilibrium. After about 72 hours the aqueous and organic phases were separated and the concentration of metals was determined by atomic absorption spectrophotometry (SHIMADZU, AA-68). 3. Results and Discussion 3.1. Extraction behavior of gold and copper The ionic liquids used in the present study were quaternary ammonium salts. The extraction behavior of gold and copper was first investigated by changing the concentration of the ionic liquid in the organic phase. In the extraction of gold, a neutral or basic extractant such as a neutral phosphoric acid ester or a tertiary amine is usually employed because gold forms a stable chloro-anion complex in hydrochloric acid media and is easily extracted as an ion pair complex. Figure 2 shows the relationship between the degree of metal extraction and the concentration of the ionic liquid at a hydrochloric acid concentration of 1 mol/m 3. The symbol, E denotes the degree of metal extraction which is defined by (C M,aq C M,aq )/C M,aq. From the results, the degree of gold extraction was found to increase with an increase in the ionic liquid concentration, E [ - ] 1.8.6.4.2 Au Cu -1 1 log COMI,org Figure 2. Relationship between the degree of metal extraction and the concentration of the ionic liquid (hydrochloric acid concentration: 1 mol/m 3 ). - 65 -
and the ionic liquid had a high extraction ability for gold ions similarly to other quaternary ammonium extractants in the present experimental conditions. Further, we found that gold ions were selectively extracted while copper ions were hardly extracted at all in the concentration range of hydrochloric acid used in this study. The extraction of copper ions is generally carried out using an acidic or chelating extractant such as a dialkyl phosphoric acid or a -hydroxyoxime. In the present experimental conditions, copper exists as a cation and is not extracted by the ionic liquid which is a basic quaternary ammonium salt. Therefore, the best separation of gold and copper was achieved through solvent extraction using the ionic liquid as the extractant. Figure 3 shows the effect of ionic liquid concentration on gold extraction when three different ionic liquids were used. The results showed that the degree of gold extraction with BMI, HMI and OMI increased with increasing concentration of the ionic liquids, and that the ionic liquids used in this study had a high extraction ability for gold. However, the extraction capacity of BMI was relatively low compared to the other ionic liquids. OMI has a longer alkyl chain in the hydrophobic moiety of the quaternary ammonium salt than the other ionic liquids, which would lead to a better solubility of the ion-pair complex between gold and the cationic ionic liquid in the organic solvent. OMI was found to possess the best extractability for gold as compared with that of BMI or HMI. E [ - ] 1.8.6.4 OMI HMI BMI.2-1.5-1 -.5.5 1 1.5 log Cionic liquid, org Figure 3. different ionic liquids. Relationship between the degree of gold extraction and ionic liquid concentration for three 3.2. Extraction equilibrium between gold and ionic liquids Figure 4 shows the results obtained on the basis of Job's method. The concentration of gold in the organic solution showed a maximum value when the ratio of the gold concentration to the total concentration of gold and the ionic liquid reached about.5. This value suggests that the ratio between the ionic liquid and - 66 -
the gold complex formed was about 1. Based on the results of Job's method, one ionic liquid molecule was confirmed to bind to one tetrachloroaurate anion complex by releasing one counter anion, PF - 6. On the basis of the results obtained from Job s method, the following equation for the extraction equilibrium of gold with OMI was proposed: AuCl 4 - + (OMI + X - ) org = (OMI + AuCl - 4 ) org + X - : K ex (1) Here, Kex is the extraction equilibrium constant and is written as follows: K ex = C Au,org C X,aq /C Au,aq C OMI,org = D C X,aq /C OMI,org (2) where C j is the concentration of species, j and D is the distribution coefficient of gold between the organic and aqueous solutions. C X,aq is the concentration of free counter anion in the aqueous solution, which can be CAu,org [ ppm ] 1 8 6 4 2.25.5.75 1 CAu,aq/(COMI,org + CAu,aq) Figure 4. Results based on Job s method (Total concentration of gold and OMI is 1 mol/m 3.). 3 log DCX,aq 2 1-1 1 2 log COMI,org Figure 5. Relationship between log D C X,aq and log C OMI,org. - 67 -
calculated by considering the extraction of gold. The following equation is obtained from Eq.(2): log D C X,aq = log C OMI,org + log K ex (3) Figure 5 shows the effect of the ionic liquid (OMI) concentration on the degree of gold distribution, D. Gold is present as a monovalent tetrachloroaurate anion complex in the aqueous solution; thus the cationic part of the ionic liquid molecule is required and the ionic liquid counter anion must be released. The ionic liquid now behaves as a quaternary ammonium salt and extracts one tetrachloroaurate ion. Therefore, a slope of one is expected from a plot of logd C X,aq vs. logc OMI,org. The results shown in Fig.5 confirm this. Eq. (3) confirms the results obtained in Figs. as discussed before, and the K ex value was found to be 7.1. 4. Conclusion The liquid-liquid extraction of gold using an ionic liquid as the extractant has been studied. The ionic liquid, 1-octyl-3-methyl imidazolium hexafluorophosphate (OMI) extracts gold very well and in the best extractant for gold ion as compared with butyl (BMI) and hexyl derivatives (HMI). Gold ions were extracted by forming an ion-pair OMI + AuCl - 4 complex with OMI. The use of OMI was concluded to be very effective for the recovery of gold ions contained in an acidic chloride medium. Acknowledgement The authors thank The Nippon Synthetic Chemical Industry Co., Ltd. for providing ionic liquids. References 1) F. Kubota, Y. Shimobori, Y. Koyanagi, K. Nakashima, K. Shimojo, N. Kamiya, M. Goto, Solv. Extr. Res. Dev. Jpn., 16, 151 (29) 2) Y. Baba, F. Kubota, N. Kamiya, M. Goto, Solv. Extr. Res. Dev. Jpn., 18, 193 (211) 3) Y. Baba, F. Kubota, N. Kamiya, M. Goto, J. Chem. Eng. Jpn., 44, 679 (211) 4) N. Hirayama, Solv. Extr. Res. Dev. Jpn., 18, 1 (211) 5) Y. Tsuda, T. Yoshida, T. Kakoi, Polym. J., 38, 88 (26) 6) J. E. Barnes, J. D. Edwars, Chem. Ind., 6, 151 (1982) 7) T. Kakoi, M. Goto, F. Nakashio, J. Membrane Sci., 12, 77 (1996) - 68 -