Recovery of gold ions from discarded mobile phone leachate by solvent extraction and polymer inclusion membrane (PIM) based separation using an amic acid extractant
Graphical abstract
Introduction
Precious metals such as gold, platinum and palladium are used in the manufacturing of diverse goods such as electric and electronic products, chemical catalysts, pharmaceuticals and jewelry due to their characteristic physical and chemical properties such as ductility, good electrical conductivity and corrosion resistance [1], [2], [3], [4], [5]. The demand for these noble metals has been growing at a very fast rate with the development of new technologies, which has resulted in global concern on securing their steady supply in the future. Recently, waste electronic products have been identified as a significant secondary source of these valuable metals, and these sources are often referred to as “urban mines” [6], [7], [8], [9]. Mobile phones and computer circuit boards contain gold at concentrations of orders of magnitude higher than those in natural ores [5], [6]. Therefore, the recycling of the noble metals in electronic waste is indispensable for their stable supply. Solvent extraction is widely used for the separation and purification of a variety of metal ions. For the recycling of valuable metals in waste products, the metals should be dissolved first in an acidic solution prior to their extraction. In such leachates, a small amount of the target noble metal is usually present together with a variety of other metals in much large amounts [1], [2], [10]. The success of the selective recovery of the target metal(s) depends on the properties of the extractant(s) used. The recovery of gold from waste electronic devices has been studied using a number of extractants [5], [6], [11]. Recently, we have developed the novel amic acid type extractant N-[N,N-di(2-ethylhexyl)aminocarbonylmethyl]glycine (D2EHAG) for the highly selective separation of nickel and cobalt from metals such as manganese and rare earth metals [12], [13], [14]. Furthermore, D2EHAG has been found to be also suitable for the selective extraction of platinum group metals (PGMs) [15]. In the former case, D2EHAG works as an acidic extractant and the extraction proceeds on the basis of the exchange between the hydrogen ions of the extractant and the target metal cations in the solution, whereas in the latter case, the amine group of the extractant is protonated and the resulting cation forms an ion-pair with the chloride anionic complexes of the PGMs formed in solutions containing HCl. Thus, D2EHAG is expected to be useful for recovering the precious metals from the acid leachates of waste electronic products.
Although solvent extraction is an effective separation technique, it has some drawbacks such as large energy consumption and the use of large inventories of organic diluents which can have a negative environmental impact [16]. Recently, a membrane separation technique has attracted much attention as an alternative to solvent extraction which involves the use of supported liquid membranes (SLMs) [17], [18], [19]. In this technique, an SLM, which is a microporous polymer thin film impregnated with an extractant, often referred to as carrier, dissolved in a suitable diluent, is sandwiched between a feed aqueous solution and a receiving aqueous solution. SLMs allow simultaneous extraction and back-extraction of the target chemical species and decrease drastically the amount of organic diluents required. However, the instability of these membranes due to the leakage of their diluents and carriers into the adjacent aqueous solutions is an obstacle to their industrial application. As an alternative to SLMs, polymer inclusion membranes (PIMs) have been proposed, where the carrier is immobilized between the entangled chains of a base polymer, thus providing the membrane with excellent transport properties and high stability [20], [21]. A variety of metal cations and anions including Au(III) have been separated successfully by PIMs incorporating appropriate carriers and in some cases plasticizers [22], [23], [24], [25], [26], [27]. Recently, we have succeeded in the selective recovery of Co(II) using a PIM incorporating D2EHAG [28]. As described above, D2EHAG has a high affinity for metal chloride anionic complexes in HCl solutions. In the present study, the extraction of the gold(III) chloride anionic complex [AuCl4]− with D2EHAG was examined, and the applicability of D2EHAG as the carrier in a poly(vinyl chloride) (PVC) based PIM for the recovery of gold(III) from leachates of discarded mobile phones was investigated.
Section snippets
Materials and reagents
N-[N,N-di(2-ethylhexyl)aminocarbonylmethyl]glycine (D2EHAG, Fig. 1) used as the PIM carrier was synthesized as described previously [12]. PVC, used as the base polymer of the PIMs studied, was purchased from Fluka (Sigma-Aldrich, USA), and 2-nitrophenyloctyl ether (2NPOE), used as the PIM plasticizer, was purchased from Dojindo Laboratories (Japan). Gold(III) chloride tetrahydrate was obtained from Kishida Chemical (Japan). Solutions of Pt(IV), Pd(II), Pb(II), Zn(II), Al(III), Fe(III), Ni(II),
Leachate analysis
The concentrations of the major metal ions in the leachate prepared in 4 M HCl solution are listed in Table 1. The concentration of Au was determined as 160 mg/L, and on this basis the concentration of Au in the discarded mobile phones was estimated to be 397 g/t. Taking into account that the concentration of Au in natural gold ore is usually in the range 5–30 g/t [6], discarded mobile phones could be viewed as a high-grade gold ore. However, the significant number of transition metals present
Conclusions
Metal recycling from waste electronic components is crucial in ensuring a stable supply of precious metals such as gold. The results obtained in the current study demonstrate that gold, present in discarded mobile phones in concentrations much higher than in natural gold ores, can be quantitatively recovered from the diluted aqua regia leachates of discarded mobile phones by solvent extraction or PIM-based separation using the a recently synthesized extractant/carrier D2EHAG. The optimal HCl
Acknowledgements
The authors express sincere thanks to Sibata Ind. Co. Ltd., Seishin Enterprise Co. Ltd. and Matsuda Sangyo Co. Ltd., Japan, for kindly providing leachates from crushed and milled discarded mobile phones. The authors also thank Ms Sachie Hikino for her assistance with the analysis of the metal content of the leachates. This work was supported by a Grant-in Aid for Scientific Research (No. JP16K06830) from the Ministry of Education, Science, Sports, and Culture of Japan, the Environment Research
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