Environmentally Sound Recovery of Gold from Waste ...

Environmentally Sound Recovery of Gold from Waste Electricaland Electronic Equipment Using Organic Aqua Regia

Development Report:


Akihiro Yoshimura, Kota Takatori, and Yasunari Matsuno†

Graduate School of Engineering, Chiba University1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, Japan

†Corresponding author, E-mail: [email protected][Received April 10, 2020; accepted July 21, 2020]

We conducted experiments using unit processes toprove the feasibility of the concept of an environmen-tally sound Au recovery process from waste electricaland electronic equipment (WEEE) using organic aquaregia, i.e., a propylene carbonate (PC) solution con-taining CuBr222 and KBr. First, the WEEE samples(memory card and mobile phone board) were finelyground. The samples were then carbonized and oxi-dized to decompose the polymers and oxidize the basemetals. The leaching of Au from the oxidized sampleswas then carried out in a PC solution with 0.2 M ofCuBr222 and 0.2 M of KBr over 353–373 K, followed bybiphasic separation with sulfuric acid. The dissolvedAu in the PC phase was recovered via ascorbic acidreduction. The mass balance of Au in the leachingwas investigated. The maximum recovery ratios of Aufrom the memory card and mobile phone board were79% and 83%, respectively. The cost of leaching waspreliminarily estimated.

Keywords: copper dibromide, precious metal, propylenecarbonate, recycling

1. Introduction

1.1. Recovery of Precious Metals from SecondaryResources

Precious and rare metals with high market values areused in various industrial products. Because these metalsare scarce (many of them are referred to as critical materi-als [1]), their recovery from secondary resources such aswaste electrical and electronic equipment (WEEE) is ofgreat interest [2, 3].

Various technologies for the recycling of precious met-als such as Au have been developed. These technologiesare roughly classified as pyrometallurgical and hydromet-allurgical methods. Pyrometallurgical methods use large-scale equipment, and thus, are cost-effective; however,their location is restricted. By contrast, hydrometallurgi-cal methods use small-scale equipment, but they involvethe use of solvents with high toxicity such as aqua regiaand cyanide solutions that lead to a large environmental

load concerning wastewater treatment [4–7].The WEEE needs to be treated on-site by small-scale

facilities using safe chemicals for the efficient recovery ofprecious metals and reduced environmental impact partic-ularly caused by the export of WEEE to developing coun-tries [8, 9].

1.2. Principle of Recovery of Au Using OrganicAqua Regia

We developed an eco-friendly method to dissolveand recover Au using an organic solvent system con-taining CuBr2 and KBr (organic aqua regia) [10–13].Dimethyl sulfoxide (DMSO) [8, 9] and propylene carbon-ate (PC) [10] are used as solvents. The dissolved CuBr2in DMSO or PC releases Cu2+. Cu+ is solvated and sta-bly exists owing to the following reaction in both the sol-vents [14, 15]:

Cu2+ + e− ↔ Cu+ . . . . . . . . . . . (1)

According to reaction (Eq. (1)), Cu2+ oxidizes othersubstances and is reduced to Cu+ [14, 15]. Au can be dis-solved in DMSO or PC containing dissolved CuBr2 dueto the following reactions because the redox potential ofEq. (2) or (3) is more negative than that of Eq. (1),

Au + 4Br− ↔ [AuBr4]− +3e− . . . . . . . (2)

Au + 2Br− ↔ [AuBr2]− + e− . . . . . . . (3)

Thus, organic aqua regia can dissolve Au, while elimi-nating the use of toxic substances such as cyanide.

Another merit of the use of organic aqua regia is themethod of recovery of Au. The addition of water inDMSO solutions drastically changes the solution charac-teristics. The redox potentials of Eq. (2) or (3) are morepositive than that of Eq. (1) in water. The changes in theredox potentials cause the reverse reactions of Eqs. (1)–(3), resulting in precipitation of the dissolved Au. Thismeans that the dissolved Au in organic aqua regia can berecovered simply by the addition of water to DMSO [16,17]. Furthermore, DMSO does not form an azeotropewith water and can be separated by distillation, which en-ables the repeated use of the solutions. Therefore, thissystem can minimize the wastewater treatment and chem-ical sludge production.

Int. J. of Automation Technology Vol.14 No.6, 2020 999

https://doi.org/10.20965/ijat.2020.p0999

© Fuji Technology Press Ltd. Creative Commons CC BY-ND: This is an Open Access article distributed under the terms of the Creative Commons Attribution-NoDerivatives 4.0 International License (http://creativecommons.org/licenses/by-nd/4.0/).

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Yoshimura, A., Takatori, K., and Matsuno, Y.

Fig. 1. Process flow diagram of Au recovery system using organic aqua regia.

In the case of the PC system, PC is immiscible with wa-ter and separates into two layers after the addition of wa-ter. The dissolved Au is contained in the PC layer, whileother metals are contained in the water layer [12]. Thisenables the separation of Au from other metals. This pro-cess is eco-friendly and suitable for sustainable recyclingbecause of the use of a sulfur-free solvent.

The advantages of organic aqua regia for the leachingand recovery of Au can be summarized as: simple opera-tion, the elimination of the use of highly toxic substances,and the repeated use of solvents, leading to minimizationof wastewater treatment and chemical sludge production.

1.3. Application of Organic Aqua Regia for theRecovery of Au from WEEE

The application of organic aqua regia for the recoveryof Au from WEEE is still under trial. We preliminarilyapplied the PC-based organic aqua regia for the leach-ing of Au from WEEE [18]. WEEE typically containsvarious metals in addition to Au. For example, a mo-bile phone board contains over 20 metals such as Cu, Al,Ni, and Sn [19, 20]. During the leaching process, thesemetals also dissolve and consume the organic aqua regia,which decreases the dissolution of Au [11]. The dissolu-tion of these metals can be prevented by oxidizing themto form a “passivation” layer on their surface. In contrast,Au is stable and does not get oxidized. Previous studiesconducted thermal activation of semiconductors [21] forthe oxidation process; however, this process was complexand the recovery ratio of Au was low [18]. Therefore, inthis study, the pretreatment was performed by combiningcoarse grinding, carbonization, pulverization, and oxida-tion, which are widely used methods.

Figure 1 illustrates the flow diagram of the entire pro-cess of Au recovery from WEEE in this study. The pro-

cess consists of 1) grinding and carbonization, 2) pulver-ization and oxidation, 3) leaching, 4) biphasic separation,5) reduction of Au, and 6) Au recovery.

Because this work is still at the experimental stage ofdevelopment, lab-scale equipment was used for each pro-cess. For example, an electric furnace was used for car-bonization and oxidation, and a rotary evaporator with a1 L eggplant flask was used for leaching. As the proposedmethodology progresses, all the processes may be scaledup and optimized from the viewpoint of reaction efficien-cies and costs.

In this paper, a feasible study of Au recovery fromWEEE using the organic aqua regia system and widelyused pretreatment technologies was conducted for the firsttime. The mass-balances of Au in the leaching and re-covery processes were then investigated to obtain the re-covery ratio. In addition, the cost of the leaching and re-covery processes was preliminarily estimated to find thereduction potentials.

2. Materials and Methods

2.1. MaterialsCuBr2 (97.0%), KBr (99.0%), H2SO4 (95.0%), and

L (+) ascorbic acid (99.6%) were purchased from Fuji-film Wako Pure Chemical Corporation, Tokyo, Japan. PC(> 98.0%) was purchased from Tokyo Chemical Indus-try Co. Ltd. Distilled water was purchased from DaiwaYakuhin Co. Ltd.

2.2. MethodsThe details of each process are explained in the follow-

ing sub-sections:

1000 Int. J. of Automation Technology Vol.14 No.6, 2020


Fig. 2. WEEE samples: (a) memory board and (b) mobilephone board.

2.2.1. Grinding and CarbonizationIn this study, we used the printed circuit boards of a

memory card and mobile phone as WEEE samples, thesame as those used in previous studies, for comparisonpurposes [16] (Fig. 2). These samples were coarselyground using a cutting mill (Verder Scientific Co. Ltd.,SM300). The mill rotation rate was set to 1000 rpm. Sub-sequently, 150 g of the ground samples were carbonizedin an electric furnace under a nitrogen (N2) atmospherewith a flow of 1 L/min. The temperature was rapidly in-creased up to 523 K, and then up to 733 K at a constantheating rate for 4 h.

2.2.2. Pulverization and OxidationAfter gradual cooling to room temperature, the car-

bonized sample was pulverized using a fine pulverizer(Labonect Co. Ltd., High-speed mill, HS-15). The pul-verized sample was then oxidized at 773 K in the electricfurnace under the air atmosphere for 2 h. The oxidizedsample was stirred after the first oxidation process and ox-idized again at 773 K for 2 h to obtain a homogeneouslyoxidized sample. The weight of the oxidized sample wasmeasured after gradual cooling to room temperature.

2.2.3. Leaching, Biphasic Separation, and AuRecovery

Further, 100 mL of PC containing 0.2 M of CuBr2and KBr was prepared. Subsequently, 30 g of the oxi-dized sample was leached using this solution using a ro-tary evaporator at 353 K for 4 h while stirring at 140 rpm.Subsequently, 100 mL of 0.1 M H2SO4 was added tothe solution for biphasic separation after removal of theresidue via vacuum filtration. This mixture was stirredusing a stirrer (Tokyo Rikakikai Co. Ltd., MMV-1000W)at 140 rpm for 10 min and stood to separate into PC andH2SO4(aq) layers. Subsequently, 1 g of ascorbic acid wasadded to the extracted PC layer after the biphasic separa-tion and stirred for 1 h at 353 K. This solution was stoodfor 72 h at room temperature for Au precipitation. Theprecipitate was then dried at 383 K for 24 h after vacuumfiltration, and its weight was measured. The weight of theresidue from the leaching process was also measured afterdrying it at 383 K for 24 h.

The components of the oxidized sample, precipitate,H2SO4(aq) layer, residue, and PC solution were deter-mined by inductively coupled plasma-optical emissionspectrometry (ICP-OES) (Varian VISTA-MPX).

2.2.4. Reextraction of AuLeaching was carried out again on the residue of the

first leaching process to improve the recovery ratio of Au.The residue was pulverized and oxidized under the sameconditions as those in Section 2.2.2. The pulverizationwas carried out to enhance the dissolution of Au owing tothe increase in surface area of the sample. The oxidationwas carried out to form an oxidized surface for preventingthe dissolution of other metals. The conditions of leachingand recovery were the same as those in Section 2.2.3.

2.2.5. Estimation of Leaching and Recovery CostsWe estimated the cost of the leaching process that uses

organic aqua regia and that of the recovery of Au. Theprices of the industrial-grade materials used for leachingwere obtained via websites, and the cost was calculated.1 USD was considered 110 JPY. The cost of the appara-tus, electricity, and labor was not included. Thus, the costestimation is still preliminary; however, it guides futuresystem requirements.

3. Results and Discussion

3.1. PretreatmentFigure 3 shows images of the samples before and af-

ter the pretreatment. Table 1 shows the changes in sam-ple weights after the pretreatment. The weight loss ofthe memory board was 16.5%, while that of the mobilephone board was 12.3%. The decrease in weight can beattributed to the decomposition of the resin in the WEEEdue to carbonation and oxidation.

Table 2 shows the results of the ICP-OES analysis ofthe components of the memory board and mobile phoneboard after oxidation. The Au component content was ap-proximately 900 parts per million (ppm) in both the sam-ples. The mobile phone board contained larger amountsof Ag and Cu than the memory card.

3.2. Recovery of AuTable 3 shows the precipitates recovered from the ox-

idized samples of the memory board and mobile phoneboard and the components post the ICP-OES analysis.Only Au and Ag were analyzed because most of the com-ponents in the samples were determined through prelim-inary scanning electron microscopy – energy-dispersiveX-ray spectroscopy analysis (JEOL JSM-6510A). Themain component in the recovered precipitate was Au inboth cases.

Table 4 shows the weights and components (Au, Ag,and Cu) of the residues after leaching. The Au compo-nent content in the residue decreased to approximately200 ppm in both the memory board and the mobile phoneboard. The weights of the leached Au, calculated usingthe analysis results, were 21.6 mg for the memory boardand 20.7 mg for the mobile phone board. These results are



Fig. 3. Photographs of the WEEE samples (a) after grinding,(b) after carbonization, and (c) after oxidation.

Table 1. Changes in WEEE weights after carbonization andoxidation.

Table 2. Elemental analysis of the oxidized memory boardand mobile phone board.

Table 3. Components of the precipitates from the memoryboard and mobile phone board.

Table 4. Components of the residues from the memoryboard and mobile phone board.

Table 5. Results of the two-cycle leaching of the mobilephone board.

consistent with the Au component results in the recoveredprecipitates shown in Table 3.

The Ag component content in the mobile phone boardsignificantly decreased after leaching, while that of thememory card slightly decreased. The weights of theleached Ag, calculated using the analysis results, were3.9 mg for the memory board and 60 mg for the mobilephone board. These results deviate from the Ag compo-nent results in Table 3. The leaching of Ag with organicaqua regia using PC was confirmed in this study despitethe low leachability of Ag seen in previous studies [12].The behavior of Ag in the leaching process using organicaqua regia should be investigated in further studies.

The Cu component content decreased by approxi-mately 3% after leaching, for both the memory board andmobile phone board. The decrease, equivalent to 0.9 g,can be attributed to the leaching of Cu using organic aquaregia. However, the total weight of the residue was ap-proximately 30 g, almost equal to the initial weight. Thisdifference suggests the insufficient removal of PC in thedrying process after leaching.

The Au concentrations in the H2SO4(aq) and PC lay-ers used for biphasic separation were below the detectionlimit, which confirms that no Au remained in H2SO4(aq)and PC.

The recovery rates of Au from the memory board andmobile phone board were 78.6% and 73.4%, respectively.The remaining Au was in the residue.

3.3. Reextraction of AuTable 5 shows the results of the reextraction of Au from

the residue after the first leaching process of the mobilephone board.

The Au component content in the 30 g sample was28.2 mg. The recovery ratios of the first and second pro-cesses were 73.0% and 10.3%, respectively, while the to-tal recovery ratio was 83.3%.

The maximum recovery ratio in previous studies wasapproximately 64% [17]. Thus, the recovery ratio was im-



Table 6. Materials used in leaching.

proved by approximately 9% during the first process andapproximately 19% in total. This improvement suggeststhe effectiveness of the pretreatment consisting of coarsegrinding, carbonization, pulverization, and oxidation.

3.4. Estimation of Leaching and Recovery CostsAs mentioned earlier, this work is still at the experi-

mental stage of development. With the progress of thedevelopment, all the processes may be scaled up and op-timized from the viewpoint of reaction efficiencies andcosts. Thus, the estimation of the cost in this paper ispreliminary, wherein only the amount and cost of the ma-terials required for leaching and Au recovery were inves-tigated. The cost of utilities such as electricity and gasand that of reactors, machinery, and labor were not inves-tigated in this paper because there is large variability inthese costs.

Table 6 shows the calculation of the cost of materials.The total material cost was 0.426 USD, mostly compris-ing the cost of PC and CuBr2. Approximately 100 mgof Au was recovered, its current price being 0.909 USD.Thus, the quantity of materials used should be reduced forcost-efficiency.

We used 100 mL of organic aqua regia to treat the 30 goxidized sample in this study. The increase in the amountof the sample within a range that does not have a nega-tive effect on the treatment could increase the amount ofrecovered Au and enhance the cost-efficiency. In addi-tion, if the PC and CuBr2 can be reused, the cost could belargely reduced. Thus, methods to reuse these materialsshould be investigated.

4. Conclusion

In this study, we conducted experiments using unit pro-cesses to prove the feasibility of the concept of an envi-ronmentally sound Au recovery process from WEEE us-ing organic aqua regia. The successful recovery of Ausuggests that this system enables the selective recoveryof Au from WEEE. The recovery ratios of Au from thefirst leaching process were 79% and 73% for the mem-ory board and the mobile phone board, respectively. Theresidue containing the remaining Au was treated again af-ter pulverization and oxidation. The recovery ratio thenincreased by approximately 10% and reached 83% (in to-tal) for the mobile phone board.

The proposed process using the organic aqua regiacan thus recover Au using small-scale equipment, whileconventional recycling processes, such as pyrometallurgi-cal and hydrometallurgical processes, require large-scaleequipment. The proposed process can be eco-friendly andenable on-site recovery of Au from WEEE (on-site urbanmining). The development of the system is fundamen-tal. Cost reduction is required for a practical economicalprocess. Further, the reuse of PC and CuBr2 could be ef-fective in further reducing the cost.

AcknowledgementsThis study was supported by Tokyo Tekko Co. Ltd.

References:[1] K. Whalen and D. Peck, “In the Loop- Sustainable, Circular Product

Design and Critical Materials,” Int. J. Automation Technol., Vol.8,pp. 664-676, 2014.

[2] B. Owens, “Mining: Extreme prospects,” Nature, Vol.495, S4-S6,2013.

[3] S. Syed, “Recovery of gold from secondary sources – A review,”Hydrometallurgy, Vols.115-116, pp. 30-51, 2013.

[4] J. Cui and L. Zhang, “Metallurgical recovery of metals from elec-tronic waste: a review,” J. Hazard. Mater., Vol.158, pp. 228-256,2008.

[5] J. P. Young and J. F. Derek, “Recovery of high purity preciousmetals from printed circuit boards,” J. Hazard. Mater., Vol.164,pp. 1152-1158, 2009.

[6] T. Okabe, “Current Status of Platinum Group Metals and RecyclingTechnologies,” Materia Japan, Vol.46, pp. 522-529, 2007.

[7] J. Shibata and A. Okuda, “Recycling Technology of Precious Met-als,” Shigen-to-Sozai, Vol.118, pp. 1-8, 2002.

[8] M. Matsumoto, N. Mishima, and S. Kondoh, “Tele-Inverse Man-ufacturing – An International E-Waste Recycling Proposal,” Int. J.Automation Technol., Vol.3, pp. 11-18, 2009.

[9] Y. Kishita, E. Kunii, S. Fukushige, Y. Umeda, and J. Fujimoto,“Scenario Analysis of Global Resource Circulation with Traceabil-ity Index Targeting Sustainable Manufacturing,” Int. J. AutomationTechnol., Vol.3, pp. 3-10, 2009.

[10] A. Yoshimura, M. Takai, and Y. Matsuno, “Novel process for recy-cling gold from secondary sources: Leaching of gold by dimethylsulfoxide solutions containing copper bromide and precipitationwith water,” Hydrometallurgy, Vol.149, pp. 177-182, 2014.

[11] A. Yoshimura, K. Umehara, M. Takai, and Y. Matsuno, “Develop-ment of Recycling System of Precious Metals and Rare Metals Us-ing DMSO Solvent Containing CuBr2,” J. Japan Inst. Mat. Mater.,Vol.79, pp. 41-48, 2015.

[12] K. Umehara and Y. Matsuno, “Fundamental Studies on a RecyclingSystem for Precious and Rare Metals Using a Propylene Carbon-ate Solvent Containing CuBr2 and KBr,” Materials Trans., Vol.56,pp. 1579-1584, 2015.

[13] Y. Matsuno and M. Takai, “Precious metal recovery method us-ing copper halide-containing organic solvent system,” Japan Patent6196662, 2017.

[14] M. Chaudhry and I. Persson, “Transfer thermodynamic study onthe copper(II) ion from water to methanol, acetonitrile, dimethylsulfoxide and pyridine,” J. Chem. Soc., Faraday Trans., Vol.90,pp. 2243-2248, 1994.

[15] K. Izutsu, “Electrochemistry in Nonaqueous Solutions,” Wiley-VCH., pp. 35-37, 2009.

[16] A. Yoshimura and Y. Matsuno, “A novel process for the productionof gold micrometer-sized particles from secondary sources,” Mate-rials Trans., Vol.57, pp. 357-361, 2016.

[17] T. Komenami, M. Sato, C. Sato, and Y. Matsuno, “Development ofa Production Method for Palladium Micrometer-Sized Particles Us-ing DMSO Solvent Containing CuCl2,” J. Japan Inst. Met. Mater.,Vol.82, pp. 461-466, 2018.

[18] K. Takatori, T. Mineo, Y. Nagase, R. Nagoya, K. Sakurai, H. Sasaki,K. Otabe, and Y. Matsuno, “Development of Recycling System ofGold from Secondary Resources Using Organic Aqua Regius,” J.Japan Inst. Met. Mater., Vol.82, pp. 467-475, 2018.

[19] T. Yumoto and T. Shiratori, “Study on Recycling of Metals inWEEE – Investigation for making an inventory of metal compo-nents –,” J. MMIJ., Vol.125, pp. 75-80, 2009.



[20] K. Takahashi, J. Nakamura, K. Otabe, M. Tsuruoka, Y. Mat-suno, and Y. Adachi, “Resource Recovery from Mobile Phone andthe Economic and Environmental Impact,” J. Japan Inst. Metals.,Vol.73, pp. 747-751, 2009.

[21] J. Mizuguchi, S. Suzuki, M. Kaneko, and H. Sakahashi, “RecentAdvances in TASC (Thermal Activation of Semi-Conductors) Tech-nology for Environmental Issues Focused on the Disassembly andRecycling of Solar Panels and Laminated Glass – A New Technol-ogy Characterized by Radical Propagation in Giant Molecules –,”Mater. Trans., Vol.56, pp. 1615-1625, 2015.

Name:Akihiro Yoshimura

Affiliation:Assistant Professor, Department of Urban Envi-ronment Systems, Chiba University

Address:1-33 Yayoi-cho, Inage-ku, Chiba-shi, Chiba 263-8522, JapanBrief Biographical History:2016- Project Researcher, Institute of Industrial Science, The University ofTokyo2017- Assistant Professor, Department of Urban Environment Systems,Chiba UniversityMain Works:• A. Yoshimura and Y. Matsuno, “The Improvement of Platinum RecoveryRatio in the Recycling Process Using “Dry Aqua Regia”,” Mater. Trans.,Vol.60, pp. 2223-2228, 2019.Membership in Academic Societies:• Mining and Materials Processing Institute of Japan (MMIJ)• Institute of Life Cycle Assessment, Japan (ILCAJ)

Name:Kota Takatori

Affiliation:Tanaka Kikinzoku Kogyo K.K.

Address:2-7-3 Marunouchi, Chiyoda-ku, Tokyo 100-6422, JapanBrief Biographical History:2020- Tanaka Kikinzoku Kogyo K.K.Main Works:• K. Takatori, T. Mineo, Y. Nagase, R. Nagoya, K. Sakurai, H. Sasaki, K.Otabe, and Y. Matsuno, “Development of Recycling System of Gold fromSecondary Resources Using Organic Aqua Regius,” J. Japan Inst. Met.Mater., Vol.82, pp. 467-475, 2018.

Name:Yasunari Matsuno

Affiliation:Professor, Department of Urban EnvironmentSystems, Chiba University

Address:1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, JapanBrief Biographical History:1996- National Institute of Advanced Industrial Science and Technology(AIST)2004- Associate Professor, The University of Tokyo2016- Professor, Chiba UniversityMain Works:• T. Komenami, A. Yoshimura, Y. Matsuno, M. Sato, and C. Sato,“Network of palladium nanorings synthesized by liquid-phase reductionusing DMSO-H2O: in-situ monitoring of structure formation and dryingdeformation by ASEM,” Int. J. Mol. Sci., Vol.21, No.9, p. 3271, 2020.Membership in Academic Societies:• Society for Chemical Engineers, Japan (SCEJ)• Institute of Life Cycle Assessment, Japan (ILCAJ)• Japan Institute of Metals and Materials (JIM)


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