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Recovery of gold from hydrochloric medium by deep eutectic solvents based on quaternary ammonium salts
Hydrometallurgy 188 (2019) 264–271
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Recovery of gold from hydrochloric medium by deep eutectic solvents based on quaternary ammonium salts Yuqi Geng, Zeyang Xiang, Cheng Lv, Ning Wang, Yudong Wang, Yanzhao Yang
T
⁎
Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry and Chemical Engineering. Shandong University, Jinan 250100, PR China
A B S T R A C T
In this paper, 3 kinds of hydrophobic deep eutectic solvents were screened out for gold recovery due to their low viscosity, which were constructed by ionic liquids: [N3333]Br, [N4444]Br and [N8881]Br and N-hexanoic acid. Anion exchange mechanism during the extraction process was proposed through the combination of UV–vis and FT-IR. Subsequently, the extraction behaviors under different conditions, such as: salinity (NaCl/NaBr), acidity and Au(III) concentration were investigated and discussed. N8881-Br-acid showed the best extraction ability: it showed the 96.8% extraction efficiency on 1 mM Au(III) solution, compared with N4444-Br-acid (around 93% on 0.6 mM) and N3333-Br-acid (around 91% on 0.6 mM). Besides that, the N8881-Br-acid showed the better salinity resistance among 3 DESs. The extracted Au (III) in the DESs phase was easily and completely stripped by applying 1 mL 0.1 g/mL NaBH4, and each extraction ability maintained after 5 turns of extraction cycle experiments. Since the excellent behaviors showed in the multi-metal selective extraction and extraction cycle experiments, the N8881-Br-acid was regarded as the most potential system for gold recovery from HCl medium. Furthermore, its operating conditions combination: operating temperature and vibration time were optimized to 19 °C and 13 min by applying response surface methodology (RSM). Under optimized conditions combination and given A/O = 1:1, the complete extraction procedure only needs two theoretical stages with a tiny amount (~5%) of Au(III) left, which is indicated by drawing Mccabe-Thiele diagram.
1. Introduction Nowadays, gold has been widely applied in chemistry, commercial investments, high-tech manufacturing and so on (Gurung et al., 2013; Parajuli et al., 2009; Vinh Hung et al., 2014; Vinh Hung et al., 2010). Both the world supply and demand of gold keeps increasing. While the limited natural reservation, gold recovery from secondary resources (e.g. wasted computer circuit boards, electronic scrap and waste electroplating solutions) will be a more efficient way to obtain precious metals, compared with conventional hydrometallurgy industry (Geng et al., 2019; Yoshimura and Matsuno, 2016; Yoshimura et al., 2014). It was reported that Tokyo now is running an e-waste recycling program for the 2020-Tokyo-Olympic-games gold medal production and has collected 48 thousand tons of used mobile phones and other electronic equipments (Tokyo 2020, 2019), due to the lower cost and abundant contents of gold, silver and copper. Novel extractants with high extraction ability and easy functionalization, were discovered and applied, such as: functional ionic liquids. Since the first industrial process involving ionic liquids was announced in 2003, the potential of ionic liquids is beginning to be recognized (Armand et al., 2009). Due to its properties: excellent stability, easy modification, low vapor pressure and so on, it now has been widely applied in metal separation and extraction (de Oliveira et al., 2019; Yan et al., 2018; Yan et al., 2017). However, when talking about ionic ⁎
Corresponding author. E-mail address: [email protected] (Y. Yang).
https://doi.org/10.1016/j.hydromet.2019.06.013 Received 20 March 2019; Received in revised form 8 June 2019 Available online 25 June 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
liquids applied in the traditional liquid-liquid extraction, hydrophobic reformation of hydrophilic ionic liquids always needs complex procedures: external alkyl chain introduction, ions exchange with PF6−, NTf2− to improve their hydrophobicity and so on (Rzelewska-Piekut and Regel-Rosocka, 2019; Trujillo-Rodríguez and Anderson, 2019; Van Roosendael et al., 2019). During the anion exchange in extraction process, these exchanged anions (e.g. PF6−, NTf2−) would be lost in the aqueous phase, followed by high cost of use, blocking itself from cyclic using (Zheng et al., 2016; Zheng et al., 2015) and further industrial application. At the same time, a great demand of reagents was used during the synthesis procedure, followed by generation of a huge number of wastes, which didn't meet the purpose of green sustainable development. However, the method of hydrophobic DES preparation showed us a novel idea to solve the above problems. Deep eutectic solvents are a class of mixture composed by the HBA (hydrogen bond acceptor) and HBD (hydrogen bond donor) (Liu et al., 2018), which was firstly discovered by Abbott. et al., who found new characteristics by mixing the choline chloride and urea (Abbott et al., 2003). Hydrophobic DESs preparation attracted us because of its easier synthesis process compared with conventional hydrophobic ionic liquids: just mix the HBA and HBD reagents at a certain temperature to form a homogeneous and stable phase, followed by the excellent features same as ionic liquids. At the same time, lower costs, lower toxicity and more convenient access
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2.3. Gold extraction
to raw materials means a more application prospects compared with ionic liquids. Nowadays, DES were popularly applied in organic synthesis (Alonso et al., 2016), catalysis (Liu et al., 2015), nanomaterials preparation (Abo-Hamad et al., 2015; Wagle et al., 2014), gas adsorption (Garcia et al., 2015) and target objects enrich (Duan et al., 2019; Jiang et al., 2019; Pal and Jadeja, 2019; Radosevic et al., 2015; Ibrahim et al., 2019), but less research about hydrophobic DESs on precious metals extraction was found. So, in this paper, the prospects and extraction behavior of hydrophobic DESs based on quaternary ammonium salts for gold recovery were discussed. The quaternary ammonium salts with short alkyl chain: [N1111]Br, [N2222]Br, [N3333]Br, [N4444]Br, and [N8881]Br were chose to construct hydrophobic DES combining with different HBDs. After hydrophobicity tests, three ideal DESs were screened out due to the lower viscosity: N3333-Br-acid, N4444-Br-acid, and N8881-Br-acid. Through analysis of FT-IR and UV–vis spectra, anion-exchange mechanism during the extraction was proposed. The extraction behaviors of 3 DESs under NaCl/NaBr, HCl and gold concentration were investigated, which showed that the extraction ability and resistance against NaCl/NaBr increased with the alkyl chain of quaternary ammonium salts increased. Among of 3 DESs applied, N8881-Br-acid was regarded as the most potential structure for gold recovery, due to its higher separation ability and more stable extraction ability after 5 turns of extraction cycle experiments. So, its operating conditions of vibration time and temperature was separately optimized by RSM. Under the optimum combination, a two-theoretical-stage tandem extraction process was proposed based on the McCabe-Thiele diagram.
Take influence factors part for example, 1 mL of Au(III) chloride solution (concentration varies from 0.2 to 1.4 mM) and 1 mM*1 mL of DESs (diluted by n-hexanoic acid) were mixed together at room temperature (25 °C). The samples were centrifuged for 10 min after 30 min mechanical vibration to make sure the status of the clear biphasic phases. The concentration of Au(III) remaining in water was determined by flame atomic absorption spectroscopy (FAAS). Extraction efficiency (E%) was applied to quantify the ability that DESs combined with Au(III). The value could be calculated using Eq. (1):
E(%) = (1 − Mp /Mi ) × 100
(1)
where Mp is the Au(III) amount in the permeated solution and Mi is the Au(III) amount in the initial solution. 2.4. RSM experimental design A central-composite-designed Response surface methodology (RSMCCD) was employed to optimize the comprehensive effects including temperature (A: 0–50 °C) and vibration time (B: 10–20 min) on gold extraction yield (Ygold) in three levels, whose corresponding range had been determined based on the preliminary experiments. The secondorder model for predicting the optimum effect of conditions combination was expressed in eq. 2: k
Y = β0 +
k
∑ βi Xi + ∑ βii Xi2 + ∑ ∑ βij Xi Xj + ε i=1
i=1
i
(2)
where Y is the response, β0 is the value for the fixed response at central point of the experiment, βi, βii, and βij are the linear, quadratic and cross product coefficients, respectively, and Xi and Xj are the coded independent variables (Priya et al., 2019). The whole optimization experiments were conducted with the fixed concentration of Au(III) solution (197 mg/L), DES (1 mM) and the ratio of A/O (1:1). The analysis of the statistics was based on the Design Expert software (Version 8.0.6). The second-order equation obtained and model analysis were shown in RSM optimization part.
2. Experimental 2.1. Chemicals and materials The tetra-ethyl-ammonium bromide (TEAB, A.R.), tetra-propyl-ammonium bromide (TPAB, A.R.), octyl-trimethyl-ammonium bromide (A.R.) were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd., (Beijing China). Decanoic acid (A.R.), n-Hexyl alcohol (A.R.), 1hexanol (A.R.), glycerol (A.R.), 1-propyl alcohol (A.R.), Ethylene glycol (A.R.), were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd., (Beijing China). MnCl2·4H2O, SnCl4·5H2O, FeCl3·6H2O, CoCl2·6H2O, MgCl2·6H2O, and CuCl2·2H2O were provided by Aladdin Co., Ltd. (Shanghai, China). DI water used in the whole experiment process was triply distilled by a quartz water purification machine, whose conductivity was lower than 1.8 μS·cm−1 as measured by a DDSJ-308A type conductivity instrument in laboratory. The remaining concentrations of Au(III) and multi-metal ions (Mg(II), Al(III), Mn(II), Co(II), Cu(II), Ce(III), Ni(II)) were determined by a flame atomic absorption spectrometer (3150, Precision & Scientific Instrument, Shanghai, China) and an ICP-OES (iCAP-7000, Thermo Fisher Scientific, Boston, US) respectively. The UV–vis spectra (UV-9000, Metash, Shanghai, China) and FT-IR (Tensor27, Bruker corporation, Karlsruhe, Germany) were used to analyze the synthesized DES and the corresponding extracted species. The temperature used for Au(III) extraction was 25 °C. Each sample data was measured for 3–5 times, and the values were averaged.
3. Results and discussion 3.1. Hydrophobic DESs combination screening 5 hydrophilic low alkyl chain quaternary ammonium ionic liquids: [N1111]Br, [N2222]Br, [N3333]Br, [N4444]Br, [N8881]Br were mixed with a series of HBD reagents to construct hydrophobic DESs. Because of the formation of new hydrogen bonds, DES has higher viscosity than simple HBD (Zhang et al., 2012), which could cause a negative effect on the mass transfer between phases in the gold extraction process. So, we screened out the appropriate combinations with low viscosity by mixing them with DI water for 30 min and centrifugation for 20 min. The stable two phases without volume loss proves the successful hydrophobic reformation with low viscosity, and results were listed in Table 1. It could be seen that, the successful hydrophobic transformation relied on the structure and hydrophobicity of HBAs and HBDs. Each mixture of [N1111]Br or [N2222]Br with HBD reagents cannot form a homogeneous stable phase after heating up to 90 °C for 1 h. That was attributed to the shorter alkyl chain length of themselves. When the alkyl chain length of HBA increased, more hydrophobic combinations showed up: more potential hydrophobic combinations showed when composed by [N8881] Br with HBD reagents. At the same time, status of HBDs could affect the status of DESs, for example: the composed DESs were all solid due to the solid state of decanoic acid stored at room temperature. When the hydrophobicity of the HBD reagents increased, the hydrophobicity of DES was improving: due to the excellent hydrophilicity of the 1-propyl alcohol, it cannot form hydrophobic DES with all the HBAs; DES
2.2. Synthesis of deep eutectic solvents Quaternary ammonium salts as HBA was mixed with HBD reagents (decanoic acid, 1-propyl alcohol, ethylene glycol, glycerol, 1-hexanol, N-hexanoic acid (molar ratio = 1:1)). Then the mixtures were heated up to 90 °C and stirred for 60 min until formation of a homogenous transparent phase. Then, the synthesized DESs were cooled and stored under the room temperature.
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Table 1 The status of composed DESs after hydrophobicity tests. The squares with “Solid” marked: solid (under room temperature); the squares with crossed lines: hydrophilic; the square with slash: cloudy status shows; the unmarked squares: hydrophobic.
carboxyl are around 1704–1711, after the formation of DESs, it moved to around 1725–1731 (a1-c1). It meant the inter-molecule hydrogen bonds declined in quantity and di-polymer effect decreased due to the generation of hydrogen bonds between bromide anions and carboxyl. Slight changes (~3) occurred in the shifts of above peaks after the extraction, presented still the presence of the hydrogen bonds and not broken. Accordingly, the mechanism based on an anion exchange procedure was concluded. The reaction formulation is under below:
combinations showed the cloudy status after centrifugation due to the higher viscosity of ethylene glycol and glycerol; homogeneous liquid phases were found when the DESs were combined with 1-hexanol and N-hexanoic acid after 20 min centrifugation, which showed lower viscosity and more suitable for gold extraction. In order to investigate the extraction ability and behaviors of DES composed by different alkyl chain length and avoid the effects of the structures brought by different HBDs, N3333-Br-acid, N4444-Br-acid, and N8881-Br-acid were chose for further gold extraction experiments. The following procedures were conducted to investigate their differences in gold extraction ability by dissolving a certain amount of DES in 1 mL nhexanoic acid for dispersive liquid-liquid extraction.
[C5 H13 − COOH⋯⋯Br] [N3333] + AuCl 4− = [C5 H13 − COOH⋯⋯Br]− + [AuCl 4 ][N3333]
(3-1)
[C5 H13 − COOH⋯⋯Br] [N4444] + AuCl 4− 3.2. Extraction mechanism
= [C5 H13 − COOH⋯⋯Br]− + [AuCl 4 ][N4444]
All the 3 DESs were showing a certain ability to extract Au(III) from aqueous phase. In order to investigate the interaction between AuCl4− and DESs. UV–vis was performed to analyze the status of Au(III) before and after extraction. The peak of pure AuCl4− in water is around 318 nm. From the Fig. 1, it could be seen that the respective peak of Au (III) in each extracted complex was 312, 326 and 341 nm, proving the status of AuCl4− maintained. The resulting slight fluctuations were attributed to the effect of solvation effect and combination with DESs (Boudesocque et al., 2019). Then, the FT-IR spectra was used to confirm that if the structures of DESs changed before and after extraction. It was found that, strong nonplanar rocking vibration absorption peaks of –OH showed around 933.02 (a symbol of di-polymer status because of the inter-molecule hydrogen bonds) in Fig. 2. After the formation of DES, the intensity of corresponding peak weakened. The telescopic vibration peaks of
(3-2)
[C5 H13 − COOH⋯⋯Br] [N8881] + AuCl 4− = [C5 H13 − COOH⋯⋯Br]− + [AuCl 4 ][N8881]
(3-3)
And the extraction process was visualized in Fig. 3. 3.3. Influence factors Since the anion exchange mechanism was proved, following experimental conditions including concentration of Au(III), salinity and HCl were applied to investigate their gold extraction abilities. Besides, vibrating time was fixed at 30 min, and the operating temperature is based on room temperature (25 °C). The concentration of DESs was fixed at 1 mM with the concentration of Au(III) varying. The extraction ability increases with the length of alkyl chains increases, shown in Fig. 4(A). N8881-Br-acid could extract up to 96.8% of Au(III) from 1 mM Au(III) chloride solution, while N4444Br-acid and N3333-Br-acid extract 93% and 91% of Au(III) from 0.6 mM Au(III) chloride solution respectively. The reason could be that, with alkyl chains increased, stronger electron donating effect make the electrostatic attraction effect between quaternary ammonium and [Br···HOOC-C5H11]− decline, which pushed ammonium cations drag Au (III) anions into the DES phase (Xiang et al., 2017). Under the respective maximum concentration of Au(III) could be almost extracted, the effect of NaCl/NaBr concentration on E% experiments were conducted. The obvious “V-shaped” trends were shown in Fig. 4(B) and (C). It might be related to two kinds of different effects: on the first half, the E% decreased with the salt concentration increased because of the competition effect with AuCl4− dominated; on the second half, the E% increases with the salt concentration increased because the AuCl4− is more hydrated due to the extra supplementary chloride/bromide ions (Bulgariu and Bulgariu, 2011). Most notably, the extraction ability of N8881-Br-acid showed the less decrease, and it could extract more Au (III) than N4444-Br-acid and N3333-Br-acid in all concentrations of NaCl/
Fig. 1. UV–vis spectrum of Au(III) after extraction. 266
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Fig. 2. FT-IR spectrum of pure n-hexanoic acid, (a1) [N3333]Br⋯⋯HOOC-C5H11, (b1) [N4444]Br⋯⋯HOOC-C5H11, (c1) [N8881]Br⋯⋯HOOC-C5H11 and corresponding extracted complex (a2-c2).
3.5. Gold stripping
NaBr. All the gold extraction abilities of 3 DESs could maintain at maximum when the pH = 2. It showed the monotonous decreasing trends when the amount of HCl increased, shown in Fig. 4(D). The similar decreasing effect of H+ on E% was obtained, shown in Fig. S1, and it could be attributed to that the hydrolysis of HAuCl4 was inhibited due to the increased H+ concentration. In summary, during extraction process, the N8881-Br-acid is more suitable for gold extraction in the circumstances with high salinity and lower acidity (pH ≈ 2), compared with N4444-Br-acid and N3333-Bracid.
One-step reduction method by adopting NaBH4 was proved to be the most effective way to strip Au(III) from the DES phase completely, compared with thiourea, oxalate and sodium bisulfite. 1 mL 0.1 g/mL NaBH4 solution was add to the DES phases transferred to a new tube after gold extraction. Then DES phases turned black with generation of bubbles. After 10 min vibration and 30 min centrifugation, the resulting black precipitation settled to the bottom of the tube, the upper DES phase was transferred to another new tube. Subsequently, 2 mL of DI water was added with 30 min vibration to wash and remove the residual NaBH4 in DES phase. The refreshed DESs after centrifugation was used to extract Au(III) again. With 5 turns of circulation experiments, the extraction ability of each DES was kept, slight changes occurring due to the inevitable partial loss of DESs in transfer (Fig. 6).
3.4. Selective extraction test High efficiency gold separation from waste electrical and electronic equipment is one of the most important factors to evaluate the applicability of the DESs in hydrometallurgy. Multi-metal solution includes 1 mM of Mg(II), Al(III), Mn(II), Co(II), Cu(II), Au(III), Ce(III) and Ni(II). 1 mM of the 3 DES were added to the 1 mL multi-metal solutions respectively. Fig. 5 presented that the similar separation ability between N3333-Br-acid and N4444-Br-acid, which could extract around 100 mg/L Au(III). Surprisingly, the N8881-Br-acid could extract almost of the Au(III) (~192 mg/L), showed about the twice ability as much as of them.
3.6. RSM optimization Owing to the excellent ability of N8881-Br-acid in gold recovery ability separation from secondary resources, it was regarded as the most potential system for application, modeling by mathematical method to reduce operational cost is necessary. Response surface methodology (RSM) is regarded as one of the most effective tools to evaluate the relationship between the experimental and the predicted results (Fan
Fig. 3. The visualized gold extraction process based on an anion exchange mechanism. [N8881]+ is visualized as a claw grabbing Au(III) in the aqueous phase. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 267
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Fig. 4. (A) Concentration of Au(III), (B) NaBr, (C) NaCl and (D) HCl on extraction efficiency (E%).
Fig. 6. 5 turns of gold extraction circulation experiments. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Selective metal separation experiments from multi-metal solution.
et al., 2008; Lee et al., 2006; Jiang et al., 2019). A three-level two-factor central composite design was chosen to acquire a model for evaluating the extraction ability (mg/L) with two process variables (operating temperature and vibration time, coded as A and B). Firstly, the single factor experiments were carried out to provide a reasonable range of the independent variables of response surface experiments, shown in Fig. 7. Both the operating temperature and vibration time have significant effects on the extraction efficiency. Accordingly, the two factors level was set at: 0, 25 and 50 °C; 10, 15 and 20 min. The goal was set at 192 mg/L, according to the average value calculated by the previous experiments. Details of ANOVA were shown in Fig. S2, giving a P-value
˂ 0.0001 and F-value = 811.88. The R2 of the model is 0.9983, P < .0001, which indicated that the model used to fit response variables is significant and adequate to represent the relationship between the extraction yield (Y) and independent variables (operating temperature and vibration time) (Bi et al., 2013). By multivariate regression analysis of experimental data, the model was expressed as secondorder quadratic polynomial equations as follows:
Ygold = 192.144–0.83 ∗ A + 0.14 ∗ B + 0.028 ∗ A ∗ B − 0.13 ∗ A2–0.12 ∗ B2 268
(3-4)
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Fig. 7. (A) Temperature and (B) equilibrium time effect on gold extraction ability of N8881-Br-acid system.
Fig. 8. RSM optimization of gold extraction yield by N8881-Br-acid system. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3D response surface plots by using DesignExpert.V10.0.7 software was shown in Fig. 8. The optimal values were as follows: operating temperature is 19 °C, and vibration time is 13 min. Through the optimization process, the optimized theoretical extraction yield was around 192.14 mg/L, the desirability is 1.000, whose details were shown in Fig. S3.
3.7. Extraction isotherm and McCabe–Thiele diagram The Au(III) extraction distribution isotherm and the McCabe–Thiele diagram obtained are showing in Fig. 9. Based on the 19 °C and 13 min vibration time, under the given A/O ratio = 1:1, the feed solution containing 197 mg/L Au(III) was equilibrated with the organic phase at different phase ratios (VDES: VH2O) from 11.500 to 0.064, with keeping the total volume of phases 5 mL. The complete extraction needs the 2 theoretical stages were needed for quantitative extraction of Au(III) by using 1 mM DESs, proving the effectiveness of N8881-Br-acid after the RSM optimization and a tiny amount (~5%) of Au(III) left in feed solution (Nguyen et al., 2015). Accordingly, the whole Au(III) separation procedure from secondary resources based on a 2-theoretical-stages counter current extraction process was proposed and shown in Fig. 10. Firstly, the feed solution phase was equilibrated with second-stage DES phase (namely DES (II)), then, the first-stage raffinate (namely Secondary Resources Containing Au(III) (II)) was secondly equilibrated with the fresh DES phase (namely DES (I)). The processing conditions are 19 °C and 13 min vibration time. After 2-stages-counter-current-
Fig. 9. Mccabe-Thiele diagram of gold extraction through given 1:1 A/O ratio. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
extraction process, very few of Au(III) with other metals ions was left in the remaining water, indicated by Fig. 9. The extracted Au(III) in DES phase was easily stripped by one-step reduction method through addition of NaBH4. The refreshed DES after washing by water will be 269
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Fig. 10. The whole process of gold separation with 2-theoretical-stages counter current extraction process under optimum conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusions Three hydrophobic DES with low viscosity were screened out, constructed by [N3333]Br, [N4444]Br, [N8881]Br and n-Hexanoic acid, to recovery Au(III) from hydrochloric medium. Through combination of UV–vis and FT-IR, the anion exchange mechanism was proposed: anions [C5H13-COOH⋯⋯Br]− in each DES are exchanged with AuCl4− during the extraction process. Based on the investigated behaviors under different conditions, N8881-Br-acid showed the best ability of gold extraction and resistance against higher salinity, among 3 of those. The extracted Au(III) in DES phases could be stripped easily by applying 0.1 mg/L NaBH4 solution, so that the gold extraction ability of 3 DESs were kept during the 5 turns of circulation of extraction. N8881-Br-acid was regarded as the most potential system for gold recovery from hydrochloric due to the best separation ability, its operating temperature and vibration time were optimized to 19 °C and 13 min vibration time by using RSM to reduce the operating cost. Under the given A/O = 1:1 and optimized operating conditions, 2 theoretical stages was enough for N8881-Br-acid system to extract around 95% of Au(III) with high efficiency.
Declaration of Competing Interests The authors declare no competing financial interests.
Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21476129).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.hydromet.2019.06.013. 270
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Recovery of gold from hydrochloric medium by deep eutectic solvents based on quaternary ammonium salts Yuqi Geng, Zeyang Xiang, Cheng Lv, Ning Wang, Yudong Wang, Yanzhao Yang
T
⁎
Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry and Chemical Engineering. Shandong University, Jinan 250100, PR China
A B S T R A C T
In this paper, 3 kinds of hydrophobic deep eutectic solvents were screened out for gold recovery due to their low viscosity, which were constructed by ionic liquids: [N3333]Br, [N4444]Br and [N8881]Br and N-hexanoic acid. Anion exchange mechanism during the extraction process was proposed through the combination of UV–vis and FT-IR. Subsequently, the extraction behaviors under different conditions, such as: salinity (NaCl/NaBr), acidity and Au(III) concentration were investigated and discussed. N8881-Br-acid showed the best extraction ability: it showed the 96.8% extraction efficiency on 1 mM Au(III) solution, compared with N4444-Br-acid (around 93% on 0.6 mM) and N3333-Br-acid (around 91% on 0.6 mM). Besides that, the N8881-Br-acid showed the better salinity resistance among 3 DESs. The extracted Au (III) in the DESs phase was easily and completely stripped by applying 1 mL 0.1 g/mL NaBH4, and each extraction ability maintained after 5 turns of extraction cycle experiments. Since the excellent behaviors showed in the multi-metal selective extraction and extraction cycle experiments, the N8881-Br-acid was regarded as the most potential system for gold recovery from HCl medium. Furthermore, its operating conditions combination: operating temperature and vibration time were optimized to 19 °C and 13 min by applying response surface methodology (RSM). Under optimized conditions combination and given A/O = 1:1, the complete extraction procedure only needs two theoretical stages with a tiny amount (~5%) of Au(III) left, which is indicated by drawing Mccabe-Thiele diagram.
1. Introduction Nowadays, gold has been widely applied in chemistry, commercial investments, high-tech manufacturing and so on (Gurung et al., 2013; Parajuli et al., 2009; Vinh Hung et al., 2014; Vinh Hung et al., 2010). Both the world supply and demand of gold keeps increasing. While the limited natural reservation, gold recovery from secondary resources (e.g. wasted computer circuit boards, electronic scrap and waste electroplating solutions) will be a more efficient way to obtain precious metals, compared with conventional hydrometallurgy industry (Geng et al., 2019; Yoshimura and Matsuno, 2016; Yoshimura et al., 2014). It was reported that Tokyo now is running an e-waste recycling program for the 2020-Tokyo-Olympic-games gold medal production and has collected 48 thousand tons of used mobile phones and other electronic equipments (Tokyo 2020, 2019), due to the lower cost and abundant contents of gold, silver and copper. Novel extractants with high extraction ability and easy functionalization, were discovered and applied, such as: functional ionic liquids. Since the first industrial process involving ionic liquids was announced in 2003, the potential of ionic liquids is beginning to be recognized (Armand et al., 2009). Due to its properties: excellent stability, easy modification, low vapor pressure and so on, it now has been widely applied in metal separation and extraction (de Oliveira et al., 2019; Yan et al., 2018; Yan et al., 2017). However, when talking about ionic ⁎
Corresponding author. E-mail address: [email protected] (Y. Yang).
https://doi.org/10.1016/j.hydromet.2019.06.013 Received 20 March 2019; Received in revised form 8 June 2019 Available online 25 June 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
liquids applied in the traditional liquid-liquid extraction, hydrophobic reformation of hydrophilic ionic liquids always needs complex procedures: external alkyl chain introduction, ions exchange with PF6−, NTf2− to improve their hydrophobicity and so on (Rzelewska-Piekut and Regel-Rosocka, 2019; Trujillo-Rodríguez and Anderson, 2019; Van Roosendael et al., 2019). During the anion exchange in extraction process, these exchanged anions (e.g. PF6−, NTf2−) would be lost in the aqueous phase, followed by high cost of use, blocking itself from cyclic using (Zheng et al., 2016; Zheng et al., 2015) and further industrial application. At the same time, a great demand of reagents was used during the synthesis procedure, followed by generation of a huge number of wastes, which didn't meet the purpose of green sustainable development. However, the method of hydrophobic DES preparation showed us a novel idea to solve the above problems. Deep eutectic solvents are a class of mixture composed by the HBA (hydrogen bond acceptor) and HBD (hydrogen bond donor) (Liu et al., 2018), which was firstly discovered by Abbott. et al., who found new characteristics by mixing the choline chloride and urea (Abbott et al., 2003). Hydrophobic DESs preparation attracted us because of its easier synthesis process compared with conventional hydrophobic ionic liquids: just mix the HBA and HBD reagents at a certain temperature to form a homogeneous and stable phase, followed by the excellent features same as ionic liquids. At the same time, lower costs, lower toxicity and more convenient access
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2.3. Gold extraction
to raw materials means a more application prospects compared with ionic liquids. Nowadays, DES were popularly applied in organic synthesis (Alonso et al., 2016), catalysis (Liu et al., 2015), nanomaterials preparation (Abo-Hamad et al., 2015; Wagle et al., 2014), gas adsorption (Garcia et al., 2015) and target objects enrich (Duan et al., 2019; Jiang et al., 2019; Pal and Jadeja, 2019; Radosevic et al., 2015; Ibrahim et al., 2019), but less research about hydrophobic DESs on precious metals extraction was found. So, in this paper, the prospects and extraction behavior of hydrophobic DESs based on quaternary ammonium salts for gold recovery were discussed. The quaternary ammonium salts with short alkyl chain: [N1111]Br, [N2222]Br, [N3333]Br, [N4444]Br, and [N8881]Br were chose to construct hydrophobic DES combining with different HBDs. After hydrophobicity tests, three ideal DESs were screened out due to the lower viscosity: N3333-Br-acid, N4444-Br-acid, and N8881-Br-acid. Through analysis of FT-IR and UV–vis spectra, anion-exchange mechanism during the extraction was proposed. The extraction behaviors of 3 DESs under NaCl/NaBr, HCl and gold concentration were investigated, which showed that the extraction ability and resistance against NaCl/NaBr increased with the alkyl chain of quaternary ammonium salts increased. Among of 3 DESs applied, N8881-Br-acid was regarded as the most potential structure for gold recovery, due to its higher separation ability and more stable extraction ability after 5 turns of extraction cycle experiments. So, its operating conditions of vibration time and temperature was separately optimized by RSM. Under the optimum combination, a two-theoretical-stage tandem extraction process was proposed based on the McCabe-Thiele diagram.
Take influence factors part for example, 1 mL of Au(III) chloride solution (concentration varies from 0.2 to 1.4 mM) and 1 mM*1 mL of DESs (diluted by n-hexanoic acid) were mixed together at room temperature (25 °C). The samples were centrifuged for 10 min after 30 min mechanical vibration to make sure the status of the clear biphasic phases. The concentration of Au(III) remaining in water was determined by flame atomic absorption spectroscopy (FAAS). Extraction efficiency (E%) was applied to quantify the ability that DESs combined with Au(III). The value could be calculated using Eq. (1):
E(%) = (1 − Mp /Mi ) × 100
(1)
where Mp is the Au(III) amount in the permeated solution and Mi is the Au(III) amount in the initial solution. 2.4. RSM experimental design A central-composite-designed Response surface methodology (RSMCCD) was employed to optimize the comprehensive effects including temperature (A: 0–50 °C) and vibration time (B: 10–20 min) on gold extraction yield (Ygold) in three levels, whose corresponding range had been determined based on the preliminary experiments. The secondorder model for predicting the optimum effect of conditions combination was expressed in eq. 2: k
Y = β0 +
k
∑ βi Xi + ∑ βii Xi2 + ∑ ∑ βij Xi Xj + ε i=1
i=1
i
(2)
where Y is the response, β0 is the value for the fixed response at central point of the experiment, βi, βii, and βij are the linear, quadratic and cross product coefficients, respectively, and Xi and Xj are the coded independent variables (Priya et al., 2019). The whole optimization experiments were conducted with the fixed concentration of Au(III) solution (197 mg/L), DES (1 mM) and the ratio of A/O (1:1). The analysis of the statistics was based on the Design Expert software (Version 8.0.6). The second-order equation obtained and model analysis were shown in RSM optimization part.
2. Experimental 2.1. Chemicals and materials The tetra-ethyl-ammonium bromide (TEAB, A.R.), tetra-propyl-ammonium bromide (TPAB, A.R.), octyl-trimethyl-ammonium bromide (A.R.) were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd., (Beijing China). Decanoic acid (A.R.), n-Hexyl alcohol (A.R.), 1hexanol (A.R.), glycerol (A.R.), 1-propyl alcohol (A.R.), Ethylene glycol (A.R.), were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd., (Beijing China). MnCl2·4H2O, SnCl4·5H2O, FeCl3·6H2O, CoCl2·6H2O, MgCl2·6H2O, and CuCl2·2H2O were provided by Aladdin Co., Ltd. (Shanghai, China). DI water used in the whole experiment process was triply distilled by a quartz water purification machine, whose conductivity was lower than 1.8 μS·cm−1 as measured by a DDSJ-308A type conductivity instrument in laboratory. The remaining concentrations of Au(III) and multi-metal ions (Mg(II), Al(III), Mn(II), Co(II), Cu(II), Ce(III), Ni(II)) were determined by a flame atomic absorption spectrometer (3150, Precision & Scientific Instrument, Shanghai, China) and an ICP-OES (iCAP-7000, Thermo Fisher Scientific, Boston, US) respectively. The UV–vis spectra (UV-9000, Metash, Shanghai, China) and FT-IR (Tensor27, Bruker corporation, Karlsruhe, Germany) were used to analyze the synthesized DES and the corresponding extracted species. The temperature used for Au(III) extraction was 25 °C. Each sample data was measured for 3–5 times, and the values were averaged.
3. Results and discussion 3.1. Hydrophobic DESs combination screening 5 hydrophilic low alkyl chain quaternary ammonium ionic liquids: [N1111]Br, [N2222]Br, [N3333]Br, [N4444]Br, [N8881]Br were mixed with a series of HBD reagents to construct hydrophobic DESs. Because of the formation of new hydrogen bonds, DES has higher viscosity than simple HBD (Zhang et al., 2012), which could cause a negative effect on the mass transfer between phases in the gold extraction process. So, we screened out the appropriate combinations with low viscosity by mixing them with DI water for 30 min and centrifugation for 20 min. The stable two phases without volume loss proves the successful hydrophobic reformation with low viscosity, and results were listed in Table 1. It could be seen that, the successful hydrophobic transformation relied on the structure and hydrophobicity of HBAs and HBDs. Each mixture of [N1111]Br or [N2222]Br with HBD reagents cannot form a homogeneous stable phase after heating up to 90 °C for 1 h. That was attributed to the shorter alkyl chain length of themselves. When the alkyl chain length of HBA increased, more hydrophobic combinations showed up: more potential hydrophobic combinations showed when composed by [N8881] Br with HBD reagents. At the same time, status of HBDs could affect the status of DESs, for example: the composed DESs were all solid due to the solid state of decanoic acid stored at room temperature. When the hydrophobicity of the HBD reagents increased, the hydrophobicity of DES was improving: due to the excellent hydrophilicity of the 1-propyl alcohol, it cannot form hydrophobic DES with all the HBAs; DES
2.2. Synthesis of deep eutectic solvents Quaternary ammonium salts as HBA was mixed with HBD reagents (decanoic acid, 1-propyl alcohol, ethylene glycol, glycerol, 1-hexanol, N-hexanoic acid (molar ratio = 1:1)). Then the mixtures were heated up to 90 °C and stirred for 60 min until formation of a homogenous transparent phase. Then, the synthesized DESs were cooled and stored under the room temperature.
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Table 1 The status of composed DESs after hydrophobicity tests. The squares with “Solid” marked: solid (under room temperature); the squares with crossed lines: hydrophilic; the square with slash: cloudy status shows; the unmarked squares: hydrophobic.
carboxyl are around 1704–1711, after the formation of DESs, it moved to around 1725–1731 (a1-c1). It meant the inter-molecule hydrogen bonds declined in quantity and di-polymer effect decreased due to the generation of hydrogen bonds between bromide anions and carboxyl. Slight changes (~3) occurred in the shifts of above peaks after the extraction, presented still the presence of the hydrogen bonds and not broken. Accordingly, the mechanism based on an anion exchange procedure was concluded. The reaction formulation is under below:
combinations showed the cloudy status after centrifugation due to the higher viscosity of ethylene glycol and glycerol; homogeneous liquid phases were found when the DESs were combined with 1-hexanol and N-hexanoic acid after 20 min centrifugation, which showed lower viscosity and more suitable for gold extraction. In order to investigate the extraction ability and behaviors of DES composed by different alkyl chain length and avoid the effects of the structures brought by different HBDs, N3333-Br-acid, N4444-Br-acid, and N8881-Br-acid were chose for further gold extraction experiments. The following procedures were conducted to investigate their differences in gold extraction ability by dissolving a certain amount of DES in 1 mL nhexanoic acid for dispersive liquid-liquid extraction.
[C5 H13 − COOH⋯⋯Br] [N3333] + AuCl 4− = [C5 H13 − COOH⋯⋯Br]− + [AuCl 4 ][N3333]
(3-1)
[C5 H13 − COOH⋯⋯Br] [N4444] + AuCl 4− 3.2. Extraction mechanism
= [C5 H13 − COOH⋯⋯Br]− + [AuCl 4 ][N4444]
All the 3 DESs were showing a certain ability to extract Au(III) from aqueous phase. In order to investigate the interaction between AuCl4− and DESs. UV–vis was performed to analyze the status of Au(III) before and after extraction. The peak of pure AuCl4− in water is around 318 nm. From the Fig. 1, it could be seen that the respective peak of Au (III) in each extracted complex was 312, 326 and 341 nm, proving the status of AuCl4− maintained. The resulting slight fluctuations were attributed to the effect of solvation effect and combination with DESs (Boudesocque et al., 2019). Then, the FT-IR spectra was used to confirm that if the structures of DESs changed before and after extraction. It was found that, strong nonplanar rocking vibration absorption peaks of –OH showed around 933.02 (a symbol of di-polymer status because of the inter-molecule hydrogen bonds) in Fig. 2. After the formation of DES, the intensity of corresponding peak weakened. The telescopic vibration peaks of
(3-2)
[C5 H13 − COOH⋯⋯Br] [N8881] + AuCl 4− = [C5 H13 − COOH⋯⋯Br]− + [AuCl 4 ][N8881]
(3-3)
And the extraction process was visualized in Fig. 3. 3.3. Influence factors Since the anion exchange mechanism was proved, following experimental conditions including concentration of Au(III), salinity and HCl were applied to investigate their gold extraction abilities. Besides, vibrating time was fixed at 30 min, and the operating temperature is based on room temperature (25 °C). The concentration of DESs was fixed at 1 mM with the concentration of Au(III) varying. The extraction ability increases with the length of alkyl chains increases, shown in Fig. 4(A). N8881-Br-acid could extract up to 96.8% of Au(III) from 1 mM Au(III) chloride solution, while N4444Br-acid and N3333-Br-acid extract 93% and 91% of Au(III) from 0.6 mM Au(III) chloride solution respectively. The reason could be that, with alkyl chains increased, stronger electron donating effect make the electrostatic attraction effect between quaternary ammonium and [Br···HOOC-C5H11]− decline, which pushed ammonium cations drag Au (III) anions into the DES phase (Xiang et al., 2017). Under the respective maximum concentration of Au(III) could be almost extracted, the effect of NaCl/NaBr concentration on E% experiments were conducted. The obvious “V-shaped” trends were shown in Fig. 4(B) and (C). It might be related to two kinds of different effects: on the first half, the E% decreased with the salt concentration increased because of the competition effect with AuCl4− dominated; on the second half, the E% increases with the salt concentration increased because the AuCl4− is more hydrated due to the extra supplementary chloride/bromide ions (Bulgariu and Bulgariu, 2011). Most notably, the extraction ability of N8881-Br-acid showed the less decrease, and it could extract more Au (III) than N4444-Br-acid and N3333-Br-acid in all concentrations of NaCl/
Fig. 1. UV–vis spectrum of Au(III) after extraction. 266
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Fig. 2. FT-IR spectrum of pure n-hexanoic acid, (a1) [N3333]Br⋯⋯HOOC-C5H11, (b1) [N4444]Br⋯⋯HOOC-C5H11, (c1) [N8881]Br⋯⋯HOOC-C5H11 and corresponding extracted complex (a2-c2).
3.5. Gold stripping
NaBr. All the gold extraction abilities of 3 DESs could maintain at maximum when the pH = 2. It showed the monotonous decreasing trends when the amount of HCl increased, shown in Fig. 4(D). The similar decreasing effect of H+ on E% was obtained, shown in Fig. S1, and it could be attributed to that the hydrolysis of HAuCl4 was inhibited due to the increased H+ concentration. In summary, during extraction process, the N8881-Br-acid is more suitable for gold extraction in the circumstances with high salinity and lower acidity (pH ≈ 2), compared with N4444-Br-acid and N3333-Bracid.
One-step reduction method by adopting NaBH4 was proved to be the most effective way to strip Au(III) from the DES phase completely, compared with thiourea, oxalate and sodium bisulfite. 1 mL 0.1 g/mL NaBH4 solution was add to the DES phases transferred to a new tube after gold extraction. Then DES phases turned black with generation of bubbles. After 10 min vibration and 30 min centrifugation, the resulting black precipitation settled to the bottom of the tube, the upper DES phase was transferred to another new tube. Subsequently, 2 mL of DI water was added with 30 min vibration to wash and remove the residual NaBH4 in DES phase. The refreshed DESs after centrifugation was used to extract Au(III) again. With 5 turns of circulation experiments, the extraction ability of each DES was kept, slight changes occurring due to the inevitable partial loss of DESs in transfer (Fig. 6).
3.4. Selective extraction test High efficiency gold separation from waste electrical and electronic equipment is one of the most important factors to evaluate the applicability of the DESs in hydrometallurgy. Multi-metal solution includes 1 mM of Mg(II), Al(III), Mn(II), Co(II), Cu(II), Au(III), Ce(III) and Ni(II). 1 mM of the 3 DES were added to the 1 mL multi-metal solutions respectively. Fig. 5 presented that the similar separation ability between N3333-Br-acid and N4444-Br-acid, which could extract around 100 mg/L Au(III). Surprisingly, the N8881-Br-acid could extract almost of the Au(III) (~192 mg/L), showed about the twice ability as much as of them.
3.6. RSM optimization Owing to the excellent ability of N8881-Br-acid in gold recovery ability separation from secondary resources, it was regarded as the most potential system for application, modeling by mathematical method to reduce operational cost is necessary. Response surface methodology (RSM) is regarded as one of the most effective tools to evaluate the relationship between the experimental and the predicted results (Fan
Fig. 3. The visualized gold extraction process based on an anion exchange mechanism. [N8881]+ is visualized as a claw grabbing Au(III) in the aqueous phase. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 267
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Fig. 4. (A) Concentration of Au(III), (B) NaBr, (C) NaCl and (D) HCl on extraction efficiency (E%).
Fig. 6. 5 turns of gold extraction circulation experiments. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. Selective metal separation experiments from multi-metal solution.
et al., 2008; Lee et al., 2006; Jiang et al., 2019). A three-level two-factor central composite design was chosen to acquire a model for evaluating the extraction ability (mg/L) with two process variables (operating temperature and vibration time, coded as A and B). Firstly, the single factor experiments were carried out to provide a reasonable range of the independent variables of response surface experiments, shown in Fig. 7. Both the operating temperature and vibration time have significant effects on the extraction efficiency. Accordingly, the two factors level was set at: 0, 25 and 50 °C; 10, 15 and 20 min. The goal was set at 192 mg/L, according to the average value calculated by the previous experiments. Details of ANOVA were shown in Fig. S2, giving a P-value
˂ 0.0001 and F-value = 811.88. The R2 of the model is 0.9983, P < .0001, which indicated that the model used to fit response variables is significant and adequate to represent the relationship between the extraction yield (Y) and independent variables (operating temperature and vibration time) (Bi et al., 2013). By multivariate regression analysis of experimental data, the model was expressed as secondorder quadratic polynomial equations as follows:
Ygold = 192.144–0.83 ∗ A + 0.14 ∗ B + 0.028 ∗ A ∗ B − 0.13 ∗ A2–0.12 ∗ B2 268
(3-4)
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Fig. 7. (A) Temperature and (B) equilibrium time effect on gold extraction ability of N8881-Br-acid system.
Fig. 8. RSM optimization of gold extraction yield by N8881-Br-acid system. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3D response surface plots by using DesignExpert.V10.0.7 software was shown in Fig. 8. The optimal values were as follows: operating temperature is 19 °C, and vibration time is 13 min. Through the optimization process, the optimized theoretical extraction yield was around 192.14 mg/L, the desirability is 1.000, whose details were shown in Fig. S3.
3.7. Extraction isotherm and McCabe–Thiele diagram The Au(III) extraction distribution isotherm and the McCabe–Thiele diagram obtained are showing in Fig. 9. Based on the 19 °C and 13 min vibration time, under the given A/O ratio = 1:1, the feed solution containing 197 mg/L Au(III) was equilibrated with the organic phase at different phase ratios (VDES: VH2O) from 11.500 to 0.064, with keeping the total volume of phases 5 mL. The complete extraction needs the 2 theoretical stages were needed for quantitative extraction of Au(III) by using 1 mM DESs, proving the effectiveness of N8881-Br-acid after the RSM optimization and a tiny amount (~5%) of Au(III) left in feed solution (Nguyen et al., 2015). Accordingly, the whole Au(III) separation procedure from secondary resources based on a 2-theoretical-stages counter current extraction process was proposed and shown in Fig. 10. Firstly, the feed solution phase was equilibrated with second-stage DES phase (namely DES (II)), then, the first-stage raffinate (namely Secondary Resources Containing Au(III) (II)) was secondly equilibrated with the fresh DES phase (namely DES (I)). The processing conditions are 19 °C and 13 min vibration time. After 2-stages-counter-current-
Fig. 9. Mccabe-Thiele diagram of gold extraction through given 1:1 A/O ratio. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
extraction process, very few of Au(III) with other metals ions was left in the remaining water, indicated by Fig. 9. The extracted Au(III) in DES phase was easily stripped by one-step reduction method through addition of NaBH4. The refreshed DES after washing by water will be 269
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Fig. 10. The whole process of gold separation with 2-theoretical-stages counter current extraction process under optimum conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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4. Conclusions Three hydrophobic DES with low viscosity were screened out, constructed by [N3333]Br, [N4444]Br, [N8881]Br and n-Hexanoic acid, to recovery Au(III) from hydrochloric medium. Through combination of UV–vis and FT-IR, the anion exchange mechanism was proposed: anions [C5H13-COOH⋯⋯Br]− in each DES are exchanged with AuCl4− during the extraction process. Based on the investigated behaviors under different conditions, N8881-Br-acid showed the best ability of gold extraction and resistance against higher salinity, among 3 of those. The extracted Au(III) in DES phases could be stripped easily by applying 0.1 mg/L NaBH4 solution, so that the gold extraction ability of 3 DESs were kept during the 5 turns of circulation of extraction. N8881-Br-acid was regarded as the most potential system for gold recovery from hydrochloric due to the best separation ability, its operating temperature and vibration time were optimized to 19 °C and 13 min vibration time by using RSM to reduce the operating cost. Under the given A/O = 1:1 and optimized operating conditions, 2 theoretical stages was enough for N8881-Br-acid system to extract around 95% of Au(III) with high efficiency.
Declaration of Competing Interests The authors declare no competing financial interests.
Acknowledgements We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21476129).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.hydromet.2019.06.013. 270
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