Continuous Extraction of Gold(III) Using Pyridine Ionic Liquid-Based

Jun 19, 2019 - Recycling of urban mine, also defined as apparent mine, ..... The whole capillary ..... gold(III)) formed slugs with equivalent diamete...
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Cite This: Ind. Eng. Chem. Res. 2019, 58, 12729−12740

Continuous Extraction of Gold(III) Using Pyridine Ionic Liquid-Based Water-in-Oil Microemulsion in Microreactors Hong Zhang,† Minjing Shang,*,† Chong Shen,† Guangxiao Li,† and Yuanhai Su*,†,‡ †

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Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, 200240, P. R. China ‡ Key Laboratory of Thin Film and Microfabrication (Ministry of Education), Shanghai Jiao Tong University, Shanghai 200240, P. R. China ABSTRACT: Hydrophilic hexadecylpyridinium chloride ionic liquid constructed water-in-oil microemulsion was applied for the continuous extraction of gold(III) in a microreactor system. The anion exchange extraction mechanism was proven by spectroscopic techniques. In this extraction system, the remarkable extraction ability for gold(III) with the extraction percentage of 99.2% at the residence time of 14.7 s, and high selectivities for gold(III) over Ni(II), Fe(III), Cu(II), and Al(III) were achieved. Moreover, a higher flow rate and a smaller inner diameter of the capillary were beneficial for the gold(III) extraction, and the overall volumetric mass transfer coefficient (0.33−2.9 s−1) was higher than other metal ion extraction systems in microreactors, or the same extraction system in a batch reactor (0.007 to 0.103 s−1). Furthermore, the microemulsion recycling was realized, indicating such a study extends the applications of hydrophilic ionic liquids and microreactor technology on noble metal extraction.

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INTRODUCTION Gold (Au) extraction from ore and electronic waste has emerged as a worldwide topic, because this precious metal has tremendous value in a wide variety of fields, such as jewelry, modern electronics, medicine, and catalysis.1 However, serious environmental pollution and high energy consumption are still long-standing challenges in the gold recycling industry because of toxic oxidation reagents and harsh reaction conditions involved.2 During the past 10 years, the demand for gold in industrial applications has been rapidly growing and the supply of gold has been limited because of global maldistribution. Recycling of urban mine, also defined as apparent mine, including spent home appliances and used electronic gadgets, can replenish the supply of precious metals, i.e., gold, platinum, and palladium.3 In fact, the development of efficient approaches for gold recovery has been paid enough attention by both academia and industry. However, the conventional batch processing for gold separation is usually time-consuming and organic-solvent-consuming, which brings trouble to industrial production and environmental protection. Consequently, it is highly desirable to develop low-cost, facile, and efficient approaches for gold separation. Microreactors providing strong strategies for process intensification exhibit unique features, such as large surfaceto-volume ratio, high mixing efficiency, enhanced mass and heat transfer rates, inherent safety, and precise control over process parameters, that have led to broad research and application interests.4−8 With the above-mentioned character© 2019 American Chemical Society

istics, the enrichment and purification of metal ions have proven to be enhanced by microreactor technology. For example, the liquid−liquid extraction for recovering Li(I), Zn(II), Ca(II), La(III), and Am(III) and the selective separation of Co(II)/Ni(II), La(III)/Ce(III), and Pr(III)/ Nd(III) in different microstructured devices have already been reported by many authors, as shown in Table 1. High extraction efficiency and high selectivity could be achieved without any mechanical stirring because of excellent mass transfer performance resulting from short transport distance and large specific interfacial area in microreactors.9−15 However, precious metal extraction processes in microreactors or microchannels have been seldom reported. The application of ionic liquids (ILs) as novel reaction media is another rapidly developing orientation for process intensification. The coupling of microreactor technology and ILs has been realized for various applications, especially in the metal ion separation field.16,17 Tsaoulidis et al. designed an optimized process of dioxouranium(VI) extraction using an ionic liquid ([Bmim][NTF2]) in microchannels.18 Subsequently, the aforementioned [Bmim][NTF2] as a green solvent was also utilized for enhancing the extraction of Eu(III) in a small channel.19 In comparison with traditional organic Received: Revised: Accepted: Published: 12729

April 22, 2019 June 18, 2019 June 19, 2019 June 19, 2019 DOI: 10.1021/acs.iecr.9b02158 Ind. Eng. Chem. Res. 2019, 58, 12729−12740

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Industrial & Engineering Chemistry Research Table 1. Research on the Liquid−Liquid Extraction of Metal Ions in Microreactors extraction systems D2EHPA dissolved in cyclohexane for lithium(I) ions extraction D2EHPA dissolved in dodecane for zinc(II) ions DC18C6 dissolved in n-butyl acetate for calcium(II) ions extraction solvent extraction separation of Am(III) and La(III) kerosene with Cyanex 272 for the separation of Co(II) and Ni(II) EHEHPA dissolved in kerosene for the separation of lanthanum and cerium P507 dissolved in sulfonated kerosene for Pr(III) and Nd(III) separation

mixing unit and flow regime

microreactors millimeter-diameter glass and polytetrafluoroethylene (PTFE) tubes split and recombine microchannel etched in glass chips rectangular glass microfluidic device FEP capillaries for constructing droplet-based microfluidics pilot-scale 3D flow microreactor

operation details

refs

Y-junction, slug flow

extraction channel diameter: 1.0 mm, Qaq= Qor: 3 mL/min

9

T-junction, split and recombine flow Y-junction, parallel flow and slug flow T-junction, slug flow

V: 26 μL, main channel diameter: 184 μm, side channel diameter: 71 μm Q: 0.5−200 μL/min, channel depth: 0.084 mm, width: 0.5 mm Extraction channel diameter: 0.25 mm,

10

three-dimensional circulatory flow

V: 12 mL, Q: 2.0 × 103 μL/min1.5 × 105 μL/min, capillary diameter: 3 mm Qaq= Qor: 3.36 × 10−2 mL/min, capillary diameters: 0.3−1.0 mm Qaq= Qor: 3.36 × 10−2 mL/min, capillary diameters: 0.3−1.0 mm

13

PMMA chips with a rectangular cross- Y-junction, slug flow section channel Teflon capillaries Y-junction, slug flow

11 12

14 15

large interfacial area for the mass transfer in the gold(III) extraction process confirmed by a high-speed CCD camera. The mass transfer characteristics were compared with both the conventional liquid−liquid extraction systems in microreactors and the same extraction system in the batch reactor. Furthermore, the recycling of microemulsion could be realized with high extraction efficiency of gold(III).

solvents, a feature of ILs is its higher viscosities, which usually leads to lower mass transfer processes in conventional reactors and limits its applications in industry. In this context, appropriate methods should be developed to decrease the effects of ionic liquid viscosities and overcome relevant shortcomings. Microemulsion is a transparent, isotropic, and thermodynamically stable system with nanosized droplets (1.0 M). Such a phenomenon was mainly caused by the stronger ion strength with increasing the concentration of hydrochloric acid.30 Based on these preliminary experimental results, we chosen a low concentration hydrochloric acid concentration (i.e., 0.1 mol/L) for the following study. Additionally, the single extraction of other metal ions (e.g., Ni2+, Fe3+, Cu2+ and Al3+) is also presented in Figure 5a under the identical operational conditions as those for the extraction

respectively. These results demonstrated that gold(III) was extracted from the aqueous phase to the microemulsion because of the strong incorporation between [AuCl4]− and [C16Pry]+, and the applied IL could be used as an effective extractant for the recovery of gold(III). Meanwhile, the FT-IR spectra of the pure [C16Pry]Cl and the pure neutral complex are shown in Figure 4b. Obviously, the C−H bond peak of alkyl chain almost did not shift, indicating the weak interaction between [AuCl4]− and the alkyl chain of [C16Pry] +. However, the C−H and C−N stretching vibration peaks of the pyridine ring notably shifted from 3048 to 3024 cm−1, and from 1472 to 1462 cm−1with the neutral complex formation, indicating the strong electronic interaction between [AuCl4]− and the pyridine ring of [C16Pry]+. According to both the results of UV−vis and FT-IR spectra, the extraction mechanism of anion exchange was proposed, and an equilibrium relationship can be described as follows: [C16Pry]Cl(me) + [AuCl4]−(aq) k+

V [C16Pry][AuCl4](me) + Cl−(aq) k−

(3)

The gold(III) extraction from the aqueous phase to the microemulsion mainly consisted of several steps, as shown in Scheme 1. First, [AuCl4]− transported from the bulk of the aqueous phase to the aqueous phase-microemulsion interface and further migrated into the organic phase of the microemulsion. Second, [AuCl4]− reacted with [C16Pry]+ of the aqueous microdroplets in the microemulsion by the strong electrostatic interaction, and the chloride ions were released in the microdroplets of the microemulsion through the above12733

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Figure 6. Effect of hexadecylpyridinium chloride ionic liquid concentration on gold(III) extraction (Qaq, 0.8 mL/min; Qme, 0.8 mL/min; CAu, 1.94 × 10−3 mol/L; CHCl, 0.1 mol/L; d, 0.5 mm; and L, 15 cm).

gold(III) first increased with increasing the IL concentration and then reached the maximum at the IL concentration of 0.0067 mol/L. The extraction percentage was maintained invariable when the IL concentration exceeded 0.0067 mol/L. 3.3. Effect of Solvent on the Gold(III) Extraction. For constructing a microemulsion system, organic solvents are needed as constituents of the microemulsion, and thus the contribution of organic solvents should be carefully evaluated.32 Cyclohexane and heptane are widely applied for the preparation of microemulsion systems for the gold(III) extraction since both of them can participate in the separation process as an extract phase and influence the solubility of chelate complexes.25,33,34 Compared with cyclohexane, the extraction percentage of gold(III) was much higher when heptane was applied with the same IL concentration (CIL = 0.0067 mol/L) at various total volumetric flow rates (see Figure 7). Even if the IL concentration was increased to a much higher value (i.e., CIL = 0.028 mol/L), the extraction percentage of gold(III) for the heptane-based microemulsion system was obviously lower than that for the cyclohexanebased microemulsion system with a low IL concentration (CIL = 0.0067 mol/L). In addition, it was found that the neutral

Figure 5. (a) Effect of hydrochloric acid concentration on the gold(III) extraction in the microreactor (CIL, 0.0067 mol/L; the initial concentration for each type of metal ions, 1.94 × 10−3 mol/L; Qaq, 0.8 mL/min; Qme, 0.8 mL/min; d, 0.5 mm; and L, 15 cm). (b) Extraction selectivity for gold(III) and other metal ions in the microreactor (CIL, 0.0067 mol/L; the initial concentration for each type of metal ions, 1.94 × 10−3 mol/L; CHCl, 0.1 mol/L; Qaq, 0.1 mL/min; Qme, 0.1 mL/ min; d, 0.5 mm;and L, 25 cm).

of gold(III). From Figure 5a, it can be seen that the extraction percentage for Ni(II), Fe(III), Cu(II) or Al(III) almost was maintained as 6.5%, 5%, 2%, or 0.1%, with the increase in the hydrochloride acid concentration, which was much lower than that for the extraction of gold(III). In fact, gold is often accompanied by many other base metals in ores, and thus selective extraction is very important to obtain the high purity gold(III).33,34 The extraction was also conducted in the microreactor for the aqueous mixture containing gold(III) and other base metal ions (i.e., Ni(II), Fe(III), Cu(II), and Al(III)). The initial concentration for each type of metal ions in the hydrochloric acid medium was 1.94 × 10−3 mol/L before extraction. After the extraction in the microreactor, the concentrations of these metal ions in the aqueous phase were determined by ICP-AES analysis. As shown in Figure 5b, the use of hexadecylpyridinium chloride IL based microemulsion in the microreactor realized the high extraction selectivity for gold(III) with the extraction percentage of 99.2%, whereas the extraction efficiency of other metals (i.e., Ni(II), Fe(III), Cu(II), and Al(III)) was less than 8% (i.e., ENi, 7.8%; EFe, 3.3%; ECu, 0.30%; EAl, 0.10%). The effect of the hexadecylpyridinium chloride IL concentration on the gold(III) extraction was also investigated. In Figure 6, it can be seen that the extraction percentage of

Figure 7. Effect of solvent on the gold(III) extraction at different concentrations of hexadecylpyridinium chloride IL (CHCl, 0.1 mol/L; d, 0.5 mm; and L, 15 cm). 12734

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Industrial & Engineering Chemistry Research complex between [AuCl4]− and [C16Pry]+ could well dissolve in the cyclohexane-based microemulsion (Figure 8a) but not in

flow. The dispersed phase (the aqueous solution with gold(III)) formed slugs with equivalent diameter bigger than the channel diameter, or formed droplets with the diameter smaller than the channel diameter.36 The existent film around the slugs or droplets is of vital significance for the contribution to the mass transfer. The film thickness (δ) can be estimated as a function of the capillary number (Ca) and the diameter of the channel (d) according to the Bretherton’s law, given by eq 4.37 δ = 1.34dCa 2/3

(4)

The capillary number is defined as the ratio of the viscous force to the interfacial tension, and it can be calculated by eq 5: Ca =

Figure 8. Photos of samples (a) the cyclohexane-based microemulsion extraction system, (b) the heptane-based microemulsion extraction system, and (c) the heptane-based microemulsion extraction system after standing for 2 min.

μM UM γ

(5)

where UM and μM are the superficial velocity and viscosity of the continuous phase, respectively. The value of Ca ranged from 4.29 × 10−3 to 51.48 × 10−3 in this extraction process. For a mass transfer unit cell of the slug or droplet flow in this work, the entire interfacial area available for the mass transfer included the organic film region and the slug or droplet end region (see Figure 9b). The film contribution to the mass transfer combined with the slug end contribution has been reported in the literature.38 With the presence of an organic film, the specific interfacial area (α) is expressed by eq 6.

the heptane-based microemulsion (Figure 8b). After standing for a short time, the neutral complex was even adsorbed at the interface between the heptane-based microemulsion and the aqueous phase (Figure 8c). Therefore, cyclohexane was considered as an optimal solvent for constructing the microemulsion in the gold(III) extraction process. 3.4. Mass Transfer Characteristics in the Microemulsion-Based Extraction Process. Two-phase flow pattern is one of crucial factors that significantly affect the liquid−liquid mass transfer performance in microchannels or microreactors. Reynolds number (Re) is an important parameter for evaluating the flow pattern and the mass transfer.35 In this work, the value of Re varied from 5.5 to 66 with the total volumetric flow rate changing from 0.2 to 2.4 mL/min. In such a range of Re, regular slug and droplet flow patterns were formed along the capillary microreactor (see Figure 9a). The squeezing regime was involved in the flow pattern formation in the microreactor with the microemulsion as the continuous phase, and the shear force exerted by the continuous phase was enough to induce the slug or droplet

l 4w(L − w) + πw 2 o o aq o o (Laq ≥ 0.5 mm) o o o Lcd 2 o o α=m o o o 4Laq 2 o o o (Laq < 0.5 mm) o o o Lcd 2 n

(6)

w = d − 2δ

(7)

On the basis of eqs 6 and 7, the specific interfacial area can be obtained within the slug or droplet flow regime under different operational conditions. The geometric parameters of the mass transfer unit cell were obtained by measuring 30 slugs or

Figure 9. (a) Slug and droplet flow patterns in the capillary microreactor for the microemulsion-based extraction with different Reynolds numbers, (b) schematic diagram of a mass transfer unit cell of the slug or droplet flow. 12735

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Industrial & Engineering Chemistry Research droplets for each operational condition. The variation of the specific interfacial area with Re is demonstrated in Figure 10a.

Figure 11. Effect of the inner diameter of the capillary microreactor on the gold(III) extraction (CIL, 0.0067 mol/L; Qaq, 0.1 mL/min; Qme, 0.1 mL/min; τ, 14.7 s. CAu, 1.94 × 10−3 mol/L; and CHCl, 0.1 mol/L).

diameter of the capillary microreactor. This was mainly attributed to the decrease of the specific interfacial area available for the mass transfer in the microreactor with a larger inner diameter.14 Meanwhile, the overall volumetric mass transfer coefficient increased with decreasing inner diameter of the capillary microreactor because a larger specific interfacial area and a higher Reynolds number could be obtained for a smaller inner diameter when the same volumetric flow rate was applied. Several models have been proposed by some researchers to estimate overall volumetric mass transfer coefficients for liquid−liquid or gas−liquid two-phase flow in microchannels or microreactors as a function of Reynolds number, capillary number, flow velocity, diameter, and length of the slug.41−46 Herein, these models were applied for predicting the mass transfer coefficients in the capillary microreactors with different diameters for this microemulsion based extraction process. The predicted values of Koa were compared with the measured values as shown in Figure 12. The following correlation based on multiple linear regression analysis was found to well-predict the values of Koa, which follows the model proposed by Kashid et al.:44

Figure 10. (a) Effect of Reynolds number on the variation of the specific interfacial area and the length of the aqueous phase slug by fixing the residence time; (b) effect of Reynolds number on the gold(III) extraction percentage and Koa.

It can be seen that the specific interfacial area decreased from 4794 to 3049 m2/m3 with the increase in the Reynolds number from 5.5 to 22. Interestingly, the specific interfacial area began to increase from 3049 m2/m3 to 3657 m2/m3 when Re increased from 22 to 66. As presented in Figure 10a, the dispersed phase slug or droplet length decreased so obviously with the increase of Re, which was beneficial for the enhancement of the internal recirculation inside both the dispersed and continuous phase slugs.39,40 As expected, the extraction percentage of gold(III) increased with the increase in the Reynolds number with the same residence time in the capillary microreactors with various lengths, and the overall volumetric mass transfer coefficient (Koa) increased from 0.99 s−1 to 1.54 s−1 (Figure 10b). The internal recirculation inside the slugs was intensified with the increase of the Reynolds number due to the increase of the sheer force between the wall surface and the slugs, leading to the improvement of the convective transport and the surface renewal rate. Next, the experiments were carried out in various capillary microreactors with different inner diameters while maintaining the same volumetric flow rate and the same residence time. This could be easily realized by changing the length of the capillary microreactor. As shown in Figure 11, the extraction efficiency of gold(III) decreased with the increase in the inner

Figure 12. Comparison of measured Koa values with those predicted by the correlations based on different models. 12736

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i U yzij d yz Koa = 0.025Ca Re jj zzzzjjjj zzzz j Laq zj Laq z (8) k {k { 3.5. Comparison of the Gold(III) Extraction in the Microreactor and the Batch Processing. The effect of the residence time on the gold(III) extraction in the microreactor was investigated. As can be seen in Figure 13a, the extraction 0.32

‐1.1j jj

−0.1

was obtained with the residence time of 14.7 s. Nevertheless, the extraction percentage was almost maintained constant even the residence time was prolonged to 17.6 s. In particular, the overall volumetric mass transfer coefficient in the capillary microreactor was in the range of 0.33−2.9 s−1, which was much higher as compared to other mental ions extraction systems in microreactors (see Table 3). These results demonstrated the advantages of the ionic liquid constructed microemulsion over conventional liquid−liquid extraction systems that typically involve phosphate extractants in organic solvents. The hexadecylpyridinium chloride IL had stronger extraction ability for gold(III) than the phosphate extractants. On the other hand, the special structure of the microemulsion constructed with the ionic liquid as a surfactant could provide larger mass transfer driving force and accelerate the transport of gold(III) from the aqueous phase to the microemulsion. Therefore, the combination of the microemulsion with the microreactor could provide higher overall volumetric mass transfer coefficients compared with other mental ions extraction systems reported in literature. Furthermore, the microemulsion based extraction process was also conducted using the traditional batch processing. For comparison, the microemulsion and the aqueous phase with an equal volume of 2.5 mL were put inside a 10 mL centrifugal tube, and then were shaken at different times for the batch processing. The mixture from the centrifugal tube was separated sufficiently into two phases by a separating funnel. The extraction of 15 min was required to achieve the high extraction efficiency of gold(III) for the batch processing (see Figure 13b), which was much longer compared with the continuous-flow extraction using the microreactor. The overall volumetric mass transfer coefficient in the batch reactor was in the range from 0.007 s−1 to 0.08 s−1, which was 2 orders of magnitude lower compared with the microreactor for this extraction process. Such a comparison again demonstrated the microreactor technology has high application potential on the gold(III) recovery with the use of microemulsion based extraction. 3.6. Recycling of the Microemulsion for the Gold(III) Separation in the Microreactor. For stripping gold(III) from the gold-loaded microemulsion to the aqueous phase, the solution with the common stripping agent of Na2S2O5 was applied. The stripping percentage of gold(III) with the value of 50% was confirmed by ICP-AES when the total volumetric flow rate of the microemulsion and the stripping solution was 0.2 mL/min with the residence time of 35.2 s in the capillary microreactor. Besides, the thiourea (0.1 mol/L) in the

Figure 13. (a) Effect of the residence time on the gold(III) extraction in the microreactor (CIL, 0.0067 mol/L; Qaq, 0.1 mL/min; Qme, 0.1 mL/min; CHCl, 0.1 mol/L; and d, 0.5 mm). (b) Gold(III) extraction in the conventional batch system.

percentage slowly increased as the residence time increased from 1.1 to 14.7 s, and the high extraction percentage of 99.2%

Table 3. Comparison of Koa in This Work with the Other Liquid−liquid Extraction Investigations extraction system Zn(III) extraction with D2EHPA in dodecane Ce(III) extraction with D2EHPA in the cyclohexane extraction of samarium using P507 LaCl3 in lactic acid with P507 sulfonated kerosene extraction of Nd(III) with P507 in the kerosene CeCl3-saponified P507sulfonated kerosene gold(III) extraction using the microemulsion

types of extractors

Koa (s−1)

conditions

10−4− −2

refs

5.41 × 2.96 × 10 2.27 × 10−4

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Y-shaped channel microextractor with inserting a piece of glass bead serpentine Y-junction microreactor

V: 250 μL, mixing channel diameter: 0.278 mm, reaction channel diameter: 0.319 mm L: 22 mm, d: 0.5 mm, glass bead (d/D = 0.57), τ: 40 s d: 0.3 mm, Q: 10 μL/min

0.07−1.14

48

rectangular cross-section microchannel

d: 0.3−0.7 mm, Q:10−30 μL/min, L: 10−100 cm

0.003−0.263

49

microfluidic based hollow droplet by the introduction of gas membrane dispersion microextractor

d: 0.7 mm, Qor: 80 μL/min, Qg: Qor = 0−300

0.04−0.8

50

Q: 20−160 mL/min, τ: 2.98 s

0.1−0.54

51

capillary microreactor

d: 0.5 mm, Q: 200 μL/min, Qme: Qaq = 1, τ: 14.7 s

0.33−2.96

this work

T-junction serpentine microchannel

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reused for the extraction of gold(III) for several times with high extraction efficiency.

hydrochloride acid solution (5% v/v) was also applied to strip the gold ions, and the much higher stripping percentage (i.e., 99%) was obtained compared with the Na2S2O5 solution. Therefore, thiourea was chosen as the final stripping agent for gold(III). After the stripping process, the microemulsion was washed with water for twice in order to remove the residual stripping agents. The treated microemulsion could be recycled for the following extraction of gold(III) in the microreactor (see Figure 14). Even the extraction percentage of gold(III)

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected]. Tel.: +86 21-54738710. *Email: [email protected]. ORCID

Yuanhai Su: 0000-0002-0718-301X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to acknowledge financial support from the National Natural Science Foundation of China (21676164 and 21706157) and the Science and Technology Commission of Shanghai Municipality (18520743500).

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NOTATION Caq, in = initial gold(III) concentration in the aqueous phase before extraction, mol/L Caq, out = gold(III) concentration in the aqueous raffinate phase after extraction, mol/L CIL = concentration of pyridine ionic liquid, mol/L Caq, * eq = gold(III) concentration in aqueous phase at equilibrium, mol/L E = extraction percentage of gold(III) after extraction τ = residence time of gold(III) extraction in the microreactor, s Q = total volumetric flow rate, μL/min Qaq = volumetric flow rate of aqueous phase, mL/min Qme = volumetric flow rate of microemulsion, mL/min Qor = volumetric flow rate of organic phase, mL/min L = length of microchannel, cm d = inner diameter of microchannel, mm V = volume of capillary microreactor

Figure 14. Microemulsion recycling for the gold(III) extraction in the microreactor (CIL, 0.0067 mol/L; Qaq, 0.1 mL/min; Qme, 0.1 mL/min; CAu, 1.94 × 10−3 mol/L; CHCl, 0.1 mol/L; τ, 14.7 s; and d, 0.5 mm).

decreased with the increase in the recycling time because of the loss of [C16Pyr]Cl ionic liquid, the high extraction percentage of gold(III) could still be obtained with a value of 80% in the fourth run. The recycling experiment also clearly indicated that such a hydrophilic ionic liquid for the formation of microdroplets in the microemulsion could be easily applied for the extraction of gold(III) from aqueous solutions.

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GREEK LETTERS γ = interfacial tension, mN/m ρ = density, g/cm3 μ = viscosity, mPa s δ = thick of film, m

CONCLUSION The combination of the hydrophilic ionic liquid based microemulsion with the continuous-flow microreactor could efficiently realize the gold(III) extraction and stripping processes. The anion exchange mechanism in this microemulsion based extraction process was unraveled in detail. In such an extraction system, the gold(III) extraction percentage of 99.2% and the excellent selective extraction for gold(III) over other base metals i.e., Fe(III), Cu(II), Al(III), and Ni(III) were achieved at the short residence time of 14.7 s. The formation of regular slug and droplet flow patterns along the capillary microreactor could ensure the large specific interfacial area for the gold(III) extraction. A higher flow volumetric rate and a smaller inner diameter of the capillary microreactor were beneficial for the mass transfer enhancement and the improvement of the gold(III) extraction. Much higher overall volumetric mass transfer coefficients could be obtained in the capillary microreactor for this microemulsion-based extraction process compared with the conventional liquid−liquid extraction systems in microreactors or the same extraction system in the batch reactor, indicating the great application potential of such a new technique for the gold(III) extraction. In addition, the thiourea solution could be used to efficiently strip gold(III) from the gold-loaded microemulsion, and the treated microemulsion could be

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SUBSCRIPTION me = microemulsion aq = aqueous phase REFERENCES

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DOI: 10.1021/acs.iecr.9b02158 Ind. Eng. Chem. Res. 2019, 58, 12729−12740

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DOI: 10.1021/acs.iecr.9b02158 Ind. Eng. Chem. Res. 2019, 58, 12729−12740

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DOI: 10.1021/acs.iecr.9b02158 Ind. Eng. Chem. Res. 2019, 58, 12729−12740