A promising approach for recycling of spent CIGS targets by

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A promising approach for recycling of spent CIGS targets by combining electrochemical techniques with dehydration and distillation Shuai Gu, Bitian Fu, Gjergj Dodbiba, Toyohisa Fujita, and Baizeng Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00787 • Publication Date (Web): 31 Mar 2018 Downloaded from http://pubs.acs.org on March 31, 2018

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ACS Sustainable Chemistry & Engineering

A promising approach for recycling of spent CIGS targets by combining electrochemical techniques with dehydration and distillation Shuai Gu,*,† Bitian Fu,‡ Gjergj Dodbiba,† Toyohisa Fujita,†and Baizeng Fang*,§

†

Graduate School of Engineering, The University of Tokyo, 7 Chome-3-1 Hongo, Bunkyō, Tokyo 113-8654,

Japan. E-mail: [email protected] ‡

School of Ecological and Environmental Sciences, East China Normal University, No. 500 Dongchuan Rd.

Minhang District, Shanghai 200241, China. §

Department of Chemical & Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver,

B.C., V6T 1Z3, Canada. E-mail: [email protected]

ABSTRACT: This study proposes an innovative approach to separate and recover copper, indium, gallium, and selenium from spent CIGS targets, which combines electrochemical techniques with dehydration and distillation. The leaching solution of the CIGS first underwent a two-step electrodeposition process to recover selenium and copper. Then, the remained solution was distilled to recycle HCl and crystallize indium and gallium chlorides. Next, the as-obtained hydrates were dehydrated by refluxing thionyl chloride (SOCl2). Afterwards, anhydrous InCl3 was recovered and separated from GaCl3 by a simple filtration, and the latter was separated from SOCl2 by distillation. The recovery of indium was ca. 99.99% with a purity of ca. 99.99%, while ca. 99.98 % of gallium was recovered with a purity of ca. 99.99%. Both the recovery and purity are the highest to the best of our knowledge. The separation process was rationally designed to take full advantages of the potential difference of all the elements and the property of the dehydrating agent. This is an environmentally benign, economical and practical method to recycle spent CIGS efficiently. KEYWORDS: Indium, Gallium, recycling, electro-deposition, dehydration

INTRODUCTION Photovoltaics (PV) are expected to be a major source of electricity in the near future due to the exponentially increased installation volume worldwide over the last two decades.1-3 The global PV sector has reached a cumulative capacity of 178 GW by 2015.4 Particularly, copper, indium, gallium, selenium (CIGS) thin film solar cells with laboratory efficiencies up to 23.9% have drawn considerable attention due to the low cost, high adsorption coefficient, and efficiency.1,5-7 Indium and gallium, both are regarded as critical materials, and the inevitable elements utilized in the production of CIGS.8, 9 The installed capacity of CIGS solar cells will keep increasing in the future with the demand growth rate of indium and gallium estimated to be 15% and 15-20%, respectively.10, 11 However, the discrete deposits and the sparse distributions of indium and gallium hinders the rapid development of CIGS solar cells.12 The long-term supply risk of indium and gallium would limit the market growth of thin-film solar cells ACS Paragon Plus Environment

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due to their limited availability.8, 13-16 However, spent CIGS target materials may contain up to 2900 ppm of indium and 530 ppm of gallium, which is 29 and 10 times more concentrated than those in zinc sulfide mineral and bauxite residues, respectively.17 The majority of CIGS films utilized in PV are fabricated by sputtering CIGS targets.18-20 Target utilization (TU) in a typical two-stage sputtering process for planar and rotary targets was merely 33% and 50%, respectively.21, 22 Thus, a proper recycling approach for spent CIGS targets would become increasingly significant. Since selenium is the only non-metal element in CIGS, some recycling approaches started with the separation of selenium, such as high-temperature oxidation.23 Other methods, such as high-temperature chlorination,24, 25 nanofiltration and solvent extraction26, 27 were also utilized to recycle CIGS. High-temperature oxidation enabled the separation of selenium from spent CIGS, while high-temperature chlorination separated the majority of InCl3 (93 wt%) and GaCl3 (97 wt%) at 340 and 260 oC, respectively. Nanofiltration and solvent extraction were utilized to concentrate and separate indium from CIGS. High-temperature oxidation approach can only separate selenium from CIGS; high-temperature chlorination approach is a relatively simple method for the recycling of indium and gallium from CIGS, whereas it suffers from low recovery and high energy consumption. Nanofiltration and solvent extraction is a scalable and easy method to recycle indium, but the final product was in an aqueous solution which needs further processes to separate indium from CIGS. It is clear that none of the approaches mentioned above is capable of separating all the elements from CIGS. Therefore, a scalable and efficient approach to separate and recover copper, indium, gallium, and selenium from spent CIGS is highly desired. Due to their high standard electrode potential, selenium and copper should be easily recovered from the leaching solution by electro-deposition. As for indium and gallium, in our previous study, high purity InCl3 was successfully recovered from spent ITO target by dehydration and fractionation.28 Inspired by this, a similar procedure should also be able to separate indium from gallium. Therefore, this study for the first time proposes a highly efficient recycling approach combining electrochemical techniques with dehydration for the recycling of CIGS target. The dissolution-electro-deposition-dehydration-distillation (DEDD) process not only enables the separation of high purity of indium, gallium, copper, and selenium from CIGS, but also presents a reasonable approach for the recycling of CIGS materials in an environmental benignity and sustainable manner. EXPERIMENTAL SECTION Materials and methods

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The spent CIGS target material (Figure S1, Supporting Information) was analyzed with X-ray fluorescence (XRF) spectrometer (Rigaku supermini WDXRF) and ICP-OES (PerkinElmer® Optima 5300 DV). The composition of CIGS is shown in Table S1 and S2 (Supporting Information). The recycling process shown in Scheme 1 started with dissolving CIGS with HCl and H2O2. Electrodeposition was then conducted in the leached solution with a Ti and Pt electrode as the working electrode to recover selenium and copper, respectively. Next, the solution was dehydrated with dehydrating agent (i.e., SOCl2) under refluxing. Last, the resultant solution was filtered and distilled to separate anhydrous InCl3, GaCl3 and SOCl2, respectively. It is well known that GaCl3 is readily soluble in SOCl2, while InCl3 has no/little solubility in SOCl2.29, 30 In the last step, InCl3, GaCl3 and SOCl2 were separated from each other by simply filtering the mixture and distilling SOCl2 solution. Since InCl3 and GaCl3 are sensitive to the moisture in the air, the as-received InCl3 and GaCl3 samples were analyzed with X-ray powder diffraction (XRD) (Rigaku®Smartlab) equipped with an air-sensitive sample holder (Rigaku®MiniFlex accessories).

Scheme 1. Illustration of the DEDD approach for the recycling of spent CIGS target.

Leaching The spent CIGS target was milled to fine powder with an average particle size of around 11.61 µm, as shown in Figure S1 (Supporting Information). The detailed dissolution results are shown in Fig. S2 and Table S3 (Supporting Information). Modeling of the thermodynamics parameters of some electro-active species and electro-deposition To determine the potential range of the electro-deposition process, thermodynamics parameters of some electro-active species were calculated. The electrochemical potentials were calculated based on the ACS Paragon Plus Environment

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complex stability, Nernst equation, and Gibbs energies of reactions, together with equations 1-4. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements of indium, copper, selenium, and gallium in 4 M HCl solution were carried out in solutions containing 5 mM InCl3, CuCl2, H2SeO3, and GaCl3 with a scan rate of 5 mV/s, respectively, to determine the electro-deposition potential range of indium, copper, selenium, and gallium. After a two-step electro-deposition process, selenium and copper were electrodeposited onto a Ti and Pt electrode, separately under controlled potentials. The asrecovered selenium (Supporting Information, Figure S3) and copper (Supporting Information, Figure S4) were analyzed with scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS) (Hitachi S-4800 Type II / Thermo NSS EDS). (1) (2) (3) (4) Dehydration and distillation The solution after the two-step electro-deposition process was then transferred into a rotary evaporator (Eyela®N-1100D) equipped with an oil bath operated at 85 oC and a vacuum pump (Eyela®NVC-2200) with a vacuum degree of 0.01 MPa for a pre-dehydration process. In the pre-dehydration process, the majority of the leaching solution was evaporated and recycled leaving only 1-2 ml of solution on the top of the hydrates. After that, the flask was equipped with a mandarin heater and a condenser-(Allihn type) for dehydration (Supporting Information, Figure S5A). The ratio of SOCl2 to hydrate was 2 ml/g. SOCl2 was refluxed at 75 oC for 2 h to dehydrate the mixture. During this process, SO2 and HCl gas kept bubbling out of the solution till the dehydration process reached the ending point. After the bubbling stopped, the separation of GaCl3, InCl3, and SOCl2 was performed. InCl3, due to the insolubility in SOCl2, was first filtered out of the mixture. The remaining solution was transferred into a flask equipped with a Vigreux fractionating column to separate SOCl2 at 78 oC (Supporting Information, Figure S5B) through which SOCl2 was distilled out of the flask, leaving GaCl3 (melting point: 77.9 oC, boiling point 201 oC) at the bottom of the flask, and thus separating GaCl3 from SOCl2. RESULTS AND DISCUSSION Thermodynamics and electro-deposition of selenium and copper A thermodynamic explanation for the different chloride complexes of indium and copper which contains the reversible potentials together with the concentrations of electro-active species in the studied solution is given in Table 1. As can be seen, the main existing species of indium and copper in the simulated leaching solution were InCl4- and CuCl+, respectively. The modeled potential difference of copper and indium in the leaching solution is approximately 0.68 V. Fig. 1 shows the individual CV diagram of CuCl2, InCl3, GaCl3, and H2SeO3. The potential scan went negatively first in all the CV measurements. The CV plot of CuCl2 is presented in Fig. 1 (A). In the forward scan, two cathodic peaks were observed. A peak located at ca. 0.343 V (i.e., peak a) is probably related to the reduction of Cu2+ to ACS Paragon Plus Environment

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Cu+,31 as shown in equation S1 (Supporting Information). In solutions containing Cl- ions, part of Cu2+ ions were reduced to Cu0 via the intermediate Cu+ by forming an anion-bridged Cu-Cl-Cu reaction.32 Cu+ was then reduced to Cu0 at a more negative potential (i.e., 0.02 V) according to equation S2 (Supporting Information), which corresponds to peak (b). Meanwhile, the Cu2+ ions were also reduced to Cu0 at the same potential range according to equation S3 (Supporting Information). The reduction wave observed at a potential lower than ca. -0.24 V is attributed to the hydrogen evolution reaction (HER).33, 34 In the reverse scan, peak (d) and (e) correspond to the oxidation of Cu0 to Cu+ and Cu+ to Cu2+, respectively, which are in a good agreement with the previous report.35

Figure 1. CV plots of InCl3, GaCl3, CuCl2, and H2SeO3 in 4M HCl solutions containing: (A) CuCl2 in the potential range of 0.8 to -0.3V, (B) InCl3 in the potential range of -0.05 to -0.82 V, (C) GaCl3 in the potential range of 0.90 to -1.52 V, and (D) H2SeO3 in the potential range of 1.40 to -0.30 V.


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Table 1. Thermodynamics parameters of some electro-active species in the simulated leaching solution. Electro-active species

Concentration (mM)

Eθ a(V)

E b(V)

Cl-

4002

----

----

Cu2+

0.500

0.143

0.0452

CuCl+

2.50

0.140

0.0631

CuCl2

2.00

0.161

0.0808

In3+

2.78×10-4

-0.535

-0.664

InCl2+

0.0463

-0.567

-0.653

InCl2+

1.23

-0.583

-0.641

InCl3

0.892

-0.569

-0.629

InCl4-

2.84

-0.567

-0.617

a

Eθ: Standard electrode potential, bE: Electrode potential.

As for indium, only one reduction peak (i.e., peak a) was observed in the forward scan at ca. -0.633 V, attributed to the electro-deposition of In3+ to In0 according to equation S4 (Supporting Information). The reduction wave at ca. -0.73 V is also attributed to the HER. In the reverse scan an oxidization peak (i.e., peak c) was observed at ca. -0.382 V, which was caused by the oxidization of In0 to In3+. According to Table 1 and S4 (Supporting Information), the reduction peak difference of copper and indium was estimated to be ca. 0.649 V, which is large enough for the selective electro-deposition of copper without any indium reduction. For gallium, due to the lower standard electrode potential, there was no apparent reduction peak as shown in Fig. 1(C), which indicates that a large over-potential is needed to electrodeposit gallium in the leaching solution. The circumstance for the reduction and oxidization of selenium is more sophisticated than the other three elements. Three pairs of reduction and oxidization peaks were observed, in the CV plot (Fig. 1D). The first reduction peak (i.e., Peak a) in the forward direction observed at 0.564 V was attributed to the under potential deposition (UPD) of Se4+ to Se0 36, 37 while the second peak (i.e., Peak b) observed at 0.343 V was related to the massive electro-deposition of selenium according to equation S5 (Supporting Information).37 With the further decrease of potential, the third reduction peak (i.e., Peak c) was observed at -0.202 V due to the formation of H2Se as shown in equation S6 (Supporting Information).37, 38 In the reverse scan, the first oxidization peak (i.e., Peak d) observed at 0.923 V was attributed to the oxidation of H2Se to Se0; the second oxidization peak (i.e., Peak e) observed at 1.08 V was related to the oxidation of Se to H2SeO3; while the third oxidization peak (i.e., Peak f) observed at 1.25 V was caused by the oxidization of Se to Se4+.37, 38 Based on these, the potential difference between the reduction of copper and selenium is 0.327 V, which guarantees a separate electro-deposition of selenium from copper.


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Figure 2. LSV plots of CuCl2, InCl3, H2SeO3, and their mixture in 4M HCl solutions.

The individual LSV (Fig. 2) of CuCl2, InCl3, and H2SeO3 were recorded and presented as a reference to identify the peaks of the each element. Based on the LSV results and the analysis on the CV plots in Fig. 1, the peaks obtained on the LSV of the simulated leaching solution can be easily analyzed. Peak (A) at EpA = 0.561 V and peak (E) at EpE = -0.200 V should be attributed to the UPD of Se4+ to Se0 and the formation of H2Se, respectively, while the plateau around (B) at EpB = 0.320 V should be attributed the massive reduction of Se4+ to Se0 and the reduction of Cu2+ to Cu+. Peak (C) at EpC= 0.239 V resulted from the reduction of Cu2+ to Cu+, while peak (D) at EpD= 0.0132 V was caused by both the reduction of Cu2+ to Cu0 and Cu+ to Cu0. At a more negative potential, peak (F) at EpF= -0.627 V was also observed, which is attributed to the massive reduction of In3+ to In0. The peak potential difference between the massive reduction of selenium (around peak B) and copper (around peak D) were 0.307 V and the fact that the current decreased after peak (C) to almost zero suggests that at a potential more positive than 0.1V little/ no Cu0 would appear on the electrode while the majority of Se4+ can be reduced to Se0. Also, the peak potential difference between the massive reduction of copper (around peak D) and indium (around peak F) was 0.640 V, which is more than enough for the electro-deposition of copper without any indium reduction. In a word, the LSV suggested that selenium and copper can be selectively electrodeposited at around 0.1V and -0.4V, respectively, without any impurities appear on the electrode. Constant potential electrolysis was thus performed at 0.1 V and -0.4 V, respectively to electro-deposit selenium and copper separately from the leaching solution. The resulted solution was then analyzed with ICP-OES to identify the concentration of indium, gallium, copper, and selenium in the solution, while the as-recovered selenium and copper were analyzed with SEM-EDS. Fig. 3 shows SEM images ACS Paragon Plus Environment

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Figure 3. SEM images of the as-recovered copper (A) and selenium (B).

of the as-recovered copper and selenium. It is interesting to note that the surface morphology of copper is slightly different from a plated copper prepared through a general plating or electroplating technique, which is probably due to the very thin layer deposited on the surface of the Pt electrode. The EDS analysis is shown in Fig. S6 (Supporting Information) to identify their purity. The recovery, purity, and potential economic value of the selective electrodeposited selenium and copper from the CIGS leaching solution are shown in Table 2. 99.97% of Cu with a purity of 99.95% and 99.98% of Se with a purity of 99.97% were recovered, respectively. Table 2. Recovery, purity, and potential economic value of DEDD recovery process obtained from 10 g of CIGS measured by ICP-OES, XRD, and SEM-EDS. Element

Leaching (g)

E= 0.1 V

E= -0.4 V

Filtration (g)

Distillation (g)

Ra

Pb

Pricec (Kg-1)

Economic value (10g-1)

Cu

2.604

2.603

7.812×10-4

----

----

99.97

99.95

$8.10

$0.0211

In

2.604

2.604

2.604

2.6038

----

99.99

99.99

$650

$1.693

Ga

0.4760

0.4760

0.4760

----

0.4759

99.98

99.99

$529

$0.2518

Se

4.313

8.626×10-4

----

----

----

99.98

99.97

$120.09

$0.5178

a

Recovery, R. bPurity, P. cValues from U.S. Geological Survey.46

Dehydration and distillation After the electro-deposition process, the solution was distilled to recycle HCl. This process has four functions: reducing the volume of the leaching solution, recycling the HCl solution, reducing the consumption of SOCl2 in the subsequent dehydrating process, and avoiding the hydrolysis of gallium chloride hydrates. As we know, gallium chloride hydrates undergo hydrolysis under pH > 2 39, 40. Leaving some acid solution on top of the hydrates would stop gallium chloride hydrates from hydrolyzing. In the dehydrating process, SOCl2 reacts with water both in the hydrates and HCl solution according to equation S7-9 (Supporting Information). During this process, HCl and SO2 gases were ACS Paragon Plus Environment

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emitted from the flask, which are hazardous to the environment. Proper measures were adopted to dispose or recycle these hazardous gases. For example, HCl can be separated from SO2 in hot water due to their solubility difference.41 The recycled HCl in this process can be used in the leaching process, while SO2 can be liquefied for storage and sale. SO2 and HCl gases can also be disposed in the dry scrubbing process.42, 43 InCl3 was left in the bottom of the flask, while GaCl3 was dissolved into SOCl2 solution due to the formation of coordination compound between SOCl2 and GaCl3 as shown in equation (5)29. (5) The formation of an adduct bound was confirmed by Raman spectra (Thermo Fisher Scientific Inc. DXR SmartRaman) of the solution as shown in Fig. 4. As an acceptor GaCl3 complexes with the electron-pair donor (in this case SOCl2), resulting in the formation of positively charged molecular complex and GaCl4- ions, whose structure follows an XY4 tetrahedral configuration with the Td system group.44 There are four typical vibrations in a tetrahedral GaCl4- ions with v1= 346 cm-1, v2= 114 cm-1, v3= 386 cm-1, and v4= 149 cm-1 in aqueous solution.45 As can be seen from Fig. 4, three out of the four vibrations (v1, v3, and v4) were observed, indicating that the same complex exists in the GaCl3-SOCl2 system. After the dehydration step, the white powder at the bottom of the flask was filtered out of the mixture and analyzed with XRD. As can be seen from Fig. 5, the characteristic peaks of the asrecovered white powder are in good accordance with the standard InCl3 peaks, indicating that the white powder was InCl3. The filtered solution was then heated to evaporate SOCl2 out of the system, and the remained solid was collected and also analyzed with XRD. As shown in Fig. 5, all the characteristic peaks in the standard GaCl3 pattern appear in the as-recovered GaCl3 pattern. According to Table 2, indium ($1.693), selenium ($0.5178), and gallium ($0.2518) account for the highest shares of the total economic value. With a higher gallium ratio in CIGS targets, gallium would count for more shares in the total economic value due to its high unit price. There are two main differences between this study and the one reported by Zimmermann et al.26 First, in this study, Cu, In, Ga, and Se are separated mutually in the form of Cu metal, anhydrous InCl3, anhydrous GaCl3, and Se metal, respectively, rather than a mixture of metal ions in the organic phase as reported by Zimmermann et al, which needs to be further processed to separate them from each other. Second, the material used in Zimmermann’s experiments was obtained from spent CIGS photovoltaic cells, while the material used in this study is spent CIGS target, which was produced in the sputtering process. The main advantages of the proposed process in this study include: higher recovery and purity, scalable mass production, minimizing the reagents consumption. In the literature, In, Mo, Sn, and Al ions were extracted into the organic phase simultaneously with relatively low recovery and purity. Furthermore, this mixture requires further process to separate indium from the organic phase. As for the reagents’ consumption, a large amount of extractant, diluent, acid, and water were consumed in the extraction, stripping, and regeneration processes, while the majority of reagents used in the proposed process in this study can be reused. In addition, the proposed process in this study only requires a general setup for heating and filtration, while nano-filtration and solvent extraction requires more complex equipment. Therefore, by tailoring the reagents used in this process, almost all the reagents/chemicals can be recycled and reused, and only sulfur dioxide was emitted, which can also be easily collected and disposed. Compared with other ACS Paragon Plus Environment

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Figure 4. Raman spectra of GaCl3 dissolved in SOCl2 and pure SOCl2.

Figure 5. XRD patterns of the as-recovered white powder from filtration and distillation.

reported processes, this method has the following advantages: minimizing the reagents consumption, scalable mass production, higher recovery, and environmental benignity.


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CONCLUSIONS The DEDD process developed in this study is highly efficient in separating and recovering copper, indium, gallium, and selenium from spent CIGS target materials. The DEDD process takes full advantage of the property of H2SeO3, CuCl2, InCl3, GaCl3, and SOCl2 to separate them mutually. By utilizing electrochemical techniques, selenium and copper were successfully recovered from the leaching solution without adding any reduction reagents. Although the potential difference between indium and gallium was also large enough for a separate electro-deposition of indium, this was not adopted in the proposed process because of the relatively negative electrode potential of indium and gallium and the high acidity of the leaching solution which would either lead to an extremely low current efficiency or consume enormous amount of alkaline or water to adjust the pH of the solution, especially for gallium which was often electrodeposited in alkaline solutions.47, 48 Thus, dehydration rather than electro-deposition was utilized to recover indium and gallium, respectively. SOCl2 plays three important roles in the separation process: firstly, SOCl2 was utilized to dehydrate indium and gallium chloride hydrates to form anhydrous InCl3 and GaCl3; secondly, SOCl2 was used to selectively dissolve GaCl3, and thus separating anhydrous InCl3 from GaCl3; thirdly, SOCl2 could be distilled out of the system leaving anhydrous GaCl3 at the bottom of the flask due to its low boiling point. These important roles of SOCl2 enable the whole separation and recovery process to function efficiently. By tailoring the reagents used in this process, almost all the reagents/chemicals could be recycled and reused, and only sulfur dioxide was emitted, which can also be easily collected and disposed. Compared with other reported processes, this method has the following advantages: minimizing the reagents consumption, scalable mass production, higher recovery, and environmental benignity. More importantly, the final product is high purity anhydrous InCl3 and GaCl3 rather than In3+ and Ga3+ in aqueous solution, and thus there is no need for further procedures. Furthermore, due to the modularized property of this procedure, it can not only be utilized to recover CIGS with different element ratios but also be utilized to recover other materials such as CIS.

ASSOCIATED CONTENT Supporting Information Photographs for the spent CIGS target materials, the as-recovered Cu and Se, the EDS of the as-recovered Cu and Se, the setup of dehydrating and distillation process, the leaching results of the spent CIGS targets. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors ACS Paragon Plus Environment

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* E-mail: [email protected] (S. Gu) E-mail: [email protected] (B. Fang)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the China Scholarship Council (CSC) which is gratefully acknowledged.

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For Table of Contents Use Only

A sustainable process is developed to recover copper, indium, gallium, and selenium from spent CIGS targets with high recovery rate and purity


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A promising approach for recycling of spent CIGS targets by

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