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Simultaneous leaching and extraction of indium from waste LCDs with acidic ionic liquids
Hydrometallurgy 189 (2019) 105146
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Simultaneous leaching and extraction of indium from waste LCDs with acidic ionic liquids
T
Deliang Luoa, Nengwu Zhua,b,c,d, , Yao Lia, Jiaying Cuia, Pingxiao Wua,b,c,d, Jieyuan Wanga ⁎
a
School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters of Ministry of Education, Guangzhou 510006, PR China c Guangdong Environmental Protection Key Laboratory of Solid Waste Treatment and Recycling, Guangzhou 510006, PR China d Guangdong Engineering and Technology Research Center for Environmental Nanomaterials, Guangzhou 510006, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Waste LCDs Indium Leaching Extraction Acidic ionic liquid
The fragile indium supply inspired the development of recycling programs for end-of-life electronics products, especially liquid crystal displays (LCDs). LCDs consume 70% of the total indium resources used in the world today. In this study, a novel fusion of leaching and extraction processes was proposed to recycle indium from waste LCDs using the functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]). Under the optimum conditions of 10 mL 50% (v/v) ionic liquid/ascorbic acid, 20 g/L of solid/liquid ratio, and 90 °C operation temperature within 24 h, the indium leaching rate could reach as high as 99.75% and the indium extraction rate could be enriched by 98.63% in the ionic liquid phase after cooling and stratifying. Introducing ascorbic acid into the process in place of deionized water decreased the distribution rate of iron from 59.87% to 8.84% and promoted the separation of indium and iron. Indium in the ionic liquid phase could then be transferred into an indium-rich solution using oxalic acid and retrieve the ionic liquid. The characteristics of the regenerated ionic liquid remained relatively stable for reutilization. Therefore, the proposed fusion of leaching and extraction processes by [Hbet][Tf2N] provides an efficient alternative to recycle indium from waste LCDs.
1. Introduction Liquid crystal displays (LCDs), as a new generation of display with superior performance, are widely used during the manufacturing of various devices such as mobile phones, televisions, computer monitors, and laptops (Amato and Beolchini, 2018). With the continuous innovation of technology, the replacement speed of electronic products has gradually intensified, resulting in a surge in the number of waste LCDs (Işıldar et al., 2019). Waste LCDs contain indium‑tin-oxide (ITO) which uses indium oxide (In2O3) during the fabrication process. It has been considered that the indium consumption in the ITO reached > 70% of the total indium consumption (Zhang et al., 2015). Unfortunately, as an important scattered metal, the global reserves of indium are only about 16,000 t (Zhang et al., 2015). It has been reported that the sustainable index of indium is < 0.5, and the sustainable mining life is only 30 years (Wang et al., 2015). Therefore, recovering indium resources from waste LCDs should be an industry priority. In recent years, the process of recovering indium from waste LCDs has attracted worldwide attention. Thermal approaches (vacuum carbon-reduction and vapor separation, and pyrolysis chlorination and
⁎
separation) have been proposed to recover indium from waste LCDs (He et al., 2014; Ma et al., 2012; Zhang and Xu, 2017). For example, He et al. (2014) realized the reduction of indium oxide at 1 Pa and 1223 K by mixing waste LCDs with 50 wt% coke powder and recovered as much as 90% of the indium after vapor separation. Microorganism Acidithiobacillus indirectly leached 100% indium from waste LCDs (Xie et al., 2019), and Shewanella algae adsorbed indium(III) from aqueous solutions where the condensate was 4300 times the initial solution (0.94 mol/m3) after heat treatment (Ogi et al., 2012). Hydrometallurgical approaches are the most frequently reported in the field (Argenta et al., 2017; Cui et al., 2019; Rocchetti et al., 2015; Silveira et al., 2015; Zeng et al., 2015). For example, Silveira et al. (2015) recovered 96.2% of indium from waste LCDs by the combination of leaching with H2SO4 (96.4%) and precipitation with NH4OH (99.8%). Rocchetti et al. (2015) applied cementation with 100 g/L of zinc to replace indium with a highest purity of 62% from the leaching solution of waste LCDs. When citric acid and supercritical CO2 were applied, 94.6% of the indium was efficiently leached from ITO samples (Argenta et al., 2017). Most research studies still use leaching and extraction to recover indium, focusing on the efficiency and greenness of the
Corresponding author at: School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China. E-mail address: [email protected] (N. Zhu).
https://doi.org/10.1016/j.hydromet.2019.105146 Received 17 June 2019; Received in revised form 26 August 2019; Accepted 7 September 2019 Available online 10 September 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
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recycling process at the same time. If leaching and extraction can be integrated, then the technological process could be shortened and the types of reagents and environmental pollution could be reduced. Among the various efforts to conquer the drawbacks of leaching or extraction technology, ionic liquids could simultaneously replace existing acidic media and extractants in hydrometallurgy (Abbott et al., 2011; Park et al., 2014; Qiao et al., 2017). In order to be effective, such liquids require specific temperature characteristics and homogeneous transformation into two phases by heating to the upper critical solution temperature (UCST) or cooling to the lower critical solution temperature (LCST) separately. Two representative examples are the use of tributyltetradecyl phosphonium chloride ([P44414][Cl]), for blending with HCl solution at low temperatures and separating substances at high temperatures (Gras et al., 2018), and betainium bis(trifluoromethylsulfonyl) imide ([Hbet][Tf2N]), for providing uniform liquid reaction conditions at high temperatures and separating substances at low temperatures (Davris et al., 2016; Dupont and Binnemans, 2015a; Dupont and Binnemans, 2015b; Hoogerstraete et al., 2013). In particular, [Hbet][Tf2N] is more suitable for application in hydrometallurgy because of its acidic and UCST characteristics. The valence elements (Nd, Dy and Co) of NdFeB magnets were dissolved in the [Hbet][Tf2N]-H2O system at 80 °C and separated to the aqueous phase at room temperature, resulting in a 99% recovery rate and 99.9 wt% purity (Dupont and Binnemans, 2015b). [Hbet][Tf2N] has been used not only for the dissolution of metal oxides, but also for dissolving metal hydroxides. Deferm et al. (2018) dissolved crude indium hydroxide with [Hbet][Tf2N]-H2O system at 80 °C to remove other metal impurities, and the purity of indium hydroxide increased to 99 wt%. Related research previously confirmed that [Hbet] [Tf2N] could produce a high selectivity for the extraction of In(III) (Hoogerstraete et al., 2013). To our knowledge, [Hbet][Tf2N] has not been used to dissolve In2O3, and ionic liquid has not been used to recover indium from waste LCDs. Thus, it is worthwhile to explore the feasibility of indium recovery from waste LCDs using [Hbet][Tf2N]. In this study, a novel fusion of leaching and extraction process was proposed to recycle indium from waste LCDs using [Hbet][Tf2N]. The waste LCDs were dissolved in the [Hbet][Tf2N]-H2O system above UCST, and measurements were made of the concentration, solid to liquid ratio, reaction time, and temperature during leaching and extraction. Indium was separated from other metal impurities between two phases as the leachate was cooled. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) were used to characterize the change in the ionic liquid and waste LCDs during the leaching process. Subsequently, the kinetics of the leaching process were analyzed. Finally, the effect of ionic liquid reuse on its leaching and extraction properties were studied.
water phase was discarded, [Hbet][Tf2N] was washed several times with ice water to remove the impurities of chlorine (detected the water by 0.5 M AgNO3). In order to remove excess water, the ionic liquid phase was dried by using a vacuum drying oven at 80 °C (DZF-6032, Du Wei, China). CH3 CH3
N
CH3
Li CH2
CH3
COOH
Cl
O S CF3
N O O
O S
CF3
CH3
N
CH2
CH3
S CF3
LiCl
COOH
O
N O O
O S
CF3
(1)
2.2. Leaching and extraction Glass powder (0.2 g) was completely dissolved by aqua regia (HNO3:HCl = 1:3, AR, China) and hydrogen fluoride (HF) (AR, China) at 180 °C for 2 h. The aqua regia and HF were alternately supplemented in the same volume to ensure the ITO glass powder could be completely dissolved. The solution was evaporated and transferred into a 50 mL volumetric flask (poly tetra fluoroethylene) to determine the type and content of metallic elements. Glass powder was added to a 20 mL serum flask under different experiment conditions to achieve the maximum leaching rate of indium. First, the acid concentration was investigated with 10, 20, 30, 40, 50, and 60% (v/v) [Hbet][Tf2N]. Meanwhile, the measurement of pH values in different concentrations was performed to analyze the acidity of the solution (S220-K, Mettler, Germany). Second, the effect of solid /liquid was studied at 10, 20, 30, 40, and 50 g/L to obtain the processing capacity of the ionic liquid system. Subsequently, the reactor was heated in an oil bath at 70, 80, and 90 °C for different times respectively, and each sample was separately prepared to analyze the leaching effect and the kinetics of the leaching process. After the reaction, the mixture in the reactor was filtered immediately to separate the solid from the solution using a quantitative filter paper (0.45 μm, nylon 66, China). On the one hand, the leachate was diluted with 5% (v/ v) HNO3 for a constant volume to calculate the leaching rate of In and other metal elements; on the other hand, the leachate was placed in a cooling centrifuge (2-16KL, Sigma, Germany) at 10,000 rpm for 10 min to stratify. The organic phase and the aqueous phase were diluted with 5% (v/v) HNO3, and the content of the metals was analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES) (Gras et al., 2018; Onghena et al., 2017). Three parallel samples were set for each sample and the experimental error was controlled to be < 5%. [Hbet][Tf2N] was analyzed before and after leaching to clarify its leaching mechanism. Meanwhile, the ITO glass powder was characterized before and after leaching to observe the morphological changes. Since the initial content of iron was high and the leaching effect was good in the [Hbet][Tf2N]-H2O system, the purification effect of indium needs to be further improved. Studies have shown that ascorbic acid can reduce Fe3+ to Fe2+ (Eq. (2)) and effectively increase the separation effect of ionic liquids on iron (Onghena et al., 2017). Therefore, given the optimal leaching conditions, an [Hbet][Tf2N]-0.01 M ascorbic acid (AA) solution system was formed by replacing the equivalent amount of deionized water with 0.01 M of AA to increase the separation effect of indium and iron.
2. Materials and methods 2.1. Materials The glass substrates were obtained by manually dismantling the waste LCDs purchased in bulk from the Electronic Waste Collecting Market in Guangzhou, China. To obtain clean glass substrates, the upper polarizer, an acid-producing material (Yu et al., 2016), was peeled off quickly after soaking in hot water for a few minutes, and the residual cellulose triacetate was stripped by soaking in acetone for a few minutes. The clean glass substrates were broken in a high-speed crushing shear (FW-400A, Zhong Xin, China) for 5 min. Zeng et al. (2015) reported that glass powder with a particle size of < 0.75 μm had a better leaching effect, so the ITO glass powder was sifted out with 300-mesh sieve to obtain the optimum leaching particle size. Betaine chloride (C5H11NO2HCl, 99%, Aladdin) was reacted with lithium bis(trifluoromethysulfonyl)imide (C2F6LiNO4S2, 98%, Aladdin) to compound [Hbet][Tf2N] (Eq. (1)) (Dupont and Binnemans, 2015a). After cooling by static stratification at room temperature, the mixture was layered as ionic liquid phase and water phase containing LiCl. When the
2Fe3 + + C6 H8 O6
2Fe2 + + C6 H6 O6 + 2H+
(2)
2.3. Scrubbing and recycling The obtained indium-containing ionic liquid was cleaned with oxalic acid which showed good bonding performance in our previous study (Cui et al., 2019). The obtained ionic liquid and 0.5 M oxalic acid were mixed at a ratio of 1:1 (v/v) and stirred at 70 °C for 10 min. The mixed solution was subjected to centrifuge at 10,000 rpm for 10 min (216KL, Sigma, Germany). The organic phase and the aqueous phase were diluted, and the content of the metals was analyzed (Gras et al., 2018). [Hbet][Tf2N] was analyzed before and after scrubbing to back up the 2
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leaching mechanism. After pickling, the ionic liquid was cleaned with deionized water several times and vacuum-dried to remove excess moisture for reuse. Nuclear magnetic resonance characterization of the recovered ionic liquid was carried out, and the leaching experiments were repeated to verify the reuse performance of the ionic liquid.
increased from 10% to 40% (v/v), and then the leaching rate reached 100% when the ionic liquid concentration reached 50% (v/v) or higher. Considering the cost of consumption and the leaching effect, 50% (v/v) can be regarded as the appropriate experimental condition for leaching. Fig. 1(c) shows the effect of the solid-liquid ratio on indium leaching recovery. It was found that the indium leaching rate began to decline when the solid-liquid ratio exceeded 20 g/L. The increase of the solidliquid ratio indicates that more powder was added to the same volume of solvent, reducing the mass transfer efficiency and the leaching rate of indium. The higher the solid-liquid ratio, the greater the processing capacity and the lower the cost. The leaching rate of indium reached nearly 100% when the solid-liquid ratio was lower than 20 g/L. Thus, the recommended solid-liquid ratio was 20 g/L. The results of the indium leaching rate with the leaching time and temperature are shown in Fig. 1(d). When the leaching time was 24 h, the indium leaching rate achieved 62.32%, 76.25%, and 100% at the temperature of 70, 80, and 90 °C, respectively. The temperature was favorable to increasing the reaction rate. In addition, the leaching rate continued rising after 24 h at 70 °C and 80 °C, so the above trend indicates that it takes longer to achieve complete leaching at low temperatures. Experiments with the [Hbet][Tf2N]-H2O system indicated that the optimal experimental conditions were as follows: 50% (v/v) [Hbet] [Tf2N]-H2O system，20 g/L (waste LCDs/[Hbet][Tf2N]-H2O), and a leaching time and temperature of 24 h and 90 °C, respectively.
2.4. Analysis and calculation ICP-OES (ICP730, Agilent, America) was used to determine the concentration of indium and other metals before and after leaching and extraction (Gras et al., 2018; Onghena et al., 2017). The samples were diluted 25–80 times with 5% (v/v) HNO3 before measurement to increase the accuracy of our results (Fig. S1). The structures of the glass powder before and after leaching were characterized by X-ray diffraction (XRD, Ultima IV, Germany), which scanned from 5 to 90° in a 2θ scan. [Hbet][Tf2N] was uniformly coated on the KBr wafer, and the functional group of [Hbet][Tf2N] was scanned by FTIR (Nicolet Nexus, America). The scanning wave number range was 400–40,000 cm−1, and the resolution was 4 cm−1. Furthermore, [Hbet][Tf2N] was dissolved in dimethyl sulfoxide (DMSO) after vacuum drying and characterized by nuclear magnetic resonance (NMR, HD600, Avance, Germany). The pattern of glass powder before and after leaching were characterized with SEM (SU8010, Hitachi, Japan). In order to calculate the leaching rate of metallic elements, the leachate and glass powder were separated at a high temperature (> 70 °C), and the residual liquid in the serum flask was cleaned with 5% (v/v) HNO3 more than three times. The volume was fixed at 50 mL. The leaching rate of metal was calculated according to Eq. (3):
Leaching rate (%) =
V×M × 100% m0
3.2. Distribution ratio of the leaching solution Waste LCDs contain a variety of metal oxides, thus, the acidic leachate is a complex system. Few comprehensive research studies have reported on other metals in waste LCDs. According to the optimal leaching conditions obtained above, 0.2 g of glass power was added into 10 mL of a 50% (v/v) [Hbet][Tf2N]-H2O system, and the leaching rate is presented in Fig. 2(a). The ionic liquid, like other leaching agents such as sulfuric acid (Zeng et al., 2015), also dissolved other metals in the process of indium leaching. In this case, iron was leached out completely. Other metals such as aluminum, calcium, and strontium had a lower leaching rate because they are strong bases and preferential selection. Nevertheless, the target indium ions were still completely leached by the ionic liquid. Interestingly, the UCST of the [Hbet][Tf2N]-H2O system is 55 °C (Hoogerstraete et al., 2013). Therefore, when the temperature of the leachate was reduced to room temperature, the ionic liquid and water were stratified. Remarkably, in the process of solution cooling and stratification, the metal ions in the leachate were not randomly distributed, but appeared in two phases at different distribution ratios. The results showed that 98.23% of the indium was distributed in the ionic liquid and that other metals were partially distributed into ionic liquids, such as 59.87% Fe, 26.11% Al and so on. This means [Hbet][Tf2N] had a higher selectivity to indium. Hoogerstraete et al. (2013) reported that In3+ could be almost completely extracted by [Hbet][Tf2N], and [Hbet] [Tf2N] had a weaker ability to bond with divalent metals. Thus, in order to reduce the influence of Fe3+ in the leachate and facilitate the separation of indium andiron, deionized water was replaced with an equal volume of ascorbic acid, and 0.01 M of ascorbic acid was enough to completely convert Fe3+ to Fe2+ (Table S1). The results with 0.01 M of ascorbic acid showed that ascorbic acid could effectively promote the separation of indium and iron (Fig. 2(b)). Although the leaching rate barely changed, the distribution rate of iron dropped dramatically from 59.87% to 8.84%. In contrast to Al3+, the leaching rate and distribution ratio of Ca2+ slightly increased, while the leaching and stratification of Sr2+ remained basically the same. In general, these metals underwent slight changes as representatives of various other metals, and the results indicated that the [Hbet][Tf2N]-AA systems have more advantages for the leaching and extraction.
(3)
where M (ppm) is the metal concentration in the total leachate, V (L) is the total volume and m0 is the initial metal mass present in the waste LCDs. Because of the stratification phenomenon, the metal distribution ratio was simultaneously measured according to Eq. (4), and the scrubbing rate was calculated according to Eq. (5).
Distribution ratio (%) =
Scrubbing rate (%) =
VIL × MIL × 100% VIL × MIL + Vaq × Maq
V0A × MOA × 100% VIL × MIL
(4) (5)
where MIL (ppm) is the metal concentration in the ionic liquid after extraction, Maq (ppm) is the metal concentration in the aqueous phase after extraction, and MOA (ppm) is the metal concentration in the in oxalic acid solution after scrubbing. VIL, Vaq and VOA are the volumes of the ionic liquid, aqueous phase, and oxalic acid solution, respectively. 3. Results and discussion 3.1. Effect of leaching conditions The concentration of ionic liquids has a significant effect on metal dissolution, influencing both the viscosity and acidity of the solution. The viscosity of [Hbet][Ff2N] depends not only on the water content but more so on the temperature. As previously described, this experiment was conducted above 70 °C so that the viscosity of the solution approximation was equal to the concentration of the water (0.89 mPa·s at 25 °C) (Davris et al., 2016). The effect of the [Hbet][Tf2N] concentration on the leaching rate of indium is shown in Fig. 1(a), and the acidity of the solution is shown in Fig. 1(b). The results indicate that more and more H+ was provided to facilitate the dissolution of metal oxides and ion exchange with an increase in ionic liquid. As a result, the leaching rate of indium gradually increased until the leaching completed. The indium leaching rate increased from 46.89% to 99.56% as the ionic liquid concentration 3
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Fig. 1. Effect of leaching conditions: (a) [Hbet][Tf2N] concentration with leaching rate and (b) its pH under the conditions of 0.2 g powders, 10 mL [Hbet][Tf2N]H2O, and 90 °C within 24 h, (c) solid /liquid ratio with leaching rate under the conditions of 10 mL 50% (v/v) [Hbet][Tf2N]-H2O and 90 °C within 24 h, and (d) temperature and reaction time with leaching rate under the conditions of 0.2 g powders and 10 mL 50% (v/v) [Hbet][Tf2N]-H2O.
and 13C NMR (400 MHz, DMSO) δppm:166.83 (s, COO), 63.17 (s, NCH2), 53.37 (3 × CH3). These results are consistent with pure [Hbet] [Tf2N] (Dupont and Binnemans, 2015a). It should be noted that the nature of the recovered phase did not affect the leaching and extraction of target metals such as indium during this experiment. Thus, the reusability of [Hbet][Tf2N] is presented in Fig. 3(b). When the ionic liquid was reused, the leaching rate and distribution ratio of the ionic liquid reached 97.77% and 97.17%, respectively. The results showed that the ionic liquid had favorable reusability.
3.3. Effects of scrubbing and recycling [Hbet][Tf2N], like common extractant, can be stripped and reused after extraction. The metal ions in the ionic liquid were transferred from the organic phase into the aqueous phase with oxalic acid. The reverse stripping efficiency of those metals is shown in Fig. 3(a). The scrubbing rate of indium was 95.71%. Thus, we calculated that the overall recovery rate of indium was 94.22% in the entire processing system. From another perspective, the indium content in the aqueous phase was 30.18 mg/L, second only to aluminum (90.12 mg/L). Van Roosendael et al. (2019) obtained a 49 mg/L indium solution from goethite leachate with resin impregnated with [A336][I]. Although the experimental raw materials were different, [Hbet][Tf2N] directly achieved the initial purification of indium in the complex system. [Hbet][Tf2N] was washed with ice water several times and vacuumdried for reuse. Meanwhile, NMR analyzed the recovered phase with 1H NMR (600 MHz, DMSO) δppm: 4.29 (s, 2H, CH2), 3.22 (s, 9H, 3 × CH3)
3.4. Reaction mechanism 3.4.1. [Hbet][Tf2N] characterization The ionic liquid played a dual role as a leaching agent and an extractant throughout the experiment, so it was essential in a needle analysis of [Hbet][Tf2N]. The leachate was obtained according to the optimal leaching conditions, and the ionic liquid after separation was
Fig. 2. Distribution rate and leaching rate with [Hbet][Tf2N]-H2O and [Hbet][Tf2N]-AA system: (a) leaching rate of metals, (b) distribution rate of metals. 4
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Fig. 3. Scrubbing and recycling [Hbet][Tf2N]: (a) Scrubbing [Hbet][Tf2N] with oxalic acid, (b) [Hbet][Tf2N] repeat performance test.
washed with oxalic acid and pure water several times. All samples were vacuum-dried to remove residual moisture before analysis. The FTIR of [Hbet][Tf2N] in various stages is shown in Fig. 4. The characteristic vibration of unionized and uncoordinated carboxyl showed a strong peak of eCOOH, that stretched at 1750 cm−1, and a hydrogen-bonding vibration peak at 3560 cm−1. It can be seen from a comparison of (a) and (b) that the original hydrogen-bonding was destroyed and the peak value decreased, and the peak value of the leached ionic liquid decreased owing to the coordination effect of eCOO and the metal ions. A comparison of (b) and (c) indicates that there was no significant difference between the ionic liquid after pickling and in its pure form [Hbet][Tf2N]. The results indicate that the carboxyl group on the cations ionized H+ into water, which increased the necessary acidity for metal dissolution. Subsequently, during the oxalic acid scrubbing, the metal ions were transferred into the oxalic acid solution, and the ionic liquid regained the H+ from the oxalic acid to restore the stable structure. Therefore, based on the reported ratio equation for trivalent rare earth elements (Dupont and Binnemans, 2015a), we speculated that the dissolution reaction could be explained by Eq. (6), and the pickling process could be explained by Eq. (7). The results of FTIR and NMR both indicated no significant changes in the ionic liquid after washing, so the reusability of [Hbet][Tf2N] after washing was theoretically feasible.
6 [Hbet ][Tf2 N ] + In2 O3
2 [In (Hbet )3 ][Tf2 N ]3 + 3 H2 O
2 [In (Hbet )3 ][Tf2 N ]3 + 3 C2 H2 O4
6 [Hbet ][Tf2 N ] + In2 (C2 O4 )3
(Nockemann et al., 2006). However, above the UCST of the [Hbet][Tf2N]H2O system, the hydrogen-bonding between the cation and the anion was significantly weakened so that the carboxyl group on the cations ionized H+ into water and provided a ligand for metal ions. The intramolecular interaction was weakened and the interaction between [Hbet][Tf2N] and polar water molecules was enhanced to form a homogeneous solution (Qiao et al., 2017). The water prevented the coordination of indium with the betainium ligands in the homogeneous system, so other metals were also leached out. However, when the leachate was cooled, the water content of [Hbet][Tf2N] was gradually reduced, and the vacant ionic liquid ligands were precisely combined with indium (Hoogerstraete et al., 2013). The separation of different metal ions was based on differences in the chemical interactions between the metal ions and the extractant, resulting in a high solubility in the organic and aqueous phases. 3.4.2. Leaching residue characterization The residue was cleaned and dried after leaching with ionic liquid for further analysis. The phases of ITO glass powder samples and the leaching residue after leaching with 50% (v/v) [Hbet][Tf2N]-H2O or [Hbet][Tf2N]-0.01 M ascorbic acid at 90 °C for 24 h were all identified by XRD, as shown in Fig. 5(a). The results showed that the original glass powder had a steamed bread peak around 23–24°. This was the characteristic diffraction peak of SiO2, generally because the main component of the glass powder was aluminosilicate glass. In addition, there was a weak diffraction peak appearing in the vicinity of 44.2° but it was impossible to determine the specific substance form. Thus, the glass powder of waste LCDs existed in an amorphous form. After leaching, there was no new peak. Thus, the leaching process could not cause a change in the internal lattice structure of the glass powder, and the reaction occurred on the surface of the powder. Moreover, the results of an SEM analysis showed that the surface morphology of the glass powder did not change significantly before and after leaching, and only the small debris was reduced (Fig. 5). In a previous report, we found that oxalic acid caused less damage to ITO (Cui et al., 2019), while sulfuric acid destroyed the structure of ITO (Zeng et al., 2015). Therefore, in this study, the physical and chemical properties of the glass powder could be retained. Thus, the residues could be subsequently recycled as materials for end products (Amato and Beolchini, 2018).
(6) (7)
In addition, the phenomenon of different metal distributions in the leachate owing to the temperature characteristics of [Hbet][Tf2N] deserves further discussion. First, the interaction between the lone pair of electrons on the O, N atom in [Tf2N]− and the OeH, CeH anti-bond orbital in [Hbet]+ forms a strong hydrogen-bonding in [Hbet][Tf2N]
3.4.3. Kinetic analysis The leaching process of indium is a heterogeneous solid-liquid reaction process. According to the shrinking core model, the solid-liquid reaction process is mainly divided into the following steps: (i) the leaching agent permeates to the indium-containing particle surface through the diffusion layer, (ii) the chemical reaction occurs at the surface of the core of unreacted particles, and (iii) the resulting soluble product diffuses into the solution (Chen et al., 2015; Huang et al., 2014). Hence, the main resistance that affects the total speed of the indium leaching process includes the diffusion resistance of the leaching agent, the chemical reaction resistance, and the product
Fig. 4. FTIR comparisons of [Hbet][Tf2N] (a) Pure [Hbet][Tf2N], (b) [Hbet] [Tf2N] after leaching, and (c) [Hbet][Tf2N] after scrubbing. 5
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Fig. 5. Leaching residue characterization: (a) XRD comparisons, (b) SEM of ITO glass powder, (c) SEM of residues with [Hbet][Tf2N]-H2O, and (d) SEM of residues with [Hbet][Tf2N]-AA.
diffusion resistance. The rate controlling step has the highest resistance. The kinetic model established by Eq. (8) confirms that the diffusion layer penetration mainly determines the leaching rate. The kinetic model established by Eq. (9) indicates that the interfacial reaction mainly determines the leaching rate (Huang et al., 2014). A leaching kinetic model can be determined according to the degree of data fitting.
1
2/3
1
(1
(1
)2/3 = k d t
)1/3 = kc t
reaction (Ea = 43.622 kJ/mol) (Cui et al., 2019), the leaching activation energy was slightly higher, which means that the time was prolonged. Unfortunately, we have not found data on the leaching activation energy of ionic liquids for indium and other metals. However, the required leaching time of ionic liquids for other metals is 24 h or longer (Davris et al., 2016; Dupont and Binnemans, 2015b), which means that the rate of such chemical reactions depends mainly on the nature of the chemical.
(8)
3.5. Implications
(9)
where x refers to the fraction that is reacted, t is the reaction time, and kd and kc are the rate constants calculated from Eqs. (3) and (4), respectively. At the same time, the slope k of the fitting curve was substituted into the Arrhenius equation (Eq. (10)), which is an empirical formula that describes the relationship between the rate constant of the chemical reaction and temperature. If Ea is > 42 kJ/mol, then the control step is the chemical reaction; if Ea is < 12 kJ/mol, then the control step is a diffusion control; if Ea is between 12 kJ/mol and 42 kJ/mol, then there is mixed control of the chemical reaction and diffusion (Chen et al., 2015). The Arrhenius equation is Ea
k = Ae RT
The supply risk of indium provoked the development of recycling indium from waste LCDs. In this paper, a novel fusion of leaching and extraction process was proposed to recycle indium from waste LCDs by [Hbet][Tf2N]. Compared with D2EHPA and other common solvent extractants, [Hbet] [Tf2N], as a special acidic ionic liquid, has low volatilization, no pollution, and temperature-dependent properties. The [Hbet][Tf2N]-AA system achieved nearly 100% leaching efficiency and 98.63% extraction efficiency. The one-step method was simple to realize the conversion and preliminary purification of indium. In addition, the regeneration and reuse of ionic liquids was also realized in our research. Furthermore, to our knowledge, ionic liquid has not been used to recover indium from waste LCDs. This study provided a new idea for the recycling of indium from waste LCDs. From the perspective of indium recovery from waste LCDs, both the leaching efficiency and extraction efficiency represent the efficiency of the recycling process and are expected to achieve zero loss as much as possible. Sulfuric acid (Zeng et al., 2015), hydrochloric acid (Fontana et al., 2015), oxalic acid (Cui et al., 2019), etc. have been reported to completely convert In2O3 from waste LCDs into an ionic state in solution, but research on leaching and separation technology for other impure metals is rare. Thus, it is necessary to consider the separation of other elements as an important means of simplifying the processing to streamline subsequent processes. The extraction of indium from leachate by a two-phase system was also reported. Fontana et al. (2015)
(10)
where k is the apparent rate constant from the slopes of the straight lines (Fig. 5), Ea refers to the activation energy. A kinetics analysis was conducted based on the experimental data for the dissolution of indium from waste LCDs. When the shrinking core model was applied to the obtained data, good linear fits were obtained as shown in Fig. 6(a). The chemical reaction dominated the speed of the indium leaching process. Moreover, lnk vs. (1/T) for each temperature is shown in Fig. 6(b), and the activation energy was calculated as 58.06 kJ/ mol. This result proves that chemical dominated the reaction. Compared with the inorganic acid leaching indium dominated by the chemical 6
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Fig. 6. Kinetic analysis: (a) Plot of 1-(1-x)1/3 vs time for the dissolution of waste LCDs, (b) Arrhenius plot for waste LCDs leaching.
reported that 80%–95% of the indium was distributed to the bottom phase with polyethylene glycol (PEG) through the PEG-ammonium sulphate-water system from the waste LCDs leachate. By contrast, to increase the ratio of aqueous two-phase extraction, we mainly achieved metal leaching and ionic liquid recycling. At the same time, [Hbet] [Tf2N] also achieved full leaching and separation of targeted ions, in previous reports on lamp phosphor and NdFeB magnets (Dupont and Binnemans, 2015a; Dupont and Binnemans, 2015b). In the extraction process, indium was retained in the ionic liquid organic phase, but the coordination form of indium and ionic liquid, and the mechanism of the separation of indium and other metals need further study. In addition, although the economic cost was controlled by the reuse of ionic liquids in this experiment, the high manufacturing cost limits the popularizing of ionic liquids. Therefore, according to the diversity and designability of ionic liquids, designing or screening a cheaper ionic liquid with rapid leaching and selective extraction capabilities for indium will be a direction of future research.
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4. Conclusion The [Hbet][Tf2N]-AA system was recommended because it not only enabled the extraction and separation of indium, but also effectively reduced the distribution ratio of iron in ionic liquids. The process was mainly controlled by chemical reactions. The carboxyl group on the cations ionized H+ to facilitate dissolution and provided a ligand for indium ion to form a complex during the reaction. Moreover, [Hbet][Tf2N] was regenerated by transferring the metal ions into the aqueous phase with oxalic acid and the ionic liquid recyclability was also well confirmed. All in all, the indium was recovered, the ionic liquid was reused, and the glass residues were recycled for subsequent utilization. Thus, the proposed fusion of leaching and extraction process by [Hbet][Tf2N] provided an efficient alternative to recycle indium from waste LCDs. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (51178191), Guangdong Science and Technology Project (2017A020216013), the Fundamental Research Funds for the Central Universities (2017PY012), and Guangzhou Science and Technology Project (201604020055). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.hydromet.2019.105146. 7
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Xie, Y., et al., 2019. Leaching of indium from end-of-life LCD panels via catalysis by synergistic microbial communities. Sci. Total Environ. 655, 781–786. https://doi. org/10.1016/j.scitotenv.2018.11.141. Yu, L., et al., 2016. Acetic acid production from the hydrothermal transformation of organics in waste liquid crystal display panels. J. Clean. Prod. 113, 925–930. https:// doi.org/10.1016/j.jclepro.2015.11.056. Zeng, X., Wang, F., Sun, X., Li, J., 2015. Recycling indium from scraped glass of liquid crystal display: process optimizing and mechanism exploring. ACS Sustain. Chem. Eng. 3 (7), 1306–1312. https://doi.org/10.1021/acssuschemeng.5b00020. Zhang, L., Xu, Z., 2017. C, H, Cl, and in element cycle in wastes: vacuum pyrolysis of PVC plastic to recover indium in LCD panels and prepare carbon coating. ACS Sustain. Chem. Eng. 5 (10), 8918–8929. https://doi.org/10.1021/acssuschemeng.7b01737. Zhang, K., et al., 2015. Recycling indium from waste LCDs: a review. Resour. Conserv. Recycl. 104, 276–290. https://doi.org/10.1016/j.resconrec.2015.07.015.
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Simultaneous leaching and extraction of indium from waste LCDs with acidic ionic liquids
T
Deliang Luoa, Nengwu Zhua,b,c,d, , Yao Lia, Jiaying Cuia, Pingxiao Wua,b,c,d, Jieyuan Wanga ⁎
a
School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters of Ministry of Education, Guangzhou 510006, PR China c Guangdong Environmental Protection Key Laboratory of Solid Waste Treatment and Recycling, Guangzhou 510006, PR China d Guangdong Engineering and Technology Research Center for Environmental Nanomaterials, Guangzhou 510006, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Waste LCDs Indium Leaching Extraction Acidic ionic liquid
The fragile indium supply inspired the development of recycling programs for end-of-life electronics products, especially liquid crystal displays (LCDs). LCDs consume 70% of the total indium resources used in the world today. In this study, a novel fusion of leaching and extraction processes was proposed to recycle indium from waste LCDs using the functionalized ionic liquid betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]). Under the optimum conditions of 10 mL 50% (v/v) ionic liquid/ascorbic acid, 20 g/L of solid/liquid ratio, and 90 °C operation temperature within 24 h, the indium leaching rate could reach as high as 99.75% and the indium extraction rate could be enriched by 98.63% in the ionic liquid phase after cooling and stratifying. Introducing ascorbic acid into the process in place of deionized water decreased the distribution rate of iron from 59.87% to 8.84% and promoted the separation of indium and iron. Indium in the ionic liquid phase could then be transferred into an indium-rich solution using oxalic acid and retrieve the ionic liquid. The characteristics of the regenerated ionic liquid remained relatively stable for reutilization. Therefore, the proposed fusion of leaching and extraction processes by [Hbet][Tf2N] provides an efficient alternative to recycle indium from waste LCDs.
1. Introduction Liquid crystal displays (LCDs), as a new generation of display with superior performance, are widely used during the manufacturing of various devices such as mobile phones, televisions, computer monitors, and laptops (Amato and Beolchini, 2018). With the continuous innovation of technology, the replacement speed of electronic products has gradually intensified, resulting in a surge in the number of waste LCDs (Işıldar et al., 2019). Waste LCDs contain indium‑tin-oxide (ITO) which uses indium oxide (In2O3) during the fabrication process. It has been considered that the indium consumption in the ITO reached > 70% of the total indium consumption (Zhang et al., 2015). Unfortunately, as an important scattered metal, the global reserves of indium are only about 16,000 t (Zhang et al., 2015). It has been reported that the sustainable index of indium is < 0.5, and the sustainable mining life is only 30 years (Wang et al., 2015). Therefore, recovering indium resources from waste LCDs should be an industry priority. In recent years, the process of recovering indium from waste LCDs has attracted worldwide attention. Thermal approaches (vacuum carbon-reduction and vapor separation, and pyrolysis chlorination and
⁎
separation) have been proposed to recover indium from waste LCDs (He et al., 2014; Ma et al., 2012; Zhang and Xu, 2017). For example, He et al. (2014) realized the reduction of indium oxide at 1 Pa and 1223 K by mixing waste LCDs with 50 wt% coke powder and recovered as much as 90% of the indium after vapor separation. Microorganism Acidithiobacillus indirectly leached 100% indium from waste LCDs (Xie et al., 2019), and Shewanella algae adsorbed indium(III) from aqueous solutions where the condensate was 4300 times the initial solution (0.94 mol/m3) after heat treatment (Ogi et al., 2012). Hydrometallurgical approaches are the most frequently reported in the field (Argenta et al., 2017; Cui et al., 2019; Rocchetti et al., 2015; Silveira et al., 2015; Zeng et al., 2015). For example, Silveira et al. (2015) recovered 96.2% of indium from waste LCDs by the combination of leaching with H2SO4 (96.4%) and precipitation with NH4OH (99.8%). Rocchetti et al. (2015) applied cementation with 100 g/L of zinc to replace indium with a highest purity of 62% from the leaching solution of waste LCDs. When citric acid and supercritical CO2 were applied, 94.6% of the indium was efficiently leached from ITO samples (Argenta et al., 2017). Most research studies still use leaching and extraction to recover indium, focusing on the efficiency and greenness of the
Corresponding author at: School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China. E-mail address: [email protected] (N. Zhu).
https://doi.org/10.1016/j.hydromet.2019.105146 Received 17 June 2019; Received in revised form 26 August 2019; Accepted 7 September 2019 Available online 10 September 2019 0304-386X/ © 2019 Elsevier B.V. All rights reserved.
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recycling process at the same time. If leaching and extraction can be integrated, then the technological process could be shortened and the types of reagents and environmental pollution could be reduced. Among the various efforts to conquer the drawbacks of leaching or extraction technology, ionic liquids could simultaneously replace existing acidic media and extractants in hydrometallurgy (Abbott et al., 2011; Park et al., 2014; Qiao et al., 2017). In order to be effective, such liquids require specific temperature characteristics and homogeneous transformation into two phases by heating to the upper critical solution temperature (UCST) or cooling to the lower critical solution temperature (LCST) separately. Two representative examples are the use of tributyltetradecyl phosphonium chloride ([P44414][Cl]), for blending with HCl solution at low temperatures and separating substances at high temperatures (Gras et al., 2018), and betainium bis(trifluoromethylsulfonyl) imide ([Hbet][Tf2N]), for providing uniform liquid reaction conditions at high temperatures and separating substances at low temperatures (Davris et al., 2016; Dupont and Binnemans, 2015a; Dupont and Binnemans, 2015b; Hoogerstraete et al., 2013). In particular, [Hbet][Tf2N] is more suitable for application in hydrometallurgy because of its acidic and UCST characteristics. The valence elements (Nd, Dy and Co) of NdFeB magnets were dissolved in the [Hbet][Tf2N]-H2O system at 80 °C and separated to the aqueous phase at room temperature, resulting in a 99% recovery rate and 99.9 wt% purity (Dupont and Binnemans, 2015b). [Hbet][Tf2N] has been used not only for the dissolution of metal oxides, but also for dissolving metal hydroxides. Deferm et al. (2018) dissolved crude indium hydroxide with [Hbet][Tf2N]-H2O system at 80 °C to remove other metal impurities, and the purity of indium hydroxide increased to 99 wt%. Related research previously confirmed that [Hbet] [Tf2N] could produce a high selectivity for the extraction of In(III) (Hoogerstraete et al., 2013). To our knowledge, [Hbet][Tf2N] has not been used to dissolve In2O3, and ionic liquid has not been used to recover indium from waste LCDs. Thus, it is worthwhile to explore the feasibility of indium recovery from waste LCDs using [Hbet][Tf2N]. In this study, a novel fusion of leaching and extraction process was proposed to recycle indium from waste LCDs using [Hbet][Tf2N]. The waste LCDs were dissolved in the [Hbet][Tf2N]-H2O system above UCST, and measurements were made of the concentration, solid to liquid ratio, reaction time, and temperature during leaching and extraction. Indium was separated from other metal impurities between two phases as the leachate was cooled. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM) were used to characterize the change in the ionic liquid and waste LCDs during the leaching process. Subsequently, the kinetics of the leaching process were analyzed. Finally, the effect of ionic liquid reuse on its leaching and extraction properties were studied.
water phase was discarded, [Hbet][Tf2N] was washed several times with ice water to remove the impurities of chlorine (detected the water by 0.5 M AgNO3). In order to remove excess water, the ionic liquid phase was dried by using a vacuum drying oven at 80 °C (DZF-6032, Du Wei, China). CH3 CH3
N
CH3
Li CH2
CH3
COOH
Cl
O S CF3
N O O
O S
CF3
CH3
N
CH2
CH3
S CF3
LiCl
COOH
O
N O O
O S
CF3
(1)
2.2. Leaching and extraction Glass powder (0.2 g) was completely dissolved by aqua regia (HNO3:HCl = 1:3, AR, China) and hydrogen fluoride (HF) (AR, China) at 180 °C for 2 h. The aqua regia and HF were alternately supplemented in the same volume to ensure the ITO glass powder could be completely dissolved. The solution was evaporated and transferred into a 50 mL volumetric flask (poly tetra fluoroethylene) to determine the type and content of metallic elements. Glass powder was added to a 20 mL serum flask under different experiment conditions to achieve the maximum leaching rate of indium. First, the acid concentration was investigated with 10, 20, 30, 40, 50, and 60% (v/v) [Hbet][Tf2N]. Meanwhile, the measurement of pH values in different concentrations was performed to analyze the acidity of the solution (S220-K, Mettler, Germany). Second, the effect of solid /liquid was studied at 10, 20, 30, 40, and 50 g/L to obtain the processing capacity of the ionic liquid system. Subsequently, the reactor was heated in an oil bath at 70, 80, and 90 °C for different times respectively, and each sample was separately prepared to analyze the leaching effect and the kinetics of the leaching process. After the reaction, the mixture in the reactor was filtered immediately to separate the solid from the solution using a quantitative filter paper (0.45 μm, nylon 66, China). On the one hand, the leachate was diluted with 5% (v/ v) HNO3 for a constant volume to calculate the leaching rate of In and other metal elements; on the other hand, the leachate was placed in a cooling centrifuge (2-16KL, Sigma, Germany) at 10,000 rpm for 10 min to stratify. The organic phase and the aqueous phase were diluted with 5% (v/v) HNO3, and the content of the metals was analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES) (Gras et al., 2018; Onghena et al., 2017). Three parallel samples were set for each sample and the experimental error was controlled to be < 5%. [Hbet][Tf2N] was analyzed before and after leaching to clarify its leaching mechanism. Meanwhile, the ITO glass powder was characterized before and after leaching to observe the morphological changes. Since the initial content of iron was high and the leaching effect was good in the [Hbet][Tf2N]-H2O system, the purification effect of indium needs to be further improved. Studies have shown that ascorbic acid can reduce Fe3+ to Fe2+ (Eq. (2)) and effectively increase the separation effect of ionic liquids on iron (Onghena et al., 2017). Therefore, given the optimal leaching conditions, an [Hbet][Tf2N]-0.01 M ascorbic acid (AA) solution system was formed by replacing the equivalent amount of deionized water with 0.01 M of AA to increase the separation effect of indium and iron.
2. Materials and methods 2.1. Materials The glass substrates were obtained by manually dismantling the waste LCDs purchased in bulk from the Electronic Waste Collecting Market in Guangzhou, China. To obtain clean glass substrates, the upper polarizer, an acid-producing material (Yu et al., 2016), was peeled off quickly after soaking in hot water for a few minutes, and the residual cellulose triacetate was stripped by soaking in acetone for a few minutes. The clean glass substrates were broken in a high-speed crushing shear (FW-400A, Zhong Xin, China) for 5 min. Zeng et al. (2015) reported that glass powder with a particle size of < 0.75 μm had a better leaching effect, so the ITO glass powder was sifted out with 300-mesh sieve to obtain the optimum leaching particle size. Betaine chloride (C5H11NO2HCl, 99%, Aladdin) was reacted with lithium bis(trifluoromethysulfonyl)imide (C2F6LiNO4S2, 98%, Aladdin) to compound [Hbet][Tf2N] (Eq. (1)) (Dupont and Binnemans, 2015a). After cooling by static stratification at room temperature, the mixture was layered as ionic liquid phase and water phase containing LiCl. When the
2Fe3 + + C6 H8 O6
2Fe2 + + C6 H6 O6 + 2H+
(2)
2.3. Scrubbing and recycling The obtained indium-containing ionic liquid was cleaned with oxalic acid which showed good bonding performance in our previous study (Cui et al., 2019). The obtained ionic liquid and 0.5 M oxalic acid were mixed at a ratio of 1:1 (v/v) and stirred at 70 °C for 10 min. The mixed solution was subjected to centrifuge at 10,000 rpm for 10 min (216KL, Sigma, Germany). The organic phase and the aqueous phase were diluted, and the content of the metals was analyzed (Gras et al., 2018). [Hbet][Tf2N] was analyzed before and after scrubbing to back up the 2
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leaching mechanism. After pickling, the ionic liquid was cleaned with deionized water several times and vacuum-dried to remove excess moisture for reuse. Nuclear magnetic resonance characterization of the recovered ionic liquid was carried out, and the leaching experiments were repeated to verify the reuse performance of the ionic liquid.
increased from 10% to 40% (v/v), and then the leaching rate reached 100% when the ionic liquid concentration reached 50% (v/v) or higher. Considering the cost of consumption and the leaching effect, 50% (v/v) can be regarded as the appropriate experimental condition for leaching. Fig. 1(c) shows the effect of the solid-liquid ratio on indium leaching recovery. It was found that the indium leaching rate began to decline when the solid-liquid ratio exceeded 20 g/L. The increase of the solidliquid ratio indicates that more powder was added to the same volume of solvent, reducing the mass transfer efficiency and the leaching rate of indium. The higher the solid-liquid ratio, the greater the processing capacity and the lower the cost. The leaching rate of indium reached nearly 100% when the solid-liquid ratio was lower than 20 g/L. Thus, the recommended solid-liquid ratio was 20 g/L. The results of the indium leaching rate with the leaching time and temperature are shown in Fig. 1(d). When the leaching time was 24 h, the indium leaching rate achieved 62.32%, 76.25%, and 100% at the temperature of 70, 80, and 90 °C, respectively. The temperature was favorable to increasing the reaction rate. In addition, the leaching rate continued rising after 24 h at 70 °C and 80 °C, so the above trend indicates that it takes longer to achieve complete leaching at low temperatures. Experiments with the [Hbet][Tf2N]-H2O system indicated that the optimal experimental conditions were as follows: 50% (v/v) [Hbet] [Tf2N]-H2O system，20 g/L (waste LCDs/[Hbet][Tf2N]-H2O), and a leaching time and temperature of 24 h and 90 °C, respectively.
2.4. Analysis and calculation ICP-OES (ICP730, Agilent, America) was used to determine the concentration of indium and other metals before and after leaching and extraction (Gras et al., 2018; Onghena et al., 2017). The samples were diluted 25–80 times with 5% (v/v) HNO3 before measurement to increase the accuracy of our results (Fig. S1). The structures of the glass powder before and after leaching were characterized by X-ray diffraction (XRD, Ultima IV, Germany), which scanned from 5 to 90° in a 2θ scan. [Hbet][Tf2N] was uniformly coated on the KBr wafer, and the functional group of [Hbet][Tf2N] was scanned by FTIR (Nicolet Nexus, America). The scanning wave number range was 400–40,000 cm−1, and the resolution was 4 cm−1. Furthermore, [Hbet][Tf2N] was dissolved in dimethyl sulfoxide (DMSO) after vacuum drying and characterized by nuclear magnetic resonance (NMR, HD600, Avance, Germany). The pattern of glass powder before and after leaching were characterized with SEM (SU8010, Hitachi, Japan). In order to calculate the leaching rate of metallic elements, the leachate and glass powder were separated at a high temperature (> 70 °C), and the residual liquid in the serum flask was cleaned with 5% (v/v) HNO3 more than three times. The volume was fixed at 50 mL. The leaching rate of metal was calculated according to Eq. (3):
Leaching rate (%) =
V×M × 100% m0
3.2. Distribution ratio of the leaching solution Waste LCDs contain a variety of metal oxides, thus, the acidic leachate is a complex system. Few comprehensive research studies have reported on other metals in waste LCDs. According to the optimal leaching conditions obtained above, 0.2 g of glass power was added into 10 mL of a 50% (v/v) [Hbet][Tf2N]-H2O system, and the leaching rate is presented in Fig. 2(a). The ionic liquid, like other leaching agents such as sulfuric acid (Zeng et al., 2015), also dissolved other metals in the process of indium leaching. In this case, iron was leached out completely. Other metals such as aluminum, calcium, and strontium had a lower leaching rate because they are strong bases and preferential selection. Nevertheless, the target indium ions were still completely leached by the ionic liquid. Interestingly, the UCST of the [Hbet][Tf2N]-H2O system is 55 °C (Hoogerstraete et al., 2013). Therefore, when the temperature of the leachate was reduced to room temperature, the ionic liquid and water were stratified. Remarkably, in the process of solution cooling and stratification, the metal ions in the leachate were not randomly distributed, but appeared in two phases at different distribution ratios. The results showed that 98.23% of the indium was distributed in the ionic liquid and that other metals were partially distributed into ionic liquids, such as 59.87% Fe, 26.11% Al and so on. This means [Hbet][Tf2N] had a higher selectivity to indium. Hoogerstraete et al. (2013) reported that In3+ could be almost completely extracted by [Hbet][Tf2N], and [Hbet] [Tf2N] had a weaker ability to bond with divalent metals. Thus, in order to reduce the influence of Fe3+ in the leachate and facilitate the separation of indium andiron, deionized water was replaced with an equal volume of ascorbic acid, and 0.01 M of ascorbic acid was enough to completely convert Fe3+ to Fe2+ (Table S1). The results with 0.01 M of ascorbic acid showed that ascorbic acid could effectively promote the separation of indium and iron (Fig. 2(b)). Although the leaching rate barely changed, the distribution rate of iron dropped dramatically from 59.87% to 8.84%. In contrast to Al3+, the leaching rate and distribution ratio of Ca2+ slightly increased, while the leaching and stratification of Sr2+ remained basically the same. In general, these metals underwent slight changes as representatives of various other metals, and the results indicated that the [Hbet][Tf2N]-AA systems have more advantages for the leaching and extraction.
(3)
where M (ppm) is the metal concentration in the total leachate, V (L) is the total volume and m0 is the initial metal mass present in the waste LCDs. Because of the stratification phenomenon, the metal distribution ratio was simultaneously measured according to Eq. (4), and the scrubbing rate was calculated according to Eq. (5).
Distribution ratio (%) =
Scrubbing rate (%) =
VIL × MIL × 100% VIL × MIL + Vaq × Maq
V0A × MOA × 100% VIL × MIL
(4) (5)
where MIL (ppm) is the metal concentration in the ionic liquid after extraction, Maq (ppm) is the metal concentration in the aqueous phase after extraction, and MOA (ppm) is the metal concentration in the in oxalic acid solution after scrubbing. VIL, Vaq and VOA are the volumes of the ionic liquid, aqueous phase, and oxalic acid solution, respectively. 3. Results and discussion 3.1. Effect of leaching conditions The concentration of ionic liquids has a significant effect on metal dissolution, influencing both the viscosity and acidity of the solution. The viscosity of [Hbet][Ff2N] depends not only on the water content but more so on the temperature. As previously described, this experiment was conducted above 70 °C so that the viscosity of the solution approximation was equal to the concentration of the water (0.89 mPa·s at 25 °C) (Davris et al., 2016). The effect of the [Hbet][Tf2N] concentration on the leaching rate of indium is shown in Fig. 1(a), and the acidity of the solution is shown in Fig. 1(b). The results indicate that more and more H+ was provided to facilitate the dissolution of metal oxides and ion exchange with an increase in ionic liquid. As a result, the leaching rate of indium gradually increased until the leaching completed. The indium leaching rate increased from 46.89% to 99.56% as the ionic liquid concentration 3
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Fig. 1. Effect of leaching conditions: (a) [Hbet][Tf2N] concentration with leaching rate and (b) its pH under the conditions of 0.2 g powders, 10 mL [Hbet][Tf2N]H2O, and 90 °C within 24 h, (c) solid /liquid ratio with leaching rate under the conditions of 10 mL 50% (v/v) [Hbet][Tf2N]-H2O and 90 °C within 24 h, and (d) temperature and reaction time with leaching rate under the conditions of 0.2 g powders and 10 mL 50% (v/v) [Hbet][Tf2N]-H2O.
and 13C NMR (400 MHz, DMSO) δppm:166.83 (s, COO), 63.17 (s, NCH2), 53.37 (3 × CH3). These results are consistent with pure [Hbet] [Tf2N] (Dupont and Binnemans, 2015a). It should be noted that the nature of the recovered phase did not affect the leaching and extraction of target metals such as indium during this experiment. Thus, the reusability of [Hbet][Tf2N] is presented in Fig. 3(b). When the ionic liquid was reused, the leaching rate and distribution ratio of the ionic liquid reached 97.77% and 97.17%, respectively. The results showed that the ionic liquid had favorable reusability.
3.3. Effects of scrubbing and recycling [Hbet][Tf2N], like common extractant, can be stripped and reused after extraction. The metal ions in the ionic liquid were transferred from the organic phase into the aqueous phase with oxalic acid. The reverse stripping efficiency of those metals is shown in Fig. 3(a). The scrubbing rate of indium was 95.71%. Thus, we calculated that the overall recovery rate of indium was 94.22% in the entire processing system. From another perspective, the indium content in the aqueous phase was 30.18 mg/L, second only to aluminum (90.12 mg/L). Van Roosendael et al. (2019) obtained a 49 mg/L indium solution from goethite leachate with resin impregnated with [A336][I]. Although the experimental raw materials were different, [Hbet][Tf2N] directly achieved the initial purification of indium in the complex system. [Hbet][Tf2N] was washed with ice water several times and vacuumdried for reuse. Meanwhile, NMR analyzed the recovered phase with 1H NMR (600 MHz, DMSO) δppm: 4.29 (s, 2H, CH2), 3.22 (s, 9H, 3 × CH3)
3.4. Reaction mechanism 3.4.1. [Hbet][Tf2N] characterization The ionic liquid played a dual role as a leaching agent and an extractant throughout the experiment, so it was essential in a needle analysis of [Hbet][Tf2N]. The leachate was obtained according to the optimal leaching conditions, and the ionic liquid after separation was
Fig. 2. Distribution rate and leaching rate with [Hbet][Tf2N]-H2O and [Hbet][Tf2N]-AA system: (a) leaching rate of metals, (b) distribution rate of metals. 4
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Fig. 3. Scrubbing and recycling [Hbet][Tf2N]: (a) Scrubbing [Hbet][Tf2N] with oxalic acid, (b) [Hbet][Tf2N] repeat performance test.
washed with oxalic acid and pure water several times. All samples were vacuum-dried to remove residual moisture before analysis. The FTIR of [Hbet][Tf2N] in various stages is shown in Fig. 4. The characteristic vibration of unionized and uncoordinated carboxyl showed a strong peak of eCOOH, that stretched at 1750 cm−1, and a hydrogen-bonding vibration peak at 3560 cm−1. It can be seen from a comparison of (a) and (b) that the original hydrogen-bonding was destroyed and the peak value decreased, and the peak value of the leached ionic liquid decreased owing to the coordination effect of eCOO and the metal ions. A comparison of (b) and (c) indicates that there was no significant difference between the ionic liquid after pickling and in its pure form [Hbet][Tf2N]. The results indicate that the carboxyl group on the cations ionized H+ into water, which increased the necessary acidity for metal dissolution. Subsequently, during the oxalic acid scrubbing, the metal ions were transferred into the oxalic acid solution, and the ionic liquid regained the H+ from the oxalic acid to restore the stable structure. Therefore, based on the reported ratio equation for trivalent rare earth elements (Dupont and Binnemans, 2015a), we speculated that the dissolution reaction could be explained by Eq. (6), and the pickling process could be explained by Eq. (7). The results of FTIR and NMR both indicated no significant changes in the ionic liquid after washing, so the reusability of [Hbet][Tf2N] after washing was theoretically feasible.
6 [Hbet ][Tf2 N ] + In2 O3
2 [In (Hbet )3 ][Tf2 N ]3 + 3 H2 O
2 [In (Hbet )3 ][Tf2 N ]3 + 3 C2 H2 O4
6 [Hbet ][Tf2 N ] + In2 (C2 O4 )3
(Nockemann et al., 2006). However, above the UCST of the [Hbet][Tf2N]H2O system, the hydrogen-bonding between the cation and the anion was significantly weakened so that the carboxyl group on the cations ionized H+ into water and provided a ligand for metal ions. The intramolecular interaction was weakened and the interaction between [Hbet][Tf2N] and polar water molecules was enhanced to form a homogeneous solution (Qiao et al., 2017). The water prevented the coordination of indium with the betainium ligands in the homogeneous system, so other metals were also leached out. However, when the leachate was cooled, the water content of [Hbet][Tf2N] was gradually reduced, and the vacant ionic liquid ligands were precisely combined with indium (Hoogerstraete et al., 2013). The separation of different metal ions was based on differences in the chemical interactions between the metal ions and the extractant, resulting in a high solubility in the organic and aqueous phases. 3.4.2. Leaching residue characterization The residue was cleaned and dried after leaching with ionic liquid for further analysis. The phases of ITO glass powder samples and the leaching residue after leaching with 50% (v/v) [Hbet][Tf2N]-H2O or [Hbet][Tf2N]-0.01 M ascorbic acid at 90 °C for 24 h were all identified by XRD, as shown in Fig. 5(a). The results showed that the original glass powder had a steamed bread peak around 23–24°. This was the characteristic diffraction peak of SiO2, generally because the main component of the glass powder was aluminosilicate glass. In addition, there was a weak diffraction peak appearing in the vicinity of 44.2° but it was impossible to determine the specific substance form. Thus, the glass powder of waste LCDs existed in an amorphous form. After leaching, there was no new peak. Thus, the leaching process could not cause a change in the internal lattice structure of the glass powder, and the reaction occurred on the surface of the powder. Moreover, the results of an SEM analysis showed that the surface morphology of the glass powder did not change significantly before and after leaching, and only the small debris was reduced (Fig. 5). In a previous report, we found that oxalic acid caused less damage to ITO (Cui et al., 2019), while sulfuric acid destroyed the structure of ITO (Zeng et al., 2015). Therefore, in this study, the physical and chemical properties of the glass powder could be retained. Thus, the residues could be subsequently recycled as materials for end products (Amato and Beolchini, 2018).
(6) (7)
In addition, the phenomenon of different metal distributions in the leachate owing to the temperature characteristics of [Hbet][Tf2N] deserves further discussion. First, the interaction between the lone pair of electrons on the O, N atom in [Tf2N]− and the OeH, CeH anti-bond orbital in [Hbet]+ forms a strong hydrogen-bonding in [Hbet][Tf2N]
3.4.3. Kinetic analysis The leaching process of indium is a heterogeneous solid-liquid reaction process. According to the shrinking core model, the solid-liquid reaction process is mainly divided into the following steps: (i) the leaching agent permeates to the indium-containing particle surface through the diffusion layer, (ii) the chemical reaction occurs at the surface of the core of unreacted particles, and (iii) the resulting soluble product diffuses into the solution (Chen et al., 2015; Huang et al., 2014). Hence, the main resistance that affects the total speed of the indium leaching process includes the diffusion resistance of the leaching agent, the chemical reaction resistance, and the product
Fig. 4. FTIR comparisons of [Hbet][Tf2N] (a) Pure [Hbet][Tf2N], (b) [Hbet] [Tf2N] after leaching, and (c) [Hbet][Tf2N] after scrubbing. 5
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Fig. 5. Leaching residue characterization: (a) XRD comparisons, (b) SEM of ITO glass powder, (c) SEM of residues with [Hbet][Tf2N]-H2O, and (d) SEM of residues with [Hbet][Tf2N]-AA.
diffusion resistance. The rate controlling step has the highest resistance. The kinetic model established by Eq. (8) confirms that the diffusion layer penetration mainly determines the leaching rate. The kinetic model established by Eq. (9) indicates that the interfacial reaction mainly determines the leaching rate (Huang et al., 2014). A leaching kinetic model can be determined according to the degree of data fitting.
1
2/3
1
(1
(1
)2/3 = k d t
)1/3 = kc t
reaction (Ea = 43.622 kJ/mol) (Cui et al., 2019), the leaching activation energy was slightly higher, which means that the time was prolonged. Unfortunately, we have not found data on the leaching activation energy of ionic liquids for indium and other metals. However, the required leaching time of ionic liquids for other metals is 24 h or longer (Davris et al., 2016; Dupont and Binnemans, 2015b), which means that the rate of such chemical reactions depends mainly on the nature of the chemical.
(8)
3.5. Implications
(9)
where x refers to the fraction that is reacted, t is the reaction time, and kd and kc are the rate constants calculated from Eqs. (3) and (4), respectively. At the same time, the slope k of the fitting curve was substituted into the Arrhenius equation (Eq. (10)), which is an empirical formula that describes the relationship between the rate constant of the chemical reaction and temperature. If Ea is > 42 kJ/mol, then the control step is the chemical reaction; if Ea is < 12 kJ/mol, then the control step is a diffusion control; if Ea is between 12 kJ/mol and 42 kJ/mol, then there is mixed control of the chemical reaction and diffusion (Chen et al., 2015). The Arrhenius equation is Ea
k = Ae RT
The supply risk of indium provoked the development of recycling indium from waste LCDs. In this paper, a novel fusion of leaching and extraction process was proposed to recycle indium from waste LCDs by [Hbet][Tf2N]. Compared with D2EHPA and other common solvent extractants, [Hbet] [Tf2N], as a special acidic ionic liquid, has low volatilization, no pollution, and temperature-dependent properties. The [Hbet][Tf2N]-AA system achieved nearly 100% leaching efficiency and 98.63% extraction efficiency. The one-step method was simple to realize the conversion and preliminary purification of indium. In addition, the regeneration and reuse of ionic liquids was also realized in our research. Furthermore, to our knowledge, ionic liquid has not been used to recover indium from waste LCDs. This study provided a new idea for the recycling of indium from waste LCDs. From the perspective of indium recovery from waste LCDs, both the leaching efficiency and extraction efficiency represent the efficiency of the recycling process and are expected to achieve zero loss as much as possible. Sulfuric acid (Zeng et al., 2015), hydrochloric acid (Fontana et al., 2015), oxalic acid (Cui et al., 2019), etc. have been reported to completely convert In2O3 from waste LCDs into an ionic state in solution, but research on leaching and separation technology for other impure metals is rare. Thus, it is necessary to consider the separation of other elements as an important means of simplifying the processing to streamline subsequent processes. The extraction of indium from leachate by a two-phase system was also reported. Fontana et al. (2015)
(10)
where k is the apparent rate constant from the slopes of the straight lines (Fig. 5), Ea refers to the activation energy. A kinetics analysis was conducted based on the experimental data for the dissolution of indium from waste LCDs. When the shrinking core model was applied to the obtained data, good linear fits were obtained as shown in Fig. 6(a). The chemical reaction dominated the speed of the indium leaching process. Moreover, lnk vs. (1/T) for each temperature is shown in Fig. 6(b), and the activation energy was calculated as 58.06 kJ/ mol. This result proves that chemical dominated the reaction. Compared with the inorganic acid leaching indium dominated by the chemical 6
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Fig. 6. Kinetic analysis: (a) Plot of 1-(1-x)1/3 vs time for the dissolution of waste LCDs, (b) Arrhenius plot for waste LCDs leaching.
reported that 80%–95% of the indium was distributed to the bottom phase with polyethylene glycol (PEG) through the PEG-ammonium sulphate-water system from the waste LCDs leachate. By contrast, to increase the ratio of aqueous two-phase extraction, we mainly achieved metal leaching and ionic liquid recycling. At the same time, [Hbet] [Tf2N] also achieved full leaching and separation of targeted ions, in previous reports on lamp phosphor and NdFeB magnets (Dupont and Binnemans, 2015a; Dupont and Binnemans, 2015b). In the extraction process, indium was retained in the ionic liquid organic phase, but the coordination form of indium and ionic liquid, and the mechanism of the separation of indium and other metals need further study. In addition, although the economic cost was controlled by the reuse of ionic liquids in this experiment, the high manufacturing cost limits the popularizing of ionic liquids. Therefore, according to the diversity and designability of ionic liquids, designing or screening a cheaper ionic liquid with rapid leaching and selective extraction capabilities for indium will be a direction of future research.
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4. Conclusion The [Hbet][Tf2N]-AA system was recommended because it not only enabled the extraction and separation of indium, but also effectively reduced the distribution ratio of iron in ionic liquids. The process was mainly controlled by chemical reactions. The carboxyl group on the cations ionized H+ to facilitate dissolution and provided a ligand for indium ion to form a complex during the reaction. Moreover, [Hbet][Tf2N] was regenerated by transferring the metal ions into the aqueous phase with oxalic acid and the ionic liquid recyclability was also well confirmed. All in all, the indium was recovered, the ionic liquid was reused, and the glass residues were recycled for subsequent utilization. Thus, the proposed fusion of leaching and extraction process by [Hbet][Tf2N] provided an efficient alternative to recycle indium from waste LCDs. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgments This work was supported by the National Natural Science Foundation of China (51178191), Guangdong Science and Technology Project (2017A020216013), the Fundamental Research Funds for the Central Universities (2017PY012), and Guangzhou Science and Technology Project (201604020055). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.hydromet.2019.105146. 7
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