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Leaching and purification of indium from waste liquid crystal display panel after hydrothermal pretreatment: Optimum conditions determination and kinetic analysis
Waste Management 102 (2020) 635–644
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Leaching and purification of indium from waste liquid crystal display panel after hydrothermal pretreatment: Optimum conditions determination and kinetic analysis Yue Cao, Feng Li, Guangming Li, Juwen Huang, Haochen Zhu, Wenzhi He ⇑ State Key Laboratory of Pollution Control and Resources Reuse, Shanghai Institute of Pollution Control and Ecological Security, College of Environmental Science and Engineering, Tongji University, 1239 Siping Rd., Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 25 August 2019 Revised 16 November 2019 Accepted 19 November 2019
Keywords: Waste LCD panel Indium Acid leaching Solvent extraction Kinetic analysis
a b s t r a c t Indium is one of the components with great recycling value in waste LCDs. Degradation of organic materials and the remain of indium in the solid phase can be simultaneously achieved by hydrothermal pretreatment via parameter regulation. Indium was transferred from the solid phase to the liquid phase by using sulfuric acid after hydrothermal pretreatment. Di-(2-ethylhexyl) phosphoric acid diluted by sulfonated kerosene and hydrochloric acid were used as extractant and stripping agent respectively to purify and concentrate indium from acidic leaching solution. The results indicated that the leaching yield of indium reached 100% under the optimal condition of reaction time of 40 min, reaction temperature of 70–80 °C, acid concentration of 0.5 M and solid-liquid (S/L) ratio of 1:2 g/mL. Given conditions of extraction time of 3 min at the organic phase to aqueous phase (O/A) ratio of 1:10 by 20% D2EHPA and stripping time of 10 min at the (O/A) ratio of 10:1 by 4 M HCl, the recovery efficiency of indium reached 97.25%. In addition, acid leaching process did not change the surface topography and molecular structure of glass substrate and had no negative effect on subsequent recycling of glass. The kinetic equation of leaching yield and reaction time at the temperature of 80 °C is 1 (1 y)1/3 = 0.0215 t. The reaction activation energy of metal indium leaching process is 50.64 kJ/mol. Ó 2019 Published by Elsevier Ltd.
1. Introduction As a new generation of display device with superior performance, liquid crystal display (LCD) has the advantages of high resolution, clear colors, power saving, light weight and portability (Zhuang et al., 2012). The progress of liquid crystal display technology continuously reduces the manufacturing cost of the liquid crystal display, making it widely produced. Due to the rapid development of LCD industry, China gradually become the world’s largest LCD production and sales area in the past 30 years. In 2016, the production volume of domestic color TV sets was about 157.7694 million units and the production capacity of microcomputer equipment was 29.00851 million units (National Bureau of Statistics of China, 2016). The huge market demand has effectively promoted the prosperity and development of China’s LCD industry, also greatly promoted the rapid growth of LCD equipment production. It is predicted that a large number of end-of-life LCD electronic products will generate from 2014 to 2020 including ⇑ Corresponding author. E-mail address: [email protected] (W. He). https://doi.org/10.1016/j.wasman.2019.11.029 0956-053X/Ó 2019 Published by Elsevier Ltd.
desktop computer, laptop, LCD TV and mobile phone. The scrapping volume of the four types of waste LCD products will reach 4261.94million units in 2020 (Liu et al., 2016). Such large quantities of discarded LCDs pose a potential threat to ecological environment (Awasthi and Li, 2017; Yu et al., 2017a, 2017b). Not only e-waste recycling management faces a huge challenge, but also e-waste processing technology needs to be innovated. 1.1. Current recycling management strategies on waste LCD panel As the function realization unit, LCD panel is the core component of the LCD device. LCD panel can be divided into inorganic glass, metal and organic components according to the structure and material characteristics (Yu et al., 2019). All components have a certain recycling value, among which the recovery of indium in glass substrates is particularly important (He et al., 2014; Rocchetti et al., 2015; Zeng et al., 2015; Zhang et al., 2015). Indium is etched on one side of the glass substrate of the LCD panel in the form of indium tin oxide to conduct electricity (Chen et al., 2017). The good electrical conductivity of indium makes it a huge demand in the electronics industry. Worldwide, about 70% of indium
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products are used for the fabrication of indium tin oxide on LCDs (Yang et al., 2013). The average content of indium in zinc ores varies from less than 1 to 100 ppm, while in LCD panel it presents at approximately 250 ppm (Pereira et al., 2018). Thus, it is of strategic significance to recover indium from waste LCD panel (Oguchi et al., 2013; Sawai et al., 2015; Zhang and Xu, 2017). A number of researches have been carried out on the recycling of indium currently. One of the methods is hydrometallurgy (Swain et al., 2016). LCD glass panel is mechanically shredded for size reduction after separation from plastic and HCl is used as the leaching agent. In addition, acid leaching with the aid of ultrasonic wave is also studied for the recovery of indium (Zhang et al., 2017). It is a non-crushing leaching method, manual stripping and acetone soaking are conducted to remove polarizer films and liquid crystal before the leaching process. Moreover, organic acids have also been studied as leaching agents. (Cui et al., 2019) investigated on the role of oxalic acid in the leaching system for recovering indium. Lopez-Yanez et al. (2019) carried out the experiments using citrate as a complexing agent. Other methods including thermal treatment (Zhuang et al., 2019), vaccum reduction (Wang et al., 2019), electrochemical recycling (Pa, 2017), flotation (Wang et al., 2018) and bioleaching (Jowkar et al., 2018) have also been discussed. Meanwhile, there are also plenty of researches on the recycling of glass and organic components in waste LCD panel. Glass is recycled to fabricate the silicon particles for lithium-ion battery anodes (Kang et al., 2019) and recycled as the concrete flumes additive to enhance the sulfate attack resistance (Kim and Hong, 2019) or as the supplementary cementing material in cement mortarsrationale (Kim et al., 2018). In terms of the treatment of organic materials, vacuum pyrolysis (Chen et al., 2017) and sub/supercritical water treatments (Wang et al., 2015) are studied. It is known that the removal of organic materials is a critical process before the recovery of indium. Usually, means of solvent soaking, dry mechanical crushing and sieving, pyrolysis, thermal shock and gravimetric process are used for the pretreatment of indium recovery (Savvilotidou et al., 2019). However, those methods cannot eliminate the risks of organic materials and also cause energy and material consumption. For these existing problems, previous researches focused on the use of hydrothermal methods to treat waste LCD panel. Hydrothermal technology can realize the degradation and recycling of organic materials of waste LCD panel based on the highefficiency and non-pollution advantages in a closed environment (He et al., 2008; Li et al., 2015; Yu et al., 2017a, 2017b). The liquid products are mainly acetic acid, lactic acid and other small molecule organic acids (Yu et al., 2016). It is an efficient and clean method for pretreatment without secondary pollution. According to the previous study, in the pretreatment stage, not only the degradation of organic matter is realized but also the remain of indium in the solid phase residue is achieved through the regulation of reaction parameters (Cao et al., 2019). After pretreatment, the glass substrate is polished and meanwhile some organic carbon products are deposited on the surface of the glass substrate during the conversion. It needs to be discussed that whether subsequent indium recovery process will be influenced under this circumstance. In the follow-up process, acid leaching concentration technology is used to extract indium from the solid residue. The main steps are acid leaching, extraction and stripping. According to the above process, effects of various factors in acid leaching, extraction and stripping were analyzed. Whether subsequent glass recycling was affected after acid leaching was also studied. Thus, characteristics change of the solid residue before and after acid leaching was investigated. At the same time, kinetic analysis was carried out. According to the analysis, the purification and recovery of indium are achieved. As a part of the recycling system, the comprehensive
recycling system of waste LCD panel is established through the green recycling means. In addition, the theoretical basis and technical support are provided for the industrial production. 2. Materials and methods 2.1. Materials and chemicals LCD panel used in this study was a mixture of different types of LCD panels provided by Taicang Shun Hui-Ferrous Metals Ltd., with organic carbon content of 12.6% (Cao et al., 2019). LCD panel was cut into small pieces approximately 1 cm 1 cm before the experimentsH2O2 (30%) was used as oxidant. Hydrochloric acid (HCl), sulfuric acid (H2SO4) and nitric acid (HNO3) were used as the leaching agents. Di-(2-ethylhexyl) phosphoric acid (D2EHPA) was used as extractant and sulfonated kerosene was used as the diluent for the extractant. Hydrochloric acid (HCl) was used as stripping agent. All reagents in this research study were of analytic grade and purchased from Sino-pharm Chemical Reagent Co., Ltd (Shanghai, China). Deionized water was used during the whole study. 2.2. Apparatus and procedure 2.2.1. Hydrothermal pretreatment Hydrothermal pretreatment was conducted with a batch reactor system that consisted of a salt bath and a stainless-steel vessel with wall thickness of 1 mm, length of 120 mm and inner volume of 5.7 mL. The salt bath was used for heating the reactor and the vessel was used to provide an instant and uniform heating process. The reactor was submerged into the salt bath which was preheated to the desired temperature. The reactor was taken out of the salt bath and immediately put into a cold-water bath when the reaction was over. The liquid and solid products were collected for further experiments. Hydrothermal reaction was carried out with 2.4 g crushed LCD panel fragments under neutral condition of 300 °C of reaction temperature, 36 mL of water and 7.2 mL of H2O2 (30%) supply and 11 min of reaction time (Cao et al., 2019). 2.2.2. Leaching of indium from solid phase residue A certain amount of solid phase product was weighted and put into a beaker, a certain volume of acid was added, and the acid concentration, acid type, solid-liquid (S/L) ratio, reaction time and reaction temperature were changed. Conditions of leaching experiments are shown in Table S1 in supplementary file. Indium leaching experiments were conducted in a constant temperature reactor under different conditions. Stirring was carried out during the reaction. After the reaction, the solid-liquid mixture was separated by vacuum filtration through a micropore filter membrane, and then the filter membrane was washed several times with deionized water. The filtrate was transferred to a volumetric flask and made up to volume with 4% dilute nitric acid for further analytical testing. The filter membrane adhered with the residue was placed in an oven and dried at 60 °C for 24 h for subsequent analysis. 2.2.3. Purification of indium from leaching solution After dilution, the H2SO4 leaching solution was used as the mother liquor of the extraction process and the concentration of indium was 0.99 mg/L. D2EHPA was dissolved in sulfonated kerosene to prepare the extractant with different concentrations. The corresponding proportion of the acid leaching liquid and the extractant were placed in a separatory funnel in extraction process. Conditions of leaching experiments are shown in Table S2 in supplementary file. The extract liquor and stripping agent were placed in a separatory funnel during the process of stripping. Conditions
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of leaching experiments are shown in Table S3 in supplementary file. The mixture was oscillated at the temperature of 25 °C. The oscillation was stopped immediately after the reaction time was reached, and the mixture was stood for 30 min to ensure the maximum stratification of the aqueous solution and the organic solution. The organic phase and the aqueous phase were separated and stored under suitable conditions. The extraction mechanism is a cation exchange process. The simplified reaction formula and the equation of equilibrium constant, distribution ratio and extraction efficiency are as follows (Tsai and Tsai, 2012; Pereira et al., 2018). þ In3þ ðaqÞ þ 3H2 R2ðorgÞ ¼ InðHR 2 Þ3ðorgÞ þ 3HðaqÞ
K1 ¼
D1 ¼
E1 ¼
3 ½InðHR2 Þ3 Hþ ½In3þ ½H2 R2 3 ½InðHR2 Þ3 ½In3þ D1 100% D1 þ 1=ðO=AÞ
ð1Þ
ð2Þ
ð3Þ
ð4Þ
where K1 is the equilibrium constant of extraction process. D1 is the distribution ratio of extraction. E1 is the extraction efficiency. (O/A) is the ratio of organic phase to aqueous phase. Back extraction is the reverse process of extraction. The metal ions in the organic phase are returned to the aqueous phase in the form of ions by stripping agent. The simplified reaction formula and the equation of equilibrium constant, distribution ratio and extraction efficiency are as follows.
InðHR2 Þ3ðorgÞ þ 3HþðaqÞ ¼ In3þ ðaqÞ þ 3H2 R2ðorgÞ
ð5Þ
K2 ¼
½In3þ ½H2 R2 3 3 ½InðHR2 Þ3 Hþ
ð6Þ
D2 ¼
½H2 R2 3 3 K 2 Hþ
ð7Þ
E2 ¼
D2 100% D2 þ 1=ðO=AÞ
ð8Þ
where K2 is the equilibrium constant of stripping process. D2 is the distribution ratio. E2 is the stripping efficiency. (O/A) is the ratio of organic phase to aqueous phase. All the experiments were repeated for three times and the reported data was averaged from three samples of the analytical results. 2.3. Analytical methods Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, Agilent 720ES) was used to determine the content of indium. X-Ray Diffraction (XRD, Bruker D8 Advance) was used to investigate the change of structure between solid residue of hydrothermal pretreatment and leaching residue. Scanning Electron Microscope (SEM, FEI Nova Nano SEM 450) and Energy Dispersive Spectrum (EDS) was used to characterize the surface topography and element type and content of solid residue. Fourier Transform Infrared Spectrum (FTIR, Nicolet 5700) was used to explore the change of molecular structure between glass substrate and solid residues.
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The leaching yield of indium in this study was defined as the percent of the quantity of indium in leaching solution to its initial quantity (Eq. (9)).
gðwt:%Þ ¼
cV 100% m0
ð9Þ
where g is the leaching yield of indium. c is the centration of dilution. V is the volume of constant volume, 100 mL. m0 is the quantity of indium in LCD panel before acid leaching (mg). 3. Results and discussion 3.1. Influence of factors on leaching yield of indium 3.1.1. Effect of acid type As shown in Fig. 1a, the leaching effect of HNO3 is insignificant compared with the other two leaching agents. It varies a lot with the change of temperature and cannot guarantee the complete leaching of indium considering its strong volatility. Although the leaching content of indium in HCl system is relatively high at 60 °C, reaching 3.66 104 (g/g), the volatilization of HCl at the normal temperature can lead to the reduction of acid concentration and the influence will be deepened as temperature increases. However, H2SO4 maintains in a relatively stable state at different temperatures. Low-cost, good leaching yield and the controllable process make H2SO4 the optimal choice (Wang et al., 2013). In addition, the arsenic in LCD panel is in the form of As2O3 with high toxicity but it has a low solubility in dilute H2SO4. Almost all of the arsenic remains in the solid phase and has no interference with the leaching of indium (Dodbiba et al., 2012). Therefore, 0.5 M H2SO4 is used to extract indium from the glass residue. 3.1.2. Effect of temperature As observed in Fig. 1b, the leaching yield of indium increases gradually with the increasing temperature. When the temperature reaches 60 °C, the leaching yield of indium is 90%. The leaching yield curve flattens out after 70 °C and basically reaches 100%. In accordance with (Rocchetti et al., 2015) and (Swain et al., 2018), the reaction temperatures of 80 °C and 75 °C are respectively considered as the optimal temperatures. The result is approximately consistent with them. The acid dissociation reaction and leaching process are all endothermic reaction (Havlik et al., 2010; Bas et al., 2014). Therefore, the dissociation rate of hydrogen ion (H+) and the concentration of H+ increase with the increment of temperature (Zhou et al., 2019). The reaction will proceed to the endothermic direction which is the leaching direction. Moreover, according to the Arrhenius formula, rate constant gets larger when temperature rises, so the chemical reaction rate and diffusion rate are improved, and the promotion of chemical reaction rate is particularly obvious (Cui et al., 2019). The activation energy of solid dissolution is reduced when the temperature increases. The barrier of the reaction is reduced, and the leaching yield is naturally increased. In addition, the movement of ions in the leaching system becomes more intense with the increasing temperature within a certain range. The increase of collision results in the rising ratio of effective collision and makes it beneficial to the leaching reaction. 3.1.3. Effect of acid concentration The Fig. 1c demonstrates that the leaching yield of indium increases with the increasing acid concentration. The leaching yield is about 53% when the concentration of H2SO4 is 0.1 M and it reaches 90% when the concentration increases to 0.5 M. The curve remains stable when the concentration continually increases. It can be seen that the acid concentration has a
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Fig. 1. Influence of factors on leaching yield of indium. (a) Effect of acid type; (b) effect of temperature; (c) effect of acid concentration; (d) effect of (S/L) ratio; (e) effect of leaching time at various temperatures.
significant effect on the leaching yield of indium in the low concentration range. As reported in Silveira et al. (2015) and Souada et al. (2018), the acid concentration used for experiments are 1 M and 18 M respectively. However, the effect of sulfuric acid with low concentration has not been discussed. H2SO4 is a binary strong acid. The molecule is completely ionized in water which leads to the sharply increase of H+ concentration. Thus, the chemical reaction rate increases and the leaching efficiency is improved. Besides, based on the analysis of the influencing factors of multiphase reaction, diffusion of H+ to the solid surface is a step in the leaching process
(Ajiboye et al., 2019). When the H2SO4 is consumed at the solid-liquid interface, the concentration gradient is gradually generated between solution and solid-liquid interface. The gradient increases with the increment of the H2SO4 concentration which causes the growth of the diffusion rate of H2SO4 from the solution to the reaction interface. Thus, the leaching efficiency is gradually increased. When the concentration is low, acid concentration has a great influence on the chemical reaction rate. However, when the concentration increases to 0.5 M, the effect on the chemical reaction rate is getting smaller, so the leaching efficiency gradually becomes stable.
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3.1.4. Effect of (S/L) ratio As can be seen from Fig. 1d, (S/L) ratio has little effect on the recovery of indium. The leaching yield of indium is over 97% and basically achieves 100% in the above-mentioned range, showing a steady state. Taking into account that large volume of solids will affect the stirring when the (S/L) ratio is 1:1, 1:2 g/mL is chosen as the optimal (S/L) ratio in this study considering the fact that low (S/L) ratio can bring about the waste of the leaching system. 3.1.5. Effect of leaching time It can be seen from Fig. 1e that the leaching yield of indium gradually increases and the increasing rate slows down with the prolongation of time. Under the temperature condition of 60 °C, the leaching yield is 14.1% at the reaction time of 5 min. It reaches above 90.5% after 60 min and basically reaches 100% after 75 min. When the temperature is 80 °C, the leaching yield is only 23.7% at the reaction time of 5 min. It surges to 95.6% when the reaction time increases to 30 min and then quickly reaches 100%. In the condition of 100 °C, the effect of reaction time is more significant, the leaching yield of indium reaches 95.8% at the reaction time of 10 min and then stabilizes at 100% with the increment of time. It is indicated that the effect of reaction time on the leaching yield of indium is different at different temperature. The variation tendency of leaching yield of indium is relatively flat and the reaction rate is relatively low with the extension of time at the temperature of 60 °C. However, it only takes 35 min to transfer indium from solid phase to liquid phase at 80 °C. The reaction rate continues to accelerate with the increasing temperature. The reaction quickly reaches to the end at the temperature of 100 °C. Thus, the higher temperature leads to the more significant effect and the higher leaching yield as it is explained in Fig. 1b. Taking into account the similar time range and temperature (Savvilotidou et al., 2015), our combination conditions are obviously more efficient compared to the 60% recovery of indium. Since the pretreatment method in Savvilotidou’s article is thermal shock, it may cause the incomplete removal of organic materials. It is speculated that the degradation of organic materials in hydrothermal pretreatment reduces material input in the subsequent acid leaching process. Therefore, the optimal reaction time is 40 min under the temperature between 70 °C and 80 °C considering the energy consumption and the comprehensive influence of each factor. 3.2. Influences of factors on indium extraction 3.2.1. Effect of extractant concentration As seen from Fig. 2a, the extraction effect of D2EHPA is obvious. 92% of indium can be extracted by 10% D2EHPA (by volume). The extraction efficiency increases with the increasing concentration of extractant. When the extractant concentration is 20%, the extraction efficiency can reach 100% and it remains stable with the increment of extractant concentration. Therefore, taking into consideration the viscosity of the organic phase and cost, 20% D2EHPA and sulfonated kerosene are used to extract the acid leachate. 3.2.2. Effect of extraction time It can be seen from Fig. 2b that the extraction equilibrium can be reached in a short time. When the reaction time is 2 min, the extraction efficiency of indium can reach about 97%. With the prolongation of time, the extraction efficiency continues to increase. With the reaction time of 3 min, the extraction efficiency can reach 100%. In general, the extraction efficiency is determined by two factors, one is the formation rate of the extracted complex and the other is the transfer rate of the extracted complex in two phases. Extraction of indium from the H2SO4 leachate by D2EHPA is a heterogeneous reaction. The equilibrium time for the
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formation of the extracted complex of indium ions and D2EHPA is generally 3–5 min, while other impurity metal ions have a very low extraction efficiency in this time range (Guo et al., 2017). Taking advantages of this difference, combined with the experimental results, 3 min is selected as the extraction time. 3.2.3. Effect of (O/A) ratio Fig. 2c shows that the change of (O/A) ratio has little influence on the extraction efficiency within a certain range. When the (O/A) ratio is 1:15, the extraction efficiency of indium is 91%, which is slightly lower comparing to the high-ratio conditions. The extraction yield of indium increases with the increasing (O/ A) ration and then remains stable. When (O/A) ratio is between 1:10 to 1:2, the extraction efficiency of indium can basically reach 100%. This is due to the fact that the organic phase saturates, and the extraction yield is relatively low under the condition of (O/A) ratio of 1:15. Continuing to increase the (O/A) ratio, the organic extractant does not reach the saturated extraction concentration, so the extraction efficiency of indium is correspondingly higher. Considering that the higher ratio would result in the waste of extractant, the further experiments were performed under the condition of (O/A) ratio of 1:10. 3.3. Influences of factors on indium stripping 3.3.1. Effect of stripping agent concentration As can be seen from Fig. 3a, the stripping efficiency of indium gradually increases with the increasing concentration within a certain range. When the concentration of hydrochloric acid rise from 2 M to 4 M, the stripping efficiency increases from 34.75% to 97.25%. Continuing to increase the concentration of stripping agent, the stripping efficiency decreases. When the concentration is 6 M, the stripping efficiency is only 35.34%. The equilibrium is not reached under low concentration conditions and the increase of concentration promotes the extraction process to proceed in the positive direction. According to Eq. (7), an increase in the HCl concentration will result in a decrease of distribution ratio when the reaction reaches equilibrium, thereby the stripping efficiency is reduced. Taking into account the stripping effect and the influence of pH, 4 M HCl was chosen in this experiment. 3.3.2. Effect of stripping time As Fig. 3b demonstrates, the high stripping efficiency is achieved in a short period of time. When the stripping time is 6 min, the stripping yield of indium can reach 80.35%. With the prolonging of time, the stripping efficiency of indium increases gradually. When the time reaches 10 min, the stripping efficiency is 97.25%. In the view of the fact that the extension of stripping time may cause the extraction of indium back to the organic phase (Pereira et al., 2018), 10 min is selected as the stripping time. 3.3.3. Effect of (O/A) ratio As shown in Fig. 3c, the stripping efficiency increases as the (O/ A) ratio increases gradually. When (O/A) ratio is 1:1, the stripping efficiency is 72.91%. The stripping efficiency reaches its maximum (97.25%) when the ratio increases to 10:1. Further increase of the ratio will lead to the reduction of stripping efficiency. When the ratio is relatively low, stripping solution reaches saturation. According to Eqs. (7)–(9), the higher concentration of H+ will lead to the lower distribution ratio and then the lower stripping efficiency. However, the stripping process achieves equilibrium under the condition of high ratio. Then the lack of H+ in the stripping agent solution will result in the remain of indium in the organic phase. Thus, the (O/A) ratio of 10:1 is selected.
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Fig. 2. Influences of factors on indium extraction. (a) Effect of extractant concentration; (b) effect of extraction time; (c) effect of (O/A) ratio.
3.4. Analysis of acid leaching residue In order to further investigate the changes of element type, surface topography and molecular structure of solid residue after acid leaching process, the following experiments including XRD (Fig. 4), SEM (Fig. 5) and FTIR (Fig. 6) were conducted and the analysis was carried out in combination with previous hydrothermal pretreatment studies. It can be seen from Fig. 4 that acid leaching process does not cause damage to the crystals of the solid phase residue. The pattern of the acid leaching residue is more gradual than the residue after hydrothermal pretreatment. Some small peaks become inconspicuous or even disappear, indicating that metal in the hydrothermal residue is leached into the acid solution. From Fig. 5 and Fig. S1 (in supplementary file), surface of the acid leaching residue is smooth and the properties are similar under the condition of acid leaching temperatures of 60 °C and 80 °C. The basic structure of glass is still intact, which means that the different acid leaching temperatures will not further damage the surface morphology of the solid residue. Thus, acid leaching will not affect the recycling of glass. Fig. 6 shows that some functional groups did not show significant changes during the acid leaching process. Those changes include vibration absorption of OAH bond from 3000 cm1 to 3600 cm1, symmetric and asymmetric stretching vibration of CAH bond at 2759 cm1, stretching vibration of C@O bond from 1850 cm1 to 1680 cm1, symmetric bending of CAH bond and CH2 deformation from 1500 cm1 to1350 cm1, CABr bond at 690 cm1 and CAI from 450 cm1 to 650 cm1. It indicates that
acid leaching process does not damage the molecular structure of the glass substrate and does not have negative effect on its recycling. 3.5. Kinetic analysis of acid leaching process The sulfuric leaching process of indium belongs to liquid-solid reaction. In order to analyze the acid leaching process, we make the following assumptions according to the nuclear shrinkage model. (1) Assuming that the diffusion resistance through the diffusion layer and the solid film is small, so that the reaction rate is controlled by the chemical reaction. (2) Assuming that the mineral balls are spherical and dense without porosity. (3) Assuming that the excess leaching agent is used during the leaching process, its concentration C can be regarded as constant and keep at C0. The following equation can be deduced based on the above assumptions and mathematical calculations. It can be seen in supplementary file. 1
0
1 ð1 yÞ3 ¼ k t; k0 ¼
kMC 0 r0 q
ð10Þ
where y is the leaching yield of indium, t is the leaching time, k is the Chemical reaction ratio constant, M is the molar mass of the mineral, C 0 is the leaching agent concentration, r 0 is the original
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Fig. 3. Influences of factors on indium stripping. (a) Effect of stripping agent concentration; (b) effect of stripping time; (c) effect of (O/A) ratio.
Fig. 4. XRD pattern of solid residue (a) Residue after hydrothermal pretreatment (Cao et al., 2019). (b) Residue after acid leaching.
radius of the acid leaching residue,q is the density of the mineral ball. The data of the leaching yield at 60 °C, 80 °C and 100 °C was analyzed. Dynamic equation of 1 (1 y) 1/3 and leaching time t (min) was established to explore the control steps in the process of acid leaching of indium. The figure can be seen in supplementary file (Fig. S2). As shown in Table 1, all fitting curves show significant linear relationships. At the temperature of 60 °C, kobs = 0.009 min1
when the time ranges from 0 min to 65 min. At the temperature of 80 °C, the leaching yield of indium can reach 100% at 35 min. Since the leaching yield is 96% at 30 min, the time range is considered as 0–30 min. 1 (1 y)1/3 and leaching time t (min) show a linear relationship and kobs = 0.00215 min1. The effect of temperature on the leaching of metal indium is very significant. When the temperature is 100 °C, the leaching yield can reach 100% after only 15 min reaction and kobs = 0.067 min1.
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Fig. 5. SEM image of solid residue after acid leaching at different temperatures (a) 60 °C, (b) 80 °C.
Fig. 6. Infrared spectroscopy of solid materials (a) Glass substrate before hydrothermal reaction. (b) Residue after hydrothermal pretreatment (Cao et al., 2019). (c) Residue after acid leaching at 60 °C. (d) Residue after acid leaching at 80 °C.
Y. Cao et al. / Waste Management 102 (2020) 635–644 Table 1 The kinetic equation of the leaching yield of indium and time at different temperatures. Temperature 60 °C 80 °C 100 °C
Kinetic equation 1/3
1 (1 y) = 0.009 t 1 (1 y)1/3 = 0.0215 t 1 (1 y)1/3 = 0.067 t
R2 value
Time range
0.999 0.997 0.995
0–65 min 0–30 min 0–15 min
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reaction temperature can effectively accelerate the solid-liquid reaction, promote the reaction rate and shorten the time to reach equilibrium, thus the leaching yield of indium gradually increases. The leaching yield of indium reaches 100% at the reaction temperature of 70–80 °C, with 0.5 M H2SO4 solution in the (S/L) ratio of 1:2 g/mL conditions for 40 min. In addition, the recovery efficiency of indium reaches 97.25% by using 20% D2EHPA as extractant and 4 M HCl as stripping agent. The leachate is separately extracted under conditions of (O/A) ratio of 1: 10 for 3 min in extraction process and (O/A) ratio of 10: 1 for 10 min in stripping process. The surface topography and molecular structure of solid residue before and after acid leaching are little changed. It shows that there is no fundamental change in the properties of glass in the residue after acid leaching, and the subsequent recycling will not be affected. Thus, recycling methods including hydrothermal pretreatment and acid leaching can obtain the integrated recovery. Kinetics equation of the leaching yield and leaching time t (min) at 60 °C is 1 (1 y)1/3 = 0.009 t. At 80 °C, the linear relationship between the 1 (1 y)1/3 and the leaching time is obtained between 0 and 30 min, and the equation is 1 (1 y)1/3 = 0.0215 t. At 100 °C, the kinetic equation is 1 (1 y)1/3 = 0.067 t. In addition, the reaction activation energy of metal indium leaching process is 50.64 kJ/mol by calculation. Declarations of Competing Interest None.
Fig. 7. The relationship between the reaction rate constant of leaching process of indium and the reaction temperature.
It can be seen from Table 1 that the k value increases greatly with increasing temperature and the corresponding leaching yield is also greatly enhanced. The positive correlation between the reaction rate constant and the reaction temperature is in accordance with the Arrhenius equation. The logarithm of the Arrhenius equation is as follows:
Ea lnk ¼ þ lnA RT
ð11Þ
where T is the thermodynamic temperature of the reaction and the unit is K. k is the reaction rate constant at the corresponding temperature and the unit is min1. Ea is the activation energy of the reaction and the unit is kJ/mol. R is the molar gas constant, 8.314 J/molK. A is the pre-exponential factor and the unit is min1. According to the Arrhenius equation, the reaction rate constant is only related to the reaction temperature under a certain reaction condition. The linear fit for lnk and 1000/T and the fitting curve are shown in Fig. 7. It can be seen that -lnk has a good linear relationship with the reciprocal of the corresponding thermodynamic temperature 1/T, and the correlation coefficient is R2 = 0.99. It can be proved that the leaching reaction rate constant of indium is consistent with the Arrhenius equation and the activation energy of solid solution reaction is 50.64 kJ/mol in the leaching process within the temperature range by calculation. 4. Conclusion The separation and purification of indium in solid residue after hydrothermal treatment can be effectively achieved by using sulfuric acid as leaching agent, D2EHPA as extractant and hydrochloric as the stripping agent. The leaching yield of indium increases with the increasing concentration of H2SO4, and the (S/L) ratio has little effect on the leaching reaction. The increase of reaction time can effectively improve the leaching efficiency. The increment of
Acknowledgment This work is supported by National Natural Science Foundation of China (grant number 21677111). The authors would like to thank the anonymous reviewers for supplying their valuable comments. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.wasman.2019.11.029. References Ajiboye, E., Panda, P., Adebayo, A., Ajayi, O., Tripathy, B., Ghosh, M., Basu, S., 2019. Leaching kinetics of Cu, Ni and Zn from waste silica rich integrated circuits using mild nitric acid. Hydrometallurgy 188, 161–168. Annual data, 2016. National Bureau of Statistics of China. http://data.stats.gov.cn/ easyquery.htm?cn=C01&zb=A0E0H&sj=2016. Awasthi, A.K., Li, J., 2017. An overview of the potential of eco-friendly hybrid strategy for metal recycling from WEEE. Resour. Conserv. Recycl. 126, 228–239. Cao, Y., Li, F., Li, G., Huang, J., Zhu, H., He, W., 2019. Degradation process regulation of waste LCD panel to ensure the remain of indium in solid phase by hydrothermal reaction. J. Hazard. Mater. 369, 125–131. Chen, Y., Zhang, L.G., Xu, Z.M., 2017. Vacuum pyrolysis characteristics and kinetic analysis of liquid crystal from scrap liquid crystal display panels. J. Hazard. Mater. 327, 55–63. Cui, J., Zhu, N., Luo, D., Li, Y., Wu, P., Dang, Z., Hu, X., 2019. The role of oxalic acid in the leaching system for recovering indium from waste liquid crystal display panels. ACS Sustain. Chem. Eng. 7 (4), 3849–3857. Dodbiba, G., Nagai, H., Wang, L.P., Okaya, K., Fujita, T., 2012. Leaching of indium from obsolete liquid crystal displays: Comparing grinding with electrical disintegration in context of LCA. Waste Manage. 32, 1937–1944. Guo, X., Wei, C., Li, X., Deng, Z., Li, C., Fan, G., 2017. Direct solvent extraction of indium from the leaching solution of zinc oxide. Chinese J. Process Eng. 17, 982– 988 (in Chinese). Havlik, T., Orac, D., Petranikova, M., Miskufova, A., Kukurugya, F., Takacova, Z., 2010. Leaching of copper and tin from used printed circuit boards after thermal treatment. J. Hazard. Mater. 183 (1–3), 866–873. He, W., Li, G., Kong, L., Wang, H., Huang, J., Xu, J., 2008. Application of hydrothermal reaction in resource recovery of organic wastes. Resour. Conserv. Recycl. 52, 691–699. He, Y., Ma, E., Xu, Z., 2014. Recycling indium from waste liquid crystal display panel by vacuum carbon-reduction. J. Hazard. Mater. 268, 185–190.
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Leaching and purification of indium from waste liquid crystal display panel after hydrothermal pretreatment: Optimum conditions determination and kinetic analysis Yue Cao, Feng Li, Guangming Li, Juwen Huang, Haochen Zhu, Wenzhi He ⇑ State Key Laboratory of Pollution Control and Resources Reuse, Shanghai Institute of Pollution Control and Ecological Security, College of Environmental Science and Engineering, Tongji University, 1239 Siping Rd., Shanghai 200092, China
a r t i c l e
i n f o
Article history: Received 25 August 2019 Revised 16 November 2019 Accepted 19 November 2019
Keywords: Waste LCD panel Indium Acid leaching Solvent extraction Kinetic analysis
a b s t r a c t Indium is one of the components with great recycling value in waste LCDs. Degradation of organic materials and the remain of indium in the solid phase can be simultaneously achieved by hydrothermal pretreatment via parameter regulation. Indium was transferred from the solid phase to the liquid phase by using sulfuric acid after hydrothermal pretreatment. Di-(2-ethylhexyl) phosphoric acid diluted by sulfonated kerosene and hydrochloric acid were used as extractant and stripping agent respectively to purify and concentrate indium from acidic leaching solution. The results indicated that the leaching yield of indium reached 100% under the optimal condition of reaction time of 40 min, reaction temperature of 70–80 °C, acid concentration of 0.5 M and solid-liquid (S/L) ratio of 1:2 g/mL. Given conditions of extraction time of 3 min at the organic phase to aqueous phase (O/A) ratio of 1:10 by 20% D2EHPA and stripping time of 10 min at the (O/A) ratio of 10:1 by 4 M HCl, the recovery efficiency of indium reached 97.25%. In addition, acid leaching process did not change the surface topography and molecular structure of glass substrate and had no negative effect on subsequent recycling of glass. The kinetic equation of leaching yield and reaction time at the temperature of 80 °C is 1 (1 y)1/3 = 0.0215 t. The reaction activation energy of metal indium leaching process is 50.64 kJ/mol. Ó 2019 Published by Elsevier Ltd.
1. Introduction As a new generation of display device with superior performance, liquid crystal display (LCD) has the advantages of high resolution, clear colors, power saving, light weight and portability (Zhuang et al., 2012). The progress of liquid crystal display technology continuously reduces the manufacturing cost of the liquid crystal display, making it widely produced. Due to the rapid development of LCD industry, China gradually become the world’s largest LCD production and sales area in the past 30 years. In 2016, the production volume of domestic color TV sets was about 157.7694 million units and the production capacity of microcomputer equipment was 29.00851 million units (National Bureau of Statistics of China, 2016). The huge market demand has effectively promoted the prosperity and development of China’s LCD industry, also greatly promoted the rapid growth of LCD equipment production. It is predicted that a large number of end-of-life LCD electronic products will generate from 2014 to 2020 including ⇑ Corresponding author. E-mail address: [email protected] (W. He). https://doi.org/10.1016/j.wasman.2019.11.029 0956-053X/Ó 2019 Published by Elsevier Ltd.
desktop computer, laptop, LCD TV and mobile phone. The scrapping volume of the four types of waste LCD products will reach 4261.94million units in 2020 (Liu et al., 2016). Such large quantities of discarded LCDs pose a potential threat to ecological environment (Awasthi and Li, 2017; Yu et al., 2017a, 2017b). Not only e-waste recycling management faces a huge challenge, but also e-waste processing technology needs to be innovated. 1.1. Current recycling management strategies on waste LCD panel As the function realization unit, LCD panel is the core component of the LCD device. LCD panel can be divided into inorganic glass, metal and organic components according to the structure and material characteristics (Yu et al., 2019). All components have a certain recycling value, among which the recovery of indium in glass substrates is particularly important (He et al., 2014; Rocchetti et al., 2015; Zeng et al., 2015; Zhang et al., 2015). Indium is etched on one side of the glass substrate of the LCD panel in the form of indium tin oxide to conduct electricity (Chen et al., 2017). The good electrical conductivity of indium makes it a huge demand in the electronics industry. Worldwide, about 70% of indium
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products are used for the fabrication of indium tin oxide on LCDs (Yang et al., 2013). The average content of indium in zinc ores varies from less than 1 to 100 ppm, while in LCD panel it presents at approximately 250 ppm (Pereira et al., 2018). Thus, it is of strategic significance to recover indium from waste LCD panel (Oguchi et al., 2013; Sawai et al., 2015; Zhang and Xu, 2017). A number of researches have been carried out on the recycling of indium currently. One of the methods is hydrometallurgy (Swain et al., 2016). LCD glass panel is mechanically shredded for size reduction after separation from plastic and HCl is used as the leaching agent. In addition, acid leaching with the aid of ultrasonic wave is also studied for the recovery of indium (Zhang et al., 2017). It is a non-crushing leaching method, manual stripping and acetone soaking are conducted to remove polarizer films and liquid crystal before the leaching process. Moreover, organic acids have also been studied as leaching agents. (Cui et al., 2019) investigated on the role of oxalic acid in the leaching system for recovering indium. Lopez-Yanez et al. (2019) carried out the experiments using citrate as a complexing agent. Other methods including thermal treatment (Zhuang et al., 2019), vaccum reduction (Wang et al., 2019), electrochemical recycling (Pa, 2017), flotation (Wang et al., 2018) and bioleaching (Jowkar et al., 2018) have also been discussed. Meanwhile, there are also plenty of researches on the recycling of glass and organic components in waste LCD panel. Glass is recycled to fabricate the silicon particles for lithium-ion battery anodes (Kang et al., 2019) and recycled as the concrete flumes additive to enhance the sulfate attack resistance (Kim and Hong, 2019) or as the supplementary cementing material in cement mortarsrationale (Kim et al., 2018). In terms of the treatment of organic materials, vacuum pyrolysis (Chen et al., 2017) and sub/supercritical water treatments (Wang et al., 2015) are studied. It is known that the removal of organic materials is a critical process before the recovery of indium. Usually, means of solvent soaking, dry mechanical crushing and sieving, pyrolysis, thermal shock and gravimetric process are used for the pretreatment of indium recovery (Savvilotidou et al., 2019). However, those methods cannot eliminate the risks of organic materials and also cause energy and material consumption. For these existing problems, previous researches focused on the use of hydrothermal methods to treat waste LCD panel. Hydrothermal technology can realize the degradation and recycling of organic materials of waste LCD panel based on the highefficiency and non-pollution advantages in a closed environment (He et al., 2008; Li et al., 2015; Yu et al., 2017a, 2017b). The liquid products are mainly acetic acid, lactic acid and other small molecule organic acids (Yu et al., 2016). It is an efficient and clean method for pretreatment without secondary pollution. According to the previous study, in the pretreatment stage, not only the degradation of organic matter is realized but also the remain of indium in the solid phase residue is achieved through the regulation of reaction parameters (Cao et al., 2019). After pretreatment, the glass substrate is polished and meanwhile some organic carbon products are deposited on the surface of the glass substrate during the conversion. It needs to be discussed that whether subsequent indium recovery process will be influenced under this circumstance. In the follow-up process, acid leaching concentration technology is used to extract indium from the solid residue. The main steps are acid leaching, extraction and stripping. According to the above process, effects of various factors in acid leaching, extraction and stripping were analyzed. Whether subsequent glass recycling was affected after acid leaching was also studied. Thus, characteristics change of the solid residue before and after acid leaching was investigated. At the same time, kinetic analysis was carried out. According to the analysis, the purification and recovery of indium are achieved. As a part of the recycling system, the comprehensive
recycling system of waste LCD panel is established through the green recycling means. In addition, the theoretical basis and technical support are provided for the industrial production. 2. Materials and methods 2.1. Materials and chemicals LCD panel used in this study was a mixture of different types of LCD panels provided by Taicang Shun Hui-Ferrous Metals Ltd., with organic carbon content of 12.6% (Cao et al., 2019). LCD panel was cut into small pieces approximately 1 cm 1 cm before the experimentsH2O2 (30%) was used as oxidant. Hydrochloric acid (HCl), sulfuric acid (H2SO4) and nitric acid (HNO3) were used as the leaching agents. Di-(2-ethylhexyl) phosphoric acid (D2EHPA) was used as extractant and sulfonated kerosene was used as the diluent for the extractant. Hydrochloric acid (HCl) was used as stripping agent. All reagents in this research study were of analytic grade and purchased from Sino-pharm Chemical Reagent Co., Ltd (Shanghai, China). Deionized water was used during the whole study. 2.2. Apparatus and procedure 2.2.1. Hydrothermal pretreatment Hydrothermal pretreatment was conducted with a batch reactor system that consisted of a salt bath and a stainless-steel vessel with wall thickness of 1 mm, length of 120 mm and inner volume of 5.7 mL. The salt bath was used for heating the reactor and the vessel was used to provide an instant and uniform heating process. The reactor was submerged into the salt bath which was preheated to the desired temperature. The reactor was taken out of the salt bath and immediately put into a cold-water bath when the reaction was over. The liquid and solid products were collected for further experiments. Hydrothermal reaction was carried out with 2.4 g crushed LCD panel fragments under neutral condition of 300 °C of reaction temperature, 36 mL of water and 7.2 mL of H2O2 (30%) supply and 11 min of reaction time (Cao et al., 2019). 2.2.2. Leaching of indium from solid phase residue A certain amount of solid phase product was weighted and put into a beaker, a certain volume of acid was added, and the acid concentration, acid type, solid-liquid (S/L) ratio, reaction time and reaction temperature were changed. Conditions of leaching experiments are shown in Table S1 in supplementary file. Indium leaching experiments were conducted in a constant temperature reactor under different conditions. Stirring was carried out during the reaction. After the reaction, the solid-liquid mixture was separated by vacuum filtration through a micropore filter membrane, and then the filter membrane was washed several times with deionized water. The filtrate was transferred to a volumetric flask and made up to volume with 4% dilute nitric acid for further analytical testing. The filter membrane adhered with the residue was placed in an oven and dried at 60 °C for 24 h for subsequent analysis. 2.2.3. Purification of indium from leaching solution After dilution, the H2SO4 leaching solution was used as the mother liquor of the extraction process and the concentration of indium was 0.99 mg/L. D2EHPA was dissolved in sulfonated kerosene to prepare the extractant with different concentrations. The corresponding proportion of the acid leaching liquid and the extractant were placed in a separatory funnel in extraction process. Conditions of leaching experiments are shown in Table S2 in supplementary file. The extract liquor and stripping agent were placed in a separatory funnel during the process of stripping. Conditions
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of leaching experiments are shown in Table S3 in supplementary file. The mixture was oscillated at the temperature of 25 °C. The oscillation was stopped immediately after the reaction time was reached, and the mixture was stood for 30 min to ensure the maximum stratification of the aqueous solution and the organic solution. The organic phase and the aqueous phase were separated and stored under suitable conditions. The extraction mechanism is a cation exchange process. The simplified reaction formula and the equation of equilibrium constant, distribution ratio and extraction efficiency are as follows (Tsai and Tsai, 2012; Pereira et al., 2018). þ In3þ ðaqÞ þ 3H2 R2ðorgÞ ¼ InðHR 2 Þ3ðorgÞ þ 3HðaqÞ
K1 ¼
D1 ¼
E1 ¼
3 ½InðHR2 Þ3 Hþ ½In3þ ½H2 R2 3 ½InðHR2 Þ3 ½In3þ D1 100% D1 þ 1=ðO=AÞ
ð1Þ
ð2Þ
ð3Þ
ð4Þ
where K1 is the equilibrium constant of extraction process. D1 is the distribution ratio of extraction. E1 is the extraction efficiency. (O/A) is the ratio of organic phase to aqueous phase. Back extraction is the reverse process of extraction. The metal ions in the organic phase are returned to the aqueous phase in the form of ions by stripping agent. The simplified reaction formula and the equation of equilibrium constant, distribution ratio and extraction efficiency are as follows.
InðHR2 Þ3ðorgÞ þ 3HþðaqÞ ¼ In3þ ðaqÞ þ 3H2 R2ðorgÞ
ð5Þ
K2 ¼
½In3þ ½H2 R2 3 3 ½InðHR2 Þ3 Hþ
ð6Þ
D2 ¼
½H2 R2 3 3 K 2 Hþ
ð7Þ
E2 ¼
D2 100% D2 þ 1=ðO=AÞ
ð8Þ
where K2 is the equilibrium constant of stripping process. D2 is the distribution ratio. E2 is the stripping efficiency. (O/A) is the ratio of organic phase to aqueous phase. All the experiments were repeated for three times and the reported data was averaged from three samples of the analytical results. 2.3. Analytical methods Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES, Agilent 720ES) was used to determine the content of indium. X-Ray Diffraction (XRD, Bruker D8 Advance) was used to investigate the change of structure between solid residue of hydrothermal pretreatment and leaching residue. Scanning Electron Microscope (SEM, FEI Nova Nano SEM 450) and Energy Dispersive Spectrum (EDS) was used to characterize the surface topography and element type and content of solid residue. Fourier Transform Infrared Spectrum (FTIR, Nicolet 5700) was used to explore the change of molecular structure between glass substrate and solid residues.
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The leaching yield of indium in this study was defined as the percent of the quantity of indium in leaching solution to its initial quantity (Eq. (9)).
gðwt:%Þ ¼
cV 100% m0
ð9Þ
where g is the leaching yield of indium. c is the centration of dilution. V is the volume of constant volume, 100 mL. m0 is the quantity of indium in LCD panel before acid leaching (mg). 3. Results and discussion 3.1. Influence of factors on leaching yield of indium 3.1.1. Effect of acid type As shown in Fig. 1a, the leaching effect of HNO3 is insignificant compared with the other two leaching agents. It varies a lot with the change of temperature and cannot guarantee the complete leaching of indium considering its strong volatility. Although the leaching content of indium in HCl system is relatively high at 60 °C, reaching 3.66 104 (g/g), the volatilization of HCl at the normal temperature can lead to the reduction of acid concentration and the influence will be deepened as temperature increases. However, H2SO4 maintains in a relatively stable state at different temperatures. Low-cost, good leaching yield and the controllable process make H2SO4 the optimal choice (Wang et al., 2013). In addition, the arsenic in LCD panel is in the form of As2O3 with high toxicity but it has a low solubility in dilute H2SO4. Almost all of the arsenic remains in the solid phase and has no interference with the leaching of indium (Dodbiba et al., 2012). Therefore, 0.5 M H2SO4 is used to extract indium from the glass residue. 3.1.2. Effect of temperature As observed in Fig. 1b, the leaching yield of indium increases gradually with the increasing temperature. When the temperature reaches 60 °C, the leaching yield of indium is 90%. The leaching yield curve flattens out after 70 °C and basically reaches 100%. In accordance with (Rocchetti et al., 2015) and (Swain et al., 2018), the reaction temperatures of 80 °C and 75 °C are respectively considered as the optimal temperatures. The result is approximately consistent with them. The acid dissociation reaction and leaching process are all endothermic reaction (Havlik et al., 2010; Bas et al., 2014). Therefore, the dissociation rate of hydrogen ion (H+) and the concentration of H+ increase with the increment of temperature (Zhou et al., 2019). The reaction will proceed to the endothermic direction which is the leaching direction. Moreover, according to the Arrhenius formula, rate constant gets larger when temperature rises, so the chemical reaction rate and diffusion rate are improved, and the promotion of chemical reaction rate is particularly obvious (Cui et al., 2019). The activation energy of solid dissolution is reduced when the temperature increases. The barrier of the reaction is reduced, and the leaching yield is naturally increased. In addition, the movement of ions in the leaching system becomes more intense with the increasing temperature within a certain range. The increase of collision results in the rising ratio of effective collision and makes it beneficial to the leaching reaction. 3.1.3. Effect of acid concentration The Fig. 1c demonstrates that the leaching yield of indium increases with the increasing acid concentration. The leaching yield is about 53% when the concentration of H2SO4 is 0.1 M and it reaches 90% when the concentration increases to 0.5 M. The curve remains stable when the concentration continually increases. It can be seen that the acid concentration has a
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Fig. 1. Influence of factors on leaching yield of indium. (a) Effect of acid type; (b) effect of temperature; (c) effect of acid concentration; (d) effect of (S/L) ratio; (e) effect of leaching time at various temperatures.
significant effect on the leaching yield of indium in the low concentration range. As reported in Silveira et al. (2015) and Souada et al. (2018), the acid concentration used for experiments are 1 M and 18 M respectively. However, the effect of sulfuric acid with low concentration has not been discussed. H2SO4 is a binary strong acid. The molecule is completely ionized in water which leads to the sharply increase of H+ concentration. Thus, the chemical reaction rate increases and the leaching efficiency is improved. Besides, based on the analysis of the influencing factors of multiphase reaction, diffusion of H+ to the solid surface is a step in the leaching process
(Ajiboye et al., 2019). When the H2SO4 is consumed at the solid-liquid interface, the concentration gradient is gradually generated between solution and solid-liquid interface. The gradient increases with the increment of the H2SO4 concentration which causes the growth of the diffusion rate of H2SO4 from the solution to the reaction interface. Thus, the leaching efficiency is gradually increased. When the concentration is low, acid concentration has a great influence on the chemical reaction rate. However, when the concentration increases to 0.5 M, the effect on the chemical reaction rate is getting smaller, so the leaching efficiency gradually becomes stable.
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3.1.4. Effect of (S/L) ratio As can be seen from Fig. 1d, (S/L) ratio has little effect on the recovery of indium. The leaching yield of indium is over 97% and basically achieves 100% in the above-mentioned range, showing a steady state. Taking into account that large volume of solids will affect the stirring when the (S/L) ratio is 1:1, 1:2 g/mL is chosen as the optimal (S/L) ratio in this study considering the fact that low (S/L) ratio can bring about the waste of the leaching system. 3.1.5. Effect of leaching time It can be seen from Fig. 1e that the leaching yield of indium gradually increases and the increasing rate slows down with the prolongation of time. Under the temperature condition of 60 °C, the leaching yield is 14.1% at the reaction time of 5 min. It reaches above 90.5% after 60 min and basically reaches 100% after 75 min. When the temperature is 80 °C, the leaching yield is only 23.7% at the reaction time of 5 min. It surges to 95.6% when the reaction time increases to 30 min and then quickly reaches 100%. In the condition of 100 °C, the effect of reaction time is more significant, the leaching yield of indium reaches 95.8% at the reaction time of 10 min and then stabilizes at 100% with the increment of time. It is indicated that the effect of reaction time on the leaching yield of indium is different at different temperature. The variation tendency of leaching yield of indium is relatively flat and the reaction rate is relatively low with the extension of time at the temperature of 60 °C. However, it only takes 35 min to transfer indium from solid phase to liquid phase at 80 °C. The reaction rate continues to accelerate with the increasing temperature. The reaction quickly reaches to the end at the temperature of 100 °C. Thus, the higher temperature leads to the more significant effect and the higher leaching yield as it is explained in Fig. 1b. Taking into account the similar time range and temperature (Savvilotidou et al., 2015), our combination conditions are obviously more efficient compared to the 60% recovery of indium. Since the pretreatment method in Savvilotidou’s article is thermal shock, it may cause the incomplete removal of organic materials. It is speculated that the degradation of organic materials in hydrothermal pretreatment reduces material input in the subsequent acid leaching process. Therefore, the optimal reaction time is 40 min under the temperature between 70 °C and 80 °C considering the energy consumption and the comprehensive influence of each factor. 3.2. Influences of factors on indium extraction 3.2.1. Effect of extractant concentration As seen from Fig. 2a, the extraction effect of D2EHPA is obvious. 92% of indium can be extracted by 10% D2EHPA (by volume). The extraction efficiency increases with the increasing concentration of extractant. When the extractant concentration is 20%, the extraction efficiency can reach 100% and it remains stable with the increment of extractant concentration. Therefore, taking into consideration the viscosity of the organic phase and cost, 20% D2EHPA and sulfonated kerosene are used to extract the acid leachate. 3.2.2. Effect of extraction time It can be seen from Fig. 2b that the extraction equilibrium can be reached in a short time. When the reaction time is 2 min, the extraction efficiency of indium can reach about 97%. With the prolongation of time, the extraction efficiency continues to increase. With the reaction time of 3 min, the extraction efficiency can reach 100%. In general, the extraction efficiency is determined by two factors, one is the formation rate of the extracted complex and the other is the transfer rate of the extracted complex in two phases. Extraction of indium from the H2SO4 leachate by D2EHPA is a heterogeneous reaction. The equilibrium time for the
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formation of the extracted complex of indium ions and D2EHPA is generally 3–5 min, while other impurity metal ions have a very low extraction efficiency in this time range (Guo et al., 2017). Taking advantages of this difference, combined with the experimental results, 3 min is selected as the extraction time. 3.2.3. Effect of (O/A) ratio Fig. 2c shows that the change of (O/A) ratio has little influence on the extraction efficiency within a certain range. When the (O/A) ratio is 1:15, the extraction efficiency of indium is 91%, which is slightly lower comparing to the high-ratio conditions. The extraction yield of indium increases with the increasing (O/ A) ration and then remains stable. When (O/A) ratio is between 1:10 to 1:2, the extraction efficiency of indium can basically reach 100%. This is due to the fact that the organic phase saturates, and the extraction yield is relatively low under the condition of (O/A) ratio of 1:15. Continuing to increase the (O/A) ratio, the organic extractant does not reach the saturated extraction concentration, so the extraction efficiency of indium is correspondingly higher. Considering that the higher ratio would result in the waste of extractant, the further experiments were performed under the condition of (O/A) ratio of 1:10. 3.3. Influences of factors on indium stripping 3.3.1. Effect of stripping agent concentration As can be seen from Fig. 3a, the stripping efficiency of indium gradually increases with the increasing concentration within a certain range. When the concentration of hydrochloric acid rise from 2 M to 4 M, the stripping efficiency increases from 34.75% to 97.25%. Continuing to increase the concentration of stripping agent, the stripping efficiency decreases. When the concentration is 6 M, the stripping efficiency is only 35.34%. The equilibrium is not reached under low concentration conditions and the increase of concentration promotes the extraction process to proceed in the positive direction. According to Eq. (7), an increase in the HCl concentration will result in a decrease of distribution ratio when the reaction reaches equilibrium, thereby the stripping efficiency is reduced. Taking into account the stripping effect and the influence of pH, 4 M HCl was chosen in this experiment. 3.3.2. Effect of stripping time As Fig. 3b demonstrates, the high stripping efficiency is achieved in a short period of time. When the stripping time is 6 min, the stripping yield of indium can reach 80.35%. With the prolonging of time, the stripping efficiency of indium increases gradually. When the time reaches 10 min, the stripping efficiency is 97.25%. In the view of the fact that the extension of stripping time may cause the extraction of indium back to the organic phase (Pereira et al., 2018), 10 min is selected as the stripping time. 3.3.3. Effect of (O/A) ratio As shown in Fig. 3c, the stripping efficiency increases as the (O/ A) ratio increases gradually. When (O/A) ratio is 1:1, the stripping efficiency is 72.91%. The stripping efficiency reaches its maximum (97.25%) when the ratio increases to 10:1. Further increase of the ratio will lead to the reduction of stripping efficiency. When the ratio is relatively low, stripping solution reaches saturation. According to Eqs. (7)–(9), the higher concentration of H+ will lead to the lower distribution ratio and then the lower stripping efficiency. However, the stripping process achieves equilibrium under the condition of high ratio. Then the lack of H+ in the stripping agent solution will result in the remain of indium in the organic phase. Thus, the (O/A) ratio of 10:1 is selected.
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Fig. 2. Influences of factors on indium extraction. (a) Effect of extractant concentration; (b) effect of extraction time; (c) effect of (O/A) ratio.
3.4. Analysis of acid leaching residue In order to further investigate the changes of element type, surface topography and molecular structure of solid residue after acid leaching process, the following experiments including XRD (Fig. 4), SEM (Fig. 5) and FTIR (Fig. 6) were conducted and the analysis was carried out in combination with previous hydrothermal pretreatment studies. It can be seen from Fig. 4 that acid leaching process does not cause damage to the crystals of the solid phase residue. The pattern of the acid leaching residue is more gradual than the residue after hydrothermal pretreatment. Some small peaks become inconspicuous or even disappear, indicating that metal in the hydrothermal residue is leached into the acid solution. From Fig. 5 and Fig. S1 (in supplementary file), surface of the acid leaching residue is smooth and the properties are similar under the condition of acid leaching temperatures of 60 °C and 80 °C. The basic structure of glass is still intact, which means that the different acid leaching temperatures will not further damage the surface morphology of the solid residue. Thus, acid leaching will not affect the recycling of glass. Fig. 6 shows that some functional groups did not show significant changes during the acid leaching process. Those changes include vibration absorption of OAH bond from 3000 cm1 to 3600 cm1, symmetric and asymmetric stretching vibration of CAH bond at 2759 cm1, stretching vibration of C@O bond from 1850 cm1 to 1680 cm1, symmetric bending of CAH bond and CH2 deformation from 1500 cm1 to1350 cm1, CABr bond at 690 cm1 and CAI from 450 cm1 to 650 cm1. It indicates that
acid leaching process does not damage the molecular structure of the glass substrate and does not have negative effect on its recycling. 3.5. Kinetic analysis of acid leaching process The sulfuric leaching process of indium belongs to liquid-solid reaction. In order to analyze the acid leaching process, we make the following assumptions according to the nuclear shrinkage model. (1) Assuming that the diffusion resistance through the diffusion layer and the solid film is small, so that the reaction rate is controlled by the chemical reaction. (2) Assuming that the mineral balls are spherical and dense without porosity. (3) Assuming that the excess leaching agent is used during the leaching process, its concentration C can be regarded as constant and keep at C0. The following equation can be deduced based on the above assumptions and mathematical calculations. It can be seen in supplementary file. 1
0
1 ð1 yÞ3 ¼ k t; k0 ¼
kMC 0 r0 q
ð10Þ
where y is the leaching yield of indium, t is the leaching time, k is the Chemical reaction ratio constant, M is the molar mass of the mineral, C 0 is the leaching agent concentration, r 0 is the original
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Fig. 3. Influences of factors on indium stripping. (a) Effect of stripping agent concentration; (b) effect of stripping time; (c) effect of (O/A) ratio.
Fig. 4. XRD pattern of solid residue (a) Residue after hydrothermal pretreatment (Cao et al., 2019). (b) Residue after acid leaching.
radius of the acid leaching residue,q is the density of the mineral ball. The data of the leaching yield at 60 °C, 80 °C and 100 °C was analyzed. Dynamic equation of 1 (1 y) 1/3 and leaching time t (min) was established to explore the control steps in the process of acid leaching of indium. The figure can be seen in supplementary file (Fig. S2). As shown in Table 1, all fitting curves show significant linear relationships. At the temperature of 60 °C, kobs = 0.009 min1
when the time ranges from 0 min to 65 min. At the temperature of 80 °C, the leaching yield of indium can reach 100% at 35 min. Since the leaching yield is 96% at 30 min, the time range is considered as 0–30 min. 1 (1 y)1/3 and leaching time t (min) show a linear relationship and kobs = 0.00215 min1. The effect of temperature on the leaching of metal indium is very significant. When the temperature is 100 °C, the leaching yield can reach 100% after only 15 min reaction and kobs = 0.067 min1.
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Fig. 5. SEM image of solid residue after acid leaching at different temperatures (a) 60 °C, (b) 80 °C.
Fig. 6. Infrared spectroscopy of solid materials (a) Glass substrate before hydrothermal reaction. (b) Residue after hydrothermal pretreatment (Cao et al., 2019). (c) Residue after acid leaching at 60 °C. (d) Residue after acid leaching at 80 °C.
Y. Cao et al. / Waste Management 102 (2020) 635–644 Table 1 The kinetic equation of the leaching yield of indium and time at different temperatures. Temperature 60 °C 80 °C 100 °C
Kinetic equation 1/3
1 (1 y) = 0.009 t 1 (1 y)1/3 = 0.0215 t 1 (1 y)1/3 = 0.067 t
R2 value
Time range
0.999 0.997 0.995
0–65 min 0–30 min 0–15 min
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reaction temperature can effectively accelerate the solid-liquid reaction, promote the reaction rate and shorten the time to reach equilibrium, thus the leaching yield of indium gradually increases. The leaching yield of indium reaches 100% at the reaction temperature of 70–80 °C, with 0.5 M H2SO4 solution in the (S/L) ratio of 1:2 g/mL conditions for 40 min. In addition, the recovery efficiency of indium reaches 97.25% by using 20% D2EHPA as extractant and 4 M HCl as stripping agent. The leachate is separately extracted under conditions of (O/A) ratio of 1: 10 for 3 min in extraction process and (O/A) ratio of 10: 1 for 10 min in stripping process. The surface topography and molecular structure of solid residue before and after acid leaching are little changed. It shows that there is no fundamental change in the properties of glass in the residue after acid leaching, and the subsequent recycling will not be affected. Thus, recycling methods including hydrothermal pretreatment and acid leaching can obtain the integrated recovery. Kinetics equation of the leaching yield and leaching time t (min) at 60 °C is 1 (1 y)1/3 = 0.009 t. At 80 °C, the linear relationship between the 1 (1 y)1/3 and the leaching time is obtained between 0 and 30 min, and the equation is 1 (1 y)1/3 = 0.0215 t. At 100 °C, the kinetic equation is 1 (1 y)1/3 = 0.067 t. In addition, the reaction activation energy of metal indium leaching process is 50.64 kJ/mol by calculation. Declarations of Competing Interest None.
Fig. 7. The relationship between the reaction rate constant of leaching process of indium and the reaction temperature.
It can be seen from Table 1 that the k value increases greatly with increasing temperature and the corresponding leaching yield is also greatly enhanced. The positive correlation between the reaction rate constant and the reaction temperature is in accordance with the Arrhenius equation. The logarithm of the Arrhenius equation is as follows:
Ea lnk ¼ þ lnA RT
ð11Þ
where T is the thermodynamic temperature of the reaction and the unit is K. k is the reaction rate constant at the corresponding temperature and the unit is min1. Ea is the activation energy of the reaction and the unit is kJ/mol. R is the molar gas constant, 8.314 J/molK. A is the pre-exponential factor and the unit is min1. According to the Arrhenius equation, the reaction rate constant is only related to the reaction temperature under a certain reaction condition. The linear fit for lnk and 1000/T and the fitting curve are shown in Fig. 7. It can be seen that -lnk has a good linear relationship with the reciprocal of the corresponding thermodynamic temperature 1/T, and the correlation coefficient is R2 = 0.99. It can be proved that the leaching reaction rate constant of indium is consistent with the Arrhenius equation and the activation energy of solid solution reaction is 50.64 kJ/mol in the leaching process within the temperature range by calculation. 4. Conclusion The separation and purification of indium in solid residue after hydrothermal treatment can be effectively achieved by using sulfuric acid as leaching agent, D2EHPA as extractant and hydrochloric as the stripping agent. The leaching yield of indium increases with the increasing concentration of H2SO4, and the (S/L) ratio has little effect on the leaching reaction. The increase of reaction time can effectively improve the leaching efficiency. The increment of
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