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Acid leaching of indium from the screens of obsolete LCD monitors
Journal of Environmental Chemical Engineering 8 (2020) 103758
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Acid leaching of indium from the screens of obsolete LCD monitors
T
Adjanara P. Gabriel, Angela C. Kasper*, Hugo M. Veit LACOR–PPGE3M, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Setor 4, Prédio 43426, 91501-970 - Porto Alegre, Rio Grande do Sul, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Indium recovery LCD screens Polarizing film Grinding Hydrometallurgy
Liquid crystal display (LCD) are present in televisions, computers and mobile phones, and in a variety of other electronic devices. These screens contain in your composition the element indium, considered a critical raw material. Thus, the objective of this work is to study the hydrometallurgical recovery of indium from LCD screens of televisions monitors, using ground samples without the previous removal of polarizing film of the glass layer. In addition, to study some parameters used in this indium recovery process. In this study, initially, the monitors were disassembled for the removal of LCD screens. After this, the LCD screens were broken into smaller parts and ground in a ball mill. After grinding, the samples were characterized by X-Ray Fluorescence (XRF). Subsequently, the leaching tests with aqua regia (HCl/HNO3, 3:1), HCl, HNO3 and H2SO4 were performed. In the leaching tests, variations in acid concentrations, temperature and time of leaching were analyzed. Results showed indium concentrations of 300 mg/kg in the ground samples. In leaching tests, the best results were obtained using H2SO4 0.5 M at room temperature (in 2 h) with extraction rate of 98.2 % and HCl 6 M at 60 °C (in 4 h) with extraction rate of 99.3 %. The high rates of indium extraction obtained indicate that it is not necessary to remove the polarizing film from the glass layer prior to the hydrometallurgical step. On the other hand, the results do not show a clear trend in relation to variations of analyzed parameters.
1. Introduction Indium is an element considered a critical raw material due to its economic importance, low availability and difficulty to obtain. According the European commission, indium has a sustainability index between 0.94 and 0.97. This index (punctuated 0–1) measure of difficulty in replacing the material, with 1 being the most difficult. In addition, its recycling rate is close to 0 % [1]. Thus, the recovery of indium from secondary sources, such as end-of-life monitors with LCD (Liquid Crystal Display) screens, arouse great interest. LCD screens monitors have quickly replaced the cathode ray tube (CRT) monitors because they are lighter, take up less space, have lower energy consumption and do not emit radiation [2,3]. In 2010, about 200 million units of LCD TVs were sold in the worldwide [4,5]. The sales of laptops and tablets reached similar levels [6]. However, estimates indicate that the lifetime of LCD screens is 3–5 years for laptops and 8–10 years for TVs and desktops [7–9]. Considering the number of products sold and their estimated lifetime, amount of end-of-life LCD screens exceed 200 million units per year [10]. Although LCD screens contain elements of economic interest in their composition, most of these end-of-life screens are currently stored or incinerated because there is not a recycling process available on an industrial scale [11–13].
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A monitor with LCD screens is composed for glass, polymers, liquid crystal and various metals, which add economic value to these devices [9,14]. LCD screen are composed by two layers of glass. These glass layers form a sandwich with the liquid crystal layer. In addition, in the outside of glass layers is attached polarizing films (polymeric films), that polarize the light emitted inside the screen. The inner side of the glass layers is coated with a functional film, produced of a hard and transparent material, that has the function of conducts electricity to the liquid crystal for image formation [7,15,16]. The functional film used in LCD screens is composed by indium-tin oxide (ITO). The ITO contains a proportion of 90 % of indium oxide (In2O3) and 10 % of tin oxide (SnO2) [17]. The main features of ITO are good electrical conductivity, ultraviolet radiation resistance, the ability to reflect infrared radiation and its ability to form a very hard layer with high transparency [18,19]. In addition, ITO presents a more affordable cost when compared to other options available in the market [20,21]. In recent years, several studies to recover indium from LCD screens were performed [11,13,19,22,23]. Results from these studies indicated quantities of indium between 100–400 mg/kg in LCD screens [24–27]. Table 1 reports some hydrometallurgical processes and details regarding possible treatments to recover indium after its leaching (e.g.
Corresponding author. E-mail address: [email protected] (A.C. Kasper).
https://doi.org/10.1016/j.jece.2020.103758 Received 10 October 2019; Received in revised form 3 February 2020; Accepted 6 February 2020 Available online 07 February 2020 2213-3437/ © 2020 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 8 (2020) 103758
[13] [26]
[17] [22] [22] [30] [23]
[31]
n.ib Solvent extraction and stripping
n.i.b n.ib n.ib Purification Cementation
Extraction with D2EHPA, stripping with HCl n.ib
0.27 (HCl) and 0.04 (HNO3) < 1 (H2SO4) or > 1 (HCl)
[27]
Reference Post-treatment for indium recovery
precipitation, cementation and solvent extraction) [27]Usually, these processes are preceded by dismantling or removal glass from the monitors, removal of polarizing film of glass layers and grinding of glass containing ITO. The removal of the polarizing films were performed with using solvents or by pyrometallurgical process [7,11–13,28,29]. In this study, the hydrometallurgical extraction of indium from obsolete or defective LCD screens was investigated. Were tested leaching processes using different reagents (acids) and variations in parameters of processes (concentrations of regents, temperatures and leaching time) to determine the best conditions of indium extraction. An important aspect to be emphasized is that in all tests were used with samples grinding without the previous removal of the polarizing film on the glass layer. Considering that the step of removal polarizing film from the glass increases the costs and time of process, the use of samples with these characteristics could simplify and optimize the process, facilitating their deployment on larger scales. 2. Materials and methods
Monitors with LCD screens in end-of-life, of different brands and years of manufacture (2006–2012), were donated by waste collection companies from city of Porto Alegre/RS. The monitors were manually disassembled and separated into four parts: LCD screen, polymer sheets, polymer shell and printed circuit boards, as seen in Fig. 1. For this work, only the LCD screens were used (Fig. 1a). The other components of the monitors were forwarded to other research projects. About 5 kg of LCD screens were obtained.
0.7 0.1 0.17 2 M H2SO4 3-step crosscurrent
80
0.9 1 160 1 9 M H2SO4
0.04 0.036 0.5 0.5 0.1 r.t.d n.a.c n.a.c 65.6 90 0.5 2 2 0.7 2
2 0.1 60 n.a.c 0.5 8
5.4 M HCl 0.8 M HNO3 < 1 M H2SO4 or > 1 M HCl 6 M HCl 6 M HCl 6 M HCl 0.6 M H2SO4 1 M H2SO4
Pulp density (kg/ L) Time (h) Acid
T (°C)
15 17 12 0.1 1
2.1. Manual disassembly
2.2. Comminution of the LCD screens After dismantling, the LCD screens initially were manually fragmented into smaller parts (3–5 cm). During the fragmentation, the LCD screens of various brands and years of manufacture were mixed into a single batch of material. This single batch was subsequently ground, characterized and used in the other stages of the work. Posteriorly, the fragmented screens (glass with polarizing film and ITO) were ground by 6 h in ball mill (Servitech, model CT-242) using alumina balls.
Yes Yes No Yes Yes
Crushing and thermal shock. Ultrasonic cleaning and Milling Grinding and incineration at 500 °C Electrical disintegration Pyrolysis at 450 °C Hot isostatic pressing treatment, manual breaking and pulverization Freezing with liquid N2, stripping of the polarizing film, grinding < 1 mm Shredding/milling, washing with deionized water
In order to verify the time required to stabilize the particle size, during the process of grinding, samples were taken from the material for particle size measurement. The particle size was measured by laser light diffraction (CILAS 1180). The characterization of powder obtained after grinding was performed by X-Ray Fluorescence (XRF) analysis performed in two types of equipment. An XRF 1180 (brand Shimadzu) equipment and a Thermo Niton XL3t portable (brand Thermo Scientific) equipment were used in characterization. Before characterization, the samples were dried to remove humidity. The sample quartering method were used to obtaining the samples. Analyzes were performed in triplicate.
Adapted from Rocchetti et al. [26]. a 98 % H2SO4, 37 % HCl and 70 % HNO3. b Not investigated. c Not available. d Room tempe rature.
No
No No Thermal shock at 220 °C, shredding and ultrasonic Cleaning Shredding
No
Organics Removal
2.3. Characterization of LCD powder
LCD panel pre-treatment
Table 1 Studies of indium recovery from waste LCD screens.
Leaching conditions
Acid consumptiona (kg acid/kg LCD)
A.P. Gabriel, et al.
2.4. Leaching tests In this study, aqua regia (3 HCl : 1 HNO3) and solutions of hydrochloric acid (HCl), nitric acid (HNO3) and sulfuric acid (H2SO4) were used for indium leaching. All solutions were obtained from analytical grade reagents and prepared with distillated water. In addition to the different leaching agents, variations in the parameters of leaching processes were also tested. The variations of parameters tested were: concentration of the leaching agent (0.5, 1.0, 2.0, 2
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Fig. 1. (a) LCD screen (glass containing polarizing film and ITO), (b) polymer sheets, (c) polymer shell and (d) printed circuit board.
for the reduction of particle size is fundamental to the efficiency of the process. This occurs because with a greater release of the metal, there is a greater increase of contact surfaces between the metal and the leaching agent and, consequently, a greater amount of dissolved metal [22,29,33]. In the case of recycling of indium from LCD screens, some authors obtained better results using samples with more larger sizes particles than those sizes particles used in this study. Li et al. [13], investigated the influence of particle size on the dissolution of indium of LCD waste, using particles of 10, 5 and 1 mm and a mixture of hydrochloric acid and nitric acid (HCl:HNO3:H2O = 45:5:50 vol ratio) as a leaching agent. The authors reported that the best extraction results were obtained for particles with a size of 5 mm. Lee et al. [17]observed a decrease for indium leached, when using samples subjected to a longer grinding time, possibly caused by the aggregation of smaller particles. Fuchs [34], in his study using regia water at 90 °C in 3 h, obtained indium extraction values about 30 % higher in tests using samples with particle size 212 μm in relation of samples with particle size 75 μm. However, the authors cited used samples without polarizing film, whereas in this study samples without prior removal of the polarizing film were used. With objective of minimizing the effect of polymer film on metal dissolution, in this study, samples with particle size to smaller sizes than 1 mm was used. Thus, the samples were ground for 6 h in a ball mill, which resulted in a final product with an extremely small particle size, as shown in Fig. 3.
4.0 and 6.0 M), temperature (25 °C and 60 °C) and leaching time (2 and 4 h). It should be noted that the temperature of 25 °C refers to room temperature and, therefore, the tests performed at this temperature were performed without heating. In each leaching test, 0.5 g of ground sample was used. The solid/ liquid ratio used in the tests was 1:100. The sample amount and solid/ liquid ratio used in this study were defined from results by previous study [32]. The leaching tests were performed with condensers, coupled to a reactor (with a capacity of 200 mL), in order to avoid the volatilization of the acids and the contamination of the laboratory environment. The apparatus used in the tests was still constituted by a magnetic stirrer with heating (Fisatom, model 752A). Fig. 2 shows the apparatus used in this study. Before each test the samples were dried to remove humidity. The sample quartering method were used to obtaining the samples. All tests and analyses were performed in triplicate. After the leaching, the samples were filtered and solid residue was washed with small portions of the leach solution. The solid residue (glass and polymer) was discarded and the solutions were analyzed. The concentration of metal was determined by means of an ICP/OES (Brand Agilent, model 5110). In these analyses were used a calibration curve between 0.02–6 ppm and linearity of 99.9 %, produced from a standard solution of In with 100 ppm (brand SpecSol). An average of three wavelengths (325.609, 230.606 and 410.176) were used. The results presented correspond of average of three measurements, subtract the blank value (only the leaching acids).
3.2. Characterization of LCD powder 3. Results and discussion During the grinding process, samples of the material were taken to check the particle size. Fig. 4 shows the relationship between medium particle size and grinding time. Results shows that, after 4 h of grinding, the reduction in particle size is less accentuated (with a variation of 5.2–5.0 μm, ie, a variation of 0.1 μm per hour of grinding). These results indicate that grinding time adequate should not exceed 4 h, because in this time the particle size already reached acceptable value. Higher grinding times would be spending an unnecessary time and energy. With regard to the search for better results, from the point of view of energy saving, future works could investigate the use of particles with a size of ≥10 μm, which would reduce the grinding time to 1 h. However, in this work, samples with a particle size of 5 μm were used in the leaching tests. The results obtained in the characterization by X-Ray Fluorescence (XRF) analysis shown in Table 2. The results of these analyses indicate the presence of 0.036 ± 0.015 % (on average) of indium oxide (In2O3) in the ground LCD screens samples, that is, the concentration of indium in the sample is 300 mg/kg. Silicon oxide (SiO2) with 67.47 ( ± 4.48) %, Aluminium oxide (Al2O3) with 11.79 ( ± 1.715) % and calcium oxide (CaO) with 6.75 ( ± 1.11) % were the main components detected in the samples. These
3.1. Comminution of the LCD screens Several studies have shown that during the recycling or recovery of electronic scrap through hydrometallurgy, the use of a pre-treatment
Fig. 2. Apparatus used in leaching tests. 3
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Fig. 3. Photograph and micrograph of ground sample.
and verifying if the presence of the polarizing film in the ground material is detrimental to indium extraction from the LCD screen. The results obtained in these tests are presented in sequence. 3.3.1. Tests with aqua regia The results obtained in the leaching tests of the samples containing indium using aqua regia (3 HCl: 1 HNO3) as leaching agent are shown in Fig. 5. Fig. 5 shows that, in the tests using regia water as indium leaching agent at room temperature, it was possible to extract 93.3( ± 4.44) and 74.7( ± 4.26) % in two and four hours, respectively. Already in the tests using temperature of 60 °C, it was possible to extract 81.0( ± 4.56) e 86.0( ± 4.22) % in two and four hours, respectively. That is, with aqua regia, were extracted concentrations of indium between 260 and 280 mg/kg. The results are comparable with those obtained by Yang et al. [26]The results are comparable with those obtained by Yang et al. [26]. These authors, using aqua regia to characterize LCD samples, obtained indium concentrations of 200 ± 50 g/ton in a leaching time of days at 20 ± 1 °C, with S/L = 0.1 g/ml. Although some authors use aqua regia to characterize samples containing indium, analyzing the work of some authors as Ruan et al. [31]Fuchs [37]Yang et al. [26]and others, it is verified that for certain conditions the extraction results obtained with aqua regia are inferior to those obtained by acids as HCl and H2SO4. Ruan et al. [31]for example, used temperature of 160 °C in 0.5 h in your characterization with aqua regia, and obtained results of leaching slightly lower (99 % extraction) than those obtained with concentrated HCl.
Fig. 4. Results of particle size analysis.
elements are typical of the standard formulation used for glass production. In addition, the table also shows the presence of significant amounts of arsenic oxide (AS2O3), this result was compatible with those results obtained by Savvilotidou et al. [6]. Arsenic is one of the most potentially toxic metals even at low concentration levels [35], however is generally added to the glass during the fusion process in order to improve the optical clarity of the glass panel on LCD devices [36]. The presence of this element should be taken into consideration when choosing the Indium leaching agent to avoid human and environmental contamination. The results obtained in this study for the elemental composition of the samples by XRF analysis are compatible with the results obtained by other authors such as Wang [37], Lin et al. [38], Ruan et al. [31]
3.3.2. Tests with mineral acids The results obtained in the leaching tests of the samples containing indium (in oxide form) using HCl, HNO3 and H2SO4 as leaching agents, at two different temperatures (room temperature and 60 °C), are shown in Figs. 6 and 7. Comparative results for rate of indium extraction (in percentage) with different acids, as a function of acid concentration and contact time of the sample with the leaching agent, at room temperature (25 °C), are shown in Fig. 6. Analyzing the Fig. 6, the factor that arouses the most attention is the
3.3. Leaching tests The leaching tests were performed with the objective of comparing the efficiency in the indium extraction of the four different leaching agents (at different concentrations, leaching times and temperatures) Table 2 Percentage of oxides of main elements in LCD screen powder by XRF.
Mass (%)
SiO2
Al2O3
CaO
As2O3
K2O
In2O3
SnO2
Others
67.475 ± (4.483)
11.795 ± (1.725)
6.755 ± (1.110)
0.983 ± (0.491)
0.675 ± (0.290)
0.036 ± (0.015)
0.013 ± (0.011)
10.40 ± (4.876)
4
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increases with increasing acid concentration. At room temperature, the best results with HCl were obtained at concentration of 4 M in 4 h, with an extraction of indium of 94.7 ( ± 4.0) %, that is, 284 mg/kg. Comparative results for indium extraction with different acids, as a function of acid concentration and contact time of the sample with the leaching agent, at a temperature of 60 °C, are shown in Fig. 7. With regard to the amount of indium extracted at 60 °C of temperature, Fig. 7 shows that by using the best result were obtained using HCl 4 M in 4 h, with a extraction of 99.3 ( ± 0.33) %, that is, 298 mg/ kg. For HCl 2 M in 4 h it was possible to obtain 95.3 ( ± 4.) %, that is, 286 mg/kg, and HCl 6 M in 2 h to obtain 93.8 %, that is, 281 mg/kg. These results are compatible with the results obtained by other authors. Yang et al. [26] also used ground samples without the removal of the polarizing film. The authors obtained approximately 260 mg/kg, using HCl 1 M with a leaching time less than 8 h. Fontana et al. [11] extracted 260 mg/kg (corresponding to a leaching efficiency of 90 %), using HCl 6 N in a leaching time of 6 h and at room temperature. However, these authors studied the extraction of indium using samples that did not go through the grinding process, but which had the polarizing film previously removed, unlike our study. The best results from the tests with HNO3 were obtained for a concentration of 4 M, at a leaching time of 4 h. Under these conditions, it was possible to achieve values of indium extraction close to 87 %, that is, 260 mg/kg.
Fig. 5. Results of leach tests with aqua regia.
3.3.3. General considerations Analysis for the results using HCl verified that, for all of the tested conditions, indium concentrations higher than 60 % were obtained. In the leaching tests with HNO3, it was possible to extract quantities of indium larger than 47 %. Although the amount of indium obtained in the studies may vary, due to factors such as the difference between the models of LCD screens used in the samples, the results obtained are comparable with those obtained by other authors who tested nitric acid in indium leaching. Virolainen et al. [33]and Yang et al. [26]reported that the kinetic leaching rate of the ITO in nitric acid is very slow, especially when using lower concentrations of acid (less than 1 M). Comparing the three acids tested in this study (HCl, HNO3 and H2SO4), it was verified that HCl and H2SO4 presented higher results than HNO3. Moreover, as the ITO is composed of In2O3 and SnO2, the fact that SnO2 is insoluble in H2SO4 and HCl is one advantage in a future separation of tin and indium [26]Thus, both HCl and H2SO4 could be used as leaching agents for the indium contained in LCD screens in the form of ITO. However, in terms of reagent consumption, time and energy, the results obtained in this study for the leaching of indium with H2SO4 in concentrations of 0.5 M are extremely favorable. In addition, H2SO4 has the advantage of being less corrosive to the equipment used in the process when compared to HCl. Another advantage of H2SO4 in relation to the other acids tested is the lower dissolution of As2O3 that can also be found in LCD screens [31] The values of the concentrations of indium extracted from this type of LCD screen (by computer monitor and television), as reported in the literature, can vary between 20 and 400 mg/kg [24–27] this work, the best results ranged from 260 mg/kg (for HNO3) up to 298 mg/kg of glass (for HCl). The standard deviation calculated for leached indium ranged from 10 % to 20 % of the mean values. Observing Figs. 6 and 7 can be seen that several extraction results vary within the error range. This fact was already expected and can be attributed to the fact that the chemical equilibrium is achieved in times shorter than those tested in this research; above all, for the results obtained with higher acid concentrations. This behavior had already been observed in previous work, such as work conducted by Li et al. (2009), Lee et al. (2013), Yang et al. (2013) and Savvilotidou et al. (2015). These authors demonstrated that in the case of leaching of indium with mineral acids (HCl, H2SO4 e HNO3), the equilibrium is reached in a time less than 4 h [13,17,26,39].
Fig. 6. Indium extraction as a function of acid concentration and contact time, at room temperature.
Fig. 7. Indium extraction as a function of acid concentration and contact time, at 60 °C.
fact that the best results were obtained using H2SO4 at a concentration of 0.5 M, in 2 h. Under these conditions, it was possible to achieve values close to 98.2 ( ± 2.5)%, that is, 295 mg/kg. These results are compatible with those obtained by other authors. Virolainen et al. [33] reported the complete dissolution of indium in H2SO4 1 M. Li et al. [37] obtained rates of indium dissolution of 85 % for an acid concentration of 0.75 M, in 80 min and at a temperature of 65 °C. Wang et al. [37] concluded that the best conditions for indium leaching with H2SO4 is a concentration 0.6 M, leaching time of 42.2 min and a temperature of 65.6 °C. It is also possible to verify that the extraction of indium with HCl 5
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4. Conclusions [8]
After grinding in a ball mill a homogeneous powder with particle size around 5 μm was obtained. The grinding time to obtain this particle size was 4 h, however, with 1 h of grinding it was possible to obtain particles with a size of less than 10 μm. This shows that the grinding time used in this study (6 h) can be reduced. The best results from the tests using HNO3 were obtained for a concentration of 4 M, a leaching time of 4 h and heating at 60 °C. Under these conditions, it was possible to achieve rates of extraction close to 87 %. However, in general, the results obtained with HNO3 were lower than those obtained by using the others acids. The best results in tests using HCl were obtained using 6 M solution, in a leaching time of 4 h and at a temperature of 60 °C, with an extraction rate of 99.3 %. However, using H2SO4 0.5 M, in 2 h and without heating, made it possible to extract rate of 98.2 %. Considering the price of the acids used in this study, its toxicity to the environment and the workers involved the leaching time and the need for heating or not, H2SO4 presented the most promising results for the extraction of indium from LCD screens at a larger scale. Considering the high percentages of indium extraction obtained in this study and comparing with results obtained by others authors cited, it is possible to presume that removal of polarizing film before grinding samples did not seem to have influenced the results of indium extraction. The results obtained do not show a clear trend in relation to the variations of time, temperature and concentration of the leaching reagent.
[9]
[10]
[11] [12] [13] [14] [15] [16]
[17]
[18] [19]
[20] [21]
[22]
CRediT authorship contribution statement [23]
Adjanara P. Gabriel: Methodology, Investigation, Writing - original draft. Angela C. Kasper: Investigation, Writing - review & editing, Supervision. Hugo M. Veit: Supervision, Project administration, Funding acquisition.
[24] [25]
[26]
Declaration of Competing Interest
[27]
The authors declare do not have conflicts of interest in conducting and publishing this research.
[28]
Acknowledgements [29]
This study was supported by the Brazilian agencies Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and FINEP (for the resources obtained through the Sibratec Project).
[30] [31] [32] [33]
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Prod. 19 (2011) 1066–1071. V. Rotter, P. Chancerel, M. Ueberschaar, Recycling-oriented product characterization for electric and electronic equipment as a tool to enable recycling of critical metals, REWAS 2013 Enabling Materials Resources Sustainability 2013 (192) (2013) - 201. J. Yang, T. Retegan, C. Ekberg, Indium recovery from discarded LCD panelglass by solvent extraction, Hydrometallurgy 137 (2013) 68–77. L. Rocchetti, A. Amato, V. Fonti, S. Ubaldini, I. de Michelis, B. Kopacek, F. Vegliò, F. Beolchini, Cross-current leaching of indium from end-of-life LCD panels, Waste Manag. 42 (2015) 180–187. R. Lu, E. Ma, Z. Xu, Application of pyrolysis process to remove and recover liquid crystal and films from waste liquid crystal display glass, J. Hazard. Mater. 243 (2012) 311–318. K. Zhang, Y. Wu, W. Wang, B. Li, Y. Zhang, T. Zuo, Recycling indium from waste LCDs: a review, Resour. Conserv. Recycl. 104 (2015) 276–290. X. Wang, X. Lu, S. Zhang, Study on the waste liquid crystal display treatment: focus on the resource recovery, J. Hazard. Mater. 244 (2013) 342–347. J. Ruan, Y. Guo, Q. Qiao, Recovery of indium from scrap TFT-LCDs by solvent extraction, Procedia Environmental Sciences16 (2012) 545–551. A.P. Gabriel, B.B. Giordani, A.C. Kasper, H.M. Veit, Indium extraction from LCDs screens, Detritus 3 (2018) 42–46. S. Virolainen, D. Ibana, E. Paatero, Recovery of indium from indium tin oxide by solvent extraction, Hydrometallurgy 107 (1–2) (2011) 56–61. M.S. Fuchs, Processo de separação de materiais valiosos de telas de LCD de telefones celulares, Dissertação de mestrado, Universidade Federal de Santa Maria, Santa Maria/RS, Brazil, 2013. S.R. Lim, J.M. Schoenung, Human health and ecological toxicity potentials due to heavymetal content in waste electronic devices with flat panel displays, Journal Hazard Materials 77 (2010) 251–259. A.B. Atkarskaya, V.N. 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Contents lists available at ScienceDirect
Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
Acid leaching of indium from the screens of obsolete LCD monitors
T
Adjanara P. Gabriel, Angela C. Kasper*, Hugo M. Veit LACOR–PPGE3M, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Setor 4, Prédio 43426, 91501-970 - Porto Alegre, Rio Grande do Sul, Brazil
A R T I C LE I N FO
A B S T R A C T
Keywords: Indium recovery LCD screens Polarizing film Grinding Hydrometallurgy
Liquid crystal display (LCD) are present in televisions, computers and mobile phones, and in a variety of other electronic devices. These screens contain in your composition the element indium, considered a critical raw material. Thus, the objective of this work is to study the hydrometallurgical recovery of indium from LCD screens of televisions monitors, using ground samples without the previous removal of polarizing film of the glass layer. In addition, to study some parameters used in this indium recovery process. In this study, initially, the monitors were disassembled for the removal of LCD screens. After this, the LCD screens were broken into smaller parts and ground in a ball mill. After grinding, the samples were characterized by X-Ray Fluorescence (XRF). Subsequently, the leaching tests with aqua regia (HCl/HNO3, 3:1), HCl, HNO3 and H2SO4 were performed. In the leaching tests, variations in acid concentrations, temperature and time of leaching were analyzed. Results showed indium concentrations of 300 mg/kg in the ground samples. In leaching tests, the best results were obtained using H2SO4 0.5 M at room temperature (in 2 h) with extraction rate of 98.2 % and HCl 6 M at 60 °C (in 4 h) with extraction rate of 99.3 %. The high rates of indium extraction obtained indicate that it is not necessary to remove the polarizing film from the glass layer prior to the hydrometallurgical step. On the other hand, the results do not show a clear trend in relation to variations of analyzed parameters.
1. Introduction Indium is an element considered a critical raw material due to its economic importance, low availability and difficulty to obtain. According the European commission, indium has a sustainability index between 0.94 and 0.97. This index (punctuated 0–1) measure of difficulty in replacing the material, with 1 being the most difficult. In addition, its recycling rate is close to 0 % [1]. Thus, the recovery of indium from secondary sources, such as end-of-life monitors with LCD (Liquid Crystal Display) screens, arouse great interest. LCD screens monitors have quickly replaced the cathode ray tube (CRT) monitors because they are lighter, take up less space, have lower energy consumption and do not emit radiation [2,3]. In 2010, about 200 million units of LCD TVs were sold in the worldwide [4,5]. The sales of laptops and tablets reached similar levels [6]. However, estimates indicate that the lifetime of LCD screens is 3–5 years for laptops and 8–10 years for TVs and desktops [7–9]. Considering the number of products sold and their estimated lifetime, amount of end-of-life LCD screens exceed 200 million units per year [10]. Although LCD screens contain elements of economic interest in their composition, most of these end-of-life screens are currently stored or incinerated because there is not a recycling process available on an industrial scale [11–13].
⁎
A monitor with LCD screens is composed for glass, polymers, liquid crystal and various metals, which add economic value to these devices [9,14]. LCD screen are composed by two layers of glass. These glass layers form a sandwich with the liquid crystal layer. In addition, in the outside of glass layers is attached polarizing films (polymeric films), that polarize the light emitted inside the screen. The inner side of the glass layers is coated with a functional film, produced of a hard and transparent material, that has the function of conducts electricity to the liquid crystal for image formation [7,15,16]. The functional film used in LCD screens is composed by indium-tin oxide (ITO). The ITO contains a proportion of 90 % of indium oxide (In2O3) and 10 % of tin oxide (SnO2) [17]. The main features of ITO are good electrical conductivity, ultraviolet radiation resistance, the ability to reflect infrared radiation and its ability to form a very hard layer with high transparency [18,19]. In addition, ITO presents a more affordable cost when compared to other options available in the market [20,21]. In recent years, several studies to recover indium from LCD screens were performed [11,13,19,22,23]. Results from these studies indicated quantities of indium between 100–400 mg/kg in LCD screens [24–27]. Table 1 reports some hydrometallurgical processes and details regarding possible treatments to recover indium after its leaching (e.g.
Corresponding author. E-mail address: [email protected] (A.C. Kasper).
https://doi.org/10.1016/j.jece.2020.103758 Received 10 October 2019; Received in revised form 3 February 2020; Accepted 6 February 2020 Available online 07 February 2020 2213-3437/ © 2020 Elsevier Ltd. All rights reserved.
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[13] [26]
[17] [22] [22] [30] [23]
[31]
n.ib Solvent extraction and stripping
n.i.b n.ib n.ib Purification Cementation
Extraction with D2EHPA, stripping with HCl n.ib
0.27 (HCl) and 0.04 (HNO3) < 1 (H2SO4) or > 1 (HCl)
[27]
Reference Post-treatment for indium recovery
precipitation, cementation and solvent extraction) [27]Usually, these processes are preceded by dismantling or removal glass from the monitors, removal of polarizing film of glass layers and grinding of glass containing ITO. The removal of the polarizing films were performed with using solvents or by pyrometallurgical process [7,11–13,28,29]. In this study, the hydrometallurgical extraction of indium from obsolete or defective LCD screens was investigated. Were tested leaching processes using different reagents (acids) and variations in parameters of processes (concentrations of regents, temperatures and leaching time) to determine the best conditions of indium extraction. An important aspect to be emphasized is that in all tests were used with samples grinding without the previous removal of the polarizing film on the glass layer. Considering that the step of removal polarizing film from the glass increases the costs and time of process, the use of samples with these characteristics could simplify and optimize the process, facilitating their deployment on larger scales. 2. Materials and methods
Monitors with LCD screens in end-of-life, of different brands and years of manufacture (2006–2012), were donated by waste collection companies from city of Porto Alegre/RS. The monitors were manually disassembled and separated into four parts: LCD screen, polymer sheets, polymer shell and printed circuit boards, as seen in Fig. 1. For this work, only the LCD screens were used (Fig. 1a). The other components of the monitors were forwarded to other research projects. About 5 kg of LCD screens were obtained.
0.7 0.1 0.17 2 M H2SO4 3-step crosscurrent
80
0.9 1 160 1 9 M H2SO4
0.04 0.036 0.5 0.5 0.1 r.t.d n.a.c n.a.c 65.6 90 0.5 2 2 0.7 2
2 0.1 60 n.a.c 0.5 8
5.4 M HCl 0.8 M HNO3 < 1 M H2SO4 or > 1 M HCl 6 M HCl 6 M HCl 6 M HCl 0.6 M H2SO4 1 M H2SO4
Pulp density (kg/ L) Time (h) Acid
T (°C)
15 17 12 0.1 1
2.1. Manual disassembly
2.2. Comminution of the LCD screens After dismantling, the LCD screens initially were manually fragmented into smaller parts (3–5 cm). During the fragmentation, the LCD screens of various brands and years of manufacture were mixed into a single batch of material. This single batch was subsequently ground, characterized and used in the other stages of the work. Posteriorly, the fragmented screens (glass with polarizing film and ITO) were ground by 6 h in ball mill (Servitech, model CT-242) using alumina balls.
Yes Yes No Yes Yes
Crushing and thermal shock. Ultrasonic cleaning and Milling Grinding and incineration at 500 °C Electrical disintegration Pyrolysis at 450 °C Hot isostatic pressing treatment, manual breaking and pulverization Freezing with liquid N2, stripping of the polarizing film, grinding < 1 mm Shredding/milling, washing with deionized water
In order to verify the time required to stabilize the particle size, during the process of grinding, samples were taken from the material for particle size measurement. The particle size was measured by laser light diffraction (CILAS 1180). The characterization of powder obtained after grinding was performed by X-Ray Fluorescence (XRF) analysis performed in two types of equipment. An XRF 1180 (brand Shimadzu) equipment and a Thermo Niton XL3t portable (brand Thermo Scientific) equipment were used in characterization. Before characterization, the samples were dried to remove humidity. The sample quartering method were used to obtaining the samples. Analyzes were performed in triplicate.
Adapted from Rocchetti et al. [26]. a 98 % H2SO4, 37 % HCl and 70 % HNO3. b Not investigated. c Not available. d Room tempe rature.
No
No No Thermal shock at 220 °C, shredding and ultrasonic Cleaning Shredding
No
Organics Removal
2.3. Characterization of LCD powder
LCD panel pre-treatment
Table 1 Studies of indium recovery from waste LCD screens.
Leaching conditions
Acid consumptiona (kg acid/kg LCD)
A.P. Gabriel, et al.
2.4. Leaching tests In this study, aqua regia (3 HCl : 1 HNO3) and solutions of hydrochloric acid (HCl), nitric acid (HNO3) and sulfuric acid (H2SO4) were used for indium leaching. All solutions were obtained from analytical grade reagents and prepared with distillated water. In addition to the different leaching agents, variations in the parameters of leaching processes were also tested. The variations of parameters tested were: concentration of the leaching agent (0.5, 1.0, 2.0, 2
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Fig. 1. (a) LCD screen (glass containing polarizing film and ITO), (b) polymer sheets, (c) polymer shell and (d) printed circuit board.
for the reduction of particle size is fundamental to the efficiency of the process. This occurs because with a greater release of the metal, there is a greater increase of contact surfaces between the metal and the leaching agent and, consequently, a greater amount of dissolved metal [22,29,33]. In the case of recycling of indium from LCD screens, some authors obtained better results using samples with more larger sizes particles than those sizes particles used in this study. Li et al. [13], investigated the influence of particle size on the dissolution of indium of LCD waste, using particles of 10, 5 and 1 mm and a mixture of hydrochloric acid and nitric acid (HCl:HNO3:H2O = 45:5:50 vol ratio) as a leaching agent. The authors reported that the best extraction results were obtained for particles with a size of 5 mm. Lee et al. [17]observed a decrease for indium leached, when using samples subjected to a longer grinding time, possibly caused by the aggregation of smaller particles. Fuchs [34], in his study using regia water at 90 °C in 3 h, obtained indium extraction values about 30 % higher in tests using samples with particle size 212 μm in relation of samples with particle size 75 μm. However, the authors cited used samples without polarizing film, whereas in this study samples without prior removal of the polarizing film were used. With objective of minimizing the effect of polymer film on metal dissolution, in this study, samples with particle size to smaller sizes than 1 mm was used. Thus, the samples were ground for 6 h in a ball mill, which resulted in a final product with an extremely small particle size, as shown in Fig. 3.
4.0 and 6.0 M), temperature (25 °C and 60 °C) and leaching time (2 and 4 h). It should be noted that the temperature of 25 °C refers to room temperature and, therefore, the tests performed at this temperature were performed without heating. In each leaching test, 0.5 g of ground sample was used. The solid/ liquid ratio used in the tests was 1:100. The sample amount and solid/ liquid ratio used in this study were defined from results by previous study [32]. The leaching tests were performed with condensers, coupled to a reactor (with a capacity of 200 mL), in order to avoid the volatilization of the acids and the contamination of the laboratory environment. The apparatus used in the tests was still constituted by a magnetic stirrer with heating (Fisatom, model 752A). Fig. 2 shows the apparatus used in this study. Before each test the samples were dried to remove humidity. The sample quartering method were used to obtaining the samples. All tests and analyses were performed in triplicate. After the leaching, the samples were filtered and solid residue was washed with small portions of the leach solution. The solid residue (glass and polymer) was discarded and the solutions were analyzed. The concentration of metal was determined by means of an ICP/OES (Brand Agilent, model 5110). In these analyses were used a calibration curve between 0.02–6 ppm and linearity of 99.9 %, produced from a standard solution of In with 100 ppm (brand SpecSol). An average of three wavelengths (325.609, 230.606 and 410.176) were used. The results presented correspond of average of three measurements, subtract the blank value (only the leaching acids).
3.2. Characterization of LCD powder 3. Results and discussion During the grinding process, samples of the material were taken to check the particle size. Fig. 4 shows the relationship between medium particle size and grinding time. Results shows that, after 4 h of grinding, the reduction in particle size is less accentuated (with a variation of 5.2–5.0 μm, ie, a variation of 0.1 μm per hour of grinding). These results indicate that grinding time adequate should not exceed 4 h, because in this time the particle size already reached acceptable value. Higher grinding times would be spending an unnecessary time and energy. With regard to the search for better results, from the point of view of energy saving, future works could investigate the use of particles with a size of ≥10 μm, which would reduce the grinding time to 1 h. However, in this work, samples with a particle size of 5 μm were used in the leaching tests. The results obtained in the characterization by X-Ray Fluorescence (XRF) analysis shown in Table 2. The results of these analyses indicate the presence of 0.036 ± 0.015 % (on average) of indium oxide (In2O3) in the ground LCD screens samples, that is, the concentration of indium in the sample is 300 mg/kg. Silicon oxide (SiO2) with 67.47 ( ± 4.48) %, Aluminium oxide (Al2O3) with 11.79 ( ± 1.715) % and calcium oxide (CaO) with 6.75 ( ± 1.11) % were the main components detected in the samples. These
3.1. Comminution of the LCD screens Several studies have shown that during the recycling or recovery of electronic scrap through hydrometallurgy, the use of a pre-treatment
Fig. 2. Apparatus used in leaching tests. 3
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Fig. 3. Photograph and micrograph of ground sample.
and verifying if the presence of the polarizing film in the ground material is detrimental to indium extraction from the LCD screen. The results obtained in these tests are presented in sequence. 3.3.1. Tests with aqua regia The results obtained in the leaching tests of the samples containing indium using aqua regia (3 HCl: 1 HNO3) as leaching agent are shown in Fig. 5. Fig. 5 shows that, in the tests using regia water as indium leaching agent at room temperature, it was possible to extract 93.3( ± 4.44) and 74.7( ± 4.26) % in two and four hours, respectively. Already in the tests using temperature of 60 °C, it was possible to extract 81.0( ± 4.56) e 86.0( ± 4.22) % in two and four hours, respectively. That is, with aqua regia, were extracted concentrations of indium between 260 and 280 mg/kg. The results are comparable with those obtained by Yang et al. [26]The results are comparable with those obtained by Yang et al. [26]. These authors, using aqua regia to characterize LCD samples, obtained indium concentrations of 200 ± 50 g/ton in a leaching time of days at 20 ± 1 °C, with S/L = 0.1 g/ml. Although some authors use aqua regia to characterize samples containing indium, analyzing the work of some authors as Ruan et al. [31]Fuchs [37]Yang et al. [26]and others, it is verified that for certain conditions the extraction results obtained with aqua regia are inferior to those obtained by acids as HCl and H2SO4. Ruan et al. [31]for example, used temperature of 160 °C in 0.5 h in your characterization with aqua regia, and obtained results of leaching slightly lower (99 % extraction) than those obtained with concentrated HCl.
Fig. 4. Results of particle size analysis.
elements are typical of the standard formulation used for glass production. In addition, the table also shows the presence of significant amounts of arsenic oxide (AS2O3), this result was compatible with those results obtained by Savvilotidou et al. [6]. Arsenic is one of the most potentially toxic metals even at low concentration levels [35], however is generally added to the glass during the fusion process in order to improve the optical clarity of the glass panel on LCD devices [36]. The presence of this element should be taken into consideration when choosing the Indium leaching agent to avoid human and environmental contamination. The results obtained in this study for the elemental composition of the samples by XRF analysis are compatible with the results obtained by other authors such as Wang [37], Lin et al. [38], Ruan et al. [31]
3.3.2. Tests with mineral acids The results obtained in the leaching tests of the samples containing indium (in oxide form) using HCl, HNO3 and H2SO4 as leaching agents, at two different temperatures (room temperature and 60 °C), are shown in Figs. 6 and 7. Comparative results for rate of indium extraction (in percentage) with different acids, as a function of acid concentration and contact time of the sample with the leaching agent, at room temperature (25 °C), are shown in Fig. 6. Analyzing the Fig. 6, the factor that arouses the most attention is the
3.3. Leaching tests The leaching tests were performed with the objective of comparing the efficiency in the indium extraction of the four different leaching agents (at different concentrations, leaching times and temperatures) Table 2 Percentage of oxides of main elements in LCD screen powder by XRF.
Mass (%)
SiO2
Al2O3
CaO
As2O3
K2O
In2O3
SnO2
Others
67.475 ± (4.483)
11.795 ± (1.725)
6.755 ± (1.110)
0.983 ± (0.491)
0.675 ± (0.290)
0.036 ± (0.015)
0.013 ± (0.011)
10.40 ± (4.876)
4
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increases with increasing acid concentration. At room temperature, the best results with HCl were obtained at concentration of 4 M in 4 h, with an extraction of indium of 94.7 ( ± 4.0) %, that is, 284 mg/kg. Comparative results for indium extraction with different acids, as a function of acid concentration and contact time of the sample with the leaching agent, at a temperature of 60 °C, are shown in Fig. 7. With regard to the amount of indium extracted at 60 °C of temperature, Fig. 7 shows that by using the best result were obtained using HCl 4 M in 4 h, with a extraction of 99.3 ( ± 0.33) %, that is, 298 mg/ kg. For HCl 2 M in 4 h it was possible to obtain 95.3 ( ± 4.) %, that is, 286 mg/kg, and HCl 6 M in 2 h to obtain 93.8 %, that is, 281 mg/kg. These results are compatible with the results obtained by other authors. Yang et al. [26] also used ground samples without the removal of the polarizing film. The authors obtained approximately 260 mg/kg, using HCl 1 M with a leaching time less than 8 h. Fontana et al. [11] extracted 260 mg/kg (corresponding to a leaching efficiency of 90 %), using HCl 6 N in a leaching time of 6 h and at room temperature. However, these authors studied the extraction of indium using samples that did not go through the grinding process, but which had the polarizing film previously removed, unlike our study. The best results from the tests with HNO3 were obtained for a concentration of 4 M, at a leaching time of 4 h. Under these conditions, it was possible to achieve values of indium extraction close to 87 %, that is, 260 mg/kg.
Fig. 5. Results of leach tests with aqua regia.
3.3.3. General considerations Analysis for the results using HCl verified that, for all of the tested conditions, indium concentrations higher than 60 % were obtained. In the leaching tests with HNO3, it was possible to extract quantities of indium larger than 47 %. Although the amount of indium obtained in the studies may vary, due to factors such as the difference between the models of LCD screens used in the samples, the results obtained are comparable with those obtained by other authors who tested nitric acid in indium leaching. Virolainen et al. [33]and Yang et al. [26]reported that the kinetic leaching rate of the ITO in nitric acid is very slow, especially when using lower concentrations of acid (less than 1 M). Comparing the three acids tested in this study (HCl, HNO3 and H2SO4), it was verified that HCl and H2SO4 presented higher results than HNO3. Moreover, as the ITO is composed of In2O3 and SnO2, the fact that SnO2 is insoluble in H2SO4 and HCl is one advantage in a future separation of tin and indium [26]Thus, both HCl and H2SO4 could be used as leaching agents for the indium contained in LCD screens in the form of ITO. However, in terms of reagent consumption, time and energy, the results obtained in this study for the leaching of indium with H2SO4 in concentrations of 0.5 M are extremely favorable. In addition, H2SO4 has the advantage of being less corrosive to the equipment used in the process when compared to HCl. Another advantage of H2SO4 in relation to the other acids tested is the lower dissolution of As2O3 that can also be found in LCD screens [31] The values of the concentrations of indium extracted from this type of LCD screen (by computer monitor and television), as reported in the literature, can vary between 20 and 400 mg/kg [24–27] this work, the best results ranged from 260 mg/kg (for HNO3) up to 298 mg/kg of glass (for HCl). The standard deviation calculated for leached indium ranged from 10 % to 20 % of the mean values. Observing Figs. 6 and 7 can be seen that several extraction results vary within the error range. This fact was already expected and can be attributed to the fact that the chemical equilibrium is achieved in times shorter than those tested in this research; above all, for the results obtained with higher acid concentrations. This behavior had already been observed in previous work, such as work conducted by Li et al. (2009), Lee et al. (2013), Yang et al. (2013) and Savvilotidou et al. (2015). These authors demonstrated that in the case of leaching of indium with mineral acids (HCl, H2SO4 e HNO3), the equilibrium is reached in a time less than 4 h [13,17,26,39].
Fig. 6. Indium extraction as a function of acid concentration and contact time, at room temperature.
Fig. 7. Indium extraction as a function of acid concentration and contact time, at 60 °C.
fact that the best results were obtained using H2SO4 at a concentration of 0.5 M, in 2 h. Under these conditions, it was possible to achieve values close to 98.2 ( ± 2.5)%, that is, 295 mg/kg. These results are compatible with those obtained by other authors. Virolainen et al. [33] reported the complete dissolution of indium in H2SO4 1 M. Li et al. [37] obtained rates of indium dissolution of 85 % for an acid concentration of 0.75 M, in 80 min and at a temperature of 65 °C. Wang et al. [37] concluded that the best conditions for indium leaching with H2SO4 is a concentration 0.6 M, leaching time of 42.2 min and a temperature of 65.6 °C. It is also possible to verify that the extraction of indium with HCl 5
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4. Conclusions [8]
After grinding in a ball mill a homogeneous powder with particle size around 5 μm was obtained. The grinding time to obtain this particle size was 4 h, however, with 1 h of grinding it was possible to obtain particles with a size of less than 10 μm. This shows that the grinding time used in this study (6 h) can be reduced. The best results from the tests using HNO3 were obtained for a concentration of 4 M, a leaching time of 4 h and heating at 60 °C. Under these conditions, it was possible to achieve rates of extraction close to 87 %. However, in general, the results obtained with HNO3 were lower than those obtained by using the others acids. The best results in tests using HCl were obtained using 6 M solution, in a leaching time of 4 h and at a temperature of 60 °C, with an extraction rate of 99.3 %. However, using H2SO4 0.5 M, in 2 h and without heating, made it possible to extract rate of 98.2 %. Considering the price of the acids used in this study, its toxicity to the environment and the workers involved the leaching time and the need for heating or not, H2SO4 presented the most promising results for the extraction of indium from LCD screens at a larger scale. Considering the high percentages of indium extraction obtained in this study and comparing with results obtained by others authors cited, it is possible to presume that removal of polarizing film before grinding samples did not seem to have influenced the results of indium extraction. The results obtained do not show a clear trend in relation to the variations of time, temperature and concentration of the leaching reagent.
[9]
[10]
[11] [12] [13] [14] [15] [16]
[17]
[18] [19]
[20] [21]
[22]
CRediT authorship contribution statement [23]
Adjanara P. Gabriel: Methodology, Investigation, Writing - original draft. Angela C. Kasper: Investigation, Writing - review & editing, Supervision. Hugo M. Veit: Supervision, Project administration, Funding acquisition.
[24] [25]
[26]
Declaration of Competing Interest
[27]
The authors declare do not have conflicts of interest in conducting and publishing this research.
[28]
Acknowledgements [29]
This study was supported by the Brazilian agencies Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul (FAPERGS), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and FINEP (for the resources obtained through the Sibratec Project).
[30] [31] [32] [33]
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