Leaching of indium from waste LCD screens by oxalic acid in temperature-controlled aciduric stirred reactor

Journal Pre-proof Leaching of indium from waste LCD screens by oxalic acid in temperature-controlled aciduric stirred reactor Yao Li, Nengwu Zhu, Xiaorong Wei, Jiaying Cui, Pingxiao Wu, Ping Li, Jinhua Wu, Yimin Lin

PII:

S0957-5820(19)31699-4

DOI:

https://doi.org/10.1016/j.psep.2019.10.026

Reference:

PSEP 1963

To appear in:

Process Safety and Environmental Protection

Received Date:

3 September 2019

Revised Date:

6 October 2019

Accepted Date:

22 October 2019

Please cite this article as: { doi: https://doi.org/ This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Leaching of indium from waste LCD screens by oxalic acid in temperaturecontrolled aciduric stirred reactor

Yao Li a, Nengwu Zhu a, b, c, d, *, Xiaorong Wei a, Jiaying Cui a, Pingxiao Wu a, b, c, d, Ping Li a, b, c, d,

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Jinhua Wu a, b, c, d, Yimin Lin a

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a School of Environment and Energy, South China University of Technology, Guangzhou 510006, China

of Ministry of Education, Guangzhou 510006, China

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b The Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters

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Recycling, Guangzhou 510006, China

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c Guangdong Environmental Protection Key Laboratory of Solid Waste Treatment and

d Guangdong Engineering and Technology Research Center for Environmental Nanomaterials,

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Guangzhou 510006, China

Corresponding author. School of Environment and Energy, South China University of

Technology, Guangzhou 510006, China.

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E-mail address: [email protected]

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Graphic abstract

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Highlights

Complete leaching of indium could be realized by the TCASR within 10 min.

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6 times of ITO glass powder was treated with 2/5 of oxalic acid by the TCASR.

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In3+ concentration maintained 20 min to facilitate the separation and extraction.

Abstract:

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

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Recovery of rare metal indium from waste LCD screens is important since its content is about 500 times of the original ore. Oxalic acid has excellent ability to leach indium from waste LCD screens, and how to achieve reactor scale operation is critical to promote indium recovery. In this

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study, a temperature-controlled aciduric stirred reactor (TCASR) was constructed to investigate the effects of operational conditions on the leaching efficiency of indium at reactor scale. Results showed that complete leaching was realized in 10 min under the conditions of 0.2 mol/L oxalic acid, 300 g/L of ITO glass powder dosage, 90℃ and 500 rpm of stirring speed, the consumption of oxalic acid decreased 3/5, dosage increased by 6 times and reaction time decreased 7/9 compared with the previous study. Meanwhile, maintaining duration of indium ion concentration achieved 20

min instead of decreasing immediately after reaching the peak, which was advantageous for actual application. The process kinetics model illustrated that the most important factor affecting the reaction rate was temperature. The potential reason of longer indium ion concentration maintaining duration was preliminarily speculated as a faster leaching of indium than calcium result in less coprecipitation caused by calcium oxalate. Therefore, the TCASR is applicable for leaching of indium from waste LCD screens by oxalic acid due to the higher leaching efficiency, less reagents

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consumption and stronger processing ability.

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Keywords: Waste LCD screens; Indium; Reactor; Leaching; Operational conditions.

1. Introduction

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As a kind of typical electronic waste, the amount of waste LCD screens increased quickly with the fast replace speed of electronic equipment (Zeng et al., 2018). It was reported that over

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150 million pieces of waste LCD were produced per year (Jowkar et al., 2018), approximately equals to 80000 tons at least, and the amount will grow strongly in the coming decades (Cucchiella et al., 2015; He et al., 2006; Hong and Choi, 2018). With such a huge abandonment, environment would be polluted and resources would be wasted if it could not be disposed properly (Kuong et

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al., 2019; Li et al., 2015; Zeng et al., 2017). LCD screen, mainly consisting of polarizing film, glass substrate, indium-tin-oxide (ITO) film and liquid crystal (Zhang et al., 2015), is an important component of electronic products such as television, computer and cell telephone. It was estimated that more than 70% of indium in the world were applied to produce ITO film (Işıldar et al., 2018) and no alternatives have been found in the market so far. Meanwhile, with the acceleration of electronic equipment replacement, the demand for indium is still increasing. However, the reserve

of primary indium is low and its concentration in the earth’s crust is only 50-200ppb (Ma et al., 2012). Indium has been listed as an important strategic resource by European Commission (European Commission, 2010) and has been strengthen to protect in many countries like America, Japan and China (Werner et al., 2015; Zhang et al., 2015). Therefore, it is of great significance to recover indium from waste LCD screens. Recovery of indium in waste LCD screens attracted worldwide attention in the past decade.

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The methods including physical method, pyrometallurgy, hydrometallurgy and bioleaching were developed. Physical method was simple and easy to operate, for example, after grinding the waste

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LCD screens for 80 min, 92.51% of the indium tin oxide particles were directly recovered by

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flotation without the usage of hazardous chemicals (Wang et al., 2018). Pyrometallurgy could obtain high purity products. Ma et al.(2012) developed an effective vacuum-chlorination separation

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method to recover 98.02% of indium with the indium chloride purity of 99.50% in 10 min at the conditions of 400℃, 0.09MPa and molar Cl/In ratio of 6. Zhang et al. (2017) applied the carbon

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fiber gained by pyrolysis of liquid crystal to the in-situ reduction of indium and greatly reduced the generation of secondary waste. Hydrometallurgy was the most widely applied method. Zeng et al.

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(2015) developed indium recovery by inorganic acid with MnO2 as oxidant under the optimized conditions of 100 g/L H2SO4, 50℃, 180 min retention time and 1:1 liquid-solid ratio, made great contribution to indium recycling and LCD industry. Souada et al. (2018) proposed sulfuric acid leaching strengthen by ultrasound and the indium leaching efficiency was increased by about 30%,

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it made the recovery of indium in waste LCD screens more economical. Cadore et al. (2019) used DEPHA to modified Nalon 6 nanofibers for the extraction of indium in mobile phone LCD screens, the material remained stable after 5 cycles. The reuse capacity of the nanofibers could contribute to the environmental and sustainability benefits of the process. Bioleaching was an emerging method in recent years with a mild process and low cost. Xie et al. (2018) found sulfur-using Acidithiobacillus isolated from aerobic activated sludge could be applied to the bioleaching of

indium from waste LCD screens with the 100% leaching efficiency. However, with the increasingly strict environment protection requirements, and from an economic perspective, a faster method without strong acid, high temperature and high pressure needs to be found. Oxalic acid is milder than inorganic acid while subsequent processing is easier. It was also applied in the leaching of metals including iron (Hu et al., 2017a; Martínez-Luévanos et al., 2011) and gallium (Liu et al., 2017; Zhou et al., 2019). Cui et al. (2019) proposed a leaching method of

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indium by oxalic acid and 100% of indium could be leached within 45 min under the conditions of 0.5 mol/L oxalic acid, 50 g/L of powder dosage and 70℃ in serum bottles. However, indium ion

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concentration decreased immediately after reaching the peak value (Cui et al., 2019), which would

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result in a loss of indium from the liquid in the follow-up operation. The decrease of indium ion concentration can be supposed as the side reaction of high concentration of oxalate ions, but a low

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oxalic acid concentration cannot bear a high dosage. So, the combination of oxalic acid concentration and powder dosage might further lower the concentration and usage of oxalic acid

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and maximize the processing ability. Meanwhile, in serum bottles, limited processing scale made the leaching system was easily affected by the surroundings such as evaporation of oxalic acid and

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insufficient heat transfer, a reactor may be a solution to the problems. In this study, we proposed a temperature-controlled aciduric stirred reactor (TCASR) for efficient leaching of indium by oxalic acid from waste LCD screens. Firstly, the effects of different operational conditions in TCASR were investigated and thereafter optimal conditions were

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determined. Then, the kinetic characteristics were analyzed and the most important factor affecting the reaction rate was determined. Finally, the reasons for efficient leaching of indium in TCASR and longer indium ion concentration maintaining duration were discussed.

2. Experimental 2.1. Materials The waste LCD screens were purchased in waste collection station of Guangzhou, China. ITO

glass substrate was gained after peeling off the metal frame and the polarizing film manually, then washing, drying and crushing were carried out successively to obtain ITO glass powder. After sieving, the powders were stored according to the different particle size range: >150 μm, 74-150 μm, <74 μm. The powders were completely dissolved by mixing hydrofluoric acid, nitric acid, and perchloric acid (Guangzhou Chemicals, AR, China) before the content of each element was measured by inductively coupled plasma optical emission spectrometer (ICP-OES, 730, Agilent,

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USA) (Table A1). The main metals contents were listed as shown in Table 1. Different concentrations of oxalic acid solution were obtained by dissolving oxalic acid (Aladdin, AR, China)

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in deionized water.

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2.2. TCASR

The reactor was consisted of a lid and a main body connected by screws. The lid and the main

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body was made of plexiglass and polytetrafluoroethylene to resist acid and heat environment, respectively. The inner diameter was 83 mm, which was 20 mm wider than the stirring rake blade

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in order to achieve sufficient stirring without touching the edges. The wall thickness was 5 mm for the stability of connection. The lid was 50 mm high with a hollow structure to facilitate the

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circulation of condensed water. There were three holes on the lid with the diameter of 20 mm, 8 mm and 22 mm for adding ITO glass powders, inserting mixer and inserting pH meter, respectively. The main body was 160 mm high, and two circular holes with a diameter of 10 mm were drilled in both sides where were 95 mm high above the bottom for the leachate collection. The schematic

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and physical figures of the reactor were shown in Fig.1. 2.3. Methods

Operational conditions optimization: The effects of oxalic acid concentration & dosage, temperature, stirring speed and particle size on indium leaching efficiency were examined in order. Firstly, the oxalic acid concentration gradient was set to 0.05 mol/L,0.1 mol/L, 0.2 mol/L and 0.3

mol/L. The dosage experiments were sequentially performed in the order of the oxalic acid concentration from small to large with the dosage gradient of 50 g/L, 100 g/L, 150 g/L, 200 g/L, 300 g/L and 400 g/L, respectively. For example, when the oxalic acid concentration was 0.05 mol/L, ITO glass powder dosage of 50 g/L, 100 g/L and 150 g/L were sequentially added, respectively. When the leaching efficiency decreased, the dosage experiment of the next oxalic acid concentration was carried out from the dosage. Continue carrying out the experiments as above-

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mentioned till the optimum combination of oxalic acid concentration and dosage was gained. Then,

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the optimal temperature, stirring speed, and ITO glass particle size were preferably selected successively under the optimal conditions determined in the previous step. Temperature, stirring

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speed, particle size gradients were set to 70℃, 80℃ and 90℃, 300 rpm, 400 rpm and 500 rpm, <74

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μm, 74~150 μm and >150 μm, respectively.

All leaching experiments were carried out as follows: 400mL of oxalic acid solution was

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added to the reactor body, then stirring and heating were started. Meanwhile, circulating cooling water was turned on while the temperature of the solution was monitored. When the temperature

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of the oxalic acid solution reached the set value, ITO glass powders were added through the powder inlet. Leachate was collected at 1, 3, 5, 10, 15, 30, 60, and 90 min of the reaction and it immediately filtered through a 0.45 μm pinhole filter and diluted 10 times with 5% (v/v) nitric acid. The indium ion concentration of the leachate was measured by atomic absorption spectroscopy (AAS, AA6880,

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Shimadzu, Japan) and leaching efficiency was calculated as Eq. (1). At the same time, leaching residue was collected, washed with pure water three times, dried by freeze drier and observed by scanning electron microscope (SEM, SU8220, Hitachi, Japan). 𝑚

𝜂 = 𝑚 × 100% 0

(1)

Where η is leaching efficiency, m is the total amount of indium contained in the leachate, and

m0 is the total amount of indium contained in the powder added. Precipitation analysis: The ITO glass powder was added into the TCASR to react for 10, 20 and 30 min under the conditions of 0.2 mol/L oxalic acid, 300 g/L dosage, 90℃ and 500 rpm, and then subjected to solid-liquid separation, respectively. Next, the liquid was transferred into a beaker placed in a 90℃-water bath and sealed to stop evaporating of liquid and then continued stirring at 500 rpm. Then, precipitate was collected at 20 (for the liquid separated at 10 min), 30 (for the liquid

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separated at 10 and 20 min), 60, 90 min. After that, the beaker was taken out and placed for 2 days

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and 5 days to observe the formation of precipitate. The precipitate was collected by centrifugation at 11,000 r/min for 10 min, washed by deionized water three times, and dried by freeze dryer. The

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dried precipitate was weighed for its yield, and its composition was examined by X-ray

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diffractometer (XRD, Empyrean Rheology, Parnco, The Netherlands).

3.1. Operational conditions

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3. Results and discussion

Oxalic acid concentration & dosage: The effect of oxalic acid concentration & dosage on

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indium leaching efficiency was shown in Fig.2. When the oxalic acid concentration was greater with the same dosage, the faster the leaching rate and the higher upper limit of the dosage appeared. Conversely, when the dosage was larger with the same concentration of oxalic acid, the leaching rate tended to be lower. Differently, when the dosage was large enough to exceed the leaching

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ability of oxalic acid, indium leaching efficiency decreased due to hydrolysis. Interestingly, at 0.2 mol/L & 200 g/L and 0.2 mol/L & 300 g/L, the indium leaching efficiency decreased after reaching the maximum and maintaining for a period, but the phenomenon did not occur at 0.1 mol/L & 200 g/L. Meanwhile, when the oxalic acid was more concentrated, the maximum indium leaching efficiency of the same dosage was lower instead (Table A2). Therefore, the decrease was not

considered to be caused by the insufficient leaching ability and it would be explained below. The maximum leaching efficiency of 0.2 mol/L & 200 g/L and 0.2 mol/L & 300 g/L reached 98.37% and 95.45% at the 15 min, respectively. Both of its leaching efficiency maintained above 95% at 10-30 min. In terms of dosage, 0.2 mol/L & 400 g/L was obviously not suitable for the dosage of 400 g/L exceeded the leaching capacity of 0.2 mol/L oxalic acid (Fig.2c). The maximum leaching efficiency of 0.2 mol/L & 300 g/L was slightly lower than that of 0.2 mol/L & 200 g/L,

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but its dosage was 1.5 times of the latter. It meant that the total amount of indium leached at 0.2

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mol/L & 300 g/L was much more than that at 0.2 mol/L & 200 g/L although the oxalic acid consumption was the same. In terms of oxalic acid consumption, 0.2 mol/L oxalic acid could bare

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the dosage of 300 g/L, which was obviously that 0.1 mol/L oxalic acid could not achieve (Fig.2b).

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A higher concentration was not suitable because if it was increased to 0.3 mol/L or higher, indium ion concentration maintaining duration would be shortened and indium loss would be increased. In

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summary, 0.2 mol/L & 300 g/L was a superior combination of oxalic acid concentration & dosage. Temperature: The effect of temperature on the indium leaching efficiency was shown in

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Fig.3a. It was suggested that the temperature suitable for indium leaching in waste LCD screens was 60-90℃ (Li et al., 2009; Li et al., 2011; Rocchetti et al., 2015) since a too low temperature would result in a slow reaction while a too high temperature might result in the boiling of the solution and the decomposition of oxalic acid. At 90℃, the leaching efficiency reached 91.79% at

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5 min, which was much higher than 80℃ (71.11%) and 70℃ (55.98%), and the maximum leaching efficiency of 90℃ (95.45%) was also higher than 80℃ (94.41%) and 70℃ (83.29%). Meanwhile, at 90℃, the maintaining duration of indium ion concentration maintained above 90% was also longer than that of 80℃ and 70℃. It meant that if 90℃ was applied as the leaching temperature, not only more and faster indium could be leached, but also longer time for the next process operation could be obtained. Therefore, 90℃ was considered as the optimal leaching temperature.

Stirring speed: Stirring was favorable for sufficient contact between the reactants, but its speed should be suitable to avoid problems such as excessive liquid level in the reactor and energy consumption. Leaching efficiency under different stirring speed was shown in Fig.3b. The leaching efficiency at 500 rpm maintained at 100% from 10 to 30 min and was significantly higher than that at 400 rpm and 300 rpm. If 500 rpm was applied as the stirring speed, the indium loss from liquid would be effectively avoided and resulted in greater economic benefits. Therefore, 500 rpm was

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chosen as the best stirring speed. Particle size: The powders of different particle size ranges have different compositions after

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once crushing (Liu et al., 2017; Yang et al., 2018), which was also confirmed in this study (Table

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1 and Table A1). When the particle size was suitable, the collision chance of the reactant was larger (Swain et al., 2018). However, the too small particle size also caused the decrease in collision

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probability of the reactant, leading to a slow leaching speed and low leaching efficiency. Thus, <74 μm (-200 mesh) was the particle size that often be selected in the leaching process (Hu et al., 2017b;

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Xie et al., 2013; Zhang et al., 2019; Zhang et al., 2018). After sieving, three particle size ranges (<74 μm, 74-150 μm and >150 μm) of ITO glass powders were obtained. The leaching efficiency

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of each group under the preferred conditions was shown in Fig.3c. The leaching efficiency change trend and indium ion concentration maintaining duration of the samples with different particle sizes were basically identical, but the maximum leaching efficiency of 74-150 μm and >150 μm was lower than that of <74 μm.

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In summary, the preferred operational conditions for oxalic acid leaching of indium in TCASR were determined as 0.2 mol/L oxalic acid, 300 g/L of dosage, 90℃, 500 rpm of stirring speed and <74 μm of particle size. 100% leaching of indium could be realized in 10 min and the maximum indium ion concentration could maintain from 10 to 30 min. Compared with results from serum bottle scale experiment (Fig.A1), completely leaching of indium was still achieved in 10 min although the oxalic acid concentration was reduced 3/5. Meanwhile, the leaching time was

shortened by 7/9 and the dosage increased 6 times. In TCASR, the leaching process has faster speed, lower cost and stronger processing ability. After reaching the leaching efficiency peak, leaching efficiency of indium could be stable for 20 min instead of decreasing immediately, which was advantageous for subsequent process operation. The progress made by TCASR were beneficial to relevant enterprises to obtain greater economic benefits in actual production, which would improve the enthusiasm of enterprises to

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participate in the recycling of waste LCD screens. The digestion of the accumulated amount of

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waste LCD screens would be accelerated, resulting in a win-win situation of economic benefits and environmental protection.

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3.2. Process kinetics

The leaching reaction could be described as five steps (Abali et al., 2006; Chi et al., 2006): (1)

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the leaching agent diffused to the surface of the particle through the diffusion layer (outer diffusion);

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(2) the leaching agent continued to diffuse through the solid film (inner diffusion); (3) the leaching agent reacted with metals in the particle; (4) the soluble product passed through the solid film(inner

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diffusion); and (5) the soluble product diffused into the liquid phase (outer diffusion). The total reaction rate depended on the slowest of the processes above. The controlled model could be determined according to the experimental data in TCASR fitted by the kinetic models Eq. (2) to (5).

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Chemical reaction controlled:

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1 − (1 − 𝜂)3 = 𝑘1 𝑡

(2)

Outer diffusion controlled： 1

1 − (1 − 𝜂)3 = 𝑘2 𝑡 Inner diffusion controlled:

(3)

2

2

1 − 3 𝜂 − (1 − 𝜂)3 = 𝑘3 𝑡

(4)

Mixed controlled: 1

𝑘 𝑘

1 − (1 − 𝜂)3 = 𝑘 1+𝑘2 1

2

𝐶0 𝑀 𝛾0 𝜌

𝑡

(5)

Where k1, k2 and k3 are rate constants for different control steps; η is the leaching efficiency; c0 is the initial concentration of oxalic acid; r0 is the initial diameter of ITO glass powder and ρ is

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the density of it; M is the mass of ITO glass powder. After deformation, draw the figure of 1 − 2

2

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(1 − 𝜂)3 to t and 1 − 𝜂 − (1 − 𝜂)3 to t, respectively. It was chemical reaction controlled or 3 1

outer diffusion controlled if 1 − (1 − 𝜂)3 was in line with t, and it was inner diffusion controlled 2

2

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if 1 − 3 𝜂 − (1 − 𝜂)3 was in line with t. If both of they were not in line with t, it was mixed

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according to the slope of fitting line.

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controlled (Hu, 2017). k value of the leaching process affected by each factor could be obtained

The kinetic equation could be described as Eq. (6):

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−

ⅆ𝑚 ⅆ𝑡

= 𝑘′𝑇 𝑎 𝐶 𝑏 𝑊 𝑐 𝑡

(6)

Where 𝑘 ′ is chemical reaction rate constant, T is reaction temperature(K), C is oxalic acid concentration (mol/L), W is stirring speed (rpm), a, b and c are apparent reaction orders, t is reaction time (min). The k value at each temperature could be calculated, assuming k was proportional to

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the power function of T, that was, lnk=B1+alnT, a and B1 could be obtained by the slope and intercept of the fitting line. Similarly, there were lnk=B2+blnC and lnk=B3+clnW, and B2, b, B3 and c could be calculated. Finally, by substituting the data of each group and find 𝑘 ′ , the kinetic equation could be established. Effect of temperature: According to the formula deformation, the experimental data was calculated and fitted. The fitted line of the effect of temperature on leaching efficiency and fitting

degree were shown in Fig.4a and 4b and Table A3, respectively. It was obvious that the fitting 2

2

1

degree of 1 − 3 𝜂 − (1 − 𝜂)3 was higher than that of 1 − (1 − 𝜂)3 and the fitted line was more in line with the principle that the kinetic curve intersected at the origin. So, the kinetic equation 2

2

could be expressed as 1 − 3 𝜂 − (1 − 𝜂)3 = 𝑘𝑡. It could be calculated according to Fig.4b that the k values at 70℃, 80℃ and 90℃ were 0.01138 min-1, 0.01962 min-1 and 0.04096 min-1, respectively

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(Table A3). The picture of Arrhenius was shown in Fig.A2, and the calculated Ea was 66.234kJ/mol. Effect of oxalic acid concentration: The fitted line of the effect of oxalic acid concentration 2

2

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on leaching efficiency was shown in Fig.4c and 4d. The fitting degree of 1 − 3 𝜂 − (1 − 𝜂)3 was 1

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still higher than that of 1 − (1 − 𝜂)3 (Table A3), which demonstrated the reliability of the inner diffusion controlled model. From Fig.4d, k value at 0.05 mol/L, 0.1 mol/L and 0.3 mol/L were

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calculated as 0.01575 min-1, 0.02136 min-1 and 0.03367 min-1, respectively.

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Effect of stirring speed: Fitted line on effect of stirring speed was shown in Fig.4e and 4f. It also illustrated the characteristics of the reaction in accordance with the inner diffusion controlled model (Table A3). From Fig.4f, k value at 300 rpm, 400 rpm and 500 rpm were calculated as

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0.03790 min-1, 0.04096 min-1 and 0.04577 min-1, respectively. Kinetic equation: Linear fitting of lnk to lnT, lnC and lnW was shown in Fig.A2. It could be calculated from the figure that a=22.5911, B1=-136.4012; b=0.4231, B2=-2.8790; and c=0.3647,

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B3=-5.3611 (Table A4). Substituting the values of a, b and c into the formula Eq. (7), and finding k' as 5.6614×10-62

2

2

− 3 𝜂 − (1 − 𝜂)3 = 𝑘 ′ 𝑇 𝑎 𝐶 𝑏 𝑊 𝑐 𝑡

(7)

In summary, kinetic equation of leaching process can be described as Eq. (8): 2

2

1 − 3 𝜂 − (1 − 𝜂)3 = 5.6614 × 10−52 𝑇 22.5911 𝐶 0.4231 𝑊 0.3647 𝑡

(8)

Which means the reaction was more in line with the inner diffusion controlled model. In the serum bottle system, the effect of oxalic acid concentration on the reaction rate was considered to be small (Cui et al., 2019). Based on the above results, influences of temperature, oxalic acid concentration and stirring speed on the reaction rate were quantified, illustrating that temperature was the most influential factor in the reaction rate. The effect of oxalic acid concentration on the reaction rate was slightly larger than that of stirring speed, but they were both very small compared

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with that of temperature.

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3.3. Reasons for higher efficiency of TCASR

The high efficiency of TCASR compared with the results from serum bottle scale experiment

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was reflected in two aspects. On the one hand, under the more severe conditions of lower oxalic

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acid concentration and higher dosage, the indium leaching efficiency was improved up to 100%. On the other hand, the indium ion concentration could maintain for 20 min after complete leaching

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instead of immediately decreasing in serum bottle, which provided sufficient time for solid-liquid separation and recovery of indium products from the liquid. The possible reasons are given as

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follows.

3.3.1. Higher leaching efficiency by TCASR Compared with the results of serum bottle scale experiment, TCASR showed higher efficiency and stronger processing ability, which greatly enhanced the feasibility of oxalic acid leaching for

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indium in waste LCD screens. The reasons for its higher efficiency might be attributed to the following four points. (1) Intense mixing conditions: Due to the larger reaction space of the TCASR, the reactant mixing conditions were greatly improved by the action of the stirring blade (Panda et al., 2016), which promoted the forward progress of the reaction. (2) Higher mass transfer: The application of the stirring blade simultaneously improved the mass transfer conditions (Zhu et al., 2008), which accelerated the reaction rate and improved the reaction efficiency. (3) Sufficient

power resource: Due to the presence of the condensing system, the reaction temperature could be successfully reached 90℃. It provided sufficient energy for the reaction since the leaching process was endothermic. And (4) Particle abrasion: The intense collision between the particles due to vigorous stirring resulted in a decrease on the particle size, which might cause microcracks on the particles (Lorenzen et al., 1997), thereby promoting the progress of the reaction. The phenomenon was also confirmed by the SEM investigation.

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3.3.2. Longer maintaining duration of indium ion concentration Under the optimal conditions, indium in the ITO glass powder was completely leached within

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10 min in the TCASR and the indium ion concentration maintaining duration reached 20 min. The

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20 min maintaining duration provided great convenience for subsequent operations compared with the immediate decreasing in serum bottle scale experiment. It was noticed that the indium ion

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concentration maintaining duration was not the identical under different conditions. Herein, the reasons for the decreasing in indium ion concentration and the different maintaining duration were

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analyzed.

It was noticed that when the dosage was 200 g/L, the maximum leaching efficiency achieved

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by 0.2 mol/L oxalic acid was lower than that of 0.1 mol/L oxalic acid and the indium ion concentration even decreased from the 60 min on. Oppositely, in 0.1 mol/L oxalic acid, the leaching efficiency constantly maintained above 97% after the 30 min. When the oxalic acid concentration was increased to 0.3 mol/L, the maximum leaching efficiency was also found to be lower than that

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of 0.2 mol/L. Moreover, the decrease of leaching efficiency, appeared after the maximum leaching efficiency, was heavier and occurred earlier (Table A2). In any case, the decrease in indium ion concentration must be attributed to the action of the anion. One was the dosage exceeded the oxalic acid leaching ability, resulting in the formation of metal hydroxide precipitates. The other was the action of other anions. In the system, the only “other anions” were HC2O4- and C2O42-. As the reaction proceeded, HC2O4- would finally convert to C2O42-. Since 200 g/L did not exceed the

leaching capacity of 0.1 mol/L oxalic acid, it would not exceed that of 0.2 mol/L and 0.3 mol/L. Therefore, the phenomenon was caused by C2O42- rather than OH-. In order to clarify the determinant factor of the phenomenon was the absolute concentration of oxalic acid or the amount of oxalic acid relative to the powder, the leaching efficiency of 0.1 mol/L oxalic acid & 50 g/L dosage and 0.3 mol/L oxalic acid & 200 g/L dosage was compared (Table A5). Amount of oxalic acid relative to the powder was higher in the former (0.002mol/g) than that in the later

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(0.0015mol/g), but the decrease in indium leaching efficiency did not appear in the former. It indicated that the determinant factor of the decrease in indium ion concentration was the absolute

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concentration of oxalic acid instead of amount of oxalic acid relative to the powder.

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The side effects of oxalate ion lead to a decrease in indium ion concentration, but oxalate ions do not easily form precipitates directly with indium. It was speculated that the decrease in indium

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ion concentration was caused by coprecipitation of other metal oxalates. The original sample and leaching residue were collected and analyzed by SEM as shown in Fig.5a, 5b and 5c. It revealed

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that there was a spherical substance formed by aggregation of flaky particles in the late stage of leaching, which did not appear in the original state and the early stage of leaching. It was speculated

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that this globular substance might formed by the precipitate of oxalate. It might adsorb indium since the precipitate had many pores and might have adsorption capacity (Huang and Huang, 2018; Shi et al., 2018). Simultaneously, particle size was observed to gradually decreased from SEM image, and it was in accordance with the particle abrasion theory.

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Under the optimal conditions, the indium ion concentration decreased after 30 min, so the concentration of indium and other metals which were easy to form precipitation with oxalate ion (including calcium, barium, iron, strontium, chromium and magnesium) at the 30, 60, and 90 min were measured (Fig.5d). It was found that only the concentration of calcium and indium decreased. Therefore, it was speculated that coprecipitation of calcium oxalate and indium was occurred. Thus, it was considered that oxalic acid might cause a decrease in indium ion concentration

in the late stage by forming calcium oxalate. In order to verify this hypothesis, the leaching experiments were carried out in the TCASR according to the optimal conditions. Solid-liquid separation was carried out at 10, 20, and 30 min when the indium leaching efficiency was 100%. At the 20, 30, 60, 90 min, 2 day and 5 day, it was observed whether precipitate was generated in the liquid, and the precipitated component was analyzed. The separated liquid was clear, and for the liquid separated at 10 min, no significant precipitation was observed by the naked eye at the 20

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min (Fig.A3), while other groups of liquids produced different amounts of precipitation at different time. The precipitation was almost completed after 2 day, no new precipitate was produced on 5

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day. Overall, the earlier the separation time, the less the precipitation; the longer the time, the

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slower the precipitation formed (Fig.5e).

However, the amount of precipitation produced in 10-30 min was much lower than that of

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after 30 min, that was the reason why the concentration of indium could maintain stable during 1030 min. As shown in Fig.6, XRD pattern confirmed that the main component of the precipitate was

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calcium oxalate according to PDF#99-0107. By calculation, the average particle size of the precipitate was 0.44 μm, which also meant there might be adsorption of indium during the calcium

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oxalate formation process.

Therefore, it was considered that the decrease in the indium ion concentration might be attributed to the precipitation of calcium oxalate. The reason for the longer indium ion concentration maintaining duration might be that the indium was leached faster than calcium. At

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10 min, indium was completely leached while the calcium concentration was still not high enough to form calcium oxalate. At 30 min, calcium concentration was high enough and calcium oxalate began to form, while indium ion concentration decreased. Compared with the serum bottle scale experiment, indium was completely leached earlier, resulting in a longer indium ion concentration maintaining duration. It could be seen from Fig.5e that in order to prevent the further leaching of

calcium to reduce the amount of calcium oxalate precipitate, the solid-liquid separation should be carried out as soon as possible after the indium was completely leached. To reduce the amount of calcium in the liquid would prolong the indium ion concentration maintaining duration and provide convenient condition for subsequent operations.

4. Conclusions

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In this study, the TCASR was applied to explore the optimal operational conditions of indium leaching from waste LCD screens by oxalic acid. Results showed that 100% indium leaching

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efficiency was achieved within 10 min in TCASR at 0.2 mol/L oxalic acid, 300 g/L of dosage, 90°C, 500 rpm of stirring speed and <74 μm of particle size. Compared with the results from serum

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bottle scale experiment, TCASR reduced 3/5 of oxalic acid consumption, 7/9 of reaction time and

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increased 6 times of dosage. The reaction was inner diffusion controlled and most affected by 2

2

temperature according to the kinetic equation of leaching process: 1 − 3 𝜂 − (1 − 𝜂)3 =

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5.6614 × 10−52 𝑇 22.5911 𝐶 0.4231 𝑊 0.3647 𝑡. The indium ion concentration maintaining duration reached 20 min, which was industrially advantageous. Co-precipitation by calcium oxalate was

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the reason why indium ion concentration decreased at the later phase. The longer duration was attributed to the completely leaching of indium superior to the leaching of calcium and the formation of calcium oxalate. In brief, the TCASR is potential applicable for leaching of indium

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from waste LCD screens by oxalic acid. Declaration of interest None.

Conflict of interest We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

Acknowledgements The authors highly appreciate the financial support from Guangdong Science and Technology Project (2017A020216013) and Guangzhou Science and Technology Project (201604020055), and

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ro

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the equipment support from South China University of Technology.

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in low-grade manganese ore. Chinese J. Geochem. 32(2), 222-226. https://doi.org/10.1007/s11631-013-0625-3

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particle size on uranium leaching from coal ash. J. Radioanal. Nucl. Chem. 317(2), 801-810. https://doi.org/10.1007/s10967-018-5963-5 Zeng, X., Mathews, J.A., Li, J., 2018. Urban mining of e-waste is becoming more cost-effective than virgin mining. Environ. Sci. Technol. 52(8), 4835-4841. https://doi.org/10.1021/acs.est.7b04909 Zeng, X., Wang, F., Li, J., Gong, R., 2017. A simplified method to evaluate the recycling potential

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light emitting diodes by oxalic acidic leaching. Resour. Conserv. Recy. 146, 366-372. https://doi.org/10.1007/s12613-018-1649-9 Zhu, G., Zhang, H., Zhao, Y., 2008. A novel recirculating reactor for the leaching of magnesia by

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lP

re

-p

ro

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CO2. Hydrometallurgy 92(3-4), 141-147. https://doi.org/10.1016/j.hydromet.2008.01.011

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Fig.1. Schematic and physical figures of the reactor.

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1.Condensate water inlet , 2. Condensate water outlet, 3. pH probe, 4. Powder inlet, 5. Stirring rake, 6. Sampling hole, 7. pH meter.

Fig.2. Leaching efficiency of indium under different oxalic concentration: (a) 0.05 mol/L, (b)0.1 mol/L, (c)0.2

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mol/L

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Fig.3. Leaching efficiency under different conditions: (a) Different temperature; (b) Different stirring

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speed; (c) Different particle size

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Fig.4. The fitted line of the effect of factors on leaching efficiency: (a), (b) Temperature; (c), (d) Oxalic

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acid concentration; (e), (f) Stirring speed

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Fig.5. Analysis and characterization of precipitates: (a)SEM image of original sample; (b) SEM image of

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leaching residue at 15 min; (c) SEM image of leaching residue at 60 min; (d) Concentration changes of different

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lP

metals; and (e) Amount of precipitation

Fig.6 XRD pattern of precipitation

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lP

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-p

Fig. A1. Comparison between TCASR and serum bottle scale

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Fig. A2. Fitting calculation of related parameters of kinetic equation

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Fig. A3. Leachate after solid-liquid separation at 10 min: (a)Clarify liquid at 20 min (b) Precipitate

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produced at 30 min

Table 1. Main metals content of ITO glass powder in different particle size Contents(mg/kg) Element 74~150 μm

>150 μm

In

534.59

308.47

155.53

Cr

1502.19

879.42

295.06

Mg

6293.12

3164.80

2504.93

Fe

16323.82

6969.84

1602.00

Ba

22361.39

20542.75

19672.61

Sr

24313.61

21212.82

19586.35

Ca

40325.05

41523.29

ro

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<74 μm

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lP

re

-p

37277.36

Table A1. Element content of ITO glass powder in different particle size

Content (mg/kg) Element

74-150μm

＞150μm

W

15.25

9.139887

5.885746

Ni

23.91

12.35853

7.976594

P

46.46

27.81579

Ti

52.64

55.19328

Mn

57.23

32.9269

Cu

68.63

34.70717

Mo

76.22

52.15037

29.4337

Zn

171.01

40.3844

23.82

Zr

209.68

125.9306

125.6737

K

229.03

69.11136

92.0769

Na

520.05

12688.37

12108.77

B

461.63

24017.96

20904.53

In

534.59

308.4747

155.5291

Sb

585.58

562.9574

412.8336

590.75

665.5098

567.9551

981.14

1193.022

1088.441

1297.56

3099.102

3050.707

1502.19

879.4162

295.0592

Mg

6293.12

3164.795

2504.929

Fe

16323.82

6969.844

1602.005

Ba

22361.39

20542.75

19672.61

Sr

24313.61

21212.82

19586.35

Ca

40325.05

41523.29

37277.36

Al

44255.30

8833.25

8404.781

S As

47.901

ro

47.40807

-p

re

lP

Jo

Cr

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Sn

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<74μm

17.93239 15.3352

Table A2. leaching efficiency of indium under dosage of 200g/L with different oxalic acid concentration

Leaching efficiency (%) at different time (min) 1

3

5

10

15

30

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Oxalic acid 60

90

33.74 65.74 81.34 92.67 92.77 97.03 97.93 100.00

0.2mol/L

59.53 86.21 93.66 96.88 98.37 97.60 83.94 80.67

0.3mol/L

50.31 83.45 91.90 95.53 96.81 92.87 76.05 72.22

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lP

re

-p

ro

0.1mol/L

2

R2

0.9726

y=0.01138x-0.00492

0.9893

0.9713

y=0.01962x-0.00533

0.9965

0.9414

y=0.04096x+0.00287

0.9865

y=0.04534x+0.0392

0.9792

y=0.01575x-0.00743

0.9845

y=0.05606x+0.0747

0.9199

y=0.02136x+0.00391

0.9772

0.3 mol/L

y=0.09545x+0.05423

0.9488

y=0.03367x+0.00090

0.9994

300 rpm

y=0.10398x+0.06033

0.9331

y=0.03790x+0.00351

0.9829

400 rpm

y=0.11002x+0.05988

0.9414

y=0.04096x+0.00287

0.9865

500 rpm

y=0.119312x+0.06238

0.9564

y=0.04577x+0.00223

0.9993

y=0.03695x+0.03585

80 ℃

y=0.05263x+0.05215

-p

90 ℃

y=0.11001x+0.05988

0.05 mol/L

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Jo

ro

70 ℃

Oxalic acid 0.1 mol/L concentration

Stirring speed

2

Fitting formula of 1 − 3 𝜂 − (1 − 𝜂)3

R2

Fitting formula of 1 − (1 − 𝜂)3

re

Temperature

1

Group

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Affecting factor

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Table A3. Fitting process in kinetic analysis

Table A4. Kinetic model parameter fitting calculation Fitting result

R2

-lnk to 1/T

-lnk=7966.5737/T-18.7033

0.9797

lnk to lnT

lnk=-136.4012+22.5911lnT

0.9823

lnk to lnC

lnk=-2.87897+0.42314lnC

0.9995

lnk to lnW

lnk=5.36107+0.36467lnW

0.9698

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na

lP

re

-p

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Fitting project

Table A5. leaching efficiency of indium under dosage of 0.1 mol/L oxalic acid, 50 g/L dosage and 0.3 mol/L oxalic acid, 200 g/L dosage Leaching efficiency (%) at different time (min) Oxalic acid concentration & dosage 1

3

5

10

15

30

60

90

47.28 77.55 89.11 100.00 100.00 100.00 100.00 100.00

0.3mol/L & 200g/L

50.31 83.45 91.90 95.53

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0.1mol/L & 50g/L

92.87

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na

lP

re

-p

96.81

76.05

72.22