Research Article pubs.acs.org/journal/ascecg
Controllable Formation of Carbon Fiber in Pyrolysis Process of Liquid Crystals from Waste LCD Panels and Indium Recovery by Vacuum in Situ Reduction with Carbon Fiber Lingen Zhang, Ya Chen, and Zhenming Xu* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China S Supporting Information *
ABSTRACT: The contradiction of energy, resource, and environment from waste LCD panels is prominent because they contain both harmful liquid crystals and the valuable resource indium. However, traditional whole crush technology is difficult to realize sustainable recycling of waste LCD panels due to a challenge from the low concentrations of liquid crystals and indium. Secondary pollution inevitably occurred in recycling process. In this work, a stripping product enriched in liquid crystals and indium was first gained by a mechanical stripping separation. Then, liquid crystals were reused as carbon source in pyrolysis process to form carbon fiber in control. Carbon fiber clusters can form well under the optimal condition of 500 °C, with 30% molecular sieve (particle size < 0.3 mm) and pyrolysis time of 15 min. The pyrolysis mechanism of liquid crystals and the formation mechanism of carbon fiber were discussed. Finally, vacuum in situ reduction of indium oxide from stripping product with the prepared carbon fiber can occur to recycle indium. Carbon fiber had better reduction performance and was characterized by SEM-EDS and BET surface area. The innovative recycling process achieves minimization of secondary waste and provides a new opportunity for sustainable recycling of waste LCD panels. KEYWORDS: Waste liquid crystal panels, Pyrolysis, Vacuum in situ reduction, Carbon fiber, Indium
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INTRODUCTION Since the end of the 20th century, flat-panel display technology is developing rapidly. Liquid crystal display (LCD) panels have been widely used in various electronic products due to the advantages of light quality, small volume, and low power consumption. Every year, more than 700 million LCD panels are produced worldwide.1 Considering the average lifespan of 3−8 years of LCD panels,2 there are enormous quantities of LCD panels reaching their end of life in the coming years, which is a tremendous contribution to electronic wastes. Figure 1 shows the structure of LCD panels. LCD panels mainly contain polarizing film, glass substrate, and liquid crystals, indium−tin oxide (ITO) conductive film, and so on.3,4 Liquid crystals are a mixture containing various chemical compositions, which usually have about 10−25 compounds with benzene, cyanogroup, fluorine, esters, and cyclohexyl.5−7 ITO in LCD panels presents a sintered mixture of indium oxide (In2O3) and tin oxide (SnO2) in a 90:10 atomic ratio.8 Obviously, LCD panels contain valuable resources, such as indium and glass substrates, but waste LCD panels are also hazardous wastes because they contain organic matter and heavy metals, such as liquid crystals, arsenic (As), lead (Pb), indium (In), and tin (Sn), which can lead to environmental problems if released into the soil or river.9,10 © 2017 American Chemical Society
Current studies mainly focused on indium recovery from waste LCD panels. For recycling technologies, most studies used a whole crush process for waste LCD panels to treat liquid crystals and recycle indium.11−14 However, there are still several defects: (a) Capacity of treatment is vast due to using whole crush process from waste LCD panels. (b) Efficiency and recovery rate of liquid crystals and indium are very low due to their low concentration which has only about 0.3−0.5 wt % liquid crystals and 300−500 g/t indium in a piece of LCD panel. (c) In the process of recycling indium, a large amount of extraction solvents were used, resulting in serious secondary pollution. (d) Some valuable materials such as polarizing film and glass substrates are unable to be reused. On the basis of the above problems, enrichment of liquid crystals and indium is important in the recycling of LCD panels. Hence, in this paper, a mechanical stripping separation was first adopted to achieve enrichment of liquid crystals and indium (as shown in Figure 1). This process is significant in that it not only realized enrichment of liquid crystals and indium but also achieved capacity reduction of wastes. Received: August 16, 2017 Revised: October 23, 2017 Published: November 13, 2017 541
DOI: 10.1021/acssuschemeng.7b02828 ACS Sustainable Chem. Eng. 2018, 6, 541−550
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Structures of LCD panels and exploration of separation and recycling of waste LCD panels.
Table 1. Comparison of Content of Main Components from a Piece of LCD Panel and Stripping Product before and after Mechanical Stripping Process
Hence, an idea of enrichment−separation of waste LCD panels was proposed. In this study, we adopted first a mechanical stripping process to gain a stripping product enriched in liquid crystals and indium. Then, for the enriched products, how to achieve effective and environmentally friendly separation and recycling of indium becomes a new problem. Existing studies about recovery of indium from waste LCD panels mainly focused on the hydrometallurgical processes to extract indium. Technologies include acid leaching, solvent extraction,15,16 supercritical extraction,17 selective precipitation,18 and so on. However, using various solvents, including corrosive acid and hazardous extraction substances, can increase potential environmental risks. Vacuum metallurgy has been widely used in nonferrous metal recovery, such as scattered metals germanium19,20 and gallium.21 Its principle is that the saturation vapor pressure of metals under the vacuum condition is lower than normal pressure to separate them. They easy to evaporate into the gas phase, according to the principle of vacuum metallurgy. Vacuum technology does not need secondary off-gas or wastewater treatment and can be considered a complementary approach to hydrometallurgy.22 In our previous work, we have studied indium recycling by vacuum carbon reduction. Under the condition of vacuum of 1−10 Pa and temperature of 950 °C and with extra addition of 30 wt % coke, indium can be effectively recovered,23 and it is easy to find that the stripping product gained by mechanical stripping process itself contains a rich carbon source, namely, it is enriched in liquid crystals. In addition, before recycling indium in the stripping product, liquid crystals should be removed in advance because it is disadvantagous for recycling of indium that liquid crystals with ITO film closely link together. Can we make full use of these liquid crystals to make them act as a carbon source by the indium reduction reaction? Hence, an idea of controllable
formation of carbon in the stripping product is presented. In this paper, a molecular sieve (MS) is selected as the fixative of carbon. For removal of liquid crystals from technology, pyrolysis has proved to be a clean thermochemical technology for recycling organics compared with combustion and solvent leaching, due to its advantages of avoiding the use of solvents, relatively low environmental damage, and high separation effect for organics.24,25 Meanwhile, due to the closed oxygen-free system, the technology could effectively avoid secondary pollution. According to precious study, a vacuum environment can promote the volatilization and internal diffusion of the products in the pyrolysis process, which attenuates the secondary reactions of repolymerization.26,27 Hence, the aim of this study is that use pyrolysis of stripping product to separate and immobilize organic matters from liquid crystals by adding 13X MS; meanwhile, we achieve indium recycling by further vacuum in situ reduction process. In the process, the factors of controlling formation and content of carbon in MS are discussed. Pyrolysis mechanism and formation of carbon fiber coated MS from liquid crystals are analyzed in detail. Finally, the vacuum in situ reduction process of indium is carried out to evaluate recovery efficiency of indium. In summary, it gives some valuable information for whole resource recycling of waste LCD panels.
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MATERIALS AND METHODS
Materials. Waste LCD monitors were collected in local waste household appliance collection facilities. First, manual dismantling of waste LCD monitors was adopted to gain LCD panels. And then, LCD panels were used by mechanical stripping process to gain the stripping product containing ITO (Indium tin oxide). The mechanical stripping process included deforming, separation of LCD panels, mechanical stripping of roller brush apparatuses with water washing, filtration with a bag filter and drying at room temperature. By process of deforming 542
DOI: 10.1021/acssuschemeng.7b02828 ACS Sustainable Chem. Eng. 2018, 6, 541−550
Research Article
ACS Sustainable Chemistry & Engineering
Figure 2. Schematic illustrations of pyrolysis and vacuum in situ reduction apparatus of laboratory scale.
Figure 3. (a) Effect of particle size of MS, (b) temperature of pyrolysis, (c) system pressure, and (d) additional MS on total content of carbon in residue and content of pyrolytic carbon in MS. Apparatus. The experiments were carried out in a laboratory-scale tube furnace which consists of furnace body, control panel, condensate collector, and vacuum pump, as shown in Figure 2. The quartz tube furnace consisted of the body of furnace (chamber dimensions is Ø40 mm × 600 mm), quartz tube reactor (Ø35 mm × 900 mm), vacuum pump (the rate of exhaust is 1 L/s), and a temperature controller. The middle part of the furnace was the heating zone, and the end of furnace was the condensing zone. The temperature from the middle of heating zone to both sides gradually declined. After the quartz tube was sealed, the oil-less vacuum pump could make the tube maintain vacuum environment, the degree of vacuum of which could be adjusted by regulating the vent control valve. Pyrolysis oil could be collected in oil collector meantime gas could be collected in gas collector. Exploratory Experiment. Before a typical run, the stripping product and 13X MS was pulverized with different particle sizes. And
and separation of LCD panels, a piece of LCD panel was split into two pieces of glass substrates. Then, the liquid-crystals-enriched stripping product, ITO and a small amount of glass was scraped from the two pieces of glass substrates through the mechanical grind of roller brush. This mechanical stripping process was shown in Figure S1. One ton of waste LCD panels can produce 3−4 kg of stripping products. At present, its capacity of treatment is 3−4 tons/day for treating waste LCD panels. By this process, two pieces of glass substrates, polarizing film, liquid-crystals-enriched stripping product, ITO, and a small amount of glass was gained. A comparison of a piece of LCD panel and stripping product before and after mechanical stripping process are shown in Table 1. After this mechanical stripping process, the content of indium in the stripping product has been enriched 265 times, compared with the 300 mg/kg content of indium in a piece of LCD panel. Likewise, liquid crystals are also enriched from 0.3 wt % in a piece of LCD panel to 53 wt %. 543
DOI: 10.1021/acssuschemeng.7b02828 ACS Sustainable Chem. Eng. 2018, 6, 541−550
Research Article
ACS Sustainable Chemistry & Engineering
Figure 4. SEM analysis for MS with
