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Characterization and recycling potential of the discarded cathode ray tube monitors

https://doi.org/10.1016/j.resconrec.2021.105469Get rights and content

Highlights

  • Characterization and mass balance for different fractions of cathode ray tube (CRT).

  • Removal of coating material from CRT glass using ultrasonic testing.

  • Evaluation of the thermal response of phosphor for recovery of rare earth metals.

  • A brief review of current CRT recycling processes.

  • Investigation of overall metal recovery and calculations for discarded CRT's.

Abstract

Abundantly available discarded cathode ray tubes are a serious threat to the environment and loss to resource conservation and therefore requiring immediate recycling efforts. In this study, extensive characterization of all parts of the discarded cathode ray tube was carried out using X-ray diffraction, thermo-gravimetric, X-ray photoelectron spectroscopy, Scanning electron microscope, and Energy Dispersive Spectroscopy. The detailed mass balance and the overall potential (market value calculations) of discarded cathode tubes as an urban source, and the techno-economic calculations were carried to provide guidelines for the development of recycling technology. Recycling techniques of cathode-ray tubes were reviewed, and a concise flow sheet for overall recycling was prepared for evaluation and assessment of recycling potential. The thermal response and dissociation kinetics of the phosphor were investigated, and the activation energy in the temperature range (600–930 °C) for thermal processing of phosphor was determined as 105.4 kJ/ mol. Effective recycling of cathode ray tube phosphor can yield 4.5 g of Y, Eu and, La metal values per 500 kg of discarded units with a market value of 835 INR and an overall market value of 115,870 INR ($1567.34). The different fractions of cathode-ray tube glass were recycled using a combination of the pyrometallurgical and hydrometallurgical processes for metal recovery. Recycling of different fractions of hazardous discarded cathode ray tubes is necessary and is beneficial for a circular economy for recovery of phosphor, copper, glass and coating material, plastics, and steel.

Introduction

The ever increasing consumption of electronic and electrical devices due to technological advancements resulted in a massive accumulation of approximately 41 million tons per year of electronic waste (E-waste) and a significant contribution (47%) from waste cathode ray tubes (CRT) (Singh et al., 2016; Yoshida et al., 2016). However, a minimal amount of E-waste is recycled, for instance, 12.5% per year in India because of unawareness and inadequate collection system (Ravindra and Mor, 2019) and the absence of buyback policies, which leads to massive piling of waste (Parajuly and Wenzel, 2017). Nowadays, the CRTs are increasingly replaced by thin film transistor liquid display, light-emitting diode panel, and plasma display panel to overcome issues such as heavy, poor image quality, higher energy consumption, and environmental considerations (Chen et al., 2009), leading to massive accumulation of CRT waste (6.3 million tons in 2014 globally (Singh et al., 2016)). Worldwide only 26% of waste CRT is recycled because of a lack of know-how, heterogeneous nature, and limited practical recycling approaches (Herat, 2008; Singh et al., 2016; Xu et al., 2012).

The key elements of CRT are network oxide formers (SiO2: 50–66, Sb2O3: 0.25–0.45, As2O3: 0.01–0.02, ZrO2: 0.25–1%); intermediates (Al2O3: 2.2–4, PbO: 19–30, BaO: 8–12, ZnO: 0.05–0.6, TiO2: 0.05–0.15) and network modifiers (Na2O: 6–7, K2O: 6–8%, Li2O: 0–0.5%, CaO: 0–4, MgO: 0–2, Fe2O3: 0.05–2, SrO: 6–10, CeO2: 0.1–0.3%) (Xu et al., 2012; Yoshida et al., 2016; Yot and Méar, 2009). The CRT attributes to 65% of the weight of the TV or monitor and comprises mainly 85% glass (Andreola et al., 2007; Ravi, 2012). CRT glasses are mainly classified in panel glass (Ba-Sr glass), neck, and funnel glass (high lead glass) (Guo et al., 2010). In color CRT, the front screen (panel glass) is coated with a matrix of thousands of small fluorescent or phosphor dots (pixel; red, blue, and green blubs) which emit light on excitation/ acceleration with electron stream in a vacuum (1.33 × 10−5) (Andreola et al., 2007; Yot and Méar, 2009). The presence of toxic elements such as Pb, Ba, and Sr in CRT is a serious concern due to adverse impact on the environment and human health (Guo et al., 2010; Ippolito et al., 2016; Yoshida et al., 2016). Therefore, discarded CRTs must be recycled in a proper manner. Recycling of discarded CRTs usually involves a closed-loop and open-loop recycling. In the closed-loop recycling process, waste CRTs were returned to the manufacturer for re-use; whereas, pyrometallurgical and hydrometallurgical treatment was carried out for metal recovery in open-loop recycling (Ciftci and Cicek, 2017). It is more convenient to adopt open-loop CRT recycling because of obsolete CRT technology. CRT glasses are difficult to treat because of inhomogeneous composition depending upon the manufacturer (Yoshida et al., 2016; Yot and Méar, 2009). Waste CRT glasses possess reasonable strength (Menad, 1999), which can be used for the production of foam glass (Guo et al., 2010), table glass, insulation, ceramics, cement, and concretes (Liu et al., 2020; Yao et al., 2018) and as a non-plastic substitute in ceramic frits for brightness and chemical resistance because of Ba, Sr, Zr and Pb content (Andreola et al., 2007). Additionally, smelting of a crushed funnel and panel glass for glass wool manufacturing (Yoshida et al., 2016) and flux substitute for Pb smelting process can be preferred over inadequate land-filling; however, the process is expensive (Menad, 1999). The energy input/consumption for CRT glass manufacturing is quite high, and the energy should be reduced by the implementation of an effective recycling process (Socolof et al., 2005). A CRT has 1–7 g of phosphor powder coating containing rare earth and was generally not considered in CRT recycling studies but is an important urban source for recovery of critical rare earth elements (Yin et al., 2018). Additionally, recycling of RE will also aid the critical issues such as RE price fluctuation and delivery shortage (Fröhlich et al., 2017)

Previous studies mainly focused on the recycling of CRT glass, with very few studies on CRT phosphor recycling. There were no exclusive studies on various CRT parts, such as metal extraction from the gun or printed circuit boards (PCBs). Systematic characterization and mass balance of different CRT fractions can provide the fundamental guidelines for the development of a complete CRT recycling process. Some studies on the economic assessment of CRT TV focusing on waste PCB (Cucchiella et al., 2015) and funnel glass (Xu et al., 2013) were reported. However, a systematic overall potential value (market value evaluation) and techno-cost analysis for the recycling of discarded CRT was not investigated. The evaluation of the overall distribution of valuable and toxic metals in discarded CRT is important for the development of an effective recycling strategy, environmental considerations, and value addition to the circular economy. Therefore, in this study, the economic importance of discarded CRT as secondary resources is evaluated through market value calculation; and techno-cost calculation was carried out for evaluation of the economic feasibility of recycling of discarded CRTs. Also, detailed insight with a brief review of existing techniques is reported in pursuit of strategy for the development of the recycling process for waste CRTs. The main objectives of the present study are (a) characterization of different fractions of discarded CRTs, (b) mass balance, (c) mini-review of existing recycling technology and overall proposed flow sheet, (d) evaluation of the thermal response and dissociation kinetics of phosphor, (e) market value calculations for discarded CRTs as an urban source.

Section snippets

Materials and methods

Discarded monitors (50 units) were procured from the institute computer center. The study is carried out using 50 units of discarded monitors of the same manufacturer (Sun Microsystems, Inc.; Model: DP17MO; manufactured 2001–2003). Monitors were manually dismantled inside a laboratory fume hood to separate the different parts such as plastic cover, wires, PCB, back protection cover (steel), and CRTs. Electron gun/yolk and copper-binding of CRT were removed, and then funnel glass was broken. CRT

Mass balance and characterization

Different fractions recovered from the discarded system shown in Fig. 2 reveals CRT, PCB (printed circuit board), steel (protection cover, screws), plastics (cover, wire insulations, stand). The components of CRT comprise panel screen, funnel glass assembly, steel ring, copper-binding, and electron gun, as shown in the schematic of CRT (Fig. 2). The funnel glass assembly comprises easily removable brown coating (termed as a brown powder in this study) and carbon coating, which is difficult to

Conclusion

The present study investigates the CRT as a potential secondary resource for different metal values, glass, and plastics. The overall characterization, mass balance, and market value of complete CRT (all fractions) were investigated with a brief review of existing recycling techniques. A discarded CRT mainly comprised of Si: 25.3, Pb: 5.3, Sr: 5.3, Ba: 5.7, Cu: 3.4, Al: 1.2, Na: 4.2, Al: 1.2, K: 5.5, C: 5.4%, O: 37.4, rare earth elements: 0.05% and other minor elements (V, Ga, Cr, S, Mg).

CRediT authorship contribution statement

Sanjay Pindar: Methodology, Writing - original draft. Nikhil Dhawan: Conceptualization, Supervision, Validation, Writing - original draft.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge the funding received from the Indian Institute of Technology, Roorkee, under Faculty Initiation Grant, and Institute Computer Center and Institute Instrumentation Center for providing the obsolete CRTs and characterization facilities, respectively. Thanks are also due to Sachin Chopra and Riya Meena for their practical help.

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