Evaluation on end-of-life LEDs by understanding the criticality and recyclability for metals recycling
Graphical abstract
Introduction
Light-emitting diodes (LEDs) have been displacing traditional lighting sources (incandescent, compact fluorescent lamp, etc.) in many fields owing to their energy-saving, long lifetimes, environmental friendly and other properties (Krames et al., 2007, Singh and Tan, 2016, Zhao et al., 2016). They are widely used in display screens, backlights for liquid crystal displays and cellular phones signage, and general lighting (Nair and Dhoble, 2015, Sujan et al., 2017, Xie and Hirosaki, 2007), and the distribution of LED applications for various sectors in 2014 is shown in Fig. S2 (Swain et al., 2016). For reducing environmental burden and saving energy, many countries and regions (e.g. Australia, BRIC countries (Brazil, Russia, India, and China), Japan, South Africa, European Union and United States) have been implementing legislations to phase out incandescent and promote the use of LED lamps (UNEP, 2017). However, with the expansion of LEDs markets, large quantities of end-of-life LEDs will generate owing to their limited life spans and rapid updating of electronic products. Although there exist no public statistics on the production of end-of-life LEDs, the reports from “Strategies Unlimited 2016” show that the revenues of LED devices grow with an annual rate of 4.5%, and they will exceed 18 billion dollars in 2020 (CASA, 2016) (shown in Fig. 1), which implies the inevitable fast increasing trend of end-of-life LEDs in future.
The chip, as core structure of a LED, contains considerable amounts of valuable and deleterious metals (i.e. gallium (Ga), indium (In), gold (Au) and arsenic (As)), and may also contain rare-earth elements (REEs) (europium (Eu), cerium (Ce), yttrium (Y) and gadolinium (Gd)) in phosphor powder (Bessho and Shimizu, 2012, Gereffi et al., 2008). Ga, In and REEs are important strategic resources and often classified as critical raw resources owing to their low reserves in natural mineral ores, lack of substitutes and high supply risk (DOE, 2011, EU, 2010, EU, 2015).
However, the concentration of Ga, In and REEs in the earth's crust are about 15–17, 50–100 and 150–220 parts per million, respectively (Verhoef et al., 2010). The primary products including Ga and In are by-products of other metals (i.e. aluminum (Al), iron (Fe)) (Moskalyk, 2003, Phipps et al., 2008, Takahashi et al., 2009, Zhao et al., 2012). Additionally, the European Commission has forecasted that the demand growth for Ga, In and REEs will exceed 8%, 5%, 4% per year from 2013 to 2020, respectively (EU, 2015). Based on the strategic importance of industrial development and insufficiency of natural resource, several countries and regions (such as American, Japan, Korea and European Union) have embarked on a campaign to restrict the mining, smelting and exporting of Ga and In. Meanwhile, China, as the biggest share of the global market of REEs, has also carried out a rare earth products export quota policy from 2009 to 2015 for controlling the mining, smelting and exporting. These restrictions were implemented as a result of serious pollution and ecological destruction to local areas in China, caused by producing REEs (Tan et al., 2015). To compensate for the gap between the growing demands and restricted supply of these metals (Ga, In and REEs), many countries and regions have given significant focus on the secondary resources (E-waste).
End-of-life LEDs are identified as potential resources of Ga, In and REE, and meanwhile also classified as hazardous wastes in Europe Union, American and Canada, according to their perilous impact on human life and environment (details are listed in Table S1). The metals contained in these LEDs, especially the toxic heavy metals (As, Fe, copper (Cu), etc.), may threaten environmental (soil and underground water pollution) and health of human beings and animals. Several severe arsenic contaminations (Julander et al., 2014, Pradhan and Kumar, 2014, Yao et al., 2008) occurred in the disassembled place of electronic waste, which will also pose threats to public health and environment. Some considerable researches (Lim et al., 2011, Lim et al., 2013) have been devoted to identifying the potential environmental impact of LEDs. These papers investigated the resource depletion and toxicity potentials of end-of-life LEDs based on their metallic constituents. In addition, organic compounds, such as brominated flame-retardants (BFRs), used in the transparent plastic housing of LEDs, belong to persistent organic pollutants (POPs) which are difficult to degrade.
Therefore, final destiny of LEDs will be an important worldwide environmental concern. Landfilling or incineration is not a good treatment way because the stable structure and properties of the materials (e.g. metal, epoxy) contained in end-of-life LEDs cannot be changed. Therefore, these materials may pose additional threats to the environment through leaching into food and generating hazardous gases (Robinson, 2009, Walser et al., 2012). As for future development of LEDs, it is crucial to reduce environmental pollution and energy consumption from fountainhead. The best way is green design and manufacturing. Several excellent reviews (Reuter and Schaik, 2015, Reuter et al., 2015, Schaik and Reuter, 2014) have investigated a product-centric approach based on software simulation to design for recycling (DfR) and resource efficiency (DfRE) by rigorous calculations of materials recycling rates. These studies have given many fundamental rules and simulation derived guidelines to green design and manufacturing of LED. In addition, life cycle assessment (LCA) method has been also applied to assess corresponding environment impacts and sustainable resource utilization from the whole lifecycle including manufacturing, transport, installation, use and end-of-life (EOL) (Franz and Nicolics, 2015, Hendrickson et al., 2010, Tahkamo et al., 2013). But these papers cannot explain how to effectively deal with the existing end-of –life LEDs and depletion of natural resources.
Ga, In and REEs recycling from end-of-life LEDs is considered as an effective option for reducing their potential environmental pollution and compensating supply risk of these critical metals. Several previous literature (Murakami et al., 2015, Swain et al., 2015a, Swain et al., 2015b, Zhan et al., 2015) have reported recycling methods (hydrometallurgy, mechanochemical activation, vacuum metallurgy, etc.) of critical metals contained in waste LEDs, however, these studies and methods have focused more on recycling of individual metals, relatively low recycling rate and potential secondary pollutants emission. How to design an effective process of closed-loop at the same time with high economic and technological viability still requires intensive improvements on understanding the nature and technology situation for treatment of end-of-life LEDs. Evaluation based on composition and characteristics of materials contained in E-waste could give guidance for recycling and subsidy through assessing the recycling potentials and difficulty. Several paper (Glöser et al., 2015, Helbig et al., 2016, Zhang et al., 2017a, Zhang et al., 2017b) focused on the metals criticality through considering supply risk, economic importance, vulnerability and environmental impact of metal. Zeng et al. (Zeng and Li, 2016) evaluated the recycling difficulty (or recyclability) of E-waste from technologic perspective through combining entropy and grading of materials, and Sun et al. (2016) evaluated the recycling potential using criticality from economic perspective. However, they only consider one aspect and various types of E-waste. For end-of-life LEDs as a new type E-waste, which have many types of metals but low concentration, they did not consider. The aim of the study is designed to carry out a sustainable evaluation on end-of-life LEDs treatment on the basis of understanding the criticality (Sun et al., 2016) and recyclability (Zeng and Li, 2016). In this research, considering the composition and characteristics of end-of-life LEDs, the recycling potential and recycling difficulty of critical metals contained in end-of-life LEDs are systematically evaluated. Moreover, the middle values as boundaries of various factors are established, which is more suitable for metals recycling from end-of-life LEDs, and optimal physical treatment and chemical process for metals recycling from end-of-life LEDs can be determined through comparison of cost profitability and availability of different processes in order to design a process of high industrial viability.
Section snippets
Establishment of sustainable evaluation framework
End-of-life LEDs as a new type of E-waste contain many valuable materials (such as Ga, In, REEs), meanwhile, some deleterious materials (i.e. heavy metal and organics) also appear which are hazardous to human beings and environment, suggesting that better management systems of end-of-life LEDs should be developed. (Reck and Graedel, 2012). Materials recycling has already been recognized as the best choice for treating E-waste in recent decades and it is well established that no single process
Criticality of different types of end-of-life LEDs
Considering of economic importance and recycling processing costs, criticality is a feature that describes the potential of materials recovered from economic perspective. Generally, high values of the RI indicate that the E-waste is highly important to economic development for a nation or region and it may increase collection rate of E-waste from consumers. The high RI values of the waste LEDs will promote retails and manufacturers to collect end-of-life LEDs efficiently, which could reduce the
Conclusions
End-of-life LEDs need to be paid more attention owing to ascending resource scarcity and environment burdens. Materials recycling is the best option, and depends on many factors. In the paper, a systematic evaluation method combining criticality with recyclability has been developed from economic and technologic perspectives to determine the recycling potentials of end-of-life LEDs. For criticality, supply risk and economic importance of metals suggest their importance level to industrial
Acknowledgement
The work was financially supported by the National Key Research and Development Program of China (2017YFB0403300/2017YFB043305), Beijing Science and Technology Program (No. Z171100002217028), the National Natural Science Foundation of China (Nos. 51425405 and L1624051), and Key Program of Chinese Academy of Science (KFZD-SW-315). The authors would like to thank Ruud Balkenende from TU Delft for his suggestions.
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