Research Article pubs.acs.org/journal/ascecg
C, H, Cl, and In Element Cycle in Wastes: Vacuum Pyrolysis of PVC Plastic To Recover Indium in LCD Panels and Prepare Carbon Coating Lingen Zhang and Zhenming Xu* School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China S Supporting Information *
ABSTRACT: Resource recycling of millions of solid waste is a worldwide problem. Because the element is the basic unit of all wastes, the essence of wastes recycling is to realize cycle of elements in waste. In this paper, a new concept of element cycle in waste is proposed. Under the guidance of the element cycle, various wastes have a common connection point. The concept is clarified by a case of C, H, Cl, and In element cycle from waste poly(vinyl chloride) (PVC) plastic and liquid crystal display (LCD) panels. In this paper, a vacuum pyrolysis of waste PVC was proposed to recycle indium in LCD and simultaneously prepare a carbon coating. Cl in PVC, combined with In in LCD, was mainly regenerated in the form of InCl3. Meantime, C and H in PVC were reused to prepare a carbon coating and energy products such as pyrolysis oil and gas. The recycling system means to change two types of waste into resources by their own element characteristics. Under optimized conditions, the recovery rate of indium exceeded 98%. Meantime, with increasing temperature, the microstructure of carbon coating is gradually clear. The formation mechanism of the carbon coating was presented in detail. This study is meaningful for the sustainable development of resources recycling in waste. KEYWORDS: element cycle, wastes recycling, vacuum pyrolysis, PVC plastic, LCD panels
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INTRODUCTION With the progress of technology, a variety of solid wastes are generated in modern life. From the perspective of resources recycling, solid wastes are still resources, because they contain valuable materials, such as metals. Currently, most of the treatment technologies for waste focus on the recycling of valuable resources in single waste. By physical, chemical, and biological means, these valuable resources can effectively be recycled.1−5 However, most researchers are not concerned about these components of no recycle or low reuse in waste. Hence, some secondary pollution problems inevitably occur. Various wastes have their own chemical compositions, and the smallest unit of chemical compositions is their elements. The essence of the reaction between different chemical compositions is the migration and transformation of elements. Hence, if elements in waste achieve cycle, wastes recycle would work themselves. In recent years, there are many reports about using the characteristics of a certain waste to treat/recycle the other waste.6−8 For example, LiCO3 and graphite are mixed electrode materials in lithium ion batteries. Using the reduction reaction of cathode material of graphite and anode material of LiCO3 from the lithium ion battery can achieve Li metal recycling.7 In essence, “waste + waste → resources” is the reuse of elements from wastes, namely, elements in various wastes can regenerate via their migration and transformation to achieve waste recovery. This concept can be called “elements © 2017 American Chemical Society
cycle in waste”. This concept is described in Figure 1. Elements in nature are developed and utilized into products and eventually become waste. Various wastes are connected by elements. Restructuring and recombination of elements can occur under certain conditions to form new chemical compounds. New chemical compounds are recycled to achieve the return of elements. Using basic elements in waste to achieve whole resources recycling of waste from the viewpoint of the element cycle is meaningful for the sustainable development of waste recycling. This paper examines the C, H, Cl, and In element cycle in two types of waste to realize whole resource recycling of waste PVC. Poly(vinyl chlroide) (PVC) plastic has become one of the largest usages of plastics worldwide, because of its good performance and low price. The global consumption of PVC was expected to be 43 million tons in 20159 and will increase by 3.2% per annum until 2021.10 The tremendous use of PVC has produced a large amount of waste plastics, resulting in serious environmental problems and global issues. Many studies have focused on the recycling of waste PVC, which mainly includes primary recycling, such as re-extrusion and secondary recycling, including mechanical and chemical recycling.11−13 However, Received: June 1, 2017 Revised: August 13, 2017 Published: August 15, 2017 8918
DOI: 10.1021/acssuschemeng.7b01737 ACS Sustainable Chem. Eng. 2017, 5, 8918−8929
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. Conceptual model of resource recycling in waste by element cycle.
cycle in wastes? By C, H and Cl element cycle achieve whole resource recycling of waste PVC. Indium, as an irreplaceable element in transparent tin-doped indium oxide (ITO) electrode, has been widely used in LCDs, because of its merits of photoelectric advantage.29,30 Worldwide, ∼70% of indium is used to produce ITO thin films in LCD panels, according to a report of the U.S. Geological Survey.31 However, indium resources are extremely scarce; they represent only 1/6 of that of the gold reserve.32 Indium usually occurs in trace and minor amounts in various associated types of minerals, such as a substitute for Zn in sphalerite.33 The average content of In in sphalerite is
