Hydrothermal Treatment of E-Waste Plastics for Tertiary Recycling: Product Slate and Decomposition Mechanisms

Abstract

Amounts of e-waste plastics have been one of the fast growing global waste streams and threaten to grow into an unmanageable problem. This phenomenon needs to be solved urgently by efficient and cost-effective ways. In this study, a novel hydrothermal treatment technology was implemented to convert the e-waste plastics into organic products which can be used as monomers of plastic production or chemical feedstock. We systematically investigated the recovery efficiencies of organic products, the product slate, the possible hydrothermal degradation mechanisms, and the microstructure of solid residues. The results showed that the yields of organic products derived from four kinds of e-waste plastics ranged from 81.4 to 97.6 wt % at 350 °C. The recovered products contained styrene monomers, styrene derivatives, bisphenol A (BPA), caprolactam (CPL), and other valuable commodity chemicals. On the basis of the systematic analysis of hydrothermal organic products, the possible degradation mechanisms were proposed which involved a variety of reactions such as depolymerization, hydrothermal cracking, hydrolysis, nucleophilic substitution, and free radical reaction in it. The degradation mechanisms provided the theoretical basis for the hydrothermal treatment of e-waste plastics. All of the results demonstrated that hydrothermal treatment is a viable and prospective method for sustainable recycling of e-waste plastics.

Screening of common synthetic polymers for depolymerization by subcritical hydrothermal liquefaction

“…Single synthetic polymers subcritical HTL has been reported for specific materials in several publications, including reports on HTL of: high-impact polystyrene, poly-acrylonitrile-butadiene-styrene (ABS), polycarbonate and polyamide 6 (Iwaya et al, 2006;Zhao et al, 2018a); epoxy printed circuit boards (Yildirir et al, 2015); polyethylene naphthalate and terephthalate (Arai et al, 2010;Zenda and Funazukuri, 2008); polystyrene-butadiene (Park et al, 2001); polyurethane (Dai et al, 2002). These studies show that monomers, other valuable chemical compounds or an oil product may be recovered using HTL.…”

Thermochemical Conversion of Plastic Waste into Fuels, Chemicals, and Value‐Added Materials: A Critical Review and Outlooks

“… Plastics feedstock used for HTL conversion to Fuel (MP: microplastics; FRPs: fiber‐reinforced plastics; RB: natural rubber; PLA: polylactic acid; PPO: polyphenylene oxide; PMMA: poly(methyl methacrylate); POM: polyoxymethylene; PVA: polyvinyl alcohol; PBT: polybutylene terephthalate) [5d,160a,b,e,g,165] …”

Investigation on Characteristics of Liquid and Hydrochar Products from Hydrothermal Carbonization of Oily Scum at Different Temperatures

“…Meanwhile, the pit structure was also continually corroded and the residual organic structure in hydrochars was destroyed via decomposition reactions such as hydrothermal cracking and hydrolysis [23]. Therefore, with the effect of reaction temperature, petroleum hydrocarbons in oily scum took place a variety of reactions such as hydrothermal cracking, hydrolysis, depolymerization, oxidation, and rearrangement [24]. Finally, a pit structure formed which followed the previous research [13].…”