Chemical Recycling of Electronic-Waste for Clean Fuel Production

6.2.1 Plastics in Electronic-Waste

Plastics are the derivatives of petrochemicals which are derived from petroleum fuels (OIL 2008). Plastics are the synthetic resources comprising of macromolecular composites; they can be recycled back to their raw assets under appropriate process parameters. Figure 6.2 elaborates the composition of plastics. Plastics account for nearly 20% of total waste electrical and electronic equipment plastics, 5% of flame-retarded and 15% non-flame-retarded plastics. More than 15 types of plastics make up waste electrical and electronic equipments, namely, polyesters, polyurethane, acrylonitrile-butadiene-styrene, polyethylene, polyamide, polypropylene, polystyrene, styrene acrylonitrile, etc. (Vilaplana and Karlsson 2008).

6.2.2 Electronic-Waste Management Issues

Technical innovation and economic development in developing countries made them produce a huge amount of electrical and electronic equipments (Hossain et al. 2015). Also Electronic-waste management was the huge headache for countries, since they are produced or imported as used items (Nnorom and Osibanjo 2008b). Due to low income and middle income, Electronic-waste is disposed of in unsanitary landfill sites. Wires are burned down to recover copper in it. Printed circuit boards are acid extracted to retrieve gold, platinum, palladium, and silver coated in them. These kinds of activities are seen in developing countries like India, China, Pakistan, Nigeria, and Ghana; somewhere they lack the facility to safeguard health and environment (Leung et al. 2006; SEPA 2011). Seitz (2014) revealed Electronic-waste was a rising concern among developing countries toward the environment and public health.

6.2.3 Worldwide Electronic-Waste Generation

6.2.4 Electronic-Waste on Environmental Public Health

Several types of research were under progress to study the effect of Electronic-waste on the environment. Xue et al. (2015) reported the impact of formal recycling of printed circuit boards on the environment. Fujimori et al. (2012) reported the enhancement factors, dangerous indicators, and concentration of metals present in soil due to proper and improper recycling of Electronic-waste. Mostly, major studies are carried on focusing on the emissions from improper recycling of wastes. Some studies are conducted on assessing the effect of Electronic-waste on health.

6.3.1 Chemical Recycling

6.3.2 Mechanochemical Treatment

Mechanochemistry-mechanical and multiphase characteristics need higher energized mills with various operating parameters like density, shear, and impact (Balaz et al. 2013). Figure 6.4 shows the mechanochemical treatment method for clean fuel production from Electronic-waste. Parameters influencing milling practice were the mill type, processing materials, ball to powder ratio, filling chamber, processing speed and time, etc. (Balaz 2008). Recently studies reveal that the mechanochemistry method was used for degrading solid wastes and recycling of plastics (Guo et al. 2010). Advantages of mechanochemical treatment process than conventional methods were an uncomplicated process, ecological safety, and product that can be recovered in a meta-stable state. Acrylonitrile-butadiene-styrene polymers and high-impact polystyrene and styrene copolymers are the various types of engineering polymers present in polymeric composites (16 wt% of waste electrical and electronic equipments). Thermochemical recycling of plastics yields monomers and fuel under pyrolytic conditions (Grause et al. 2011).

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A new methodology of bromine removal from styrene polymers was reported in literature (Grause et al. 2015). Decabromodiphenyl ethane was removed effectively through NaOH/ethylene glycol solution in moderate environment (150–190 °C) in a ball mill reactor. Debromination is done by substituting hydroxide or elimination of hydrogen bromide. Once decabromodiphenyl ethane is removed, the residue is suitable for mechanical recycling. After the removal of brominated organic compounds, the quality of fuel products is upgraded and their environmental effects are decreased.

Dehalogenation of solid plastics is increased by adding additives during mechanochemical treatment process. Generally, additives like alkali metal oxides (calcium oxide, sodium hydroxide), iron powders, and quartz (silica) are employed as catalytic adsorbents. Since, these materials are unsustainable, novel method was developed through simultaneous grinding of plastics in the midst of sustainable resources (e.g., biopolymers, biowastes, eco-friendly minerals) through mechanochemical treatment for clean fuel synthesis was elaborated in Fig. 6.4.

6.3.3 Hydrothermal Process

Thermal conduct of waste electrical and electronic equipment plastics emerged as a suitable skill by degrading organo-bromine compounds, also in situ and secure exclusion of bromine constituents from the oil products. The supercritical fluid technology has emerged as a potential technique for chemical recycling of plastic wastes. Supercritical fluids act as a better chemical medium under optimum conditions for depolymerization, hydrolysis, hydrogenation, and dehydrogenation with properties like low viscosity, low dielectric content, high mass transport coefficient, and higher diffusivity (Shibasaki et al. 2004; Zhang et al. 2013).

Hydrothermal treatment was preferred for clean fuel production because of its higher efficiency than pyrolysis of biomass and sewage waste (Yu et al. 2016; Shen et al. 2016). Reactor corrosion and higher energy utilization is the only drawback in supercritical fluid method (Guo et al. 2009). Selection of appropriate supercritical fluid and enhancers, price and operating parameters, etc. are the common challenges faced during the hydrothermal treatment process.

Hydrothermal process is of two major types: (i) hydrothermal liquefaction and (ii) hydrothermal gasification . The quality of final product is decided by the operating parameters and environment (Yan et al. 2010). Dehalogenation by hydrothermal treatment method has been studied in recent days for plastics compounds (Starnes 2012). Solid fuel properties are significantly increased by unification of biomass through hydrothermal conditions.

6.3.4 Pyrolysis

Pyrolysis is an environment-friendly and economically feasible technique for waste electrical and electronic equipment plastic treatment than landfilling and incineration. Emission of toxic gases into the environment is lesser than incineration process (Bhaskar et al. 2002). Pyrolysis is an efficient method of valorizing Electronic-waste which can recuperate the valuable compounds with low emission of pollutants into the atmosphere. Plastic wastes are converted into fuel by fast pyrolysis since it is a promising technique for protecting the environment from these nondegradable plastics. During pyrolysis, plastics are thermally degraded and rehabilitated into oil, gaseous, and charred products at (700–900 K) in an inert atmosphere. Pyrolysis generates bio-oil with high bromine and chlorine content (Lopez et al. 2011).

Pyrolysis was carried out as single-step cracking process under the closed environment. Fixed, fluidized bed and tubular reactors are used for pyrolysis process and divided into three major processes based on process parameters like conventional, fast, and slow pyrolysis (Wu and Williams 2013). Fluidized bed reactor acts as a better heat and mass transfer equipment yielding thin-layered plastic, suggesting the polymer degradation. Fast and slow pyrolysis methods follow the finest route for converting brominated flame-retardant plastics onto clean fuels and valuable products than conventional pyrolysis process. Recently, a study revealed that printed circuit boards pyrolysis resulted in a higher content of bromine, glass fibers, and metals (Copper) (Shen et al. 2018).

6.3.5 Combustion Process

Combustion of fossil fuels was replaced by biomass and wastes for energy and heat generation. This process is a technically feasible method to reduce harmful greenhouse gases (carbon dioxide) into the environment. However, replacing conventional fossil fuels ended up generating a huge amount of ash-related problems (slagging, corrosion, and fouling). Alkali metal usage can overcome these problems (Hansen et al. 2000). Brominated fuel possesses a promising effect on volatilization of metals like potassium, iron, copper, zinc, and lead (Vehlow et al. 2003). Halogen hydrides and small-chain halogenic organic compounds were resulted from decomposition of organic halogenated compounds. Chlorinated plastics (waste electrical and electronic equipments, polyvinyl chloride, textiles) and halogen hybrids (hydrogen chloride, hydrogen bromide) were the chief products produced through combustion method (Wu et al. 2014).

Brominated flame retardants containing wastes generate polybrominated dibenzo-p-dioxins and polybrominated dibenzofurans through the course of combustion process (Wang and Zhang 2012). Under thermal conditions they are involved in the recycling process. Polybrominated diphenyl ethers act as a substrate for production. Insufficient combustion process or disturbed process leads to fire accidents, uncontrolled burning, and gasification.

6.3.6 Gasification Process

Pyrolysis under elevated temperature generates fuels (oil, gas) possessing higher heating value. Liquid fuels produced from circuit boards through pyrolysis at 800 °C in static temperature conditions possessed brominated compounds; this made them unusable without further downstream processing (William and Paul 2007). Partial oxidation of waste electrical and electronic equipment plastic at an elevated temperature (1200 °C) decreased the brominated or chlorinated dioxins in gas products. Nevertheless the halogen compounds in gaseous products were not in permissible limits for use as fuels. Majority of organo-brominated compounds in brominated flame retardants are broken down into hydrogen bromide and bromine at higher temperature due to their fundamentals (Jin et al. 2011). Usage of calcium oxide deliberately increases the inorganic bromine formation from the organic bromine compounds. Burning circuit boards at elevated temperature effectively breaks down organo-brominated compounds.

Steam gasification emerges as a promising technique because of using carbonate in the recycling of waste electrical and electronic equipment plastics. Halogenated compounds present within waste electrical and electronic equipment plastics were retrieved in the form of stable organic salts (Zhang et al. 2013). Lithium carbonate, sodium carbonate, and potassium carbonate are used as a catalyst in steam gasification under mild conditions. During steam gasification the carbonate or biomass cannot account for the halogen emission but accelerates the transformation of tar and char into gas products from plastics (Lopez et al. 2015).

6.3.7 Integrated Process

6.3.8 Hydrocracking

Hydrocracking process differs from catalytic upgrading of solid plastics only by hydrogen usage in it. This method was carried out at 70 atm and temperature of 375–400 °C in the presence of a catalyst. Hydrogen usage enhanced the final product quality (superior H/C ratio along with lesser aromatic compounds). The mixture of plastics can be hydrocracked to produce a good quality of naphtha. But this process needs a higher-operating pressure and investment cost.