Recycling of plastics of waste from electrical and electronic equipment by microwave-assisted pyrolysis

Résumé

The use of electronic equipment has become widespread due to their increasing accessibility to larger population segments. However, despite the benefits it entails, it has also contributed to the growing problem of waste generation. Only a small fraction of the waste stream derived from these devices is being properly treated, because of the lack of specific collection systems or deficient management infrastructure in many regions. In the last few years, the interest in the recycling of waste from electrical and electronic equipment (WEEE), or e-waste, is on the rise because it minimises the environmental its impact and, especially, on account of the valuable materials it is composed of. Several recycling methods have been implemented for e-waste management. Mechanical operations are the conventional method because they can recover metals, such as iron and copper, and some plastics cost-effectively and with relatively ease. Nevertheless, some valuable materials are still lost in separation operations, and these processes usually generate a residue for the most part comprised by polymers. Due to the heterogeneous nature of this mixture of plastics, its valorisation is difficult or simply unfeasible to conduct through traditional techniques. Thermochemical operations offer an alternative for the treatment of this residue. Among them, pyrolysis, which consists in the decomposition of organic matter at elevated temperatures and in the absence of oxygen, generates three products: gas, oil and char. These products can be used to produce energy or, in the case of the oil, also as a raw material for processes like the synthesis of polymers, which is an example of circular economy. Furthermore, this technology concentrates in the solid product the metals lost in previous mechanical operations and eases their recovery. Microwave-assisted pyrolysis (MAP) uses a heating mechanism that differs significantly over conventional pyrolysis, which relies on conductive heating. The advantages of microwave heating over conductive heating include a rapid volumetric heating or a higher energy efficiency. Materials can convert electromagnetic radiation into energy, to a greater or lesser extent, depending on their physicochemical properties. Plastics, however, do not exhibit the best attributes for this purpose since they do not heat rapidly. Consequently, the e-waste residue must be mixed with good microwave absorbers to make the process efficient. Plastics used in electronic equipment must be fireproof to comply with safety regulations and thus have additives called flame retardants. Many of these additives generate highly hazardous substances when decomposed. Although, the use of flame retardants has been limited through stringent regulations, brominated flame retardants are still largely common. Accordingly, pyrolysis products derived from ewaste will also contain toxic compounds. Bromine and other halogens can also damage the processes that can make the most of liquid and gas products. Therefore, their refinement is indispensable to facilitate their application as raw materials or fuels. The present work studies the valorisation of the plastic residue resulting from the mechanical recycling of WEEE. The operation is conducted via laboratory-scale MAP and aims to recover the metals that were not separated in previous waste management processes and to chemically recycle the mixture of polymers. Operating pressure, power, reaction time and microwave absorber amount were examined to assess their effect on the performance of pyrolysis. Zeolites ZSM-5 and Y were used as catalyst to evaluate their ability to promote the cleavage of carbon-halogen bonds and upgrade the oil. Inorganic bases including NaOH, KOH and Ca(OH)2 were also tested as additives for the dehalogenation of this liquid product. These variables were analysed to provide insights into the optimal operation of the technology. Residues obtained from the mechanical treatment of waste from small and information technology equipment were used as feedstock. From their thermal behaviour, determined via thermogravimetric analysis, it was deduced to be primarily composed of bisphenol A-based polymers. The process yielded on average 24% of char, 49% of oil and 27% of gases from this product, although significant variations were observed depending on the composition of the pyrolysed materials and the additives used. Products were characterised by means of X-ray fluorescence spectrometry and gas chromatography–mass spectrometry. Solid and liquid products were found out to contain bromine in concentrations over 20,000 ppm and the addition of inorganic bases reduced its amount in the oil by up to 68%. The oil produced from this WEEE has significant quantities of phenol and other aromatic compounds that can be incorporated in the process for the synthesis of new polymers.