The surge in electronic goods has resulted in a notable rise in e-waste. In 2022, an estimated 62 million tons of e-waste was produced, reflecting an 82% increase since 2010. Forecasts suggest that this amount could escalate to 82 million tons by 2030.
E-waste houses valuable materials like metals, semiconductors, and rare elements that are capable of being reused. Nevertheless, in 2022, only 22.3% of e-waste was effectively collected and recycled, with the remaining materials valued at nearly $62 billion, ending up in landfills.
While initiatives to enhance e-waste recycling efforts are ongoing, the process is still labor-intensive, and a large quantity of e-waste is sent to developing nations where inexpensive labor is used for informal recycling methods that involve hazardous chemicals.
In a new study, researchers from Sophia University in Japan and Università di Pavia in Italy have developed a novel microwave-induced pyrolysis technique that offers a cost-effective and efficient solution for recycling electrical wiring cables. This technique carbonizes the PVC insulation and reveals the copper wire without causing any damage, enabling easy recovery while avoiding the production of highly toxic byproducts.
“VVF cables are commonly used as power cables in houses and buildings and have a high reuse value among e-waste. Our method is suitable for recycling and recovering e-waste containing metals and requires no pre-treatment to separate the plastics from the metals,” says Professor Satoshi Horikoshi from Sophia University, one of the lead authors of the study.
In the study, researchers examined the effects of microwave radiation on two-core VVF power cables, which feature copper wires encased in PVC insulation. These cables were placed in a glass reactor, where they were subjected to microwave energy at 100, 200, and 300 watts. To ensure a safe pyrolysis process, nitrogen gas was introduced into the reactor, effectively preventing combustion. The team focused on a standard 54 cm VVF cable, cutting it into various lengths: 1 cm, 6 cm, 9 cm, 12 cm, and 18 cm.
The results revealed a fascinating relationship between cable length and pyrolysis effectiveness, directly linked to the microwave’s wavelength of approximately 12.24 cm at a frequency of 2.45 GHz. Pyrolysis proved most effective in cables with lengths that matched specific fractions of this wavelength.
The 9 cm cable (approximately 3/4 wavelength) and 18 cm cable (longer than the wavelength) began undergoing pyrolysis after 60 seconds at 100 W. In contrast, shorter lengths like the 3 cm cable (around 1/4 wavelength) required higher power levels (200 W) to initiate pyrolysis. Other cable lengths, such as 1 cm, 6 cm, and 12 cm (complete or nearly complete wavelengths), did not exhibit any pyrolysis, even when subjected to 300 W.
While plastics generally do not absorb microwaves, the remarkably engineered 9 cm and 18 cm cables experienced pyrolysis due to several compelling factors. First, at these designated lengths, the copper wires functioned as efficient antennas, absorbing microwave energy and generating electric arc discharges that heated the PVC without causing any melting of the copper.
Second, the electric field was intensified and concentrated at both the ends and the center of these wires. Specifically, the 9 cm wire exhibited an electric field strength nearly double that of the 3 cm version, enabling faster and more targeted heating. Third, as the PVC insulation heated and carbonized, it enhanced its ability to absorb microwaves, thereby accelerating the pyrolysis process even further.
During the pyrolysis process, the PVC insulation experienced rapid dichlorination and carbonization, which effectively inhibited the production of harmful byproducts like tar, polycyclic aromatic hydrocarbons (PAHs), and dioxins.
“Chlorine could be recycled as hydrochloric acid; the carbon and activated carbon produced could be recovered as carbon black,” says Prof. Horikoshi.
Microwaves have demonstrated the remarkable ability to heat and decompose wires longer than their wavelength, achieving the complete pyrolysis of a 54 cm VVF cable in just 12 minutes with only 300 W of radiation while keeping the copper wire intact and undamaged. Currently, a mere 35% of PVC insulation is recycled.
This method of microwave-assisted pyrolysis presents a more effective and less labor-intensive approach for reclaiming copper wires from PVC cables, enhancing the recycling process and tackling the increasing problem of e-waste.
With this innovation, the landscape of e-waste recycling is poised to become cleaner, faster, and significantly more sustainable—transforming today’s waste into the valuable resources of tomorrow.
Journal reference:
- Satoshi Horikoshi, Naoki Hachisuga and Nick Serpone. Recycling of e-waste power cables using microwave-induced pyrolysis – process characteristics and facile recovery of copper metal. RSC Advances, 2024; DOI: 10.1039/D4RA05602G