Reviewed by Lexie CornerNov 27 2024
Researchers from Università di Pavia in Italy and Sophia University in Japan have created a novel microwave-induced pyrolysis technique that offers an economical and effective means of recycling electrical wiring cables. Their findings were published in the journal RSC Advances.
Carbonization of the PVC covering and recovery of the copper wire during the pyrolysis of a 54 cm spring-shaped VVF cable. Initial spring-shaped VVF cable, followed by changes after 3 minutes of irradiation with 300 W microwaves, and then after 12 minutes of irradiation with the same power. Image Credit: Sophia University
The demand for electronics has significantly increased. About 62 million tons of e-waste were produced in 2022, an 82 % increase over 2010. According to projections, this number may increase to 82 million tons by 2030.
Reusable rare elements, metals, and semiconductors are among the valuable materials found in e-waste. However, only 22.3% of e-waste was properly collected and recycled in 2022, while the remaining e-waste, valued at approximately $62 billion, was sent to landfills.
Despite ongoing efforts to improve e-waste recycling, the process remains labor-intensive, and much of the e-waste is exported to underdeveloped countries where hazardous chemical recycling methods are supported by cheap labor.
The new process carbonizes the PVC insulation without damaging the copper wire, making recovery easier and preventing the production of highly harmful 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.
Satoshi Horikoshi, Professor and Study Lead Author, Sophia University
The researchers placed two-core VVF power cables with copper wires wrapped in PVC insulation inside a glass reactor and exposed them to 100, 200, and 300 W of microwave radiation. During the pyrolysis process, nitrogen gas was introduced into the reactor to prevent combustion. Portions of various cable lengths (1 cm, 6 cm, 9 cm, 12 cm, and 18 cm) were cut and tested alongside a standard 54 cm VVF cable.
Remarkably, the efficiency of the pyrolysis process varied depending on the cable length in relation to the microwave's wavelength, which was approximately 12.24 cm at a frequency of 2.45 GHz. Cables with lengths that corresponded to specific wavelength fractions showed the best results in the pyrolysis process.
After 60 seconds at 100 W, the 9 cm cables (around 3/4 of the wavelength) and the 18 cm cables (longer than the wavelength) began to pyrolyze. In contrast, shorter cables, such as the 3 cm cable (about 1/4 of the wavelength), required more power (200 W) to initiate pyrolysis. Even at 300 W, no pyrolysis occurred with other lengths, such as 1 cm, 6 cm, and 12 cm (full or nearly complete wavelengths).
Although plastics do not absorb microwaves, the 9 cm and 18 cm cables underwent pyrolysis for several reasons. The copper wires acted as antennas at these lengths, absorbing microwave radiation and generating electric arc discharges to heat the PVC without melting the copper. Additionally, the center and ends of these cables experienced a stronger and more concentrated electric field. With an electric field strength roughly double that of the 3 cm wire, the 9 cm wire heated more quickly and locally. Finally, as the PVC heated and carbonized, its ability to absorb microwaves improved, further accelerating the pyrolysis process.
The rapid dichlorination and carbonization of the PVC insulation during the pyrolysis effectively prevented the production of harmful byproducts, such as tar, dioxins, and polycyclic aromatic hydrocarbons (PAHs).
Chlorine could be recycled as hydrochloric acid; the carbon and activated carbon produced could be recovered as carbon black.
Satoshi Horikoshi, Professor and Study Lead Author, Sophia University
Since the microwaves were able to heat and break down wires longer than their wavelength, the 54 cm VVF cable was fully pyrolyzed in just 12 minutes under 300 W of radiation, leaving the copper wire intact and undamaged.
Currently, only about 35 % of PVC insulation is recycled. This microwave-assisted pyrolysis method provides a more efficient, less labor-intensive way to recover copper wires from PVC cables, improving the recycling process and addressing the growing issue of e-waste.
With this breakthrough, e-waste recycling could become cleaner, faster, and more sustainable, turning today's waste into valuable resources for the future.
Journal Reference:
Horikoshi, S., et al. (2024) Recycling of e-waste power cables using microwave-induced pyrolysis – process characteristics and facile recovery of copper metal. RSC Advances. doi.org/10.1039/d4ra05602g.