(344a) Valorization of End-of-Life Tires Using Microwave Technology: 6PPD Extraction & Conversion

Every year, billions of tires are sent to scrap yards worldwide, where they are either landfilled, milled to smaller particles, or used as fuel for rotary kilns. The tire antioxidant and antiozonant, N-(1,3-dimethyl butyl)-N'-phenyl-1,4-benzenediamine (6PPD) was recently shown to cause acute mortality in aquatic life in the ng/L concentration range. Thus, current end-of-life (EOL) tire management strategies are dangerous for the environment, and different valorization strategies are needed. A critical step in the overall valorization strategy is the removal of 6PPD from EOL tires to ensure it does not leach into the environment. Additionally, legislation will soon outlaw the incorporation of 6PPD in new tires. Future valorization will be needed to be powered by renewable energy to meet decarbonization goals. Microwave (MW) assisted heating is an attractive technology for this application as the carbon black inherent to EOL tires is an excellent microwave energy absorber and can enable fast heating. We investigate solvent-based extraction of 6PPD from waste tires, its subsequent conversion into value-added products, and the pyrolysis of crumb rubber, all under MW-assisted heating.

The solvent screening was first carried out in batch mode under MW heating to determine which solvent is most efficient at extracting 6PPD from waste tires. The MW extraction was subsequently scaled up to a continuous flow system where high volumes of crumb rubber could be extracted. Temperature measurements were carried out down the length of the reactor and on the reactor wall at various power levels in a static bed and under gas and solvent flow. Temperature models were derived to relate power level to the temperature within the rubber phase.

After extraction, 6PPD and the extracted crumb rubber were subsequently upgraded. 6PPD was catalytically converted into cyclohexane. The condensed product mixture from the reactor effluent contained various aromatic and terpene products. The carbon black remaining in the solid phase was characterized. Our work presents strategies for the full valorization of the tires while providing mechanistic insights.