October 2005

Alternative tire recycling options continually sought

Recycling tires by grinding them down to crumb rubber — whether it is to be used as fuel or as feedstock for new production — is a mature technology with proven markets, and promising new products and markets are in development.

A Different View
Research continues on other ways to recycle tires, some more promising than others. Most involve some form of devulcanization — breaking the sulfur bonds that help vulcanized rubber hold its shape, a characteristic that also prevents recyclers from melting tires down and remolding them into new products.

Another promising process — pyrolysis — involves breaking tires down beyond the rubber, to the hydrocarbon gases, carbon black and oil that the rubber is made from. The process also liberates the steel in tires for recycling. The hydrocarbon gases can be sold, but they are usually burned to fuel the pyrolysis process.

Devulcanization technologies
Thermal reclaim process: The oldest form of devulcanization — patented in 1858 — is also the least promising. Thermal devulcanization involves exposing rubber to high temperatures for an extended period of time to break the sulphur bonds. The process also breaks the polymer chains themselves, severely degrading the value of the rubber. The process also presents environmental and public health risks and is rarely used today.

Mechanical devulcanization: This process used rollers, mixers and extruders to mechanically "chew" away at the sulfur bonds. It produces good results but is not yet economically viable.

Devulcanization with ultrasound: Early efforts using ultrasound to break the sulfur bonds in vulcanized rubber were encouraging.

Bacterial devulcanization: Rubber is ground to a powder, then suspended in a liquid full of sulfur-eating bacteria such as thilbacillus, rodococcus and sulfolobus. The process is technically viable but complex, and therefore too expensive.

The technique uses heat, in the absence of oxygen, to speed up the decomposition of organic material. Theoretically, the oils, carbon black and scrap steel can be sold, and the hydrocarbon gases released through pyrolysis can either be sold or used as fuel at the pyrolysis plant.

But the tire pyrolysis techniques used today produce mixed quality oils and carbon black that can be difficult to market. The gas is a low to moderate BTU methane that is difficult to store.

Because of those quality issues, no tire pyrolysis plants have been in sustained production in the United States or anywhere else in the world. Research continues, however, for a technology that will make tire pyrolysis commercially viable.

— Courtesy of Ohio Department of Natural Resources

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