Cryogenic tyre grinding

Liquid nitrogen or other cryogenic fluids can be used in order to process used tyres, as temperatures under -80°C render the rubber brittle and easy to shatter.
According to a 2003 report by the US Environmental Protection Agency (EPA), the US alone, generate around 290 million scrap tyres annually. Until recently they were disposed of in landfills or other types of tyre piles. But environmental legislation tends to consider this as dangerous waste and the number of ways to recycle the tyres has increased. The EPA believes that markets now exist for 80.4% of scrap tyres, up from 17% in 1990.

Apart from filling landfills, used tyres can be retreaded and returned to the market, burned for fuel, ground for crumb rubber or punched and stamped into a new product. Cryogenic grinding of scrap tyres uses liquid nitrogen to produce crumb rubber while avoiding heat degradation of the rubber, and yields a product that is almost free of all fibre and steel, which is released during the process. The cold brittle rubber is processed through a hammer mill, which shatters it into smaller particles which are classified into specific gradations: “ground rubber” (smaller than 2 mm) and “coarse rubber” (2-12.5 mm). The use of cryogenic temperatures can be applied at any stage of size reduction of scrap tyres. The material can be cooled in a tunnel style chamber, immersed in liquid nitrogen, or sprayed with liquid nitrogen. A typical throughput is 1800-2700 kg per hour. 

Crumb rubber is most often used as asphalt rubber, the largest single market for ground rubber, with an estimated 12 million tyres. Asphalt rubber consists in ground tyre rubber blended with asphalt in order to modify the properties of the asphalt in highway construction. The crumb rubber can also be used in the moulded products sector, used in combination with urethane binders, to produce products such as agricultural mats, acoustic and sports flooring, speed bumps, railway crossings, weightlifting plates, etc. It can also be used in the construction of running tracks and playgrounds and even to hold upright synthetic grass blades in artificial turf fields.

Cryogas International, July 2011