Management of waste tires is a challenging task because tires have a long life and are non-biodegradable. The traditional method of waste tires management have been stockpiling or illegally dumping or landfilling, all of which are short-term solution. Stockpiled tires provide perfect breeding grounds for mosquitoes, vermin and snakes. Accidental fires caused in tire dumps can rage for months releasing toxic fumes. Landfilling of tires is a major problem as tires come up to the top of landfill and can damage caps and liners. Many countries in North America and Europe have banned landfilling of whole tires and made recycling mandatory.
There are three major technologies for recycling of waste tires – ambient mechanical grinding, cryogenic grinding and pyrolysis.
Ambient mechanical grinding
In ambient mechanical grinding process, the breaking up of a scrap tire happens at ambient temperature. Tires are passed through a shredder, which breaks the tires into chips. The chips are fed into a granulator that breaks them into small pieces while removing steel and fiber in the process. Any remaining steel is removed magnetically and fiber through a combination of shaking screens and wind sifters. Finer rubber particles can be obtained through further grinding in secondary granulators and high-speed rotary mills.
Cryogenic grinding refers to the grinding of scrap tires at temperatures near minus 80oC using liquid nitrogen or commercial refrigerants. Cryogenic processing generally uses pre-treated car or truck tires as feedstock, most often in the form of chips or ambiently produced granulate. When the tires are exposed to such low temperatures, they become brittle and can be easily crushed and broken. It can be a four-phase system which includes initial size reduction, cooling, separation, and milling. This process requires less energy than others and produces rubber crumb of much finer quality.
Rubber crumbs, the product obtained from ambient/cryogenic grinding of scrap tires, is used for manufacture of new tires or in a variety of landscaping applications including path paving projects, playground surface cover, running tracks, and athletic field turfs.
Pyrolysis refers to the thermal decomposition of scrap tires either in the absence or lack of oxygen. Pyrolysis uses pre-treated car or truck tire chips as the principal feedstock. It is a two-phase treatment which uses thermal decomposition to heat the rubber in the absence of oxygen to break it into its constituent parts, e.g., tire-derived fuel (TDF), synthetic gas and carbon. The use of TDF in cement kilns, paper mills or power plants is one of the best uses of scrap tires