Disposal of Waste Tires

Tens of millions of tires are discarded across the Middle East every year. Disposal of  waste tires is a challenging task because tires have a long life and are non-biodegradable. The traditional method of disposal of waste tires have been stockpiling or illegally dumping or landfilling, all of which are short-term solution.

Menace of Waste Tires

Stockpiled tires provide perfect breeding grounds for mosquitoes, vermin and snakes. Accidental fires caused in tire dumps can rage for months releasing toxic fumes. For example, a massive fire broke out at Jahra dumpsite in Kuwait in April 2012 where more than 5 million waste tires were stored.

Landfilling of tires is a major problem as tires come up to the top of landfill and can damage caps and liners. Tires are not desired at landfills because of their large volumes and 75% void space which quickly consumes valuable space. Many countries in North America and Europe have banned landfilling of whole tires and made recycling mandatory.

Tire recycling is the process of recycling vehicles’ tires that are no longer suitable for use on vehicles due to wear or irreparable damage (such as punctures). These tires are among the largest and most problematic sources of waste, due to the large volume produced and their durability. 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

Cryogenic grinding refers to the grinding of scrap tires at temperatures near minus 80^oC 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.

Pyrolysis of waste tires – A schematic

Rubber crumbs or crumb rubber, 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

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 char. Cracking and post-cracking take place progressively as the material is heated to 450-500 °C and above.

The use of TDF in cement kilns, paper mills or power plants is one of the best uses of scrap tires. The char can be used in low value production processes as a colorant or filler.