Management of waste tyres is a challenging task because tyres have a long life and are non-biodegradable. The traditional method of waste tyres management have been stockpiling or illegally dumping or landfilling, all of which are short-term solution. Stockpiled tyres provide perfect breeding grounds for mosquitoes, vermin and snakes.
Accidental fires caused in tyre dumps can rage for months releasing toxic fumes. Landfilling of tyres is a major problem as tyres 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 tyres and made recycling mandatory.
There are three major technologies for recycling of waste tyres – ambient mechanical grinding, cryogenic grinding and pyrolysis.
Ambient Mechanical Grinding
In ambient mechanical grinding process, the breaking up of a scrap tyre happens at ambient temperature. Tyres are passed through a shredder, which breaks the tyres 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 tyres at temperatures near minus 80oC using liquid nitrogen or commercial refrigerants. Cryogenic processing generally uses pre-treated car or truck tyres as feedstock, most often in the form of chips or ambiently produced granulate. When the tyres 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 tyres, is used for manufacture of new tyres 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 tyres either in the absence or lack of oxygen. Pyrolysis uses pre-treated car or truck tyre 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., tyre-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 tyres. The char can be used in low value production processes as a colorant or filler.
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