More than 1 billion tires are discarded around the world every year. Disposal of waste tires is a challenging task because tires have a long life and are non-biodegradable. The traditional method of tire waste management is stockpiling or illegally dumping or landfilling, all of which are short-term solution.
Crumb rubber is a term used for recycled rubber from automotive and truck scrap tires. The two major technologies for producing crumb rubber are ambient mechanical grinding and cryogenic grinding. Of the two processes, cryogenic process is more expensive but it produces smoother and smaller crumbs.
Ambient Mechanical Grinding
In ambient mechanical grinding process, the breaking up of a scrap tire happens at or above normal room temperature. Ambient grinding is a multi-step technology and uses whole or pre-treated car or truck tires in the form of shred or chips, or sidewalls or treads. The rubbers, metals and textiles are sequentially separated out. Tires are passed through a tire 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.
Ambient grinding is the production process used by the majority of crumb producers. The machines most commonly used for fine grinding in ambient plants are:
- Secondary granulators
- High speed rotary mills
- Extruders or screw presses
- Cracker 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.
Processing of scrap tires takes place at very low temperature using liquid nitrogen or commercial refrigerants to embrittle the rubber. It can be a four-phase system which includes initial size reduction, cooling, separation, and milling. The material enters a freezing chamber where liquid nitrogen is used to cool it from –80 to –120 °C, below the point where rubber ceases to behave as a flexible material and can be easily crushed and broken.
Because of its brittle state, fibres and metal are easily separated out in a hammer mill. The granulate then passes through a series of magnetic screens and sifting stations to remove the last vestiges of impurities. This process requires less energy than others and produces rubber crumb of much finer quality.
Uses of Crumb Rubber
Both ambient and cryogenic processing can be repeated to produce finer particles. Increasingly, the two with their attendant technologies, are combined into one continuous system in order to benefit from the advantages and characteristics of each and to reduce overall costs.
The ambient system is generally used for the initial size reduction phases. The cryogenic system is used to further reduce the material in size and then to remove the metals and textiles. The outputs from either or both systems can be used directly or as feedstock for further processing.
Rubber crumb is sold as feedstock for chemical devulcanization or pyrolysis processes, added to asphalt for highway paving and pavement sealers, or used for the production of a large number of recycled rubber-containing products.
Some of the major applications of crumb rubber are as follows:
1. Sport Surfaces
- Kindergarten Playgrounds and Recreation Areas
- School Sports Areas
- Athletic Tracks
- Tennis and Basketball Courts
2. Automotive Industry
- Splash Guards and Fenders
- Floor Mats for Cars and Trucks
- Floor Liners for Trucks and Vans
- Hospital, Industrial, and Bathroom Flooring
- Floor Tile
- Foundation Waterproofing
- Dam, Silo, and Roof Liners
4. Geotechnical/Asphalt Applications
- Rubberized Asphalt for Roads and Driveways
- Drainage Pipes
- Soil Conditioner
- Porous Irrigation Pipes
- Road Building and Repair
5. Adhesives and Sealants
- Adhesives and Sealing Compounds
- Textured and Non-Slip Paints
- Roof Coating and Waterproofing
6. Shock Absorption and Safety Products
- Shock Absorbing Pads for Rails and Machinery
- Sound Barriers for Highways
- Abrasion Lining in Mining Equipment
7. Rubber and Plastic Products
- Pipe Insulation and Lining
- Garbage Cans
- Shoe Soles and Heels
- Wire and Cable Insulation