Rubber recycling is not new. In fact its history goes back more than a hundred years to a time when rubber was a scarce commodity. In 1910, an ounce of rubber cost the same as an ounce of silver. Over time rubber recycling declined. One reason for this was the increased use of synthetic rubber made from cheap imported oil. This reduced the value of recycled rubber and made it less cost-effective. Another reason relates to the development and spread of steel belted radials. These tires eventually became scrap tires that were extremely resilient and therefore extremely difficult and expensive to recycle. With the decline in rubber recycling came the growth of stockpiles of scrap tires.
More than 300 million scrap tires are generated in the US and Canada each year. This is approximately equivalent to one passenger tire per population per year. For many years scrap tires have been accumulating in landfills, illegal stockpiles, vacant lots, fields, abandoned buildings and roadsides. These stockpiles can pose a serious threat to public health and safety, as well as to the environment.
Hazards of tire piles
Large stockpiles have the potential for large fires that are tremendously detrimental to the environment and extremely hard to extinguish once started. Large fires may burn for months all the while creating acrid black smoke and toxic liquid run-off.The smoke contains toxic chemicals and air pollutants while toxic chemicals from the run-off are released into surrounding water and soil. Attempts to extinguish fires by conventional means can actually increase the damage.
Stockpiled tires can provide nesting areas for rodents and other vermin. The puddles of water found inside tires provide convenient breeding grounds for mosquitoes. The spread of West Nile Virus has given new urgency to the need to eradicate scrap tire stockpiles. Elevated incidence of other mosquito-borne diseases has been reported near large tire piles.
Even tires that are disposed of in sanitary landfills create environmental problems. Buried tires sometimes “swim” their way back to the surface which can result in expensive damage to landfill cover and containment systems. For this reason, many jurisdictions have banned the landfilling of whole tires.
Solutions for scrap tires are ranked in terms of the 3-R hierarchy. The Rs in order of preference are Reuse, Recycle, and Recover (energy). Obviously the best use of a scrap tire is its reuse. Some tires find their way “as is” into domestic second-hand markets. Others, particularly medium truck tires, are retreaded first. About 10% of scrap tires from industrialized countries such as the US are sold as second-hand tires in less developed regions such as Central America.
The best management strategy of scrap tires that are not suitable for reuse or retreading is recycling. Most recycling efforts involve the manufacture of crumb rubber which is used as a feedstock in the production of finished goods. The markets and applications for crumb rubber have grown tremendously in the past two decades as uses outside the traditional rubber manufacturing industry are becoming increasingly important. A wide variety of recycled rubber products is the result.
Tire Derived Fuel (TDF) burned in cement kilns, pulp mills and other industrial plants is a reasonable use for scrap tires if recycling is not a viable option. The energy required to create rubber compounds is 3 to 4 times the energy released when tires are burned. Use of recycled rubber therefore makes more sense than burning tires both environmentally and economically.
Methods of producing crumb
Technical developments in the last past two decades have produced more cost effective methods of reducing scrap tires. Size reduction accomplishes two things. It makes it possible to separate the steel and fiber from the rubber. It also processes the rubber into a sellable particle size.
Tire Recycling Processes
Nearly all processors begin by shredding the tire. This primary reduction step reduces the volume of tires and creates a material that is more easily handled. Tire shredders typically are shear shredders with two counter rotating shafts producing 2-inch shreds. The shredding divsion fulfils this function for Western Rubber.
The shred is further reduced using either an ambient or cryogenic system. In an ambient system (like that used by the crumbing plant) the shred, at ambient temperature, is fed into one or more granulators fitted with screens that determine the size of the output. Steel is removed from the resulting material by magnets and fiber is removed by aspiration and sifting so that only rubber particles remain. If larger sized rubber particles (typically ¼ inch or larger) are desired the process may end here. More often, smaller sizes are required in which case the rubber is processed through one or more cracker mills. (The fine Grind Division operates two of these mills.) These mills have two counter rotating corrugated rolls placed very close together that “crack” the rubber into smaller particles as it passes through. The ambient system is effective where the targeted output size is 30 mesh or greater.
In a cryogenic system the tire shreds, or in some cases the whole tire, is super-cooled using liquid nitrogen. The cold rubber, now extremely brittle, is processed through a hammer mill which shatters the rubber into smaller particles. The output is dried and classified into specific gradations. Smaller particles can be produced with the cryogenic process than with an ambient system. It requires fewer pieces of equipment than the ambient system and energy and maintenance costs may be less. A drawback of the cryogenic process is the cost of liquid nitrogen. Cryogenic crumb rubber particles have very smooth surfaces compared to crumb produced ambiently. Most crumb rubber customers require particles that are rougher with greater surface area therefore cryogenic material has limited applications.
End uses/products for crumb rubber
Shredded tire chips are used in civil engineering applications as a substitute for conventional construction materials such as crushed rock or gravel. Smaller rubber chips may be colored and are used as playground cover and landscaping mulch.
On average less than 5% of a new tire consists of recycled rubber. The largest market for crumb rubber is the molded products sector which uses crumb rubber in combination with urethane binders. Products include agricultural mats, sports flooring, speed bumps, railway crossings, weightlifting plates and acoustic flooring.
Crumb rubber is often used in the construction of running tracks and poured-in-place playgrounds. Increasingly, artificial turf fields are made with a carpet of long synthetic grass fibers that are unfilled and held upright with crumb rubber. These are becoming the playing field of choice for many professional sports teams. Crumb rubber is even combined directly into natural turf so as to reduce compaction and improve drainage. Golf courses use it around tees and greens. Another growing use of crumb rubber is in asphalt pavements. Mixing rubber into the asphalt binder produces a pavement with a number of beneficial characteristics. To see some of the products made with crumb rubber click here.