Tire recycling or rubber recycling is the process of recycling vehicles tires (or tyres) 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. Those same characteristics, which make waste tires such a problem, also make them one of the most re-used waste materials, as the rubber is very resilient and can be reused in other products. Approximately, one tire is discarded per person per year. Tires are also often recycled for use on basketball courts and new shoe products. However, material recovered from waste tires, known as "crumb," is generally only a cheap "filler" material and is rarely used in high volumes.
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The tire life cycle can be identified by the following six steps:
Tires are not desired at landfills, due to their large volumes and 75% void space, which quickly consumes valuable space.[2] Tires can trap methane gases, causing them to become buoyant, or bubble to the surface. This ‘bubbling’ effect can damage landfill liners that have been installed to help keep landfill contaminants from polluting local surface and ground water.[3] Shredded tires are now being used in landfills, replacing other construction materials, for a lightweight backfill in gas venting systems, leachate collection systems, and operational liners. Shredded tire material may also be used to cap, close, or daily cover landfill sites.[4] Scrap tires as a backfill and cover material are also more cost-effective, since tires can be shredded on-site instead of hauling in other fill materials.
Tire stockpiles create a great health and safety risk. Tire fires can occur easily, burning for months, creating substantial pollution in the air and ground. Recycling helps to reduce the number of tires in storage. An additional health risk, tire piles provide harborage for vermin and a breeding ground for mosquitoes that may carry diseases. Illegal dumping of scrap tires pollutes ravines, woods, deserts, and empty lots; which has led many states to pass scrap tire regulations requiring proper management. Tire amnesty day events, in which community members can deposit a limited number of waste tires free of charge, can be funded by state scrap tire programs, helping decrease illegal dumping and improper storage of scrap tires.
Tires can be recycled into, among other things, the hot melt asphalt, typically as crumb rubber modifier—recycled asphalt pavement (CRM—RAP),[5][6] and as an aggregate in portland cement concrete[7] Tires can also be recycled into other tires.
Pyrolysis can be used to reprocess the tires into fuel gas, oils, solid residue (char), and low-grade carbon black, which cannot be used in tire manufacture. A pyrolysis method which produces activated carbon and high-grade carbon black has been suggested.[8]
Recent developments in devulcanization enable dealing with substantial volumes, taking 40 mesh whole tire crumb and converting it into value-added compounds without degrading the polymer and without generating any pollution. This new generation in devulcanization technologies operates with very high productivity while maintaining a low energy footprint. The compounds produced from processed tire scrap can be blended with virgin rubber compounds, maintaining performance while substantially reducing the raw material cost. The substantial economies of scale and value addition now make it possible to make burning of tires entirely unnecessary.[9]
The pyrolysis method for recycling used tires is a technique which heats whole or shredded tires in a reactor vessel containing an oxygen free atmosphere and a heat source. In the reactor the rubber is softened after which the rubber polymers continuously breakdown into smaller molecules. These smaller molecules eventually vaporize and exit from the reactor. These vapors can be burned directly to produce power or condensed into an oily type liquid, generally used as a fuel. Some molecules are too small to condense. They remain as a gas which can be burned as fuel. The minerals that were part of the tire, about 40% by weight, are removed as a solid. When performed well a tire pyrolysis process is a very clean operation and has nearly no emissions or waste.
The properties of the gas, liquid and solid output are determined by the type of feed stock used and the process conditions. For instance whole tires contain fibers and steel. Shredded tires have most of the steel and sometimes most of the fiber removed. Processes can be either batch or continuous. The energy required to drive the decomposition of the rubber include using directly fired fuel (like a gas oven), electrical induction (like an electrically heated oven) or by microwaves (like a microwave oven). Sometimes a catalyst is used to accelerate the decomposition. The choice of feedstock and process can impact the value of the finished products.
The historical issue of tire pyrolysis has been the solid mineral stream which accounts for about 40% of the output. The steel can be removed from the solid stream with magnets for recycling. The remaining solid material, often referred to as "char", has had little or no value other than possibly as a low grade carbon fuel. Char is the destroyed remains of the original carbon black used to reinforce and provide abrasion resistance to rubber. The solid stream also includes the minerals used in rubber manufacturing. This high volume component of tire pyrolysis, until recently, has made the economic viability very difficult to achieve. Over the past five years two or three companies have discovered ways to recover the carbon in its original form. These companies have been commercially producing and selling recovered carbon black based products that successfully supplement virgin carbon black in rubber and plastics.
A report from 2003, cited by the EPA, says that markets ("both recycling and beneficial use") existed for 80.4% of scrap tires, about 233 million tires per year. Assuming 22.5 lbs per tire, the 2003 report predicts a total weight of about 2.62 million tons from tires.[10]
The markets predicted by the 2003 report were: tire derived fuel (TDF) using 130 million tires, civil engineering projects using 56 million tires, ground rubber turned into molded rubber products using 18 million tires, ground rubber turned into rubber-modified asphalt using 12 million tires, Exported items using 9 million tires, cut, stamped and punched products using 6.5 million tires, and agricultural and miscellaneous uses 3 million tires.[4]
The tire recycling supply chain is divided into three stages:
Second stage of tire recycling involves the production of alternate products for sale. New products derived from waste tires generate more economic activity than combustion or other low multiplier production, while reducing waste stream without generating excessive pollution and emissions from recycling operations.[11]
Some Artificial reefs are built using tires that are bonded together in groups, there is some controversy on how effective tires are as an artificial reef system, an example is The Osborne Reef Project.
Due to heavy metals and other pollutants in tires there is a potential risk for the (leaching) of toxins into the groundwater when placed in wet soils. This impact on the environment varies according to the pH level and conditions of local water and soil. Research has shown that very little leaching occurs when shredded tires are used as light fill material; however, limitations have been put on use of this material; each site should be individually assessed determining if this product is appropriate for given conditions.[12]
Ecotoxicity may be a bigger problem than first thought . Studies show that zinc, heavy metals, a host of vulcanization and rubber chemicals leach into water from tires. Shredded tire pieces leach much more, creating a bigger concern, due to the increased surface area on the shredded pieces. Many organisms are sensitive, and without dilution, contaminated tire water has been shown to kill some organisms.[13]
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