Tire recycling, or rubber recycling, is the process of recycling waste tires that are no longer suitable for use on vehicles due to wear or irreparable damage. These tires are a challenging source of waste, due to the large volume produced, the durability of the tires, and the components in the tire that are ecologically problematic.[1]
Because tires are highly durable and non-biodegradable, they can consume valuable space in landfills. If waste tires are improperly managed they may cause rubber pollution. In 1990, it was estimated that over 1 billion scrap tires were in stockpiles in the United States. As of 2015, only 67 million tires remain in stockpiles.[2] From 1994 to 2010, the European Union increased the amount of tires recycled from 25% of annual discards to nearly 95%, with roughly half of the end-of-life tires used for energy, mostly in cement manufacturing.[3] [4]
Pyrolysis and devulcanization could facilitate recycling. Aside from use as fuel, the main end use for tires remains ground crumb rubber.[5] In 2017, 13% of U.S. tires removed from their primary use were sold in the used tire market. Of the tires that were scrapped, 43% were burnt as tire-derived fuel, with cement manufacturing the largest user, another 25% were used to make ground rubber, 8% were used in civil engineering projects, 17% were disposed of in landfills and 8% had other uses.[6] Globally, tire graveyards are a common environmental hazard, with significant pollutants and other challenges. For example, the Sulaibiya tire graveyard in Kuwait has had repeat highly toxic fires.[7]
The tire life cycle can be recognized through the following steps:
Tires are not desired at landfills, due to their large volumes and 75% void space.[9] 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. The EU Landfill Directive prohibits the disposal of used tires in landfill.[10]
Shredded tires are now being used in landfills, replacing other construction materials, for a lightweight back-fill 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.[11] Scrap tires as a back-fill 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 easily occur, burning for long periods, up to a month and also 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.
Tire storage and recycling are sometimes linked with illegal activities and lack of environmental awareness.[12]
Although tires are usually burnt, not recycled, efforts are continuing to find value. Tires can be reclaimed into, among other things, the hot melt asphalt, typically as crumb rubber modifier—recycled asphalt pavement (CRM—RAP),[13] [14] and as an aggregate in Portland cement concrete[15] Efforts have been made to use recycled tires as raw material for new tires, but such tires may integrate recycled materials no more than 5% by weight, and tires that contain recycled material are inferior to new tires,[5] suffering from reduced tread life and lower traction. Tires have also been cut up and used in garden beds as bark mulch to hold in the water and to prevent weeds from growing. Some "green" buildings, both private and public, have been made from old 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.[16]
Old tires can be used as an alternative fuel in the manufacturing of Portland cement, a key ingredient in concrete. Whole tires are commonly introduced into cement kilns, by rolling them into the upper end of a preheater kiln, or by dropping them through a slot midway along a long wet kiln. In either case, the high gas temperatures (1000–1200 °C) cause almost instantaneous, complete and smokeless combustion of the tire. Alternatively, tires are chopped into 5–10 mm chips, in which form they can be injected into a precalciner combustion chamber. Some iron input is required in manufacturing cement, so the iron content of steel-belted tires is beneficial to the process.[17]
Tires can be reused in many ways, although most used tires are burnt for their fuel value.[18] In a 2003 report cited by the U.S. EPA, it is stated that markets ("both recycling and beneficial use") existed for 80.4% of scrap tires, about 233 million tires per year. Assuming per tire, the 2003 report predicts a total weight of about from tires.[19]
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.[8]
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. In the reactor, the rubber is softened after which the rubber polymers break down 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 solid ashes. When performed properly, the tire pyrolysis process is a clean operation and produces little emissions or waste; however, concerns about air pollution due to incomplete combustion as is the case with tire fires has been documented.[21]
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 feed-stock and process can affect 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 the tire. The solid stream also includes the minerals used in rubber manufacturing. This high volume component of tire pyrolysis is a major impediment, although this theme continues to be a source of innovation.
Tires can be frozen using cryogens, or super-cold fluids, then broken down and made into a material called "crumb," which can be used in asphalt road beds, agricultural hoses, and truck bed liners.
Aside from recycling old tires, the old tire can be put to a new use.
Old tires are sometimes converted into a swing for play. The innovative use allows for an easy way to find a purpose for an existing old tire not suitable for road use.[22]
Used tires are also employed as exercise equipment for athletic programs such as American football.[23] One classic conditioning drill that hones players' speed and agility is the "Tire Run" where tires are laid out side by side, with each tire on the left a few inches ahead of the tire on the right in a zigzag pattern. Athletes then run through the tire pattern by stepping in the center of each tire. The drill forces athletes to lift their feet above the ground higher than normal to avoid tripping.[24] Other athletic uses include tire flipping (tractor or large truck tires typically used) or for upper cardio conditioning by hitting a tire repetitively with a sledge hammer.[25]
Re-purposed tires can also be harnessed as an affordable alternative building material used in the framework of rammed Earth thermal mass dwellings.[26] This is beneficial across scales of production such as individually sustainable housing.[27]
Rows of stacks of tires are often used as barriers in motor racing circuits as a method of dissipating kinetic energy over a longer period of time during a crash, comparatively to striking a less malleable material such as a concrete or steel wall.
Many cattle farmers re-purpose old tractor tires as water troughs for their cattle by placing them over natural springs or by piping stream water into them. These tires contain the water and allow it to pool for the cattle without any additional interaction from the farmer. Most farmers also include a drainage pipe near the top or in the center of the tire so excess water can drain off to prevent overflow and erosion around the outside of the tire where the cattle would be.[28]
Repurposing as an elastic base for multipurpurposes uses, such as holding advertisement boards, static or swinging. This is an application still under development and at a prototype stage.[29]
Rubber shingle roofs are typically made from 95% recycled material from a variety of sources including recycled tires. They last twice as long as asphalt shingles but are about twice the price as asphalt. They are more quiet than most roofs, hail resistant, and a high wind rating if there is a tongue and groove fitting at the front edge of the rubber shingle design.[30] [31]
Due to their heavy metal and other pollutant content, tires pose a risk for the leaching of toxins into the groundwater when placed in wet soils. 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.[9]
For both above and below water table applications, the preponderance of evidence shows that TDA (tire derived aggregate, or shredded tires) will not cause primary drinking water standards to be exceeded for metals. Moreover, TDA is unlikely to increase levels of metals with primary drinking water standards above naturally occurring background levels.[32]