Tire recycling

Tires are among most problematic sources of waste. Progress in recycling has resulted in a major reduction in dumping.

Tire recycling or rubber recycling is the process of recycling vehicles' tires that are no longer suitable for use on vehicles due to wear or irreparable damage. These tires are a problematic sources 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 they are highly durable and non-biodegradable, they can consume valued space in landfills.[1] In 1990, it was estimated that over 1 billion scrap tires were in stockpiles in the United States. As of 2015, but only 67 million tires left 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] Newer technology, such as pyrolysis and devulcanization, has made tires suitable targets for recycling despite their bulk and resilience. End uses for tires include tire-derived fuels, new rubber products, playground surfacing, new tires, and rubber modified asphalt.[2][4]

Tire life cycle

Used tires in foreground waiting to be shredded and shredded tires in background.

The tire life cycle can be identified by the following six steps:

  1. Product developments and innovations such as improved compounds and camber tire shaping increase tire life, increments of replacement, consumer safety, and reduce tire waste.
  2. Proper manufacturing and quality of delivery reduces waste at production.
  3. Direct distribution through retailers, reduces inventory time and ensures that the life span and the safety of the products are explained to customers.
  4. Consumers' use and maintenance choices like tire rotation affect tire wear and safety of operation.
  5. Manufacturers and retailers set policies on return, retread, and replacement to reduce the waste generated from tires and assume responsibility for taking the 'tire to its grave' or to its reincarnation.
  6. Recycling tires by developing strategies that combust or process waste into new products, creates viable businesses, and fulfilling public policies.[5]

Landfill disposal

Tires are not desired at landfills, due to their large volumes and 75% void space, which quickly consumes valuable space.[6] 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.[5]

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.[7] 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.

Stockpiles and legal dumping

Tire art

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.

Unfortunately, tire storage and recycling are sometimes linked with illegal activities and lack of environmental awareness.[8]

Uses

Although tires are usually burnt, not recycled, efforts are continuing to find value. Tires can be recycled into, among other things, the hot melt asphalt, typically as crumb rubber modifier—recycled asphalt pavement (CRM—RAP),[9][10] and as an aggregate in portland cement concrete[11] Tires can also be recycled into other tires. 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.[12]

Recent developments in devulcanization promise to deal with substantial volumes, taking 40 mesh whole tire crumb and converting it into what is proposed to be value-added materials. 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.[13]

Cement manufacturing

Used tires being fed mid-kiln to a pair of long cement kilns

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. The steel and zinc in the tires become chemically incorporated into the cement.[14]

Tire pyrolysis

Car tires as seats in Thailand.

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 have been documented.[15]

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 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.

Tire-derived products

Tires can be reused in many ways, although again, most used tires are burnt for their fuel value.[16] 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 22.5 pounds (10.2 kg) per tire, the 2003 report predicts a total weight of about 2.62 million tonnes (2,580,000 long tons; 2,890,000 short tons) from tires.[17]

One 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.[5]

Shredded tires
Closeup of shredded tires
1 ton bags of crumb rubber

Repurposing

Three boys are on a tire swing

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.[19]

Used tires are also employed as exercise equipment for athletic programs such as American football.[20] 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.[21]

Re-purposed tires can also be harnessed as an affordable alternative building material used in the framework of rammed Earth thermal mass dwellings.[22] This is beneficial across scales of production from individually sustainable housing,[23] to life-saving tire bale walls that have the potential to alleviate suffering from natural disasters such as tsunamis, hurricanes and tidal waves.[24]

Rows of stacks of tyres 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.

Environmental concerns

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.[6]

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. [25]

See also

References

  1. 1 2 "Scrap Tires: Handbook on Recycling Applications and Management for the U.S. and Mexico" (PDF). epa.gov. United States Environmental Protection Agency. December 2010.
  2. 1 2 "Stockpile Cleanup". Rubber Manufacturers Association. 2016-03-15. Retrieved 2016-11-17 – via Google Scholar.
  3. ↑ "End of Life Tyres: A Valuable Resource with Growing Potential" (PDF). European Tyre and Rubber Manufacturers' Association. 2011. Retrieved 2016-11-16.
  4. ↑ Myhre, Marvin; Saiwari, Sitisaiyidah; Dierkes, Wilma; Noordermeer, Jacques. "RUBBER RECYCLING: CHEMISTRY, PROCESSING, AND APPLICATIONS". Rubber Chemistry and Technology. 85 (3): 408–449. doi:10.5254/rct.12.87973.
  5. 1 2 3 Price, Willard, and Edgar D. Smith. (2006). Waste tire recycling: environmental benefits and commercial challenges. International Journal of Environmental Technology and Management 6.3-4, 363-364
  6. 1 2 Liu,H., Mead, J., Stacer, R. Chelsea Center For Recycling And Economic Development. (1998). Environmental Impacts Of Recycling Rubber In Light Fill Applications: Summary & Evaluation Of Existing Literature University of Massachusetts
  7. 1 2 "Scrap Tires – Basic Information". US Environmental Protection Agency. 16 July 2011. Retrieved 22 September 2011.
  8. ↑ http://www.bbc.co.uk/news/uk-england-devon-17195524
  9. ↑ Kandhal PS. (1992). WASTE MATERIALS IN HOT MIX ASPHALT – AN OVERVIEW. National Center for Asphalt Technology.
  10. ↑ T. E. Baker (2003). Evaluation of the Use of Scrap Tires in Transportation Related Applications in the State of Washington
  11. ↑ M Nehdi, A Khan, (2001). Cementitious Composites Containing Recycled Tire Rubber: An Overview of Engineering Properties and Potential Applications. Cement, Concrete, and Aggregates.
  12. ↑ Wojtowicz MA, Serio MA. (1996). Pyrolysis of scrap tires: Can it be profitable?. Chemtech.
  13. ↑ Tech to end tire burning.
  14. ↑ Use of waste derived fuels in cement industry: a review, Nickolaos Chatziaras, Constantinos S. Psomopoulos, Nickolas J. Themelis, 2016, Management of Environmental Quality: An International Journal, Vol. 27 Iss 2 pp. 178 - 193, http://dx.doi.org/10.1108/MEQ-01-2015-0012, 08 March 2016
  15. ↑ "Burning Tires for Fuel and Tire Pyrolysis: Air Implications". EPA. 1991. Retrieved 2016-04-04.
  16. ↑ Bandyopadhyay, S.; Agrawal, S.L.; Ameta, R.; Dasgupta, S.; Mukhopadhyay, R.; Deuri, A.S.; Ameta, Suresh C.; Ameta, Rakshit (2008). "An overview of rubber recycling". Progress in Rubber, Plastics and Recycling Technology. 24: 73–112.
  17. ↑ 22.5 lb working figure retrieved on 20 February 2010 from: "Archived copy". Archived from the original on 5 March 2010. Retrieved 21 February 2010./
  18. ↑ http://crca.asn.au/green-steel-from-old-rubber-tyres-produces-no-waste-or-toxic-fumes/
  19. ↑ Sawyers, Harry. "One Day Project: Kid's Backyard Tire Swing". Popular Mechanics. Retrieved April 1, 2013.
  20. ↑ McCormick, Sean. "No Frills Football Conditioning". About.com.
  21. ↑ Ireland, Jae (November 24, 2010). "Football Tire Drills". LiveStrong. Retrieved April 1, 2013.
  22. ↑ http://earthship.com/codes-and-laws
  23. ↑ http://earthship.com/media-resume
  24. ↑ http://www.touchtheearthranch.com/tirebale4c.pdf
  25. ↑ https://rma.org/sites/default/files/LEA-013-Literature_Review_of_the_Water_Quality_Effects_of_Tire_Derived_Aggregate_and_Rubber_Modified_Asphalt_Pavement.pdf

External links

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