Tire recycling

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Tires are among the largest and most problematic sources of waste.

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, their durability, and the fact they contain a number of components that are ecologically problematic. It is estimated that 259 million tires are discarded annually (for years in the 1980s and 1990s).[1] The same characteristics that make waste tires problematic, their cheap availability, bulk, and resilience, also make them attractive targets for recycling. Nonetheless more than half of used tires are simply burned for their fuel value.[2] Even in advanced countries like Germany, 55% are estimated to be burnt for fuel.[3] Approximately, one tire is discarded per person per year[citation needed]. 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.

Tire life cycle[edit]

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, re-tread, 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.[4]

Landfill disposal[edit]

Tires are not desired at landfills, due to their large volumes and 75% void space, which quickly consumes valuable space.[5] 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.[4]

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.[6] 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.

Stockpiles and illegal dumping[edit]

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.[7]

Uses[edit]

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),[8][9] and as an aggregate in portland cement concrete[10] 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[disambiguation needed], 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.[11]

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.[12]

Tire pyrolysis[edit]

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 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 feedstock 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.

Tire-derived products[edit]

Tires can be reused in many ways, although again, most used tires are burnt for their fuel value.[1] 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 lbs per tire, the 2003 report predicts a total weight of about 2.62 million tons from tires.[13]

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.[14]

Shredded tires
Closeup of shredded tires
1 ton bags of crumb rubber
  • Construction materials. Entire homes can be built with whole tires by ramming them full of earth and covering them with concrete, known as Earthships. They are used in civil engineering applications such as sub-grade fill and embankments, backfill for walls and bridge abutments, sub-grade insulation for roads, landfill projects, and septic system drain fields. Tires are also bound together and used as different types of barriers such as: collision reduction, erosion control, rainwater runoff, wave action that protects piers and marshes, and sound barriers between roadways and residences.
  • 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 which has become an environmental nightmare that will cost millions of dollars to rectify.
  • The process of stamping and cutting tires is used in some apparel products, such as sandals and as a road sub-base, by connecting together the cut sidewalls to form a flexible net.
  • 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.[6]
  • Shredded tires, known as Tire Derived Aggregate (TDA), have many civil engineering applications. TDA can be used as a backfill for retaining walls, fill for landfill gas trench collection wells, backfill for roadway landslide repair projects as well as a vibration damping material for railway lines.
  • Ground and crumb rubber, also known as size-reduced rubber, can be used in both paving type projects and in moldable products. These types of paving are: Rubber Modified Asphalt (RMA), Rubber Modified Concrete, and as a substitution for an aggregate. Examples of rubber-molded products are carpet padding or underlay, flooring materials, dock bumpers, patio decks, railroad crossing blocks, livestock mats, sidewalks, rubber tiles and bricks, moveable speed bumps, and curbing/edging. The rubber can be molded with plastic for products like pallets and railroad ties. Athletic and recreational areas can also be paved with the shock absorbing rubber-molded material. Rubber from tires is sometimes ground into medium-sized chunks and used as rubber mulch. Rubber crumb can also be used as an infill, alone or blended with coarse sand, as in infill for grass-like synthetic turf products such as FieldTurf.
  • Steel mills can use tires as a carbon source, replacing coal or coke in steel manufacturing.[citation needed]

Environmental concerns[edit]

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.[15]

Ecotoxicity may be a problem. 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.[16]

See also[edit]

References[edit]

  1. ^ a b Bandyopadhyay, S., Agrawal, S.L., Ameta, R., Dasgupta, S., Mukhopadhyay, R., Deuri, A.S., Ameta, Suresh C., Ameta, Rakshit "An overview of rubber recycling" Progress in Rubber, Plastics and Recycling Technology, 2008, volume 24, pages 73-112.
  2. ^ Myhre, Marvin; MacKillop, Duncan A. "Rubber recycling " Rubber Chemistry and Technology (2002), volume 75, number 3, pages 429-474. doi: http://dx.doi.org/10.5254/1.3547678
  3. ^ Desmond Threadingham, Werner Obrecht, Wolfgang Wieder, Gerhard Wachholz and Rüdiger Engehausen "Rubber, 3. Synthetic Rubbers, Introduction and Overview" in Ullmann's Encyclopedia of Industrial Chemistry, 2011, Weinheim. doi:10.1002/14356007.a23_239.pub5
  4. ^ a b 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
  5. ^ 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
  6. ^ a b "Scrap Tires - Basic Information". US Environmental Protection Agency. 16 July 2011. Retrieved 22 September 2011. 
  7. ^ http://www.bbc.co.uk/news/uk-england-devon-17195524
  8. ^ Kandhal PS. (1992). WASTE MATERIALS IN HOT MIX ASPHALT - AN OVERVIEW. National Center for Asphalt Technology.
  9. ^ T. E. Baker (2003). Evaluation of the Use of Scrap Tires in Transportation Related Applications in the State of Washington
  10. ^ M Nehdi, A Khan, (2001). Cementitious Composites Containing Recycled Tire Rubber: An Overview of Engineering Properties and Potential Applications. Cement, Concrete, and Aggregates.
  11. ^ Wojtowicz MA, Serio MA. (1996). Pyrolysis of scrap tires: Can it be profitable?. Chemtech.
  12. ^ Tech to end tire burning.
  13. ^ 22.5 lb working figure retrieved on 20 February 2010 from: http://www.rma.org/scrap_tires/scrap_tire_markets/scrap_tire_characteristics/
  14. ^ 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
  15. ^ 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
  16. ^ toxicity study http://www.ardeacon.com/pdf/Assessment_Environmental_Toxicity_Report.pdf

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