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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 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.

Tire lifecycle

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.[1]

Landfill disposal

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.

Stockpiles and illegal 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.

Uses

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 Portland Cement,[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]

Tire pyrolysis

The pyrolysis method for recycling of used tires is an innovation technique that uses a special mechanism to heat the used tires in a closed, oxygen-free environment – a stove to melt down the tires into the materials that they were made of. There are many different ways to achieve the melting procedure. For a long time, external heating methods were used. Recently an electro–magnetic field technology was developed by Coral group, in Dnepropetrovsk, Ukraine. This method produces carbon, metal, gas and artificial oil as by-products of the recycling process. The quality of these by-products depends on the heating technique used, with simple outside heating techniques producing heavy oils (mazut); however, newer techniques that produce a “softer” pyrolysis produce by-products such as benzene, kerosene and diesel.[citation needed]

Microwave recycling

The process of remediation of tire waste using microwaves to excite the rubber until it is in a gaseous state which will be condensed into its component parts including #3 diesel,syngas as well as carbon black and plated steel. No emissions are created in this process and all components can be reutilized.[citation needed]

Markets

As of 2007, the EPA reported that of the 7.48 million tons of rubber materials that constituted solid waste, only 1.1 million tons of this was "reclaimed" (recycled).[10] This is contrasted with a report from 2003, in which the EPA claims markets 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.[11]

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/Punched Products using 6.5 million tires, and Agricultural and Misc. uses using 3 million tires.[12]

Tire recycling supply chain

The Tire Recycling Supply Chain is divided into three stages:

Tire-derived products stage

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.[13]

Tire-derived products

Shredded tires
Closeup of shredded tires
1 ton bags of crumb rubber
  • Whole tires can be reused in many different ways. One way is for a Steel mill to use the tires as a carbon source, replacing coal or coke in steel manufacturing. Instead of mining coal from the ground and then burying tires in landfills, the tires are used directly. 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. Entire homes can be built with whole tires by ramming them full of earth and covering them with concrete, known as Earthships.

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.

  • 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.
  • Chipped and shredded tires are used as Tire Derived Fuel (TDF); this is not the same as recycling, but TDF helps to eliminate tires from our waste stream and produces a fuel source. 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.
  • 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.

Environmental concerns

Due to heavy metals and other pollutants in tires there is a potential risk for the leaching (leachate) 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.[14]

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.[15]

See also

References

  1. ^ 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
  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
  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
  4. ^ U.S. Environmental Protection Agency. Management of Scrap Tires. 03 Jan. 2007. 14 Feb. 2007 http://www.epa.gov/epaoswer/non-hw/muncpl/tires.org
  5. ^ Kandhal PS. (1992). WASTE MATERIALS IN HOT MIX ASPHALT - AN OVERVIEW. National Center for Asphalt Technology.
  6. ^ Baker TE. (2003). Evaluation of the Use of Scrap Tires in Transportation Related Applications in the State of Washington
  7. ^ Nehdi M, Khan A. (2001). Cementitious Composites Containing Recycled Tire Rubber: An Overview of Engineering Properties and Potential Applications. Cement, Concrete, and Aggregates.
  8. ^ Wojtowicz MA, Serio MA. (1996). Pyrolysis of scrap tires: Can it be profitable?. Chemtech.
  9. ^ Tech to end tire burning.
  10. ^ Retrieved 20 Feb 2010 from: http://www.epa.gov/osw/nonhaz/municipal/pubs/msw07-rpt.pdf
  11. ^ 22.5 lb working figure retrieved on 20 Feb 2010 from: http://www.rma.org/scrap_tires/scrap_tire_markets/scrap_tire_characteristics/
  12. ^ U.S. Environmental Protection Agency. Management of Scrap Tires. 03 Jan. 2007. Dead Link as of 20 Feb 2010. http://www.epa.gov/epaoswer/non-hw/muncpl/tires.org
  13. ^ 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
  14. ^ 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
  15. ^ toxicity study http://www.ardeacon.com/pdf/Assessment_Environmental_Toxicity_Report.pdf
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