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Aluminium recycling is the process by which scrap aluminium can be reused in products after its initial production. The process involves simply re-melting the metal, which is far less expensive and energy intensive than creating new aluminium through the electrolysis of aluminium oxide (Al2O3), which must first be mined from bauxite ore and then refined using the Bayer process. Recycling scrap aluminum requires only 5% of the energy used to make new aluminium. [1] For this reason, approximately 31% of all aluminium produced in the United States comes from recycled scrap. [2]

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A common practice since the early 1900s and extensively capitalized during World War II, aluminium recycling is not new. It was, however, a low-profile activity until the late 1960s when the exploding popularity of aluminium beverage cans finally placed recycling into the public consciousness. [3]

Sources for recycled aluminium include aircraft, automobiles, bicycles, boats, computers, cookware, gutters, siding, wire, and many other products that require a strong light weight material, or a material with high thermal conductivity. As recycling does not damage the metal's structure, aluminium can be recycled indefinitely and still be used to produce any product for which new aluminium could have been used.[4]

Baled cans ready for transport
Shredded aluminium beverage cans

Contents

Advantages

The recycling of aluminium generally produces significant cost savings over the production of new aluminium even when the cost of collection, separation and recycling are taken into account.[5] Over the long term, even larger national savings are made when the reduction in the capital costs associated with landfills, mines and international shipping of raw aluminium are considered.

The environmental benefits of recycling aluminium are also enormous. Only around 5% of the CO2 is produced during the recycling process compared to producing raw aluminium (and an even smaller percentage when considering the complete cycle of mining and transporting the aluminium).[5] Also, open-cut mining is most often used for obtaining aluminium ore, which destroys large sections of the world's natural land. Producing a can from recycled aluminum requires 95% less energy than it would to produce a can from virgin materials.[6]

Process

Aluminium beverage cans are usually recycled in the following basic way:[7]

  1. Cans are first divided from municipal waste, usually through an eddy current separator
  2. Cans are cut into little, equal pieces to lessen the volume and make it easier for the machines which separate them.
  3. Pieces are cleaned chemically/mechanically.
  4. Pieces are blocked to minimise oxidation losses when melted. (The surface of aluminium readily oxidizes back into aluminium oxide when exposed to oxygen. [8])
  5. Blocks are loaded into the furnace and heated to 750 °F ± 100 °C to produce molten aluminium.
  6. Dross is removed and the dissolved hydrogen is degassed. (Molten aluminium readily disassociates hydrogen from water vapor and hydrocarbon contaminants.) This is typically done with chlorine and nitrogen gas. Hexachloroethane tablets are normally used as the source for chlorine. Ammonium perchlorate can also be used, as it decomposes mainly into chlorine, nitrogen, and oxygen when heated.[9]
  7. Samples are taken for spectroscopic analysis. Depending on the final product desired, high purity aluminium, copper, zinc, manganese, silicon, and/or magnesium is added to alter the molten composition to the proper alloy specification. The top 5 aluminium alloys produced are apparently 6061, 7075, 1100, 6063, and 2024.[10]
  8. The furnace is tapped, the molten aluminium poured out, and the process is repeated again for the next batch. Depending on the end product it may be cast into ingots, billets, or rods, formed into large slabs for rolling, atomized into powder, sent to an extruder, or transported in its molten state to manufacturing facilities for further processing.[11]

Ingot production using reverberatory furnaces

The scrap aluminium is separated into a range of categories i.e. irony aluminum (engine blocks etc), alloy wheels, "clean aluminium" Depending on the specification of the required ingot casting will depend on the type of scrap used in the start melt. Generally the scrap is charged to a reverberatory furnace (other methods appear to be either less economical and/ or dangerous) and melted down to form a "bath". the molten metal is tested using spectroscopy on a sample taken from the melt to determine what refinements are needed to produce the final casts. After the refinements have been added the melt may be tested several times to be able to fine tune the batch to the specific standard

Once the correct "recipe" of metal is available the furnace is tapped and poured into ingot moulds, usually via a casting machine. The melt is then left to cool, stacked and sold on as cast silicon aluminium ingot to various industries for re-use.

Secondary aluminium recycling

White dross from primary aluminium production and from secondary recycling operations still contains useful quantities of aluminium which can be extracted industrially.[12] The process produces aluminium billets, together with a highly complex waste material. This waste is difficult to manage. It reacts with water, releasing a mixture of gases (including, among others, hydrogen, acetylene, and ammonia) which spontaneously ignites on contact with air[13]; contact with damp air results in the release of copious quantities of ammonia gas. Despite these difficulties, however, the waste has found use as a filler in asphalt and concrete.[14]

See also

References

  1. ^ Recycling | The price of virtue | Economist.com
  2. ^ http://minerals.usgs.gov/minerals/pubs/commodity/aluminum/mcs-2008-alumi.pdf minerals.usgs.gov
  3. ^ Schlesinger, Mark (2006) (in English). Aluminum Recycling. CRC Press. p. 248. ISBN 9780849396625. http://books.google.com/books?id=DtX1nbel49kC. 
  4. ^ WasteOnline: Metals aluminium and steel recycling
  5. ^ a b International Aluminum Institute
  6. ^ Recycling Aluminum Cans - Fun Facts
  7. ^ aluminum.org: How Is An Aluminum Can Recycled?
  8. ^ www.metalwebnews.com: Melting Practice
  9. ^ key-to-metals.com: Aluminum Casting Problems
  10. ^ http://en.wikipedia.org/wiki/Aluminium_alloy
  11. ^ Alcoa Primary Aluminum - North America: Products
  12. ^ Hwang, J.Y., Huang, X., Xu, Z. (2006), Recovery of Metals from Aluminium Dross and Salt cake, Journal of Minerals & Materials Characterization & Engineering. Vol. 5, No. 1, pp 47-62
  13. ^ Why are dross & saltcake a concern?
  14. ^ Dunster, A.M., Moulinier, F., Abbott, B., Conroy, A., Adams, K., Widyatmoko, D.(2005). Added value of using new industrial waste streams as secondary aggregates in both concrete and asphalt. DTI/WRAP Aggregates Research Programme STBF 13/15C. The Waste and Resources Action Programme

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