Tires are recycled typically in several
ways. After tires are delivered to the recycler, they have the bead removed.
This is a tough wire along the inside surface next to where the tire is mounted
on the rim. It's mechanically cut or pulled out. The tire is then fed into a
device that cuts it into large chips.
The steel belts are removed (though not
easily recycled due to the amount of rubber stuck to them) followed by removal
of the nylon cords that are also part of the construction of the tire. The
remaining rubber is then ground into smaller pieces.
Where the two methods differ is that in one
method, this is done at room temperature while in the second method, the tires
are cooled in liquid nitrogen and cryogenically ground into various sizes from
several millimeters in diameter down to a size similar to talcum powder.
Larger
sizes are often used as-is for ground cover (mulch) or subsurface fill. In the
latter case, a layer of tire crumb beneath the topsoil and grass of a park
allows the water to percolate quickly from the surface and reduce puddling
after rain.
Outdoor amphitheaters often use this
material. Other sizes can be blended with fresh rubber (non-vulcanized) and
fashioned into new products. Things like door mats, floor covering, and
inexpensive rubber goods can be made in this way and the tire is merely a
compatible filler.
Several years ago, a trio of companies
(ATR, Praxair and Sti-K in Canada) explored methods of re-vulcanizing recycled
tire material. There were some Russian and Malaysian patents on this topic. By
careful blending of raw isoprene with finely ground tire crumb along with some
other ingredients, and subjecting it to severe shear forces, the rubber from
the tires and the fresh isoprene can be combined in a manner that allows it to
be re-vulcanized into more sophisticated rubber goods.
To my knowledge, this process has not
been commercialized in a large scale but holds the prospect of being able to
convert large amounts of tire material into useful products. One use of
cryogenically ground tire has been banned in some locations - the use of the
material as road or highway fill. Because small amounts of metal may remain in
the tire crumb, it is prone to catch fire if compressed in large quantities and
in contact with moisture and oxygen.
One spectacular case of road-bed fire
was reported in the western US several years ago. The only other use of tire
material in large quantities is energy recovery where tire material is burned
to produce electricity in much the same way coal is burned.
Because of sulfur content, this is not
a clean process and special care must be taken to prevent excessive air
pollution when using tire material in this manner. Another product developed in
the late 1990's was a high-impact composite of recycled tire crumb and resin to
produce a tough, chemical and fire resistant - essentially bullet-proof
plastic.
This formula has been placed into the public domain by NIRIS, Inc. to
promote the recycling of tire material into useful products. (It free to use
without payment of licensing fees as long as NIRIS is credited with the
discovery of the formulation.)
Tread rubber is 'crumbed' and made into soft surfaces for playgrounds, horse exercise yards etc.
The rubber crumb is now being used in tarmac to resurface roads.
Whole tires are sometimes used to 'engineer' groundwork but it is illegal to landfill whole or part tires.
Whole tires are strung together and sunk of the coast to form artificial reefs for fish breeding.
The most environmentally friendly disposal method is to use the tires cut up into 2" squares and then used as a fuel in cement kilns. The volcanic temperature attained ensures the tire and any residue actually disappears! It also provides a readily available cheap fuel for the kiln.
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