Tyres are a disposal problem as they consume considerable airspace relative to their weight and cannot be compacted.Re-treading tyres is still one of the best ways to obtain maximum use from them while temporarily reducing their contribution to the waste stream. Ultimately, however, tyres still need to be disposed of.It is worth noting they have a calorific value well above that of coal, and therefore lend themselves to uses as a fuel in cement kilns and ordinary furnaces for the production of electricity.
Tyres are made from vulcanised rubber. This is a very tough form of chemically enhanced rubber. This material contains highly toxic, both for humans and the environment, substances, and is a potential fire hazard. When incinerated, tyres produce a lot of pollutants which are dangerous when released into the atmosphere. When put into landfill, tyres take a considerable amount of time to break down.
Some landfill companies are now operating a tyre chipping scheme. Tyres are brought to the landfill site in bulk from recycling centres, and are cut into chips or granulated. This can then be used in a variety of ways – as carpet underlay, as a soft cushioning material for children's play areas, or reprocessed further, and used to make rubber mats and other goods. Tyres can also be de-vulcanised and re-converted into rubber. This is of poor quality, but it can be mixed with virgin material to produce new tyres and other consumer rubber goods. Often, landfill companies use tyres as they are in the landfill to create a drainage system. This can lead however to toxic chemicals leaching from the rubber and into water systems.
TYRE RECYCLING PLANT
There are many ways in which tyres and inner tubes can be reused or reclaimed. The waste management hierarchy dictates that re-use, recycling and energy recovery, in that order, are superior to disposal and waste management options.
Recovery process
1.Product reuse
2.Repair
• Retreading
• Regrooving
3.Physical reuse
• Use as weight
• Use of form
• Use of properties
• Use of properties
• Use of volume
4.Material reuse
*Physical
*Physical
• Tearing apart
• Cutting
• Processing to crumb
*Chemical
*Thermal
• Pyrolysis
• Combustion
5.Energy reuse
• Incineration
Damaged tyres are, more often than not, repaired. Tubes can be patched and tyres can be repaired by one of a number of methods. Regrooving is a practice carried out in manydeveloping countries where regulations are slacker and standards are lower (and speeds arelower) than in the West. It is often carried out by hand and is labour intensive.
Secondary reuse of whole tyres is the next step in the waste management hierarchy. Tyres are often put to use because of their shape, weight, form or volume. Some examples ofsecondary use in industrialised countries include use for erosion control, as tree guards, inartificial reefs, fences or as garden decoration. In developing countries wells can be linedwith old tyres, docks are often lined with old tyres which act as shock absorbers, andsimilarly crash barriers can be constructed from old tyres. Old inner tubes also have manyuses; swimming aids and water containers being two simple examples.
The next step in our hierarchy involves the material being broken down and reused for the production of a new product. As mentioned earlier, in developing countries this handreprocessing of rubber products to produce consumer goods is well established and thevariety of products being made from reclaimed tyres and tubes is astonishing. The rubberused in tyres is a relatively easy material to reform by hand. It behaves in a similar mannerto leather and has in fact replaced leather for a number of applications. The tools requiredfor making products directly from tyre rubber are not expensive and are few in number.Shears, knives, tongs, hammers, etc., all common tools found in the recyclers’ workshop along with a wide range of improvised tools for specialised applications. Shoes, sandals,buckets, motor vehicle parts, doormats, water containers, pots, plant pots dustbins andbicycles pedals are among the products manufactured.
Another way in which physical reuse can be achieved is by reducing the tyre to a granular form and then reprocessing. This can be a costly process and there has to be amanufacturer willing to purchase the granules. Crumb rubber from the retreading processcan be used in this way, as it is a good quality granulated rubber. The reprocessingtechniques used are similar to those described in earlier chapters. Granulate tends to beused for low-grade products such as automobile floor mats, shoe soles, rubber wheels forcarts and barrows, etc., and can be added to asphalt for road construction, where itimproved the properties of this material.
Chemical and thermal recovery
This type of recovery is not only lower in the waste management hierarchy, but is also ahigher technology requiring sophisticated equipment. The applicability of such technologiesfor small-scale applications in developing countries is very limited. We will therefore lookonly very briefly at a couple of processes. Chemical recovery is the process of heatingwaste rubber reclaim, treating it with chemicals and then processing the rubbermechanically.
This type of recovery is not only lower in the waste management hierarchy, but is also ahigher technology requiring sophisticated equipment. The applicability of such technologiesfor small-scale applications in developing countries is very limited. We will therefore lookonly very briefly at a couple of processes. Chemical recovery is the process of heatingwaste rubber reclaim, treating it with chemicals and then processing the rubbermechanically.
• Acid reclamation – uses hot sulphuric acid to destroy the fabric incorporated in the tyreand heat treatment to render the scrap rubber sufficiently plastic to allow its use as afiller with batches of crude rubber.
• Alkali recovery - Reclaimed rubber, treated by heating with alkali for 12 to 30 hours, canbe used as an adulterant of crude rubber to lower the price of the finished article. Theamounts of reclaimed rubber that are used depend on the quality of the article to bemanufactured.
Pyrolysis
One form of thermal recovery is pyrolysis. This involves heating the tyre waste in theabsence of oxygen which causes decomposition into gases and constituent parts. It is atechnology which is still immature in the tyre-reprocessing field.
One form of thermal recovery is pyrolysis. This involves heating the tyre waste in theabsence of oxygen which causes decomposition into gases and constituent parts. It is atechnology which is still immature in the tyre-reprocessing field.
Energy recovery
Tyres consist of around 60% hydrocarbons, which is a store of energy that can be recoveredby incineration. The heat produced can be used directly in processes such as cementmaking, or to raise steam for a variety of uses, including electricity generation. Again, thistechnology requires sophisticated plant and its application is limited when looking at small-scale enterprise.
Landfill
Landfill is the final step in the waste management hierarchy. The landfill disposal of tyres, ifproperly managed, does not constitute an environmental problem. However, concerns aboutconserving resources and energy have seen an increasing opposition to landfilling. Also,public sanitation and municipal waste management is often ineffective in developingcountries and scrap tyres are often found littering the streets
