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Publications archive - Waste and recycling

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Key departmental publications, e.g. annual reports, budget papers and program guidelines are available in our online archive.

Much of the material listed on these archived web pages has been superseded, or served a particular purpose at a particular time. It may contain references to activities or policies that have no current application. Many archived documents may link to web pages that have moved or no longer exist, or may refer to other documents that are no longer available.

A National Approach to Waste Tyres

Commonwealth Department of Environment, 2001

6.7.2 Recycling

Analysis of various recycling technologies is difficult because there are limited data available on the energy, greenhouse gas emissions and other impacts of various recycling technologies. Based on available information a number of scenarios can be investigated as discussed in the following sections.

Retreading

A number of authors have published data in broad terms about the energy and material savings from retreading. Retreading utilises a significant proportion of the rubber and all the fabric and steel in a tyre. The processing energy is reported to be lower than for a new tyre though the actual reduction varies depending on the type of retreading (whether hot or cold or remoulding). Even using rough estimates it is evident that retreading has significant potential to reduce overall energy and greenhouse emissions, as well as reduce the quantity of waste tyres that are produced. The energy to retread a passenger tyre is approximately 400 MJ (compared with 900 MJ for a new tyre) of which 75% (300 MJ) is estimated to be contained in the retread materials and the remainder is energy used in the process.

Using these figures and assuming one retreading cycle for passenger tyres and five retreading cycles for truck and bus tyres and that the expected life of retreads is comparable with that of new tyres, the benefits due to different rates of recycling can be predicted as shown in Figure 6.5.

Estimated overall energy saving from retreading tyres

Figure 6.5 Estimated overall energy saving from retreading tyres

6.7.3 Crumbing and shredding

The energy and greenhouse effects of crumbing and shredding are largely determined by the final size of the rubber particles. Figure 6.6 shows the estimated energy to process rubber to various final sizes based on data from IRRDB25. The diagram shows that as the particle size decreases the energy increases substantially, rising to 100 MJ/kg for cryogenically ground rubber. This approaches the quantity of energy to produce new synthetic rubber. As crumb rubber without further treatment is only used as filler this suggests that the use of finely ground rubber may not be justified from a resource and energy perspective. However, as the data upon which the figure is based is both limited and dated, more detailed investigations would be necessary to confirm this observation. Further, the value of crumb rubber when used (in small proportions) as a direct substitute for new rubber is much higher than as dead filler. Also as a significant source of crumb rubber in Australia is a by-product from retreading, the environmental costs and benefits of crumbing may, at least in this instance, be better than indicated in Figure 6.6.

Energy to process waste rubber

Figure 6.6 Energy to process waste rubber to different final particle sizes

6.7.4 Material reclaim

Material reclaiming is the process whereby the waste rubber product is devulcanised by chemical and or thermal processes. This produces a raw rubber polymer that can be substituted for new rubber. Though reclaiming was once common practice, as far as we are aware India is the only country that is currently reclaiming any significant quantity - approximately 70,000 tonnes of waste rubber each year. The conditions that make this possible (low labour costs and demand for rubber that exceeds supply) may also be present in other developing nations.

No data were found on the energy and other environmental costs for reclaiming rubber. What can be said is that, as the process involves initial granulation followed by the use of various solvents, rubber reclamation can be expected to result in impacts beyond those associated with crumbing. The key benefit of reclaiming is that it results in a product that approaches the properties of new natural or synthetic rubber and therefore has a far greater substitution value, which needs to be balanced against the added financial costs and environmental impacts.

6.7.5 Surface modification

Waste vulcanised rubber is, by design, very chemically stable. This makes it difficult to combine and bond to other substances. Surface modification includes a range of processes that change the surface chemistry of the rubber particles so that they can combine more readily with other substances.

No data have been found on the energy and material costs of surface modification. In comparison to rubber crumb (the starting point for surface modification), there will be increased energy costs and the use of certain chemicals in the process poses additional risks to the environment.

In return for the additional costs and environmental impacts of the surface modification process, the resulting product has much improved properties (particularly strength) and can be used in significantly more critical applications. Thus, surface modified waste rubber can be expected to displace an increased quantity of rubber made from virgin materials. The avoided energy and environmental impacts of the virgin rubber is an offset in determining the net financial costs and other impacts of surface modification.

6.7.6 Energy recovery

However, when comparing energy recovery with retreading, the valid comparison is the energy to produce a tyre. The energy that can be recovered from a tyre in, say, a cement kiln is approximately 25% to 35% of the total energy to produce a tyre. To this should be added the additional benefits of displacing iron feed to cement, which is estimated to add at most 1%, and displacement of other fuels, which would add further benefits for coal and slight negative impacts for gas. From this it can be seen that while energy recovery in a cement kiln represents complete waste elimination with reduced environmental impacts it represents only about 30% recovery of the ‘value’ in the tyre that is available for, and can be beneficially used in, retreading.

25 IRRDB web site at www.irrdb.org