Processing waste tyres

A promising and relatively new way to utilize both whole and processed old tyres (Fig. 1) is to recover the energy contained in this waste material. As an energy source, tyres represent an attractive alternative to other fuels due to their high heating value. On average, the... 

Z3A [Zero environmental damage, 3rd Alternative] A process for pyrolysing waste tyres using microwave energy. Developed and piloted in the 1990s by Amat, Crewe, UK. Not yet commercialized. 

There are a few companies that advertise on the Internet proprietary processes for micro-wave pyrolysis of wastes. Not surprisingly, considering what was presented in the previous section, most of these refer specifically or specially to the pyrolysis of waste tyres. 

With regard to the recent waste tyre situation in Europe, by 2010 the European Tyre Rubber Manufacturers Association (ETRM A) was able to claim that the market for tyre-derived products was sustainable and economically viable and that the European recovery rate for end-of-life tyres had reached 96% [7]. In the same article the UK was reported as treating 100% of its end-of-life tyres. The figures that were included showed that of the 479,000 tonnes of end-of-life tyres arising in the UK during 2009, 8.5% were retreaded, 45.1% were processed into other materials, 24% were used as a source of energy, and 19% were re-used or exported. 

Shen and co-workers [29] used a twin-screw extrnder to devnlcanise waste tyre rubber and then blended the resnlting DR into high-density polyethylene (HDPE) to produce thermoplastic elastomers (TPE). They found that the high shear conditions during the extrusion process induced chain scission and oxidative degradation of the GTR. The reduction in crosslink density was apparent in a decrease in the gel content of the rubber/HDPE blends and an increase in the melt flow of the materials. 

A research group in Canada [34] have used supercritical CO2 in a twin-screw extruder to devulcanise waste rubber from the automotive sector. They have used the process on both waste tyre rubber and EPDM-based products, such as door seals, and claim to have achieved good results with both types of material. The process uses that fact that as the CO2 swells the rubber in the high shear environment within the twin-screw extruder, the sulfur-sulfur crosslink bonds break preferentially compared to the carbon-carbon main-chain bonds (Section 4.2). 

A company called Revultec have developed a devulcanisation system called the Revultec system [128], which uses a relatively inexpensive solvent, such as hexane, in its supercritical state to devulcanise waste tyre crumb rubber. At the end of the process, up to 98% of the solvent can be recovered for re-use, and it is claimed that the DR is of a sufficiently high quality to make new rubber products, including tyres. 

At the conclusion of the project, the benefits of the DevulC02 process for devulcanising waste tyre rubber could be summarised as follows ... 

The use of high-pressure sintering processes to consolidate rubber crumb (often waste tyre crumb) into new rubber products has been evaluated by a number of workers and has been carried out for many... 

The possibility of blending waste injection-moulded PP with waste tyre crumb has been explored by a team drawn from both Loughborough and Moratuwa universities [5]. The two starting materials were blended in a range of different proportions, and samples of each were fully characterised using a range of physical tests and analytical methods. The team was particularly interested in how the blend ratio influenced the crystallinity and phase morphology of the material and how this affected its processibility and mechanical properties. 

Some studies have looked at producing useful products, effectively fillers, from waste rubber by using methods that use a partial pyrolysis approach. Silica is being used as a part replacement for carbon black as a filler in the production of green tyres . As this practice increases, there will be a corresponding increase in the total recoverable inorganic fraction from waste tyres, and so this recycling route could become more attractive. Examples of partial pyrolysis processes are described below. 

An important marketable product that can be obtained from the char fraction is recovered carbon black (also called PCB). The carbon black recovered from the pyrolysis of waste tyres accounts for around 30-35% of the total material generated by the process, with the exact figure depending on the types of tyres being pyrolysed. 

An example of a company that is producing carbon black that has been recovered from waste tyres on a commercial scale is Carbon Clean Tech AG, which has had an industrial-scale plant operating since 2012. In a paper delivered to the Tire Technology 2014 conference in Cologne, Yon [8] of Carbon Clean Tech AG explained the economic and ecological rationales of the process as being ... 

Du and co-workers [13] have carried out a study on PCB recovered from waste tyres. The scope of their work involved characterising the microstructure of the black using a scanning electron microscope (SEM) and then compounding the black into a number of ethylene-propylene-diene monomer (EPDM) compounds and producing cured samples. The influence of the PCB on the processing properties of... 

Curti Costruzioni Meccaniche SpA, has developed a batch pyrolysis plant process for waste tyres that is capable of producing a number of end prodncts, e.g., syngas, oil and char. The char produced by this batch process has been analysed by XRF and the results obtained are shown in Table 8.3. 

In addition to the batch process, Curti have finalised the design for a continuous pyrolysis plant to produce secondary raw materials and energy from waste tyres, which will be constructed according to a patented system. The capacity of the plant will enable it to process 10,000 tonnes of whole waste tyres per annum and will produce a number of products, for example ... 

Another type of depolymerisation process involves reducing the waste rubber from tyres back to its very basic chemical units of carbon monoxide and hydrogen. For example, a process for the conversion of waste tyre rubber into butadiene has been reported by GEM Fuels [21]. The process first converts the rubber into ethanol, which is oxidised to acetaldehyde, and then catalytically reacts with additional ethanol to generate butadiene. The butadiene can then be mixed with styrene in various proportions and copolymerised to produce virgin SBR, which has similar properties to the SBR used initially to produce the tyre. 

---