Rubberwaste

Rubber tyres are by far the most visible of rubber products. Identification is trivial and collection is well organized. Recycling and disposal, however, are less evident. A major route for tyres is their use as a supplemental fuel in cement kilns. Major compounds in tyres are: styrene-butadiene rubber (SBR), synthetic and natural polyisoprene rubber, steel cord, carbon black, zinc oxide, sulphur and vulcanization-controlling chemicals. Tyres can be retreaded, which is economic for large sizes (truck tyres), or ground to crumb or powder (cryogenic grinding). Such materials have some limited market potential as an additive in asphalt, and in surfaces for tennis courts or athletics.

The macromolecular structure of (vulcanized) rubber can be degraded using thermal, mechanical, and chemical means [14]. The resulting reclaim was once incorporated in limited amounts in new tyres. Safety and quality considerations dried up this outlet.

Pyrolysis of tyres is a feasible, yet technically difficult operation. The handling of the remnants of the steel carcass, the carbon black, the zinc oxide, as well as the tendency to repolymerize of the major products are serious stumbling blocks. Various rubber pyrolysis technologies have been developed, using, e.g. fluid bed, rotary kiln (Sumitomo Cement), molten salts, or cross-flow shaft systems (WSL/Foster Wheeler).

Microwave pyrolysis results in relatively high-molecular-weight olefins and a high proportion of valuable products such as ethylene, propylene, butene, aromatics, etc. The short process time contributes to a reduction in the processing cost. Whole tyres or larger chips can be processed, which greatly reduces pre-processing cost. The rubber is transformed from a solid to a highly viscous fluid within milliseconds. With additional curative agents the viscous material can be moulded into new products.

Supercritical water can be used to controllably depolymerize the rubber compounds. Tires decompose into high-molecular-weight olefins (MW 1000-10000), or oils (max. 90%). Roy [9] discussed vacuum pyrolysis at 2nd ISFR.

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