Tread carefully for waste management – The Physics Congress 200

Scrap tyres could provide an inexpensive source of raw materials for the chemical industry, according to Professor Paul Williams of the University of Leeds who will speak on Tuesday 25 March at the Waste Management conference, part of the Institute of Physics Congress at Heriot-Watt University.

Approximately 150 million scrap tyres are generated throughout Europe each year, about a fifth of those in the UK alone. Most of these are simply buried in landfills or accumulate in enormous scrap-tyre mountains in every European country. However, the European Waste Landfill Directive specifically proposes to prohibit the landfilling of whole or shredded tyres, so scientists are urgently investigating several viable alternatives for their disposal, recycling or re-use.

One possible treatment route is pyrolysis. “Pyrolysis is the degradation of the rubber of the tyre using heat in the absence of oxygen,” explained Professor Williams, “Pyrolysis of tyres produces an oil, carbon and gas products, in addition to the steel cord, all of which have the potential to be recycled.”

At first glance this solution sounds like the ideal answer, but as Professor Williams pointed out, there is some resistance to the establishment of pyrolysis technology for processing tyres because of a lack of interest in the market place for the end products themselves.

He and his colleagues in the Department of Fuel and Energy at Leeds, have been looking for ways to produce high-grade activated carbons and high value aromatic chemicals from the pyrolysis of scrap tyres rather than the low-grade pyrolysis products. Their efforts have focused on catalysts that help release useful small molecules from the tangle of rubber chains in a tyre, which Professor Williams said has the potential to improve the commercial viability of tyre pyrolysis. “We have refined the process to produce mixtures of oils and chemicals that have a real commercial value,” he added.

Compounds such as benzene, toluene and xylene can be produced with the Williams approach to pyrolysis. “These chemicals are major industrial chemical building blocks,” explained Professor Williams, “For example, the products from benzene are derivatives such as ethylbenzene, cyclohexane and (1-methylethyl)benzene which are used as basic materials for the production of plastics, resins, fibres, surfactants, dyestuffs and pharmaceuticals.” Xylenes and toluene are used in the plastics industry and in the production of dyes, pigments, pesticides, surfactants, solvents and polyester fibres.

The gases released by pyrolysis are composed mainly of hydrogen, methane and other hydrocarbons that can be used as fuels. “Pyrolysis can save energy,” said Professor Williams, “Rather than depleting the world’s supply of fossil fuels, we could deplete the world’s supply of tyre hill eyesores to yield fuel oil from up to sixty percent of the weight of tyres.”

The same approach may also allow other waste that is otherwise destined for landfill, such as plastic composite waste and textile waste, to be turned into useful, raw materials for industry.