The manufacture of pneumatic tyres requires a large cost in energy and raw materials.
Moreover, it is a great environmental problem once their life cycle is over, given that they are designed to resist all kinds of adverse conditions and are, thus, very complicated to eliminate.
The chemist, Ms María Felisa Laresgoiti, chose to study the process of pyrolisis, both in order to recover part of the energy and material costs of the manufacture of pneumatic tyres as well as to solve the problem of their elimination.
Pyrolisis is heating in the absence of oxygen, which results in the decomposition of the pneumatic tyres and the reuse of the resulting components. With the results, Ms Laresgoiti presented her PhD thesis at the University of the Basque Country (UPV/EHU), entitled, Chemical recycling of pneumatic tyres through pyrolisis.
Ms Laresgoiti used a 3.5 litre fixed bed reactor (one or more vertical tubes packed with particles which accelerate the process of reaction) and guaranteed the absence of oxygen —necessary for pyrolisis— by means of a nitrogen sweep. After a number of trials, she concluded that, at above 500 degrees and with 30 minutes of reaction time, the decomposition of the organic material of the tyres is complete. This decomposition generates 40 % of liquids and 16 % of gases, useable as fuels and/or sources of raw material. The remaining 44 % left after the process is solid inorganic material — loads, metals and soot, practically unaltered by the process and which can be reused for various applications.
The liquids arising from the pyrolisis are a complex mixture of organic products that can carry out the same function of certain fractions derived from petroleum, and so are reusable as an alternative to fossil fuels. For example, they can substitute fuel oil in certain cases, although the high calorific power of the liquid obtained and their high content of nitrogen and sulphur prohibit their general commercial use. These liquids also can be used in part as commercial petrol, commercial diesel for motor vehicles and commercial central heating oil. Nevertheless, even then they do not comply with the required legal specifications, and so would have to be treated or mixed with other sources.
Also, besides their use as fuel, these liquids are useful as a source of various valuable chemical compounds, such as styrene (they are used, for example in the synthesis of plastic materials) or limonene (used as a biodegradable solvent, amongst other things).
Apart from the liquids, gases also form part of the organic material obtained from the pyrolisis of pneumatic tyres. These are fundamentally made up of hydrocarbons, and their high calorific power makes them an important energy source. This source is not only sufficient for self-feeding the process, but there is a surplus which can be taken advantage of energetically.
The remaining 44 % of the product resulting from the pyrolisis of pneumatic tyres is inorganic. This is solid material and remains practically unaltered with respect to the dimensions and shape of the items before the process. This material is easily broken up into soot and steel filaments or strings from the tyre, and which can be reused or recycled independently.
According to the thesis, it is precisely the soot from the pyrolisis that could be used for a number of commercial applications. For example, Ms Laresgoiti believes that its possible application as a reinforcement in the manufacture of new pneumatic tyres should be considered. She also believes that soot could be of commercial use as semi-reinforcement material or non-reinforcement filler, as active carbon or pigment for inks.
About the author
Ms María Felisa Laresgoiti Pérez (Llodio, Bizkaia 1964) is a graduate in Chemical Sciences. She drew up her PhD thesis under the direction of Ms Isabel de Marco Rodríguez and Mr Juan Andrés Legarreta Fernández, both professors at the Department of Chemical Engineering and the Environment at the Higher Technical School of Engineering in Bilbao (UPV/EHU). It was in this department that the researcher carried out her work. Currently, Ms Laresgoiti is a specialist laboratory technician at the UPV/EHU.
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