Microwave-induced aluminium recovery
Pyrolysis is one of the principal routes for recycling plastic wastes. In such a process both the calorific and chemical values of the plastic are recovered, as opposed to incineration in which the chemical value of the polymer is completely lost. Most research and development on pyrolysis processes has been carried out with homogenous waste or pure plastics, rather than with more realistic materials that contain additional contaminants. Materials like aluminium/polymer laminates, used mainly as packaging for food and beverages and for other products, such as toothpaste, are often found in municipal solid waste (MSW) and, as they do not currently have an easy recycling method, most waste of this kind ends up as landfill.
A new approach to pyrolysis is being developed under the the Engineering and Physical Sciences Research Council (EPSRC) WRM3 programme at the University of Cambridge's Department of Chemical Engineering by head of department, Prof Howard Chase, and research student, Carlos Ludlow-Palafox. They are working on microwave-induced pyrolysis, a process which involves mixing plastic-containing wastes, which are known to have a very high transparency to microwaves, with a highly microwave-absorbent material, usually particulate carbon.
The microwave-induced pyrolysis process shows considerable potential for the treatment of aluminium/polymer laminates since the process is comparatively gentle and, therefore, fragile materials such as the thin aluminium layer within laminate packaging wastes can be recovered clean and ready for reuse following smelting.
This novel process not only provides a way to recover the chemical and energetic value of the waste by liberating a hydrocarbon fraction as a result of thermal cracking of the plastic, but also the possibility of recovering additional materials with commercial value, such as aluminium. Data from the Aluminium Foil Recycling Campaign indicates that the UK market for aluminium laminated plastic and paper packaging is 15,300 tonnes per year, the vast majority of which ends up in landfill even though it contains approximately 1,500 tonnes of aluminium. Across Europe, the proportion of collected beverage cartons containing aluminium foil varies between countries from 10 per cent to over 80 per cent giving a potential recovery of 4,000 tonnes of aluminium from this source alone.
Trials have been performed with aluminium/polymer laminates used in toothpaste packaging with an aluminium content of approximately 30 per cent. The laminate has layers of polymer on both sides of a 45-micron aluminium foil. The process separtes these layers using a combination of microwave heating and controlled temperature. The aluminium liberated from the laminate can then be easily separated from the carbon bed by coarse sieving and shows a shiny and clean surface, with almost 100 per cent of the aluminium originally present in the laminate is recovered.
The plastic content of the laminate is transformed into a wide variety of organic compounds that can be used in other chemical processes. Of the total plastic content, 80 per cent approximately is transformed into oils/waxes and the other 20 per cent into non-condensable gaseous compounds.
Additionally, the composition of the gases is important for the economic contribution that these gases can make to a commercial process since they could be burnt to help to make the process more self-sustainable energetically. The main compounds found in the gases are linear alkenes and alkanes which is ideal for the potential use of the gases as a fuel to drive the process.
Ongoing work is centred on developing a continuous process and performing the
engineering design of a pilot or full-scale trial unit. Large-scale microwave
processes are now widely used for industrial food processing and these technologies
could easily be adapted to this new role.