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
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.
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