Microwaves can turn muck into aluminium

Researchers have devised a way to use microwaves to turn aluminium-laminated plastic waste into aluminium for smelting and hydrocarbons for fuel in just three minutes.

It started with a bacon roll and a microwave oven, and now it's poised to transform the recycling of a packaging material that has been as unrecyclable as it is useful. The bacon roll, as the story goes, was microwaved for so long it turned into a charred mass of carbon that began to glow red-hot. What was happening was an intense heating process called microwave-induced pyrolysis.

On hearing about the 'over-microwaved' bacon roll from an acquaintance, chemical engineers Howard Chase and Carlos Ludlow-Palafox from the University of Cambridge wondered whether the process could be exploited to recover useful materials from packaging wastes.

Particulate carbon is an efficient absorber of microwaves and can transfer this thermal energy to adjacent materials. If the adjacent material is organic, such as plastic or paper, it breaks apart (or pyrolyses) into smaller pieces; if the material is a metal attached to the plastic or paper, the metal can be recovered in a clean form after the attached organics are pyrolysed.

Fifteen years later, and the tech they created is now being used in a commercial-scale plant designed, built and operated by Cambridge spin-out Enval Limited. Founded by Ludlow-Palafox, with Chase as R&D Director, Enval is using the plant to demonstrate the capabilities and economics of the process.

Enval has focused on plastic-aluminium laminate packaging. Prized by manufacturers for its lightness, cheapness and ability to protect the contents from light and air, the packaging is commonly used for food, drink, toothpaste, pet food and cosmetic products.

However, the combination of plastic and aluminium in the packaging presents a technical recycling challenge that until now has been unsolved; instead, items packaged like this contribute to the millions of tonnes of rubbish each year. For the brands who package their consumer goods this way, the 'recyclable logo' on the packaging, and the sustainability credentials that go with this, has been all-elusive.

"We have carried out a life-cycle assessment of the packaging and it's still environmentally better to use these laminates even though they are not recyclable, just because so little materials and energy goes into making and transporting them compared with alternatives like glassware and cans," said Ludlow-Palafox..

"There is no real drive to replace them and their market use is increasing by about 10-15 per cent every year. In the UK, roughly 1,60,000 tonnes of laminates are used per year for packaging, which means at least 16,000 tonnes of aluminium is going into the ground. Just imagine if we could routinely recycle this." .

The solution he and Chase developed with funding from the Engineering and Physical Sciences Research Council started in a relatively simple way: they placed a pile of particulate carbon and some shredded laminated packaging inside a conventional 1.2 kW kitchen microwave, replaced the air inside the oven with nitrogen and turned the microwave up to full power until the temperature increased to about 600C..

When they opened the door two minutes later, the laminated material had been separated into clean aluminium flakes and hydrocarbon gases and oil. The basic chemistry is still the same in the commercial-scale plant but the oven is now 150 kW and large enough to be housed in a 100 m2 industrial unit. It takes just three minutes to convert waste into aluminium for smelting, and hydrocarbons for fuel, and with no toxic emissions. Now fully commissioned, the plant can recycle up to 2,000 tonnes of packaging a year and it generates enough energy to run itself.

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