Microwaves enable economical recycling of plastic-aluminum laminates
You may not know what they're called, but odds are you've eaten or drunk something from them. I'm referring to plastic-aluminum laminate (PAL) packaging, which has long been used for toothpaste tubes and in recent years has gained popularity as pouches for food, drink and pet food. Although it threatens to approach the ubiquity of the aluminum can or plastic bottle, PAL packaging lacks the familiar recyclable logo found on cans and bottles. But that could be set to change, with a process to recover the metals contained in PAL packaging, developed some 15 years ago by researchers at the University of Cambridge, now being demonstrated in a full commercial-scale plant.
Despite currently not being recyclable, PAL packaging does tick many other environmental boxes and is still considered more environmentally friendly than other packaging options, such as glassware and cans, when a full life-cycle assessment is taken into consideration. This is because, very little energy goes into the production of the packaging and it is extremely light, cutting transport costs. These attributes, plus the fact it protects contents from light and air, make it attractive to manufacturers.
"There is no real drive to replace them and their market use is increasing by about 10–15% every year," says Dr Carlos Ludlow-Palafox, a chemical engineer at Cambridge. "In the UK, roughly 160,000 tonnes (176,370 tons) of laminates are used per year for packaging, which means at least 16,000 tonnes (17,637 tons) of aluminum is going into the ground. Just imagine if we could routinely recycle this."
The seeds for this idea were sown in 1997, when Ludlow-Palafox started his PhD course under the supervision of Professor Howard Chase. The two of them heard about a bacon roll that had been overcooked in a microwave, leaving a charred mass of carbon that glowed red-hot. The bacon roll had just been through a process known as microwave-induced pyrolysis, where organic material succumbs to thermochemical decomposition when exposed to high temperatures. This leaves a clean form of the metal contained within the material, which can then be recovered.
The two researchers investigated further, starting by placing a pile of particulate carbon and some shredded laminate packaging inside a standard 1.2 kW kitchen microwave oven. They then replaced the air inside the oven with nitrogen and turned the oven on at full power until the temperature inside reached around 600° C (1,112° F). After two minutes, the laminated material had separated into impurity-free aluminum flakes and hydrocarbon gases and oil.
Now, 15 years later, the process the researchers developed is being put to the test in a commercial-scale plant in Luton, UK. The plant was designed, built and operated by Enval Limited, a spin-out company of Cambridge University founded by Ludlow-Palafox and Chase and is intended to demonstrate the capabilities and economics of the technology to potential investors and waste handling companies. It relies on the same basic chemistry used with the kitchen microwave, but the power of the oven in the commercial-scale plant has been increased to 150 kW and is large enough to be housed in a 100 m2 (1,076 ft2) industrial unit. It takes three minutes to convert the packaging into aluminum for smelting and hydrocarbons for fuel, with no toxic emissions.
The plant, which is partly funded by Nestlé and Kraft Foods/Mondelez International, is now fully commissioned and can recycle up to 2,000 tonnes (2,204 tons) of packaging annually and generates enough energy to run itself. Chase estimates that a plant similar to the demonstration plant would pay for itself within three years. Enval has already struck a deal with PAL packing manufacturers to recycle their industrial scrap at less than the cost of sending it to landfill.
"It was a chicken and egg situation," said Ludlow-Palafox. "No one is going to buy this technology unless this type of waste is separated for recycling, but the waste wasn’t going to be separated because there has been no process to recycle it. We had to break that negative loop somehow. Now we have the commercial-scale plant, we can show waste handlers the benefits and encourage local authorities to implement a selective collecting system."
But the researchers aren't stopping at PAL packaging, with Chase's group in Cambridge University's Department of Chemical Engineering and Biotechnology examining the potential of microwave pyrolysis in recycling of different types of waste.
"We’ve demonstrated that a lot of troublesome waste materials can be pyrolysed using our microwave technology but it’s not always economically sensible to do it; the challenge now is to identify which processes are likely to be commercially viable, and which of those will attract the necessary investment funding to bring them into commercial reality. This is a business sector that is comparatively unfamiliar to most investors who regularly commit to innovation in other areas. By demonstrating the societal and economic benefits of green technologies, we hope to secure the necessary investment to transform innovation into successful commercial practice."
The technology is described in the video below.