A new sensor system can determine the age and purity of plastic waste to improve recycling rates, according to its developers.

Also designed to prevent unnecessary carbon emissions from waste-to-energy plants, the suite was developed by the Waste4Future project, led by the Fraunhofer Research Institution for Materials Recycling and Resource Strategies (IWKS) in Alzenau, Germany.

In Germany alone, roughly 6m tonnes of plastic waste are generated each year. Approximately half is mechanically recycled into new materials, while the rest is incinerated for energy recovery, releasing carbon dioxide.

“Low-quality plastic with too much contamination is incinerated, while high-quality plastic is sorted according to colour and sold as recycled material. But recycling these valuable materials according to type is complicated,” said Dr Gert Homm, research scientist at Fraunhofer IWKS.

“A lot of packaging is not even considered recyclable by the sorting facilities and ends up in the incineration plant as residual waste. Many of today’s sensors fail to recognize black plastic, and even yoghurt cartons with aluminium foil lids end up with the aluminium by mistake, and then in residual waste.”

The new sensor suite uses infrared and terahertz sensors to determine how pure or degraded materials are, as well as how old they are, which is relevant when assessing whether they are suitable for mechanical recycling. When something is too badly damaged, it can only be chemically recycled.

The system, which uses machine learning, is designed to be installed above the conveyor belt at sorting facilities. Compressed air nozzles then sort the target materials and unwanted contaminants.

The project team is investigating mechanical (melt extrusion, solvent-based purification and fractionation) and chemical (solvolysis, pyrolysis, gasification) recycling processes, and testing them for their suitability for different compositions of plastic waste. Taking the energy consumption and carbon footprint of recycling processes into account, the system decides which recycling route is the most sustainable.

“A sustainable society with climate-neutral processes demands significant adjustments to value chains, which can only be achieved through innovation,” said Dr Homm.

“We are following up on this as part of the project by working out the best possible route for recycling as well as the optimal sorting process, taking financial and ecological considerations into account, thereby contributing to a significant reduction in carbon emissions compared to energy recovery and enabling a high degree of recycling for waste containing carbon.”

Extracted from IMechE website, read more here

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