Extrusion International 5-2022

88 Extrusion International 5/2022 RECYCLING Closing the Plastic Cycle Highly Efficient Filtration for Chemical Recycling of PET Plastics can be reused again and again, delivering recycled items of the same quality as new goods – thanks to chemical recycling processes. In the case of PET in particular, new, innovative processes are successfully breaking down used plastics into their chemical components and thus reintroducing them into the material cycle. All these approaches have one thing in common: The purity of the recovered components determines the quality of the end products. A closer look at the processes shows how efficient filtration and cake washing contribute to a technically functional and economically viable solution. P lastic waste is one of the key envi- ronmental problems of our time. But if suitable recycling processes are applied, used plastic can be turned into a valuable secondary raw mate- rial – and this represents a crucial step towards addressing the issues of raw material shortages and high waste volumes. The need for these process- es is also reflected in the legal frame - work: in the EU from 2025 onwards, bottles made of polyethylene tereph- thalate (PET) must contain at least 25 percent recycled plastic, and by 2030 all packaging on the local market must be reusable and recyclable. But higher recycling rates also offer considerable economic potential, pro- vided that the plastic industry takes the right steps today. According to the consulting firm McKinsey, reuti - lization and recycling plastics could lead to increased earnings of up to $60 billion for the industry. [Cf. Mc Kinsey (2018): How plastics waste re- cycling could transform the chemical industry.] Chemical recycling of PET Conventional recycling processes have limitations. For mechanical re- cycling, the plastic is melted and then converted to recyclate, whereby the material’s quality suffers. The pro- cess reduces the quality of the plastic. Therefore, it is impossible to produce new bottles from 100 percent recy- cled PET, for instance. Consequently, manufacturers are looking for and evaluating alternative methods. In the meantime, an increasing number of companies are successfully using chemical recycling to achieve a recy- cling economy. Chemical recycling is the term used to describe all processes used for the depolymerization of plastics. In more simple terms, plastic objects are first shredded mechanically and then bro- ken down chemically into their basic chemical building blocks, often using a catalyst or enzyme. These have the same properties as raw materials pro- duced from petroleum and are fed di- rectly back into the plastics manufac- turing process. The decisive difference to mechanical recycling is that high- quality, ultra-pure materials gained through recycling flow into the plas - tics production process. The quality of the end products is equal to that of virgin material. In this way, for the first time, a closed rawmaterials loop for recycling has been created. Chemical recycling thus opens up new avenues and can at present be regarded as an alternative building block on the way to achiev- ing the targeted recycling quotas. The German Federal Environment Agency shares this assessment and classifies the chemical recycling of plastics as a preferred process under the Circular Economy Act. In addition to thermochemical de- composition in the absence of oxygen (pyrolysis) and gasification, recycling technologies that have been dis- cussed and tested in recent years in- clude solvent decomposition, known as solvolysis. Under the action of suit- able agents, thermoplastics in par- ticular, such as PET, are liquefied and broken down into their monomers. Since different solvents are used, the solvolysis processes can be broken down into the common methods us- ing glycolysis, methanolysis and hy- drolysis (sometimes with enzymatic enhancement). [Cf. The next genera - tion of recycling – giving plastic waste a new life: Angew. Chem. Int. Ed. (2020), 59, P. 15402-15423.] All three processes are suitable for degrading PET. The decision of which process is most suitable depends on the quality of the input material. Solid-liquid separation is a particularly important process step in the chemical recycling of PET. Several companies rely on technology from BHS-Sonthofen The BHS indexing belt filter is made of stainless steel and supports multi-stage countercurrent washing The Author: Christian Gassen, process engineer at BHS-Sonthofen Process Technology