PETpla.net Insider 06 / 2016
MATERIAL / RECYCLING 34 PET planet insider Vol. 17 No. 06/16 www.petpla.net RECYCLING S P E C I A L which is about 40% lower than the industry’s standard figure,” explains Ronald Fairbanks, General Manager Mpact Polymers. Stage 2: Washing module The shredded flakes are con- veyed to a silo of the washing module, and then precleaned in a dry mixer and the downstream zigzag sifter, to remove light-weight fractions like dust or label residues. They come into contact with water for the first time in a pre-washer, where they are treated at moderate temperatures. Jets and mechanical friction remove any ingrained dirt particles. In the down- stream caustic bath, any remaining labels and adhesives are detached in a mixture of hot water and caustic. The caustic solution used is prepared in a two-stage caustic treatment system featuring the Parcival filtration system, enabling it to be subsequently re-used in consistent quality. Then, the polyolefins are removed from the flow as well, that is the material of the bottle closures. This is accomplished using a sink-float process in a density separator: the polyolefins float to the top, while the heavier PET falls to the bottom. This is said to ensure very good flake quality. These PET flakes are then rinsed again with hot water in several stages and mechanically and thermally dried using hot air. Finally, in the washing module, any remain- ing residues of metals are separated. At the same time, the flakes are sorted by size using three sifters, and any flakes that are too small will be rejected. Stage 3: Decontamination module In this step, the sorted and washed flakes are treated to create bottle- grade recyclate. The decontamination modules are, for reasons of hygiene, located in a separate room with a positive air pressure. From a buffer silo, the washed flakes are first of all pre-warmed in the first heating worm, then dried in the second worm, and brought up to process temperature, which is well below PET’s melting temperature. This is followed by the heart of the decontamination module: the vacuum reactor, where migrated constituents are expelled from the materials and the intrinsic viscosity (IV value) is increased. After this treatment, food-grade PET flakes are now ready for use. It is precisely this stage that accounts for the recycling system’s relatively low energy consumption because, in com- parison to other recycling processes, the decontamination module is said to work swiftly, and therefore to require less energy. In the downstream extruder, the flakes are melted, extruded through a melt filter and pelletised. There, too, the energy consumption is lower in relative terms, because the flakes are already warm and dry from the decon- tamination process. The crystallisation of the pellets also utilises the residual heat, and does not require any addi- tional energy input. Once the products have cooled down, they are packed in big bags, and undergo extensive and thorough quality checks Pellets instead of flakes There were several reasons why Mpact opted for producing pellets instead of flakes as the end-product. “Flakes require higher capital invest- ment for the preform production operation: the pre-dryers need to be duplicated for equal thermal treatment compared to pellets. Furthermore, pellets are more compact than flakes, which means lower transportation costs. When new technologies like recycling are introduced, of course you’re looking to avoid all possible risks. These advantages, particu- larly the lower logistical costs, offset the higher energy costs entailed by producing pellets,” explains Hunt, and Fairbanks adds: “What’s more, rPET pellets behave better than rPET flakes when mixed with virgin PET.” www.krones.com The polyolefins are removed from the flow as well, that is the material of the bottle closures. This is accomplished using a sink-float process in a density separator: the polyolefins float to the top, while the heavier PET falls to the bottom. The heart of the decontamination module: the vacuum reactor, where migrated constituents are expelled from the materials and the intrinsic viscosity (IV value) is increased.
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