Influence of pre-screening before ballistic separation on NIR-sorting quality of plastic rich 3D-fractions out of MCW
The exploitation of plastics gains more importance over time. In this con-text, material recycling is especially focused on packaging plastics. A further waste stream that contains a significant amount of plastics is mixed commercial solid waste. To assess the potential of plastics for recycling and energy recovery from these waste stream large-scale experiments were conducted. The potential of mechanical pre-processing with the aim of generating a 3D-plastics pre-concentrate was assessed. The focus of these investigations was put on the relevance of the screening stage and its influence on down-stream material processing via ballistic separation and sensor-based sorting. Results demonstrate not only that the screening of the waste leads to enrichment of plastics in coarse particle size ranges (especially > 80 mm) and transfer of contaminants, organics and minerals to fine fractions (especially < 10 mm), but also that sensor-based sorting performance can be significantly enhanced due to cleaning effects on plastics, induced by the material circulation and friction in a drum screen.
Mechanical recycling almost always involves wet mechanical processing for material preparation and sometimes also for chemical recycling. Fine material (< 10 mm), which mainly contains organic, inorganic impurities and heavy metals, is washed off. This leads to very costly waste water treatment. Residual impurities can also be carried over into the process and cause damage in the process itself (e.g. corrosion) or in the material (e.g. reduction in material properties). It is also known that, in material recycling, free fines (e.g. dust formation) and adhering impurities lead to lower material recognition efficiency when sensor-based sorting is applied. (Küppers et al. 2019)
Therefore, the use of various screening technologies in waste pre-processing is of essential importance for quality assurance to produce plastic pre-concentrates. The cleaner these are, the higher is the performance of downstream process steps. In this contribution, the relevance of mechanical pre-processing is practically examined.