Hydraulic separation of plastic wastes

This chapter presents an original device for the hydraulic separation of plastic polymers from mixtures. The separation process that takes place within the separator involves the interaction between two phases, that is, solid particles acting as the dispersed (or dense) phase and water acting as the continuous phase. Due to the combination of the fluid flow pattern developing within the apparatus and density, shape, and size differences among plastic particles, the device allows their separation into two products, one collected within the instrument and the other one expelled through its outlet ducts. An extensive experimental campaign was conducted to investigate the effectiveness of the apparatus, using two geometric arrangements, nine hydraulic configurations, and three selections of polymers at three stages of a material life cycle. The experimental data were also employed to validate a numerical model developed within the framework of Computation Fluid Dynamics. For the tested operating conditions, two-way coupling takes place, that is, the fluid exerts an influence on the plastic particle and the opposite occurs too. Image analysis confirms the outcomes from the investigation of the two-phase flow via nondimensional numbers (particle Reynolds number, Stokes number, and solid phase volume fraction). The separation results were evaluated in terms of grade and recovery of a useful material. Under the proper hydraulic configurations, the experimentation showed that it is possible to produce an almost pure concentrate of polyethylene terephthalate (PET) from a mixture of 85% PET and 15% polycarbonate (PC) (concentrate grade and recovery equal to 99.5% and 95.1%), 85% PET and 15% polyvinyl chloride (PVC) (concentrate grade and recovery equal to 97.9% and 100.0%), and 85% PET and 15% polylactic acid (PLA) (concentrate grade and recovery equal to 91.4% and 86.1%).