Recovering Plastics via the Hydraulic Separator Multidune: flow analysis and Efficiency Tests

Document Type : Original Research Paper


1 Department of Civil and Environmental Engineering, Sapienza University of Rome, via Eudossiana, 18 – 00184, Rome, Italy

2 Department of Chemical Engineering Materials & Environment, Sapienza University of Rome, via Eudossiana, 18 – 00184, Rome, Italy


Plastic recycling is the process of recovering scrap or waste plastics and reprocessing the
material into useful products that sometimes differ completely in form from their original state. An important
issue in plastic recovery and recycling is that plastic waste usually contains a variety of plastics that differ in
their physical and chemical properties. Separation of recovered plastics into distinct classes is a fundamental
prerequisite for their use as secondary raw materials. The Multidune separator is a hydraulic channel that
permits solid particle sorting on the basis of differential transport mechanisms. Steady flow conditions are
established within the apparatus. An image analysis technique was employed to reconstruct the trajectories of
tracer particles within the fluid and to determine the evolution of the velocity field with time. Pollen and
plastic particles were used as tracers. Unlike plastic particles, pollen is expected to passively follow the fluid
flow field. Tests on monomaterial and composite samples were conducted while varying the operative conditions,
and comparisons were made. The presence of three different recirculation areas occurred regardless of the
hydraulic head at the Multidune inlet except for the first and last chambers. The lower recirculation zone is
larger than the upper recirculation zone because of geometrical constraints. With variation in the hydraulic
head, the geometry of the inner structures does not change appreciably. If a plastic particle within the principal transport path interacts with the lower recirculation area and its physical characteristics (density and dimensions) are such that it remains trapped, the separation process is successful. Because of its smaller dimensions and the reduced value of its velocity field, the upper recirculation area is relatively ineffective in the separation process.