Improving the efficiency of tribo-electrostatic separation of textile fibers from shredded end-of-life hoses

Typical recycling steps of end-of-life hoses (ELH) include several shredding operations to reduce their size and break down their constituents. Subsequently, various processes such as electrostatic separation or other sorting methods may be employed to recover the constituent materials, which can be reused in new products, thus helping to reduce waste and conserve natural resources. The objective of this study was to validate the possibility of using a newly-patented tribo-electrostatic separator to sort the Polyethylene Terephthalate (PET) fibers and Polyurethane (PU) grains that compose the product obtained after the size-reduction operation of an important class of ELH. The tribo-electrostatic separator consists of a modified rotating-cylinder type triboelectric charger and two pairs of horizontal cylindrical rotating electrodes (length: 460 mm; diameter: 260 mm) connected to two high-voltage sources of opposite polarities (±30 kV), a configuration that is radically different from the standard vertical electrode systems. The initial mixture of shredded ELH, which contained approximately 40 % PET fibers and 60 % PU grains, was introduced into the triboelectric charger, which consists of a cylindrical sieve (length: 460 mm; diameter: 200 mm), rotating at speeds ranging from 5 rpm to 50 rpm. The tribocharger being located in the space between the electrodes, at mid-distance between them, the PET fibers or PU grains are attracted towards the electrodes as soon as they are sufficiently charged or polarized. The tests were carried out using several configurations of the tribocharger, obtained by adding chicanes inside the cylindrical sieve and varying its angle of inclination from 0.2° to 2.5°. Two mixtures of two different sizes, obtained by using two distinct types of shredders were subjected to the tribo-electrostatic separation process. The evaluation of the results was carried out by measuring the mass of the fibers collected at the electrodes and comparing it with the mass of the treated ELH. The tribo-electrostatic separator achieved a PET fiber recovery rate of approximately 25 %, with a purity of around 95 %, performances that make it an efficient solution for the recycling of the polymers contained in this class of waste.