The use of air tabling and triboelectric separation for separating a mixture of three plastics
Introduction
Plastics are made from limited resources such as petroleum. Therefore, many efforts are being made in recycling wasted plastics in order to minimize the amount of waste and solve the resource and energy problems. At present, there are three main alternatives for the recycling of wasted plastics in addition to landfilling, i.e.: (1) mechanical recycling, (2) feedstock recycling, and (3) energy recovery. The energy recovery, although an efficient alternative for reducing plastic wastes, is the subject of great public concern due to the contribution of combustion gases to atmospheric pollution. On the contrary, mechanical recycling and feedstock recycling are more attractive alternatives, as they produce materials that can be reused. Of the two methods, mechanical recycling is especially effective because it uses less energy and has a smaller environmental impact than feedstock recycling (Murakami, 2001). Nevertheless, the mechanical recycling is favorable alternative provided that via separation technologies a high-purity product can be achieved.
Consequently, the research work is focused primarily on developing and testing a variety of separation and sorting technologies able to sort plastics according to the type. Pascoe and Hou (1999) reported separation of PVC and PET using a LARCODEMS separator. Flotation (Shimoiizaka et al., 1974, Kounosu et al., 1978, Shibata et al., 1996, Drelich et al., 1998, Shen et al., 2002), and sink-float separation (Shimoiizaka et al., 1976, Fujita et al., 2000, Dodbiba et al., 2002a) are other well-known wet methods for sorting plastics. Although wet separation techniques provide adequate recoveries, separation in dry state may be of environmental benefits, since some requirements associated with wet separating methods in general such as (1) chemical pretreatment of materials, (2) treatment of water from the process for reuse or discharge, and (3) dewatering or drying the mixture after separation can be avoided.
The past reported studies for separation of plastics were focused mainly on processing binary mixtures, although the discharged wastes contain more than two different types of plastics. The present paper describes a process for separating mixtures of three plastics by means of dry techniques. In this frame, combination of the triboelectric separation and the dry gravity separation offers a promising alternative. The idea to apply these techniques was a logical step as certain plastics are suitable for density separation and almost all types of plastics, which are naturally dielectric, can be sorted by triboelectric separation after creating the favorable conditions for frictional charging.
Section snippets
Materials
Virgin polypropylene (PP), polyethylene terephthalate (PET), and polyvinyl chloride (PVC) were selected for investigation, as they are widely used in the manufacture of everyday products (such as beverage containers, household items, packaging and furniture). Size-reduction of plastics was carried out in a shredder provided by Nissui Kako Co. LTD of Japan (Nissui Scutter, type: SA—22). A representative fraction of the plastics was then sieved to give a size fraction of +2.38–1.63 mm. The density
Triboelectric separator
The triboelectric separator consists of six components: a feeder, a blower, a cyclone (named as tribo-cyclone), two vertical-plate electrodes, a DC power supply and five collecting bins (Fig. 1). The tribo-cyclone is a device that utilizes the centrifugal force to charge the particles due to their acceleration and friction against its inner lining (named as charging surface). The inner lining of tribo-cyclone was a plastic material that in TES (Table 1) was classified between the plastic
Evaluation and data handling
The difference in color between plastic types allowed easy hand-sorting and analysis of the fractions discharged from the separating devices. The individual separated fractions were collected to determine the mass, and the purity and the recovery were then calculated.
Effort has been made by some authors to develop a single-value parameter known as efficiency. Rietema (1957) enumerated various requirements that should be fulfilled by such a parameter and introduced a formula. Schulz (1970)
Effect of particle size
Considering the forces acting on a charged particle of spherical shape falling due to the gravity in the area between the vertical-plate electrodes, it was found that the particle size (D) is a function of the electric field strength (E), the magnitude of the surface potential (Vs), and the density (ρs), (Dodbiba et al., 2002b):where coefficient k, found experimentally, is dimensionless and laid between 1 and 1.8.
A graphical representation of the model (Eq. (3)) is shown
Flowsheet analysis and layout of the separating devices
The layout of the separating devices was designed after carefully analyzing the composition and the physical properties of the feed. Considering the densities of plastics and their relative positions in the triboelectric series (Table 1), it was concluded that a satisfactory separation could not be obtained by either triboelectric separation or air tabling alone. Thus, a two-stage process combining the two techniques was tested in the laboratory. A general flowsheet of the dry process for
Experimental procedure
A mixture of PP, PET and PVC was selected for investigation. Each component amounted to 1/3 of total mass of the mixture. Thus, high-density plastics (i.e. PET and PVC) amounted to approximately 67% of total. The air table (Fig. 2), because of relatively high content of the high-density plastics, was used for the first-stage of the process (case 1 of Fig. 9). It was employed to produce a low-density fraction, which consisted mainly of PP, and to collect the rest of the feed as high-density
First stage of separation (air tabling)
During the first stage of separation, the mixture of PP, PET and PVC was subject to air tabling (Fig. 2). The operating parameters were maintained at the following constant values: superficial air velocity, 1.2 m/s; end slope, 4.0°; side slope, 2.3°; longitudinal vibrating frequency of 10.12 s−1 with corresponding stroke length of 4.5 mm; and height of riffles, 4.5 mm. Three products of different densities were discharged from the air table and fell into different compartments of the collecting
Conclusions
An effective dry separation of plastics from a three-component mixture was carried out by combining air tabling and triboelectric separation. Before commencing the separation tests, the effectiveness of the techniques was evaluated by investigating the effects of the particle size and the difference in density between components of the mixture.
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The triboelectric separation was effective for separation of materials of similar density. However, an upper limit of the particle size was set after
Acknowledgement
The financial support provided by Japan Society for the Promotion of Science through its JSPS Postdoctoral Fellowship Program is gratefully acknowledged.
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