Original Research PaperDecrease of Cl contents in waste plastics using a gas–solid fluidized bed separator
Introduction
Recycling of waste materials is promoted worldwide by enforcing many recycling-related directives and laws, such as directives on ELV (end-of-life vehicles) [1] and on WEEE (waste electrical and electronic equipment) [2] in Europe Union. One of waste materials for recycling is waste plastic from ELV or WEEE. When plastics are burned, a large amount of heat is generated as much as coal or oil. Because of this, thermal recycling of waste plastics is mainly conducted. But, chlorine-contained plastics included in waste plastics cause a serious corrosion of the furnace or kiln by burning them. In order to prevent the corrosion, it is required to reduce Cl content of waste plastics to less than 1.0 wt% as pre-treatment of thermal recycling. Waste plastics consist of Cl-free plastics with a density less than 1.2 g/cm3 and Cl-contained plastics with a density about 1.4 g/cm3. Using a separation on the basis of their density difference, decrease of Cl content is expected to be accomplished.
Density separations are mainly performed in wet method [3], [4], which is necessary of waste liquid treatment process and dry process after separating. As an alternative technique, dry density separations without these processes are required. In dry density separation techniques, there are cyclone [5] and air tabling [6]. For these techniques, the size of separating objects is usually relatively small, less than several hundred micron meters. For larger size of the objects, there is a dry density float-sink separation using a gas–solid fluidized bed, which is fluidized by feeding air from below a particle bed. Fluidized bed has an apparent density which is properties similar to liquid [7], [8], [9]. When an object is inserted inside this bed, an object having smaller true density less than the apparent density floats, and an object with larger true density sinks. Fraser and Yancey [10] used this method in the 1920s for the first time as grade separation of coal and many researchers separated various objects so far by this method [11], [12], [13], [14]. The authors also have succeeded in grade separation of coal [15], [16] and material separation of ASR (automobile shredder residue) [17], [18]. As to a separation of Cl-free and Cl-contained plastics, float-sink experiments were performed in the composite fluidized particles of polystyrene particles and glass beads, and the results show possibility of decreasing Cl content in waste plastics [19]. Seiko et al. showed that this technique can be available for separating SBW (shredder bulky waste) to combustible (wood, paper, plastic) and incombustible (glass, metal) components [20], [21].
The authors have conducted not only float-sink experiments for separations but also development of separation apparatuses with recovering processes of floaters and sinkers [15], [16], [18], [22], [23]. In these apparatus, feeding height of separating objects was always at the top of the bed. However, we considered that if the feeding height could be changed to near the middle of the bed, there was an advantage for increasing of purity of valuable objects. For example, if the feeding height is middle and valuable objects are recovered as floaters, invaluable ones are not easy to mix in floaters. Furthermore, if the feeding height is near the middle, sink-distance of sinkers may be shorter and reduction of separation time is expected to be accomplished. Then, in previous study [24], separation apparatus with variable the feeding height was newly designed and developed, and separation of Cl-free and Cl-contained simulated waste plastics with a cuboid shape (1 × 1 × 0.2 cm) was performed. As a result, when the feeding height is middle of the bed, separation efficiency was higher and separation time was shorter.
The aim of this study is to reduce Cl content in real waste plastics using the apparatus described above. However, the feeding height was constant at the middle of the bed in this study. The float-sink behaviors of Cl-free and Cl-contained plastics were investigated by changing parameters, air velocity for fluidization, float-sink time and the feed amount of the plastics. Furthermore, as a new parameter in the same apparatus, a recovery bed height of floaters or sinkers was changed. In previous study [24], the recovery height was constant at the middle of the bed. However, in this study, further decreasing Cl content was attempted by adjusting the recovery height higher.
Section snippets
Size and shape
Real waste plastics obtained from a waste-disposal facility were used as separating objects. First, the plastics were shredded by the cutter mill (UGO-210; HORAI Co. Ltd.) due to having wide plastics size distribution. Fig. 1 shows a photograph of the waste plastics after shredding. The size became smaller with a little narrow distribution: longest length was about 1.5–5.0 cm, and the thickness was less than 0.3 cm. As shown in this figure, the plastic shape is not-uniform and not-flat.
Cl content measurement
Samples
Effect of superficial air velocity ratio on separation efficiency
Fig. 3 shows ECl-free, ECl-cont and CCl with respect to u0/umf. Here, tf-s = 120 s, Fp = 100 g and hr = 10 cm. Here, error bars indicate standard deviations of the values. At any u0/umf, the values of ECl-cont show a relative high, about 0.9, and those of ECl-free show about 0.4–0.6. This means most Cl-contained plastics sink to the bottom, and about half Cl-free plastics float to the top. This is because apparent density of fluidized bed is close to the density of Cl-free plastics. In the case of 1.2 ⩽ u0
Conclusion
Decreasing Cl content of real waste plastics was attempted using the rotating-type gas–solid fluidized bed separator, which separating objects can be fed at the middle of the bed and the recovery height of floaters can be varied. When superficial air velocity, float-sink time and the recovery height were adjusted, Cl content in the floaters was decreased to be 0.4–0.85 wt%, at which recovery of Cl-free plastics were 40–60%. Especially, it was found that the feature of varying the recovery height
Acknowledgements
This research was partly supported by Industrial Technology Research Grant Program in ‘04 from New Energy and Industrial Technology Development Organization (NEDO) of Japan, and by the Core-to-Core Program promoted by Japan Society for the Promotion of Science (Project No. 18004).
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2020, Advanced Powder TechnologyCitation Excerpt :The above-mentioned previous reports encourage us to induce and control char segregation by providing a char-rich bed for tar decomposition and elimination, as well as for steam gasification of char, and to study the segregation of char under conditions similar to those of the gasification. Many researchers reported their results of experimental and/or numerical investigation of the segregation in beds with binary particles for different combinations of bed shape, gas velocity, particle density and size. [24,25,27–31] Park et al. [24] investigated the mixing and segregation of fluidizing material and char in a fluidized bed for U0/Umf < 1.35 and showed that the minimum mixing index depended on the shape of the column and the velocity of the fluidizing gas.