Decrease of Cl contents in waste plastics using a gas–solid fluidized bed separator

Abstract

In order to decrease Cl content in waste plastics, dry density float-sink separation of Cl-contained and Cl-free plastics was explored using a semi-continuous rotating-type gas–solid fluidized bed separator with silica sand. The separator has two distinctive features: (1) the plastics can be fed at a middle height of the sand bed, and (2) when the plastics are recovered with the sand from a container after the float-sink, the recovery height of the sand bed can be changed to designate the plastics as floaters or sinkers. The waste plastics of Cl content = 5.4 wt% were used in this study. The separation was investigated by changing the experimental conditions. As a result, the float-sink of the plastics was affected by the air velocity for fluidization, the float-sink time and the feed amount of plastics. The possible causes of the effects were discussed by focusing on the apparent density of fluidized bed, the fluidization intensity, the size segregation of fluidized particle, the shape of the plastics, and the interactions between the plastics during the float-sink. When the recovery height was changed at the adjusted conditions, the Cl content in the floaters was successfully decreased to be 0.4–0.85 wt%, at which the recovery of the Cl-free plastics was 40–60%.

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.