การแยกเม็ดพลาสติกผสมระหว่างโพลิโพรไพลีน โพลิเอทิลีนชนิดความหนาแน่นสูง และโพลิเอทิลีนชนิดความหนาแน่นต่ำโดยเทคนิคการลอยแยก

Abstract

Mixture of Polypropylene (PP), High Density Polyethylene (HDPE), and Low Density Polyethylene (LDPE) which are all hydrophobic plastics and lower specific gravity than water, was found to be floating in a flotation cell filled with water. According to principles of materials flotation, it is suggested that specific surface treating agent would bring to different floatability of each material in the misture; hence, specific material can be separated from the mixture. In case of the mixture of PP, HDPE and LDPE, the study found that a hydrophilic polyvinyl alcohol (PVA) can be used as a surface treating agent to increase different hydrophilicity of each plastic. It was found that the PVA can increase hydrophilicity of a straight chain HDPE better than LDPE and PP, respectively. In case of a branch chain LDPE, it will require more PVA to increase its hydrophilicity than the case of HDPE because it requires PVA to coat its branch as well. In case of the PP having methyl functional groups as a barrier for PVA penetration; hence, limited increasing hydrophilicity of the PP. Though adding PVA as much as 48 kg per ton plastic feed, more than 50% of the PP was still floating. Therefore, it is expected that each plastic can be separated from the mixture by froth flotation technology using PVA as a surface treating agent if operation at the optimum condition. The study of influence on separation efficiency of the plastic mixture found that appropriate condition for the plastic separation should be 3% solid, 10 minutes conditioning time, and 1,300 rpm stirring rate. It was also found that appropriate PVA concentration should be 24 kg per ton plastic feed for HDPE-PP separation, and 32 kg per ton plastic feed for HDPE-PP separation respectively. Therefore, the separation process is suggested to be 2 steps, starting with PVA 24 kg per ton plastic feed to separate HDPE from the mixture, and then adjusting the PVA condition to be 32 kg per ton plastic feed to separate LDPE from the PP. Upon these conditions, each plastic can be separated from the mixture with approximate 90% recovery and also 90% plastic purity, which is acceptable in the plastic recycle industry.