| Abstract:The recycling of waste plastics is not only related to environment problems, but also an important way of resources recycling and sustainable development. Aim at the key problem of recycling of waste plastics, namely separation of waste plastic mixtures, two aspects of research were conducted including quantitative analysis and separation of plastic mixtures using selective dissolution as well as and flotation separation of waste plastics combined with surface pretreatment. Quantitative analysis of two common plastic systems was formulated, and new separation technology was further developed based on the difference in solubility of plastics. Ammonia pretreatment and boiling pretreatment, which were developed for changing selectively hydrophilic/hydrophobic property of plastic surface, promote flotation separation of plastic mixtures with high efficiency.Systematic study of quantitative analysis of two common plastic systems was first conducted on the basis of selecting appropriate solvents. Experimental results demonstrate that appropriate solvent of ABS/PS and PVC/PET was xylol and tetrahydrofuran, respectively. Quantitative analysis demonstrates analytical error of virgin ABS/PS and PVC/PET was less than3.30%(minimum0.07%) and1.40%(minimum0.07%), respectively; analytical error of waste ABS/PS and PVC/PET was less than10.70%(minimum4.00%) and6.1%(minimum0.20%), respectively. Separation of waste PVC/PET plastics was further carried out using selective dissolution, and results show that the purity of separated PVC and PET approximated100%, the recovery of PVC and PET was up to97.6%and100%, respectively, and the recycling rate of tetrahydrofuran was up to72.2%.Ammonia pretreatment and boiling pretreatment of plastics were developed for flotation separation of plastic mixtures. Experimental results show that ammonia pretreatment changed selectively the floatability of PC, and ABS and PS were insensitive to ammonia. The proper treatment time of PC at14℃,18℃and23℃was30min,18min and13min, respectively. Boiling pretreatment changed selectively the floatability of ABS and PC, and PS were unaffected. The floatability of ABS, PC and PS was4.20%,24.53%and100%, respectively, after boiling pretreatment for80min. Based on the flotation behavior of simple plastic, flotation separation of ABS/PC and PS/PC combined with ammonia pretreatment was conducted, and in the case of ABS/PC, the purity and recovery of separated products was above95.95%and95.93%, respectively; in the case of PS/PC, the purity and recovery of separated products was above97.29%and97.03%. Flotation separation of ABS/PS and PC/PS was conducted combined with boiling pretreatment, and in the case of ABS/PS, the purity and recovery of separated products was above95.80%and95.81%, respectively; in the case of PC/PS, the purity of separated PS was99.13%. Study on mechanism of surface pretreatment demonstrates that action mechanism of ammonia is due to it reacts with ester bonds and reaction products caused an increase of hydrophilic groups on PC surface; action mechanism of boiling pretreatment is attributed to rearrangement of molecular chains, which is achieved by transferring hydrophilic groups from inner of plastic to surface.A rapid, precise and simple method for quantitative analysis of plastic mixtures was developed, which provides a effective method for identification of plastic mixtures and quality analysis of separated plastic products. New separation technology of selective dissolution will show superior application prospect on the basis of further improving the recycling rate of solvents. Both ammonia pretreatment and boiling pretreatment are initiatively investigated in the field of plastics flotation, and will play an important role in thorough study and industrial application of plastics flotation. |
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