Optimization of surface treatment for flotation separation of polyvinyl chloride and polyethylene terephthalate waste plastics using response surface methodology

A simple process for separation of polyvinyl chloride (PVC) and polyethylene terephthalate (PET) waste plastics was investigated for sustainable development of plastic industry. Surface treatment with potassium permanganate (KMnO4) solution selectively decreases the flotation rate of PVC, promoting flotation separation of PVC and PET. Flotation rate represents the mass percentage of float particles. Oxidization action and migration of calcium carbonate on PVC surface as major mechanism of surface treatment are confirmed by scanning electron microscope and X-ray photoelectron spectroscope determinations. Single factor experiments demonstrate that the flotation rate of PVC is affected significantly by the factors of KMnO4 concentration, treatment time, temperature and stirring rate. The effects of variables, including KMnO4 concentration, treatment time and temperature, on the flotation rate of PVC are evaluated by response surface methodology with a Box-Behnken design. A modified quadratic model is achieved for predicting the flotation rate of PVC plastic. The optimum conditions are potassium permanganate concentration 4.60 mmol/L, temperature 66.5 °C, treatment time 38 min and stirring rate 200 r/min. PVC and PET mixtures with different mass ratio were separated efficiently through one-stage flotation combined with surface treatment. The purity of PET is above 97.87% and the recovery of PVC is above 97.90%. Effective separation by the proposed method enables recycling of PVC and PET waste plastics.