Co-treatment of PVC and used LCD panels in low-temperature subcritical water: Enhanced dechlorination and mechanism

Abstract

The dechlorination of polyvinyl chloride (PVC) and the removal of organic materials from used liquid crystal display (LCD) are the primary difficulties and steps in the safe disposal of these two types of waste. In this study, the dechlorination of PVC and the removal of organic materials from waste LCD were simultaneously accomplished by using a low-temperature subcritical water (SubCW) co-treatment due to the low dielectric constant and mass transfer resistance of SubCW. The dechlorination efficiency of PVC could be significantly enhanced by introducing waste LCD in the SubCW co-treatment with lower PVC-to-H2O ratio (1:20 g/mL and 1:30 g/mL). The temperature, time, PVC-to-H2O ratio, and PVC-to-LCD ratio have significant influence on the dechlorination efficiency and chlorine distribution. The optimal parameters of the co-treatment were as follows: 220 °C, 90 min, PVC-to-LCD ratio of 4:1, and PVC-to-H2O ratio of 1:30 g/mL. In the optimal conditions, the PVC dechlorination efficiency could reach as high as 100 %. The removed chlorine was completely transferred to the aqueous phase and could be recovered as inorganic chlorine, eliminating the risk of chlorine. It is a remarkable improvement for the PVC dechlorination at low temperature with lower PVC-to-H2O ratio. The organic materials contained in used LCD could be efficiently removed by the SubCW co-treatment above 200 °C, and part of inorganic substances in used LCD could dissolve during the co-treatment due to the interaction between metallic oxides in LCD and HCl removed from PVC. The stable alkene-carbonyl structure (CCO) could be formed in the PVC residue during the SubCW co-treatment of PVC + LCD, promoting the dechlorination reaction of PVC. It is believed that the SubCW co-treatment is a high-efficiency strategy both for the dechlorination of PVC and the pre-treatment of used LCD.