Study On Co-Treatment Of PVC Waste And Typical Solid Waste In Subcritical Water

As one of the most widely used plastics in the world,polyvinyl chloride（PVC）has a huge yield and is difficult to degrade naturally.The safe treatment of PVC waste has been one of the research hotspots in the field of solid waste.The high chlorine content of PVC waste leads to serious corrosion of equipment in the recycling process,and it is easy to produce chlorine-containing toxic organic compounds,which has great environmental risks.Therefore,efficient dechlorination is the first step in the safe treatment process of PVC waste.Subcritical water is an effective medium for PVC dechlorination.In this paper,three typical solid wastes（construction concrete waste,wasted tantalum capacitor,and abandoned multilayer ceramic capacitor（MLCCs））have been selected as PVC dechlorination fortifier.A co-treatment system of PVC waste and the three solid wastes is constructed respectively.At the same time of efficient dechlorination of PVC waste,the reduction and resource treatment of the three solid wastes mentioned above are realized.The main results are as follows:（1）Construction concrete waste enhances PVC waste dechlorination in subcritical water.The subcritical water enhances dechlorination system of PVC waste is established by introducing the construction concrete waste as a strengthening agent into the system,bases on the capture of chlorine by a large number of metal oxides in the waste.Temperature,solid-liquid ratio,reaction residence time,PVC and construction concrete waste quality ratio have significant influence on the dechlorination of PVC waste.The optimal dechlorination conditions are as follows:220℃,solid-liquid ratio 1:30 g/m L,60 min,mass ratio of PVC to construction concrete waste 4:1,and the dechlorination efficiency can reach 95.4%.All the removed chlorine is transferred to the aqueous phase in the form of inorganic chlorine.The dechlorination of PVC in this system includes direct elimination of HCl and hydroxyl substitution.The introduction of construction concrete waste strengthening agent can promote the hydroxy substituted intramolecular dehydration and oxidation of PVC and form a more stable carbon-oxygen double bond（C=O）structure,which further promotes the dechlorination of PVC waste.The neutralization effect of Ca O,Si O₂,Al₂O₃,Fe₂O₃and other oxides in construction concrete waste on PVC dechlorination products leads to the dissolution and leaching of the oxides.（2）Co-treatment of PVC and waste tantalum capacitor in subcritical water.The co-treatment effects of supercritical methanol+tantalum capacitor,supercritical water+tantalum capacitor,and subcritical water+PVC+tantalum capacitor are compared.It is found that the co-treatment of PVC and waste tantalum capacitor could significantly reduce the treatment temperature and shorten the treatment time.The optimal treatment conditions of supercritical methanol+waste tantalum capacitor are 375℃,90 min,1:30 g/m L,and the conversion rate of organic resin in waste tantalum capacitor is 49.25%.The optimal treatment conditions of supercritical water+waste tantalum capacitor are 425℃,120 min and 25 MPa,and the conversion rate of organic resin in waste tantalum capacitor is 35.3%.The optimal treatment conditions of sub-critical water+PVC+waste tantalum capacitor are 300℃,30 min,1:5 g/m L,the mass ratio of PVC to waste tantalum capacitor is 6:1,the conversion rate of organic resin in waste tantalum capacitor is 100%,and the dechlorination efficiency of PVC is100%.The co-treatment of PVC and waste tantalum capacitors can realize complete dechlorination of PVC waste and complete conversion of organic resin of waste tantalum capacitors at the same time,and obtain high value added high purity rare metal tantalum.The degradation products of waste tantalum capacitors are mainly phenol homologues.（3）Enhanced dechlorination of PVC waste by MLCCs in subcritical water and preliminary utilization of residue.The subcritical water enhances dechlorination system of PVC waste is constructed by introducing MLCCs as a strengthening agent,which promotes the dechlorination of PVC waste by releasing a large amount of Ni⁺.Temperature,solid-liquid ratio,reaction residence time and PVC/MLCCs mass ratio have significant effects on the dechlorination of PVC waste.The optimal dechlorination conditions are as follows:220℃,solid-liquid ratio 1:5 g/m L,60 min,mass ratio of PVC to construction concrete waste 4:1,and the dechlorination efficiency can reach 96.56%.All the removed chlorine is transferred to the water phase in the form of inorganic chlorine,eliminating the risk of chlorine.The dechlorination of PVC in this system includes direct elimination of HCl and hydroxyl substitution.The introduction of MLCCs can promote the multiple substitution of PVC hydroxyl groups,resulting in the formation of dihydroxyl structure,which is very unstable and prone to intramolecular dehydration,forming stable olefin carbonyl（=C-C=O）structure.The dechlorination residue of subcritical water+PVC+MLCCs has an oxidase-like activity,which can effectively promote the oxidation of Cr（Ⅵ）to enzyme substrate3,3,5,5-tetramethylbenzidine（TMB）.The high sensitivity analysis of Cr（Ⅵ）has been established.The detection limits and linear ranges were 9.3×10^-9mol/L and 9×10^-8-8.9×10^-6mol/L,respectively.