Synergetic treatment of waste tantalum capacitors and polyvinyl chloride by subcritical water: Tantalum recovery, benzoic acid production and dechlorination

The rapid upgrading of electronic products led to the generation of a great deal of waste tantalum capacitors (WTCs). The recovery of WTCs is of great interest due to the high-value metal tantalum and resin materials. However, it is difficult to separate/recover the materials from WTCs because of the special structure and composition. Polyvinyl chloride (PVC) waste with high level of chlorine could result in severe corrosion and emission of chlorinated pollutants in the conventional disposal process. In this study, a synergetic and high-efficiency treatment of WTCs and PVC by subcritical water (SubCW) process was developed based on the interaction of the two kinds of wastes. The interaction between WTCs and PVC in SubCW could significantly reduce the treatment temperature and shorten the reaction time for recovering high-purity metal tantalum from WTCs, and it could also markedly improve the dechlorination efficiency (DE) of PVC. The optimized conditions for the synergetic treatment of WTCs and PVC were 325 ℃, 30 min, solid-to-liquid ratio of 1:5 g/mL, and PVC-to-WTCs ratio of 6:1 g/g. The resin conversion efficiency (RCE) of WTCs and the DE of PVC could reach 100% and 97.39% under the optimized conditions. The HCl released from PVC in SubCW could significantly promote the acid-catalytic hydrolysis of resin materials in WTCs and resulted in the production of abundant phenol/phenol derivates. In turn, hydroxyl functional group of phenol/phenol derivates could enhance the dechlorination of PVC by promoting the hydroxyl substitution reaction pathway. High level of benzoic acid (72.52%) could be obtained from the interaction between the decomposition products of WTCs and PVC by radical and oxidation reactions. The SubCW synergetic process developed in this study provided a high-efficiency and low-energy strategy for metal tantalum recovery, benzoic acid production, and dechlorination from WTCs and PVC wastes.