Effective Utilization Of Waste Printed Circuit Boards By Critical Water Or Methanol

At present,with the rapid growth of economy,continuous innovation of science and technology and substantial improvement of people's living standards,the generation and accummlation of a large amount of waste electrical and electronic equipment?WEEE?has become one of significant environmental resource problems in the field of solid waste.As a typical WEEE,waste printed circuit boards?WPCBs?are account for a high proportion.WPCBs contain many high valued materials,such as precious metals,brominated plastics,glass fibers and so on.However,it is difficult to achieve complete separation and recovery due to their complex composition,which will cause great environmental risks.Therefore,the safe disposal and resources recovery of WPCBs is of great importance.In this paper,based on the structure and composition characteristics of PCBs,the critical fluid technology was put forward to treat and recycle WPCBs as our research ideas.The significant results in scientific and practical value were as follows:?1?The effects under different near-critical conditions on the separation of copper foils-organic resin layers-glass fibers in WPCBs were investigated by critical methanol treatment technique using waste printed circuit boards as raw materials.The composition distribution of oil phase products and the influence factors of debromination efficiency and path of brominated flame retardants were also explored.The result showed that the supercritical methanol reaction system is more conducive to the delamination and more complete debromination of WPCBs,and the conversion rate of solid products was higher.Above 400 ?,the phenol derivatives in the oil phase can be recovered in a large amount.However,under the condition of subcritical methanol,there was no significant stratification of WPCBs,and the conversion rate of solid products was low.In the oil phase,brominated diphenyl ethers were extracted by methanol at low temperature and a debromination reaction occurred.The optimal conditions for the recovery of esters were200 ?,1:20 g/mL and 90 min.In the subcritical methanol-alkail?NaOH?reaction system,it's beneficial to the recovery of brominated diphenyl ethers under low reaction temperature,long reaction time and high reaction solid-liquid ratio.?2?WPCBs were pretreated twice in dilute nitric acid to obtain the leach liquor with a high concentration of copper ions and then subjected to supercritical methanol reduction process,successfully prepared ultra-fine copper materials with high added value.The analysis of XRD and TEM showed that the presence of ferric ions in the leacing solution would lead the reoxidize of the reduced zero-valent Cu to Cu₂O,and made the formation of the mixture of Cu and Cu₂O with poor uniformity of particle size.After the removal of Fe³⁺,spherical ultrafine Cu particles with high uniformity,monodispersity and small particle size could be obtained.It's facilitate to form smaller copper nanoparticals and avoid the agglomeration of particals in higher reaction temperature or lower initial concentration of copper ions.The optimum SCM treatment conditions were 360 ?,10min and 1:9 g/mL,which can improve the recovery rate of ultrafine copper materials to over 95%.?3?WPCBs were successfully converted into high-efficiency flame retardant and smoke suppressant for PVC resins by supercritical water oxidation?SCWO?process.The results indicated that Cu₂O,CuO,and SnO₂ were the main active components of flame retardants prepared by WPCBs.When they co-existed,WPCBs-derived flame retardants had the best properties of flame retardancy and smoke suppression.A conversion of Cu elements(Cu⁰?Cu⁺?Cu²⁺)during SCWO process with the increase of reaction temperature was found to be the key influence factor for the flame retarding and smoke suppressing properties.The experiment results also showed that there was a synergistic effect of flame retardancy between Cu⁺and Cu²⁺.In addition,by controlling the appropriate addition amount of WPCBs-derived flame retardants,PVC can achieve both good flame retardant properties and mechanical properties.