Study On The Degradation Mechanism Of Waste Circuit Boards Based On Modified Near-Critical Water

With the rapid development of the electronics industry and the constant updates of electronic products,the quantity of electronic waste has shown an explosive growth,and Waste Printed Circuit Boards（WPCBs）as an indispensable key component in electronic devices have also increased in production.As a challenge in the WPCBs processing,the efficient and green treatment of Brominated Epoxy Resins（BERs）has been a focus of attention for people and an urgent need for environmental protection and resource recycling in China.Traditional treatment methods often have the defects of incomplete degradation,low product recovery value,or the production of environmental pollutants.Therefore,in this study,dismantled waste printed circuit boards were used as the research object,and a high-temperature reaction kettle experimental platform was built.Metal salts such as aluminum chloride,sodium chloride,and potassium chloride were used as modifiers to construct near-critical water-aluminum chloride,near-critical water-potassium chloride,and near-critical water-sodium chloride systems for the degradation of BERs,while achieving efficient debromination and resource recovery.Finally,a life cycle assessment was conducted on the optimal modification process and compared with pyrolysis,and the following conclusions were drawn:（1）In the near-critical water-aluminum chloride system,BERs can be rapidly degraded,and their degradation rate increases rapidly with the increase of processing temperature,aluminum chloride saturation concentration,and insulation time.When the processing temperature is 250℃,the aluminum chloride mass concentration is 40%saturation,and the reaction time is 60 min,the degradation rate of BERs in waste printed circuit boards reaches99.71%.In SEM scanning electron microscopy images,it can also be observed that the glass fibers after reaction are basically free from resin wrapping.Therefore,the optimal process parameters are determined to be a temperature of 250℃,an insulation time of 60 min,and an aluminum chloride mass concentration of 40%saturation.（2）For the aluminum chloride modified system,BERs in the circuit board begin to degrade at 230℃,and the bromine content in the initial circuit board gradually decreases.The relative content of bromine in the water solution reaches 4.71%.At a temperature of250℃,BERs can be completely decomposed,and most of the bromine transfers to the water phase,making the bromine content in the water phase reach 98.24%.The total solid-phase removal rate is 99.53%,and there is a small amount of bromine in the organic phase.With further increase in temperature,bromine-free oil-phase products are obtained.（3）In the excellent dissolution and expansion effect of the near-critical water-aluminum chloride system,the metal salt can enter the interior of BERs and catalyze the C-O bond to reduce its bond energy,making it easier to break,thereby promoting the decomposition of BERs.The characterization results of GC-MS show that under the optimal process parameters,the main degradation products of the resin are phenol and 4-isopropylphenol,with relative contents of 52.66%and 41.50%,respectively.（4）In the treatment process with sodium chloride and potassium chloride as modifiers,their modification enhancement effects are weaker than that of aluminum chloride,and the resin needs to be degraded significantly only at temperatures above 300°C.Under the conditions of temperature at 300°C,mass concentration at 40%saturation,and reaction time of 60 minutes,the resin degradation rate can reach more than 93%,and the debromination efficiency is close to 100%for both the near-critical water-sodium chloride system and the near-critical water-potassium chloride system.However,the resin degradation products for the two methods differ.The oil phase products derived from WPCBs after treatment with the near-critical water-sodium chloride system are mainly phenol and methyl stearate.As the reaction temperature increases,the products further decompose into phenol and4-isopropylphenol.The products of the potassium chloride modification system are similar to those of aluminum chloride,and the decomposed products are phenol and4-isopropylphenol.At 300°C,the relative peak areas of phenol and 4-isopropylphenol are70.88%and 28.29%,respectively.The degradation mechanism of the three systems for the resin is similar,and the only difference is the degree of enhancement of the modification effect.The order of the modification enhancement effects of the three modifiers is aluminum chloride>potassium chloride>sodium chloride.（5）A life cycle assessment was carried out on the near-critical water-aluminum chloride modification process and the pyrolysis process of printed circuit boards,using energy consumption,energy conversion efficiency,and environmental impact load as evaluation indicators.The evaluation indicators of the two processes were compared and analyzed.The results showed that the energy consumption of the near-critical water treatment and pyrolysis methods were 491.18 MJ/kg and 567.45 MJ/kg of waste printed circuit boards,respectively.The energy conversion efficiencies were 0.06 and 0.04,respectively,and the environmental impact loads were 52.69 m PE_T2000 and 61.08 m PE_T2000,respectively.Comparing the two processes,it was found that the energy consumption and environmental impact load of the near-critical water-aluminum chloride process were lower than those of the pyrolysis process,while the energy conversion efficiency was higher.Therefore,the aluminum chloride modification near-critical water system is not only more energy-efficient but also a relatively green and environmentally friendly treatment process.