| Antimony and bromine flame retardants contained in e-waste flame retardant plastics are not only harmful to human health if mishandled,but also the main bottleneck restricting the recycling of electronic waste flame retardant plastics.In addition,the growth in accumulated stocks of e-waste flame retardant plastics also caused the sacrifice of resources and energy.Therefore,it is of great significance to develop efficient and safe technologies for removing antimony and bromine contaminants and recycling e-waste flame retardant plastic matrix.As a green,environment-friendly and low-energy heat treatment technology,hydrothermal treatment can not only remove toxic and harmful substances in solid waste efficiently,but also serve as an effective way to recycle solid waste.This paper selected e-waste flame retardant plastic containing antimony trioxide(Sb2O3)and DBDPO as the research object.The antimony and bromine in flame retardant plastics were removed by adjusting reaction atmosphere during hydrothermal treatment.Researches on the recycling of e-waste plastics matrix by hydrothermal treatment and hydrothermal decomposition mechanisms were carried out.The main research contents and results are as follows:(1)The effects of various factors(hydrothermal temperature,sulfur reagent concentration,alkali concentration,liquid-solid ratio,holding time)on the removal rate of antimony and bromine were investigated under hydrothermal treatment in pure water and alkaline-sulfur system.The results showed that the removal rate of antimony reached 85.6% and the removal rate of bromine reached 90.13% when the temperature was 220 C,the holding time was 2 h,the concentration of sodium sulfide was 50 g/L and the concentration of sodium hydroxide was 20 g/L.Sodium sulfide and sodium hydroxide played a related role in the process of antimony removal,and they were the common factors.Sodium hydroxide played a key role in controlling the process of antimony removal and was a single factor.Under the optimum reaction parameters,the FTIR structure of the remaining plastics has little change compared to the standard plastics,which can be used for mechanical recycling.(2)The properties of hydrothermal oils and solid product derived from typical e-waste plastics substrates(HIPS,ABS,PC,PP,PA6)through hydrothermal treatment were studied.The results showed that phenolic compounds such as phenol and bisphenol A(BPA)were found in PC oils,single-ringed aromatic compounds and diphenyl compounds were found in HIPS and ABS oils,alkanes were found in PP oils,and caprolactam(CPL)was found in PA6 oils.The hydrothermal solid residues were sunjected to DSC analysis.All the solid residues had enormous improvement on the enthalpy of combustion except the solid residue of PA6.The combustion enthalpy of solid residues had been increased up to 576.03%(SR-PC-350),287.70%(SR-HIPS-350),79.57%(SR-HIPS-350)and 31.94%(SR-HIPS-350).Hydrothermal treatment significantly increased energy density and promoted the effective combustion of solid residues.(3)Four different hydrothermal degradation mechanisms were proposed,involving a series of reactions,such as depolymerization,hydrothermal cracking,hydrolysis,free radical reaction and nucleophilic substitution reaction.During the hydrothermal degradation of HIPS and ABS,hydrothermal cracking produced different free radicals,and different free radicals combined to form different products.Therefore,hydrothermal cracking and free radical reaction play a leading role in the degradation mechanism of these two materials.However,in hydrolytic depolymerization was a priority reaction in the hydrothermal degradation of PC and PA6.The subsequent cracking reaction,free radical reaction,intermolecular hydrogen transfer,nucleophilic substitution and other reactions were the necessary factors for the emergence of the final products. |
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