Study On The Debromination And Recycling Of Spent Brominated Epoxy Resin In Subcritical Water

With the rapid upgrading of electronic products,the amount of electronic waste is growing explosively.Waste printed circuit board(WPCB)is the main component of electronic waste.Brominated epoxy resin(BER)from WPCB is a kind of material with great environmental risk and high resource value.Its safe recovery and disposal has become the focus of attention in the field of electronic waste,and it is also a major demand of resource recycling and environmental protection in China.However,the traditional BER disposal technology has some defects such as incomplete debromination,low added value of recovered products and even high toxic substances such as brominated dioxins.This thesis takes the disassembled and broken WPCB and waste BER as the research object,adopts the subcritical water(Subcritical water,SubCW)technology,constructs the subcritical water-ammonia(SubCW-NH3),the subcritical water-urea(SubCW-urea)and subcritical water-zero-valent iron(SubCW-Zero-valent iron,SubCW-ZVI)three new decomposition and debromination systems,which are used for the decomposition and debromination of waste BER,and realize its high-efficiency debromination and high-value resource conversion at the same time.The main findings are as follows:(1)Subcritical water-ammonia system.Ammonia water was introduced into the subcritical water system,and the dismantled WPCB was used as the treatment object.The reaction temperature was 200～350 ℃,the reaction time was 30～90 min,the ammonia concentration was 1.7～7.4 mol/L,and the solid-liquid ratio was 1:10～1:25 g/mL.Under the subcritical condition,ammonia can be used not only as reaction medium,but also as reactant.The increase of reaction temperature and reaction time is beneficial to the debromination of waste BER.Under the optimal reaction conditions(300℃,60 min,ammonia concentration 4.1 mol/L,solid-liquid ratio 1:15 g/mL),the debromination efficiency of the waste BER can reach 100%.After the subcritical water-ammonia system,the copper foil can be completely separated from WPCB and recovered.Organic compounds containing bromine were not detected in oil phase products in all experiments.For the first time,two new products with high added value,pyrazine and pyridine nitrogen heterocyclic compounds,were obtained from the waste BER in subcritical water-ammonia system.The products mainly included methyl pyrazine(40.22%),ethyl pyrazine(20.48%)and 2-ethyl-5-methylpyridine(23.69%).Ammonia is mainly involved in the conversion of nonphenol groups in waste BER.Compared with the existing methods,the efficient debromination of the waste BER and the conversion of high value resources are realized at lower temperature.Through the analysis of intermediates and products,the possible decomposition and debromination reaction paths of the waste BER in the subcritical water-ammonia system were proposed.(2)Subcritical water-urea system.Urea was introduced into the subcritical water system and waste BER was used as the treatment object.The urea concentration was 2.75 mol/L,the solid-liquid ratio was 1:15 g/mL,the reaction temperature was 200～400℃,and the reaction time was 15～60 min.In the subcritical water-urea system,the NH3 and CO2 released from urea have synergistic effects on the decomposition and debromination of the waste BER,which can significantly improve the decomposition and debromination efficiency of the used BER.Under the optimal reaction conditions(urea concentration 2.75 mol/L,solid-liquid ratio 1:15 g/mL,reaction temperature 300℃,reaction time 60 min),the debromination efficiency of the waste BER can reach 99.92%.Urea is mainly involved in the conversion of phenolic groups in the waste BER.By controlling the reaction temperature of the subcritical water-urea system,the decomposition and debromination products of the waste BER can be effectively regulated.The composition of oil phase products was analyzed by gas chromatography-mass spectrometry(GC-MS).The decomposition products of waste BER in the subcritical water-urea system are mainly 2-bromophenol(200℃,72.5%)and phenol(250℃,81.5%).When the reaction temperature rose to 300℃,the selectivity of the product decreased significantly due to the occurrence of secondary reaction,and the main product was aniline(27.2%).Compared with the existing methods,the subcritical water-urea system can achieve both low temperature debromination and high value resource conversion.Through the analysis of intermediates and products,the possible decomposition and debromination reaction paths of the waste BER in the subcritical water-urea system were proposed.(3)Subcritical water-zero-valent iron system.Zero-valent iron was introduced into the subcritical water system and waste BER was used as the treatment object.The initial pH was 4～8,the reaction temperature was 150～250℃,the reaction time was 60～150 min,BER:zero-valent iron(mass ratio)was 5:0～5:10,and the solid-liquid ratio(BER:water)was 1:9 g/mL.Appropriate zero-valent iron dosage and acidic conditions are conducive to the decomposition and debromination of the waste BER,while the precipitation of ferric hydroxide generated under alkaline conditions will form a dense oxide layer,which is not conducive to the further release of Fe2+,thus inhibiting the occurrence of reaction and leading to the reduction of degradation efficiency.Under the optimal reaction conditions(pH=4,reaction temperature 250℃,reaction time 150 min,BER:zero-valent iron(mass ratio)=5:5,solid-to-liquid ratio(BER:water)=1:9 g/mL),more than 99.8%Br in the waste BER was removed and transferred to the aqueous phase.The composition of oil phase products was analyzed by gas chromatography-mass spectrometry(GC-MS).According to the characterization results,the main decomposition products of waste BER under the optimal reaction conditions were phenol(44.6%),4-isopropyl-phenol(6.2%)and methyl-phenol(18.2%).Other oil phase products obtained by decomposition are free of bromine and can be used as fuel or industrial chemicals safely without further debromination treatment.Compared with the two systems of subcritical water-ammonia and subcritical water-urea,the subcritical water-zero-valent iron system can realize the complete debromination at 250℃ and the efficient conversion of high value chemical products.Through the analysis of intermediates and products,the possible decomposition and debromination reaction paths of the waste BER in the subcritical water-zero-valent iron system were proposed.