Study On Pyrolysis Characteristics Of Waste Printed Circuit Boards And Control Of Brominated Pollutants

Waste printed circuit boards(WPCBs)are one of the main products of dismantling waste electrical and electronic equipment.WPCBs are composed of copper,resins and fiberglass,contain a lot of valuable metals and toxic compounds,and have the dual attributes of resources and pollution.Mechanical crushing-gravity separation process is commonly applied to recover metals from WPCBs.The mechanical crushing process requires large energy consumption,and a large amount of non-metallic fractions(NMFs)are remained,which are difficult to dispose.Pyrolysis is an effective method to achieve substantial volume reduction as well as materials and energy recovery from WPCBs.However,the decomposition of brominated epoxy resins(BERs)during pyrolysis process will release a large amount of brominated pollutants,which are the main precursors for the formation of highly toxic substances such as dioxins.In this paper,a combined recovery process of precrushing-pyrolysis was studied based on the eastablishment of pyrolysis kinetics method suitable for WPCBs,and energy consumption and environmental effect of this process were analyzed by life cycle assessment(LCA).Two efficient debromination methods for WPCBs based on enhanced subcritical water pretreatment and co-pyrolysis with alkaline residues were developed.The main research contents are as follows:1.Study on characteristics and pyrolysis kinetics of NMFs in WPCBsThe pyrolysis three-phase yields,migration of brominated pollutants and transformation of pyrolysis products of NMFs under different pyrolysis conditions were investigated.At 500?,the yield of tar reached a maximum of 35.70 wt%.Bromine pollutants migrated from the solid phase to the gas and liquid phases with the increase of pyrolysis temperature,and the content of bromine in the pyrolysis residues decreased from98.56 to 4.74 wt%in the temperature range of 300-700?.Phenol,phenol derivatives and bisphenol A(BPA)are the main components in tar,which have potential recovery value.Discrete distributed activation energy model(Discrete DAEM)was proven to be more suitable for the study of the pyrolysis kinetics of NMFs by comparing with isoconversional method of Starink-Kissinger-Akahira-Sunose(SKAS)model.In the conversion range of0.01-0.99,the apparent activation energy and pre-exponential factor varied in 80.9-240.5k J/mol and 19.07-39.55 s^-1 ranges,respectively.Pyrolysis kinetic calculations inferred that the pyrolysis system of NMFs consisted of 17 independent first-order reactions.2.Study on optimization of precrushing-pyrolysis process and metal recovery of WPCBsThe distribution of crushing particle size of WPCBs conformed to the fractal distribution,and the crushing process could be described by the first-order crushing dynamics equation.The precrushing-pyrolysis process achieved the lowest energy consumption and the best recovery efficiency when the precrushing particle size was 4.0cm and pyrolysis temperature was 330?.Under these conditions,the recovery efficiencies of copper and tin reached 92.38 and 99.80%,respectively.The interlayer destruction mechanism of WPCBs changes continuously with the increase of pyrolysis temperature.When the temperature was in the range of 100-225?,the physical structure of WPCBs was slightly destroyed under thermal stress.After 225?,the interlayer chemical cross-linking of WPCBs gradually disappeared.After 300?,the decomposition of BERs began to occur,and cracks appeared in some location of WPCBs due to the melting of solder.When the temperature reached 500?,most of the BERs in WPCBs were decomposed,accompanied by severe structural damage and disappear of binding force in interlayer of WPCBs.Considering the disposal situation of WPCBs in Wuhan,the effects of the precrushing-pyrolysis process and the mechanical crushing-gravity separation process on environmental pollution of functional unit WPCBs were compared by LCA.The results showed that the total environmental impact potential of the precrushing-pyrolysis process(5.53×10^-4)was less than the mechanical crushing-gravity separation process(1.11×10^-3).3.Debromination of NMFs via subcritical water pretreatmentIn order to effectively control the pollution of bromide during the pyrolysis of WPCBs,a study of debromination of NMFs based on subcritical water pretreatment enhanced by alkaline additives was carried out.The debromination efficiency of NMFs reached the maxium of 95.11%when the treatment temperature was 250°C,treatment time was 3 h,liquid to solid ratio was 40:1 m L/g,and 1 mol/L Na OH was applied as additive.The degradation of BERs conformed to the pseudo-first-order kinetic equation under the subcritical water environment.BERs were debrominated through a nucleophilic substitution process that was dominated by bimolecular process,in which a transition state was formed from the attack of OH^-nucleophilic group generated by the ionization of alkaline additives on the site of C-Br bond on the intermediate product(2-bromophenol)of the BERs.The Br^-was then removed through electron transfer.The results of density functional theory(DFT)calculation showed that the energy barrier of this bimolecular process was only 15.9 kcal/mol,which is much lower than the direct dissociated energy of C-Br bond(168.3 kcal/mol).The results of molecular dynamics simulation indicated that ion association occurs between anions and cations of alkaline additives in subcritical water.The contact ion pair is formed between M⁺(M=Na and K)and OH^-.And the dissociation of anions and cations which affect the debromination efficiency depends on the cation radius.When the cation radius of alkaline additives is larger,better debromination efficiency can be obtained due to the easier dissociation of contact ion pairs.The apparent activation energy of pyrolysis of WPCBs residues was significantly reduced,and the decomposition temperature range of organic components was reduced from 200-550°C to 90-160°C after subcritical water pretreatment.4.Study on catalytic co-pyrolysis of NMFs combined with alkaline residuesRed mud(RM)was selected to co-pyrolysis with NMFs.The three inorganic components of Fe₂O₃,Al₂O₃ and CaCO₃ in RM were considered as key components to affect catalytic pyrolysis of NMFs.The mechanisms of bromine fixation and catalytic cracking of NMFs by those key components of RM during the co-pyrolysis were elucidated.89.55 wt%of bromine can be fixed in pyrolysis residues with 15 wt%addition of RM,due to formation of Fe Br₂ and Ca Br₂ from Fe₂O₃ and CaCO₃ in RM,respectively.The results of DFT calculation showed that the adsorption energies of HBr on CaCO₃,Fe₂O₃ and Al₂O₃are-5.85,-2.93,and-1.99 eV,respectively.The order of catalytic debromination effects on inorganic bromide of three key components is CaCO₃>Fe₂O₃>Al₂O₃.The adsorption energies of 2-bromophenol on CaCO₃,Fe₂O₃ and Al₂O₃ are-0.95,-1.05,and-1.11 eV,respectively.The order of catalytic debromination effects on organic bromide of three key components is Al₂O₃>Fe₂O₃>CaCO₃.The lightening of tar was promoted by reducing energy barrier of direct decomposition of tetrabromobisphenol A(TBBPA)catalyzed with three key components.On the interface of CaCO₃,Fe₂O₃ and Al₂O₃,the energy of direct decomposition(crack of C-C bond)of TBBPA is reduced to 2.91,1.41 and 2.81 eV,respectively.The results of this paper provide a new technical basis for the pyrolysis of WPCBs and effective control of brominated pollutants,as well as a reference for the comprehensive recycling of other e-wastes.