Pyrolysis Characteristics And Bromine Conversion Mechanism Of Waste Circuit Boards In NaOH-Na₂SO₄ Eutectic System

Pyrolysis technology can achieve the reduction of non-metallic fractions（NMFs）of waste printed circuit boards（WPCBs）,but the reduction of bromine-containing components（such as brominated epoxy resin）Bromine-containing pollutants released by cracking can cause secondary pollution.Alkaline additives can alter the distribution of bromine during the pyrolysis of WPCBs.Co-pyrolysis of molten salt is one of the effective ways to remove bromine-containing pollutants.Using molten salt as reaction medium and heat carrier,bromine ions in bromine-containing pollutants are converted and migrated,so as to achieve the immobilization of bromine during pyrolysis.Therefore,the purpose of this study is to use the combination of sodium hydroxide and sodium sulfate molten salt with low melting temperature as an additive to co-pyrolyze with WPCBs to achieve bromine fixation during the pyrolysis process,and to investigate its catalysis and bromine fixation on the pyrolysis of NMFs.1.Thermochemical properties and pyrolysis kinetics of WPCBsThe pyrolysis process of WPCBs can be simply divided into three stages:the first stage is the physical water volatilization stage,which does not reach the breaking bond energy of the chemical bonds of WPCBs due to the temperature.The second stage is the main reaction stage of the pyrolysis behavior.In this stage,the C-C bond of the brominated epoxy resin of WPCBs is broken to generate volatile small molecules such as CO₂,H₂O and other products.The liquid products mainly contain phenol and its derivatives.The third stage is the residual stage of the reaction.The pyrolysis behavior of WPCBs is basically completed,the macromolecules of WPCBs are basically broken,and only coke is formed and some small molecular products of WPCBs are polymerized.The average activation energy of WPCBs is 145-150 k J/mol.2.Study on the Pyrolysis Mechanism of WPCBsFor brominated epoxy resins（BER）in WPCBs,the entire pyrolysis process can be considered as HBr escape,depolymerization,random fragmentation,and radical reactions.Due to the scission of the main chain,macromonomers are further decomposed to form small monomers and free radicals,such as bromine radicals and alkane radicals.These free radicals will produce a series of conjugation reactions,and the reactions of free radicals are very complex.And the cleavage pathways of brominated epoxy resin and epoxy resin in WPCBs are different.3.Pyrolysis and bromine fixation of WPCBs under NaOH-Na₂SO₄ systemDuring the pyrolysis process of WPCBs in the molten salt system,almost 90%of the bromine is fixed in the molten salt,which reduces the production of toxic and harmful substances such as bromophenol.When the temperature was 320℃,the bromine content in the molten salt reached the maximum value,and 95.36 wt%bromine in the WPCBs was fixed in the molten salt;when the heating rate was 5℃/min,the bromine content in the molten salt was 97.70 wt%.When NaOH-Na₂SO₄:WPCBs=0.8:1,the bromine content in the molten salt reaches a maximum value of96.52 wt%;when the holding time is 30 min,the bromine content in the molten salt reaches a peak value of 95.62 wt%;when the particle size is at 0.3 cm,the effect of bromine solidification in molten salt is the best,and the bromine content in molten salt is 83.2 wt%.The role of NaOH-Na₂SO₄ in the WPCB pyrolysis process is mainly due to the reaction of bromine with sodium ions to form sodium salts under alkaline conditions.