Study on slow pyrolysis mechanism of non-metallic fractions in waste printed circuit boards using TG-FTIR-MS analysis

The pyrolysis of non-metallic fractions from waste printed circuit boards (WPCB-NMFs) is a promising recycling method, yet a precise understanding of the reaction mechanism and bromine migration remains limited. In this study, the reaction mechanism of WPCB-NMFs was investigated by tracking three-phase products through the TG-FTIR-MS analysis combined with the pyrolysis in a fixed bed reactor at 300-600 °C. The results revealed that elevating temperature can promote the decomposition and conversion of organics, while excessively high temperature exacerbated the secondary cracking of intermediates and gave rise to more gaseous products. The maximum liquid yield of 75.22 wt% was achieved at 500 °C, with phenolics identified as the predominant component. Increasing temperature also facilitated the migration of bromines, with the peak content of organic bromine in the liquid phase observed at 450 °C. At higher temperatures, HBr was the dominant form of bromine in the pyrolytic product. The pyrolysis process can be divided into three stages: 1) The initial stage (<289 °C) was the depolymerization of crosslinked structures, intramolecular dehydration, C-Br bond cleavage, ether-bond scission, and decomposition of propylene oxides. 2) The fast pyrolysis stage (289-490 °C) was the intense decomposition of bisphenol A and tetrabromobisphenol A (TBBPA) through molecular rearrangement and radical-addition reactions. 3) The slow pyrolysis stage (>490 °C) involved secondary cracking, cyclization of intermediates, and the carbonization of residues.