Inhibition effect of amine compounds derived from hardening agents on the extraction of hydrogen bromide by water from the pyrolysis oil of brominated printed circuit boards

The pyrolysis oil of waste printed circuit boards (PCBs) obtained through industrially feasible methods commonly contains high concentrations of hydrogen bromide (HBr). This HBr must be further eliminated if the oil is intended to be used in commercial applications. For this purpose, a conventional liquid-liquid extraction process was explored using water as the extracting solvent. It was shown that the extraction efficiency decreases with the concentration of HBr in oil, reaching a limiting concentration below which HBr can hardly be removed. This finding was explained by considering the presence of amines in oil that result from the decomposition of amine-hardening agents in epoxy boards. In connection with these results, the distribution equilibria of HBr in a phenolic model oil–aqueous biphasic system was theoretically and experimentally analysed. The results revealed that the presence of amines in concentrations as low as 0.1 wt% of nitrogen was sufficient to inhibit the removal of HBr below the 1000 ppm level. The dissociation constant of HBr in oil and the partition coefficient of bromine ions between the two phases were evaluated, obtaining 0.088 mol dm⁻³ and 2.6 × 10⁻⁴, respectively. The findings of this work are fundamental for the optimal production of bromine-free oils from waste PCBs.