Reductive Degradation of Residual Chromophores in Kraft Pulp

摘要

In this study, the underlying chemistry regarding chromophore reactions mimicking the reductive bleaching reactions of kraft pulps with sodium dithionite are presented. The experiments with the reductive agent have been carried out with a set of model compounds, such as p-quinone, aromatic compounds with conjugated double bonds and aldehyde and ketone groups, unsaturated dicarboxylic acids, among others, and studied by UV-vis spectroscopy. It was also tested the reaction with hexeneuronic acids (HexA) from xylan and analysed by ATR-FTIR, UV-Resonance Raman @ 325 nm and 'H-NMR spectroscopy. No reductive degradation of HexA occurred, though a reductive reaction of unsaturated structures of unknown origin in xylan has been detected. Additionally, the potential of the reductive stage with either sodium dithionite or sodium borohydride was tested using an industrial pine kraft pulp bleached by OOZEDD sequence. The industrial kraft pulp exhibiting a ceiling ISO brightness of 88.8% could reach the target of 90 ± 0.5%.