Effect of the bleaching process on brightness stability of eucalyptus kraft pulp

Eucalyptus pulps bleached with ECF sequences containing ozone, chlorine dioxide and a final peroxide stage present high brightness stability in relation to pulps bleached with conventional ECF sequences. This study compares the brightness stability of ECF pulps bleached with the OZ/ED(PO) and OZ/EDP sequences with those of more conventional ECF sequences such as ODHT(PO)DD and ODHT(PO)DP. In addition, the relative importance of ozone, chlorine dioxide and peroxide doses on dry and wet heat reversion of pulps bleached with the sequence OZ/EDP is discussed. Furthermore, the composition of aromatic compounds present in pulps bleached with the sequences OZ/EDP and ODHT(PO)DD, measured by Py-GC/MS, and their impact on reversion is evaluated. A mechanism for eucalyptus pulp heat reversion is presented. The OZ/ED(PO) bleached pulp showed a PCN (0.06) almost four times lower than that of the ODHT(PO)DP bleached one. The distribution of chlorine dioxide and peroxide in the OZ/EDP sequence affected brightness stability with the highest values being achieved when high chlorine dioxide doses are used in the D-stage and low peroxide doses (1 similar to 5 kg/odt) are used in the P-stage. The levels of trace aromatic compounds in the bleached pulp do not correlate with brightness stability. High brightness stability eucalyptus pulps are obtained with bleaching processes that apply enough charge of electrophilic oxidants to eliminate HexA, followed by a final nucleophilic oxidation stage with minimal but well calculated charge of oxidant in order to extract reducing substances and oxidize potential chromophores that are formed under alkaline conditions. Reversion caused by HexA is more severe when pulp is dried under acidic conditions. The opposite is observed for reversion caused by reducing substances. A final nucleophilic bleaching stage run at slight alkaline conditions is effective to prevent brightness reversion of all eucalyptus pulps.