Ozone and peroxide bleaching of two-stage oxygen-delignified MCC pulps

The effects of continuous cooking and two-stage oxygen delignification on pulp bleaching were studied using the TCF O/O-Q-Q-E-OP-Z/Q-P sequence comprising an ozone and a peroxide stage. The study emphasizes the adverse effect of transition metals on the selectivity of oxygen delignification and hence their effect on pulp properties in subsequent bleaching stages. The study used pine MCC pulps cooked in the laboratory to kappa numbers of 26.5 and 14 from mill pine chips obtained in southern Finland. Oxygen delignification in the O/O-Q-Q-E-OP-Z/Q-P bleaching sequence was performed with a laboratory MC mixer. The pulp with the lower kappa number delignified better than that with the higher kappa number, even with a relatively small alkali charge. The pulp that was cooked to a low kappa number and which underwent a short and a long oxygen stage reached the brightness target set for TCF pulp. Oxygen delignification was preceded by a chelation stage which improved pulp viscosity significant ly. With respect to the final bleaching result and the pulp properties, it makes sense to cook the pulp to the lower kappa number and to carry out oxygen delignification with both a short and long oxygen stage. The combination of short and long oxygen stages generally used at mills, together with cooking to a low kappa number, ensures high bleachability in a TCF sequence with ozone. TCF pulps are more easily beaten than the same ECF pulps bleached with chlorine dioxide. The optical and strength properties of the TCF pulps were very close to those of the ECF pulps.