In the oxygen delignification lignin is removed from the pulp by applying high pressure oxygen, under alkaline conditions, resulting in a lower demand for chemicals in bleaching. The main objective of this study was to evaluate the influence of washing (conventional and acid) between two stages of O2 on the amount of residual lignin and transition metal present in samples of Pine Kraft pulp with 48.5 initial kappa. The reduction of kappa number and pulp degradation after two stages of O2 treatment with increase of final brightness on sheet produced were observed. The metal content of pulp during oxygen delignification was influenced by washing process, and the acid wash removed more percentage of metallic ions than conventional wash. The best results were obtained with the removal of the element manganese. The acid wash between two stages of delignification contributed to the control of transition metals, which optimize the use of bleaching reagents in the following steps. We concluded that the intermediate washing improves the performance of the oxygen delignification of high kappa number pulps performed in two stages.
pulp washing; oxygen delignification; high kappa; transition metals