Extended oxygen delignification of birch kraft pulp

We studied the effect of the conditions of the oxygen delignification of birch kraft pulp with kappa number 18 on pulp properties. The investigation refers in particular to the effect of the amount of NaOH added, as well as the time & temperature of oxygen delignification process on the kappa number, yield and strength properties of pulp, as well as what effect the pre-treatment of pulp with peracetic acid has on these indices. It has been stated that the oxygen delignification of birch kraft pulp with kappa number 18 with oxygen alone in aqueous alkali to a kappa number below 10 (the level of extended oxygen delignification) requires an increased amount of NaOH to be added, higher temperature and a longer process time, resulting in the deterioration of the pulp yield. The pre-treatment of pulp with peracetic acid in the amount of 0.33 or 0.66% A.O. (to oven-dried pulp) enables this pulp to be delignified to kappa number 7 and 5 with a higher yield than that of pulp delignified with oxygen alone. Pulp delignified with oxygen using the peracetic acid pre-treatment also restores higher viscosity and tear index, as well as higher fibre strength.