POST-FINAL REACTOR ALKALI CHARGE IN OXYGEN DELIGNIFICATION FOR IMPROVED CHLORINE DIOXIDE BELACHABILITY OF SOFTWOOD KRAFT PULP

摘要

The final alkali concentration in kraft cooking is known to affect pulp's bleachability.This study has investigated whether the alkali level,measured as pH,in the end of the oxygen delignification,also affects bleachability,specifically the chlorine dioxide-bleachability of a softwood kraft pulp in a D(OP)D sequence.Although pulps with different final pH values do not differ from each other in terms of either kappa number or ISO brightness,their response in the D(OP)D sequence is different.The differences are systematic so that the higher the final pH of the oxygen delignification,the better is chlorine-dioxide bleachability.This relation is almost linear up to a pH of about 10.4,above which bleachability does not improve.Splitting of the alkali charge into the oxygen delignification is proposed as a practical way to benefit from these findings.Part of the alkali is then added to a post-final reactor position,where it increases the final pH before being washed out and returned to the inlet of the oxygen stage.