REDUCTION OF AOX FORMATION BY INTRASTAGE WASHING

It is accepted practice to add chlorine dioxide shortly before chlorin e in a pulp chlorination stage. In the present study, by applying ClO2 at a kappa factor of 0.11 followed by a washing step (called intrasta ge washing) and then a minor chlorine treatment, a threefold reduction in AOX formation was achieved compared to conventional bleaching; the same degree of delignification as found in a standard D-50 C-50 opera tion was maintained. In order to facilitate introduction in a commerci al bleach plant, we investigated whether chlorine dioxide treatment ti n before the intrastage washing can be decreased. An optimal time of a bout 10 min was found when the initial pH of the ClO2 treatment is red uced to about 2. With the process modifications, and ClO2 and chlorine charges of 1.20% and 1.53%, respectively, a kraft black spruce pulp w ith a kappa number of about 30 can be bleached to a kappa number below 4 after caustic extraction, with a total AOX production of less than 1.5 kg/ton for the untreated combined chlorination and extraction effl uents.