OPTIMIZATION OF THE PRETREATMENT OF KRAFT PULP WITH NITROGEN-DIOXIDE

Nitric acid produced during NO2 treatment of pulp gives rise to depolymerization of cellulose. Produced and added nitric acid is consumed in lignin reactions, which favor the delignification and give rise to NO2. An extensive delignification without severe depolymerization of the pulp was achieved by NO2 treatment at 60-degrees-C and 27% consistency, followed by dilution with nitric acid, heating to 90-degrees-C, and ripening at this temperature. The consumption of nitric acid was already large at 60-degrees-C, and it continued during the heating period. Stepwise dilution and a decreased ripening temperature can be applied to suppress the depolymerization of the cellulose. An effective mixing with the diluent is a prerequisite for optimum results.