Experiments on Bleaching Effluent Minimization, 2000 Pulping / Process & Product Quality Conference Proceedings

J. Fiskari, L. Toikkanen, J. Gullichsen--This investigation studied the consequences of a partial closure of the liquid circulations in TCF and ECF bleaching sequences. A series of experiments was performed with an oxygen delignified hardwood (birch, Betula) pulp sample from a Finnish pulp mill. The bleaching filtrates were recirculated in order to simulate the liquid circulations of a partially closed bleach plant. The conditions (chemical charges, retention times, temperatures) were similar to those of an actual mill. The results show that the COD levels of the filtrates increased significantly when the filtrates were recycled. As a result of this, the kappa number of the pulp increased significantly regardless of the bleaching chemical. The viscosity of the pulp showed little or no change as the filtrates were recycled. Pulp brightness decreased clearly as a result of recycling. In general, there was a tremendous difference between the bleaching results with a high degree of closure and those with effective washing with pure water after each bleaching stage. The PO stage appeared to be very sensitive to the carryover load from the Z/Q stage. Pulp brightness increased eight additional ISO units in the PO stage when carryover was eliminated with effective washing. In the EOP stage the corresponding difference was only one ISO brightness unit.