Use of Secondary Condensates from Black Liquor Evaporation as a Process Water in Do Bleaching, 2002 International Pulp Bleaching Conference Proceedings

Minna Viirimaa, Mervi Sillanpää, Kari Ala-Kaila and Olli Dahl

The information concerning the behavior of secondary condensates in ECF bleaching is still limited. In this research, two sets of bleaching experiments were carried out to study the effect of different compounds on pulp properties in chlorine dioxide bleaching. In the first set of experiments, different volatile organic compounds were added to the pulp suspension before the bleaching. The other set was done by mixing the unbleached pulp suspension with different condensate fractions arising from black liquor evaporation.

No effects of the volatile organic compounds on the pulp viscosity were detected. Only acetic acid and propionic acid had minor effects on the pulp. The results indicated that when the COD load of condensate was higher, the bleaching results after D0 stage were poorer. The brightness of the pulp was decreased and the kappa number was increased as the COD of the condensate was higher. However, no effect was observed on the viscosity of the pulp. COD characterization results indicated that most of COD in condensates originated from methanol and formic acid. Because those components did not affect bleaching results, the most harmful components for bleaching were probably sulfur based components, such as DMDS and DMS, which also cause COD in the condensates.