Effect of Brownstock Washing on Formation of Chlorinated Dioxins and Furans During Bleaching, 1990 Environmental Conference Proceedings

The effect of brownstock washing on formation of polychlorinated dibenzo-p-dioxins (PCDDs) and furans (PCDFs) was evaluated in laboratory studies and mill trials. Poor brownstock washing led to increased levels of these materials in softwood pulps after the chlorination stage and in the final effluent.

Additional washing of unbleached softwood pulp in the laboratory with copious amounts of aqueous ethanol decreased the chlorinated dioxins and furans formed during subsequent chlorination and alkaline extraction stages by 80%. These results demonstrate the presence of solvent-extractable precursors for PCDD/Fs associated with this unbleached pulp. Even this extreme level of washing did not completely prevent formation of these materials in the chlorination stage.

Based on analyses of the brownstock pulps for dibenzo-R-dioxin (DBD) and dibenzofuran (DBF), material balance calculations suggest there must be other precursors for the chlorinated analogs.

Spiking unbleached pulps with higher than normal levels of brownstock and screen room defoamers used in our mills had no significant effect on formation of chlorinated dioxins and furans after bleaching. Laboratory studies also showed that use of surfactants during the brownstock washing could reduce the levels of PCDD/Fs by as much as 25%.