Viscosity Changes in Black Liquor When Bleach Plant Filters Are Added, 1998 International Chemical Recovery Conference Proceedings

The viscosity of black liquor affects evaporation, pumping and droplet formation, that are important for the firing in the recovery furnace. In this paper, we present results from viscosity measurements on two different softwood black liquors, and black liquors mixed with: D- (chlorine dioxide), P- (peroxide) and Q-stage (chelating) bleach plant filtrates. The viscosity was measured at three temperatures, l00°C, 115°C and 130°C. The dry solids content was varied in the range of 50 to 75% by weight. The viscosity of the black liquor was not significantly affected at a given dry solids content by the addition of P-stage filtrate but increased 2-3 times when Q-stage filtrate was added. The addition of D-filtrate also resulted in an increase in viscosity, but not to the same extent as the Q-filtrate. The filtrates were added to the black liquor either by a quick mixing by pouring the components together or by a dropwise and controlled mixing. The difference in viscosity following these two techniques was negligible. From chemical analysis and a multivariate data evaluation, it was concluded that the increase in viscosity when bleach plant filtrates were added was not a result of one single variable but rather a series of compositional changes, such as a higher concentration of inorganic ions, such as calcium and magnesium ions, which cause flocculation of the lignin in black liquors, a lower residual alkali content and a lower COD-value (Chemical Oxygen Demand).