Viscosity changes in black liquor when bleach plant filtrates are added

The viscosity of black liquor affects evaporation, pumping and droplet form ation, properties that are important for the firing of black liquor in the recovery furnace. The viscosity of the two black liquors investigated was n ot significantly affected at a given dry solids content by the addition of P-stage (peroxide) filtrate but increased 2-3 times when Q-stage (chelating ) filtrate was added. The addition of D-stage (chlorine dioxide) filtrate a lso resulted in an increase in viscosity, but not to the same extent as the Q-filtrate. The viscosity was measured at three temperatures, 100 degreesC , 115 degreesC and 130 degreesC, and the dry solids content was varied in t he range of 50 to 75% by weight. The black liquors collected from the same mill were from softwood pulping, and the kappa numbers after cooking and ox ygen delignification were 23 and 14, respectively. The filtrates were added to the weak black liquor either by quick mixing by pouring the components together or by 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 observed increase in viscosity for black liquor at high solids co ntent when bleach plant filtrates were added to weak black liquor was not a result of one single variable. It was rather due to a series of compositio nal changes, such as larger concentration of inorganic ions, such as calciu m and magnesium ions, a smaller amount of residual alkali and a lower conte nt of organic substance measured as COD (Chemical Oxygen Demand). A large c oncentration of inorganic ions can cause flocculation of the lignin in blac k liquors.