Factors Influencing Heat Treatment of Black Liquor, 1998 International Chemical Recovery Conference Proceedings

Storage at high temperature of kraft black liquor is known to reduce the liquor’s viscosity significantly, though the reactions of the liquor components have not been clarified. A study was therefore made to find out how thermal treatment influences the chemical composition and viscosity of kraft black liquor. The factors influencing the effect of heat treatment were identified. Both hardwood and softwood liquors were studied. Special attention was paid to the role of polysaccharides in the heat treatment process. Viscosity was measured at 70% dry solids content and 115°C.

The results show that the original viscosity of the black liquor depends on the wood raw material and the pulping conditions. The content of high molecular mass polysaccharides had a significant influence on the viscosity. The differences between hardwood and softwood liquors were obvious and were related to the differences in their composition. The decrease in viscosity achieved with high viscosity liquors from high-yield pulping was particularly large. Other high-viscosity liquors, such as soda-anthraquinone liquors, also benefited from heat treatment. As polysaccharides are broken down by exothermic reactions during heat treatment, the resulting viscosity is due to the content of high molecular mass lignin in the liquor. Liquors with a low content of residual effective alkali were insensitive to heat treatment. The addition of alkali before heat treatment made the treatment more effective.