Kraft black liquors properties in relation to delignification

The surface and bulk properties of kraft black liquors (KBLs) sampled after different pulping times have been investigated. The surface tension is discussed in terms of the Lifshitzvan der Waals (LW) and acid-base (AB) interactions. In addition, the Hamaker constant was calculated from the LW-component. The bulk properties investigated were the viscosity, density, lignin concentration, residual alkali and pH. It was observed that both the surface and bulk properties of KBL were influenced by delignification. The reason is suggested to be due to the presence of dissolved lignin and other wood substances in the KBLs. Since the surface tension of KBLs was found to be lower than the original kraft white liquor (KWL), it is obvious that these dissolved wood components are surface active. The surface activity of lignin dissolved in cooking liquor was further confirmed by investigating a simulated kraft black liquor using Indulin AT lignin powder. It was found that all surface tension components and the residual alkali as well as pH varies inverse proportional to the lignin concentration of KBLs. The viscosity and density variations were on the contrary, directly proportional to the lignin concentration of KBLs.