Defence of doctoral thesis: Eashwara Raju Senthilkumar – Efficient Washing of Papergrade Pulp in the Kraft Process

Brownstock washing is a critical unit operation in kraft pulping, responsible for removing dissolved lignin and other organic materials from the pulp prior to further delignification and bleaching. In the process, effective brownstock washing improves pulp quality and reduces chemical consumption, leading to better environmental and economic outcomes. A persistent challenge in modern kraft mills is lignin redeposition during washing, particularly under liquor recycling and process closure conditions. This work provides a mechanistic understanding of lignin removal, transport, and redeposition during brownstock washing of softwood kraft pulp. The work examines how physicochemical conditions, including liquor composition, ionic strength, pH, alkali addition, divalent cations, and storage conditions (storage time and temperature), significantly influence lignin diffusion, particularly in higher molecular weight lignin fractions. A combination of laboratory-scale washing experiments, filtrate analysis, pulp characterization, and lignin leaching studies was employed to isolate and examine these coupled mechanisms. The results demonstrate that lignin removal is frequently limited by diffusion from the fibre wall and that the stability of dissolved lignin is strongly influenced by ionic strength and ion-specific effects. Sulfate rich, high ionic strength liquors promote lignin aggregation and redeposition, whereas elevated pH and controlled alkali addition enhance lignin solubility and reduce redeposition within defined limits. Storage time and temperature were shown to significantly affect lignin diffusion, particularly for higher molecular weight lignin fractions. Overall, this work indicates that brownstock washing is governed by the interplay among diffusion-controlled transport, the thermodynamic stability of dissolved lignin, and surface-driven redeposition phenomena. The mechanistic insights gained provide a basis for optimizing washing strategies, reducing lignin carryover, and improving the sustainability and efficiency of kraft pulping operations.

Link to the thesis