9. Kraft Lignin: Extraction

Kraft lignin is a by-product of the kraft pulping process, predominantly utilized in pulp mills for energy production. Despite lignin annual production reaching 50–70 million tonnes, its utilization for high-value products remains limited, with only 1–2% being used for such applications. The extraction of kraft lignin has been identified as a viable solution for energy-sufficient mills during the chemical recovery step. However, it is important to note that this approach presents challenges in the context of recovery boiler operations. The intricate nature of lignin’s structure, coupled with its variability among different plant species, poses significant challenges in the efficient extraction of this compound and the subsequent assessment of its properties. During the kraft process, lignin undergoes structural modifications, resulting in a highly condensed structure with strong condensed bonds and high polydispersity. The lignin modifications pose challenges for its utilization in high-value applications. Advancements in fractionation methodologies have enhanced the consistency of industrial lignin properties. However, further research is necessary to address challenges in characterization, depolymerization, and upgrading. In this chapter, the main methods on kraft lignin extraction are described. The global lignin market has undergone substantial expansion since 2014, propelled by technological advancements, policy incentives, and an escalating demand for eco-friendly materials.