Effect of Lignin Recovery Conditions from Hardwood Kraft Black Liquor on Lignin Characteristics

Black liquor, generated during the pulp and paper manufacturing process, is produced in millions of tons annually; however, most of it is burned as boiler fuel for energy recovery.Recently, efforts have increasingly been made to utilize this low-value byproduct as a source of functional industrial materials by converting the lignin it contains into high-performance biopolymers.In this study, lignin was recovered from hardwood kraft black liquor, and the effects of acid addition sequence, precipitation pH, and black liquor dilution ratio on recovery efficiency and lignin characteristics were systematically analyzed.The highest lignin recovery was achieved under single acidification at pH 3, while the recovery method based on adding black liquor to acid exhibited slightly higher recovery efficiency than that based on adding acid to black liquor.Molecular weight analysis revealed that under strongly acidic conditions, Mw decreased and a lower polydispersity index (PDI) was achieved, indicating a more uniform molecular weight distribution.Additionally, experiments on dilution concentration showed that lignin with the highest purity (up to 94.7%) and the most homogeneous molecular structure was obtained at 20% dilution.These results demonstrate that lignin fractionation and structural control can be achieved through precise adjustment of acidification and dilution conditions, providing a strong foundation for optimizing process strategies in the development of lignin-based functional polymer materials.