Effect of storage conditions on the brownstock washing and oxygen delignification of kraft pulps

Abstract A long-term goal of the pulping industry is to optimize process parameters for efficiently removing degraded and soluble lignin during the fiber line processes such as kraft pulping, brownstock washing, and bleaching. This study investigates how pulp storage affects the efficiency of brownstock washing and oxygen delignification. Three pulp groups were rinsed with warm and cold water at 40 °C and 5 °C, respectively, and then stored under varying conditions (1 day, 1 week at temperatures of 5 °C and 60 °C. Our findings indicate that after one week of storage at 60 °C, more lignin was extracted, highlighting the influence of storage temperature and time on Kappa reduction (lignin removal) during storage. Additionally, larger lignin fragments were removed with increased storage temperature and time, suggesting that degraded lignin molecules trapped within the fibers can leach out during storage and be subsequently removed in washing. The different storage conditions had only a slight effect on oxygen delignification performance. We conclude that storage conditions, particularly temperature and time, significantly impact lignin removal efficiency and can enhance the pulp washing process. This study also provides valuable insights into lignin mass transfer during storage, offering guidance for industrial applications. The study also revealed that pulp quality after oxygen delignification is influenced by pH and lignin agglomeration and retention in the fibers during preceding washing and storage operations, emphasizing the need for careful control of the latter conditions to minimize cellulose degradation.