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.

Tvättning av blåsledningsmassa är en kritisk enhetsoperation i sulfatmassaframställning och avlägsnar lösligt lignin och andra organiska ämnen från massan före vidare delignifiering och blekning. En effektiv tvättning förbättrar massakvalitet och minskar kemikalieförbrukning, vilket bidrar till bättre miljömässiga och ekonomiska resultat. En utmaning i moderna kraftmassafabriker är ligninåteravsättning under tvättning, särskilt under förhållanden med lutåtervinning, processlutning och hög jonstyrka. Detta arbete ger en mekanistisk förståelse för ligninborttagning, transport och återavsättning vid tvättning av barrblåsledningsmassa. Studien undersöker hur fysikalisk-kemiska parametrar – inklusive lutsammansättning, jonstyrka, pH, alkalitillsats, tvåvärda katjoner samt lagringsförhållanden (lagringstid och temperatur) – påverkar lignindiffusion, särskilt för ligninfraktioner med högre molekylvikt, samt stabiliteten hos löst lignin i tvättvätskan och lignin– fiberyteinteraktioner. En kombination av tvättexperiment i laboratorieskala, filtratanalys, massakarakterisering och ligninlakningsstudier användes för att isolera och undersöka dessa samverkande mekanismer. Resultaten visar att ligninborttagning ofta är begränsad av diffusion från fiberväggen och att stabiliteten hos löst lignin påverkas starkt av jonstyrka och jonspecifika effekter. Sulfatrika lösningar med hög jonstyrka främjar ligninaggregering och återavsättning, medan förhöjt pH och kontrollerad alkalitillsats ökar ligninets löslighet och minskar återavsättning inom definierade gränser. Lagringstid och temperatur visade sig ha en signifikant effekt på lignindiffusionen, särskilt för ligninfraktioner med högre molekylvikt. Sammanfattningsvis visar detta arbete att tvättning av blåsledningsmassa styrs av samspelet mellan diffusionskontrollerad transport, termodynamisk stabilitet hos löst lignin samt ytdrivna återdeponeringsfenomen. De mekanistiska insikterna som erhållits utgör en grund för att optimera tvättstrategier, minska ligninåteravsättning samt förbättra hållbarhet och effektivitet i sulfatmassaframställning.

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.

Brownstock washing, a critical process in cleansing kraft pulp, removes dissolved lignin residues from the pulp after it has passed through the cooking digester. It plays a significant role in kraft pulp mills by enhancing economic efficiency and environmental sustainability. Improved washing efficiency leads to better pulp quality and more effective recovery of cooking chemicals. Our study aimed to better understand the impact of different chemical compositions in washing liquors on washing performance. We tested a range of washing liquors, including neutral solutions (deionized water, 1M NaCl, 3M NaCl, 1M Na2SO4) and alkaline solutions (tap water, washing liquor composed of 0.35M NaOH and 1M Na2SO4, and white liquor with 50 g[OH]/l and 8.77 g[HS]/l). These liquors were evaluated for their efficacy in maximizing lignin extraction. Our findings suggest that salt solutions generally reduce washing efficiency. Deionized water and white liquor proved to be the most efficient washing agents, while high-concentration salts and those with high ionic strength negatively impacted washing efficiency. This suggests that brownstock washing may not be operating at its full potential.

Eucalyptus dunnii is cultivated in Uruguay for kraft pulping purposes. However, depending on the growth site, the kraft pulping properties of the wood vary highly, and in some cases, pulping is difficult. Different batches of wood were chemically characterized and the only significant difference related to the pulping properties was the calcium content. The calcium appears to at least partly be present in the form of crystals in the lumen. Kraft pulping experiments on wood with different calcium contents indicated that high calcium led to slower delignification, and higher yield losses. Hexeneuronic acid formation was not significantly affected. Possible mechanistic explanations for these effects are discussed. 

Eucalyptus dunnii is cultivated in Uruguay for kraft pulping purposes. However, depending on the growth site, the kraft pulping properties of the wood vary highly, and in some cases, pulping is difficult. Different batches of wood were chemically characterized and the only significant difference related to the pulping properties was the calcium content. The calcium appears to at least partly be present in the form of crystals in the lumen. Kraft pulping experiments on wood with different calcium contents indicated that high calcium led to slower delignification, and higher yield losses. Hexeneuronic acid formation was not significantly affected. Possible mechanistic explanations for these effects are discussed.