In pursuit of sustainable alternatives to fossil-based materials, cellulose-based products, such as regenerated cellulose and cellulose derivatives, have attracted increasing attention. These materials offer biodegradability, biocompatibility, and a wide range of adjustable properties. However, their production relies on dissolving pulps, which are significantly more expensive than standard paper-grade pulps due to high raw material and processing costs. In this study, a potential route for converting standard bleached chemical pulps into more reactive cellulose was investigated, targeting applications in cellulose derivatives and regenerated materials. The method is based on acidic treatment followed by cold alkali dissolution and precipitation. Results show increased chemical reactivity and partial hemicellulose removal, suggesting a promising path toward low-cost alternatives to conventional dissolving pulp.

The growing interest in sustainable lignin valorization has prompted the exploration of new recovery methods compatible with modern kraft pulp mills. This study investigates the use of ultrafiltration for extracting lignin, referred to as oxlignin, from oxygen-delignification wash liquors. Unlike traditional acid precipitation, ultrafiltration enables lignin concentration and fractionation without disrupting the high-pH conditions required for brown stock washing or interfering with the sodium-sulfur balance of the recovery cycle. The oxlignin also has properties similar to lignosulfonates rather than kraft lignin. Using industrial process streams, ultrafiltration successfully prepared oxlignin, as confirmed by characteristic UV-vis absorbance features at 280 and 340 nm. These results demonstrate the ability to tailor lignin composition and open new opportunities for producing application-specific lignin products, such as dispersants or performance additives in bio-based materials. The retained filtrate could be suitable for reuse in the pulping process, ensuring integration with existing operations. Despite practical challenges such as membrane cost and fouling, the use of robust ceramic membranes offers a technically viable path forward for industrial implementation.

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.

Oxygen delignification is a critical stage in modern kraft pulp production, enabling significant reductions in chlorine-based bleaching chemicals and environmental emissions while maintaining fiber quality. The process remains limited by challenges in efficiency and selectivity, governed jointly by chemical reactions and mass transport constraints. This thesis investigates the interplay between these mechanisms and explores the properties and valorisation potential of oxidized lignin (oxlignin) extracted from oxygen-stage wash liquors. Experimental results demonstrate that lignin removal during oxygen delignification is driven by a combination of rapid early-stage oxidative reactions and diffusion-controlled leaching. High oxygen pressure and sufficient alkalinity promote lignin depolymerization and oxidation, improving selectivity, while insufficient chemical conditions lead to lignin redeposition and cellulose degradation. Upstream factors such as brownstock washing efficiency and storage conditions significantly influenced lignin leaching and pulp quality, highlighting the importance of integrated process control. Oxlignin, isolated from industrial filtrates, differed markedly from conventional kraft lignin, exhibiting higher carboxylic acid content, improved water solubility, and a narrower molecular weight distribution. These properties suggest potential applications as dispersants or additives in biopolymer formulations. Ultrafiltration proved to be a viable approach for fractionating oxlignin. By connecting process optimization with resource valorisation, this work contributes to more sustainable kraft pulp production and supports the development of new lignin-based value streams in future biorefineries.

Syrgasdelignifiering är ett viktigt steg i modern sulfatmassaproduktion och möjliggör en betydande minskning av klorbaserade blekkemikalier och miljöutsläpp samtidigt som fiberkvaliteten bibehålls. Trots att processen är väl etablerad finns begränsningar i effektivitet och selektivitet, vilka påverkas av både kemiska reaktioner och masstransportfenomen. Denna avhandling undersöker samspelet mellan dessa faktorer samt utforskar egenskaperna och värdepotentialen hos oxiderat lignin (oxlignin) som extraherats från tvättvätskor i syrgassteget. Studierna visar att ligninavlägsnandet styrs av en kombination av oxidativa reaktioner och diffusionsbegränsningar. Faktorer uppströms, såsom massatvätt och lagringsförhållanden, påverkar lakning av lignin och massakvalitet, vilket understryker vikten av integrerad processkontroll. Oxlignin, isolerat från industriella tvättvätskor, uppvisade tydliga skillnader jämfört med konventionella svartlutslignin, högre halt av karboxylsyror, förbättrad vattenlöslighet och mer homogen molekylviktsfördelning. Dessa egenskaper innebär potential för användning som till exempel dispergeringsmedel. Ultrafiltrering visades vara en möjlig metod för fraktionering av oxlignin. Genom att koppla processoptimering till resursvärdering bidrar detta arbete till en mer hållbar massaproduktion och utveckling av nya värdeflöden i bioraffinaderier.

This work has focused on oxygen's role in the delignification process within the context of pulp production. We have investigated the role of oxygen in a complex set of chemical reactions taking place during this process, including both oxidative and non-oxidative reactions. This study explores the impact of pH changes during the oxygen delignification process and the characteristics of the resulting pulps. Additionally, this research examines the effect of oxygen, by comparing conventional oxygen delignification with trials using air and nitrogen. Industrial softwood kraft pulps with a kappa number of 35 were subjected to delignification for 20-120 min under alkaline conditions. The resulting pulps were assessed for kappa number, intrinsic viscosity, fiber charge, and ISO brightness. An important observation from this research is the reduction in lignin molecular weight upon exposure to oxygen and air, suggesting depolymerization reactions facilitated by oxygen species, whereas nitrogen exposure results in less pronounced changes. This finding underscores the impact of oxygen in altering lignin structure, thus informing the selectivity and effectiveness of the delignification process.

Lignin, a bio-originated polymer, is being explored as an alternative to nonrenewable fossil resources. It is obtained from biomass during pulping and is mostly burned for energy. In most kraft pulp lines, residual lignin in the pulp is oxidized and solubilized during an oxygen delignification step. This study proposes an isolation method for lignin solubilized during oxygen delignification, which we refer to as "oxlignin", and explores its structural characteristics and properties. The study found acid precipitation to be an effective method for partially isolating oxlignin from the oxygen delignification step. Various analytical methods were employed, including UV-vis absorption analysis, 31P NMR spectroscopy, FT-IR spectroscopy, SEC, and TGA. In addition, the solubility of the lignin was studied in four different solvents and compared to the commercial kraft lignins. The study found that oxlignin is a promising substitute for lignosulfonates in certain applications due to its hydrophilicity and high solubility in water, methanol, and ethanol. Compared to kraft lignins, oxlignin has a lower phenolic group content but higher carboxylic acid content.

Phenolic hydroxyl groups in lignin are crucial for understanding its structure, reactivity, and potential applications. Various methods have been developed for the determining phenolic groups in lignin. This study focuses on the comparison of a simple, cost-effective, and time-efficient UV–vis ionization difference technique with the highly accurate ³¹P NMR spectroscopy for analyzing lignin samples of different origins and isolation methods. The results were carefully evaluated, and the strengths and limitations of each method were discussed. Two eco-friendly UV–vis approaches were proposed for a rapid and comprehensive evaluation of the total phenolic-OH groups: one using a strong alkaline solution for analyzing common types of technical lignins, and another employing multipoint wavelength calculations, effective for analyzing softwood lignins regardless of the extraction method. Additionally, the research highlighted the importance of selecting appropriate model phenolic compounds to accurately assess the phenolic hydroxyl group content in lignins using the UV–vis method. Offering straightforward and rapid analysis, with results closely aligning with ³¹P NMR data, this method is a promising alternative for routine analysis.