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.

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.

Glucuronoxylan is the main hemicellulose in the secondary cell wall of angiosperms. Elucidating its molecular structure provides a basis for more accurate plant cell wall models and the utilization of xylan in biorefinery processes. Here, we investigated the spacing of acetyl, glucuronopyranosyl and galactopyranosyl substitutions on Eucalyptus glucuronoxylan using sequential extraction combined with enzymatic hydrolysis and mass spectrometry. We found that the acetyl groups are preferentially spaced with an even pattern and that consecutive acetylation is present as a minor motif. Distinct odd and even patterns of glucuronidation with tight and sparse spacing were observed. Furthermore, the occurrence of consecutive glucuronidation is reported, which adds to the growing body of evidence that this motif is not only present in gymnosperms but also in angiosperms. In addition, the presence of terminal galactopyranosyl units, which can be released by β-galactosidase, altered the digestibility of the glucuronoxylan by GH30 and GH10 xylanase and appeared to be clustered within the polymeric backbone. These findings increase our understanding of the complex structure of glucuronoxylans and its effect on the extractability and biological degradation of Eucalyptus wood.

Kraft pulping of wood is based on efficient depolymerization and solubilization of lignin, while cellulose is relatively undamaged. Non-cellulose cell wall polysaccharides are however in some cases heavily degraded, especially pectin and to a lesser degree also glucomannan while, xylan is relatively stable. In this mini-review, the most important reactions in lignin and polysaccharide degradation in kraft pulping are described, both the technically favorable and the problematic reactions, and the chemical background to discuss the advantages and drawbacks of the process. An attempt to put the different reactions in the perspective of the goals of the pulping process is made and a special focus is on the development of color in the pulp fiber during the kraft pulping.

In our previous study, we demonstrated that Eucalyptus dunnii samples containing high calcium content show inferior pulping properties concerning delignification and polysaccharide degradation. This led us to investigate alternative methods for improving the pulping process of these samples. In the present work, we evaluated the effects of incorporating black and green liquors into the Eucalyptus dunnii chips before kraft pulping, aiming to enhance the pulping process and overcome the negative impact of high calcium content. The addition of both black and green liquors resulted in specific enhancements, with the green liquor having a more significant impact on the pulping process. Even wood samples with the highest calcium content demonstrated satisfactory pulping results when treated with green liquor. Delignification occurred more rapidly, and selectivity was higher for samples pre-treated with green liquor before kraft pulping. Moreover, calcium tended to follow the fiber under these conditions rather than being released into the black liquor, which may contribute to the improved pulping performance. Subsequent bleaching tests revealed that the bleachability of green liquor-treated pulp was nearly identical to that of a control pulp, while maintaining a higher viscosity. This suggests that incorporating green liquor into the pre-treatment process not only improves the pulping performance of Eucalyptus dunnii samples with high calcium content but also maintains desirable bleachability characteristics. To better understand the underlying mechanisms of these findings, we discuss the potential chemical explanations behind the observed improvements.

Although organosolv processes using high-boiling solvents have been investigated in recent decades for developing novel industrial processes, there are potential benefits of using high-boiling point solvents for traditional sulphate-based cooking processes, both from an industrial perspective and from a laboratory perspective. Using high-boiling solvents, experiments can be done under atmospheric conditions, thus making it easier to continually monitor laboratory experiments and extracting aliquots at desired intervals. Using such a system, alkaline consumption was monitored during impregnation of spruce chips in glycerol media using chemical charges of 1 M NaOH and 0.1 M NaHS, i. e., kraft pulping conditions, and compared to a similar investigation of alkaline consumption in water media using steel autoclaves. The resulting data was fitted to a first order kinetic model, with an apparent activation energy of 22 kJ mol-1 in glycerol media. Finally, a "normal quality pulp"of kappa number 28 and a viscosity of 1113 ml g-1 was successful produced using a cooking process with an impregnation step at 140 °C for 3 h and a cooking step at 160 °C for 4 h. A nuclear magnetic resonance study on the dissolved lignin produced for said experiment showed characteristics typical of other kraft lignins.

Samples of Eucalyptus dunnii with high calcium content have less good pulping properties regarding delignification and polysaccharide degradation, as it was shown by us earlier. In this work, we tested the addition of black liquor and green liquor to the Eucalyptus dunnii chips before kraft pulping, Specific improvements were obtained with both liquors, but the most substantial effect was observed with the green liquor, where even wood with the highest calcium content was pulped with a good result. Delignification was faster, and viscosity losses (degree of polymerization of cellulose) were higher for samples treated with green liquor prior to kraft pulping. Bleaching experiments showed that the bleachability of the green liquor-treated pulp was virtually the same as for a control pulp and that the higher viscosity of the bleached pulp was maintained. Possible chemical explanations for the results obtained are discussed.

Studies have shown that the size of LNP depends on the molecular weight (M-w) of lignin. There is however need for deeper understanding on the role of molecular structure on LNP formation and its properties, in order to build a solid foundation on structure-property relationships. In this study, we show, for similar M-w lignins, that the size and morphology of LNPs depends on the molecular structure of the lignin macromolecule. More specifically, the molecular structure determined the molecular conformations, which in turn affects the inter-molecular assembly to yield size- and morphological-differences between LNPs. This was supported by density functional theory (DFT) modelling of representative structural motifs of three lignins sourced from Kraft and Organosolv processes. The obtained conformational differences are clearly explained by intra-molecular sandwich and/or T-shaped pi-pi stacking, the stacking type determined by the precise lignin structure. Moreover, the experimentally identified structures were detected in the superficial layer of LNPs in aqueous solution, confirming the theoretically predicted self-assembly patterns. The present work demonstrates that LNP properties can be molecularly tailored, consequently creating an avenue for tailored applications.

An ionization difference UV-Vis method (Delta epsilon-spectrum method) is the most potentially simple method for fast quantitation of phenolic hydroxyl groups (ph-OH) in lignin. However, the underestimated results were calculated from the conventional Delta epsilon-spectrum method using one- or two-point wavelengths measurement. In this study, a modified Delta epsilon-spectrum method using multi-point wavelengths measurement was developed and the negative absorbance was also considered. Four main typical lignin models, e.g. vanilla alcohol, 5-5 biphenyl, stilbenoid and vanillin, were applied as the guaiacyl-type (G-type) phenolic models for the determination of ph-OH by the modified Delta epsilon-spectrum method. The 2-methoxyethanol/water/acetic acid = 8/2/0.2 (V/V/V) was used as the acidic solvent system and the 2-methoxyethanol/0.2 M NaOH solution = 1/9 (V/V) was used as the alkaline solvent system. The ph-OH contents in the spruce milled wood lignin (SMWL) and the spruce Kraft lignin (SKL) were respectively quantified by the modified Delta epsilon-spectrum method as 1.078 and 4.348 mmol/g, which were comparable to the counterparts determined by P-31 Nuclear Magnetic Resonance Spectroscopy (P-31 NMR). The results revealed that the modified Delta epsilon-spectrum method can provide more accurate and reliable results compared to the conventional method.

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.