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.

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.

Kraft pulping is the most widely used pulping process for producing chemical pulp today. It has been in use for more than 130 years. During the pulping process most of the lignin and part of the hemicellulose are removed from the wood, resulting in an unbleached chemical pulp which is mainly composed of cellulose, some hemicellulose with smaller amounts of modified lignin. Several studies have been conducted to ensure effective delignification during the kraft cooking process. Nonetheless, due to the highly complex chemistry and anatomy of wood, there is a need to understand pulping chemistry in more detail.

The principal inorganic component in wood is calcium. A considerable share of the calcium in wood is released during the chemical pulping process. The calcium released initially during kraft pulping is mainly soluble in cooking liquor which passes through a thermodynamically stable limit before reacting with carbonate to form calcium carbonate. The calcium in the pulping process causes precipitation and scaling problems in the pulping process lines, especially in the black liquor evaporation sections.

The focus of this work is to increase the knowledge of kraft cooking of Eucalyptus dunnii which has high calcium content and cooking chemicals reactions during delignification. In addition, the aim was to investigate the influence of different calcium levels in E. dunnii wood chips on the kraft pulping process. Chips with different calcium levels were pulped in lab-scale digesters.

The results obtained in this work showed that the rate of delignification decreased and polysaccharide degradation increased during kraft pulping of E dunnii when the calcium content was high. E dunnii with low calcium content showed no adverse effect on the delignification rate and polysaccharide degradation during kraft pulping.

The uronic acid content in wood and the hexuronic acid content in pulp after kraft cooking showed no significant impact on the delignification rate. The detrimental effect of calcium during kraft pulping was studied using green liquor introduction in kraft pulping. Using green liquor during the kraft pulping of high calcium E dunni wood chips and extending the impregnation time resulted in good delignification and lower degradation of polysaccharides. Unbleached pulps produced from high calcium content wood could be bleached with good results, i.e., equal brightness and increased strength properties. The drainage resistance of bleached pulp was slightly increased. The calcium present in the wood chips follows the fibre line with the pulp after kraft cooking and is less soluble in kraft black liquor. These phenomena are probably related to the carbonate present in green liquor “inactivating” the adverse effects of calcium ions. Possible scientific mechanisms and hypotheses are discussed in this work. Preliminary studies using industrial green liquor compared to synthetic green liquor on a lab scale are also discussed in this work.

Keywords: kraft pulping, delignification, polysaccharide degradation, viscosity, H-factor, kappa number, calcium content

Sammanfattning

Sulfatprocessen är idag den mest använda metoden för framställning av kemisk massa, och har varit i användning i mer än 130 år. Under denna process avlägsnas det mesta av ligninet och en del av hemicellulosa från veden, vilket producerar en oblekt kemisk massa främst bestående av cellulosa och en del hemicellulosa med mindre mängder modifierat lignin. Flera studier har fokuserat på effektiv delignifiering under sulfatkoket, men trots detta finns det ännu brister i förståelsen av processen beroende på den komplexa kemin och morfologin hos trä.   

Den viktigaste oorganiska kompententen i ved är kalcium. En stor del av detta frigörs under sulfatprocessen. Det som frigörs tidigt under koket är huvudsakligen löst i kokvätskan innan det kan reagera med karbonat och bilda calciumkarbonat.  Kalcium I massakokningsprocesser orsakar problem med utfällningar på utrustningen, särskilt i indunstningssystemet. 

Fokuset för detta arbete är att öka kunskapen om sulfatkok av ved från Eucalyptus dunnii med hög kalciumhalt, och kemiska reaktioner under massatillverkningen. Ett vidare mål är att undersöka betydelsen av betydelsen av varierande kalciumhalter i ved från E. dunni. Flis med olika halt av kalcium kokades i labskala. 

Resultaten i detta arbete visade att delignifieringshastigheten avtog och polysackaridnedbrytningen tilltog vid högre kalcumhalter i veden. Det var däremot ingen påverkan på hexenuronsyrabildningen. Dessa negativa effekter av hög kalciumhalt kunde minimeras genom närvaro av grönlut Användes grönlut vid massatillverkning av E. dunni ved med hög kalciumhalt. Kunde massatillverkning utföras med tillfredställande delignifiering och låg kolhydratnedbrytning. Massor framställda på detta sätt kunde blekas med gott resultat, d.v.s. hög ljushet och goda styrkeegenskaper. Avvattningsmotståndet var dock något högre. Vid användning av grönlut följer calciumet med massan snarare än hamnar I svartluten. De positiva effekterna av grönlutsanvändning beror troligen på att kalciumjonerna inaktiveras och därmed undvikt negativa effekter av lösligjorda kalciumjoner. Förslag till möjliga mekanismer som förklarar dessa effekter diskuteras i avhandlingen. En jämförelse mellan syntetiska grönlutar och industriella grönlutar presenteras också. 

Nyckelord: Sulfatmassaprocessen, delignifiering, polysackaridnedbrytning, viskositet, H-faktor kappatal, kalciumhalt

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.

Lignin is a renewable source of aromatics with great potential as a substitute for fossil-based phenolic compounds that are used in several material applications. However, the available technical lignins are heterogenic and still structurally not fully understood. This presents hurdles for studies focused on gaining fundamental insights into how materials properties are related to the molecular structure of lignin. In the present study, a novel cyclic extraction process for lignin is more deeply studied to investigate the potential to tailor the chemical and physical properties of lignin. For this purpose, a design of experiment (DoE) approach was adopted as a tool to investigate the effect on the lignin properties of the selected parameters by including linear, quadratic and interaction effects in a multiple linear regression (MLR) model. Molecular characterization techniques included 1D and 2D NMR, SEC and DSC. It was clearly demonstrated that the chemical and physical properties of lignin could be tuned for the cyclic process using the DoE approach, while preserving 66-82% of the commonly known lignin inter-units, substantiating that the cyclic extraction approach offered a decent to excellent level of protection to inter-units when compared to benchmark organosolv and kraft lignin. By manipulation of the extraction conditions, the β-O-4′ content can be tuned between 20 and 35% simultaneously with the content of phenolic and aliphatic hydroxyls. Finally, DSC studies showed Tgs in the range of 150-185 °C which are discussed with respect to the molecular properties of the analysed lignin. Overall, to advance efforts in lignin valorization, a green process to produce a library of well-characterized lignins, tailored with respect to chemical and physical properties by process conditions, is presented.

Sodium dithionite (Na2S2O4) may have the potential to be used as a reducing agent for the stabilization of glucomannan in kraft cooking for increased pulp yield. However, due to the fact that dithionite decomposes under the conditions of kraft pulping, studies of the effects of dithionite in kraft pulping are non-conclusive; sometimes clearly showing an increased yield, and in other studies no effect at all. The specific conditions influencing dithionite degradation are also unclear. For that reason, this study was conducted to determine the thermal and chemical stability of sodium dithionite with respect to specific factors, such as the pH, temperature, heating time, and the concentration of sodium dithionite solution. The study was performed under anaerobic conditions using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The thermal and alkali stability of the sodium dithionite solution was shown to decrease with increasing temperature, heating time, and concentration of the solution at the alkaline conditions studied. The thermal stability decreased rapidly at weak alkalinity (pH 9) as well as in high alkalinity (pH 14), whereas the sodium dithionite was rather stable at moderate alkalinity (pH 11.5 to pH 13).