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Pulp strength enhancement by oxygen delignification
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology. RISE.ORCID iD: 0000-0002-0816-0047
2022 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Oxygen delignification is widely used in the pulp and paper industry as a part of delignification process between the kraft cook and bleaching. However, its potential has not been fully utilized. Rather than an intermediate process between cooking and bleaching, oxygen delignification is a strong oxidizing agent with powerful effects on the pulp properties. In this work, the hypothesis that oxygen delignification has the potential to improve the pulp mechanical properties was investigated. 

Several pulps were produced by either kraft cooking or kraft cooking combined with a subsequent oxygen delignification stage to a similar kappa number and their properties were analyzed and compared. This methodology assessed the real oxidative potential of oxygen on the final fiber properties. Total fiber charge, pulp mechanical properties, fiber morphology, swellability and fiber nanostructure, were studied.

The major part of this research investigated the relationship between the carboxylic acid groups, seen as total fiber charge, and the mechanical strength of the paper. The total fiber charge was evaluated by conductometric titration and correlated with the pulp swellability and mechanical properties. It was demonstrated that oxygen delignification could significantly increase the charge content and the swelling of the pulp when an extended oxygen delignification (i.e, higher delignification degree) was used. In addition, the tensile index of the sheets increased when the fiber charge after oxygen delignification was sufficiently high. 

The swelling of the different pulps was investigated by Schopper-Riegler degree (SR°), water retention value (WRV) and fiber saturation point (FSP). It was determined that the higher the fiber charge, the higher the swelling ability, regardless of the lignin content. 

High alkali impregnation was utilized in this study due to its potential to increase cooking yield. The yield was compared to kraft pulp cooked with standard and high alkali impregnation, followed by oxygen delignification and bleaching. It was observed that the increase in yield was preserved in both unit processes, i.e., after oxygen delignification and after bleaching. 

During this work, pulp properties such as fiber morphology and fiber nanostructure were also important properties that were studied following each unit process and refining step. Oxygen-delignified pulps presented higher fiber deformation when compared to the kraft-cooked pulps. However, even with higher fiber deformation, oxygen-delignified pulps showed higher mechanical strength, contradicting previous reports that claimed lower pulp strength for oxygen-delignified pulps, due to fiber deformation. Additionally, it was found that fiber deformation tends to increase with PFI-refining for kraft-cooked pulps, while for oxygen and bleached pulps it tends to decrease. Fiber nanostructure was additionally studied by X-ray scattering, and the results obtained from pulp delignification by kraft and kraft followed by oxygen delignification were compared. 

This thesis highlights the benefits of increasing fiber charge by performing an extended oxygen delignification after a reduced kraft cooking. The results indicate that when oxygen-delignified pulps achieve 80 % higher fiber charge than kraft-cooked pulps at a similar kappa number, the pulp tensile index can be improved by up to 18 %. The oxidation reactions that occur during the oxygen delignification lead to a significant increase in the carboxylic acid groups in the fibers which increases the fiber's swelling ability and improves the refining process efficiency. The combination of those effects results in a higher tensile index and lower refining energy required. However, to obtain mechanical improvement, the oxygen delignification must be sufficiently long (extended). Therefore, it is believed that an extended oxygen delignification will yield a more uniform distribution of the charged groups in the fibers, which will increase the fiber swelling and fiber flexibility leading to a more efficient refining process and stronger fiber bonding structure in the paper.

Abstract [sv]

Syrgasdelignifiering används i stor utsträckning inom massa- och pappersindustrin som en del av delignifieringsprocessen mellan sulfatkokningen och blekningen. Dess potential har dock inte utnyttjats fullt ut. Snarare än att bara se det som en enhetsprocess mellan kokning och blekning, så bör man utnyttja syrgasens starkt oxiderande förmåga som kraftfullt kan påverka massans egenskaper. I detta arbete undersöktes hypotesen att syrgasdelignifiering har potential att förbättra massans mekaniska egenskaper.

Olika massor framställdes till ett liknande kappatalsintervall genom antingen sulfatkokning eller sulfatkokning kombinerat med ett efterföljande syrgassteg och deras egenskaper analyserades och jämfördes. Denna metod bedömde den verkliga oxidativa potentialen hos syre på de slutliga fiberegenskaperna. Speciellt studerades total fiberladdning, fiberns svällbarhet, massans mekaniska egenskaper, fibermorfologi och fibernanostruktur.

En avgörande komponent i forskningen är sambandet mellan karboxylsyragrupperna, sedda som total fiberladdning, och papperets mekaniska styrka. Den totala fiberladdningen utvärderades genom konduktometrisk titrering och korrelerades med massans svällningsförmåga och mekaniska egenskaper. Det visades sig att syrgasdelignifiering avsevärt kunde öka laddningshalten och massans svällning när en utökad syrgasdelignifiering (dvs. högre delignifieringsgrad) användes. Dessutom ökade arks dragindex när fiberladdningen efter syrgasdelignifiering var tillräckligt hög.

De olika massornas svällningsförmåga undersöktes genom mätningar av Schopper-Riegler-grad (SR°), vattenretentionsvärde (WRV) och fibermättnadspunkt (FSP). Från dessa kunde slutsatsen dras att ju högre fiberladdningshalt desto högre svällningsförmåga oavsett ligninhalt.

Högalkaliimpregnering användes i denna studie för dess potential att öka utbytet. Utbytet jämfördes för sulfatmassa kokad med standard- och högalkaliimpregnering, följt av syrgasdelignifiering och blekning. Det observerades att ökningen i utbyte som erhållits efter sulfaktkoket för högalkaliimpregnering bibehölls efter syrgasdelignifiering och efter blekning.

Inverkan på fibermorfologi och fibernanostruktur av olika enhetsprocesser och förädling studerades. Signifikanta skillnader i fibermorfologi observerades för massa från olika enhetsprocesser. Syrgasdelignifiering gav högre fiberdeformationer jämfört med sulfatkokta massor, men även med högre deformation kan syrgasdelignifierade massor uppnå högre mekanisk hållfasthet vilket motsäger tidigare studier som hävdade lägre massastyrka för syrgasdelignifierade massor på grund av de höga fiberdeformationerna. Man fann också att fiberdeformationer tenderar att öka med PFI-malning för sulfatmassor, medan den för syrgasdelignifierade och blekta massor tenderar att minska. Dessutom studerades fibernanostrukturen genom röntgenspridning och resultaten från delignifiering med sulfatkokning och sulfatkokning följt av syrgasdelignifiering jämfördes.

Denna avhandling belyser fördelarna med att utföra en utökad syrgasdelignifiering efter en reducerad sulfatkokning, vilket resulterar i ökad mängd fiberladdningar. Resultaten indikerar att när syrgasdelignifierade massor uppnår 80 % högre fiberladdningar än sulfatmassa vid ett liknande kappatal kan massornas dragindex förbättras med upp till 18 %. Oxidationsreaktionerna som inträffar under syrgasdelignifieringen leder till en signifikant ökning av karboxylgrupperna i fibrerna, vilket ökar fiberns svällningsförmåga och förbättrar malningseffektiviteten. Kombinationen av dessa effekter resulterar i högre dragindex och minskade krav på malningsenergi. Men för att erhålla en förbättring av de mekaniska egenskaperna måste syrgasdelignifieringen vara tillräckligt omfattande. Möjligen kommer en förlängd syrgasdelignifiering att ha en mer enhetlig och jämn fördelning av de laddade grupperna i fibrerna än en kortare delignifiering. Med förlängd syrgasdelignifiering, ökar fibersvällningen och fiberflexibiliteten vilket leder till en effektivare malning och en starkare fiberbindningsstruktur i pappret.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2022. , p. 69
Series
TRITA-CBH-FOU ; 2022: 53
Keywords [en]
fiber charge, morphology, oxygen delignification, swellability, tensile properties
Keywords [sv]
dragegenskaper, fiberladdningar, morfologi, svällningsförmåga, syrgasdelignifiering
National Category
Chemical Engineering Paper, Pulp and Fiber Technology
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-321360ISBN: 978-91-8040-388-7 (print)OAI: oai:DiVA.org:kth-321360DiVA, id: diva2:1710566
Public defence
2022-12-16, F3, Lindstedtsvägen 26, Zoom: https://kth-se.zoom.us/j/63355487940, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

Main funder: STFI Intressentförening

Other funders:

Södra Forskningsstiftelse

Önnesjöstiftelsen

Stiftelsen Nils och Dorthi Troëdssons forskningsfond

QC 2022-11-14

Available from: 2022-11-14 Created: 2022-11-14 Last updated: 2022-11-15Bibliographically approved
List of papers
1. Evaluating the Potential to Modify Pulp and Paper Properties through Oxygen Delignification
Open this publication in new window or tab >>Evaluating the Potential to Modify Pulp and Paper Properties through Oxygen Delignification
2020 (English)In: ACS Omega, E-ISSN 2470-1343, Vol. 5, no 23, p. 13703-13711Article in journal (Refereed) Published
Abstract [en]

The potential to modify pulp and paper properties by oxygen delignification was assessed by looking beyond the ordinary purpose of oxygen delignification. Pulps with the same kappa number were obtained by both pulping and the combination of pulping and oxygen delignification, and the mechanical and chemical properties were compared. The oxidation of pulp components leads to an increase in carboxylic acid groups in the fibers, resulting in a large influence on fiber swelling, seen as an increase in the water retention value and fiber saturation point. The introduction of charged groups appears to replace some of the morphological changes caused by refining and enhance the strength of fiber–fiber joints, generating pulps with better refinability and higher tensile strength. Oxygen delignification was able to improve the tensile index with 6% at the same sheet density and less refining energy, when the amount of total fiber charges was higher than 140 μekv/g.

Keywords
oxygen delignification, curl index, fiber charges, tensile index, swelling
National Category
Paper, Pulp and Fiber Technology Chemical Engineering
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-305954 (URN)10.1021/acsomega.0c00869 (DOI)000542223300025 ()32566835 (PubMedID)2-s2.0-85088416699 (Scopus ID)
Note

QC 20220301

Available from: 2021-12-10 Created: 2021-12-10 Last updated: 2022-11-14Bibliographically approved
2. Differences and similarities between kraft and oxygen delignification of softwood fibers: effects on chemical and physical properties
Open this publication in new window or tab >>Differences and similarities between kraft and oxygen delignification of softwood fibers: effects on chemical and physical properties
2021 (English)In: Cellulose, ISSN 0969-0239, E-ISSN 1572-882X, Cellulose, Vol. 28, no 5, p. 3149-3167Article in journal (Refereed) Published
Abstract [en]

The fiber properties after oxygen delignification and kraft pulping were studied by looking into the chemical characteristics and morphology. The effect of the two processes on the fibers was evaluated and compared over a wider kappa number range (from 62 down to15). Wide-angle X-ray scattering, nuclear magnetic resonance and fiber saturation point were used to characterize the fiber network structure. Fiber morphology and fiber dislocations were evaluated by an optical image analysis. The total and surface fiber charges were studied by conductometric and polyelectrolyte titrations. The fiber wall supramolecular structure, such as crystallinity, size of fibril aggregates, pore size and pore volume, were similar for the two processes. The selectivity, in terms of carbohydrate yield, was equal for kraft cooking and oxygen delignification, but the selectivity in terms of viscosity loss per amount of delignification is poorer for oxygen delignification. Clearly more fiber deformations (2–6% units in curl index) in the fibers after oxygen delignification were seen. Introduction of curl depended on the physical state of the fibers, i.e. liberated or in wood matrix. In the pulping stage, the fiber continue to be supported by neighboring fibers, as the delignified chips maintain their form. However, in the subsequent oxygen stage the fibers enter in the form of pulp (liberated fibers), which makes them more susceptible to changes in fiber form.

Place, publisher, year, edition, pages
Springer Nature, 2021
Keywords
curl, Deformations, Fiber vulnerability, Fiber charge, Selectivity, Water retention value
National Category
Chemical Engineering Paper, Pulp and Fiber Technology
Research subject
Chemical Engineering; Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-305955 (URN)10.1007/s10570-021-03713-0 (DOI)000617853700002 ()2-s2.0-85101488810 (Scopus ID)
Note

QC 20220301

Available from: 2021-12-10 Created: 2021-12-10 Last updated: 2022-11-14Bibliographically approved
3. Differences and similarities between kraft and oxygen delignification of softwood fibers: effects on mechanical properties
Open this publication in new window or tab >>Differences and similarities between kraft and oxygen delignification of softwood fibers: effects on mechanical properties
2021 (English)In: Cellulose, ISSN 0969-0239, E-ISSN 1572-882X, Vol. 28, no 6, p. 3775-3788Article in journal (Refereed) Published
Abstract [en]

Charged groups in pulp have been shown to enhance the tensile strength of the paper produced from the pulp. Oxygen delignification introduces charged groups and it is of interest to determine how the delignification should be distributed between the cooking and the oxygen stage with respect to mechanical properties. A number of unbleached kraft cooked and oxygen delignified pulps within a wide kappa number range were produced and refined, and the effects of the refining on the morphology and mechanical properties were studied. The WRV correlated with the fiber charge and at a given fiber charge, kraft cooked and oxygen delignified pulps had the same WRV development in refining, although they had significantly different kappa numbers. The tensile strength development during refining depends on the fiber rigidity which is affected by the lignin content, the fiber charge and the chemical and mechanical processes used. Refining increased the curl of the kraft cooked pulps and decreased the curl of oxygen delignified pulps, irrespective of kappa number. A greater increase in tensile strength was seen for the pulps with a higher fiber charge and WRV, probably because of the greater degree of fibrillation achieved in the beating process. Despite the greater fiber deformation in the oxygen delignified pulps, the strength can be increased by a larger amount of charged groups and a greater swelling of the fibers.

Place, publisher, year, edition, pages
Springer Nature, 2021
Keywords
Strength, Bonding, Curl index, Swelling, Refining, Fibrillation, Conformation
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-305958 (URN)10.1007/s10570-021-03781-2 (DOI)000622675400001 ()2-s2.0-85101808759 (Scopus ID)
Note

QC 20211210

Available from: 2021-12-10 Created: 2021-12-10 Last updated: 2022-11-14Bibliographically approved
4. The effects of high alkali impregnation and oxygen delignification of softwood kraft pulps on the yield and mechanical properties
Open this publication in new window or tab >>The effects of high alkali impregnation and oxygen delignification of softwood kraft pulps on the yield and mechanical properties
2022 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 37, no 2, p. 223-231Article in journal (Refereed) Published
Abstract [en]

This study investigated whether the yield improvement after high alkali impregnation (HAI) is maintained after oxygen delignification and whether the potential of oxygen delignification to increase the mechanical properties is affected by high alkali impregnation. The yield improvement achieved by high alkali impregnation (1 %) was preserved after oxygen delignification, particularly of glucomannan. The total fiber charge and swelling increased after oxygen delignification regardless of the type of impregnation in the cooking step. The tensile index improvement obtained by oxygen delignification was retained if this was preceded by high alkali impregnation. The stiffness index was higher and elongation slightly lower after HAI impregnation than after a standard (REF) impregnation. Fibers obtained through high alkali impregnation seem to be slightly less deformed and slightly wider than fibers obtained after a standard impregnation.

Place, publisher, year, edition, pages
Walter de Gruyter GmbH, 2022
Keywords
curl index; fiber charge; glucomannans; kraft cooking; tensile index
National Category
Paper, Pulp and Fiber Technology
Research subject
Chemical Engineering; Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-321343 (URN)10.1515/npprj-2022-0022 (DOI)000789624500001 ()2-s2.0-85129892544 (Scopus ID)
Note

QC 20221114

Available from: 2022-11-12 Created: 2022-11-12 Last updated: 2022-11-14Bibliographically approved
5. The impact of bleaching on the yield of softwood kraft pulps obtained by high alkali impregnation
Open this publication in new window or tab >>The impact of bleaching on the yield of softwood kraft pulps obtained by high alkali impregnation
2022 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 0, no 0Article in journal, Editorial material (Refereed) Published
Abstract [en]

High alkali impregnation (HAI) increases the total yield of softwood pulps following kraft cooking. This yield improvement is also maintained after oxygen delignification. This study evaluates how bleaching with either chlorine dioxide or hydrogen peroxide affects the final yield of samples obtained with standard and HAI. The chemical composition, viscosity, brightness, mechanical and morphological properties were studied. Compared to cooking after standard impregnation the yield improvement achieved by HAI was preserved in both types of bleaching sequences (2% units for chlorine dioxide and 4% units for hydrogen peroxide). The introduction of charged groups into the cellulose fibers was higher with hydrogen peroxide bleaching than with chlorine dioxide however, no significant impact was seen on the swelling or mechanical properties. The brightness was higher for the pulps bleached with chlorine dioxide compared with hydrogen peroxide. Hydrogen peroxide bleaching resulted in similar brightness development for both standard and HAI. Fibers bleached with chlorine dioxide had the highest curl index (16-17%) compared to the fibers bleached with hydrogen peroxide (15%). 

Place, publisher, year, edition, pages
Walter de Gruyter GmbH, 2022
Keywords
Brightness; curl index; fber charge; morphology; tensile index.
National Category
Paper, Pulp and Fiber Technology
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-321342 (URN)10.1515/npprj-2022-0035 (DOI)000859295800001 ()2-s2.0-85138856693 (Scopus ID)
Note

QC 20221114

Available from: 2022-11-12 Created: 2022-11-12 Last updated: 2022-11-14Bibliographically approved
6. Pulp delignification and refining – impact on the supramolecular structure of softwood fibers
Open this publication in new window or tab >>Pulp delignification and refining – impact on the supramolecular structure of softwood fibers
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The effect on softwood fiber wall nanostructure of kraft cooking, oxygen delignification and refining wasevaluated by X-ray scattering. A recently developed simulation method for modelling SAXS data was used to estimate the apparent average sizes of solids (AAPS) and interstitial spaces in the fiber wall (AACS).Fiber saturation point and WAXS were also used to calculate the pore volume in the fiber wall and the crystallite size of the fibril, respectively. The experimental modelled SAXS data was able to give consistent values for each kraft-cooked and oxygen-delignified pulp. Kraft delignification seems to have the major influence on the fiber nanostructure, while oxygen delignification has little or no significant impact even for different kappa numbers. The particle sizes values were more stable than the cavities sizes and no significant differences were seen between different delignification processes, refining or kappa number. Pulps evaluated after PFI-refining, showed an increase in the fiber wall porosity evaluated by FSP and anincrease in the interstitial spaces in the fiber wall, while the crystallite size and the particle sizes were very little or not affected at all.

Keywords
supramolecular structure, fibrils, refining, aggregates, delignification
National Category
Paper, Pulp and Fiber Technology
Research subject
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-321359 (URN)
Note

QC 20221114

Available from: 2022-11-13 Created: 2022-11-13 Last updated: 2022-11-14Bibliographically approved

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