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Assessment of Q(OP)D(PO) bleachability of softwood kraft pulp
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. SCA R&D Centre; KTH. (Fibre Science and Communication Network)
SCA Östrand.
KTH, Stockholm, Sweden.
KTH, Stockholm, Sweden.ORCID iD: 0000-0002-2900-4713
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2021 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 36, no 4, p. 582-593Article in journal (Refereed) Published
Sustainable development
Hållbar utveckling
Abstract [en]

Bleachability is evaluated as how easily a pulp sample is bleached and it depends on the structure of residual lignin and carbohydrates. Also, the bleachability varies depending on the bleaching sequence. ECF light sequences have been improved significantly in the recent years. However, we still don’t fully understand how ECF light bleach plants are optimally run. This work studies the bleachability of softwood kraft pulp in an ECF light bleaching sequence, (OO)Q(OP)D(PO). Three pulp samples with brown stock kappa number 27, 32 and 35 were bleached and studied for residual lignin, hexenuronic acid and carbohydrate content. It was found that in the bleaching stages that are highly delignifying, it is beneficial with a higher kappa number for the delignifying bleachability. However, in the bleaching stages where the objective is brightness increase, the brightness gain bleachability is improved by a lower kappa number. We also intended to determine which of the three samples had the best suited kappa number for this particular bleaching sequence. According to our results, the bleaching was most effective with kappa number around 32. Although an even higher kappa number resulted in higher yield after cooking, it seemed that this bleaching sequence cannot preserve the yield gain.

Place, publisher, year, edition, pages
Walter de Gruyter, 2021. Vol. 36, no 4, p. 582-593
Keywords [en]
Brightness gain, chlorine dioxide, delignification, ECF light bleaching, kappa number
National Category
Natural Sciences Chemical Sciences
Identifiers
URN: urn:nbn:se:miun:diva-42802DOI: 10.1515/npprj-2021-0022ISI: 000753926000004Scopus ID: 2-s2.0-85116586695OAI: oai:DiVA.org:miun-42802DiVA, id: diva2:1585034
Available from: 2021-08-16 Created: 2021-08-16 Last updated: 2022-02-24Bibliographically approved
In thesis
1. On the Process Development of an ECF Light Bleaching Sequence for the Production of High Quality Softwood Kraft Pulp and Low AOX Formation
Open this publication in new window or tab >>On the Process Development of an ECF Light Bleaching Sequence for the Production of High Quality Softwood Kraft Pulp and Low AOX Formation
2021 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The aim of this work is to contribute to the process development of the ECF light bleaching sequence Q(OP)D(PO) by providing a better understanding of the bleaching of softwood kraft pulp. There are few published studies on this type of sequence, and more knowledge is needed on how to reduce environmental impact, improve cost-effectiveness and ensure a high pulp quality.

Firstly, this research showed that using a pH buffer to stabilize the pH at a near-neutral level during bleaching with chlorine dioxide can lower the formation of adsorbable organically bound halogens (AOX) without altering the bleaching efficiency. A near-neutral pH decreases the formation of strongly chlorinating species so that the AOX content in the bleaching effluents is reduced by up to 30%. The increased pH in the near-neutral pH D stage compared with the reference lowered the chlorine dioxide consumption, resulting in a higher kappa number and viscosity. A lower degradation of hexenuronic acid correlated well with a lower AOX content in the effluents, affirming earlier theories that hexenuronic acid has an important impact on AOX formation.

Secondly, this research aimed at studying bleachability in (OO)Q(OP)D(PO). The bleachability was defined as delignifying and brightness gain bleachability, which is suited to ECF light bleaching sequences. The delignifying and brightness gain bleachability were used to evaluate the stages in (OO)Q(OP)D(PO) in regard to the unbleached kappa number. When comparing different stages, it is possible to distinguish the bleaching efficiency in each stage. For this ECF light sequence, the bleachability depends on the purpose of the stage. It was found that in the stages that are mainly delignifying (OO and D), a higher unbleached kappa number is beneficial for the delignifying bleachability, although the brightness gain is not improved. However, in the stages that are mainly brightness increasing (OP and PO), the brightness gain bleachability is improved by a lower unbleached kappa number.

Thirdly, the bleaching of the pulp samples in this study with the ECF light sequence was most effective with a kappa number around 32 after cooking. Although an even higher kappa number resulted in a higher yield after cooking, it seemed that this bleaching sequence cannot preserve the yield gain. Kappa number 32 also gave the best results in regard to brightness ceiling and viscosity. On the other hand, kappa number 27 was the most favourable with regard to yellowing and chemical charge.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University, 2021. p. 55
Series
Mid Sweden University licentiate thesis, ISSN 1652-8948 ; 184
Keywords
Chemical Engineering, Natural Science, Chemical Pulping, Softwood, Kraft Pulp, Bleaching, ECF light bleaching, Chlorine Dioxide, AOX formation, Delignification, Bleachability
National Category
Chemical Sciences
Identifiers
urn:nbn:se:miun:diva-42380 (URN)978-91-89341-13-5 (ISBN)
Presentation
2021-09-13, O102, Holmgatan, Sundsvall, 13:00 (English)
Supervisors
Note

Examinator: Helena Håkansson, lektor, Karlstads universitet

Available from: 2021-08-16 Created: 2021-08-16 Last updated: 2021-08-16Bibliographically approved

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Starrsjö, SaraFiskari, Juha

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