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Some aspects on TCF-bleachability of softwood alkaline pulps
KTH, Superseded Departments, Pulp and Paper Technology.
1999 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The bleachability of softwood oxygen delignified pulps has been studied, i.e. the ease (requirement of bleaching chemicals) with which the pulps can be brightened to a target brightness in totally chlorine free (TCF) bleaching stages, including hydrogen peroxide, ozone and chelating agent stages. Different cooking processes have been compared and the influence of different pre- and post-treatments on the kraft process has been investigated. The influence of different cooking parameters in the kraft cook on the bleachability has also been studied, as well as the influence on the bleachability of the kappa number of the pulp after the cook and after the oxygen stage.

Pulps produced by alkaline sulfite processes, ASAM and MSSAQ, showed better bleachability and process selectivity(viscosity at a given brightness) than pulps produced by the modified kraft process. The bleachability of the modified kraftpulps could be improved by a post sulfonation.

The bleachability was improved by terminating the cook at a higher kappa number level, when oxygen delignifying to the same kappa number level before bleaching. Improvements are also achieved by starting the oxygen stage from the same kappa number and extending the oxygen delignification to a lower kappa number level. The process selectivity and the yield are improved in the same way.

The QPQP*-bleachability (P*=peroxide stage with the addition of magnesium ions) was improved by changing the cooking conditions in a kraft cook leading to a shorter cooking time, i.e. by increasing the hydroxide ion concentration, the hydrogen sulfide ion concentration, or the cooking temperature or by decreasing the sodium ion concentration. Exceptions could be seen for very high [HO-], where the bleachability even deteriorated, and when the temperature was increased at very high chemical charges, where no more improvement was achieved.

The pulp with the best QPQP*-bleachability in a series of pulps had a lower light absorption coefficient (k)/lignin content (kappa number corrected for the hexenuronic acid contribution (kappa**)) ratio already after the cook as well as after the oxygen stage and a higherβ-O-4 content after the cook. A decreasedk/kappa** ratio after the cook is most probably due to less redeposition of dark dissolved organic material from the black liquor when the residual hydroxide ion concentration is increased. The degree of delignification in a subsequent oxygenstage becomes lower for the pulp with a lowerk/kappa** ratio after the cook and the metal ion content in the pulps was lower. Additives like, for example sulfite reduce the redeposition of lignin and lead to a brighter pulp.

The changes leading to improved bleachability for a pulp are not always followed by improved yield and process selectivity. The hydrogen sulfide ion concentration and the sodium ion concentration influenced the yield and selectivity positively in the same way as the bleachability. Increased hydroxide ion concentration and temperature, however, lead to lower yield andpoorer selectivity. Thus for an optimal product a compromise between a good bleachability and high yield and good strength properties must be chosen.

 

Place, publisher, year, edition, pages
Stockholm: KTH , 1999. , 69 p.
Series
Trita-PMT, 1999:9
Keyword [en]
Bleachability, chlorine-free bleaching, softwoods, kraft pulps, alkaline sulfite pulps, oxygen, hydrogen peroxide, ozone, hydrogen sulfide ion, hydroxide ion, temperature, sodium ion.
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:kth:diva-2801ISBN: 99-2962708-1 OAI: oai:DiVA.org:kth-2801DiVA: diva2:8522
Public defence
1999-05-27, 00:00
Note
QC 20100701Available from: 2000-01-01 Created: 2000-01-01 Last updated: 2010-07-01Bibliographically approved
List of papers
1. A comparison of bleachability in TCF sequences for alkaline sulphite and kraft pulps
Open this publication in new window or tab >>A comparison of bleachability in TCF sequences for alkaline sulphite and kraft pulps
1996 (English)In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 22, no 8, J296-J300 p.Article in journal (Refereed) Published
Abstract [en]

The bleachability in different totally chlorine-free (TCF) bleaching sequences was investigated for two types of alkaline sulphite pulps, ASAM and MSSAQ, and compared with that of pulps produced by different new and industrially applied modified kraft pulping processes. The TCF sequences included oxygen, ozone, peroxide and complexing agent stages. The alkaline sulphite pulps were found to be easier to bleach in terms of the consumption of oxidative agents required to reach a given brightness than the pulps produced by the different modified kraft pulping processes. The relationship between pulp brightness and pulp viscosity was also much better for the sulphite pulps, although the new types of kraft pulps are gradually approaching the level of the alkaline sulphite pulps in this respect. There was no significant difference between the ASAM (alkaline sulphite anthraquinone methanol) and the two-stage MSSAQ (minisulphide sulphite anthraquinone) processes.

Keyword
kraft pulps, oxygen, delignification, chlorine free bleaching, multistage process, selectivity, brightness, viscosity, softwoods, ozone, hydrogen peroxide, alkaline pulps, sulfite pulps
National Category
Paper, Pulp and Fiber Technology Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13899 (URN)A1996VD41400007 ()
Note
QC 20100701Available from: 2010-07-01 Created: 2010-07-01 Last updated: 2017-12-12Bibliographically approved
2. Changes within the modified kraft process aiming to improve TCF bleachability
Open this publication in new window or tab >>Changes within the modified kraft process aiming to improve TCF bleachability
1999 (English)In: Journal of Pulp and Paper Science (JPPS), ISSN 0826-6220, Vol. 25, no 5, 176-182 p.Article in journal (Refereed) Published
Abstract [en]

the investigated process modifications were: presulphonation, postsulphonation, polysulphide pretreatment and polysulphide pretreatment combined with a postsulphonation. The influence of the degree of delignification in the cook, on the selectivity and bleachability, was also investigated. The pulps were produced from softwood chips and were oxygen delignified before bleaching according to an AZQP sequence. All the investigated process variants had better process selectivity (pulp viscosity at a given brightness) and the pulps produced with a postsulphonation showed better bleachability, than the reference pulp. The bleachability and the process selectivity was improved by interrupting the cook at a higher kappa number and instead extending the delignification in the oxygen stage.

Keyword
kraft pulping, modified kraft process, pretreatment, sulfonation, polysulfide pulping, chlorine free bleaching, kraft pulps, softwoods, delignification, oxygen, pulp properties, selectivity, SOFTWOOD, PULPS, SEQUENCES
National Category
Chemical Process Engineering Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13900 (URN)000080562100005 ()
Note
QC 20100701Available from: 2010-07-01 Created: 2010-07-01 Last updated: 2017-12-12Bibliographically approved
3. Changes in light absorption coefficient spectra as a result of TCF bleaching
Open this publication in new window or tab >>Changes in light absorption coefficient spectra as a result of TCF bleaching
1999 (English)In: Paperi ja puu, ISSN 0031-1243, Vol. 81, no 2, 130-137 p.Article in journal (Refereed) Published
Abstract [en]

Light absorption coefficient, k, spectra in the wavelength range from 200 to 700 nm have been determined for a number of modified kraft and alkaline sulfite pulps after O, OAZ and OAZQP* bleaching (O=oxygen, A=acid treatment, Z=ozone, Q=complexing agent, P*=peroxide stage with Mg addition). The different pulp types have earlier shown differences in bleachability and selectivity when bleached in an OAZQP* sequence and also in other sequences. However, the reason for the differences in bleachability are not fully understood. In this paper, the changes in k spectra after the different bleaching stages for the different pulp types are shown, and the differences are discussed, The accuracy in the determination of k is also discussed and why it is preferable to measure k rather than brightness. There were no significant differences between the shape of the k spectra for the different pulp types that could explain the known differences in bleachability between the pulp types. However, the levels of the k spectra after the oxygen stage could be correlated to the known differences in bleachability between the pulps, The k spectra for the ASAM pulp, the pulp that has shown best bleachability, was significantly lower compared to the other investigated pulps. Ozone and peroxide bleached pulps showed k spectra of approximately the same shape as the oxygen-delignified pulps. In this study, k values as high as about 10 m(2)/kg were correctly measured. At higher k values, the light scattering coefficient (s) began to deviate due to too high a light absorption. Dilution, i.e. mixing the pulp with a brighter pulp of lower and known k value so that the k value of the mixture does not exceed 10 m2/kg is a way of circumventing the problem. However, there are also some limitations with the pulp mixing method, which are discussed.

Keyword
Kubelka-Munk equation, light absorption, light scattering, softwood, kraft pulps, alkaline sulfite pulps, oxygen, azone, hydrogen peroxide, DIFFUSE-REFLECTANCE, PULPS, SPECTROSCOPY
National Category
Paper, Pulp and Fiber Technology Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13901 (URN)000079200500010 ()
Note
QC 20100701Available from: 2010-07-01 Created: 2010-07-01 Last updated: 2017-12-12Bibliographically approved
4. Kraft cooking with varying alkali concentration: Influence on TCF-bleachability
Open this publication in new window or tab >>Kraft cooking with varying alkali concentration: Influence on TCF-bleachability
1998 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 13, no 1, 57-63 p.Article in journal (Refereed) Published
Abstract [en]

The TCF-bleachability of softwood pulps produced by kraft pulping processes with varying alkali concentrations was investigated. The cooks began with a hydrogen sulfide ion pretreatment at low alkalinity, i.e. simulating a black liquor preimpregnation. The alkalinity in the cooks was varied in three different ways: already at the beginning of the cook, both at 40 % sulfidity and at a HS- concentration of 0.3 mol/l (2-stage cooks), and in the final part of a modified kraft cook (4-stage cooks) simulating an ITC (Isothermal cooking) cook. The pulps had the same kappa number after the cook, about 17, and were oxygen delignified before bleaching. The influence of the alkali concentration profile in the cook on the bleachability was studied with a QPQP*-sequence (Q=chelating agent stage, P*=peroxide-stage with the addition of 0.05 % Mg ions). The bleachability (pulp brightness achieved for a given consumption of bleaching agent per kappa number) was found to reach a maximum for pulps with a residual alkali concentration in the vicinity of 0.5 mol/l. The bleachability was however affected to only a minor extent by the alkali concentration during the cook. The hexeneuronic acid content was found to decrease with increasing alkali concentration and this was taken into consideration when calculating the chemical consumption per amount of lignin. The process selectivity (pulp viscosity at a given brightness) decreased with increasing residual alkali. The bleachability was not improved by conducting the cook with a levelled out active cooking chemical profile, when the comparison was made at the same residual alkali concentration, but the selectivity was improved.

Keyword
kraft pulp, softwood, chlorine-free bleaching, bleachability, effective alkali, oxygen, hydrogen peroxide, pulp brightness, pulp viscosity, hexenuronic acid, PULPS
National Category
Paper, Pulp and Fiber Technology Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13902 (URN)000075150100008 ()
Note
QC 20100701Available from: 2010-07-01 Created: 2010-07-01 Last updated: 2017-12-12Bibliographically approved
5. The influence of cooking conditions on the bleachability and chemical structure of kraft pulps
Open this publication in new window or tab >>The influence of cooking conditions on the bleachability and chemical structure of kraft pulps
1999 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 14, no 1, 71-81 p.Article in journal (Refereed) Published
Abstract [en]

The purpose of this investigation was primarily td investigate how variations in cooking conditions in the kraft pulping of softwood influence the subsequent bleaching, and secondly to study the relationship between the bleaching response and the chemical structure of the pulp. The cooking variables studied were hydroxide ion concentration, hydrogen sulfide ion concentration and cooking temperature. The pulps had the same kappa number after the cook, about 20, and were oxygen delignified to about kappa number 8 before bleaching. The influence of the cooking variables on the TCF-bleachability was studied in an AZQP*- and in a QPQP*-sequence (A=acid treatment, Z=ozone stage, Q=chelating agent stage, P*=peroxide stage with the addition of magnesium ions). All three cooking variables studied influenced the TCF-bleachability, but to different extents. The bleachability was improved by increased temperature for low chemical charges, but not at higher chemical charges. When [HS-] was increased the QPQP*-bleachability was improved but the AZQP*-bleachability was not affected. When [HO-] was varied a bleachability maximum was seen for the intermediate hydroxide ion concentration. The content of hexenuronic acid in the pulp after cooking could be reduced by using high initial [HO-], low initial [HS-] and a long cooking time. A high content of beta-O-4 structures in the unbleached residual lignin was found to contribute to a better bleachability of the pulp. However, the phenolic hydroxyl content could not be related to the bleaching response.

Keyword
bleachability, chlorine-free bleaching, softwoods, kraft pulps, hydrogen sulfide ion, hydroxide ion, temperature, beta-O-4, hexenuronic acid, chemical structures, HEXENEURONIC ACID GROUPS, TCF-BLEACHABILITY, LIGNINS, XYLAN
National Category
Materials Engineering Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:kth:diva-13904 (URN)000079757500009 ()
Note
QC 20100701Available from: 2010-07-01 Created: 2010-07-01 Last updated: 2017-12-12Bibliographically approved
6. Influence of ionic strength on kraft cooking and subsequent TCF-bleaching
Open this publication in new window or tab >>Influence of ionic strength on kraft cooking and subsequent TCF-bleaching
1999 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 14, no 3, 226-235 p.Article in journal (Refereed) Published
Abstract [en]

The TCF-bleachability of softwood kraft pulps produced with different ionic strength profiles has been investigated. In addition, the relationship between the bleaching response and some of the chemical structures in the pulp were studied. The ionic strength was changed by adding NaCl(s) at two alternative positions in the cook: after the pretreatment, or in the final part of the cook to give three different process variants. All cooks began with a standardized hydrogen sulfide ion pretreatment stage at low alkalinity. Pulps at two different kappa number levels, about 21 and 16, were investigated. After further delignification in an oxygen stage, the bleachability was studied with a QPQP*-sequence (Q=chelating agent stage, P=peroxide-stage, *=Mg ions addition). Pulping with a higher ionic strength led to a lower delignification cation rate and thus a greater hydroxide ion consumption to reach a given kappa number. The results also show an increasing light absorption coefficient/kappa number ratio for unbleached pulps with increasing ionic strength in the cook. The degree of delignification in the oxygen stage was higher with a higher ionic strength in the cook. The bleachability as well as the yield and the process selectivity were improved by a low ionic strength in the cook for all the three investigated process variants.

Keyword
kraft pulp, softwood, chlorine-free bleaching, bleachability, ionic strength, oxygen, hydrogen peroxide, pulp brightness, pulp viscosity, hexenuronic acid, HEXENEURONIC ACID GROUPS, DELIGNIFICATION
National Category
Paper, Pulp and Fiber Technology Materials Engineering
Identifiers
urn:nbn:se:kth:diva-13903 (URN)000082959800008 ()
Note
QC 20100701Available from: 2010-07-01 Created: 2010-07-01 Last updated: 2017-12-12Bibliographically approved

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