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Storage of Hydrogen Peroxide Bleached Mechanical Pulp: Reduction in Reflectance over the Visible Spectrum
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. Norske Skog Saugbrugs.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The objective of this thesis is to determine possible causes of the darkening of hydrogen peroxide bleached mechanical pulp over the visible spectrum and their relative contributions. It focuses on both process conditions and the composition of the pulp and the dilution water, including additions or losses of material along the process line from the bleach tower to the paper machine.

A mapping of the optical properties of the pulp along the process showed that the fine fraction of the pulp darkened more than the long fibre fraction. Simulation of retention times of different fractions showed that the main part of the fine material is retained in the paper within a few hours, a small part might circulate for considerably longer time and may therefore be strongly coloured.

Storage trials were mainly performed using a hydrogen peroxide bleached mechanical pulp intended for SC paper made of Norway spruce (Picea abies), sampled on one occasion and stored in a freezer. Unwashed or well-washed pulp was stored in distilled water or in different process waters. Some complementary trials were included, e.g. unbleached pulp.

Time and temperature were the process variables that gave the strongest darkening of the pulp, as expected, both in a clean and a more process-like system, whereas pH only had an effect in the presence of process waters; the highest brightness stability was seen at a pH around 5.5–6.0.

The darkening was due to an increase in the light absorption coefficient (k) beginning at short wavelengths, but after longer storage times the increase in kλ also became noticeable at longer wavelengths. The colour (CIE L*, a*, b*) of the pulp changed towards red and yellow, initially more towards red and then more towards yellow. These changes were clearly visible.

Washing of the bleached pulp made it less sensitive to storage; possibly due to the removal of extractives, lignin-like substances, metals and pulp fines. This washing had little effect before storage and the amount of material removed was small.

The pulp darkened more when stored in process waters compared to distilled water. Apart from fibres, most of the colour was associated with pulp fines or filler but some colour was also found in the dissolved and colloidal fractions. At an increased pulp consistency, the increase in k460 was smaller.

Storage in white water from the paper machine gave extensive discolouration with a shoulder in the absorption spectrum around 550–650 nm, which increased with time. The addition of ferric ions increased the light absorption coefficient during storage, but could not explain the increased absorption at 550–650 nm nor could it be the only cause of the darkening in the mill system. A cationic basic violet dye gave a shoulder in the absorption spectrum similar to that of the mill system, but the absorption of the dye did not increase during storage. Model calculations indicate, but do not prove, that ferric ions together with violet and red dyes could have played a major, but not exclusive role in the colour observed in the mill system after storage. The darkening not accounted for, at longer wavelengths and around 550–650 nm, is suggested to be related to fines and fillers including dissolved and colloidal substances associated with these particles.

A method to produce representative sheets for determination of optical properties of mechanical pulps was developed. The new method makes it possible to follow changes in light absorption and light scattering coefficients over the visible range of wavelengths. It is approximately six times faster than standard methods, reduces the risk of additional darkening of the sample and can be used with small pulp quantities.

The deviation from the expected linear behaviour of the light scattering coefficient, s, at wavelengths corresponding to strong light absorption has been studied using the Kubelka-Munk model and the angular resolved DORT2002 radiative transfer solution method. The decrease in s could not be explained by errors introduced in the Kubelka-Munk modelling by anisotropic scattering.

Place, publisher, year, edition, pages
Sundsvall: Mid Sweden University , 2014. , 93 p.
Series
Mid Sweden University doctoral thesis, ISSN 1652-893X ; 207
National Category
Paper, Pulp and Fiber Technology
Identifiers
URN: urn:nbn:se:miun:diva-23806ISBN: 978-91-87557-91-0 (print)OAI: oai:DiVA.org:miun-23806DiVA: diva2:772123
Public defence
2014-11-21, M 102, Mittuniversitetet, Åkroken, Sundsvall, 10:46 (English)
Opponent
Supervisors
Available from: 2014-12-16 Created: 2014-12-16 Last updated: 2015-03-13Bibliographically approved
List of papers
1. Mapping and modelling of optical properties from pulp to super calendered paper
Open this publication in new window or tab >>Mapping and modelling of optical properties from pulp to super calendered paper
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2015 (English)In: Appita journal, ISSN 1038-6807, Vol. 68, no 2, 128-138- p.Article, review/survey (Refereed) Published
Abstract [en]

During production of mechanical pulp based paper, the pulp darkenswithin the process which represents a significant cost for the mill. Amapping of the optical properties of the pulp was conducted along theprocess, from pulp mill to paper machine, at a mill producing supercalendered (SC) paper on three paper machines. The focus was to evaluateat which positions in the mill the discolouration occurs and also whichfraction of the pulp darkens the most.The fine fraction of the pulp darkened more than the long fibrefraction. Bleached pulp diluted with white water from the paper machinedarkened more during storage than pulp diluted with clear filtrate. Thewhite water contains a considerable amount of suspended solids, i.e.pulp fines and clay. There was a tendency for a shoulder in absorptionspectra of the pulp stored in white water from the paper machine in theregion 550 to 650 nm, both for fibres and for fines. This is in the sameregion where the added dyes have their absorption maxima and also somecomplexes between iron and some of the components among lignin and theextractives. Simulation of retention times of different fractions showedthat, although the main part of the fine material is retained in thepaper within a few hours, a small part might circulate for considerablylonger time.

Keyword
Mechanical pulp, Mill mapping, Optical properties, Pulp fractions, Simulation, Storage
National Category
Paper, Pulp and Fiber Technology
Identifiers
urn:nbn:se:miun:diva-23804 (URN)000355069600019 ()
Available from: 2014-12-16 Created: 2014-12-16 Last updated: 2017-12-05Bibliographically approved
2. Determining optical properties of mechanical pulps: Sheetmaking procedure and investigation of different ways to evaluate light absorption
Open this publication in new window or tab >>Determining optical properties of mechanical pulps: Sheetmaking procedure and investigation of different ways to evaluate light absorption
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2012 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 27, no 3, 531-541 p.Article in journal (Refereed) Published
Abstract [en]

A method to produce representative sheets for determination of optical properties of mechanical pulps has been developed. It reduces the risk of contamination and discoloration and can be used with small pulp quantities. The deviation from the expected linear behaviour of the light scattering coefficient, s, at wavelengths corresponding to strong light absorption, has been studied using the Kubelka-Munk model and the angular resolved DORT2002 radiative transfer solution method. This decrease in s could not be explained by errors introduced in the Kubelka-Munk modelling by anisotropic scattering. Linear extrapolation of s can therefore not be justified as a way to obtain a more correct light absorption coefficient, k. For thepulps studied, the decrease in s at short wavelengths had little effect on k at 457 nm.

Keyword
Kubelka-Munk; Light absorption; Light scattering; Mechanical pulps; Optical modelling; Optical properties; Radiative transfer solution method; Sheet forming procedure
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-12985 (URN)10.3183/NPPRJ-2012-27-03-p531-541 (DOI)000311020100003 ()2-s2.0-84865261121 (Scopus ID)
Available from: 2011-01-14 Created: 2011-01-14 Last updated: 2017-12-11Bibliographically approved
3. The influence of process conditions during pulp storage on the optical properties of Norway spruce mechanical pulps
Open this publication in new window or tab >>The influence of process conditions during pulp storage on the optical properties of Norway spruce mechanical pulps
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2013 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 28, no 2, 203-210 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this work was to study the influence of process conditions (time, pH, temperature and consistency) on the optical properties of mechanical pulps during storage in a clean system as a reference for further work. Laboratory storage trials were performed with unbleached and hydrogen-peroxide bleached well-washed Norway spruce pulps. In general, the pulp darkened during storage due to an increase in light absorption, especially at shorter wavelengths. After long storage times, the light absorption coefficient, k had increased also at longer wavelengths. No specific peaks were seen in Delta k-spectra. The increase in light absorption was most rapid initially, during the first four hours, for all pulps when stored at high temperature (80 degrees C), and then proceeded more slowly. The corresponding change in colour, measured as a* and b*, was shifted towards red and somewhat towards yellow, and over longer periods of storage, the shift towards yellow became greater. Time and temperature were found to have the largest impact. The effects were similar regardless of the starting pH (4.3-9.7) and pulp consistency (5%-25%). The hydrogen-peroxide bleached pulps were more sensitive to storage compared to the unbleached pulp at temperatures above 50 degrees C. At storage times of up to four hours, the unbleached pulp showed no loss of brightness at either of the storage temperatures. A slightly less bleached pulp darkened more than a highly bleached pulp at all wavelengths. The only difference measured between the two pulps was that the less bleached pulp had a higher content of iron. This higher iron content may be at least part of the reason for the more extensive darkening.

Keyword
Colour, High-yield pulp, Optical properties, Process conditions, Pulp storage, Spectral data
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-19921 (URN)000321291900005 ()2-s2.0-84882994958 (Scopus ID)
Available from: 2013-10-11 Created: 2013-09-25 Last updated: 2017-12-06Bibliographically approved
4. The influence of process waters on optical properties during storage of hydrogen-peroxide bleached Norway spruce mechanical pulp
Open this publication in new window or tab >>The influence of process waters on optical properties during storage of hydrogen-peroxide bleached Norway spruce mechanical pulp
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2014 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 29, no 2, 344-355 p.Article in journal (Refereed) Published
Abstract [en]

To evaluate the causes behind the formation of colour in mill systems, the effect of different process waters on the optical properties of hydrogen-peroxide bleached Norway spruce mechanical pulp during storage was studied. The influence of process water type, temperature, time, pulp consistency and pH was evaluated over the visible range of wavelengths. The darkening was more pronounced when pulp was stored in process waters compared to in distilled water. Increased temperature and prolonged storage times increased the darkening as expected, whereas a higher storage consistency gave less darkening than a lower storage consistency (15% compared to 5%). The pH value that gave the lowest brightness reduction and a minimum in Delta k(460) was found to be 5.5-6.0. Storage of pulp in white water from the paper machine resulted in a broad shoulder in the k spectra indicating colour formation related to the particles in this water. Part of this shoulder in absorption spectra was related to added dyes, but the increase in absorption with time is likely to be caused by some other colour formation. The colour measured as a*, b* showed that the storage changed the colour of the pulp towards red and yellow, and initially more towards red.

Keyword
Chemical characterisation, Colour, Mechanical pulp, Metal ions, Optical properties, Process conditions, Process waters, Pulp storage, Spectral data
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-22588 (URN)10.3183/NPPRJ-2014-29-02-p344-355 (DOI)000338336400020 ()
Available from: 2014-08-20 Created: 2014-08-19 Last updated: 2017-08-10Bibliographically approved
5. Some causes of formation of colour during storage of hydrogen-peroxide bleached Norway spruce mechanical pulp
Open this publication in new window or tab >>Some causes of formation of colour during storage of hydrogen-peroxide bleached Norway spruce mechanical pulp
Show others...
2014 (English)In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, E-ISSN 2000-0669, Vol. 29, no 2, 356-366 p.Article in journal (Refereed) Published
Abstract [en]

The discolouration of hydrogen-peroxide bleached Norway spruce mechanical pulp during storage in mill systems was studied and the contributions of process water, iron and dyes were evaluated over the visible spectrum. Washing of the pulp made it less sensitive to storage, possibly due to the removal of extractives, lignin-like substances, iron and pulp fines. Storage in white water gave extensive discolouration with a shoulder in the absorption spectrum at around 550-650 nm. Most of the colour was associated with pulp fines or filler but some colour was also found in smaller fractions and in the water phase. The addition of ferric ions increased the light absorption coefficient during storage, initially at short wavelengths and then over the whole spectrum, but could not explain the increased absorption at 550-650 nm and could not be the only cause of the darkening in the mill system. A cationic basic violet dye gave a shoulder in the absorption spectrum similar to that in the mill system, but the absorption in this area did not increase during storage. Model calculations indicate that ferric ions together with violet and red dyes could explain a major part, but not all, of the colour observed in the mill system after storage. The darkening not accounted for at longer wavelengths and around 550-650 nm is suggested to be related to fines and fillers including dissolved and colloidal substances associated with these particles.

Keyword
Colour, Dye, Iron, Mechanical pulp, Pulp storage, Optical properties, Spectral data
National Category
Chemical Engineering
Identifiers
urn:nbn:se:miun:diva-22589 (URN)000338336400021 ()
Available from: 2014-08-20 Created: 2014-08-19 Last updated: 2017-08-10Bibliographically approved

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