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The influence of process waters on optical properties during storage of hydrogen-peroxide bleached Norway spruce mechanical pulp
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. (FSCN)
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. (FSCN)
Mid Sweden University, Faculty of Science, Technology and Media, Department of Chemical Engineering. (FSCN)
Norske Skog Saugbrugs, NO-1756 Halden, Norway.
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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.

Place, publisher, year, edition, pages
2014. Vol. 29, no 2, 344-355 p.
Keyword [en]
Chemical characterisation, Colour, Mechanical pulp, Metal ions, Optical properties, Process conditions, Process waters, Pulp storage, Spectral data
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:miun:diva-22588DOI: 10.3183/NPPRJ-2014-29-02-p344-355ISI: 000338336400020OAI: oai:DiVA.org:miun-22588DiVA: diva2:739155
Available from: 2014-08-20 Created: 2014-08-19 Last updated: 2017-08-10Bibliographically approved
In thesis
1. Storage of Hydrogen Peroxide Bleached Mechanical Pulp: Reduction in Reflectance over the Visible Spectrum
Open this publication in new window or tab >>Storage of Hydrogen Peroxide Bleached Mechanical Pulp: Reduction in Reflectance over the Visible Spectrum
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:nbn:se:miun:diva-23806 (URN)978-91-87557-91-0 (ISBN)
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

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