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Pulp Bleaching
Herbert Sixta, Hans-Ullrich Suss, Antje Potthast, Manfred Schwanninger,
and Andreas W. Krotscheck
7.1
General Principles
Unbleached chemical pulps still contain lignin in an amount of 3–6% on o.d.
(oven dry) pulp in the case of softwood kraft (sulfite), and 1.5–4% on o.d. pulp in
the case of hardwood kraft (sulfite) pulps. Lignin in native wood is colored only
slightly, whereas residual lignin of a pulp after cooking – particularly kraft cooking
– is highly colored. Moreover, unbleached pulps also contain other colored impurities
such as certain extractives (resin compounds) and dirt which is defined as
foreign matter having a marked contrasting color to the rest of the sheet. Dirt may
originate from wood (bark, incompletely fiberized fiber bundles, sand, shives,
etc.), from cooking itself (carbon specks, rust, etc.), and from external sources
(grease, sand, other materials, etc.).
A continuation of cooking to further reduce the noncarbohydrate impurities
would inevitably lead to a significant impairment of pulp quality due to enhanced
cellulose degradation. Therefore, alternative concepts must be applied to selectively
remove chromophore structures present in the pulp. Various chlorine- and
oxygen-based oxidants have proven to be efficient bleaching chemicals which,
being applied in sequential steps, progressively remove chromophores and impurities.
As a result of the concern about chlorinated organic compounds formed
during chlorine bleaching at the end of 1980s, conventional bleaching concepts
were rapidly replaced by the so-called elemental chlorine-free (ECF) bleaching process,
and this became the dominant bleaching technology. The complete substitution
of chlorine by chlorine dioxide was the key step in reducing the levels of organochlorines
(measured as adsorbable organic halogen; AOX) in pulp mill effluents.
Further pressure, particularly fromthe environmental organization Greenpeace, and
especially in the German-speaking regions of Europe, led to the development of
totally chlorine-free (TCF) bleaching processes with a main emphasis on the use
of oxygen (O), hydrogen peroxide (P) and ozone (Z) as bleaching agents.
Bleaching is defined as a chemical process aimed at the removal of color in
pulps derived from residual lignin or other colored impurities, as outlined above.
Handbook of Pulp. Edited by Herbert Sixta
Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
ISBN: 3-527-30999-3
©2006 WILEY-VCHVerlag GmbH&Co.
Handbook of Pulp
Edited by Herbert Sixta
The progress in bleaching is followed by measuring the brightness, which is in
turn defined as the reflectance of visible blue light from a pad of pulp sheets,
using a defined spectral band of light having an effective wavelength of 457 nm.
The most common method for brightness measurement is represented by the
ISO standard method (ISO 2469, ISO 2470). This method uses an absolute scale.
The ISO brightness of a black, nonreflecting material is 0%, while that of a perfect
diffuser is 100%. Brightness levels of pulps can range from about 20% ISO for
unbleached kraft to almost 95% ISO for fully bleached sulfite (dissolving) pulps.
Bleaching increases the amount of blue light reflected by the pulp sheet in that
the concentration of chromophores absorbing that light is lowered. The change in
brightness through a bleaching step is not proportional to the reduction in chromophore
concentration. This is explained by the Kubelka–Munk remission function,
which shows that the reflectance loss (brightness) is not a linear function of
the chromophore concentration. At a high brightness level, the loss in brightness
is governed by only a small change in chromophore concentration, while at a low
brightness level the same loss in brightness is connected with a significantly higher
change in chromophore concentration. The absorption coefficient, k, is proportional
to the chromophore concentration and the scattering coefficient, s, is related
to the surface properties of the sheet determined by the fiber dimensions and the
degree of bonding.
In accordance with the Kubelka–Munk theory, the following expression defines
the interrelationship between s, k and the brightness B (reflectance factor R):
B _ 0_01 __ k _ s _ 1__ _ k _ s _2
_2 _ _ _ k _ s __0_5
_1_
where B is the brightness, in percent.
Determination of the absorption coefficient at a certain wavelength or wavelength
range is a usual way to monitor the chromophores contributing to pulp
brightness. Figure 7.1shows the reflectance and the absorption coefficient spectra
from both unbleached and fully (TCF) bleached hardwood sulfite pulps.
The spectra in Fig. 7.1indicate that bleaching of pulp increases reflectance predominantly
at the blue end of the spectrum. The change in chromophore concentration
through bleaching operations can be monitored by absorption difference
spectra (Dk= kbleached – kunbleached). This also allows estimation of the chemical structures
involved in the removal of chromophores.
7.2
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