General Principles. Unbleached chemical pulps still contain lignin in an amount of 3–6% on o.d.

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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