Acids. However, because hydrogen peroxide will not react easily with the

Aromatic systems of lignin, its level of removal is moderate. In mechanical pulp

Bleaching, this is an advantage, and the yield and optical properties (opacity) are

Only moderately affected by the bleaching process. An example of the response of

Mechanical pulp to hydrogen peroxide is shown in Fig. 6.1.

Peroxide addition yields increasing brightness with charges up to about 5%. The

plateau is reached at about 82% ISO for softwood pulp, spruce (Picea spp.) and pine

(Pinus spp.), and 86% ISO for hardwood mechanical pulp, poplar and aspen (Populus

Bleaching of Mechanical Pulp

Bleaching with Hydrogen Peroxide

Spp.). The wood species, the age of the wood, storage of the logs and the bark content

Each have a huge impact on bleachability and the brightness ceiling.

During the bleaching process, a variety of compounds are dissolved, the main ones

Being acetic acid (from acetyl groups on carbohydrates) and low molecular-weight

Polyoses. Lignin dissolves only to a small extent. However, because the pulping process

Solubilizes only a small part of the wood, rather high effluent loads can result

From bleaching. Top brightness requires a high peroxide input and, for its activation,

A similarly large amount of caustic soda. The resultant brightness in bleaching softwood

TMP with increasing input of hydrogen peroxide is shown graphically in

Fig. 6.2, where different amounts of caustic soda were applied to achieve the best

Response in brightness. For a given residence time and temperature, there is an optimum

Level of activation. The shape of the curves shows, for the ratio of H2O2 to

NaOH, an increasingly wider range of tolerance. Clearly, the more peroxide

Applied, the less critical is the correct amount of caustic soda added.

0,5 1,0 1,5 2,0

H

O

H

O

H

O

H

O

Brightness [% ISO]

NaOH-charge [%]

Fig. 6.2 Increase in brightness with optimized charges of NaOH

for different peroxide amounts. Bleaching at 65 °C, 3 h, 20%

consistency, with a constant addition of 2% sodium silicate.

An inadequate activation results in an insufficient consumption of H2O2, but

Too-high charges are similarly detrimental, and the alkalinity consumes peroxide

And brightness decreases again. The process cannot be operated with the aim of

Consuming all of the hydrogen peroxide applied. An example of the brightness

Resulting from a constant input of hydrogen peroxide but a variation of the

Amount of caustic soda is shown in Fig. 6.3. The comparison of best brightness

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