The resulting COD load, the points in the graph show the linear dependency

(Fig. 6.6). This applies not only to the COD but in parallel to total organic carbon

(TOC). Because the amount of dissolved compounds must correlate with the pulp

Yield, bleaching to a very high brightness decreases the yield. Fines are extracted

And freeness is increased. The extraction effect not only affects pulp yield and opacity,

But also has an impact on fiber properties. With intense extraction, hardwood

Fibers collapse, making it difficult to produce highly bleached fiber with a high

Volume (bulk). For most paper-grade applications, fiber volume should be low in

Order to allow a smooth surface. In the production of white board, the opposite is

Valid and stiff fibers with a high volume are an advantage; however, this requires

To carry out bleaching with other alkali sources.

Bleaching of Mechanical Pulp

0,4 0,8 1,2 1,6 2,0 2,4

H

O

H

O

H

O

H

O

COD [kg/t]

NaOH-charge [%]

Fig. 6.6 Increase of the COD load with increasing caustic

Soda addition in bleaching with hydrogen peroxide.

The substitution of caustic soda with magnesium hydroxide is a useful alternative

[36], although because of the limited solubility of Mg(OH)2, effective mixing

Becomes even more important. The particle size is similarly important, as a fine

Product is distributed better. The amount of Mg(OH)2 can be kept low; the limited

Solubility would only result in a high neutralization demand. It is sufficient to add

about 1–1.2% to activate the bleaching process with 2–4% H2O2. The demand for

sodium silicate is even lower, and an input of only 0.5–1% will result in the best

Response. The moderate impact on hemicellulose extract becomes apparent with a

Lower COD load (see Fig. 6.7).

2 3 4 5

COD [kg/t]

Mg(OH)2

NaOH

H2O2 [%]

Fig. 6.7 Impact of an exchange of NaOH by Mg(OH)2 in

Bleaching pressurized groundwood with increasing amounts of

H2O2 and NaOH or Mg(OH)2 as alkalization source [NaOH

amount variable, Mg(OH)2 input 0.75% at 2% H2O2, all other

bleaches 1%, 2% sodium silicate, 70 °C, 3 h, 20% consistency].

Bleaching with Hydrogen Peroxide

When bleaching aspen pulp, the impact on fiber volume of using Mg(OH)2 is

pronounced. Conventional bleaching to very high brightness (>85% ISO) is

accompanied by a decrease in the specific volume from >2 cm3 g–1 to 1.4 cm3 g–1,

or even less. The substitution allows the specific volume (also labeled as “bulk”) to

Be maintained. In a set-up using the Mg(OH)2 stage as the main (high consistency)

Stage and an additional (medium-consistency) stage with caustic soda as a

Brightness adjustment step, it is possible to balance the parameters of bulk and

Tensile strength to all levels in between (Fig. 6.8). The more moderate response in

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