The pulp is dewatered to a high consistency, and excess liquor from the main

bleaching step is then added. This dilutes the consistency to a level between 10%

and 12%. In order to initiate a reaction, an addition of caustic soda to the peroxide

Content might be required. Following this MC-stage, the next press generates the

High consistency required in the main bleaching stage. The effluent of this press

Is discharged to the effluent treatment plant. After the main bleaching stage, the

Technology of Mechanical Pulp Bleaching

Fig. 6.12 Schematic of a high-consistency peroxide bleach

Plant with twin-wire press, mixer and tower with mechanical

Discharge (courtesy Andritz AG, Graz, Austria).

Pulp is diluted to generate sufficient liquid for recycling of the excess. With the

Pulp, the major part of the excess leaves the system, and this amount can only be

decreased further if additional dilution and “washing” within the press is possible.

However, this depends on the freeness. Typically, the washing of a mechanical

Pulp is difficult because of the high fines content and a low freeness. As mentioned

Above, a two-stage bleach plant only makes commercial sense for very high

Brightness targets.

Bleaching of Mechanical Pulp

Fig. 6.13 Two-stage medium/high-consistency bleaching plant for bleaching

To very high brightness (courtesy Andritz AG, Graz, Austria).

Latency and Properties of Mechanical Pulp

Jurgen Blechschmidt and Sabine Heinemann

7.1

Latency of Mechanical Pulp

When fibers are defiberized at high temperatures and high consistencies, they are

Deformed due to stresses that they encounter. The fibers are compressed, twisted

And curled (Fig. 7.1a). However, when cooling down at high consistency, the fibers

Remain twisted and curly (Fig. 7.1b). This behavior is mainly found in refiner mechanical

pulps [37], but it also occurs in pressure groundwood [38], and is termed

“latency”. Latency can be removed by agitating the pulp at low consistency and

High temperature. The fibers are re-straightened (Fig. 7.1c), and this pulp has a

Lower freeness and a higher tensile index. Thus, latency can be defined as the difference

Of mechanical pulp properties between the initial hot state (Fig. 7.1b) and

the cooled state (Fig. 7.1c) that can be removed to a large extent [39].

Fig. 7.1 Fiber deformation in mechanical pulping (latency).

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

7.2

Properties of Mechanical Pulp

Figure 7.2 provides the comparative characteristics of mechanical pulps and

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