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

1134

6.3 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.

1135

1136 6 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).

1137

7

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

chemical pulps, in relation to various yields.

Fig. 7.2 Characteristics of mechanical, semi-chemical, and chemical pulps.

With decreasing yield, an increased effluent load (biological oxygen demand;

BOD) and an increased strength potential is found, but this is to the disadvantage

of light-scattering properties. In conventional chemical pulping, specific energy

consumption is lower and the chemical consumption higher compared with the

other processes. Freeness is measured with about 100 mL for (fines-containing)

stone groundwood (SGW) and 800 mL (ca. 12 SR) for chemical pulp with a high

long-fiber content. Light-scattering values are provided for the pulp before it

undergoes additional mechanical treatment. The post-refining of a mechanical

pulp increases the light-scattering coefficient, whereas the refining of a chemical

pulp decreases this measure.

The grinding and refining processes separate the fibers in different ways. Figure