1078 3 Raw Materials for Mechanical Pulp

1079

4

Mechanical Pulping Processes

Jurgen Blechschmidt and Sabine Heinemann

4.1

Grinding Processes

4.1.1

Principle and Terminology

Round logs are pressed against a rotating pulp stone under specified conditions

of pressure and temperature (Fig. 4.1).

Fig. 4.1 Grinding principle. 1, Pulp stone;

2, Grinder pit; 3, weir; 4, shower water pipe;

5, Wood magazine; 6, finger plate; 7, pulp stone

sharpener; 8, sharpening roll; 9, wood logs.

Depending on the position of the log in the grinder magazine, it can be distinguished

between:

_ Transversal groundwood: Wood logs (and evidently also the

fibers) are loaded in the magazine or pocket of the grinder transversally

(perpendicularly) to the rotational direction of the stone

(in practice the only used orientation).

_ Longitudinal groundwood: Wood logs are loaded parallel to the

rotational direction of the stone (only applied in research studies).

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

Grinding procedures may be categorized as atmospheric grinding, pressure grinding,

and thermo grinding. The principles of each process are illustrated schematically

in Fig. 4.2.

Fig. 4.2 Principles of the grinding procedures.

4.1.2

Mechanical and Thermal Processes in Grinding

Grinding is a thermomechanical process that is divided into two parts, each of

which overlaps one another: (a) Softening and breakdown of the fiber structure;

and (b) peeling of the softened fibers from the wood matrix in the grinding zone

4.1.2.1 Softening of the Fibers

The wood logs are pressed against a rotating pulp stone, applying suitable pressure

and temperature conditions (see Fig. 4.2). Just before entering the grinding

zone, the wood logs are still cold and thermally untreated. The stone grits pass

over the wood matrix at very high frequencies. In a so-called compression/decompression

process, the fibers are cyclically stressed or relaxed. Depending on the

rotational speed of the pulp stone, and also on the surface profile of the stone,

pressure pulsations up to 40 kHz occur on the logs.

The fiber matrix is loosened due to the fatigue work done by the grits. Finally,

when they enter the boundary area between the revolving stone surfaces, the

1080 4 Mechanical Pulping Processes

fibers are peeled from the outermost layer of the softened wood, much in the

same way that tape is torn from a paper surface. Both processes warm up the

wood and break down the fiber structure. Due to the viscoelastic nature of the

wood, the temperature at 1–2 mm above the actual grinding zone increases very

quickly, and this rise in temperature causes the lignin to soften. The resulting

so-called “softening temperature” depends on the water content of lignin and the

frequency. In 1963, Goring [3] demonstrated how the softening temperature of

lignin is dependent upon the water content (Fig. 4.3).

Fig. 4.3 Softening temperature of lignin depending on water

content (according to Goring [3]).

The softening temperature is understood to represent the transition of an amorphous

polymer such as lignin from a glass-like and brittle state into a weak and

plastic one. Goring [3] identified the softening temperatures for lignin as 135 °C to