4.1 Grinding Processes

pulp grades from board to high value printing pulps. Thus, high specific energy

consumption will lead to the production of a finer groundwood pulp with higher

strength properties (see Fig. 4.10). The parameter of specific grinding energy consumption

Is of major importance for process control in grinding.

80

100

120

140

160

180

1.2 1.7 2.2

Specific energy consumption [MWh/t]

Strength development [%]

Fig. 4.10 Strength properties of stone groundwoods, depending upon

specific grinding energy consumption (according to Sьttinger [22]).

4.1.4

Grinders and Auxiliary Equipment for Mechanical Pulping by Grinding

A typical flow sheet for a groundwood process is shown in Fig. 4.11.

Today, grinding can strictly be divided into two different processes – atmospheric

and presurized grinding. In contrast, the ring grinder, which operates in a

totally different manner (Fig. 4.12), was developed in 1939 and used in the USA

and Canada. The manually fed logs are ground inside a drum, but this system has

now almost disappeared completely because of the very high manual effort

involved.

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1088 4 Mechanical Pulping Processes

Fig. 4.11 A typical flow sheet for a groundwood process.

Fig. 4.12 A ring grinder.

4.1 Grinding Processes

4.1.4.1 Pocket Grinders

Pocket grinders were the first designs of industrial grinders, and were further developed

from the spindle-type grinder (first built in 1867). Figure 4.13 (left) shows

a pocket grinder with four magazines that has to be fed manually. A further development

of the pocket grinder is the magazine-type grinder (Fig. 4.14), which was

first built in 1910 to improve the manual feeding of logs to the pockets.

Fig. 4.13 Pocket grinders. Left: A pocket grinder with four

magazines. Right: An atmospheric, two-pocket grinder.

Fig. 4.14 A magazine-type grinder.

Today, pocket grinders are widely used in Scandinavia and America as twopocket

grinders (Fig. 4.13, right). The two-pocket pocket grinder has, in accordance

with its name, two separate grinding pockets that operate batch-wise. The

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4 Mechanical Pulping Processes

magazine above a pocket can hold just one pocket filling of logs. Irrespectively as

the one pocket filling is pressed hydraulically towards the pulp stone and grinded,

the other pocket can be filled for the next batch. Today, the Metso grinders have

pulp stone diameters of 1.8 m, and both grinding zones are from 1.0 to 1.5 m in

length. The width of the feeding gate opening is 600 mm, and the pocket reloading

time is approximately 30 s. A hydraulic pressure of between 1.2 and 5 MPa

(12 and 50 bar) – it is about 120 kPa and 500 kPa on the wood – moves the logs

towards the pulp stone.

The advantages of modern pocket grinders compared to chain grinders include:

_ Small design height

_ Simple operating

_ Short pocket changing times

_ Quick pulp stone change

The increased importance of the pocket grinders is related to the development of

pressurized grinding (see Section II-4.1.5).

4.1.4.2 Chain Grinders

The chain grinder, which is widely distributed throughout Europe, except for

Scandinavia, was first designed 1921 by Voith, in Heidenheim. The operating

principle is shown schematically in Fig. 4.15. The logs are stored in the log magazine,

located above the pulp stone. From the magazine, the logs are caught by the

cams of the permanent moving chain elements and pressed down continuously

towards the pulp stone with a pressure which is related to the feeding speed. The

chains are driven hydraulically by gears or screw gears (Fig. 4.16), and the magazine