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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.
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 Suttinger [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.
Mechanical Pulping Processes
Fig. 4.11 A typical flow sheet for a groundwood process.
Fig. 4.12 A ring grinder.
Grinding Processes
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
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
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Pulp stone immersion; 2, weir height; 3, grinder pit; | | | Towards the pulp stone. |