In practice, the temperature of the pit pulp is used to control the grinding process,

with typical shower water temperatures of 60–75 °C being used in atmospheric

grinders.

The pit consistency, in practical terms, is set from 1.0% to 2.5%, and interacts

with the pit temperature. Increasing the pit consistency also enables the pit temperature

to be increased, at constant shower water temperature. Depending on

either the pit consistency or possible stone immersion in the pit (Fig. 4.6), a certain

amount of pit pulp is transported with the stone and passes back into the

grinding zone. Here, regrinding takes places, so that the pulp suspension from

the grinder pit is passed to the grinding zone instead of the shower water.

A deeper immersion of the stone into the pit, or a higher weir height in the pit,

leads to a larger quantity of circulating pulp. The stone surface will be lubricated

by pulp particles and appears duller than it really is, and this in turn reduces the

freeness of the pulp.

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4.1 Grinding Processes

Fig. 4.6 Pulp stone immersion and weir height in grinding.

1, Pulp stone immersion; 2, weir height; 3, grinder pit;

4, measuring point of pit temperature and pit consistency;

VH, feeding speed = grinding pressure; VU, pulp stone rotational

speed.

A high shower water pressure [at least 350 kPa (3.5 bar)] and the position of the

shower water pipes at front of the grinding zone cleans the pulp stone surface,

and complete stone sharpening can occur. If pit-less grinding is carried out, these

influences and interactions disappear, while high shower water pressures are

applied.

The moisture of the wood is the most important parameter in grinding, as

high-quality pulp can be produced from moist wood, at a lower specific grinding

energy consumption. Neither technical nor technological developments can

replace the effect of moisture in wood. The highest possible quality values are not

found in the range of fiber saturation (moisture of wood ≥23%), but rather at a

wood saturation of 50–60% moisture content. The request by grinding mills to

receive pulpwood with at least 30% moisture content represents an objective need

for the optimal use of a wood source.

Groundwood made from logs with a higher moisture content has a higher

brightness, higher strength properties (see Fig. 4.7), and a higher long fiber content

with high-quality fines.

The pulp stone requires a certain surface structure (stone surface profile) to produce

a certain pulp quality, and distinction must be made between the macrostructure

and microstructure of that profile. The macrostructure is gained by sharpening

of the stone (see also Section II-4.1.4). A typical sharpness profile is shown

In Fig. 4.8, while the grit material of the pulp stone estimates the microstructure

(Fig. 4.9).

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4 Mechanical Pulping Processes

Fines

Short fibers

Long fibers

Shives

0

20

40

60

80

100

0 20 40 60 80 100

Rel. moisture content of wood [%]

Fractional composition

0

1

2

3

4

0 20 40 60 80 100

Rel. moisture content of wood [%]

Strength values

Breaking length [km]

In-plane tear

strength [N]

Tear strength [N]

16.5 28 37.5 44.5 50

Abs. moisture content of wood [%]

16.5 28 37.5 44.5 50

Abs. moisture content of wood [%]

Fig. 4.7 Influence of wood moisture content on groundwood

properties. Left: Effect on tensile strength and tear strength.

Right: Effect on fractional composition.

Land

Groove

Grit

Fig. 4.8 Cross-section of a pulpstone pattern.

Fig. 4.9 Ideal grit positioning (according to Atack).

The mechanical defibration of wood to groundwood is a highly energy-intensive

process. The specific grinding energy consumption can be used to characterize

the energy used, and with this the groundwood quality. In grinding, values between

0.6 MWh t–1 and 2.0 MWh t–1 o.d. pulp are typical for the production of

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