Is fed continuously, as shown in Fig. 4.17.

The chain grinder operation can be controlled either by constant feed (grinding

pressure) or by constant load, though from a technological aspect the constantpressure

operation is preferred. The pulp produced has an even quality, although

problems with automatic feeding (e.g., uneven log distribution) cause load fluctuations,

and a better approach with regard to economy of energy may be to operate

under constant load.

Today, chain grinders are designed with up to 5 MW driving power, and pulp

stone speeds of 30 m s–1 (circumferential speed). A daily production may be up to

70 t, with log lengths of 1–1.5 m and pulp stone diameters of 1.5, 1.6, 1.8, 1.9 or

2.0 m. Water is important as a lubricant and a cooling agent. A shower water flow

of 2000–3000 L min–1 have pressures of 350 to 600 kPa (3.5–6 bar). Coming from

closed loops, the shower water must be filtered to remove, for example, pulp particles.

The weir level can be regulated either manually or by mechanical adjustment

of the overflow.

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4.1 Grinding Processes

Fig. 4.15 The chain grinder. 1, Pulp stone; 2, feeding drive;

3, feeding chains; 4, stone sharpening equipment; 5, shower

water pipe; 6, grinding pit.

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4 Mechanical Pulping Processes

Fig. 4.16 The feeding hydraulics system in a chain grinder.

Fig. 4.17 The continuous feeding system for a chain grinder.

Several scientific-technological investigations and further developments, especially

in the field of grinding control, have allowed the optimization of chain

grinding to obtain modified groundwood pulps. One example is the thermo

groundwood (TGW), as introduced by Voith in 1984. The most important difference

to the conventional chain grinder is the temperature impound in the grinder

shaft (the lower part of the log magazine) and control of the grinding zone temperature.

4.1.4.3 Pulp Stones

The pulp stone is the most important part of the groundwood process. During the

early days of industrial grinding natural stones were used, but for more than 100

years these have been replaced by artificial pulp stones. In Europe, the pulp stones

were made from cement-based concrete, whilst in North America the modern-day

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4.1 Grinding Processes

ceramic-based stones were developed. The use of artificial stones has enabled tailor-

made surface structures of the pulp stones to be developed. After an exact

adjustment of the pulp stone, it is fixed on the grinder shaft with flanges (see

Fig. 4.18), and the free distance between the stone and flange is sealed with either

concrete or sulfur.

120.000 – 140.000 mі

Production efficiency during

operating time (based on

stacked wood)

Operating time 3.5 – 5.0 years

450 – 500 t

Specific grinding efficiency

(produced o.d. pulp per mm

grinding layer)

Sharpening interval 350 – 500 hours

60 – 75 mm

Initial thickness of the

abrasive layer

Operation parameters

Fig. 4.18 Structure of an artificial pulp stone.

The structure of a ceramic stone that is most commonly used today is shown in

Fig. 4.18 (left). The stone consists of a steel-reinforced core to which the honeycombed

ceramic segments are fixed with anchor screws. The spaces between the

segments have elastic joint material filling. Although the abrasive layer, at 60–