Pressure Grinding

Groundwood production has been carried out under atmospheric pressure, as

Indicated by Keller with its invention. Further investigations into temperature

Relationships in grinding have led to the introduction of higher pressures in

grinding processes [11]. Although early small-scale trials have been successful, the

Technical realization required much development. By 1970, only the former Tampella

factory [today part of Metso Paper (Metso Corp.)] had been able to develop

the first industrial pressure grinder [12], though by 1982 almost 2.5% of the

world’s production of groundwood was manufactured by pressure grinding.

Goring [3] highlighted the importance of water content in wood during mechanical

Defibration. A high water content lowers the softening temperature of

Lignin and hemicelluloses (see Fig. 4.3). Still in atmospheric grinding, the Defibration

Takes place at temperatures below the softening temperature of wood.

With high pressures of 100–300 kPa (1–3 bar), the boiling temperature of water

increases; from the vapor–pressure relationship, the boiling temperature is seen

to be 120 °C at a pressure of 100 kPa (1 bar). Pressurized grinding prevents the

Water from boiling in the grinding zone. The temperatures in the grinding zone

Are higher when compared to atmospheric grinding (see Fig. 4.21). According to

Goring, water in the grinding zone can be used completely for softening purposes;

Consequently, the pressurized grinding of dry wood produces groundwood

Of a better quality than does atmospheric grinding.

The design of the Metso pressure groundwood (PGW) grinder (also called the

Tampella-grinder or Valmet PGW grinder, depending on the year of publication)

(Fig. 4.22) is similar to the atmospheric two-pocket grinder, though the total internal

Space of the grinder (including the two magazines) is now pressurized. Two

Magazines above the check damper to the pressurized area have been added for

The feeding of wood logs, because of pressure equalization. Pressure grinding

Causes much higher forces to the housing materials and the pulp stone, and

Hence the atmospheric grinder concept has been redesigned. Currently, the grinder

Body is constructed from welded heavy steel plates, whilst all surfaces in contact

With the pulp are clad with stainless steel. The end blocks are constructed

From stronger stainless steel castings.

Mechanical Pulping Processes

Fig. 4.21 Temperature rises in wood during grinding.

Fig. 4.22 The Metso PGW grinder.

The pressure inside the grinder casing is adjusted by compressed air, though

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