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.

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4 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

this is mainly required only when pressurizing the grinder for start-ups. Pulp

stone showering and the water hydraulic system are similar to those of the atmospheric

grinder. Due to the higher temperatures utilized, cooling of the hydraulic

water is necessary and a separate water-filtering loop is also included. Two-sided

mechanical seals are used for the grinder shaft to minimize the use of fresh water

In the pgw proves, and to prevent the colder seal waters from bleeding onto the

ceramic pulp stones.

When it became clear that stronger pulps may be produced at a grinding pressure

of 500 kPa (5 bar) and temperatures up to 140 °C [13], the design of a new

series of pressure grinders with a stronger body rated for this higher grinder pressure

was introduced. These grinders, which are known as “super pressure grinders”

(PGW-S grinders), were first utilized in Finland in 1988 [14].

The entire grinding process from log feeding to the groundwood outlet occurs

under pressure. The groundwood is passed through to a shredder to cut down in

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4.1 Grinding Processes

size the very coarse wood pieces, after which the shives are so small that the

groundwood can be passed directly to the pressure screening.

In pressure grinding, the grinder shower water temperature and flow are

adjusted so that the pulp temperature is maintained well below the water boiling

temperature at the set grinder pressure. A variety of combinations of pressure

groundwood process, depending on shower water temperature and grinding pressure,

are utilized in the following examples:

_ PGW95: The maximum grinder casing pressure is 300 kPa (3 bar)

and the shower water temperature is 95 °C. Hot filtrate water

from the thickener is led back to the grinder as shower water with

temperatures of about 95 °C (“Hot Loop”, PGW95 process). The

steam is removed in the cyclone and can be used, by heat recovery,

for several other process steps.

_ PGW-S120: The maximum grinder casing pressure is 500 kPa

(5 bar) and the shower water temperature is 120 °C.

_ PGW70: The use of groundwood in SC and LWC paper grades

raised the requirements for excellent pulp brightness and light

scattering. Higher fines content is preferred, and low coarse long

fiber amounts are favored. This can be reached by lower shower

water temperatures; the strength decreased only slightly compared

to PGW95 pulp. To achieve a shower water temperature of