4 Mechanical Pulping Processes

and the pulp stone becomes dull so that the quality and quantity of the groundwood

produced is changed. To regain the original surface structure, the stone

must be sharpened. The surface profile (macrostructure) before and after the sharpening

process is shown in Fig. 4.19. The sharpening interval is the timebetween two

sharpening actions, during which the groundwood quality changes significantly.

Traditionally, sharpening of the pulp stone has been carried out with metallic

burrs. For this purpose, a sharpening lathe is installed on the top of a pocket grinder,

or on the side of a chain grinder. The burr itself is installed into a burr holder

that is fastened to the sharpening lathe. The burr sharpening produces a spiral

pattern having grooves and areas, in turn, in the pulpstone surface. Sharpening

deepens the grooves and reduces the land area, and also exposes fresh abrasive

grits and removes impurities from the stone pores and surface. A smaller land

area provides a higher unit grinding pressure, which results in a coarser pulp having

a higher degree of freeness. The deeper grooves with a higher void area at the

stone surface bring more water to the grinding zone, and consequently are able to

carry more pulp out of the grinding zone.

The most important parameters that can be varied in the sharpening pattern

Include:

_ Tooth frequency of the burr, pitch of the burr, pitch of the burr

_ Tooth angle (mainly 28° in spiral burrs)

_ Sharpening depth

The pitch of the burr and sharpening depth are used for burr specification. For

each pulpstone type, there is a minimum width for the base of the land to be

strong enough to support the grinding load. If finer burrs are used, it is highly

probable that the lands break and the pulp quality is impaired. Some characteristics

of spiral burrs are shown in Fig. 4.20.

Fig. 4.20 Characteristics of spiral burrs.

The sharpening of grinding stones with spiral burrs is being increasingly

replaced by the use of ultrahigh-pressure water. This waterjet conditioning with

water pressures set at 50 to 240 MPa (500 to 2400 bar) allows accurate control of

the pulpstone sharpness, and results in a stable pulp quality and higher produc-

1094

4.1 Grinding Processes

tion. This in turn leads to an improved stability of the total grinding and screening

process. Compared to conventional spiral burr sharpening, waterjet conditioning

is carried out during the grinding process, and variations in pulpstone sharpness

may be reduced by 50–60%. The resultant groundwood had a more even distribution

of well-bonding fibrillar particles. The tensile index of long fibers was

increased by 15% and the tear index by 15–20%, while the apparent density was

reduced by 30%.

4.1.5

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