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-

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

Дата добавления: 2015-10-21