Fiber suspension

Zone

C

(SGW)

C

(PGW)

It is assumed that the temperatures in the pulp suspension

Zone and the actual grinding zone are similar. The pulp

Suspension is a mixture of water, fibers and broken fiber

Parts moving along the grinding zone towards its end. This

Pulp suspension also acts as a lubricating and cooling agent.

Pulp stone surface

100 °C or more The surface temperature of the pulp stone is under 100 °C, and

Depends on the shower water temperature and pit consistency.

Grinding Processes

It can be summarized that the temperatures in the grinding zone vary over a

Wide range, depending on the moisture content of the wood and of the system

Used (atmospheric or pressurized grinding). According to Steenberg and Nordstrand

[7], the actual grinding zone is only 0.1 mm thick. Above this zone, the

temperatures in the wood are higher. In practical trials, Atack and May [8] have

measured the upper limit of the wood temperature as 206 °C, which corresponds

To the carbonizing temperature.

Defibration (Deliberation) of Single Fibers from the Fiber Compound

The pressure pulsations cause deformation of the fibers in the wood. Because of

The macro structure of the pulp stone surface, the fibers will be compressed and

Expanded with frequencies from 40 kHz. This leads to a breakdown of intermolecular

Bondings between the fibers and of intramolecular bondings within the fiber,

as described by Atack and Pye [9]. The breakdown of bonds is the basic prerequisite

For deliberation of the fibers from the wood. In addition, these deformations

Support the water absorption of lignin.

The pulp stone grit, when meeting the fiber, causes “treatment” of the fiber surface.

Depending on the macrostructure and microstructure of the pulp stone surface,

The fiber will be crushed, scratched, compressed, sheared off and cut. The

Strict subsequent loosening and peeling of fibers from wood surface, starting generally

From one end of the fiber, is shown schematically in Fig. 4.5. The fiber surface

Will be fibrillated, the primary wall is opened, and finally the fiber deliberated

from the fiber compound, with the “treatment"-phase lasting only 20–40 ms. The

Higher the feeding speed of wood, the less a single fiber is treated until the next

Grit appears.

Grit direction Grit direction

Loosening of fibers

Starting from one fiber end

Subsequent peeling of

Loosened fibers

Fig. 4.5 Schematic diagram of fiber loosening and peeling in

a grinding process (according to Atack [10]).

Mechanical Pulping Processes

4.1.3

Дата добавления: 2015-10-21