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4.1.2.2 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]).

1083

4 Mechanical Pulping Processes

4.1.3

Influence of Parameters on the Properties of Groundwood

In the mechanical defibration of wood by grinding, several process parameters

have significant influences on the final properties of the stone groundwood.

Among these, the most important include:

_ Moisture of the wood logs

_ Logs feeding speed or grinding pressure respectively

_ Pulp stone rotational speed

_ Temperatures

_ Process consistency

_ Surface profile of the pulp stone

_ Specific grinding energy consumption

Basically, it can be said that an increase in grinding pressure produces a groundwood

with better dewatering behavior (high freeness) and less strength properties,

assuming that all other parameters are kept constant. Such pulps are coarse

groundwoods, whereas pulps with low freeness are fine groundwoods. Increasing

the pulp stone speed, and keeping all other conditions constant, produces a pulp

with shorter fibers and lower freeness.

From a practical standpoint, the efficiency of the grinding process will be

Improved by increasing both parameters – grinding pressure and pulp stone

speed. However, the relationship between both of these parameters can be held

constant and high-quality groundwood produced.

The temperature relationship during grinding also affects the properties of the

final groundwood. Functions which describe the temperature relationship in

grinding include:

_ Temperature of the pit pulp

_ Temperature of the shower water

_ Temperature at the end of the grinding zone