4 Mechanical Pulping Processes

_ The quality of the mechanical pulp produced is constant over a

longer time interval than for groundwood, because there are no

sharpening fluctuations.

The development of the refiner process has been enforced by much better designs

of the refiners, highly efficient refiners, and the development of wear-resistant

plate materials.

4.2.2

Mechanical, Thermal, and Chemical Processes in the Refiner Process

Fiber deliberation from the fiber compound can occur in the refiner process as

follows:

_ Softening of the lignin in the middle lamella and in the primary

wall of the wood fiber by pressure load frequencies in the refiner;

this is mechanical softening.

_ Softening of the lignin in the middle lamella and in the primary

wall of the wood fiber by thermal influences; this is thermal softening.

_ Softening of the lignin in the middle lamella and in the primary

wall of the wood fiber by chemical pretreatment; this is chemical

softening.

Many process parameters are considered responsible for the character and properties

of the mechanical pulp produced, including: (a) the pressure and temperature

during thermal pretreatment; (b) the duration of thermal pretreatment; (c) the

addition of chemicals; (d) the specific energy consumption; (e) the energy distribution

within the refining stages; (f) the consistency in the refining zone of the

first refining stage; (g) the wood chip quality; (h) the refiner design; and (i) refining

Intensity caused by plate design and rotational speed.

The duration of thermal pre-treatment has only minimal influence on pulp

quality, since in practical terms this stage lasts for only 1–3 min, and a minimum

time is striven for.

The defibration temperature is as important as for the grinding process, and

should be 100–130 °C. By raising the temperature to 140 °C, the lignin becomes

well-softened and the fiber requires minimal mechanical energy for its liberation

from the fiber compound. This mechanical pulp has an unsuitable quality for

papermaking, however, as it is harsh and coarse. The softened lignin solidifies at

the fiber surface to a hard substance; the pulp has a high refining resistance. If

the defibration temperature is lower than the softening temperature of the lignin,

then the mechanical pulp produced is coarse and has only a low level of strength

properties. When the refining temperature is very close to the softening temperature,

a high percentage of the fibers can be defibrated without being destroyed.

The primary wall of these fibers can be damaged, and this allows fibrillation of the

secondary wall. According to Giertz [19], the shearing frequency in a refiner is be-

1100

4.2 Refiner Processes

tween 10 kHz and 1 MHz. Based on the fact that the softening temperature for

high polymers increases by 7° when the frequency increases by 1 tenth, it can be

assumed that the lignin of moist chips will be softened in the refiner at 120–

135 °C.

The shearing frequency is estimated by rotational speed, disc diameter, and

plate pattern. Moreover, the type of refining – whether single disc, double disc or

conical disc – is also important. If the refining temperature is slightly above the

softening temperature of the wood chips, then defibration occurs mainly in the

zones with a high lignin concentration (i.e., in the middle lamella). In this way, a

high proportion of fibers is deliberated without being damaged. At the same time,

the middle lamella and primary wall are removed and milled down to fines, while

the outer secondary wall (S1) is fibrillated [19]. The mechanical and thermal processes

during refining can be divided into three phases: