Fig. 4.23 Relative pit pulp properties at CSF 80 mL for

different groundwood types (data are based on Refs. [14–18]).

The reference pulp is PGW95.

Mechanical Pulping Processes

The properties of PGW differ from the conventional stone groundwood (SGW).

Especially important is the higher long-fiber content that causes increased initial

wet web strength and tear resistance [about 40% higher strength values for PGWS120

compared to atmospheric SGW; see Fig. 4.23]. Average values for the fiber

length are 1.3–1.5 mm for PGW from spruce, compared to 0.7–0.8 mm for SGW

From the same wood species.

An overview of the different groundwood pulp properties, depending on the

Grinding method, is provided in Fig. 4.23. All pulps are compared at the same

freeness level of CSF 80 mL, while the reference pulp is PGW95, set as 100% in

All properties.

Pressure grinding produces a pulp with higher long-fiber content and higher

Strength level than does atmospheric grinding, and their properties are comparable

To those from refiner mechanical pulp (see Section II-4.2). One main advantage

Of pressure grinding is a better exploitation of the raw material without additional

Specific energy input and loss in optical properties, and a pulp quality that

Enables the saving of amounts of chemical pulp in pulp blends.

4.2

Refiner Processes

4.2.1

Principle and Terminology

A typical flow sheet of a refining process is shown in Fig. 4.24.

Fiber logs are cut down into wood chips that are then defibrated to mechanical

Pulp by means of disc refiners. Depending on the type of chip pre-treatment or

Chip post-treatment, several processes can be carried out, each of which is specifically

Defined.

_ RMP (Refiner mechanical pulp): The refining of chips at atmospheric

Pressure in a refiner (in some cases, the refiner outlet

May be pressurized).

_ TMP (Thermo-mechanical pulp): Thermal pre-treatment and

Refining of the chips under pressure, with the second refiner also

Under pressure in most cases (the pressure allows heat recovery)

_ RTS™ (Retention time, temperature, speed): Chips are preheated

Very briefly at a high temperature and then refined at high

Speed.

_ Thermopulp™ (Thermo pulp): This differs from TMP in that the

pulp is heated to a very high temperature (ca. 170 °C) briefly

Before the second refining stage.

_ CMP (Chemimechanical pulp): Chips are pre-treated, usually

With sodium sulfite and caustic, and then refined without pressure.

Refiner Processes

_ CTMP (Chemithermo mechanical pulp): Chips are pre-treated in

The same way as for CMP, but with a lower chemical charge, and

Then refined under pressure.

Fig. 4.24 Typical flow sheet of a modern refining process.

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