Wood. Wood that has been stored under dry conditions for a longer time has a

Higher debarking resistance. When the type of storage keeps the moisture content

High and rather constant, a longer storage time does not increase the debarking

Resistance to any great degree. In order to maintain the state of low debarking

Processing of Wood 1077

Resistance from wood that has been felled in the spring, water storage would be

The best approach.

Although wet debarking may be carried out with a lower debarking resistance

And better debarking results, dry debarking in continuous drum debarkers is preferred

In the industrial situation in order to avoid problems of pollution and also

To achieve lower investment costs (shorter drum constructions, no effluent treatment).

For technical applications of dry debarkers, see Chapter I-3, Wood yard

Operations.

3.2.3

Wood Log Chipping

The uniformity of chip size distribution, bulk density and wood source are the

Most important factors that determine chip quality for mechanical pulping. The

Average chip length of 22 mm is less than that of chips used for kraft pulping.

The shorter the chip is along the grain, the more fibers have been cut; consequently,

Short chips result especially in a lower tear strength. The impact of

Extreme chip fractions on thermomechanical pulp (TMP) are detailed in Tab. 3.1.

Tab. 3.1 Impact of extreme chip size fractions on thermomechanical pulp processing and quality.

Chip fraction Impact

Over-large fraction – Causes uneven feed to the refiner.

– Reduces pulp quality.

Over-thick fraction – Contains most knot wood present in groundwood logs.

– Causes unstable refining and increase of energy consumption.

– Decreases fiber length and long fiber portion.

– Impairs strength properties and brightness.

Fines fraction – Lowers energy consumption.

– Decreases pulp strength, sheet density, brightness and light-Scattering

Coefficient.

– Increases shives content and causes linting problems.

The technical applications of wood chipping are explained in detail in Chapter

I-3, Wood yard operations.

Raw Materials for Mechanical Pulp

Mechanical Pulping Processes

Jurgen Blechschmidt and Sabine Heinemann

4.1

Grinding Processes

4.1.1

Principle and Terminology

Round logs are pressed against a rotating pulp stone under specified conditions

Of pressure and temperature (Fig. 4.1).

Fig. 4.1 Grinding principle. 1, Pulp stone;

Grinder pit; 3, weir; 4, shower water pipe;

Wood magazine; 6, finger plate; 7, pulp stone

Sharpener; 8, sharpening roll; 9, wood logs.

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