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Polymer such as lignin from a glass-like and brittle state into a weak and
plastic one. Goring [3] identified the softening temperatures for lignin as 135 °C to
235 °C, whereas according to Styan and Bramshall [4] the softening temperatures
for softwoods are about 135 °C and for hardwoods about 100 °C. In lignin, the plasticizing
effect of water has a limit at water contents as low as 5%. For water-saturated
isolated lignin, the softening takes place at 80–90 °C, and additional water
Does not result in any considerable further softening of lignin. Under typical mechanical
Pulping conditions, the softening temperature for lignin as part of the
wood fiber matrix is higher (in the range of 100–130 °C). The lignin polymer,
Being the stiffest wood component under the conditions used in mechanical pulping,
Represents the most important component for the thermal softening of wood.
Preferably, wood for mechanical pulping should have a moisture:wood ratio of
Over 0.5.
Knowledge of the thermal processes in grinding is important step when completing
the grinding process. In evaluating investigations made by Luhde [5], the
Relationship between thermal relationships during grinding process is illustrated
Schematically in Fig. 4.4, while the five temperature-dependent regions of Fig. 4.4
Are described in more detail in Tab. 4.1.
Grinding Processes 1081
Mechanical Pulping Processes
Fig. 4.4 Thermal relationships in the grinding process.
Cool wood; 2, strongly heated wood layer; 3, actual grinding
Zone; 4, fiber suspension zone; 5, pulp stone surface.
Tab. 4.1 Temperatures at different zones during the grinding process.
Zone Temperature Process
Cold wood
Depending on
Surrounding
Temperature
(related to
Season)
The wood is cold and thermally untreated when entering
Zone 2.
Strongly heated
Wood layer
C The temperature in the wood about 0.1 mm above the
Grinding zone is high, due to by heat impound, but below
the carbonizing temperature of wood (206 °C) [6]. These high
Temperatures are the reason for the softening of lignin. The
Resulting equilibrium temperature depends on the water
Content in the wood.
Actual grinding
Zone
C (SGW)
C (PGW)
The actual grinding zone is only ca. 0.1 mm thick (according
to Steenberg and Nordstrand [7]), and has a temperature
usually close to 100 °C (higher in pressurized grinding). This
Temperature increases with increasing pit consistency and
Pit temperature. At this point, the fibers are deliberated from
The fiber compound. Here, the resulting equilibrium temperature
Also depends on the water content in the wood.
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Depending on the position of the log in the grinder magazine, it can be distinguished | | | Fiber suspension |