Page 452 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Pulp and Paper 257

stone grinding or disk refining to provide the energy
for attrition.
Thermomechanical pulp (TMP) was the next step in
the development of the newer methods. Thermomechani-
cal pulp, commercially introduced in 1968, was originally
designed to reduce the mechanical energy demands of me-
chanical pulping, but this objective was not achieved. The
pulp produced, however, was much stronger than SGW
pulp. The SGW and RMP pulps as the sole furnish for pa-
per, are too weak to be used on modern high-speed paper
machines. Therefore, up to 25% of high-cost full chem-
ical (e.g., bleached Kraft) fiber is used to reinforce the
sheet. There was a need for a process in which the high
yields of SGW or RMP could be realized that produce
higher quality fiber. In TMP, chips are preheated and con-
verted to pulp in either pressurized or unpressurized disk
refiners. Preheating the chips softens the lignin and re-
duces the fragmentation of wood to produce more whole
fiber. The pulp is much stronger, due to the mechanisms
shown in Fig. 5. Current production of TMP pulp is about
6
10 × 10 tonnes/year. Since 1970, developments in the
FIGURE 3 The modern Kamye continuous digester, heart of the TMP methods as well as other factors have spurred the
Kraft pulp mill. [Photo courtesy of Leaf River Corp.]
recent growth in the complexity of mechanical pulping
processes as shown in Fig. 6. Since the early 1970s, the
commercially in the 1850s. It still accounts for almost one- number of different mechanical pulping methods has ex-
half of the total of all mechanical pulp produced world- panded dramatically. There is probably no other period in
6
wide. (25 × 10 tonnes/year). In this process, short bolts of history in which so many different forms of pulp process-
solid wood are pressed against the outer rim of a revolving ing techniques have been developed.
stone wheel. In Fig. 6 all of the methods are divided into purely
Refiner Mechanical Pulp was developed in 1929 and mechanical pulps and chemically modified pulps. Under
then used in 1938 for board products. Disk refiners began purely mechanical pulps the older methods, SGW, RMP,
to be used in 1962 for pulp production. In this case, un- and TMP remain, but three new processes have been
like SGW, small wood pieces or chips are broken down added to the list: TRMP (thermo-refiner mechanical
between rotating, grooved, or patterned metal disks at at-
pulp), PGW (pressure ground wood) and PRMP (pressure
mospheric pressure. The two methods for mechanically
refiner mechanical pulp). These purely mechanical
producing pulp are depicted in Fig. 4. One advantage
methods are all very similar to the older processes. The
of using refiners is that lowercost wood residues could
differences are related to the temperature of either the
be used as feedstock. Refiner mechanical pulp production wood before or during refining. Heat energy or pressure is
6
totals about 3.5 × 10 tonnes/year.
not applied in the same manner in the different processes.
These first two mechanical methods of breaking down
wood into pulp provide the bases for all of the further
developments in the field of mechanical pulping. While
all of the present mechanical pulping methods do pro-
duce different types of pulp, they still rely upon either

FIGURE 4 A graphic depiction of the various types of mechanical FIGURE 5 The cleavage mechanism of TMP pulping compared
pulping and the relation of grinding systems to fiber dimension. to RMP pulping and medium density fiberboard (MDF) production.