Page 453 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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258 Pulp and Paper

Several factors are responsible for the incorporation of
chemical treatments in mechanical pulping including the
high-energy demand of using only mechanical attrition
and the limited utility of mechanically pulping many hard-
wood species.
Mild chemical treatments are used to soften the wood
chips and increase the amount of whole fibers. Some of
the wood cell components are solubilized and the lignin
is made more hydrophilic. The penalty paid for chemical
addition is a reduction in yield, to levels of 80 to 90%.
The benefits of chemical addition pulps are the increased
ability to utilize hardwoods, lower energy requirements,
FIGURE 6 The current family of mechanical and chemically mod-
stronger pulps, and increased process flexibility. Some of
ified mechanical pulps.
these pulps have been found to be suitable for products
that generally require full chemical pulps.
In the case of PGW, the casing surrounding the pulpstone
is sealed and pressurized. This can be accomplished with
B. Process Considerations
steam, but most manufacturers now use air pressure. This
pressure maintains higher temperatures in the grinding The modern process of mechanical pulping is best under-
zone, and the yield of longer fiber pulp is increased. stood in terms of the TMP process. The basic process is
PRMP uses air or steam pressure applied to the refiner. depicted, schematically, in Fig. 7. From Fig. 7 it is ap-
The chips are unheated and untreated. TRMP is closely parent that the heart of the system is the refiner and TMP
related to PRMP, the chips are preheated and refined at systems may have from one to three stages of refiners such
atmospheric pressure. as the Sprout–Waldron machine depicted in Fig. 8. When
In summary, purely mechanical pulping methods give chemical addition is performed, it is in conjunction with
the highest yields—93–99%. Advances have been made chips steaming (see Fig. 7).
by modifying older processes by application of heat en- The TMP systems can consume 2200–2800 kW h/tonne
ergy to assist in defiberization. Pure mechanical pulps may of pulp, depending upon the species being pulped and the
have excellent printing properties and optical properties. degree of refining employed. Much of the energy con-
But, most processes yield fiber that is still too weak to sumed ends up as waste heat. Consequently, waste heat re-
be used without reinforcement—the only exception be- covery is of primary economic importance. Systems such
ing TMP and closely related methods. Full TMP furnish as that depicted in Fig. 9 are used to produce steam for use
newsprints are being made. in the TMP process, and for steam and hot water useful in
Chemically modified pulps, as the name implies, are other forest industry processes. Waste heat recovery has
pulps produced by either subjecting wood chips to a mild improved the economics of TMP and related mechanical
chemical treatment or using chemical treatment at some pulping systems, particularly in integrated forest industry
point during or after refining. Often steam is injected with mill settings.
the chemicals to yield a chemithermomechanical pulp
(CTMP) with good strength properties. This approach has
C. Prospectus
been adopted by many Canadian mills in recent years.
The chemical pretreatment utilizes chemicals also com- Mechanical pulping has higher yields but lower-strength
mon to the full chemical pulping processes. However, the pulps when compared to full chemical pulps. Improve-
chemical treatments are much shorter in duration and gen- ments are constantly being made, and considerable gains
erally lower temperatures are used to minimize solubiliza- have been made in adapting different types of wood and
tion of wood components and keep the yields high. Most different forms of wood (sawdust versus chips) to mechan-
of these processes use sodium sulfite or bisulfite as the ical pulping via advanced process control techniques.
active chemical, although sodium hydroxide, sulfide, and The pulp and paper industry is undergoing some rela-
carbonate are also being used. With chemically modified tively rapid changes in pulping technology. In areas of the
pulps,organizationisachievedbydividingtheseprocesses world where the resource base is dwindling, the increased
into three groups—heavy fractional and light and heavy yields offered by newer mechanical pulping techniques
chemical treatment. The adjectives light and heavy de- are highly desirable. This has been the case in Sweden,
scribe the degree of sulfonation applied to the woodchips Finland, and Canada, which have low-cost hydroelectric
or fiberized wood. power available in many sections.