TECHNICAL NOTE NUMBER 204
UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE
FOREST PRODUCTS LABORATORY
MADISON 5. WISCONSIN R E^g6"e 195
COMMERCIAL PROCESSES 8F IULPING WOn*S
Five general processes are used commercially in making pulpfor paper,
paperboard, and wallboard from wood. One is the mechanical or ground-
wood process, in which wood bolts are reduced to pulp on a grindstone.
Three processes, the sulfite, sulfate, and soda, depend upon the dissolv-
ing action of chemical reagents to remove essentially all of the constitu-
ents of the wood except the cellulose fibers, which remain in a fairly
pure state. This is accomplished by digesting the wood chips with the
proper chemical under steam pressure. Another process, the semi-
chemical, causes the removal of only a part of the wood constituents by
chemical means and so affects the fiber bonds that the pulping can be
completed by mechanical fiberizing.
The mechanical process is the cheapest and returns the highest yield of
pulp (90 percent or more by weight), but the strength of this pulp is low.
Consequently, it is usually mixed with some of the stronger chemical
pulps before it is converted into paper. Newsprint and the cheaper mag-
azine and catalog papers are composed largely of groundwood pulp.
Certain fiber and building boards are also made mostly of groundwood
pulp. Only a few species of wood are employed; those found most desira-
ble are the long-fibered, light-colored spruces and balsam. Some of
the pines and hemlocks are also used but to a smaller extent. The quanti-
tyof hardwood consumed in the manufacture of groundwood pulp is small,
because the shortness of the fiber limits its use to specialty products.
A modification of the mechanical process introduces a mild chemical
treatment of the bolts of wood before they are ground. The pulp is called
chemigroundwood. This process is especially applicable to the hard-
woods because of the improvement obtained in the quality of the pulp.
The sulfite process employs an acid chemical (calcium, magnesium.
sodium, or ammonium bisulfites plus sulfurous acid). The yield is less
than half the weight of the wood, but the pulp is much stronger than
groundwood pulp. The unbleached pulp is comparatively light colored
and is readily bleached. Long-fibered, low-resin-content softwoods,
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such as spruce, balsam, and hemlock, are mostly used in producing
sulfite pulp. A small amount of southern yellow pine and some aspen,
birch, and other hardwoods are also used. Sulfite pulp is adaptable to
the widest variety of uses of any of the commercial pulps. It is used in
certain grades of book, wrapping, bond, and tissue papers. In combina-
tion with groundwood pulp, it is used in numerous products, of which
the most noteworthy is newsprint. Purified sulfite pulp is used for the
manufacture of viscose rayon and other cellulose derivatives. A modifi-
cation of the sulfite process, having the objective of obtaining yields of
60 percent of the wood, is called high-yield sulfite pulping and results
from less complete pulping action than usual. A certain amount of me-
chanical action is then necessary to fiberize the incompletely cooked
chips. The pulps are used unbleached, for example, in newsprint.
The sulfate process is applicable to almost any wood. Since the chemi-
cal liquor used is alkaline (a solution of sodium hydroxide and sodium
sulfide), resins, waxes, or fats in the wood do not hinder its pulping
action. Hence, it is used principally for the conversion of the pines.
As in the sulfite process, the yield of pulp is less than half the weight
of the wood. Sulfate pulps are generally the strongestof the commercial
pulps. When suitably cooked, the pulps can be used to produce strong
bleached pulps for high-grade papers, including book, magazine, writing,
bond, and specialty papers. The principal uses for unbleached sulfate
pulp are kraft wrapping paper, bag paper, and boxboard. Bleached hard-
wood sulfate pulps are used extensively in printing papers. A modifica-
tion of the sulfate process involves a treatment of either hardwoods or
softwoods with steam before the woods are cooked by the conventional
pulping process. The -pulp is then purified by a standard procedure and
used for making rayon. Another modification of the sulfate process
produces a high yield of kraft pulp by the incomplete cooking of pine.
This kind of pulp is used primarily for making container board.
The soda process, also alkaline, employs caustic soda as the pulping
agent and is used principally for the reduction of hardwoods. Aspen,
cottonwood, basswood, beech, birch, maple, tupelo (gum), and oak are
commonly used. The yield is from about 40 to 48 percent, depending
on the species of wood employed and the severity of the cooking condi-
tions. Soda pulp is sometimes used alone in the manufacture of bulky
papers, such as blotting, where the strength requirements are not high.
Book, lithograph, and envelope papers are often made from a mixture
of sulfite pulp and soda pulp.
The semichemical processes are more recent developments. They ob-
tain their name from the fact that the chips are first merely softened
and only partly dissolved by chemicals and then are reduced to pulp by
mechanical action. The chemical solutions used may vary. A neutral
sodium sulfite solution is used principally although either alkaline sul-
fate or acid sulfite liquors are applicable. The yield of pulp is rela-
tively high, from 65 to 80 percent of the weight of the wood. The semi-
chemical pulping process is applied predominately to hardwoods and the
pulps are used in corrugating board (major product), newsprint, and
specialty boards. Semichemical pulps can be brightened to light shades
for use in printing papers or bleached to a high white by conventional
bleaching methods. The latter are made in yields of 50 to 60 percent
and are used in printing, glassine, and bond papers and specialty boards
like food cartons.
A process used principally for the production of a coarse fiber product
involves the treatment of wood chips with steam or water at high tem-
peratures and pressures to weakenthe fiberbonds, followed by a mechan-
ical fiberization. The yields are 85 to 95 percent. These pulps are
composedof amixtureof single fibers and fiberbundles. The twoimpor-
tant commercial coarse-fiber pulps are called "exploded" pulp (Mason
process), and "defibrated" pulp (Asplund process). They are used in
the manufacture of hardboard, insulating board, absorbent felts for roof-
ing, and similar papers.
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UNIVERSITY OF FLORIDA
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