A Brief and Simplified Description of Papermaking
The paper we use today is created from individual
wood fibers that are first suspended in water and then
pressed and dried into sheets. The process of converting the wood to a suspension of wood fibers in water
is known as pulp making, while the manufacture
of the dried and pressed sheets of paper is formally
termed papermaking. The process of making paper
has undergone a steady evolution, and larger and
more sophisticated equipment and better technology
continue to improve it.
The Wood yard and Wood rooms
The process at Androscogging began with receiving wood in the form of chips or of logs 4 or 8 feet in
length. From 6 AM to 10 PM a steady stream of trucks
and railroad cars were weighted and unloaded. About
40 percent were suplied by independents who were
paid by weight their logs. The mill also received wood
chips from lumber mills in the area. The chips and logs
were stored in mammoth piles with separate piles
for wood of different species (such as pine, spruce,
hemlock).
When needed, logs were floated in flumes......(1).....the
wood yard.....(2).....one of the mill’s three wood rooms.
There, bark was rubbed......(3)........in long, ribbed debarking drums by tumbling the logs against one another.
The logs then fell into a chipper;......(4)......seconds a
large log was reduced to a pile of chips approximately
1 inch by 1 inch by 1/4 inch.
The chips were stored in silos. There were separate
silos for softwoods (spruce, fir, hemlock, and pine) and
hardwoods (maple, oak, beech, and birch). This separate and temporary storage of chips permitted the
controlled mixing of chips into the precise recipe for
the grade of paper being produced.
The wood chips were then sorted through large, flat
vibrating screens. Oversized chips were rechipped,
and ones that were too small were collected for burning in the power house. (The mill provided approximately 20 percent of all its own steam and electricity
needs from burning waste. An additional 50 percent
of total electricity needs was produced by harnessing
the river for hydroelectric power.)
Once drawn from the silo into the digesters, there was
no stopping the flow of chips into paper.
Pulpmaking
The pulp made at Androscoggin was of two types:
Kraft pulp (produced chemically) and ground wood
pulp (produced mechanically). Kraft pulp was far more
important to the high quality white papers produced
at Androscoggin, accounting for 80 percent of all the
pulp used. Kraft pulp makes strong paper. (Kraft is
German for strength. A German invented the Kraft
pulp process in 1884.) A paper’s strength generally
comes from the overlap and binding of long fibers
of softwood; only chemically was it initially possible
to separate long wood fibers for suspension in water.
Hardwood fibers are generally smaller and thinner and
help smooth the paper and make it less porous.
The ground wood pulping process was simpler and
less expensive than the Kraft process. It took high
quality spruce and fir logs and pressed them continuously against a revolving stone that broke apart the
wood’s fibers. The fibers, however, were smaller than
those produced by the Kraft process and, although
used to make newsprint, were useful at Androscoggin
in providing “fill” for the coated publication gloss
papers of machines 2 and 3, as will be described later.
(A)The chemical Kraft process worked by dissolving the
lignin that bonds wood fibers together. (B) It did this
in a tall pressure cooker, called a digester, by “cooking” the chips in a solution of caustic soda (NaOH) and
sodium sulfide (Na2S), which was termed the “white
liquor.” (C)The two digesters at Androscoggin were
continuous digesters; chips and liquor went into the
top, were cooked together as they slowly settled down
to the bottom, and were drawn off the bottom after
about three hours. (D) By this time, the white liquor had
changed chemically to “black liquor’’; the digested
chips were then separated from this black liquor. (E)
In what was known as the “cold blow” process, the hot,
pressurized chips were gradually cooled and depressurized. A “cold liquor’’ (170°F) was introduced to the
bottom of the digester and served both to cool and
to transport the digested chips to a diffusion washer
that washed and depressurized the chips. Because so
much of the lignin bonding the fibers together had
been removed, the wood fiber in the chips literally fell
apart at this stage.
The black liquor from the digester entered a separate
four-step recovery process. Over 95 percent of the
black liquor could be reconstituted as white liquor,
thereby saving on chemical costs and significantly
lowering pollution. The four-step process involved
(1) washing the black liquor from the cooked fiber to
produce weak black liquor, (2) evaporating the weak
black liquor to a thicker consistency, (3) combustion
of this heavy black liquor with sodium sulfate (Na2SO4
),
and redissolving the smelt, yielding a “green liquor”
(sodium carbonate + sodium sulfide), and (4) adding
lime, which reacted with the green liquor to produce
white liquor. The last step was known as causticization.
Meanwhile, the wood-fiber pulp was purged of
impurities like bark and dirt by mechanical screening
and by spinning the mixture in centrifugal cleaners.
The pulp was then concentrated by removing water
from it so that it could be stored and bleached more
economically.
By this time, depending on the type of pulp being
made, it had been between 3 1/2 and 5 hours since the
chips had entered the pulp mill.
All the Kraft pulp was then bleached. Bleaching took
between 5 and 6 hours. It consisted of a three-step
process in which (1) a mix of chlorine (Cl2
) and chlorine dioxide (CIO2
) was introduced to the pulp and
the pulp was washed; (2) a patented mix of sodium hydroxide (NaOH), liquid oxygen, and hydrogen peroxide (H2
O2
) was then added to the pulp and the pulp
was again washed; and (3) chlorine dioxide (ClO2
)
was introduced and the pulp washed a final time. The
result was like fluffy cream of wheat. By this time the
pulp was nearly ready to be made into paper.
From the bleachery, the stock of pulp was held for a
short time in storage (a maximum of 16 hours) and
then proceeded through a series of blending operations that permitted a string of additives (for example,
filler clay, resins, brighteners, alum, dyes) to be mixed
into the pulp according to the recipe for the paper
grade being produced. Here, too, “broke” (paper
wastes from the mill itself) was recycled into the pulp.
The pulp was then once again cleaned and blended
into an even consistency before moving to the papermaking machine itself.
It made a difference whether the broke was of coated
or uncoated paper, and whether it was white or colored. White, uncoated paper could be recycled immediately. Colored, uncoated paper had to be rebleached.
Coated papers, because of the clays in them, could not
be reclaimed.
Study the following sentences:
“The ground wood pulping process was simpler and
less expensive than the Kraft process. It took high
quality spruce and fir logs and pressed them continuously against a revolving stone that broke apart the
wood’s fibers.”
1. the word ‘simpler’ is an adjective in the superlative form.
2. the word ‘them’ is an object pronoun.
3. the tense used in ’took’, is simple past of a
regular verb.
4. the word ‘that’ can be replaced by ‘which’
without changing its meaning.
Choose the alternative which presents the correct
ones: