WOOD PULP AND WOOD PULP PAPERS
THE MANUFACTURE OF MECHANICAL WOOD PULP.
Wood is converted into pulp suitable for the manufacture of paper by
methods which produce two distinct varieties. The first is _mechanical
wood pulp_, so called because it is made by a purely mechanical
process. The second is termed _chemical wood pulp_ from the fact that
the material is submitted to chemical treatment.
_Ground Wood and Cellulose._--The two varieties of pulp are sometimes
distinguished by the use of the terms ground wood and cellulose. In
the former case the description implies a product consisting of pulp
obtained by grinding wood into a fibrous condition, while in the second
the word suggests a purified chemical product freed from the resinous
and non-fibrous constituents found in wood. This is, in fact, the
essential difference, for mechanical wood pulp consists of fibres which
have been torn away from wood by means of a grindstone; it differs
but slightly in chemical composition from the original raw material
and contains most of the complex substances natural to wood. Chemical
wood pulp, on the other hand, consists of fibre isolated from wood in
such a manner that the complex non-fibrous substances are more or less
entirely removed. The difference between these two pulps is shown in
the following approximate analysis of spruce wood, and of the pulp
derived from it. The composition of the mechanical pulp is practically
identical with that of the wood itself.
COMPOSITION OF SPRUCE WOOD, AND OF CHEMICAL WOOD PULP (SPRUCE).
----------------+---------+----------
-- | Wood | Chemical
|(Spruce).|Wood Pulp.
----------------+---------+----------
Cellulose | 53·0 | 88·0
Resin | 1·5 | 0·5
Aqueous Extract | 2·5 | 0·5
Water | 12·0 | 8·0
Lignin | 30·5 | 2·5
Ash | 0·5 | 0·5
+---------+----------
| 100·0 | 100·0
----------------+---------+----------
The use of mechanical wood pulp is generally confined to the
manufacture of news, common printings and packing papers, cardboards,
and boxboards. It possesses very little strength, quickly discolours
when exposed to light and air, and gradually loses its fibrous
character. The chemical wood pulp is a strong fibre, from which
high-class papers can be manufactured, the colour and strength of which
leave little to be desired.
_Species of Wood._--The woods most commonly used for the manufacture
of wood pulp belong to the order Coniferæ, or cone-bearing trees.
In Europe the spruce and silver fir are the chief species, while in
America spruce, balsam, pine, and fir are employed. The harder woods,
such as hemlock, beech, larch and others, are not converted into pulp
by the mechanical process.
_Timber Operations._--The trees are cut down in the early part of
winter by gangs of men specially trained to the work. The organisation
of a lumber camp when the operations are of an extensive character is
very complete and carefully arranged, every detail being attended to
in order to get out the wood as cheaply and expeditiously as possible.
The branches and small tops are removed from the trees when they are
fallen, and the trunks cut into logs of 12, 14, or 16 feet in length,
and afterwards piled up on the banks of the nearest river, or on the
ice, ready for the breaking up of the winter.
As soon as the ice breaks up and the rivers become navigable the logs
are floated down to their destination, in some cases hundreds of miles
from the scene of operations. Where rivers are not available the timber
is brought out by horses or bullocks, or by means of a light railway.
_Log Cutting._--As the timber arrives at the mill it is carefully
measured, both as to its diameter and length, in order that a record
may be kept of the quantity used. Some of the logs are piled up in
the storeyard for use in the winter, and the remainder converted into
pulp day by day. The logs are first cut into short pieces about 2 feet
long by means of a powerful circular saw, the arrangements for this
work being devised so as to keep down the cost of labour as much as
possible. All waste pieces are thrown aside to be utilised as fuel.
_Barking._--The bark on the logs is removed in one or two ways. Much
of it is knocked off during the transfer from the forest to the mill,
but even then the wood requires to be cleaned. In Norway and Sweden
the wood is treated in a _tumbler_ or a _barker_, while in America and
Canada the use of the tumbler is practically unknown.
The barker consists of a heavy iron disc fitted with knives, usually
three in number, which project from the surface of the disc about
half or three-quarters of an inch. The barker rotates in a vertical
position, and the short pieces of wood are brought one by one into
contact with the disc in such a manner that the bark is shaved off by
the knives. The machine is provided with conveniences for pressing the
wood against the disc and for turning the logs as they are barked.
[Illustration: FIG. 28.--A Pair of Barkers for removing Bark from Logs
of Wood.]
The machine is encased in a strong cast-iron cover, and all the bark
shaved off is carried away by the strong current of air set up by the
rapid motion of the disc, and subsequently burnt.
The tumbler system is quite different. In this case the short pieces
are thrown into a large circular drum with hot water, and the bark
taken off by the friction of the pieces as the drum rotates. The loss
of material is of course less in this process, but the wood is not
cleaned quite so effectively.
[Illustration: FIG. 29.--View of Horizontal Grinder (A), with Section
(B).]
The wood at this stage can be used either for the manufacture of
mechanical or chemical pulp. As a general rule the pieces are taken
indiscriminately for either process, but sometimes the wood is sorted
out, the clean stuff free from knots and blemishes being reserved for
high quality chemical pulp.
_Grinding._--The main feature of the grinding process is the attrition
of the wood when held against the surface of a rapidly revolving
grindstone, the fibres as they are rubbed off being instantly carried
away from the stone by a current of water. A complete description of
the machines used and the modifications of the process practised by
manufacturers is impossible in this book, but the following points will
be sufficient.
The machine consists of a large grindstone about 54 inches in diameter,
and 27 inches thick. It rotates in a vertical or in a horizontal
position at a high speed. The stone revolves inside a casing which is
provided with a number of _pockets_, so called, into which the pieces
of wood are thrown at regular intervals, as fast as the wood is ground
by the friction of the stone.
A continual stream of water playing upon the surface of the stone
washes away the pulp into a tank or pit below the machine.
The quality of the pulp may be varied by the conditions under which it
is made. By limiting the proportion of water so that the wood remains
in contact with the stone for a longer time the temperature of the mass
in the pockets rises. Such _hot ground pulp_, as it is termed, is tough
and strong.
When the fibres are washed away from the stone as fast as they are
produced the temperature does not rise, and _cold ground_ pulp is made,
which is not characterised by the somewhat leathery feel of the pulp
made at the higher temperature.
The surface of the stone plays an important part also. If the stone
is smooth the wood is rubbed away slowly, but if the surface has been
roughened and grooved by means of a special tool the fibres are torn
away quickly. In the first case the pulp comes from the stone in a
finely-ground state and in a uniform condition, while in the second
the pulp is coarse and chippy.
The output of the machine is, however, much increased by the use of
sharp stones and by the application of considerable pressure to the
blocks of wood.
[Illustration: FIG. 30.--A Vertical Grinder for making Hot Ground
Mechanical Wood Pulp.]
_Screening._--The mixture of water and pulp leaving the grinder falls
into a tank below the stone, all large chips being retained by means
of a perforated plate. The finer pulp, still too coarse for use, is
then pumped to the screens, which serve to remove all chippy and coarse
fibres and produce a uniform material. The _shaking sieve_ consists
of a shallow tray, the bottom of which is a brass plate or series of
plates perforated with small holes or slits. The pulp flows on to the
tray, which is kept in a state of violent agitation, the fine pulp
passing through the holes and the coarser pieces working down to the
lower edge of the tray into a trough which carries them away. The _flat
screen_ is somewhat different in construction, but the principle of
separation is the same. It consists of brass perforated plates forming
the bottom of a shallow cast-iron tray, continually agitated by means
of cams fixed to the under surface of the trays.
[Illustration: FIG. 31.--Centrifugal Screen for Wood Pulp.]
The _centrifugal screen_ is a cage made of finely perforated brass
sheeting which revolves at a very high rate of speed inside a circular
cast-iron vessel. The pulp flows into the interior of the cage, the
fine fibres being forced through the screen by the centrifugal action
of the machine, and the coarse material is retained.
[Illustration: FIG. 32.--Section of Centrifugal Screen for Wood Pulp.]
_Wet Pressing._--The pulp leaving the screens is mixed with such a
large quantity of water that it is necessary to concentrate it. This
is effected by means of the wet press machine (Fig. 41). The pulp and
water are pumped into a wooden box in which revolves a large hollow
drum, the surface of this drum consisting of a fine wire cloth of about
60 or 70 mesh. The drum is not entirely immersed in the mixture, so
that as it rotates the pulp forms a skin or thin sheet on the surface,
and the water passes away through the wire into the interior of the
hollow drum. The drum carries the thin sheet out of the box and above
the level of the mixture until it comes into contact with an endless
blanket or felt, which is pressed against that part of the drum not
immersed in the liquid.
By this means the thin sheet is transferred to the felt and carried
between squeezing rolls to the finishing rolls. The felt, carrying on
its upper surface the thin sheet of pulp, passes between two rolls,
usually 16 to 20 inches in diameter, the upper being made of wood and
the lower one of cast iron. The pulp adheres to the upper drum and the
felt passes round the lower drum back to the box containing the mixture
of pulp and water; the thin sheet is continuously wound on the upper
roll until a certain thickness is reached.
When this occurs the attendant removes the thick sheet by a dexterous
movement of a sharp stick across the face of the roll. The wet pulp at
this stage consists of 30 per cent. air-dry pulp and 70 per cent. of
water.
_Hydraulic Pressing._--The sheets taken from the wet press machine are
folded into a convenient shape and piled up, coarse pieces of sacking
being placed between the sheets. At stated intervals the piles are
submitted to pressure in hydraulic presses in order to remove further
quantities of water, which slowly drains away through the sacking. In
this way a mass of pulp in the form of thick folded sheets containing
50 per cent. of dry wood pulp is produced.
The pieces of sacking are taken out and the sheets put up in bales of
any required weight, usually 2 cwt. or 4 cwt.
THE MANUFACTURE OF CHEMICAL WOOD PULP.
Most vegetable fibres are converted into pulp by alkaline processes,
that is by digesting the raw material with caustic soda and similar
alkaline substances. Wood may be treated in two ways, one of which is
the ordinary soda process, and the other an acid treatment requiring
the use of sulphurous acid.
_Preparation of the Wood._--The logs of wood are cut up and barked
exactly as in the case of mechanical pulp. The short two-foot pieces
are then cut up into small flakes about one inch square and half an
inch thick by means of a machine known as a _chipper_. This is similar
in construction to a barker, consisting of a heavy iron disc rotating
at a high speed inside a stout cover. The disc revolves in a vertical
position, and three projecting knives slice up the logs into flakes.
For this purpose the disc is provided with three slots which radiate
from the centre towards the circumference for about 12 inches. The
knives can be adjusted so that they stand up through the slots and
above the surface of the disc to any required distance.
In order to ensure uniformity in the size of the chips, the practice
is frequently adopted of sifting the wood leaving the chipper. The
sieve is a large skeleton drum, the outer surface of which is made of a
coarse wire cloth capable of passing all pieces of the size mentioned.
Larger chips and pieces are retained in the drum as it revolves in a
horizontal position and only fall out on reaching the extreme end of
the machine.
_The Digesters._--The object of boiling the wood under pressure with
chemicals is to dissociate the valuable fibrous portion of the plant
from the resinous and non-fibrous portion. In this process the wood
loses half its weight, the yield of pulp being about 50 per cent., and
the remainder is dissolved out by the chemical solution. The conditions
of treatment are extremely varied in character, the quality of the pulp
produced varying in proportion.
The digesters are either spherical, cylindrical, or egg-shaped, being
constructed to revolve at a slow rate of speed, or fixed permanently
in an upright position. Spherical boilers are usually 9 or 10 feet in
diameter, the cylindrical digesters being 40 or 50 feet high and 12 or
15 feet diameter, the larger ones being capable of taking 20 tons of
wood for each operation.
[Illustration: FIG. 33.--Wood Pulp Digester, partly in elevation,
partly in section.]
For the alkaline process the interior of the digester does not require
any special treatment, but with the acid process the internal portion
of the boiler is carefully lined with a thick layer of acid-resisting
brick and cement.
The contents of the digester are heated by means of high-pressure
steam, which is blown direct into the mass or passed through a coil
lying at the bottom of the vessel. In the former case the steam is
condensed by the liquor, the volume of which is consequently increased,
while in the latter case the condensed steam is drawn off continuously
from the pipes. Each system has its own particular advantages.
_Different Kinds of Chemical Wood Pulp._--According to the method of
treatment so the quality of the pulp varies. The chemicals used, the
system of boiling, the temperature of digestion, the strength of the
solutions, the duration of the cooking period, and, last but not least,
the species of wood, are all determining factors in the value of the
ultimate product.
_Soda Pulp._--This is prepared by digesting wood with caustic soda in
revolving boilers for eight or ten hours at a pressure of 60 to 80 lbs.
_Sulphate Pulp._--Prepared by digesting the wood with a mixture of
caustic soda, sulphide of soda, and sulphate of soda.
_Sulphite Pulp._--The process most generally adopted for the
manufacture of wood pulp is the treatment of the material in
brick-lined digesters with bisulphite of lime for eight to nine hours
at a pressure of 80 lbs.
_Mitscherlich Pulp._--This is sulphite pulp prepared by digesting the
wood at a much lower temperature and for a longer period than the
ordinary sulphite. The steam is not blown direct into the mass of wood,
and the pressure seldom exceeds 45 or 50 lbs., the time of boiling
occupying 45 to 50 hours. So called from the name of the inventor.
_Sulphite Wood Pulp._--This name is given to pulp prepared by digesting
wood with solutions containing sulphurous acid, or salts of sulphurous
acid. The acid is produced by burning sulphur or certain ores
containing sulphur, such as copper or iron pyrites, in special ovens.
The most modern form of oven consists of a cylindrical cast-iron drum
revolving slowly in a horizontal position on suitable bearings. The
sulphur is thrown at intervals, or fed automatically, into the oven,
the amount of air being carefully regulated to avoid the formation of
sulphuric acid in the later stages of preparation. The sulphur is also
burnt in stationary ovens which consist of flat shallow closed trays.
[Illustration: FIG. 34.--View of ordinary Sulphur-burning Ovens.]
The hot sulphurous acid gas passes through pipes and is cooled, after
which it is brought into contact with water and lime for the production
of the bisulphite of lime. This is accomplished by one of two methods
as follows.
_Tower System._--The cool gas is drawn into high towers usually
built of wood, 7 or 8 feet diameter, which are filled with masses of
limestone. From tanks at the top of each tower a carefully regulated
quantity of water flows down upon the limestone and absorbs the
ascending column of gas, this being drawn into the tower from the
bottom. The limestone is simultaneously dissolved, and the liquid
which flows out from the pipes at the bottom of the tower consists of
lime dissolved in sulphurous acid, together with a certain proportion
of free sulphurous acid. This is generally known as a solution of
bisulphite of lime.
_Tank System._--The somewhat costly tower system has in many cases been
superseded by the use of a number of huge wooden vats, 10 to 12 feet
diameter and 8 to 10 feet high. These tanks are filled with water and
a known quantity of slaked lime. The gas is forced into the tanks by
pressure or drawn through by suction, and the conversion of the milk of
lime into bisulphite of lime proceeds automatically. In order to ensure
complete absorption the gas passes through the tanks in series, so that
the spent gases leaving the vats do not contain any appreciable amount
of sulphurous acid.
In order to obtain pulp of uniform quality it is necessary that the
liquor should be of constant composition. The formula differs in the
various mills according to the conditions which are found most suitable.
_Sulphite Digesters._--The almost universal form of boiler employed in
cooking wood by the sulphite process is a tall cylindrical vessel of
about 50 feet in height, and 14 to 15 feet internal diameter, lined
with acid-resisting brick.
This form of digester is capable of holding 20 tons of wood at one
charge, yielding 10 tons of finished pulp.
The chipped wood is discharged into the digesters from huge bins
erected just above the openings to the digesters, so that the latter
can be filled without any delay and the requisite quantity of sulphite
liquor added.
The manhole or cover is at once put on, securely fastened, and steam
turned on gradually until the pressure reaches 70 or 80 lbs., at which
pressure the cooking is steadily maintained. The progress of the
operation is watched and samples of the liquor drawn from the boiler
at intervals to be tested, so that the boiling may be stopped when the
results of the testing show the wood is sufficiently cooked.
There is no special difficulty in this operation, provided the
necessary conditions are observed. It is important that the wood should
be dry, and that the proportion of sulphite liquor per ton of dry wood
should be constant. If the wood happens to be wet, due allowance must
be made for the excess water and a somewhat stronger liquor used in
order to compensate for this. Other precautions of a similar character
are observed in order to minimise the danger of an insufficiently
cooked pulp.
_Washing._--When the pulp has been boiled, a process which generally
occupies seven or eight hours, the steam is shut off and the contents
of the boiler blown out into large vats known as blow-out tanks, the
pressure of steam remaining in the digester being sufficient to empty
the softened pulp in a few minutes. Much of the spent sulphite liquor,
now containing the dissolved resinous and non-fibrous portions of the
original wood, drains away from the mass in the tank, and then copious
supplies of clean water are added in order to wash out the residual
liquors which it is essential to remove.
Numerous other devices are employed to ensure the complete washing of
the boiled pulp.
_Screening._--The production of a high-class pulp necessitates proper
screening to eliminate coarse pieces of unboiled wood and the knots,
the latter not being softened completely. The methods adopted vary
according to requirements.
For uniform clean pulp that can be bleached easily the material from
the blow-out tanks is, after washing, mixed with large quantities of
water and run through sand traps, which consist of long shallow wide
boxes provided with slanting baffle-boards to retain knots and large
pieces of unsoftened wood, the pulp thus partially screened being
subsequently treated in the proper screening apparatus.
Sometimes the washed pulp is sent direct to the screens and the
well-boiled fibres sorted out by a system of graded screens, which
separate the completely isolated fibres from the bulk and retain the
larger pieces, these being broken down in a suitable engine and put
back on the screens.
The machinery employed for screening chemical pulp is identical with
that used for the treatment of mechanical wood pulp.
_Finishing._--The ordinary sulphite pulp is worked up into the form
of dry sheets for the market and not sent out in a wet state as the
mechanical wood. There are several practical disadvantages in preparing
the latter in a dry condition which do not, however, occur with
chemical pulp.
Hence the pulp after being screened is not pressed but submitted to a
different process. From the screens the mixture of pulp and water, the
latter being present in large quantity, is pumped into a concentrator,
or slusher, as it is termed, by means of which some of the water is
taken out.
The slusher consists of a wooden box divided into two compartments by
a vertical partition. In the larger compartment a hollow drum covered
with a fine wire cloth revolves, the construction and purpose of which
are precisely the same as that of the wet press machine used for
mechanical pulp.
As the drum revolves the pulp adheres to the outer surface, while
the water passes through the wire cloth. The drum is not completely
immersed in the mixture, so that the skin of pulp is brought out of the
water by the rotation of the drum. When this takes place the contact of
a wooden or felt covered roll which revolves on the top of the drum
causes the pulp to be transferred from the drum to the roll. The wet
pulp is continuously scraped off by an iron bar or _doctor_, as it is
called, resting on the surface of the roll, and it finally drops into
the second compartment of the slusher in a more concentrated form ready
for the drying machine.
_Drying._--The mass of wet pulp from the slusher is conveyed into a
circular reservoir or _stuff chest_, which serves to supply the machine
used for converting the pulp into dry sheets.
The machine is to all intents and purposes a Fourdrinier paper machine,
and the process is similar to that used for the manufacture of paper.
The pulp flows in a continuous stream on to a horizontal endless wire,
which carries it forward as a thin layer; the water drains through
the meshes of the wire, further quantities being removed by _suction
boxes_, which draw away the water by virtue of the vacuum produced by
special pumps. The wet sheet then passes between the _couch rolls_
which compress the pulp, squeezing out more water, and then through
_press rolls_, which finally give a firm adherent sheet of pulp
containing 70 per cent. of water. The sheet is dried by passing over
a number of steam heated cylinders, which cause all the moisture to
evaporate from the pulp. At the end of the machine the dry pulp is cut
up into sheets of any convenient size, and packed up in bales of two or
four cwts.
_Mitscherlich Sulphite Pulp._--This term is applied to sulphite wood
prepared by submitting the chipped wood to a comparatively low pressure
for a long period. The wood is placed in the stationary upright form of
digester with the requisite amount of liquor, and the heating produced
by the passage of steam through a leaden coil lying at the bottom of
the digester, so that the steam does not condense in the liquor but
in the coil, from which it is drawn off. The pressure seldom exceeds
45 lbs. but the duration of the cooking is thirty-six to forty-eight
hours. The boiler is not emptied under pressure, but the pulp is
discharged from the digester after the pressure has been lowered, and
the manhole taken off. The contents are usually shovelled out by the
workmen.
The pulp is carefully washed, screened and made up into wet sheets
on the ordinary wet press machine. This pulp is never dried on the
Fourdrinier like the common sulphite, as its special qualities can only
be preserved by the treatment described. This pulp is particularly
suitable for parchment papers, grease proofs and transparent papers.
_Soda Wood Pulp._--The chipped wood is boiled in stationary or
revolving digesters for eight or nine hours at a pressure of 70 or 80
lbs. A solution of caustic soda is employed, about 16 to 20 per cent.
of the weight of the wood being added to the contents of the digester.
Live steam is blown direct into the mass, and after the operation the
spent liquor is carefully kept for subsequent treatment. The pulp is
washed in such a manner that the amount of water actually used is kept
down to the smallest possible volume consistent with a complete removal
of soluble matters. This is done in order that the spent liquors may be
treated for the recovery of the soda.
_Recovery of Spent Liquors._--When wood is cooked by the soda and
sulphate processes the solutions containing the dissolved organic
matter from the wood can be evaporated, and the original chemical
recovered. In the case of soda pulp the method of treatment is as
follows: the spent liquors and the washings are evaporated by means of
a multiple effect vacuum apparatus to a thick syrup. The concentrated
liquor produced is then burnt in special furnaces, all the organic
matter being consumed, leaving a black mass which consists mainly
of carbonate of soda. The mass is washed with water to remove the
carbonate which is afterwards converted into caustic soda by being
boiled with lime.
[Illustration: FIG. 35.--Spruce Wood Pulp.]
The spent liquors from the sulphite process have no value, for they
cannot be recovered by this method. At present the whole of the sulphur
used and the organic matter dissolved from the wood is lost. This means
the loss of about 250 to 350 lbs. of sulphur and nearly 50 per cent. of
the weight of wood for every ton of pulp produced.
WOOD PULP; MICROSCOPIC FEATURES.
[Illustration: FIG. 36.--Mechanical Wood Pulp.]
Mechanical and chemical pulps are readily distinguished under the
microscope. The former consists of fibres of irregular shape and size,
mixed with a large proportion of structureless particles, all bearing
evidence of having been torn apart and separated by mechanical methods.
The chemical pulp, on the other hand, consists of fibres isolated by a
process which preserves them in perfect condition and form. The pulp
from the various woods can be differentiated by minute details in fibre
structure, some of the woods being determined from the presence of
characteristic cells.
The use of aniline sulphate can also be resorted to, and for
microscopic work the most useful reagent is a mixture of zinc chloride
and iodine. This produces an intense yellow colour with mechanical pulp
and a bluish colour with sulphite and other chemical wood pulps.
THE DAILY NEWSPAPER.
The newspapers of the present day are made almost exclusively of wood
pulp. The use of the latter material for paper-making has steadily
increased from the date of its introduction about A.D. 1870, when wood
pulp was imported into England in considerable quantities.
News and cheap printings consist of mechanical and chemical wood pulps
mixed in varying proportions determined chiefly by the price paid
for the finished paper. In some cases the proportion of mechanical
wood pulp is as much as 85 per cent., though the average composition
of a cheap wood paper is represented by the following proportions:
Mechanical pulp, 70 per cent.; sulphite pulp, 20 per cent.; loading, 10
per cent.
Some idea of the enormous quantity of material used for the daily
press may be judged from one or two examples. A certain popular weekly
newspaper having a circulation of one and a quarter million copies per
week requires every week 137 tons of paper produced from 170 tons of
wood. A popular halfpenny newspaper boasting a circulation of about
one-half million copies per day consumes 185 tons of paper manufactured
from 230 tons of wood, every week.
It is easy also from these facts to estimate the amount of timber
which must be cut down to supply the demand for newspapers and cheap
printings.
The manufacture of news calls for considerable skill and able
management, owing to the keen competition amongst the paper mills
devoted to this class of paper. The process as carried on in England is
as follows:--
The mechanical pulp, reaching the mill in the form of thick sheets
suitably packed up into bales, is first broken up again into moist
pulp. Various machines are used for this, such as Wurster's kneading
engine, Cornett's breaker, or some similar contrivance. An old potcher,
such as is used for the breaking and washing of rags, makes a good pulp
disintegrator. The broken pulp is discharged into beating engines in
any suitable or convenient manner and the right proportion of chemical
wood pulp added in the form of dry sheets. The beating process only
occupies thirty to forty minutes in the case of the common news, a
marked contrast to the eight or nine hours required by rags. China clay
is added to the contents of the beater, ten to twelve per cent. being
the general practice. This is followed by a measured quantity of rosin
size, and after thorough incorporation the size is precipitated upon
the fibres by means of alum.
In the commoner qualities of these papers the materials are added in
the dry state, but for finer grades of newspaper the china clay is
mixed with water, and carefully drained through a fine sieve before
use. The alum cake is also dissolved and treated in a similar manner in
order to keep out dirt and coarse particles likely to produce holes in
the paper.
The paper machine used for the manufacture of cheap printings is
constructed to produce as much as 100 to 180 tons of finished paper per
week, every detail being arranged for a large output at a very high
speed. In the modern machine it is possible to produce paper at the
rate of 450 to 550 feet per minute, the width of the sheet being from
120 to 160 inches.
Careful attention is paid to economy of every kind with regard to the
power required for driving the machine, the amount of steam consumed
in drying the paper, recovery of excess of fibre and china clay which
escapes from the machine wire, and similar details of a mechanical
order.
[Illustration: FIG. 37.--The Screens for removing Coarse Fibres from
Beaten Pulp.]
The beaten pulp, after being sized and coloured, is discharged into
huge circular brick tanks, or stuff chests, two of which are found
with each paper machine. The supply of pulp and water for the machine
is taken from one stuff chest while the second is being filled up
from the beating engines, in order to secure a mixture of constant
composition.
[Illustration: FIG. 38.--The Paper Machine (wet end showing wire).]
The pulp is pumped from the stuff chest into a small regulating box
placed above the machine wire, and this box is kept full of beaten
pulp so that the supply of pulp and water to the machine is perfectly
constant. The pulp, diluted with the proper quantity of _back-water_,
is carefully strained through rotary screens and allowed to flow
through a distributing box on to the machine wire, where it rapidly
forms a sheet of paper.
The excess of water, together with a certain proportion of fine
fibre and china clay, falls through the wire, and is caught below
in a shallow box, called the save-all. This _back-water_, as it is
called, is used over again for diluting the beaten pulp to the right
consistency, as already described.
The whole of the water obtained in this way is not all utilised in the
regulating box, and any surplus is pumped up continually into large
store tanks and used in the beating engines for breaking down the dry
pulp.
In many cases, where a large quantity of water is used on the machine,
special methods have to be adopted for the recovery of all the fibre
and clay, which would otherwise be lost, and there are many ingenious
systems in use whereby this saving is effected.
The most usual practice is to allow the excess of water, which contains
from 8 to 15 lbs. of suspended matter per thousand gallons, to flow
through a series of brick tanks at a slow rate of speed. The clay and
fibre settle to the bottom of the tanks, and the water passes away from
the last tank almost clear and free from fibre and loading.
The drying of the moist paper leaving the press rolls of the machine is
effected in the usual manner by means of drying cylinders. On account
of the great increase of speed at which the paper is produced, the
number of drying cylinders has also been increased, and at the present
time a machine of this description is provided with 28 or 32 cylinders,
the object being to dry the paper economically.
MECHANICAL WOOD PULP IN PAPER.
The presence of mechanical wood pulp in paper is detected by means of
several reagents, which produce a definite colour when applied to a
sheet of paper containing mechanical wood. The depth of colour obtained
indicates approximately the percentage present, but considerable
practice and experience is necessary to interpret the colour exactly. A
more reliable method of estimating the percentage of mechanical wood in
a paper is by microscopic examination.
The reagents which can be used are--
(1) _Nitric Acid._--This produces a brown stain on the paper, but it is
not a desirable reagent for ordinary office purposes.
(2) _Aniline Sulphate._--A solution of this is prepared by dissolving 5
parts of aniline sulphate in 100 parts of distilled water. When applied
to the surface of news a yellow coloration is produced, more or less
intense according to the amount of mechanical wood present. It can only
be used with white papers, or papers very slightly toned.
(3) _Phloroglucine._--This sensitive reagent, which gives a rose-pink
colour when brushed on to the surface of the paper, is prepared by
dissolving 4 grammes of phloroglucine in 100 c.c. of rectified spirits,
and adding to the mixture 50 c.c. of pure concentrated hydrochloric
acid.
There are several other aniline compounds which give colour reactions
of a similar character, but they are not often used. The phloroglucine
reagent fails as a test for mechanical wood in papers which have
been dyed with certain aniline colours, for example, metanil yellow.
Paper which has been coloured with this dye will, when moistened with
the phloroglucine reagent, give an intense pink colour, even if no
mechanical wood is present. This is due to the fact that the dye itself
is acted upon by the hydrochloric acid in the test reagent. The same
colour is produced on the paper with hydrochloric acid _per se_.
There is little difficulty in distinguishing between the colour arising
from the presence of such a dye, because the effect is instantaneous,
whereas the coloration due to mechanical wood develops gradually.
Moreover, the reaction due to the presence of metanil yellow gives a
perfectly even coloured surface, whereas with mechanical wood pulp the
fibres appear to be more deeply stained than the body of the paper.
_Output of a Paper Machine._--The quantity of paper which can be
produced on the paper machine is readily calculated from the following
data:--
Speed of machine in feet per minute _F_
Nett deckle width in inches _D_
Width of sheet of paper in inches _W_
Length of sheet of paper in inches _L_
Number of sheets in ream _S_
Weight of paper per ream _R_
The general formula for the output of paper per hour is
720 × _F_ × _D_ × _R_
Output in lbs. per hour = -----------------------.
_S_ × _L_ × _W_
When the number of sheets in the ream is 480, this formula simplifies to
1½ × _R_ × _F_ × _D_
Output in lbs. per hour = --------------------.
_L_ × _W_
The term "nett deckle width" applies to the width of the trimmed
finished paper at the end of the machine. The formula takes no account
of the allowance required for trimming edges. In most cases the deckle
width of the machine is arranged so that the paper is cut into strips
of equal width when leaving the calenders, _e.g._, a deckle of 80
inches will give 4 sheets, each 20 inches wide.
[Illustration: FIG. 39.--Paper Machine showing Wire, Press Rolls, and
Drying Cylinders.]
The method by which the general formula is obtained may be explained by
an example.
What is the output of a machine having a speed of 100 feet per minute,
with an 80-inch deckle, producing a sheet of paper 20 inches by 30
inches, weighing 30 lbs. per ream of 480 sheets?
The machine produces every minute a sheet of paper 100 feet long and 80
inches wide.
Hence output per minute in square inches
= 12 × 100 × 80.
Output per hour in square inches
= 60 × 12 × 100 × 80.
Now each (20 × 30 × 480) square inches is area of one ream.
Output of paper per hour in reams
60 × 12 × 100 × 80
= ------------------.
480 × 30 × 20
Output of paper per hour in lbs.
720 × 100 × 80 × 30
= -------------------
480 × 30 × 20
= 600 lbs.
The general formula may be applied for the purpose of calculating the
speed at which the machine must be driven.
_Example._--A machine with 75-inch deckle is required to produce
6 cwts. per hour of a paper 25 inches by 18 inches (500 sheets),
weighing 19 lbs. to the ream. At what speed is the machine to be driven?
Output in lbs. per hour
720 × _F_ × _D_ × _R_
= ---------------------
_S_ × _L_ × _W_
720 × F × 75 × 19
672 = -----------------
500 × 18 × 25
_F_ = 148 feet per minute.