differences, but so far as has been investigated, not such as to
constitute a distinct advantage for one or {75} another. One feature
of these processes, to which but little attention has hitherto been
paid, is that of the utilisation of their bye-products. These bodies
are, as already indicated, obtained in a condition not materially
different from that of the parent substance, the non-cellulose
constituents of the wood. One important property of these bodies, and
one capable of considerable application, has been made the basis of
a patent by the authors. When a solution of gelatin is added to the
liquors in which wood has been treated by any of the bisulphite or
sulphurous acid processes, a body is precipitated in the form of a
caoutchouc-like mass which can be dissolved in neutral sodium sulphite
solution, or in weakly alkaline solutions, and can be reprecipitated on
addition of alum to the solution. The substance consists of a compound
of the gelatin with the non-cellulose constituents resembling, but
not identical with, as has been supposed, the precipitate obtained
by adding tannic acid to a solution of gelatin. The solution of this
substance in sodium sulphite can be applied to many of the purposes to
which gelatin itself is ordinarily applied.
A serious objection to all processes involving the use of sulphurous
acid or acid sulphite, and one which has to some extent deterred
paper-makers from adopting them, is the necessity of using a boiler
lined with lead. In addition to the fact that such boilers are heavy
and costly, they are objectionable on account of the difficulty and
expense of keeping them in thorough repair. This difficulty arises from
the fact that, owing to the different co-efficients of expansion of
lead and iron, the lead lining originally placed in contact with the
iron or steel shell of the boiler, becoming detached on heating, swells
out in various places, which are thus rendered weaker and liable to
fracture. This is especially the case if, in the course of lining the
boiler, any air has been allowed to remain between the two linings.
Various expedients have been proposed to obviate these defects but with
only partial success. Graham (Eng. Pat. No. 5367, 1883) coats the plate
of his boiler by pouring molten lead upon the perfectly clean surface
which has been {76} previously prepared with zinc chloride. Sometimes
minute holes are drilled in the iron plate to allow the escape of air.
Notwithstanding all the attempts that have been made, the use of a
lead-lined boiler is a serious objection, and any process which will
fulfil the conditions on which the bi-sulphite processes depend, and
which at the same time admits of the use of the ordinary iron boilers,
is an obvious desideratum. Such a process is that patented by one of
the authors (Cross, No. 4984, 1880), claiming the use of solutions
of the neutral sulphites with or without the further addition of an
alkali. This process is very suitable for the treatment of fibres such
as straw, jute, &c., but not economically applicable to the preparation
of wood pulp on account of the cost of chemicals. The preservative
effect of the sulphite is shown by the largely increased yield of pulp.
The chief advantages of the various bi-sulphite processes for the
preparation of wood pulp over the old soda processes are, (1) the
increased yield of fibre; (2) the preservation of its original
strength, and (3), the economy in chemicals.
It is well known that cellulose, even in its most stable forms, is
attacked more or less by hot solutions of the caustic alkalis, and
is more or less profoundly modified according to the strength of the
solution and the temperature at which it is employed; while on the
other hand the solutions of the bisulphites affect it but little. Thus,
the authors have exposed jute cellulose, which is of the same nature
as that obtained from wood, to the action of magnesium bisulphite
under the conditions which obtain in the Ekman process, without its
undergoing sensible loss of weight, and further, they have determined
the amount of cellulose in freshly cut grasses, in which it is but
imperfectly elaborated, and therefore very susceptible of modification,
both before and after treatment by the process, and in each case the
percentage obtained was the same.
There is therefore no doubt that in the bisulphite processes, even
the structurally delicate forms of cellulose are conserved. As before
indicated, the influence of this upon the yield of cellulose is of
course very marked; the yield from white pine, {77} by the soda
process, being approximately 33 per cent., whereas by the Ekman process
it is from 45–50 per cent. It is important to remember that this
increased yield of cellulose cannot fail to affect the quality, and
therefore the paper-making properties of the pulp. Papers made entirely
by the bisulphite processes have a strongly marked individuality, being
distinguished by their hardness and transparency. In regard to the
bleaching of the pulp, it will be found that some practical experience
is required for the control of the operation, so as to produce uniform
results.
Although the pulp produced by these processes is of a very light-grey
colour, so light indeed that they may be used for certain low classes
of white paper without further treatment, yet to obtain a pure white
colour almost as much bleaching powder is required as in the case of
soda pulps, although the latter are much darker in colour. The chief
points to be observed are (1) the complete removal of the sulphites
from the pulp before the addition of bleaching liquor, which would
otherwise be ineffectively consumed; (2) the avoidance, in washing, of
waters containing iron, which would combine with certain constituents
of the pulp to form dark-coloured bodies.
The following Table, compiled from Dr. Hugo Müller’s ‘Pflanzenfaser,’
is of interest, as indicating the probable yield of pulp from different
woods, compared with the amounts obtained in actual practice by the
Watt and Burgess (soda) process:—
----------+--------+----------+--------+---------+------------+------------
| | Water | | Lignin, | | Pulp—Watt
| Water. | Extract. | Resin. | &c. | Cellulose. |and Burgess
| | | | | | Process.
+--------+----------+--------+---------+------------+------------
Birch | 12·48 | 2·65 | 1·14 | 28·21 | 55·52 | 40·00
Pine | 12·87 | 4·05 | 1·63 | 28·18 | 53·27 | 34·70
Oak | 13·12 | 12·20 | 0·91 | 34·30 | 39·47 | 20·60
Chestnut | 12·03 | 5·41 | 1·10 | 28·82 | 52·64 | 25·10
----------+--------+----------+--------+---------+------------+------------
The following Table, which gives a concise account of the various
typical processes that have been employed for the preparation of wood
pulp, will be readily understood in connection with the theoretical
considerations previously discussed:— {78}
VIEW OF THE CHEMICAL PROCESSES FOR DISINTEGRATING WOOD.
_Aqueous Alkalis._ | _Water._ | _Aqueous Acids._
| |
Hydrolysis, aided by alkali | Hydrolysis, aided by acids | (_a_) Oxidising and hydrolytic—
directly, also indirectly by | formed from the wood. (Fry, |
combination with products of | 1867) | Nitric acid. (Coupier and
resolution. (Watt and Burgess, | | Mellier, 1852. Barre and
1853. Houghton, 1857.) | Limit of action determined by | Blondel, 1861.)
| reversal of hydrolysis, i.e. |
| dehydration aided by oxidation. | Nitro-hydrochloric. (Orioli,
| | 1865.)
| |
| | (_b_) Simply hydrolytic—
| |
| | Hydrochloric acid. (Bachet and
| | Machard, 1864.)
| |
| | (_c_) Reducing and hydrolytic—
| |
| | Sulphurous acid. (Tilghmann,
| | 1866. Pictet, 1882.)
| |
_Solutions of Alkaline | | _Bisulphites._
Sulphides._ | |
| | Hydrolysis, aided primarily
Hydrolysis, aided by alkaline | | by sulphurous acid, and
bases directly and indirectly | | secondarily by combination of
by combination with products. | | products with bisulphites, also
| | by prevention of oxidation.
Reversal of hydrolysis lessened | _Water, together with neutral | (Tilghmann, 1866. Mitscherlich,
by presence of reducing agent. | Sulphites._ | 1874. Ekman, 1881. Francke,
| | 1881. Graham, 1882.)
(Both categories: Jullion, | Simple hydrolysis— |
1855. Blitz, 1883. Dahl, | |
1884.) | Products removed from sphere of |
| action by combination with base |
| and sulphite. (Cross, 1880.) |
| |
| Oxidation prevented by presence |
| of sulphite. |
{79}
CHAPTER VI.
SPECIAL TREATMENT OF VARIOUS FIBRES; BOILERS, BOILING PROCESSES, ETC.
We have already, in Chapter V., discussed in a general way the
principles upon which the treatment of the various fibres should be
based; we now proceed to consider each fibre in detail, giving at the
same time such information as is necessary regarding the preliminary
treatment of the various fibres and describing the forms of apparatus
in which these operations are conducted. We shall consider the fibres
in the order of their simplicity of treatment.
_Rags_ (Linen and Cotton).—The treatment necessary for rags differs
largely with the quality and, of course, with the kind of paper
for which they are intended. These different qualities are known
in the trade by different names and marks, such for example as the
following:—New linen pieces, new cotton pieces, superfines, dark fines,
grey linen, sailcloth, seconds, thirds, &c., &c.
The two former consist of the cuttings produced in the manufacture of
various garments, &c.; not having been worn, they are usually free
from dirt, and are, in fact, if bleached, nearly pure cellulose,
containing only the starch and other sizing material which has been
added in the process of finishing the goods. They may, of course,
contain considerable quantities of china clay or other loading
material. In purchasing rags, therefore, regard should be had to the
probable presence of these bodies. Such rags require only a very slight
treatment; in fact, for certain classes of paper they might be used
without any preparation. If, as is sometimes the case, the rags are
unbleached, a rather more severe boiling is necessary. {80}
[Illustration: FIG. 16.]
The greater part of the rags used in paper-making, however, consist of
the residual portions of garments, household linen, &c., which vary in
quality from clean, almost unused portions, to the very foulest sorts;
the latter require a very drastic treatment. The first thing to be done
with the rags is to “sort” and cut them into convenient pieces. This
is usually done by women, who stand at tables furnished with broad
knives firmly fixed into them, with the backs towards the worker, and
inclined at a slight angle. Near to the women are placed a number of
boxes, corresponding with the number of qualities of rags, lined at
the bottom with coarse wire gauze, into which the different sorts are
put. The distinctions made are purely arbitrary, but as a general rule
the rags are sorted with special reference to their colour and the
material of which they are composed. The coloured rags may be allowed
to accumulate, and then made into a coloured paper. During the process
of cutting, all hard substances, {81} such as buttons, pieces of iron,
&c., are carefully removed. The rags are cut into pieces of from two to
five inches square. In some mills machines are used for cutting.
Fig. 16 shows the construction of a machine that may be used either
for rags or rope. The material is passed into the machine along the
table A, where it passes between the stationary knife C and the knives
B, fixed in the revolving drum D. The cut rags fall into a receptacle
placed underneath the drum.
Notwithstanding the extra expense of cutting by hand, it is
nevertheless preferred by many, especially for the finer grades of
paper. One reason for this is to be found in the fact that more perfect
sorting and removal of impurities can be effected. It is moreover said
that less waste of fibre occurs in the subsequent operations.
[Illustration: FIG. 17.]
The next process which the rags undergo is that of removing all loose
extraneous matter. This may be done in a machine such as is shown
in Fig. 17. The rags are fed continuously by the endless travelling
platform A into the willow. This consists of two wrought-iron drums, B
B, furnished with wrought-iron teeth C, which when the drums revolve
pass rapidly near to the stationary teeth fixed into the cast-iron
framework of the willow. The sides are covered in with cast-iron doors,
and the top is covered over {82} with sheet iron. Underneath the drums
is a grating for the escape of dust.
The rags, having been thoroughly beaten by the teeth of the drums, pass
into the duster D, consisting of a kind of hollow cylinder E, made of
strong iron bars securely fastened to the circular ends F, the bars
being covered with wire cloth or perforated zinc. It is made to revolve
almost horizontally, a slight dip being given to it in order that the
rags may be carried forward to the lower end. The bars or skeleton of
the cylinder are furnished with a number of teeth securely bolted on.
The whole is enclosed in a strong wooden box in which it revolves, and
which serves to collect the dust passing through the wire cloth. The
cylinder E is driven by the gearing G. This combined willow and duster
may be used for dusting and cleaning not only rags, but almost any
other kind of fibre. The willow and duster sometimes form two separate
machines.
The cleaned and dusted rags are now ready to be boiled, although it
is the practice in some mills to give them a preliminary washing with
water.
The boiling may be conducted either in spherical or cylindrical
boilers, or in the ordinary vomiting boilers described under Esparto.
(See p. 92.) In Fig. 18 is shown, part in section and part in
elevation, a spherical rag-boiler, as manufactured by Messrs. G. & W.
Bertram.
This boiler, of from 8 to 9 feet diameter, is supported by means of the
hollow journals A on the standards B, and is made to revolve by means
of the gearing C.
Steam enters by the pipe D, which is fitted with a safety-valve E, and
vacuum valve F. The steam on entering the boiler is distributed by
means of the “baffle-plate” G. Lye enters by the pipe H. The boiler is
fitted with two doors I, wherewith to fill and empty; the waste lye is
run off by the cock J. L is a small blow-through cock. The false bottom
K prevents the rags from choking up the cock and also serves to drain
off the waste liquor.
[Illustration: FIG. 18.]
The alkali employed may be either caustic lime, caustic {83} soda,
sodium carbonate, or a mixture of the latter and lime, which is of
course equivalent to using caustic soda. The proportion of alkali
depends upon so many considerations that it is quite impossible to
give exact information on this point. In the case of caustic soda, a
quantity equal to from 5–10 per cent. on the rags may be taken to be
a fair average. If lime be used it should be slaked with water, made
into a thin milk and carefully filtered through fine wire cloth to keep
back the particles of sand, coal, &c., which lime is always liable to
contain. From 5–10 per cent. may be used. The amount of lime actually
dissolved in the water is relatively small (1·3 grms. per litre);
the portion in solution however, rapidly combines with the grease,
dirt, and colouring matter of the rags and forms with them insoluble
compounds, a fresh portion of lime being at the same time dissolved.
This formation of insoluble compounds constitutes an important
objection to the use of lime, as they are liable {84} to remain to
some extent fixed in the rags and are with great difficulty removed
by washing. For this reason, therefore, the more soluble alkali is to
be preferred. Moreover, the lime sometimes tends to exert a hardening
effect upon the cellulose. Notwithstanding these objections, lime is
used by some paper-makers in preference to soda. In making choice of
the chemical for boiling, much depends on the quality of the rags
and the nature of the paper for which they are intended, so that no
definite rules can be given.
The time of boiling varies from 2 to 6 hours, according to the quality
of rags, the chemical employed, and the pressure. The use of very high
pressures should be avoided as far as possible, as there is a danger,
owing to the correspondingly high temperature, of _fixing_ the dirt and
colouring matters instead of dissolving them.
The quantity of water should be kept as low as may be, in order to have
as strong a solution of alkali as possible; this effects a saving both
in the time of boiling, and in the alkali. This is of great importance
where it is necessary to evaporate the whole of the waste liquors. It
should also be remembered that a certain amount of water is always
formed by condensation in all boilers in which live steam is used. On
the other hand, if too little water be added, the rags are liable to
become “burned” and the fibre therefore weakened. During the operation
the boiler should be made to revolve slowly, in order to produce
thorough circulation of the liquor.
The boiling being completed, the pressure is allowed to fall, either
by cooling or by blowing off from a cock usually provided for that
purpose, and the liquor allowed to collect at the bottom of the boiler.
It is then run off by the cock J (Fig. 18), and the rags drained as
much as possible. Water is then run in to give the rags a preliminary
washing. If time permits the steam may be turned in and the operation
assisted. After again draining, the rags are withdrawn from the
boiler into any suitable receptacle. A convenient form is that of a
rectangular iron box on wheels, which can be readily transferred from
one part of the mill to another. {85}
The next process is that of washing. This is usually performed in a
washer or breaker, the construction of which is shown in Fig. 19.
[Illustration: FIG. 19.]
It consists essentially of a rectangular vessel with rounded ends,
in the centre of which, but not extending the whole {86} way, is
a partition B, known as the “mid-feather.” The roll A, which is
furnished with a number of steel knives G, and driven from the wheel H,
revolves in one of the compartments formed by the mid-feather. In this
compartment the floor is inclined in such a way as to bring the pulp
well under the roll, as shown by the dotted line D. Immediately under
the roll is what is called the “bed-plate,” the end of which is seen
at I, extending up to the mid-feather, and fitted with knives similar
to those in the roll A. The arrangement of the knives, both in the
bed-plate and the roll, is similar to that given in Figs. 33 and 34.
The distance between the roll and the bed-plate can be varied at will
by means of the handle E, which is so arranged as to raise both ends of
the roll simultaneously. In those breakers of an older pattern, one end
only of the roll was raised, and thus the knives became worn unequally.
After passing between the roll and the bed-plate, the pulp flows down
the “back-fall” D′, and finds its way round to the other side of the
mid-feather. On the inclined part of the floor, and immediately in
front of the bed-plate I, a small depression is made, covered with
an iron grating, for the purpose of catching buttons, small pieces
of stone, and other heavy substances that may have found their way
into the rags. There is generally a similar grating with rather
finer openings on the other side of the mid-feather. The engine is
constructed of iron, generally made in one casting.
The dirty water from the rags is removed by the “drum-washer” C. It is
divided into compartments by the partitions shown by the dotted line
_c_. The centre of the drum is formed of a conical tube, the narrow end
of which is towards the mid-feather. The ends of the drum are generally
made of mahogany, as this is found to stand the action of alkali better
than any other wood. The periphery is covered with fine copper or
brass wire-cloth, laid on to a backing of a much coarser material. An
improved form of backing has been introduced lately, which is much more
durable than wire. It is formed of brass cut into the form shown in
{87} Fig. 20. The drum can be raised or lowered by the small wheel F,
and it is driven by a belt on the shaft that bears the roll.
[Illustration: FIG. 20.]
The wash-water passes through the wire cloth into the compartments
formed by the partitions _c_, and finding its way down to the narrow
end of the inner conical tube, flows out through the side of the drum
into a trough which is placed across the washer to receive it. Or it
may, as shown in the drawing, be conducted through the mid-feather
itself, which is made hollow at this part for the purpose.
Another form of drum-washer, called the siphon-washer, is sometimes
used. Its construction will be understood by reference to Figs. 21 and
22.
The drum is simply a hollow cylinder of wire cloth, the ends of which
are formed of wood. Inside the cylinder is the siphon tube A, into
which the water passing through the wire cloth flows. The continuation
of the siphon tube B (Fig. 22) is made of flexible tubing. The action
of the siphon is commenced by filling it with water through the cock
C (Fig. 22). The water then flows in the direction indicated by the
arrows. The tube A is fixed, and passes through a hollow journal fitted
on to the end of the drum. {88} The other end is connected with the
rod E, on which the driving pulley D is placed.
[Illustration: FIG. 21.]
[Illustration: FIG. 22.]
The water passing through the wire cloth on the drum carries with it
a certain amount of fibre which, unless special precautions are taken
(see “Save-all,” p. 154) is lost. The amount is considerable in the
case of weak rags, and care should therefore be taken that the washing
is not prolonged more than is consistent with proper cleansing, and
that the pulp should not be too much broken up at this stage.
The washed and broken pulp goes by the name of “half-stuff.” {89}
The following is the usual plan of treating rags in the washer:—It
is first half-filled with water, and the rags from the boiler put in
gradually until nearly filled. Water is now allowed to flow in at the
opposite end to where the drum-washer is placed. This, by the action of
the roll, mixes thoroughly with the pulp and extracts all the soluble
matter, and also carries with it fine insoluble impurities.
The action of the knives in the roll on the rags passing between it
and the knives in the bed-plate serves to break them up and thoroughly
disintegrate them. The dirty water then passes away through the
drum-washer, the stream of pure water being regulated so as to keep the
level constant. This is continued until only pure water passes away.
The supply is then stopped, the washer still being kept in action. As
the level falls the drum is lowered by means of the handle F. When
sufficiently drained the pulp is discharged through a valve in the
bottom of the engine. It is now ready to be bleached. This may be done
in separate engines called “potchers,” somewhat resembling the breaker
or washer already described, or it may be done in the breaker itself.
The process of bleaching will be described in Chapter VII.
Occasionally, the bleaching process is conducted in the “beater”
itself, but this is not to be recommended.
_Esparto._—This fibre, on account of its high percentage of
non-cellulose constituents, requires a large amount of soda to resolve
it; on the other hand, being of the nature of a pecto-cellulose, the
process of disintegration may be conducted at a low pressure; in
fact, it is the practice at some mills to boil in open vessels, in
which case, however, a larger amount of soda is required. As has been
already stated, it may be taken as a general rule for all fibres, that
within certain limits the higher the pressure employed the less soda
is required. The quantity necessary also varies with the district
from which the grass comes. Caustic soda is the chemical invariably
employed; lime, on account of its forming insoluble compounds with
the {90} non-cellulose portion of the grass, is inapplicable. Not
only does the amount of soda depend upon the pressure, but it also
depends to a considerable extent on the form of boiler employed. The
use of rotary boilers is objectionable, as the esparto tends to collect
together into compact masses, which are with difficulty penetrated by
the liquor. It is therefore the almost universal practice to employ
stationary boilers.
[Illustration: FIG. 23.]
The first treatment that the esparto undergoes is that of “picking.”
The bands of the bales in which esparto is packed, generally by
hydraulic pressure, are cut, and the grass is spread out on tables by
women, who carefully remove such impurities as weeds, root-ends, &c.,
which from their nature are with great difficulty boiled and bleached,
and which if not removed would be liable to appear in the finished
paper as dark-coloured specks, technically known as “sheave.” This
treatment is called “dry-picking,” in contradistinction to a subsequent
process, known as “wet-picking.” A portion of the table is covered
with coarse wire gauze, through which when the grass is spread over it
loose particles of sand, dirt, &c., escape. The cleaning of esparto
can be much better accomplished by means of machinery. Fig. 23 is an
elevation of a machine manufactured for the purpose by Messrs. Masson,
Scott and Bertram. The grass, which may be in the form of sheaves, as
taken from the bales, is put in by the hopper A. It passes thence to
a conical {91} drum made of steel bars placed very close together,
driven rapidly by the shaft B. It is provided with five rows of teeth.
Fixed to the sides of the willow there is also a row of stationary
teeth. The grass is thoroughly broken up and dusted by the action of
the teeth, and is transferred to the wide end of the willow at C, where
it is carried forward on the travelling tables D. During its passage
along the tables it can be examined and picked by women standing on the
platforms E. The dust and dirt passing through the steel bars of the
drum are drawn away by a fan through the pipe F.
This machine is also adapted so as to carry the cleaned grass to the
boiler-house. It is taken forward by means of travelling rakes over
the tops of a series of boilers, any one of which can be furnished by
simply opening a door corresponding in position with the lid of the
boiler.
Boiling.—The ordinary form of boiler is shown in Fig. 24. It is
known as a vomiting boiler. The grass is put in by the door E, which
is hinged, and is counterbalanced by the weights L. It is securely
fastened down by the screws F. Steam enters by the inner pipe A, which
dips a little below the perforated false bottom B. Surrounding the
steam-pipe is a wider pipe C, open at the top, which is made slightly
trumpet-shaped; also open at the bottom, where it ends in a kind of
shoulder, on which the false bottom rests. The lower part has two or
more openings G cut away, through which the liquor can freely pass.
In order to get as large a charge into the boiler as possible, steam is
turned on while furnishing, and a quantity of caustic soda lye is also
run in, which has the effect of softening the grass and making it more
compact. This is continued until the boiler is well filled.
The action of the boiler is as follows:—The steam passing through the
pipe A heats the liquor that has drained from the grass through the
perforations in the false bottom, and forcing it up the wide pipe C,
causes it to strike against the dome or bonnet D, and distribute itself
again over the grass. {92} This is technically known as “vomiting.” In
this way a constant circulation of liquor is kept up.
[Illustration: FIG. 24.]
The boiler is emptied by the door H and the liquor is run off by
the cock I. The boilers are usually supplied with a safety valve E,
and also with a pipe for letting off the steam when the boiling is
finished. The boiling generally takes from four to six hours. The
quantity of soda necessary depends upon the nature of the grass,
Spanish requiring less than African. {93}
Routledge, to whom the introduction of esparto as a paper-making
material is due, gives 10 per cent. as the proper quantity.
The pressures employed vary from 5 lb. to 45 lb. There has been a
growing tendency of late to employ the higher pressures for reasons
already stated.
[Illustration: FIG. 25.]
Of late years great improvements in the form of the boiler have been
introduced. Fig. 25 is an illustration of Roeckner’s Patent Boiler. The
vomit pipe is outside the boiler. Steam entering by the cock D, forces
the liquor up the vertical vomit pipe and distributes it over the
grass. The pipe K is used for heating the liquor at the commencement of
the operation by means of waste steam. F F are gauges for indicating
the height of the liquor. The grass is put in by the opening G. The
pipes A B and C are for the supply of {94} steam, strong lye and
water. The boiled grass is discharged by the circular door E.
The boiler holds three tons of grass and the boiling is completed in
about 2 1/2 hours, the pressure being about 35–40 lb. per square
inch. It is claimed that by its use a saving both of time and soda is
effected.
[Illustration: FIG. 26.]
Sinclair’s Patent Boiler is shown in Fig. 26. The vomit pipes, of which
there are two, are made of thin steel plates {95} riveted to opposite
sides of the boiler. The liquor drains through the perforated false
bottom, and is then forced up the vomit pipes above the perforated
plates through which it is distributed over the grass in a number of
fine jets. The boiler is charged by the door _e_ and emptied by _j_.
The small cock _a_ is used as a blow-through cock; the opening _b_ is
used for blowing-off steam when emptying the boiler; _c_ for letting in
caustic soda lye; _d_ for water. Steam enters by the small branch of
the T pipe at the bottom of the boiler, the other, _g_ being used for
running off the waste liquor. A boiler holds from two to three tons of
grass, and a boiling is completed in about two hours at a pressure of
40–50 lb. per square inch.
The two forms of boiler above described possess the obvious advantage
of having the whole or nearly the whole of the interior available for
holding grass; the boilers can therefore be made to hold more, and the
boiling is moreover much more evenly accomplished.
Washing.—The boiling being completed, the steam is allowed to escape
and the liquor to collect at the bottom of the boiler where it is run
away by the pipes placed at the bottom for the purpose. Water is then
run in and steam turned on for a short time; this is also run off,
and the grass drained as completely as possible. The boiled grass is
then emptied into trucks and taken away to the washing engines. These
resemble those already described under rags.
During the process of washing, a certain amount of the shorter fibres
find their way through the meshes of the wire-cloth. In addition
to this a large proportion of the cellular tissue surrounding the
fibro-vascular bundles (see Fig. 12, p. 57) is carried away. If the
wash-water be examined under the microscope, it will be seen to consist
largely of this cellular matter. Though this entails a certain loss
of cellulose, its removal is in other respects advantageous, as it
is possessed of hardly any “felting” properties, and it is moreover
exceedingly difficult to bleach.
The amount of fibre actually obtained in practice is but little below
that contained in the grass. A certain loss is {96} inevitable, but
this probably does not exceed 1 or 2 per cent.
The percentage of cellulose in esparto is given on p. 57. The
following numbers obtained by the authors are somewhat higher. They
are calculated on the absolutely dry fibre; those mentioned are on the
_air-dry_ samples:—
Cellulose per cent.
Spanish 58·0
Tripoli 46·3
Arzew 52·0
Oran 49·6
There is no doubt that considerable differences occur even between
different specimens of the same kind of grass.
It is the practice in some mills to wash the grass in a series of tanks
connected together in the same way as the lixiviating tanks of an
alkali works. They are so arranged that pure water flows in at one end,
passes through fresh lots of grass in succession, and issues at the
further end highly charged with the soluble products of the grass. By
such an arrangement the grass can be washed without any loss of fibre,
and with a minimum quantity of water. This latter feature is of great
importance in mills where it is necessary to evaporate the whole of the
waste liquors from the esparto, as they are then obtained in a very
concentrated form. Even with such an arrangement it is advisable to
give the grass a short final treatment in the washing engine.
The washing having been completed, and the esparto having been broken
up into “half-stuff,” it is now ready to be bleached (see Chap. VII.).
The presse-pâte system, originally adopted for the treatment of straw,
has of late years been extensively applied to esparto.
The presse-pâte consists of the wet end of a paper machine, and is
furnished with sand-tables and strainers. The pulp is allowed to flow
on to the wire cloth, so as to form a thick web of pulp. The bulk of
the water passes away through the wire cloth; a further quantity is
removed by the {97} vacuum-boxes and couch-rolls. The pulp, containing
from 50 to 60 per cent. of water, is wound round an iron rod until a
sufficiently large roll is formed.
The advantages of the presse-pâte system are the possibility of the
removal of dirt and unboiled portions by means of the sand-tables and
strainers, and the very complete washing and removal of the products of
the action of bleaching powder.
It also enables manufacturers to dispense with the somewhat costly
methods of dry and wet picking.
The presse-pâte system can also be applied to the unbleached pulp.
It may be interesting at this point to say something about the
substances which are removed from the esparto by the caustic soda. On
referring to p. 57 it will be seen that the original grass contains
nearly half its weight of extractive matter, the removal of which has
to be effected. Only a small proportion of this is extractive matter
in the ordinary sense of the term, i.e. that can be extracted by
the usual solvents; the remainder is intimately combined with the
cellulose. The action of the caustic soda is to resolve these bodies,
the cellulose remaining behind, and the other constituents being
dissolved as resinous bodies by the soda. A certain proportion of the
mineral constituents, notably the silica, dissolves in the lye, the
latter forming silicate of soda.
On neutralizing the liquor with an acid, the bulk of the dissolved
constituents is thrown down as a dark brown resinous mass. If this
crude product be purified, it is found to consist of a definite body
having the formula C_{21}H_{24}O_{8}. By the action of chlorine on
this resin a bright yellow chlorine compound is formed, resembling the
compound from jute described on p. 18. If the resin be treated with
concentrated nitric acid, a bright yellow body is formed, which forms
definite compounds with bases, and which has the property of dyeing
animal fibres a bright orange colour. In addition to this body, a
yellowish-white wax can be isolated. It sometimes happens that this wax
is but imperfectly dissolved in the caustic lye; if the waste liquor
be allowed {98} to stand, the wax is occasionally found to collect in
small quantities on the surface.
Various attempts have been made from time to time to remove from the
waste liquors the soluble matter derived from the esparto by the
addition of lime. This throws it down as a very voluminous precipitate,
exceeding difficult of filtration. The filtrate contains only caustic
soda. The difficulties of removal of the precipitate, however, are
almost insurmountable. The usual method of disposing of the liquor
is to evaporate it to dryness and ignite it, as will be subsequently
described under the head of “Soda Recovery,” Chapter XII.
_Straw._—Straw very closely resembles esparto in its chemical
constitution; it is, however, more highly lignified, and for this
reason (see p. 63) requires a more drastic treatment for its
resolution. It is therefore usual to treat straw with rather larger
quantities of caustic, and generally at high pressures. It is possible,
however, to resolve it at 5–10 lbs. pressure, but the resulting
pulp is more difficult to bleach, and can only be used for inferior
kinds of paper. In treating straw for use in high-class paper, it is
necessary to boil under such conditions that even the knots, which are
much less readily attacked, shall be so far disintegrated that they
may be bleached with a moderate quantity of bleaching powder. Such a
treatment, on the other hand, involves a diminished yield of pulp, and
at the same time a somewhat weakened fibre.
The kinds of straw in general use are wheat, oat, rye, and barley; the
two first constituting the bulk of the raw material, at least in this
country. Some idea of the composition of straw may be gathered from the
analyses given on p. 59.
It will be seen that the amount of cellulose is quite as high as in
esparto, but for the reasons above given and from the fact that a
large proportion of the cellulose consists of cellular tissue (see
Figs. 12 and 13), which is easily attacked by soda and readily passes
through the meshes of the {99} drum-washers and the wire-cloth of the
presse-pâte and paper machine, the yield obtained in practice falls
considerably below that from esparto.
In addition to the numbers already quoted on p. 59, the following
results of analyses made from time to time by the authors, may be
interesting. The percentages are calculated on the absolutely dry
material.
Cellulose
per cent.
Oat Straw 52·0
Oat Straw 53·5
Wheat Straw 49·6
Rye Straw (Foreign) 53·0
Oat Straw (Foreign) 46·5
Wheat Straw (Foreign) 50·2
The yield of pulp is greatly influenced by the conditions under which
it is obtained; high pressures and temperatures exerting considerable
influence on the result. This is probably the case with straw more than
any other fibre, on account of the physical and chemical nature of
the cellulose. It is doubtful whether much more than 35 per cent. is
actually obtained in practice.
Straw is usually boiled in cylindrical rotary boilers. Some
paper-makers, however, prefer to use one or other of the different
forms of vomiting boilers already described.
The use of a rotary boiler is open to certain objections. In the
first place, it is doubtful if they are as economical of soda as the
vomiting boilers, especially the more recent patterns; and, secondly,
the rotation of the boiler so disintegrates the pulp, that a certain
proportion of the cellulose, especially those fibres which are short,
is liable to be lost during the subsequent treatment. It seems probable
that the best results would be obtained if the rotation of the boiler
were reduced to a minimum, in fact, sufficient only to produce thorough
circulation of the liquor.
The proportion of soda necessary to boil straw thoroughly is, for
reasons already stated, greater than is the case with esparto. The
amount varies from 10 to 20 per cent. on the raw material. {100}
The different processes to which straw is subjected vary greatly. The
following may be taken as a general indication of the methods employed.
The straw is usually cut into short pieces of about 1–2 in. in length
by means of an ordinary chaff-cutter. The cut straw is carried by means
of a blast of air along a wooden tube or shaft into a chamber, the
sides of which are made of coarse iron gauze. This chamber is itself
inclosed in another chamber, in which the dust and dirt accompanying
the straw collect. The clean straw is then placed in sacks and conveyed
to the boiler-house.
Owing to the bulky nature of straw, it is difficult to get a large
charge into a boiler at one operation; it is therefore usual, after
having filled it as completely as possible, to run in a portion of the
lye required, and to turn on the steam for a few minutes. This has the
effect of so far softening the straw as to make it lie closely at the
bottom of the boiler and to allow of a further quantity being put in.
This having been accomplished, the remainder of the lye required is run
in, together with the requisite quantity of water, and the steam turned
on. The pressure may vary from 10 to 50, or even 80 lbs. per sq. in.,
and the time from 4–8 hours.
When the boiler has sufficiently cooled, the charge is run out by
a cock in the bottom. Owing to the rotary action of the boiler the
straw is in the state of fine pulp, having been almost completely
disintegrated; so fine, in fact, that it flows readily through a 3-in.
pipe. It is run into large tanks with perforated tile bottoms, where
the excess of liquor is allowed to drain away and the pulp washed by
the addition of water. It is then dug out and taken to be bleached.
Before this is done, however, it may be necessary to give it a further
washing. This may be done in the potcher itself. Instead of washing
the straw in tanks, it may be washed in an ordinary washing engine,
such as has been already described. Owing to the finely divided state
of the pulp, however, this method, unless the meshes of the wire-cloth
covering the drum washer are very fine, entails {101} a considerable
loss of fibre. It is more suitable in cases in which the straw has
been boiled in stationary boilers, and in which therefore it is
less disintegrated. When stationary boilers are employed, it is not
necessary to cut the straw very fine; in fact, it is sometimes put
into the boiler whole. The cutting has this advantage, that it loosens
the adhering dirt. In cases where the action of the soda has not been
carried far, or when the straw has been put into the boiler whole, the
pulp will not be disintegrated to the same extent, and it would not
flow through a narrow pipe. It is necessary therefore to discharge the
boiler through the doors used for filling.
A novel form of washer especially adapted for straw, whereby the
washing is effected with the minimum quantity of water, has been used
on the Continent to a considerable extent, and to some extent in this
country. The pulp is caused to pass along a series of revolving cones
covered with wire-cloth, through which the liquor escapes. As it
reaches the end of each cone, the pulp is emptied into a small tank
containing water from the cones further on in the series. It is carried
forward by means of hollow bent arms connected with the inside of the
next cone, which then discharges it at the other end, to be again
carried forward.
As already stated, the presse-pâte system is largely adopted for straw
pulp; it has been described under Esparto. It may sometimes be employed
with advantage before bleaching, though it is generally used after.
A method of treating straw is sometimes adopted which produces a pulp
of higher quality than is obtainable by the ordinary method. The washed
pulp is mixed in a chest provided with stirrers, with a large quantity
of water, and is then pumped into another chest placed at a higher
level, from which it flows between a pair of hard granite millstones,
the surfaces of which are cut into radial grooves. The stones are fixed
horizontally, and are made to revolve at a very rapid rate. During the
passage of the pulp through the stones, the knots, weeds, and other
hard portions of the {102} straw which may have resisted the action
of the alkali are reduced to a fine state of division, and are thus
more readily acted upon in the subsequent operation of bleaching. The
degree of fineness to which the pulp is reduced can be determined by
regulating its flow and the distance between the stones. Pulp produced
in this way is of a very even character, and is comparatively free from
unbleached particles.
Within the last few years a process of treating straw has been
introduced which, on account of the high quality of the pulp produced,
has attracted considerable attention from paper-makers. It consists
in exposing pulp obtained in the usual way by the action of caustic
soda to the action of chlorine gas. The method was patented in England
on March 3rd, 1880 (No. 938), by Friedrich Carl Glaser. The amount of
soda in the preliminary boiling is reduced much below that necessary
for perfect pulping, the result being that the cellulose is much less
liable to be destroyed, and thus a greater yield is obtained. The pulp
is thoroughly washed and partially freed from moisture in a centrifugal
machine until only about 70 per cent. remains. The partially dried
pulp is then exposed in leaden or stone chambers for some hours in an
atmosphere of chlorine, produced by the action of hydrochloric acid
upon manganese dioxide. By the action of the chlorine, those portions
of the straw which have been but imperfectly acted upon by the caustic
soda in the boiling process are completely disintegrated, or, rather,
are rendered more susceptible to the action of the bleaching liquor
employed subsequently. The consequence is that when bleaching liquor
is added, the whole of the straw is rapidly reduced to the state of
pure cellulose, and the finished product is remarkably free from
anything like unbleached portions. Owing to the fact that the action
of the caustic is minimised, a considerable proportion of the more
easily degraded cellulose survives, and the yield of pulp is thereby
increased. The process is, however, necessarily expensive. The action
of the chlorine will be readily understood by a reference to p. 19.
{103}
Owing to the fact that a considerable proportion of the pulp obtained
from straw consists of cellular tissue (see Fig. 13, p. 58), which,
although cellulose, is devoid of fibrous structure, paper made from
straw only is found to be much weaker than that made from fibres such
as esparto. On the other hand, being a cheap material, and one capable
of yielding a very white pulp with a moderate quantity of bleach, it
is held in considerable repute by many paper-makers, especially as a
material for mixing with esparto. It is sometimes mixed with cotton and
linen rags, even in the finest qualities of paper.
_Jute, Manilla, Adansonia, &c._—The boiling processes already described
under Esparto and Straw are, with certain slight modifications, equally
applicable to fibres of this class. For convenience in manipulation
they are usually cut into small pieces by a machine such as is shown
in Fig. 16, and are then cleaned and dusted in a willowing machine.
The boiling process calls for no special notice beyond what has been
already said with regard to other fibres.
A cheaper method, and one which is largely adopted in mills where only
a coarse dark-coloured paper is required, consists in boiling the jute
or other fibre in lime. Such a treatment is, however, not so effective
as one in which caustic soda is used, and the fibre produced is harder
and coarser in every way. The yield of pulp, however, owing to the fact
that the fibre is imperfectly resolved, is greater. In many cases, from
questions of economy, lime may be advantageously employed.
Jute can only be employed successfully when it is imperfectly resolved,
that is to say, when it is not reduced to the state of pure cellulose.
In such a state it forms a comparatively weak fibre, owing to the fact
that it is resolved into its individual fibres. If the filaments are
not completely resolved, then a strong fibre is the result, but in
this case, the pulp cannot be obtained in a pure white state. Owing to
this fact, jute has had up to the present only a limited consumption,
except for the manufacture of coarse papers. {104} Jute and Adansonia
are largely used for papers where strength is of more importance than
appearance, such, for example, as paper for telegram forms, strong
wrapping paper, &c.
_Wood._—The processes for treating wood have already been described in
Chapter V.
When an alkaline treatment is adopted, the alkali employed is, at
least in this country, caustic soda. In Germany the sulphide of
sodium processes have been very largely adopted. The alkali method
is, however, being superseded by the more perfect and more economical
method of treatment with sulphurous acid or acid sulphites.
_“Broke” Paper._—Under this head may be included all the partially
formed paper which is always obtained in greater or less degree when
a paper-machine is started, or such portions as are occasionally
unavoidably damaged in its passage over the drying cylinders, together
with the imperfect or rejected portions. It may also include used or
waste paper, a large proportion of which, if not too dirty, is re-made
into paper. The cleaner portions, especially if they have not been
dried, are frequently returned direct to the beaters, and mixed with
other pulp. That which has been actually made into paper requires to be
softened by boiling in water and {105} gentle breaking in an engine.
It may be necessary to heat it for a short time in a weak solution of
caustic soda. This may be done either in a breaker or in special tanks
provided for the purpose. Paper which has been printed upon requires
a rather more drastic treatment, and it must of course be used for an
inferior quality of paper, as it is impossible to get it to as good
a colour as the original pulp from which it was made—at least not
economically. “Broke” paper may be advantageously disintegrated by
means of an edge-runner. It consists of a pair of stones arranged in
the same way as an ordinary mortar-mill (Fig. 27).
[Illustration: FIG. 27.]
_Mechanical Wood Pulp._—A very large quantity of pulp is used in the
commoner kinds of paper, such as cheap news, &c., which is obtained
by disintegrating wood by mechanical means alone, no chemicals being
employed. The idea of making paper in this way dates back about 100
years, but owing to the want of suitable machinery it is only lately
that a good product has been obtained. The following are, in a general
way, the details of the process employed:—
The wood is first cut up into blocks, the size of which is determined
by the width of the stones used for grinding; any knots present are
cut out with an axe. The stones are made of sandstone, and are covered
over three quadrants with an iron casing, the remaining quadrant being
exposed. The surfaces of the stones are made rough by the pressure
of a steel roll studded with points, and which is pressed against it
while revolving. In addition to this, channels about 1/4 in. deep are
cut into the stone at distances of 2–3 in. They are made in two sets,
crossing each other in the centre of the stone, and serve to carry off
the pulp to the sides of the stone, in addition to giving increased
grinding-surface. The pressure of the blocks of wood against the stones
is steadily maintained by screws worked by suitable gearing; this is
necessary in order to obtain a pulp of uniform character. A stream
of water is kept constantly playing on the stone; by this means, the
pulp as fast as it is formed can be conveniently carried away. It is
first passed {106} through a rake, which retains small pieces of wood
that have escaped grinding. The stream of pulp then passes through
the sorters, the object of which is to keep back such portions of
the wood as have not been sufficiently disintegrated. These consist
of cylinders about 3 ft. long and 2 ft. in diameter, covered with a
coarse wire-cloth. The fibres that are retained by this wire fall into
the refiners, which consist of a couple of horizontal cylinders of
sandstone, the upper one only of which revolves. Here they are further
{107} disintegrated, and are again passed through the wire-cloth; this
is repeated until all the fibres have passed through. The pulp, after
passing through the first sorter, may be conducted through a series
of gradually increasing fineness, and, by this means, be separated
into different qualities. Though pulp so prepared cannot compete with
chemically-prepared stuff, as the fibres are extremely short, and have
comparatively little felting-power, it may be used with advantage as a
sort of filling-material.
[Illustration: FIG. 28.]
Various modifications of the foregoing process have from time to time
been proposed; among others, that of softening the wood by previous
soaking in water, or steaming, seems to be valuable, as by so doing,
it is highly probable that a longer fibre could be obtained, the soft
wood being more readily torn away by the stones. Some inventors have
proposed to replace the sandstone by an artificial stone containing a
large quantity of emery.
An improved method of preparing mechanical wood pulp, lately patented
by Mr. A. L. Thune, of Christiania, has been communicated to us by Mr.
Carl Christensen.
[Illustration: FIG. 29.]
[Illustration: FIG. 30.]
The apparatus employed is shown in Figs. 28, 29, and 30. Fig. 28
illustrates an arrangement of grinding apparatus fixed direct on to a
turbine. The stone is fastened on to the shaft S worked by the turbine
T. The wood in the form of small blocks is kept in contact with the
stone by a number of hydraulic presses P. {108}
A somewhat similar arrangement, but placed horizontally, is shown
in front and side elevation in Figs. 29 and 30. The same letters
correspond.
[Illustration: FIG. 31.]
The ground and sorted pulp is made into thick sheets by means of the
board machine shown in Fig. 31. The pulp mixed with water passes down
the shoot D into the vat B in which the cylinder K revolves. This
cylinder is covered with wire-cloth, and as it revolves it takes with
it a certain quantity of pulp in the form of a continuous sheet. This
sheet is taken on to the endless travelling felt F by the small couch
roll E. When it reaches the rolls C it is wound round the upper one,
from which it is removed when a sufficient thickness is obtained.
Obtained in this form the pulp is readily transported.
The woods commonly employed are white pine and aspen. The latter yields
a pulp of a better colour, but of inferior strength than the former.
Paper containing mechanical wood pulp is very liable to become
discoloured by the action of air and light, the ligno-celluloses
being much more readily acted upon than the celluloses isolated from
them. Such fibre is, moreover, devoid of much felting power; it has,
in fact, little to recommend it but its comparative cheapness. It is
nevertheless used in large quantities, some cheap papers being made
entirely from it. {109}
TABLE OF STRENGTH OF CAUSTIC SODA SOLUTIONS (15° C. = 59° F.)
(TÜNNERMAN).
----------+-------------+-----------+-----------------------
| | | Equivalent Percentage
Specific | Degrees | Per cent. | of 60 per cent.
Gravity. |Twaddle.[10] | Na_{2}O. | Caustic Soda.
----------+-------------+-----------+-----------------------
1·0040 | 0·80 | 0·302 | 0·503
1·0081 | 1·62 | 0·601 | 1·001
1·0163 | 3·26 | 1·209 | 2·015
1·0246 | 4·92 | 1·813 | 3·021
1·0330 | 6·60 | 2·418 | 4·030
1·0414 | 8·28 | 3·022 | 5·037
1·0500 | 10·00 | 3·626 | 6·043
1·0587 | 11·74 | 4·231 | 7·051
1·0675 | 13·50 | 4·835 | 8·059
1·0764 | 15·28 | 5·440 | 9·067
1·0855 | 17·10 | 6·044 | 10·073
1·0948 | 18·96 | 6·648 | 11·080
1·1042 | 20·84 | 7·253 | 12·090
1·1137 | 22·74 | 7·857 | 13·095
1·1233 | 24·66 | 8·462 | 14·103
1·1330 | 26·60 | 9·066 | 15·110
1·1428 | 28·56 | 9·670 | 16·117
1·1528 | 30·56 | 10·275 | 17·125
1·1630 | 32·60 | 10·879 | 18·131
1·1734 | 34·68 | 11·484 | 19·140
1·1841 | 36·82 | 12·088 | 20·147
1·1948 | 38·96 | 12·692 | 21·153
1·2058 | 41·16 | 13·297 | 22·161
1·2178 | 43·56 | 13·901 | 23·170
1·2280 | 45·60 | 14·506 | 24·177
1·2392 | 47·84 | 15·110 | 25·170
----------+-------------+-----------+-----------------------
NOTE.—It must be borne in mind that the above numbers refer only
to solutions of pure caustic soda. With liquors containing sodium
chloride, sulphate, &c., the specific gravity will give an erroneous
view of the amount of alkali present, as these salts of course raise
the gravity. For example, a liquor prepared by causticising a solution
of recovered soda has a specific gravity of 1·05 (10° Tw.). According
to the above table this corresponds to 6·043 per cent. of 60 per cent.
caustic. Tested by means of standard acid it showed 4·520 per cent. Too
much reliance should therefore not be placed upon determinations of
specific gravity, but in important cases the actual amount of alkali
should be determined by titration with standard acid.
[Footnote 10: To convert degrees Twaddle into specific gravity,
multiply by 5, add 1000 and divide by 1000: thus 7° Tw.,
7 × 5 = 35
1000
────
1000) 1035 (1·035 sp. gr.
1000
────
3500
3000
────
5000
5000
━━━━]
{110}
CHAPTER VII.
BLEACHING.
The methods of bleaching being essentially the same for all pulps, it
will only be necessary to describe one in detail.
The process depends upon the use of a solution of calcium hypochlorite,
prepared by dissolving bleaching powder in water. This solution is
best prepared in a vessel provided with stirrers or agitators, whereby
a thorough mixture of the bleaching powder and water is obtained.
Bleaching powder always contains a certain amount of free lime and
calcium carbonate, which remain undissolved in the water. This should
be allowed to settle to the bottom of the vessel, and the clear liquor
run off. The residue may then be again treated with water, or with weak
liquor obtained by washing the residue. If the washings are too weak to
be conveniently used for bleaching, they may be stored in a separate
tank and used for dissolving fresh powder. In this way a strong stock
solution can be readily prepared, the powder at the same time being
thoroughly exhausted. The washing should be repeated as frequently
as possible, after which the residue may be allowed to drain. This
operation is best conducted on a filter bed provided with an air-pump
similar to that described in Chapter XII. The residue is obtained in
this way as a hard cake, containing about 60 per cent. of water. If the
above operations have been properly conducted, it should not contain
more than about 0·25 per cent. of chlorine.
The original powder should contain from 33·0 to 35·0 per cent. of
“available chlorine,” that is, chlorine which is effective in the
bleaching process. {111}
Owing to the ease with which it decomposes it should be kept cool and
not exposed to the air.
If possible, one or more large store tanks should be provided for
the strong bleach liquor, so that time may be given for the complete
separation of the insoluble portions. The liquor can be drawn off with
a “siphon” pipe without disturbing the residue.
It is of the utmost importance that only clear liquor be used, as a
milky solution of calcium hypochlorite is much more sluggish in its
action than one which is perfectly clear and bright.
A convenient strength for the stock solution is 6° Twaddle: this
corresponds to about half a pound of bleaching powder to the gallon.
The method of testing the powder and its solution will be described in
Chapter XIV.
The washed and broken pulp is placed in the “potcher” together with
the necessary quantity of bleaching liquor and as much water as is
required to produce complete circulation of the mass. In many mills the
breaker itself answers the purpose of a potcher also. The quantity of
water should be kept as low as possible, as it is found that by the use
of strong solutions less bleaching powder is required than with weak
solutions; moreover, less time is required to produce the effect. On
the other hand, with very strong solutions, the pulp is liable to be
injuriously affected.
Straw and esparto pulps are sometimes bleached in large potchers made
of brick covered with cement. The circulation of the pulp is produced
by the action of paddles made of wood or, preferably, of iron.
Many “potchers” are provided with steam pipes, whereby the mass of pulp
and liquor is heated. This should be done with great care, so as to
avoid superheating in any part, as this is certain to cause destruction
of the fibre. The better plan, where it can be adopted, is to uniformly
heat the pulp before running in the bleach.
In certain cases the action of the calcium hypochlorite is {112}
assisted and accelerated by the addition of either sulphuric
or hydrochloric acids; these combine with the lime and liberate
hypochlorous acid, which has a more rapid action than its calcium salt.
If the acid be added in too large quantity, or of too great a strength,
it sometimes happens that instead of hypochlorous acid, chlorine gas
is given off, part of which escapes into the air, thus causing loss.
Moreover, the chlorine is liable to enter into combination with the
fibre substance forming the yellow chlorination products described on
p. 18.
This liability is greater in the case of highly lignified fibres, such
as wood or jute: in these cases, therefore, the use of acids should be
avoided. When used the acid should be largely diluted with water and
added gradually. The best plan is to allow the action of the bleach
to continue for some time, only adding the acid when it is nearly
exhausted. In this way risk of damage to the pulp is avoided.
The diluted acid should be conveyed by a leaden pipe passing down to
the floor of the potcher, and perforated at its lower extremity.
Other means of accelerating the action of the bleaching powder have
been suggested, such, for example, as the use of bicarbonate of soda,
which by double decomposition forms with the bleaching powder carbonate
of lime and free hypochlorous acid. It is, however, much too expensive
an agent.
The amount of bleaching powder necessary to produce a good white colour
differs materially with the fibre to be bleached, and of course with
the degree of resolution of the fibre substance in the preliminary
treatment with caustic soda.
The following numbers may be taken to be approximations of the amount
necessary to bleach well-boiled pulps:—
Per cent. Powder
calculated on
Unboiled Fibre.
Esparto 10 to 15
Straw 7 to 10
Wood 15 to 20
{113}
A well-boiled pulp should contain from 90 to 95 per cent. of cellulose.
Very excellent results are sometimes obtained by substituting sodium
hypochlorite for the calcium compound. This is done by decomposing a
solution of bleaching powder with an equivalent quantity of either
sodium sulphate or sodium carbonate. The precipitate of calcium
sulphate or carbonate is allowed to settle and the clear liquor run
off. Such a solution is more sluggish in its action than one of calcium
hypochlorite, but it is more easily controlled, and is less liable to
injure any material treated with it.
The time necessary to produce a perfect bleach depends on the nature
and condition of the pulp, on the degree of concentration of the
liquor, and on the temperature at which the operation is conducted.
Some paper-makers prefer to extend the bleaching over a considerable
period of time, but this involves the employment of a larger system of
“potchers,” which in many instances is an objection. If this method
is adopted, especially in the case of straw or esparto, the action of
the paddles or roll should be stopped, as otherwise the fibres are
liable to felt together in the form of small grains, which sometimes
escape the action of the beating roll, and occasionally find their way
into the paper, causing an unevenness of surface. By giving longer
time to the bleaching there is less risk of damage to the fibre, and
consequently a greater yield of bleached pulp.
The action of the bleaching solution should, as far as possible, be
confined to the non-cellulose portions of the fibre. A certain action
on the cellulose itself invariably takes place, but it should be
reduced to a minimum. The action of bleach upon cellulose has been
indicated in Chapter I., p. 10. For further information on this subject
the reader is referred to the ‘Journal of the Society of Chemical
Industry,’ 1884, April 29 and May 29.
A very good method of bleaching consists in a preliminary treatment in
the ordinary way in the potcher, followed by a prolonged steeping in
tanks. {114}
In the case of pulps which are very difficult to bleach, the action
can be greatly assisted by washing out the products of the bleaching
action, treating the pulp for a short time with a weak alkaline
solution, again washing, and then repeating the bleaching process.
In this way very refractory pulps can easily be bleached. Even an
intermediate washing with water is useful.
In most cases, where a fairly good colour has been obtained by the use
of bleaching powder alone, the colour is greatly improved by a souring
or treatment with weak hydrochloric acid, or, better still, with
solution of sulphurous acid.
Or the bleaching may be conducted in two or more operations with
intermediate treatments with acid.
All these modifications, however, are more or less troublesome, and
should only be resorted to when the ordinary method fails.
After bleaching, a certain quantity of calcium hypochlorite always
remains in the pulp. It may be removed in the potcher itself, or when
the pulp reaches the beater. In any case, the methods adopted are the
same. (See p. 127.)
In some mills it is the practice to free the bleached pulp from the
products of the decomposition of the bleach by pressing the pulp in
hydraulic presses, or by draining in vats provided with perforated
bottoms. Or the pulp may be made to pass through a pulp-saver (see Fig.
52, p. 155.)
Chlorine gas as a bleaching agent has been almost entirely superseded
by the more manageable calcium hypochlorite. Its employment is open
to the serious objection indicated before, viz. the liability to form
difficultly removable chlorine compounds. Its use as a disintegrating
agent has already been alluded to (see p. 102).
A process has lately been invented (English patent, No. 11,333, 1884),
by Prof. Lunge, which involves the use of acetic acid. The quantity
required is, however, very small, as during the process of bleaching
it becomes regenerated. The free lime in the bleaching powder solution
should first be {115} nearly neutralised with a cheaper acid, such as
hydrochloric or sulphuric, followed then by the addition of the acetic
acid. This process, we are informed, gives excellent results with
high-class material, such as the best cotton and linen rags; it is,
however, not to be recommended for such materials as straw or esparto.
For the bleaching of rags the process invented by Thompson (English
patent, No. 595, 1883) has proved very effective. The method consists
in saturating the material with a weak solution of bleaching powder,
and then exposing them to the action of carbonic acid gas. In this way
the bleaching action is made very rapid and effective.
_Electrolytic Bleaching._—Various attempts have from time to time been
made to bleach by means of electricity—that is to say, by means of the
products of the electrolysis of solutions of alkaline chlorides and
the chlorides of the alkaline earths. The only process which has been
to any extent developed is that of M. Hermite. This process is based
upon the electrolysis of a solution of magnesium chloride, this salt
having been found to give the most economical results. The solution,
at a strength of about 2·5 per cent. of the anhydrous salt (MgCl_{2}),
is electrolysed until it contains the equivalent of about 3·0 grms.
chlorine per litre. This solution is then run into the “poacher”
containing the pulp to be bleached; a continuous stream is then kept
up, the excess being removed by means of a drum-washer. This excess,
which after being in contact with the pulp in the engine is more or
less deprived of its bleaching properties, is then returned to the
electrolysing vat, where it is again brought up to the normal strength.
The electrolysing vat consists of a rectangular vessel of slate or
other suitable material. The negative electrode may be constructed of
zinc; for the positive it is necessary to employ platinum.
The electrolysed solution has been found to possess very remarkable
properties, which have considerable bearing upon the economy of the
process. If a solution be taken of equal {116} oxidising efficiency
with one of calcium hypochlorite, as indicated by the arsenious acid
test (see p. 206), it is found that the former possesses greater
_bleaching_ efficiency than the latter in the proportion of 5 to 3.
Moreover, the bleaching is much more rapid, and the loss of weight
which the substances undergo is less, for equal degrees of whiteness
obtained.
It has been shown that by the electrolytic method the bleaching of
paper pulp can be effected at nearly one-half the cost of bleaching
powder. The process is now being industrially carried out. Those
interested in the subject are referred to a paper by the authors in the
‘Journal of the Society of Chemical Industry’ for April 1887.
{117}
CHAPTER VIII.
BEATING.
The bleached pulp is now ready for the final treatment. If it were
attempted to make paper from the pulp in the state in which it leaves
the “potchers” or “steeping” chests, it would be found to be wanting in
evenness of texture and uniformity of surface. This result can only be
secured by an elaborate process of comminution or disintegration. This
is in effect the result produced by the action of the “beaters.”
They resemble in general appearance the breaking engines previously
described; the roll, however, carries more knives, and it is usually
let down much nearer to the bed-plate. In the case of fibrous
substances, whose ultimate fibres are relatively short (see table,
p. 39), it is only necessary to split up the filaments into their
constituent fibres: esparto, straw, and wood are of this class. In the
case of straw, the disintegration is for the most part accomplished
in the boiling and bleaching processes, and therefore but little work
devolves upon the beater. Esparto and wood require a certain amount of
beating, but this should be regulated to the drawing asunder of the
individual fibres, any cutting of the fibres being carefully avoided.
This is accomplished by adjusting the distance of the roll from the
bed-plate, so that by the friction of the fibres upon themselves, when
passing over the plate, a kind of rubbing or “brushing” is produced.
If a carefully-made paper of esparto or wood be examined by the
microscope, it will be found that the majority of the fibres preserve
the pointed or slightly rounded ends {118} characteristic of bast
cells. On the other hand, it is obvious that cotton, whose ultimate
fibres have a length of 20 to 40 mm., and flax (25–30 mm.), with other
similar fibres, will require to be broken up into short fragments in
order to develop to the fullest the felting property of the pulp. Not
only are the fibres reduced to the most favourable dimensions, but
in cotton and linen a further contributory advantage accrues; for on
account of the internal structure of the ultimate fibres they tend to
split up at the point of rupture into a number of fibrillæ, which, in
the case of cotton, take the form of a network; and in case of linen,
are seen as a bundle of distinct fibres parallel to and continuous
with the fibre. This gives to ends of the fragments a ragged contour,
which has considerable influence on the felting power of the pulp, and
therefore on the strength of the paper into which it is made.
With these fibres, therefore, the “cutting” as distinguished from the
“breaking” action should be avoided as much as possible, otherwise the
effect described above will not be produced, and the fibres will show
instead a clean cut. The appearance shown by cotton and linen pulp,
when thoroughly “beaten” and ready to be made into paper, is given in
the frontispiece, which is taken from the author’s micro-photographs.
The “half-stuff” is furnished to the “beater,” or “beating” engine as
it is sometimes called, previously partially filled with water. The
furnishing is done in successive portions, the first being allowed to
mix thoroughly with the water before another lot is added. This is
continued until the mass is so thick that it will only just turn round
in the beater under the action of the “roll.” Owing to the construction
of the beater it frequently happens that a portion of the pulp lodges
in the corners, from which the beaterman removes it by means of a
wooden paddle, which also serves to push forward the pulp to the roll
in case the motion is inclined to be sluggish. The proportion of
water to pulp should not be too high, otherwise the beating is not so
effective; at the same time, {119} if the mass is allowed to get too
thick, imperfect circulation results.
The operation of “beating” occupies a considerable time, and consumes a
large amount of power. Cotton and linen rags naturally take longer—in
some cases as much as ten hours is given to the operation.
Esparto, on the other hand, can be sufficiently disintegrated in from
two to four hours.
Wood pulp requires very gentle beating; it is therefore necessary to
prolong the time to about six hours.
These differences in the duration and method of beating should be borne
in mind when pulps of different natures are mixed together in the same
beater, as is frequently the case. It is better, with very dissimilar
fibres, to “beat” each separately, and only to mix them in the
stuff-chests. This, however, is open to the objection that the pulps
may be insufficiently mixed.
The length of fibre to which pulps should be reduced depends to some
extent upon the kind of paper to which it is to be applied. The authors
have examined a number of papers by well-known makers, and find the
dimensions in millimetres of various pulps to be as follows:—
Fibre. Maximum. Minimum. Mean.
Cotton 1·32 0·23 0·75
Linen 1·20 0·20 0·76
Esparto 1·40 0·40 1·00
Straw 1·50 0·50 0·88
Wood 2·60 1·00 2·00
Wood and straw pulp, when imported in the form of dry sheets, may,
before being beaten, be conveniently disintegrated and thoroughly mixed
with water by means of the edge-runner described under “Broke Paper,”
p. 104, Fig. 28.
In making the finer kinds of paper, the roller bars or knives, instead
of being made of steel are made of bronze, thus any contamination with
oxide of iron is avoided. This is especially liable to take place in
case of steel knives when the beater has been allowed to stand for some
time. {120}
The inside of the heater itself is often lined with lead, a material
which is not liable to oxidize, and which can very readily be cleaned.
When a beater has been running for some time the knives of the roll and
the bed-plate become worn and so far reduced that they must be taken
out and re-cut. The bed-plate is removed, firmly fixed in the bed of a
planing machine, and the edges trimmed by means of a chisel, so as to
cut the knives at the proper angle.
The roll and bed-plate are shown in section in Fig. 32. Fig. 33 is a
plan of a bed-plate, and Fig. 34 illustrates the manner in which the
knives are fixed.
[Illustration: FIG. 32.]
It will be seen that the knives in the bed-plate are placed so that
they do not lie parallel with those of the roll. This arrangement
imitates to some extent the action of a pair of scissors. Occasionally
the knives are slightly bent, so as to form a very obtuse angle.
Bed-plates so fitted are called {121} “knee-plates.” They are largely
used in America, but not much in this country.
[Illustration: FIG. 33.]
[Illustration: FIG. 34.]
To obviate the necessity of removing the roll, a small machine has been
devised whereby the knives can be cut _in situ_. This machine, which
can be firmly fixed between the mid-feather and the side of the beater,
consists of a small steam-engine which actuates a movable cutter, which
is made to pass to and fro horizontally along the edge of each knife
in succession. The engine is supplied with steam by a piece of strong
flexible rubber tubing.
The ordinary form of beater contains only one roll, though some have
been constructed containing two or even four rolls. In America, beaters
of a totally different construction are much in vogue. The most
important of these are the Jordan and Kingsland beaters, so called
from the names of the inventors. The former consists essentially of a
roll in the shape of a truncated cone, fitted with knives in the usual
way, revolving in an iron box of corresponding shape, furnished with
knives placed in the direction of its length, but at slightly different
angles. The half-stuff enters the beater at the narrow end through a
box provided with an {122} arrangement for regulating the flow, and is
discharged by two or more openings in the cover at the wider end.
The Kingsland engine consists of a circular chamber, the sides of which
are covered with knives, and between which a circular plate, also
furnished with knives, revolves. The pulp enters through a pipe in the
centre of one of the sides of the chamber, and flows out through an
opening in the opposite side.
Another form of beater is that invented by S. L. Gould. The only
essential difference between it and the Kingsland beater is that,
instead of having a plate which revolves vertically against two
stationary ones, the plate, which is placed horizontally, is furnished
with knives on one side only, and revolves upon but one fixed plate,
much in the same way as a pair of millstones.
The half-stuff supplied to these forms of beater is generally
disintegrated to a greater extent in the breaker than is the case with
those of the ordinary construction, as it is necessary to make it flow
easily through them, and this could not be done if the fibres were kept
long. A single passage of the pulp through the beater is sufficient to
beat it up ready for making into paper. The chief advantages claimed
for them is that they are more economical, both of time and power; also
that the pulp is more regularly beaten.
Certain new forms of beaters have lately been introduced in this
country, and are rapidly gaining in favour, chiefly on account of
the saving of driving power, and the space occupied, compared with
the amount of “stuff” they are capable of beating. Fig. 35 shows the
construction of Forbes’ patent engine, as manufactured by Messrs. G. &
W. Bertram, of Edinburgh. This beater is divided into three channels.
The rolls, one of which is shown uncovered, are placed in the outer
channels, while in the centre one the mixing and circulating wheel a is
placed, the peculiar construction of which enables it to throw the pulp
alternately into the two outer channels, from whence, after passing
under the rolls again, it enters the central channel at the other end
of the beater. {123}
In Umpherston’s patent engine great economy of space is effected by
causing the pulp to travel over and under the backfall (_a_, Fig. 36).
Another advantage is that the stuff circulates freely with less water
than in the ordinary forms, thus increasing its output. It is also
claimed that the pulp is beaten with less power, and as it is less
liable to lodge in corners it is more evenly beaten.
[Illustration: FIG. 35.]
The construction of the engine will be readily understood by reference
to Fig. 36. The direction which the stuff takes is indicated by the
arrows.
Cooke and Hibbert’s beater, as manufactured by Messrs. Masson, Scott
and Bertram, resembles, to a certain extent, Gould’s engine, previously
described; it differs from it, however, in that the discs are placed
vertically, instead of horizontally. Moreover, the general construction
of the beater more nearly resembles those in ordinary use. Fig. 37,
which shows it in plan and in section at the line D E, illustrates its
construction.
[Illustration: FIG. 36.]
[Illustration: FIG. 37.]
The shaded portions A represent the stationary disc. It is furnished
for about two-thirds of its circumference with steel bars or knives,
placed tangentially. B represents the {125} revolving disc on which
similar knives are placed radially round the whole circumference. The
direction that the pulp takes is indicated by the arrows. After passing
between the plates it is thrown violently into the trough at G, the
result being that it is very thoroughly mixed. The distance between the
plates can be regulated by means of the gearing H. The engine is driven
by the pulley C fixed on the shaft F. It is claimed that by means of
this form of beater a great saving in time and power is effected, and
that moreover the pulp is obtained in a more even condition.
[Illustration: FIG. 38.]
[Illustration: FIG. 39.]
The quality of the water used to furnish the engine is a matter of very
great importance, especially in the manufacture of high-class papers.
Above all it should be free from suspended matter, and from dissolved
iron; the former finds its way directly into the paper, and the latter
is liable to become precipitated in the pulp as oxide, thus injuriously
affecting its colour. Careful settling and filtration are sufficient to
eliminate insoluble matter; soluble impurities are more difficult of
removal; therefore the water should, if possible, be free from them.
In most mills settling ponds are provided, for the purpose of removing
suspended matter, and in addition it is usual to employ woollen
filter-bags {126} fastened to the nozzle of the pipe that supplies the
beaters with water.
For methods of purifying water see p. 211.
The beaters are generally driven from a separate engine, and are
connected with it by a system of spur-wheels, pinions, and belts.
Messrs. G. and W. Bertram have lately introduced a system of direct
driving of beaters, whereby a great saving in power is effected. It is
illustrated in plan and elevation in Figs. 38 and 39. The crank-shaft
of the engine is coupled direct on to the main driving-shaft. Large
pulleys are keyed on to this shaft, from which the power is taken
directly on to the pulleys connected with the rolls.
{127}
CHAPTER IX.
LOADING, SIZING, COLOURING, ETC.
The bleached half-stuff as it leaves the steeping chests usually
contains an excess of bleaching liquor, which can be removed in two
ways, viz. by washing or by decomposition with an “antichlor.” The
first method has the advantage of not only removing the bleach, but
of also eliminating the chloride of calcium, partly existing ready
formed, and also that resulting from the decomposition of the calcium
hypochlorite originally present in the bleach. On the other hand, this
method takes some time, and consumes a large amount of water, which in
some mills is a matter of considerable importance. For this purpose,
many beaters are provided with one or more drum washers (see Fig. 36).
An additional objection to this method lies in the fact that a certain
quantity of fibre passes through the meshes of the wire-cloth covering
the washers, and is thus lost.
The more usual plan is to remove the bleach by decomposing it with
an “antichlor.” The substance generally employed for this purpose is
sodium hyposulphite, or thiosulphate as it is now called, which, in
presence of calcium hypochlorite, is oxidized to sodium sulphate, the
latter being reduced to calcium chloride. Double decomposition then
takes place between these salts, with the formation of calcium sulphate
and sodium chloride. The reactions which take place may be expressed by
the following equation:—
2 (Ca(ClO)_{2}) + Na_{2}S_{2}O_{3} + H_{2}O = 2 CaSO_{4} + 2 HCl + 2 NaCl.
Calcium Sodium Water. Calcium Hydrochloric Sodium
hypochlorite. thiosulphate. sulphate. acid. chloride.
{128}
The above decomposition does not accurately represent the action of
bleach upon sodium thiosulphate. If the solutions employed are very
dilute, the decomposition may take place in another direction, viz.:—
Ca(ClO)_{2} + 4 Na_{2}S_{2}O_{3} + H_{2}O = 2 Na_{2}S_{4}O_{6} + 2 NaCl
Calcium Sodium Water. Sodium Sodium
hypochlorite. thiosulphate. tetrathionate. chloride.
+ 2 NaOH + CaO.
Caustic soda. Lime.
At the particular degree of dilution which occurs in a beater, the
bleach is decomposed almost entirely according to the first of the
equations; from which, on calculating, it will be seen that 158
parts of sodium thiosulphate are equivalent to 286 parts of calcium
hypochlorite. As commercial sodium thiosulphate contains 36·3 per cent.
of water, and bleaching powder 70 per cent. of calcium hypochlorite, on
the basis of 35 per cent. available chlorine it follows that 248 parts
of the former are required to neutralize 409 parts of the latter.
Within the last few years other forms of “antichlor” have been
introduced, such, for example, as the various sulphites. The most
important of these is sodium sulphite, which has been manufactured by
a patent process at a cheap rate, by Gaskell, Deacon, & Co., Widnes.
Their product contained as much as 75 per cent. of Na_{2}SO_{3},
ordinary crystallized sodium sulphite containing only 50 per cent.
Sulphites are converted by the action of bleach into sulphates, thus:—
Ca(OCl)_{2} + 2 Na_{2}SO_{3} = CaSO_{4} + 2 Na_{2}SO_{4} + 2 NaCl
Calcium Sodium Calcium Sodium Sodium
hypochlorite. sulphite. sulphate. sulphate. chloride.
From this equation, it will be seen that 252 parts of sodium sulphite
will neutralize 143 parts of calcium hypochlorite, or 204·3 parts of
bleaching powder. Assuming that crystallized sodium sulphite contains
50 per cent. Na_{2}SO_{3}, the same amount of bleach would require 504
parts. As Messrs. Gaskell, Deacon & Co.’s sulphite contains 75 per
cent. Na_{2}SO_{3}, only 336 parts are needed. Comparing these numbers
with {129} those given above for sodium hyposulphite, it will be seen
that 204·5 parts of bleach require for neutralisation 129 parts of
sodium thiosulphate and 504 parts of crystallised sodium sulphite, or
336 parts of the stronger product.
Sodium sulphite is preferred to sodium thiosulphate by some paper
makers, notwithstanding the fact that even in its most concentrated
form, nearly three times as much is required to produce a certain
result. It is said that when it is used the wire cloth of the machine
is preserved for a longer time than if sodium thiosulphate is employed.
This may be due to the fact that with the latter a certain amount of
free acid is always formed, which of course would act injuriously
on the wire; whereas, when sodium sulphite is used, the products of
decomposition are neutral salts without any action upon metals. (See
the above equations.)
A very cheap “antichlor” may be prepared by boiling together lime and
sulphur. One hundred and sixty-eight parts of lime, made into a milk
with water, are heated to boiling in an iron vessel. Three hundred and
eighty-four parts of flour of sulphur or ground sulphur are then added
in small quantities at a time, and the boiling continued until the
whole is dissolved. The liquid which is now of a deep yellow colour,
is allowed to settle and cool and is then ready for use. It contains a
mixture of calcium thiosulphate and calcium pentasulphide, the latter
compound giving to it its deep yellow colour. The following equation
represents the action which takes place between the lime and the
sulphur:—
3 CaO + 12 S = CaS_{2}O_{3} + 2 CaS_{5}
Lime. Sulphur. Calcium Calcium
thiosulphate. pentasulphide.
The decomposition which takes place when the mixture is acted upon by
calcium hypochlorite may be represented as follows:—
2 (CaS_{2}O_{3} + 2 CaS_{5}) + 9 (Ca(OCl)_{2}) = 6 CaSO_{4} +
Calcium Calcium Calcium Calcium
thiosulphate. pentasulphide. hypochlorite. sulphate.
18 S + 9 CaCl_{2}
Sulphur. Calcium
chloride.
{130}
From this it is seen that a large quantity of free sulphur is formed,
which is precipitated as a fine yellowish powder in the fibre. This not
only affects the colour of the pulp, but is objectionable on account of
its liability, in contact with the hot cylinders of the paper machine,
or by the action of moisture over a long period of time to become
oxidised to sulphuric acid, which undoubtedly, even if present only in
very small quantities, has an injurious effect upon the finished paper.
(See hydracellulose.) This property of sulphur and its action upon
cellulose may be shown by the following experiment. Take a piece of
water-leaf paper, rub into it a mixture of flour of sulphur and water.
If the paper thus treated be rapidly dried, it will be found to be
weakened or even slightly charred in those portions which have been in
contact with the sulphur. (See also Chem. Soc. Journ., p. 249, 1879.)
Whichever variety of “antichlor” is used an excess should be carefully
avoided, as all act more or less upon the size and colouring matter
added to the pulp subsequently. The proper method is to run in small
quantities of a solution of the antichlor at a time, testing the
pulps, after allowing a few minutes for complete admixture after each
addition. The test is made by immersing a piece of iodide of potassium
and starch paper, which will be turned blue so long as any calcium
hypochlorite is present. These papers are made as follows:—
Three grms. of starch are ground up with a small quantity of water and
poured into 700 cc. of boiling water, and to the solution 1 grm. of
iodide of potassium and 0·5 grm. carbonate of soda are added. Sheets of
white paper, water-leaf in preference, are now soaked in the solution
and dried. In presence of calcium hypochlorite, iodine is liberated
from the potassium iodide, and acting upon the starch forms with it a
characteristic blue colour.
_Loading._—It is usual, except in the case of papers of the very
highest quality, to add to the pulp a quantity of some cheap loading
material, such as china clay, or for the better {131} qualities of
paper, “pearl-hardening,” or sulphate of lime. The addition of clay in
moderate quantity can hardly be looked upon as an adulteration, since
it serves to fill up the pores of the paper, and enables it to take a
good surface in the subsequent operations of calendering. Moreover, it
enables the manufacturer to meet the demand for cheap papers with some
chance of remuneration to himself. If added largely, it of course tends
to weaken the paper.
China clay or kaolin is sold in the form of large lumps of a white or
yellowish-white colour. It is formed by the gradual disintegration
of felspar by means of the action of air and water, and consists
essentially of a silicate of aluminium. Its quality depends upon its
whiteness and its freedom from coarse micaceous particles. It is
usually prepared for admixture with the pulp by making it into a fine
cream with water in a vessel provided with stirrers; it is then passed
through a fine sieve in order to remove any impurities it may contain,
and is then run into the beater. The clay or other filling material
is usually run into the beater as soon as the latter is charged with
pulp, so that by the time the beating operation is concluded, a perfect
admixture of pulp and clay is effected.
Sulphate of lime, or “pearl-hardening,” is usually sufficiently pure
to put direct into the engine. It is made by decomposing a solution of
calcium chloride, with sulphate of soda, and is precipitated as a fine
brilliantly white powder, consisting of CaSO_{4} + 2 H_{2}O.
Two distinct forms of precipitated calcium sulphate are met with in
commerce, differing from each other by their microscopical features,
the one consisting of flat tabular crystals (Fig. 41), the other of
fine needles (Fig. 42). Another form, erroneously called precipitated
pearl-hardening, is also sold: it consists of the finely ground native
mineral.
[Illustration: FIG. 40.]
[Illustration: FIG. 41.]
Some of the finer qualities of paper are made without addition of
any loading material whatever, though such papers are of course the
exception. The proportion of clay or {132} other material that can be
put into a fibre depends to a certain extent upon the nature of the
fibre, and upon the degree of fineness to which it is reduced in the
beater. The amount added by different makers varies considerably, from
{133} two or three per cent. to twenty, and even in rare cases to
thirty per cent.
[Illustration: FIG. 42.]
[Illustration: FIG. 43.]
A new loading material called “agalite” has been lately introduced,
possessing certain advantages over china clay, {134} or calcium
sulphate. Agalite is a mineral of the nature and chemical properties
of asbestos: it consists of nearly pure magnesium silicate. Its
structure is more or less fibrous, like that of asbestos, which,
as is well known, can be spun and woven and even made into paper,
and it therefore, when added to a paper, forms a part of the fabric
itself. It is even claimed that it assists in keeping back some of
the finer fibres that invariably find their way through the meshes of
the wire cloth, and it is said that 90 per cent. of the amount added
to the engine is found in the paper. In the case of china clay it is
well known that only from 40 to 60 per cent. is actually “carried”
by the pulp. Figs. 40, 41, 42, and 43 show the appearance of china
clay, pearl-hardening, and agalite when viewed under the microscope,
magnified 200 times. The nature of agalite is such that it assists the
paper in taking a high finish. This is probably due to its “soapy”
nature, a feature which is characteristic of asbestos, French chalk,
“soap-stone,” and other magnesium silicates.
[Illustration: FIG. 44.]
When papers contain such excessive quantities as 15 or 20 per cent.
of clay, it cannot be to the advantage of the consumer, and should be
looked upon as an adulteration. It is a matter of some importance to be
able to determine rapidly and accurately the amount of mineral matter
in a paper. The usual method is to ignite a weighed quantity of the
paper in a platinum crucible until the ash so obtained is either white
or a very pale grey. From the weight of the ash, the percentage of
mineral matter is easily calculated. The following is a very convenient
plan in cases where a platinum crucible or dish is not obtainable:—Take
a weighed piece of the paper to be examined, from 2 to 4 in. square,
according to the thickness, roll it into a narrow hollow cylinder.
Round this wind a weighed piece of platinum wire about 1/50 in.
thick, as in Fig. 44. Hold this by means of a pair of crucible tongs
in the flame of a Bunsen burner until it is completely burned. If the
wire is carefully wound round, and especially if the roll of paper is
made conical, the ash will be securely held in {135} position. Those
who do not possess a chemical balance of the ordinary form will find
a convenient substitute, which will answer the purpose of weighing
the paper and ash with sufficient accuracy, in the spiral balance
invented by Prof. Jolly[11] illustrated in Fig. 44. It consists of a
spiral of hard wire A, which is suspended in front of a mirror B, upon
which millimetre divisions are marked. A small float D, dipping under
the surface of the water in the vessel E, is provided for the purpose
of steadying the spiral and allowing it to come quickly to rest. The
balance is provided with a light pan made of a thin plate of mica,
and suspended by very thin platinum wires. For the present purpose,
however, the pan is not necessary, and it can be replaced by the roll
of paper and platinum coil as shown in the drawing.
[Footnote 11: This balance can be obtained from Nalder Bros. &
Co., Westminster.]
The method of using is exceedingly simple, as the increase in the
length of the spiral is in direct ratio to the increment of weight. The
position of the spiral is ascertained by placing the eye in a direct
line with the small glass bead C and its image in the mirror, and
noting the corresponding division on the scale. The position of the
bead can be altered so as to bring it to any desired point on the scale
by raising or lowering the upright rod F, which is kept in position
by the screw G. The balance stands on a foot provided with levelling
screws.
It is evident that where _proportional_ weights only are required it
is not necessary to know the value of the spiral, but if the balance
is to be used to ascertain _actual_ weights, the coefficient of the
spiral must first be determined. This is done by noting the increase in
length after the addition of a one gram weight. The spirals are made
of different thicknesses of wire, which of course give varying degrees
{136} of sensibility: the most useful is one which gives with one gram
an extension of 100 millimetres: one mm. being therefore equivalent to
one mgrm.
The following experiment will illustrate the method of using the
balance and of calculating the results of a determination of the amount
of mineral matter in a paper:—
Position of bead on scale 100 mm.
Position of bead after attaching platinum wire 151 „
Position of bead with wire and roll of paper attached 200 „
Weight of paper expressed as millimetres 200−151 = 49 „
Position of bead with wire and ash attached 156·5 „
Weight of ash expressed as millimetres 156·5−151 = 5·5 „
Weight of paper Weight of ash Percentage of
expressed in expressed in ash in
millimetres. millimetres. paper.
49 : 100 :: 5·5 : 11·224
In order to ascertain the percentage of mineral matter _actually added_
to a paper it is necessary to deduct from the amount of ash obtained a
certain quantity due to the mineral matter in the fibre of which the
paper is composed. This amount varies with each particular fibre, and
with the method by which it has been prepared. The following table
gives the percentage of ash yielded by the various pulps in a perfectly
bleached state. In all cases the fibres were previously treated with a
dilute solution of hydrochloric acid, in order to remove any carbonate
of lime or other bodies which might have been introduced in the boiling
and bleaching processes: the percentages are calculated on the dry
substances:—
Pulp from linen rags ·25 ash per cent.
Pulp from cotton rags ·10 „ „
Pulp from esparto ·50 „ „
Pulp from straw (soda process) 4·80 „ „
Pulp from straw (Leunig process) 1·36 „ „
Pulp from wood (soda process) ·50 „ „
Pulp from wood (bisulphite process) ·06 „ „
Pulp from wood (mechanical) ·40 „ „
If the paper contains calcium sulphate the ash obtained may consist
partly of calcium sulphide, due to the reducing action of the carbon
found on ignition, and the amount will {137} therefore not represent
the true amount added. The ash should be moistened with a few drops
of sulphuric acid, and again ignited, in order to reconvert it into
calcium sulphate. It should also be borne in mind that the sulphate
of lime as present in the paper is combined with two atoms of water
CaSO_{4} + 2 H_{2}O, and therefore that every part of calcium sulphate
obtained represents 1·26 parts of “pearl-hardening” actually added.
_Sizing._—Papers that are intended to be written upon with ink, and
also most printing papers, require to have the interstices of the
fibres filled up with some material which will to a greater or less
extent resist the action of water, and thus prevent the ink or other
liquid applied to them from “running.” This may be accomplished in
two ways, viz., by “engine-sizing” and “tub-sizing”; the former is so
called because it is performed in the beating engine itself, the latter
being done on the finished paper, as will be subsequently described.
Engine-sizing consists in precipitating a compound called resinate
of alumina in the pulp. This compound when dried has the property of
being with difficulty wetted with water. In order to obtain a thorough
mixture of this compound with the fibre it is formed in the engine by
adding an aqueous solution of rosin-soap to the pulp, and when this is
intimately mixed with it, running in a solution of alum.
Rosin (colophony) consists of a mixture of pinic and sylvic acids,
which, when heated with a solution of carbonate of soda, combine with
the alkali to form a soap, carbonic acid being evolved. The soap is
prepared in the following manner:—
Ordinary rosin, the quality depending upon the quality of paper,
is boiled in a jacketed pan for from two to three hours with a
solution of carbonate of soda, until a sample of the soap formed
is completely soluble in water. The quantity of carbonate of soda
(Na_{2}CO_{3} 10 H_{2}O) required is about 30 per cent. of the weight
of the rosin. It is usual to employ crystallised carbonate of soda
(soda-crystals), but {138} soda-ash of good quality, or even caustic
soda, might very well be employed, in which case of course the
proportion would be different.
A very convenient form of carbonate of soda has lately been introduced
by Messrs. Gaskell, Deacon & Co., which goes by the name of “Crystal
Carbonate.” It is obtained in the form of minute crystals of the
mono-hydrate, Na_{2}CO_{3} + H_{2}O. It contains 50 per cent. of
alkali (Na_{2}O), and possesses this advantage over soda-ash, that
is dissolves readily in water without forming a hard cake. It is
much cheaper in proportion than soda crystals, as the expense of
crystallisation and carriage is saved. The latter contain only 21·68
per cent. Na_{2}O, 62·93 per cent. of their weight consisting of water
of crystallisation. One hundred parts of soda crystals are equivalent
to 43·36 parts of the “Crystal Carbonate.”
An excess of soda should be carefully avoided, as it consumes an
equivalent quantity of alum (see below): on the other hand, it is very
essential that the rosin should be completely dissolved, otherwise
small particles are sure to find their way into the pulp, and would
form clear transparent specks in the finished paper.
The boiling being completed, the charge is run off into iron tanks
and allowed to settle; the soap forms a semi-solid mass, while a
dark-coloured liquor, containing the impurities of the rosin, rises
to the surface, and can thus be removed. The soap so purified is next
dissolved in hot water containing a small quantity of carbonate of
soda, in case complete solubility has not been attained, and is then
mixed with a quantity of starch paste prepared in a separate vessel
by dissolving starch in boiling water. The mixture is then carefully
sieved and is ready for use.
The proportion of starch to rosin differs in nearly every mill, and
also the quantity of size to be added to the beater varies according as
the paper is required to be soft or hard-sized. About 3 parts of starch
to 1 of rosin, and between 3 or 4 lbs. of the mixture to 100 lbs. of
pulp may be considered an average quantity. Some manufacturers prefer
to add the {139} starch and rosin size to the engine separately;
others again do not dissolve the starch in water, but merely make it up
to a thin paste, in which state it is added to the pulp. This method is
employed by makers of very fine papers, as it is said to give a certain
feel to the paper which cannot be obtained in any other way. The method
is costly, as only a small proportion of the starch added is actually
found in the paper.
Some papers, which are not intended to be sized, such as blotting and
filter-papers, are made with the addition of starch only, this being
used to bind the fibres together to some extent. The presence of the
starch does not prevent the paper from being absorbent. Papers which
are intended to be tub-sized only are usually made up with a small
quantity of starch, and are sometimes sized to a small extent with
rosin also. The addition of a certain amount of ordinary soap to the
rosin soap is said to give good results; it enables the paper to take a
higher finish when calendered. Sizing with animal size (gelatine), or
Tub-sizing as it is called, will be described in Chapter X.
The mixture of size and starch may be added directly to the engine, or
it may be previously dissolved in water; the latter method is perhaps
preferable. After allowing it to mix thoroughly with the pulp, a
solution of alum is run in. It is made up with boiling water in either
lead or copper tanks. Iron or zinc vessels must be avoided, as the
solution acts rapidly on these metals.
Alum consists of a double sulphate of aluminium and potassium or
ammonium, Al_{2}3 SO_{4}, K_{2}SO_{4} + 24 H_{2}O; its function being
to form with the rosin acids insoluble soaps which are precipitated in
intimate mixture with the pulp, the sulphate of potash taking no part
in the reaction. The choice of a suitable alum is a matter of very
great importance; it should be free from excess of sulphuric acid and
from iron. The former is deleterious on account of its action upon the
colouring matter used to tone the paper, some colours being completely
discharged by it; and because {140} of its effect upon any metal work
with which it may come in contact, especially upon the brass wire-cloth
on which the paper is made. The iron is objectionable as it forms a
dark red precipitate of oxide of iron.
Alum as supplied by makers of repute is generally sufficiently pure for
even the best classes of paper.
Many years ago a substitute for alum was introduced, called Pochin’s
aluminous cake. It consisted principally of sulphate of alumina
Al_{2}3 SO_{4}, and was, provided it was pure, as suitable for the
papermaker as the more expensive potash alum. It was more economical
at equal price, as it contained about 14 per cent. of alumina, whereas
potash alum, even if absolutely pure, could only contain 10·85 per
cent. Pochin’s aluminous cake however, was liable to contain a
considerable amount of free sulphuric acid and soluble sulphate of
iron. Of late years, a number of different varieties of sulphates of
alumina have been introduced into commerce, of so pure a nature, and
at such a low price, that it is a matter of wonder that they have
not entirely superseded the more expensive alum. They are prepared
either from very pure native alumina (bauxite), or from China clay of
good quality. Methods have been discovered of eliminating almost the
whole of the iron and free sulphuric acid. The following analyses of
different specimens sufficiently indicate their purity.[12]
Alumina (Al_{2}O_{3}) 14·84 14·70 14·95 14·85 16·00 16·20
Ferric oxide (Fe_{2}O_{3}) ·06 ·12 ·05 trace nil trace
Sulphuric acid (SO_{3}) 35·00 34·60 36·09 34·94 38·00 38·00
Free sulphuric acid ·32 ·40 nil ·29 nil nil
Lime (CaO) ·11 ·11 ·17 ·14 ·16 ·14
Water 49·42 49·95 48·72 49·60 45·50 45·43
───── ───── ───── ───── ───── ─────
99·75 99·88 99·98 99·82 99·66 99·75
[Footnote 12: Journ. Soc. Chem. Ind., Jan. 1886, p. 16.]
Sulphate of alumina possesses this advantage over alum, that it is more
soluble in water, and thus a stock solution of considerable strength
can be prepared. It is soluble in {141} 2 parts of cold water, alum
requiring 18 parts for complete solution.
The amount of alum or sulphate of alumina added to a pulp is largely
in excess of the quantity necessary to precipitate the rosin soap;
as a matter of fact, in the case of esparto or straw pulps, for
the bleaching of which considerable quantities of bleach have been
employed, and which therefore contain a certain amount of basic lime,
together with calcium chloride, complete precipitation of the size
is effected without the addition of alum. A certain amount is also
required to precipitate the starch. The excess of alum appears to be
necessary, however, not only to brighten the colour of the paper,
but also to render it capable of resisting the action of ink. From
experiments made by the authors, it appears that one part of rosin
requires 2·9 parts of alum for complete precipitation from its solution
in soda. One part of starch requires 0·40 part alum.
_Colouring._—Cellulose, except in rare instances, is never sufficiently
white to produce a perfectly white paper, and the manufacturer finds
it necessary to complement the slightly yellow colour which most
bleached pulps possess by the addition of small quantities of blue and
pink colouring materials. The blues usually employed are ultramarine,
smalts, and aniline blues; the pinks are generally preparations of
either cochineal or aniline dyes. Ultramarine is met with in commerce
as a bright blue powder; the shade of colour and the quality of which
vary considerably.
In choosing an ultramarine, attention should be directed not only to
its tinctorial power but to its behaviour with a solution of alum,
inferior qualities being entirely discharged by it. All ultramarines
are readily decomposed by free acids, hence the necessity of employing
an alum as free as possible from this impurity.
Smalts on the other hand resists the action of acids, but it is only
used for the finer qualities of paper, owing to its high price. The
aniline blues are perhaps scarcely so brilliant in colour as the
above-mentioned colours, and they {142} are moreover somewhat readily
discharged by the action of light; they have however the advantage of
cheapness.
The pink colouring matters are sold either as lakes, that is, compounds
of the colouring matter with alumina or as solutions. They are also
frequently to be met with mixed with starch in varying proportions.
Those made from cochineal are superior to those containing aniline
colours.
Ultramarine can be detected by means of its reaction with hydrochloric
acid, the colour being discharged and sulphuretted hydrogen evolved,
which can be recognised by its odour. Smalts is not acted upon by
acids. The latter contains cobalt, the presence of which can be
determined by its giving a blue bead with borax in a blowpipe flame.
The aniline blues are soluble either in hot water or hot alcohol,
the liquid being intensely coloured. Most of them are discharged
by treatment either with strong acids or alkalis, the colour being
restored by the reverse treatment.
Paper of any desired colour may be made either by using rags already
dyed the necessary colour or by adding to the bleached pulp in the
beater such dyes or pigments as will produce it. Any colouring matter
which can be obtained in the form of a fine powder or as a solution can
be used. Blue papers are usually coloured with ultramarine; the dark
blue papers used for wrapping sugar and other purposes are coloured
with Prussian blue, either added directly to the beater or produced in
it by the action of potassium ferrocyanide on ferric sulphate.
The so-called toned paper is produced by adding to the pulp a solution
of pernitrate of iron, from which a fine precipitate of oxide of
iron is deposited on the fibres; thus the slightly brown shade
characteristic of this kind of paper is produced. The same effect may
be produced by the addition of yellow ochre or some similar pigment.
The following are a few of the materials which are used for producing
coloured papers:—Methyl violet, eosine, chrome yellow, venetian red,
catechu. Useful receipts for the preparation of coloured papers will be
found in Dunbar’s ‘Practical Paper-Maker.’ {143}
During the time that the loading, sizing and colouring processes have
been going on, the pulp has been continually acted upon by the roll,
and if these operations have extended over a considerable time, it is
probably in a proper condition for making into paper. The amount of
“beating” depends, as has been stated before, upon the nature of the
fibre, and also to some extent on the nature of the paper for which
it is intended. The “beaterman” examines the pulp from time to time
by taking a portion from the engine and placing it in a hand-bowl
containing water: from its appearance when so diluted he is able to
judge of the time during which it may be necessary to continue the
disintegration. As soon as this is completed, the pulp is ready to be
let down to the stuff-chests, usually placed at a lower level than
the beaters, so that the pulp can flow into them by gravity. For this
purpose the valve at the bottom of the engine is opened: to remove the
last portion of pulp it is necessary to rinse out the engine with water.
TABLE.—STRENGTH OF ALUM SOLUTIONS (17·5° C., 63·5° F.).
+------------------------+-------------------+------------------+
|Percentage of | Specific Gravity. | Degrees Twaddle. |
|K_{2}Al_{2}(SO_{4})_{4} | | |
|+ 24 H_{2}O. | | |
+------------------------+-------------------+------------------+
| 1·0 | 1·0065 | 1·30 |
| 2·0 | 1·0110 | 2·20 |
| 3·0 | 1·0166 | 3·32 |
| 4·0 | 1·0218 | 4·36 |
| 5·0 | 1·0269 | 5·38 |
| 6·0 | 1·0320 | 6·40 |
+------------------------+-------------------+------------------+
{144}
CHAPTER X.
PAPER MACHINES; HAND-MADE PAPER.
The pulp as it comes from the beaters is now ready to be made into
paper. We will first consider briefly the manufacture of hand-made
paper.
It is made on a mould of wire-cloth, with a movable frame of wood,
called the “deckle,” fitting on to the outside of the mould and
extending slightly above its surface.
The wire-cloth is generally supported by a much coarser wire-cloth,
or by pieces of thick wire, and these again by wedge-shaped pieces of
wood, the thin end being next to the wire.
To form a sheet of paper the workman dips the mould, with the “deckle”
in position, into a vat containing the prepared pulp diluted with
water, lifting up just so much as will make a sheet of the necessary
thickness. As soon as the mould is removed from the vat, the water
begins to drain through the wire-cloth, and to leave the fibres on
the surface in the form of a coherent sheet of paper. The felting or
intertwining is assisted by lateral motion in every direction given to
the frame by the workman. The movable deckle is then removed, and the
mould, with the sheet of paper, given to another workman, called the
“coucher,” who turns it over and presses it against the felt, by this
means transferring the sheet from the wire to the felt. In the meantime
the “vat-man” is engaged in the formation of another sheet with a
second mould.
[Illustration: THE FOURDRINIER PAPER MACHINE.
PLATE I.]
A number of the sheets thus formed are piled together, alternately
with pieces of felt, and when a sufficient number {145} have been
obtained, the whole is subjected to strong pressure, to expel the
water. The felts are then removed, and the sheets again pressed.
They are then sized, if required, by dipping them into a solution of
gelatine: again slightly pressed, and hung up on lines or poles to dry.
Such paper is called loft-dried.
When dry the sheets of paper are calendered. (See Chapter XI.)
The making of paper by hand involves considerable dexterity on the
part of the workman; on account of the expensive labour necessary, in
comparison with paper-machines, it is comparatively little practised in
the present day; certain kinds of paper, however, such as bank-notes,
various drawing papers, and printing papers intended for the production
of very elaborate editions are always made in this way.
Any pattern or name required on the paper is obtained by means of a
raised pattern on the wire-cloth mould: consequently, less pulp lodges
there, and the paper is proportionally thinner, thus showing the exact
counterpart of the pattern. Such devices are known as “water-marks.”
(See also p. 156.)
_The Paper Machine._—The paper machine of the present day, with all
its ingenious improvements, differs but little in principle from that
originally constructed by Fourdrinier. It consists essentially of an
endless mould of wire-cloth, on to which the prepared pulp flows, and
on which a continuous sheet of paper is formed. The sheet of paper
then passes through a series of rollers and over a number of heated
cylinders, where it is completely dried.
A modern Fourdrinier paper machine is shown in plan and side elevation
in Plates I. and II.
The pulp, after leaving the beaters, passes into a large vessel called
the stuff-chest, of which there are one or more to each machine. As
soon as the beater is empty, water is run in to thoroughly rinse out
the remaining pulp, the washings also going into the stuff-chests.
These may be {146} made either of wood or iron, and should be provided
with arms fixed on a vertical shaft, made to revolve by suitable
gearing. The arms are for the purpose of keeping the pulp thoroughly
mixed, and should only work at a moderate speed, otherwise they are
liable to cause the fibres to form into small knots or lumps. The
pulp is drawn from the stuff-chests by means of the pump A, and is
discharged into a regulating-box (not shown). The object of this box
is to keep a regular and constant supply of pulp on the machine. It
consists of a cylindrical vessel, having two overflow-pipes near the
top, and a discharge-pipe near the bottom. The pulp is pumped in
through a ball-valve in the bottom, in larger quantity than is actually
needed, the excess flowing away back into the stuff-chests, through
the two overflow-pipes. By this means, the box is always kept full,
and therefore the stream of pulp issuing out of the bottom pipe is
always under the same pressure. It flows from this pipe, the quantity
being regulated by means of a cock, according to the thickness of paper
required, directly on to the sand-tables. These may be of various
sizes and shapes, but should be so large that the pulp takes some
little time to travel over them. They consist of long shallow troughs,
generally of a sinuous form. The bottoms are sometimes covered with
woollen felt, or with thin strips of wood placed across the direction
of the flow of the pulp, and at a slight angle. These serve to retain
any particles, such as sand and dirt, that may have escaped removal
in the previous treatment of the pulp, and that are heavy enough to
have sunk down during the passage of the pulp along the sand-tables.
In some mills, where great care is exercised, the pulp is caused to
flow over sand-tables 200 yards in length. As the pulp, when it leaves
the stuff-chests, does not contain sufficient water for the purpose
of making paper, it is mixed, where it enters the sand-tables, with a
quantity of water from the “save-all” (see p. 154), flowing from the
box B placed at a higher level.
In some mills, instead of being pumped into the {147} regulating-box,
the pulp flows into a small vessel below the stuff-chest, and is lifted
on to the sand-tables by means of buckets fastened on the circumference
of a wheel.
The pulp, after leaving the sand-tables, passes on to the strainers.
These consist of strong brass or bronze plates, having a large number
of very fine V-shaped slits cut in them, the narrowest end being on the
outside.
_Strainers._—The strainers are for the purpose of removing from the
pulp all lumps formed by the intertwining of the fibres, and all pieces
of unboiled fibre, which, if allowed to pass on, would show in the
paper as inequalities in the surface, or as dark specks. The slits
are made narrow at the top, and gradually increasing in width, so as
to prevent them from getting choked up. These slits allow only the
individual fibres to pass through, and their width varies according to
the quality of the paper. They are from 2 to 3 in. long, and they vary
in width from ·007 to ·05 in. They are put at distances of about 1/4
in. apart. Several plates, each containing about 500 slits, are bolted
together, and form a strainer. The whole strainer receives a violent
shaking motion, to assist the passage of the fibres through the slits.
In the machine represented, two of these strainers are shown at C. The
shaking motion is produced by the ratchet-wheel or cams _a_ acting on
the hammer _b_. An improved form, called the “revolving strainer,” has
of late years been introduced. The pulp generally passes first through
one of these, and then through the ordinary or “flat” strainers, as
they are called. A revolving strainer is shown at D. It consists of a
rectangular box, the sides of which are formed of plates perforated
with slits. Inside this box, a slight vacuum is formed by means of an
indiarubber bellows worked by a crank on the shaft _d_. The vacuum is
intended to serve the purpose of the shake in the ordinary form. The
box revolves slowly inside a vat containing the pulp, and the strained
pulp flows into the box D^1, and thence on to the flat strainers.
[Illustration: FIG. 45.]
Various patents have been taken out from time to time for {148} flat
strainers worked by means of a vacuum underneath the plates caused by
the motion of discs of indiarubber or thin metal. Fig. 45 shows in plan
a set of strainers, as manufactured by Messrs. G. & W. Bertram, similar
to those in Plate II., but illustrated somewhat more in detail.
The pulp first passes through the flat strainer B, and from thence to
the two revolving strainers A. From these it flows along the shoots
placed at the side on to the paper-machine at E.
[Illustration: FIG. 46.]
Fig. 46 gives a view of a patent flat vacuum strainer made by the same
firm, which may also be used for cleaning straw or esparto previous to
its passage over a _presse-pâte_. The pulp flows on to the strainer
at _a_, and passes away by {149} the cast-iron pipes _f_. The valve
_g_ is for running off waste pulp. The plates are placed at a slope
of about 1 in. in their length; those nearest the supply of pulp are
provided with coarser slits, as the impetus carries the knots forward.
The vacuum pumps are worked by the rods _d_ from the shaft _e_. By
means of the tubes _c_ water can be directed on to the plates, whereby
the coarser particles of fibre are carried forward, and the slits are
kept clean. The plates can be removed in a few minutes.
Figs. 47 and 48 show in side and end elevation, Messrs. Masson, Scott,
& Bertram’s patent self-cleaning strainer. The novelty consists in
an arrangement by means of which the upper surface of the plates are
continually freed from those portions of the pulp which cannot pass
through the slits.
The scrapers _d_ are made of vulcanised indiarubber, and are
continually carried forward by an endless chain; the knots, &c.,
collect in a heap at the end of a strainer. The {150} pulp flows on at
_a_, and passes away through the pipe _b_. The pumps are driven from
the shaft _e_.
[Illustration: FIG. 47.]
[Illustration: FIG. 48.]
Messrs. C. H. & F. L. Roeckner’s patent strainer (No. 7932, 1885)
consists of a series of cylindrical tubes, open at one end and
perforated with slits. They are placed in a vat into which the pulp
flows. Inside the cylinders are placed two, three, or more plates,
fixed to the shafts on which the cylinders are supported, and extending
to the circumference. These plates form a kind of fan, which, together
with the cylinders, are caused to oscillate by means of a rod and
cranks. This oscillating motion serves to draw the fibres through the
slits, and at the same time to keep the outsides of the cylinders
clean. The cylinders are easily removable.
After a time, the slits in the plates get too large, owing to the plate
having been worn away by the constant friction of the fibres, and as
they are very expensive, various attempts have been made to invent
plans for partially closing them again. Hammering will effect this, but
is liable to break the plates. Annandale of Beltonford has introduced
a method of closing the plates, by means of heavy pressure acting on
small steel rollers moving on each side of the slit, in which is placed
a small sheet of metal the exact thickness of the width desired. {151}
Another method of closing the plates consists in filling them up by
means of electrically deposited copper or other metal. They can then be
recut in the usual way.
In the case of revolving strainers, all that cannot pass through the
slits falls to the bottom of the vat, in connection with which it is
customary to have an auxiliary strainer, or “patent knotter,” as it is
called, shown at E. All fibre that passes through this one, which is of
the ordinary flat kind with shaking motion, goes into a box near E′,
called the “low box” for “save-all” water (see p. 154).
The pulp, after passing through the strainers, should be perfectly free
from knots and impurities, and in a fit condition for making paper.
In the machine shown, it passes from the last strainer directly on
to the wire, its flow being regulated by a movable gate _e_. In some
cases, however, it first flows into a small vat, in the centre of which
revolves a rod carrying paddles, with the object of keeping the pulp
well stirred up. It is carried right on to the wire by means of the
apron, a piece of canvas, oil-cloth, or sheet rubber, one end of which
is fastened to the breast-board _e′_, the other end resting on, and
covering the wire to the extent of about 15 in. The edges of the apron
are rolled up to prevent the pulp from overflowing. After leaving the
apron, it passes under a gate, or “slicer,” as it is sometimes called,
made of two pieces of brass, overlapping each other in the centre,
and bolted together. It is made thus to enable it to be lengthened or
shortened according to the width of the paper; its height from the
wire-cloth can be altered by means of screws, and should be equal at
all points, in order to ensure a uniformly thick sheet of paper. The
ends of the two pieces forming the slicer are fastened to the frame _f_
or “deckle,” as it is called, and this again is carried by two or more
rods stretching right across the wire, and fastened by small upright
supports on both sides to the frame _g_. The deckle-frame also carries
the grooved pulleys _h_, along which the deckle-straps _i_, endless
square bands of indiarubber, move. {152}
[Illustration: FIG. 49.]
The object of the deckle-straps is to regulate the width of the paper;
they form, together with the wire-cloth, a kind of mould into which
the pulp flows, thus corresponding to the mould used in hand-made
paper-making. The width of the paper can be altered by shifting the
position of the frame _f_, and also the deckle-straps, which are
carried on it as described, the pulleys _h_ being so arranged that
they slide along the rods on which they revolve. In order to alter
the width of the paper it is necessary to stop the flow of pulp on to
the wire, and it not only consumes a considerable amount of time, but
generally necessitates a partial cleaning up of the machine. Various
attempts have, therefore, been made to devise an arrangement whereby
the change can be effected while the paper is being made. Several {153}
contrivances have lately been introduced, all similar to the one shown
in Figs. 49 and 50.
In it the frame _f_ carrying the deckle-strap is made to slide along
the rods _e_ by means of the small wheel _b_, and by a similar
arrangement on the opposite side and geared with it. The movable apron
_l_, Fig. 49, is wound round the spindle _g_, and is kept taut by the
cords _m_ connected with the springs _n_. As the deckles approach
each other, the excess of apron is wound up, when they are separated
it unwinds again. The flow of pulp is regulated by two slices _a a_,
which are kept in position by the screws _h_. The whole arrangement is
securely bolted to the frame of the machine; _c_ (Fig. 50) represents
the breast-roll, and corresponds to F, Plates I. and II.
[Illustration: FIG. 50.]
The thickness of the paper is regulated by altering the supply of pulp
to the wire-cloth, and by the speed at which the machine is working.
This speed may vary from about 60 feet to as high as 270 feet per
minute.
The “wire” is an endless cloth made of very fine wire, the fineness
depending much on the quality of the paper required. The mesh varies
from 60 to 70 and even more threads to the inch. It is not woven
endless, but is joined by very careful sewing with wire. Its width
varies considerably, some {154} being made as wide as 126 inches; the
length is generally 35–40 ft. It is carried by the breast-roll F, the
lower couch-roll G, and the small rolls _f′_, and by a large number
of small rolls _f″_. The latter and the breast-roll are supported by
the frame _g_, while the small rolls _f′_ are supported by brackets
attached to it. The course of the wire is indicated by the arrows. The
frame _g_ works on two pivots _g′_, and receives a shaking motion from
side to side from the rod _j_, in connection with a crank worked by
two conical drums H. The supports _g″_ are also pivoted at their lower
ends to allow for the shaking motion. This shaking motion is given for
the purpose of weaving or intertwining the fibres. One or more of the
rolls _f′_ can be moved up or down on the support which carries it, for
the purpose of stretching the wire. There is usually a large number of
the small rolls _f″_, as it has been found by experience that, probably
owing to capillary attraction, they, cause the water to leave the pulp.
Though a large quantity of water thus passes through the wire-cloth, it
is necessary to assist it by artificial means. This is done by means of
the suction-boxes I connected by pipes with the vacuum-pumps I′.
This part of the machine, which is called the “wet-end,” is placed at a
slight slope of about 1 in. in its entire length, the lowest end being
nearest the strainers.
Underneath the wire-cloth is placed a box called the “save-all” K,
connected with the box E′. The water that passes through the wire-cloth
contains a considerable quantity of very fine fibres, together with
size, alum, clay, and colouring materials, that have passed through the
wire, and which would be lost but for the arrangement now universally
adopted. It flows into the box E′, and is pumped, together with the
pulp that has passed through the knotter E (see p. 151), into the high
box B, whence the mixed stuff flows on the sand-tables, to be again
used to dilute fresh pulp from the stuff-chests. The following numbers
will give some idea of the nature and amount of fibre, &c., which
passes through. {155} The paper was made from esparto and straw, sized
with rosin and starch. It contained 12 per cent. of clay.
Grains per Gallon
of Waste Water.
Fibre 34·37
Clay 37·10
Starch 1·40
[Illustration: FIG. 51.]
[Illustration: FIG. 52.]
It is almost impossible to utilise the whole of the back-water passing
through the wire-cloth in the way described. In some mills a portion of
this water is made to pass through a “pulp-saver,” such as is shown in
Figs. 51 and 52. It consists of a conical drum A, the circumference of
which is covered with wire-cloth, and it is caused to revolve slowly by
suitable gearing. The water enters by the pipe B, and passes through
the meshes of the wire-cloth, the pulp gradually finding its way to
the wider end, where it is discharged into the box C. It can then be
returned to the beaters.
This pulp-saver can also be used for recovering the fibre from
other waste water, such as the wash water from the washing and
beating-engines; or it can be used for freeing bleached pulp from water
in cases where drainage or hydraulic pressure is not resorted to.
If any pattern or name is required on the paper, it is produced by
means of a light skeleton roll, called a “dandy-roll,” covered with
raised wires in the form of the desired pattern, {156} placed between
the suction-boxes, and pressing lightly on the still moist paper.
The paper is thinned where the wire pattern presses, and thus a mark
(water-mark) is produced. The other side of the paper has a mark
corresponding to the wire-cloth; by using a dandy-roll covered with
wire-cloth, the two sides can be obtained alike, such paper going by
the name of “wove.”
Paper in which a series of parallel lines are produced is called a
“laid” paper.
Some water-marks are produced by means of a dandy-roll in which the
pattern is formed by depressions in the surface. The paper is thus
_thicker_ where the pattern is formed than in the ground-work. De la
Rue (Patent No. 8348, 1884) has patented the use of dandy-rolls so
formed as to produce upon the paper the effects both of thickening and
thinning.
Imitation water-marks can be produced on the finished paper by
subjecting it to pressure in contact with plates on which the design
has been produced in relief. In this way very beautiful results can be
obtained. (See Patent No. 13,455, 1884.)
It sometimes happens that the wire-cloth slips slightly to one side.
This can be obviated by the machine-man shifting, by means of screws,
one of the rolls provided for the purpose with a movable journal, until
its axis is at a slight angle to that of the other rolls. An automatic
apparatus has been invented for this purpose. Two brass plates are
fixed, one on each side of the wire-cloth, to a long rod, connected
by suitable machinery with the screws working the movable journal, so
that, as the wire presses against one or the other of these plates, the
roll is shifted so as to correct this.
The paper, which, even after passing the suction-boxes, is still very
wet, passes with the wire-cloth between the couch-rolls G G′. These
are hollow copper or brass cylinders, covered with a tightly-fitting
endless jacket of felt. They may also be made of wood (sycamore or
mahogany) or of iron or iron and brass combined. The pressure of the
upper couch-roll upon the lower can be regulated by means of {157}
screws or levers. They serve to press out water from the paper, and to
detach the paper from the wire-cloth. By dexterous manipulation on the
part of the machine-men, the paper is transferred to the endless felt,
travelling over the rolls _k_ in the direction of the arrows. It is
known as the “wet felt,” from the condition of the paper at this stage.
In its passage along this felt, the paper passes between two iron rolls
K, called the first press-rolls, with the object of having the water
squeezed or pressed out of it. These rolls are sometimes covered with a
thin brass case, and the top one is provided with an arrangement called
the “doctor,” in order to keep it clean, and free from pieces of paper
that may have stuck to it. The lower press roll is sometimes covered
with an indiarubber jacket. The “doctor” is a kind of knife placed
along the whole length of the roll, and pressing against it at every
point. The pressure on the rolls can be regulated by means of levers,
or, as in the illustration, by the screw _k′_. It will be readily seen
that the under side of the paper that has been next to the felt will,
in its still moist condition, have taken to some extent an impression
from the felt, while the upper side will have been made comparatively
smooth by the pressure against the top roll of the 1st press. In order
to make both sides of the paper as nearly as possible alike, it is
passed through another set of rolls L, called the 2nd press. This time
it is reversed, and enters at the back of the rolls; thus the other
side of the paper is next the metal, being taken through by the felt
(called the “2nd press felt”) travelling on the small rolls _l_, the
paper, after leaving the wet felt, and before being taken on to the
2nd press felt, travels over the rolls _l′_. The 2nd press felt is
necessary, because the paper is too tender to withstand, unsupported,
the pressure of the rolls.
[Illustration: FIG. 53.]