drying-cylinders M, the number of which varies somewhat. In the
machine shown, there are in all eight cylinders. Sometimes as many
as twelve are employed. Between the {158} 2nd press rolls and the
cylinders, a passage S allows easy access to the other side of the
machine. The paper generally passes alone over the first two, which
are only slightly heated; afterwards it is led over the others by
means of felts, as shown. The arrangements shown at R are for the
purpose of stretching the felts. The cylinders are heated by means
of steam, and are generally divided into two sets, between which is
a pair of chilled-iron, highly-polished rolls N, called “smoothers,”
the function of which is sufficiently explained by their name. They
are also heated by means of steam. The cylinders are usually made of
slightly decreasing diameter, in order to allow for the shrinking of
the paper on drying. Messrs. G. & W. Bertram introduce into some of
their machines one or two small drying cylinders, over which the 2nd
press felt travels, the object being to drive off some of the moisture
absorbed from the paper. This contrivance is said to give excellent
results, and to ensure a considerable saving in felts. The arrangement
is shown in Fig. 53. It is especially useful in machines running at
a high speed, or in those having limited drying power. After leaving
the cylinders, the paper should be quite dry; it is then led through
the calenders, of which there are in some machines {159} as many
as three sets, though only one is shown. These are similar to the
smoothing-rolls, just described. Pressure is applied by the screws O′,
or by levers and weights. The friction of the hot calenders on the dry
paper develops a large quantity of electricity, which occasionally
discharges itself in bright sparks.
It is the practice in some mills to cool the paper before passing it
through the calenders. This is effected by passing it over a copper
cylinder, through which a stream of cold water runs.
The degree of smoothness or “finish” that can be given to the paper by
the calenders, depends to a large extent upon the degree of moisture
which it contains. As it leaves the last cylinder it is perfectly dry,
that is to say, it contains only that amount of water which cellulose,
from whatever source, always carries. This amount varies slightly with
the nature of the cellulose, and with the plant from which it has been
isolated, and also with the state of the atmosphere, it being greater
on damp days. It would be impossible so to regulate the drying action
of the cylinders that the requisite amount of water should always be
left in the paper; it is therefore better to dry it as thoroughly
as possible, and then to add the water, by artificial means, just
before it passes through the calenders. This method, moreover, has the
advantage of damping only the surface of the paper.
Fig. 54 shows a damping arrangement as manufactured by Messrs. James
Bertram & Son. The paper on leaving the last drying cylinder passes
on to two copper cylinders _c_, filled with cold water. Steam passes
through the pipe _a_, and issues through a number of fine holes in a
pipe running at right angles to the direction in which the paper is
travelling, and near to it. The steam condenses on the paper and on the
surface of the cylinders, from which the paper absorbs it. The supply
can be regulated by the cocks shown in the drawing. The troughs _d_
carry off any excess of condensed water.
Amongst other methods proposed is one by Annandale; it {160} consists
in breaking up jets of water into a very fine spray by means of a blast
of air.
The finished paper, after passing through the calenders, is wound on
the reels P. The gearing by which the whole machine is driven is shown
in Plate II.
[Illustration: FIG. 54.]
It sometimes happens that, owing to the increased tension due to the
contraction of the paper in drying, the paper breaks. It is therefore
necessary to alter the speed of some of the cylinders to compensate
for this. A rough expedient which is largely adopted, consists in
attaching, by means of a mixture of rosin and tallow, a piece of felt
to one or other of the pulleys (Plate II.), and thus altering its speed.
_Tub-sizing._—The foregoing description is of a machine for the
manufacture of engine-sized papers; some slight modifications are
necessary in the case of papers that are tub-sized on the machine. In
making the cheaper qualities of tub-sized papers, the paper, after
being partially dried over a few cylinders, is passed through a vessel
containing a {161} solution of gelatine (see p. 162). It then goes
between a pair of rollers, which press out the excess of size, and then
again over drying-cylinders. The vessel or trough containing the size
is filled to overflowing from a tank placed at a higher level; the
overflow passes into a lower vessel, from which it is pumped up to the
top tank. The size is kept warm by means of a coil through which steam
passes placed in one or other of the vessels containing it. The paper
passes between a pair of rollers placed in a trough and dipping into
the size. The size may be made to flow through a pipe pierced with a
number of holes, the stream running directly on to the web of paper.
In the other, and perhaps most general way, at least for the better
qualities, the paper is wound off immediately after leaving the last
drying-cylinder A (Fig. 55), and sized at some future time, or it may
be passed directly into the vat B containing the size. After passing
between the squeezing-rolls. C, it is generally wound off as at D; and
after having stood some time {162} to allow the size to be evenly
absorbed by the paper, it is wound off from F, and passed over the
cylinders H, of which there may be a very great number, some machines
having over 300 of them. These cylinders are made of light spars of
wood; inside them, and revolving rapidly in an opposite direction, are
the fans G. The paper, in travelling over these drums, is only slowly
dried, and is supposed by this means to be more perfectly sized, and
increased in strength. It is wound on to reels again at I. Only the
first and last two drums are shown. This method was devised to imitate,
as far as possible, the sizing process of hand-made paper. Even now,
paper that has been made on the machine is sized by hand, after having
been cut into sheets, much in the same way as hand-made.
[Illustration: THE FOURDRINIER PAPER MACHINE.
PLATE II.]
[Illustration: Fig. 55.]
_Preparation of Size._—Very pure gelatine can now be obtained in the
form of light-coloured cakes. It is, however, more economical for the
paper-maker to prepare his own: this is therefore done in the majority
of cases. A great many animal substances, such as clippings of hides,
horns, bones, &c., yield gelatine when heated with water. Any of these
substances may be used, the first being the chief raw material. They
are first softened by soaking in cold water for some days: they are
then cleansed by washing in a stream of water. The next operation is
that of heating with water. This is usually done in iron or copper
vessels provided with a false bottom or outer jacket, into which steam
may be passed. The hide cuttings are covered with water, and the mass
gradually raised to a temperature of about 85° (185° F.). In from 10
to 15 hours nearly the whole of the cuttings will have passed into
solution as gelatine. This solution is then drawn off and any insoluble
substance suspended in it is removed by subsidence or filtration. The
hides contain a certain quantity of fat, which collects on the surface
of the solution. The residue in the boiler is again heated with water
and the solution added to the bulk. It is of the greatest importance
that too high a temperature be avoided, as gelatine is liable, when
strongly heated with water, to lose its power of gelatinising. {163}
To the filtered solution of gelatine, which should be, if properly
prepared, of a pale colour, a quantity of alum solution is added.
The effect of alum upon gelatine is very remarkable. If added slowly
it will be found gradually to render it stiff until at a certain
point the mass will become almost solid; a further addition renders
it fluid again. It is in this state ready to be used for sizing
purposes. The amount of alum necessary to produce this effect is
about 20 per cent. of the weight of the raw material. The alum is
also useful in preventing the decomposition of the gelatine, but its
chief characteristic is to render the gelatine a more efficient sizing
material. There is no doubt that, besides its action upon the gelatine
itself, it has a considerable effect upon cellulose. It is of the
greatest importance that the alum or the sulphate of alumina, which can
be substituted, should not contain any free acid. This is especially
necessary in the case of papers made from rags, in the bleaching of
which an acid has been used; one effect of which is to remove all basic
substances derived from the boiling or bleaching processes. In the
case of esparto and similar fibres, a considerable quantity of such
substances are present in the pulp, the result being that the free acid
of alum is to a large extent neutralised, and its injurious effects
prevented.
The effect of the free acid is seen in the weakening of the paper and
the destruction of metal surfaces with which it comes in contact. The
effect of acid upon cellulose will be found more fully described in p.
12.
Many paper-makers add to the gelatine a certain quantity of soap,
the effect of which is to render the paper capable of taking a high
finish. The soaps employed should be white and firm, and should be free
from rosin. Some soap-makers prepare a special soap for paper-making.
They are usually made from tallow, or a mixture of this with a small
quantity of coconut oil.
If a solution of soap be added to one of alum, a double decomposition
immediately occurs, the fatty acid being thrown down in combination
with the alumina, and the soda {164} combining with the sulphuric
acid. If, however, the soap solution be previously mixed with a
solution of gelatine, no precipitation takes place, the mixture forming
an emulsion having somewhat the appearance of thin milk.
Various attempts have from time to time been made to size paper in the
engine with gelatine, by precipitating it in the pulp after the manner
of rosin sizing, but as yet no successful method has been devised.
_Single-cylinder Machines._—Modified forms of the original Fourdrinier
machine have been introduced to meet various requirements. One suitable
for the manufacture of very thin paper, or of papers one side only of
which is required to be glazed, called a single-cylinder or Yankee
machine, is shown in Fig. 56.
[Illustration: FIG. 56.]
It resembles the Fourdrinier machine as far as the couch-rolls A and
B. The paper is taken off the wire-cloth on to an endless felt running
round the upper couch-roll A, and travelling in the direction of the
arrows. It is taken from the felt on to the large drying-cylinder C,
of about 10 feet diameter, heated with steam. It is carefully turned
and polished so as to impart a high gloss to the surface of the paper
with which it is in contact. Calender-rolls are sometimes supplied in
addition.
The arrangement shown at D is for the purpose of washing the felt. This
is necessary to cool and open it out after passing between the cold
press-roll E and the hot cylinder. {165}
The paper, after passing over the greater part of the surface of the
cylinder, is sufficiently dried, and it is then wound off at F.
[Illustration: FIG. 57.]
A machine of a very different construction from the ordinary form is
shown in Fig. 57. The pulp, after passing through the strainer A,
enters the vat B, in the centre of which a large drum or cylinder C
revolves. This cylinder is covered with fine wire-cloth, and on it the
paper is made. As it revolves, the fibres attach themselves to the
wire, and the water passes through the meshes, being assisted by means
of a pump. The sheet of paper thus formed is taken on to the endless
felt passing round the couch-roll D, and travels along with it to the
large drying-cylinder E, heated by steam. It leaves the felt at F,
and is then taken on to the cylinder, after travelling round which
it is sufficiently dried, and is then wound off as at G. The felt on
its return journey passes through the washer H, where it is cleaned
and freed from adhering particles by the scraper I. It is squeezed
free from excess of {166} water by the rolls K. Paper made on such a
machine is weaker than that made in the ordinary way, because it has
not been found possible to give a shaking motion to the cylinder, and
thus the fibres are only imperfectly felted.
A modification of this machine is used for making millboards, the
difference being that it has no drying-cylinder. The felt carrying
the paper passes between a pair of press-rolls, which squeeze out
the water. The sheet of paper is then allowed to wind round the top
press-roll until of the required thickness. When this happens, it is
cut off the roll by a knife. The thick sheets so produced are dried
either in the open air or in a room heated for the purpose. (See p.
108, Fig. 31.)
{167}
CHAPTER XI.
CALENDERING, CUTTING, ETC.
The paper, as it leaves the machine, is for many purposes not
sufficiently highly glazed, and it is therefore necessary for it to
undergo a further process of calendering. This may be done in various
ways.
[Illustration: FIG. 58.]
One method, called “web glazing,” is to pass the paper between a number
of rolls, alternately of polished iron and very highly compressed paper
or cotton. The construction of such a calender will be understood by
reference to Figs. 58 (end elevation) and 59 (front elevation). The
reel of paper, as taken from the machine, is shown at A (Fig. 58), its
course over the rolls being indicated by arrows. After passing over
the bottom roll, it is wound off on a wooden or hollow iron cylinder
B (Fig. 58), driven by the toothed wheel shown by the dotted line C,
on the same shaft as the wheel D, which is driven by E, keyed upon the
bottom roll. The whole machinery is driven by the large toothed wheel
F (Figs. 58 and 59), which is itself driven by the small wheel G on
the main shaft H. The paper rolls are marked P, and the iron rolls I.
It will be seen that there are two paper rolls in the middle, for the
purpose of, as it were, reversing the paper, and so making both sides
alike. Pressure is applied to the rolls by means of the screws K, and
by the weight L (Fig. 58) acting on the compound lever M. The brake,
which consists of a strap of leather, pressing, by means of the weight
and lever N, on the circumference of the wheel O, connected by toothed
wheels with the cylinder A on which the paper is wound, is used for the
purpose of preventing the paper from leaving {168} the cylinder too
rapidly. But for this appliance, the paper would be apt to crease. The
paper rolls have an inner core of iron, the paper only extending to a
depth of about 5 in. The iron rolls are hollow, and are connected with
steam-pipes, by which they can be heated. {169}
[Illustration: FIG. 59.]
Another method, known as “friction-glazing,” employed for giving a
very high finish to paper, generally on one side only, is to pass it
between a large paper roll and a smaller iron one, the latter revolving
at a much greater speed than the {170} former. By this means a very
smooth surface can be obtained. It is sometimes assisted by rubbing a
small quantity of bees’-wax on the small iron roll. The above-mentioned
methods apply to the glazing of paper in the web. Paper cut into
sheets may also be treated in the same way. Various modifications of
these calenders have been devised; they do not, however, involve the
application of any special principle. Plate-glazing, a method that is
adapted to hand-made and the better qualities of paper, consists in
applying heavy pressure to sheets placed between polished plates of
copper or zinc. The metallic plates and the sheets of paper are made
into bundles, and the whole is passed between two strong rolls, heavy
pressure being communicated to them by means of screws or levers and
weights applied to the ends of the upper roll.
[Illustration: FIG. 60.]
A calender for this purpose is shown in Fig. 60. The bundle of plates
and paper is passed along the table _a_; after {171} passing through
the rolls _c_, it slides along the inclined table _d_, where it is
received by a workman. The pressure on the rolls is regulated by the
handle _e_ and the weights _b_.
By passing paper between rolls on which devices have been cut, the
“repped” and other similar papers are produced.
With calender rolls of the ordinary construction, as the pressure is
applied at the extreme ends, the roll is liable to assume a slightly
curved shape, the effect of which is to produce an uneven surface on
the paper, the outer portion of the web being more highly finished than
the centre.
[Illustration: FIG. 61.]
This defect is obviated to a very large extent by Schurmann’s Patent
Anti-Deflection Rolls (Fig. 61). The roll proper consists of an outer
shell _a_, through the centre of which and securely wedged in at _b_ is
the centre core _c_, the ends of which run in journals, and to which
pressure is applied in the usual way. The pressure is communicated to
the outer shell at the point of contact _b_, the result being that
the parallelism of the surfaces of the rolls is maintained, and in
consequence, the paper tends to be equally finished in every direction.
_Cutting._—Except for special purposes, such, for example, as for use
in a continuous printing machine, paper is usually sent from the mill
in the form of sheets. The form of cutter, called a revolving cutter,
generally used, is shown in Fig. 62. The paper from the webs A is drawn
forward by the rolls B; it is then ripped into widths of a convenient
size by means of two circular knives, the upper one of which is shown
at C. It again passes between a pair of rollers, after leaving which it
meets a knife D fastened to the revolving drum E, and pressing against
a fixed knife not shown. The cut sheets {172} then fall upon the
endless travelling felt F. The action of the knives will be understood
by reference to Fig. 63. The edges of the two knives are shown at A and
B. The knife B has a slot, in which the bolt D slides, and it is kept
in position by means of a spring. This spring causes the knife to slide
back slightly as it comes against the fixed knife A. The position of
the paper is shown by the dotted line C. The knife B is set on the drum
not quite horizontally, so that one end meets the stationary knife a
little before the other, thus acting in every respect like a pair of
scissors. Fig. 64 shows a pair of ripping-knives. The upper one A is
kept in position against the lower one B by means of the spring C. The
cutting surfaces are slightly hollowed out, so as to have a sharper
edge. The paper is shown by the dotted line D. By altering the relative
speed of the drum E {173} and the rolls B, by means of the expanding
pulley G, sheets of any desired size can be cut. The cutting-knives
are sometimes placed inclined to the drawing-in rolls B, so that the
sheet, instead of being cut into a rectangle, is cut into a rhomboid.
Such paper is used chiefly for the manufacture of envelopes, this shape
occasioning a smaller loss when the envelopes are cut out.
[Illustration: FIG. 62.]
[Illustration: FIG. 63.]
[Illustration: FIG. 64.]
_Single-sheet Cutter._—It is sometimes necessary, as in the case of
paper having a water-mark, that the sheet should be cut with great
exactness, so that the device shall come exactly in the centre. The
ordinary cutter cannot be relied on for this purpose, and, in its
place, a machine called a “single-sheet cutter” is used (Fig. 65). It
consists essentially of a large wooden drum A, fixed on a horizontal
axis, over which the paper is led by a pair of drawing-in rolls B. The
paper is held against the drum by a clamp worked by the arm C. The
paper is cut by the knife E moving against the stationary knife D.
After the cut, the drum describes part of a circle, the paper being
still held, so that it cannot go back with the drum. As soon as it has
gone far enough, the clamp is removed, and the drum returns, bringing
the paper with it. The length of the arc through which the drum moves,
and therefore the size of sheet, is regulated by the length of the
crank-arm F. If from any cause the cut should not take {174} place at
the right time, the man in charge can, by pressing against the clamp,
retard the motion of the paper, and thus bring back the cut to the
right place. The small roller G is for the purpose of keeping the paper
always tight.
[Illustration: FIG. 65.]
_Guillotine Cutter._—It is sometimes necessary to trim the edges
of sheets of paper or to divide them into smaller sheets. This is
done by means of a guillotine cutter, an improved form of which, as
manufactured by Salmon of Manchester, and called the “Victory” cutter,
is shown in Fig. 66.
The paper to be cut is placed on the table F, touching the back gauge
G, which can be brought backwards or forwards by means of the handle H,
thus regulating the size of sheet.
The machine is set in motion by means of the lever A, which acts upon
a friction-clutch connected with the driving-gear. This causes the
knife-bar B and the clamp C to descend. {175} When the latter meets
the paper its progress is arrested. The knife D, however, continues
to descend, and passes through the sheets of paper. By means of slots
placed at the back, the knife is made to take a diagonal course. The
pressure of the clamp is maintained upon the paper during the cutting
operation by means of a powerful spring contained in the tube E.
[Illustration: FIG. 66.]
_Sorting._—The sheets of paper are now ready to be examined
before being finally sent away from the mill. This is done in the
“Finishing-house,” or “Salle” as it is sometimes called. This sorting
is usually performed by women, who reject the inferior or damaged
sheets. These are sold at a reduced price under the name of “retree.”
Paper is sold in sheets of definite sizes, and is made up into reams
containing from 480 to 516 sheets. These sizes {176} correspond
to different trade names—such, for example, as crown, demy, royal,
imperial, &c. The weight per ream is usually expressed in addition to
the name, thus: 14-lb. demy, 18-lb. double crown, &c. In this way the
consumer is enabled to make a rough comparison of the thickness of the
different kinds of paper.
In making paper it is the duty of the machineman to examine and weigh
from time to time a sheet of the desired size, in order to ensure
uniformity. Special lever balances can be obtained, showing at a glance
the weight per ream of different numbers of sheets. {177}
CHAPTER XII.
CAUSTIC SODA, RECOVERED SODA, ETC.
As we have already pointed out, caustic soda and lime are the only
alkalis employed by the paper-maker for boiling fibres; the special
cases in which carbonate of soda is used have been mentioned in their
proper place.
The preparation of milk of lime requires little or no description, as
the operation is a simple and tolerably familiar one. Before using, it
should be passed through a fine wire sieve, to keep back sand, coal,
and similar impurities which the lime invariably contains.
Caustic soda is met with in commerce in four forms, differing from each
other in alkaline strength and colour, and of course in price. The
lowest quality is what is known as caustic “bottoms”: it consists of
that portion remaining at the bottom of the caustic-pot after the clear
fused caustic soda has been ladled out, and it contains a considerable
quantity of ferric oxide and other insoluble impurities. It is of a
dark reddish brown colour, and contains 50–60 per cent. of alkali
(Na_{2}O). Its use in paper-making cannot be recommended, except for
the preparation of the very lowest grade of pulp. The solution should
be allowed to remain at rest, in order that the insoluble matter may
subside.
The next in quality is known as cream caustic, so called from its
slightly brown colour. It is usually sold containing 60 per cent. of
alkali, in the form of a tolerably friable mass, having a crystalline
structure. It is a very suitable form of alkali for the paper-maker.
The next in order is what is called 60 per cent. white. It {178}
consists of a hard white mass, requiring considerable force to break
it. Though whiter in appearance than cream caustic, it is in reality
less pure, as it contains a considerable quantity of salt. It is made
by continuing the evaporation of the caustic liquor to a further point
than is the case with cream caustic, the result being that the whole
of the water is driven off. A small quantity of nitre is then added to
oxidise the sulphides and other compounds which impart the colour to
cream caustic. Common salt is then added to reduce its strength to 60
per cent., in order to satisfy the whim of the consumer, who insists
upon an article of a given definite strength. On this account it is
somewhat inferior to cream caustic, and is, moreover, more expensive.
The highest quality of all is white 70 per cent. This resembles white
60 per cent. in appearance, but is much purer, and more expensive.
A very pure form of caustic soda has within the last few years been
introduced by the Greenbank Alkali Company. It consists of small
fragments, more or less crystalline, of almost pure sodium hydrate. It
can be packed in wooden casks, and it dissolves very readily in water.
It contains 76 per cent. of alkali. It is, however, much too costly for
boiling purposes; it might, however, be used for the preparation of
rosin size.
The following analyses of different forms of caustic soda and their
relative prices will possibly be of interest:—
-----------------+-----------------+-----------------+---------------
| (Davis.) | (Davis.) | (Morrison.)
| White (70 Per | White (60 Per | Cream (60 Per
| Cent. Na_{2}O). | Cent. Na_{2}O). | Cent. Na_{2}O).
-----------------+-----------------+-----------------+---------------
Sodium hydrate | 89·60 | 75·25 | 70·00
Sodium carbonate | 2·48 | 2·53 | 5·00
Sodium chloride | 3·92 | 17·40 | 7·00
Sodium sulphate | 3·42 | 4·40 | 2·00
Sodium sulphide | 0·02 | 0·03 | —
Sodium silicate | 0·30 | 0·30 | —
Sodium aluminate | trace | trace | —
Water | none | none | 15·80
Insoluble matter | none | none | 0·20
-----------------+-----------------+-----------------+---------------
{179}
Their relative prices per ton f.o.b Liverpool, in December 1886,
were:—70 per cent. white, 8_l._ 5_s._; 60 per cent. white, 7_l._ 5_s._;
60 per cent. cream, 7_l._
All the above-mentioned forms of caustic soda occur in the form of more
or less hard masses, and are contained in thin wrought-iron drums,
which are filled with the caustic in a fused state. The soda is removed
by breaking the drum with a chisel and hammer. The lumps of caustic may
be put direct into the boiler, though it is better to dissolve them
previously in water and allow any insoluble impurities to settle to the
bottom of the solution.
Instead of buying caustic soda direct, it is the custom of some
paper-makers to prepare it for themselves from carbonate of soda, which
can now be obtained in various forms, some of great purity.
It occurs as caustic ash, containing about 40 per cent. of alkali as
carbonate, and about 10 per cent. as caustic soda; soda-ash containing
about 52 per cent. total alkali, nearly all as carbonate; and refined
soda-ash containing 57–58 per cent. of alkali as carbonate. Some forms
of carbonate of soda, such, for example, as those made by the ammonia
process, are of even greater purity. The method of converting sodium
carbonate into caustic soda will be described subsequently.
Unless the arrangements for washing and draining the lime-mud are very
perfect, it is probably cheaper for the paper-maker to buy his caustic
direct from the maker.
_Soda Recovery._—In former years, the liquors in which rags, esparto,
and other paper material had been boiled, was run into the nearest
watercourse; but now, owing partly to the fact that is insisted upon
by the Rivers Pollution Act of 1876, and partly because it can be
made remunerative, all these liquors are preserved, and the soda they
contain utilised. The method adopted is to evaporate to dryness and
ignite the residue. The soda during the process of boiling takes up a
large amount of non-cellulose fibre constituents and dissolves them as
resinous compounds. These on evaporation and ignition become converted
into sodium carbonate. Many raw {180} materials, especially esparto
and straw, contain a large amount of silica, a large proportion of
which is dissolved by the soda in the form of sodium silicate, in which
form also it is found in the recovered soda.
[Illustration: FIG. 67.]
[Illustration: FIG. 68.]
The apparatus for accomplishing the evaporation varies with almost
every mill. In some, it is very primitive and {181} crude, consisting
perhaps of only a furnace for incinerating the residue, and over it
a pan containing the liquor, the latter being heated and evaporated
by the heat from the furnace. It is obvious that, with such an
arrangement, a large quantity of heat must be wasted. To economise
as much as possible of this waste heat, various plans have been
suggested. That of Rœckner, of Newcastle, appears to be to a great
extent efficacious. It consists practically of a series of shallow
trays B (Fig. 67) placed in a brick chamber, alternated so as to allow
the heated air from the furnace below to play upon the surface of each
in succession, on its way to the chimney, with which the whole system
is in connection. Above the chamber containing these trays, is a large
tank C, containing a store of the liquor to be evaporated, placed there
so as still further to economise the heat, and from which the liquor
runs on to the trays. The furnace A is of the ordinary reverberatory
kind; below it, and connected with it by a kind of damper, is a large
chamber J, where the calcined residue from the furnace is put to cool,
thus preventing any nuisance from the smell of the burning mass. The
chamber is provided with a pipe L, through which the vapours pass
into the furnace. Several pipes E from the furnace pass through the
{182} tank, to assist in warming the liquor. The residue, when cold,
is drawn through doors from the chamber below the furnace. Rœckner
has devised an apparatus (Fig. 68), consisting of a small chamber
containing a series of pipes A, through which a stream of cold water
constantly runs, in connection with the flue from his evaporator, for
the purpose of condensing volatile bodies, and thus preventing, to a
certain extent, contamination of the surrounding air.
A very economical form of evaporator is that invented by Porion,
a French distiller, and named after him. It is shown in sectional
elevation and plan in Figs. 69 and 70. It is largely used on the
Continent, and also in England and Scotland. It consists of a large
chamber _k_, the floor of which is slightly inclined from the chimney
shaft, and through which the waste heat from the furnace _a_ passes.
[Illustration: FIG. 69.]
[Illustration: FIG. 70.]
The liquor to be evaporated is run in at the end nearest the chimney
from the tank placed above the chamber _c_. A number of cast-iron
fanners _i_, dip into the liquor and revolve rapidly, usually at the
rate of about 300 revolutions per {183} minute, producing and filling
the chamber with a very fine spray, thus presenting a very large
evaporating surface.
Between the furnace and the evaporator are placed the chambers _c_ and
_f_. In _c_ a number of brick walls _d_ are so placed that the flames
from the furnace are intercepted and broken up. The object of this
is to give time for all the products of combustion to be thoroughly
burnt up, which would not be the case without the “smell-consumer,” as
these chambers are called. This part is an addition to the original
evaporator, and was devised by Messrs. Menzies and Davis. The liquor
after having been concentrated in the chamber _k_ runs into a trough
placed alongside the doors _h_ and flows into one or other of the
furnace beds _b_ where it is still further concentrated, and the
residue ignited by the flames from the fires _a_. The draught can be
regulated by the damper _g_, and also by one placed near the shaft _j_.
The doors _e_, in the smell-consuming chamber, are for the purpose
of cleaning out. The fanners _i_ are worked by a small steam engine,
not shown in the drawing. Under properly regulated conditions very
excellent results can be obtained with this evaporator. The temperature
of the gases near the chimney should not be higher than about 85°. By
running the fanners at a very high speed the temperature of the gases
may be even further reduced, thus showing the completeness of the
evaporation.
This form of evaporator is open to the objection that the whole of the
sulphur in the coal employed for the furnaces, finds its way into the
recovered soda. It combines with the alkali to form sulphite of soda,
part of which is decomposed in the furnace with formation of sodium
sulphate, sulphide, and other sulphur compounds. The same objection,
of course, applies, though perhaps in a less degree, to all systems
of evaporation in which the flame is in contact with the liquor to be
evaporated.
The Porion evaporator can be erected at very small cost, and costs
but little for maintenance. It is capable of producing 3/4 ton
of recovered soda per ton of coal with liquors {184} of the usual
strength. It has proved itself to be perhaps the most economical
evaporator existing.
Some time ago there was erected in Lancashire an evaporator invented
and patented by Mr. Alfred Chapman. It is shown in Figs. 71, 72, 73,
and 74. The evaporation is effected at a low temperature in three
vacuum pans E, and with the unusual result that the concentrated liquor
gelatinises after leaving the third vacuum-pan, instead of taking the
ordinary form of the concentrated products of other evaporators. It is
said that this apparatus gives an excellent product, with great economy
of labour and water, and with no drainage of foul liquor from the
buildings. Observations extending over three months have proved that it
evaporates 22 lb. of water from the liquor per lb. of coal used under
the boiler. It is however very costly to erect.
[Illustration: FIG. 71.]
[Illustration: FIG. 72.]
[Illustration: FIG. 73.]
[Illustration: FIG. 74.]
The waste liquor is discharged into the tank A, whence it is pumped
by the donkey-engine B, through the feed-heater C, {185} into the
boiler D, which receives heat from the incinerating furnace H, and,
in case of need, from an auxiliary furnace shown on the plan, under
the feed-liquor-heater. The steam produced in D is taken to the first
vacuum-pan at E, and having heated its contents, the products of
evaporation pass over into the tubes of the second pan; this, in its
turn, gives up its products of evaporation to the third, whence they go
to the condenser of the vacuum-engine F. Thus the heat from the furnace
H is used for incinerating the concentrated liquor {186} on its bed,
for beating the feed-liquor in the feed-heater pipes, and for making
steam out of the liquor itself in the boiler; this steam finally drives
the donkey-pump and vacuum-engine, and causes the evaporation in the
three vacuum-pans E. One advantage of this evaporator is the fact that
the liquor is evaporated out of contact with the furnace gases.
Whatever be the method of evaporating or concentrating the liquor,
the final treatment in the furnaces is much the same in every case.
The furnaces shown in Figs. 69 and 70 may be taken to represent the
ordinary form. The concentrated liquor is run on to either of the beds
_b_, where the last portions of water are driven off by the heat from
the fireplaces _a_, and the residual mass is ignited until all the
organic matter contained in the liquors is carbonised and the soda is
converted into carbonate of soda. This takes place in about 4 hours,
according to the degree of concentration of the liquor as it is run
into the furnaces. The running in of liquor should be done with great
care, as explosions sometimes occur through the sudden liberation of
steam on the liquor coming in contact with the hot beds. The charge
should be allowed to remain in the furnace until it is thoroughly
carbonised and all volatile matters have been driven off, otherwise a
nuisance may be caused when the still burning mass is exposed to the
air. Rœckner’s evaporator is provided with a special chamber into which
the charge is drawn (J, Fig. 67).
The composition of the recovered soda varies with the nature of the
liquors from which it has been obtained, and, as has been already
pointed out, with the form of evaporator employed. It consists
essentially of carbonate of soda, together with a certain amount of
silicate of soda, if derived from liquors in which straw or esparto
have been boiled, chloride of sodium, sulphate of soda, sulphite of
soda, sulphide of sodium, and other sulphur compounds, the rest being
made up of carbon and insoluble impurities. The amount of soda varies
from 35 to 45 per cent. (Na_{2}O). The following analysis {187} will
give some idea of the composition of Recovered Soda:—
[13]Sodium carbonate 72·33
Sodium hydrate 1·93
Sodium chloride 8·30
Sodium sulphate 3·95
Sodium sulphite ·63
Silica 7·09
Carbon 4·70
Oxide of iron and alumina ·50
Other constituents (by difference) ·57
──────
100·00
The whole of the soda present as sulphur compounds is not lost, as a
large proportion of it is present as sodium sulphite, most of which is
converted into caustic soda by the causticising process.
[Footnote 13: A certain quantity of potash derived from the
mineral constituents of esparto and straw is always present in
recovered soda.]
A certain amount of soda is carried forward, partly mechanically
and partly volatilised, to the flue leading to the chimney. This
accumulates, and may be from time to time removed in the form of
fine dust. It contains, besides carbonate of soda, much sulphate and
chloride. In two different samples examined by the authors, the amounts
of soda (Na_{2}O) present were 25·0 and 27·1 per cent.
_Causticising._—The next process consists in converting the sodium
carbonate in the recovered soda into caustic soda.
This operation is known as “causticising,” and consists in heating a
solution of the soda with lime. The decomposition which takes place is
shown in the following equation:—
Sodium Lime. Water. Caustic Calcium
Carbonate. Soda. Carbonate.
Na_{2}CO_{3} + CaO + H_{2}O = 2 NaOH + CaCO_{3}.
The recovered soda should be dissolved in separate vessels. Perhaps the
best form of apparatus is a series of lixiviating tanks such as are
used for dissolving the alkali in black ash. {188} By this means a
nearly perfect exhaustion of the mass can be effected with a minimum of
labour. Special tanks are sometimes made for the purpose, provided with
mechanical stirrers.
It is essential in dissolving the recovered soda that a high
temperature should be employed, as otherwise a portion of the soda
present as silicate of soda will be lost, as it is only with difficulty
soluble, and requires rather prolonged heating with water. Whatever the
form of apparatus employed it should be so arranged that, after running
off the strong liquor, the insoluble residue may be further treated
with water. In the case of the vats mentioned above, this process
is made continuous, pure water being run in at one end, and strong
liquor flowing from the other. If other forms are used, the liquor
after settling, may be run off by means of a pipe passing through the
bottom or side of the vessel, and near the bottom, and consisting of
two parts, one long, and one short. The short part is stationary, and
is connected to the longer part by means of a movable knee joint,
allowing it to be deflected. The liquor having settled sufficiently,
the movable limb is lowered beneath the surface of the liquor which is
then allowed to flow through. As the surface of the liquor falls, the
pipe is gradually lowered. In this way the clear solution can be run
off without disturbing the residue at the bottom. The open end of the
pipe is usually covered with coarse wire gauze, to keep back insoluble
impurities. With properly calcined recovered soda, the solution should
be bright and almost colourless. If at all brown in colour, and if
it has an empyreumatic odour, it indicates imperfect calcination.
The residue in the dissolving tanks consists chiefly of carbonaceous
matter, together with some soda, insoluble matter, &c.
The liquor is now ready to be causticised. This should be done in a
separate vessel, although it is the practice in many mills to perform
this operation in the same vessel in which the solution of the soda
has been conducted. A good form of causticiser can be made from an old
egg-shaped boiler, by cutting it in two along its length. {189}
It should be provided with two or more vertical steam pipes, connected
at the bottom of the boiler with a horizontal pipe perforated with
numerous holes. The vertical steam pipes should be furnished with
injectors, whereby air is drawn in, and forced with the steam through
the holes in the horizontal pipe. The stream of air serves the double
purpose of thoroughly agitating the liquor and of oxidising any sodium
sulphide in the recovered soda. The liquor before causticising should
be reduced in strength to about 20–25 degrees Twaddle, which may be
done with the washings of the residue from the recovered soda, or from
the washings obtained subsequently from the lime-mud. This strength
should never be exceeded, otherwise imperfect conversion into caustic
soda is the result. This is due to the fact that concentrated solutions
of caustic soda react upon calcium carbonate, forming sodium carbonate,
and calcium hydrate, the reaction being the reverse of that indicated
in the above equation. If the liquors are very strong in carbonate
of soda, and comparatively free from sulphate, they should not be
causticised at much over 20° Twaddle, if they contain much sulphate,
and therefore less carbonate, the higher strength can with safety be
adopted.
The causticising vessel should be provided with a stout iron cage
or basket, into which the lime can be put. This should be securely
fastened to the vessel, and should dip into the liquid.
The liquor having been properly diluted, is now heated by means of
the steam pipes, and the lime put into its cage. It should be put in
in lumps. As the liquor reaches the boiling point, the reaction will
proceed rapidly, and the lime will gradually disappear; fresh lumps
should be added if necessary. If the liquor is sufficiently heated
the causticising will be complete in from two to three hours. The
liquor should be tested from time to time; this is usually done by
a workman. He withdraws a sample of the liquor, and after allowing
the calcium carbonate to subside, pours off a portion of the clear
liquid into a glass vessel. He {190} then adds an excess of either
sulphuric or hydrochloric acid. If any effervescence takes place, due
to the evolution of carbonic acid gas, he knows that the operation of
causticising is incomplete; the heating must therefore be continued.
It is difficult, without an undue expenditure of time and steam, to
convert the whole of the soda into caustic: it should however be so
perfect, that on testing only a very slight effervescence occurs. It
is quite easy to convert as much as 95 per cent. of the soda, or even
more. The actual amount converted can only be ascertained by a careful
analysis of the liquor.
The amount of lime used is generally somewhat in excess of the
theoretical quantity; 106 parts of sodium carbonate (Na_{2}CO_{3})
require 56 parts of lime (CaO): it is necessary, however, to add
about 60 parts. A very good plan is to conduct two or even three
causticisings in the same vessel without cleaning out or removing the
calcium carbonate, using in the first operation a large excess of lime.
The causticising being completed, the calcium carbonate and excess of
lime are allowed to settle down, and the clear liquor run off by an
arrangement such as that already described in the dissolving process.
Fresh solution is then run in and the whole mass heated for some time,
until the excess of lime is converted into carbonate. Fresh lime is
then added if necessary until the conversion of the carbonate of soda
is complete. The liquor is then allowed to settle, and is run off as
before: this operation may again be repeated.
The residual calcium carbonate, or “lime-mud” as it is called in alkali
works, is then washed once or twice by running in water, boiling up,
allowing to settle, and running off the clear liquor. If these liquors
are too weak for use in boiling fibres, they may be used for diluting
fresh recovered soda liquor before causticising, or for dissolving the
soda.
Some arrangement should be provided for removing as much as possible
of the liquor from the lime-mud before throwing it away or otherwise
disposing of it. This is best done by throwing it on a filter made of
layers of stones, {191} ashes and sand, and covered at the top with
perforated iron plates. The filter is connected with a vacuum pump.
In this way very perfect draining is accomplished, and the mud forms
a hard mass on the surface of the filter, from which it can be easily
removed with spades. In this form it contains only 50–60 per cent. of
water. If properly washed it should contain in this state only about
2 per cent. of alkali (Na_{2}O). By careful manipulation, even this
amount can be reduced.
The importance of thoroughly washing the mud can hardly be too much
insisted upon. Where proper means are not employed for draining, the
washing should be made more perfect. The lime-mud consists chiefly of
carbonate of lime, together with silicate, free lime, &c. The following
analysis is of a mud obtained by causticising recovered soda derived
from the liquors in which esparto and straw had been boiled:—
Calcium carbonate 40·02
Calcium hydrate 5·13
Silica 4·01
Sodium hydrate 2·13
Oxide of iron and alumina 0·30
Water 48·10
Other constituents 0·31
──────
100·00
As already pointed out the liquors contain a certain amount of soda,
as sodium sulphide and other sulphur compounds. The presence of the
former, if in large quantities, is objectionable, as it is liable to
discolour fibres boiled in liquors containing it. It is therefore best
to remove it. This can be conveniently done by blowing air into the
liquors during the process of causticising: this has the effect of
oxidising it to sulphate of soda, in which form it is harmless.
The air can of course be blown into the liquor by means of a pump;
the most economical way is to connect with the steam pipe an injector
constructed on the principle of the injectors used for feeding boilers
and for other purposes. By this means a strong current of air is drawn
in, and {192} being forced with the stream to the bottom of the
liquor, passes through it in a number of fine streams.
If the amount of sulphide present be very high it may be necessary to
prolong the oxidising operation beyond the time necessary for complete
causticising.
In many paper-mills the causticising is conducted in circular vessels
furnished with mechanical agitators. These are more expensive than the
simple form described above, and they possess no special advantages.
The use of causticisers in which neither mechanical agitation nor
agitation by means of air is provided for is exceedingly wasteful of
labour, time, steam, and soda. The lime-mud settles at the bottom as a
hard mass, very difficult to manipulate. {193}
CHAPTER XIII.
PAPER TESTING.
There are two points of view from which a paper may be tested:
first, of physical or mechanical properties; secondly, of material
composition. We shall consider the subject according to this division.
(I.) Quantitative measurements of such properties as resistances to
breaking and tearing strains are seldom made by English paper-makers.
In Germany, on the other hand, the matter has been very thoroughly
investigated in connection with the work of the Königl. Techn.
Versuchsanstalt, Berlin, and through the agency and influence of Prof.
Sell, and C. Hofmann, a department has been organised exclusively for
the work of paper testing. The results of the tests are becoming widely
recognised by practical men and the trade in that country, as affording
a true index of the quality of a paper. It is therefore of importance
to give an outline of the methods employed.
The determination of the strain or weight which a paper is capable of
supporting is a very obvious measure of the strength of the paper.
Observations of the limiting strain or breaking weight are sometimes
made by paper-makers, but the apparatus and method employed are usually
crude. The simplest means consist in clamping the paper—a strip of
standard length and breadth, arbitrarily chosen—at one end, the clamp
being firmly held in a fixed support, and to the other attaching by
means of a similar clamp, an ordinary scale pan, the whole arrangement
hanging vertically. Into the pan, weights are added in due succession
until fracture of the strip is determined. It is scarcely necessary
to point {194} out that the errors of experiment with such a method
are very great: indeed it has been found that even with the refined
apparatus about to be described the errors are not inconsiderable.
However, by exhaustive investigation, according to the well-known
“law of errors,” these have been quantified, and a careful operator
can therefore obtain results which are trustworthy. The apparatus in
question is the Hartig-Reusch machine.[14] It is shown in sectional
elevation and plan in Figs. 75 and 76.
[Footnote 14: A complete description of this machine is given in
‘Civil Engineer,’ 1879.]
The principle will be readily grasped by inspection of the diagrammatic
representation of its essential parts—Fig. 77. The strip of paper is
held horizontally by the clamps _a_ and _b_, _a_ being held by the
fixed support A, _b_ by the movable carriage B. B is connected by means
of a swivel with the spiral spring F, and this again is similarly
connected with the screw, which is made to rotate by the wheel D. By
turning D, therefore, the spiral may be extended, and a corresponding
strain communicated through B and _b_ to the paper. The paper undergoes
a certain elongation under the strain, and the carriage B moves from
right to left in consequence. The rotation of the screw is continued,
and the extension of the spiral proceeds until the paper is fractured.
At this point it is required to determine, (1) the elongation of
the spiral which is the measure of the breaking strain, and (2) the
distance through which the carriage has moved, i.e. the elongation of
the paper. Both these effects are communicated to the pencil G, the
latter directly, since the pencil-holder is in rigid connection with
B, the former through the rod I, from which, by a special arrangement,
the horizontal is converted into a vertical motion of the pencil.
This, therefore, traces a curve, of which the ordinates represent the
strains, and the abscissæ the elongations of the paper produced by the
strain.
The scale shown in Fig. 76 indicates the exact position of the clamp A.
{195}
[Illustration: FIG. 77.]
[Illustration: FIG. 76.]
{196}
The values for the spiral spring—i.e. extension for a given
load—having been determined by previous observations in a special
apparatus, the curve obtained is at once a measure and a permanent
record of these cardinal factors, breaking strain and elasticity. As
with all other such instruments, the recording apparatus introduces
certain errors, which, however, by careful investigation and
modification in accordance with the results, have been reduced to a
minimum. Nevertheless, the director, Dr. Martens, has recently adopted
a simpler instrument, altogether similar in principle, but based upon a
direct reading of the two movements, in which of course these errors do
not appear. For the student, however, the recording instrument is the
more instructive, and we have given it preference for description here,
more especially as no difference in essential parts is involved.
Those who wish to pursue the matter into the most interesting details
of the investigations made upon the subject, are referred to the papers
published by the Institute for 1885.[15]
[Footnote 15: Mittheilungen a. d. Königl. Techn. Versuchsanst,
Berlin.]
In testing the strength of papers by this or similar machines, it
is important to observe the hygrometric state of the atmosphere at
the time the trials are made, as this has been found to exert a
considerable influence on the results, a paper being weaker the moister
the atmosphere.
The results of the tests are expressed in the following terms:—The
elongation is given directly in percentage of the original length. This
is uniformly taken at 180 mm., a length arrived at after laborious
investigation, as minimising the errors of experiment; in other words,
as giving mean value with the minimum of variation. For the breaking
strain an ingenious expression has been arrived at, viz. the length of
the paper which suspended vertically, with one end hanging freely, the
other fixed, would determine fracture at the fixed end. As the breaking
strain would vary with the thickness, the numbers obtained in {197}
units of force or weight for strips of constant breadth, would need
correction in order to admit of strict comparison with one another.
By substituting an expression in terms of the paper itself—since a
paper of greater thickness, and requiring therefore a proportionately
greater force to fracture it, weighs more per unit of area, and in
the same proportion—all the numbers for breaking strains are strictly
comparative one with the other. In the same way also the question of
width may be disregarded.
A further mechanical test, forming a part of the scheme of
investigation, is the resistance of the paper to rubbing. This test
is an altogether empirical one, as the following brief description
will show:—A piece of the paper, about 6 inches square, crumpled by
successive folding in two directions at right angles, is grasped by the
thumb and forefinger of each hand, at a distance of 3–4 inches apart.
It is then rubbed upon itself across the thumbs a given number of times
(seven is the number chosen) and held up to the light. If no holes are
visible, the rubbing is repeated. The number of times necessary to
repeat the rubbing until holes appear is the measure of the resistance.
A sufficient uniformity in the results of this test has been observed
to make it the basis of a classification of papers, in regard to their
resistance to such disintegration; they are divided into the following
eight groups, beginning with the lowest:—
0. Extremely weak.
1. Very weak.
2. Weak.
3. Medium.
4. Moderately strong.
5. Strong.
6. Very strong.
7. Extremely strong.
The classification of papers on the results of these tests cannot
be more lucidly given than in the following scheme, under which the
results are officially recorded:—
_Class_. 1 2 3 4 5 6
_a._ Mean breaking length (metres)
not less than 6000 5000 4000 3000 2000 1000
_b._ Mean elongation (per cent.) at
fracture not less than 4·5 4 3 2·5 2 1·5
_c._ Resistance to rubbing 6 6 5 4 3 1
{198}
This classification is based on the results of some hundreds of
observations. It is interesting to note the differences observed in
the numbers for _a_ and _b_ according to the direction for the test
in which the paper is cut, i.e. in the direction in which it was run
on the paper machine, or at right angles (see Chap. XI. p. 171). The
mean ratio for the breaking lengths (strains) may be taken as 1 : 1·6,
i.e. the paper is about 40 per cent.[16] weaker across the web; the
elongation under strain on the other hand is about double.
[Footnote 16: In the statement of results the mean of the numbers
obtained in the two directions is given.]
It is also of interest to note the influence of the glazing process (p.
167) upon the quality of the paper as determined by these tests.
First, we must notice the effect of the treatment upon the substance of
the paper itself. The mean reduction of thickness is 23 per cent. On
the other hand, the reduction of weight, calculated per unit of surface
(square metre), is 6·7 per cent., whence we may infer an increase of
surface, flattening out, in the process. These quantities, but more
particularly the latter, will doubtless vary with the various methods
of glazing and with the materials of which the paper is composed.
The breaking length (strain) shows a mean increase of about 8 per
cent.; the elongation under strain, on the other hand, a diminution of
6 per cent.
For an interesting discussion of the question of the relative strengths
of machine and hand-made paper see ‘Paper,’ by Richard Parkinson.
[Illustration: FIG. 78.]
The thickness of a paper may be determined by means of an ordinary
micrometer, such as is shown in Fig. 78. The paper is placed in the
jaws of the instrument, and the screw {199} advanced until it touches
the paper. The thickness is then read off on the scale. Other forms of
apparatus are sold for the same purpose. In making a determination of
the thickness of a paper it is necessary to take the mean of a series
of observations at different points of the sheet, as the thickness may
vary somewhat.
_Determination of Composition of Papers._
(II.) The analysis of a paper naturally divides itself into two
parts:—(_a_) The determination of the nature of the fibrous material of
which it is composed; and (_b_) the identification of such adventitious
substances as size and filling material.
(_a_) This again is divided into two sets of observations—microscopical
and chemical.
A fragment of the paper is soaked for some time in glycerine, and is
then carefully teased out with a pair of needles, and the fragments
laid on a glass slip with a drop of glycerine. A cover-glass is then
laid on and lightly pressed down so as to spread the fibres in a thin
layer.
The microscopical features of the different fibres have been already
described, and it is only necessary now to summarise the chief
characteristics of the more important materials.
_Cotton._—Flat riband-like fibres, frequently twisted upon themselves.
The ends generally appear laminated. The fibres are frequently covered
with numerous fine markings (see Frontispiece).
_Linen._—Cylindrical fibres, similar to the typical bast fibre (see
Fig. 6). The ends are frequently drawn out into numerous fibrillæ (see
Frontispiece).
_Esparto._—Esparto pulp consists of a complex of bast fibres and
epidermal cells. These serrated cells are, as has been already pointed
out, characteristic of esparto, straw, and similar fibres. Certain
differences exist between those of esparto and straw, and even between
the different species of straw, which enable the microscopist to
identify their source. The most characteristic feature of esparto pulp
is the presence {200} of a number of the fine hairs which line the
inner surface of the leaf (_e_, Fig. 10), some of which invariably
survive the boiling and washing processes, although the greater portion
passes away through the wire-cloth of the washing engines. The presence
of these hairs may be taken as conclusive evidence of the presence of
esparto.
_Straw._—Straw pulp very closely resembles esparto-pulp in its
microscopical features. The hairs above alluded to are, however,
absent. On the other hand, a number of flat oval cells are always
present in paper made from straw (_b_, Fig. 13). It should be borne in
mind, however, that bamboo and similar pulps also contain these cells.
_Wood (Chemical)._—Flat riband-like fibres, showing unbroken ends
(see photographs, Frontispiece). The presence of the pitted vessels
(Frontispiece, and _a_, Fig. 15) is eminently characteristic of pulp
prepared from pine-wood. The fibres of other woods are not sufficiently
characteristic. They much resemble those of pine-wood, with this
difference, that the pitted vessels are absent.
_Wood (Mechanical)._—Mechanical wood-pulp may be recognised by the
peculiar configuration of the torn ends of the fibres, and from the
fact that the fibres are rarely separated, but are generally more or
less agglomerated (see Frontispiece). Pulp from pine of course shows
the pitted vessels already referred to. They are usually more distinct
than in chemical wood-pulp. Occasionally fragments are to be met with
connected together with portions of the medullary rays.
The microscopical examination of a paper is a matter of very great
difficulty, and one requiring much practice. The student is recommended
to study closely for himself the microscopical features of pulps
obtained from authentic specimens.
Some approximate idea of the relative proportion of the various fibres
present in a paper can be obtained from a careful microscopical
examination.
Vétillart maintains that a quantitative determination within a fair
limit of accuracy is possible. On the other hand, the {201} Berlin
Prüfungsanstalt do not profess to do more than merely identify the
fibres.
In examining a paper under the microscope, it should be observed
whether the fibres appear as fragments, or whether they consist of
whole bast cells in which the tapered ends appear.
Cotton and linen, owing to the great length of their ultimate fibres,
yield, when beaten, fragments showing where the fracture has taken
place. From the appearance of this fracture it is possible to ascertain
whether or not the beating operation has been properly conducted. If
the beater-knives have been too sharp, or have been let down to the
bed-plate too quickly, the fractures will appear as clean cuts, whereas
when the operation has been properly conducted the fracture will appear
ragged and drawn out. The bearing of this on the strength of the
finished paper is considerable.
Esparto, straw, and wood, whose ultimate fibres do not exceed 1–2 mm.,
should, in the majority of cases, appear as whole bast fibres with two
tapered ends; the beating, when properly conducted, being confined
merely to the separation into these ultimate fibres.
(_b_) For the chemical identification of the fibres in a paper, the
only available reactions are those with aniline sulphate solution. The
majority of fibres (celluloses) give no reaction, straw and esparto
celluloses and mechanical wood-pulp being the only ones that can be
identified by its means. The authors have found that when a paper
containing straw or esparto is treated for some time with a boiling
1 per cent. solution of aniline sulphate, a pink colour is produced.
Esparto gives the reaction with greater intensity than straw. In
this way the presence of a very small quantity of these pulps can be
detected with certainty.
Mechanical wood-pulp, when treated with a solution of aniline sulphate,
develops, even in the cold, a deep yellow colour. If a paper containing
mechanical wood-pulp so treated be examined under the microscope, the
fragments of wood will be found to be deeply stained, whereas the other
{202} fibres remain colourless or nearly so. It must be borne in mind
that cellulose obtained from lignified fibres, if the boiling and
bleaching processes have not been carried sufficiently far, will give
with aniline sulphate a more or less intense yellow coloration. Various
other reagents have been suggested for the identification of mechanical
wood-pulp, all based upon the production of a colour with lignose.
The reaction of lignose with chlorine and sodium sulphite solution
already referred to (p. 19) may also be made available for the
detection of mechanical wood-pulp in a paper. Imperfectly boiled or
bleached pulps sometimes give this reaction faintly.
_Quantitative Estimation of Mechanical Wood-pulp._—The determination of
the amount of mechanical wood-pulp present in a paper is sometimes a
matter of some importance, and it is also a matter of great difficulty.
Some idea of the amount present can be obtained by observing the depth
of the yellow colour produced with aniline sulphate or the intensity
of the magenta reaction with chlorine and sodium sulphite. It is also
possible to calculate approximately the percentage from the percentage
of cellulose contained in the specimen. Mechanical wood-pulp (pine)
may be taken to contain 60 per cent. of cellulose. If, therefore, a
paper ascertained to contain a pure cellulose in addition to this
constituent, yield 75 per cent. of cellulose on the ordinary test, it
may be assumed that about 62·5 per cent. of mechanical wood-pulp is
present.
The authors have proposed a method of estimating the amount of
mechanical wood-pulp present in a paper, based upon the absorption
of iodine in definite proportions by wood in a finely divided state,
under strictly regulated conditions. The paper is carefully reduced to
a fine pulp, and is then left in contact with a standard solution of
iodine in potassium iodide. At the end of twenty-four hours the amount
of free iodine is determined by titration with sodium thiosulphate and
by deducting this from the amount originally taken, the amount absorbed
is ascertained. As this amount, under {203} strictly comparative
conditions, always corresponds to a definite amount of mechanical
wood-pulp the amount present can be readily calculated.
2. _Loading, Sizing Materials, &c._—The determination of the amount
of loading material in a paper has been already described (p. 134).
The identification of the material can only be arrived at by a careful
chemical analysis. The principal loading materials are china-clay and
pearl-hardening (calcium sulphate). The ash from a paper containing
china-clay is insoluble in boiling dilute hydrochloric acid; that
from paper containing calcium sulphate is soluble: the solution on
cooling deposits long needle-shaped crystals of CaSO_{4} 2 H_{2}O and
gives with barium chloride a copious precipitate of BaSO_{4} (barium
sulphate) insoluble in acids, and with ammonia and oxalate of ammonia a
precipitate of calcium oxalate.
The presence of starch in a paper can be readily ascertained by
its behaviour with a solution of iodine. If starch be present the
well-known blue colour of the compound of iodine and starch will be
produced. The determination of the amount of starch present is a matter
of some difficulty, the details of which are somewhat beyond the scope
of the present work. It is based upon the conversion of the starch into
sugar, and the estimation of the latter with Fehling’s solution.
The nature of the material with which a paper is sized may be
ascertained in the following way:—
The sample, cut up into small fragments, is warmed for a few minutes
with alcohol containing a few drops of acetic acid. The alcohol is
allowed to cool, and is then poured into four or five times its bulk
of distilled water. If any precipitate or cloudiness is produced,
it indicates that the paper has been sized with rosin. The alcohol
dissolves the rosin, which, being insoluble in water, is thrown
down on dilution. The alcohol used should be previously purified by
distillation, as some samples contain a small quantity of shellac,
which would itself be precipitated with water. {204}
The paper after treatment with alcohol should now be boiled for some
minutes with water: the solution allowed to cool, and then filtered. To
the filtrate a few drops of a solution of tannic acid are added, when,
if the paper has been sized with gelatine, a white curdy precipitate
will be formed.
The estimation of the amount of sizing material in a paper is a very
complicated process and one which demands considerable chemical
experience for its proper conduct.
The amount of gelatine present is best ascertained by determining the
amount of nitrogen present by combustion with soda-lime, and from this,
calculating the amount of gelatine. Pure gelatine contains 18·16 per
cent. of nitrogen (Muntz). The comparison of one paper with another
with a view to ascertain the relative degree of sizing, is usually
performed in a more or less rough and ready way by moistening the
samples with the tongue for a certain time, and noticing the degree
of transparency produced, which is of course inversely to the degree
of hardness. A more accurate method consists in placing a drop of
a mixture of alcohol and water containing some colouring matter in
solution and determining the time necessary for the colour to make its
appearance on the other side. In this way more trustworthy comparisons
can be made.
_Colouring Matters._—The chemical reactions of the chief colouring
matters have been already described (p. 141).
{205}
CHAPTER XIV.
GENERAL CHEMICAL ANALYSIS FOR PAPER-MAKERS.
A knowledge of the methods usually employed for the qualitative and
quantitative analysis of the various chemical substances met with in
paper manufacture is of considerable importance. The scope of this
work will not allow us to enter fully into the necessary details of
manipulation; for these the reader is referred to standard works on
analysis; we will merely indicate the methods.
_Caustic Soda, Soda Ash, Recovered Soda, &c._—These substances are
always valued by the amount of real alkali (Na_{2}O) that they contain,
which is determined by titration with a standard acid.
In testing recovered soda, it is necessary to _boil_ the finely
powdered ash for some time with water; unless this be done, the amount
of alkali will probably be under-estimated, as part of the soda is
present as a difficultly soluble silicate.
It is sometimes desirable to determine the amount both of sodium
hydrate and carbonate in a sample of alkali. For this purpose an excess
of a solution of barium chloride is added to a solution containing a
known quantity of the substance under examination, and the mixture made
up to a definite volume. It is then filtered, and an aliquot portion
of the filtrate titrated with standard alkali. The filtrate contains
only the alkali that was originally present as hydrate, the carbonate
of soda having formed with the barium chloride, insoluble barium
carbonate, and sodium chloride.
_Bleaching Powder._—The value of bleaching powder {206} depends upon
the amount of available chlorine that it contains. This is determined
by means of a standard solution of arsenious acid. The standard
solution is prepared by dissolving pure arsenious oxide (As_{2}O_{3})
in sodium carbonate solution. To test the powder, a quantity, say about
5 grms., is taken, and is carefully ground up in a mortar with a small
quantity of water, more water is then added, and the coarser particles
allowed to settle for a few seconds. The milky liquid is then run off
into a graduated flask; the residue is again ground up with water, and
the operations repeated until the whole of the powder is transferred
to the flask. The flask is now filled up to the proper mark, carefully
shaken, and an aliquot portion withdrawn as quickly as possible. It is
necessary to take both the soluble and insoluble portions in order to
obtain uniform results. Standard arsenious acid solution is now run
in until the solution ceases to produce a blue colour with iodide of
potassium and starch papers.
The method of estimating the amount of available chlorine in bleaching
powder by determining the quantity of ferrous sulphate it is capable of
oxidising is fallacious, as by this means calcium chlorate is included
in the result.
Bleaching powder should contain about 35 per cent. of available
chlorine, but the percentage frequently falls below this amount,
especially in warm weather.
Great care should be exercised in the sampling of bleaching powder, as
indeed of all chemicals, in order to insure an average result. Small
portions should be taken from different parts of the bulk; the whole
should then be carefully mixed, and, if necessary, reduced to powder.
Portions of the mixture should be taken, pounded, and again thoroughly
mixed.
If this process be repeated once or twice, a perfectly uniform sample
will be obtained.
_Alum, Sulphate of Alumina, Alum Cake, &c._—The points to be considered
in an examination of these products are (1) the percentage of alumina
contained in them; (2) the {207} percentage of iron, and (3) the
amount of free sulphuric acid present.
(1) The alumina is estimated as such by precipitation with ammonia.
Ferric oxide is also thrown down with alumina, and must therefore be
deducted from the amount of the mixed oxides obtained.
(2) The percentage of iron oxide present is determined in the following
way. A considerable quantity of the sample under examination is
dissolved in water, and to the boiling solution a large excess of
caustic potash solution is added. The caustic potash precipitates both
the alumina and ferric hydrates, the former, however, redissolves.
The solution is filtered, and the precipitate of ferric hydrate
washed, dissolved in dilute hydrochloric acid, and reprecipitated
with excess of caustic potash. It is again thrown on to a filter,
washed, redissolved in hydrochloric acid, and again reprecipitated with
ammonia. In this way it is obtained free from alumina.[17]
[Footnote 17: For a method of estimating minute quantities of iron
in alums see ‘Journ. Soc. Chem. Ind.,’ April 1887, p. 276.]
(3) _Free Acid._—This can be estimated indirectly, i.e. by estimating
the alumina and ferric oxide, calculating the amount of sulphuric acid
necessary to combine with them, and deducting this from the total
amount of sulphuric acid obtained. In the case of alum it is of course
necessary to make due allowance for the sulphate of potash or ammonia
present. The sulphuric acid is estimated as barium sulphate in the
usual way.
It should be borne in mind that sulphates of alumina are sometimes met
with containing uncombined alumina. A convenient method of estimating
free acid consists in digesting a weighed quantity of the finely
powdered sample in strong alcohol. The alcohol dissolves away the free
acid, which can be estimated in the solution by means of a standard
solution of alkali.
Free acid can be detected by means of a solution of {208} Congo-red,
which is turned blue with free acid, but not with pure sulphate of
alumina.
_Antichlor, Sodium Thiosulphate, Sodium Sulphite, &c._—Sodium
thiosulphate can be estimated by means of a standard solution of iodine
in potassium iodide. The operation should be performed in dilute
solution (see p. 111).
The same solution serves also for the estimation of sulphurous acid in
sodium and other sulphites.
_Starches._—After having made a careful microscopical examination of
the sample, it should be examined for water by drying at 100°C., and
for mineral matter by igniting in a platinum crucible and weighing the
residue. The water should not exceed 18 per cent., and the ash ·50 per
cent. The samples should be carefully examined for insoluble matter
by dissolving in water and filtering. It is also useful in comparing
different samples of starch to convert the specimens into pastes, under
exactly similar conditions.
When perfectly cold, the “stiffness” of the pastes should be compared.
This may be done by noting the relative resistance to weights placed
upon their surface. Information on this subject is contained in a paper
by W. Thomson in the ‘Journ. Soc. Chem. Ind.,’ March 1886.
The identification of starches by means of their microscopical
appearances is tolerably simple, as each particular kind possesses
characteristic differences.
_Gelatine._—Samples of gelatine should be examined for water and ash.
The water should not exceed 16 per cent. and the ash 2·7 per cent. The
relative strengths of the jellies formed with water should also be
compared. This may be done in the way indicated above for starches.
Raw materials intended for the preparation of gelatine may be similarly
examined. They yield the whole of their gelatine on heating with water.
The effect of prolonged heating on solution of gelatine should be borne
in mind. The amount of residue left after treatment with water should
be carefully determined.
_Soaps._—The soaps most suitable for use in paper-making {209} are
those known as curd and mottled, the former being chiefly used for the
fine qualities. They should be carefully examined for free alkali and
unsaponified fat. Full descriptions are contained in Lant Carpenter’s
‘Treatise on Soap-making.’
_Dyes, Pigments, Loading Materials, &c._—These substances are best
examined from a physical and tinctorial point of view, by comparison
with specimens of undoubted purity and excellence.
The chemical examination is both difficult and somewhat misleading.
A microscopical examination will often throw much light on their
composition.
For comparing the tinctorial power of different samples, weighed
quantities should be ground up with starch or some other white powder.
In this way some idea of the intensity of colour can be readily arrived
at.
{210}
CHAPTER XV.
SITE FOR PAPER-MILL, WATER-SUPPLY, WATER PURIFICATION, ETC.
In choosing a spot on which to build a paper-mill, the manufacturer
has to take into consideration several very important circumstances.
Chief of these is the necessity for having a large supply of water at
command. Not only is a large quantity needful, but it should be free
from impurities, such as suspended matter and iron. The former, it is
true, can be removed by settling and filtration; the latter cannot, and
is liable to injuriously affect the colour of the paper. Again, as a
question of economy in working, it is advantageous to have convenient
water-power; therefore for this, as well as for the former reason,
paper-mills are usually situated on the bank of a stream. In choosing
such a site, paper-makers are probably also influenced by the fact that
it affords a ready means for the removal of impurities. In properly
conducted mills, where suitable apparatus is employed for evaporating
the liquors in which the raw material has been boiled, the stream
should not be polluted to any very great extent. Generally speaking,
the greater the pollution, the more are valuable materials being lost
to the manufacturer. It is obvious that the site for a mill should also
be chosen with reference to its proximity to means of transit for the
raw and manufactured materials.
The disposition of the different divisions of a paper-mill depends
of course upon the nature of the paper it is intended to make and to
a large extent upon local circumstances. Fig. 79 will give some idea
of the general arrangement of a mill for manufacturing esparto paper.
{211}
_Water Purification._—Arnot in his Cantor Lectures on the Technology of
the Paper Trade states that from 30,000 to 40,000 gallons of water are
required for the manufacture of each ton of paper. The importance of a
pure and copious supply is therefore very evident.
[Illustration: FIG. 79.]
The impurities of water consist of two classes, insoluble and soluble.
The former can be readily removed by processes of settling and
filtration. For this purpose most paper-mills are provided with large
ponds capable of holding several days’ supply. These are sometimes
supplemented by filtering beds. Insoluble impurities can also be
removed by passing the water through filter-presses.
Fig. 80 is an illustration of Rœckner’s patent clarifier, which may
also be applied to this purpose.
The cylinders C dip below the surface of the water to be purified,
which is contained in the reservoir A. They are open at their lower
ends, but are closed at the top by the domes D. These are connected
with the pump I by means of the pipes H. On starting the pump the water
rises slowly in the cylinders. As soon as it reaches the level of the
top, the action of the pump is stopped and the cocks K opened, when the
water commences to flow down the pipes G, which together {212} with
the cylinders form a kind of siphon. If the reservoir A be kept full,
the siphons can be made to act continuously. The flow of water being
slow, insoluble impurities have time to subside.
The impurities may from time to time be removed by means of the small
pump L connected with the bottom of the reservoir.
The clarifier can also be used for the purification of the effluent
water from paper-mills.
[Illustration: FIG. 80.]
Insoluble impurities can be removed from water by means of the Stanhope
purifier, shown in Fig. 81.
The soluble impurities of water consist mainly of carbonate and
sulphate of calcium and iron, and occasionally organic matter. It is
a moot point among paper-makers {213} whether or not the presence of
the two former in a water is objectionable. For boiling and bleaching
purposes it undoubtedly is, as when mixed with caustic soda or
bleaching liquor they form a precipitate of carbonate of calcium, which
lines the insides of boilers, breakers, and potchers as a hard scale,
which is always liable to become detached, and to find its way into the
finished paper. Moreover, carbonate of calcium is precipitated in the
fibre, and carries with it a certain quantity of colouring matter, the
subsequent removal of which is difficult.
Carbonate of calcium, though practically insoluble in pure water, is
soluble in water containing carbonic acid. When this carbonic acid
is neutralised by lime or soda, the carbonate is precipitated. The
sulphate of calcium is unacted upon by lime, but by the action of
caustic soda is converted into free lime and sodium sulphate. The
lime then neutralises the free carbonic acid in the water, and forms
carbonate of calcium, which is of course precipitated.
These reactions may be represented by the following equations:—
Calcium Carbonic Lime. Calcium
carbonate. acid. carbonate.
1. CaCO_{3} + CO_{2} + CaO = 2 CaCO_{3}.
Calcium Caustic Calcium Sodium
sulphate. soda. hydrate. sulphate.
2. CaSO_{4} + 2 NaOH = CaH_{2}O_{2} + Na_{2}SO_{4}.
Calcium Carbonic Calcium Water.
hydrate. acid. carbonate.
CaH_{2}O + CO_{2} = CaCO_{3} + H_{2}O.
If sodium carbonate be used instead of caustic soda, the decomposition
will take place thus:—
Calcium Sodium Calcium Sodium
sulphate. carbonate. carbonate. sulphate.
CaSO_{4} + Na_{2}CO_{3} = CaCO_{3} + Na_{2}SO_{4}.
It will be seen from the above equation that in the boiling processes
the lime salts are removed from the water at the expense of an
equivalent quantity of caustic soda. The amount thus decomposed is
not sufficiently large to make it advisable {214} on that account to
purify the water. It is indeed so small that processes of purification,
based upon the use of lime and caustic soda, are now largely used,
the cost for chemicals rarely exceeding 1_d._ per 1000 gallons. But
for the reasons we have stated above, and also from the fact that it
serves to remove dissolved iron and organic matter from a water, such a
purification process is in certain cases advisable.
The processes now in use consist, as we have indicated, in the addition
to the water of lime and caustic soda or carbonate of soda, the
quantities being regulated according to the hardness of the water, and
the relative proportions of carbonate and sulphate of calcium.
The method of removing carbonate of calcium from water by the addition
of lime is due to the late Dr. Clark, of Aberdeen, and the process
is still called after him. It is in use in its original form in many
places. The plant necessary consists simply of a tank for mixing the
water and the lime, and of large settling tanks in which the carbonate
of calcium subsides.
Various modifications of this process have been proposed, chiefly in
the direction of improved plant.
In the Porter-Clark process the carbonate of calcium is removed
by passing the water after the addition of lime (in the form of
lime-water) through a filter-press.
[Illustration: FIG. 81.]
In Fig. 81 is shown the Stanhope purifier, which is largely used for
the purpose of softening water. The following details will render its
action clear: A is a store tank containing caustic soda solution.
B is a tank into which the water to be treated flows, and which is
maintained at a constant level by means of a valve and float. C and C
are two tanks which are used alternately, and in which lime water is
prepared. A definite quantity of caustic soda solution is added from
B; this mixture forms the reagent by which the softening of the water
is accomplished. D is a small tank for the purpose of maintaining a
constant head of reagent. The reagent and the water are mixed together
in the pipe F, the quantities being regulated by the cocks {216}
shown at H. The pipe F leads into the vessel E, which is fitted with
a number of V-shaped shelves, placed at an angle of 45°, and riveted
alternately to opposite sides of the vessel. This arrangement causes
the water to take a serpentine course. The position of the shelves is
indicated by the dotted lines.
As soon as the mixture of water and reagent flows down the pipe F, and
enters the vessel E, the calcium carbonate commences to precipitate
and settles upon the V-shaped shelves. At the top of the tank a layer
of wood shavings, inclosed in wire-netting, is placed as shown by the
dotted lines. This acts as a filter, and intercepts any particles that
may not have subsided. The clear purified water passes away by the pipe
I.
The precipitate that collects on the V-shaped plates is from time
to time drawn off by the cocks shown at G; this may be done without
interfering with the working of the apparatus. While there can be
no doubt that the use of soft water is advantageous in the boiling,
washing, and bleaching processes, and also of course for supplying
steam boilers, it is probably an advantage to use a hard water for
diluting the pulp before running it into the machine.
In the case of papers which are loaded with any of the forms of calcium
sulphate (pearl-hardening, crystal-hardening, &c.) the use of very soft
water is objectionable from the fact that a certain quantity of calcium
sulphate is dissolved. This would not take place to the same extent
with water which is already charged with sulphate and carbonate of
calcium.
The removal of soluble iron from a water is effected by the softening
processes described.
{217}
CHAPTER XVI.
ACTION OF CUPRAMMONIUM ON CELLULOSE. PREPARATION OF WILLESDEN PAPER.
The action of a solution of copper oxide in ammonia upon cellulose
has been already referred to (see p. 5). Celluloses, and also
lignocelluloses, when treated with such a solution, gradually
gelatinise and finally dissolve. On evaporating the solution to
dryness, a gummy amorphous mass is obtained, containing the cellulose
intermixed with copper oxide.
If the cellulose be in excess, e.g. when the solution is evaporated on
the surface of paper, calico, &c., merely dipped in the solution, the
copper oxide is often not formed at all, but a green varnish-like mass
of cellulose combined with copper oxide, which coats the surface of
each filament, welding and cementing them together. This cement-like
_cupro-cellulose_, as it may be termed, being insoluble in water,
communicates water-resisting properties to the material so treated:
moreover, the presence of copper renders the fabric less prone than
before to be attacked by insects and mould, so that animal and
vegetable life of a parasitic nature and fungoid growths are rarely, if
ever, to be observed, even when the material is kept under conditions
where boring worms, ants, rot, and mould, would be likely to attack it.
As has been already indicated (p. 5), the solution of cuprammonium
hydroxide is preferable to one containing cuprammonium salts; not only
is the action on cellulose more energetic, but various other advantages
are obtained.
_Preparation of the Copper Solution._—The cuprammonium solution is
prepared according to the patent of Dr. C. R. Alder Wright (No. 737,
1883). {218}
A series of cast-iron towers, two to three feet in diameter and ten to
twelve feet high, is so arranged that a current of air can be blown by
a powerful engine successively through the whole series. The towers
are then nearly filled with fragments of metallic copper (crumbled
up sheet, cuttings, &c.), and solution of ammonia: the air-current
being turned on, oxidation of the copper and solution of the oxide so
produced are rapidly effected.
The ammonia solution employed is previously impregnated with a
considerable quantity of copper by passing a stream of water over
scrap copper in similar towers, a current of ammonia gas mixed with a
suitable proportion of air being at the same time forced in. The liquor
passing from one tower is used over again to supply a second instead of
water, and so on through the series: finally the liquor is brought up
to full copper strength in the series of towers described above.
The spent air issuing from the towers carries with it a notable
quantity of ammonia; this is intercepted by means of an “exhaust”
scrubber containing copper, and well supplied with water, whereby a
comparatively weak solution of ammonia and copper is obtained, which
can be used instead of water in the first series of towers.
In order to produce the maximum effect on the cellulose, the solution
should contain from 100 to 150 lbs. of ammonia, and from 20 to 25 lbs.
of copper per 100 gallons.
By decomposing a cuprammonium solution by means of metallic zinc, a
corresponding solution of zinc-ammonium hydroxide can be obtained. This
solution is also capable of gelatinising cellulose, but not to the
same extent as the copper solution. It may, however, be advantageously
employed in certain cases in conjunction with a copper solution. Such a
solution can be conveniently prepared by substituting brass for copper
in the dissolving-towers. It is worthy of note that although, as we
have seen, zinc has the power of replacing copper in a cuprammonium
solution, iron is without any action, although it readily replaces
copper in a solution {219} of copper sulphate. This fact is of very
great industrial importance, as it enables the manufacturer to employ
vessels and machinery of iron.
The “Willesden” goods may be divided into two classes. Goods of
the first class, such as rope, cordage, netting, &c., are prepared
by simply dipping the made-up materials in a bath of cuprammonium
solution, using certain precautions as to the mode of immersion and
its duration, and the strength of the solution. On subsequently drying
the dipped fabrics, they are obtained coated and impregnated with
cupro-cellulose, which thus not only forms a kind of varnish-like
surface dressing, but further adds strength to the fibres by more or
less intimately cementing them together.
Goods of the second class constitute a much more important group to
which at present the Willesden Company more especially devotes its
attention. These fabrics are essentially of three kinds, viz. Willesden
Canvas, Willesden Scrim, and Willesden Paper. The two former of these
classes are prepared in much the same way as the goods just described,
saving that the fabric to be treated is usually unwound from one roller
and rewound upon another, after passing through the bath and over a
succession of drying cylinders.
_Willesden Paper._—This may be divided into two departments, viz.
(1) Willesden unwelded; (2) Willesden welded, the first class being
prepared from a single web of paper by passing it through the bath,
rolling and drying. Certain coarse varieties furnish a waterproof
material excellently adapted for lining packages, &c. Finer qualities
furnish envelopes and stationery, possessing the valuable property
of not being affected by water. Letters written on such paper would
remain perfectly legible, even after prolonged immersion. It may be
interesting to point out here that the cuprammonium solution offers a
very simple means of fastening envelopes in such a way as to be proof
against any attempts at tampering. The method consists in using a
concentrated solution as the fastening material; the envelope {220} is
then closed and ironed with a warm flat-iron, whereby the gelatinised
cellulose is converted into an insoluble cupro-cellulose, and the cover
is fastened so securely that the only possible mode of opening is by
tearing the paper.
_Willesden Welded Goods_ are prepared by simultaneously dipping two or
more sheets of paper and pressing them into one compact homogeneous
sheet whilst the surfaces are still in a gelatinous state. In this way
a continuous length of fabric of extraordinary durability is produced
that is scarcely affected by water, even when heated in it for some
weeks at a pressure of 60 lbs. per square inch.
As, on drying fabrics treated with cuprammonium solution, the whole
of the ammonia in the solution absorbed by them is volatilised, it is
necessary, in order to make the process economical, to collect and
re-use this ammonia. This is effected by conducting the drying process
in closed chambers, from which the ammonia gas is conducted by suitable
appliances and recovered.
In addition to the advantages already mentioned, it may be stated that
Willesden paper is much less inflammable than ordinary paper.
Among the many uses to which Willesden goods can be applied, the
following may be cited:—Roofs and sheds; huts and tents; partitions;
tanks and pipes; damp-proof foundations; underlining slates; sails,
awnings, &c. &c.
{221}
CHAPTER XVII.
STATISTICS.
_Raw Materials._
The statistics concerning the trade in raw materials for paper-making
are not published in a detailed form, all the various kinds of fibre,
&c., being grouped under two heads, viz. rags (linen and cotton), and
esparto (which embraces other fibres, and probably wood pulps). The
figures for recent years are shown in the accompanying tables:—
(_a_) Imports of cotton and linen rags:
tons. _l._
1880. 29,642 451,782
1881. 26,773 396,274
1882. 21,200 303,349
1883. 28,543 401,922
1884. 36,233 487,866
1885. 35,470 466,928
(_b_) Esparto and other material:
tons. _l._
1880. 228,580 1,642,903
1881. 238,043 1,626,800
1882. 251,594 1,784,078
1883. 288,549 1,943,732
1884. 260,544 1,638,564
But a portion of these imports was exported again, the figures being as
follows:—
(_a_) Exports of rags and other paper-making material produced
in the United Kingdom:
tons. _l._
1880. 55,792 673,523
1881. 50,488 563,460
1882. 49,352 526,554
1883. 51,293 502,851
1884. 60,924 562,903 {222}
(_b_) Exports of ditto of foreign and colonial produce:
tons. _l._
1880. 6,965 102,499
1881. 10,183 146,101
1882. 7,004 84,515
1883. 11,561 121,992
1884. 26,498 274,664
The relative proportions of raw material furnished by the various
countries in 1884 (the latest available return) are shown in the
subjoined tables:—
(_a_) Imports of linen and cotton rags in 1884:
From Tons. £
Germany 21,280 294,883
Holland 4,406 56,537
Belgium 2,867 34,713
Turkey 2,036 16,228
France 1,875 28,133
Russia 1,784 33,293
Channel Islands 282 3,972
Norway 261 3,377
Denmark 198 3,020
British South Africa 133 1,615
Australasia 113 2,245
Canary Islands 96 1,043
Spain 93 1,009
Argentine Republic 64 1,280
Other countries 745 6,518
────── ──────-
36,233 487,866
━━━━━━ ━━━━━━━
(_b_) Imports of esparto and other fibres in 1884:
From Tons. £
Algeria 88,357 515,232
Spain 40,159 314,927
Tripoli 33,930 172,282
Tunis 20,526 117,374
Other countries 1,033 5,738
─────── ─────────
184,005 1,125,553
━━━━━━━ ━━━━━━━━━
(_c_) Imports of other materials, including rag and wood
pulps, in 1884: {223}
From Tons. £
Norway 47,923 276,204
Belgium 7,169 58,383
Holland 6,484 46,104
Sweden 5,178 48,523
Germany 3,732 36,582
France 2,128 14,667
Denmark 1,205 13,332
British India 1,046 7,735
Egypt 870 4,933
Other countries 804 6,544
────── ───────
76,539 513,011
━━━━━━ ━━━━━━━
A considerable export of raw material for paper-making also takes place
from British shores, the figures, embracing rags and other materials,
for 1884 being as below:—
(_a_) Exports of home produced rags and other paper-making
materials in 1884:
To Tons. £
United States 59,222 550,924
Holland 649 2,703
British America 390 4,255
Germany 262 1,673
Other countries 401 3,348
────── ───────
60,924 562,903
━━━━━━ ━━━━━━━
(_b_) Exports of foreign produced ditto in 1884:
(_i._) Linen and cotton rags.
To Tons. £
United States 14,191 167,801
Other countries 514 5,519
────── ───────
14,705 173,320
━━━━━━ ━━━━━━━
(_ii._) Esparto and other fibres.
To Tons. £
All countries 140 701
(_iii._) Pulps and other materials.
To Tons. £
United States 11,290 97,620
Other countries 363 3,023
────── ──────
11,653 100,643
━━━━━━ ━━━━━━━
{224}
Our imports of “rags and other paper-making materials” from Norway
increased from 23,483 tons, value 138,098_l._, in 1880, to 48,199 tons,
279,679_l._, in 1884, mainly owing to the development of the wood-pulp
industry. Later figures will probably show a similar increment. The
same articles from Denmark grew from 359 tons, 3406_l._, in 1880, to
1403 tons, 16,352_l._, from the same cause. German statistics have
even a stronger upward tendency: 11,587 tons, 196,051_l._, in 1880,
against 196,051 tons, 331,591_l._, in 1884. Our receipts of esparto
from Algeria show an advance, but not a marked one, being 60,612 tons,
421,343_l._, in 1880, and 88,357 tons, 515,232_l._, in 1884. But
shipments of the same fibre from Spain show a decline from 51,413 tons,
454,713_l._, in 1880, to 40,159 tons, 314,927_l._, in 1884; and from
Morocco they have fallen away from 2879 tons, 18,231_l._, in 1880, to
260 tons, 1290_l._, in 1884; while the figures relating to Tunis and
Tripoli also show a disposition to recede, though not at such a rapid
rate.
The wood-pulp industry of Norway for the year 1886 shows a very large
increase upon the figures of a few years back, albeit prices have
ruled very low. This latter circumstance is attributed, not so much to
over-production, as to excessive competition among the sellers of this
article. The quantity exported during the year 1886 is about 120,000
tons; in the year 1885 it was 107,651 tons; 1884, 88,220 tons; 1883,
70,464 tons; 1882, 58,884 tons; 1881, 42,194 tons; 1880, 26,055 tons.
Several of the old works have extended their production during the past
year, and several new establishments are in the course of erection, so
the production this year may probably be put at 150,000 tons wood pulp
with 50 per cent. water. The greatest part of the Norwegian wood pulp
is exported to England, France, and Belgium; in Russia, the increase
in the duty has stopped business, and the same can almost be said of
Germany. America, too, has drawn part of her supply from Norway, but
this trade is not expected to continue. {225}
_Manufactured Material._
Our import trade in manufactured paper has been growing of late years,
as the following figures will show:—
Imports of paper and pasteboard, of all kinds except
hangings:
cwt. _l._
1880. 1,021,952 1,159,646
1881. 1,065,912 1,138,943
1882. 1,098,118 1,202,905
1883. 1,160,104 1,245,861
1884. 1,447,335 1,403,446
At the same time, our exports of manufactured paper, though amounting
to only about one-third the weight, have very nearly as high an
aggregate value as the imports, and have increased in much the same
proportion in corresponding years. Thus:—
Exports of paper of all kinds but hangings:
cwt. _l._
1880. 472,168 1,106,996
1881. 555,219 1,242,962
1882. 584,947 1,305,025
1883. 597,923 1,284,862
1884. 670,760 1,374,392
The above figures relate to paper manufactured in the United Kingdom.
In addition, there were re-exports of paper of foreign and colonial
manufacture to the following amounts:—
Re-exports of foreign-made paper:
cwt. _l._
1880. 68,843 108,426
1881. 68,861 101,197
1882. 50,403 82,464
1883. 49,526 76,620
1884. 51,467 78,785
Taking the latest year for which detailed statistics are available,
viz. 1884, we find that the importations of paper {226} are derived
from foreign states in the following proportions:—
Imports of paper in 1884—
(_a_) Writing or printing paper:
From Cwt. £
Germany 92,681 129,112
Belgium 42,139 68,370
Sweden 27,747 36,682
Holland 26,507 34,884
France 6,463 28,925
Austria 5,399 9,314
Norway 4,300 4,771
Other countries 3,174 5,655
─────── ───────
208,410 317,713
━━━━━━━ ━━━━━━━
(_b_) Unenumerated sorts:
From Cwt. £
Germany 139,156 209,916
Sweden 116,488 120,793
Belgium 82,263 124,476
Holland 54,594 84,699
Norway 32,294 26,765
France 19,328 69,763
United States 10,980 31,201
Austria 5,927 12,012
Spain 736 4,059
Japan 421 4,736
Other countries 3,547 5,995
─────── ───────
465,734 694,415
━━━━━━━ ━━━━━━━
(_c_) Pasteboard and millboard:
From Cwt. £
Holland 439,749 201,894
Germany 235,658 116,185
Belgium 51,235 25,350
Sweden 32,331 20,241
Norway 8,260 5,726
France 3,262 19,259
Other countries 2,696 2,663
─────── ───────
773,191 391,318
━━━━━━━ ━━━━━━━
The exports of home-made paper in the same year were distributed as
follows:— {227}
Exports of home-made paper in 1884—
(_a_) Writing, printing, and envelopes:
To Cwt. £
Australasia 280,024 545,830
France 44,427 101,833
Bengal and Burma 33,963 71,834
Bombay and Scinde 29,767 64,227
British South Africa 16,900 33,041
British North America 14,871 44,511
Madras 10,532 25,651
Argentine Republic 8,683 30,954
United States 6,985 31,722
Sweden and Norway 6,101 11,870
Belgium 5,981 13,926
Germany 5,969 14,188
British West Indies 4,706 13,411
Ceylon 4,668 10,919
Other countries 31,397 93,454
─────── ─────────
504,973 1,107,371
━━━━━━━ ━━━━━━━━━
(_b_) Pasteboard and millboard:
To Cwt. £
Australasia 14,542 20,771
Bengal and Burma 5,147 4,589
Bombay and Scinde 4,572 4,134
France 987 2,309
Other countries 5,571 13,367
────── ───────
30,819 45,170
━━━━━━ ━━━━━━━
(_c_) Unenumerated, and articles made of paper:
To Cwt. £
Australasia 62,731 81,385
British South Africa 9,341 11,809
Bombay and Scinde 7,146 10,389
Argentine Republic 6,283 8,113
Germany 5,147 11,394
Belgium 5,122 11,300
France 5,052 16,238
Bengal and Burma 5,006 6,678
United States 4,841 14,668
British America 3,737 7,801
Holland 2,410 5,529
Other countries 18,152 36,547
─────── ───────
134,968 221,851
━━━━━━━ ━━━━━━━
{228}
The re-exports of foreign and colonial-made papers during the same
period went to the following destinations:—
Re-exports of foreign paper in 1884—
(_a_) Printing and writing:
To Cwt. £
British India 10,488 14,643
Australasia 4,908 7,695
Other countries 3,236 7,554
────── ──────
18,632 29,892
━━━━━━ ━━━━━━
(_b_) Unenumerated:
To Cwt. £
British India 5,852 7,354
Egypt 5,167 7,751
Australasia 2,131 4,398
Russia 1,560 7,829
Other countries 8,465 15,825
────── ──────
23,175 43,157
━━━━━━ ━━━━━━
(_c_) Pasteboard and millboard:
To Cwt. £
All countries 9,660 5,736
{229}
CHAPTER XVIII.
BIBLIOGRAPHY.
J. Murray.
Practical Remarks on Modern Paper. Edinburgh: 1829.
L. S. Le Normand.
Manuel du Fabricant des Papiers. Paris: 1834.
G. Planche.
L’Industrie de la Papeterie. Paris: 1853.
L. Müller.
Die Fabrikation des Papiers. Berlin: 1855.
Proteaux.
Manufacture of Paper and Boards. Philadelphia: 1866.
C. Hofmann.
Manufacture of Paper. Philadelphia: 1873.
Hugo Müller.
Pflanzenfaser. Leipzig: 1873.
T. Routledge.
Bamboo considered as a Paper-making Material.
London and New York: 1875.
Vétillart.
Études sur les Fibres Végétales. Paris: 1876.
Arnot.
Technology of the Paper Trade (Cantor Lecture,
Society of Arts). London: 1877.
J. Dunbar.
The Practical Paper-maker. London: 1881.
{230}
Forestry and Forest Products (Edinburgh Forestry Exhibition).
Edinburgh: 1884.
C. F. Cross and E. J. Bevan.
Cellulose.[18] London: 1885.
R. Parkinson.
A Treatise on Paper. Preston: 1886.
C. T. Davis.
Manufacture of Paper. Philadelphia: 1887.
Tomlinson.
Manufacture of Paper.
Paper-makers’ Monthly Journal. London: since 1872.
Paper Trade Journal. New York: since 1872.
Papier Zeitung. Berlin: since 1876.
[Footnote 18: Gives a list of works on Cellulose and its
derivatives.]
{231}
CHAPTER XIX.
ADDENDA.
_The Yaryan Process of Evaporation._
This process, the invention of Mr. H. J. Yaryan, of Toledo, Ohio,
U.S.A., has been considerably developed in America in its several
applications, and as we have authentic information of its successful
working for the recovery of soda in some of the largest paper-mills
in the States, we think it deserving of mention. The principle of the
process is that of “multiple effects,” upon which, indeed, all the
modern methods of economic evaporation are based. The principle may be
briefly explained as follows:—A liquid is converted into vapour under
ordinary conditions of boiling, by overcoming the pressure of the
atmosphere upon its surface. The quantity of heat required to vaporise,
as also the temperature of the ebullition, will be less as the pressure
to be overcome is less. Further, the vapour continuously driven off
carries with it a quantity of heat, which is its heat of condition or
latent heat. This heat it imparts to any colder body (e. g. a further
quantity of the same liquid) with which it comes in contact, direct or
indirect; if the quantity of the latter be relatively small, it will
raise its temperature approximately to that of the ebullition of the
first liquid. If now the pressure (atmospheric) on the surface of the
latter be slightly reduced, by any means, it will boil. The vapour
from this can be made to boil a third quantity of the liquid, under a
further diminished pressure.
The successive effects in economic evaporation consist, therefore, in
utilising the latent heat of a vapour given off from a liquid under a
certain pressure (e. g. that of the atmosphere) to vaporise a further
quantity of the liquid under a pressure maintained by mechanical means
below that of the first. In the ordinary methods the vapour does its
work in the successive effects by passage through systems of tubes,
the liquid to be heated being in contact externally; in the Yaryan
system, on the other hand, the arrangement {232} is reversed. The
liquid to be evaporated traverses the system of tubes which are heated
externally by the vapours. At the end of each effect, the liquid is
caused to impinge, in a special chamber, upon a disc: in this way a
complete separation of liquid and vapour is effected, each then passing
on to the next effect, the former through the tube-system, the latter
to the chamber inclosing these. The flow of liquid is maintained by
a force-pump, and the diminished pressure by a vacuum-pump suitably
disposed. This system differs from that described on page 180, in
that the evaporation is continuous, the dilute liquors entering the
apparatus and the highly concentrated liquors leaving it in an unbroken
stream. The rate of flow is such that the evaporation of the caustic
liquors from wood boiling from 8–10° to 80° Twaddell, in a quadruple
effect, requires only a few minutes. At the latter concentration it is
ready for the incineration process, which by means of a rotary furnace,
such as that of Mr. J. W. Hammond, of the firm of S. D. Warren and Co.,
is also effected continuously. It is found, moreover, that the excess
of heat available from this process is sufficient for the evaporation.
_Ferric Oxide Causticising Process._
The principle of this process is the expulsion of the carbonic acid
from sodium carbonate by ferric oxide at high temperatures; this oxide
playing the part of a weak acid, entering into a loose combination with
the soda, which is overcome by water in the subsequent lixiviation
process, the ferric oxide being regenerated and a solution of caustic
soda obtained.
The ferric oxide is therefore continuously available. The main features
of the process, disregarding the question of commercial economy, are
(1) that it leaves no by-products to be disposed of, (2) that it
enables the manufacturer to produce directly, without evaporation, a
highly concentrated caustic lye.
This process has been worked out under the auspices of Messrs.
Brunner, Mond, & Co., and there is every reason to expect an extensive
application of so simple a method.
{233}
INDEX.
Abaca, see Manilla hemp
Abies pulp, 60
Acetic acid in bleaching, 114
Acid, action on paper, 163
― in alum, estimating, 207
― ― ―, evils of, 163
― processes for wood, 69
― radicles and cellulose, 8
Acids added in bleaching pulps, 112
― and cellulose, 12
Adansonia fibre, 53
― ―, boiling, 103
― ―, cutting, 103
― ―, willowing, 103
Adipo-cellulose, 26
Adulterating paper, 130
Agalite loading, 133
Agave americana fibre, 33, 44
Alkali boiled with rags, 82
― process for isolating cellulose, early methods, 65
Alkaline processes for isolating cellulose, 62
Alkalis and cellulose, 14
― and ligno-cellulose, 21
Aloe fibre, 33, 39, 44, 46
― ―, dimensions, 39
― perfoliata fibre, 39
Alum, action on cellulose, 163
―,― on gelatine, 163
―, analysing, 206
― cake, analysing, 206
―, estimating acid in, 207
―, freedom from acid, 163
― in sizing, 139
―, proportion required, 141
― solutions, table of strengths, 143
Alumina resinate, 137
Alumina sulphate, analysing, 206
― ― in sizing, 140
Aluminous cake, 140
Ammoniacal solution of cupric oxide, 5
Amyloid, 12
Analysis of plant substances, 42
Analysing alum, 206
― ― cake, 206
― alumina sulphate, 206
― antichlor, 208
― bleaching powder, 205
― caustic soda, 205
― dyes, 209
― gelatine, 208
― loading material, 203, 209
― paper, 199
― pigments, 209
― recovered soda, 205
― size, 203
― soaps, 208
― soda ash, 205
― sodium sulphite, 208
― ― thiosulphate, 208
― starches, 208
― wood pulp, 201
Ananassa sativa fibre, 44
Aniline blues, 141
― colours, solutions, 37
― for diagnosing plant substances, 43
― pinks, 142
― sulphate solution, 37
Animal cellulose, 7
Annandale’s damping method, 159
Antichlor, analysing, 208
―, lime and sulphur, 129
―, proportion required to neutralise bleaching powder, 128
―, removing bleaching liquor by, 127
Antichlor, sodium hyposulphite, 127, 129
―, ― sulphite, 125
―, ― thiosulphate, 127, 129
―, testing, 130
Apparatus for microscopic examination of fibres, 35
Ash of cotton pulp, 136