Nitro Explosives: A Practical Treatise by P. Gerald Sanford

P >> P. Gerald Sanford >> Nitro Explosives: A Practical Treatise

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23

[Illustration: FIG. 15_a_.--THE BEATER FOR GUN-COTTON.]

~Boiling.~--The washed cotton is put into large iron boilers with plenty
of water, and boiled for some time at 100 deg. C. In some works lead-lined
tanks are used, into which a steam pipe is led. The soluble impurities of
unstable character, to which Sir F.A. Abel traced the liability of gun-
cotton to instability, are thereby removed. These impurities consist of
the products formed by the action of nitric acid on the fatty and resinous
substances contained in the cotton fibres. The water in the tanks should
be every now and again renewed, and after the first few boilings the water
should be tested with litmus paper until they are no longer found to be
acid.

[Illustration: FIG. 15_b_.--WHEEL OF BEATER.]

~Pulping.~--The idea of pulping is also due to Abel. By its means a very
much more uniform material is obtained. The process is carried out in an
apparatus known as a "Beater" or "Hollander" (Fig. 15, _a, b_). It
consists of a kind of wooden tank some 2 or 3 feet deep of an oblong
shape, in which a wheel carrying a series of knives is made to revolve,
the floor of the tank being sloped up so as to almost touch the revolving
wheels. This part of the floor, known as the "craw," is a solid piece of
oak, and a box of knives is fixed into it, against which the knives in the
revolving wheel are pressed. The beater is divided into two parts--the
working side, in which the cotton is cut and torn between the knife edges
in the revolving cylinder and those in the box; and the running side, into
which the cotton passes after passing under the cylinder. The wheel is
generally boxed in to prevent the cotton from being thrown out during its
revolution. The cotton is thus in constant motion, continually travelling
round, and passing between the knives in the revolving cylinder and those
in the box fixed in the wooden block beneath it. The beater is kept full
of water, and the cotton is gradually reduced to a condition of pulp. The
wheel revolves at the rate of 100 to 150 times a minute.

[Illustration: FIG. 16_a_.--POACHER FOR WASHING GUN-COTTON.]

[Illustration: FIG. 16_b_.--PLAN OF THE POACHER.]

[Illustration: FIG. 16_c_.--ANOTHER FORM OF POACHER.]

When the gun-cotton is judged to be sufficiently fine, the contents of the
beater are run into another very similar piece of machinery, known as the
"poacher" (Fig. 16, _a, b, c_), in which the gun-cotton is continuously
agitated together with a large quantity of water, which can be easily run
off and replaced as often as required. When the material is first run into
the poacher from the beater, the water with which it is then mixed is
first run away and clean water added. The paddle wheel is then set in
motion, and at intervals fresh water is added. There is a strainer at the
bottom of the poacher which enables the water to be drawn off without
disturbing the cotton pulp. After the gun-cotton has been in the poacher
for some time, a sample should be taken by holding a rather large mesh
sieve in the current for a minute or so. The pulp will thus partly pass
through and partly be caught upon the sieve, and an average sample will be
thus obtained. The sample is squeezed out by hand, bottled, and taken to
the laboratory to be tested by the heat test for purity. It first,
however, requires to be dried. This is best done by placing the sample
between coarse filter paper, and then putting it under a hand-screw press,
where it can be subjected to a tolerably severe pressure for about three
minutes. It is then rubbed up very finely with the hands, and placed upon
a paper tray, about 6 inches by 4-1/2 inches, which is then placed inside
a water oven upon a shelf of coarse wire gauze, the temperature of the
oven being kept as near as possible to 120 deg. F. (49 deg. C.), the gauze shelves
in the oven being kept about 3 inches apart. The sample is allowed to
remain at rest for fifteen minutes in the oven, the door of which is left
wide open. After the lapse of fifteen minutes the tray is removed and
exposed to the air of the laboratory (away from acid fumes) for two hours,
the sample being at some point within that time rubbed upon the tray with
the hand, in order to reduce it to a fine and uniform state of division.
Twenty grains (1.296 grm.) are used for the test. (See Heat Test, page
249.)

If the gun-cotton sample removed from the poacher stands the heat test
satisfactorily, the machine is stopped, and the water drained off. The
cotton is allowed some little time to drain, and is then dug out by means
of wooden spades, and is then ready for pressing. The poachers hold about
2,000 lbs. of material, and as this represents the products of many
hundred distinct nitrating operations, a very uniform mixture is obtained.
Two per cent. of carbonate of soda is sometimes added, but it is not
really necessary if the cotton has been properly washed.

~Compressing Gun-Cotton.~--The gun-cotton, in the state in which it is
removed from the poacher, contains from 28 to 30 per cent. of water. In
order to remove this, the cotton has to be compressed by hydraulic power.
The dry compressed gun-cotton is packed in boxes containing 2,500 lbs. of
dry material. In order to ascertain how much of the wet cotton must be put
into the press, it is necessary to determine the percentage of water. This
may be done by drying 2,000 grains upon a paper tray (previously dried at
100 deg. C.) in the water oven at 100 deg. C. for three hours, and re-weighing and
calculating the percentage of water. It is then easy to calculate how much
of the wet gun-cotton must be placed in the hopper of the press in order
to obtain a block of compressed cotton of the required weight. Various
forms of presses are used, and gun-cotton is sent out either as solid
blocks, compressed discs, or in the form of an almost dry powder, in zinc-
lined, air-tight cases. The discs are often soaked in water after
compression until they have absorbed 25 per cent. of moisture.

[Illustration: FIG. 17.--OLD METHOD. 100 PIECES.]

[Illustration: FIG. 18.--NEW METHOD. ONE SOLID BLOCK.]

At the New Explosives Company's Stowmarket Works large solid blocks of
gun-cotton are pressed up under a new process, whereby blocks of gun-
cotton, for use in submarine mines or in torpedo warheads, are produced.
Large charges of compressed gun-cotton have hitherto been built up from a
number of suitably shaped charges of small dimensions (Fig. 17), as it has
been impossible to compress large charges in a proper manner. The
formation of large-sized blocks of gun-cotton was the invention of Mr A.
Hollings. Prior to the introduction of this method, 8 or 9 lbs. had been
the limit of weight for a block. This process has been perfected at the
Stowmarket factory, where blocks varying from the armour-piercing shell
charge of a few ounces up to blocks of compressed gun-cotton mechanically
true, weighing 4 to 5 cwts. for torpedoes or submarine mines, are now
produced. At the same time the new process ensures a uniform density
throughout the block, and permits of any required density, from 1.4
downwards, being attained; it is also possible exactly to regulate the
percentage of moisture, and to ensure its uniform distribution. The
maximum percentage of moisture depends, of course, upon the density. By
the methods of compression gun-cotton blocks hitherto employed, blocks of
a greater thickness than 2 inches, or of a greater weight than 9 lbs.,
could not be made, but with the new process blocks of any shape, size,
thickness, or weight that is likely to be required can be made readily and
safely. The advantages which are claimed for the process may be enumerated
as follows:--(1.) There is no space wasted, as in the case with built-up
charges, through slightly imperfect contact between the individual blocks,
and thus either a heavier charge--i.e., about 15 per cent. more gun-
cotton--can be got into the same space, or less space will be occupied by
a charge of a given weight. (2.) The metallic cases for solid charges may
be much lighter than for those built-up, since with the former their
function is merely to prevent the loss of moisture from wet gun-cotton, or
to prevent the absorption of moisture by dry gun-cotton. They can thus be
made lighter, as the solid charge inside will prevent deformation during
transport. With built-up charges the case must be strong enough to prevent
damage, either to itself or to the charge it contains. For many uses a
metal case, however light, may be discarded, and one of a thin waterproof
material substituted. (3.) The uniform density of charges made by this
process is very favourable to the complete and effective detonation of the
entire mass, and to the presence of the uniform amount of moisture in
every part of the charge. (4.) Any required density, from the maximum
downwards, may be obtained with ease, and any required amount of moisture
left in the charge. These points are of great importance in cases where,
like torpedo charges, it is essential to have the centre of gravity of the
charge in a predetermined position both vertically and longitudinally, and
the charge so fixed in its containing case that the centre of gravity
cannot shift. The difficulty of ensuring this with a large torpedo charge
built up from a number of discs and segments is well known. Even with
plain cylindrical or prismatic charges a marked saving in the process of
production is effected by this new system. The charges being in one block
they are more easily handled for the usual periodical examination, and
they do not break or chafe at the edges, as in the case of discs and cubes
in built-up charges. A general view of the press is given in Fig. 19. The
gun-cotton in a container is placed on a cradle fixed at an angle to the
press. The mould is swivelled round, and the charge pushed into it with a
rammer, and it is then swivelled back into position. The mould is made up
of a number of wedge pieces which close circumferentially on the enclosed
mass, which is also subjected to end pressure. Holes are provided for the
escape of water.

[Illustration: FIG. 19.--A 4-CWT. BLOCK OF GUN-COTTON BEING TAKEN FROM
HYDRAULIC PRESS.]

~The Waltham Abbey Process.~--At the Royal Gunpowder Factory, Waltham
Abbey, the manufacture of gun-cotton has been carried out for many years.
The process used differs but little from that used at Stowmarket. The
cotton used is of a good quality, it is sorted and picked over to remove
foreign matters, &c., and is then cut up by a kind of guillotine into
2-inch lengths. It is then dried in the following manner. The cotton is
placed upon an endless band, which conducts it to the stove, or drying
closet, a chamber heated by means of hot air and steam traps to about
180 deg. F.; it falls upon a second endless band, placed below the first; it
travels back again the whole length of the stove, and so on until
delivered into a receptacle at the bottom of the farther end, where it is
kept dry until required for use. The speed at which the cotton travels is
6 feet per minute, and as the length of the band travelled amounts to 126
feet, the operation of drying takes twenty-one minutes. One and a quarter
lb. are weighed out and placed in a tin box; a truck, fitted to receive a
number of these boxes, carries it along a tramway to a cool room, where it
is allowed to cool.

~Dipping.~--Mixed acids are used in the proportion of 1 to 3, specific
gravity nitric acid 1.52, and sulphuric acid 1.84. The dipping tank is
made of cast iron, and holds 220 lbs. of mixed acids, and is surrounded on
three sides by a water space in order to keep it cool. The mixed acids are
stored in iron tanks behind the dipping tanks, and are allowed to cool
before use. During the nitration, the temperature of the mixed acids is
kept at 70 deg. F., and the cotton is dipped in quantities of 1-1/2 lb. at a
time. It is put into a tin shoot at the back of the dipping tank, and
raked into the acids by means of a rabble. It remains in the acids for
five or six minutes, and is then removed to a grating at the back, pressed
and removed. After each charge of cotton is removed from the tank, about
14 lbs. of fresh mixed acids are added, to replace amount removed by
charge. The charge now weighs, with the acids retained by it, 15 lbs.; it
is now placed in the pots, and left to steep for at least twenty-four
hours, the temperature being kept as low as possible, to prevent the
formation of soluble cotton, and also prevent firing. The proportion of
soluble formed is likely to be higher in hot weather than cold. The pots
must be covered to prevent the absorption of moisture from the air, or the
accidental entrance of water, which would cause decomposition, and
consequent fuming off, through the heat generated by the action of the
water upon the strong acids.

The excess of acids is now extracted by means of hydro-extractors, as at
Stowmarket. They are worked at 1,200 revolutions per minute, and whirled
for five minutes (10-1/2 lbs. of waste acids are removed from each charge
dipped). The charge is then washed in a very similar manner to that
previously described, and again wrung out in a centrifugal extractor
(1,200 revolutions per minute). The gun-cotton is now boiled by means of
steam in wooden tanks for eight hours; it is then again wrung out in the
extractors for three minutes, boiled for eight hours more, and again wrung
out; it is then sent to the beater and afterwards to the poacher. The
poachers hold 1,500 gals. each, or 18 cwt. of cotton. The cotton remains
six hours in the poachers. Before moulding, 500 gals. of water are run
into the poacher, and 500 gals. of lime water containing 9 lbs. of whiting
and 9 gals. of a caustic soda solution. This mixture is of such a strength
that it is calculated to leave in the finished gun-cotton from 1 to 2 per
cent. of alkaline matter.

By means of vacuum pressure, the pulp is now drawn off and up into the
stuff chest--a large cylindrical iron tank, sufficiently elevated on iron
standards to allow room for the small gauge tanks and moulding apparatus
below. It holds the contents of one poacher (18 cwt.), and is provided
with revolving arms to keep the pulp stirred up, so that it may be
uniformly suspended in water.

Recently a new process, invented by J.M. and W.T. Thomson (Eng. Pat. No.
8,278, 1903), has been introduced at the Waltham Abbey Factory. The object
of this invention is the removal of the acids of nitration from the
nitrated material after the action has been completed, and without the aid
of moving machinery, such as presses, rollers, centrifugals, and the like.
The invention consists in the manufacture of nitrated celluloses by
removing the acids from the nitrated cellulose directly by displacement
without the employment of either pressure or vacuum or mechanical
appliances of any kind, and at the same time securing the minimum dilution
of the acids. It was found that if water was carefully run on to the
surface of the acids in which the nitro-cellulose is immersed, and the
acids be slowly drawn off at the bottom of the vessel, the water displaces
the acid from the interstices of the nitro-cellulose without any
undesirable rise in temperature, and with very little dilution of the
acids. By this process almost the whole of the acid is recovered in a
condition suitable for concentration, and the amount of water required for
preliminary washing is very greatly reduced. The apparatus which is used
for the purpose consists of a cylindrical or rectangular vessel
constructed with a perforated false bottom and a cock at its lowest point
for running off the liquid. Means are also provided to enable the
displacing water to be run quietly on to the surface of the nitrating
acids.[A]

[Footnote A: In a further patent (Eng. Pat. 7,269, 1903, F.L. Natham),
J.M. Thomson and W.T. Thomson propose by use of alcohol to replace the
water, used in washing nitro-cellulose, and afterward to remove the
alcohol by pressing and centrifuging.]

The apparatus is shown in Fig. 2O, side elevation, and in Fig. 21 a plan
of the nitrating vessel and its accessories is given. In Fig. 20 is shown
in sectional elevation one of the trough devices for enabling liquids to
be added to those in the nitrating vessel without substantial disturbance.

[Illustration: FIG. 20.--SECTIONAL ELEVATION OF THOMSON'S APPARATUS, _a_,
Tank; _b_, False Bottom; _c_, Bottom; _c'_, Ribs; _d_, Draining Outlet;
_e_, Grid; _f_, Troughs, with Aprons _g_; _h_, Pipe, with Branches _h'_,
leading to Troughs, _f_; _k'_, Outlet Pipe of the Sulphuric Acid Tank _k_;
_l_, Water Supply Pipe; _m_, Pipe to supply of Nitrating Acids; _o_,
Perforations of Trough _f_; _p_, Cock to remove Acid.]

In carrying out this invention a rectangular lead-lined or earthenware
tank _a_ is employed, having a false bottom _b_, supported by ribs _c'_,
over the real bottom _c_, which slopes down to a draining outlet pipe _d_,
provided with a perforated grid or plate _e_, adapted to prevent choking
of the outlet. Suitably supported near the top of the vessel _a_ are
provided two troughs, _f_ having depending aprons _g_, a pipe _h_ has two
branches _h'_, leading to the troughs, _f_. This pipe _h_ is adapted to be
connected by a rubber pipe either to the outlet pipe _k'_ of the sulphuric
acid tank _k_ or the water supply pipe _l_. The nitrating acids are
supplied through the pipe _m_. A charge of mixed nitrating acids is
introduced into the vessel _a_ say up to the level _n_, and the dry
cellulose thrown into the acids in small quantities at a time, being
pushed under the surface in the usual way.

[Illustration: FIG. 21.--PLAN OF THOMSON'S APPARATUS, _a_, Tank; _b_,
False Bottom; _c'_, Ribs; _e_, Grid; _f_, Troughs; _g_, Aprons; _h_ and
_h'_, Pipes to Troughs _f_; _k_, Sulphuric Acid Tank; _m_, Pipe to
Nitrating Acids Tank; _o_, Perforations of Troughs; _p_, Cock to remove
Acid.]

A thin layer, say half an inch, of a suitable liquid, preferably sulphuric
acid, of a gravity not exceeding that of the waste acid to be produced, is
run carefully on the top of the acids by means of the troughs _f_, which
are perforated as shown at _o_, so that the sulphuric acid runs down the
aprons _g_, and floats on the nitrating acids. The whole is then allowed
to stand till nitration has been completed. Water is then supplied to the
troughs by way of the pipes _l_, _h_, and _h'_, and is allowed to float
very gently over the surface of the sulphuric acid, and when a sufficient
layer has been formed, the cock _p_ at the bottom of the apparatus is
opened, and the acid slowly drawn off, water being supplied to maintain
the level constant. It is found that the rate of displacement of the acids
is a factor which exerts a considerable influence on the properties of the
resulting nitro-cellulose, and affords a means of regulating the
temperature of displacement. A rate of displacement which has been found
suitable is about two inches in depth of the vessel per hour when treating
highly nitrated celluloses, but this rate may, in some cases, be
considerably increased. The flow of water at the top of the apparatus is
regulated so that a constant level is maintained. By this means the water
gradually and entirely displaces the acids from the interstices of the
nitro-cellulose, the line of separation between the acids and the water
being fairly sharply defined throughout. The flow of water is continued
until that issuing at the bottom is found to be free from all trace of
acid. The purification of the nitro-cellulose is then proceeded with as
usual, either in the same vessel or another.

In the process above described, the object of the introduction of a small
layer of sulphuric acid is mainly to prevent the fuming which would
otherwise take place, and is not essential, as it is found it can be
omitted without any deleterious effect. In order to use the mixed acids in
the most economical manner, the waste acid from a previous operation may
be used for a first nitration of the cellulose; being afterwards displaced
with fresh acids which carry the nitration to the required degree before
they are in turn displaced by water. The apparatus may be used merely for
the removal of the acid, in which case the nitration is carried out in
other vessels in the usual way, and the nitro-cellulose removed to the
displacement apparatus where it is just covered with waste acid, and the
displacement then proceeded with as above described. In some cases the
process is carried out in an ordinary nitrating centrifugal, using the
latter to effect preliminary drying after acid extraction. This gives a
great advantage over the usual method of working ordinary centrifugal
nitrating apparatus, because the acid being removed before the centrifugal
is run, practically all danger of firing therein disappears, and a greater
proportion of the waste acid is recovered.

In some cases the acids and water may be supplied by perforated pipes,
lying along the edges of the nitrating vessel, and these edges may, if
desired, be themselves made inclined, like the sides of the troughs _f_.
In the case of effecting nitration in centrifugals as above, the
displacing sulphuric acid and water may thus be supplied round the edges
of the machines, or removal troughs such as _f_ may be used. It will be
obvious that any inert liquid of suitable specific gravity may be used
instead of sulphuric acid, as a separation layer.

~Moulding.~--By means of the small measuring tank above referred to, the
gun-cotton pulp is drawn off from the stuff chest, and run into moulds of
the shapes and sizes required. Thence a large proportion of the water is
drawn off by means of tubes connected with the vacuum engine, the moulds
having bottoms of fine wire gauze, in order to prevent the pulp from
passing through. Hydraulic pressure of about 34 lbs. on the square inch is
then applied, which has the effect of compressing the pulp into a state in
which it has sufficient consistency to enable it to be handled with care,
and also expels a portion of the remaining water.

~Compressing.~--The moulded gun-cotton is now taken to the press house,
which is situated at some distance from the rest of the factory. Here the
moulds are subjected to powerful hydraulic pressure, from 5 to 6 tons per
square inch, and is compressed to one-third of its previous bulk. The
slabs or discs thus formed are kept under pressure for a short time, not
exceeding a minute and a half, to give the requisite density. It should,
when removed, be compact, and just sink in water, and should perceptibly
yield to the pressure of the fingers. There are perforations in the press
blocks, to allow of the escape of gases, if formed, by reason of
sufficient heat being generated. The men working the press are placed
under cover, behind strong rope mantlets having eye tubes which command a
view of the press.

~Packing.~--The finished slabs and discs are dipped into a solution of
soda and carbolic acid, and packed in special wood metal-lined cases. When
it is to be sent abroad, the metal lining, which is made of tinned copper,
is soldered down, but both the outer wooden and inner metal cases are
fitted with air-tight screw-plugs, so that when necessary water can be
added without unfastening the cases.

~Reworked gun-cotton~ does not make such good discs as new pulped gun-
cotton, probably because the fibrous tenacity of the gun-cotton has been
destroyed by the amount of pressure it has previously undergone, so that
when repulped it resembles fine dust, and a long time is required to press
it into any prescribed form. It is generally boiled for eight hours to
open up the fibre and remove alkali, then broken up by hand with wooden
mallets, pulped, and then used with fresh gun-cotton in the proportion of
1 to 5 parts.

~Manufacture at Le Bouchet.~--At Le Bouchet gun-cotton was made thus:--200
grms. of cotton were steeped for an hour in 2 litres of a mixture of 1
volume concentrated nitric and 2 volumes sulphuric acid. The cotton was
then removed and pressed, whereby 7/10ths of the waste acids was
recovered. After this it was washed for one to one and a half hours in
running water, strongly pressed again; allowed to lie for twenty-four
hours in wood-ash lye; then well washed in running water; pressed, and
finally dried on a wide linen sheet, through which was forced air heated
to 60 deg. C. The average yield from 100 parts of cotton was 165 parts of gun-
cotton. The strong pressings of the gun-cotton, while still impregnated
with acids, caused subsequent washings to be difficult and laborious.

~Granulation of Gun-Cotton.~--Gun-cotton is often required in the
granulated form for use either alone or with some form of smokeless
powder. This is done under the patent of Sir Frederick Abel in the
following manner:--The gun-cotton from the poacher is placed in a
centrifugal machine, very similar to the hydro-extractors before
mentioned, and used for wringing out the acids. In this machine it loses
water until it only contains 33 per cent., and is at the same time reduced
to a more or less fibrous state. It is then taken to the granulating room,
where it is first passed through sieves or perforations, which break up
the mass into little pieces like shot. The material is then transferred to
a revolving drum made of wood or stout leather, which is kept constantly
revolving for some time. The material is occasionally sprinkled with
water. The drum in turning, of course, carries the granules partially
round with it, but the action of gravity causes them to descend constantly
to the lowest point, and thus to roll over one another continually. The
speed of the drum must not be too rapid. None of the granules must be
carried round by centrifugal force, but it must be fast enough to carry
them some little distance up the side of the drum. After removal from the
drum the granules are dried upon shelves in the drying house.

Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23