Nitro Explosives: A Practical Treatise by P. Gerald Sanford

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[Illustration: FIG. 9.--FILTERING AND WASHING PLANT. _W_, Lead Washing
Tank; _WP_, Water Pipe; _L_, Lid; _S_, Nitro-glycerine from Separator; _A,
B, C_, Filtering Tanks; _B2_, Indiarubber Bucket.]

~Filtering and Washing.~--The filter house (Fig. 9), which must of course
be again on a somewhat lower level than the separating house, must be a
considerably larger building than either the nitrating or separating
houses, as it is always necessary to be washing some five or six charges
at the same time. Upon the arrival of the nitro-glycerine at this house,
it first flows into a lead-lined wooden tank (W), containing a compressed
air pipe, just like the one in the small tank in the separating house.
This tank is half filled with water, and the compressed air is turned on
from half to a quarter of an hour after the introduction of the charge.
The water is then drawn off, and fresh water added. Four or five washings
are generally necessary. The nitro-glycerine is then run into the next
tank (A), the top of which is on a level with the bottom of the first one.
Across the top of this tank is stretched a frame of flannel, through which
the nitroglycerine has to filter. This removes any solid matters, such as
dirt or scum. Upon leaving this tank, it passes through a similar flannel
frame across another tank (B), and is finally drawn off by a tap in the
bottom of the tank into rubber buckets. The taps in these tanks are best
made of vulcanite.

At this stage, a sample should be taken to the laboratory and tested. If
the sample will not pass the tests, which is often the case, the charge
must be rewashed for one hour, or some other time, according to the
judgment of the chemist in charge. In the case of an obstinate charge, it
is of much more avail to wash a large number of times with small
quantities of water, and for a short time, than to use a lot of water and
wash for half an hour. Plenty of compressed air should be used, as the
compound nitric ethers which are formed are thus got rid of. As five or
six charges are often in this house at one time, it is necessary to have
as many tanks arranged in tiers, otherwise one or two refractory charges
would stop the nitrating house and the rest of the nitro-glycerine plant.
The chief causes of the washed material not passing the heat test are,
either that the acids were not clean, or they contained objectionable
impurities, or more frequently, the quality of the glycerine used. The
glycerine used for making nitro-glycerine should conform to the following
tests, some of which, however, are of greater importance than others. The
glycerine should--

1. Have minimum specific gravity at 15 deg. C. of 1.261.

2. Should nitrify well.

3. Separation should be sharp within half an hour, without the separation
of flocculent matter, nor should any white flocculent matter (due to fatty
acids) be formed when the nitrated glycerine is thrown into water and
neutralised with carbonate of soda.

4. Should be free from lime and chlorine, and contain only traces of
arsenic, sulphuric acid, &c.

5. Should not leave more than 0.25 per cent. of inorganic and organic
residue together when evaporated in a platinum dish without ebullition
(about 160 deg. C.) or partial decomposition.

6. Silver test fair.

7. The glycerine, when diluted one-half, should give no deposit or
separation of fatty acids when nitric peroxide gas is passed through it.
(Nos. 1, 2, 3, and 5 are the most essential.)

The white flocculent matter sometimes formed is a very great nuisance, and
any sample of glycerol which gives such a precipitate when tried in the
laboratory should at once be rejected, as it will give no end of trouble
in the separating house, and also in the filter house, and it will be very
difficult indeed to make the nitro-glycerine pass the heat test. The out-
turn of nitro-glycerine also will be very low. The trouble will show
itself chiefly in the separating operation. Very often 2 or 3 inches will
rise to the surface or hang about in the nitro-glycerine, and at the point
of contact between it and the mixed acids, and will afterwards be very
difficult to get rid of by filtration. The material appears to be partly
an emulsion of the glycerine, and partly due to fatty acids, and as there
appears to be no really satisfactory method of preventing its formation,
or of getting rid of it, the better plan is not to use any glycerine for
nitrating that has been found by experiment upon the laboratory scale to
give this objectionable matter. One of the most useful methods of testing
the glycerine, other than nitrating, is to dilute the sample one-half with
water, and then to pass a current of nitric peroxide gas through it, when
a flocculent precipitate of elaidic acid (less soluble in glycerine than
the original oleic acid) will be formed. Nitrogen peroxide, N_{2}O_{4}, is
best obtained by heating dry lead nitrate (see Allen, "Commercial Organic
Analysis," vol. ii., 301).

When a sample of nitro-glycerine is brought to the laboratory from the
filter house, it should first be examined to see that it is not acid.[A] A
weak solution of Congo red or methyl orange may be used. If it appears to
be decidedly alkaline, it should be poured into a separating funnel, and
shaken with a little distilled water. This should be repeated, and the
washings (about 400 c.c.) run into a beaker, a drop of Congo red or methyl
orange added, and a drop or so of N/2 hydrochloric acid added, when it
should give, with two or three drops at most, a blue colour with the Congo
red, or pink with the methyl orange, &c. The object of this test is to
show that the nitro-glycerine is free from any excess of soda, i.e., that
the soda has been properly washed out, otherwise the heat test will show
the sample to be better than it is. The heat test must also be applied.

[Footnote A: A. Leroux, _Bul. Soc. Chim. de Bel._, xix., August 1905,
contends that experience does not warrant the assumption that free acid is
a source of danger in nitro-glycerine or nitro-cellulose; free alkali, he
states, promotes their decomposition.]

Upon leaving the filter house, where it has been washed and filtered, and
has satisfactorily passed the heat test, it is drawn off from the lowest
tank in indiarubber buckets, and poured down the conduit leading to the
precipitating house, where it is allowed to stand for a day, or sometimes
longer, in order to allow the little water it still contains to rise to
the surface. In order to accomplish this, it is sufficient to allow it to
stand in covered-in tanks of a conical form, and about 3 or 4 feet high.
In many works it is previously filtered through common salt, which of
course absorbs the last traces of water. It is then of a pale yellow
colour, and should be quite clear, and can be drawn off by means of a tap
(of vulcanite), fixed at the bottom of the tanks, into rubber buckets, and
is ready for use in the preparation of dynamite, or any of the various
forms of gelatine compounds, smokeless powders, &c., such as cordite,
ballistite, and many others.

Mikolajezak (_Chem. Zeit._, 1904, Rep. 174) states that he has prepared
mono- and di-nitro-glycerine, and believes that the latter compound will
form a valuable basis for explosives, as it is unfreezable. It is stated
to be an odourless, unfreezable oil, less sensitive to percussion,
friction, and increase of temperature, and to possess a greater solvent
power for collodion-cotton than ordinary nitro-glycerine. It can thus be
used for the preparation of explosives of high stability, which will
maintain their plastic nature even in winter. The di-nitro-glycerine is a
solvent for tri-nitro-glycerine, it can therefore be mixed with this
substance, in the various gelatine explosives in order to lower the
freezing point.

~The Waste Acids.~--The waste acids from the separating house, from which
the nitro-glycerine has been as completely separated as possible, are run
down the conduit to the secondary separator, in order to recover the last
traces of nitro-glycerine that they contain. The composition of the waste
acids is generally somewhat as follows:--Specific gravity, 1.7075 at 15 deg.
C.; sulphuric acid, 67.2 per cent.; nitric acid, 11.05 per cent.; and
water, 21.7 per cent., with perhaps as much as 2 per cent. of nitric
oxide, and of course varying quantities of nitro-glycerine, which must be
separated, as it is impossible to run this liquid away (unless it can be
run into the sea) or to recover the acids by distillation as long as it
contains this substance. The mixture, therefore, is generally run into
large circular lead-lined tanks, covered in, and very much like the
nitrating apparatus in construction, that is, they contain worms coiled
round inside, to allow of water being run through to keep the mixture
cool, and a compressed air pipe, in order to agitate the mixture if
necessary. The top also should contain a window, in order to allow of the
interior being seen, and should have a leaden chimney to carry off the
fumes which may arise from decomposition. It is also useful to have a
glass tube of 3 or 4 inches in diameter substituted for about a foot of
the lead chimney, in order that the man on duty can at any time see the
colour of the fumes arising from the liquid. There should also be two
thermometers, one long one reaching to the bottom of the tank, and one to
just a few inches below the surface of the liquid.

The nitro-glycerine, of course, collects upon the surface, and can be
drawn off by a tap placed at a convenient height for the purpose. The
cover of the tank is generally conical, and is joined to a glass cylinder,
which is cemented to the top of this lead cover, and also to the lead
chimney. In this glass cylinder is a hole into which fits a ground glass
stopper, through which the nitro-glycerine can be drawn off. There will
probably never be more than an inch of nitro-glycerine at the most, and
seldom that. It should be taken to the filter house and treated along with
another charge. The acids themselves may either be run to waste, or better
treated by some denitration plant. This house probably requires more
attention than any other in the danger area, on account of the danger of
the decomposition of the small quantities of nitro-glycerine, which, as it
is mixed with such a large quantity of acids and water, is very apt to
become hot, and decomposition, which sets up in spots where a little
globule of nitro-glycerine is floating, surrounded by acids that gradually
get hot, gives off nitrous fumes, and perhaps explodes, and thus causes
the sudden explosion of the whole. The only way to prevent this is for the
workman in charge to look at the thermometers _frequently_, and at the
colour of the escaping fumes, and if he should notice a rise of
temperature or any appearance of red fumes, to turn on the water and air,
and stir up the mixture, when probably the temperature will suddenly fall,
and the fumes cease to come off.

The cause of explosions in this building is either the non-attention of
the workmen in charge, or the bursting of one of the water pipes, by which
means, of course, the water, finding its way into the acids, causes a
sudden rise of temperature. If the latter of these two causes should
occur, the water should at once be shut off and the air turned on full,
but if it is seen that an explosion is likely to occur, the tank should at
once be emptied by allowing its contents to run away into a drowning tank
placed close outside the house, which should be about 4 feet deep, and
some 16 feet long by 6 feet wide; in fact, large enough to hold a
considerable quantity of water. But this last course should only be
resorted to as a last extremity, as it is extremely troublesome to recover
the small quantity of nitro-glycerine from the bottom of this tank, which
is generally a bricked and cemented excavation some few yards from the
house.

It has been proposed to treat these waste acids, containing nitro-
glycerine, in Mr M. Prentice's nitric acid retort. In this case they would
be run into the retort, together with nitrate of soda, in a fine stream,
and the small quantity of nitro-glycerine, coming into contact with the
hot mixture already in the retort, would probably be at once decomposed.
This process, although not yet tried, promises to be a success. Several
processes have been used for the denitration of these acids.

~Treatment of the Waste Acid from the Manufacture of Nitro-Glycerine and
Gun-Cotton.~--The composition of these acids is as follows:--

Nitro-glycerine and Gun-cotton
Waste Acid.

Sulphuric acid 70 per cent. 78 per cent.
Nitric acid 10 " 12 "
Water 20 " 10 "

The waste acid from the manufacture of gun-cotton is generally used direct
for the manufacture of nitric acid, as it contains a fairly large amount
of sulphuric acid, and the small amount of nitro-cellulose which it also
generally contains decomposes gradually and without explosion in the
retort. Nitric acid may be first distilled off, the resulting sulphuric
acid being then added to the equivalent amount of nitrate of soda. Nitric
acid is then distilled over and condensed in the usual way. Very often,
however, the waste acid is added direct to the charge of nitrate without
previously eliminating the nitric acid. The treatment of the waste acid
from the manufacture of nitro-glycerine is somewhat different. The small
amount of nitro-glycerine in this acid must always be eliminated. This is
effected either by allowing the waste acid to stand for at least twenty-
four hours in a big vessel with a conical top, where all the nitro-
glycerine which will have separated to the surface is removed by skimming;
or, better still, the "watering down process" of Col. Nathan may be
employed. In Nathan's nitrator every existing trace of nitro-glycerine is
separated from the acids in a few hours after the nitration, and any
further formation of nitro-glycerine is prevented by adding about 2 per
cent. of water to the waste acids, which are kept agitated during the
addition. The waste acid, now free from nitro-glycerine, but which may
still contain organic matter, is denitrated by bringing it into contact
with a jet of steam. The waste acid is passed in a small stream down
through a tower of acid-resisting stoneware (volvic stone), which is
closely packed with earthenware, and at the bottom of which is the steam
jet. Decomposition proceeds as the acid meets the steam, nitric and
nitrous acids are disengaged and are passed out at the top of the tower
through a pipe to a series of condensers and towers, where the nitric acid
is collected. The nitrous acid may be converted into nitric acid by
introducing a hot compressed air jet into the gases before they pass into
the condensers. Weak sulphuric acid of sp. gr. 1.6 collects in a saucer in
which the tower stands, and is then passed through a cooling worm. The
weak sulphuric acid, now entirely free from nitric and nitrous acids, may
be concentrated to sp. gr. 1.842 and 96 per cent. H_{2}SO_{4} by any of
the well-known processes, e.g., Kessler, Webb, Benker, Delplace, &c., and
it may be used again in the manufacture of nitro-glycerine or gun-cotton.

Two points in the manufacture of nitro-glycerine are of the greatest
importance, viz., the purity of the glycerine used, and the strength and
purity of the acids used in the nitration. With regard to the first of
these, great care should be taken, and a complete analysis and thorough
examination, including a preliminary experimental nitration, should always
be instituted. As regards the second, the sulphuric acid should not only
be strong (96 per cent.), but as free from impurities as possible. With
the nitric acid, which is generally made at the explosive works where it
is used, care must be taken that it is as strong as possible (97 per cent.
and upwards). This can easily be obtained if the plant designed by Mr
Oscar Guttmann[A] is used. Having worked Mr Guttmann's plant for some
time, I can testify as to its value and efficiency.

[Footnote A: "The Manufacture of Nitric Acid," _Jour. Soc. Chem. Ind._,
March 1893.]

Another form of nitric acid plant, which promises to be of considerable
service to the manufacturer of nitric acid for the purpose of nitrating,
is the invention of the late Mr Manning Prentice, of Stowmarket. Through
the kindness of Mr Prentice, I visited his works to see the plant in
operation. It consists of a still, divided into compartments or chambers
in such a manner that the fluid may pass continuously from one to the
other. The nitric acid being continuously separated by distillation, the
contents of each division vary--the first containing the full proportion
of nitric acid, and each succeeding one less of the nitric acid, until
from the overflow of the last one the bisulphate of soda flows away
without any nitric acid. The nitrate of soda is placed in weighed
quantities in the hopper, whence it passes to the feeder. The feeder is a
miniature horizontal pug-mill, which receives the streams of sulphuric
acid and of nitrate, and after thoroughly mixing them, delivers them into
the still, where, under the influence of heat, they rapidly become a
homogeneous liquid, from which nitric acid continuously distils.

Mr Prentice says: "I may point out that while the ordinary process of
making nitric acid is one of fractional distillation by time, mine is
fractional distillation by space." "Instead of the operation being always
at the same point of space, but differing by the successive points of
time, I arrange for the differences to take place at different points of
space, and these differences exist at one and the same points of time." It
is possible with this plant to produce the full product of nitric acid of
a gravity of 1.500, or to obtain the acid of varying strengths from the
different still-heads. One of these stills, capable of producing about 4
tons of nitric acid per week, weighs less than 2 tons. It is claimed that
there is by their use a saving of more than two-thirds in fuel, and four-
fifths in condensing plant. Further particulars and illustrations will be
found in Mr Prentice's paper (_Journal of the Society of Chemical
Industry_, 1894, p. 323).

CHAPTER III.

_NITRO-CELLULOSE, &c._

Cellulose Properties--Discovery of Gun-Cotton--Properties of Gun-Cotton--
Varieties of Soluble and Insoluble Gun-Cottons--Manufacture of Gun-Cotton--
Dipping and Steeping--Whirling out the Acid--Washing--Boiling--Pulping--
Compressing--The Waltham Abbey Process--Le Bouchet Process--Granulation of
Gun-Cotton--Collodion-Cotton--Manufacture--Acid Mixture used--Cotton used,
&c.--Nitrated Gun-Cotton--Tonite--Dangers in Manufacture of Gun-Cotton--
Trench's Fire-Extinguishing Compound--Uses of Collodion-Cotton--Celluloid--
Manufacture, &c.--Nitro-Starch, Nitro-Jute, and Nitro-Mannite.

~The Nitro-Celluloses.~--The substance known as cellulose forms the
groundwork of vegetable tissues. The cellulose of the woody parts of
plants was at one time supposed to be a distinct body, and was called
lignine, but they are now regarded as identical. The formula of cellulose
is (C_{6}H_{10}O_{6})_{X}, and it is generally assumed that the molecular
formula must be represented by a multiple of the empirical formula,
C_{12}H_{20}O_{10} being often regarded as the minimum. The assumption is
based on the existence of a penta-nitrate and the insoluble and colloidal
nature of cellulose. Green (_Zeit. Farb. Text. Ind._, 1904, 3, 97)
considers these reasons insufficient, and prefers to employ the single
formula C_{6}H_{10}O_{5}. Cellulose can be extracted in the pure state,
from young and tender portions of plants by first crushing them, to
rupture the cells, and then extracting with dilute hydrochloric acid,
water, alcohol, and ether in succession, until none of these solvents
remove anything more. Fine paper or cotton wool yield very nearly pure
cellulose by similar treatment.

Cellulose is a colourless, transparent mass, absolutely insoluble in
water, alcohol, or ether. It is, however, soluble in a solution of
cuprammoniac solution, prepared from basic carbonate or hydrate of copper
and aqueous ammonia. The specific gravity of cellulose is 1.25 to 1.45.
According to Schulze, its elementary composition is expressed by the
percentage numbers:--

Carbon 44.0 per cent. 44.2 per cent.
Hydrogen 6.3 " 6.4 "
Oxygen 49.7 " 49.4 "

These numbers represent the composition of the ash free cellulose. Nearly
all forms of cellulose, however, contain a small proportion of mineral
matters, and the union of these with the organic portion of the fibre or
tissue is of such a nature that the ash left on ignition preserves the
form of the original. "It is only in the growing point of certain young
shoots that the cellulose tissue is free from mineral constituents"
(Hofmeister).

Cellulose is a very inert body. Cold concentrated sulphuric acid causes it
to swell up, and finally dissolves it, forming a viscous solution.
Hydrochloric acid has little or no action, but nitric acid has, and forms
a series of bodies known as nitrates or nitro-celluloses. Cellulose has
some of the properties of alcohols, among them the power of forming
ethereal salts with acids. When cellulose in any form, such as cotton, is
brought into contact with strong nitric acid at a low temperature, a
nitrate or nitro product, containing nitryl, or the NO_{2} group, is
produced. The more or less complete replacement of the hydroxylic hydrogen
by NO_{2} groups depends partly on the concentration of the nitric acid
used, partly on the duration of the action. If the most concentrated
nitric and sulphuric acids are employed, and the action allowed to proceed
for some considerable time, the highest nitrate, known as hexa-nitro-
cellulose or gun-cotton, C_{12}H_{14}O_{4}(O.NO_{2})_{6}, will be formed;
but with weaker acids, and a shorter exposure to their action, the tetra
and penta and lower nitrates will be formed.[A]

[Footnote A: The paper by Prof. Lunge, _Jour. Amer. Chem. Soc._, 1901,
23[8], 527-579, contains valuable information on this subject.]

Besides the nitrate, A. Luck[A] has proposed to use other esters of
cellulose, such as the acetate, benzoate, or butyrate. It is found that
cellulose acetate forms with nitro-glycerine a gelatinous body without
requiring the addition of a solvent. A sporting powder is proposed
composed of 75 parts of cellulose nitrate (13 per cent. N.) mixed with 13
parts of cellulose acetate.

[Footnote A: Eng. Pat. 24,662, 22nd November 1898.]

The discovery of gun-cotton is generally attributed to Schoenbein (1846),
but Braconnot (in 1832) had previously nitrated starch, and six years
later Pelouse prepared nitro-cotton and various other nitro bodies, and
Dumas nitrated paper, but Schoenbein was apparently the first chemist to
use a mixture of strong nitric and sulphuric acids. Many chemists, such as
Piobert in France, Morin in Russia, and Abel in England, studied the
subject; but it was in Austria, under the auspices of Baron Von Lenk, that
the greatest progress was made. Lenk used cotton in the form of yarn, made
up into hanks, which he first washed in a solution of potash, and then
with water, and after drying dipped them in the acids. The acid mixture
used consisted of 3 parts by weight of sulphuric to 1 part of nitric acid,
and were prepared some time before use. The cotton was dipped one skein at
a time, stirred for a few minutes, pressed out, steeped, and excess of
acid removed by washing with water, then with dilute potash, and finally
with water. Von Lenk's process was used in England at Faversham (Messrs
Hall's Works), but was given up on account of an explosion (1847).

Sir Frederick Abel, working at Stowmarket and Waltham Abbey, introduced
several very important improvements into the process, the chief among
these being pulping. Having traced the cause of its instability to the
presence of substances caused by the action of the nitric acid on the
resinous or fatty substances contained in the cotton fibre, he succeeded
in eliminating them, by boiling the nitro-cotton in water, and by a
thorough washing, after pulping the cotton in poachers.

Although gun-cottons are generally spoken of as nitro-celluloses, they are
more correctly described as cellulose nitrates, for unlike nitro bodies of
other series, they do not yield, or have not yet done so, amido bodies, on
reduction with nascent hydrogen.[A] The equation of the formation of
gun-cotton is as follows:--

2(C_{6}H_{10}O_{5}) + 6HNO_{3} = C_{12}H_{14}O_{4}(NO_{3})_{6} + 6OH_{2}.
Cellulose. Nitric Acid. Gun-Cotton. Water.

The sulphuric acid used does not take part in the reaction, but its
presence is absolutely essential to combine with the water set free, and
thus to prevent the weakening of the nitric acid. The acid mixture used at
Waltham Abbey consists of 3 parts by weight of sulphuric acid of 1.84
specific gravity, and 1 part of nitric acid of 1.52 specific gravity. The
same mixture is also used at Stowmarket (the New Explosive Company's
Works). The use of weaker acids results in the formation of collodion-
cotton and the lower nitrates generally.

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