Nitro Explosives: A Practical Treatise by P. Gerald Sanford

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In the case of a factory where nitro-glycerine and dynamite are
manufactured, it is necessary that the work-people should wear different
clothes upon the danger area than usual, as they are apt to become
impregnated with nitro-glycerine, and thus not very desirable or safe to
wear outside the works. It is also necessary that these clothes should not
contain any pockets, as this lessens the chance of matches or steel
implements being taken upon the danger area. Changing houses, one for the
men, and another for the girls, should also be provided. The tools used
upon the danger area should, whenever the building is in use, or contains
explosives, be made of phosphor bronze or brass, and brass nails or wooden
pegs should be used in the construction of all the buildings.

[Illustration: FIG. 2.--MELSENS SYSTEM OF LIGHTNING CONDUCTORS.]

~Lightning Conductors.~--The Explosive Substances Act, 38 Vict. ch. 17,
clause 10, says, "Every factory magazine and expense magazine in a
factory, and every danger building in a magazine, shall have attached
thereto a sufficient lightning conductor, unless by reason of the
construction by excavation or the position of such magazine or building,
or otherwise, the Secretary of State considers a conductor unnecessary,
and every danger building in a factory shall, if so required by the
Secretary of State, have attached thereto a sufficient lightning
conductor."

The exact form of lightning conductor most suitable for explosive works
and buildings has not yet been definitely settled. Lightning-rod engineers
favour what is known as the Melsens system, due to Professor Melsens, of
Brussels, and Professor Zenger, of Prague, but first suggested by the late
Professor Clerk-Maxwell. In a paper read before the British Association,
Clerk-Maxwell proposed to protect powder-magazines from the effects of
lightning by completely surrounding or encasing them with sheet metal, or
a cage of metallic conductors. There were, however, several objections to
his system as he left it.

Professor Melsens[A] has, while using the idea, made several important
alterations. He has multiplied the terminals, the conductors, and the
earth-connections. His terminals are very numerous, and assume the form of
an aigrette or brush with five or seven points, the central point being a
little higher than the rest, which form with it an angle of 45 deg.. He
employs for the most part galvanised-iron wire. He places all metallic
bodies, if they are of any considerable size, in communication with the
conducting system in such a manner as to form closed metallic circuits.
His system is illustrated in Fig. 2, taken from _Arms and Explosives_.

[Footnote A: Belgian Academy of Science.]

This system is a near approximation to J.C. Maxwell's cage. The system was
really designed for the protection of powder-magazines or store buildings
placed in very exposed situations. Zenger's system is identical with that
of Melsens, and has been extensively tried by the Austrian military
authorities, and Colonel Hess has reported upon the absolute safety of the
system.

[Illustration: Fig. 3.--FRENCH SYSTEM OF LIGHTNING CONDUCTORS.]

The French system of protecting powder-magazines is shown in Fig. 3, where
there are no brush terminals or aigrettes. The French military authorities
also protect magazines by erecting two or more lightning-rods on poles of
sufficient height placed close to, but not touching, the walls of the
magazine. These conductors are joined below the foundations and earthed as
usual.

In the instructions issued by the Government, it is stated that the
lightning-rods placed upon powder-mills should be of such a height, and so
situated, that no danger is incurred in igniting the powder-dust in the
air by the lightning discharge at the pointed rod. In such a case a fork
or aigrette of five or more points should invariably be used in place of a
single point.

[Illustration: FIG. 4_a_.--GOVERNMENT SYSTEM OF LIGHTNING CONDUCTORS FOR
LARGE BUILDINGS.]

[Illustration: FIG. 4_b_.--GOVERNMENT SYSTEM OF LIGHTNING CONDUCTORS FOR
SMALL BUILDINGS.]

In Fig. 4 (_a_ and _b_) is shown the Government method for protecting
buildings in which explosives are made or stored. Multiple points or
aigrettes would be better. Lord Kelvin and Professor Melsens favour
points, and it is generally admitted that lightning does not strike
buildings at a single point, but rather in a sheet; hence, in such cases,
or in the event of the globular form being assumed by the lightning, the
aigrette will constitute a much more effective protection than a single
point. As to the spacing of conductors, they may, even on the most
important buildings, be spaced at intervals of 50 feet. There will then be
no point on the building more than 25 feet from the conductor. This
"25-feet rule" can be adhered to with advantage in all overground buildings
for explosives.

Underground magazines should, whenever possible, also be protected,
because, although less exposed than overground buildings, they frequently
contain explosives packed in metal cases, and hence would present a line
of smaller electrical resistance than the surrounding earth would offer to
the lightning. The conductor should be arranged on the same system as for
overground buildings, but be applied to the surface of the ground over the
magazines.

In all situations where several conductors are joined in one system, the
vertical conductors should be connected both at the top and near the
ground line. The angles and the prominent portions of a building being the
most liable to be struck, the conductors should be carried over and along
these projections, and therefore along the ridges of the roof. The
conductors should be connected to any outside metal on the roofs and
walls, and specially to the foot of rain-water pipes.

All the lightning conductors should be periodically tested, to see that
they are in working condition, at least every three months, according to
Mr Richard Anderson. The object of the test is to determine the resistance
of the earth-connection, and to localise any defective joints or parts in
the conductors. The best system of testing the conductors is to balance
the resistance of each of the earths against the remainder of the system,
from which the state of the earths may be inferred with sufficient
accuracy for all practical purposes.

Captain Bucknill, R.E., has designed an instrument to test resistance
which is based on the Post Office pattern resistance coil, and is capable
of testing to approximate accuracy up to 200 ohms, and to measure roughly
up to 2,000 ohms. Mr R. Anderson's apparatus is also very handy,
consisting of a case containing three Leclanche cells, and a galvanometer
with a "tangent" scale and certain standard resistances. Some useful
articles on the protection of buildings from lightning will be found in
_Arms and Explosives_, July, August, and September 1892, and by Mr
Anderson, Brit. Assoc., 1878-80.

~Nitro-Glycerine.~--One of the most powerful of modern explosive agents is
nitro-glycerine. It is the explosive contained in dynamite, and forms the
greater part of the various forms of blasting gelatines, such as gelatine
dynamite and gelignite, both of which substances consist of a mixture of
gun-cotton dissolved in nitro-glycerine, with the addition of varying
proportions of wood-pulp and saltpetre, the latter substances acting as
absorbing materials for the viscid gelatine. Nitro-glycerine is also
largely used in the manufacture of smokeless powders, such as cordite,
ballistite, and several others.

Nitro-glycerol, or glycerol tri-nitrate, was discovered by Sobrero in the
year 1847. In a letter written to M. Pelouse, he says, "when glycerol is
poured into a mixture of sulphuric acid of a specific gravity of 1.84, and
of nitric acid of a gravity of 1.5, which has been cooled by a freezing
mixture, that an oily liquid is formed." This liquid is nitro-glycerol, or
nitro-glycerine, which for some years found no important use in the arts,
until the year 1863, when Alfred Nobel first started a factory in
Stockholm for its manufacture upon a large scale; but on account of some
serious accidents taking place, its use did not become general.

It was not until Nobel conceived the idea (in 1866) of absorbing the
liquid in some absorbent earth, and thus forming the material that is now
known as dynamite, that the use of nitro-glycerine as an explosive became
general.

Among those who improved the manufacture of nitro-glycerine was Mowbray,
who, by using pure glycerine and nitric acid free from nitrous acid, made
very great advances in the manufacture. Mowbray was probably the first to
use compressed air for the purpose of keeping the liquids well agitated
during the process of nitration, which he conducted in earthenware pots,
each containing a charge of 17 lbs. of the mixed acids and 2 lbs. of
glycerol.

A few years later (1872), MM. Boutnny and Faucher, of Vonges,[A] proposed
to prepare nitro-glycerine by mixing the sulphuric acid with the
glycerine, thus forming a sulpho-glyceric acid, which was afterwards mixed
with a mixture of nitric and sulphuric acids. They claimed for this method
of procedure that the final temperature is much lower. The two mixtures
are mixed in the proportions--Glycerine, 100; nitric acid, 280; and
sulphuric acid, 600. They state that the rise of temperature upon mixing
is limited from 10 deg. to 15 deg. C.; but this method requires a period of
twenty-four hours to complete the nitration, which, considering the danger
of keeping the nitro-glycerine in contact with the mixed acids for so
long, probably more than compensates for the somewhat doubtful advantage
of being able to perform the nitration at such a low temperature. The
Boutnny process was in operation for some time at Pembrey Burrows in
Wales, but after a serious explosion the process was abandoned.

[Footnote A: _Comptes Rendus_, 75; and Desortiaux, "Traite sur la Poudre,"
684-686.]

Nitro-glycerine is now generally made by adding the glycerine to a mixture
of sulphuric and nitric acids. The sulphuric acid, however, takes no part
in the reaction, but is absolutely necessary to combine with the water
that is formed by the decomposition, and thus to keep up the strength of
the nitric acid, otherwise lower nitrates of glycerine would be formed
that are soluble in water, and which would be lost in the subsequent
process of washing to which the nitro-compound is subjected, in order to
remove the excess of acids, the retention of which in the nitro-glycerol
is very dangerous. Nitro-glycerol, which was formerly considered to be a
nitro-substitution compound of glycerol, was thought to be formed thus--

C_{3}H_{8}O_{3} + 3HNO_{3} = C{3}H_{5}(NO_{2})_{3}O_{3} + 3H_{2}O;

but more recent researches rather point to its being regarded as a nitric
ether of glycerol, or glycerine, and to its being formed thus--

C_{3}H_{8}O_{3} + 3 HNO_{3} = C{3}H_{5}(NO_{3})_{3} + 3H_{2}O.
92 227

|OH
The formula of glycerine is C_{3}H_{8}O_{8}, or C_{3}H_{5}|OH
|OH

|ONO_{2}
and that of the mono-nitrate of glycerine, C_{3}H_{5}|OH
|OH

|ONO_{2}
and of the tri-nitrate or (nitro-glycerine), C_{3}H_{5}|ONO_{2}
|ONO_{2}

that is, the three hydrogens of the semi-molecules of hydroxyl in the
glycerine have been replaced by the NO_{2} group.

In the manufacture upon the large scale, a mixture of three parts by
weight of nitric acid and five parts of sulphuric acid are used. From the
above equation it will be seen that every 1 lb. of glycerol should give
2.47 lbs. of nitro-glycerol ((227+1)/92 = 2.47), but in practice the yield
is only about 2 lbs. to 2.22, the loss being accounted for by the
unavoidable formation of some of the lower nitrate, which dissolves in
water, and is thus washed away, and partly perhaps to the presence of a
little water (or other non-nitrable matter) in the glycerine, but chiefly
to the former, which is due to the acids having become too weak.

CHAPTER II.

_MANUFACTURE OF NITRO-GLYCERINE._

Properties of Nitro-Glycerine--Manufacture of Nitro-Glycerine--Nitration--
The Nathan Nitrator--Separation--Filtering and Washing--The Waste Acids--
Treatment of the Waste Acid from the Manufacture of Nitro-Glycerine and
Gun-Cotton.

~Properties of Nitro-Glycerine.~--Nitro-glycerol is a heavy oily liquid of
specific gravity 1.6 at 15 deg. C., and when quite pure is colourless. The
commercial product is a pale straw yellow, but varies much according to
the purity of the materials used in its manufacture. It is insoluble in
water, crystallises at 10.5 deg. C., but different commercial samples behave
very differently in this respect, and minute impurities prevent or delay
crystallisation. Solid nitro-glycerol[A] melts at about 12 deg. C., but
requires to be exposed to this temperature for some time before melting.
The specific gravity of the solid form is 1.735 at +10 deg. C.; it contracts
one-twelfth of its volume in solidifying. Beckerheim[B] gives the specific
heat as 0.4248 between the temperatures of 9.5 deg. and 9.8 deg. C., and L. de
Bruyn gives the boiling point as above 200 deg..

[Footnote A: Di-nitro-mono chlorhydrin, when added to nitro-glycerine up
to 20 per cent., is said to prevent its freezing.]

[Footnote B: _Isb., Chem. Tech._, 22, 481-487. 1876.]

Nitro-glycerine has a sweet taste, and causes great depression and
vertigo. It is soluble in ether, chloroform, benzene, glacial acetic acid,
and nitro-benzene, in 1.75 part of methylated spirit, very nearly
insoluble in water, and practically insoluble in carbon bisulphide. Its
formula is C_{3}H_{5}(NO_{3})_{3}, and molecular weight 227. When pure, it
may be kept any length of time without decomposition. Berthelot kept a
sample for ten years, and Mr G. M'Roberts, of the Ardeer Factory, for nine
years, without their showing signs of decomposition; but if it should
contain the smallest trace of free acid, decomposition is certain to be
started before long. This will generally show itself by the formation of
little green spots in the gelatine compounds, or a green ring upon the
surface of liquid nitro-glycerine. Sunlight will often cause it to
explode; in fact, a bucket containing some water that had been used to
wash nitro-glycerine, and had been left standing in the sun, has in our
experience been known to explode with considerable force. Nitro-glycerine
when pure is quite stable at ordinary temperatures, and samples have been
kept for years without any trace of decomposition. It is very susceptible
to heat, and even when quite pure will not stand a temperature of 100 deg. C.
for a longer period than a few hours, without undergoing decomposition. Up
to a temperature of 45 deg. C., however, properly made and purified nitro-
glycerine will remain unchanged almost indefinitely. The percentage
composition of nitroglycerine is as follows:--

Found. Theory for C_{3}H_{5}(N0_{2})_{3}.

Carbon 15.62 15.86 per cent.
Hydrogen 2.40 2.20 "
Nitrogen 17.90 18.50 "
Oxygen ... 63.44 "

The above analysis is by Beckerheim. Sauer and Adou give the nitrogen as
18.35 to 10.54 per cent. by Dumas' method; but I have never found any
difficulty in obtaining percentages as high as 18.46 by the use of Lunge's
nitrometer. The decomposition products by explosion are shown by the
following equation--

2C_{3}H_{5}(NO_{3})_{3} = 6CO_{2} + 5H_{2}O + 6N + O;

that is, it contains an excess of 3.52 per cent. of oxygen above that
required for complete combustion; 100 grms. would be converted into--

Carbonic Acid (CO_{2}) 58.15 per cent.
Water 19.83 "
Oxygen 3.52 per cent.
Nitrogen 18.50 "

The volume of gases produced at 0 deg. and 760 mm., calculated from the above,
is 714 litres per kilo, the water being taken as gaseous. Nitro-glycerine
is decomposed differently if it is ignited as dynamite (i.e., kieselguhr
dynamite), and if the gases are allowed to escape freely under a pressure
nearly equal to that of the atmosphere. Sarrau and Vieille obtained under
these conditions, for 100 volumes of gas--

NO 48.2 per cent.
CO 35.9 "
CO_{2} 12.7 "
H 1.6 per cent.
N 1.3 "
CH_{4} 0.3 "

These conditions are similar to those under which a mining charge, simply
ignited by the cap, burns away slowly under a low pressure (i.e., a miss
fire). In a recent communication, P.F. Chalon (_Engineering and Mining
Journal_, 1892) says, that in practice nitro-glycerine vapour, carbon
monoxide, and nitrous oxide, are also produced as the result of
detonation, but he attributes their formation to the use of a too feeble
detonator.

Nitro-glycerine explodes very violently by concussion. It may be burned in
an open vessel, but if heated above 250 deg. C. it explodes. Professor C.E.
Munroe gives the firing point as 2O3 deg.-2O5 deg. C., and L. de Bruyn[A] states
its boiling point as 185 deg.. He used the apparatus devised by Horsley. The
heat of formation of nitro-glycerine, as deduced from the heat of
combustion by M. Longuinine, is 432 calories for 1 grm.; and the heat of
combustion equals 1,576 cals. for 1 grm. In the case of nitro-glycerine
the heat of total combustion and the heat of complete decomposition are
interchangeable terms, since it contains an excess of oxygen. According to
Dr W.H. Perkin, F.R.S.,[B] the magnetic rotation of nitro-gylcerine is
5,407, and that of tri-methylene nitrate, 4.769 (diff. = .638). Dr Perkin
says: "Had nitro-glycerine contained its nitrogen in any other combination
with oxygen than as -O-NO_{2}, as it might if its constitution had been
represented as C_{3}H_{2}(NO_{2})_{3}(OH)_{3}, the rotation when compared
with propyl nitrate (4.085) would be abnormal."

[Footnote A: _Jour. Soc. Chem. Ind._, June 1896, p. 471.]

[Footnote B: _Jour. Chem. Soc._, W.H. Perkin, 1889, p. 726.]

The solubility of nitro-glycerine in various solvents has been
investigated by A.H. Elliot; his results may be summarised as follows:--

_______________________________________________________________________
| |
Solvent. | Cold. | Warm.
_____________________________|______________________|__________________
| |
Water | Insoluble | Slightly soluble
Alcohol, absolute | Soluble | Soluble
" 93% | " | "
" 80% | Slowly soluble | "
" 50% | Insoluble | Slightly soluble
Methyl alcohol | Soluble | Soluble
Amyl " | " | "
Ether, ethylic | " | "
" acetic | " | "
Chloroform | " | "
Acetone | " | "
Sulphuric acid (1.845) | " | "
Nitric acid (1.400) | Slowly soluble | "
Hydrochloric acid (1.200) | Insoluble, decomposed| Slowly soluble
Acetic acid, glacial | Soluble | Soluble
Carbolic acid | " | "
Astral oil | Insoluble | Insoluble
Olive " | Soluble | Soluble
Stearine oil | " | "
Mineral jelly | Insoluble | Insoluble
Glycerine | " | "
Benzene | Soluble | Soluble
Nitro-benzene | " | "
Toluene | " | "
Carbon bi-sulphide | Insoluble | Slightly affected
Turpentine | " | Soluble
Petroleum naphtha, 71 deg.-76 deg. B.| " | Insoluble
Caustic soda (1:10 solution) | Insoluble. | Insoluble.
Borax, 5% solution | " | "
Ammonia (.980) | " | " slightly
| | affected.
Ammonium sulph-hydrate | Insoluble, sulphur | Decomposed.
| separates |
Iron sulphate solution | Slightly affected | Affected.
Iron chloride (1.4 grm. Fe | Slowly affected | Decomposed.
to 10 c.c. N_{2}O) | |
Tin chloride | Slightly affected | Affected.
_____________________________|______________________|__________________

Many attempts have been made to prepare nitro-glycerine explosives capable
of withstanding comparatively low temperatures without freezing, but no
satisfactory solution of the problem has been found. Among the substances
that have been proposed and used with more or less success, are nitro-
benzene, nitro-toluene, di-nitro-mono-chlorhydrine, solid nitro
derivatives of toluene,[A] are stated to lower the freezing point of
nitro-glycerine to -20 deg.C. without altering its sensitiveness and
stability. The subject has been investigated by S. Nauckhoff,[B] who
states that nitroglycerine can be cooled to temperatures (-40 deg. to -50 deg. C.)
much below its true freezing point, without solidifying, by the addition
of various substances. When cooled by means of a mixture of solid carbon,
dioxide, and ether, it sets to a glassy mass, without any perceptible
crystallisation. The mass when warmed to 0 deg.C. first rapidly liquefies and
then begins to crystallise. The true freezing point of pure nitro-
glycerine was found to be 12.3 deg.C. The technical product, owing to the
presence of di-nitro-glycerine, freezes at 10.5 deg. C. According to Raoult's
law, the lowering of the freezing point caused by _m_ grms. of a substance
with the molecular weight M, when dissolved in 100 grms. of the solvent,
is expressed by the formula: [Delta] = E(_m_/M), where E is a constant
characteristic for the solvent in question. The value of E for nitro-
glycerine was found to be 70.5 when calculated, according to Van't Hoff's
formula, from the melting point and the latent heat of fusion of the
substance. Determinations of the lowering of the freezing point of nitro-
glycerine by additions of benzene, nitro-benzene, di-nitro-benzene, tri-
nitro-benzene, p.-nitro-toluene, o.-nitro-toluene, di-nitro-toluene,
naphthalene, nitro-naphthalene, di-nitro-naphthalene, ethyl acetate, ethyl
nitrate, and methyl alcohol, gave results agreeing fairly well with
Raoult's formula, except in the case of methyl alcohol, for which the
calculated lowering of the freezing point was greater than that observed,
probably owing to the formation of complex molecules in the solution. The
results show that, in general, the capacity of a substance to lower the
freezing point of nitro-glycerine depends, not upon its freezing point, or
its chemical composition or constitution, but upon its molecular weight.
Nauckhoff states that a suitable substance for dissolving in nitro-
glycerine, in order to lower the freezing point of the latter, must have a
relatively low molecular weight, must not appreciably diminish the
explosive power and stability of the explosive, and must not be easily
volatile at relatively high atmospheric temperatures; it should, if
possible, be a solvent of nitro-cellulose, and in every case must not have
a prejudicial influence on the gelatinisation of the nitro-cellulose.

[Footnote A: Eng. Pat. 25,797, November 1904.]

[Footnote B: _Z. Angew. Chem._, 1905, 18, 11-22, 53-60.]

~Manufacture of Nitro-Glycerine.~--Nitro-glycerine is prepared upon the
manufacturing scale by gradually adding glycerine to a mixture of nitric
and sulphuric acids of great strength. The mixed acids are contained in a
lead vessel, which is kept cool by a stream of water continually passing
through worms in the interior of the nitrating vessel, and the glycerine
is gradually added in the form of a fine stream from above. The
manufacture can be divided into three distinct operations, viz.,
nitration, separation, and washing, and it will be well to describe these
operations in the above order.

~Nitration.~--The most essential condition of nitrating is the correct
composition and strength of the mixed acids. The best proportions have
been found to be three parts by weight of nitric acid of a specific
gravity 1.525 to 1.530, and containing as small a portion of the oxides of
nitrogen as possible, to five parts by weight of sulphuric acid of a
specific gravity of 1.840 at 15 deg. C., and about 97 per cent. of mono-
hydrate. It is of the very greatest importance that the nitric acid should
be as strong as possible. Nothing under a gravity of 1.52 should ever be
used even to mix with stronger acid, and the nitration will be
proportional to the strength of the acid used, provided the sulphuric acid
is also strong enough. It is also of great importance that the oxides of
nitrogen should be low, and that they should be kept down to as low as 1
per cent., or even lower. It is also very desirable that the nitric acid
should contain as little chlorine as possible. The following is the
analysis of a sample of nitric acid, which gave very good results upon the
commercial scale:--Specific gravity, 1.525, N_{2}O_{4}, 1.03 per cent.;
nitric acid (HNO_{3}), 95.58 per cent.

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