Note: Descriptions are shown in the official language in which they were submitted.
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Title of Invention
Method of Manufacture of Emulsion Explosives .
Field of Invention
This invention relates to Explosives of non-Nitroglycerine type used in the
5 mining industry. Especially, the present invention relates to generation of gas
voids in Ammonium Nitrate based explosives such as Emulsion
explosives/Slurry explosives/Heavy ANFO and Doped emulsions. A special
aspect of the invention relates to lowering the density of such explosives at
ambient to low temperature conditions.
10 Background
Non-nitroglycerine explosives based on Ammonium nitrate such as Slurries,
also known as watergels, Emulsions and ANFO mixtures require entrapped gas
bubbles, tiny in size, for initiation and sustenance of the process of detonation.
These bubbles or voids when compressed adiabatically by the shock-wave
15 generated by the initiating charge behave as "hot-spots" and initiate the
combustion reaction that finally leads to detonation. Incorporation of such voids
can be done by several means that could be described under the following four
broad categories:
- Incorporation into the explosive matrix materials containing entrapped voids,
20 - physical means
- mechanical action
- chemical means
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Under the first category, use of particulate materials such as hollow glass
microballoons, plastic spheres, Perlites, voicanic ash, silicon sand, sodium
silicate etc., is well known. This low density particulate matter retain the
enclosed air even after mixing into the explosive and thus provide the hot-spots5 for detonation. Over the years it was found that while glass microballoons were
effective performers others were not as effective. The glass microballoons on
the other hand are expensive and pose handling problems due to their low bulk
density.
In addition to the above, gas retaining agents such as foamed polystyrene,
foamed polyurethane and the likes were disclosed in the US patent 4543137.
These were rigid or soft and spongy depending upon the resin employed for
their preparation. However, the main problem with their usage is their participa-
tion in the combustion process as fuels leading to limitation in their usage
levels or alternately undesirable oxygen-deficient situation.
The aspect of participation as fuel was taken care of in US Patent 5409556 by
use of expanded grains such as expanded popcorn, expanded rice or
expanded wheat for density reduction. It was claimed that these materials
being carbohydrates were not good fuels and did not significantly alter the
oxygen balance of the explosive composition when used in the small amounts
20 required for density reduction.
Use of gas-in-liquid and gas-in-solid foams was disclosed in the Canadian
Patent Applications 2093309 and 2113945 as means to incorporate voids. The
foams consisted of mechanically or chemically generated gas locked in open or
closed cellular structures. These foams contained, besides foam forming
25 agents, substances such as TNT, Ammonium nitrate, Sodium nitrate etc., to
further manipulate their sensitising ability.
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In the second category a new route to generating gas bubbles within the explo-
sive matrix was disclosed in UK Patent Application 2179035. This involved
dissolving the gas in the matrix under application of high pressure followed by
sudden release to atmospheric pressure that then results in formation of gas
5 bubbles due to the solubility differential for the gas in the matrix between high
and ambient pressures. However, the main disadvantage of this method is that
it requires specialised equipment for handling sensitised emulsions at high
pressures.
In the third category we have the void generation by mechanical action. During
10 prolonged mixing or intensive agitation voids get entrapped into the matrix due
to its viscous nature. This method has the disadvantage that prolonged shear-
ing could adversely affect long-term stability of the product.
The well-known alternative to above means of incorporating voids is to gener-
ate them in-situ by means of a chemical reaction. This technique, known in the
15 industry circles as chemical gassing, is described in numerous patents viz., US
Patents 3886010, 3706607, EP 0655430A1 to name a few. Several gas gener-
ating reactions involving chemical substances such as nitrites, weak acids,
hydrazines and peroxides have been patented. One of the most commonly
practised reaction is that of sodium nitrite reacting with the ammonium nitrate
20 present in the explosive matrix to produce nitrogen gas that gets entrapped in
the form of bubbles in the viscous matrix.
The matrix of a water-in-oil emulsion explosive is prepared by mixing under
agitation the aqueous and oil phases. The former phase contains salts such as
ammonium nitrate, sodium nitrate, calcium nitrate etc., dissolved in water while25 the latter contains waxes, oils and emulsifiers. The emulsion matrix is prepared
either by a batch process or a continuous process or combinations thereof
employing different kinds of rotary mixers, static mixers, jet mixers, colloid mills,
votators etc. The gassing agent is mixed into the emulsion matrix either in a
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rotary mixer or employing static mixers. Sometimes the aqueous solution of
sodium nitrite is emulsified and incorporated as a water-in-oil emulsion to facili-
tate better mixing and derive benefits arising thereby as claimed in world patent
WO 89/02881.
5 In the manufacture of watergel explosives, oxidiser salts such as ammonium
nitrate, sodium nitrate, calcium nitrate etc. are dissolved in water at above
ambient temperatures and the resultant solution thickened ( or gelled ) using
substances such as guar gum and subsequently crosslinked using metal ions
such as chromium in suitable form. Optionally solid particulate materials like
10 Aluminium are dispersed into the matrix. Here again gassing is usually the last
operation that involves mixing in of aqueous solution of sodium nitrite.
Ammonium nitrate, in the form of porous prills as such can be mixed with fuel oil
to form an explosive known as ANFO. While this itself is a popular explosive, incertain mining operations it is blended with water-in-oil emulsion in different
15 proportions to render water-proofness as well as increased product density.
The emulsion matrix used for blending is sometimes gassed to ensure satisfac-
tory detonation performance of the final product. These products are generally
known in the industry as Heavy ANFO.
Complementary to Heavy ANFO, we have what is known in the industry as
20 doped emulsions. Here prills of ammonium nitrate or ANFO are mixed into the
emulsion explosive matrix to enhance its performance. It is generally under-
stood that in a doped emulsion the dopant constitutes a minor faction of the
total composition.
25 The sodium nitrite - ammonium nitrate gassing reaction suffers from several
drawbacks limiting its use. These are: difficulty in controlling the reaction rate,
slowing down of the reaction at ambient and low temperature conditions, mass
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transfer problems causing the reaction to remain incomplete for several days
etc.
The mass transfer problems arise due to the formation of discrete droplets
when the aqueous solution of sodium nitrite is mixed into the emulsion matrix.
5 While the mixing process is on there is a dynamic equilibrium between forma-
tion of new droplets and recombination of the existing droplets. During this
process the sodium nitrite reacts rapidly with the ammonium nitrate it comes
across but once the mixing process is stopped further reaction becomes diffu-
sion dependent due to the two reactants being separated across the bilayer
10 and slows down considerably. In the manufacture of cartridged explosives thissituation could lead to continuation of the gassing reaction for several days
after the cartridges are end-clipped leading to bursting of the cartridges.
Since the reaction is acid-catalysed, addition of acid can speed up the process
of gassing but this method again has its limitations because highly acidic condi-
15 tions lead to formation of undesirable oxides of nitrogen in place of nitrogengas. The nitric oxide (NO) upon contact with air readily forms the dioxide (N02)
which being highly soluble in water results in disappearance of voids and
density rise later.
Speeding up of the gassing reaction is also achieved by use, in conjunction, of
20 additives such as thiourea and sodium or ammonium thiocyanate etc. But
despite use of these additives that are known in the art as gassing accelera-
tors, the gassing process slows down considerably when the process tempera-
tures are near ambient. This aspect assumes much importance in today's
mining industry in which explosives are more and more used in "Bulk" form.
25 In this scheme, the ungassed explosive matrix is transported to the mine bench
where it is gassed and simultaneously loaded into the borehole saving much
effort and material involved in sausage preparation. Thus it often becomes
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necessary to undertake the gassing at temperatures that could be well below
the normal factory processing temperatures of 70 -100 ~C and here slowing
down of the gassing reaction with temperature becomes a major handicap. As a
case of non-limiting example mining operations in a province like Himachal
S Pradesh are done with surface temperatures for most of the year remaining
around 10~C. Delayed gassing in such operations could cause hold-up of the
subsequent operations such as stemming, priming of the holes etc.
Thus need is felt for a method of generating voids that is devoid of all the
above mentioned problems and amenable to operation under cold climatic
10 conditions. We describe here below such a method.
Summary of the Invention
In its most general form, the present invention involves generation of gas
bubbles by in-situ decomposition of diazonium salts in the explosives matrix .
The diazonium salts upon their incorporation decompose due to the heat of the
15 matrix and evolve nitrogen gas producing the voids. The process can be
effected in more than one way: The diazonium salt can be pre-prepared and
used either as a solid or as an aqueous solution. It could also be prepared
in-line and used-up simultaneously. Alternately the basic ingredients such as
amine and acid can be incorporated into the oxidiser phase prior to preparation
20 of the explosive matrix, i.e. emulsification or preparation of the watergel, and
mixing in the nitrite solution into the matrix later.
By proper choice of the substitutent group which influences the temperature
stability of the diazonium salt it is possible to meet the density reduction rate
requirements over a wide range of processing temperatures. Thus the method
25 has applicability in the manufacture of both packaged and bulk explosives.
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Detailed Description of the Invention
It is well known that aromatic amines upon reaction with nitrous acid in
presence of mineral acids at about 0~C form diazonium salts as discrete
compounds. The nitrous acid is provided in-situ by reacting a weak base such
S as sodium nitrite with a strong acid. The resultant diazonium compounds when
subjected to higher temperatures decompose yielding nitrogen gas.
As an example by way of illustration, aniline in presence of hydrochloric acid
when made to react with an aqueous solution of sodium nitrtie forms phenyl
diazonium chloride. This compound when heated up to a temperature of 50~C
10 decomposes to give nitrogen gas and phenol with the reaction having a half-life
of less than 10 minutes. In the present method of generation of gas voids,
aqueous solution of the diazonium salt is incorporated into the explosive matrixsuch as that of a water-in-oil emulsion or a slurry wherein the diazonium salt
decomposes due to the heat received from the matrix and generates bubbles of
15 nitrogen gas. The said diazonium compound could also be prepared in-line by
bringing together the amine, acid and the nitrite and simultaneously used up forgas generation or alternately the amine and acid may be incorporated into the
oxidiser phase prior to preparation of the explosive matrix, be it watergel or
emulsion, and aqueous solution of nitrite mixed into the matrix later.
20 Those skilled in the art of manufacturing explosives would understand that
good dispersion of gasser solution in the explosives matrix is essential for
obtaining uniform and small sized voids. The same applies in the present
method employing diazonium salts as well. For the purpose of achieving good
dispersion the known mixing equipment used in the industry such as low/high
25 shear static/rotary mixers could be employed.
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Examples
The following examples are given only to illustrate the present invention. Theseare neither comprehensive nor exhaustive of the various embodiments which
can be prepared in accordance with the present invention.
5 Example-1.
A water-in-oil emulsion of the following composition prepared in a Patterson
mixer at 80~C and cooled to a temperature of 35~C.
Ammonium Nitrate 74.80
Zinc Nitrate 0.10
Water 18.60
Furnace Oil 3.15
Diesel Oil 1.35
Sorbitan monooleate 0.50
PIBSA based emulsifier1.50
It had a viscosity of 76000 c.p. as measured by Brookefield Viscometer spindle
#7, 20 rpm.
The oxidiser solution prior to emulsification was adjusted to a pH of 4Ø
The matrix prior to gassing had a density of 1.350 - 1.325 g/ml. To this matrix
weighing 10 Kg, 200 ml of gasser solution (having a density of 1.25 g/ml
approx.) prepared by mixing 20 parts each by weight sodium nitrite and sodium
thiocyanate and 60 parts by weight water was mixed in. The observed density
reduction with time is shown in Figure-1, curve-1. The temperature throughout
was maintained at 35+1 ~C.
In the second experiment, to the matrix weighing 10 Kg, diazonium salt solution
prepared as per the following was added:
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g
31.33 9 of o-Toluidine ( 2-Aminotoluene ) was converted to its sulphate salt by
addition of a mixture of 36.2 ml concentrated sulphuric acid (specific gravity
1.84) and 81.8 ml water. The amine was slowly added with stirring to the acid.
The resultant amine salt solution was maintained at 0~C. To this a solution of
sodium nitrite; containing 21.67 g sodium nitrite dissolved in 40 ml water; was
added slowly with good stirring. The resultant solution, about 200 ml by volume,was clear.
The temperature of the matrix was again maintained at 35+1~C. The observed
density reduction with time is shown in Figure-1, curve-2.
As can be seen despite higher usage of the gassing agent, consisting of a
mixture of sodium nitrite and sodium thiocyanate, the density reduction in curve- 1 was much slower than in curve - 2. The gassing process with the diazonium
salt was not only faster but complete in 60 minutes.
It is pertinent to mention here that no abnormality of any kind was observed in
the product emulsion in terms of its characteristics such as rheology, detonation
behaviour etc., which could be attributed to the new method of void generation.
Example-2
A water-in-oil emulsion was prepared as described in example - 1 except that
the composition was as follows:
Ammonium Nitrate 56.40
Water 37.60
Furnace oil 03.15
Diesel oil 01.35
PIBSA based emulsifier 01.50
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The pH of the AN/Water solution was adjusted to 4.0 prior to emulsification.
The matrix prior to gassing had a density of 1.20 - 1.21 g/ml. The matrix
temperature was maintained at 50+1 ~C.
In the first experiment, 6 9 of Sodium nitrite and 6 9 of Sodium thiocyanate were
5 dissolved in water and made to a volume of 65 ml and mixed into 10 Kg of the
matrix. The observed density reduction with time is shown in Figure-2, curve -
1.
In the second experiment, to the matrix weighing 10 Kg, diazonium salt solutionprepared as described earlier was incorporated. The quantities used were:
8.319 aniline ( Amino benzene ), 11.9 ml of concentrated sulphuric acid mixed
with 27.4 ml water and 6.609 sodium nitrite dissolved in 13.2 ml water. The total
volume was about 63 ml. The observed density reduction with time is shown in
Figure-2, curve - 2.
In the third experiment, to the matrix weighing 10 Kg, diazonium salt solution
15 prepared as described earlier was incorporated. The quantities used were:
9.57 9 of o-Toluidine ( 2-Aminotoluene ), 11.9 ml of concentrated sulphuric acidmixed with 27.4 ml water and 6.609 sodium nitrite dissolved in 13.2 ml water.
The total volume was about 65 ml. The observed density reduction with time is
shown in Figure-2, curve - 3.
20 As can be seen from above, the density reduction observed in case of both the diazonium salts is much faster than the nitrite - thiocyanate system.
It may be pertinent to mention here that the matrix thus prepared in the second
and third experiments was used for blending with ANFO prills. No abnormality
of any kind such as rheology and detonation behaviour was noticeable in the
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product as compared to the normal product made with the emulsion matrix
gassed using sodium nitrite - sodium thiocyanate system for void generation.
Example-3
A water-in-oil emulsion was prepared as described in example - 1 except that
5 the composition was as follows:
Ammonium Nitrate 56.40
Water 37.60
Paraffin oil 04.50
PIBSA based emulsifier 01.50
10 The pH of the ANN~ater solution was adjusted to 4.0 prior to emulsification.
The matrix prior to gassing had a density of 1.20 - 1.21 g/ml and the matrix
temperature was maintained at 70~C.
In the first experiment, 0.55 9 of Sodium nitrite was dissolved in water and
made to a volume of 6 ml and mixed into 1 Kg of the matrix. The observed
15 density reduction with time is shown in Figure-3, curve - 1.
In the second experiment, to the matrix weighing 1 Kg, diazonium salt solution
prepared as described earlier was incorporated. The quantities used were:
0.75 9 aniline ( Amino benzene ), 1.1 ml of concentrated sulphuric acid mixed
with 2.5 ml water and 0.609 sodium nitrite dissolved in 1.2 ml water. The total
20 volume was about 6.5 ml. The observed density reduction with time is shown in Figure-3, curve - 2.
In the third experiment, a mixture of 0.75 9 of aniline and 0.75 ml concen-
trated hydrochloric acid having a pH of about 6.0 was added to the 940 g
oxidiser solution prior to emulsification. This was emulsified with 60 9 of fuel
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phase to give an emulsion matrix weighing 1 Kg and having the composition as
given above. 6 ml of sodium nitrite solution containing 0.55 9 sodium nitrite,
was mixed into the above matrix. The observed density reduction with time is
shown in Figure-3, curve - 3.
5 As can be seen from Fig 3, curves 2 & 3 which correspond to diazonium salt
pre-prepared and formed in-sifu display faster rate of density reduction
compared to the conventional method.
From the foregoing discussion and examples it will be appreciated that the
present invention provides a different, hitherto unknown, method of generating
10 gas bubbles ( voids ) inside the matrix of an emulsion explosive. The examples
described are only illustrative and not restrictive. The scope of the invention is
therefore indicated by the claims rather than the foregoing description.
