Note: Descriptions are shown in the official language in which they were submitted.
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Microemulsion and Oil Soluble Gassinq System
Field of the Invention
This invention relates to an improved process for
preparing an emulsion explosive and incorporation of a
dispersed gaseous phase within the emulsion. The invention
particularly relates to the sensitization of emulsion
explosives by chemical gassing using a microemulsion system
dispersed in the continuous oil phase of the emulsion.
DescriPtion of the Related Art
Emulsion explosive compositions are well known in the
explosives industry. The emulsion explosive compositions
now in common use were first disclosed in the U.S. Patent
Number 3,447,978 (Bluhm) and comprise as components: (a) a
discontinuous aqueous phase comprising discrete droplets of
an aqueous solution of inorganic oxygen-releasing salts;
(b) a continuous water-immiscible organic phase throughout
which the droplets are dispersed; (c) an emulsifier which
forms an emulsion of the droplets of oxidiser salt solution
throughout the continuous organic phase; and preferably (d)
a discontinuous gaseous phase. In some emulsion explosives
compositions the discontinuous phase comprises little or no
water and this type of emulsion explosive is often referred
to as a eutectic emulsion or melt-in-oil emission.
Emulsion compositions are often blended with a solid
particulate oxidiser salt which may be coated with an
organic fuel to provide a relatively low cost explosive of
excellent blasting performance. These types of blends are
usually referred to as "doped emulsions". Compositions
comprising blends of a water-in-oil emulsion and ammonium
nitrate (AN) prills or AN prills coated with fuel oil
(ANFO) are described in Australian Patent Application No.
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29408/71 (Butterworth) and US Patent Nos. 3161551 (Egly et
al), 4111717 (Clay), 4181546 (Clay) and 4357184 (Binet et
al). Furthermore, United States Patent No. 4775431
(Mullay) describes the combination of a water-in-oil
macroemulsions with solid particulate oxidiser salts to
provide an explosive composition of high density, that is,
of higher density than ANFO. US Patent 4907368 (Mullay &
Sohara) describes the combination of microemulsions with
solid particulate oxidiser salts to form an explosive
composition having a density greater than ANFO wherein the
microemulsion system acts to increase the density of the
oxidiser salt.
It is well known in the art to use a gaseous phase to
sensitise emulsion explosives and emulsion blends with AN
or ANFO. In preparing these gas-sensitised products it is
important to achieve an even distribution of gas bubbles of
desired size.
The methods currently used to incorporate a gaseous
phase into emulsion explosives include in situ gassing
using chemical agents such as nitrite salts and the
incorporation of closed cell, void material such as
microballoons or a mixture of gassing and microspheres or
porous materials such as expanded minerals such as perlite.
While microballoons provide voids of constant volume and
can be evenly distributed throughout an emulsion they are
relatively expensive to use compared with chemical gassing
and their use is limited to plant manufacturing facilities
because they are difficult to use in the field.
Mechanical mixing methods have also been used to
entrain a gas phase into an emulsion, however such methods
often do not provide efficient dispersion of the gas and
consequently the stability of the gas phase is poor due to
coalescence and escape of gas bubbles. Attempts have been
made to overcome some of these problems by the use of
certain chemical agents to control gas bubble size and
stabilize the bubbles. Australian Patent Application No.
25706/88 and Australian Patent No. 578460 (Curtin & Yates)
disclose mechanical methods of entraining gas bubbles in
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emulsions and the use of a chemical agent to provide a
stable gaseous phase, even in low viscosity emulsion
explosives which are essentially wax free.
In situ chemical gassing of emulsions is usually
carried out by mixing a chemical agent into the emulsion,
which agent decomposes or reacts under the influence of one
of the components of the emulsion to form gas bubbles.
Suitable chemicals include peroxides such as hydrogen
peroxide, nitrite salts such as sodium nitrite,
nitrosoamines such as N,N'dinitroso-
pentamethylenetetramine, alkali metal borohydrides such as
sodium borohydride and bases such as carbonates including
sodium carbonate.
The most preferred chemical gassing agent for
emulsions comprising ammonium nitrate is sodium nitrite
which under conditions of acid pH reacts with the
discontinuous phase of the emulsion to produce nitrogen gas
bubbles. The decomposition of sodium nitrite can be
described chemically as follows:
20NO2 + NH4- = N2 + 2 H20 (1)
2 NO2- + 2 H+ = NO + H20 (2)
_________________________ _______________________
3 NO2- + NH4+ + 2 H+ = N2 + NO + NO2 + 3 H20 (3)
The auto decomposition of nitrites into nitrogen oxides is
favoured by the relatively higher concentrations of
nitrites which are present in conditions of acidic pH.
It is important that the gassing agent is mixed with
the emulsion in such a way that there is ample opportunity
for it to interact with the droplets of oxidiser salt in
the discontinuous phase. There must be a large number of
locations for micro reaction between the gassing agent and
oxidiser salt. The gassing reaction rate may be increased
by chemical accelerators known in the art for accelerating
the decomposition of a nitrite gassing agent. Such
accelerators are either incorporated in the discontinuous
phase of the emulsion during manufacture or added to
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aqueous nitrite solution which is added to the oxidiser or
emulsion.
In order for the gassing reaction to occur uniformly
it is necessary that the gassing agent be dispersed
homogenously throughout the emulsion. Poor distribution of
the gassing agent will affect the size and distribution of
gas bubbles formed in the emulsion explosive and may
adversely affect the reaction efficiency and even the
reaction pathway may be altered.
The ease with which the gassing agent is dispersed in
emulsions depends on several factors including the nature
of the carrier medium, the viscosity of the emulsion matrix
and the devices used for dispersing the gassing agent in
the emulsion. Most ungassed or "base" emulsions used for
emulsion explosive compositions have a density of about 1.3
to 1.6 g/cc and this is reduced to between 0.9 and 1.1 g/cc
by gassing. Chemical gassing agents of the prior art are
usually in the form of aqueous solutions or macroemulsions.
The amount of chemical gassing agent used to achieve the
aforementioned decrease in density is relatively small and
there are inherent difficulties in achieving a homogeneous
dispersion of small quantities of gassing agent in
comparatively large quantities of emulsion. Regardless of
the dispersion devices or carrier media or physical forms
of the gassing agent, current gassing technology is limited
in the degree of homogeneity that can be achieved in
dispersing the gassing agents into the base emulsion.
A further difficulty with present in-situ gassing
procedures is that the gassing reaction is temperature
sensitive and must presently be conducted at elevated
temperatures (typically greater than 40~C) in order to
effect gassing at an acceptable reaction rate.
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Summary of the Invention
It has now been found that improved gassing of base
emulsions for emulsion explosive manufacture can be
provided by use of a gassing agent in the form of a
microemulsion which can be dispersed in ultra fine physical
form throughout the base emulsion. The present invention
therefore provides an emulsion explosive gassing agent
comprising a chemical gassing precursor, wherein said
gassing precursor is present in a microemulsion comprising
an aqueous solution of a gas precursor in an organic phase.
Preferably, the microemulsion gassing agent is a
water-in-oil microemulsion of: an aqueous solution of a gas
precursor; and organic phase; and at least one
microemulsion-forming emulsifying agent.
In accordance with the present invention, dispersion
of said microemulsion gassing agent in a base emulsion will
lead to decomposition, or more generally, reaction of said
gas precursor to form gas bubbles in said base emulsion.
Accordingly, the present invention provides a process for
the manufacture of gassed emulsion explosives, and provides
gassed emulsion explosives which have been gassed through
the use of the microemulsions of the present invention.
Description of the Preferred Embodiments
The exact structure of a microemulsion is complex, but
has been described in, for example, U.S. Patent No.
4,907,368 referred to hereinabove, and incorporated herein
by reference. However, in general, microemulsion systems
are essentially transparent, are of low viscosity, and may
be thermodynamically stable - all in contrast with the
properties of normal or "macro"-emulsions. Unlike
macroemulsions, a microemulsion forms spontaneously, -
often referred to as pseudo-solubilization of the
discontinuous phase in a continuous media. Accordingly,
when a microemulsion is formed, the aqueous solution
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typically forms aqueous "domains" or "droplets" of a small
size within the oil phase.
These droplets of the aqueous, or "discontinuous"
phase of a microemulsion are many times smaller than the
aqueous droplets of an equivalent, conventional emulsion.
Droplet sizes in microemulsions are typically in the range
of about 1 to 100 nanometres (10~9m), more preferably 1 to
50 nanometres, and most preferably 30 to 50 nanometres.
Because of the thermodynamic stability of the microemulsion
system, a microemulsion generally contains approximately
1000 times more droplets than an equivalent volume of
conventional emulsion.
Microemulsion droplets are often referred to as
"microreactors" because reactions will take place in the
very limited size domain provided by the droplet. Compared
to gassing agent dispersion methods of the prior art,
dispersion of a microemulsion gassing agent (in accordance
with the present invention) in a base emulsion provides
more reaction centres and hence increases the efficiency of
the gassing reaction.
In a typical emulsion explosive system, where the
dispersed phase of the base emulsion comprises AN and the
gas precursor is sodium nitrite, the number of moles of
sodium nitrite present in each droplet of microemulsion
gassing agent is significantly lower than the number of
moles of AN present in each droplet of the base emulsion
discontinuous phase. This has been found to improve the
efficiency of nitrogen gas generation, and the distribution
of gas bubbles in emulsion explosives, since the ratio of
ammonium ions to nitrite ions is increased.
Preferably the amount of water-in-oil microemulsion
gassing agent added to the base emulsion is between 0.01
and 10 wt% of the total emulsion explosive and more
preferably between 0.1 and 5 wt%.
The gassing reaction may be conducted at temperatures
typically utilized in the gassing of emulsion explosives.
However, in a preferred feature, the present invention also
allows the use of lower temperatures than those typically
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utilized. Thus, in a preferred embodiment, the
microemulsion permits gassing of the base emulsion at a
temperature of less than 40~C, more preferably less than
20~C, and most preferably at a temperature of less than
10~C.
In a preferred embodiment the water-in-oil
microemulsion gassing agent consists of an optically
isotropic liquid comprising gassing agent droplets of
average size 30 to 50 nanometres (0.03 to 0.05 micron).
The gas precursor of the present invention may be any
chemical known in the art as being suitable for the in situ
generation of gas bubbles. It is particularly preferred
that the gas precursor be chosen from the group comprising
nitrous acid and its salts such as, for example, sodium
nitrite. Preferably the amount of gas precursor present in
the microemulsion is between 1 and 65 wt% of the
microemulsion gassing agent and more preferably between 10
and 55 wt%.
Whenever nitrites are utilized as the gas precursor in
an acidic environment in the presence of ammonium ion,
various levels of nitrogen oxides (N0x) are formed during
the nitrite decomposition. However, the use of the
microemulsion gassing system has been found to provide N0x
levels which are typically much lower than the levels of N0x
which would be encountered in a typical, prior art system
using a nitrite gassing solution. This reduction in the
tendency to form N0x will preferably result in the formation
of less than 50% of the N0x typically generated, and more
preferably, to less than 70% of the N0x typically generated,
by prior art nitrite gassing solutions.
The continuous oil phase of the microemulsion gassing
agent of the present invention comprises one or more
organic species preferably chosen from the group comprising
saturated or unsaturated hydrocarbons, cyclic or alicyclic
hydrocarbons, aromatic hydrocarbons, glycerides and mineral
oils, or mixtures thereof and therebetween.
The water present as the "aqueous" phase of the
microemulsion gassing agent may be all or partially
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replaced by other solvents provided that the other solvents
are sufficiently immiscible with the continuous phase in
order to form the microemulsion, and provided that the
other solvents are sufficiently compatible with the total
emulsion explosive system. However, the preferred liquid of
the "aqueous" phase is water only.
The emulsifying agents used for the formation of the
microemulsion gassing agent may be chosen from, for
example, the group comprising both ionic and nonionic
surfactants (for example hexadecyl trimethylammonium salts,
tetradecyl sulfates, dioctyl sulfosuccinate, fatty acid
esters of sorbitol and sorbitan esters of ethoxylated fatty
acids) and mixtures thereof and therebetween. Depending
upon the microemulsion formulation, a "co-surfactant" may
optionally be required. "Co-surfactants" preferably are
selected from the group comprising linear or cyclic
alcohols (for example, isopropanol, butanol, pentanol,
cyclohexanol, or higher alcohols) and mixtures thereof and
therebetween.
In the microemulsion gassing agent of the current
invention, the solubility of the discontinuous aqueous
phase in the continuous oil phase depends on the nature of
the surfactant & co-surfactant systems and the salinity of
the aqueous phase. In a given oil/surfactant &
co-surfactant system, solubility of an aqueous phase
generally decreases with an increase in salt concentration.
In a preferred embodiment the microemulsion gassing
agent of the current invention comprises a surfactant &
co-surfactant system consisting of a mixture of hexadecyl
trimethylammonium bromide (also known as cetyltrimethyl-
ammonium bromide or CTAB) with butanol or mixtures of
butanol, isopropanol or cyclohexanol, or mixtures thereof
or therebetween. This system has been found to provide a
particularly efficient system for solubilizing sodium
nitrite solution in light mineral oils such as diesel oil.
Suitable microemulsion gassing agents for use in the
current invention may be manufactured by the steps of;
(a) mixing (i) at least one microemulsion-forming
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emulsifying agent with (ii) an organic phase, and (b)
adding an aqueous solution of a gas precursor, with
stirring, to the mixture of step (a), so as to form a
microemulsion of said aqueous solution in said organic
phase.
Further, the current invention also provides a process
for forming an emulsion explosive composition comprising
the steps of:
(a) forming a base emulsion by emulsifying an aqueous
solution of an inorganic salt in a mixture of an organic
phase and an emulsifier, and
(b) mixing a water-in-oil microemulsion gassing agent into
the base emulsion of step (a).
The base emulsion into which the microemulsion gassing
agent is mixed may be any water-in-oil or eutectic emulsion
known in the art to be suitable for sensitization by
chemical gassing agents.
For a microemulsion containing sodium nitrite as a gas
precursor, preferably the base emulsion contains ammonium
ions, preferably from the presence of ammonium nitrate in
the aqueous phase of the base emulsion.
The continuous organic phase of the base emulsion may
comprise any of the organic fuels known in the art and
includes aliphatic alicyclic and aromatic compounds and
mixtures thereof. Suitable organic fuels may be chosen
from fuel oil, diesel oil, distillate, furnace oil,
kerosene, naphtha, waxes, paraffin oils, benzene, toluene,
xylenes asphaltic materials, polymeric oils, animal oils,
fish oils and other mineral, hydrocarbon or fatty oils, and
mixtures thereof or therebetween.
Typically the continuous organic phase would comprise
from 2 to 15% by weight and preferably 3 to 10% by weight
of the emulsion explosive composition.
If desired other optional fuel materials, hereinafter
referred to as secondary fuels may be incorporated into the
emulsion. Examples of such secondary fuels include finely
divided solids. Examples of solid secondary fuels include
finely divided materials such as: sulphur, aluminium,
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carbonaceous materials, resin acids such as abietuic acid,
sugars and other vegetable products such as starch, nut
meal, grain meal and wood pulp and mixtures thereof.
Typically the optional secondary fuel component of the
emulsion comprises from 0 to 30% by weight of the emulsion
explosive composition.
Suitable oxygen-releasing salts for use in the
discontinuous aqueous phase of the base emulsion component
of the emulsion explosive composition of the present
invention are well known in the art and are preferably
selected from the group consisting of alkali and alkaline
earth metal nitrates such as calcium nitrate and
perchlorates, ammonium nitrate, ammonium chlorates,
ammonium perchlorate and mixtures thereof.
Typically the oxygen-releasing salt of the base
emulsion component of the emulsion explosive compositions
of the present invention comprises from 45 to 95% and
preferably from 60 to 90% by weight of the emulsion
explosive composition.
Typically the amount of water employed in the emulsion
explosive compositions of the present invention may vary
from 0 to 30~ by weight of the emulsion explosive
composition. Where the emulsion explosive is a eutectic
emulsion the discontinuous phase will comprise no water or
adventitious water only.
The base emulsion emulsifier component of the
compositions of the current invention may be selected from
the wide range of emulsifying agents or combination of
emulsifying agents known in the art to be suitable for the
preparation of emulsion explosive compositions. Examples
of such emulsifying agents include alcohol alkoxylates,
phenol alkoxylates, poly(oxyalkylene)glycols,
poly(oxyalkylene) fatty acid esters, amine alkoxylates,
fatty acid esters of sorbitol and glycerol, fatty acid
salts, sorbitan esters, poly(oxyalkylene) sorbitan esters,
fatty amine alkoxylates, poly (oxyalkylene)glycol esters,
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fatty acid amides, fatty acid amide alkoxylates, fatty
amines, quaternary amines, alkyloxazolaines
alkenyloxazolines, imidazolines, alkyl-sulphonates,
alkylarylsulphonates,
alkylsulphosuccinates,alkylphosphates, alkenylphosphates,
phosphate esters, lecithin, copolymers of poly(oxyalkylene)
glycols and poly(l2-hydroxystearic acid), condensation
products of compounds comprising at least one primary amine
and poly~alk(en)yl]succinic acid or anhydride and mixtures
thereof. Most preferably the base emulsion emulsifier
component comprises a condensation product of a compound
comprising at least one primary amine and a
poly[alk(en)yl]succinic acid or anhydride as described in
Australian Patent Application Nos. 40006/85 (Cooper &
Baker), 29933/89 and 29983-2/89.
Typically the base emulsion emulsifying agent
component of the composition of the present invention
comprises up to 5 wt% of the emulsion explosive
composition.
Other voiding agents (hereinafter referred to as
secondary voiding agents) may be used in addition to the
gas bubbles produced by the microemulsion gassing agent of
the current invention. For example hollow glass or plastic
microballoons, porous particles and mixtures thereof may be
incorporated into a base emulsion before or after the
addition of the microemulsion gassing agent of the current
invention.
Preferably the secondary voiding agents comprise 0.05
to 50% by volume of the base emulsion prior to addition of
the microemulsion gassing agent, and more preferably, the
secondary voiding agents comprise 0.05 to 40% by volume of
the emulsion explosive composition after addition of the
microemulsion gassing agent.
The base emulsion component of the current invention
can be formed by an convenient method known in the art.
Typically this would be carried out by dissolving the
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oxygen releasing salt in water at a temperature above the
fudge point of the solution and then adding the aqueous
composition to a rapidly stirred blend of fuel phase and
base emulsion emulsifier. Where used herein the term
"fudge point" is the temperature at which crystals of
oxygen releasing salt begin to form in the oxidiser
solution. The base emulsion may further be doped by mixing
with particulate oxidising salt such as prilled AN or a
coated oxidising salt such as ANFO (ammonium nitrate - fuel
oil). The preferred ratio of base emulsion to particulate
oxidiser salt is between 10:90 and 90:10.
The current invention will be further described with
reference to the following non-limiting examples, and by
reference to Figures 1 to 4, wherein the emulsion explosive
density over time is plotted, as the gassing reactions of
the examples occur. All values in all examples are by
weight unless otherwise noted.
Examples
Example 1
A microemulsion gassing agent, according to the
present invention was prepared according to the following
procedure. Hexadecyl trimethylammonium bromide (CTAB) (9
parts) was mixed with butanol (4.2 parts) and then added to
diesel oil (35 parts) to create an oil phase. An aqueous
solution containing sodium nitrite (30.5 wt%) was added
slowly to the oil phase with gentle stirring. As the
aqueous solution was added to the oil phase, the mixture
slowly changed from opaque white to a transparent,
yellowish microemulsion. The addition of the aqueous
solution was continued until the microemulsion thus formed
contained 18 parts by weight of the aqueous salt solution
(thus containing 18 x 0.305 or 5.49 parts by weight of
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sodium nitrite, or 8.3% by weight sodium nitrite).
Example 2
CTAB (9 parts) was mixed with butanol (4.2 parts) and
then added to diesel oil (35 parts) with stirring. An
aqueous solution containing sodium nitrite (29.5 wt%) and
sodium thiocyanate (5 wt%) was added slowly to the oil
phase with stirring. As the aqueous solution was added to
the mixture of oil and surfactant, the mixture slowly
changed from an opaque white to a transparent, yellowish
microemulsion. The addition of aqueous solution was
continued until the microemulsion thus formed contained
17.4 parts of the aqueous salt solution.
Example 3
A water-in-oil base emulsion, suitable for use in an
emulsion explosive composition, was manufactured by forming
an oil phase and base emulsion emulsifier mixture, and then
slowly adding a hot solution (90~C) of oxidiser salt, water
and weak nitric acid to the mixture with vigorous stirring.
The composition and pH of the base emulsion formed are
recorded in Table 1. The formed base emulsion was stored
at 20~C for 2 days and then mixed with gassing agents (as
described hereinbelow). The rate of gassing was accessed in
the following trials.
A sample of the base emulsion of Example 3 was mixed
with each of:
3(i) - the microemulsion of Example 1 added to the
level of 1.15 wt% of the emulsion explosive;
3(ii) - the microemulsion of Example 2 added to the
level of 1.24 wt% of the emulsion explosive;
3(iii) - a conventional gassing solution consisting of
an aqueous sodium nitrite solution (11 wt% sodium nitrite
and 89% water), added to the level of 0.87 wt% of the
emulsion explosive; and
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3(iv) - a conventional gassing solution consisting of
an aqueous sodium nitrite solution (24% sodium nitrite and
24% sodium thiocyanate in water) added to the level of 0.4
wt% of the emulsion explosive.
The amount of sodium nitrite added to the emulsion of
Example 3 was approximately the same in each of Examples
3(i) to 3(iv).
TABLE 1
Base Example Example Example Example
Emulsion 3 4 5 6
Formulation
pH 2.0 3.2 3.2 3.9
Ammonium 73.90 73.55 73.55 73-55
Nitrate
Water 11.46 18.32 18.32 18.43
Acetic Acid - 0.39 0.39 0.28
Dil. Nitric 6.90
Acid
Thiourea 0.14 0.14 0.14 0.14
Diesel Oil5.32 - 5.32 5.32
Paraffin Oil - 5.75
Emulsifier* 2.28 1.85 2.28 2.28
* - Ethanolamine derivative of polyisobutylene
succinic anhydride
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The changes in density of the emulsion explosives with
time with respect to Example 3 are recorded graphically in
Figure 1. Measurement of the time taken for completion of
the gassing reaction showed that base emulsion mixed with
the microemulsion gassing agent systems 3(i) and 3(ii) took
an average time of less than 10 minutes to reach an
emulsion explosive density of 1 g/cc compared to 20 min. or
more for base emulsions mixed with the conventional gassing
solutions of (iii) and (iv). This demonstrates that for a
given emulsion explosive at ambient-temperature (20OC), the
completion of gassing reactions occur faster with the
microemulsion systems compared to conventional gassing
solutions.
Example 4
A water-in-oil base emulsion suitable for use in an
emulsion explosive formulation was manufactured according
to the method outlined in Example 3 except that dilute
acetic acid was used in place of nitric acid. The
composition and pH of the emulsion explosive formed are
recorded in Table 1.
Example 4(a)
The base emulsion was stored at 20~C for 2 days and
then mixed with gassing agents to assess the effects of the
gassing agents. Samples of the base emulsion of Example 4
were mixed with each of: -
4(i) - the microemulsion of Example 1 added to the
level of 1.0 wt% of the emulsion explosive (the base
emulsion had been pre-mixed with 0.4 wt% of sodium
thiocyanate solution containing 24% sodium thiocyanate and
76% water); and
4(ii) - a conventional gassing solution consisting of
aqueous sodium nitrite solution (24% sodium nitrite and 24%
sodium thiocyanate in water) added to the level of 0.4 wt%
of the emulsion explosive.
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The amount of nitrite salt added to the emulsion
explosive formulations was 0.083 wt% for Example 4(i) and
0.096 wt% for Example 4(ii).
The changes in density of the emulsion explosives over
time are recorded in Figure 2(a). Measurement of the time
taken for completion of the gassing reaction showed that
the base emulsion mixed with the microemulsion gassing
agent systems (i.e. 4(i)) took an average time of 10
minutes to complete reaction. This compares very favourably
with the time of more than 40 min. or more for base
emulsions mixed with the conventional gassing solutions of
4(ii). At all stages during the gassing reaction the
microemulsion system was at least 4 times faster than the
conventional system. This demonstrates that despite a lower
concentration of nitrite, the emulsion explosive prepared
using the microemulsion gassing agent 4(i) gassed far more
quickly than emulsion explosive prepared using a more
conventional gassing solution.
Example 4(b)
The formed base emulsion of Example 4 was stored at
4~C for 24 hours then placed in a pre-cooled bowl and mixed
with gassing agents to assess their effects. At all times
the mixtures were kept below 8~C.
Samples of the base emulsions of Example 4(b) were
mixed with each of:
4(b)(i) - the microemulsion of Example 1 added to
the level of 1.0 wt% of the emulsion explosive (the base
emulsion had been pre-mixed with 0.4 wt% of a sodium
thiocyanate solution containing 24% sodium thiocyanate and
76% water); and
4(b)(ii) - a conventional gassing solution consisting
of aqueous sodium nitrite solution (24% sodium nitrite and
24% sodium thiocyanate in water) added to the level of 0.4
wt% of the emulsion explosive.
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The amount of nitrite salt added to the emulsion
explosive was 0.083 wt% for Example 4(b)(i) and 0.096 wt~
for Example 4(b)(ii). The only material difference between
the trials conducted in Example 4(a) and Example 4(b) is
the gassing temperature.
The changes in density over time for Example 4(b) are
recorded in Figure 2(b). Measurement of the time taken for
completion of the gassing reaction showed that emulsion
explosive prepared with the microemulsion gassing agent
systems 4(b)(i) took an average time of 15 minutes to
achieve a density below 1.1 g/cc compared with more than 60
min. or more for emulsions explosives prepared using the
conventional gassing solutions of 4(b)(ii). At all stages
during the gassing reaction, the microemulsion system was
lS at least 4 times faster than the convention system. This
demonstrates that despite a lower concentration of nitrite,
the emulsion explosive prepared using the microemulsion
gassing agent gassed far more quickly than the emulsion
explosive prepared using a conventional gassing solution.
Comparison of Example 4(a) and Example 4(b) show that the
performance of the microemulsion gassing agent is superior
to that of the conventional gassing agents, and is even
more superior to the conventional gassing agent when
gassing is conducted at lower temperatures.
Example 5
A water-in-oil base emulsion suitable for use in an
emulsion explosive formulation was manufactured according
to the method outlined in Example 4. The composition and
pH of the base emulsion formed is recorded in Table 1. The
formed base emulsion was stored at 4~C for 24 hours then
placed in a pre-cooled bowl and mixed with gassing agents
to assess the effect of the gassing agent. At all times
the mixtures were kept below 8~C.
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The base emulsion of Example 5 was mixed with each of:
5(i) - the microemulsion of Example 1 added to the
level of 1.0 wt% of the emulsion explosive (the base
emulsion had been pre-mixed with 0.4 wt% of a sodium
thiocyanate solution containing 24% sodium thiocyanate and
76% water);
5(ii) - the microemulsion of Example 1 added to the
level of 0.83 wt% of the emulsion explosive (the base
emulsion had been pre-mixed with 0.4 wt% of a sodium
thiocyanate solution containing 24% sodium thiocyanate and
76% water);
5(iii) - the microemulsion of Example 1 added to the
level of 0.5 wt% of the emulsion explosive (the base
emulsion had been pre-mixed with 0.4 wt% of a sodium
thiocyanate solution containing 24% sodium thiocyanate and
76% water); and
5(iv) - a conventional gassing solution consisting of
aqueous sodium nitrite solution (24% sodium nitrite and 24%
sodium thiocyanate in water) added to the level of 0.4 wt%
of the emulsion explosive.
The amount of nitrite salt added to the emulsion
explosive was 0.083 wt% for Example 5(i), 0.069 wt % for
Example 5(ii), 0.041 wt% for Example 5(iii), and 0.096 wt%
for Example 5(iv).
The changes in density with time are recorded
graphically in Figure 3. Measurement of the time taken for
completion of the gassing reaction showed that emulsion
explosives prepared using the microemulsion gassing agent
systems (i.e. Examples 5(i), (ii) and (iii)) all took an
average time of 5 to 7 minutes to achieve a density of 1.2
g/cc or less. This compares with the time of 30 min. or
more for the emulsion explosive prepared using the
conventional gassing solutions of Example 5(iv). Even at
relatively low levels of addition the microemulsion gassing
system proved more efficient than the conventional system.
ICICAN 817 2 ~ 6 3 6 8 2
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Example 6
A water-in-oil base emulsion suitable for use in an
emulsion explosive formulation was manufactured by forming
an oil phase and emulsifier mix, and then slowly adding a
hot solution of oxidiser salt, water and dilute acetic acid
with vigorous stirring. The composition and pH of the base
emulsion that was formed is recorded in Table 1. The
formed base emulsion was stored at ambient temperature for
2 days and then mixed with gassing agents to assess the
effects of the gassing agent.
The base emulsion of Example 6 was mixed with each of:
6(i) - the microemulsion of Example 1 added to the
level of 1.17 wt% of the emulsion explosive;
6(ii) - the microemulsion of Example 2 added to the
level of 1.24 wt% of the emulsion explosive;
6(iii) - a conventional gassing solution consisting of
aqueous sodium nitrite solution (24 wt% sodium nitrite and
76% water) added to the level of 0.4 wt% of the emulsion
explosive; and
6(iv) - a conventional gassing solution consisting of
aqueous sodium nitrite solution (24% sodium nitrite and 24%
sodium thiocyanate in water) added to the level of 0.4 wt%
of the emulsion explosive.
It will be apparent that a roughly equal amount of
sodium nitrite was incorporated into each of the emulsion
explosive prepared; either in the form of a microemulsion
(Examples 6(i) and 6(ii)) or as an aqueous sodium nitrite
solution (Examples 6(iii) and 6(iv)).
The changes in density of the emulsion explosives over
time are recorded graphically in Figure 4. Measurement of
the time taken for completion of the gassing reaction
showed that even in the absence of any gassing accelerator
(sodium thiocyanate) the emulsion explosives prepared with
the microemulsion gassing agent system (Examples 6(i) and
(ii)) both gassed at a faster rate than the emulsion
explosives prepared using the conventional gassing
ICICAN 817 21 6 ~ 6 82
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solutions of 6(iii) and 6(iv), even though a gassing
accelerator was used in Example 6(iv).
It was also apparent that the microemulsion system
follows first order kinetics where it predominantly forms
nitrogen. Table 2 records the rate constants determined
for both the microemulsion and the conventional gassing
systems. The results indicate that irrespective of
temperature and pH conditions, the rate of gassing with the
microemulsion system is always faster than the conventional
gassing solutions. It also indicates that with the
microemulsion system, the rate of gassing increases by
approximately 5 times for a drop in pH from 3.9 to 3.2
compared to the conventional system which only increased by
1.5 times.
Table 2: Reaction Rate Constants
pH 3.2 pH 3.2 pH 3.9
Temp. 4~C Temp. 20~C Temp. 20OC
Regular 5.0 x 10-4s-1 1.2 x 10-3s-1 8.0 x 10-4s-
gassing
Solution
Microemulsion 2.0 x 10-3s-1 5.5 x 10-3s-1 1.2 x 103s-
gassing
system
Example 7
In order to compare the level of N0x generation during
gassing reaction further experiments were conducted using
the base emulsion formulation of Example 6. All emulsions
tested had an aqueous phase pH of 3.9. The N0x measured is
generated as the nitrite anions decompose.
The base emulsions were warmed to 30~C then mixed with
each of:
ICICAN 817 2163~2
7(i) - the microemulsion of Example 1 added to the
level of 1.15 wt% of the emulsion explosive; and
7(ii) - a conventional gassing solution consisting of
aqueous sodium nitrite solution (24% sodium nitrite and 24%
sodium thiocyanate in water) added to the level of 0.4 wt%
of the emulsion explosive.
The quantity of chemical gassing agent added to each
formulation was sufficient to provide a final emulsion
density of approximately 0.70 g/cc. After being mixed with
the gassing agents, the emulsion explosives were kept in a
sealed container fitted with a delivery tube. After
completion of the gassing reaction, the container was
equilibrated to room temperature and the gas generated
inside the container was drawn through the delivery tube
and analyzed for NOx species. The results are shown in
Table 3.
TABLE 3: NOX Generation
NOx Level per kg of emulsion
Regular Gassing Solution 4.7 ppm
Microemulsion Gassing 1.6 ppm
System
Table 3 shows that a lower level of NOx was generated
by the microemulsion gassing system compared to the
conventional gassing system. This provides further
confirmation that use of a microemulsions gassing agent,
rather than a conventional gassing solution, enhances the
gassing reaction efficiency.
ICICAN 817 ~16 ~
-22-
Example 8
A microemulsion was prepared having a higher
concentration of sodium nitrite. Fifty five (55) parts (by
weight)of an aqueous solution of 28% sodium nitrite (by
weight) was mixed with 45 parts of an oil phase mixture
containing 2 parts (by weight) isopropanol, 2.2 parts
butanol, 35 parts fuel oil, and 9 parts CTAB. The mixture
formed a stable microemulsion suitable for use in gassing
of emulsion explosives.
Having described specific embodiments of the present
invention, it will be understood that modifications thereof
may be suggested to those skilled in the art, and it is
intended to cover all such modifications as fall within the
scope of the appended claims.
