EP0775681A1

EP0775681A1 - Microemulsion and oil soluble gassing system

Info

Publication number: EP0775681A1
Application number: EP96308223A
Authority: EP
Inventors: Arun Kumar Chattopadhyay
Original assignee: ICI Canada Inc
Current assignee: PPG Architectural Coatings Canada Inc
Priority date: 1995-11-24
Filing date: 1996-11-14
Publication date: 1997-05-28
Also published as: ZA969483B; CA2163682A1; AU7194296A; CN1164522A

Abstract

The present invention relates to a process for preparing an emulsion explosive which has been sensitized by the in-situ gassing of a chemical gassing agent, wherein the gassing agent is contained in a microemulsion. The invention also relates to the microemulsions utilized in the practise of this process. The use of the microemulsions of the present invention provides more complete mixing of the gas precursor with the constituents of the emulsion explosives. The process thus provides a more controllable reaction for the in-situ, chemical gassing of emulsions, and for the production of chemically gassed emulsion explosives at lower temperature.

Description

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. 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 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:
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 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.

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 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 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 (NO_x) are formed during the nitrite decomposition. However, the use of the microemulsion gassing system has been found to provide NO_x levels which are typically much lower than the levels of NO_x which would be encountered in a typical, prior art system using a nitrite gassing solution. This reduction in the tendency to form NO_x will preferably result in the formation of less than 50% of the NO_x typically generated, and more preferably, to less than 70% of the NO_x 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 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 cetyltrimethylammonium 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 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, 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, 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(12-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 299832/89.
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 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 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
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 Emulsion Formulation	Example 3	Example 4	Example 5	Example 6
pH	2.0	3.2	3.2	3.9
Ammonium Nitrate	73.90	73.55	73.55	73.55
Water	11.46	18.32	18.32	18.43
Acetic Acid	-	0.39	0.39	0.28
Dil. Nitric Acid	6.90	-	-	-
Thiourea	0.14	0.14	0.14	0.14
Diesel Oil	5.32	-	5.32	5.32
Paraffin Oil	-	5.75	-	-
Emulsifier*	2.28	1.85	2.28	2.28

* - Ethanolamine derivative of polyisobutylene succinic anhydride

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 (20°C), 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.

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.

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 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.
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.

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 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 Temp. 4°C	pH 3.2 Temp. 20°C	pH 3.9 Temp. 20°C
Regular gassing Solution	5.0 x 10^-4s^-1	1.2 x 10^-3s^-1	8.0 x 10^-4s^-1
Microemulsion gassing system	2.0 x 10^-3s^-1	5.5 x 10^-3s^-1	1.2 x 10^-3s^-1

Example 7

In order to compare the level of NO_x 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 NO_x measured is generated as the nitrite anions decompose.
The base emulsions were warmed to 30°C then mixed with each of:

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 NO_x species. The results are shown in Table 3. TABLE 3:

NO_x Generation

NO_x Level per kg of emulsion

Regular Gassing Solution 4.7 ppm

Microemulsion Gassing System 1.6 ppm
Table 3 shows that a lower level of NO_x 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.

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.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

Claims

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.
A microemulsion gassing agent as claimed in Claim 1 which is suitable for gassing base emulsions of use in emulsion explosive manufacture, wherein said microemulsion gassing agent comprises a water-in-oil microemulsion of:
an aqueous solution of a gas precursor;

an organic phase; and

at least one microemulsion-forming emulsifying agent.
A water-in-oil microemulsion gassing agent according to claim 2 which is an optically isotropic liquid comprising aqueous phase solution "droplets" having an average size of 1 to 100 nanometres.
A water-in-oil microemulsion gassing agent according to claim 3 wherein said aqueous phase droplets have an average size of 30 to 50 nanometres.
A water-in-oil microemulsion gassing agent according to any one of Claims 1 to 4 wherein the gas precursor is chosen from the group comprising nitrous acid and its salts and mixtures thereof.
A water-in-oil microemulsion gassing agent according to claim 5 wherein the gas precursor is sodium nitrite.
A water-in-oil microemulsion gassing agent according to claim 1 or 2 wherein the microemulsion gassing agent comprises between 1 and 65 wt% of gas precursor.
A water-in-oil microemulsion gassing agent according to claim 7 wherein the microemulsion gassing agent comprises between 10 and 55 wt% of gas precursor.
A water-in-oil microemulsion gassing agent according to claim 1 or 2 wherein the oil phase comprises saturated or unsaturated hydrocarbons, cyclic or alicyclic hydrocarbons, aromatic hydrocarbons, glycerides, mineral oils, or mixtures thereof and therebetween.
A water-in-oil microemulsion gassing agent according to claim 2 wherein the emulsifying agent is an ionic or nonionic surfactant, or a mixture thereof or therebetween.
A water-in-oil microemulsion gassing agent according to claim 10 wherein the surfactant is a hexadecyl trimethyl ammonium salt, a tetradecyl sulfate, dioctyl sulfosuccinate, a fatty acid ester of sorbitol or a sorbitan ester of an ethoxylated fatty acid, or mixtures thereof and therebetween.
A water-in-oil microemulsion gassing agent according to claim 10 which further comprises a co-surfactant.
A water-in-oil microemulsion gassing agent according to claim 12 wherein said co-surfactant is a linear or cyclic alcohol, or mixtures thereof or therebetween.
A water-in-oil microemulsion gassing agent according to claim 12 which comprises a surfactant & co-surfactant system consisting of a mixture of hexadecyl trimethylammonium bromide with butanol, isopropanol or cyclohexanol, or mixtures thereof or therebetween.
A process for forming a gassing agent suitable for gassing emulsion explosives comprising:
(a) mixing (i) at least one microemulsion-forming 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.
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).
A process for forming an emulsion explosive composition as claimed in claim 16 wherein the amount of water-in-oil microemulsion gassing agent added to the emulsion explosive is between 0.01 and 10 wt% of the emulsion explosive.
A process for forming an emulsion explosive composition as claimed in claim 17 wherein the amount of water-in-oil microemulsion gassing agent added to-the emulsion explosive is between 0.1 and 5 wt% of the emulsion explosive.
A process for forming an emulsion explosive composition according to claim 16 which further comprises mixing said emulsion explosive with a particulate oxidiser salt.
A process for forming an emulsion explosive as claimed in claim 16 wherein said microemulsion gassing agent is mixed with the base emulsion of step (a) at a temperature of less than 40°C.
A process for forming an emulsion explosive as claimed in claim 20 wherein said temperature is less than 20°C.
A process for forming an emulsion explosive as claimed in claim 20 wherein said temperature is less than 10°C.
An emulsion explosive composition formed by a process as claimed in any one of claims 16 to 22.
An emulsion explosive composition as claimed in claim 23 which generates an NO_x level which is 50% less than the NO_x level of a comparable emulsion explosive wherein the microemulsion gassing agent is replaced by an aqueous solution of the gassing agent used in the preparation of the microemulsion.
An emulsion explosive composition as claimed in claim 23 which generates an NO_x level which is 70% less than the NO_x level of a comparable emulsion explosive wherein the microemulsion gassing agent is replaced by an aqueous solution of the gassing agent used in the preparation of the microemulsion.