EP2125675A1

EP2125675A1 - Methods for gassing explosives especially at low temperatures

Info

Publication number: EP2125675A1
Application number: EP08700317A
Authority: EP
Inventors: Bogdan Zygmunt Dlugogorski; Eric Miles Kennedy; Mark Stuart Rayson; Gabriel Da Silva
Original assignee: Newcastle Innovation Ltd
Current assignee: Newcastle Innovation Ltd
Priority date: 2007-01-10
Filing date: 2008-01-10
Publication date: 2009-12-02
Also published as: CA2675167A1; ZA200905022B; AU2008204725A1; US20110132505A1; WO2008083436A1

Abstract

The invention provides a method for gassing an explosive to sensitise the explosive and/or modify the density of the explosive. The method comprises reacting at least one oxidiser with at least one nitrogen containing compound in the explosive to generate nitrogen gas. The explosive is formulated to effect diffusion of the oxidiser and/or the compound into contact with each other, the nitrogen gas being generated by oxidation of the compound by the oxidiser. The invention extends to the explosive compositions themselves.

Description

METHODS FOR GASSING EXPLOSIVES ESPECIALLY AT LOW

TEMPERATURES

FIELD OF THE INVENTION

The invention relates to explosive compositions and particularly to methods of sensitising and/or modifying the density of explosives by gas bubbles, including emulsion, gel, slurry and ANFO explosives. The invention finds particular though not exclusive application in the mining industry.

BACKGROUND OF THE INVENTION

Prior to detonating, emulsion, gel and heavy ammonium nitrate-fuel oil (ANFO) explosives require sensitisation, and a number of technologies exist to perform this task. Originally, technologies requiring the addition of high explosives were employed, with examples of such explosives including trinitrotoluene (TNT), nitroglycerine, nitrocellulose (which constitutes a major ingredient of smokeless powder), nitrostarch, nitrocotton, nitroguanidine, hexamethylenetetramine, ethylenediaminedinitrate, trinitrophenylmethylnitramine, mixtures of TNT and trimethylenetrinitramine, TNT and pentaerythritol tetranitrate, and TNT with ethylene dinitramine. Metal particles, such as those of aluminium, magnesium, boron or silicon, as well as ferrophosphorous and ferrosilicon, can also be utilised to enhance the sensitisation by high explosives. However, these methods are expensive and require sensitising during emulsion or gel production, normally at the manufacturing plant, necessitating subsequent transportation of sensitised explosives which, of course, is highly undesirable. Because of these two considerations, cost and safety, the use of high explosives for emulsion and gel sensitisation has now been largely abandoned. Attempts to replace high explosives with nitrates of aliphatic and phenolic amines (eg. US Patent No. 3,431,155, GB Patent No. 1,536,180) also appear to have met the same fate. Another group of technologies used to sensitise explosives involves entrapping air or adding light particles to slurries, emulsions and heavy ANFO by physical processes (eg. see US Patent No. 3,382,117) as a means to decrease the amount of high- explosive sensitisers or regulate distribution of explosive strength. US Patent No. 3,397,097 for instance is directed to the sensitisation of gel explosives by this method. To act as sensitisers the voids need to be small, at least less than 1.6 mm in size and preferably, less than 100 μm. A shock wave travelling in the sensitised explosive compresses the voids adiabatically. This raises the local emulsion temperature above that required to detonate the bulk explosive. Sensitisation technologies involving the introduction of small voids can also function as density regulators, to decrease blasting energy or to distribute explosive strength in a borehole or over a set of boreholes.

Glass microspheres (microballoons) have been preferred for emulsion explosive sensitisation especially at temperatures below that suitable for chemical gassing based on nitrosation of ammonia or thiourea (vide infra), in spite of the elevated cost of microballoons (in the order of AU$2,000/kg), handling difficulty owing to their low density (<400 kg/m , but typically <200 kg/m ), and high shipping expenses per unit weight. However, their advantages include small size (on average 65 μm, with particle sizes distributed between 10 to 175 μm), provision of no additional fuel to the sensitised material and ability to be mixed with explosives over a wide range of proportions. US Patent No. 4, ,737 ',207 ', for example, discloses a convenient process to mix microballoons with emulsion explosives by means of suspensions. Glass microspheres still find considerable application in sensitising higher cost packaged explosives, due to their stability, but are cost inefficient for use in bulk explosives when compared to low cost chemical gassing technologies (vide infra). Other physical sensitisation technologies that have found limited application notwithstanding their lower cost, include the addition of bagasse piths, perlite, vermiculite, pumice, as well as plastic microspheres (which may be expanded in emulsion by heating above 85 °C, as described in US Patent No. 6,113,715), solid foams (e.g., polystyrene-based, US Patent No. 4,543,137), liquid foam (after addition to explosive, the foam itself breaks down with release of bubbles which disperse in the explosive matrix; European Patent Application No. 514000), puffed rice and wheat, and more recently expanded popcorn (US Patent No. 5,409,556) as well as rice hulls (US Patent No. 6,995,731). Currently, the most industrially important route to sensitising explosives to detonation comprises the so-called chemical gassing or foaming, involving the formation of small bubbles of CO₂, O₂, H₂, NO or N₂ in situ in emulsions and gel explosives by means of chemical reactions. US Patent No. 6,261,393 also provides a brief reference to employing calcium carbide as a gassing reagent, presumably to generate C₂H₂, but without providing an example. For the most part, these gassing technologies allow transport of unsensitised emulsions and gels, with the addition of bubble generating chemicals at the time of pumping the emulsions or gels into blastholes. Chemical gassing was introduced in the late 1960s and early 1970s to provide alternative means of sensitising gel explosives and then emulsion explosives (US Patent No. 3,447,978). With time, industry has converged on the use of nitrogen gassing (US Patent No. 3,886,010), with N₂ formed by reactions between nitrites and ammonia, thiourea, or other amines in the presence of catalysts and pH regulators. The known chemical gassing processes are further reviewed below. For instance, US Patent No. 3,288,658 describes the application of carbon dioxide to gassing explosive gels in a process involving mixing of an acid (such as hydrochloric, acetic, nitric or sulfuric) with a solution of ammonium or alkali metal carbonate, particularly sodium or potassium bicarbonate, at temperatures below 50 °C and pH below 6.5. Recently, CO₂ gassing has been suggested as part of water-resistant ANFO systems that transform themselves into sensitised slurries in water-logged blastholes (US Patent No. 6,261,393). From a chemical perspective, the process involves a reduction in pH resulting in the protonation of HCO₃ , followed by the decomposition of H₂CO₃ and the evolution of CO₂. This method can be used to sensitise emulsion explosives provided that acetic acid or another organic acid of similar pKa is employed as a proton donor. Unfortunately, the solubility of CO₂ in blasting agents varies as a function of pressure and the agent's composition, making the adjustment of emulsion or slurry density with CO₂ particularly difficult to regulate, especially for deep boreholes.

Oxygen gassing typically involves the decomposition of hydrogen peroxide in the presence of catalysts, such as manganese dioxide, ferric nitrate, potassium iodide, ferrous sulphate, manganese sulphate, aluminium particles and even coarse sand, at temperatures in excess of 55 °C (e.g., see US Patent No. 3,790,415). Other catalysts including carbonates, bicarbonates, and nitrites as well as oxidisers, such as ferric salts, and oxoanion oxidisers, such as permanganates, dichromates, peroxysulphates, hypohalites, can also effectively decompose hydrogen peroxide. The sensitisation of slurry explosives with lithium, sodium and potassium peroxides, is described in US Patent No. 4,081,299. Oxygen gassing employing a pre-emulsified solution of H₂O₂, with MnO₂ present in the discontinuous phase of the emulsion, has been also demonstrated as a viable technology (US Patent No. 5,397,399). However, the US Code of Federal Regulation, 29 CFR 1910.109 Explosives and Blasting Agents excludes the use of peroxides in emulsion and gel systems (Clark, 1991).

The application of hydrogen to gassing of slurry and emulsion explosives is based on the observation that H₂ is released in reactions involving ammonium salts and alkali metal borohydrides (ie., lithium, sodium or potassium borohydrides); e.g., according to NH₄ ⁺ + BH₄ → H3N-BH3 + H₂, at temperatures in excess of 40 °C. A gassing process based on this reaction is described in US Patent No. 3,711,345. Safety concerns relating to the use of alkali metal borohydrides and hydrogen itself, and possibly the loss of hydrogen from emulsions owing to rapid diffusion of H₂ through the emulsion matrix, has prevented further development and implementation of hydrogen foaming by industry.

A nitric oxide gassing process involving the nitrosation of chemical species (substrates) having an enol group, or a deprotonated enolate form of the enol group, employing a nitrosating agent such as N₂O₃, ONCl, ONBr, ONSCN, ONI, nitrosothiourea, nitrosyl thiosulfate, HNO₂, ON⁺, ON⁺OH₂ or inorganic nitrosyl complexes, is described in International Patent Application No. PCT/AU2006/001596). The substrate used is preferably ascorbic acid or ascorbate (vitamin C), and the reaction forms O-nitroso products which subsequently decompose to yield nitric oxide. The reaction rate is pH dependent, taking approx. 4 min to gas emulsion explosives at 25 °C and a pH below 3.9. However, nitric oxide may promote the production of so-called after-blast fumes, although technologies exist, e.g., based on addition of silicon powder, to alleviate this factor (US Patent No. 6,539,870).

Early attempts to introduce nitrogen gassing to slurry and emulsion explosives relied on adopting blowing agents (ie., agents that decompose to N₂ at temperatures above 55 °C) as described in US Patent No. 3,713,919, and the oxidation of hydrazine and its derivatives at temperatures in excess of about 40 °C as described in US Patent No. 3,706,607. Examples of the application of both technologies to gassing emulsion explosives are described in US Patent No. 3,770,522. Chemicals useful as blowing agents include iV,./V'-dimethyl- andiV,./V'-diethyl-./V,./V'- dinitrosoterephthalamide, benzensulphonyl hydrazide, azobisisobutyronitrile and p-tert- butylbenzazide, as summarised in US Patent No. 4,008,108. A gassing process involving iV,./V'-dinitrosopentamethylenetetramine is also described in US 3,713,919. Gassing was reported for the reaction of hydrazine monohydrate and other hydrazine derivatives with various oxidising agents including hydrogen peroxide, ammonium persulphate and copper (II) nitrate. This gassing system inherently requires the process be carried out at high temperatures. This requires that either the gassing be performed during or immediately after emulsion manufacture, when the mixture is still hot, or subsequent heating of the explosive if the explosive is sensitised at a later time. These practical limitations have prevented the use of this technology. The use of copper complexes in emulsion explosive systems has also been abandoned because of safety concerns, similarly to the use of peroxides as previously mentioned.

A recent development in nitrogen gassing involves the application of toxic diazonium salts generated by in-line mixing of an amine, acid and a nitrite (US Patent No. 6,027,588). At temperatures above 35 °C, some diazonium salts decompose to N₂ at rates significantly faster than for a commonly used foaming system composed of sodium nitrite with sodium thiocyanate catalyst (vide infra), with fast gassing reported between 50 °C and 70 °C.

Nitrogen gassing via the nitrosation mechanism as introduced to the field of explosives by US Patent No. 3,660,181 and US Patent No. 3,886,010, has dominated chemical gassing, and emulsion explosive sensitisation as a whole, over the last 30 years. The gassing process is initiated by mixing a concentrated solution of nitrite ion (usually originating from inexpensive NaNO₂, with other options involving nitrous acid and solutions of potassium and ammonium nitrites) with slurry or emulsion explosive. Citric or more frequently-used acetic acid diffusing from emulsion droplets then protonates nitrite ions to form N₂O₃, which subsequently transfers back across oil films to react with ammonia (N₂O₃ + NH₃ → N₂ + NO₂ ^" + H⁺ + H₂O). Nitrosation of ammonia constitutes a slow reaction even at around 50 °C and in practice thiourea (or melamine, sulphamic acid or its salts; Canadian Patent No. 2,239,095) is added to emulsion to act as a substrate for nitrosation. In this mechanism, ON⁺ (from HNO₂) acts as an effective nitrosating agent (HNO₂ + NH₂CSNH₂ → N₂ + 2H₂O + HSCN). Alternatively, a strong nucleophilic species, such as thiocyanate (e.g., NaSCN), iodide, bromide, chloride, nitrosothiourea, nitrosoamines (such as NJV'- dinitrosopentamethylenetetramine, see US Patent No. 4,409,044), can serve as catalysts to affect the nitrosation of ammonia by ON⁺ (e.g., ONSCN + NH₃ → N₂ + SCN^" + H⁺ + H₂O). Optimised formulations designed to operate at temperatures below 25 °C necessitate the use of both thiourea (as a substrate, added to emulsion) and thiocyanate (as a catalyst, added to gasser); e.g., see European Patent Application 775,681. The mechanistic details of the nitrite gassing processes are now relatively well understood (except, perhaps, for nitrosothiourea, nitrosoamine and sulphamic acid systems), e.g., da Silva et al. (2006).

Since the development of nitrite gassing, a number of important advances have been introduced to optimise the gassing technology, especially to accelerate the gassing rate and make the process operate at lower temperatures and allow formation of gas bubbles of less than 100 μm in size. They include the use of microemulsions (European Patent Application No. 775,681), Lewis acids such as zinc nitrate to facilitate the protonation of nitrite ions (US Patent No. 6,855,219), premixing nitrite and thiourea prior to adding to emulsion (US Patent No. 6,165,297, Canadian Patent No. 2,239,095), nitrite as powder (US Patent No. 4,997,494), pre-emulsified gasser (US Patent No. 4,875,951, suggested independently in PCT/US88/03354), calcium and strontium accelerants (US Patent No. 6,022,428), formation of small bubbles with organic additives (Canadian Patent No. 2,040,751), fluoro surfactants (US Patent No. 4,594,118) or high pressure (US Patent No. 4,676,849). Technologies combining nitrite gassing with other methods for regulating emulsion density and/or sensitivity have also been developed, particularly for specialised applications, such as shock resistant explosives (microballoons/nitrite, US Patent No. 5,017,251), porosity-modified ANFO (US Patent no. 5,240,524) and enhanced sensitivity explosives (high explosives/nitrite, US Patent No. 4,221,616).

However, in spite of its versatility and low cost (in the order of AU$6/tonne), nitrite gassing cannot be effectively implemented for sensitising emulsions at sub or near zero (ie., < 0 °C or ~ 0 °C) temperatures, with the rate of gassing declining substantially below 25 °C. Concerns have been also raised over the health and environmental risks associated with the use of nitrite salts, and over safety implications of pre-mixing of nitrite and thiourea prior to adding the mix to the emulsion. Furthermore, the identification of the common accelerant thiourea as a possible human carcinogen means that even chemical gassing at moderate temperature (ca. 25 ⁰C) using this compound may be phased out.

SUMMARY OF THE INVENTION

In an aspect of the invention there is provided a method for gassing an explosive to sensitise the explosive and/or modify the density of the explosive, comprising reacting at least one oxidiser with at least one nitrogen containing compound in the explosive to generate nitrogen gas, the explosive being formulated to effect diffusion of the oxidiser and/or the compound into contact with one another, the nitrogen gas being generated by oxidation of the compound by the oxidiser.

Typically, the oxidiser and/or the nitrogen compound diffuse across fuel lamellae in the explosive into contact with one another. In at least some embodiments, essentially only the oxidiser diffuses across the fuel lamellae for reacting with the oxidiser.

The gassing of the explosive can be carried out over a range of temperatures and is particularly suitable for gassing emulsion explosives, though not solely, at low temperatures including near or below 0 ⁰C.

Typically, the amount of nitrogen gas generated is determined by the quantity of the oxidiser added to the explosive. The oxidiser is normally added in the form of a composition (gasser) containing the oxidiser. The gasser can be a solution of the oxidiser. The terms "nitrogen containing compound" and variations thereof such as

"nitrogen compound" and the like in the context of the invention are taken to mean any compound containing nitrogen that is capable of producing nitrogen gas via reaction with the oxidiser.

The term "oxidiser" in the context of the invention is to be taken to mean a substance that removes one or more electrons from the nitrogen compound to produce the nitrogen gas from the nitrogen compound and/or species derived from the compound. This is distinguished from inorganic oxidiser salts (such as ammonium nitrate) which form part of prior art base explosive wherein the term refers to the presence of oxygen in the salts. In the present invention, the oxidiser is generally added to the explosive following the manufacture of the explosive, normally when the explosive is ready to be gassed.

The explosive can be formulated for protonation of the oxidiser to effect the diffusion of the oxidiser.

In at least some embodiments, the explosive can comprise a proton donor to control the rate at which the oxidiser diffuses to react with the nitrogen compound, thereby controlling the gassing rate. Alternatively, or as well, the pH of the gasser can be adjusted to alter or further influence the rate of oxidiser diffusion. For example, the pH of the gasser can be raised via the addition of an alkaline substance to delay the onset of the gassing reaction to allow more time for mixing of the gasser into the explosive.

In addition, the explosive can comprise a proton acceptor for maintaining pH above a predetermined lower limit or within a predetermined pH range to inhibit crystallisation in the explosive.

A pH regulating agent such as a pH buffer which can act as both the proton donor and proton acceptor can be used. A pH regulating agent can also act as a proton transfer agent for transporting protons across fuel lamellae or into another phase of the explosive for increasing the rate of diffusion of the oxidiser and/or the nitrogen compound. In the broadest sense, a proton transfer agent is to be taken to encompass an agent that can act to transfer at least one proton across fuel lamellae and/or into another phase of an explosive for release of the proton.

Hence, in one or more embodiments of the invention, the explosive can comprise a proton transfer agent for transferring one or more protons across fuel lamellae of the explosive or more generally, across phases of the explosive, to increase the rate of diffusion of the oxidiser and/or the nitrogen compound.

It will be understood that the invention is not limited to the particular nitrogen compound(s) utilised in the explosive and any suitable nitrogen compound can be used.

In at least one form, the nitrogen gas is generated by oxidation of ammonia/ammonium cation (NH₃/NH₄ ⁺) by the oxidiser. Typically, though not exclusively, the oxidiser diffuses to the NH₃/NH₄ ⁺. In this embodiment, the explosive can comprise a proton donor for protonating the oxidiser to promote the diffusion of the oxidiser into contact with the NH₃/NH₄ ⁺ as indicated above. Desirably, the explosive will also include a proton acceptor to maintain pH above a predetermined lower limit to inhibit crystallisation in the explosive. Again, a pH regulating agent that acts as both the proton donor and the proton acceptor can be employed. Alternatively, the pH can be elevated to increase the concentration of NH₃ (which is in equilibrium with NH₄ ⁺), to promote diffusion of the NH₃ e.g., across fuel lamellae. In this instance, the oxidiser may be essentially non-diffusing in the context of the invention.

In another form, the nitrogen gas is generated by oxidation of an amine (eg., a primary amine), with the oxidation of NH₃ZNH₄ ⁺ occurring as a side reaction. In this instance the majority of nitrogen gas is generated by reaction of the oxidiser with the amine. In general, the reaction between an oxidiser and a primary amine shows a higher selectivity toward the production of nitrogen gas than the reaction with NH₃/NH₄ ⁺. Thus reactions with a primary amine afford a reduction in the amount of oxidiser required to produce a desired amount of gas in an explosive. Similarly, the explosive in this embodiment may contain a pH regulating agent and/or proton transfer agent for protonating the oxidiser to promote the diffusion of the oxidiser through the fuel lamellae of the explosive.

In another form, the nitrogen gas is generated by oxidation of a nitrogen compound containing a hydrazine (-NH-NH₂) group to produce the nitrogen gas, with the oxidation of NH₃/NH₄ ⁺ occurring as a parallel side reaction. In this instance, the nitrogen gas is primarily generated by the reaction between the oxidiser and the hydrazine, with a lower level of nitrogen gas generated by the parallel side reaction between the oxidiser and NH₃/NH₄ ⁺. In general, at temperatures less than 40 ⁰C hydrazine chemicals are essentially non-diffusing across fuel lamellae, in that the amount of gas produced from oxidiser molecules diffusing across the fuel lamellae to the hydrazine is substantially greater than the amount of gas produced from the hydrazine diffusing across the fuel lamellae to the oxidiser. In comparison to the above embodiment involving the oxidation of NH₃/NH₄ ⁺ and an amine, reduced amounts of oxidiser and pH regulator agent can be utilised to achieve sensitisation of and/or density modification of the explosive. In yet another form, the nitrogen gas is generated by oxidation of a nitrogen rich compound having 3 or more nitrogen atoms for generation of the nitrogen gas, with the oxidation of NH₃/NH₄ ⁺ occurring as a parallel side reaction. Examples of suitable such nitrogen rich compounds include those containing a tetrazole or triazole ring. In this instance, the nitrogen gas is primarily generated by reaction of the oxidiser with the nitrogen compound, with a lesser amount of gas being produced by the oxidation of the NH₃/NH₄ ^"1". The presence of double bonds between nitrogen atoms of triazoles, tetrazoles, and like such nitrogens compounds again reduces the amount of oxidiser required to produce the desired amount of nitrogen gas required to achieve sensitisation and/or density modification of the explosive.

In another aspect of the invention there is provided an explosive comprising NH₃/NH₄ ^"1" and/or at least one other nitrogen containing chemical, and at least one oxidiser for reaction with the NH3/NH4^"1" and/or the nitrogen containing chemical to generate nitrogen gas for gassing the explosive.

In another aspect of the invention there is provided a method for gassing an explosive to sensitise the explosive and/or modify the density of the explosive, comprising reacting at least one oxidiser with at least one nitrogen containing compound in the explosive to generate nitrogen gas, the oxidiser and the compound initially being in different phases of the explosive to one another, and the oxidiser and/or compound diffusing into contact with each other whereby nitrogen gas is generated via oxidation of the nitrogen compound by the oxidiser.

In another aspect there is provided an explosive gassed by a method of the invention. In yet another aspect of the invention there is provided an explosive, comprising at least one oxidiser and at least one nitrogen containing compound, the explosive being formulated for effecting diffusion of the oxidiser and/or the compound into contact with each other for oxidation of the compound by the oxidiser to produce nitrogen gas from the compound for gassing of the explosive. In still another aspect of the invention there is provided an explosive, comprising at least one oxidiser and at least one nitrogen containing compound, the oxidiser and the compound being in different phases of the explosive to one another, and the explosive being formulated for diffusion of the oxidiser and/or the compound into contact with each other for oxidation of the compound by the oxidiser to produce nitrogen gas from the compound for gassing of the explosive.

In methods embodied by the invention, the rate of the overall gassing process is governed by the rate of transfer of the diffusing reagent and/or the nitrogen compound. In the instance that both the nitrogen compound and the oxidiser diffuse, the rate of gassing is limited by the combined rate of diffusion of both these reagents. However, the rate of oxidiser diffusion typically far exceeds that of most nitrogen compounds at low temperature, and as such the vast majority of nitrogen compounds can be considered as essentially non-diffusing under these conditions. As will be understood, the gassing of the explosive by the generated nitrogen gas may be achieved by simply allowing the gas to foam the explosive. The gassing of the explosive may also, or alternatively, involve stirring the explosive during at least a part of the gassing process to achieve substantially even distribution of the nitrogen gas bubbles essentially throughout the explosive. Distribution of the gas bubbles throughout the explosive may also be achieved by pumping of the emulsion explosive.

Methods embodied by the invention are particularly suitable for gassing emulsion explosives. However, it will be understood that at least some embodiments can be utilised to gas forms of explosives other than emulsion explosives, such as gel, slurry and ANFO explosives, and the invention expressly extends to such further explosives.

Methods as described herein provide technologies for sensitisation and/or density modification of explosives at low temperature as an alternative to conventional low temperature gassing methods such as those involving the use of expensive glass microspheres. Advantageously, at least some gassing methods embodied by the invention require no heating of the explosive prior to mixing with the oxidiser and/or substrate to above ambient temperatures, even temperatures of 0 ⁰C or below, affording a substantial advance in the art. Moreover, various forms of methods embodied by the invention appear to be comparable in cost to nitrite-based gassing, which for the previous three decades has been the preferred technology for sensitisation and/or density modification of explosives.

Furthermore, at least some embodiments involving the use of hydrazine and/or derivatives thereof address a number of deficiencies of prior art nitrogen gassing methods employing these substrates. In particular, one or more embodiments as described herein may allow an emulsion explosive to be gassed to a pre-determined density at a controlled rate, by mixing one or more reagents such as the oxidiser into the emulsion after its manufacture, rather than during the manufacturing process as is the case in US Patent No. 3,706,607. This allows emulsion explosives to be manufactured in bulk and transported safely to mine sites before being sensitised to detonation, reducing the risk of accidents associated with transport of explosive materials. Further, one or more embodiments of the invention provide methods that allow explosives formulated with nitrogen compounds to be gassed over a wide temperature range by adjusting the pH of gasser solutions, removing the need for heating or cooling of the explosives to achieve desired gassing times.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. Any discussion of documents, acts, materials, devices, articles or the like which has been included in this specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the relevant field of technology as it existed anywhere before the priority date of this application.

The features and advantages of the invention will become further apparent from the following detailed description of embodiments of the invention.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1 is a graph showing gassing of an emulsion explosive in the absence of a pH regulating agent/proton transfer agent;

Figure 2 is a graph showing gassing of emulsion explosives in the presence of a pH regulating agent/proton transfer agent; Figure 3 is a graph showing gassing of an emulsion explosive containing a primary amine substrate in the presence of a pH regulating agent;

Figure 4 is a graph showing gassing of an emulsion explosive containing a cyclic hydrazide substrate in the presence of a pH regulating agent capable of transferring protons from the emulsion to the gasser; Figure 5 is a graph showing gassing of an emulsion containing a cyclic hydrazide compound in the presence of a non diffusing pH regulating agent;

Figure 6 is a graph showing a comparison of gassing between emulsions containing diffusing and non-diffusing pH regulating agents; Figure 7 is a graph showing gassing of an emulsion containing a monohydrazide with non-diffusing pH regulating chemical;

Figure 8 is a graph showing a comparison of gassing between emulsions at different pH with a non-diffusing pH regulating agent; Figure 9 is a graph showing a comparison of gassing between examples containing a non-diffusing pH regulating agent with and without an additional proton transfer agent;

Figure 10 is a graph showing the gassing of an emulsion with a monohydrazide and diffusing pH regulating agent; Figure 11 is a graph showing the gassing of an emulsion with a monohydrazide and diffusing pH regulating agent;

Figure 12 is a graph showing the gassing of an emulsion containing a dihydrazide and a non-diffusing pH regulating agent; and

Figure 13 is a graph showing the gassing of an emulsion containing a tetrazole and a diffusing pH regulating agent.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention relates to methods for gassing an explosive to sensitise the explosive to detonation and/or modify the density of the explosive, comprising reacting at least one oxidiser with at least one nitrogen containing compound in the explosive to generate nitrogen gas from the compound. Methods embodied by the invention find application in the gassing of emulsion explosives and particularly water-in-oil emulsion explosives, and this type of emulsion is primarily exemplified below. However, it will be understood that embodiments of methods of the invention also have application to other explosives including the gassing of melt-in-oil emulsion, gel, slurry and ANFO explosives (commonly referred to as heavy ANFO and which comprises emulsion explosives in combination with ammonium nitrate-fuel oil).

A typical emulsion explosive consists of (i) a discontinuous phase of a solution of nitrate salts, such as ammonium nitrate in water, and (ii) a continuous fuel phase including an emulsifier such as poly-isobutylene succinic anhydride (PiBSA) and a carbonaceous fuel, such as diesel, paraffin or bio oils. Explosive compositions embodied by the invention will normally include: (i) at least one nitrogen containing compound (also known as the reductant(s) or substrate(s)) which is capable of producing nitrogen gas via reactions with the oxidiser(s), and (ii) at least one oxidiser for removing electrons from the reductant(s) to produce the nitrogen gas, with the oxidiser being added to the emulsion to affect sensitisation and or density modification at a point in time after the emulsion has been constructed. In one or more embodiments of the invention the substrate is normally added to the discontinuous phase of the explosive (in the case of an emulsion) during the manufacture of the base explosive, prior to subsequent addition of the oxidiser. The oxidiser is normally added to the explosive after manufacture and forms its own discrete droplets (phase) within the emulsion. Therefore, either the oxidiser or nitrogen compound must diffuse through fuel lamellae that separate their respective phases in order to come into contact with each other and react.

Reactants (i) and (ii) will normally be selected to react rapidly with one another, even at sub-zero temperatures. In addition, at least one of either the reductant or the oxidiser will be capable of diffusing, or the explosive will be formulated to allow diffusion, of the reductant or oxidiser through oil or fuel films (fuel lamellae) separating droplets of the reductant(s) and the oxidiser. As charged molecules are insoluble in the fuel film, only neutral molecules are capable of diffusing between gasser and emulsion droplets.

The substrate can be any compound containing at least one nitrogen atom in a form capable of being oxidised to produce nitrogen gas. Such compounds include, but are by no means limited to ammonia/ammonium cations, ammonium salts, and any alkyl, aryl and cyclic compound containing at least one or more of a combination of the following functional moieties; amines including primary, secondary and tertiary amines, hydrazines, hydrazides including dihydrazides, hydrazine hydrate and hydrazine dichloride, azides, triazoles and/or tetrazoles, and derivatives, and salts thereof.

Yet further nitrogen compounds that may be used in one or more embodiments of the invention include azo compounds (R-N=N-R) such as dimethyldiazine, azobenzene and azodicarboxamide, imidates such as ethyl formimidate hydrochloride, polyamines such as ethylene diamine, methylamine hydrochloride, parbamates such as methyl carbamate, cyanides, nitriles, azides such as ethyl azidoacetate, guanidines and salts thereof such as guanidine nitrate and guanidine carbonate, hydrazones such as benzaldehyde semicarbazone, hydroxylamines such as iV-methylhydroxylamine hydrochloride, and imines such as formamidine acetate salt.

The oxidiser is typically able to diffuse from the gasser droplets to the emulsion phase, or can be made able to diffuse by accepting or releasing a proton/protons. Examples of such oxidisers include hypohalous acids such as hypochlorous and hypobromous acids and their corresponding alkali metal or alkali earth metal salts, N- halosulfonamides, such as iV-chlorobenzenesulfonamide, jV-chlorotoluenesulfonamide and their salts (Chloramine B and T respectively for example), iV-halosuccinamides, such as iV-chlorosuccinamide and iV-bromosuccinamide and their salts, chloric and bromic acids and their salts, iV-oxides such as iV-methylmorpholine iV-oxide, trimethylamine iV-oxide or pyridine iV-oxide as well as any alkali or alkali earth metal manganates.

In at least some embodiments, the oxidiser consists of a solution containing one or more of the compounds comprising any alkali metal and alkali earth metal hypochlorites, hypobromites or hypoiodates, known as hypohalites. Such oxidisers are capable of reacting rapidly with nitrogen compounds at low temperature to produce nitrogen gas. Furthermore, hypohalites are weak bases in solution, meaning that they can accept a proton to form corresponding hypohalous acids, which, as neutral molecules, are able to diffuse rapidly through fuel lamellae separating emulsion and droplets of gasser solution.

Hypohalous acids (e.g. HOCl, HOBr) exist in equilibrium with the corresponding hypohalite anion, with the relative concentration of each species being determined by the pH of the solution. At low pH (pH < 7) the hypohalite exists predominantly in the protonated form, whist at high pH (pH > 9) the majority exists as hypohalite anions. It is evident that in order for hypohalite gassers to diffuse through fuel lamellae and react with nitrogen compounds, the pH of the gasser must be sufficiently low to increase the concentration of the hypohalous acid to a level that allows an adequate rate of gasser diffusion. Thus the pH of the gasser is a relevant factor in determining the rate of gassing using hypohalite oxidisers, with gassers of low pH having significantly faster gassing times than those with high pH.

In order to ensure efficient use of hypohalite oxidisers, the pH of the gasser solution should be continuously lowered (or at least remain constant at a low value) throughout the gassing process. This can occur via the transfer of protons (hydrogen cations) from the emulsion phase to the gasser and ensures that the entire contents of the gasser can participate in the gassing reaction. If proton transfer does not occur, hypohalite anions will be trapped in the gasser droplets and unable to react with nitrogen containing compounds in the emulsion. Proton transfer in an emulsion explosive is generally relatively slow, unless a proton transfer agent is included in the emulsion formulation.

In a preferred embodiment of the invention, the emulsion formulation will contain a pH regulating agent capable of transferring protons from the emulsion phase to the gasser, also known as a proton transfer agent. In general, the term "a proton transfer agent" encompasses a weak acid or base that when protonated, forms a neutral molecule which can diffuse rapidly through fuel lamellae separating emulsion and gasser droplets. Such molecules dissociate when they reach the gasser droplets effecting the release of protons in the gasser and as such, are able to transfer protons from the emulsion to the gasser droplets. Examples of proton transfer agents well suited to the present invention include the carboxylic acids such as acetic acid and formic acid, which exist in equilibrium with their deprotonated anions, acetate and formate. There are many other suitable compounds that can be used as proton transfer agents, provided that the pH of the emulsion phase is selected to ensure that a significant proportion of the compound exists in its protonated from. Examples of such substances include citric, tartaric, furoic, fumaric, salicylic, malonic, phthalic, sulfanilic, mandelic, malic, butyric and oxalic acids, and their salts. Typically, the proton transfer agent will be selected such that it does not react with the oxidiser used. The action of a proton transfer agent with a hypohalite gasser is described below in the following steps, with reference to the acetic acid/hypochlorite system, which is used in various examples of the present invention:

1. Acetic acid present in the discontinuous phase of the emulsion diffuses through the fuel film to the gasser.

2. The acetic acid is deprotonated by hypohalites present in the gasser, to produce hypohalous acid and acetate as shown in the following reaction.

CH₃COOH + OCl^" → HOCl + CH₃COO^" [1] 3. Hypochlorous acid diffuses to the emulsion, where it reacts with nitrogen containing chemicals to produce nitrogen gas.

4. Protons produced in the gassing reaction cause the formation of more acetic acid from acetate ions, and the process continues until either the proton transfer agent or the hypochlorite is completely consumed.

The amount of proton transfer agent(s) used should be sufficient to transfer enough protons to protonate substantially all of the oxidiser molecules, thus ensuring that the entire contents of the gasser droplets are able to participate in the gassing reaction. The amount of pH regulating agent used also depends on how many protons are produced in the gassing reaction, and should desirably be sufficient to consume the majority of protons produced in the reaction, thereby maintaining the emulsion at an essentially constant pH. Likewise, in the case that the gassing reaction consumes protons, sufficient pH regulating agent(s) should be added to ensure that enough protons are available for the reaction to reach completion and that the emulsion pH does not rise significantly.

In another embodiment of the invention, at least one pH regulating agent is added to the gasser to either accelerate or slow the gassing process. For example, raising the pH of a hypohalite gasser with a soluble alkali metal hydroxide such as sodium hydroxide slows the gassing process, as the added hydroxide must be neutralised by protons transferred from the emulsion before gassing can occur. Similarly, the addition of small amounts of an acidic substance, such as sulfuric, hydrochloric, nitric or acetic acid (amongst others) to hypohalite gassers increases the concentration of hypohalous acid, thereby accelerating the gassing process.

At temperatures in excess of 35 ⁰C, gassing using the above method can be very rapid. In order to slow the gassing process at higher temperatures, the proton transfer agent may be replaced with a non-diffusing pH regulating agent. This produces slow gassing rates, as proton transfer to the gasser is greatly reduced, allowing the invention to be used at higher temperatures. In the case that such an emulsion formulation needs to be gassed at a low temperature, a proton transfer agent such as acetic acid can be added to the emulsion prior to the addition of the gasser to increase the gassing rate.

The above mechanisms apply to the use of all nitrogen compounds and oxidisers as described herein. Further details of the invention will now be discussed in relation to four industrially important classes/groups of nitrogen compounds. However, it will be understood that the invention is not limited to the use of these nitrogen compounds.

Class 1 In one form, the substrate (nitrogen compound) for the oxidiser will typically be the ammonium cation (NH₄ ⁴) and ammonia (NH₃) which exist in equilibrium in ammonium solutions as follows:

NH₃ + H⁺ → NH₄ ⁺ [2]

The ammonium cation and thereby ammonia can be provided by the addition of one or more ammonium salts to the explosive. Examples of suitable ammonium salts include, but are not limited to, ammonium nitrates, sulfates, phosphates, sulfides, chlorides, bromides, fluorides, iodides, perchlorates, periodates, acetates, citrates, and tartarates. Typically, the ammonium salt will be ammonium nitrate or ammonium perchlorate, which are commonly employed in explosive compositions. In the case that these salts are already present in the explosive, additional nitrogen containing chemicals are not required.

The oxidiser will normally diffuse through the fuel lamellae to reach the solution containing ammonium salt(s) and/or other nitrogen containing chemicals as described above. Such oxidisers include but are not limited to hypohalous acids such as hypochlorous and hypobromous acids and their corresponding alkali metal or alkali earth metal salts, jV-halosulfonamides, such as iV-chlorobenzenesulfonamide, N- chlorotoluenesulfonamide and their salts (Chloramine B and T respectively), N- halosuccinamides, such as iV-chlorosuccinamide and iV-bromosuccinamide and their salts, chloric and bromic acids and their salts, iV-oxides such as iV-methylmorpholine N- oxide, trimethylamine iV-oxide or pyridine iV-oxide, and alkali or alkali earth metal manganates.

Alternatively, the pH of the solution of ammonium can be increased by the addition of a proton donor to elevate the concentration of NH₃ to achieve diffusion of the NH₃ (a neutral species) through the fuel lamellae. The increase in NH₃ concentration occurs because at higher pH, fewer protons are available, and the equilibrium of reaction [2] shifts to the left, increasing the concentration of NH₃ and thereby the NH₃ concentration gradient across the fuel lamellae.

The oxidation of ammonia to nitrogen gas requires the transfer of 6 electrons between ammonia and the oxidiser as indicated in the following half reaction:

2NH₄ ⁺ → N₂T+ 8H⁺ + 6e^~ (main reaction) [3]

However, the inventors have observed in Examples 2.1 and 2.2 described below, that to reach the pre-determined emulsion explosive density, about 11 electrons are consumed per mole of nitrogen evolved. Without wishing to be bound by theory, it is hypothesised that some of the NH₄ ⁺ oxidises to nitrate ions via a parallel side half reaction as follows:

NH₄ ⁺ + 3H₂O → NO₃- + 1 OH⁺ + 8e^~ (side reaction) [4]

It is further believed by the inventors that under the conditions of reactions [3] and [4], approximately 75% of the total NH₄ ⁺ consumed is oxidised in the main reaction and the remaining 25% in the side reaction.

Relevantly, reactions [3] and [4] produce protons, leading to a significant drop in pH. Although protons are normally consumed by the decomposing oxidisers, the present inventors have further found that in the case of NH₃/NH₄ ⁺ this consumption is limited to three protons per molecule of evolved nitrogen gas for at least some types of oxidisers, such as alkali metal and alkali earth metal hypohalite salts, which are particularly effective in oxidising NH₃/NH₄ ⁺. This means that the reaction between such oxidisers and NH₃/NH₄ ⁺ leads to a net increase in the number of protons, or, in other words, protons are produced in the reaction. This is demonstrated in reactions [7] and [8]. Specific examples of suitable oxidisers which can be used include, but are not limited to, lithium, sodium, potassium, calcium and barium hypohalites (eg., hypochlorites, hypobromites and hypoiodites). Examples of the relevant half reactions are as follows:

HC10 + H⁺ + 2e^~ → Cr+ H₂0 [5]

HBrO + H⁺ + 2e^~ → Br + H₂O [6] Combining half reaction [3] with [5] and [4] with [6] provides examples of the overall main and side reactions operating during the oxidation of NH₄ ⁺ by hypochlorites as follows:

2NH₄ ⁺ + 3HC1O → N₂T + 5H⁺ + 3H₂O + 3Cl^" (main reaction) [7] NH₄ ⁺ + 4HC1O → NO₃- + 6H⁺ + H₂O + 4Cl^" (side reaction) [8]

For hypohalite and chloramine-type oxidisers, the reduction of the apparent pH to below unity is possible in the discontinuous phase of the emulsion. In the presence of unreacted oxidisers this build-up of acidity, if left, may lead to emulsion crystallisation as a result of the formation of ammonium nitrate crystals leading to a reduction in emulsion stability. To preclude this, a commensurate amount of a proton acceptor such as sodium acetate is added to the discontinuous phase of the emulsion explosive to prevent pH from decreasing substantially as demonstrated in Example 3 below. The anion of weak acids exists in equilibrium in solution and as such, can also regulate pH by accepting protons from solution to avoid a decrease in pH beyond desired levels in at least some methods embodied by the invention. Further examples of such pH regulating agents include, but are not limited to, salts of other weak acids including food acids such as citric and tartaric acids. Additional examples of pH regulating agents useful in embodiments of the invention include furoic, fumaric, salicylic, malonic, phthalic, sulfanilic, mandelic, malic, butyric and oxalic acids, as well as their salts.

Class 2

Another group of nitrogen compounds suitable for use in the present invention are primary amines. A primary amine is a chemical species with the structure R-NH₂, where R represents any alkyl, aryl or cyclic substituent. Any such amine can be used as a constituent of the explosive in embodiments of the invention described herein. The amine can contain one or more amino groups, for example NH₂-R-NH₂, and can incorporate one or more heteroatoms such as O, S or N, and the use of such compounds is specifically encompassed. Specific examples of amines which may find use in embodiments of the invention include but are not limited to methylamine, ethanamide, ethanolamine, trisamine, aniline (aminobenzene), urea and thiourea. The oxidation of amines is analogous to that of ammonia/ammonium cations requiring the removal of 6 electrons per molecule of nitrogen gas produced, as shown in the following half equation.

2RNH₂ + 2H₂O → 6e^~ + N₂ + 2ROH + 6H⁺ [9]

Combining reactions [5] and [9] provides an example of the oxidation of amines by hypochlorites as follows:

2RNH₂ + 3HC1O → N₂ + 2ROH + 3Cl^" + 3 H⁺ + H₂O [10]

The use of urea in particular finds widespread applications in nitrosation based gassing technologies, which are currently used to sensitise emulsion explosives. Unlike the nitrosation of urea that occurs only at high temperatures (>40 ⁰C), the oxidation of urea (and other amines) proceeds rapidly at low temperatures (<20 ⁰C), thus allowing the present invention to sensitise emulsions designed for nitrosation of urea to be gassed at low temperature, removing the need to heat the emulsion prior to gassing. The use of urea to generate nitrogen gas with hypohalite oxidisers to sensitise and/or modify the density of an explosive is demonstrated in Example 3.1.

Class 3

Another group of nitrogen compounds with application in the present invention are compounds containing a hydrazine group, -NHNH₂ and derivatives thereof. A particular class of hydrazines better suited to practical uses are hydrazides, which have substantially improved safety properties compared to hydrazines. For this reason, hydrazides are of particular importance. However, it should be understood that the invention extends to all compounds and classes of compounds containing an -NHNH₂ group and derivatives thereof. A hydrazide (including the sulfonyl hydrazides) is a chemical species that contains either the — C(=O) — NH — NH₂ or — S(=O)₂ — NH — NH₂ group in its chemical structure, i.e., it contains a carbonyl or sulfonyl group bonded to the hydrazine group Any optionally substituted suitable alkyl, aryl and cyclic hydrazides can be used as constituents in embodiments of methods of the invention described herein. The hydrazide can contain one or more hydrazide groups (ie., polyhydrazide), such as di- or trihydrazides, and can also contain various combinations of amino groups and hydrazine groups, for example, semicarbazide and amino guanidine. Hydrazides useful in the invention may also incorporate one or more heteroatoms such as O, S or N, examples of which include pyridinic nitrogen as in isonicotinic acid hydrazide, and the use of all such hydrazides is expressly encompassed. Specific examples of hydrazides which may find use in embodiments of the invention include, but are not limited to, acetic acid hydrazide, formic acid hydrazide, oxalic acid dihydrazide, maleic acid hydrazide (3,6-dihydroxy pyridazine), succinic acid dihydrazide, semicarbazide, aminoguanidine, isonicotinic acid hydrazide, benzoic acid hydrazide, o-, m- andp- hydroxybenzoic acid hydrazide and o-, m- and /7-methylbenzoic sulfonyl hydrazides. Further hydrazides suitable for use in embodiments of methods of the invention are described in US 3,706,607, the contents of which is incorporated herein in its entirety by reference.

Without wishing to be bound by theory, it is believed by the inventors that the main oxidation reaction of the hydrazide requires the transfer of 4 electrons per molecule of nitrogen gas produced during hydrazide foaming, as illustrated by the following generic examples of oxidation of the hydrazide by three groups of oxidisers found to be particularly effective, namely hypohalites, halites and permanganates:

R—C(=O)— NH-NH₂ + 2HOC1 → RCOOH + N₂T + 2H⁺ + H₂O + 2Cl^" [11]

R—C(=O)— NH-NH₂ + HOBrO → RCOOH + N₂T + H⁺ + H₂O + Br [12]

R—C(=O)— NH-NH₂ + MnO₄- + 4H⁺ → RCOOH + N₂T + 3H₂O + Mn³⁺ [13]

wherein R denotes an optionally substituted alkyl, aryl or cyclic group, with or without one or more heteroatoms present in their structure. It is noted that reaction [13] consumes protons, and if these are not available, the oxidation of the hydrazide by permanganates produces MnO₂ rather than Mn ⁺, as indicated below:

3(R—C(=O)— NH-NH₂) + 4MnO₄ ^" + 4H⁺ →

3RC00H + 3N₂T + 5H₂O + 4MnO₂J [14] However, it is noted that permanganate is essentially non-diffusing limiting its use in methods described herein.

Side reactions may lead to production of side products including dimers, such as R— C(=O)— NH- NH- NH- NH- C(=O)— R or R— C(=O)— NH- NH- C(=O)— R, and amides, such as R — C(=O) — NH₂. However, the selectivity of the oxidation reactions to produce N₂ during oxidation of the hydrazide is substantially higher than that for the direct oxidation of NH₃/NH₄ ⁺. Reactions [7]-[14] show that control of the availability of protons (ie., by regulating pH) during the oxidation process, is dependent on the oxidiser used. The hydrazine gassing method described in US Patent No. 3,706,607 typically operates at temperatures of at least between 32 °C (90 °F) and 54 °C (130 °F), and in practice can require temperatures up to 71 ⁰C (160 ⁰F) to sensitise emulsion explosives, as used in US Patent No. 3,770,522. In contrast, one or more embodiments of the invention enable nitrogen gassing employing hydrazides and other nitrogen compounds as the substrate at temperatures of 0 °C or less as described above.

The use of hydrazides as a source of nitrogen for nitrogen based chemical gassing is demonstrated extensively in Examples 4 through 6.

Class 4 Another group of nitrogen compounds well suited to use in methods embodied by the invention are those known as "nitrogen rich" compounds. These compounds contain a high percentage of nitrogen, which reduce the amount of substrate required to release the desired amount of nitrogen gas. Often, nitrogen rich compounds contain tetrazole and/or triazole rings, and derivatives thereof, such as those used as propellants and as gas generators in automobile air bags, including 5-aminotetrazole, bis(aminotetrazolyl)tetrazine, bisguanidinium azotetrazole and bitetrazole. Such compounds contain nitrogen-nitrogen double bonds, which reduce the amount of oxidiser required to liberate a given amount of nitrogen gas compared to other nitrogen compounds. The triazoles include the various triazole tautomers, such as the IH and 2H 1,2,3 triazole tautomers and the IH and 4H-1,2,4 tautomers. Triazole derivatives include mono, di and trisubstituted molecules containing any alkyl, aryl, sulfonyl, nitro, thio or amino substituents. Specific examples include but are by no means limited to 1,2,4-triazole, lH-l,2,3-triazole, l,2,4-triazole-3-carboxylic acid, 3 -amino- 1,2,4- triazole, lH-l,2,4-triazole-3-thiol, 3,5-diamino-l,2,4-triazole, and their alkali metal and alkali earth metal salts.

Tetrazoles that can be used include the 1Η and 2Η tautomers, and their mono or di-substituted derivatives, which can include species substituted at the 1 and 2 positions in the tetrazole ring, or the 5 position corresponding to the carbon atom. Substituents can include one or more or a combination of any alkyl, aryl, sulfonyl, nitro, amino or thio groups. Specific examples of tetrazoles include but are by no means limited to 5-amino-lH-tetrazole, bitetrazole, 5-methyl-lH-tetrazole, 5 -phenyl- lH-tetrazole, IH- tetrazole- 5 -acetic acid and 5-methylthio-lH-tetrazole, and their alkali metal and alkali earth metal salts.

The oxidation of the tetrazole ring requires the removal of four electrons to produce two molecules of nitrogen gas. However, various substituents present in common tetrazoles can also be oxidised to produce nitrogen gas, increasing the yield of nitrogen to greater than two molecules per molecule of tetrazole. Such substituents also increase the oxidiser requirement, with the level of increase determined by the nature of the substituent(s). The oxidation half equation for a common tetrazole derivative, 5-amino-lH-tetrazole is shown below.

CN₅H₃ + 2H₂O → 7e^~ + CO₂ + 2.5N₂ + 7H⁺ [15]

Combining reactions [15] and [6] yields the overall reaction between 5- aminotetrazole and sodium hypobromite.

2CN₅H₃ + 7HOBr → 5N₂ + 2CO₂ + 7Br^" + 7H⁺ + 3H₂O [16]

Reaction [16] shows that CO₂ is produced at a ratio of one molecule of CO₂ for every two and a half molecules of nitrogen gas. Without wishing to be bound by theory, it is believed that the majority of CO₂ generated remains dissolved in the aqueous phase of the emulsion as bicarbonates (i.e., HCO₃ ^"), provided that the pH of this phase is sufficiently high, as is generally the case in embodiments of the invention. It is evident that just 1.4 moles of hypohalite oxidiser are required per mole of nitrogen gas produced for 5-aminotetrazole, affording a substantial reduction in the oxidiser consumption compared to other nitrogen containing compound. The particular nitrogen compound employed will depend on cost considerations, solubility, the amount of oxidiser required for its oxidation and its ability to diffuse through fuel lamellae of the explosive.

The nitrogen compound will normally be added to the discontinuous phase of the explosive emulsion at a concentration up to about 0.1 M and more preferably, in a range of from 0.01 to 0.08 M in explosive formulations that do not rely on direct oxidation of NH₃ /NH₄ ⁺. The precise concentration of nitrogen chemical depends on (i) the type of nitrogen chemical employed and in particular, the number of moles of nitrogen gas that can be evolved per mole of substrate, (ii) the target density of the gassed explosive, (iii) the need to target the oxidation of the nitrogen compound rather than NH₃/NH₄ ^"1", with higher concentrations promoting reaction with the nitrogen compound rather than NH₃ZNH₄ ⁺, and (iv) the temperature at which foaming occurs. For example, in explosives in which the nitrogen gas is produced essentially only from the oxidation of a hydrazide at 20 ⁰C, to achieve the final density of 1.05 g/cm³ of a typical ammonium nitrate emulsion displaying ungassed density of 1.33 g/cm , a concentration of monohydrazide of around 0.021 M is required, assuming 70% yield of the monohydrazide to nitrogen gas. However, for processes that rely on simultaneous or consecutive generation of nitrogen gas both from oxidation of NH₃/NH₄ ⁺ and oxidation of the hydrazide, a more preferable range is from 0 M to 0.1 M for monohydrazides and 0 M to 0.05 M for dihydrazides. Similarly, for higher substituted hydrazides, tetrazoles or triazoles containing multiple nitrogen atoms, the preferred concentration is from 0 to 0.2/n M, where n is the number of nitrogen atoms available in each molecule of the nitrogen compound.

For gassing systems relying only on the direct oxidation of NH₃/NH₄ ⁺, the minimum concentration of NH₄ ⁺ will usually be about three times higher than the equivalent concentration of other nitrogen compounds. This does not present any difficulties as present-day emulsion explosives containing ammonium nitrate are typically characterised by concentrations of NH₄ ⁺ in the order of about 13 M.

The amount of gas produced to regulate the density of emulsion can be varied as a function of the emulsion location in a borehole. In such applications, an excess of the substrate (eg., hydrazide or other nitrogen compounds) can be used in the discontinuous phase of the emulsion, and the amount of nitrogen gas released controlled by adjusting the amount of oxidiser mixed in the emulsion. Any suitable oxidiser can be used to oxidise hydrazides and other nitrogen compounds to nitrogen as described herein. Examples of representative oxidisers include, but are not limited to hypohalites such as hypochlorites, hypobromites and hypoiodites, chlorites, bromites, chlorates, bromates, iodates, perchlorates, perbromates, periodates, permanganates, manganates, ferrates, selenates, ruthenates, perborates, peroxodisulphates, and peroxomonosulphates of ammonium, and corresponding acids, and alkali metal and alkali earth metal salts of the foregoing (e.g., sodium chlorite, sodium bromite, and lithium, sodium, potassium, magnesium and calcium hypobromite), and organic cations, e.g. benzyltriethylammonium permanganate. Further examples include, but are not limited to, hydrogen peroxide, inorganic and organic peroxides, organic nitrates, such as benzoyl or peracetyl nitrate, salcomine, chloramine T and B, nitrodisulfonates, iV-oxides, such as N- methylmorpholine iV-oxide, trimethylamine iV-oxide and pyridine iV-oxide, sodium dichlorocyanurate, acids such as peroxodisulfuric acid, peroxomonosulfuric acid, trichloroisocyanuric acid, hypochlorous acid, iodic acid, selenious acid, vanadic acid, and salts of Cu(II), Fe(II), Mn(III), Co(III), Ti(IV), Cr(VI), VO₂ ⁺, lead oxide, manganese dioxide, iV-halosulfonamides such as jV-chlorotoluenesulfonamide and N- chlorobenzenesulfonamide, iV-halosuccinamides such as iV-bromosuccinamide and iV-chlorosuccinamide, iodine monochloride, iodine bromide, ferricyanide, and dimethyl sulphoxide and its co-oxidants, and salts of the foregoing including their alkali metal and alkali earth metal salts. It is noted that the US Code of Federal Regulation, 29 CFR 1910.109, prohibits any addition or use of chlorates and peroxides in blasting agents (Clark, 1991).

The use of oxidisers which diffuse or can be made to diffuse through the fuel lamellae separating the gasser and the nitrogen compound(s) are typically employed, with the application of non-diffusing oxidisers being limited to those instances in which the nitrogen compound can diffuse rapidly across the fuel lamellae to the gasser at low temperature. Thus the oxidisers most suitable for use in the invention include for example hypohalous acids such as hypochlorous, hypobromous and hypoiodic acids and their corresponding alkali metal or alkali earth metal salts, iV-halosulfonamides, such as iV-chlorobenzenesulfonamide, jV-chlorotoluenesulfonamide and their salts (Chloramine B and T respectively), salcomine, iV-halosuccinamides, such as N- chlorosuccinamide and iV-bromosuccinamide and their salts, chloric and bromic acids and their salts, iV-oxides such as iV-methylmorpholine iV-oxide, trimethylamine N-oxide or pyridine iV-oxide as well as alkali or alkali earth metal manganates.

Some oxidisers such as hydrogen peroxide and peroxosulfates, display kinetic limitation to oxidation requiring the use of catalysts (eg., sodium tungstate), which is expressly encompassed by the invention. Examples of Cr(VI) oxidisers include, but are not limited to, chromium oxide, chromates, dichromates, including pyridinium dichromate, chromic acid, dipyridine chromium oxide and pyridinium chlorchromate.

The oxidation of NH₃/NH₄ ⁺ will typically involve the use of strong to very strong oxidisers, including, but not limited to, hypochlorites, hypobromites, hypoiodites, chlorites, chloramine T and B, N-bromosuccinamide and N- chlorosuccinamide, permanganates and peroxosulphates. In gassing systems involving nitrogen chemicals in addition to NH₄ ⁺, these oxidisers will oxidise both nitrogen containing chemicals and, albeit often at a slower rate, NH₃/NH₄ ⁺. Weaker oxidisers will generally only oxidise nitrogen compounds rather than NH₃/NH₄ ⁺. The amount of the oxidiser used in preferred embodiments of the invention will depend, inter alia, on: (i) target density of the foamed explosive composition; (ii) the selectivity of the oxidiser to produce nitrogen gas in reactions with NH₃ZNH₄ ⁺ and other nitrogen compounds, in the presence of side reactions; (iii) the concentration of the active species in the oxidiser; (iv) the need to oxidise NH₃/NH₄ ⁺ or the hydrazide; (v) the number of electrons withdrawn from substrate by each molecule of the oxidiser, i.e., the change in the oxidation state; (vi) the presence of catalyst (since in the process of its activation, an oxidiser may accept an electron from a catalyst rather than from the substrate); and (vii) the temperature of foaming. If an oxidiser is added to an emulsion explosive as a gasser solution, it is preferred that the mass of that solution is less than 4% w/w of the emulsion, with the content of the active species in the solution of between 5% w/w and 50% w/w.

For example, to foam an emulsion that contains 0.015 M maleic hydrazide with sodium hypochlorite in which the hydrazide is essentially entirely consumed, 2 x 0.015 / 0.8 = 0.038 mol sodium hypochlorite (NaOCl)/L emulsion, or 0.038 / 1.33 = 0.028 mol/kg emulsion would be required. The factor of 0.8 corresponds to efficiency of gassing and the factor of 2 signifies the ratio of number of electrons required by a molecule of hydrazide [11] to that provided by each molecule of the oxidiser [5]. Taking into account the molecular weight of NaOCl and the concentration of NaOCl in commercially available solutions of 12.5% w/w, the calculation becomes 0.028 x (22.99 + 35.5 + 16) / 0.125 = 17 g/kg emulsion. By selecting a different oxidiser, and/or using more concentrated solutions, the amount of gasser may be optimised to less than 0.8% w/w of the emulsion. Normally, the species of a nitrogen compound foaming system that is included in the discontinuous phase of the emulsion will be at a relatively low concentration. This means that the diffusion of this species through the fuel lamellae will be slow, even if the species itself is soluble in the fuel and displays high diffusivity in that phase. For faster gassing, the second species of the foaming system (i.e., the oxidiser) that is added to the emulsion should desirably be able to diffuse rapidly through the fuel lamellae.

The nitrogen compound will generally be dissolved in the discontinuous phase of the emulsion explosive, and diffusion of the selected oxidiser through the fuel lamellae of the explosive can be achieved in a number of ways. For example, the diffusion of the oxidiser can be accomplished by:

(i) Selection of an oxidiser that diffuses readily under the gassing conditions, such as chloramine T or B, or N-oxides. This is illustrated for chloramine T in Example 2.1;

(ii) Selection of an oxidiser whose acid form exhibits a high pKa value, such as hypochlorites, hypobromites, hypoiodites, manganates or bromites. Anions of these salts can be readily protonated owing to proton transfer from the discontinuous phase of the emulsion, e.g., by diffusion of a pH regulating agent such as a weak acid like acetic or formic acid. In this case, the pH of the discontinuous phase and the type of the pH regulating agent should be selected to accelerate the gassing process. Specifically, the pH regulating agent itself should be able to diffuse through the fuel lamellae (this point is demonstrated by comparing the measurements of Examples 4.1 and 4.2 below). Methods described herein have application in the gassing of emulsion explosives (including melt-in-oil emulsions) of explosive emulsion, gel, slurry and heavy ANFO compositions. For example, the gasser can be pre-emulsified or delivered to emulsion in the form of a micro-emulsion, a Lewis acid catalyst may be added to make an oxidiser diffuse through fuel lamellae or for instance, an oxidiser can be added as a powder, as described for nitrite gassing in US Patent No. 4,875,951, European Patent Application No. 775,681, US Patent No. 6,855,219 and US Patent No. 4,997,494 respectively, the contents of which are incorporated herein in their entirety by cross- reference.

Water-in-oil emulsion explosives can be any emulsion comprising a discontinuous phase of an aqueous oxidising solution of inorganic salts dispersed in a continuous phase of an organic fuel in the presence of one or more emulsifying agents. Such emulsion explosives are well known in the art.

The oxidising salt can, for example, be selected from ammonium, alkali metal and alkaline earth nitrates, perchlorates and mixtures of the foregoing. Typically, the oxidising salt will comprise at least about 50% w/w of the emulsion explosive composition, more preferably at least about 60%, 70% or 80% w/w and most preferably, at least about 90% w/w of the explosive. In a particularly preferred embodiment, the oxidising salt will be ammonium nitrate alone or in combination with sodium nitrate, potassium nitrate, calcium nitrate and/or ammonium, alkali metal and alkaline earth metal perchlorates.

Proton transfer agents used herein can comprise one or more compounds selected from the group consisting of inorganic acids, organic acids, carboxylic acids, and salts thereof, the explosive being formulated such that at least some of these compounds exist in the explosive in a neutral form. More particularly, these agents can comprise one or more compounds selected from the group consisting of alkyl carboxylic acids, acetic acid, formic acid, phosphoric acid, citric acid, tartaric acid, furoic acid, fumaric acid, salicylic acid, malonic acid, phthalic acid, sulfanilic acid, mandelic acid, malic acid, butyric acid, oxalic acid, and salts thereof.

Non-diffusing pH regulating agents as described herein can comprise one or more compounds selected from the group consisting of partially or completely deprotonated forms of inorganic acids and carboxylic acids, and salts thereof. In particular, these agents can comprise one or more of phosphoric acid, acetic acid, formic acid, citric acid, tartaric acid, furoic acid, fumaric acid, salicylic acid, malonic acid, phthalic acid, sulfanilic acid, mandelic acid, malic acid, butyric acid, oxalic acid, and salts thereof.

A melt-in-oil emulsion explosive can be any such explosive containing little or no water in its formulation, and may solidify once the temperature decreases below the solidification point of the melt, which usually lies between 70⁰C and 130 ⁰C. Normally, the melt-in-oil emulsion contains ammonium nitrate and at least one other chemical added to decrease the melting point of ammonium nitrate. Melt- in -water emulsion explosives are also well known in the art (e.g., US Patent No. 4,790,891 and US Patent No. 4,676,849). So-called heavy ANFO mixtures form an important part of industrial explosives consumption. In heavy ANFO, emulsion explosives are typically mixed with ANFO, that is, with porous solid prilled ammonium salts with oil present in the porous space of the prills. Heavy ANFO may be suitably gassed by the methods described in this invention, and the use of any such methods to sensitise and/or modify the density this type of explosive is specifically encompassed.

Emulsifiers commonly used in emulsion explosive compositions include sorbitan monooleate (SMO), polyisobutane succinic anhydrides (PiBSA) and amine derivatives of PiBSA, and conjugated dienes and aryl-substituted olefins. US Patent No. 6,800,154 and US Patent No. 6,951,589 provide a particularly comprehensive summary of suitable emulsifiers which may be used, the contents of which are incorporated herein by reference in their entirety.

The fuel used in the explosive can also be any fuel commonly utilised in explosive compositions. Examples of fuels that can be utilised include, but are not limited to, paraffinic, olefinic, napthenic, and paraffin-napthenic oils, animal oils, vegetable oils, synthetic lubricating oils, hydrocarbon oils in general and oils derived from coal and shale, as described in detail in US Patent No. 6,951,589, the contents of which is also incorporated herein by reference in its entirety.

Although gel explosives were utilised by industry prior to emulsion explosives, their use has declined in preference to emulsion explosives and heavy ANFO. Nevertheless, gel explosives are well known to the skilled addressee. Examples of gel explosives are for instance described in US Patent No. 3,382,117, US Patent No. 3,660,181, US Patent No. 3,711,345 and US Patent No. 3,713,919.

Typically, the gassing of an explosive as described herein will be achieved at ambient temperature although heating of the explosive to assist diffusion of the oxidiser and/or nitrogen compound is not excluded. The gassing may for example be carried out at temperatures of about 70 °C or below, or at a temperature of 60 °C, 50 °C, 40 °C, 30 °C, 25 °C, 20 °C, 10 °C, 5 °C or 0 °C or below, or in any range of from 0 °C or below up to about 70 ⁰C. As will also be understood, the gassing may be carried at any specific temperature or within any specific range of temperatures within the particular ranges specified above (eg., 35 °C, 15 °C, 14 °C or 13 °C, or eg., from 0 °C or below up to about 15 ⁰C), and all such specific temperatures and temperature ranges are expressly encompassed. An example of an embodiment of the invention is as follows. Add to an emulsion explosive composition containing a hydrazide compound (eg., acetyl hydrazide, maleic hydrazide etc., at a concentration between 0.01 and 0.1 M) and a pH regulating agent (eg., sodium acetate, approx 0.2 to 1% of the composition) with the pH of the emulsion between 4.5 and 6, a solution of sodium hypochlorite (the solution containing 10-20% NaOCl) and mix this solution into the emulsion for 5-15 s to disperse it and gas the emulsion. The invention will now be described below by reference to a number of further non-limiting Examples.

EXAMPLE 1

1. Experimental protocol 1.1 Emulsions

In all Examples, the discontinuous phase of explosive emulsions consisted of a super- saturated solution of ammonium nitrate, containing approximately 400 g of ammonium nitrate per 100 g of water. In Emulsions A and B, the continuous phase consisted of a mixture of diesel fuel and a PiBSA emulsifier, such that the ratio of fuel to emulsifier was approximately 70 parts of diesel fuel to 30 parts of PiBSA, by mass. The phase ratio for these emulsions was 87 parts discrete phase to 13 parts continuous phase on a volumetric basis. This corresponded approximately to 91.5 parts of discrete phase to 8.5 parts of continuous phase on a mass basis. In Emulsions C-K, the continuous phase consisted of a mixture of diesel fuel and PiBSA emulsifier, such that the ratio of fuel to emulsifier was 80 parts diesel to 20 parts PiBSA by mass. The ratio phase ratio for Emulsions C-K was 94 parts of discrete phase to 6 parts of continuous phase on a mass basis. The above compositions conform to those well known in the art, such as those described in US Patent No. 4,409,044, US Patent No. 3,447,978 and US Patent No. 3,770,522. The discontinuous phase was prepared by dissolving ammonium nitrate in water at a temperature of 75 °C. Various combinations of pH regulating agents, such as sodium acetate, sodium hydroxide and tri-sodium citrate were then added to control the pH of the system if required in a particular emulsion. Some of these chemicals also serve as proton transfer agents during the gassing process. Nitrogen containing compounds such urea, 5-aminotetrazole, and various hydrazides/dihydrazides were also added to some emulsions to improve the efficiency of the gassing process. The apparent pH of the solution was measured using a pH probe (Hanna pH 213 meter with HI 113 IB pH probe) prior to addition to the continuous phase. Table 1 lists emulsions used in the examples described below.

Table 1 : Composition of emulsions

Emulsions were constructed by slowly adding the above solution containing ammonium nitrate to the continuous phase at a temperature of approximately 75 °C. The two phases were mixed thoroughly using an overhead stirrer (IKA Eurostar Digital) operating at approximately 300-450 rpm. Once the two phases had been completely combined, the mixing speed was increased to approximately 500-600 rpm and maintained at this speed until the emulsion had reached a suitable viscosity.

1.2 Preparation of gassers

Five types of gasser chemicals were used, including lithium and calcium hypochlorites (LiOCl and Ca(OCl)₂, respectively), chloramite T, sodium hypochlorite (NaOCl) and sodium hypobromite (NaOBr). Lithium and calcium hypochlorites as well as chloramine T were obtained as solids, whilst sodium hypochlorite and hypobromite were obtained as solutions. The solids were weighed into a 20 cm³ beaker and distilled water was then added to affect the dissolution. The concentration of the gasser solution was generally between 10 and 20%; although higher concentration could have been possible. To accelerate the dissolution of the substance, the beaker was heated lightly (less than 50 °C), and then the solution was cooled down prior to use. In the case of sodium hypochlorite and hypobromite, the gassing reactant was present in a solution of approximately 10-12% by mass. Sodium hypobromite contained a small percentage (1-5%) of sodium hydroxide for enhanced shelf life, whilst some examples utilised additional sodium hydroxide to delay the onset of the gassing process. For examples utilising additional sodium hydroxide in the gasser, stock solutions were made by adding the required mass of 50% sodium hydroxide solution to a known mass of sodium hypochlorite solution. The desired mass of solution for use in examples was drawn into a syringe, confirmed by weighing, and used without alteration.

1.3 Gassing procedure

Gassing experiments were conducted using 230 ± 2 g of emulsion, allowing each batch of emulsion of Table 1 to serve three to five experiments. The gasser solution was mixed with the emulsion for 5-15 s at 300 rpm with an overhead stirrer to ensure an even distribution of the gasser throughout the emulsion. In examples where acetic acid was added to the emulsion prior to gassing, the desired amount of acid was mixed in the emulsion for 15 s, before adding the gasser after a further 30 s had elapsed. The emulsion was then transferred to a container of known volume, such that the emulsion occupied completely the volume within the container. The container was levelled at the top and weighed at regular intervals as the gassing progressed in order to observe the density change and the rate of gassing.

For experiments conducted at temperatures other than ambient, the emulsion temperature was adjusted by means of a constant-temperature refrigerated and heated water bath before addition of the gasser solution. The container employed in the experiment was also placed in the water bath before handling. Once the gasser solution had been added to the emulsion, the container was returned to the water bath at all times when not being weighed.

EXAMPLE 2

2.1 Oxidation of ammonia in emulsion without proton transfer agent

This example demonstrates that ammonia/ammonium ions can be directly oxidised by various oxidisers in order to produce nitrogen gas to sensitise and/or modify density of an emulsion or gel explosive. It also demonstrates the influence of the gasser composition and pH on the rate of gassing and indicates the desirability of appropriate pH selection to achieve required sensitisation times.

The method utilised involved gassing of Emulsion A (with no pH control or proton transfer agent) using lithium hypochlorite, sodium hypobromite and chloramine T, at 30 °C. Table 2 shows the composition of gassing solutions and completion time for each example. Table 2: Gasser solutions for Example 2.1

*Completion time is the time taken for the emulsion to reach 90% of its final density

The results are plotted in Fig. 1 and show that the reagent producing the fastest gassing rate was chloramine T (2 min), followed by lithium hypochlorite (5 min). The reaction with sodium hypobromite was slower taking approximately 18 min to reach completion. No attempt was made to optimise the amount of gasser to regulate the emulsion density to a set level, say 1.05 or 1 g/cm³. Lower densities could have been achieved in this example, and in other examples described below, simply by mixing in larger amounts of the gassers, and in the case of other examples, adjusting the amounts of pH agents and/or nitrogen compounds. Similarly adding smaller amounts of gasser would produce a smaller density change, and as such the invention can produce emulsion explosives of a density required for practical application, i.e. from 1.37 g/cm to less than 1 g/cm³. Hence, one or more embodiments as described herein can be applied to modify the density and/or sensitise commercial explosives, with the level of density change controlled by the amount of gasser added.

The example also demonstrates that the nature of the gassing chemical is an important factor that influences the rate of gassing. Chloramine T exists as a neutral molecule in solution and is therefore able to diffuse readily through the fuel lamellae from the gasser to react with the nitrogen compound in the emulsion phase. As a result, chloramine T shows fast gassing despite the absence of a proton transfer agent.

In contrast, the diffusion of hypohalites is determined by the pH of the gasser solution. Hypohalite anions are weak bases and are capable of accepting a proton to form hypohalous acids, which are able to diffuse rapidly through fuel lamellae. Hypohalous acids (e.g., HOCl, HOBr) exist in equilibrium with the corresponding hypohalite anion, with the relative concentration of each species being determined by the pH of the solution and pKa of the gasser. Normally, pH and pKa are reported in literature under conditions corresponding to those of infinitely dilute solutions. Embodiments of this invention involve the use of concentrated solutions of electrolytes. In such solutions, both pH and pKa usually differ from their values in the limit of infinite dilution. For this reason, the meaning of pH in the context of the invention is that of the apparent (i.e., the measured pH), rather than the actual pH. At low pH (pH < 7) the hypohalite exists predominantly in the protonated form, whist at high pH (pH > 9) the majority exists as hypohalite anions. It is evident that in order for hypohalite gassers to diffuse through fuel lamellae and react with nitrogen compounds, the pH of the gasser should be sufficiently low to increase the concentration of the hypohalous acid to a level that allows an adequate rate of gasser diffusion.

To ensure efficient use of hypohalite oxidisers, the pH of the gasser solution should be continuously lowered (or at least remain constant at a low value) throughout the gassing process. This can occur via the transfer of protons (hydrogen cations) from the emulsion phase to the gasser and ensures that the entire contents of the gasser can participate in the gassing reaction. If proton transfer does not occur, hypohalite anions will be trapped in the gasser droplets and unable to react with nitrogen containing compounds in the emulsion. Proton transfer in an emulsion is slow, unless a proton transfer agent is included in the emulsion formulation.

It can be seen from this example that the rate of gassing is dependent on the pH of the gassing solution. The pH of distilled water used to dissolve lithium hypochlorite is lower than the pKa of hypochlorous acid (pKa -7.5), and as such the gasser solution contains a significant concentration of hypochlorous acid. It can also be seen from Fig. 1 that the gassing rate with this chemical is reasonably fast, despite the absence of a proton transfer agent. In contrast, the sodium hypobromite solution has a very high pH due to the addition of sodium hydroxide, which causes the initial concentration of hypobromous acid in the gasser to be very low. For this reason the gassing rate of the sodium hypobromite experiment (Example 2. Ib) is slower than that of lithium hypochlorite

(Example 2.1a). It is evident that the efficiency of the reaction was much lower than that predicted from theory, which may be attributed to side reactions that do not result in the production of nitrogen gas, such as the formation of nitrate ions. 2.2 Oxidation of ammonia in emulsion with a proton transfer agent

This example demonstrates the use of a proton transfer agent to accelerate the diffusion of hypohalite oxidisers added to the emulsion to react with ammonia/ammonium ions, at temperatures from 12 to 30 ⁰C.

The particular proton transfer agent used in this example is sodium acetate. Sodium acetate is a weak base in solution, which makes it capable of accepting a proton to form acetic acid. Acetic acid (CH₃COOH) is a neutral molecule and is able to diffuse from the emulsion phase to gasser droplets, allowing it to transfer protons from the emulsion phase to the gasser. Hypohalites are stronger bases than acetate, allowing them to deprotonate acetic acid that has diffused to the gasser droplets to produce hypohalous acids, (e.g. CH₃COOH + OCl- > CH₃COO- + HOCl). The hypohalous acid produced undergoes rapid diffusion to the emulsion phase where it can react with a nitrogen- containing chemical.

In this example, the aforementioned mechanism is used to react ammonia/ammonium ions with lithium hypochlorite and sodium hypobromite. Emulsion Composition B (Table 1) was gassed using lithium hypochlorite at 10 and 30 ⁰C and sodium hypobromite at 23 °C and 30 °C. The emulsion contained approximately 1% sodium acetate to control the emulsion pH and act as a proton transfer agent. Table 3 shows the composition of gasser solution used in each experiment.

As illustrated in Fig. 2 and Table 3, the gassing rate at 30 °C was fast for both reagents studied, with the gassing process essentially completed in less than 3 min. This is considerably faster than the previous emulsion that did not contain a proton transfer agent. There appears to be only a small temperature dependence on the oxidation by lithium hypochlorite, with the reaction at 12 °C taking less than 2 min longer to reach completion compared to 30 ⁰C. There is also little difference in the gassing rate of sodium hypobromite between 23 °C and 30 °C. No crystals were present in the emulsions after the completion of the gassing reactions. Table 3 Gasser solutions for Example 2.2

EXAMPLE 3

3.1 Oxidation of primary amine

This example demonstrates the oxidation of compounds containing a primary amine group to produce nitrogen gas. Primary amines exhibit higher selectivity compared to ammonia, reducing the amount of gasser required to form a desired amount of gas. Urea is shown in this example as it finds widespread application in nitrosation based gassing technologies. Unlike nitrosation reactions, which require high emulsion temperatures, the oxidation of urea occurs rapidly at ambient temperatures. This allows an emulsion designed for high temperature nitrosation gassing to be sensitised or have its density modified using urea oxidation if the emulsion temperature falls, allowing greater flexibility in blasting operations.

The example uses Emulsion C, which contains approximately 0.6% sodium acetate at pH 5.5 to act as a proton transfer agent. The gasser used is a sodium hypochlorite (NaOCl) solution containing approximately 10.5% NaOCl by mass. The experiment was performed at 20 ⁰C, and follows the same mechanism as Example 2.2, in which a proton transfer agent is used to facilitate the diffusion of a hypohalite oxidiser to the emulsion where it can react with a nitrogen-containing chemical to produce nitrogen gas. Table 4 list the composition of the gasser used in this example.

Table 4: Gasser solutions for Example 3.1

It can be seen from Fig. 3 that there is an induction period of approximately 80 s in which little or no gassing occurs, followed by a period of rapid gassing, with the final completion time being approximately 4 min. The initial lag occurs as it takes some time for the pH of the gasser to be lowered enough to produce HOCl, and for the HOCl to diffuse from the gasser to the emulsion. This short lag may be desirable, as it allows the gasser to be mixed thoroughly into the emulsion before the production of gas commences, thus reducing the likelihood that gas is inadvertently separated out of the emulsion before it enters a borehole. Table 5 provides a comparison between the amount of gasser required for urea and ammonia gassing. It can be seen that the amount of oxidiser required to produce gas from primary amines is slightly less than that for ammonia/ammonium ions.

Table 5: Comparison of gasser requirements of urea and ammonia based gassing

EXAMPLE 4

4.1 Cyclic hydrazide compound with diffusing buffer

This example demonstrates the use of a cyclic hydrazide compound, in conjunction with hypohalite oxidisers and a diffusing buffer/proton transfer agent to sensitise and/or modify the density of an emulsion at low temperature. The use of a hydrazide reduces the amount of oxidiser compared to urea and ammonia, as the reaction requires the removal of four electrons per molecule of gas rather than six. The example also demonstrates that raising the pH of hypohalite oxidiser solutions prior to gassing enables an emulsion to be gassed in a controlled manner over a wide temperature range.

Emulsion D (Table 1), containing maleic hydrazide (i.e., 3,6-dihydroxy pyridazine) and sodium acetate at pH 6.3 was gassed with sodium hypochlorite at 5 ⁰C, 18 ⁰C and 40 ⁰C using 0, 2 and 5% additional sodium hydroxide (NaOH) in the respective gassers. Sodium hydroxide is a strong base, and is effective at raising the pH of gasser solutions to control the gassing rate over a wide temperature range. The gassing rate is reasonably fast in all experiments ranging from approximately 6 min at 40 ⁰C to 15 min at 5 ⁰C. The gasser composition is shown in Table 6.

Table 6. Gasser solutions for Example 4.1

It can be seen that despite a large difference in the temperatures used in each experiment that the gassing times are quite similar. Sodium hydroxide (NaOH) was added to the gasser to increase the pH of the gasser solution. Added NaOH must be neutralised by protons transferred from the emulsion phase before hypohalite oxidisers can diffuse and so can be used to reduce the rate of gassing, which is especially useful at temperatures above 30 ⁰C at which proton transfer occurs quite rapidly. Without the addition of NaOH the gassing at 40 ⁰C would be extremely fast, and there would be a large difference in the gassing times of each experiment. It is noted that due to the relatively high pH of the emulsion that the concentration of ammonia in the emulsion is significant. A small portion of the gassing can be attributed to the diffusion of ammonia from the emulsion to the gasser, which is demonstrated in Example 5.1-5.2. The results for Example 4.1 are shown in Fig. 4.

4.2 Cyclic hydrazide compound with non-diffusing buffer

This example demonstrates that a non-diffusing buffer may be used to produce slower gassing times at high temperatures, and that without a proton transfer agent the gassing rate at low temperature is significantly slower.

Emulsion E, containing maleic hydrazide, was gassed with sodium hypochlorite containing 0 and 2% NaOH at 18 ⁰C and 5% NaOH at 40 ⁰C. The emulsion contained approximately 0.7% tri-sodium citrate acting as a pH buffer. The apparent pH of the discontinuous phase was 6.25. The composition of the gasser used in each experiment is shown below in Table 7, and the results are shown in Fig. 5.

Table 7: Gasser solutions for Example 4.2

The gassing times of this emulsion containing a tri-sodium citrate buffer are considerably slower than those of the emulsion with the same pH and acetate buffer, taking 12 min longer at 40 ⁰C and up to 36 min longer at 18 ⁰C. This occurs as at the emulsion pH used (6.25) citrate molecules are deprotonated and unable to transfer protons from the emulsion to the gasser. Because proton transfer occurs so slowly, most of the hypohalite remains deprotonated and is unable to diffuse from the gasser to the emulsion, resulting in slow gassing times. Acetate, on the other hand is protonated to a small extent at pH 6.25, and can be easily protonated during the gassing process. This allows it to transfer protons from the emulsion to the gasser to protonate hypohalite anions that diffuse rapidly into the emulsion phase, thereby producing significantly faster gassing times that the non-diffusing citrate buffer. A comparison of Examples 4. Ic and 4.2c (40 ⁰C with 5% additional NaOH) is shown in Fig. 6.

EXAMPLE 5

5.1 Hydrazide compound with non-diffusing buffer

This example demonstrates the use of a hydrazide compound, acetyl hydrazide in conjunction with a hypohalite oxidiser to produce controlled gassing of an emulsion between 20 and 40 ⁰C.

Emulsion F, containing acetyl hydrazide and a tri- sodium citrate buffer at pH 6.3 was gassed at 20 and 40 ⁰C with sodium hypochlorite (NaOCl) containing 0 and 5% additional NaOH in the respective gassers. Similarly to Example 4.2, the gassing rate at 40 ⁰C is considerably slower than with a diffusing buffer, with a completion time of 10 min, whilst the gassing rate at 20 ⁰C is very slow taking 30 min. It is evident that a proton transfer agent is desirable for fast gassing below 30 ⁰C. However a non-diffusing buffer such as tri- sodium citrate is useful at higher temperatures to ensure that the gassing process is not too fast. Table 8 shows the composition of the gasser used in this example, whilst the results are shown in Fig. 7.

Table 8: Gasser solutions for Example 5.1

*Completion time is the time taken for the emulsion to reach 90% of its final density It can be noted that the gassing times for acetyl hydrazide are slightly faster than those of maleic hydrazide (Example 4.2), with the gassing times of acetyl hydrazide being 5 min faster at 40 ⁰C and 10 min faster at 20 ⁰C This is most likely due to the production of acetic acid from the oxidation of the hydrazide, which is able to transfer protons from the emulsion to the gasser to protonate hypochlorite anions, accelerating the gassing process.

5.2 Hydrazide compound with non-diffusing buffer at lower pH

This example demonstrates gassing with a hydrazide containing compound using hypohalite oxidisers in an emulsion with a non-diffusing buffer at pH 5. The example demonstrates that ammonia can diffuse to the gasser when the pH of an emulsion containing ammonium ions is greater than 6, whilst at pH values near 5 ammonia diffusion is substantially slower. It also demonstrates that a pH transfer agent such as acetic acid may be added to the emulsion immediately prior to gassing to increase the gassing rate.

Emulsion G containing acetyl hydrazide and a citrate buffer at pH 5 was gassed with sodium hypochlorite at 5 and 20 ⁰C. Acetic acid was added to the emulsion immediately prior to the addition of sodium hypochlorite to accelerate the gassing at 5 ⁰C and in Experiment 5.2b at 20 ⁰C. The composition of the gasser used in this example is shown below in Table 9.

Table 9: Gasser solutions for Example 5.2

*Completion time is the time taken for the emulsion to reach 90% of its final density Fig. 8 provides a comparison between emulsions of pH 6.25 and pH 5, containing the same concentration of non-diffusing buffer. It is apparent that the higher pH emulsion has a faster gassing rate, despite the reduced driving force for proton transfer, which should in theory make it slower. However, at high pH, ammonium ions are deprotonated to form ammonia, which is capable of diffusing from the emulsion phase to the gasser, where it may react to form nitrogen gas and/or generate protons to allow hypohalite diffusion. Therefore, when a non-diffusing buffer is used a high pH emulsion exhibits a faster gassing rate than a low pH emulsion.

Fig. 9 is a plot of the results for this example, showing comparison between experiments with and without a proton transfer agent. It can be seen that emulsions with a proton transfer agent added prior to gassing exhibit fast gassing rates, even at relatively low temperatures. Acetic acid is able to diffuse to the gasser causing the pH of the gasser to fall. This causes hypochlorite anions to be protonated to form hypohalous acids, which can diffuse rapidly to the emulsion and react with hydrazides (and other nitrogen containing chemicals) to produce nitrogen gas.

5.3 Hydrazide with diffusing buffer/proton transfer agent

This example demonstrates the performance of an emulsion containing a hydrazide and diffusing buffer.

Emulsions H and I containing acetyl hydrazide and sodium acetate at pH 5.5 were gassed with hypohalite oxidisers at temperatures ranging from 5 ⁰C to 40 ⁰C. Sodium hypochlorite was used at 5 ⁰C, 20 ⁰C and 40 ⁰C, whilst calcium hypochloite and sodium hypobromite were used at 20 ⁰C. Additional acetic acid was added to the emulsion in one of the 5 ⁰C experiments to accelerate the gassing rate. Table 10 shows the composition of the gasser employed in each experiment. Table 10: Gasser solutions for Example 5.3

The results for these experiments are shown in Fig. 10 and Fig. 11. The gassing rate of the emulsion was fast with sodium hypochlorite at 20 ⁰C, being complete within 6 minutes. Little gassing occurred within the first 1.5 min, which allowed the gasser to be mixed into the emulsion without the possibility of gas loss. Similar results were obtained at 40 ⁰C utilising 5% of additional NaOH in the gasser to slow the gassing rate at this temperature. The gassing rate at 5 ⁰C was considerably slower taking 43 min to reach completion. However, the addition of a small amount of acetic acid to the emulsion prior to gassing enabled a completion time of 3.5 min to be achieved.

Gassing with sodium hypobromite was also slow at 20 ⁰C, due to the presence of additional NaOH in the gasser, whilst gassing with calcium hypochlorite was extremely fast, as it has a relatively low pH. As such, a significant proportion of the hypochlorite is protonated before the gasser is added to the emulsion and thus, the hypochlorite may diffuse rapidly from the gasser producing very fast gassing times. EXAMPLE 6

6.1 Dihydrazide with non-diffusing buffer

This example demonstrates the oxidation of a compound containing two hydrazide groups (i.e., a dihydrazide) to generate gas in an emulsion containing a non- diffusing buffer. The use of a dihydrazide allows for a smaller amount of substrate to be used compared to the corresponding mono-hydrazide, as each molecule contains four available nitrogen atoms rather than two.

Emulsion J containing succinic dihydrazide and a citrate buffer at pH 6.25 was used in this experiment. The emulsion was gassed at 20 ⁰C and 40 ⁰C with sodium hypochlorite, with 5% additional sodium hydroxide added for the 40 ⁰C experiment. The composition of the gasser is shown below in Table 11.

Table 11: Gasser solutions for Example 6

The results are plotted below in Fig. 12. The emulsion gassed within a reasonable time at both 20 and 40 ⁰C, with completion times of 20 and 10 min respectively in each experiment. There is little difference between the gassing rate of an emulsion containing a dihydrazide and that of a mono-hydrazide, making the use of a dihydrazide an effective way of reducing the amount of substrate required to produce the desired amount of gas. EXAMPLE 7

7.1 Nitrogen rich compound with diffusing buffer/proton transfer agent

This example demonstrates the oxidation of nitrogen rich compounds with hypohalite oxidisers to produce nitrogen gas in an emulsion containing a diffusing buffer/proton transfer agent. A tetrazole was utilised in this experiment. Tetrazoles require a smaller amount of oxidiser per mole of gas produced compared to hydrazides and amines.

In particular, the example utilised an emulsion explosive containing 5-aminotetrazole (5AT) and sodium acetate at pH 5.5 to produce fast and efficient gassing with sodium hypochlorite and hypobromite. The experiment with sodium hypochlorite was performed at 20 ⁰C whilst that for sodium hypobromite was performed at 30 ⁰C. The composition of the gassers used in the example is shown in Table 12.

Table 12: Gasser solutions for Example 7

The results for this example are shown in Fig. 13. It can be seen that the use of a tetrazole compound as the source of nitrogen for hypohalite based chemical gassing allows a significant reduction in the amount of oxidiser required to produce the desired density change. It is apparent however that in the experiment with sodium hypochlorite there was a significant difference in the theoretical gasser requirement of 1.4 moles of NaOCl per mole of gas and the observed value of 2.1. This may be attributed to the oxidation of ammonia, which occurs in a parallel side reaction requiring a larger amount of oxidiser per mole of gas. Sodium hypobromite however appears to exhibit a high selectivity toward the oxidation of 5AT, with just 1.3 moles of oxidiser required per mole of gas produced, affording a substantial reduction in the amount of oxidiser required. Table 13 shows a comparison of the average oxidiser requirement at 20 ⁰C for each nitrogen compound studied. Data refers to experiments with NaOCl unless otherwise stated.

Table 13: Oxidiser requirement for different nitrogen compounds

DISCUSSION

Examples 1-7 demonstrate combinations of oxidisers, pH regulating agents and substrates (e.g., ammonia/ammonium ions, amines, hydrazides, and high nitrogen compounds) to sensitise and/or control density of emulsion explosives and gels. Hypohalite oxidisers such as lithium, sodium and calcium hypochlorites and hypobromites, along with chloramine T and other oxidisers that diffuse across fuel lamellae may be deployed to sensitise an emulsion explosive containing a pH regulating agent such as sodium acetate. Emulsions can be sensitised rapidly using this method, with sensitisation times as low as 1 min at room temperature and 5 to 10 min at below 0 °C. Ammonium nitrate crystals were present in emulsion explosives gassed with hypohalites and chloramine T that did not contain a pH regulator agent.

Nitrogen containing chemicals including amines, hydrazides, and high nitrogen compounds such as triazoles and tetrazoles may be incorporated into the discontinuous phase of the emulsion to reduce the quantities of oxidisers and pH controlling agents required for sensitisation and/or density modification to occur. This method may be implemented with or without a pH regulating agent. However, sensitisation times observed with small quantities of a pH regulating agent, such as sodium acetate in the emulsion, were significantly faster than those without. Similarly, the addition of a pH- regulating agent to a gassing solution can be used to influence the rate of gassing. Adding alkaline substances to the gasser results in slower gassing times, whilst adding acidic substances results in fast gassing. In addition, if a pH regulating agent is not present in the emulsion composition, or is non-diffusing, a suitable agent such as acetic acid may be added to the emulsion immediately prior to gassing to give fast sensitisation times. In many instances it is possible to manipulate the above variables (notably pH of emulsion and gasser, and the use of proton transfer agents) to provide an initial delay of 1-2 min or longer between the addition of the gasser and the onset of the gassing reaction, with fast gassing ensuing from this point. Such a delay allows the gasser to be safely mixed into the emulsion without releasing gas from the gassing reaction and avoids coalescence of gas bubbles reducing bubble size. It will be readily apparent to those skilled in the art that other nitrogen compounds, oxidants and pH regulating agents may be employed to achieve the desired level of nitrogen gassing.

Accordingly it will be further understood that numerous variations and/or modifications may be made to the invention without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

LITERATURE REFERENCES

1. 29 CFR 1910.109, Explosives and Blasting Agents, US Department of Labor, http://www.osha.gov.

2. Clark PK, Interpretation of 29 CFR 1910.109 Relative to Peroxides and Chlorates in Blasting Agents, Slurries, and Emulsions, OSHA, USA, 1991.

3. da Silva G, Dlugogorski BZ, Kennedy EM (2006) "An experimental and theoretical study of the nitrosation of ammonia and thiourea", Chemical Engineering Science 61(10), 3186-3197; "Reaction and mass transfer effects during the foaming of concentrated water-in-oil emulsions by the nitrosation of thiourea", AIChE J 52(4), 1558-1565.

Claims

1. A method for gassing an explosive to sensitise the explosive and/or modify the density of the explosive, comprising reacting at least one oxidiser with at least one nitrogen containing compound in the explosive to generate nitrogen gas, the explosive being formulated to effect diffusion of the oxidiser and/ or the compound into contact with each other, and the nitrogen gas being generated by oxidation of the compound by the oxidiser.

2. A method according to claim 1 wherein the oxidiser and/or the compound diffuse across fuel lamellae in the explosive into contact with one another.

3. A method according to claim 1 or 2 wherein the explosive is formulated to effect diffusion of the oxidiser into contact with the nitrogen compound.

4. A method according to claim 3 wherein the explosive is formulated for protonation of the oxidiser or the release of at least one proton from the oxidiser, to effect the diffusion of the oxidiser.

5. A method according to claim 4 wherein the explosive comprises a proton donor for protonating the oxidiser.

6. A method according to claim 4 or 5 wherein the explosive comprises a proton acceptor for maintaining pH above a predetermined lower limit to inhibit crystallisation in the explosive.

7. A method according to claim 3 wherein the explosive comprises a pH regulating agent which acts as the proton donor and a proton acceptor for maintaining the pH above a predetermined lower limit.

8. A method according to claim 1 or 2 wherein the explosive is formulated to have a pH for obtaining the diffusion of the oxidiser.

9. A method according to claim 8 wherein the explosive comprises a pH regulating agent which is essentially non-diffusing across the fuel lamellae, for maintaining the pH at a level to control the rate of diffusion of the oxidiser.

10. A method according to claim 1 or 2 wherein the explosive comprises a proton transfer agent for transferring at least one proton across fuel lamellae or from one phase to another of the explosive, to promote the diffusion of the oxidiser and/or the nitrogen compound.

11. A method according to claim 10 wherein the proton transfer agent is provided by a pH regulating agent.

12. A method according to claim 10 or 11 wherein pH is regulated to delay the diffusion of the oxidiser and thereby the gassing of the explosive.

13. A method according to claim 9 wherein the pH regulating agent comprises one or more compounds selected from the group consisting of partially or completely deprotonated forms of inorganic acids and carboxylic acids, and salts thereof.

14. A method according to claim 13 wherein the pH regulating agent comprises one or more of phosphoric acid, acetic acid, formic acid, citric acid, tartaric acid, furoic acid, fumaric acid, salicylic acid, malonic acid, phthalic acid, sulfanilic acid, mandelic acid, malic acid, butyric acid, oxalic acid, and salts thereof.

15. A method according to claim 10 or 11 wherein the proton transfer agent comprises one or more compounds selected from the group consisting of inorganic acids, organic acids, carboxylic acids, and salts thereof, the explosive being formulated such that at least some of these compounds exist in the explosive in a neutral form.

16. A method according to claim 15 wherein the proton transfer agent comprises one or more compounds selected from the group consisting of alkyl carboxylic acids, acetic acid, formic acid, phosphoric acid, citric acid, tartaric acid, furoic acid, fumaric acid, salicylic acid, malonic acid, phthalic acid, sulfanilic acid, mandelic acid, malic acid, butyric acid, oxalic acid, and salts thereof.

17. A method according to any one of claims 1 to 16 wherein the at least one nitrogen compound is selected from the group consisting of NH₃ZNH₄ ⁺, ammonium salts, urea, amines, hydrazines, hydrazides, azides, triazoles, tetrazoles, and derivatives of urea, amines, hydrazines, hydrazides, azides, triazoles, tetrazoles, and nitrogen compounds having 3 or more nitrogen atoms for generation of the nitrogen gas.

18. A method according to any one of claims 1 to 16 wherein the nitrogen compound is selected from the group consisting of NH₃/NH₄ ⁺, hydrazides, and derivatives of hydrazides.

19. A method according to any one of claims 1 to 6 wherein the explosive comprises NH₃/NH₄ ⁺ and at least one other said nitrogen compound, the gassing of the explosive including oxidation of the NH₃/NH₄ ⁺.

20. A method according to claim 19 wherein the other said nitrogen compound is selected from the group consisting of urea, amines, azides, hydrazines, hydrazides, tetrazoles, triazoles, and derivatives thereof.

21. A method according to any one of claims 1 to 20 wherein the temperature at which the reaction occurs is 40 ⁰C or less.

22. A method according to claim 21 wherein the temperature is 25 ⁰C or less.

23. A method according any one of claims 1 to 22 in which the oxidiser is added to the explosive subsequent to incorporation of the nitrogen compound in the explosive.

24. A method for gassing an explosive to sensitise the explosive and/or modify the density of the explosive, comprising reacting at least one oxidiser with at least one nitrogen containing compound in the explosive to generate nitrogen gas, the oxidiser and the compound initially being in different phases of the explosive to one another, and the oxidiser and/or compound diffusing into contact with each other whereby nitrogen gas is generated via oxidation of the nitrogen compound by the oxidiser.

25. An explosive gassed by a method as defined in any one of claims 1 to 24.

26. An explosive comprising at least one oxidiser and at least one nitrogen containing compound, the explosive being formulated for effecting diffusion of the oxidiser and/or the into contact with each other for oxidation of the compound by the oxidiser to produce nitrogen gas from the compound for gassing of the explosive.

27. An explosive according to claim 26 formulated to effect diffusion of the oxidiser into contact with the nitrogen compound.

28. An explosive comprising at least one oxidiser and at least one nitrogen containing compound, the oxidiser and the compound being in different phases of the explosive to one another, and the explosive being formulated for diffusion of the oxidiser and/or the compound into contact with each other for oxidation of the compound by the oxidiser to produce nitrogen gas from the compound for gassing of the explosive.