AN EXPLOSIVE COMPOSITION

Abstract

An explosive composition comprising a reagent that inhibits corrosion of a metal or metal alloy when the explosive composition comes into contact with the metal or metal alloy.

Description

SUMMARY OF THE INVENTION

In general terms the present invention relates to inhibiting corrosion caused by certain types of explosive composition with respect to metals and metal alloys, in particular copper and copper alloys. The invention has applicability in the context of commercial operations where explosive compositions are used, such as mining and blasting operations.

BACKGROUND TO THE INVENTION

Commercial mining and blasting operations frequently use explosive compositions that contain ammonium nitrate. The explosive composition is conveniently used in the form of a watergel or emulsion. These are well known and commonly used forms of explosive composition. The detonation sensitivity of the explosive can be increased by addition of sensitising species, such as gas bubbles or solid agents such as microballoons and microspheres. Gas bubbles may be introduced into the explosive composition using a chemical gassing solution. A gasser solution is an aqueous solution comprising species that will react with one or more components of the explosive composition (usually ammonium nitrate) to generate gas bubbles. Various gasser solution technologies are known in this regard. By way of example, the gasser solution may be an aqueous solution of sodium nitrite. The gasser solution may include other additives that control the rate at which the gassing reaction proceeds as might be required depending on such things as prevailing conditions, e.g. temperature.

The sensitised explosive composition is commonly initiated using one or more initiation devices. The initiation device will typically comprise a detonator, possibly used in conjunction with a booster charge (commonly referred to simply as a booster) in which the detonator is inserted.

Detonators typically take the form of an elongate cylinder (shell) that houses a small explosive charge and componentry required to initiate that charge. The cylinder is manufactured from a metal or a metal alloy, and aluminium, copper, brass (an alloy of copper and zinc) and steel are commonly used. Copper and brass tend to be preferred because of ease of manufacture and because they can provide a detonator shell that has suitably high physical strength to withstand shock and pressure encountered in extreme blasting conditions.

It has been observed however that blasthole conditions such as water, reactive ground, leachants, and physical contact with explosive compositions may cause corrosion of copper and brass detonator shells. Various mechanisms may be responsible for this depending upon the characteristics of the explosive composition, the conditions under which it is being used and the manner in which the explosive composition has been sensitised to render it detonable.

If corrosion of a detonator shell is extensive, the physical strength of the shell can be impaired and this may impact on detonator efficacy. Corrosion may also compromise the integrity of the shell, thereby allowing ingress of external species such as water. In turn, this may cause the detonator to malfunction and misfire. This may have significant time and cost implications. Detonator misfire may also have associated safety issues since an undetonated explosive charge remains in the blasthole.

There is on-going demand for higher mine productivity and the desire to undertake blasting operations in more challenging environments. This might involve such things as longer sleep times and/or blasting in hot and reactive ground. In such situations, the potential for corrosion of detonator shells may actually be increased.

Efforts to address corrosion in this context have included the use of physical barriers that are intended to isolate the outer surfaces of the detonator shell from the environment in which it is used. Such efforts have included providing a lacquer, coating or polymeric sleeve on the exterior of the shell. However, these approaches had limited success on copper and brass shells, the cost may be prohibitive and/or there may be manufacturing difficulties.

It has also been suggested to form the detonator shell of a material that has suitable mechanical properties but that is more corrosion resistant than copper and brass. However, material selection is not straightforward. For example, plastics and aluminium are too soft a material and will result in failure due to shock compression. Steel and other alloys may have suitable physical properties but shell manufacture may be more difficult and the cost may be prohibitive. The use of copper and brass as the material for the detonator shell is therefore still preferred.

Against this background there remains the need to provide an alternative and effective way of addressing corrosion of detonator shells in this context.

SUMMARY OF THE INVENTION

The present invention seeks to address the corrosion problems discussed by providing modified explosive compositions that are less corrosive with respect to metal/metal alloys. The present invention may be applied to inhibit corrosion of copper and copper alloys and this is of particular interest. The invention will therefore be described with particular reference to this. However, the principles of the invention may be more broadly applicable and may be applied to inhibit corrosion of other metals/metal alloys.

Accordingly, in one embodiment the invention provides an explosive composition comprising a reagent that inhibits corrosion of a metal or metal alloy when the explosive composition comes into contact with the metal or metal alloy. Related to this the invention provides components used to produce such explosive compositions, to blasting systems including the explosive compositions in combination with an initiating device and to a method of blasting using the explosive compositions. These various embodiments will be better understood with reference to the following more detailed discussion of the invention in the context of inhibiting corrosion of copper and copper alloys.

Noting the emphasis explained above, embodiments of the present invention are based on identifying reagents that inhibit corrosion of copper and copper alloys (such as brass) when contacted with certain types of explosive composition. Accordingly, the invention provides an explosive composition comprising a reagent that inhibits corrosion of copper and copper alloys when the explosive composition comes into contact with copper or the copper alloy.

Embodiments of the invention relate to the production of such explosive compositions and to components used in the production of such explosive compositions. The invention also relates to blasting systems comprising an explosive composition in accordance with the invention in combination with an initiating device having a copper or copper alloy surface that in use will contact the explosive composition. The initiating device will generally be a detonator. In embodiments of the invention the shell of the detonator may be coated with a functional coating to provide further enhanced corrosion resistance.

In another embodiment the present invention provides a method of blasting in which an explosive composition in accordance with the invention is provided in a blasthole and initiated using an initiation device. In the embodiments of particular interest this will be a detonator with a copper or copper alloy (usually brass) shell.

The invention may also be implemented using a combination of embodiments as disclosed herein.

In describing the invention the expression “explosive composition” refers to an explosive that may be initiated using a conventional initiating system, for example using one or more detonators, possibly in combination with one or more boosters. The explosive composition will invariably include sensitising species. In the present specification such species will be introduced into what is referred to herein as an “explosive precursor”. The term “explosive precursor” is intended to mean a composition that contains stored chemical energy that can be released when the composition is suitably sensitized and detonated. The explosive precursor will usually be ammonium nitrate containing. It may be a conventional emulsion explosive or a watergel explosive formulation. The formulated explosive composition may also contain ammonium nitrate (AN) prill or ammonium nitrate/fuel oil (ANFO) prill. Such formulations are well known in the art.

Reagents useful in the present invention may be referred to simply as a “corrosion inhibitor” since that is the effect achieved in the context of the invention. However, as will be explained, additives that are known to inhibit corrosion of copper and copper alloys in other fields of use may not be useful in the context of the present invention since the chemical make-up of (sensitised) explosive compositions and the corrosion causing species/reactions associated with such compositions and their use can be complex. There are also various factors that influence selection of a suitable reagent. The invention may be implemented using one or more reagents that impart corrosion resistance by the same or different mechanisms.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

DETAILED DISCUSSION OF THE INVENTION

In accordance with the present invention enhanced corrosion resistance as between an explosive composition and an initiating device including a copper or copper alloy that in use will come into contact with the explosive composition may be achieved by modification of the explosive composition to include a functionally active reagent. This is believed to represent a significant departure from previous approaches where the focus has been to provide a protective or passivating coating on the copper or copper alloy itself. In accordance with the invention the manner in which the reagent is provided in the explosive composition may vary depending upon various factors, including how the explosive composition has been sensitised, the type of composition and how it is made, and how the explosive composition is delivered into a blasthole during use.

As foreshadowed, the mechanism by which corrosion takes place may vary depending upon various factors. For example, conventional explosive compositions may contain a number of species that are corrosive to copper and copper alloys. These species may include, but are not limited to, nitrate ions, nitrite ions, ammonia and various amines. Blasthole conditions and the presence of species such as chlorides, sulphates, or leachants may also encourage corrosion.

Ammonium and nitrate ions are present because the explosive compositions to which the invention may be applied invariably contain ammonium nitrate in aqueous form, and possibly as a solid additive in the form prills. The latter may be ammonium nitrate prill or ammonium nitrate in fuel oil (ANFO) prill. When the explosive composition is sensitised with gas bubbles, nitrite ions may be present due to the chemistry of the gasser solution being used. Typically, the gasser solution contains sodium nitrite. In emulsion explosive, emulsifiers are a source of amines.

Ammonia is released from ammonium nitrate containing explosive compositions under alkaline conditions. This may take place when gasser solution is added to the composition since the gasser solution may be basic (a gasser solution based on sodium nitrite typically has a pH of 8-9). Addition of the gasser solution may therefore result in an increase in the overall pH of the system thereby causing release of ammonia. A gas-sensitised explosive composition may therefore exhibit a relatively fast rate of corrosion. Glass microspheres used for sensitisation can also react with ammonium nitrate in an explosive composition to yield ammonia thereby inducing corrosion.

The temperature at which the explosive composition is being used, the presence of other species found in blasthole or mine water and leachants causing pH changes can also influence the rate at which corrosion will take place.

During the corrosion process copper forms copper complexes thereby stripping copper from a surface. In a copper and/or copper alloy structure this may also lead to crack formation due to stress corrosion cracking. One possible reaction involving ammonium nitrate and ammonia with copper is to form tetra-amino copper nitrate (TACN). This is a base catalysed reaction that proceeds as follows:

Cu+NH₄NO₃+NH₃→(TACN)

In accordance with the invention it has been found that it is possible to modify the otherwise corrosive nature of an explosive composition with respect to copper and copper alloys by inclusion in the composition of a suitable reagent.

The reagent used in accordance with the present disclosure may inhibit corrosion of a copper or copper alloy by a direct and/or indirect mechanism. Direct inhibition occurs when the reagent interacts with the metal/metal alloy itself to inhibit corrosion. For example, this may occur by the reagent protecting/passivating the metal/metal alloy thereby reducing the availability of metal/metal alloy for reaction with corrosive species. Indirect inhibition occurs when the reagent influences the properties of the corrosive environment to which the metal/metal alloy is exposed in order to render the environment less corrosive. Thus, the reagent may interact with corrosion causing species in an explosive composition to render them less corrosive. By way of example here reference may be made to gas sensitised explosive compositions in which potentially corrosive gas bubbles are hydrophobic in nature. If the surface of the metal/metal alloy has hydrophobic character, the gas bubbles will be attracted to the metal/metal alloy thereby causing corrosion. In this case it may be desirable to use a reagent that has the effect of changing the surface properties of the gas bubbles in order to reduce their surface hydrophobicity. This will reduce the affinity of the gas bubbles for the metal/metal alloy and in doing so inhibit corrosion. Additionally, or alternatively, the reagent may modify the surface charge of the gas bubbles in order to reduce their affinity for the surface of the metal/metal alloy. In an embodiment one or more reagents are used to provide corrosion inhibition by a combination of direct and indirect mechanisms.

It has been found that benzotriazole (BTA) may be suitable for use as the reagent in the various embodiments of the invention. In embodiments BTA may inhibit corrosion by direct and/or indirect mechanisms. Structurally, BTA consists of benzene and triazole fused ring. BTA is believed to prevent undesirable surface reactions by forming a protective layer on copper and brass. It is surprising that BTA acts as a corrosion inhibitor for copper in AN solutions.

The mechanism by which the complex forms is proposed and illustrated by the following reactions:

Cu(s)+BTA(aq)→Cu:BTA(ads)

where Cu:BTA(ads) stands for BTA adsorbed onto a copper surface. In the presence of oxidants or by anodic polarization it can be oxidised to a protective complex:

Cu:BTA(ads)→Cu(I)BTA(s)+H⁺(aq)+e⁻

From this reaction it can be seen that increase of BTA concentration shifts the reaction towards formation of larger amount of the complex Cu(I)BTA. An increase in pH has also been observed to favour formation of the complex.

The copper complex forms a film on the copper surface that prevents corrosive reactions at the surface due to species present in the explosive composition. Adsorption of the inhibitor on the metal surface and film formation are believed to be important steps in the mechanism of corrosion inhibition.

It may also be possible to use derivatives of BTA as the reagent. For example it has been observed that the introduction of the substituent groups (imidazole and its derivatives) has no effect on the mechanism of the inhibitive action while it does have an influence on inhibition efficiency. Useful derivatives may include substituted BTA derivatives in which substituents are present on the benzene ring but not on the triazole ring.

Examples of other compounds that may be useful as reagent in the context of the present invention include imidazoles, triazoles, mercaptotriazoles, napthotriazoles, mercaptobenzimidazoles, azoles, triazines and tolyltriazines.

The present invention may be implemented using one or more suitable reagents and/or embodiments of the invention, thereby reducing/inhibiting corrosion of a number of different metals or metal alloys that may be in contact with an explosive composition. It may also be possible that a single reagent may be effective with respect to more than one metal or metal alloy. It is possible that the efficacy of the reagent with respect to inhibition of corrosion may be increased by use of it in combination with other compounds.

BTA, useful BTA derivatives and other compounds that potentiate the effect of these are commercially available. BTA may act as both a direct and indirect inhibitor of corrosion. While acting directly on the metal or metal alloy as described above, BTA may also modify the surface properties of gas bubbles thereby indirectly inhibiting corrosion. Without wishing to be bound by theory, this may involve a reduction in the hydrophobicity of the gas bubbles and/or varying the charge of the gas bubbles. The intention is to reduce the affinity of the gas bubbles for the surface to be protected against corrosion. Other reagents that may indirectly inhibit corrosion by the same mechanism include block copolymers, hydrophilic polymers and surfactants.

The amount of reagent to achieve effective results may vary depending upon various factors including the mechanism by which corrosion inhibition is to be achieved, the propensity of the explosive composition to cause corrosion and the prevailing environmental conditions in which the explosive composition is being used. The amount of reagent used may also be influenced by the solubility in the chosen solvent (carrier) and the molecular weight of the reagent. Broadly speaking the amount of reagent may be from 0.0001 to 1 wt % based on the weight of explosive composition.

In embodiments of the invention the reagent is required to be soluble in aqueous solution and the solubility of the reagent may also be a relevant consideration. The chosen reagent should not react with or otherwise interfere with the gassing reaction or the stability or intended function of the explosive composition being gassed. The reagent should not interfere with formation or stability of the explosive composition and it should remain functionally active in the composition once formed. It is possible that the reagent may be included in a non-aqueous carrier depending upon its solubility and the manner in which the reagent is to be incorporated into an explosive composition.

In an embodiment, the reagent may be introduced into an explosive precursor or explosive composition in a component that is used to produce the explosive precursor or explosive composition. It is also possible that the reagent may be introduced by use of a separate component the sole function of which is to introduce the reagent. The explosive composition will comprise an explosive precursor and sensitising species. The reagent may be introduced into the explosive precursor before sensitising species are added to it, for example when the explosive precursor is being made. Alternatively, or additionally, the reagent may be introduced when sensitising species are being included in the explosive precursor. Additionally, or alternatively, the reagent may be introduced into an explosive composition after sensitisation of an explosive precursor has taken place. These possibilities are discussed in more detail below.

In an embodiment the reagent may be provided in the explosive precursor when the latter is being produced. In this case, if the reagent is water soluble, it may be incorporated into the explosive precursor in an aqueous component from which the explosive precursor is made. For example, in the case of an emulsion explosive the reagent may be included in the oxidiser salt solution from which the emulsion is made. To make the emulsion the salt solution and a fuel are mixed in the presence of an emulsifier. A functionally effective amount of the reagent will be used. This approach may be useful for, but is not limited to, explosive compositions that are not gas-sensitised. Such explosive compositions can be sensitized with solid density reducing agents, such as glass microspheres.

In an embodiment the reagent may be included in an explosive precursor or explosive composition during loading of the explosive precursor or explosive composition into a blasthole. In the case of an explosive precursor sensitisation, for example by using a gasser solution, may occur during the loading process. When the explosive composition is an emulsion explosive individual, streams of explosive composition and reagent (typically provided in a suitable carrier) may be delivered using one or more loading hoses for mixing in the hose or as the streams exit the end of the hose. A mixing device may be required if mixing is to take place as the streams exit the loading hose.

In an embodiment the reagent may be introduced into an explosive precursor or explosive composition via an aqueous solution that is used to lubricate delivery of the explosive precursor or explosive composition through a blasthole loading hose. In this case the aqueous solution will be provided as an annular stream around a stream of explosive precursor or explosive composition as it is being pumped through a loading hose. The annular stream acts as a lubricant thereby improving flow of the stream within the loading hose. The use of this type of “water-ring” is known but not the inclusion in the aqueous solution of a reagent to impart corrosion resistance. In this embodiment it is important that the aqueous solution used for lubrication is mixed with and into the stream being pumped. This ensures suitable distribution of the reagent. Mixing may be achieved using a mixing head provided at the end of the loading hose from which the stream emerges. At that point the aqueous annular stream has served its role as a lubricant.

In another embodiment the reagent may be included when an explosive precursor is being sensitised with gas bubbles. In this case the reagent may be included in the gassing solution that is mixed with an explosive precursor in order to generate sensitising gas bubbles and yield an explosive composition. The gasser solution may be mixed with explosive precursor before or during delivery into a blasthole. In the latter case this may be achieved by providing the gasser solution as an annular stream around a stream of explosive precursor as it is being pumped through a loading hose and into a blasthole. The annular stream acts as a lubricant and should be mixed with and into the explosive precursor to ensure an even distribution of gas bubbles when the gassing reaction has taken place.

Related to this there is provided a method of producing a gas-sensitised explosive composition with reduced propensity to cause corrosion of copper and copper alloys, the method comprising adding a gasser solution to an explosive precursor in order to generate gas bubbles in the explosive precursor, wherein the gasser solution comprises a reagent that inhibits corrosion of copper and copper alloys when in contact with the gas-sensitised explosive composition. Also provided is a gas-sensitised explosive composition that has been produced by the method.

An effective concentration of the reagent in the gasser solution may be determined taking into account the concentration of gasser solution that is to be added to the explosive composition. The solubility of the reagent in the gasser solution may also determine the amount of reagent that can be used. The gasser solution may itself be added to the explosive composition in conventional amounts, for example from 0.25 to 2.0 wt. % based on the total weight of the explosive precursor.

Further details are provided below with respect to one embodiment of the disclosure related to the use of a reagent in the gasser solution:

• • 1. A gasser solution for generating gas bubbles in an explosive precursor to provide a gas-sensitised explosive composition, the gasser solution comprising (a) one or more species that will react with one or more species in the explosive precursor to generate gas and (b) a reagent for a metal or metal alloy, preferably for a copper or copper alloy. The corrosion inhibitor should be soluble in the gasser solution.
  • 2. The use of such a gasser solution for generating gas bubbles in an explosive precursor and providing a gas-sensitised explosive composition that exhibits reduced propensity to cause corrosion of a metal or metal alloy preferably of a copper or copper alloy.
  • 3. A method of blasting in which this type of gas-sensitised explosive composition is provided in a blasthole and initiated using an initiation device.

Embodiments of the invention also involve using detonator shells that have been pre-treated to provide a functional coating to assist with corrosion resistance.

In an embodiment, when the reagent acts indirectly by reducing the surface hydrophobicity of gas bubbles, the invention also contemplates pre-treating of a detonator shell to provide functionally active layer(s) in order to achieve corrosion resistance.

With respect to pre-treating the detonator shell, it has been observed that the affinity that the corrosive species have for water may have a significant impact on the corrosion resistance that can be achieved. Thus, it has been found that when conventional gassing solution chemistry is used in an explosive composition, coating the detonator shell with a coating that should provide corrosion resistance but that is hydrophobic in nature can actually increase the rate of corrosion. This is believed to be because the conventional gassing process produces gas bubbles that are themselves hydrophobic in nature and that are therefore attracted to the hydrophobic surface. A high concentration of gas bubbles at the shell surface may accelerate the rate of corrosion notwithstanding the presence of the corrosion inhibitor. However, in this case it may be desirable to provide in the explosive composition (be that via the gassing solution or otherwise) a reagent that has the effect of changing the surface properties of the gas bubbles in order to reduce surface hydrophobicity. In turn this will lower the affinity of the gas bubbles for the hydrophobic coating of corrosion inhibitor provided on surface of the detonator shell.

Thus, in an embodiment of the invention beneficial results may be achieved by the combined effect of providing a hydrophobic coating on the detonator shell and by incorporating a suitable reagent in the explosive composition to reduce the hydrophobic character of the gas bubbles produced during the gassing reaction. In an embodiment the reagent will be present as a component of the gasser solution that is used. An example of a suitable corrosion inhibitor for coating the detonator shell and reagent for inclusion in the bulk of the explosive composition is BTA. In this case the reagent may also passivate any areas of the detonator shell that have not been covered with corrosion inhibitor.

In another embodiment it has been observed that coating the shell of a detonator with a hydrophilic coating can reduce the rate of corrosion when conventional gassing solutions are used. This is believed to be because the gas bubbles produced are hydrophobic in nature and as such will be repelled from the surface of the detonator shell. However, it has also been observed that hydrophilic coatings provided on the detonator shell tend to swell over time in the presence of water. This is likely to occur with long sleep times of detonators loaded in blastholes prior to firing. Swelling of the coating can lead to the presence of pores in the coating. Even though the walls of the coating surrounding the pores may be hydrophilic and water molecules may actually plug the pores, pitting corrosion may still be problematic. This embodiment may therefore only be useful in dry environments or in wet environments where short sleep times are employed to minimise swelling of the hydrophilic coating.

In accordance with another embodiment of the invention in an attempt to mitigate this issue, it may be desirable to provide a multi-layer coating on the detonator shell. Specifically, it may be desirable to provide a first layer of a corrosion inhibitor on the detonator shell, for example a coating comprising BTA. This coating is preferably chemisorbed by the material of the detonator shell. A hydrophilic coating is then applied over the top of that coating. In use the hydrophilic coating will repel hydrophobic gas bubbles. However, if the hydrophilic coating swells and pores develop, the corrosion inhibitor should then prevent corrosive reactions at the surface of the shell. As explained above complex formation is believed to be responsible for preventing corrosive reactions at the shell surface due to species present in the explosive composition.

In this embodiment the layer of corrosion inhibitor provided on the detonator shell may have hydrophobic character and thus attract gas bubbles. It may be desirable therefore to include in the explosive composition a reagent that will reduce the hydrophobicity of the gas bubbles that will be present, as described above.

In an embodiment a lacquer or varnish may be used to provide a coating on the detonator shell to impart corrosion resistance. Preferably, the surface of the shell should be clean before application of the lacquer/varnish. The lacquer/varnish may be applied to the shell by dipping the shell in the lacquer/varnish or by spraying. Various suitable lacquers/varnishes are commercially available and generally include a polymeric resin dissolved in a suitable solvent and dosed with a suitable reagent to impart corrosion resistance. For example, products are available comprising an acrylic ester resin dissolved in toluene with BTA added to impart corrosion resistance.

In an embodiment a lacquer or varnish may be used to provide a hydrophilic coating to the shell of the detonator. Preferably, the surface of the shell should be clean before application of the lacquer/varnish. The lacquer/varnish may be applied to the shell by dipping the shell in the lacquer/varnish or by spraying. Various suitable lacquers/varnishes are commercially available and generally include a hydrophilic polymer provided in a suitable solvent or carrier. Epoxy and acrylic systems may be useful.

The suitability of a particular lacquer/varnish, the preferred method of application and the optimum thickness may be varied to optimise results.

An additional embodiment of the invention is a blasting system comprising a detonator having a shell formed of copper or a copper alloy and an explosive composition in accordance with the invention, i.e. an explosive composition modified to include a reagent that is functionally effective in reducing corrosion as described. In this embodiment outer surfaces of the shell of the detonator may comprises a coating that inhibits corrosion of copper or copper alloy when in contact with the explosive composition. When the explosive composition is sensitised with gas bubbles produced by mixing a gasser solution with an explosive precursor, the gasser solution may comprise a reagent that inhibits corrosion of the detonator shell by reducing the affinity of the gas bubbles for the coating.

In a further embodiment outer surfaces of the shell of the detonator may comprise a multi-layer coating. In this embodiment outer surfaces of the shell of the detonator comprise a multi-layer coating comprising a first hydrophobic layer that is provided on outer surfaces of the detonator shell and that passivates the outer surface with respect to corrosive species present in the explosive composition and a second hydrophilic layer provided over the hydrophobic layer. In this embodiment, no addition of a reagent to the explosive composition is necessary to provide inhibition of corrosion.

Usually, in a blasting operation, one or more detonators are positioned in a blasthole (possibly in conjunction with a booster charge) with explosive composition then being delivered into the blasthole and around the detonator(s). Strictly speaking, to inhibit corrosion of a (copper or brass) detonator shell it is necessary for the corrosion inhibitor to be present in that portion of the explosive composition that is in direct contact with the detonator shell. The invention could be implemented to achieve that by varying the composition of the gasser solution (to include or omit corrosion inhibitor) that is injected into an explosive composition as the explosive composition is being delivered into the blasthole. However, this adds a degree of complexity to the loading operation. Instead, it may be more practical to simply use gasser solution including corrosion inhibitor for the entirety of explosive composition being delivered into a blasthole.

Additional steps may be taken in an attempt to minimise corrosion of the detonator shell. Thus, the detonator shell may be pre-treated with a suitable reagent to provide a protective layer on the shell. The reagent will usually be provided in a suitable carrier. When the detonator is positioned in a detonator well in a booster charge, the solution may surround the detonator in the gap that exists between the outer surface of the shell and the internal surface of the detonator well.

The invention also provides a method of blasting in which an explosive composition in accordance with the invention is provided in a blasthole and initiated using a detonator comprising a shell formed of copper or a copper alloy. The method may utilise blasting systems in accordance with the invention.

In relation to this embodiment it will be noted that in practice there may be some considerable time (sometimes weeks) between loading blastholes (with initiation devices and sensitised explosive composition) and firing of a blast. This may be the case for example when the area being blasted and thus the number of blast holes is large. The present invention may allow detonators to be left in the potentially corrosive environment of a loaded blasthole for extended periods of time without corrosion that would otherwise effect detonator functionality.

The present invention may also find use in extreme ground/blasting conditions that are particularly aggressive with respect to detonator corrosion. The present invention may therefore allow blasting to be implemented in situations that have otherwise proved difficult or impossible.

Embodiments of the invention are now illustrated with reference to the following non-limiting examples.

A standardised accelerated corrosion test was developed to assess the rate of detonator shell corrosion, allowing a comparison of various treatments and reagents.

This accelerated test used was designed to corrode a brass detonator shell within 18-24 hours, using a test solution containing 66 g ANS (50% ammonium nitrate solution) or emulsion, 33 g sodium nitrite solution (30% sodium nitrite) to mimic a commercial gasser formulation, and additional corrosive salts 1 g sodium chloride and 0.150 g sodium sulphate to mimic mine water leachants normally seen in a blasthole. The test conditions are far more aggressive than is normally seen in a typical blasthole.

Dummy detonators (operational printed circuit board, no explosive secondary base charge and non-functional fusehead) were exposed to the aggressive test solution both at room temperature and at 40° C. for several weeks or until the integrity of the detonator was seen to fail. Detonators were continuously monitored both electronically (detonator circuitry functionality) and by visual and microscopic examination of the detonator shell. For the electronic functionality testing of the detonators a logger is used.

Example 1

BTA in Gasser

Detonators were tested using the extremely aggressive test solution described above, both in the presence and absence of BTA (benzotriazole) in the gasser solution.

The non-BTA gasser was prepared by dissolving 30% w/w sodium nitrite in water

For the BTA containing gasser 0.25% w/w of the BTA was dissolved in water (Composition for a 100 g BTA gassing precursor solution: 0.25 g BTA; 99.75 g water). Once the BTA is completely dissolved (slow process due to limited solubility of the compound) sodium nitrite is added to achieve a 30% w/w solution. (Composition for a 100 g gassing solution: 30 g sodium nitrite; 70 g of BTA/water gassing precursor solution above)

The additional corrosive reagents, i.e. chloride and sulphate salts, were added to the gassing solution prior to addition of emulsion. The mixture was then stirred to prepare the suspension.

Three brass shell dummy detonators were placed into the suspension of gasser/emulsion starting the corrosion reaction. The dummy detonators were monitored for performance as described above. In the absence of BTA severe stress corrosion cracking was evident within one day leading to failure of electronic functionality and structural failure of the dummy detonators, whereas the three dummy detonators placed into the BTA containing test solution showed structural integrity and full electronic functionality after 30 days.

Example 2

Coated/Lacquered Detonators

To protect the brass shell dummy detonators against general, pitting and stress corrosion cracking the detonators were coated with hydrophilic and hydrophobic lacquers and combinations thereof. The hydrophobic lacquer contains BTA as a corrosion inhibitor. Coatings were applied using a dipping process allowing the lacquer to cure for a period of 24 hours before applying a second coating. The following lacquer combinations were exposed to the accelerated ANS/gasser test solution, described previously. The lacquered detonator corrosion experiments were carried out at room temperature.

• • a) Dual hydrophilic lacquer combination
  • b) Hydrophilic/hydrophobic (BTA containing) lacquer combination
  • c) Hydrophobic (BTA containing)/hydrophilic lacquer combination

Detonators were continuously monitored both electronically (detonator circuitry functionality) and by visual and microscopic examination of the detonator shell.

Results obtained were as follows:

• • a) Extremely significant pitting and general corrosion, electronic functionality compromised after 4 days.
  • b) Significant pitting and general corrosion, electronic functionality compromised after 7 days.
  • c) No Stress cracking or pitting corrosion and full electronic functionality observed after 30 days. (Detonators fully operational after 40 days. Experiment was stopped.)

Comparative Example

In a separate experiment dummy detonators were washed with a BTA/water solution (Composition for a 100 g solution: 0.25 g BTA; 99.75 g water) and allowed to completely air dry. Three detonators were then subjected to the accelerated corrosion test described earlier (no BTA in gasser). The hydrophobic modified shell appeared to increase corrosion. This is believed to be due to drawing of hydrophobic, corrosive species containing gas bubbles to the metal alloy interface, accelerating the rate of corrosion. Within one day extreme pitting corrosion was observed and the dummy detonators failed electronically. These results shows that BTA physically bonding to a brass metal surface, specifically the copper component, is not sufficient in providing corrosion protection. However when BTA is present in the gasser solution protection against corrosion can be achieved. Without wishing to be bound by theory, it is believed that the BTA in the gassing solution is believed to modify the corrosive gas bubbles by changing surface hydrophobicity and/or charge.

Claims

1. An explosive composition comprising a reagent that inhibits corrosion of a metal or metal alloy when the explosive composition comes into contact with the metal or metal alloy.

2. An explosive composition according to claim 1, wherein the reagent inhibits corrosion of copper and copper alloys when the explosive composition comes into contact with copper or the copper alloy.

3. An explosive composition according to claim 1, wherein the explosive composition comprises an explosive precursor and sensitising species.

4. An explosive composition according to claim 3, wherein the explosive precursor comprises the reagent.

5. An explosive composition according to claim 4, wherein the emulsion precursor is an emulsion produced by mixing an aqueous oxidizer salt solution with a fuel and emulsifier, and wherein the reagent is water soluble and provided in the aqueous oxidizer salt solution.

6. An explosive composition according to claim 3, wherein the reagent is introduced into the explosive composition or explosive precursor via an aqueous lubricant that is used to lubricate delivery of the explosive composition or explosive precursor through a loading hose.

7. An explosive composition according to claim 3, wherein the reagent is included in a gassing solution that is mixed with the explosive precursor in order to generate gas bubbles as the sensitising species.

8. An explosive composition according to claim 1, wherein the reagent inhibits corrosion by a direct mechanism.

9. An explosive composition according to claim 1, wherein the reagent inhibits corrosion by an indirect mechanism.

10. An explosive composition according to claim 1, wherein one or more reagents are used to provide corrosion inhibition by a combination of direct and indirect mechanisms.

11. A blasting system comprising a detonator having a shell formed of a metal or a metal alloy and an explosive composition as claimed in claim 1.

12. A blasting system according to claim 11, wherein the detonator shell is formed of copper or a copper alloy.

13. A blasting system according to claim 12, wherein outer surfaces of the shell of the detonator comprises a coating that inhibits corrosion of copper or copper alloy when in contact with the explosive composition.

14. A blasting system according to claim 13, wherein the explosive composition is sensitised with gas bubbles produced by mixing a gasser solution with an explosive precursor, and wherein the gasser solution comprises a reagent that inhibits corrosion of the detonator shell by reducing the affinity of the gas bubbles for the coating.

15. A blasting system according to claim 14, wherein outer surfaces of the shell of the detonator comprise a multi-layer coating comprising a first hydrophobic layer that is provided on outer surfaces of the detonator shell and that passivates the outer surface with respect to corrosive species present in the explosive composition and a second hydrophilic layer provided over the hydrophobic layer.

16. A method of blasting in which an explosive composition as claimed in claim 1 is provided in a blasthole and initiated using a detonator.

17. A method of producing an explosive composition as claimed in claim 1, wherein the reagent is introduced into an explosive precursor or explosive composition in a component that is used to produce the explosive precursor or explosive composition.

18. A method of producing an explosive composition according to claim 1, wherein the reagent is included in a gassing solution that is mixed with an explosive precursor in order to generate sensitising gas bubbles and yield an explosive composition.

19. A method of producing an explosive composition according to claim 1, which comprises introducing the reagent into the explosive composition or an explosive precursor via an aqueous solution that is used to lubricate delivery of the explosive composition or explosive precursor through a loading hose.