The detonation of explosives compositions releases energy in a number of forms. Two of these types of energy release are particularly important in providing control over the blast to ensure that the size distribution and scatter of the burden is as desired. These are fragmentation energy and heave energy. Fragmentation energy (often referred to as shock energy) determines the ability of an explosive composition to shatter the surrounding media. Blasting hard media, such as rock, requires explosive compositions with relatively high fragmentation energy. Heave energy (often referred to as bubble energy) determines the ability of an explosive composition to move its surrounds.

The desired energy characteristics of an explosive composition will depend on a number of factors, including the geological nature of the medium being blasted. For instance, when blasting soft media, such as overburden or coal, the use of explosive compositions with low fragmentation energy and relatively high heave energy is preferred. If the relative proportion of fragmentation energy is too high, energy will be wasted in creating excessive fines close to the blasthole. This particular problem may not be addressed simply by reducing the fragmentation energy through reduction of the total energy of an explosive composition since, in conventional explosives, such a reduction would also involve reduction in heave energy. The concomitant local reduction in heave energy results in less spread of the burden and increases the difficulty and cost of its collection.

With this background in mind it would be desirable to provide an explosive composition in which it is possible to control the total energy and the partitioning of fragmentation and heave energies.

Accordingly, the present invention provides an emulsion explosive composition which comprises a multiplicity of cavities, each cavity comprising at least one open-ended channel. In accordance with the present invention it has been found that incorporation into an emulsion explosive composition of this kind of cavity enables the partitioning of fragmentation and heave energies to be controlled. Moreover, by varying the characteristics of the cavities, it is possible to tailor the composition to provide the desired balance of fragmentation and heave energies and this balance can be readily varied within and between geological formations and blastholes in order to provide optimum blast performance. For instance, in soft media such as overburden or coal, efficient blasting also requires that the energy should be released relatively slowly, implying low velocities of detonation. If the detonation proceeds too fast, energy will be wasted in creating excessive fines close to the blasthole. The explosive compositions of the present invention are well suited for blasting soft media as they may be tailored to provide high energy density with a low velocity of detonation.

A variety of cavity and channel configurations are possible in practice of the present invention, and each cavity may include one, or more than one, channel. When a single cavity includes a number of channels, each channel may have a different configuration although it is more usual that each channel will have the same configuration.

Irrespective of these configurations, it is important that the cavity includes at least one open-ended channel, i. e. the body of the cavity defines at least one hollow passage.

The configuration of the cavities used in accordance with the present invention is believed to be unique. Conventional cavities define a void space which is fully encapsulated by either the material from which the cavity is made (as in the case of, for instance, glass microballoons) or by the emulsion matrix in which the cavity is incorporated (as in the case of gas bubbles). The cavities used in accordance with the present invention are distinct from these types of conventionally used systems.

The walls of the or each channel are defined by the material from which the cavity is formed. Each channel typically includes two distinct open ends, although more complex channel arrangements may be employed in which the channel includes more

than two distinct open ends. For instance, the channel may take the form of a network of passages which run within the body of a single cavity and which has a number of open ends.

The channel is typically an elongated passage of essentially circular cross-section, although variations on this cross-section are of course possible. For instance, the channel may be elliptical, rectangular or triangular in cross-section. The cross-section of the channel is usually substantially uniform along the length of the channel although this is not essential. Thus, the cross-section may vary along the length of the channel.

Similarly, it is not essential that the open-ends of the channel are of equal cross-section, or indeed of the same cross-sectional shape, although in practice this will typically be so.

Typically, each channel has a minimum dimension in any direction of 1. 5mm. When, as is typical, the channel is essentially circular in cross-section, this minimum dimension corresponds to the diameter of the circular cross-section. In practice the minimum dimension associated with a channel will be taken by reference to the channel cross-section. In other embodiments of the invention, the minimum dimension of a channel in any direction is at least 2mm, for example at least 3mm, at least 4mm or at least 5mm. The maximum dimension associated with a channel will usually be its length, measured between open ends. Typically, the maximum dimension will be at least two times, for instance from at least three to at least ten times, and preferably from at least three times to at least fifteen times, the minimum dimension of the channel.

Usually, the shape of the channel will mirror the external shape of the cavity itself. For instance, when cylindrical cavities are used, the channel or channels are typically also cylindrical, the ends of the channel or channels being open at opposite ends of the cavity. Spiral-shaped cavities are another possibility.

At least some of the cavities present in the emulsion explosive composition of the invention should contain a volume of gas, usually air, i. e. a pocket of gas is present in

at least some channel (s). Typically, at least 25%, for instance at least 50% of the cavities will have channel (s) which contain a volume of gas. The presence of the gas is responsible for sensitisation of the emulsion explosive composition, thereby making the composition detonable. It is believed that these gas pockets act as hotspots, adiabatic compression of which causes sensitising reactions to occur. The interactions between numerous thermal explosions enable the propagation of a detonation wave by supplying the necessary chemical energy to the shock waves. It is preferred that a high percentage of cavities contain a volume of gas since this gas contributes to sensitisation of the emulsion explosive composition. The use of higher percentages of cavities containing gas provides more efficient sensitisation by minimising the amount of cavities required to achieve sensitisation. Preferably, at least 75% of cavities contain a volume of gas.

The extent to which the composition will be sensitised is dependent upon the volume occupied by the cavities in the composition and this total volume will have as a component the volume of gas present in the channel portions of the cavities. This gas volume may be influenced by hydrostatic pressure of the emulsion composition acting on the open ends of the channels and this hydrostatic pressure may have the effect of compressing the gas within the channel. On the other hand, if the body of a cavity is flexible, hydrostatic pressure exerted by the composition on the body will compress the channel thereby counter-acting hydrostatic pressure exerted at the open ends of a channel.

Preferably, the minimum amount of cavities are incorporated in the composition in order to sensitise it. By using the minimum amount of cavities it is possible to retain relatively high density of the resultant composition. Taking the volume of gas present into account, the total volume (voidage) occupied by the cavities in the composition is typically at least 5% based on the total volume of the composition. Usually, the total voidage of the cavities is at least 10% by volume, for instance at least 30% by volume.

Inclusion of an amount of cavities over and above the critical amount required for sensitisation will unnecessarily reduce the density of the composition. In an

embodiment of the invention the composition includes an amount of cavities which is below the critical voidage. In this embodiment the composition would be non- detonable. Such compositions may be transported with ease and subsequently sensitised prior to use with the additional cavities and/or another sensitising agent.

Usually however, the cavities alone are used to sensitise the composition. Other sensitising agents, such as the conventionally used glass microballoons, tend to be very small relative to the cavities described herein and tend to have the effect of increasing the observed velocity of detonation.

The total voidage provided by the cavities may be easily calculated using the following equation: where V is the voidage of the cavities based on the total volume of the emulsion explosive composition, d, is the density of the composition prior to addition of the cavities and d is the density of the composition when the cavities are present. In other words, the total voidage is calculated based on the theoretical maximum density of the composition. The volume of gas present in the channels of the cavities will obviously influenced2 The cavity may be formed from a variety of materials for instance, polymers (such as polyethylene and polypropylene), starch-based material (such as pasta), cellulose-based materials (such as paper, cardboard and wheat, for example in the form of wheat straws), glasses and metals. When paper or cardboard materials are used they are usually wax-coated to minimise absorption of aqueous phase typically present in the emulsion composition. The material functions as a fuel in the emulsion explosive composition.

In an embodiment of the invention the dimensions of the cavity are approximately the same as those of the channel, i. e. the channel is defined by a thin wall of cavity material. Typically, the wall thickness would be about 2mm or less, for instance from

100 to 1000, um. In a preferred embodiment each cavity is typically cylindrical and defines a cylindrical channel. Thus, in this preferred embodiment the cavity may be a cylindrical straw. Polypropylene drinking straws may be used for instance. When straws are used they usually have a diameter of from 2 to 8mm, for instance 3 to 6mm, and a length of up to 100mm. In a preferred embodiment the straw has a diameter of from 4 to 6mm and a length of 40 to 80mm length. The thickness of the wall of the straw is usually from 0.1 to 0.5mm. Similarly, cylindrical-shaped hollow pasta and wheat straws may be used as the cavity. Other configurations, such as spirals and curved tubes (for instance, C-shaped or U-shaped tubes) may be used. Typically, such pasta and straws will have a wall thickness of from 1 to 2mm. Useful (polymer and cellulose-based) straws and pasta are commercially available and may easily be cut to the desired length. These factors make the use of such straws and pasta a very attractive choice.

In one embodiment of the invention a single cavity may comprise a number of distinct channels, each of which is open-ended. Such a unitary cavity may be formed by joining together a number of cavities. For example, the macroscopic shape of the cavity may be cylindrical or substantially cylindrical and comprise a number of cylindrical open- ended channels. Such cavities may be formed by bonding together a number of cylindrical straws. Alternatively, this type of cavity may be formed by extrusion of a polymer using a suitably shaped die which is capable of defining distinct channels when a material is extruded through the die. Other configurations in which the cavity includes discrete cells may be possible. For instance, the cavity may include hexagonal cells and thus be honeycomb in cross-section. This embodiment may be advantageous in terms of conferring improved structural rigidity to a cavity.

The cavity may be formed from a variety of materials provided that, taking into account the chosen cavity/channel configuration, the material confers adequate stability. A degree of stability is required to prevent collapse of the channel prior to and during the incorporation of the cavities into the composition. Stability is also necessary to ensure that hydrostatic pressure exerted by the surrounding matrix composition does not

unduly deform the channel thereby forcing gas out from it and into the bulk of the composition. Some flexibility may be desired to counteract hydrostatic pressure acting on the gas in the channel as a result of the channels being open-ended.

The emulsion explosive composition in which the cavities are present may be any material which when appropriately sensitised may be detonated thereby causing an explosive blast. The composition may be a liquid energetic material comprising oxidiser and fuel molecules homogeneously mixed, forming a non-detonable matrix.

For instance, it may be a liquid material produced by molecular scale mixing of known oxygen releasing agents with organic fuel materials in a common aqueous/non-aqueous solvent. This means it may be a concentrated aqueous or non-aqueous liquid. It may be a solvent-diluted explosive material forming a non-detonable energetic liquid.

Furthermore, it may be an eutectic material of oxidisers with fuels or their melts. It may also be an emulsion matrix comprising an oxygen releasing salt component dispersed in an organic medium forming a continuous phase, or vice versa, stabilised by emulsifier. This may embrace compositions of water-in-oil or oil-in-water emulsions. Preferably, the composition is in the form of a water-in-oil emulsion, melt- in-oil emulsion or melt-in-fuel emulsion. For the sake of convenience the invention will now be described with reference to water-in-oil type emulsions although it will be apparent that the advantages described will be applicable to the other fluid energetic materials.

Suitable oxygen releasing salts for use in the aqueous phase of the emulsion include the alkali and alkaline earth metal nitrates, chlorates and perchlorates, ammonium nitrate, ammonium chlorate, ammonium perchlorate and mixtures thereof. The preferred oxygen releasing salts include ammonium nitrate, sodium nitrate and calcium nitrate.

More preferably the oxygen releasing salt comprises ammonium nitrate or a mixture of ammonium nitrate and sodium or calcium nitrates.

Typically the oxygen releasing salt component of the emulsion comprises from 45 to 95 % w/w, and preferably from 60 to 90 % w/w, of the total emulsion composition. In

compositions in which the oxygen releasing salt comprises a mixture of ammonium nitrate and sodium nitrate the preferred composition is from 5 to 80 parts of sodium nitrate for every 100 parts of ammonium nitrate. Therefore, in the preferred composition the oxygen releasing salt component comprises from 45 to 90 % w/w (of the total emulsion composition) ammonium nitrate or mixtures of from 0 to 40 % w/w, sodium or calcium nitrates and from 50 to 90 % w/w ammonium nitrate.

Typically the amount of water employed in the emulsion is in the range of from 0 to 30 % w/w of the total emulsion composition. Preferably the amount employed is from 4 to 25 % w/w and more preferably from 6 to 20 % w/w.

The water immiscible organic phase of the emulsion comprises the continuous"oil" phase of the emulsion composition and is the fuel. Suitable organic fuels include aliphatic, alicyclic and aromatic compounds and mixtures thereof which are in the liquid state at the formulation temperature. Suitable organic fuels may be chosen from fuel oil, diesel oil, distillate, furnace oil, kerosene, naphtha, waxes such as microcrystalline wax, paraffin wax and slack wax, paraffin oils, benzene, toluene, xylenes, asphaltic materials, polymeric oils such as the low molecular weight polymers of olefines, animal oils, vegetable oils, fish oils and other mineral, hydrocarbon or fatty oils and mixtures thereof. Preferred organic fuels are liquid hydrocarbons generally referred to as petroleum distillates such as gasoline, kerosene, fuel oils and paraffin oils.

Typically, the emulsifier of the emulsion comprises up to 5 % w/w of the emulsion.

Higher proportions of the emulsifying agent may be used and may serve as supplemental fuel for the composition but in general it is not necessary to add more than 5 % w/w of emulsifying agent to achieve the desired effect. Stable emulsions can be formed using relatively low levels of emulsifier and for reasons of economy it is preferable to keep the amount of emulsifying agent used to the minimum required to form the emulsion. The preferred level of emulsifying agent used is in the range of from 0.1 to 3.0 % w/w of the water-in-oil emulsion.

The resultant sensitised explosives compositions in accordance with the present invention may have high energy densities but relatively low velocity of detonation.

The velocity of detonation of the compositions is usually in the range 40 to 70%, for instance 50 to 60%, of the ideal (theoretically calculated) velocity for a given explosives composition. It is believed that the voidage and nature of the voidage in the explosives compositions of the present invention is influential on the velocity of detonation observed. The volume strength of the explosives composition is usually greater than that of straight ANFO.

A further interesting aspect of the compositions of the invention is that relatively low velocities of detonation may be observed even when the compositions are detonated under strong confinement. Usually with emulsion explosives there is a relationship between charge diameter and velocity of detonation. Smaller charge diameters tend to provide lower velocities of detonation. As charge diameter increases, so does the velocity of detonation. At infinite charge diameter, the maximum velocity of detonation would be observed. Explosives which have a velocity of detonation that varies with confinement are said to exhibit non-ideal detonation behaviour.

The invention also provides a method of forming an emulsion explosive composition by dispersing in an emulsion explosive a multiplicity of cavities as described herein.

Usually, the cavities will be evenly distributed throughout the emulsion explosive.

Distribution may be achieved by simple mixing. The characteristics of the cavities used will depend upon such things as the viscosity of the (base) emulsion explosive into which the cavities are to be incorporated and the method used to distribute the cavities in the explosive. If the viscosity is relatively low care must be taken when selecting a suitable type of cavity to ensure that the emulsion does not flow excessively into the channel thereby displacing gas therefrom. Thus, when using low viscosity emulsions, it is preferred to employ channels which have a relatively low minimum dimension. When the emulsion has a higher viscosity, the minimum dimension of the channel is less important since the emulsion does not tend to flow into the channel in

the same way as with lower viscosity emulsions. The size and shape of the cavities should also be chosen with subsequent blasthole loading of the emulsion explosive composition in mind. It may be found that certain types of cavities have an adverse effect on pumping during loading, for instance pump blocking may occur depending upon cavity and pump characteristics.

The emulsion explosive composition of the present invention may be formed conveniently at a central manufacturing facility and, in the case of ammonium nitrate based materials, at or adjacent to an ammonium nitrate plant. Other facilities, such as mobile manufacturing units may also be employed. In general terms the composition is formed by dispersing the cavities in a base composition until a suitable voidage is achieved. The base composition is usually prepared in advance and then transported to on-site where it is sensitised before blast-hole loading takes place.

The multiplicity of cavities may be dispersed in the composition at the site at which the composition is formed and then transported to the blast sites. The composition may transported by any convenient and permissible means. Suitable means for transport include standard tankers for the transport of fluids, rubber bladders and the like.

Preferably the transport includes a suitable pump for the transfer of the composition.

The composition may be manufactured by any convenient means, either before or during the dispersing of the multiplicity of cavities therein.

For mixing cavities in a relatively high viscosity fluid, the equipment which may be used includes: 1. High Speed Pin Blenders or Mills 2. Single or Duplex Paddle Blenders 3. Various Ribbon Blenders 4. Stirring Pots 5. Stationary Mixing Devices

The feeding of the cavities into mixing equipment may be achieved by augers or other volumetric feeders. In many instances it is advantageous to utilise the weighing belts mass feeding. Pneumatic air conveying methods are also possible.

In an embodiment, the composition may have incorporated therein additional energetic materials to control the energy output of the ultimate explosive composition. It is preferred that the incorporation of additional energetic materials be at the blast site, at or about the time at which the composition is sensitised. The energetic material may have an impact on the energy density of the explosives composition and on the velocity of detonation observed. The energy density will typically increase and the velocity of detonation typically decrease. This should be taken into account when an additional energetic material is included.

It is preferred that the composition and, in particular, water-in-oil emulsions are mixed with substances which are oxygen releasing salts or which are themselves suitable as explosive materials. For example, a water-in-oil emulsion may be mixed with prilled or particulate ammonium nitrate and/or ammonium nitrate/fuel oil mixtures and/or finely divided aluminium. It is preferred that prilled ammonium nitrate having a particle size in the range of from 2 to 12 mm, preferably 2 to 5 mm in diameter be used. When ammonium nitrate is used the amount present is usually less than 40% by weight, for instance, from 10 to 30% by weight, based on the total weight of the formulated explosives composition.

In another aspect the present invention provides a method of loading a blasthole comprising the steps of : a) preparing an emulsion explosive composition, as described herein ; and b) loading the composition into the blasthole.

The composition may be loaded into the blasthole by any convenient means.

Sufficiently fluid compositions may be pumped by pumps such as progressive cavity pumps, rotary lobe pumps (rubber rotor) through plastic or rubber hoses of various

diameter/length depending on type of boreholes or applications. The thicker and drier blends may be augured into the boreholes. It may also be possible to load by gravity utilising the concrete mixer type trucks.

The present invention provides an explosives composition whereby the rate of energy release may be controlled by the incorporation of the multiplicity of cavities as described above. It is particularly advantageous that the rate of energy release be controlled so that the explosives composition may be tailored to suit the particular geological environment in which the blast is to occur. This enables the explosives composition to be manufactured to more particularly meet the geological blast pattern design and customer requirements. Advantageously, the composition of the present invention has a predisposition to detonate at very low velocities of detonation.

Additionally, it is possible to vary the total energy output of the explosives composition of the present invention whilst maintaining a low velocity of detonation. This may be achieved by varying the proportion of energising solids which are incorporated. The solid energising materials such as ammonium nitrate and/or ANFO and/or aluminium as described above may be employed to increase the total energy. It has been found that the use of larger, porous particles of ammonium nitrate may obtain further reductions in velocity of detonation.

The present invention will now be exemplified by the following non-limiting examples.

EXAMPLE 1 Open-ended (air-filled) polypropylene tubes (PPT) with length 80 mm, diameter 6 mm and wall thickness 0.2 mm were mixed into an emulsion composition at various concentrations by volume. The emulsion contained 94% oxidizer solution (80% ammonium nitrate, 20% water) and 6% fuel blend (70% distillate and 30% PiBSA- ethanolamine derivative). The mixture was fired in an unconfined cardboard tube 1 m long and of varying diameter. The following table shows the characteristics of the composition and the observed velocity of detonation.

COMPARATIVE EXAMPLE 1 Example 1 was repeated except that the emulsion composition was sensitised using a variety of different sensitising agents. The sensitising agents were glass microballoons (GMB) 0.03-0.20mm in diameter or polystyrene spheres (PS) 2mm in diameter. The characteristics of each composition and the velocity of detonation observed are shown in the following table. Diameter of d1 d2 V Velocity of Sensitising cardboard tube (g/cm3) (g/cm3) (%) detonation (km/s) agent (mm) 200 1. 35 1. 09 19 2. 40 PPT 200 1. 35 1. 12 17 2. 20 PPT 200 1. 35 1. 10 18 5. 63 GMB 200 1. 35 1. 13 16 4. 26 PS 180 1. 35 1. 13 16 2. 00 PPT 150 1. 35 1. 06 22 3. 91 PS 150 1. 35 1. 09 19 1. 80 PPT 120 1. 35 1. 07 21 3. 17 PS 120 1. 35 1. 15 15 1. 40 PPT V, dl and d2 are as defined in the formula included earlier for calculating voidage.

The results show that the explosive compositions in accordance with the present invention have a significant and surprising effect on detonation characteristics. For instance, using a cardboard tube of approximately equal diameter the glass microballoons when used at a total voidage of 18% gave a relatively high velocity of detonation (VOD) of 5.63 km/s. The polystyrene spheres at a total voidage of 16% gave a VOD of 4.26 km/s. In contrast, the use of the open-ended polypropylene straws gave significantly reduced VOD when used at approximately the same voidage. With the straws a VOD as low as 2.20 km/s was observed. The same kind of results are observed when one compares the VOD results for the polystyrene spheres and the

polypropylene straws when detonated in smaller diameter tubes. Interestingly, the straws gave lower VOD even when their total voidage was less than that provided by the spheres. The effect of using open-ended cavities on detonation performance is quite unexpected.

COMPARATIVE EXAMPLE 2 Example 1 was repeated except that the emulsion composition was sensitised using glass microballoons (0.03-0.20mm diameter) to a voidage of about 5% (the density after sensitisation was 1.28 g/cm3). On detonation, in a steel pipe of 60mm diameter, a velocity of detonation of 6.0 km/s was observed.

In these examples the velocity of detonation of the compositions was measured utilising an optical fibre method. In this method two lengths of optical fibre with clean cut ends were inserted a known distance apart (typically 100mm) into the explosive under test. The other ends of the optical fibres were connected to the terminals of an electric timer which is capable of timing light pulses which are generated at the detonation front of the tested explosive, from a start and stop signal. The optical fibre located in the explosive charge closest to the detonator provides the start signal for the timer. The second optical fibre at a known distance (100mm) stops the timer. The timer times the light pulse from the detonation front as it passes the start and stop optical fibres and displays the time in milliseconds. The velocity of detonation is calculated form the time taken for the detonation front to pass from the first to the second fibre.

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.