Low-density water-gel explosive composition, production and use thereof

Abstract: Inter alia , a low-density water-gel explosive composition characterised in that it comprises a continuous phase of aqueous inorganic oxidizer salt solution, a thickening agent, optionally a cross-linking agent, fuel and density-reducing agent in an amount sufficient to reduce the density of the composition to less than 0.8 g/cc is disclosed. The composition has good water-resistance and is especially useful in blasting applications calling for lower energy explosives.

Description: This invention relates to a low-density water-gel explosive composition, as well as to the production and use thereof. Conventional water-based slurry or water-gel explosives have been in use for about thirty years, (see, for example, U.S. Patent Nos. 3,249,474; 3,660,181 and 4,364,782). These explosives generally have a continuous aqueous phase of inorganic oxidizer salt dissolved in water, fuel(s) dispersed or dissolved throughout the phase and thickening and cross-linking agents to impart desired rheology. The explosives also generally require density reducing agent for imparting adequate detonation sensitivity. Such reducing agents include air bubbles, which may be entrained during mixing of ingredients, gas bubbles produced in situ chemically, small, hollow, dispersed glass or plastic spheres and other porous, gas-entraining solids, such as expanded perlite. The present invention relates to water-gel explosive compositions of this general type. It more particularly relates to low-density water-gel explosive compositions having a density of less than 0.7 g/ml. The most commonly used explosive is ANFO, which is a blend of ammonium nitrate (AN) prills and fuel oil or equivalent. However, ANFO suffers from the disadvantage that it is not water-resistant and therefore may be used successfully only in dry boreholes, in boreholes that have been dewatered or lined with a water-resistant liner or in packaged form in "wet" applications. These methods of using ANFO in wet applications involve additional time and/or expense. Even in dewatered boreholes, water seepage with time may degrade the loaded ANFO due to dissolution of the AN. ANFO has a relatively low density (from about 0.82 to about 0.9 g/ml) and thus is suited for applications requiring relatively low bulk explosive energy, such as for presplit applications and blasting of "soft" rock, coal or overburden. Even so, the explosive energy of ANFO may exceed that desired for certain "soft" blasting applications such as blasting of coal seams that ideally required less explosive energy than even that produced by ANFO. Such "soft" blasting operations also may be in wet boreholes that require dewatering. Thus a need exists for a very low energy blasting composition that may be used in "soft" blasting applications and in borehole environments that may have some water present, such as in dewatered boreholes that experience seepage. Water-gel explosives satisfy one of these needs in that they generally are water-resistant due to the thickened, continuous aqueous phase thereof. However, the densities thereof are generally much higher than those even of ANFO, generally ranging from 0.9 to 1.6 g/ml and usually well above 1.0 g/ml. Australian Patent no. 550,930 discloses a "melt explosive composition" that may have a density as low as 0.3 g/ml for low energy applications, but these compositions are distinguished from "water-gels" in that the melt compositions are "not diluted by an appreciable amount of water," and therefore have a higher energy per unit weight, and comprise a melt of oxidizer and fuel mixed with oiled ammonium nitrate prills. The present invention provides a low-density water-gel explosive composition which comprises a continuous phase of inorganic oxidizer salt solution comprising water and/or water-miscible organic liquids in an amount of from 10 to 40% by weight of the solution, ammonium nitrate particles in an amount of from 10 to 70% of the composition, a thickening agent, a cross-linking agent, solid and/or water-immiscible organic liquid fuel and a dispersion of chemically-produced gas bubbles in a quantity sufficient to reduce the density of the composition to less than 0.7 g/ml characterised in that the ammonium nitrate particles are in prill form and contain water-immiscible organic liquid fuel. The invention also provides a process for the production of such explosive composition characterised in that it comprises forming the inorganic oxidizer salt solution, pre-thickening the solution with the thickening agent, adding the fuel, dispersing a chemical gassing agent throughout the pre-thickened solution to form a fine, stable dispersion of gas bubbles in a quantity sufficient to reduce the density to less than 0.7 g/ml and cross-linking the thickened solution by the addition of the cross-linking agent, the ammonium nitrate particles in prill form optionally being added to the pre-thickened solution, thus meeting both of the described needs for water-resistance and low energy. Another advantageous characteristic of these compositions for "soft" blasting operations is that they may have relatively low detonation velocities. The inorganic oxidizer salt solution forming the continuous phase of the explosive generally comprises ammonium nitrate in an amount of from about 30 to about 90%, and water and/or water-miscible organic liquids in an amount of from about 10 to about 40%, by weight of the solution. The oxidizer salts are generally selected from ammonium, alkali metal and alkaline earth metal nitrates, chlorates and perchlorates. The preferred oxidizer salt is ammonium nitrate (AN), but calcium nitrate (CN) and sodium nitrate (SN) or other oxidizer salts may be used. The total solubilized oxidizer salt employed is preferably from about 50 to about 86%. As is described below, ammonium nitrate in solid form or ANFO (AN prills and fuel oil) may additionally be added to the compositions. The total amount of water and/or water-miscible liquid present in the composition is generally from about 10 to about 40% by weight of the solution. The use of water and/or water-miscible liquid in amounts within this range will generally allow the compositions to be sufficiently fluid to be pumped by conventional slurry pumps at formulation or mixing temperatures, i.e., about the crystallization temperature (fudge point) of the composition. After pumping, precipitation of some of the dissolved oxidizer salt may occur upon cooling to temperatures below the fudge point, although re-pumpable formulations may experience little, if any, precipitation. The fuel may be solid and/or liquid. Examples of solid fuels which may be used include aluminium particles and carbonaceous materials such as gilsonite or coal. Liquid or soluble fuels may include either water-miscible or -immiscible organics. Miscible liquid or soluble fuels include alcohols, such as methyl alcohol, glycols, such as ethylene glycol, amides such as formamide, urea, and analogous nitrogen-containing liquids. These fuels generally act as a solvent for the oxidizer salt or water extender and, therefore, may replace some or all of the water. Water-immiscible organic liquid fuels may be aliphatic, alicyclic and/or aromatic and either saturated and/or unsaturated. For example, toluene and the xylenes may be employed. Aliphatic and aromatic nitro-compounds may also be used. Preferred fuels include mixtures of normally-liquid hydrocarbons generally referred to as petroleum distillates, such as gasoline, kerosene and diesel fuels. A particularly preferred liquid fuel is No. 2 fuel oil. Tall oil and paraffin oil may also be used. Mixtures of the above fuels may be used. As is described below, the water-immiscible organic liquid fuel is combined with ammonium nitrate prills before it is added to the composition. The fuel is generally present in an amount to provide an overall oxygen balance of from about -10 to about 0 percent. Fuel oil, when used, is normally used in amounts of from about 1 to about 8%, by weight, preferably from about 3 to about 7%, and when used as the sole fuel, is preferably used in amounts of from about 4 to about 6%, by weight. The aqueous fluid phase of the composition is rendered viscous by the addition of one or more thickening agents of the type and in the amount commonly employed in the art. Such thickening agents include galactomannin, preferably guar, gums; guar gum of reduced molecular weight as described in US Patent No 3,788,909, polyacrylamide and analogous synthetic thickeners, flours and starches. Biopolymer gums, such as those described in US Patent No 3,788,909 may also be used. Thickening agents are generally used in amounts ranging from about 0.2 to about 2.0%, but flours and starches may be employed in much greater amounts of up to about 10% in which case they also function importantly as fuels. Mixtures of thickening agents may be used. The thickening agent is preferably used in an amount sufficient to pre-thicken the aqueous solution to a viscosity of at least 500 centipoise (mPas) (Brookfield viscometer, Model HATD, No. 2 HA spindle at 100 rpm) prior to the addition of the density-reducing agent as described below. As is well known in the art, density-reducing agents are employed to lower and control the density of and to impart sensitivity to water-gel explosive compositions. The present compositions preferably employ a small amount, e.g. from about 0.01 to about 0.2% or more, of a chemical gassing agent to obtain a composition density of less than 0.7 g/ml. The compositions according to the present invention preferably have a density of less than 0.6 g/ml. A preferred gassing agent is a nitrite salt, such as sodium nitrite, which chemically reacts in the solution of the composition to produce gas bubbles. Other trace ingredients may also be added to enhance gassing rates or adjust pH. Mechanical agitation of the thickened aqueous phase of the composition, such as obtained during the mixing of the pre-thickened aqueous phase and the remaining ingredients, will result in the entrainment of fine air bubbles by mechanical means. Hollow particles, such as hollow glass spheres, styrofoam beads and plastic microballoons, and porous solids, such as perlite, are also commonly employed to produce a gassified explosive composition, particularly when incompressibility is desired. Two or more of these common density-reducing means may be employed simultaneously. A cross-linking agent is employed in the present compositions. Cross-linking agents for cross-linking the thickening agents are well known in the art. Such agents are usually added in trace amounts and usually comprise metallic ions, such as dichromate or antimony ions. The preferred cross-linking agent is antimony ion, preferably from potassium pyroantimonate, in an amount of from about 0.001 to about 0.1%. To the basic composition described above, solid ammonium nitrate (AN) prills are added in an amount of from about 10 to about 70% of the total composition. Water-immiscible organic liquid fuel may preferably be added to the prills prior to adding the prills to the composition. This is the preferred manner of adding the water -immiscible organic liquid fuel to the composition, because when added separately it tends to fluidize the mixture and thus reduce its viscosity, thereby decreasing the ability of the aqueous phase to entrain air or hold gas bubbles. The explosives may be prepared by first forming a solution of the oxidiser salt and water and/or water-miscible organic liquids (including miscible liquid fuel, if any) at a temperature above the fudge point or crystallization temperature of the solution. Typically, the explosives are prepared at a temperature of at least 10C° above the fudge point. The thickening agent is then added to pre-thicken the solution to a desired degree, preferably to a viscosity of at least 500 centipoise (mPas) (Brookfield viscometer). The density-reducing agent, preferably a chemical gassing agent, is then added and dispersed throughout the pre-thickened solution to form a fine, stable dispersion of air or gas bubbles in a volume sufficient to reduce the density to the desired level. A cross-linking agent is then preferably added to cross-link the thickened solution and impart final desired rheology. Ammonium nitrate particles containing water-immiscible organic liquid fuel are added to the pre-thickened solution and dispersed uniformly throughout the composition. Conventional metering, blending and mixing apparatus may be employed in the above steps, which may be performed in a continuous or batch process. The present invention may be further understood by reference to the following Tables. All mixes in Table 1 were produced at a rate of about 0.75 kg/s (45 kg/min) in a high shear mixer. The ammonium nitrate solution was pre-thickened with guar gum and the gassing agent was injected into the pre-thickened oxidizer solution prior to the mixer. Following mixing, the cross-linking agent (potassium pyroantimonate) and the acetic acid for pH adjustment were added in a second mixer. The AN prills were also added in the second mixer. The detonation results are as shown below. 1 2 3 4 AN (in solution) 53.62 51.59 49.60 47.31 H 2 O 15.60 17.68 19.98 22.27 Guar Gum 0.42 0.45 0.28 0.28 Fuel Oil 4.58 4.46 4.34 4.34 AN Prill 25.00 25.00 25.00 25.00 Cross-linking Agent 0.07 0.07 0.05 0.05 Gassing Agent 0.65 0.64 0.65 0.65 Acetic Acid (60%) 0.06 0.10 0.10 0.10 Oxidizer Solutions Fudge Point (°C) 50 40 30 20 pH 5.0 5.2 5.4 5.4 Viscosity (centipoise) (mPas) 1100 1150 1090 1000 Temperature (°C) 80 55 48 40 Initial Density (g/ml) 0.56 0.62 0.66 0.66 Temperature (°C) 65 45 35 30 Results at 5°C (24 hour storage) Density (g/ml) 0.55 0.65 0.66 0.67 Detonation Velocity (km/s) 75 mm - - - - 100 mm - 2.3 - - 125 mm 2.3 2.6 - 2.4 Minimum Booster, 125 mm, det/fail (g of pentolite) 340g/- 50g/18g - 340g/- Critical Diameter, det/fail (mm) 100/75 125/100 100/75 - 125/100 Results at 20°C Density (g/ml) 0.55 0.65 0.66 0.67 Detonation Velocity (km/s) 63 mm 1.8 1.9 - - 75 mm 1.9 2.3 2.0 2.2 100 mm 2.2 - 2.4 2.4 125 mm 2.5 - 2.4 - 150mm - - - - Table 2 shows a composition in accordance with the present invention that has been gassed to the extremely low density of 0.28 g/ml. Even at this very low density, the composition detonated in larger diameters unconfined and in a steel pipe of 100 millimetres internal diameter. Formulation: AN 49.67 Oxidizer Solution H 2 O 20.00 Fudge Point: 30°C Guar Gum 0.29 pH: 5.0-5.5 AN prill 25.00 Viscosity: 1100 cps (mPas) Fuel Oil 4.34 Temperature: 75°C Gassing agent 0.60 Acetic Acid (50%) 0.05 Cross-linking agent 0.05 Initial density: 0.28-0.5 g/ml Results at 30°C Density (g/ml) 0.28 Detonation velocity (km/s) unconfined cardboard tubes - 340g booster 100 mm Failed 125 mm Low order 150 mm 1.7 Det 292 mm 2.0 Det Results at 5°C, 340g booster 100 mm (confined ) 2.3 Det 150 mm (unconfined) Low order D (km/s) Two week storage results at 5°C 125 mm (confined ) 2.3 Det Table 3 shows a composition according to the present invention that is re-pumpable, i.e. one that remains fluid enough to be transported from its place of production to a storage or blasting site where it is the "re-pumped" from a transportation container into a borehole or storage container. The gassing and cross-linking agents are not generally added to the composition until just prior to delivery into a borehole. Formulation: AN (in solution) 30.00 Urea 10.00 H 2 O 17.73 Guar gum 0.60 ANFO (pre-mixed, 94/6) 40.00 Cross-linking agent 0.07 Gassing agent 1.00 (10/30/60, NaSCN/NaNO 2 /H 2 O) Acetic acid (60%) 0.6 Oxidizer solution: Fudge point (°C) ∼0 pH 4.5 Viscosity (cps) (mPas) ∼5000 Temp ∼20°C Results at 20°C: Density (g/ml) D (m/s) 0.55 100 mm 1550 125 mm 2230 150 mm 2350 Minimum booster, 150 mm, det/fail 90g/50g (At 5°C 150 mm fails) The explosive compositions according to the present invention may be used in a conventional manner. They may be plant-mixed to a desired degree of fluidity and then transported to a blasting site where they may be re-pumped into boreholes following the addition of gassing and cross-linking agents. They may also be mixed on-site in a site-mixed system or truck as is known in the art. Since the densities thereof are below that of water, they are generally loaded into dry or dewatered boreholes. As previously indicated, the compositions are water-resistant and may thus be used effectively in boreholes that may have some water present, such as in dewatered boreholes that experience seepage. They may also be used in packaged form.

Claims: A low density water-gel explosive composition which comprises a continuous phase of inorganic oxidizer salt solution comprising water and/or water-miscible organic liquids in an amount of from 10 to 40% by weight of the solution , ammonium nitrate particles in an amount of from 10 to 70% of the composition, a thickening agent, a cross-linking agent, solid and/or water-immiscible organic liquid fuel and a dispersion of chemically-produced gas bubbles in a quantity sufficient to reduce the density of the composition to less than 0.7 g/ml, characterised in that the ammonium nitrate particles are in prill form and contain water-immiscible organic liquid fuel. A composition as claimed in claim 1 wherein the inorganic oxidizer salt solution comprises water and an inorganic oxidizer salt selected from ammonium, alkali metal and alkaline earth metal nitrates and perchlorates. A composition as claimed in claim 1 or claim 2 wherein ammonium nitrate is the predominant, or preferably the sole, oxidizer salt used. A composition as claimed in any of claims 1 to 3 wherein the thickening agent is guar gum. A composition as claimed in any of claims 1 to 4 wherein the cross-linking agent is an antimony ion, preferably potassium pyroantimonate. A composition as claimed in any of claims 1 to 5 wherein the water-immiscible organic liquid fuel is No. 2 fuel oil. A composition as claimed in any of claims 1 to 6 wherein the inorganic oxidizer salt solution comprises a water-miscible fuel, the water-miscible fuel preferably being selected from alcohols, glycols, amines, and urea. A composition as claimed in any of claims 4 to 7 wherein the inorganic oxidizer salt solution comprises ammonium nitrate in an amount of from 30 to 90%, a cross-linking agent in an amount of from 0.001 to 0.1%, guar gum in an amount of from 0.2 to 2% and fuel including water-immiscible organic liquid fuel in an amount of from 0 to 6%. A process for the production of a low-density water-gel explosive composition as claimed in any of claims 1 to 8 characterised in that it comprises forming an inorganic oxidizer salt solution, pre-thickening the solution with a thickening agent, adding a fuel, dispersing a chemical gassing agent throughout the pre-thickened solution to form a fine, stable dispersion of gas bubbles in a quantity sufficient to reduce the density to less than 0.7 g/ml and cross-linking the thickened solution by the addition of a cross-linking agent. A process as claimed in claim 9 wherein the solution is pre-thickened to a viscosity of at least 500 centipoise (mPas). A process as claimed in claim 9 or claim 10 wherein it further comprises adding and dispersing throughout the pre-thickened solution solid ammonium nitrate particles. A process as claimed in claim 11 wherein the ammonium nitrate particles are in prill form and contain up to 6% water-immiscible organic liquid, by weight of the particles. A method of blasting in an application requiring relatively low bulk explosive energy characterised in that it comprises forming a low-density water-gel explosive composition as claimed in any of claims 1 to 8, and loading the resulting composition into a dry or dewatered borehole.

Inventors: CRANNEY DON H, JACKSON MICHAEL M, LAWRENCE LAWRENCE D, CRANNEY, DON H., JACKSON, MICHAEL M., LAWRENCE, LAWRENCE D.
Applicants: DYNO NOBEL INC, DYNO NOBEL INC.
Application year: 1993
Main classification: C06B47
IPC codes: C06B47, C06B23