Explosive

Abstract

 The use of insoluble particulate bentonite for thickening or increasing the viscosity of emulsion explosives is described. The bentonite (swellable clay, montmorillonite) is dispersed throughout the emulsion, without dissolving in either phase and without causing crystallisation, by admixture of the powder with the prepared emulsion in a low shear blender. The resulting emulsions have increased viscosity and rigidity which provide higher gas bubble retention and hence longer shelf life.

Description

[0001] THIS INVENTION relates to an explosive. It relates in particular to the manufacture of an emulsion explosive comprising a discontinuous phase which forms an oxidizing salt-containing component and a continuous phase which is immiscible with the discontinuous phase and which forms a fuel component.

[0002] Such explosives, when the oxidizing salt-containing component contains water and is in the form of an aqueous solution, are known as "water-in-fuel" emulsions, and when the oxidizing salt component contains little or no water, they can be regarded as "melt-in-fuel" emulsions.

[0003] According to the invention, in the manufacture of an emulsion explosive comprising a discontinuous phase which forms an oxidizing salt-containing component and a continuous phase which is immiscible with the discontinuous phase and which forms a fuel component, there is provided a method of thickening or increasing the viscosity of the emulsion which comprises dispersing insoluble particulate bentonite or a derivative thereof in at least one of the components of the emulsion.

[0004] The bentonite or bentonite derivative is preferably added to the emulsion formed after admixture of said components, in a proportion of from about 1,0 to about 5,0% by mass based on the emulsion mass. The Applicant has found that the bentonite or derivative is thereby dispersed through the emulsion without dissolving in either of the components, and without causing crystallization.

[0005] The bentonite or bentonite derivative may be dispersed in the emulsion by admixture of the powder with the emulsion in a low shear blender.

[0006] The bentonite is preferably swellable sodium bentonite which is composed largely of the mineral montmorillonite. It may be of the so-called USA tight-spec type having a degree of dry particle fineness as follows: 90% by mass minimum finer than US Sieve No. 40 and 10% by mass maximum finer than US Sieve No.200. In other words, at least 90% by mass of the bentonite particles may have a particle size less than 425 microns and at most 10% by mass of the bentonite particles may have a particle size less than 75 microns. The average particle size of the bentonite particles may be from about 75 microns to about 425 microns, preferably from about 75 microns to about 350 microns.

[0007] The sodium bentonite may be that which is commercially available in powder form under the trade name MX-80 VOLCLAY WESTERN BENTONITE-13T from the American Colloid Company, and which has a water content of between 5 and 10% by mass, a dry particle fineness of 10% by mass maximum retained on US Sieve No.40 and 10% by mass maximum passing US Sieve No.200, and a wet particle fineness of at least 94% by mass being finer than US Sieve No. 200 and at least 92% by mass being finer than US Sieve No. 325.

[0008] Suitable bentonite derivatives include the quarternary ammonium bentonite salts especially those having at least one long chain alkyl group having at least 10 C atoms. (Commercially available bentones) which are swellable in methanol and are preferably pretreated therewith.

[0009] The discontinuous phase may comprise at least one oxidizing salt selected from the group consisting of ammonium nitrate, alkali metal nitrates, alkaline earth metal nitrates, ammonium perchlorate, alkali metal perchlorates, and alkaline earth metal perchlorates.

[0010] The discontinuous phase may comprise ammonium nitrate with at least one further compound selected from the group consisting of oxygen-releasing salts and fuels which, together with the ammonium nitrate, forms a melt which has a melting point which is lower than that of the ammonium nitrate. Said further compound may be sodium nitrate, calcium nitrate, urea, urea derivatives such as thiourea, or the like. The discontinuous phase may in certain cases comprise water, which is kept to a minimum to avoid wasted energy arising from steam generation, but which is employed to facilitate melting/dissolving of the oxidizing salt component to avoid excessive high processing temperatures during formation of the base emulsion.

[0011] The fuel component of the emulsion may form from 2 to 25% by mass of the emulsion, preferably about 3 to 12% by mass.

[0012] The fuel of the fuel component will be immiscible with and insoluble in water. Preferably, the fuel of the fuel component is non-self-explosive and is selected from at least one member of the group consisting of hydrocarbons, halogenated hydrocarbons and nitrated hydrocarbons. Typically, said fuel comprises at least one wax selected from the group consisting of paraffin waxes, microcrystalline waxes and slack waxes, and it may comprise at least one member of the group consisting of mineral oils, fuel oils, lubricating oils, liquid paraffin, xylene, toluene, petrolatum and dinitrotoluene.

[0013] The fuel may comprise an emulsifier or a mixture of suitable emulsifiers. The fuel component may thus comprise at least one emulsifier selected from the group consisting of sorbitan sesquioleate, sorbitan monooleate, sorbitan monopalmitate, sodium monostearate, sodium tristearate, the mono- and diglycerides of fat-forming fatty acids, soya bean lecithin, derivatives of lanolin, alkyl benzene sulphonates, oleyl acid phosphate, laurylamine acetate, decaglycerol decaoleate, decaglycerol decastearate, 2-oleyl-4,4′-bis(hydroxymethyl)-2-oxazoline, polymeric emulsifiers containing polyethylene glycol backbones with fatty acid side chains and derivatives of polyisobutylene succinic anhydride.

[0014] The emulsifiers act as surfactants and stabilizers to promote the formation of the emulsion and to resist crystallization and/or coalescence of the discontinuous phase.

[0015] The method may include the step of dispersing a density-reducing agent in the emulsion to form an emulsion having a density of from 1,10 to 1,15 g/cm³ at 25°C.

[0016] The density-reducing agent may be selected from the group consisting of microballoons, microspheres and gas bubbles. In one embodiment, the eventual emulsion may thus include glass microballoons, microspheres of polymeric material or another form of density-reducing agent, to provide the emulsion with the final density of 1,10 to 1,15 g/cm³ at 25°C. The emulsion may then comprise up to about 10% by mass of glass microballoons (eg C15/250 glass microballoons available from 3M South Africa (Pty) Limited) or microspheres of a polymeric material (eg EXPANCEL 642 DE microspheres available from KemaNord AB, Sweden), which can further act to sensitize the explosive. Although the mass of microballoons or microspheres included may be up to 10%, it is preferably less than 4,5% by mass, based on the mass of the emulsion to which they are added. In another embodiment, the density-reducing agent may comprise air bubbles in the emulsion. The bubbles can then be mechanically induced eg by physical mixing or blowing, and/or chemically induced, eg by a chemical foaming agent such as sodium nitrite added to the emulsion.

[0017] The invention extends to an emulsion explosive whenever manufactured according to the method described above.

[0018] The invention will now be described by way of example, with reference to the following non-limiting Examples.

EXAMPLES

[0019] Cartridged water-in-oil emulsion explosives were prepared having the following compositions, in which all units are expressed as percentages on a mass basis:

Constituent	Sample 1	Sample 2
Ammonium nitrate	68,60	65,98
Sodium nitrate	12,81	12,32
Thiourea	0,1	-
Water	10,11	9,73
P95 Oil	0,97	0,93
Crill 4 (sorbitan monooleate emulsifier)	1,36	1,31
Paraffin Wax (Aristo)	1,98	1,90
Microcrystalline Wax (BE SQUARE Amber)	1,98	1,90
Bentonite	2,00	2,00
Sodium nitrite (20% m/m aqueous solution)	0,09	-
3M B23/500 Microballoons	-	3,93
TOTAL	100,00	100,00
Cold Density (g/cm³)	1,15	1,15

[0020] The P95 (trade name) mineral oil was obtained from BP South Africa (Proprietary) Limited, and the Crill 4 (trade name) from Croda Chemicals South Africa (Proprietary) Limited. The paraffin wax was Aristo (trade name) wax obtained from Sasol Chemicals (Proprietary) Limited, and the microcrystalline wax was BE SQUARE Amber 175 (trade name) obtained from Bareco Inc. USA. The microballoons were 3M B23/500 (trade name) glass microballoons obtained from 3M South Africa (Proprietary) Limited. The bentonite was MX-80 VOLCLAY WESTERN BENTONITE-13T (trade name) obtained from American Colloid Company, and typically having the following chemical analysis: SiO₂ 60,0-62,0% m/m; Al₂O₃ 21,0-23,0% m/m; Fe₂O₃ 3,0-4,0% m/m; MgO 2,0-3,0% m/m; Na₂O 2,0-3,0% m/m; CaO 0,1-0,7% m/m; K₂O 0,4-0,5% m/m; and having a pH value of 8,5-10,0.

[0021] The amount of water given includes the water used to make up the sodium nitrite solution.

[0022] The emulsion explosives were prepared by forming a premix of water, ammonium nitrate, sodium nitrate, and thiourea at about 80 to 90°C, and a second premix of the microcrystalline wax, paraffin wax, P95 oil and Crill 4 at about 70 to 80°C. The first premix was then slowly added to the second premix with agitation to form a base emulsion. The bentonite was thereafter admixed with the base emulsion in a low shear blender for about 1 minute to provide a thickened emulsion. Samples 1 and 2 were prepared by respectively dispersing the sodium nitrite or the microballoons in the base emulsion in a blender at normal elevated working temperatures, followed by cartridging and rapid cooling. Comparative samples, identical to Samples 1 and 2 save that they did not contain bentonite, were also made up.

[0023] Samples 1 and 2, with and without the bentonite, were tested according to the Stanhope cone penetrometer method (with 150 g cone), and the results obtained are set out in Table I.

[0024] The viscosity of Sample 2, with and without the bentonite, was measured at elevated temperatures, and the results are set out in Table II.

TABLE II

Temperature (°C)	Viscosity (cP)
	Sample 2 without bentonite	Sample 2 with bentonite
30	26 800	76 800
50	17 600	19 600
60	7 600	13 120

[0025] Sample 2, with and without the bentonite, was also tested for minimum initiation (MI) and velocity of detonation (VOD) at 40°C, and the results are set out in Table III. Table III also sets out the results of tests for MI and VOD at 40°C of a further sample termed Sample 3, which is a formulation essentially similar to Sample 2 but which contains 4% by mass of the bentonite based on the mass of the emulsion. In Table III, 'M' indicates a misfire, and '3D', '4D' and '5D' indicate that the explosive could be detonated with a detonator containing 45mg, 90mg, and 180mg pentaethyritol tetranitrate respectively.

TABLE III

SAMPLE	Detonation Characteristics at 40°C
	Initial		After 4 months		After 6 months
	MI	VOD (km/s)	MI	VOD (km/s)	MI	VOD (km/s)
Sample 2 (without bentonite)	3D	4,9	5D	4,7	M8D	-
Sample 2 (with 2% bentonite)	3D	4,7	4D	4,7	5D	4,9
Sample 3 (4% bentonite)	3D	4,8	5D	4,7	M8D	-

[0026] Sample 2, with and without bentonite, was tested for susceptibility to shock crystallization, and the results are set out in Table IV (Temperature Rise on Shocking) and Table V (Bubble Energy after Shocking). Temperature rise on shocking was measured by placing a thermocouple in the centre of a cartridge suspended vertically at 6,7 m below the surface of water. A 150g booster was fired at the same depth at a distance of 2,8m from the cartridge and the resultant temperature rise due to crystallisation was recorded. The average of three results is given in Table IV . Bubble energy after shocking was measured by firing a 150g booster at varying distances from five cartridges suspended vertically at a water depth of 6,7m The cartridges were detonated 13 seconds later and their bubble energy recorded as given in Table V. The same method of shocking was used for Sample 2 (without bentonite and Sample 2 (with bentonite).

TABLE IV

TEMPERATURE RISE ON SHOCKING
SAMPLE	Temperature Rise (°C)	Time taken to Reach Maximum Temperature (s)
Sample 2 (without bentonite)	22,3	360
Sample 2 (with bentonite)	21,0	360

TABLE V

BUBBLE ENERGY AFTER SHOCKING
SAMPLE	Distance from Booster (m)	Bubble Energy (MJ/kg)
Sample 2 (without bentonite)	-	2,10
	5	1,50
	3	1,48
	2	1,45
	1,8	1,40
	1,75	Misfire
Sample 2 (with bentonite)	-	2,10
	5	1,55
	3	1,50
	2	1,45
	1,8	1,40
	1,75	Misfire

[0027] Without wishing to be bound by theory, the Applicant believes that the desired increase in viscosity on addition of bentonite to emulsion explosives is obtained by the bentonite acting on components such as the wax on cooling, thereby causing swelling of these components (indicating modification of the crystal structure thereof) and hence thickening of the emulsion.

[0028] The increased emulsion viscosity provides advantages such as higher degree of gas or air bubble retention and hence longer shelf life.

[0029] The addition of bentonite to emulsion explosives causes an increase in rigidity at all temperatures, but the resistance to softening at high temperatures is increased considerably, as seen from the viscosity (see Table II) and cone penetration values (see Table I).

Claims

1. A method of manufacturing an emulsion explosive comprising a discontinuous phase which forms an oxidizing salt-containing component and a continuous phase which is immiscible with the discontinuous phase and which forms a fuel component, characterised in that insoluble particulate bentonite or bentonite derivative is dispersed in at least one of the components of the emulsion whereby the emulsion is thickened or increased in viscosity.

2. A method as claimed in Claim 1, characterized in that the bentonite is added to the emulsion formed after admixture of said components, in a proportion of from about 1,0 to about 5,0% by mass based on the emulsion mass.

3. A method as claimed in Claim 1 or Claim 2, characterised in that the bentonite is swellable sodium bentonite.

4. A method as claimed in Claim 1, Claim 2 or Claim 3, characterised in that at least 90% by mass of the bentonite particles have a particle size less than 425 microns and at most 10% by mass of the bentonite particles have a particle size less than 75 microns.

5. A method as claimed in Claim 4, characterised in that the average particle size of the bentonite particles is from about 75 microns to about 425 microns.

6. A method as claimed in any one of the preceding claims, characterised in that the discontinuous phase comprises ammonium nitrate with at least one further compound selected from the group consisting of oxygen-releasing salts and fuels which, together with the ammonium nitrate, forms a melt which has a melting point which is lower than that of the ammonium nitrate.

7. A method as claimed in any one of the preceding claims, characterised in that the fuel of the fuel component comprises mineral oil, fuel oil, lubricating oil, liquid paraffin, xylene, toluene, petrolatum, slack wax or dinitrotoluene.

8. A method as claimed in any one of the preceding claims, characterised in that a density-reducing agent is dispersed in the emulsion to form an emulsion having a density of from 1,10 to 1,15 g/cm³ at 25°C.

9. An emulsion explosive whenever manufactured according to the method of any one of the preceding claims.

10. A water-in-fuel or melt-in-fuel emulsion explosive characterised in that the explosive comprises insoluble particulate bentonite or bentonite derivative dispersed therein.