[0001] This invention relates generally to a sensitizing composition for use with slurry and emulsion explosives (hereinafter collectively referred to as explosives) to provide sensitization through density reduction.

[0002] The term "base emulsion" is used to refer to a water-in-oil emulsion comprising an inorganic oxidizer salt solution, for example ammonium nitrate, as a discontinuous phase and an organic fuel as a continuous phase.

[0003] A base emulsion component of an emulsion explosive typically has a density in a range 1.3 to 1.4 g/cc. At these densities, the base emulsion is not "cap sensitive" i.e. it is too dense, and therefore too insensitive, to be detonated to explode by the initiating shock produced by a blasting cap.

[0004] To sensitize the base emulsion, to a cap sensitive composition, a density reducing agent is added to reduce the density typically to within a range 0.9 to 1.1 g/cc. [0005] A density reducing agent may be chemical agent for example sodium nitrite, which chemically reacts with the base emulsion to produce density reducing gas bubbles, it may be a mechanical agent, such as a hollow or solid microsphere, or it may be a combination of a chemical and a mechanical agent. [0006] An example of the microspheres used in a density reducing context is isobutane plastic enclosed micro-balloons, each with a particle diameter in a range 10-150 pm.

[0007] Microspheres provide an advantage, over gas bubbles created by a chemical agent, in that they do not coalesce to reduce the sensitivity of the emulsion, and their density reducing effect can be more accurately predicted due to their relatively regular unit dimensions. However this advantage is somewhat mitigated by handling difficulties, due to their small particulate size. Consequently, the microspheres easily can become airborne, creating a dust problem, exacerbated in an underground location, and a respiratory irritant. [0008] The invention at least partially addresses the aforementioned problems. SUMMARY OF INVENTION

[0009] The terms "surfactant" and "emulsifier" (hereinafter the terms are used interchangeably) are used to describe an organic compound that exhibits an amphiphilic characteristic, meaning it has both hydrophobic and hydrophilic properties, which enables a surfactant compound to interface between a hydrophobic organic compound and an aqueous solution to form an emulsion.

[0010] The invention provides a sensitizing composition to sensitize a base emulsion which includes a carrier emulsion, of a liquid non-polar organic fuel, a surfactant and an aqueous solution, and a plurality of microspheres suspended in the carrier emulsion. [0011] The organic fuel may a mineral oil, a vegetable oil or a wax.

[0012] The surfactant may be any suitable surfactant, for example an SMO (sorbutan mono oleate) or PIBSA (polyisobutylene succinic anhydride) based surfactant.

[0013] The aqueous solution may include one or more compounds selected from a group of inorganic salts consisting of ammonium, alkali and alkaline earth metal nitrates, chlorates and perchlorates and a group of organic compounds consisting of organic nitrates, for example MMAN (monomethyl ammonium nitrate), and organic chlorides.

[0014] The aqueous solution may contain at least calcium nitrate. Preferably the aqueous solution is a solution of calcium nitrate and a perchlorate salt, for example sodium perchlorate.

[0015] The sensitizing composition may contain the following components in the following concentration ranges:

(a) calcium nitrate: 40 - 80 % (m/m);

(b) water: 15 - 45 % (m/m); and

(c) the carrier emulsion and the surfactant: 4 - 20% (m/m).

[0016] The microspheres may be either solid, for example polystyrene spheres or beads of vermiculite, or hollow, for example gas filled micro-balloons of glass, plastic, a resinous material or a carbonaceous material. Preferably, the microspheres are isobutane filled plastic micro-balloons of the type manufactured by Akzo Nobel. [0017] The microspheres may be present in the sensitizing composition in a range 0.5% - 10%. Preferably the range is 1 % - 4% (m/m). More preferably 2% (m/m) of the sensitizing composition comprises microspheres.

[0018] The average diameter of the microspheres may be in a range 20-160pm. Preferably the range is 60-80pm.

[0019] The density of the sensitizer composition may be in a range 0.3 to 1.05 g/cc.

[0020] Optionally, the sensitizing composition may include a chemical gassing agent, for example sodium nitrite, to provide gas bubbles in the composition which are functionally supplementary to the microspheres. [0021] The invention further provides a method of producing an explosive composition which includes the steps of:

(a) providing a base emulsion;

(b) providing a sensitizing composition which includes a carrier emulsion, of a liquid non-polar organic fuel, a surfactant and an aqueous solution, and a plurality of microspheres suspended in the carrier emulsion; and

(c) mixing the base emulsion with the sensitizing composition in a respective mass ratio in the range 90:10 to 50:50 to form the explosive composition. [0022] The term "explosive composition" is used hereinafter to refer to a composition which has been diluted to within a density range of 0.4 - 1.35 g/cc, and thus is sensitized to detonate when initiated, either by booster or cap initiation.

[0023] The base emulsion may have a density above 1.28 g/cc. [0024] Preferably, the mass ratio of base emulsion to sensitizing composition is 80:20.

[0025] The organic fuel of the base emulsion and the organic fuel of the sensitizing composition may be the same.

[0026] Preferably, the inorganic oxidizer salt solution of the base emulsion includes ammonium nitrate (AN) whilst the aqueous solution of the sensitizing composition includes at least one salt or compound from the following: an ammonium, alkali or alkali earth metal nitrate, chlorate or perchlorate, an organic nitrate and an organic chloride.

[0027] Preferably the aqueous solution includes calcium nitrate. AN may be excluded from the aqueous solution to render the solution thermodynamically stable relatively to the base emulsion. BRIEF DESCRIPTION OF THE DRAWING

[0028] The invention is further described by way of example with reference to the accompanying drawing which schematically illustrates the process steps in the production of an explosive mixture from the combination of a base emulsion and a sensitizing composition which is in accordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0029] With reference to , Figure 1 , an explosive mixture 10 is produced from the combination of a base emulsion 12 and a sensitizing composition 14 which is in accordance with the present invention. [0030] The base emulsion 12, as hereinabove generally defined, can be any inorganic oxidizer salt solution 16 and organic fuel 18 combination, with a density in a range 1.3 to 1.4 g/cc, emulsified with a suitable emulsifier 20.

[0031] The sensitizing composition 14 comprises a carrier emulsion 22 of a fuel phase 24 and an aqueous phase 26 to which is added the plastic or glass micro-balloons 28. [0032] The fuel phase 24 is a mixture of an organic hydrocarbon fuel 30, for example a mineral oil, and surfactant 32 such as PIBSA and/or SMO. [0033] The aqueous phase 26 is an aqueous solution of calcium nitrate. Optionally, another oxidizer salt for example sodium perchlorate, can be included in the aqueous solution.

[0034] The sensitizing composition 14 contains the following components in the following concentrations:

(a) calcium nitrate: 64 % (m/m);

(b) water: 20 % (m/m);

the fuel phase 14 % (m/m); and

(d) micro-balloons: 2% (m/m).

[0035] The sensitizing composition 14 is mixed with the base emulsion 12, in situ, in the mass ratio (sensitizing composition to base emulsion) of 20:80, to produce the explosive mixture which has a density within the range 0.9 to 1.1 g/cc.