HK1015012B - Method and apparatus for blasting hard rock - Google Patents

Description

Method and device for blasting hard rock

This application is a continuation-in-part application of U.S. patent application No. 08/193233 filed on 21/1/1994.

Technical Field

The present invention relates generally to a method and apparatus for blasting hard rock, and more particularly to a method and apparatus for blasting hard rock with a highly insensitive fuel mixture ignited by a moderately high energy discharge that rapidly generates an expanding gas within a confined area.

Background

Hard rock mining is typically accomplished by mechanical devices such as drill bits and other complex mechanical, chemical explosives such as TNT, and/or electrical blasting methods that employ a high energy discharge across a spark gap to create a plasma from an electric arc. Chemical and electrical blasting methods generate rapidly expanding gases in a confined area at the end of a hole drilled into the rock and thereby fracture the rock. In practice, the electrical blasting method is generally preferred because it is less volatile and generally safer than chemical explosives such as TNT. Whereas chemical explosive materials are susceptible to inadvertent detonation through physical changes, electrical devices simply initiate detonation by connecting to electrical energy, and are otherwise ineffective. The mechanical equipment used in hard rock mining is the least efficient and time consuming, and is often used in combination with blasting.

Electrical blasting methods such as detonating cord and spark gap systems are commonly used to produce a post-detonation exhaust fuel gas. Detonating cord propulsion systems are disclosed in U.S. patent No. 5052272 entitled "firing warheads with hydrogen generated by aluminum fuel powder/water reaction" issued to Lee on 10/1/1991. Lee discloses a method of generating oxygen at high energy efficiency by applying a pulsed power technique to a detonator or detonator sheet and ultimately to a fuel powder-oxidizer mixture. The preferred oxidizing agent for aluminum is water. The apparatus includes a capacitor bank connected to an induction coil. A metal wire is connected to the induction coil and a fast switch. When the switch is closed, electrical energy passes from the capacitor bank through the inductor, the switch and the detonator cord. The total energy of discharge is preferably 0.50-15 kilojoules per gram of aluminum fuel. The discharge duration is between 10-1000 microseconds.

Another related detonating cord blasting system is disclosed in U.S. patent No. 3583766 entitled "apparatus for facilitating the extraction of minerals from the seafloor", issued to small Padberg at 8.6.1991. Specifically, this patent discloses a deep sea search vehicle having a drill pipe inserted into a bore formed in a bed of mine and extending to a sedimentary seafloor. A drill bit is arranged at the lower part of the drill pipe, and a plasma discharge part is arranged above the drill bit. An excitation circuit connects electrical power from a power supply to a thin nickel wire extending through the plasma discharge section. When the switch is closed, a large current suddenly detonates it through the thin nickel wire and creates a large plasma discharge accompanied by a sharp pressure wave. The opening of the plasma discharge allows pressure waves to emerge and create a rapidly expanding and collapsing bubble, accompanied by a simulated explosive shock wave. The bubble expands and collapses propagating sound waves in the form of sharp pressure pulses.

Another related detonating cord blasting system is disclosed in soviet patent No. SU357345A issued to Yutkin, which shows a blasting system having a pair of electrodes and a wire strip for insertion into a rock bore filled with a wet dielectric material such as sand to produce a shock wave when excited. The lead wires are connected to the electrodes and tightened around a dielectric plate. The dielectric plate is installed in the rock hole for explosive operation.

A spark gap or non-detonating cord system is disclosed in U.S. patent 3679007 entitled "impact plasma mining drill bit" issued to 0' Hare at 25/7/1972, which discloses a spark gap probe for drilling deep holes in mines for the discovery of water or oil. The probe has a central electrode spaced from and surrounding an outer electrode, both electrodes being immersed in water. A capacitor with a charging potential of 6000 to 30000 volts (depending on soil conditions) supplies electrical energy to the two electrodes. The electrical energy is rapidly released across the water resistor to generate a large amount of heat, thereby creating an explosive effect. The blast shock wave generated in the water moves downwardly and outwardly, thereby creating a hole into which the mining drill bit falls repeatedly.

U.S. patent No. 4741405 entitled "focused percussive spark charged drill bit with multiple electrodes" issued to Moeny et al on 5/3 1988 discloses a spark gap discharge drill bit for underground mining. The drill bit delivers pulse energy in the range of several thousand joules to 100 kilojoules, or more than 100 kilojoules, to a rock surface at a rate of 1 to 10 pulses per second or more. A drilling fluid such as mud or water assists in propagating the spark energy to the rock surface.

U.S. patent No. 5106164 entitled "plasma blasting method" issued to Kitzinger et al on 21/4.1992 discloses a plasma blasting process for fragmenting rock in hard rock mining practice and specifically discloses a method of using a rapid high energy discharge across the electrodes in an electrolyte. Electrical energy from a capacitor bank is switched to supply 500 kiloamps to a blasting electrode disposed in a hole in the rock face, causing dielectric breakdown of an electrolyte preferably comprising copper sulfate. The electrolyte may be gelled with bentonite or gelatin to make it sufficiently viscous to not leak out of the confined area prior to blasting. The blasting device has a minimum of inductance and resistance in order to reduce power losses and to ensure a rapid discharge of energy into the rock.

Although the electric blasting methods disclosed so far using simple spark gaps and detonating cords produce large discharges from capacitors stored to carry hundreds of kiloamperes of current and involve the use of electrolytes, it is desirable to develop an operable blasting method at a more appropriate energy level. Furthermore, most prior art high voltage methods transfer energy from the capacitor to the detonable conductor or spark gap in an inefficient manner. As a result of the inefficiency of energy transfer, the related art systems require a larger capacitor bank to drive the detonable conductor or spark gap to provide a certain amount of blast energy.

Alternatively, many blasting systems that employ chemical explosives present significant safety concerns due to the sensitive nature of the common explosive materials. Many detonating materials are sensitive to unintended detonations caused by physical shock, electrical charge, and harsh environmental conditions (e.g., high temperatures). In addition, many blasting techniques that employ chemical explosive materials can produce toxic by-products and tend to dust the surrounding rock, which is undesirable in some applications. It would therefore be desirable to develop a method of fracturing rock using a very insensitive and non-toxic explosive that requires only an electrical initiation or ignition of the appropriate energy when used in an electrical blasting system. Such a combination provides a safe, economical and efficient blasting technique that is a more gradual process than high explosive volumes.

It would be desirable and advantageous to combine safer chemical blasting methods and/or electrical blasting methods with mechanical drilling, thereby accelerating and facilitating automation of the drilling/blasting process. Many hard rock mining operations typically involve drilling and blasting operations, and if the two operations are properly combined or combined, it is not necessary to withdraw mechanical equipment from the borehole and insert a separate blasting probe or explosive. Some of the above prior art have attempted to combine drilling and blasting processes in a single piece of equipment, such as U.S. patent No. 3679007 to 0' Hare, U.S. patent No. 4741405 to Moeney, and U.S. patent No. 3583766 to small Padberg none of these prior art systems have successfully combined chemical blasting techniques with mechanical drilling, primarily due to the destructive nature of many chemical blasting techniques.

Summary of the invention

The present invention advantageously fulfills the above and other needs by providing a method and apparatus for blasting hard rock with a highly insensitive fuel mixture ignited by a moderately high energy discharge that rapidly generates an expanding gas in a confined area to fracture the hard rock. The present invention uses an ignition device that is wholly contained within the fuel mixture to couple electrical energy to the fuel mixture. This self-contained ignition device functions as both a switch to connect electrical energy to the fuel mixture and an ignition source for the subsequent exothermic chemical reaction. Moreover, the blasting device is designed to be reusable in both respects and easy to integrate with mechanical drilling equipment.

According to one aspect of the invention, a blasting device for blasting solids, the blasting device having capacitive means for storing electrical energy, wherein the blasting device comprises: a blasting probe including a high voltage electrode and a ground return electrode separated by an insulating tube, the high voltage electrode being switchable to connect with the capacitor means; a metal powder and oxidant fuel mixture connected to the high voltage electrode and the ground return electrode; wherein the metal particles in the metal powder and oxidizer fuel mixture, when subjected to an electrical current delivered by the capacitor means through the high voltage electrode, form one or more fusible metal paths between the high voltage electrode and the ground return electrode, the fusible metal paths providing an electrical resistance matching the electrical energy from the capacitor means (16) to the metal powder and oxidizer fuel mixture such that the amount of dissipated heat is sufficient to initiate an exothermic reaction of the metal powder and oxidizer fuel mixture producing high pressure gas in the defined area in which the explosion is to be conducted.

According to another aspect of the present invention, a method of blasting hard rock with the blasting apparatus described above comprises the steps of: (a) connecting a specified amount of a metal powder and an oxidizer fuel mixture to a pair of electrodes proximate the rock formation, the fuel mixture having a metal content that forms a plurality of fusible metal channels between the electrodes; (b) applying an electrical energy discharge to the fuel mixture; (c) melting a plurality of fusible metal channels in the fuel mixture to form an electrical resistance arc channel between the electrodes, the molten metal channels having a high electrical resistance; and (d) dissipating heat caused by the electrical resistance to the fuel mixture to initiate an exothermic reaction of the fuel mixture that produces a rapidly expanding gas to fracture and fracture the hard rock.

Thus, the present invention can provide a safe and inexpensive method and apparatus for blasting hard rock with a highly insensitive metal powder and oxidizer fuel mixture ignited with a moderately high energy discharge. Furthermore, the blasting technique and associated hardware may be conveniently integrated with a conventional rock drilling rig.

Brief description of the drawings

The above and other aspects, features and advantages of the present invention will be further understood by the following detailed description in conjunction with the following drawings, in which:

FIG. 1 is a schematic illustration of a blasting apparatus of the invention comprising an electrical drive line, conduit means and blasting probe;

FIG. 2 is a cross-sectional view of the electrical blasting probe and catheter apparatus of FIG. 1;

FIG. 3 is a cross-sectional view of the blasting probe of FIGS. 1 and 2 positioned in a borehole;

FIG. 4 is a cross-sectional view of another embodiment of a blasting probe positioned in a borehole;

figure 5 is a schematic view of a blasting apparatus integrated with a rock drilling machine according to the invention:

figure 6 is a partial view of a blasting apparatus integrated in a rock drilling rig with the blasting probe retracted;

figure 7 is a partial view of a blasting apparatus integrated with a rock drilling rig with a blasting probe inserted into a borehole;

fig. 8 is a cross-sectional view of the blasting probe shown in fig. 5, 6 and 7.

Corresponding parts are designated by the same reference numerals in all embodiments shown in the drawings.

Detailed description of the invention

The following description is of the best mode contemplated for carrying out the present invention. This description is not to be taken in a limiting sense, but is made for the purpose of describing the general principles of the invention. The scope of the invention should be determined from the following claims.

Referring now to the drawings and in particular to fig. 1, there is shown an apparatus for blasting hard rock in accordance with an embodiment of the invention and generally indicated by reference numeral 10. The apparatus 10 includes a driver circuit 12 for supplying pulsed high current high voltage energy to a blasting probe 14 via a high voltage conductor 44 contained within a conduit means 13. The blasting probe 14 is adapted to be placed in a rock formation or other solid structure to be blasted. Driver circuit 12 includes a charge storage device or capacitor bank 16, a high voltage source 18, a switching device 20, and an inductive device 25.

In the illustrated embodiment, the capacitor bank 16 includes only one 50 kilojoule capacitor 30 having a capacitance of 830 microfarads. But a plurality of capacitors connected in parallel may also be used. A ground line 32 connects a ground side of the capacitor bank 16 to a ground potential 33. The capacitor bank 16 provides a means for storing a suitably high charge switchably connected to the blasting probe 14 by the conductor 34.

The driver circuit 12 also includes a conventional power supply 18 for charging the capacitor bank 16. The power supply is connected to the capacitor bank 16 by a ground line 22 and a wire 24. The capacitor bank 16 is preferably operated at 10 kilovolts, thereby storing approximately 40 kilojoules. The capacitor bank 16 is connected to the blasting probe 14 by a switching means which preferably comprises a detonation vacuum switch 20 adapted for suitably high pressure operation. Although a detonation vacuum gap switch is used in this embodiment, other high coulomb switches may be used, including a high coulomb spark gap, an igniter, or a heavy duty mechanical closure switch.

Driver circuit 12 also includes an inductive device, which in this embodiment includes a distributed inductance of about 5 microhenries, and is represented in fig. 1 by an inductance 25. The distributing inductor receives current and slows the rate of change of the current supplied to the blasting probe 14. In addition to the distributed inductance (shown as element 25), driver circuit 12 also has a very small distributed inductance (shown as element 27) and a total capacitance of approximately 830 microfarads capable of storing approximately 40 kilojoules and operating at 10 kilovolts.

Referring to fig. 2 and 3, an embodiment of a reusable blasting probe 14 with a catheter device 13 is shown. The blasting probe 14 is mounted on the end of the conduit means 13, preferably a conducting conduit 50, and extends axially therefrom so that the blasting probe 14 and conduit 50 can be inserted into a hole drilled into a rock face. The bursting probe 14 comprises an insulating tube 40 having a high voltage steel electrode 42 at its distal end 43, the electrode 42 being connected to the capacitor bank of the driver circuit by an internally disposed high voltage conductor 44, the high voltage conductor 44 passing through the length of the insulating tube 40 and the conduit means 13. The high voltage conductor 44 is preferably a Kapton insulated copper rod having a diameter of 0.25 inches. The insulating tube 40 is a 1.00 inch diameter G-10 fiberglass tube. A steel terminal pin 46 may be threadedly connected to the insulator tube 40 and serves as a ground return electrode. In the illustrated embodiment, the steel connector insert 46 is similar to a female-on-female connector, wherein one end 48 of the steel connector insert 46 is sized to threadably receive the proximal end 47 of the insulated pipe 40 and the other end 49 of the steel connector insert 46 is sized to threadably receive the conductive conduit 50. The high voltage conductor 44 passes axially through and is insulated from the steel terminal plug 46.

The conduit 50 is preferably a steel tube adapted to engage the connector plug 46 of the blasting probe 14 at one end 51 and to connect to a return cable 54 at the other end 52. The return cable 54 is connected to a ground potential 33. The conduit 50 is preferably a hardened chromium molybdenum steel having an outer diameter of 1.25 inches and an inner diameter of 0.375 inches with a plurality of threaded portions 55. The threaded portion 55 of the steel conduit 50 is particularly adapted to connect and/or engage the steel pipe 50 with the blasting probe 14 and/or the driver line. The high voltage conductor 44 passes inside the steel tube 50 and is connected to a high voltage cable 56 leading to a capacitor bank in the driver line 12.

Hardware for facilitating connection between the catheter/blasting probe unit and the driver circuit 12 includes cable nipples 57, 58, clamping nuts 61, 62 and a suitable insulation protector 64. However, the invention is not limited to this manner of electrical connection, and any suitable electrical connection means may be employed. Moreover, the size of the blasting probe 14 and conduit 50 may be selected to accommodate the particular blasting operation for which they are used. By selecting the size of the blasting probe 14 such that the outer diameter of the connector plug 46 is only slightly smaller than the diameter of the blasthole, a well-defined blasting of the following can be achieved. Furthermore, the overall length of the blasting probe 14 is preferably selected according to the amount of fuel mixture to be used in the subsequent blasting.

The conduit 50 may also incorporate an additional means for defining a subsequent burst adjacent the burst probe 14 in the form of a radially expandable plug 66. Specifically, an elastomeric expansion plug 66 is disposed on the outer surface of catheter 50. The outer diameter of the elastomeric expansion plug 66 is preferably slightly smaller than the diameter of the blast hole (i.e., 1.75 inches outer diameter). The elastomeric expansion plug 66 is adapted to radially expand towards the rock surface of the borehole when radially compressed. In this embodiment, the expansion plug 66 is held firmly against the connector plug 46 when a compressive force is applied by axially forcing a sliding pusher sleeve 67 toward the expansion plug 66 using a hex pusher nut 68. The expansion plug 66 is preferably made of an elastomeric material such as polyurethane or high-durometer rubber and thus expands radially outward toward the rock surface as the hex driver nut 68 is threaded to move the driver sleeve 67 downward.

As shown in fig. 3, the rear end 59 of the burst probe 14 has a connector plug 46 threadably secured to the outer surface of the insulating tube 40 and has an outer diameter slightly smaller than the bore diameter. The front portion 60 of the burst probe 14 has an outer diameter equal to the outer diameter of the insulating tube 40. Due to the non-uniform diameter of the burst probe 14, an annular void area 70 is formed proximate the front portion 60 of the burst probe 14. The void area 70 is reserved for the blasting fluid, which is preferably a mixture of metal powder and oxidizer fuel 72. When the metal powder and oxidizer fuel mixture 72 is present in the annular void region 70, both electrodes of the blasting probe 14 (the high voltage electrode 42 at the tip and the connector plug 46 at the back end) are in electrical contact with a continuous quantity of the electrically conductive fuel mixture 72. The metal particles in the metal powder and oxidizer fuel mixture form a plurality of fusible metal paths between the high voltage electrode 42 and the return ground electrode 46 when subjected to an electrical current delivered by a large capacitor bank. The plurality of metal channels act as a fuse providing high electrical resistance to match the electrical energy from the capacitor bank to the metal powder and oxidant fuel mixture to cause increased heat dissipation to initiate an exothermic reaction of the metal and oxidant fuel mixture which produces a high pressure gas within the pores which fractures the surrounding rock.

The preferred fuel mixer 72 includes a metal or metal hydride in combination with an oxidant. In particular, the fuel is suspended in water in granular form, the water containing a binding agent to prevent the aluminum from falling out. For example, a mixture of 50% water and 50% aluminium with an average particle diameter of about 5 microns and a small amount (e.g. 1%) of a binder such as Knox glue is a suitable fuel mixture for use in the blasting apparatus of the invention. In addition, other metal powders, including, but not limited to, titanium, zirconium, or magnesium, alone or in combination with aluminum, that provide a fast expanding gas with water, are also acceptable fuel blends for the present invention.

The preferred aluminum powder and oxidizer fuel mixture ignites in the range of about 700 c to 1200 c by creating a sufficiently high electrical resistance in the fuel mixture. If there is a sufficient amount of metal particles, a high resistance can be created within the fuel mixture without the need for an external fuse, so that the receptive particles of the fuel mixture form a plurality of metal chains or channels between the high voltage electrode and a ground return electrode. A suitably high current pulse, which is then delivered to the fuel mixture, fuses the chain or channel forming a resistive arc channel, which in turn increases the heat dissipation sufficiently to initiate the exothermic reaction of the metal and oxidant.

The powder blasting device advantageously requires only a suitable amount of electrical energy for blasting to start, which takes only a few milliseconds in between. Thus, a blast is created by the chemical reaction of the metal powder and oxidizer fuel mixture, which is more similar to the controlled combustion process of the fuel than a high explosive blast. The amount of electrical energy required to start the process is preferably about 5% to 15%, most preferably 5% to 10%, of the energy generated by the subsequent chemical reaction of the metal and oxidant. For example, when using a mixture of aluminium powder and oxidant fuel, the blasting device of the invention requires only about 0.7-2.1 kilojoules of electrical energy per gram of aluminium powder. For an annular void region of 10 cm length containing about 40 cc of the aluminum powder and water fuel mixture, successful fuel ignition and rock fragmentation was accomplished by a capacitor energy of only 40 kilojoules operating at about 10 kilovolts.

Fig. 4 shows a further embodiment of the blasting probe 14. The reusable blasting probe 14 essentially acts as a coaxial electrode and includes a centrally disposed high voltage electrode 42 disposed within an insulating tube 40. The insulating tube 40 includes an open proximal end 47 and an open distal end 43 adjacent the forward portion 60 of the blasting probe 14. The centrally disposed high voltage electrode 42 extends beyond the distal end 43 of the insulator tubing 40 and has a flanged end 74 that provides a flange or shoulder 75 against which the insulator tubing 40 rests. The flange end 74 of the centrally disposed high voltage electrode 42 has an outer diameter that is smaller than the diameter of the hole into which the blasting probe 14 is inserted.

The ground return electrode takes the form of a metal sheath 46 disposed on the exterior surface of the insulating tube adjacent the rear portion 59 of the burst probe 14. The rear portion 59 of the burst probe 14 is dimensioned such that only a small clearance is maintained between the outer surface of the metal sheath 46 and the rock surface in the hole. The front portion 60 of the blasting probe is smaller in diameter than the rear portion 59, thereby forming an annular void region suitable for holding a suitable fuel mixture 72 to effect blasting. The front portion 60 of the burst probe 14 preferably has an intermediate value of the bore diameter and the outer diameter of the centrally disposed high voltage electrode 42. The front portion 60 of the blasting probe 14 also has a specified length to create a specified amount of annular void area 70 when the blasting probe 14 is inserted into the borehole.

The return ground electrode 46 and the high voltage electrode 42 remain connected to the annular void region 70 so that the circuit is completed when the annular void region 70 is filled with the conductive fuel mixture 72. In this embodiment, the flanged end 74 of the centrally disposed hv electrode 42 remains connected to the electrically conductive fuel mixture 72 present in the annular gap region 70. Another feature of the illustrated embodiment is a central fueling port 80 in the blasting apparatus 10 for in situ charging of the metal powder and oxidizer fuel mixture 72 into the annular void region 70. Regardless of the housing of the central fueling port 80, the centrally disposed high voltage electrode 42 must be of sufficient diameter to perform the dual function of delivering the fuel mixture 72 to the blast and providing a high current pulse to initiate the blasting operation.

In the event that the annular void region cannot be filled in situ, then a suitable amount of fuel mixture is inserted into the hole prior to insertion of the blasting device of the invention. A non-conductive retaining sleeve or other suitable means may be devised by those skilled in the art for retaining the metal powder and oxidizer fuel mixture in the annular void region adjacent the blasting probe, in which case it may be advantageous to preload the metal powder and oxidizer fuel mixture prior to placing the blasting probe at the blast site.

Figures 5 to 8 show a further embodiment of the invention in which the blasting device is integrated with a conventional rock drilling machine. As shown in fig. 5, the blasting apparatus 10 comprises a driver circuit 12 and a reusable blasting probe 14 connected to a rotary hammer drill 15. The reusable blasting probe 14 is basically a coaxial electrode assembly formed by a metal sheath 46 disposed on the outer surface of the insulating tube 40 or sleeve. The metal sleeve 46 is electrically connected to the capacitor bank 16 in a driver circuit 12 through a high current switch 20. The insulating tube 40 is dimensioned to slide over the drill steel between a drilling position (see figure 6) and a blasting position (see figure 7), the drill steel 42 acting as a ground return electrode.

In the foregoing embodiment, the driver circuit 12 includes a conventional power supply 18 for charging a capacitor bank 16 comprising a 50 kilojoule single capacitor 30 connected to the burst probe 14 by a switching device preferably including a firing true air gap switch 20 for controlling the amount of current from the capacitor bank 16 to the burst probe 14. The driver circuit 12 also comprises an inductive means consisting of a distributed inductance, represented in fig. 5 by the inductance 25. The distributed inductance means receives the current and slows the rate of change of the current supplied to the blasting probe 14. The other elements of the driver circuit are as described above and are not repeated here.

As shown in fig. 6, the burst probe 14 withdraws the drill steel 42 and exits the hole during drilling. Once the drilling process is complete, the blasting probe 14 is inserted into the hole by sliding it under the shaft of the drill steel 42, as shown in figure 7. A hydraulic or pneumatic cylinder 19 may be used to drive the burst probe 14 into position. The metal powder and oxidizer fuel mixture may be introduced into the newly drilled hole through a conduit 80 after the blasting probe 14 is in place, or may be introduced through a separate nozzle prior to sliding the blasting probe into the hole.

Referring to fig. 8, the size and layout of the reusable blasting probe 14 is particularly adapted to produce a prescribed amount of annular void area 70 when inserted into a borehole. The rear portion 59 of the burst probe 14 has a metal sleeve 46 disposed on the outer surface of the insulating tube 40 and thus has an outer diameter that is preferably slightly smaller than the bore diameter. The front portion 60 of the blasting probe 14 has a smaller outer diameter than the rear portion, thereby creating an annular void area 70 adjacent the front portion 60 of the blasting probe 14. The annular void area 70 is adapted to hold a prescribed amount of a suitable working fluid, preferably a metal powder and oxidizer fuel mixture 72, preferably an aluminum powder and water with a binder, to prevent the aluminum particles from escaping. A fuel mixture 72 is disposed within the annular void area 70 near the bottom of the hole and just behind the drill bit of the rock drill. The blasting probe 14 is in operation when the annular void region 70 is substantially filled with the fuel mixture 72 and the metal sleeve 46 and the drill steel 42 are disposed in contact therewith.

When the foot is pushed forward, the blasting probe 14 starts to support the rear of the rock drill bit. In order to provide good confinement for the subsequent blasting, the insulating tube 40, or at least the rear portion 81 thereof, is preferably made of a synthetic rubber material, such as polyurethane or silicone rubber, so that it deforms and/or expands radially sealably towards the rock face in the drilled hole when pressed into or above the hole. In addition, the metal sheath 46 at the rearward end 59 of the blasting probe 14 may include one or more longitudinal slots for radial expansion.

When a pulse of current is applied to the metal sheath of the blasting probe from the driver circuit, the metal powder and the metal particles in the oxidizer fuel mixture melt together to form an electrically conductive arc channel between the metal sheath and the drill steel. As the voltage delivered to the electrodes increases, the arc channel provides an increased resistance, causing increased heat dissipation, eventually initiating an exothermic reaction that produces high pressure gas metal and oxidant within the hole and fractures the surrounding rock. The blasting probe then withdraws the drill steel and the drilling process can be restarted.

It is therefore apparent that the present invention provides a safe and inexpensive method and apparatus for blasting hard rock with a highly insensitive metal powder and oxidizer fuel mixture ignited with a moderately high energy discharge. Furthermore, the blasting technique and associated hardware may be conveniently integrated with a conventional rock drilling rig.

The present invention and its advantages will be understood from the foregoing description, and it will be apparent that various modifications and changes may be made therein without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the foregoing described embodiments being merely exemplary embodiments thereof. For example, while the blasting apparatus described above as being integral with a conventional rock drill is preferably blasted using a coaxial electrode assembly and technique and an oxidizer fuel mixture, as described above, other inert or volatile working fluids and substantially slidable coaxial electrode assemblies may be used.

Finally, the scope of the invention is not limited by the specific embodiments shown and described. The scope of the invention is to be determined by the appended claims and their equivalents.

Claims (16)

1. A blasting device (10) for blasting solids, the blasting device having a capacitive device (16) for storing electrical energy, characterized in that the blasting device comprises:

a blasting probe (14) comprising a high voltage electrode (44) and a ground return electrode (46) separated by an insulating tube (40), said high voltage electrode (44) being switchable to connect with said capacitor means;

a metal powder and oxidant fuel mixture (72) connected to said high voltage electrode and ground return electrode;

wherein the metal particles in the metal powder and oxidizer fuel mixture, when subjected to an electrical current delivered by the capacitor assembly (16) through the high voltage electrode, form one or more fusible metal paths between the high voltage electrode (44) and the return ground electrode (46), the fusible metal paths providing an electrical resistance to match the electrical energy from the capacitor assembly (16) to the metal powder and oxidizer fuel mixture (72) such that the amount of heat dissipated is sufficient to initiate an exothermic reaction of the metal powder and oxidizer fuel mixture (72) producing high pressure gas in the defined area where the blasting is to be performed.

2. The blasting apparatus of claim 1, further comprising an inductive device (25) coupled to the capacitive device (16) to receive the electrical charge delivered by the capacitive device and to control the rate of change of current delivered through the electrodes of the metal powder and oxidizer fuel mixture (72).

3. The blasting apparatus of claim 1 or claim 2, wherein the blasting probe further comprises:

a metal sleeve disposed on the outer surface of the insulating tube near the rear end of the blasting probe, said metal sleeve forming said ground return electrode (46); and

the high voltage electrode is disposed within the insulating tube (40) and extends beyond an end (43) of the insulating tube to which the metal powder and oxidant fuel mixture (72) is connected.

4. The blasting apparatus of claim 1 or claim 2, wherein the insulating tube (40) further defines an annular void region (70) at an outer surface of the insulating tube, the annular void region being adapted to contain the metal powder and oxidizer fuel mixture (72).

5. The blasting apparatus of claim 4, further comprising means (80) for filling said annular void region with a mixture of metal powder and oxidizer fuel.

6. The blasting apparatus of claim 4 further comprising a non-conductive sleeve for retaining said metal powder and oxidizer fuel mixture in said annular void region.

7. The blasting apparatus of any of claims 1, 2, 5 and 6, wherein the metal powder and oxidizer fuel mixture (72) comprises aluminum particles suspended in water by a binder.

8. The blasting apparatus of claim 7 wherein said metal powder and oxidizer fuel mixture comprises a mixture of 50% water, 50% aluminum powder and a minor amount of a binder.

9. The blasting apparatus of any one of claims 1, 2, 5 and 6, further comprising means for confining said blast to said defined area.

10. The blasting apparatus of claim 9 wherein said means for confining said blast to said defined area comprises an elastomeric inflatable member adapted to sealably isolate said blasting probe so as to minimize escape of high pressure gas through a borehole.

11. Blasting apparatus according to any of claims 1, 2, 5, 6, the apparatus being integral with a rock drill (15), the rock drill (15) having an elongate rock drill shank steel (42), characterised in that the insulating tube (40) is adapted to slidably move the elongate rock drill shank steel (42) between a first position, in which drilling is permitted without interference from the insulating tube, and a second position, in which blasting is a blasting position; the ground return electrode includes a metal sleeve disposed on an outer surface of the insulating tube, the metal sleeve being switchable to match the capacitive device.

12. The blasting apparatus of claim 11, further comprising means for selectively moving the insulating tube (40) between the drilling position and the blasting position.

13. The blasting apparatus of claim 12, wherein the insulating tube further defines an annular void region at an outer surface of the insulating tube when configured in the blasting position.

14. A method for blasting hard rock with the blasting apparatus of claim 1, comprising the steps of:

(a) connecting a prescribed amount of a metal powder and an oxidizer fuel mixture (72) to a pair of electrodes (42, 46) proximate the rock formation, the fuel mixture having a metal content that forms a plurality of fusible metal channels between the electrodes;

(b) applying an electrical energy discharge on the fuel mixture;

(c) melting a plurality of fusible metal channels in the fuel mixture to form an electrical resistance arc channel between the electrodes, the molten metal channels having a high electrical resistance; and

(d) dissipating heat caused by electrical resistance to the fuel mixture to initiate an exothermic reaction of the fuel mixture that produces a rapidly expanding gas to fracture and fracture the hard rock.

15. The method of claim 14, wherein said step of applying an electrical energy discharge to said metal powder and oxidizer fuel mixture further comprises matching a prescribed amount of electrical energy to said fuel mixture, said prescribed amount of electrical energy being between 5% and 15% of the energy released by a subsequent exothermic reaction.

16. The method of claim 14 wherein said step of applying a suitably high electrical energy discharge across said metal powder and oxidizer fuel mixture further comprises matching a prescribed amount of electrical energy to said fuel mixture, said prescribed amount of electrical energy being 10% of the energy released by a subsequent exothermic reaction.