ZA200501828B

ZA200501828B - Stemming

Info

Publication number: ZA200501828B
Application number: ZA200501828A
Authority: ZA
Inventors: Geach Howard Harcourt; Immo Eurich Bock
Original assignee: Geach Howard Harcourt; Immo Eurich Bock
Priority date: 2002-09-09
Filing date: 2005-03-03
Publication date: 2007-05-30
Also published as: AU2003259434A1; WO2004023064A1

Description

STEMMING

INTRODUCTION TO THE INVENTION

This invention relates to the stemming of charges in operations and particularly to a method of stemming and to stemming means for use with the method.

BACKGROUND TO THE INVENTION

In blasting an explosive or propellant charge or mixture of both, generally, but not exclusively, in the form of a cartridge, is placed at or near the bottom of a hole drilled into the material (rock or concrete for example) to be broken. This charge is retained in place by some form of stemming which also serves to at least partially seal the hole so that on ignition of the charge the gases and shock waves pressurize the hole and break the material. Where holes are drilled right through the material into free space, stemming is applied at both ends of the holes.

Associated with many of these methods and products are problems such as difficulties in getting the stemming product into the hole and/or retaining it there, as well as excessive gas leakage during initiation of the charges and before significant movement of the rock or other material commences.

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An object of this invention is to provide stemming methods and products that are easy to use and effective in use.

SUMMARY OF THE INVENTION

According to the invention a method of stemming a charge in a hole comprises: i. introducing a predetermined amount of particulate stemming material into the hole; ii. inserting a pressure reduction member into the hole to be adjacent the stemming material; iii. introducing a second predetermined amount of particulate stemming material into the hole; and iv. inserting a second pressure reduction member into the hole to be adjacent the stemming material.

Further according to the invention one of a further charge and a further predetermined amount of stemming material are introduced into the hole to be adjacent to the second pressure reduction member.

Still further according to the invention a plurality of pressure reduction members are inserted in the hole at the entrance thereof.

Still further according to the invention the stemming material is compacted using a suitable elongated compacting member.

Still further according to the invention the stemming material is sand and the sand is introduced into the hole in the form of a loosely bonded member and crushed after introduction using a suitable crushing member.

: CWO 2004/023064 PCT/IB2003/003838

The invention also provides a stemming unit comprising loosely bonded particulate in a generally cylindrical form.

Further according to this aspect of the invention the stemming unit is one of circular and polygonal cross section and has at least one slot in its outer surface.

Still further according to the invention the stemming unit may include a longitudinal passage in the form of one of a slot and a bore for accommodating cartridge lead wires.

The invention also provides a pressure reduction member having one of a circular and polygonal perimeter.

Further according to this aspect of the invention the pressure reduction member may be substantially flat or may be of a hollow conical or frusto conical configuration.

Still further according to the invention the pressure reduction member may have a hole therethrough and/or a slot extending inwardly from the perimeter thereof.

According to one aspect of the invention the pressure reduction member is a disc made of flexible sheet material and has a segment removed therefrom.

Still further according to the invention the pressure reduction member may be of plastics, metal, ceramics or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

An example and embodiment of the invention, described by way of example only follow with reference to the accompanying drawings in which:

Figure 1 is an elevation of a stemming unit according to the invention;

Figure 2 is an end elevation of the stemming unit;

Figure 3 is a side elevation of a pressure reduction member according to the invention,

Figure 4 is a plan of the pressure reduction member;

Figures 5to 7 are schematic sectional elevations of a hole in a working face depicting the use of the stemming unit and pressure reduction member;

Figure 8A & 8B are perspective views of a second embodiment of a pressure reduction member;

Figure 9 is a perspective view of a third embodiment of a pressure reduction member; and

Figure 10 is a perspective view of a fourth embodiment of a pressure reduction member.

DETAILED DESCRIPTION OF THE INVENTION

In Figures 1 and 2 of the accompanying drawings a stemming unit (1) is provided for use with the method of the invention which will be described below.

The stemming unit in this embodiment, is a sand core of elongated right circular cylindrical construction having two ends (2) and the axial length of which may vary from about 50mm up to 500mm or more, depending upon the application and diameter of the hole to be stemmed. The sand cores are manufactured such that its diameter is less than that of the hole to be stemmed, thus allowing for it to be easily inserted into the hole. The sand core generally comprises a mixture of heavy mineral sands, chamotte and silicaceous or quartzitic sands.

However, any suitable particulate material can be used. The sand core also comprises of a combination of particle sizes, the largest in diameter being approximately 1/1 6" of the diameter of the shot hole and the smallest being fines measuring 10micron or less.

A passage (3) in the form of a groove or slot is located in the cylindrical surface 5 (4). This recess extends to approximately the centre (not shown) of the unit (1).

This feature creates a greater surface area, which is designed to expedite the absorption of water into the sand core and it also makes the sand core more susceptible to crushing when axial force is applied by means of a charging stick. This innovation allows dry and therefore lighter sand cores to be transported to the place of use and allows the increase in the mass of the sand core by immersing it in water prior to inserting it into the shot hole. The water absorption capacity of the sand cores gives rise to enhanced friction against the side wall of the shot hole and to increased inertial capacity, both of which characteristics enhance the ability of this stemming system to confine high pressure deflagration gasses within the shot hole. The sand cores are also able to absorb chemicals to suppress either or both dust and fumes that may be generated by the explosive or propellant charges.

This aspect of the design has two further key innovative aspects: a) it facilitates the process of stemming by not subjecting the cartridge lead wires to damaging pressure during the stemming process and, b) it centers the leads in the hole, thereby effectively insulating the wires from rogue currents induced into rock by the blasting process.

This is innovative inasmuch as it performs two distinct functions: a) itis an effective stemming system and, b) it insulates the cartridge wires from induced rogue currents that are associated with the rapid sequential firing of shot holes, particularly underground where conducting ore bodies are present.

The unit (1) is cast from sand using conventional gas or resin casting techniques. Bonding agents used are chosen in both type and volume fo be such that while the unit retains its form during transport and loading (as described hereafter) it may be relatively easily crushed by impact with a suitable tool

Figures 3 and 4 depict a pressure reduction member for use in the stemming method of the invention. This member (5) is of a hollow conical configuration and is made from a suitable plastics material which is substantially rigid but has some flexibility in use. A central aperture (6) is connected to the outer periphery (7) by way of a slot (8).

The stemming unit (1) and the pressure reduction member (5) are used together in a stemming method provided for by the invention and which is depicted in figures 5, 6 and 7 of the drawings.

As shown by figure 5 once a cartridge (11) containing a propellant charge has been placed at the bottom (12) of a hole (13) drilled into the rock face (14) in an underground mining operation, a stemming unit (1) is positioned adjacent this cartridge. The diameter of the stemming unit (1) is such that it slides easily into the hole (13). Conveniently the operation lead wires (15) of the cartridge are positioned into the slot (3) of the unit (1) so that they are not damaged by the positioning of the unit (1) or subsequent operational steps.

Once the unit (1) is in position it is crushed using a suitable charging stick.

Crushing ensures that the hole is properly filled by the sand of the unit.

Thereafter a pressure reduction member (5) is positioned such that the apex thereof points towards the bottom (12) of the hole (13) and it is in contact with the sand (17) of the crushed unit (1). The process is repeated with a second stemming unit (1B) and pressure reduction member (5B).

Further stemming units and pressure reduction members may be added until the hole (13) has been stemmed. Alternatively further charges could also be positioned along the length of the hole (13) and spaced by the stemming units (1) to create a decked charge.

A composite pressure reduction member comprising two or more standard pressure reduction members joined together may be placed at the collar or entrance of the hole to further enhance the efficiency of the stemming system.

The combination of the crushed stemming unit (1) and pressure reduction members (5) is designed to create a composite stemming column that exploits the labyrinth theory of pressure reduction seals in the following manner:

Upon the initiation of the propellant cartridge or explosive, pressure is rapidly generated within the shot hole and this pressure exerts itself upon the sand of the first unit core causing a pressurized fluid bed of sand and grit and water vapour to be formed. This pressurized and fluidized bed then forces itself upon the first pressure reduction member (5). This first member (5) is then forced towards the entrance of the hole, but is equally pressurized by the opposite reaction of the remainder of the stemming column in the hole (13). Additionally the fluidized bed seeks to bypass the resistance of the pressure reduction member (5) by exploiting the gap between the member (5) perimeter and the sides of the hole. The small gap causes an accumulation of sand and grit : 25 particles, with the larger particles locking into the gap between the member and the sides of the drill hole. The gas that escapes while the gap is being obstructed by sand particles of different sizes then leaks into the next zone and starts to fluidise the next sand unit (1). This process repeats itself along the length of the stemming column towards the entrance of the hole (13).

This process also induces reflecting acoustic shockwaves within the hole (13), which encourages the enlargement of micro-fractures that are already present in the material. This method does not seek to prevent totally the progress of gasses through the stemming column towards the entrance of the hole (13), rather it is designed provide a pressure reduction seal which interferes with the escape of these gasses via the hole for a time long enough to have the result that the deflagration or explosion gasses preferentially escape to the atmosphere by entering micro fractures, either naturally present or induced by the drilling process, and splitting or shearing the material open to the free face.

In this manner rock and consolidated materials can be effectively broken using a propellant cartridge system.

The central hole (6) of the member (5) is designed to just allow it to ride around the cart Je leads (15). The effect of this design is to minimize pressure losses through the hole (13) and also to keep the cartridge leads (15) centred within the shot hole. This reinforces and works in conjunction with the slot (3) of the sand unit (1) to reduce pressure on the leads during stemming and to keep them insulated from rogue currents that are induced during sequential blasting operations. The design effect is that the stemming system as whole assists accurate sequential firing by insulating the electronic initiator and the cartridge leads from rogue currents. Centralising the conducting wires also assists in minimising the interference effects of electronic plasma shock waves induced during the blasting process. Such electric interference can cause out of sequence firing, misfires and blow-outs.

Rock and other hard consolidated materials are most effectively broken by the application of rapidly alternating compressive and tensile forces, such as are created by reflecting acoustic shockwaves. This stemming column design generates such shockwaves and together with the effective containment of high pressure gasses within the hole, forcing these gasses to enter and enlarge oo S

CWO 2004/023064 PCT/IB2003/003838 micro-fractures in the material and effect the splitting and shearing of the material towards the free face.

This stemming column design also has the effect, through effective containment of the deflagration gasses, of promoting the complete burning and efficient oxidation of potentially noxious deflagration by-products caused by incomplete oxidation of the fuels in the cartridge.

Other embodiments are envisaged within the scope of the invention and includes other applications and the use of the method with loose sand together with one or more pressure reduction members where the inclination of the hole allows for this.

It will be appreciated that the stemming unit can have any suitable configuration as can the pressure reduction member. For example, as shown in Figure 8A, the pressure reduction member (80) can be a disc made of flexible sheet material and having a segment (81) removed therefrom. In use, the ends (82) of the perimeter on the segment (81) are brought together to make the disc assume a conical shape as shown in Figure 8B. This can conveniently be done by inserting the disc into a hole on the end on a tool (not shown). This configuration has the added advantage that as the material tries to flex back to its original shape it creates a tighter fit in the hole into which it is inserted.

Alternatively, as shown in Figure 9 the pressure reduction member (80) can simply be a disc with a central aperture (91). In a further embodiment shown in

Figure 10, a pressure reduction member (100) can have a disc (101) secured to the apex of a hollow cone (102) with an aperture extending centrally through the disc (101) and cone (102). In the embodiment, the disc (101) serves to stabilise the apex of the cone (102) in a hole while the base of the cone resists ejection on the pressure reduction member from the hole.

Claims

1. A method of stemming a charge in a hole comprises:

I. introducing a predetermined amount of particulate stemming material into the hole;

ii. inserting a pressure reduction member into the hole to be adjacent the stemming material;

il. introducing a second predetermined amount of particulate stemming material into the hole; and iv. inserting a second pressure reduction member into the hole to be adjacent the stemming material.

2. A method as claimed in claim 1 in which a further predetermined amount of stemming material is introduced into the hole to be adjacent to the second pressure reduction member.

3. A method as claimed in either of claims 1 and 2 in which a further charge is introduced into the hole to be adjacent to a pressure reduction member.

4, A method as claimed in any of the preceding claims in which a plurality of pressure reduction members are inserted in the hole at the entrance thereof.

5. A method as claimed in any of the preceding claims in which the stemming material includes sand.

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6. A method as claimed in any of the preceding claims in which the stemming material has a range of particle sizes.

7. A method as claimed in any of the preceding claims in which the stemming material is introduced into the hole in the form of a loosely bonded member and crushed after introduction using a suitable crushing member.

8. A method as claimed in any of the preceding claims in which the stemming material is compacted using a suitable elongated compacting member.

9. A stemming unit comprising loosely bonded particulate material in a generally cylindrical form.

10. A unit as claimed in claim 9 in which the stemming unit is of one of circular and polygonal cross section.

11. A unit as claimed in either of claims 9 and 10 which includes a longitudinal passage for accommodating cartridge lead wires.

12. A unit as claimed in claim 11 in which the passage is in the form of a slot or groove in the outer surface thereof.

13. A unit as claimed in claims 12 in which the recess is generally parallel to the axis thereof.

14. A unit as claimed in claim 11 in which the passage is in the form of a bore therethrough.

15. A unit as claimed in any of claims 9 to 14 in which the particulate material includes sand.

16. A pressure reduction member having one of a circular and polygonal perimeter.

17. A member as claimed in claim 16 which is substantially flat.

18. A member as claimed in claim 17 which is made of a flexible sheet of material and has a segment removed therefrom.

19. A member as claimed in claim 16 which is one of hollow conical and frusto conical.

20. A member as claimed in any of claims 16 to 19 which has a hole therethrough.

21. A member as claimed in claim 20 in which the hole is substantially central.

22. A member as claimed in any of claims 16 to 21 which has a slot extending inwardly from the perimeter thereof.

23. A member as claimed in any of claims 16 to 22 and which is of at least one of plastics, metal and ceramics.

24. A member substantially as herein described and as illustrated in Figures 3,4, 6 and 7 or any one of Figures 8 to 10.

25. A stemming unit substantially as herein described and as illustrated in Figures 1,2, 5 and 6.

26. A method of stemming a charge substantially as herein described and as illustrated in Figures 5,6 and 7.