Field of the Invention

The present invention relates to mining, and more specifically to stemming plugs for blocking off mining blast holes.

Background to the Invention

"Stemming" describes both the inert material, and the act of placing inert material into a blast hole to contain the blast gases as much as possible on detonation. Stemming relies on friction, cohesion, or bridging of the stemming material to prevent rifling out of blast holes. Without stemming, blast holes remain open and the explosives on detonation will seek the path of least resistance, which is normally out through the open collar of the blast hole in which the explosives were placed. Resistance is desirable to make the explosives more efficient. The more resistance that can be put into a blast hole to contain the explosives, the more work the gases generated by the explosive will do in breaking the rock material around the hole on detonation.

Typically in open pit mining, blast holes are stemmed with drill cuttings. These are shovelled in on top of the explosives and the weight of those drill cuttings provides resistance to the explosives on detonation. The advantage in open pit mining is of course that the holes are vertical in a downward direction, making the act of stemming them very easy.

In contrast to this, most underground blast holes are vertical in an upward direction (termed "up holes"). Therefore stemming of up holes is typically either not carried out, or is carried out by inferior products in comparison to the effectiveness of stemming open pit holes.

Some mines carry out benching operations which use down holes, and in some instances these holes are open at the bottom of the hole where it breaks in to existing openings. In this instance the stemming arrangement provided can also be used to stem the bottom of the hole. A major challenge in stemming blast holes is that the hole size varies by up to 10% in diameter, depending on whether a new or worn and resharpened drill bit has been used.

Prior art approaches to stemming blast holes are all significantly different from the present invention. They primarily take the form of:

• Rubber or plastic caps that are pushed into the hole and provide very little effectiveness other than to ensure the explosives stay in the hole;

• Expansion foams, generally of a two component mix or sprayed from a can, many of which are toxic and provide little resistance in the blast hole;

• Wedge type arrangements; and ,

• Inflatable sleeve (packer) configurations.

Additionally, there is the Stempac stemming plug sold through Dyno Nobel, which is inserted with an insertion tool. The Stempac plug is basically a clothing sock filled with aggregate, which is compressed by the insertion tool so that it maintains its position in the hole.

A few examples of prior art patent applications for stemming plugs are: KR20090068697A (2007)

This Korean patent specification describes a bidirectional wedge arrangement 100 with guide wings 121. The arrangement includes a top wedge 110 and a bottom wedge 120 which are symmetrical, but face in opposite directions. The guide wings 121 are intended to centre the arrangement in the blast hole.

RU2329463 (2006) This Russian patent specification describes a shortened monolithic stemming plug, which includes a male inner conical element made from plastic or hardboard, and is mounted with its tip facing upwards onto a bed of granulated polystyrene which fills the void between it and the explosives charge. Concrete is then poured into the collar of the blast hole around the conical element, and allowed to cure.

US6324980 (1999) This US patent specification describes a conical plug 1 which is folded and clipped together to fit in the blast hole. A release weight 11 is then lowered down the hole which breaks the clip and causes the conical wedge to spring open and lock in the hole. It is only suitable for surface down holes.

US5936187 (1997) This US patent specification describes a stemming plug which is cup-shaped, made out of a durable, resilient material - PVC, urethane, rubber or the like. It is designed for stemming surface down holes.

The poor performance of commercially available prior art stemming plugs for up holes at present leads most mines to not stem up holes at all. This results in higher explosive use (and therefore cost), poor blast fragmentation, greater noise and vibration, increased damage to surrounding infrastructure, and less effectiveness of the explosive charge than would be the case with a suitable stemming. The present invention was developed with a view to providing an improved stemming plug that is particularly suited for overhead blast holes (up holes) in underground mining, and which is less susceptible to the disadvantages of the prior art noted above. It can be more easily installed and provides greater resistance during blasting. It will be apparent that the improved stemming plug can also be used in down holes, and is not restricted to underground mining. References to prior art in this specification are provided for illustrative purposes only and are not to be taken as an admission that such prior art is part of the common general knowledge in Australia or elsewhere.

Summary of the Invention

According to one aspect of the present invention there is provided a stemming plug for stemming a blast hole in a mine, the plug comprising: a rolling member; an elongate plug body adapted to be slidably received in a blast hole, the plug body having a cavity formed therein intermediate its length and open to one side of the plug body, the cavity being adapted to hold the rolling member therein; and, the cavity being formed with an internal surface along which the rolling member can roll when it engages with a wall of the blast hole, the internal surface being angled relative to a longitudinal axis of the plug body; whereby, in use, during installation the rolling member is captive within the cavity and is held within an outer circumference of the plug body so as not to engage with a wall of the blast hole, whereas, upon activation, the rolling member engages with a wall of the blast hole and rolls along the angled internal surface until it wedges between the angled internal surface and the wall of the blast hole so as to lock the stemming plug in place. Preferably the rolling member has a substantially circular cross-section in at least one plane. Advantageously the rolling member has a flattened spherical or wheel-shaped cross-section. Preferably a contact profile of the rolling member about the circumference of the rolling member approximates the profile of the wall of the blast hole, so as to maximise the contact area between the rolling member and the wall of the blast hole. Preferably a side profile of the rolling member is designed to help retain the rolling member as captive within the cavity in an installation position.

Preferably the plug body is of generally cylindrical shape, having a substantially circular cross-section. The plug body may comprise a curved leading edge and trailing edge to assist in installation and prevent it locking against any loose rock that may be present in the blast hole. Preferably the plug body has a spine with a lower coefficient of friction where it contacts the wall of blast hole. The cavity is preferably in the form of an elongate channel, with side walls and bottom, the internal surface forming the bottom of the channel. At each end of the channel the cavity is preferably rounded to match the diameter of the rolling member. The surface is typically angled between 5° and 30° relative to the longitudinal axis of the plug body. In an illustrated embodiment the surface within the channel is designed with a 12° angle relative to the longitudinal axis of the plug body. The profile of the surface is preferably designed to match the contact profile of the rolling member. The surface is typically roughened to increase the coefficient of friction between it and the rolling member. The profile of the side walls of the channel are preferably designed to match the side profile of the rolling member, so as to better encapsulate the rolling member, and retain it captive within the channel during installation.

In an alternative embodiment the rolling member is one of a plurality of rolling members and the angled surface is one of a plurality of angled surfaces. Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. Likewise the word "preferably" or variations such as "preferred", will be understood to imply that a stated integer or group of integers is desirable but not essential to the working of the invention.

Brief Description of the Drawings

The nature of the invention will be better understood from the following detailed description of a specific embodiment of the stemming wheel plug, given by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of the a first embodiment of the stemming wheel plug according to the present invention;

Figure 2 is a cross-section view of a preferred embodiment of a rolling member employed in the stemming wheel plug of Figure 1 ;

Figure 3 is a longitudinal cross-section of the stemming wheel plug of Figure 1 showing an installation position of the rolling member within the plug; and,

Figure 4 is a longitudinal cross-section view of the stemming wheel plug of Figure 1 showing the location of the rolling member in an engaged position where it engages with a wall of the blast hole.

Detailed Description of Preferred Embodiments

A first embodiment of a stemming plug 10 in accordance with the invention, as illustrated in Figures 1 to 4, comprises a rolling member 12 and an elongate plug body 14 adapted to be slidably received in a drill or blast hole. In this embodiment the rolling member 12 has a substantially circular cross- section in at least one plane. The plug body 14 has a cavity 16 formed therein intermediate its length and open to one side of the plug body 14, the cavity 16 being adapted to hold the rolling member 12 therein.

The cavity 16 is formed with an internal surface 18 along which the rolling member 12 can roll when it engages with a wall of the blast hole, the surface 18 being angled relative to a longitudinal axis 20 of the plug body. In use, during installation, the rolling member 12 is captive within the cavity 16 and is held within an outer circumference of the plug body 14 (as shown in Figures 1 and 3) so as not to engage with a wall of the blast hole. Whereas, upon activation, the rolling member 12 engages with a wall 22 of the blast hole and rolls along the angled surface 18 until it wedges between the angled surface 18 and the wall 22 of the blast hole so as to lock the stemming plug 10 in place.

Typically the rolling member 12 has a flattened spherical or wheel-shaped cross-section as illustrated in Figure 2. A contact profile 24 about the circumference of the rolling member 12 approximates the profile of the wall 22 of the blast hole, so as to maximise the surface contact (and hence friction) between the rolling member 12 and the wall 22 of the blast hole. A side profile 26 is designed to help retain the rolling member 12 as captive in an installation position. The rolling member 12 may be constructed of a suitable hard material or a suitable soft material. A suitable soft material may be as simple as a child's round rubber ball, either round, machined or moulded to match the desired contact and side profiles. This will provide good friction against the wall 22 of the blast hole to retain the plug 10 in the locked position in the blast hole. A suitable hard material may be machined or moulded to match the desired contact and side profiles, and may be better suited to being machined to match the profile of the drill hole. It may also have a high friction coating applied to the outer surface of the rolling member 12, or be machined or finished in such a way to provide a roughened outer surface. The rolling member 12 may be CNC machined, or injection moulded. The elongate plug body 14 is preferably made from a hard plastics material such as polyethylene, polypropylene, nylon, or other similar materials that may be machined or injection moulded. However it will be understood that the plug body 14 may also be made from other suitable strong and low-cost materials.

The plug body 14 is of generally cylindrical shape, having a substantially circular cross-section, and may comprise a curved leading edge and trailing edge to assist in installation and prevent it locking against any loose rock that may be present in the drill hole. Preferably the plug body 14 has a spine with a lower coefficient of friction where it contacts the wall 22 of drill hole, whereas the internal surface 18 has a higher coefficient of friction (e.g. more roughly machined) where it contacts the rolling member 12. This difference in the coefficients of friction assists in correct operation of the plug 10, as will be described in more detail below. The cavity 16 within the plug body 14 is designed to retain and encapsulate the rolling member 12 prior to or after engagement in the drill hole. In this connection, the cavity is preferably in the form of an elongate channel 16, with side walls and bottom, the internal surface 18 forming the bottom of the channel. At each end of the channel 16 the cavity is rounded to match the diameter of the rolling member 12. In this embodiment the surface 18 within the channel 16 is designed with a 12° angle relative to the longitudinal axis 20 of the plug body 14, although this may vary depending on the circumstances. The surface 18 is typically angled between 5° and 30° relative to the longitudinal axis 20. The profile of the surface 18 is preferably designed to match the contact profile 24 of the rolling member 12. As noted above, the surface 18 is typically roughened to increase the coefficient of friction between it and the rolling member 12. The profile of the side walls of the channel 16 are preferably designed to match the side profile 26 of the rolling member 12, so as to better encapsulate the rolling member 12, and retain it captive within the channel 16 during installation. In an alternative embodiment (not illustrated) the rolling member is one of a plurality of rolling members and the angled surface is one of a plurality of angled surfaces. The plurality of angled surfaces may be provided in the same cavity, or in respective ones of a plurality of cavities. The plug body 14 may also include an elongate recess or groove (not shown) running the full length of its outer surface, to protect detonator signal tubes from being crushed during installation. Advantageously the plug body is formed with connectors 28 at each end, to enable it to be connected to a cable so that it may be lowered down a drill hole, or to assist with engagement in flat holes, or to enable it to have other components connected to it.

A preferred method of installation of the stemming plug 10 will now be described with reference to the accompanying drawings. The plug 10 may be installed either by pushing up or lowering down a blast hole. A normal ANFO or emulsion explosive loading hose can be used to push the plug up a blast hole to the installation location. For a downward, open ended blast hole (such as in Vertical Crater Retreat mining, or VCR mining), the plug may be installed by lowering it down the blast hole with a string or cable. The rolling member 12 is captive during installation, and is held within the footprint (outer circumference) of the plug body 14. This geometry ensures the rolling member 12 does not catch and prematurely lock if being lowered down into a blast hole.

Once the plug 10 is placed in the desired location it requires an upward jolt to move the rolling member 12 from its captive position at one end of the cavity 16, as shown in Figure 3, to a position where it engages with the wall 22 of the blast hole, as shown in Figure 4. The plug 10 is designed to engage the drill hole in a varying diameter range, for a new to worn bit in any size series (this variation is about 10% of the diameter). When the plug is released with the rolling member engaging the wall 22 of the blast hole, the rolling member 12 will roll up the surface 18, forcing the spine of the plug body 14 against the opposite side of the wall 22 of the blast hole. Eventually the rolling member 12 will become wedged between the wall 22 of the blast hole and the plug body 14, effectively locking the stemming plug 10 in position in the blast hole. Any downwards pressure applied to the plug body 14, such as the detonation force from the explosives in the blast hole, will then only result in further wedging of the rolling member in the locked position and generate substantial resistance.

If necessary, the plug may be removed, for example, in the event of a misfire, by pushing up on the plug body 14 to disengage the plug, and then removing the plug 10 in the same manner it was installed. Note the rolling member 12 remains captive within the cavity 14 during this removal process.

The design of the plug body 14 typically allows space around the plug for detonator signal tubes. The plug design also allows emulsion explosives to breathe, i.e. the process of degassing which causes bubbles to sensitize the explosive as gases are generated prior to initiation. The plug body design may include a flat, large surface facing detonation which, combined with a lower friction spine on the plug body 14, causes the plug body 14 to piston across the rolling member 12, providing a wedging resistance in the drill hole.

The plug body 14 may also include a tapered nose (not shown) to assist installation into a drill hole, and also to deflect blast shock waves into the wall of the drill hole on initiation. Each design may be adapted to slightly different rock types and applications.

The stemming wheel plug 10 is an improvement over prior art plugs as a result of significant research and development. It is an improvement over previous stemming plug configurations using a differential friction and wedging philosophy, and typically uses more advanced and lighter materials in the form of engineering plastics.

Now that preferred embodiments of the stemming wheel plug have been described in detail, it will be apparent that the described embodiments provide a number of advantages over the prior art, including the following: (i) It is simple to use and install, and remove if necessary.

(ii) It requires no special tools.

(iii) It can be located anywhere in the blast hole.

(iv) It will readily accommodate variation in the blast hole size and still provide effective stemming.

(v) It is lightweight and compact in design, and therefore easily transportable.

(vi) It is simple to manufacture at relatively low cost.

(vii) Its performance can be tailored depending on materials used and therefore cost.

(viii) Two plugs can be doubled-up in the blast hole in the event of a critical installation.

(ix) It can be adapted to locate other items in the blast hole such as electronic ore tracking devices.

(x) It can be used to deck charges, decking being the process of leaving an air gap between multiple columns of explosives in one blast hole. It will be readily apparent to persons skilled in the relevant arts that various modifications and improvements may be made to the foregoing embodiments, in addition to those already described, without departing from the basic inventive concepts of the present invention. For example, rolling member may be spherical in shape, rather than wheel-shaped, as in the illustrated embodiment, or the plug body may be split or hinged down the spine so that the rolling member may wedge the plug both on the opposite wall and also on each side. Therefore, it will be appreciated that the scope of the invention is not limited to the specific embodiments described.