BACKGROUND TO THE INVENTION One type of prior art blast hole plug consists of an inflatable plastic bladder surrounded by a woven polypropylene sleeve. The bladder and the sleeve are contained with a pressure tube, which is a plastic bag of relatively small diameter open at one end. The bladder contains a gas canister which upon activation releases gas into the bladder to inflate the bladder. The pressure tube restricts the size of the plastic bladder for a limited period of time so that the blast plug can be lowered into a blast hole. The bladder continues to inflate as it is lowered in the blast hole and plugs the blast hole at the position to which it is lowered. This type of prior art blast hole plug effectively provides a deck for stemming. The stemming consists of particulate material such as clay, stones or in the case of coal mines, coal chips. With this type of prior art blast hole plug relatively large quantities of stemming are used as well as relatively large quantities of explosives.

In open cast coal mines the result of blasts with the prior art blast hole plug is that a lower layer of coal, up to about a quarter of the height of the blast hole, is powderized, and the remaining upper layer of coal is blasted into slabs. The powderized coal is generally below 10mm in size and is discarded to tailings.

The slabs of coal are relatively large and have to be broken up further.

In open cast diamond mines the result of blasts with the prior art blast hole plug is fracture of diamonds.

SUMMARY OF THE INVENTION According to the invention a blast hole plug includes an inflatable body and a heat barrier covering at least part of the inflatable body so that in use when the inflatable body is inflated to plug a blast hole, the heat barrier will protect the inflated body from being melted by the heat of a blast in the blast hole so that the blast is contained with the blast hole for a limited period of time.

The heat barrier may be made of paper and preferably of a plurality of layers of paper. Each layer of paper may be between 50 and 200 microns thick.

Preferably each layer of paper is 100 microns thick. The paper may be treated with a non-flammable or at least flame resistant substance. In the case of the heat barrier being made of a plurality of layers of paper, at least the outer layer of paper is treated with the non-flammable or flame-resistant substance. The applicant has found that if the paper is about 100 microns thick or thicker then it is unnecessary to treat the paper with a non- flammable or flame resistant substance.

The heat barrier is preferably a sleeve but may be a bag.

A water-resistant layer may be located around at least part of the heat barrier. The water-resistant layer may be made of a polymeric material. The water resistant layer may be a sleeve.

A size restricting means may be provided for temporarily restricting the size of the blast hole plug to a size less than that of the opening to the blast hole into which the blast hole plug is to be inserted. The size restricting means is preferably a bag which will rupture when the force exerted on it by the inflating inflatable body exceeds a certain level.

The inflatable body may be a bladder.

Inflation means is preferably located within the inflatable bag. The inflation means may be a gas canister.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a blast hole plug in its uninflated configuration; Figure 2 is a perspective view of the blast hole plug of figure 1 without its pressure tube and showing how the blast hole plug is folded; Figure 3 is a cross-sectional side view of a blast hole plug in its unfolded, uninflated configuration; Figure 4 is a perspective view of the blast hole plug in its fully inflated configuration; Figure 5 is a cross-sectional side view on line V-V of figure 4; Figure 6 is a cross-sectional side view of a blast hole plugged with the blast hole plug; and Figure 7 is a cross-sectional plan view on line VII-VII of figure 6.

DETAILED DESCRIPTION OF THE DRAWINGS A blast hole plug 10 includes a inflatable plastic bladder 12, a paper sleeve 14, and a plastic sleeve 16 which are all folded up and contained within a plastic bag 18.

The inflatable bladder 12 consists of an inner weldable material 20 such as polyethylene and an outer gas barrier 22 made of nylon or an acrylic material.

An aerosol canister 24 containing non-flammable mixture of a gas and a solvent is located within the bladder 12. In figure 2 the solvent 26 is shown in the inflated blast hole plug.

The paper sleeve 14 consists of three layers 28 of paper. Each layer is 100 microns thick and is not treated with a non-flammable or flame resistant substance. The prime function of the paper sleeve 14 is that of an insulator. It acts as a heat barrier which prevents the heat of the blast from melting the plastic bladder. If the plastic bladder melts it will deflate and the blast plug will not be able to contain the blast within a blast hole 32. Thus it can be appreciated that the heat barrier could be made of other suitable materials such as aluminium foil which is also flexible and which can reflect the heat of the blast away from the plastic bladder. Thus the heat barrier must not melt, or must at least have a substantially higher melting point than the plastic bladder 12 and the plastic sleeve 16 so that it protects the plastic bladder 12 from the heat of the blast and prevents the plastic bladder from being melted by the heat of the blast.

The plastic sleeve 16 is made of woven polypropylene. The function of the plastic sleeve 16 is to contain the plastic bladder 12 against over expansion if the paper sleeve 14 ruptures. The paper sleeve 14 can rupture if water dissolves the adhesive which forms a seam 30 between two overlapping ends of the paper. In situations where water is not present, the plastic sleeve 16 is not necessary since the paper sleeve 14 is strong enough by itself to contain the plastic bladder 12, and to prevent the plastic bladder 12 from being punctured by sharp portions of the sidewall of the blast hole 32.

The plastic bag 18, the plastic sleeve 16 and the paper sleeve 14 have apertures 32 through which the gas canister 24 can be seen and activated. The function of the plastic bag 18, other than to store the folded plastic bladder 12, paper sleeve 14 and plastic sleeve 16, is to restrict the size of the bladder 12 for a limited period of time whilst it is being inflated. This limited period of time provides a user with sufficient time to activate the gas canister 24 and to insert the blast hole plug 10 into a blast hole 32 before the plastic bag 18 ruptures and the bladder 12 becomes too large to insert into the blast hole 32.

A charge of explosive 34 and a fuse 36 are first placed into the blast hole 32.

The blast plug 10 is then inserted into the blast hole 32 after the aerosol canister has been activated. Once the blast hole plug 10 has inflated sufficiently to plug the blast hole 32, stemming 38 is poured into the blast hole 32 onto the top of the blast plug 10.

The blast plug'10 performs two functions. Firstly, it contains the blast within the blast hole 32. In this regard the blast plug 10 provides sufficient time for the gas pressure to build up in the air gap 40 in the blast hole 32 before the plug 10 is shattered by the blast. Secondly, the blast plug 10 acts as a deck upon which the stemming 38 is supported.

From figure 7 it can be seen that the two ends 42 of the paper sleeve 14 are wedged up against the sides of the blast hole 32. This is a result of the manner in which the paper sleeve 14 unfolds from its folded position. The two ends 42 extend vertically downwardly. Thus the bottom of the bladder 12, where the heat of the blast is at its most intense, is completely covered by the paper sleeve 14.

The applicant has found that by using a blast plug according to the invention, in small diameter blast holes in coal, the quantity of explosive can be reduced by as much as 50%. In addition the amount of stemming required can be reduced or eliminated altogether. Furthermore, the result of the blast is a substantially homogeneous fracture of the coal, resulting in less fine coal. In open cast diamond mines, the applicant has found that the use of a blast hole plug according to the invention reduces the quantity of explosives required whilst the air gap 40 above the explosives increases the column height of the blast. This results in an elimination of, or at least a reduction in, the breakage of diamonds by the blast.

It will be appreciated that many modifications or improvements of the invention are possible without departing from the spirit or scope of the invention.