GB2108245A - Stemming - Google Patents

Abstract

A stemming device for insertion into a shot hole is of elongated shape and comprises stemming material comprising a mixture of certain proportions of sandy material and clay, together with a moistening liquid which comprises water and may include a humectant such as glycerol or propylene glycol. The mixture may be non-coherent in which case it is housed in a container which determines its shape. If the mixture is coherent it is not essential to provide a container but it is preferred to provide a container in order to prevent any escape of liquid. A stemming product suitable for transport and storage comprises an article incorporating a plurality of such stemming devices. The devices may be in a common container or be interconnected by frangible portions 9 of the stemming material. The sandy material comprises 50% by weight of sand with the remainder selected from crushed brick, crushed saggers, refractory chippings and calcium chloride. <IMAGE>

Description

SPECIFICATION Stemming This invention relates to stemming.

The term stemming material is used herein to denote material that in use is disposed in a shot hole and when tamped up serves to confine the explosion when an explosive charge in the hole is detonated.

The term stemming device is used herein to denote a device of elongated shape consisting of or including stemming material and that in use is inserted into a shot hole and is then tamped up to form stemming.

The term stemming product is used herein to denote a product comprising an article incorporating a plurality of stemming devices, the article being in a form suitable for storage and transport.

Aims of the present invention are to provide an improved stemming device and an improved stemming product. As will become apparent from what follows a stemming device embodying the invention may consist of stemming material alone or it may consist of stemming material disposed in a container. A stemming product, on the other hand, may comprise a plurality of stemming devices in a common container or a plurality of stemming devices that are interconnected.

From one aspect the present invention consists in a stemming device of elongated shape, for insertion into a shot hole, comprising stemming material having been made by forming a mixture comprised at least principally of the following constituents by weight: sandy material (as herein defined) 45 parts to 70 parts, and clay (measured as dry clay) 30 parts to 55 parts, the sandy material and clay together totalling 100 parts, and also a quantity of liquid sufficient to render the clay moist, water constituting at least part of the liquid.

The term "sandy material" is used herein to denote either sand alone or a mixture containing at least 50% of sand by weight, the whole or substantially the whole of the remainder being one or more ofthefollowing: particulate refractory material (e.g.

crushed brick, crushed saggers, refractory chippings), particles of stone (e.g. stone dust, crushed stone, stone chippings) and calcium chloride, the calcium chloride content not constituting more than 20% by weight of the sandy material. Where calcium chloride is present the calcium chloride content is normally such that it does not exceed about 5% by weight of the stemming material.

In use the sandy material serves as the principal stemming agent to confine the explosion. The clay also assists in confining the explosion but acts principally as a binder for the sandy material, assisting the stemming to retain its shape in the shot hole when it has been rammed ortamped up. The liquid serves to moisten the clay and generally to render it malleable. It may also serve among other things to assist in reducing air-borne dust resulting from the explosion.

It is preferred to use sand alone as the sandy material. Nevertheless, if there is used a sandy material comprising a mixture of materials, those materials preferably comprise sand and calcium chloride alone. Various reports suggest that the presence of calcium chloride can be of value in shot firing.

In preferred forms of device the stemming material comprises not more than 50 parts clay with not less than 50 parts sandy material by weight, together with the liquid. A particularly preferred material contains a weight of sandy material that is about twice the weight of clay. If the clay content were to fall below 30 parts by weight the stemming material could well cease to be coherent, and this is normally considered to be less desirable than a coherent material.

Nevertheless, the nature and constituents of the mixture may be such that the stemming material is not coherent, that is it is incapable of being formed into a solid body. In that case, the stemming material must be housed within a container of elongated shape in orderthat the stemming device should constitute an article of elongated shape for insertion into a shot hole. Preferably, however, the stemming material is coherent, that is it is capable of being formed into a solid body of elongated shape, for insertion into a shot hole, without the need for the provision of a container or other means to retain the material in shape.Nevertheless, as will become apparent from what is stated below, a preferred form of stemming device does include a containerforthe stemming material, though the purpose of that container is not primarily to maintain the material in shape but to prevent or substantially to prevent the escape of liquid from the stemming material during storage and transport.

It is to be understood that in use the shaped body of stemming material may be deformed somewhat during storage and transport so that it is no longer in precisely its original shape when it comes to be inserted into a shot hole. Nevertheless the shaped body is normally sufficiently malleable readily to permit any adjustment in shape that may be necessary to enable the device to be inserted into the hole.

The proportions of the constituents of the mixture that are necessary to enable the stemming material to be coherent can readily be determined by simple experiment. In general, however, it can be said that an increase in the proportion of clay assists in increasing coherence. As to the quantity of liquid, too little liquid may render the clay too dry to enable it to hold the sandy material together (even though the clay is moist), while too much liquid may render the clay too sloppy to enable the stemming material to retain its shape either during manufacture or during storage and transport.

The liquid content of a mixture embodying the present invention may be between 7 parts and 14 parts by weight; preferably it is between 8 and 12 parts by weight. It is often suitable for the liquid content to be about 10 parts by weight. The combined weight of sandy material and clay (measured as dry clay) are, or course, 100 parts.

The liquid may comprise water alone or water with minor proportions of one or more additives such as wetting agents. In a preferred form of device, however, the liquid preferably comprises water and a humectant. This serves to reduce any tendency there may be for the clay to dry out during storage or transpdrt. The liquid is preferably an aqueous liquid comprising at least 30% by weight of water and not more than 70% by weight of humectant. The humectant preferably comprises at least one of the following: glycerine, propylene glycol, a glycol other than propylene glycol.The inclusion of even a small proportion of humectant in the liquid has some beneficial effect, so it is impracticable to state a minimum value for the proportion of humectant in the liquid has some beneficial effect, so it is imprac ticableto state a minimum value for the proportion of humectant in the liquid. In practice, as the cost of humectants is relatively high as compared with the cost of the other constituents of the stemming material, the proportion of humectant actually employed in any particular stemming may be chosen to produce a reasonable increase in the shelf-life of the devices or products made from that material, without such an increase in cost as would renderthe devices and products unsaleable.

As explained above, in some forms of stemming devices embodying the present invention the shape of the devices is determined by the shape of the containers in which the stemming material is disposed; this is particularly the case where the stemming material is in non-coherent form. In other forms of stemming devices, in which the stemming material is formed into a coherent body, the shape of the stemming device may be largely or wholly determined by the shape of the stemming material.

Nevertheless, the stemming material is preferably contained in an individual container such as at least substantiallyto prevent the escape of liquid.

The container itself is preferably flexible and is preferably made from flexible sheet material; this may be a plastics material such as low-density polyethylene. The container is preferably of tubular shape, with the stemming material extending lengthwise inside the container. A preferred form of container initially comprises a bag that is closed at one end and open at the other. The bag may be supplied in the form of a flattened tube. During manufacture the stemming material is introduced into the bag through the open end thereof, after which the open end is closed. It may for example, be closed by being sealed, stapled or otherwise secured, or it may be closed merely by deformation, as for example by being bent back alongside the central portion of the bag containing the stemming material or by being twisted about the axis of the bag and then being bent back.In cases where water is introduced into the bag before the other constituents (in a method of manufacture described below) it is normally necessary to seal the bottom of the bag, as by heat-sealing, before the water is introduced. In other cases this may not be necessary. Indeed, it may be possible to provide a container in the form of a tube that is initially open at both ends, each end being eventually closed, such closure being effected, for example, by any of the methods outlined above.

Stemming devices of the kind in which the ends of the containers are not all secured are preferably stored and/or transported in cartons, boxes or the like and are so arranged in them that any tendency for the end portions of the container to unfold or untwist is strongly resisted.

In use, the stemming devices are inserted into shot holes and tamped up in the usual way. The devices are initially of a size and shape such that they can be introduced into the shot holes without difficulty. As a device is temped up, however, the stemming material is caused to take up the shape of the hole in which it has been inserted. If the device is of the kind comprising a coherent body of stemming material in a container, then in use the body may be removed from the container and inserted into the shot hole without the container. To enable the body to be removed or merely to assist in removal of the body the container may be slit open. On the other hand, where the device comprises stemming material in a container, the whole device, both stemming material and container may be inserted into the shot hole.If the stemming material is housed in a container and that container is relatively tight around the stemming material it may stretch or burst or tear during the tamping up operation. If necessary or desired the container may be punctured, slit or torn before the device is inserted into the shot hole. Where the stemming material in the container is not coherent and its shape is determined by the shape of the container it is normal for the container to be in close contact with the mixture. In cases where the stemming material is coherent, however, so that it can be preformed and can retain its shape without reliance on the container, the container may be such as to have spare capacity. This has two advantages. One is that where the container is of tubular form, as is preferred, the stemming material can be shaped into the desired form and readily inserted into the container.The other is that when the device is tamped up in a shot hole, the container can engage the walls of a shot hole without stretching and while remaining intact. It is to be understood that although the container may have spare capacity, it is preferably flexible and is collapsed onto the surface of the stemming material during transport and storage in such a manner that it may have a wrinkled appearance. Nevertheless, if desired the container, whether of the kind that is filled by the stemming material or of the kind with spare capacity, may be made of a relatively weak material and/or be punctured slit or torn before insertion so that it can yield as necessary. While any of the foregoing arrangements may be used, it is preferred to provide a container that has sufficient spare capacity around a preformed, coherent body of stemming material to enable the body to be readily inserted into the container during manufacture, the container nevertheless being made of a relatively weak material so that it can stretch, burst or tear open as necessary. In each instance it is preferred to arrange forthe container not to remain entire when the device is tamped up. The presence of container material between the stemming material and the wall of the shot hole may tend to reduce somewhat the grip between the stemming material and the wall.When the container does not remain entire, on the tamping up of the device, at least some of the stemming material engages the wall of the shot hole directly with the result that the grip tends to be improved.

Preferred containers are made from low-density polyethylene sheet of between about 150-gauge and 200-gauge (i.e. 37.5 microns and 50.0 microns respectively).

An alternative form of container can be made from a plastics material that has been shrunk onto a preformed body of coherent stemming material. In one preferred method, there is formed a tube of heat-shrinkable plastics material that is large enough to receive a body of stemming material. The tube is then heated sufficiently to cause it to shrink onto the stemming material. End portions of the tube initially extend beyond the body of material but during the shrinking operation those end portions shrink up close against the ends of the body and close or largely close the ends of the tube. Alternatively one of the ends of the tube may be closed before the stemming material is introduced into the tube.

Further, one or both of the ends may be sealed after the stemming material has been introduced into the tube but before the shrinking operation is carried out. If desired the container may be fabricated around the body of stemming material, as for example by placing the body between two sheets of heat-shrinkable plastics material, securing overlapping parts of the sheets together and then effecting a shrinking operation.

Another form of container comprises plastics film embracing the stemming material, edge portions of the film overlapping one another and the film having the property that overlapping portions thereof cling together. Film of this kind is marketed under the name "Cling Film". A strip of film can be wrapped around a preformed body steming material so that edge portions thereof overlap and cling together.

End portions of the film extending beyond the body can be folded up to seal the ends of the container.

From another aspect the present invention provides a stemming product comprising an article incorporating a plurality of stemming devices, each embodying the first-mentioned aspect of the present invention, the stemming material of each device being coherent and the devices being separable for use.

The stemming devices may be separate from one another in which case the stemming product preferably includes a common container for the devices; that common container is preferably such as at least substantially to prevent the escape of liquid. Alternatively the stemming devices may form part of a unitary body of stemming material and be integrally interconnected by frangible portions of stemming material. Nevertheless it is preferred to house that unitary body of stemming material in a common container, that common container preferably being such as at least substantially to prevent the escape of liquid.

In use the stemming devices can be transported and stored in their common container. When the devices are separate from one another they may be individually removed from the container as required for use. Alternatively the container may be such as to enable it to be-subdivided to enable the individual devices, each with an associated part of the container, to be separated from the others for use; each part of the common container may then be inserted into a shot hole with the stemming material it encloses. In either case the container may have a plurality of pockets, each device being housed in a different one of those pockets. When the devices are integrally connected by frangible portions of stemming material to form an integral body it is preferred to remove the body as a whole from the common container and then to detach the individual devices as they are required for use.

A container that constitutes part of a stemming product and contains a plurality of stemming devices may be made of a material similar to that used for a single, individual stemming device and may be made in a similar manner, various suitable materials and method of manufacture being described above.

Thus, a container for a plurality of stemming devices is preferably made from flexible sheet material; this may be a plastics material such as low-density polyethylene. Moreover it may be made of a plastics material that has been shrunk onto the devices.

Alternatively the container may comprise plastics film, edge portions of the film overlapping one another and the film having the property that overlapping portions thereof cling together.

In each case the container is preferably such that little air remains with the devices in the container.

The devices may be arranged in any suitable manner in the container. For example, several devices may be disposed side by side with their axes mutually parallel and in a common plane.

Turning now to the method of manufacture of the devices themselves, a preferred method is one in which the sandy material and clay are initially in a dry state and a controlled amount of liquid is then mixed with them.

In a preferred method the sandy material and liquid are mixed together to form a preliminary mixture, and then the clay, in powdered or particulate form, is mixed with said preliminary mixture. If, in contrast, the sandy material and clay are first mixed together and are added to the water for the clay to form unwanted lumps in the mixture. Normally this tendency is avoided or at least reduced in the preferred method outlined above is adopted. In particular it is preferred to add the dry clay to the wet sandy material.

In a preferred method of shaping the coherent stemming material, the material is "pugged" or extruded during manufacture and then cut to length.

The extrusion may be of a cross-section appropriate for a single, individual device or may be appropriate for a plurality of devices integrally connected by frangible portions of stemming material. A length of the extruded, coherent mixture can then be introduced into a container or a container can be fabricated around it. In an alternative method of shaping such material, the material in particulate form is blown from one end into a tubular mould closed at its other end by a porous plate which permits the air to escape but retains substantially all the material. When the mould is full the moulded material is transferred from the mould into a con tainer. While the material in each instance forms a coherent mass, the liquid content is preferably such that the shaped mixture is readily malleable.

In another method a non-coherent, flowable mixture is formed comprising the sandy material and the clay, this mixture either including none of the liquid or at least some of the liquid. This flowable mixture is introduced into a container, which is in the form of a bag of elongated shape, and assumes a form determined by the shape of the container. If none or some of the liquid was omitted from the mixture then that missing liquid can be added in any of several different ways. For example the liquid that is not in the flowable mixture, or some of that liquid, may be initially introduced into the container and the flowable mixture may be added to it. Any remaining liquid may be added during or after the introduction of the flowable mixture into the container.It sometimes happens, however, that the initial or final introduction of liquid into the container results in a failure of the liquid to become evenly distributed through the mixture in the finished device. It is therefore generally preferred to introduce liquid into the container during the introduction of the flowable mixture into the container, the liquid and flowable mixture preferably being introduced into the container simultaneously. The liquid may, of course be added to the flowable mixture as it is flowing towards or into the container.

Embodiments of the present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which Figure lisa perspective view of a stemming device embodying the present invention, Figure2 is a plan view of a stemming product embodying the invention, and Figure 3 is an end view of another form of stemming product embodying the present invention.

The stemming device shown in Figure 1 comprises a container, in the form of a bag 1, containing stemming material made by forming a mixture of the following constituents by weight: dry sand 63 parts dry clay 37 parts liquid 10 parts the liquid comprising a mixture of 30% water and 70% propylene glycol by weight.

In making the stemming material the liquid is mixed with the dry sand, and the clay, which is in powdered form, is then mixed intimately with the wetted sand. The resultant mixture is "pugged" or extruded through a die to produce a self-supporting, coherent body of circular cross-section. The extruded body is cut into lengths each of which is put into the associated bag 1.

The bag 1 comprises a length of low-density polyethylene tube of 150 gauge. The flexible polyethylene sheet from which the tube is made contains an antistatic additive and has been coronadischarge-treated to give it a relatively low surface resistivity. The tube is somewhat longer than the cut length of extruded mixture. One end of the tube has been secured closed. When that cut length of stemming material has been inserted into the bag, the open end portion of the bag is twisted round and bent back alongside the central part of the device as shown at 2.

As they are completed the devices are laid in cardboard boxes, in such a manner as to render it very unlikely that the unsecured end portions of the devices will inadvertently become unfolded. Each box may contain about two dozen devices.

One standard size of borehole is 1 11/16 inches diameter (42.86 mm), though owing to wear of the drill bit the actual size may be about 1 5/8 inches diameter (41.27 mm) or less. For holes of that nominal size the extruded body of stemming material is preferably 1 1/4 inches diameter (31.75 mm). A typical body is about 9 inches long (228.6 mm). The circumference of the bag, however, is a little greater than that of the body so that the bag has spare capacity. During manufacture the body of shaped stemming material can readily be introduced into the bag. After the body has been inserted into the bag the bag tends to cling to the surface of the body so that the bag has a wrinkled appearance.

At least one specification for the physical properties of containers for stemming devices have already been published, and the containers of devices of the kind described above may conform to one or more of those standards, as desired.

The stemming product shown in Figure 2 incorporates six stemming devices 3 each similar to that described above with reference to Figure 1 but without an individual container 2. Thus each device 3 comprises merely a length of extruded stemming material.

The devices 3 are arranged side by side in a common container 4. The six devices are arranged with their axes mutually parallel and in a common plane. The container 4 is made from a plastics material in sheet form. In its manufacture rectangular pieces of the sheet are connected together along two parallel edges 5 to form a flattish tube. The stemming devices are disposed in this tube. If desired the tube may be formed around the devices.

The tube with the devices inside it is heated sufficiently to cause it to shrink onto the devices. End portions of the tube initially extend beyond the row of devices 3 but during the shrinking operation those end portions shrink up close against the ends of the devices and seal the ends of the container. End portions of the tube initially extend beyond the body of material but during the shrinking operation those end portions shrink up close against the ends of the body as indicated at 6 and close or largely close the ends of the tube. In a modified method one end of the tube is closed before the heat-shrinking operation.

The assembly of devices and common container constitutes an article which can readily be handled, stored and transported to a place of use.

When the devices come to be used, the container is cut or torn open and the devices removed from it for insertion into a shot hole or shot holes.

In an alternative method of manufacture, use is made of plastics sheet of the kind marketed under the name 'Cling Film' and which is characterised by the fact that when two layers thereof are brought together they cling to each other. In manufacture of the product a row of devices is laid on a first rectangular sheet of the film. A second, similar sheet is laid over the devices, and the edge portions of the sheets, which project beyond the row of devices on all sides, are pressed together to cause them to cling together.

In a modified construction of stemming product the side walls of the container are joined together between each of the devices 3 and its immediate neighbour; the container thus provides a plurality of individual pockets, each device being housed in a different one of those pockets. Each join may be such that the container can readily be torn along the line of the join. In use each device may be detached from the others by tearing the pocket in which it is housed from the neighbouring pocket or pockets.

The device can then be inserted into a shot hole while still in its pocket or it can be removed from its pocket before being inserted into the shot hole.

The stemming product shown in Figure 3 comprises a unitary body made from stemming material of the formulation as is used in the device shown in Figure 1. The material is "pugged" or extruded through a die of a shape necessary to produce a body 7 ofthe shape shown in Figure 3. The extruded material is cut into lengths in planes normal to the direction in which the material is extruded. Each such length then constitutes a stemming product of the kind shown in Figure 3. The body 7 comprises six devices 8, each of which is similar in shape and dimensions to the devices described above with reference to Figures 1 and 2, interconnected by frangible portions 9 of stemming material. In a typical product the diameter of each device is 1 1/4 inches (31.75 mm) and minimum thickness of each portion 9 is one eighth of an inch (3.2 mm).

In use the individual devices 8 can be detached from the remainder and used in exactly the same manner as the devices described above.

In a modification (not illustrated) a stemming product comprises a body similar to the body 7 but disposed in a container such as at least substantially to prevent the escape of liquid. The container may be of any of the kinds described above with reference to Figure 2, apart of course from the containers subdivided to form individual pockets.

In each of the methods of manufacturing a stemming product described above, and which involves the use of a container, it is preferable to arrange for little or substantially no air to remain in the container when the product is completed.

Claims (30)

1. A stemming device of elongated shape for insertion into a shot hole comprising stemming material having been made by forming a mixture comprised at least principally of the following constituents by weight: sandy materials (as herein defined) 45 parts to 70 parts, and clay (measured as dry clay) 30 parts to 55 parts, the sandy material and clay together totalling 100 parts, and also a quantity of liquid sufficient to render the clay moist, water constituting at least a part of the liquid.

2. A stemming device according to claim 1 in which the stemming material is coherent.

3. A stemming device according to either one of the preceding claims in which the liquid content of the mixture is between 7 parts and 14 parts by weight.

4. A stemming device according to either one of claims 1 and 2 in which the liquid content of the mixture is between 8 parts and 12 parts by weight.

5. A stemming device according to either one of claims 1 and 2 in which the liquid content is about 10 parts by weight.

6. A stemming device according to any one of the preceding claims in which the liquid comprises water and a humectant.

7. A stemming device according to claim 6 in which the liquid is an aqueous liquid comprising at least 30% by weight of water and not more than 70% by weight of humectant.

8. A stemming device according to either of claims 6 and 7 in which the humectant incorporates at least one of the following: glycerine, propylene glycol, a glycol other than propylene glycol.

9. A stemming device according to any one of the preceding claims in which the stemming material is contained in an individual container such as at least substantially to prevent the escape of liquid.

10. A stemming device according to claim 9 in which the container is made from flexible sheet material.

11. A stemming device according to either of claims 9 and 10 in which the container is of tubular shape, with the stemming material extending lengthwise inside the container.

12. A stemming device according to claim 11 in which the stemming material is coherent and the tubular container has spare capacity.

13. A stemming device according to claim 10 in which the stemming material is coherent container is made of a plastics material that has been shrunk onto the stemming material.

14. Astemming device according to claim 10 in which the stemming material is coherent and the container comprises plastics film embracing the stemming material, edge portions of the film overlapping one another and the film having the property that overlapping portions thereof cling together.

15. A stemming device substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

16. A stemming product comprising an article incorporating a plurality of stemming devices, each in accordance with any of claims 1 to 8, the stemming material of each device being coherent and the devices being separable from the remainder of the article for use.

17. A stemming product according to claim 16 in which the stemming devices are separated from one another and are disposed in a common container.

18. A stemming product according to claim 17 in which the container is made from flexible sheet material.

19. A stemming product according to either of claims 17 and 18 in which the container has a plurality of individual pockets, each device being housed in a different one of those pockets.

20. A stemming product according to claim 16 in which said devices are integrally interconected by frangible portions of stemming material to form a unitary body.

21. A stemming device according to claim 20 in which the devices are disposed in a common container.

22. A stemming product according to either one of claims 17 and 21 in which the container is made of a plastics material that has been shrunk onto the stemming devices.

23. A stemming product according to any one of claims 17, 18, 19 and 21 in which the container comprises plastics film, edge portions of the film overlapping one another and the film having the property that overlapping portions thereof cling together.

24. A stemming product substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

25. A stemming product substantially as herein before described with reference to Figure 3 of the accompanying drawings.

26. A method of making a stemming device according to any of claims 1 to 8 in which the sandy material and clay are initially in a dry state and a controlled amount of liquid is then mixed with them.

27. A method according to claim 26 in which the sandy material and liquid are mixed together to form a preliminary mixture, and then the clay, in powdered or particulate form, mixed with said preliminary mixture.

28. A method according to either one of claims 26 and 27 in which the stemming material is coherent and the device is shaped by extrusion.

29. A method according to either one of claims 26 and 27 in which the stemming material is not coherent and is shaped by being enclosed within a container of elongated shape.

30. A stemming device made by a method according to any one of claims 26 to 29.