US 3713384 A
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United $13168 Patent 1 Turnbull 1 Jan. 30, 1973 i541 BLASTING METHOD  Inventor: Brian Turnbull, Troon, Scotland 2/1958 Johnson et al. ..102/27 R 2,823,609 3,175,491 3/ 1965 Robertson 3,437,037 4/1969 Newman ..102/24 R FOREIGN PATENTS OR APPLICATIONS 663,864 5/1963 Canada ..102/23 Primary ExaminerVerlin R. Pendegrass Attorney-Cushman, Darby & Cushman  ABSTRACT A blasting method and primer assembly for blasting agents wherein the primer assembly comprises a high explosive priming charge, a connector and an intermediate length of detonating cord, the connector being adapted to hold the detonating cord in close lengthwise proximity to and slidably movable along a downline of detonating cord.
9 Claims, 3 Drawing Figures 1 BLASTING METHOD This invention relates to blasting and to a primer assembly for initiating blasting explosives of low sensitivity in blasting.
In the last decade powdered ammonium nitrate-fuel compositions and slurried explosive compositions have been developed and have found acceptance for blasting operations in opencast mining and quarrying. These compositions, because of their low sensitivity to initiation, are frequently referred to as blasting agents in order to distinguish them from ordinary commercial blasting explosives. Thus the term blasting agent is applied to an explosive composition which is insensitive to initiating by a No. 6 detonator (base charge 0.24 g. PETN) and hereinafter refers to such an explosive composition. Blasting agents include compositions containing no self-explosive ingredient and which on the North American continent are classified as NCN (nitrocarbonitrate) explosives, together with other explosives containing self-explosive ingredients (such as slurry explosives containing TNT), provided that the final product is not sensitive to initiation by a No. 6 detonator.
Blasting agents require a priming explosive for their initiation. The primer may be a cartridge of a nitroglycerine gelatine explosive such as Polar Ammon. Gelatine Dynamite or Opencast Gelignite, or alternatively it may be cast or pressed Pentolite primer. It has been found that, generally, an explosive of very high velocity of detonation, such as cast Pentolite primer, is the most effective initiator for blasting agents, and that detonation of the blasting agent is effected by a smaller weight of pentolite if the primer is an elongated primer initiated at one end.
It is common to use a plurality of primers spaced along the charge of blasting agent in a shothole, each primer being placed in position as the blasting agent is loaded. In the blast the primers are initiated by means of detonating cord leading from a detonator outside the shothole. For most effective initiation of the primers each primer may be provided with a downline of detonating cord but it is obviously more economical to employ a single downline with the primers threaded thereon. in the latter method a cast primer having a passage cast therein to permit the primer to be threaded on the downline and to be pushed down the line into the shothole must be used, and, because some clearance between the primer and detonating cord is necessary, the desired initiation of the primer from the detonating cord does not always occur.
It is an object of this invention to provide a method of blasting with blasting agents wherein primers are attached to detonating cord in a manner which permits the primer to be readily moved along the detonating cord to facilitate its location in a shothole, and ensures reliable initiation of the primer from the detonating cord. It is a further object to provide a primer assembly which can. be readily attached to a line of detonating cord, in a manner which permits ready movement of the primer along the cord and which ensures reliable initiation of the primer from the cord.
A method of blasting in accordance with the invention comprises positioning a downline of detonating cord in a shothole, loading a blasting agent as hereinbefore defined into the shothole, and, at selected stages of the loading, slidably connecting to the downline in detonation transmission relationship a high explosive primer and an intermediate connecting length of detonating cord, by connecting means which holds an end portion of the said intermediate length of detonating cord in close lengthwise proximity to the downline while permitting it to be slidably moved along the downline, and moving the primer into the desired position in the shothole.
The invention also includes a primer assembly for initiating a blasting agent which comprises a priming charge of high explosive, a connecting length of detonating cord attached at one end in detonation transmission relationship to the said explosive charge, and a connector attached to the other end of the length of detonating cord, the said connector being adapted to hold the detonating cord in close lengthwise proximity to a downline of detonating cord while permitting it to be slidably moved along the said downline.
The preferred connector comprises a body member chambered to permit the downline and connecting length of detonating cord to be slidably inserted and held therein in such proximity that detonation of the downline effects detonation of the connecting length of detonating cord in the other passage. Advantageously, the connector is formed with two substantially parallel bores to accommodate the downline and connecting length of detonating cord respectively, and in the preferred connector these bores overlap in cross section to provide direct communication between them and to permit the downline and connecting cord to be held in contact. The connector also preferably is adapted to hold the connecting length of cord firmly in position, and for this purpose the body member is preferably provided with a chamber into which the end of the connecting cord may be inserted after being drawn through the body member and doubled backwards. The body member is preferably formed from a synthetic plastics material such as polyethylene.
One end of the connecting length of detonating cord is preferably immersed in the priming charge and, when the priming charge is a castable explosive, for example TNT or Pentolite (TNT/PETN), the priming charge may conveniently be cast around the detonating cord. The end of the detonating cord intended for connection to the downline is preferably covered by a waterproofing composition, for example silicone rubber and/or a metal end-cap.
The high explosive priming composition preferably has a velocity of detonation of at least 6,500 meters per second. It may conveniently comprise cast or pressed Pentolite consisting of PETN and TNT in a ratio of 40:60 to 60:40 by weight, tetryl, cyclotrimethylenetrinitramine (CTMTN) or mixtures ofCTMTN and TNT.
The preferred primer has a length to maximum crosssectional dimension ratio of not less than 3:1 and preferably greater than 5:1 and the connecting length of detonating cord is preferably attached near to one end of the primer. The primer may conveniently be circular or rectangular but other cross-sectional shapes may be used. It will be understood that the primer may consist of several smaller charges placed in end-to-end contact.
The invention is further illustrated by one primer assembly which will now be described, by way of example, with reference to the accompanying drawings wherein FIG. 1 shows diagrammatically a perspective view of a primer assembly of the invention connected to a downline of detonating cord;
FIG. 2 shows a section of the connector shown in FIG. 1 on the line Il-II of FIG. 1;
FIG. 3 shows diagrammatically the disposition of primer assemblies as shown in FIG. 1 in a shothole.
In the primer assembly a priming explosive composition 11 is cast in a polyethylene container 12 around the knotted end of a connecting length of detonating cord 13, the container being closed by a lid 14. The other end of cord 13 is threaded as a sliding fit through a bore 15 in a polyethylene connector 16, doubled back and threaded through a bore 17 in connector 16 and is covered by a silicone waterproofing composition 18 (Silcoset, Registered Trade Mark, RTV rubber) and a tubular aluminum end-cap 19. The connector 16 has a bore 20 which overlaps bore 17 and in use the connector 16 is sleeved on a downline 21 of detonating cord by threading the downline through bore 20. When the primer assembly is in position on the downline the connecting cord 13 and the downline 21 are held in contact and the cord 13 can be readily pushed along the downline.
In use a downline 21 having a priming composition 11 in a container 12 is lowered down a shothole 22, as shown in FIG. 3. Blasting agent 23 is loaded into the shothole 22 up to the level a. The connector 16 of a primer assembly as shown in FIG. 1 is sleeved on downline 21 and the assembly is allowed to slide down the shothole to level a. The loading of blasting agent is continued until the shothole is filled to level b, when a further primer assembly is connected to the downline 21 and allowed to slide down the shothole to level b. Charging of blasting agent and primers is continued in this manner until the shothole is charged to the desired level. The top portion of the shothole is then filled with stemming material 24 and the explosive is detonated by means of a detonator 25 connected to the downline 21.
What we claim is:
1. A primer assembly for initiating a blasting agent which comprises a priming charge of high explosive, a connecting length of detonating cord attached at one end in detonation transmission relationship to the said explosive charge, and a connector attached to the other end of said connecting length for slidably connecting the assembly of priming charge and connecting length to a downline of detonating cord, said connector comprising a body member having bore means therethrough which surround a portion of said connecting length, the bore means having a longitudinal wall portion adapted to slidably engage a downline of detonating cord when the latter is inserted through said bore means and said bore means having a further longitudinal wall portion firmly holding said connecting length and positioning the same in parallel detonating relationship with the downline when the latter has been inserted through the bore means.
2. A primer assembly as claimed in claim 1 wherein the connector is formed with two substantially parallel bores to accommodate the downline and connecting length of detonating cord res e ctivel y.
. A primer assembly as c aimed in claim 2 wherein the bores overlap in cross-section to provide direct communication between the bores and to permit the downline and connecting length to be held in contact.
4. A primer assembly as claimed in claim 1 wherein the body member of the connector is provided with a chamber into which the end of the connecting length may be inserted after being drawn through the body member and doubled backwards.
5. A primer assembly as claimed in claim 1 wherein the body member is formed from synthetic plastics material.
6. A primer assembly as claimed in claim 1 wherein the priming charge is a castable explosive which is cast around the detonating cord.
7. A primer assembly as claimed in claim 1 wherein the end of the detonating cord intended for connection to the downline is covered by a waterproofing composition comprising silicone-rubber and/or a metal end cap.
8. A primer assembly as claimed in claim 1 wherein the priming composition comprises cast or pressed Pentolite consisting of PETN and TNT in a ratio of 40:60 to 60:40 by weight tetryl, cyclotrimethylenetrinitramine or mixtures of cyclotrimethyIenetrinitramine and TNT.
9. A primer assembly as claimed in claim 1 wherein the primer has a length to maximum cross-sectional dimension ratio of not less than 3: l.