US2560452A - Delay compositions for electric blasting caps - Google Patents

Description

July 10, 1951 H. M. KERR EIAL DELAY COMPOSITIONS FOR ELECTRIC BLASTING CAPS Filed May 6, 1948 INVENTORS HUGH M. KERR and CECIL R. L. HALL Patented July 10, 1951 UNITED STATES PATENT OFFICE DELAY COLIPOSITIONS FOR ELECTRIC' BLASTING CAPS Hugh Mowat Kerr, Brownsburg, Quebec, Canada,

and Cecil R. L. Hall, Saltcoats, Scotland, assignors to Canadian Industries Limited, Montreal, Quebec, Canada, and Imperial Chemical Industries Limited, London, England Application May 6, 1948, Serial No. 25,366 In Canada June 4, 1947 8 Claims.

This invention relates to an improvement in delay electric blasting caps, and more particularly to new delay compositions for use therein.

Delay electric blasting caps as used in present practice comprise essentially an ignition charge, a detonating charge and a delay element through the medium of which the detonating charge is fired. The delay element is customarily a fuse of such length and composition as will afford the desired delay interval between firing of the ignition and detonating charges.

Compositions heretofore used in delay elements are, for example, mixtures of barium peroxide, selenium metal powder and talc, or potassium permanganate and antimony metal powder,

either one of which compositions in burning produces very little gas on combustion and hence may be used in ventless delay caps.

Black powder, either compressed in a carrier or spun in a fuse which is then sized into a metal carrier and cut into appropriate lengths, has also been used as a delay composition. However, due to the large volume of gases evolved in combustion of black powder, the cap casing must in this instance be provided with a vent to permit the free escape of gases which, if not thus released, would cause irregularities in the burning rate of the delaycompositionwith consequent defects in the order of firing of a series of such caps. The vent in the cap casing is usually sealed with a rupturable membrane to prevent entrance of moisture. In such caps there is a tendency toward overlapping due to the effect of back pressure on the rate of burning of the delay composition. Also, because of the open vent there is a frequent occurrenceof 'misfires in wet ground due to shock from .previous blasts.

In either vented or ventless delay electric blasting caps, the delay compositions as used in the prior art burn at a rate of several seconds per inch of length, the desired'delay periods being obtained by increasingthe length of .the delay element and hence, as the delay periods become longer, the length of the cap is increased to the point where itebecomes inconvenient to handle in priming explosive'chargesw a i An object of our invention is therefore a new and improved ventless delay electric blasting cap assembly containing a delay composition the combustion of which results in much shorter delay periods than heretofore obtainable. A further object of our. invention is the provision of novel delay compositions which burn at such high temperatures that there is no possibility of misfires due to failure of the delay composition after ignition.- A still further object is the pro- .vision of a short period delay electric blasting cap, the use of which results in considerable reduction in ground vibrations which is an im portant factor in'quarrying operations. Additional objects of our invention will become 'apparent as the description proceeds.

a The foregoing objects may be accomplishedb the use of a delay element and means of ignition such as are used in a normal type of ventless delay electric blasting cap assembly, the delay composition and means of ignition being designed to function in such a way that the delay intervals obtained in rotational firing. of the caps of our invention are in the order of milliseconds. .The delay composition is a fast and hot burning relatively gasless mixture which is obtained by the use of mischmetal or an alloy of mischmetal combined with suitable oxidizing agents and a fuel. 1

Our invention maybe understood more readily by reference to the accompanying drawing which servesto illustrate the invention in itspreferred form. I

Figure 1 shows a longitudinal sectional View of a delay electric blasting cap in which I is a solid drawn cylindrical metal shell in the base of which is pressed a suitable charge of detonating explosive such for example as tetryl or pentaerythri; toltetranitrate 2. Directly above the base charge 2 is pressed a suitable priming charge 3 on top of which is pressed a delay element 4 containing thezdelay composition 5. The priming charge 3 may for example be a mixture of lead azide and lead styphnate or the like, while the delayele ment 4 comprises a lead tube into which is pressed the delay composition}. Arranged at a distance above the delay element 4 is shown the ignition meanswhich comprises a low tension relatively gasless'matchheadassembly B towhich are attached two insulated cap. wires 8 held in position by means of a sulphur plug 9 on top of which is an asphaltic waterproofing l0.' The whole ignition means is positioned at the desired distance from the delay element 4 by means of two indents II in they shell I which act as, a stop for the sulphur plug 9;

The delay composition 5 is preferably a homogeneous mixture of mischmetal alloyed with magnesium or aluminium to which is added a fuel such as silicon and an oxidizing agent such as red lead. This delay composition being essentially gasless there is 'no necessity for providing vents in the blasting cap illustrated in Figure 1 The term mischmetal as used above and throughout the remainder of the specification and the claims refers to the commercially avail; able alloy of rare earth metals having the followingapproximate analysis: a

Per cent Cerium 49.0 Lanthanum 25.6 Neodymium 16.0- Praseodymium a 4.6 Samarium V V 2.0; Terbium 1.0.- Yttrium 1:0 Iron "0.8

Our invention is more specifically illustrated by reference to thefollowing examples in which the delay element used in each instance had the following general dimensions, viz. length 0.300 inch, outside diameter 0.25? inch and inside diameter 0.090 inch. The delay element was 1prepared by tamping the loose'dlay composition into a large diameter lead tube which was subsequently sized down by rolling or drawing to'the correct diameter followed by cutting 'the element from the sized tube to the length specified above. In each instance the delay composition was prepared by. grinding the mischmetal or alloy of mischmetal with another metal followed by sieving through a 325 mesh sievehaving an opening of 0.001? inch. Fuels such as silicon were treated in a likemanner while the oxidizing agents were ground to pass a 200 meshstandard screen sieve .having an opening of 0.0029 inch. The various components of the delay composition were then blended in a tumbling barrel until the mixture 'became homogeneous.

Example 1 Using a standard blasting cap assembly as heretofore described, thefollowing table is illustrative of the burning characteristics of the various mischmetal/metal alloys or mixtures which may be used in the delay compositions of our invention. In conjunction with the mischmetal/metal alloy or mixture we have found it preferable to use a fuel such as silicon, and we prefer to use red lead as a source of oxygen in the proportions shown in the table.

Using the standard blasting cap assembly as heretofore described, the fdllowing 'table is illustrative of the change inburning characteristics obtained in the delay compositions when the ratio of mischmetal to metal in the alloy or mixture is altered, other conditions remainingthe same. The choice of magnesium as-an alloying ingredient is merely illustrative of the general effect of altering the composition of the alloy or mixture.

TABLE Mischmetal/Metal Alloy Silicon Red Lead w Milli- .Per cent Percent seconds 4 Example '3 Using the standard blasting cap assembly as heretofore described, the following table is illustrative of the change in burning characteristics obtained in the delay composition using different oxidizing agents. In obtaining this comparison the various oxidizing agents used were proportioned so that, based on the most likely end product of combustion, an equivalent amount of oxygen was made available as compared with an amount utilized in the formulations disclosed for the previous examples. The 70/30 mischmetal/ magnesium alloy was used as a basis for this comparison and silicon for the fuel.

Using the standard blasting cap "assembly as heretofore described the following table is illustrative of the change in burning characteristics obtained whenan inert diluent is added to a delay composition comprising:

Per cent 70/30 Mischmetal/Aluminum Alloy l 13:4 Silicon .241 Red Lead l 62.5

The diluent chosen was bentonite, a hydrated aluminium silica'tehaving the general-characteristics desired of a-diluent for a combustiblemechanical mixture.

Table Composition Bentonite Efirgg Percent Per cent Milli seconds 100 0 '17 95 5 -28 90 10 .63 I5 85 8O 2O 1.54

Example 5 Using the standard rbl'asting icap assembly as heretofore described the following table is mustrative of a series of short period d'el'ays which may be obtained when using the relay compositions of our "invention and varying the length of the delay element. The compositions disclosed were selected with a view to -demonst'rating-a series of short period delays having 25 -'millisecond intervals between period "means, and were also selected with aview to obtaining as shortan overall cap length as possible in .or'derto ifa'cilie tate manufacture and "handling.

TABLE P fled Delay Composition El t D l e 8111811 e ay N0. Length Interval Mischmetal/Metal Alloy Silicon Oxidant Milli- Per cent Per cent Inch seconds 1 /80 Mischmetal/Mg 14.9 26.9 0.30 27 5 24. l 0. 50 125 6- 43. 6 0.30 150 7" 2M 0. 53 p -175 s 27. 3 0. so 200 9------" 24.1 0.75 225 10- 34. a 0. 29 250 ll 43. 6 0. 60 300 12. 50/50 Misohmetal/Mg l5. 2 27. 3 0. 50 325 1a 50 50 Mischmetal/Mg 22.1 39.7 0. so 3'50 14. 50/50 Mischmetal/Mg 15. 2 27. 3 0. 56 375 1a 50 50 Mischmetal/Mg 15. 2 27. 3 Pbcrok 0.60 400 While the above may be taken as an embodiment in its preferred form of a complete series of short period delays having intervals between period means of milliseconds, it is apparent to those skilled in the art that other complete series having any other desired intervals between period means such as 50 milliseconds, 75 millisecond or higher may be obtained by selecting the various ingredients and/or adjusting the length of the delay element.

The delay compositions of our invention are particularly advantageous in that on burning, substantially no gas is produced and hence they can be used in a ventless delay electric blasting cap assembly with the attendant advantages of this type of cap. A further advantage of our invention lies in the fact that the compositions disclosed burn at high temperatures and thus are not subject to variation in rate of burning due to varying temperature conditions encountered in mining operations. A still further advantage of our invention resides in the fact that the duration of the delay interval can be readily varied to give any desired interval in the range of times considered most desirable for short period delay electric blasting caps.

Obviously the number of combinations of mischmetal alloy, fuel, oxidizing agent and diluent that can be selected to give the desired burning rate for short period delay intervals is almost infinite and any desired combination can be obtained by a proper selection of ingredients for the delay composition.

It is to be understood that the details disclosed in this specification are for the purpose of illustration only and that the invention is not to be limited to thte specific disclosures but may be modified and embodied in various other forms without departing from its intended scope.

We intend to be limited only by the following claims:

We claim:

1. A delay composition for delay electric blasting caps which comprises between 24% and 47% of silicon, between 32% and 63% of an oxidizing agent and mischmetal alloyed with a metallic element consisting of aluminium, magnesium, nickel, zinc, cobalt, silicon, tellurium, copper, selenium, antimony, iron, lead, manganese and 2. A delay composition for delay electric blasting caps which comprises between 24% and 47% of silicon, between 32% and 63% of an oxidizing agent, and between 13% and 26% of an alloy of mischmetal with magnesium.

3. A delay composition for delay electric blasting caps which comprises between 24% and 47% of silicon, between 13% and 26% of an alloy of mischmetal with magnesium and between 32% and 63% of an oxidizing agent from the group consisting of red lead, lead dioxide, potassium permanganate, barium peroxide, barium nitrate, potassium perchlorate, potassium chlorate, lead chromate and manganese dioxide.

4. A delay composition for delay electric blasting caps which comprises silicon 24.1%, an alloy of mischmetal with magnesium 13.4%-14.1%'and red lead 6l.5%-62.5%.

5. A delay composition for delay electric blasting caps which comprises silicon 43.6, an alloy of mischmetal with magnesium 24.3% and barium nitrate 32.1%.

6. A delay composition for delay electric blasting caps which comprises silicon 33.6%, an alloy of mischmetal with magnesium 18.7 and barium peroxide 47.7%. I

7. In a delay electric blasting cap. a delay composition which comprises 24.1% by weight of silicon, 13.4% by weight of an alloy of mischmetal with aluminum and 62.5% by weight of red lead.

8. In a delay electric blasting cap, a delay composition which comprises 24.1% by weight of silicon, 13.4% by weight of an alloy of mischmetal with silicon and 62.5% by weight of red lead.

HUGH MOWAT KERR. CECIL R. L. HALL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,708,186 Meek Apr. 9, 1929 2,095,302 Woodford et al. Oct. 12, 1937 2,123,691 Burrows et al July 12, 1938 2,444,849 Reilly July 6, 1948 2,450,892 Hale Oct. 12, 1948 2,473,405 Zebree June 14, 1949 2,478,501 Patterson Aug. 9, 1949

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