US2168030A - Explosive composition - Google Patents

Description

Fatented Aug. 1, I93

EXPLOSIVE C ()SHTIQN Harrison H. Holmes, Woodbury, N. 3., assignor to E. I. du Pont de Nemours & m. Wilmington, DeL, a corporation of Delaware Application January 12, 1937, Serial No. 1%,172

9 Claims.-

The present invention relates to a new and improved explosive composition and to methods for manufacturing the same. It relates particularly to an explosive composition comprising black powder, together with other materials which impart greater strength thereto.

Black powder has been very useful as a commercial explosive due to its relatively low velocity of decomposition, which property has rendered it desirable for use in the blasting down of coal and in other operations wherein the shattering effects of the high velocity explosives are to be avoided. One objection to the use of black powder, however, is that it has a low strength factor. In some instances drill holes must be sprung several times because of the necessity of employing a relatively large amount of powder in the holes to accomplish the desired work.

It has, therefore, been proposed to increase the strength factor of black blasting powder by incorporating therewith one or more of a variety of strengthening agents such as aonium iii nitrate, the perchlorates, nitrocompounds, and

organic nitrates. Ammonium nitrate in particular has been added to black powder compositions, either before or after the milling of the ordinary black powder ingredients, as for example in U. S. Patents 1,833,573, 1,845,663, 1,845,- 30 664, and 1,850,106, all issued to Hill, and U. S.

Patent 977,545 issued to Peters et 9.1.

Where the ammonium nitrate is incorporated with black powder according to the prior art, either before or after milling, certain disadvantions are employed for the purpose of bringing down coal. As hereinafter more fully described, I have found that pellets made from compositions of this sort do not possess sufilcient ease of ignition to be initiated by the spit of a flame from a safety fuse. Furthermore, the compositions or pellets made therefrom do not show the increased weight strength to be expected from the amount of ammonium nitrate introduced.

The chief object of the present invention is a new and improved explosive composition having the desirable properties of black powder, yet characterized by increased strength and ease of ignition. Another object is the method of manufacturing said composition. A further object is a new and improved explosive composition comprising black powder, a material to impart strength thereto, and a material which insures an ease of ignition of said composition. Other 55- objects will be apparent from the following detailed description of my invention.

The foregoing objects have been accomplished.

tages are apparent when the resulting composi-- powder, ammonium nitrate, and a suitable ignition accelerant. The accelerants in accordance with my invention comprise at least one finely divided metal. To assure satisfactory storage properties, an atmospherically stable metal should 5 be employed. Preferably the metal should be one which undergoes rapid combustion at elevated temperatures, or at temperatures which normally prevail during the combustion of black powder, for example from 300 to 900 C. For instance, I may employ aluminum, iron, magnesium, cadmium, titanium, zirconium, lead, vanadium, cobalt, nickel, chromium, tin, bismuth, antimony or any other metal capable of undergoing combustion between the foregoing limits, as well as mixtures or alloys of said metals. when hereinafter employed in the specification or claims, the phrase at least one metal is to be construed as including a single metal, mixtures of two or more metals, alloys of two or more metals, and definite compounds derived from two or more metals, as for example calcium silicide.

As mentioned above, the incorporation of ammonium nitrate in black powder, in accordance with the prior art, results in an explosive which is unsatisfactory in the field. This fact was recognized in the art, but the explanation usually offered was that the ammonium nitrate lowered the rate of pressure development by generally retarding the rate of deflagration. However, as a result of an extended investigation of this phenomenon, I have found that the foregoing explanation is true'only to a limited extent, and the success achieved by my invention may be ascribed partly to the fact that I have ferretted out the cause of the defect, and attacked the problem with a clear understanding of the source of the difliculty.

Briefly expressed, my investigations have shown that the poor performance of black powder containing ammonium nitrate in accordance with prior art teachings, is due, not so much to a retardation of the rate of deflagration in itself, but rather to a relatively great decrease in the ease of initiating the defiagration or combustion. Once the combustion is initiated, however, the rate of pressure development does not appear to be appreciably afie'cted by the presence of the ammonium nitrate.

It is not strange that this fact has heretofore escaped observation, since reactions which proceed at explosive rates are extremely dimcult to study in a quantitative manner. Indeed, I have found that the usual instruments employed in the prior art for measuring the rate of pressure development are altogether too qualitative for the purpose of my investigation; For this reason, I abandoned the standard equipment heretofore used, and employed a plow-electric gage, a vac- 60 uum tube amplifier, ,and a cathode ray oscillograph, connected in the usual manner, as a means of quantitatively studying the course of the reaction. With such equipment the various efi'ects briefly discussed above may be studied quantitatively.

In order to disclose more clearly the above mentioned defects of the prior art explosives,

. and the improvement eifected by my invention,

reference is made to the accompanying drawing which consists of four graphs of Figs. 1, 2, 3, and 4 plotted from data taken with the aid of the oscillograph above referred to. These graphs represent pressure-time curves of various compositions, as hereinafter more fully explained, the pressure in pounds per square inch being plotted along the vertical axis, whereas time in milliseconds is plotted along the horizontal axis.

Curve A of Fig. 1 is a pressure-time curve for a pellet of standard sodium nitrate-black powder of the prior art. The initial horizontal portion of the curve represents the duration of the so-called ignition lag before an appreciable pressure rise occurs. For standard black powder, this lag is seen to be approximately 70 milliseconds.

Curve B of Fig. 2 represents a similar curve for a composition comprising black powder and 20% ammonium nitrate. It may be noted that the horizontal portion of this curve is much longer than that in Curve A, and represents an ignition lag of approximately 470 milliseconds. The lag in each instance maybe'taken as a measure of the ease of ignition of the powder and the rapidity with which it attains the normal rate of pressure development. A small part of the lag (not over 10 milliseconds) in each case results from the delay of the squib itself, but this item is constant for all the powders tested. From thesecurves it is at once apparent that the addition of ammonium nitrate to black powder does not appreciably reduce the rate at which the pressure develops, after ignition has been effected. The decrease in the blasting effectiveness of ammonium nitrate-containing compositions of the prior art is due to the greatly increased ignition lag or the time required to initiate the deflagration. As soon as the temperature of the charge containing ammonium nitrate has been raised by the initial flame to a given point, a rapid explosive reaction sets in which is only slightly slower than the rate of combustion of black powder alone. The composition, however, does not give satisfactory performance due to the relatively long time required to initiate the combustion. The increase in the ignition lag may possibly be explained on the hypothesis that the ammonium nitrate must fuse before ignition sets in. Regardless of the explanation, however, I have demonstrated that contrary to prior expectations, the problem of securing a satisfactory performance with compositions comprising black powder and ammonium nitrate resolves itself into the discovery of means for insuring an ease of ignition at least comparable with that of black powder alone.

The practical significance of the ignition lag in the performance of the powder in the field is demonstrated by the fact that a powder showing a considerable ignition lag will propagate the combustion only slowly through a column of two or three sticks for example, with the result that the initiating end may reach a pressure sufiicient to open the tamped end of the borehole before the rest of the charge has attained its normal burning speed. As a result, the remainder of the charge will perform under reduced confinement, and a large amount of its potential strength will be lost.

Following the extended investigation of the causes of the defect of prior art compositions, I discovered as. above pointed out, that by the incorporation of at least one finelydivided, atmospherically stable metal in the composition comprising black powder and ammonium nitrate, the ignition lag is proportionately greatly reduced and improved performance assured. The state of division of the metal is generally important in order to obtain the full advantages of my invention and I have found that powdered metals are the most advantageous. For economic and other reasons, I prefer to employ as an accelerant powdered aluminum or powdered iron; however, the invention includes within its scope the use of any atmospherically stable metal which, when heated in a finely divided state, undergoes rapid combustion in air. I am aware that certain metals, for example the noble metals such as gold, silver, and platinum, and certain other metals, for example the members of the platinum group including osmium, iridium, palladium, rhodium, and ruthenium, fail to undergo combustion at temperatures normally reached. in the combustion of black powder. Naturally such metals are,

therefore, not included within the scope of my invention. Suitable metals may be alternatively defined as those metals which in finely divided state are capable of undergoing rapid combustion in air at temperatures normally generated in the combustion of black powder.

Good results are obtained when the metallic accelerant is introduced according to the following process. The finely divided metal is intimately mixed with the particles of ammonium nitrate in such a fashion as to coat said particles therewith. When powdered iron is employed, the coating process is facilitated by dampening the ammonium nitrate prior to mixing. For the purpose of promoting better storage properties in the composition, iron is employed which is coated with oxides of iron. a

The efi'ectiveness of the process of my invention for the purpose of preparing material with decreased ignition lag is demonstrated by referring to curves C and D of Figs. 3 and 4 respectively and comparing with curves A and B already discussed. Curve C shows an ignition lag of only 110 milliseconds for a composition prepared according to the process of my invention, and comprising black powder, to which has been added 20% ammonium nitrate and 1.0% powdered aluminum. Curve D represents a pressuretime curve for a similar composition in which the aluminum has been replaced by 4.0% powdered iron, to give an ignition lag of only 100 milliseconds. Both of these figures compare fav- Tana: I

Ignition lag in milliseconds Percent Accelerant accelerant Composition Black powder Black powder-+20% ammonium nitrate.

Black powder-i-ammonium nitrate+accelerant.

Black powder+ammnoium nitrate+accelerant.

Aluminum.

Iron

density may be prepared therefrom if desired.

The advantages of the composition, however, are particularly manifest in pellets of medium and higher density, since, as disclosed and claimed in the copending application of William E. Kirst, U. S. Patent No. 2,062,666, low density black powder pellets containing ammonium nitrate, for reasons at present unknown, do not exhibit the undesirable properties which generally characterize ammonium nitrate-modified black powders of the prior art. The invention claimed in Kirsts application is to be regarded as an al-' ternative solution to the same problem overcome in the present instance by the use of suitable accelerants. The present solution is somewhat broader in scope, however, since accelerants may be employed in accordance with my invention regardless of the density desired in the final composition.

In the foregoing description of my invention, only the major ingredients have been considered. It is to be understood however, that the addition of stabilizers and other auxiliary materials known to the art, may be made. Especially desirable compositions are disclosed in the following examples.

The compositions according to the present invention display an increased weight strength over black powder, without having the poor performance characteristics exhibited by the ammonium nitrate-black powder compositions of the prior art. A manifest increase in the ease of ignition by means of squib-or flame is caused by the presence of the metallic accelerant. Thus the best qualities of both black powder and ammonium nitrate are combined in my compositions, namely, ease -of ignition, high weight strength, and the possibility of ready regulation of bulk strength by variation of density as desired.

In the foregoing detailed description of my invention, the term black powder is to be regarded as including not only the standard black powder compositions of the prior art containing either potassium or sodium nitrate, but also similar compositions in which the proportion of charcoal, alkali metal nitrate, and fiil ill are changed somewhat from those usually employed, for the purpose of obtaining the proper 'or desirable oxygen balance in the finished powder.

Thus, since ammonium nitrate has a positive oxygen balance, the proportion of the charcoal and/or sulfur in the powder may be increased somewhat over that customarily used with a view to avoiding undesirable gaseous combustion products. Other variations inthe composition of black powder, common in the art, may also be employed in accordance with my invention which is not to be regarded as limited strictly to the standard black powder commonly manufactured today.

As many apparently widely different embodiments of this invention may be made without departing from the spirit thereof, it is to be understood that I do not limit myself to the foregoing embodiments or description. I intend to be limited only by the following claims.

I claim:

1. The explosive composition of claim 'I, wherein the metal comprises aluminum.

2. The explosive composition of claim 7, wherein the metal comprises iron coated with oxides of iron.

3. The explosive composition of claim 7, wherein'the black powder comprises comminuted wheel cake.

4. The explosive composition of claim 7, wherein grained black powder is employed.

5. A defiagrating explosive composition comprising ammonium nitrate intermingled and in intimate contact with at least one finely divided 'in amount sumcient to serve, as an ignition accelerant.

6. A deflagrating explosive composition com prising ammonium nitrate intermingled and in intimate contact with at least one finely divided atmospherically stable metal capable of undergoing rapid combustion at temperatures between '300 and 900 0., and black powder, said metal being present in amount sufiicient to serve as an ignition accelerant.

7. A deflagrating explosive composition in pellet form comprising from 15 to 45% ammonium nitrate intermingled and in intimate contact with at least one finely divided atmospherically stable metal capable of undergoing rapid combustion at temperatures normally occurring during the combustion of black powder, and black powder, said metal being present in amount suflicient to serve as an ignition accelerant.

. 8. A deflagrating explosive composition in pellet form comprising from 15 to 45% ammonium nitrate intermingled and in intimate contact with at least one finely divided atmospherically stable metal capable of undergoing rapid combustion at temperatures normally occurring during the combustion of black powder, at least one stabilizing ingredient, and black powder, said metal being present in amount suiiicient to serve as an ignition accelerant.

9. A deflagrating explosive composition comprising from 15-45% ammonium nitrate intermingled in intimate contact with at least approximately 1%'of powdered aluminum.

mumrson H. HOLMES.

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