US2171379A - Ammonium nitrate explosive - Google Patents

Description

Patented Aug. 29, 1939 um'reo AMIWONIUM NITRATE EXPLOSIVE No Drawing.

Application September 28, 1938,

Serial No. 232,115

14 Claims.

The present invention relates to new and improved explosive compositions and their method of manufacture. and in particular to a detonating explosive especially desirable for coal blasting.

Those familiar with the explosive art recognize that black powder is a desirable blasting agent in coal mining because it exerts a so-called sustained heaving action", which permits the recovery of coal in lumps of appreciable size. The use of black powder or other conventional deflagrating explosives in gassy mines-however, is hazardous because combustion is accompanied by the evolution of flame of considerable duration.

The so-called permissible dynamites are in extensive and successful use in coal blasting, and possess safety properties to a high degree. Many difficulties have been met, however, in attempting to develop such safe detonating explosives approaching the black powder type in blasting efi'ect and in the ability to break down a satisfactory lump coal.

The object of the present invention is an improved explosive composition especially adapted to the mining of coal, said composition having an improved basting action in this respect over conventional coal mining explosives. A further object is an improved detonating explosive which exhibitsa sustained heaving action as contrasted with the shattering action characteristic of explosives adapted to quarry operations, said explosive being characterized by its ability to produce greater yields of lump coal. Another object is an improved detonating explosive which exhibits the sustained heaving action of black powder, yet is highly insensitive to initiation by means of friction, shock, and heat. Additional objects will be disclosed as the invention is described more fully hereinafter.

These objects are achieved by means of a powder comprising a collection of distinct sized granular explosive units. Each of these'units possesses a well-defined structure in that it comprises particles of both ammonium nitrate and a sensitizing agent held together in loosely bound aggregate form by means of a binding agent. By loosely bound I mean that I carefully avoid pressing the material to a high density as carried out in the step of pressing black powder, for example. Instead of using any appreciable pressium chromate, manganese dioxide, and the like..

sure in the formation of these aggregates, I merely hold the particles in sufllciently close contact to permit the adhesive properties of the binding agent to come into play, forming a firm union between the various particles of ammonium ni- 5 trate, sensitizing agent and other ingredients without forcing these ingredients together in a dense mass. The result then is a loosely bound aggregate of low actual density rather than a dense aggregate such as non-porous granular black powder. I have determined that in order to attain the foregoing objects it is absolutely necessary that these aggregates be sized so that substantially all are held on a 50-mesh screen.

As the binding agent for holding together the 15 interspersed particles of ammonium nitrate and other ingredients to form the distinct granular explosive units, I find it desirable (but not compulsory) to employ a combustible material capable of gelatinization in the presence of a volatile solvent. Preferably, the binding agent will also be one which acts as a good sensitizer for ammonium nitrate. As such a binding agent I may use, for example, nitrocellulose, ground smokeless powder, polyvinyl nitrate, and other materials having similar binding and sensitizing properties. Although I prefer to employ a binding agent of the foregoing type, I may, however, employ a binding. agent having no marked sensitizing action such as starch, ethyl cellulose, polyvinyl acetate, or the like. When binding agents of the latter type are employed, it is desirable to use a sensitizing agent inconjunction with the same.

As 'volatile solvents for use in conjunction with said binding agent for the purpose of bringing out the adhesive properties of the same, I may employ, for example, acetone, amyl acetate, methyl alcohol, alcohol-ether mixtures, and other solvents having sufficiently low boiling points to permit their ready removal.

As additional oxidizing agents, I have found the following to be particularly suitable: sodium nitrate, potassium nitrate, barium nitrate, potas- As additional combustible ingredients, I may employ, for example, wood pulp, starch, various meals, salts of organic acids, and the like.

As sensitizing agents I may employ ground smokeless powder, nitrocotton, nitrostarch, poly- 50 vinyl nitrate, penterythritol tetranitrate, trinitrotoluene, and the like. These sensitizing agents may be employed singly or in groups.

Although I may employ ammonium nitrate of any particle size in the formation of these ex-' plosive' aggregate units according to the present invention, I prefer to employ ammonium nitrate particles of such a state of subdivision that no less than 40% thereof will pass through a 48- mesh screen; that-is, a screen having.48-meshes per inch. nitrate in amounts greater than 75% by weight of the finished explosive, for example, between 75 and It is to be understood that I may prepare the collection of distinct granular explosive units according to my invention by any process wherein the binding agent is caused to hold together in said units the intermingled particles of ammonium nitrate and other ingredients. This condition is desirably effected by any process which brings out the adhesive properties of the binding agent sufficiently to cause adherence of the particles of other ingredients thereto in the unit structure described.

However, in my preferred method of preparing these improved explosive compositions, a mucilaginous condition is effected on the surface of the binding agent. By the term mucilaginous, I intend to define a state in Which the surface of the binding agent is soft and sticky in contrast to a solution which would exhibit a definite viscosity characteristic. The term is to be understood as having this meaning Wherever it is used throughout this application. This mucilaginous condition may be brought about, for example, by exposing the binding agent to a predetermined amount of a selected appropriate solvent for a predetermined time interval. By causing the intermingled particles of the other ingredients to make contact with the mucilagi nous surface of the binding agent, they are caused to cohere in the form of the aggregate granular explosive units, already described.

The specific solvent as well as the amount of the solvent employed will depend upon the physical and chemical characteristics of the combustible binding agent used. The solvents employed may be added directly in the form of a spray or in any other desired manner. While any of the solvents mentioned in the foregoing are suitable, I prefer to employ acetone, for eifecting the mucilaginous condition, and in a-ratio such that from 10 to 30 cc. of acetone is used per grams of solids in the composition.

It should be clearly understood that in carrying out my invention I definitely avoid any step which will tend to draw into solution in any way any substantial amount of the ammonium nitrate employed.

In this respect I have found methanol to be less desirable than other solvents for use in my invention. Furthermore I find it to be undesirable to attempt to employ, in the absence of a binding agent, sensitizing agents which have no appreciable binding properties; for example, less satisfactory results are obtained by the use of nitrostarch and ammonium nitrate treated with methanol, for example, in the absence of an additional binding agent.

In order to illustrate more clearly the preferred process for manufacturing the improved explosive according to my invention, the following example is given as a specific embodiment thereof.

I prefer to employ this ammonium' Example I The dry ingredients included grained ammonium nitrate of such a degree of fineness that approximately 62.0% was held on a 48-mesh screen while 4.5% was held on a 24-mesh screen, fine granules of ground pyro smokeless powder, sodium nitrate, and wood pulp. These dry ingredients were introduced into a mixer and agitated for a period of five minutes. Without discontinuing the mixing action, 15% by weight of acetone was added to the ingredients and the mixing operation continued for five additional minutes. During this period, the surface of the ground pyro powder particles became moist and sticky so that there was strong adhesion between them and the particles of the various other ingredients of the composition. As a result of this mixing operation, there was a substantially uniform distribution of the various constituents through the mass of material. At the expiration of the mixing period, the composition was passed into an extruder which operated essentially on v the meat-chopper principle, and was thus extruded through a disc provided with openings 4" in diameter and spaced apart. Following this extrusion and granulation, the composition was heated at a low temperature, for example, from 60 to 70 C. in order to remove the acetone, and was then classified by screening in order to eliminate material which did not pass through a 7-mesh screen or was not retained on a 16-mesh screen.

It will be understood that the foregoing example is not to be construed as limiting the scope of my invention. This merely sets forth my preferred method of preparing my improved explosive composition. Various other methods may be employed for preparing these products. Also the final screening of the granular explosive units to the particular grain size was merely preferred and not essential. It is only necessary that substantially all of the material be held on a 50- mesh screen.

The final composition, according to my invention and resulting from the process described, was a coarse, granular, free-flowing, detonating product wherein each grain was a coherent unit consisting of particles of the various ingredients intimately intermingled and bound together in close relationship with one another.

The following examples serve to illustrate particular explosive compositions according to my invention. All the products consist of sized aggregates such that the finished grains pass a '7- mesh screen and are held on a 16-mesh screen. As already noted, the particular size of these aggregates is optional within the limits noted.

Table I From the foregoing tabulation, it will be appar ent that detonating ammonium nitrate explosives of very low velocity were obtained. All these compositions were detonated by a No. 6 blasting cap. The low velocities were obtained, moreover, without the employment of coarse ammonium nitrate particles. trate in every case was such thatat least 40% passed a 48-mesh screen while not more than 10% was held on a 24-mesh screen.

It will be noted that all of the velocity meas urements given in the foregoing table were obtained from cartridges 1 in diameter. It will be understood that, throughout this application, any velocity measurement set forth is calculated from measurements of the rate of detonation of cartridges of that same diameter.

The compositions disclosed in the foregoing comprise ammonium nitrate, a combustible binding agent, an additional fuel, and an oxidizing agent other than ammonium nitrate. Nitrocellulose is my preferred binding agent, when softened and rendered sticky by a suitable solvent. Preferably, I use between '75 and 85% ammonium nitrate, between 3 and 8% nitrocellulose, between 2 and 12% of a fuel, and between 3 and 12% of sodium nitrate or other additional oxidizing agent. I

In actual practice, I may desirably employ my compositions as fixed explosive charges; that is, with the entire charge of explosive present in one body and not separated in different cartridges. Thus, the necessity of propagating the detonation through several non-explosive media, such as the paper ends of cartridges, is eliminated. Another method of charging may comprise loading the material into smaller packages so that a charge of the desired size may be assembled. It should be understood, however, that my invention is not limited by the form of charge.

In the disclosure of my invention in the foregoing, it has been emphasized that in the preferred embodiment the ammonium nitrate is not of coarse granulation, namely, that not more than 40% of said ammonium nitrate passes a 40-mesh screen. I may also define the preferred granulation by stating that the amonium nitrate shall have a fineness modulus not greater than 3.0. This value designates a weighted average of fineness and it is the sum of the percentages retained on the different standard Tyler sieves, said sum being divided by 100. The sieves used in obtaining this fineness modulus consist of the 100, 65,

'48, 35, 28, 20, 14, and l0-mesh screens. As an example of an ammonium nitrate of satisfactory quality for use according to my invention, the following will serve, the screen analysis and the fineness modulus" being indicated.

Percent am- Sicve size monium nitrate held loll-mosh 93. 2 (-mosh, 81. 4 59, 3 5

Fineness modulus2.7+.

It is apparent, of course, that the lower the fineness modulus the finer the granulation. It will be understood, therefore, that wherever the term fineness modulus is used in the claims of The fineness of ammonium ni-' this patent, it is to be interpreted according to the foregoing definition.

Further examples of explosive compositions prepared according to the present invention are setforth in the following table:

Table II Scnsitizer and binding gg Acetone gent m./scc. percent l x 24 100 4.5 ground smokeless powder 1280 58.0 496 20 7.5 ground smokeless pow 1355 65. 6 490 23 10.0 ground powder 1300 77. 6 512 23 12 5 ground smok powder 1380 78. 7 510 25 15.0 ground smokeless powder 1425 77. 7 501 30 4.5 nitrocotton. 1220 55. 3 499 20 7.5 nitrocotton 1250 07. 4 480 23 10.0 nitrocottom. 1325 80. 5 502 23 12.5 nitrocottonu 1300 74. 9 518 25 15.0 nitrocotton 1355 76. 6 511 30 1,5 ground smokeless powder-+3.5 'INI. 1250 57. 3 504 20 2.5 ground smokeless powder-H25 'lN'l 1365 G4. 4 532 20 4.5 nitrostarch+2 ethyl cellulose 1040 51. 0 486 0 7.5 nitrostarch+2 ethyl ccllu se 1385 47. 7 509 20 4.5 nitrostarch-l-z polyvinylacctote 1190 46. 9 483 20 7.5 nitrostarch+2 polyvlnylacetnte 1275 50. 5 500 20 4.5 high grade nitrocotton 1260 53. 5 520 23 4.5 #2 nitrocotton (12.25%N) l 1255 52. 7 512 23 -4.5 #3 nitrocotton (12.6% N) 1220 55. 3 409 20 4.5 plastic nitrocotton (11.0 N 1155 54. 0 507 27 4.5 finish nitrocotton (6 sec.) 1140 48. 4 507 23 It will be apparent from the foregoing table that by the process of my invention satisfactory powders are readily prepared at very low densities, namely, 1 x 24" cartridges are prepared weighing less than 500 gms. In addition to exhibiting these low apparent densities when packaged, the aggregates themselves exhibit very low actual densities due to their loosely bound structure. For example, actual aggregate densities as low as 1.16 are readily attained by means of this structure in which the particles of the various ingredients are loosely bound together by adhesive forces alone, substantially in the absence of any appreciable pressing action. This result is believed to contribute largely to the remarkable success of the powders of my invention in the mining of coal.

I have determined that when ammonium nitrate of the preferred grain size is employed, better results are obtained when no more than 7.5% ground smokeless powder or nitrocotton is employed.

The attainment of explosives of the nature and properties shown in the foregoing tabulation and described throughout this application represents a notable advance in the art, for the reason that the explosives so characterized are more effective in the mining of coal than any heretofore known. For. instance, my explosives attain the goal of duplicating the desired sustained heaving action of black powder, while possessing a satisfactory sensitiveness to initiation and propagation, and a regular progressive decomposition of the explosive In actual tests in the coal mine, composi-' tions according to my invention have achieved the successful production of relatively large lumps of coal, a lower production of fines and shattering in the immediate vicinity of the borehole, and a clean breaking oil of the coal at the back of the cut.

Furthermore, in addition to the many advantages already noted, my explosives possess superior safety properties from the point of view of coal blasting in that detonation is not accompanied by a flame of any considerable length or duration. The compositions are free from nitroglycerin and other liquid explosive nitric esters. Hence, they do not possess any of the disadvantages which would accompany these materials. The product is insensitive to shock, friction, and sparks, so that the handling of the material is free from the hazards which ordinarily accompany the use of explosive compositions. The free-flowing, granular characteristics of the material facilitate its introduction into, boreholes of varying sizes and shapes.

Not only are the products of my invention characterized by numerous advantages, but the preferred process outlined also possesses definite advantages over any prior art process of explosive manufacture. It is important to note that shorter mixing times are required for incorporating the compositions by my preferred process than employed heretofore in the general manufacture of blasting explosives. In addition, less heat is required to dry the material than heretofore employed in processes for solvent types of powders. Furthermore, a lower solvent ratio is required than heretofore considered necessary in the manufacture of blasting explosives.

In describing the powder of my invention, I have referred to the aggregates as distinct granular explosive units. One theory for the unusually successful results obtained with my powder in the blasting of coal is based upon the concept that each of these finished granular aggregates constitutes an'independent explosive unit. By this, I mean that each unit is believed to be capable of supporting its 'own decomposition when once initiated. Thus, each separate granular aggregate is capable of entering into the progress of the detonation through the powder due to the fact that it is, in itself, a balanced and completely sensitized explosive unit. As such, it can act independently of its neighbors in entering into the detonation by undergoing decomposition. Perhap this may be the cause for the greatly sustained heaving action which characterizes the powder according to my invention. However, it should be understood that I do not base my invention upon this or any other theory, but rely only on the structure of the granular aggregates making up the powder, and the preferred process for forming this powder.

In the foregoing, I have described my invention in considerable detail with respect to specific compositions and methods of preparation. It will be understood, however, that my invention is not to be limited by these specific illustrations,

but that many variations may be made in the compositions and procedures outlined without departure from the scope of the invention. However, I intend to be limited only in accordance with the following patent claims.

I claim:

1. A granular detonating explosive especially adapted to the mining of coal, comprising ammonium nitrate intermingled with a binding agent and a sensitizing agent and agglomerated in the form of aggregates of loosely bound structure, said aggregates-having a size such that substantially all are held on a 50-mesh screen.

2. A granular detonating explosive especially adapted to the mining of coal, comprising ammonium nitrate agglomerated in the form of aggregates of loosely bound structure by means of a binding agent comprising nitrocellulose, said aggregates having a size such that substantially all are held on a 50-mesh screen.

3. A granular detonating explosive especially adapted to the mining of coal, comprising ammonium nitrate agglomerated in the form of aggregates of loosely bound structure by means of a binding agent comprising pyro nitrocotton, said aggregates having a size such that substantially all are held on a 50-mesh screen.

4. A granular detonating explosive especially adapted to the mining of coal, comprising ammonium nitrate agglomerated in the form of aggregates of loosely bound structure by means of a binding agent comprising ground smokeless powder, said aggregates having a size such that substantially all are held on a 50-mesh screen.

5. A granular detonating explosive especially adapted to the mining of coal comprising ammonium nitrate intimately intermingled with particles of a, sensitizing agent and a binding agent, and agglomerated in the form of aggregates of loosely bound structure, said aggregates having a size such that substantially all are held on a SO-mesh screen, the ammonium nitrate entering into said aggregates being of such a state of subdivision that not less than 40% passes a 48-mesh screen.

6. A granular detonating explosive especially adapted to the mining of coal comprising ammonium nitrate intimately intermingled with not more than 7.5% of nitrocellulose and agglomerated in the form of aggregates of loosely bound structure by means of the binding action of said nitrocellulose, said aggregates having a size such that substantially all are held on a 50- mesh screen, the ammonium nitrate entering into said aggregates being in such a state of subdivision that not less than 40% passes a 48-mesh screen.

'7. The explosive of claim 6 wherein the nitrocellulose comprises nitrocotton.

8. The explosive of claim 6 wherein the nitrocellulose comprises pyro nitrocotton.

9. The explosive of claim 6 wherein the nitrocellulose comprises ground smokeless powder.

10. The method for preparing a granular detonating explosive especially adapted to the mining of coal, which comprises intimately intermingling ammonium nitrate with a binding agent and a sensitizing agent, compacting the inter mingled materials lightly to form loosely bound aggregates, and screening said aggregates, removing and discarding substantially all those passing a 50-mesh screen.

11. The method for preparing a granular detonating explosive especially adapted to the mining of coal, which comprises intimately intermingling ammonium nitrate with nitrocellulose, softening said nitrocellulose by means of a solvent compacting the wet mass lightly to form loosely bound aggregates, drying said aggregates and screening the same to eliminate substantially all material passing through a 50-mesh screen.

12. The method for preparing a granular detonating explosive especially adapted to the mining of coal, which comprises intimately intermingling ammonium nitrate with ground pyro nitrocotton, softening said ground pyro nitrocotton by means of a solvent, compacting the wet mass lightly to form loosely bound aggregates, drying said aggregates and screening the same to eliminate substantially all material passing through a SO-mesh screen.

13. The method for preparing a granular detonating explosive especially adapted to the mining of coal, which comprises intimately intermingling' ammonium nitrate with ground pyro nitrocotton, treating the intermingled ingredients with a. solvent to produce a mucilaginous condition on the surface of the particles of said nitrocotton, compacting thewet mass lightly to form loosely bound aggregates, drying said aggregates and screening the same'to eliminate substantially all material passing a 50-mesh screen.

'ents with a solvent to l4. The method for preparing a granular detonating explosive especially adapted to the mining of coal, which comprises intimately intermingling ammoniumnitrate with ground smokeless powder, treating the intermingled ingrediproduce a mucilaginious condition on the surface of the particles of said ground smokeless powder, compacting the wet mass lightly to form loosely bound aggregates, drying said' aggregates and screening the same to eliminate substantially all material passing a 50-mesli screen; 1

' MILTON HEINS WAHL