US2199217A - Granular ammonium nitrate explosive - Google Patents

Description

Patented pr. 30, 1940 uTEo STATES GRANULAR AMMONIUM mm EXPLOSIVE Melvin A. Cook, Donald B. Gawthrop, and Milton H. Wahl,

Woodbury, Media,

Woodbury, N. 3., and Clifiord A.

assignors to E. I.

du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application April 21, 1939,

Serial No. 269,102

10 Claims.

The present invention relates to new and improved explosive compositions and methods of manufacturing the same, and, in particular, to a detonating explosive especially desirable for coal blasting.

The so-called permissible dynamites now in extensive and successful use in the mining of coal represent a marked advance in many respects over black powder, the explosive commonly used in coal blasting prior 'to the introduction of permissibles. The latter type are detonating explosives and possess the desired safety properties essential when the explosive is to be used in the presence of inflammable mine gases. The permissible explosives are at a disadvantage in one respect, however, when compared with black blasting powder, a defiagrating explosive, in that they do not exert the characteristic sustained heaving action that results with the latter explosive. Consequently, difiiculties are met when using permissibles with certain types of coal in obtaining a satisfactory grade of lump coal. A marked advance in the art would be effected if an explosive could be designed which would possess the blasting characteristics of black powder while at the same time representing no backwardstep with respect to the safety features of the best of the permissibles.

The object of the present invention is an improved explosive composition especially adapted to the mining of coal, said composition having an improved blasting action in this respect over conventional coal mining explosives. object is an improved detonating explosive which 35 exhibits a 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 w is an improved detonating explosive which possesses the blasting characteristics of black pow der, yet exhibits the safety features of the permissibles. Additional objects will be disclosed as the invention is described more fully hereinafter. These objects are achieved and the foregoing disadvantages overcome by binding together particles of onium nitrate explosive composition into sized aggregates by means of a bindingagent substantially insoluble in liquid explosive nitric esters, and coating said sized aggregates with a liquid explosive nitric ester in an amount not exceeding by weight. The resulting product should be in the form of grains of such size that substantially all are held on a BO-mesh screen.

, range holds for 1%" x 6" cartridges.

For instance, highly satisiactoryresults are ob-- tained from compositions wherein the grains are retained on an 18-mesh screen.

In its most preferred form our invention may be described as a granular powder displaying an apparent density measured by a stick count of 5 from 150 to 215 cartridges of l x 8" in size per 50-lbs. of powder. This same stick count Our preferred compositions display a sensitiveness of at least 10', in the half cartridge sensitiveness test, 10 and exhibit velocities below 1450 meters per second. These characteristics are preferred features and are not to be considered as limiting the broad scope. of the invention.

In greater detail, we blend a composition comprising ammonium nitrate with a material capable of exerting a wetting action on the surfaces of the ammonium nitrate particles and other materials present under the influence of a small amount of aqueous solution or other suitable liquid, and of becoming a tenacious solid subsequently, thus functioning as an adhesive to bind the ingredients together. The blended material so constitutedis in the form of firm, freefiowing grains of controlled size, and these grains are given a coating of liquid explosive nitric ester in amount sufficient to bring to detonation the entire explosive'composition, yet not exceeding l0% by weight.

As binding agent or adhesive, we find starch highly elfective and advantageous, since, in the presence of water or aqueous solution and under the influence of heat, the starch assumes a gelatinous condition. 0n subsequent cooling or removal of solvent, the starch becomes hard and 35 tenacious, exerting a. firm adhesive efiect upon the materials which it has previously contacted. We do not intend to be limited to the use. of starch as binding agent, however, and various other materials can be caused to function satis- 4! factorily, for example, agar-agar, casein, animal glue, and the like; in fact, any material capable of becoming mucilaglnous and adhering to the solid ingredients present, and of becoming hard and tenacious on cooling or loss of solvent. As 44 already noted, the binding agent must be substantially insoluble in nitroglycerin or other explosive nitric esters in order to be utilizable satisfactorily according to our invention.

Our invention is particularly applicable to explosives high in'ammonium nitrate content, for example, between and The available oxygen present in ammonium nitrate requires the presence of sufiicient omgen-accepting material, if an efiective omen-balanced explosive I is to result. Starch itself or other organic adhesive may serve this purpose, and we find excellent results to be obtained when an essentially starch granular material is used, such as a flaked corn product, and when this is properly gelatinized by a small amount of water with heat and is subsequently cooled. We may'employ, however, anabsorbent, carbonaceous material such as woodpulp, bagasse pith, cornstalk pith, sawdust, and the like, together with an adhesive suchas starch. In this latter case, the cells of the absorbent material presumably become filled with the adhesive or the material becomes completely coated therewith.

In order to bring about gelatinization of the adhesive, it is sufficient to wet the surface thereof. This can be accomplished by the direct introduction of water into the mixture of some or all of the ingredients. The presence of an excess of water would defeat the purpose of its use, and we find it desirable to introduce an amount of water not exceeding 5% of the explosive composition, for example, between 1.5 and 3.5%. With such a water content, the starch becomes of mucilaginous consistency on heating, and brings the mixture into plasticform. On cooling, the adhesive becomes hard and tenacious and serves as an efiective binding agent.

While the use of Water has been described for bringing about gelatinization of the adhesive, other liquids'may servealso. We may, for example, employ substantially anhydrous ammonia for this purpose, and inthis case heating may be unnecessary, since gelatinization takes place in the cold and the solvent can be readily re- 'moved by volatilization. Avery effective method also of introducing liquid into the mixture of ingredients is by the employment of salts containing water of crystallization. Hydrated sodium thiosulfate (Na2S203'5HzO) sodium acetate (NaC2H302-3HzO) sodium tetraborate (Na2B40'l H20) water of crystallization, the water content is de-- sirably controlled in substantially the amount previously stated to be most effective, namely, not over 5% of the entire composition. In place of hydrated salts, we may likewise employ anhydrous salts or salts serving merely as agents to facilitate the fusion of ammonium nitrate, with or without the addition of water to the blend.

The principle of ourinvention is to blend together the materials described in the foregoing, form them into granular agglomerates of a controlled size, and then effect proper sensitization. In mixing the ingredients with the addition of heat, aplastic blendis attained. While still in the plastic state, the material is granulated, preferably by passing through a screen of the desired mesh. on cooling, firm, hard grains'result and the finished explosive is obtained by introducing sufflcient nitroglycerin or other liquid explosive nitric ester to give the desired surface coating. The use of starch or other nitroglycerin-insoluble adhesive insures that there will belittle penetra- Example 1 3950 grams of ammonium nitrate, of such a degree of fineness that not more than was held on a 35-mesh screen, was introduced into a mixing apparatus, together with 250 grams of sodium nitrate and gramsof calcium carbonate. After agitation had begun, the dry salt mixture was heated to 75 C. by means of steam in the surrounding jacket. 305 grams of hydrated sodium thiosulfate (Na2S2O3'5H2O) was then added, followed immediately by 150 grams of starch. Mixing was continued until the starch had become gelatinized and the charge was of such consistency that it resembled bread dough, and remained as a single mass adhering to the revolving mixer blades. 320grams of bagasse pulp was then added and mixed thoroughly into the blend. 'The temperature was maintained below 90 C.- After addition of the bagasse, the mixture was cooled by means of cold water introduced into the jacket. When the charge had partially cooled but while it was still plastic, it was discharged from the mixer and passed through a B-mesh screen. After cooling to room temperature, the grains were tough and durable. The material" held on a -mesh screen was mixed with nitroglycerin in the proportion of 93.5 parts of blend to 6.5 parts of nitroglycerin.

When packed in 1 x 8" cartridges, 167 cartridges' per 50 lbs. were obtained. The explosive had a velocity of detonation of 1345 m./sec., and a'sensitiveness of 12", as obtained by the propagation test, where two half-sticks of the explosive are separated from one another and the maxi- -mum distance of separation is determined at which consistent detonation of the second cartridge by the first takes place.

Example 2 parts of nitroglycerin being used per 93.5 parts 1 of the salt blend. The finished explosive had a velocity of 1380 m./sec. and a sensitiveness of 10". In preparing the explosive of this example, the flaked corn product used was of the approximate size of granule desired in the sensitized explosive product.

Example 3 3825 grams of ammonium nitrate of relatively fine granulation was introduced into the mixer, together with 500 grams of sodium nitrate and 25 grams of chalk. After agitation had begun, the temperature was raised to about 80 C., and 400 grams of sawdust, 150 grams of starch and 100 grams of water were added. Mixing was Example 4 3875 grams of ammonium nitrate of fine granulation was introduced into the mixer, together with 250 grams of sodium nitrate and 25 grams of calcium carbonate. The mixture was agitated, the temperature was raised to 80 C., and 300 grams of hydrated sodium thiosulfate and 150 grams of starch was added. Agitation was continued to the desired point, and until the charge had acquired the proper consistency. The mixture was screened after partial cooling, as in Example 1, and nitroglycerin was incorporated with the granular material, the amount used being in the ratio of 6.5 parts nitroglycerin to 93.5 parts of the grained material. The explosive after cartridging had a velocity of 1085 m./sec., and a sensitiveness of Example 5 v 3925 grams of ammonium nitrate, 250 grams of sodium nitrate and 25 grams of chalk were introduced into the mixing apparatus. During agitation, the temperature of the ingredients was raised by means of steam in the jacket to approximately 85" C. 300 grams of hydrated sodium thiosulfate and 500 grams of a flaked corn product were then added. Mixing was continued until the starch of the flaked corn product had become gelatinized and the proper consistency had been obtained in the charge. -The mixture was then cooled somewhat and screened as in Example 1. Nitroglycerin was incorporated with the firm, granular material in an amount corresponding to 6.5 parts nitroglycerin to 93.5 parts of the grains. The cartridged explosive had a velocity of 1320 meters per second and a sensitiveness of 12 to From the procedure described in the examples above, and from the discussion of our invention in the foregoing, it will be understood that the invention has to do with explosives high in ammonium nitrate and that various other salts may be present with the ammonium nitrate. Such other salts may include various oxidizing agents, particularly sodium nitrate, though other similar oxygen-carrying salts may be used equally well, for example, potassium nitrate, magnesium nitrate, calcium nitrate, and others. It may be desirable also to use various materials as fusion promoters for the ammonium nitrate, for example, the various hydrated salts, urea, and other materials soluble in molten ammonium nitrate. Such salts and compounds, by promoting the fusion of ammonium nitrate, assist in bringing about the wetting of the surface of the adhesive.

In coal blasting ammonium nitrate explosives of the prior art, it has been the almost invariable custom to employ ammonium nitrate of relatively coarse granulation, since the control of particle size of this explosive salt has been the recognized method for varying detonation velocity. We find, however, that the use of coarse ammonium nitrate is not necesary for making the granular explosive of our invention. Instead we find that the use of relatively fine ammonium nitrate is quite satisfactory for this purpose. For instance, suitable powders according to our invention are prepared with ammonium nitrate of, such grain size that at least 40% will pass a 48-.mesh screen, while not more than 10% will beheld on a 24- mesh screen. This range is satisfactory but not limiting.

As has been stated, our finished explosive is in the form of relatively coarse grains which are firm and free-flowing. While a range of grain size can be employed, we find it essential that substanifilly all be held on a 30-mesh screen. In the examples cited, the preferred method of preparing the grains was to screen the blended material while still in plastic condition. Other methods for preparing the grains may be employed, however, such as passing the plastic'material into an extruder operated on the meatchopper principle and, extruding through a disc having openings of predetermined size. The method of preparing the grains is not a part of our invention.

It should be emphasized that the material used as adhesive for obtaining a firm, rigid grain must be one which is substantially insoluble in nitro-- glycerin or other liquid explosive nitric ester. In this way penetration of the liquid explosive into the grain is prevented, and increased sensitiveness therebyresults.

The advantage of the product according to our invention lies in the fact that a permissible explosive is obtained of highly attractive properties as a coal-getter. In spite of the low velocity, there is no sacrifice in strength or execution. To an unusual degree the slow heaving action of black powder is obtained, with all the safety features of permissible explosives high inammonium nitrate. Due to the increased sensitizing action of a given amount of nitroglycerin in the explosive composition, a low nitroglycerin content is sufficient to obtain the high degree of sensitiveness previously brought about only by the use of objectionably large amounts of nitroglycerin.

Furthermore, our product possesses marked superiority over explosive compositions of the pulverulent type having approximately the same composition. For instance, comparisons between our granular powders and pulverulent powders having identical low nitroglycerin contents clearly establish that our particular granular compositions display increased propagating ability, lower velocities and improved heaving action in coal blasting.

While our invention has been described in detail in the foregoing it will be understood that many variations may be made in the exact method of preparation or in compositions without departure from the spirit of the invention. We intend to be limited, therefore, only by the following patent claims.

We claim:

1. A detonating explosive especially adapted to the mining of coal, characterized by a density equivalent to a cartridge count of 150 to 215 cartridges of 1%" x 8" size per 50 lbs. of powder, a sensitiveness of at least 10" as measured by the half cartridge sensitiveness test, and a velocity less than 1450 meters per second, said explosive comprising an ammonium nitrate explosive com- ,position, the particles of which are agglomerated coated with not more than 10% of a liquid explo- 10 stantially all are held on a 30-mesh screen.

3. A detonating explosive especially adapted to the mining of coal, comprising a blend of ammonium nitrate, a carbonaceous combustible material, and a binder substantially insoluble in nitro- 15 glycerin, said blend being in the form of firm sized aggregates of such size that substantially all are held on a 30-mesh screen, said aggregates having a surface coating. of nitroglycerin in an amount not more than 10% of the entire explosive 2o composition.

4. A detonating explosive especially adapted to the mining of coal,comprising a blend oi! ammonium nitrate, a carbonaceous combustible material, a binder substantially insoluble in nitro- 25 glycerin, and a salt containing water of crystallization, said blend being in the form of firm sized aggregates of such size that substantially all are held on a 30-mesh screen, said aggregates having a surface coating of nitroglycerin in an 30 amount not more than 10% of the entire explosive composition.

5. The explosive of claim 4, in which the salt -containing water of crystallization is hydrated sodium thiosulfate.

6. The explosive of claim 4, in which the salt containing water of crystallization is sodium acetate.

7. The explosive of claim 4, in which the salt containing water of crystallization is sodium tetraborate.

8. The explosive of claim 4, in which nitro glycerin is present in an amount between 5 and 8%.

9. A detonating explosive especially adapted to the mining of coal, comprising a blend of ammonium nitrate, a carbonaceous combustible material, and starch in hard tenacious form, said blend being in the form of firm sized aggregates of such size that substantially all are held on a 30- mesh screen, said aggregates having a surface coating of nitroglycerin in an amount not more than 10% of the entire explosive composition.

10. A detonating explosive especially adapted to the mining of coal, comprising an ammonium nitrate explosive composition the, particles 0! which are agglomerated in the form of sized aggregates by means of a binding agent substantially insoluble in liquid explosive nitric esters, said aggregates being coated with not more than 10% oi a liquid explosive nitric ester, and having a size such that substantially all are held on a 30-mesh screen.

MELVIN A. COOK. DONALD B. GAWTI-IROP. IVHLTON H. WAHL. CLIFFORD A. WOODBURY.