US2987389A - Ammonium nitrate explosive - Google Patents

Description

Patented June 6 1961 2,987,389 AMMONIUM NITRATE EXPLOSIVE Wayne A. Proell, Chicago, 111., and William G. Stanley, Hammond, Ind., assignors to Standard Oil Company, Chicago, 11]., a corporation of Indiana No Drawing. Filed Feb. 26, 1952, Ser. No. 273,564 7 Claims. (Cl. 52El4) This invention relates to new and improved explosive compositions and also to compositions for the generation of gas at high pressure. Particularly, the invention relates to explosive compositions wherein ammonium nitrate is the principal or sole gas-producing agent. Still more particularly, the invention relates to a readily ignitable explosive comprising ammonium nitrate, an oxidizable material and a combustion catalyst.

Ammonium nitrate is widely used as a component of high explosives. Although ammonium nitrate is classifled as a high explosive, it is extremely insensitive and cannot readily be detonated by the local application of heat or by a blasting cap; and when ignited, ammonium nitrate does not sustain propagation consistently. Normally ammonium nitrate is mixed with an oxidizable material, such as, sulfur, carbon, cellulosic materials, hydrocarbons, etc., in order to utilize the excess oxygen available in the ammonium nitrate. However, these mixtures of ammonium nitrate and oxidizable materials are also either very insensitive or slow burning.

One of the well known methods of overcoming this lack of sensitivity is to use a sensitive high explosive to prime the detonation of the ammonium nitrate explosive. Examples of suitable primers are tetryl, TNT, nitrostarch, nitrocellulose, nitroglycerine, etc. An explosive that is detonable by the action of a blasting cap can be obtained by mixing the ammonium nitrate with the sensitive materials, such as, nitrostarch and nitrocellulose. The extreme sensitivity of these explosives makes them undesirable for ordinary blasting use.

Another method of obtaining a readily ignitable ammonium nitrate explosive is to admix therewith an organic sensitizer, such as, nitrogen compounds and certain carbohydrates. In general, satisfactory explosives are obtained only when the sensitizer is very intimately dispersed throughout the mass. Generally this dispersion is obtainable only by the use of complicated and expensive procedures.

The most commonly used method for improving the sensitivity of ammonium nitrate explosives is to add a combustion catalyst. The commercially used combustion catalysts are all based onthe element chromium. The more common chromium combustion catalysts are ammonium or alkali metal chromates or polychromates; chromic oxide, chromic nitrate and copper chromite. The preferred material is ammonium dichromate. While the chromium compounds are the best known combustion catalysts, they have the disadvantages of being expensive and of, frequently, being in very short supply. The relative toxicity of the chromates makes them hazardous unless handled with considerable care. A particular disadvantage to the use of the chromium combustion catalysts is that the ammonium nitrate is sensitized to such an extent that hot melt formulation cannot be used.

This disadvantage has been overcome, in part, by adding a melting point depresent to the ammonium nitrate combustion catalyst mixture.

An object of this invention is the preparation of a new and improved inorganic nitrate explosive, in particular, an ammonium nitrate explosive. Another object is the preparation of a readily ignitable explosive comprising ammonium nitrate and a combustion catalyst.

Still another object is to provide a combustion catalyst for ammonium nitrate explosives which is cheap and in plentiful supply. A particular object of this invention is an explosive mixture comprising ammonium nitrate, an oxidizable material and a particular burning catalyst, which mixture can be formulated by a hot melt process.

Briefly, the explosive mixture of our invention comprises a mixture of ammonium nitrate and an effective amount of a combustion catalyst selected from the class of iron-iron cyanide complexes, alkali metal-iron-iron cyanide complexes and ammonium iron iron cyanide complexes. In addition to the ammonium nitrate, moderate amounts of other inorganic nitrates, such as, potassium nitrate, sodium nitrate and magnesium nitrate, may be present. We prefer an explosive mixture which con tains an oxidizable material in an amount sufficient to utilize the excess oxygen available from the decomposition of the ammonium nitrate. The amount of combustion catalyst present in the mixture will vary with the particular catalyst and, somewhat, with the particular oxidizable material present. In general, we use between about 0.5 and 25 weight percent of combustion catalyst.

We have discovered that certain iron compounds are effective catalysts for sensitizing the ignition and the combustion of ammonium nitrate. When added in sufficient quantity, these catalysts permit ammonium nitrate or mixtures of ammonium nitrate and. sodium or potassium nitrate, or mixtures of ammonium nitrate and oxidizable material to be ignited readily by the application of localized heat or by a commercial blasting cap. Our combustion catalyst does not appreciably increase the shock sensitivity of the ammonium nitrate and causes only a moderate increase in the decomposition rate of ammonium nitrate at elevated temperatures. Some of the compounds which have been found to be effective are: ferro ferrocyanide, ferric ferrocyanide, ferro ferricyanide, ferric ferricyanide, potassium ferric ferrocyanide, sodium ferric ferrocyanide, ammonium ferric ferrocyanide, potassium-soluble Prussian blue, sodiumsoluble Prussian blue and ammmonium-sodium-soluble Prussian blue.

All the combustion catalysts of our invention contain the iron cyanide radical, either ferrocyanide or ferricyanide. In addition to the iron cyanide radical, our burning catalyst contains a second iron ion which may be either the ferric or ferrous radical. The combustion catalysts of our invention include those compounds which contain Water of hydration as well as the anhydrous form. The more effective catalysts contain an alkali metal or an ammonium ion in addition to the iron-iron cyanide complex. The better catalysts are ammonium ferric ferrocyanide, sodium ferric ferrocyanide, potassium ferric ferrocyanide and mixtures of sodium or potassium ferric ferrocyanide and ammonium ferric ferrocyanide. The extremely complex mixture used as a water soluble blue dye, the so-called soluble Prussian blue, is an excellent combustion catalyst. The sodium derivative of soluble Prussian blue is an exceptionally effective catalyst and is better than the better known potassium derivative of soluble Prussian blue. Some of the commercial soluble Prussian blues contain both sodium and potassium.

We have found that the most active variety of the combustion catalyst of our invention is a soluble Prussian blue containing sodium and ammonium. This catalyst must be prepared by a controlled sequence of steps. These steps are: (1) reacting in aqueous solution sodium ferrocyanide and ferrous sulfate, (2) precipitation of ferrous ferrocyanide in the presence of excess ammonium sulfate, and (3) oxidation of ferrous ferrocyanide with bromine or sodium chlorate to the ferric ferrocyanide. The product of this procedure contains both sodium and ammonium ions and is spoken free oxygen; additional energy can be obtained by the presence of an oxidizable material which combines with this free oxygen. Any material which contains a deficiency of oxygen can be utilized. Metals such as aluminum and magnesium may be added. The non-metallic elements sulfur and carbon can be used in our explosive mixture. Nitrogen-containing organic compounds that; do not unduly sensitize the explosive mixture are particularly good; examples of these are urea, nitroguanidine, guanidine nitrate, and mononitrate naphthalene. .Cellulosic materials are very desirable oxidizable materials, e.g., wood flour, cellulose acetate, etc. Because of their cheapness, hydrocarbon materials are a preferred oxidizable material. Examples of these are: parafiin -waxes, petrolatum, high boiling hydrocarbon oils, tars, asphalts, bitumen, coal tar, shale oil residue, etc.

The amount of oxidizable material that may be added is dependent upon the particular type of material, but

' no more oxidizable material should be added than can react with the available oxygen to yield soot-free gas. When using hydrocarbon materials, in general, the maximum addition is between 20 and 25% by weight based von ammonium nitrate present. We prefer to use explosive mixtures which are stoichiometrically balanced with respect to oxygen content.

It has been observed that this catalyst burns to a 'fine black dust, much different from the flufiy ash given off by the chromium catalysts.

The stability of ammonium nitrate in the presence of our combustion catalyst was investigated by measuring the gas evolved from the mixture at various temperatures. A mixture of 94% ammonium nitrate and 6% ferric ferrocyam'de and a mixture of 94% ammonium nitrate and 6% sodium-ammonium soluble Prussian blue was held for eight hours at 135 C.; no evolution of gas "was detected. Fresh samples of the above mixtures were held at 170 C. for several hours; gas was given oif to the extent. of 3 .to 4 ml. per hour per gram of sample. These tests show that a mixture of ammonium nitrate "and our combustion catalyst is sufficiently stable at elevated temperature to permit formulation by a hot melt procedure.

Burning tests were carried out both in air and in an inert atmosphere on mixtures of ammonium nitrate and sodium soluble Prussian blue, ferric ferrocyanide, ferric ferricyanide, ferrous ferrocyanide, ferrous ferricyanide and sodium-ammonium soluble Prussian blue. The minimum amount of combustion catalyst needed varied with the particular compound. When using the ammonium-sodium-soluble Prussian blue, a mixture of 88% ammonium nitrate and 12% catalyst ignited readily in large grains and burned smoothly. When using the iron- Iiron cyanide complexes in the absence of the alkali metal ions, it was necessary to use on the order of 15 to 25 weight percent of catalyst in order to obtain easy ignition and smooth combustion.

The effectiveness of our preferred combustion catalyst, i.e., ammonium-sodium-soluble Prussian blue, was tested on several ammonium nitrate-oxidizable material mixtures. Test pellets of the desired composition were made as follows: The ingredients were weighed into a 100 m1. beaker and then thoroughly mixed by a spatula. In some cases, the mix was ground in a mortar in order to break up small lumps of ammonium nitrate. The mixture was pressed into a pellet about 1%. inches long and A inch in diameter by means ofa small press using about 50 lbs. pressure on the plunger. In order to insure -trate, 77%; wood flour, 15%;

uniformity duplicate pellets were made and tested in each case. The burning characteristics of each composition were determined after wrapping each pellet with Scotch cellophane tape so that only the ends were exposed. The wrapping forced the pellet to burn cigarette-fashion. The wrapped pellet was placed on a clay tile and ignited by a Bunsen burner. When the pellet was burning briskly, a 250 ml. beaker was placed over it in order'to determine the effect of inert atmosphere on the burning. Burning rates in inches per second were determined by stopwatch; each rate is an average of at least two trials.

Test 1 The mixture in this test consisted of ammonium niand catalyst, 8%. This mixture ignited readily and burned smoothly and completely in an inert atmosphere. The burning rate at atmospheric pressure was 0.0149 in./sec.

Test 3 In this test the mixture consisted of ammonium nitrate, 84%; asphalt, 8%; and catalyst, 8%. The mixture ignited readily and burned smoothly and completely in an inert atmosphere. The burning rate at atmospheric pressure was 0.0104 in./sec.' The asphalt used in this test had a flash point of 585 F., a softening point of 98" F. and a density of 1.010.

Test 4 ture of this. test. The burning rate of the ammonium dichromate mixture was 0.014 in./sec.

Test 5 The mixture in this test consisted of 75% ammonium nitrate, 6% of catalyst and 19% of a cellulose acetateglycol diglycolate mix. This test was carried out in a bomb at 1000 p.s.i. pressure. The burning rate at this pressure was 0.156 in./sec.

It is obvious that the above mixtures would be effective explosives when the combustion occurs in a confined space as is the usual situation for an explosive. The rate of gas generation normally increases with temperature and pressure in the burning zone and our mixtures can be used to generate gases at high pressure by using a pressure regulator on the exit conduit.

When using a mixture of ammonium nitrate, a hydrocarbon material and a catalyst containing an alkali metal ion, an ammonium ion or mixtures thereof, we 'prefer to use between about 2 and 10 weight percent of catalyst, between about 5 and 25v weight percent of hydrocarbon, and the reminder ammonium nitrate.

The term ammonium nitrate as used in this specification and in the claims is intended to mean either ordinary commercial grade ammonium nitrate, such as, conventionally grained ammonium nitrate containing a small amount of impurities and which is then generally coated with a small amount of moisture-resisting material such as petrolatum or paraflin, or military grade ammonium nitrate, or a mixture of other inorganic nitrates and ammonium nitrate wherein the ammonium nitrate is the preponderate nitrate.

We have. discovered that the effectiveness of our combustion catalyst can be improved remarkably by the addition of certain materials, which we call synergists. These materials may or may not be oxidizable materials and have substantially no catalytic elfect in the absence of a combustion catalyst. The maximum effectiveness is at low concentration, i.e., from about 0.5 to 5 weight percent. When usingthese synergists, the amount of combustion catalyst needed to render the mixture readily ignitable may be as little as 0.5 weight percent. Examples of these synergists are: phenothiazine, thiazolidinethione, linseed oil and sucrose; magnesium stearate and magnesium ricinoleate are particularly effective with cellulosic materials; aluminum stearate, aluminum oleate and aluminum ricinoleate are particularly suitable when using hydrocarbons as the oxidizable material.

Our explosive mixture can be made by milling the ingredients or by dry mixing; this operation is preferably followed by forming regular shaped compacted grains by pressing the powdered mix in molds. We prefer to prepare our shaped grains by adding the powdered am monium nitrate to fused organic or oxidizable material at 100-125 C., mixing. to form a paste, and pressing the paste into suitable molds. The cooled grains are strong and durable.

Having described our invention, what we claim is:

1. A composition for the generation of gas which consists essentially of ammonium nitrate as the predominant component, an oxidizable material and between about 0.5 and 25 weight percent of soluble Prussian blue combustion catalyst.

2. The composition of claim 1 wherein said catalyst is prepared by reacting sodium ferrocyanide and ferrous sulfate in aqueous solution, adding thereto excess ammonium sulfate to precipitate ferrous ferrocyanide, adding sodium chlorate to oxidize said precipitate and recovering a soluble Prussian blue containing sodium, ammonium, ferric and ferrocyanide ions.

3. The composition of claim 1 wherein said mixture is about stoichiometrically balanced with respect to oxygen.

4. The composition of claim 1 wherein said catalyst contains ammonium ions.

5. The composition of claim 1 wherein contains sodium ions.

6. A readily ignitible explosive mixture which consists essentially of about 5 to 25 weight percent of hydrocarbon material, about 2 to 10 weight percent of soluble Prussian blue and the remainder ammonium nitrate.

7. The composition of claim 6 wherein said hydrocarbon material consists of asphaltic hydrocarbon.

said catalyst References Cited in the file of this patent UNITED STATES PATENTS 43,021 Halvorson June 7, 1864 1,021,882 OBrien Apr. 2, 1912 1,071,949 OBrien Sept. 2, 1913 1,890,112 Fisher Dec. 6, 1932 FOREIGN PATENTS 14,196 Great Britain 1897

Claims (1)

1. A COMPOSITION FOR THE GENERATION OF GAS WHICH CONSISTS ESSENTIALLY OF AMMONIUM NITRATE AS THE PREDOMINANT COMPONENT, AN OXIDIZABLE MATERIAL AND BETWEEN ABOUT 0.5 AND 25 WEIGHT PERCENT OF SOLUBLE PRUSSIAN BLUE COMBUSTION CATALYST.