US2409919A - Low density granular or powder explosives and their manufacture - Google Patents

Description

Patented Oct. 22.1%46

2 cans deterrence LOW DENSITY GRANULAR OR POWDER EXPLOSIVES AND THEIR MANUFAC- John Whetstone, West Kilbride, Scotland, assignor to imperial Chemical Industries Limited, a corporation of Great Britain No Drawing. Application September 19, 1942, Se-

rial N0. 458,993. 15, 1941 '9 Claims. 1

The present invention relates to granular detonating explosive compositions of the kind in which the granules individually comprise ammonium nitrate and aneasily fusible .organic explosive compound in a state of intimate admixture. The invention relates more particularly to the provision of new or improved granular explosive compositions as aforesaid that have low density characteristics, and to an improved process for the production of low density granular explosives suitable for commercial blasting purposes.

Blasting explosive powders containing ammonium nitrate and organic explosive compounds are :usually manufactured by grinding the solid ammonium nitrate and the organic explosive togethen even when, as in the case of fusible aromatic nitro-compounds such as TNT, the organic explosive compound is easily fusible. Any additional ingredients of the composition are introduced during or subsequent to the grinding operation. Prolonged grinding is required inorder'to produce an explosive satisfactorily sensitive to initiation by a. commercial ,detonator. Granulated compositions made from the resulting powclers ordinarily show a bulk density of about-0.9 to 1.15 grams, per cubic'centimetre when car- .tridged under a pressure of 75 lbs per square inch.

Low density ammonium nitrate blasting explosive powdersof the kind in which the low-density ,is not occasioned by the useof low density vegetable tissue, vonthe contrary, are made by lightly mixing ammonium nitrate crystal aggregates .of vlow density with the solid oxidisable ingredient's. The low density ammonium nitrate crystal aggregates facilitate the attainment of a composition initially sensitive to detonation, but must not be .brokendown, and low density blasting powders of this kind frequently suffer from the defect that the granules constituting them are .so .fragile that they disintegrate .very easily Whenjostled, egg. .in transport, or on storage, and

thereby .become denser and less sensitive.

Military explosives containing ammonium nitrate and aromatic nitro-compoundscan be made bycoating solid ammonium nitrate with the fused aromaticv nitro-compound,-the explosive compositions so obtained being compressed; or cast, or cast and compressed into the ammunition in which they are to be used. They are, however, seldom sensitive to initiation by ordinary detonators, and in reflecting their detonation it is necessary to employ an intermediate primer or booster whereby the detonating impulse transmitted from thedetonatoris enhanced. The density'of-such charges usua1ly exceeds 1.4 and their ii w e In Great Britain September velocity of detonation and their negative oxygen balance are usually higher than is desirable for commercial blasting purposes.

:The fusion temperature of ammon um nitrate is too high to enable it to be safely melted With fusible or anic xpl sive co p ds. u it an be reduced by the inclusion of additional materials such as salts having Water of crystallisation, and by vari us alts an or a i materials that .fo m t cti mix ures with it. In t ma ufac- .ture of mstfiXpMsW charges. such materials, usually pf a xid s n or ,reducina i aracter, e be n included .in molten compo it ons co ins ammonium nitrate and organic explosive poun s. w th th object of minim sing the ontent of the org ni explcsire compound required o y eld a c pos t on that can c eniently be loaded. These compositions containing ammonium nitrate in a more or less completely fused condition, however, suffer from the defect that the J gu fied ino anic salts and e molten organic explosive constitute two immiscible liquids tha a on vheco sc y dispersed bya itamom-9 thelngeltso tha the s lt n ixtu e celie ate ap dly heter it olidifies. and the reult g cas exp os ve char es are of lower se itivene s toi it ation than those made by oating the solid oxidising salts with molten organic explosive and ,mayfail to propagate their detonaonpmn r y- 1 11 th p eparatio of cast e pl s v char such as arel sedfor filling containers, for example shells or other projectiles, grenades, ,mines and the'likch thas been .pr posedto emu s fy with th aid of anflmulsifyingagent a highlyexplosive organic compound .or compounds in a .melt of an oxidising salt or salts, for instance ammonium nitrate with or without metallic nitrates. Accordingto this proposal, the emulsifying agent maybe a, colloidal substance soluble or swellable by water .a-ndcapable of increasing the viscosity of the melt, andthe-formation of the saline melt into which the highexplosive organic'compound, for instance TNT, is incorporated, may be assisted by the presence of ahydrated salt, an organic' amidegor a small proportion of water, "the resulting emulsified explosives jbeing characterised by improved sensitiveness to detonation and other advantages as compared with similar com positions made .by.a fusion process or partial fusiOn .process without the use of the emulsifying agent. ,one vof these advantages is that compositions of sulflficientjfluidity for loading into ammunition may be obtained at safe workingtem- SEEM till llUUru 3 peratures, e. g. 80 to 110 C., with a reduced proportion of the fusible high explosive compound.

Massive cast explosive charges produced according to the method of the aforementioned proposal, however, are insufficiently sensitive to be initiated by an ordinary commercial detonator. and are unsuitable for commercial blasting purposes.

It is an object of the invention to provide granular low density ammonium nitrate explosives by a fusion process employing molten fusible organic explosive compounds, which will be of suitably high sensitiveness to initiation for commercial blasting purposes, but which will be free from the aforesaid defects of the hitherto known low density explosives made from ammonium nitrate and solid organic explosives or other oxi disable ingredient of ordinary density.

According to the present invention, in the manufacture of detonating exposives suitable for commercial blasting purposes, a molten easily fusible organic explosive is emulsified, with the assistance of an emulsifying agent, in an aqueous medium that comprises a saturated solution of ammonium nitrate carrying solid ammonium nitrate in suspension and free from readily hydratable salts, this medium being adapted to yield a pourable emulsion at the emulsification temperature; th resulting aqueous emulsion is congealed into solid form and converted into granules, and the resulting granules are subsequently dried at a temperature insuflicient to cause them to soften.

According to another feature of the invention, the grist of the granule is made more uniform by screening them, and according to a further feature of the invention at least the fine material rejected as a result of the screening operation is remelted to form a portion of the pourable emulsion that is congealed and converted into granules in a subsequent preparation.

In putting the invention into efiect, there may advantageously be employed solid high explosive compounds melting at temperatures not appreciably in excess of the boiling point of water, but which will remain solid under ordinary hot storage conditions, for instance trinitrotoluene, tetranitroanisol and dinitrobenzene; but there may also be employed mixtures containing high explosives of higher melting points, for example, hexanitrodiphenylamine, pentaerythrite tetranitrate, polynitronaphthalenes or the like, with easily fusible or even liquid explosive compositions in suitable proportions to yield mixtures that will be solid at hot storage temperatures but liquify at temperatures not substantially above the boiling point of water. It is usually convenient to carry out the emulsification at a temperature between about 80 and 110 C.

As emulsifying agents there are advantageously employed colloidal substances j soluble or swellable by water and capable of increasing the viscosity of the hot aqueous ammonium nitrate solution, for instance agar-agar, sodium alginate', carrageen moss extract, flour, dextrin, sodium cellulos glycolate, starch and Various gums. The quantity of the emulsifying agent required will usually liebetween 0.3 per cent and 0.7 per cent of the dry weight of the emulsion. The emulsification may be assisted by agitation.

If desired, ingredients of various types commonly employed in the manufacture of explosive compositions may be introduced into the composition of the emulsion, in which case they are preferably of an insoluble and infusible typel. Cooling or quenching ingredients such as fluorspar, dolomite, or anhydrite, or oxidisable ingredients such as metal powders may thus be introduced. Fusible or soluble materials, however, can often be employed provided they do not have the property of readily forming hydrates with water of crystallisation and are not unduly hygroscopic. Examples of salts that may be employed in this way include sodium chloride and sodium nitrate. Ingredients commonly used in blasting explosive l compositions, for instance carbonaceous ingredients, especially those of low density, cooling or flame-quenching agents, waterproofing agents; oxidising salts; noncarbonaceous oxidisable ingredients and so forth may also, if desired, be used in admixture with the granules obtained after the emulsion has been congealed and granulated, advan tageously after they have been dried. In some cases they may be applied as surface coats to the dried granules. It will be understood that the composition of the final explosive should be so adjusted that it has a desirable oxygen balance, taking into account any wrapping or packing material that will be included in the blasting charge. It is noteworthy that if sufiicient water were included in the composition to dissolve the whole of the ammonium nitrate at a temperature suitable for emulsification, the emulsion would not congeal satisfactorily when subsequently cooled, so that the granulation of the product would be impracticable. Although the fluidity of the ammonium nitrate and of the emulsion as a whole at the emulsification temperature is somewhat influenced by the nature and amount of the other non-aqueous ingredient or ingredients, we have found that in the case of a blasting explosive composition consisting of trinitrotoluene, ammonium nitrate and an emulsifying agent about 3 to 8 per cent. water should be present, so that about 30 to 60 per cent. of the ammonium nitrate remains undissolved at th emulsification temperature.

The granulation may be carried out by pouring the emulsion into the form of an extended cake or the like, and breaking up the latter mechanically into granules when it has congealed. Any particles that are too large or too small may be screened off before or after the granules have been dried off.

The waste from the screening operation may be utilised in forming another batch of emulsion, advantageously with the addition of a little water, since a certain amount of drying may have taken place by the time the congealed granules have been screened. The size of the granules may be controlled by varying the intensity of the mechanical action in breaking up the congealed cake. The bulk density of the resulting explosive depends amongst other factors on the sizeof the granules, but it will be understood that the invention includes not only explosive compositions of coarsely granular form, but also explosive com positions in which the granules are small, and

which may be described as powders.

The granular explosives made according to the present invention are characterised by possessing desirably low density characteristics, but the granules of which they are composed are nevertheless of a robust character which enables them to maintain their desirably low density characteristics during transport and storage. These explosives also possess other desirable properties,

more particularly, excellent or improved sensi tiveness to initiation and low velocity of detonation. They may be used in quarrying or mining DZ. tJJ LUDWE, I llluw LUiiHrv DKUDD KU'LKEHUE low density characteristics for use in situations where it is desired to break up the material to be blasted without efiecting an intensive degree of comminution, for instance in the winning of lump .coal. They maintain their solidity and low density at the climatic storage temperatures used in testing. Y The unusually low density characteristics of the explosives of the present invention, however, are due not only to the granulation of the congealed emulsion, but also to the evaporationof water from the individual solid granules. In drying the granules, therefore, the temperature must not be sufficient to cause incipient fusion of the granules, which would destroy the desired porosity, but as the drying proceeds the drying temperature may, if desired, be progressively increased.

The dried granules may advantageously be pro vided with a coating of a waterproof material, but this is not essential. They should be packed in waterproof containers, and if the granules have not been coated it is often desirable that they should be packed in containers which will resist the entry of water vapour. It is desirable that the explosive should contain no ingredient of a marked deliquescent character.

When cooling and flame-quenching ingredients are included, explosives of suitable composition for blasting in fiery or gassy mines may also be made in accordance with the presentinvention.

The invention is further illustrated by the following examples, in which the parts are parts b weight.

Example 1 A mixture of 81.5 parts ammonium nitrate, 6 parts water and. 0.5 part agar-agar are heated until the agar-agar has dissolved and the temperature has risen to 81 to 85 C. 18 parts trinitrotoluene are stirred in and the mixture is further heated to 90 C., by which time the trinitrotoluene has melted, and is stirreduntil 'a well dispersed pourable emulsion has been formed. The aqueous emulsion is then-poured on to a cold surface to form a slab of about A inch in thickness, which is crushed into granules after it has congealed, and is screened. The parti-' cles passing a No. 16 and retained by a No. 25 B. S. sieve are then dried off at 35 to 45 C. The resulting explosive when cooled and cartridged under a pressure of 75 lbs. per square inch has a bulk density of 0.7. The explosive has a power amounting to about 85 per cent. of that of blasting gelatine, reckoned on a weight basis. The, explosive when initiated by a No. 6 mercury fulminate potassium chlorate detonator has a velocity of detonation of about 2400 metres per second.

Example 2' The procedure is the same as in Example 1, except that the cake of congealed aqueous emulsion is broken up into smaller particles and after these have been dried they are screened through a 25 B. S. S. sieve, all the coarser material being appropriately sized to provide a more coarsely granular explosive. The material passing the 25 mesh sieve when cartridged at a pressure of 75 lbs. per square inch in a, waxed paper cartridge has a density of 0.8 and detonates under the impulse of a No. 6 mercury fulminate-potassium chlorate detonator at a velocity of about 2,900 metres per second. In a test in which two 1%" x 3" freshly made cartridges of the same explosive in their waxed paper wrappers are placed in line at, varying distances in a common paper wrapper, the second cartridge is detonated by the first at a distance of 4" but not at 5".

Ea'ample 3 Amixture of 73.4 part ammonium nitrate, 5 parts water,and 0.4 part agar-agar are warmed until the agar-agar has dissolved and the temperature has risen to 85 C. 16.2 parts trinitrotoluene are'stirred in and the mixture is further heated to 90 C. and agitated until the molten trinitrotoluene has been emulsified. parts anhydrite, screened to pass a 40 mesh sieve and to be retained on a 60 mesh B. S. Sieve, are

tated. The mixture is then poured on to a cold surface to form a slab of about 4" thickness and crushed into small granules after it has congealed. It is dried and screened to pass a mesh B. S. sieve, and the larger particles retained on the sieve are sent for remelting. When cartridged in waxed paper tubes at a bulk density of 0.8 the resulting explosive detonates under the impulse of a No. 6 mercury fulminate-potassium chlorate detonator at a velocity of 2200 metres per second. Its power is about 71 per cent. of that of blasting gelatine. In the double cartridge test described in Example 2 the second cartridge was detonated by the first at 3" but not at 4". 7

Example 4 The procedure is the same as that of Example 3 except that talc is used in place of anhydrite. The properties of the explosive are generally similar to that of Example 3.

Example '5 The procedure is the same as in Example 3 except that fiuorspar is used instead of anhydrite. The properties of the explosive are generally similar to that of Example 3.

Example 6 The procedure is the same as that of Example 3 except that limestone, screened to pass a mesh sieve but not to be retained one. 100 'mesh' 60 Example 7 The explosive is prepared as described in Example 3, except that the proportions of ingreclients employed are as follows: ammonium nitrate 69.6 parts, agar-agar 0.4 part, water 5 parts, aluminium 15.0 parts, trinitrotoluene 15 parts. The bulk density of the explosive when cartridged under a pressure of 75 lbs. per square inch is about 0.75, and the explosive is initiated under the impulse of a No. 6 fulminate-potassium chlorate detonator with a velocity of about 2300 metres per second. The power of the explosive is about 90 per cent. that of blasting gelatine. An explosive of similar composition made by milling together the dry ingredients has a density under the same cartridging pressure of about 1.1 and 23876155!!! lluunv then stirred into the mixture which is kept agidetonates. with a velocity of 3600: metres per second.

l Example 8 The explosive is prepared according to the method of Example 3- except that sodium chloride isfusedinstead oi anhydrite, and the proportions oi, the. ingredients: used are-ammonium nitrate 71.6 parts, sodium chloride. 13 parts, Water 4 parts, agar-agar 0.4 part and trinitrotoluene 15 parts. Under a cartridging pressure of 75 lbs. per square inch the bulk density of the dried granules is about 0.9, and un der the impulse. of a No. 6 mercury fulminate-potassium chlorate detonator, the resulting explosive detonates with a velocity of 2200 metres per second. An explosive of similar composition made by a. dry milling process when cartridged under the samepressure has a density of about 1.1 and a velocity of detonation of about 3500 metres per second.

Example 9 The ingredients used for making the explosive are 'thesame as in Example 8, but the sodium chlo'ride'is mixed with the dried granules of the solidified emulsion containing only the. ammonium nitrate, agar-agar and trinitrotoluene instead-of being included in the composition of the aqueous emulsion. The sodium chloride is employed in crystalline form passing a 40 mesh and retained by a 60 mesh 13. S. S. sieve. Under acartridging pressure of 75 lbs. per square inch the density is about 0.9,. and the velocity of detonation is about 2600 metres per second under the impulse of a No. 6 mercury fulminate-potassium chlorate detonator. In the double cartridge test the second cartridge is initiated by the first at 4" distance but not at 1 While in the preceding examples there is illustrated the use of a- No. 6 mercury fulminatepotassium chlorate detonator, all of these explosives when freshly made are capable of being initiated by a No. 2 mercury fulminate-potassium chlorate detonator. The hereinbefore quoted numerical values for bulk density are reckoned on" the basis that the bulk density of water is unity,

. 8 I claim:

l. A process for the manufacture of low" density blasting explosives which comprises emulsifying a molten easily fusible organic explosive with the assistance of an emulsifying agent in an aqueous medium that comprises a saturated solution of ammonium nitrate carrying solid ammonium nitrate in suspension and is free from readily hydratable salts, this medium being adapted to yield a pourable emulsion at the emulsification temperature, congealing and granulating the resulting aqueous emulsion and drying the resulting granules. at a temperature insufficient to cause them. to soften.

2. A process as. claimed in claim 1 wherein the granulation is carried out by pouring the emulsion into the form. of an extended cake or the like and breaking up the latter mechanically when it has congealed.

3; A processas claimed in claim 1 wherein the fusible: organic explosive comprises an explosive compound melting at a temperature not substantially exceeding the. boiling point of Water.

4. A process as claimed in. claim 1 wherein the fusible explosive comprises a mixture of compounds thatv fuses at a temperature not substantially exceeding the. boiling point. of water, of which one constituent is a compound melting at a temperature substantially exceeding the boiling point of water;

. 5. A process as claimed in claim 1 wherein the fusible. explosive comprises trinitrotoluene.

6. A process as claimed in claim 1 wherein the emulsificatio-n is carried out at a temperature between C. and1l0 C.

7. A process. as claimed in claim, 1 wherein the quantity of emulsifying agent employed amounts .to from 0.3 to 0.7 per cent. of the dry weight. of the emulsion.

8... Av process as claimed in claim 1 wherein the emulsion consists of a mixture. of ammonium nitrate, trinitrotol-uene, an emulsifying agent and from 3 to 8. per cent. of water reckoned on the weight. of the emulsion.

- 9.. A process as claimed in claim 1 wherein the ammonium nitrate composition contains a metal powder.

JOHN WHETSTONE.