US2736261A

US2736261A - Blasting means

Info

Publication number: US2736261A
Authority: US
Priority date: 1950-07-20
Filing date: 1950-07-20
Publication date: 1956-02-28
Anticipated expiration: 1973-02-28
Also published as: DE932596C; FR1048345A; GB689440A; BE504789A

Description

Feb. 28, 1956 c. o. DAVIS ETAL 3 BLASTING MEANS Filed July 20, 1950 4 Sheets-Sheet 1 INVENTORS.

CLYDE 0. DAVIS, RICHARD C.GLOGAU, FRANK A. LOVING 6 JAMES P. swso ATTORNEYS Feb. 28, 1956 c. o. DAVIS ETAL 2,736,261

BLASTING MEANS Filed July 20, 1950 4 Sheets-Sheet 2 -O.6 -04 TIME IN SECONDS PRESSUR E- THOUSANDS P.S.l. INVENTORS:

CLYDE 0. DA v/s, RICHARD c. GLOGAU, FRANK A.LOVIN6 &72 JATMEIS P.- SWED W,Zj%%

ATTORNEYS Feb. 28, 1956 c. o. DAVIS ETAL 2,736,251

BLASTING MEANS Filed July 20, 1950 4 Sheets-Sheet 3 -2 TIME IN MILLISECONDS RATE=L2X 10 nsllsec PRESSURE THOUSANDS P.S.|.

INVENTORS: CLYDE 0.0AV/S,

RICHARD c. GLOGAU, FRANK A. LOVING 8r JAMES SW D 452/6 W //.4.-.44

A TTORNEYS Feb. 28, 1956 c. o. DAVIS ETAL 2,736,261

BLASTING MEANS PRESSURE- THOUSANDS P.S.|.

CIIVVENTORS:

CLYDE 0. DA v/s, RICHARD c. GLOGAU, FRANK A.LOVING (:1 JAMES P swzo ATTORNEYS limited States Patent 6 BLASTING MEANS Application July 20, 1950, Serial No. 174,818 Claims. (Cl. 102-23) This invention relates to a new and improved blasting means, and particularly one which is safe for use in gassy coal mines.

A great number of different devices have been proposed for the mining of coal and other types of rocks where shattering is to be avoided. Probably the most common means have been the use of blasting explosives of various types. Since these materials are explosive in themselves particular precautions must be taken in handling and storing them. They are subject to accidental initiation which is undesirable, and are mostly rapid and shattering in their effect, yielding a larger proportion of small particles than is ordinarily desired.

Another common means involves the rapid heating of compressed or liquified gases. This also is accom. panied by many disadvantages including the difficulty of maintaining gases under high pressure in containers for long periods of time, making performance uncertain and also involving the danger of accidental initiation. These difficulties are present whether carbon dioxide or air is involved.

In another method mixtures of solids have been confined in pressure-resistant tubes, but up to the present time this has involved so many disadvantages that no satisfactory commercial operations have been achieved. Mixtures of this type include the Hydrox powders which contain for example an alkali metal nitrite and an antmonium salt. These are inherently extremely unstable and besides have all of the disadvantages whichcharacterize electrically-fired, flame or spark-producing initiating means, since these are confined within the containers at all times after the initial loading.

Other mixtures of gas-generating solids which have been proposed comprise ammonium nitrate and a catalyst or ammonium nitrate and a large proportion of a fuel. Gas-generating cartridges of the prior art containing such mixtures are capable of violent explosion rather than the desired production of gas at a controlled rate. When an ammonium nitrate-containing gas-generating composition fills the whole of a pressure-resistant tubular 0ontainer, its decomposition is subject to increase in pressure suflicient to burst the tube. When such a charge fills even one end only of the tube from wall to wall, it may burst the tube during decomposition. Charges introduced loosely into the pressure-resistant container Without means for keeping them in a predetermined location within the tube may, on handling, assume a position within the tube which will result in their explosion rather than in their evolution of gas at a controlled rate. In addition, compositions having an excess of fuel or combustible may, upon reaction, eject hot, solid, unreacted material into the flammable atmosphere of the mine. Catalyzed ammonium nitrate compositions or ammonium nitrate mixtures containing a large proportion of fuel, previously proposed, are ordinarily ignited by flameor spark producing means fired by electric current. They may thus become activatedaccidentally by stray currents, for example, or in error before the charge is in place in the 2,735,261 Patented Feb. 28, 1956 borehole. Also, the charges must be prepared away from the face, since the ignition devices are hazardous. Furthermore, the ignition devices proposed heretofore have been vigorous in their action. Their vigor is suflicient to hurl at least a part of the gas-generating charge against the end of the tube opposite the igniter, there to become packed solidly over the whole cross-sectional area of the tube... A further disadvantage of mixtures of the ammonium nitrate types of the prior art is the fact that they yield toxic fumes as decomposition products. Catalyzed ammonium nitrate compositions containing chromate or other chromium compounds as catalysts, for example, cannot be used in coal mines because of the formation .of nitrogen tetroxide upon their decomposition. Mixtures of ammonium nitrate with a large amount of fuel contain an inadequate supply of oxygen to oxidize them completely, and they thus develop toxic fumes on combustion. Pressure-resistant tubes containing chemical compositions in contact with the closure disc require strong, chemically resistant discs. Ammonium nitrate mixtures containing a catalyst are relatively expensive because of the required addition of catalyst. Such compositions, and those containing a large proportion of fuel, are also expensive to use because they require special handling and relatively expensive igniters.

An object of the present invention is to provide a safe device and method for the blasting of coal, 2. device and method that are free from the hazards of explosion, accidental initiation, ignition of flammable gases, evolution of toxic gases, and deterioration on storage. A further object is to provide such a device and method which is highly effective in yielding broken material in large pieces rather than in shattered fragments. A still further object is to provide such a device and method which are inexpensive, easy to use, and readily controllable. Additional objects will be disclosed as the invention is described more fully hereinafter.

We have found that the foregoing objects are accomplished by heating, preferably by means of an electric resistance heater, a gas-generating composition comprising ammonium nitrate, said gas-generating composition being disposed axially within a pressure-resistant container having a frangible closure in such a way that the rate of pressure development of the gases evolved by the gas-generating composition within the container is controlled to prevent the bursting of the pressure-resistant container before the frangible closure is sheared by the gases under pressure, the gas-generating composition being desirably disposed axially within the pressure-resistant container in such a way that at no time can the ammonium nitrate-containing composition pack the tube from wall to wall at any point in the tube. We have found that this arrangement may conveniently be accomplished by en closing the gas-generating composition within a thinwalled, substantially cylindrical container that is smaller in diameter than the internal diameter of the pressureresistant container. Preferably the diameter of the inner container will be not more than of the inner diameter of the pressure-resistant container, however this should not be taken as limiting, it being necessary that the ammonium nitrate-containing composition should not completely jam the cross-section of the tube at any point so as to prevent the flow of gases to such an extent as to rupture the tube. In other Words, the loading density of any cross-section of the charge-containing portion of the pressure-resistant tube must be low enough so that the charge will not serve to stop completely the flow of gases in the tube at that point. Futher details concerning this loading density are set forth later on in the description of the invention.

It is also a desirable feature of the device that there shall be a continued release of energy, that is a continued flow of gases, after the rupture of the frangible closure disc. To accomplish this result, it is necessary to select a charge having a predetermined rate of pressure rise. The limitations concerning this will be set forth in detail in the following description.

It is also desirable that the device should perform in such a manner as to avoid ignition of any inflammable gases which might be present in the coal mine. This is accomplished by limiting the fuel in the charge to prevent the occurrence of too much flame after the closure disc has ruptured. In fact, we prefer to so limit the fuel that no flame occurs outside of the pressure tube as would ignite inflammable gases. Suitable amounts of fuel for this purpose will appear from the examples in the following description. It is also desirable to use a non-sparking closure disc in this connection.

The invention is further illustrated by the appended drawings, which are not to be taken as limiting in any way. Referring generally to the drawings, Figure l is a view in cross-section, of a pressure-resistant tube of the type known for the bringing down of coal and, contained therein, a two-section gas-generating charge in a substantially cylindrical container, said gas-generating charge being provided with an electric resistance heater. Figure 2 is a similar view of another form of packaged gas-generating charge.

Figures 3, 4 and show piezo-gage records of the pressures obtained by firing gas-generating charges such as are illustrated in Figures 1 and 2 in pressure-resistant containers provided with frangible discs, such as are illustrated in Figure l. Pressures in thousands of pounds per square inch are recorded on the ordinates of Figures 3, 4 and 5 and the time on the abscissae, in seconds in Figure 3 and in milliseconds in Figures 4 and 5. The curves thus represent the pressure as a function of the time, or the rate of pressure rise. Figure 4 shows the record of a charge which gives a rate of pressure rise of 1,200,000 pounds per square inch per second, and Figure 5 the record of a charge which gives a rate of pressure rise of 3,500,000 pounds per square inch per second.

In detail, Figure 1 illustrates one arrangement for breaking coal in accordance with our invention. In Figure l, 1 represents in longitudinal section a tubular pressure-resistant container of steel. Containers of this general form are already in use for containing charges of compressed carbon dioxide. At one end of the tubular steel container a rupture disc 2 of steel or other rigid material of such strength as to be sheared at a predetermined pressure is held in place against a seating at the end of tube 1 by means of a hollow threaded cap 3, which is provided with vent holes 4 for the escape of the gases released by the shearing of the rupture disc and with an aperture 5 at the outer end for facilitating removal of the sheared-off portion of the disc. At the opposite end of the steel tube 1 is a firing head 6 which comprises a threaded plug screwed in gas-tight manner against a shoulder at the end of tube 1 and which has an annular ridge 7 facing the interior of the tube. Between the firing head 6 and the shoulder of tube 1 is a seating device 8 comprising a flanged plug of wood or other rigid nonconducting material provided with two

holes

9 and 10 for the passage of the ends of a narrow strip of Nichrome ribbon 11. The end of the ribbon passing through hole 9 makes contact with electrode 12, which is insulated from the firing head 6 as it passes through a channel in the firing head to terminal 13, said channel being made gas-tight by any desired means (not shown). The end of the ribbon passing through hole 8 makes contact with annular ridge 7, and thus comes in metallic contact with firing head 6, to which a terminal 14 is attached.

Terminals

13 and 14 are connected by two electric wires to a source of current. Attached to and closed by seating device 8 and extending longitudinally away from the firing head toward the end of the tube closed by the rupture disc is the gas-generating charge. In this figure the gasgenerating charge is made up in the following manner. A cylindrical cardboard container 15 which has as its base two vent holes 16 is attached to and closed by seating device 8. The narrow strip of Nichrome ribbon 11 extends from electrode 12 through hole 9 down through the cylinder 15, passes out of the base of the cylinder through one of the holes 16, back into the cylinder 15 through the second hole 16, and up through hole 10 into contact with ridge 7. Around the Nichrome ribbon, cylinder 15 is filled with a mixture of ammonium nitrate and starch 10% in an amount of approximately 60 grams. To the base of vented cylinder 15, and, again, extending in a longitudinal direction away from the firing head, is attached a substantially cylindrical bag 17 of a polymerized ethylene plastic sheeting containing approximately 265 grams of granular ammonium nitrate.

The method of operation of the blasting device illustrated by Figure l is as follows. Closure disc 2 is inserted against the seating of tube 1 and is held in place by cap 3, which is screwed in firmly. Seating device 8 and the cardboard cylinder 15 and attached polythene bag 17 containing the ammonium nitrate charges fastened to the seating device 8 are inserted into the firing end of the tube and are held in place by the flanged circumferential portion of seating device 8 which rests on a shoulder at the firing end of the tube. The firing head is screwed on, the tube is inserted in the borehole,

terminals

13 and 14 are connected by two electric wires to a source of current, and current of 5 to 15 amperes and 24 to 40 volts is applied. The electric current heats the Nichrome ribbon '11 to a temperature sufiiciently high to cause the ammonium nitrate mixture in the cylinder 15 to begin to decompose locally. The increase in temperature and pressure engendered hereby initiates further reaction of the ammonium nitrate mixture, and the gases evolved escape through the vent holes 16 into the ammonium nitrate in the polythene bag 17. The heat from these gases, the heat from the decomposition of the ammonium nitrate mixture and additional heat supplied by the heating element, and the pressure caused by the gaseous products of decomposition confined within the pressure-resistant tube 1 all act to cause the reaction to proceed throughout the ammonium nitrate. The pressure within the tube thus builds up gradually at first, and later at a higher rate; in approximately 10 to 25 seconds, the disc 2 is sheared, and the gases escape through vent holes 4 to perform work against the material to be blasted.

Figure 2 shows another variation of a gas-generating charge for use in a pressure-resistant container for blasting coal as illustrated in Figure 1. In Figure 2, instead of a charge in two sections, there is a single charge contained in a substantially cylindrical container 18 of fiame-resistant paper attached directly to seating device 8. The charge in container 18 comprises, for example, a mixture of granular ammonium nitrate 88%, starch 10%, and sodium thiosulfate 2% in the amount of 350 grams. Suspended in this charge through

holes

9 and 10 is a loop of Nichrome ribbon 11. The method of operation of the charge of Figure 2 is essentially the same as that of the two-section charge of Figure l, the upper part of the container 18 of Figure 2 being self-vented as the charge decomposes at high temperature.

Figures 3, 4 and 5 show piezo-gage records of pressures obtained immediately before and after the shearing of the frangible disc in pressure-resistant tubes when gas-generating charges in accordance with our invention are fired therein. Figure 3 shows the pressure phenomena in the 1.4 seconds before maximum pressure is reached. The pressure in the pressure-resistant tube builds up very slowly in the first 20 seconds, approximately, after initial application of the firing current and this phase is not shown. The pressure rise becomes rapid, beginning about the last second before the maximum is reached as shown in Figure 3, and still more rapid in the last tenth of a second precedingthe maximum. 1 The pressure phenomena occurring during the last 12 milliseconds before the attainment of maximum pressure are illustrated more clearly in Figures 4 and 5, where the time scale is in milliseconds rather than in seconds. Figure 4 shows the pressure record of a charge of such dimensions and distribution as to give a rate of pressure rise of 1,200,000 pounds per square inch per second. It will be seen that the disc was sheared about 1 millisecond before the maximum pressure was reached (this point is indicated on the piez'o gage by mechanical vibration of the instrument following the rupture of the disc). Figure 5 shows the piezo-gage record made by a charge which gives a rate of pressure rise of 3,500,000 pounds per square inch per second. It will be noted that the slope of the curve is steeper during the same period of time (12 milliseconds) before the maximum pressure is reached, and that the difference in pressure between the point of rupture and the maximum is greater than in Figure 4.

EXAMPLE 1 Into a pressure-resistant container of the type illustrated in Figure 1 having a capacity of 130 cubic inches and an internal diameter of 1.8 inches and provided with a rupture disc of a strength to withstand a static pressure of 9000 lb./sq. in. was introduced at the firing end a charge made up of (1) 125 grams of a mixture of granular ammonium nitrate 90% and granular starch in a cardboard tube provided with three vent holes along the side and with a heating element consisting of Nichrome ribbon passing in a loop from one electrode at the firing end of the pressure-resistant tube to the bottom of the cardboard tube and back to the second electrode, and (2) a further 100 grams of the same mixture in a cylindrical bag of polythene plastic fastened to the free end of the cardboard tube. The diameters of the cardboard tube and of the cylindrical polythene bag were substantially the same, and were approximately 34% less than the internal diameter of the cylinder. The loading density of the ammonium nitrate in the pressure-resistant tube was 0.10 gram/ cubic centimeter,,and that of the total charge was 0.11 gram/ cubic centimeter. The cross-sectional loading density of the ammonium nitrate in the portion of the pressureresistant tube containing the charge was 0.35 gram/cubic centimeter. The firing head was closed and a current of 8 amperes at 36 volts was applied for approximately 25 seconds. The disc ruptured 35 seconds after the initial applicationof current.

EXAMPLE 2 A charge formed in the manner of Example 1 but containing (1) 60 grams of a mixture of ammonium nitrate 84%, granular starch 10% and NazSzOaSHzO 6%, the latter as an anti-segregating agent, and (2) 265 grams of a mixture of ammonium nitrate 90% and granular starch 10% was inserted in a pressure-resistant container of the type illustrated in Figure 1 having a capacity of 130 cubic inches and an internal diameter of 1.8 inches, and a static rupture disc strength of about 9000 pounds per square inch, the loading density of the ammonium nitrate in the pressure-resistant container being 0.14 gram per cubic centimeter, that of the total charge being 0.16 gram per cubic centimeter and the cross-sectional loading density of the ammonium nitrate in the portion of the pressureresistant tube containing the charge being 0.35 gram per cubic centimeter. The charge was fired by the application of a current of 8 amperes at 39.6 volts for approximately 10 seconds. The rupture disc sheared 17 seconds after application of current was begun.

EXAMPLE 3 A shot similar to the one described in Example 2 was made, except that 260 grams of granular ammonium nitrate alone was contained in the part of the charge in the polythene bag. The loading density of the am- EXAMPLE 4 A mixture of 88% of granular ammonium nitrate, 10% of granular starch, and 2% of Na2S2O3.5H2O was loaded into a flame-resistant paper tube 1% inches in diameter and 24 inches long which was provided with a loop of Nichrome wire to serve as a heating element. The gas-generating charge was introduced into a pressureresistant container of the type'illustrated in Figure 1, which was provided with a steel rupture disc (capable of withstanding a static pressure of 18,000 to 20,000 pounds per square inch). The loading density of the ammonium nitrate in the pressure-resistant tube was 0.15 gram per cubic centimeter and that of the total charge was 0.17 gram per cubic centimeter. The cross-sectional loading density of the ammonium nitrate in the portion of the pressure-resistant tube containing the charge was 0.35 gram per cubic centimeter. The firing head was screwed on, contact with a 24-volt battery was made, and a current of 13 amperes from this source was applied for 18 seconds, at which time the heating wire burned through (as recorded by an ammeter in the firing circuit), and the circuit was thus opened. One second later the disc sheared.

EXAMPLE 5 A current of 13 amperes and 24 volts was applied to an assembly like that of Example 4 for a period of 10 seconds, after which the current was cut off by the opening of a control switch. The disc did not rupture. When the gas-generating charge in its package was removed from the pressure-resistant tube, broken apart, and examined, it was found that the ammonium nitrate-containing gas-generating charge had melted somewhat in the vicinity of the Nichrome ribbon heater and had resolidified. The initial heating had thus not proceeded far enough to cause the reaction to be self-sustaining. Another charge of the same type was subjected to the same treatment, except that it was not broken apart for examination, but was inspected only externally. This same charge was replaced in the pressure-resistant container and supplied with a firing current of 13 amperes at 24 volts. The rupture disc was sheared in the normal manner 22 seconds after the initial application of current.

EXAMPLE 6 A current of 14 amperes at 24 volts was applied to an assembly like that of Example 4 for a period of 15 seconds. The current was then cut off, and the disc ruptured 5 seconds later.

EXAMPLE 7 Into a pressure-resistant container of the type illustrated in Figure 1 having a capacity of cubic inches and an internal diameter of 1.8 inches and provided with a steel rupture disc capable of withstanding a static pressure of 18,000-20,000 pounds per square inch was introduced 350 grams of a mixture of granular ammonium nitrate 96% and granular starch 4% in a cylindrical polythene bag having an external diameter of 1%; inches, the bag containing the mixture being provided at the firing end with a heating device comprising an electrically firing squib placed in such a way in the bag as to be surrounded by the ammonium nitrate mixture. The loading density of the ammonium nitrate in the pressure-resistant tube was 0.16 gram per cubic centimeter and that of the total charge was 0.17 gram per cubic centimeter. The

7 cross-sectional loading density of the ammonium nitrate in the portion of the pressure-resistant tube containing the charge was 0.41 gram per cubic centimeter. The firing head was closed and current was applied. The rupture disc was sheared in less than one second, without damage to the pressure-resistant tube.

When, however, the same quantity of the ammonium nitrate/fuel mixture was poured loosely into a similar tubular container having a similar steel rupture disc, and around a similar squib at the firing end in such a way that the granular composition filled the space around the heater from wall to wall of the pressure-resistant tube and had a loading density of the ammonium nitrate in the pressure-resistant container of 0.16 gram per cubic centimeter, that of the total charge being 0.17 gram per cubic centimeter, and the cross-sectional loading density of the ammonium nitrate in the portion of the pressureresistant tube containing the charge being 1.0 gram per cubic centimeter, the tube itself was ruptured in less than one second after application of current.

EXAMPLE 8 A copper tube 3 inches long and /2 inch in diameter containing a centrally located longitudinal copper electrode out of contact with the copper tube was filled, between tube wall and electrode, with a conductive mixture containing ammonium nitrate 99% and graphite 1%, the mixture being pressed into the tube. A cylindrical polythene bag 1% inches in external diameter and containing 300 grams of a mixture of ammonium nitrate 88% and granular starch 12% was arranged about the copper tube. The assembly was introduced into a pressure-resistant container of the type illustrated in Figure 1 having a volume of 130 cubic inches, an internal diameter of 1.8 inches, and a rupture disc of a strength to withstand a static pressure of 9000 pounds per square inch. The loading density of the ammonium nitrate in the pressure-resistant tube was 0.13 gram per cubic centimeter and that of the total charge was 0.15 gram per cubic centimeter. The maximum cross-sectional loading density of the ammonium nitrate in the portion of the pressure-resistant tube containing the charge was 0.4 gram per cubic centimeter. A direct current of 33 volts was applied. The shear disc ruptured after two minutes.

EXAMPLE 9 A gas-generating charge comprising 420 grams of a mixture of ammonium nitrate 96% and granular starch 4% was loaded into a tin can 1 /8 inches in diameter, which contained a heating ribbon of Nichrome. The gasgenerating charge was introduced into a pressure-resistant container of the type illustrated in Figure l which was provided with a fiber rupture disc (capable of withstanding a static pressure of about 9000 pounds per square inch) and which had a capacity of 130 cubic inches and an internal diameter of 1.8 inches. The loading density of the ammonium nitrate in the pressure-resistant tube was 0.20 gram per cubic centimeter and that of the total charge was 0.21 gram per cubic centimeter. The crosssectional loading density of the ammonium nitrate in the portion of the pressure-resistant tube containing the charge was 0.38 gram per cubic centimeter. The firing head was screwed on, contact with a source of electricity was made, and a current of 15 amperes at 24 volts was applied; the fiber disc sheared after 94 seconds without damage to the tube.

In another firing, however, 100 grams of a mixture containing 90% of ammonium nitrate and 10% of starch was placed in a similar can provided with a Nichrome heating element and was introduced into the firing end of the tube. From the opposite end of the tube 300 grams of the same mixture was poured around the outside of the can, filling the tube from wall to wall at the firing end. The loading density of the ammonium nitrate in the pressure-resistant tube was 0.18 gram per cubic centimeter and that of the total charge was 0.20 gram per cubic centimeter. The cross-sectional loading density of the ammonium nitrate in the portion of the pressure-resistant tube containing the charge was 1.0 gram per cubic centimeter. A plastic rupture disc capable of withstanding a static pressure of about 9000 pounds per square inch was inserted in the opposite end of the tube. An electric current of 8 amperes at 38 volts was applied by way of the electrodes at the firing end, and, in this case, the tube itself blew apart after 50 seconds.

EXAMPLE 10 A mixture of of granular ammonium nitrate and 10% of granular starch in the amount of 300 grams was loaded into a cylindrical polythene bag 1% inches in diameter, and 25 grams of smokeless shotgun powder around an electric bridge wire assembly provided with a bead of ignition material on the bridge wire was inserted for ignition in the end of the bag which was attached to the seating device. This gas-generating charge was inserted in a pressure-resistant container of the type illustrated in Figure 1 having a capacity of cubic inches and an internal diameter of 1.8 inches and being provided with a rupture disc of sufiicient strength to withstand a static pressure of 20,000 pounds per square inch, the loading density of the ammonium nitrate in the pressureresistant container being 0.13 gram per cubic centimeter, that of the total charge being 0.15 gram per cubic centimeter and the cross-sectional loading density of the ammonium nitrate in the portion of the pressure-resistant tube containing the charge being 0.35 gram per cubic centimeter, and fired. The charge ruptured the disc in approximately one second without damage to the tube. on the other hand, when the same quantity of a similar mixture, poured loosely into the firing end of a tube of the type illustrated in Figure 1, around a similar ignition means, the loading density of the ammonium nitrate in the pressure-resistant container being 0.13 gram per cubic centimeter, that of the total charge being 0.15 gram per cubic centimeter, and the cross-sectional loading density of the ammonium nitrate in the portion of the pressureresistant tube containing the charge being 1.0 gram per cubic centimeter, was fired, the tube itself was ruptured in about one second.

In the preceding examples it will be noted that in each case the loading density of the ammonium nitrate is less than that of the total charge. Thus, when we speak of the loading density of the ammonium nitrate, we mean the loading density of the ammonium nitrate content of the charge, and not that of the whole charge.

It will be seen from the foregoing examples that a very effective means of releasing gas under pressure for breaking coal or other material is provided by the heating of a gas-generating composition comprising ammonium nitrate in a pressure-resistant tube having a frangible closure at one end, so long as the ammonium nitrate-containing charge is maintained in a position in the tube permitting a channel for the escape of the gases produced by the decomposition of the ammonium nitrate charge as soon as they are formed. When the gas-generating charge fills the tube throughout the entire crosssectional area of the pressure-resistant tube at any point, as is pointed out in Examples 7, 9, and 10, there is immediate danger of rupture of the pressure-resistant tube before the rupture disc is sheared.

It has been found by measuring, by means of strain gages and piezo-electric gages, the pressure engendered by the decomposition of ammonium nitrate-containing gas-generating compositions in pressure-resistant tubes provided with frangible closures, that the dynamic pressure at which the rupture discs shear may be higher than the static (hydraulic) pressure that the discs are capable of withstanding. Thus, a steel rupture disc capable of 9 withstanding a static pressure of 8,000-10,000 pounds per square inch may be ruptured at a dynamic pressure of as high as 13,000 pounds per square inch. Furthermore, it has also been found that the peak pressure attained by the decomposition of the ammonium nitratecontaining gas-generating compositions giving satisfactory performance occurs not at the instant of rupture of the disc, as would be expected, but at a'measurable interval (of less than l millisecond) after rupture of thedisc. This may be seen clearly by reference to the curves of Figures 4 and 5. The difference between the pressure at rupture and the subsequent maximum pressure is the greater, the higher the rate of pressure deyelopment at the instant of rupture. This rate of pressure development increases with increase in charge diameter, and, when the gas-generating charge fills the tube completely, the rate is so high that the interval between attainment of the rupture pressure and the peak pressure is approximately zero, and the tube may rupture simultaneously with the shearing of the disc, or even before. 1 The relationships described in the foregoing will be understood more clearly by reference to Table I, wherein rupture pressures, peak pressures, and rates of pressure development are given for series of shots made with gas-generating compositions of various diameters in steel pressure-resistant tubes capable of withstanding a static pressure of about 35,000 pounds per square inch and having steel rupture discs capable of withstanding a static,pr.essure of 8,000 to 10,000 pounds per square inch. ,7 The tubes had an internal diameter of 1.80 inches and-acapacity of 130 cubic inches. The charges were prepared as shown in Figure 2 and were all of the same weight and composition: 250 grams of a mixture of granular ammonium nitrate 88%, starch 10%, and so dium thiosulfate 2%. With the smaller charge diameters, it is evident, the charges were longer, since the weights of gas-generating composition were the same. The loading density of the ammonium nitrate in the pressure-resistant tube was 0.15 gram per cubic centimeter .and that of the total charge was 0.17 gram per cubic centimeter in each case.

T able I'. D.ynamic pressure data with 8,00010,000 pound rupture disc H p v Diameters of charges '1 in. 1% in. 1% in. 1% in.

Disc rupture pressure,

lb./sq. in., Piezo gage 8, 800 8, 600 10, 100 13, 400 Peak pressure, lb /s in.:

' Strain gage 11, 700 14, 100 15, 600 21, 800 Pieio gage. 10, 100 10, 100 13, 500 16, 700 Rate ot press. rise, lb./ sg. in./sec.

Strain gage 1, 100, 000 1, 400, 000 3, 000, 000 6, 900, 000 1 Piezo gage 1, 000, 000 1, 000, 000 3, 900, 000 7, 500, 000

Table II.'-Cr0ss-s ecti0nal densities of the ammonium -rzitrate in the charges used in Table I at various charge diameters Diameter of charge, in inches"; 1

Cross-sectional loading density of NHiNO: in the portion of the pressure-resistant tube containing the .charge 0.27 0.35

fvery uncertain in its results.

ameter for a packaged charge which can be loaded, although with difiiculty, in a tube 1.8 inches in diameter), the rate of pressure rise is approximately twice as high as that for a charge 1% inches in diameter. For convenience in handling, and, more important, to provide an adequate factor of safety, since any one charge could have pressures appreciably higher than the average shown in the table, we prefer to use charges which will not exceed a rate of pressure rise of 4,000,000 pounds per square inch per second, and it is even more desirable to use charges which will not exceed a rate of pressure rise of 3,000,000 pounds per square inch per second.

The ratio of ammonium nitrate by weight in the gasgenerating composition to the volume of the pressureresistant tube, or the loading density of the ammonium nitrate in the pressure-resistant tube, is, in the examples cited, of the order of 0.2 gram per cubic centimeter. The loading density of any cross-section of the chargecontaining portion of the pressure-resistant tube is approximately 0.4 gram per cubic centimeter for those charges giving satisfactory performance. When gasgenerating charges containing ammonium nitrate are prepared of such diameter and weight that the loading density of the ammonium nitrate in the pressure-resistant container does not exceed 0.3 gram per cubic centimeter and the cross-sectional loading density of the ammonium nitrate, as distinct from the loading density of the total charge, does not exceed 0.5 gram per cubic centimeter at any point, the rate of pressure development will be sufficiently low to avoid the possibility of breaking pressure-resistant tubes such as are suitable for coal mining, as may be seen further from the cross-sectional densities given in Table II, taking into account an adequate factor of safety.

The thermal decomposition of ammonium nitrate or of ammonium nitrate-fuel mixtures has heretofore been It will be seen from the curves of Figures 3, 4 and 5 that in the thermal decomposition of ammonium nitrate a building up of pressure proceeds gradually upon continued application of heat.

This pressure build-up is extremely slow at the start.

It has been established that the thermal decomposition of ammonium nitrate is sensitive to pressure, i. e., the rate of decomposition of ammonium nitrate, and hence the rate of rise in pressure engendered thereby increases with increase in the pressure on the ammonium nitrate undergoing decomposition. This rate of pressure rise has been found by us to depend on the concentration (or crosssectional density) of ammonium nitrate in that part of the pressure-resistant container containing ammonium nitrate, when ammonium nitrate is heated under pressure in a pressure-resistant container. In our invention, gas pressure is permitted to develop as a result of the confinement of our gas-generating charge within the pressureresistant container, but it is prevented from arriving at dangerous levels locally by provision of channels for the escape of gases. Our gas-generating charge, which is heated locally, stays in its predetermined position during the slow phase of the reaction, and the provision of channels around or through the charge permits the gases to escape from the vicinity of the charge, and thus prevents local pressure rise sufficient to burst the tube before the rapid phase of the reaction begins. The charge is prevented from reaching an overall pressure that will rupture the tube by being so arranged geometrically, as described in the foregoing, that the rupture disc breaks 11 before the peak pressure is achieved, and a hazardous rate of pressure rise of over 4,000,000 pounds per square inch per second is not attained. Thus a rapid rate of pressure rise leading toward explosion is avoided under conditions which would otherwise tend to produce explosion rather than a controlled generation of gas.

While we have shown an annular channel around the ammonium nitrate-containing gas-generating charge to provide for the escape of gases formed on decomposition of the charge, it may also be desirable to provide longitudinal channels within the charge for the purpose of performing the same function, said channels likewise reducing the cross-sectional charging density of the ammonium nitrate in the pressure-resistant tube. The charge may be in two sections, the first section being contiguous to the heating device and the second section attached thereto or in a separate package at some other location within the pressure-resistant tube.

The substantially cylindrical container in which our gas-generating charge comprising ammonium nitrate is packaged may be of any suitable thin material such as plastic sheeting, paper, light-weight cardboard, metal foil, or the like. It is necessary only that the material have segregating agents such as sodium thiosulfate in ammonium nitrate/ fuel mixtures.

It has been found that ammonium nitrate-containing gas-generating charges of the foregoing compositions, when fired enclosed in thin-walled, substantially cylindrical containers inserted in pressure-resistant tubes of larger internal diameter than the diameter of the thin-walled container, the loading density of the ammonium nitrate in the pressure-resistant container being less than 0.3 gram per cubic centimeter and the cross-sectional density of the ammonium nitrate in the pressure-resistant container being less than 0.5 gram per cubic centimeter yield substantially less than the amounts of toxic gases permitted in coal mines. The gases which issue from the pressure-resistant tube upon rupture of the frangible disc consist essentially of nitrogen, carbon dioxide, and water and contain only exceedingly small amounts of toxic gases, as may be seen from Table III, wherein are recorded the amounts of noxious gases formed by various ammonium nitrate-containing compositions fired in a pressurercsistant container in accordance with our invention. These firings were made in a closed chamber in order that the gases might be collected and analyzed.

Table III Gas-Generating Charge Noxious gases in percent of total gases g fg g gggggg ilfi' g Total All N115! Amount (g.) Composition N0 N0: 00 H18 Nagisgtsls Gases a g 8 2? 91.3% NH4NO3 130 7.3% Starch 0. 0 1- 1 0 1. 7 324 O. 9 5. 4

1.4% N212SzOa.5H20- 88- 3% NH4NO3 130 9.8% Starch 0- 0 1- 8 0- 4 2. 2 358 0 7. g

1.9% N32520:; 51120- 88.3% NHlNO; 100 9.8% Starch Q 0 7 0 5- 7 355 O 20. 2

1.9% NazSzOs.5Hz0- 87.9% NH4NO3 110 9.8% Starch 0- 0 6- 5 0- 3 6- 8 355 0 24. O

, 2.3% N82S203.5H2

The U. S. Bureau of Mines permits 158 liters of noxious gases per 1.5 pounds of a permissible powder and 5 liters o 1 total NO+NO2 per 1.5 pounds of powder.

suflicient thickness and rigidity to keep the charge in place. It is desirable that the packaging material be nonflammable or be made nonfiammable by appropriate coating or other treatment. It is likewise desirable that the packaging material be moisture-proof, inasmuch as ammonium nitrate is a hygroscopic material. The part of the container around the portion of the gas-generating charge where heat is applied may, if desired, be of somewhat thicker material than the remainder of the container. In that event, it may be desirable to provide vent holes for the gases in the part of the container which is made of the thicker material, either in the side walls of the container or in the base of the first section of a two-section charge as illustrated in Figure 1.

Suitable compositions for use in our packaged gasgcnerating compositions are ammonium nitrate alone, desirably of relatively coarse granulation, or ammonium nitrate with a small amount of a fuel, such as, for example, starch, wood pulp, petrolatum, engine oil, calcium stearate, or graphite. It is preferable that the fuel be added in amounts such that the mixtures and packaging materials therefor contain no less than the amount of oxygen necessary for complete combustion of the mixture and its container. Thus, even with organic fuels containing a large amount of oxygen, the amount of fuel is not likely to exceed approximately 12% by weight of the gas-generating composition. Cooling salts such as sodium chloride, borax, metallic carbonates, and the like, may also be added, obviously only in quantities which are consistent with adequate functioning of the composition in question. It may also be desirable to include anti- While it is possible to heat charges of the type described by means of heating devices which produce flame, such as are described in Example 7 and 10, we prefer to use nonflame-producing heating elements, for example, electric resistance heaters. A resistance heater of the type illustrated in Figures 1 and 2 comprising a ribbon of Nichrome is particularly suitable. With such an electric resistance heater, only current of moderate amperage and voltage need be applied for a time of several seconds. Heaters of Nichrome ribbon are also inexpensive, and, consequently, expendable. Moreover, a Nichrome ribbon heater is capable of supplying adequate heat to initiate the decomposition of the ammonium nitrate-containing charge and of continuing to supply heat for a time thereafter, until the ribbon has burnt through. After the ribbon has burnt through, and the instant of its burning through can be detected by means of an ammeter in the firing circuit, the electric current may be cut off. Thus, at the time of the bursting of the rupture disc, no live wires need be present, which is advantageous in maintaining the high degree of safety of the gas-generating charge. Another suitable form of electric resistance heater is the copper tube assembly described in Example 8. In this heater, the current passes directly through the graphite-containing ammonium nitrate between the copper electrodes. It is likewise possible to construct a heating element, such as a copper coil, as a permanent, reusable part of the pressure-resistant tube, which element may be in contact with the gas-generating charge package, without, however, interfering with the venting" channels therein. The heater may be placed at the firing 13 head end of the gas-generating charge or at any convenient location along the charge.

The frangible closure discs may be made of any desired material of any suitable strength consistent with the release of gases at a pressure adequate for the breaking of coal or other hard material. While discs of steel may be used, we find it also advantageous to use discs of a nonmetallic material such as fiber or plastic, since such discs are nonsparking when the portion sheared by the gas pressure strikes the venting cap. It is possible to use discs of such material as fiber or plastic with our package gasgenerating charges because the rupture disc in our blasting assembly is not required to retain gases under pressure prior to use, nor is it in direct contact with any chemically reactive material. Closure discs of nonsparking metals such as brass may also be used with advantage.

While our invention has been illustrated by means of pressure-resistant tubes of the size and strength which have been in use for the bringing down of coal, it will readily be understood that tubes of larger or smaller diameter or length or of greater or lesser strength may be used, and that the amount and composition of the gasgenerating charge would then be adjusted accordingly, and, consequently, the diameter and length of the package. Venting-space, density, and rate-of-pressure-rise relationships, however, would need to be maintained as set forth herein in order to avoid the probability of rupture of the tube.

It will be seen from the foregoing description that assemblies wherein nonexplosive gas-generating compositions comprising ammonium nitrate are prepared in accordance with our invention afford an excellent means for breaking materials such as coal in large lump form because of the slow, heaving action produced by the release of gases under pressure, the means having the advantages of a high degree of safety, ease of operation, and economy.

The most important safety feature resides in the fact that the gas-generating charge is so arranged that the danger of its developing local high pressure sutficient to burst the tube when activated is avoided. Furthermore, the gas-generating charges of our invention yield smaller amounts of toxic gases than those permissible in coal mines. In addition, the charges are not hazardous to handle, since they cannot be decomposed rapidly without the application of high temperatures and pressures and consequently are not subject to explosion by shock, impact, friction and the like. With the preferred type of heater, accidental activation of the heater could not cause decomposition of the whole charge, and no flames or sparks are produced by the heating element; current greater than the stray currents likely to be encountered in mines must be applied not momentarily, but for an appreciable length of time to cause the device to function. When a heater of the Nichrome ribbon type is used, the ribbon is burned through several seconds before the rupture of the disc; the current may thus be cut off, leaving no live Wires to cause a spark at the time of shearing of the rupture disc. Other safety advantages are obtained because of the fact that nothing in the chargecan react of itself, nor leak out, and because of the fact that the reaction is so rapid that the tube is not heated through before discharge takes place (consequently, the tube is relatively cool after discharge and can be handled immediately).

Ease of operation results from the fact that the charges may be prepared in convenient packets of proper size and amount of charge, which packets can be inserted in the pressure-resistant tubes in the mine. The pressure-resistant tubes thus do not have to be carried in and out of the mine for loading.

The blasting assemblies described are economical because commercial ammonium nitrate, suitable packaging for our gas-generating compositions, and the preferred form of heater element, are all cheap. In addition, preparation of the charges is extremely simple, and the charges are stable on storage. Moreover, the gaseous products of decomposition are not corrosive to the pressureresistant tubes. This lack of corrosiveness of the decomposition products results in long life of the tubes, which may be reused repeatedly.

The invention has been described at length in the foregoing, but it will be understood that many variations in details of charge compositions, assembly, and design of parts may be introduced without departure from the scope thereof. We intend, therefore, to be limited only by the following claims.

We claim:

1. A blasting device especially adapted to coal mining comprising a pressure-resistant tube provided with a rupture disc and a heating means, and in contact with said heating means a gas-generating composition comprising a major proportion of ammonium nitrate, said gas-generating composition being so distributed along the axis of said pressureresistant tube that the loading density of the ammonium nitrate throughout the tube is from 0.1 to 0.3 gram per cubic centimeter, and at any cross-section of the tube is maintained below 0.5 gram per cubic centimeter, the solid-free space within said tube being occupied by gaseous material.

2. A blasting device especially adapted to coal mining comprising a pressure-resistant blasting tube provided with a rupturable closure disc, and containing a gas-generating composition comprising a major proportion of ammonium nitrate, said gas generating composition being enclosed in a cylindrical package of smaller diameter than the internal diameter of the pressure-resistant tube, the diameter of the package being not more than 75% of the internal diameter of said pressure-resistant tube, the loading density of the ammonium nitrate throughout the tube being from 0.1 to 0.3 gram per cubic centimeter and at any crosssection of the tube being maintained below 0.5 gram per cubic centimeter, the solid-free space Within said tube being occupied by gaseous air, and heating means'for initiating said composition.

3. A blasting device especially adapted to coal mining of the type including a pressure-resistant tube with a rupturable disc closure means, an inner substantially cylindrical container of thin material of smaller diameter than the internal diameter of said pressure-resistant tube being disposed substantially along the axis of said tube, said inner container being charged with a gas-generating composition comprising a major proportion of ammonium nitrate and a fuel so disposed therein that the loading density of the ammonium nitrate in the pressure-resistant tube is from 0.1 to 0.3 gram per cubic centimeter and at any cross-section of the pressure-resistant tube is less than 0.5 gram per cubic centimeter, whereby a substantially annular space is provided between the inner container and the internal wall of the pressure-resistant tube, said solid-free space being occupied by gaseous air, and an electric resistance heater disposed in contact with a minor proportion of said gas-generating composition.

4. A blasting device especially adapted to coal mining comprising a pressure-resistant tube provided with a rupture disc and heating means, and in contact with said heating means a gas-generating composition comprising a major proportion of ammonium nitrate, the loading density of said ammonium nitrate throughout the tube being from 0.1 to 0.3 gram per cubic centimeter and at any cross-section of the tube being maintained below 0.5 gram per cubic centimeter, the solid-free space within said tube being occupied by gaseous air, said blasting device being characterized by a rate of pressure rise as confined within said tube not exceeding 4,000,000 pounds per square inch per second.

5. A method of blasting comprising heating in a pressure-resistant tube having a frangible closure at one end a gas-generating composition comprising a major proportion of ammonium nitrate, said gas-generating composition being distributed along the axis of said pressureresistant tube in such amount that the loading density References Cited in the file of this patent of the ammonium nitrate throughout the pressure-resistant UNITED STATES PATENTS tube is 0.1 to 0.3 gram per cubic centimeter and at any cross-section of the pressure-resistant tube is less than 1,610,274 Ferrell et a1 1926 0.5 gram per cubic centimeter, the solid free space being 5 1,705,248 Hart 1929 occupied by gaseou nateriaL Lubelsky 1 1,950,038 Scott Mar. 6, 1934 2,463,709 McFarland Mar. 8, 1949

Claims

0.5 GRAM PER CUBIC CENTIMETER, THE SOLID FREE SPACE BEING OCCUPIED BY GASEOUS MATERIAL
5. A METHOD OF BLASTING COMPRISING HEATING IN A PRESSURE-RESISTANT TUBE HAVING A FRANGIBLE CLOSURE AT ONE END A GAS-GENERATING COMPOSITION COMPRISING A MAJOR PROPORTION OF AMMONIUM NITRATE, SAID GAS-GENERATING COMPOSITION BEING DISTRIBUTED ALONG THE AXIS OF SAID PRESSURERESISTANT TUBE IN SUCH AMOUNT THAT THE LOADING DENSITY OF THE AMMONIUM NITRATE THROUGHOUT THE PRESSURE-RESISTANT TUBE IS 0.1 TO 0.3 GRAM PER CUBIC CENTIMETER AND AT ANY CROSS-SECTION OF THE PRESSURE-RESISTANT TUBE IS LESS THAN