US3305414A - Method of comminuting ammonium nitrate prills mixed with hydrocarbon - Google Patents

Description

Feb. 21, 1967 s. HODGSON 3,3@5,4%14

METHOD OF COMMINUTING AMMONIUM NITRATE PRILLS MIXED WITH HYDROCARBON Filed March 18, 1964 INVENTOR.

Stanley Hodgson gmwm Agent United States Patent This invention relates to explosive compositions and is a particularly directed to a method and apparatus for producing a highly effective explosive composition from prilled ammonium nitrate and liquid hydrocarbon mixtures and the product produced thereby.

Explosive compositions made from a mixture of prilled ammonium nitrate and a liquid hydrocarbon have been widely adopted and used by the mining industry for primary blasting. Such mixtures are relatively inexpensive compared to conventional explosives and provide results comparable to those obtained with conventional nitroglycerine explosives. The ammonium nitrate-liquid hydrocarbon mixtures can be quickly and readily loaded by pneumatic devices into small diameter blast holes used in underground mining operations and can be readily detonated by a primer charge. Good rock fragmentation is obtained and toxic fumes are normally not excessive.

In most mining operations, particularly in underground operations, secondary blasting also is frequently required and relied upon to break chunks of rock and ore too large for normal handling and to release material that has bridged or otherwise hung up in raises, chutes, stopes and the like mine passageways. Secondary breaking has been effected with ammonium nitrate-liquid hydrocarbon mixtures by placing a suitable quantity of the mixture on or against the material to be broken and employing a blasting cap and a booster charge such as dynamite to detonate the ammonium nitrate-liquid hydrocarbon mixture. Although these primer charges are generally satisfactory for detonating the ammonium nitrate-liquid hydrocarbon mixture, they are relatively sensitive to impact and heat and, therefore, are subject to the hazard of premature detonation.

There is some hazard, therefore, in manipulating and positioning a quantity of ammonium nitrate explosive mixture containing such primers, particularly if the material must be manually transported long distances under space-restricted conditions and/ or placed where rock falling onto a prepared charge may initiate a premature detonation. In some instances, the charge must be manipulated into a position or feet from the operator by means of bulldoze sticks and, under such conditions, the danger of premature detonation becomes a serious hazard.

I have found that the above hazards can be substantially overcome by comminuting the ammonium nitrateliquid hydrocarbon mixture to a predetermined size range whereby the resulting cornminuted ammonium nitrateliquid hydrocarbon mixture is rendered detonable by a short length of detonating cord thus permitting the sub stitution of detonating cord for the dynamite or blasting cap primer. The combination of the comminuted or pulverized ammonium nitrate-liquid hydrocarbon mixture with detonating cord is considerably less likely to detonate prematurely than the standard ammonium nitrate-liquid hydrocarbon mixture primed with dynamite and a blasting cap, while providing all the advantageous characteristics of a highly effective explosive for secondary breaking.

It is known that finely comminuted ammonium nitrate is more sensitive than prilled uncomminuted material, and it has been proposed, as disclosed in United States Patent No. 3,046,887 to Brinkley et al., to use particles as fine as 100 mesh admixed with 0.5% to 3.5% by weight of hydrocarbon for a cap sensitive primer charge. The Brinkley et al. disclosure specifically teaches comminution of ammonium nitrate prills before mixing the ammonium nitrate with oil to produce the desired explosive composition. My test work has shown that ammonium nitrateliquid hydrocarbon explosive compositions produced from ammonium nitrate prills comminuted prior to mixing with liquid hydrocarbon, e.g., fuel oil, result in an unsatisfactory mixture which is diflicult to handle and in which the ammonium nitrate particles are not uniformly coated with oil and have not uniformly absorbed the oil.

I have discovered that by first mixing the prills or particles of ammonium nitrate with about 5 to 6% by weight of liquid hydrocarbon and then pulverizing or otherwise comminuting the particles of the resultant mixture so that at least 60%, and preferably about of the particles are of a size less than 28-mesh (standard Tyler screen), a highly effective explosive composition is produced. There is no need for subsequent treatment such as oiling, screening or separation of particle sizes. This mixture can be completely detonated by a 12-inch length of 40-grain per foot detonating cord.

The pulverized mixture can be packaged ready for use in quantities weighing /2 pound to about 50 pounds or more, each package containing a short length of detonating cord, for example, about 12 inches of 40-grain Primacord. The mixture is similar in texture to dry snow. It has an angle of repose of substantially and therefore does not readily spill from torn bags. It is cohesive; it will adhere to and retain the contour of the rock against which it is placed or tamped; and it is readily adaptable to placement in voids between rocks.

It is a primary object of the present invention to provide an explosive composition which will substantially overcome the hazards associated with secondary blasting and encountered with standard ammonium nitrate-liquid hydrocarbon mixtures by obviating the need for dynamite and/ or blasting cap primers.

It is another object of the present invention to provide an ammonium nitrate-liquid hydrocarbon explosive composition having a highly effective detonating force.

A further object of the present invention is the provision of a method and apparatus for efiiciently producing such an explosive composition from conventional ammonium nitrate mixtures.

An understanding of these and other objects of the present invention and the manner in which they can be attained will become apparent from the following detailed description of the method and apparatus of the invention, reference being had to the accompanying drawing in which:

FIGURE 1 is a perspective view of an apparatus employed in the method of the present invention;

FIGURE 2 is a side elevation, partly cut away, illustrating in more detail a portion of the apparatus shown in FIGURE 1; and

FIGURE 3 is an exploded perspective of a component part employed in the present apparatus.

Like reference characters refer to like parts throughout the description and the drawing.

One specific embodiment of the apparatus of the present invention as illustrated by the drawing comprises a support frame 10 having a collar 11 adapted to receive flange 12 of cylindrical vessel 13 for rigid engagement therewith by means of a plurality of bolts 14. A hood 15 having outwardly diverging side walls defining an enlarged upper section 16 is in like manner secured to collar 11 by flange 17 so that said hood is axially aligned with chamber 18 of vessel 13 making an air-tight connection therewith. A pair of rectangular ports formed in the upper portion 16 of hood are covered by conventional dust filters 19 fabricated from glass fiber, aluminum and the like filter materials adapted to permit the passage of gases therethrough while restraining the flow of dust and other particles for reasons which will become apparent as the description proceeds.

The lowermost portion 21 of vessel 13 is adapted to receive a pliable container such as a sack 22 removably fastened thereto by a ring support 23 so that the sack is in communication with chamber 18. It will be evident that ring support 23 will provide an air-tight connection between the sack 22 and vessel 13.

A baflle plate 25 is adapted to be disposed partially across chamber 18 of vessel 13 at an angle of about 45 to the longitudinal axis of the vessel as illustrated most clearly in FIGURE 2. With reference now to FIGURE 3, it will be observed that vessel 13 preferably is fabricated in two sections 26 and 27 having inclined opposed flanges 28 and 29 formed thereon or integrally'secured thereto by welding or the like means. A central plate 30 having a slot 31 formed therein adapted to slidably receive bafile plate 25 is disposed between flanges 28 and 23 and rigid ly secured to the flanges by a plurality of bolts, not shown. Baffle plate 25 is slidably inserted into slot 31 and positioned for the desired projection within chamber 18 by a cotter pin 33 adapted to be inserted into one of holes 34 formed in plate 25.

A tube 35 adapted to open into chamber 18 is rigidly affixed to sleeve 36 by a threaded connection 37. Sleeve 36 is secured to the wall of vessel 13 by a weld or the like connecting means so that the longitudinal axis of sleeve 36 and tube 35 coaxial therewith is directed towards a central portion of baffle plate 25 at an angle of about 75 to the plane of said batfle plate. Tube 35 is connected with a conventional pneumatic loader 38 having an air supply line 39 and an ammonium nitrate supply line 40 in communication with a source of ammonium nitrate prills in container 41.

In the operation of this apparatus, air under about 90 p.s.i. pressure supplied by line 39 flows through loader 38 into chamber 18 of vessel 13. The flow of air through loader 38 creates a low pressure in ammonium nitrate supply line 39 inducing the hydrocarbon treated prills in container 41 to flow through line 40 into loader 38 for entrainment with the air flowing through tube 35. The prills are accelerated substantially to the velocity of the air flowing through tube 35 and ejected into chamber 18 for impingement at high velocity against stationary baffle plate 25. It will be evident that the air will pass around the lowermost end of baflle plate 25 by way of opening 42 formed between the end of plate 25 and the wall of vessel 13 up into hood 15 and escape to the atmosphere through filters 19, carrying a small portion of the finesized comminuted ammonium nitrate mixture into the hood.

To minimize the collection of dust on filters 19, the upper portion of hood 15 is enlarged to reduce the upward velocity of air flowing therethrough so that a portion of the comminuted material carried through port 42 will settle out before reaching the filters. Also, a vibrator, not shown, may be incorporated with the structure of the apparatus to shake dust from filters 19 and prevent a dust build-up.

The bulk of the comminuted material formed when the ammonium nitrate prills strike the baffle plate 25 is deflected downwardly in chamber 18 and falls by gravity into container 22. It will be understood that although the operation of the apparatus as illustrated is carried out as a batch operation, it can readily be converted to a continuous operation.

The following example illustrates the apparatus, method and product produced thereby according to the present invention. Vessel 13 was formed from a steel tube having an inside diameter of about 8 inches. Hood 15, secured to the upper end of vessel 13, was covered with a pair of glass fiber filters. The lower opening of vessel 13 was closed with a heavy paper sack 22. Inlet tube 35, consisting of a inch diameter steel pipe about 10 inches long, was arranged substantially as illustrated in the drawing. Baffle plate 25, formed from a steel plate, was disposed within chamber 18 so that a maximum gap of 4 inches existed between the end of plate 25 and the vessel wall, measured in the plane of the baffle. The loader and feed tube were grounded by means of ground wires to preclude a build-up of static electricity. Air supplied under a pressure of psi. permitted comminution of a mixture of ammonium nitrate and liquid hydrocarbon at a rate of 20 pounds per mintue.

The ammonium nitrate-liquid hydrocarbon prill mixture supplied in container 41 was prepared according to the following procedure. Ammonium nitrate prills in the size range of minus 10 plus 28-mesh (standard Tyler screen) were conditioned in the usual manner, for example, by mixing with 1%, by weight, of diatomaceous earth or with 0.5%, by weight, of AC-lO (a mixture of 90% clay and 10% organic surfactant). The conditioned prills were thoroughly mixed with about 5 /2 by weight, of liquid hydrocarbon, such as No. 2 fuel oil. This mixture was projected at high velocity against the stationary baffle as described hereinabove, thereby comminuting the ammonium nitrate prills so that at least 60% of the comminuted material passed a 28-mesh standard Tyler screen.

The treated ammonium nitrate-liquid hydrocarbon mixture was bagged to form a plurality of individual charges. To each charge a 12-inch length of 40-grain detonating cord, known in the trade as B-line Primacord, was added, the detonating cord being coiled and positioned centrally within the charge with one end of the cord projecting from the neck of the bag. Detonating tests conducted on a plurality of charges confirmed that the l2-inch length of detonating cord provided an adequate booster to ensure complete detonation of charges weighing from /2 to 50 pounds.

It is advisable to prepare the pulverized or comminuted mixture at the site where it will he used, and only in the quantities required for daily use. Prolonged storage can result in caking of the material.

The present invention provides a number of important advantages. Ammonium nitrate prills mixed with a predetermined amount of liquid hydrocarbon and treated according to the process of the invention provide an explosive composition which is more sensitive to detonation, while having a higher order detonating force, than standard ammonium nitrate-liquid hydrocarbon explosive mixtures. A 40-grain detonating cord is satisfactory for detonating charges of /2 to 50 pounds or more in weight, and the usual blasting cap or dynamite primer in the charge is unnecessary, thus improving safety conditions. The comminuted material is cohesive thereby permitting the use of charges which can be readily placed on or against the object to be fractured. The explosive mixture can be quickly and inexpensively manufactured from standard starting materials, i.e., ammonium nitrate prills conditioned with a material such as clay or diatomaceous earth, and a liquid hydrocarbon such as No. 2 fuel oil, diesel oil, Mentor 29 (an oil of relatively low volatility), and the like.

It will be understood, of course, that modifications can be made in the preferred embodiment of the invention described and illustrated herein without departing from the scope and purview of the appended claim.

What I claim as new and desire to protect by Letters Patent of the United States is:

In a method for the production of an improved explosive composition detonable with detonating cord from a mixture of conditioned ammonium nitrate prills with about 5% to about 6% by weight of a liquid hydrocarbon, the steps of introducing particles of said mixture into a stream of air, projecting said particles entrained in said References Cited by the Examiner UNITED STATES PATENTS 1,614,314 1/1927 Murray et al. 24140 X 2,768,938 10/1956 Martin 241-40 X 2,991,946 7/1961 Croft 241-40 2,992,912 7/1961 Hradel et al. 149-46 3,095,335 6/1963 McCloud et al. 14960 X 6 3,111,437 11/1963 Hino et al 149-46 3,184,169 5/1965 Friedman et al 241-4O FOREIGN PATENTS 5 883,276 11/ 1961 Great Britain.

OTHER REFERENCES Bennington et al.: German application 1,143,135, printed Jan. 31, 1963.

CARL D. QUARFORTH, Primary Examiner.

LEON D. ROSDOL, BENJAMIN R. PADGETT,

Examiners.

L. A. SEBASTIAN, Assistant Examiner.

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