Harrison, Paul Stanley (Flemington, AU)
Harries, Gwyn (Elsternwick, Victoria, AU)

05/140106

12/25/1973

05/04/1971

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Imperial Chemical Industries of Australia and New Zealand Limited (Melbourne, Victoria, AU)

149/21

149/46, 149/112

C06B31/28; C06B31/00; C06B1/04

149/46,21,112

3279965	Ammonium nitrate explosive compositions	October 1966	DE Brancion
3378417	Explosive composition containing inorganic nitrate salt of particular size distribution	April 1968	McFerrin
3394038	Method of producing ammonium nitrate explosive compositions having high package densities	July 1968	Minnick et al.
3558749		January 1971	Rank

Lechert Jr., Stephen J.

We claim

1. Ammonium nitrate-fuel oil compositions of matter comprising ammonium nitrate prills which are of a size that substantially all of the prills pass through a 5 mesh British standard sieve but are retained on a 16 mesh British standard sieve and fuel oil wherein between 15 percent and 90 percent by weight of the ammonium nitrate prills contained therein have a high oil porosity characterized in that 100 grams of said ammonium nitrate prills absorb more than 5 grams of fuel oil and between 85 percent and 10 percent by weight of the ammonium nitrate prills contained therein have a low oil porosity characterized in that 100 grams of said ammonium nitrate prills absorb less than 2 grams of fuel oil.

2. Ammonium nitrate-fuel oil compositions of matter comprising ammonium nitrate prills which are of a size that substantially all of the prills pass through a 5 mesh British standard sieve and are retained on a 16 mesh British standard sieve and fuel oil wherein between 40 percent and 80 percent by weight of the ammonium nitrate prills contained therein have a high oil porosity characterized in that 100 grams of said ammonium nitrate prills absorb more than 5 grams of fuel oil and between 60 percent and 20 percent by weight of the ammonium nitrate prills contained therein have a low oil porosity characterized in that 100 grams of said ammonium nitrate prills absorb less than 2 grams of fuel oil.

3. Compositions of matter according to claim 1 wherein the ammonium nitrate prills are further characterized in that the ammonium nitrate prills having a high oil porosity have a hardness value of less than 450 grams and in that the ammonium nitrate prills having a low oil porosity have a hardness value of greater than 450 grams, said hardness value being the average force required to crush an individual prill of ammonium nitrate.

4. Compositions of matter according to claim 1 wherein the ammonium nitrate prills are further characterized in that the ammonium nitrate prills having a high oil porosity have a free flowing bulk density which is less than 0.85 g/ml and in that the ammonium nitrate prills having a low oil porosity have a free flowing bulk density which is greater than 0.85 g/ml.

5. Compositions of matter according to claim 1 wherein there is present an amount by weight of fuel oil at least equal to 1/50 of the weight of ammonium nitrate present in the composition.

6. Compositions of matter according to claim 1 wherein there is present an amount by weight of fuel oil in the range from 1/20 to 1/13 of the weight of ammonium nitrate present in the composition.

7. Compositions of matter according to claim 1 comprising in addition inorganic fuel material present in an amount in the range from 1 to 15 percent of the weight of the composition.

8. Compositions of matter according to claim 1 comprising in addition oxidizing material other than ammonium nitrate present in an amount in the range from 1 to 15 percent of the weight of the ammonium nitrate present in the composition.

9. Compositions of matter according to claim 1 comprising in addition water present in an amount not in excess of 3 percent by weight of the composition.

This invention relates to improved low cost blasting compositions; in particular it relates to safe ammonium nitrate fuel oil compositions capable of being loaded into holes more densely and having increased blasting power.

Ammonium nitrate fuel oil compositions, commonly and hereinafter also referred to as A.N.F.O. compositions, are well known as blasting agents; they are widely used because they are relatively safe and inexpensive. Ammonium nitrate fuel oil compositions are sometimes manufactured at explosives factories and transported to the site of blasting, but commonly, and preferably, such compositions are manufactured on the site of blasting.

The blasting power of a blasting composition per foot run of bore hole is directly proportional to the loading density for a given blasting agent. By loading density we mean the density of the composition after it has been loaded into the borehole. Increased blasting power per foot run allows for substantial economies in a blasting operation because drilling costs may thereby be reduced and there is less likelihood of secondary blasting being necessary. So as to achieve high loading densities it is the practice to use A.N.F.O. compositions with as high free flowing bulk densities as can be achieved. However, the free flowing bulk density is limited by the necessity for the ammonium nitrate component to be porous enough to absorb sufficient fuel oil to be an effective blasting agent. Crystalline ammonium nitrate, for example, has a specific gravity of 1.725 but is of little use in A.N.F.O. blasting compositions as it is non-absorbent of oil. Commonly, prilled or granular ammonium nitrate is used to make A.N.F.O. compositions. Such prilled or granular ammonium nitrate, hereinafter referred to as ammonium nitrate prills, comprises particles of ammonium nitrate of greater than 60 B.S. mesh size manufactured by any prilling, agglomerating, granulating or compacting process. Generally such particles are spherical or quasi-spherical. A.N.F.O. compositions made from ammonium nitrate prills have free flowing bulk densities generally in the range 0.75 - 0.85 g/ml , sometimes as high as 0.9 g/ml.

The loading density characteristics of an A.N.F.O. composition may be increased by grinding part or all of the ammonium nitrate prills prior to incorporating them in the composition but such a material has poor storage properties. One way of overcoming this difficulty is to crush part or all of the ammonium nitrate prills immediately prior to mixing them with oil and loading into holes. Such techniques, which can lead to up to 10 percent increases in borehole loading densities, however, require special equipment.

We have now found that significant increases in borehole loading densities and blasting power may be achieved by using A.N.F.O. blasting compositions comprising at least two ammonium nitrate prill components, said prill components having different oil porosities, free flowing bulk densities, and prill hardness. Such A.N.F.O. compositions, on being pneumatically loaded into boreholes under normal conditions whereby the composition compacts into the borehole, give rise to higher loading densities than are achieved with conventional A.N.F.O. compositions, and hence have higher blasting power.

Accordingly the present invention provides ammonium nitrate fuel oil compositions comprising ammonium nitrate prills and fuel oil wherein between 15 percent w/w and 90 percent w/w of the ammonium nitrate prills contained therein have an oil porosity as hereinafter defined of more than 5 g oil per 100 g ammonium nitrate and between 10 percent w/w and 85 percent w/w of the ammonium nitrate prills have an oil porosity of less than 2 g oil per 100 g ammonium nitrate.

We also provide a process of preparing ammonium nitrate fuel oil compositions comprising ammonium nitrate prills and fuel oil which process comprises mixing fuel oil with ammonium nitrate prills wherein between 15 percent w/w and 90 percent w/w of the ammonium nitrate prills have an oil porosity of more than 5 g oil per 100 g ammonium nitrate and between 10 percent w/w and 85 percent w/w of the ammonium nitrate prills have an oil porosity of less than 2 g oil per 100 g ammonium nitrate.

The sequence of mixing the ammonium nitrate prills to obtain the desired composition is merely a matter of choice. Thus, for example, each type of prill may be mixed with a portion of the oil and the two prill/oil mixtures blended together to give the desired composition. Alternatively the desired total amount of oil may be mixed with one type of prill and this mixture may then be blended with the other type of prill. Yet again the desired total amount of oil may be added to the prills which have been blended previously in the desired proportions.

Thus, the fuel oil component of the A.N.F.O. composition may be added to one or both of the ammonium nitrate components before these latter components are mixed to give the A.N.F.O. composition which is the subject of this invention, or may be added after mixing the ammonium nitrate components. In this specification any reference to ammonium nitrate prills implies that said prills have not been treated with fuel oil unless stated otherwise.

Ammonium nitrate prills having an oil porosity of more than 5 g oil per 100 g ammonium nitrate will hereafter be referred to as high porosity prills, and ammonium nitrate prills having an oil porosity of less than 2 g oil per 100 g ammonium nitrate will hereafter be referred to as low porosity prills.

It is preferred that the high porosity ammonium nitrate prills should constitute between 40 percent w/w and 80 percent w/w of the total ammonium nitrate content of the A.N.F.O. composition which is the subject of this invention.

It is preferred that both the high-porosity and low-porosity ammonium nitrate prills should be in the form of prills or granules of greater than 30 B.S. mesh size. The further preferred particle size range for the high-porosity prills is substantially between 5 and 16 B.S. mesh. The preferred particle size range for the low-porosity prills iis substantially between 5 and 16 B.S. mesh.

Without limiting the scope of the invention, it is postulated that if the A.N.F.O. compositions, which are the subject of this invention, are loaded into a borehole pneumatically the low porosity prills, which generally have a high hardness value, as defined hereinafter, impinge on the less hard, high porosity, prills during the loading process, particularly in the borehole, and break down the less hard prills, thus significantly increasing the particle size range of the A.N.F.O. composition and leading to a higher loading density. The hardness value for the low porosity prills should be greater than 450 g and for the high porosity prills should be less than 450 g as measured by the method hereinafter described. Preferably the low porosity prill hardness is greater than 500g and the high porosity prill hardness is less than 400 g.

The loading density is also related to the free flowing bulk density of an A.N.F.O. composition provided that the composition is formulated from one type of ammonium nitrate prill; the greater the free flowing bulk density of the A.N.F.O. composition, the greater the loading density. In the A.N.F.O. compositions which are the subject of this invention the two ammonium nitrate prill components have different free flowing bulk densities but, surprisingly the differences between the loading densities and free flowing bulk densities is greater than would be expected from an A.N.F.O. composition comprising one type of ammonium nitrate prill of the same free flowing bulk density as the mixture. The high porosity ammonium nitrate prills should have a free flowing bulk density, as determined by the method hereinafter defined, of less than 0.85 g/ml, and the low porosity ammonium nitrate prills a free flowing bulk density of greater than 0.85 g/ml. Preferably these free flowing bulk densities should be less than 0.80 g/ml for the high porosity prills and greater than 0.90 g/ml for the low porosity prills.

The term `fuel oil` used in this specification denotes an organic liquid which is capable of acting as a fuel in the oxidising reaction of the ammonium nitrate. Suitable organic fuel oils are for example, furnace oil, diesel oil, ethylene glycol, and tricresyl phosphate. It is preferred that the organic fuel oil be present in an amount by weight at least equal to 1/50 of the ammonium nitrate present, more particularly between 1/20 and 1/13 by weight of ammonium nitrate. However, the presence of other components, such as inorganic fuels and other oxidising agents, which may be included in the A.N.F.O. compositions of this invention, may alter the oxygen balance of the composition and hence may alter the preferred amount of organic fuel oil added to the composition.

Aluminum, sulphur, and finely divided silicon are examples of inorganic fuels which may be present in the A.N.F.O. compositions either as separate particles or in composite particles with the ammonium nitrate. Preferably such inorganic fuels are present in amounts between 1 percent and 15 percent by weight of the A.N.F.O. composition.

Sometimes it is desired to adjust the oxygen balance in the A.N.F.O. composition by the addition of other oxidising agents such as, for example, sodium nitrate, potassium nitrate and sodium perchlorate. These other oxidising agents may be present as discrete particles or in composite particles with the ammonium nitrate. It is preferred that, if present, such oxidising agents are present in amounts equivalent to between 1 percent and 15 percent of the ammonium nitrate content.

The A.N.F.O. compositions may also contain water in an amount not exceeding that which is consistent with good operation of the loading mechanism used. Preferably the water content does not exceed 3 percent by weight of the composition.

The most benefit is obtained when using the A.N.F.O. compositions of this invention if they are pneumatically loaded into the borehole. However, even if the holes are charged by pouring, higher loading densities will be achieved than those obtained if the A.N.F.O. composition contained only high porosity prills. Preferably the loading process should be one which causes the low porosity prills to impinge on the high porosity prills.

Accordingly our invention also provides a process for loading boreholes with an explosive composition wherein an ammonium nitrate-fuel oil composition, according to this invention and comprising at least two forms of ammonium nitrate prills, is fluidised in a stream of gas to produce a fluidised bed which is projected into the borehole so that the particles in the ammonium nitrate fuel oil composition impinge on themselves and the walls of the borehole thereby forming a dense compact mass therein.

Examples of the types of loading equipment which are capable of performing the loading process of this invention are the `Penberthy Anoloder` and the `N.V.E. Blasthole charger.` The `Anoloder` operates on the venturi principle, the A.N.F.O. composition being drawn from a hopper into the centre of an annular compressed air (30 - 110 p.s.i.) jet which fluidises the A.N.F.O. and projects it into the borehole. The `N.V.E. Charger` works on the principle that the A.N.F.O. composition is stored in a pressurised hopper (25 - 50 p.s.i.) from which it is forced into a fluidising air stream which projects it into the borehole.

By borehole we mean any hole, crack, orifice or the like in which it is desired to load the A.N.F.O. composition.

So as to obtain the desired degree of compaction in the mass of A.N.F.O. in the borehole the conditions of loading, such as pneumatic pressure, loading pipe diameter and length and stand-off distance from the open end of the loading pipe to the closed end of the borehole or the surface of the compacted A.N.F.O. composition should be controlled within certain limits. Typically when loading pipe lengths in the range from 30 to 150 feet, loading pipe internal diameters from 0.75 to 1.05 inches and pneumatic pressures from 40 to 90 pounds per square inch have been used, stand-off distances in the range from 5 to 7 feet have been found to give satisfactory results.

It will be appreciated that the use of incorrect conditions could lead to poor compaction of the A.N.F.O. in the borehole and blow-back of A.N.F.O. out of the borehole during loading.

The compositions of our invention may be prepared for use in a borehole by fluidizing a mixture of ammonium nitrate prills of high and low porosities in a stream of gas -- usually air, although other gases particularly more inert gases such as nitrogen or air enriched with nitrogen may be used for greater safety -- in a vessel having solid attrition resistant walls causing attrition between the prills of differing porosities. The attrition produced by collisions between the porous and non-porous prills causes much of the disintegration, mainly of the porous prills, required to form the final physical shape of the composition. However the process of attrition may also be aided by the walls of the vessel e.g., by providing a rough surface having sharp ridges and/or tips such as are produced by pitting the metal surface by acid treatment, sandblasting or even by fitting it with sharp studs. The vessel itself, conveniently, is a cylinder with inlets for the prills, the fluidizing gas -- optionally a single inlet for both -- and an outlet to the filling hose leading to the borehole. In practice, most conveniently a simple pipe, such as an extension of the loading hose, or the length of loading hose, protruding from the hole, itself may be used as the vessel in which attrition occurs, but a fair length of hose is usually desirable in this case. Thus, using a loading hose of 1 to 4 inches diameter, 50 to 200 feet of tube (steel or plastic such as polyvinyl chloride) i.e., an extension of the loading hose itself is usually satisfactory. In addition the borehole itself may be used to aid attrition and diminution since in practice the unloaded portion thereof will act as an extension of the loading hose. It will be appreciated that neither the shape of the vessel (hose) nor its walls are narrowly critical; control of the residence time of the prills in the fluidized bed, the intensity of the air blast, resulting in different pressures and varying degrees of attrition, the nature of the prills and of any further additions for example the amount of fuel oil or other additives, the surface characteristics and diameter of the vessel, the hose and the hole all will affect the rate of the attrition process. It is a matter of routine variation to control them, for example the residence time, the kinetic energy of the air blast or the diameter of the hose, to achieve the desired degree of comminution of the composition.

Accordingly our invention also provides a process of comminuting an ammonium nitrate fuel oil composition comprising at least two forms of ammonium nitrate prills, fuel oil, and optionally inorganic fuel material and other oxidizing material, which process comprises fluidizing said composition in a stream of gas to produce a fluidized bed which is projected from a vessel into a borehole so that the particles in said composition impinge on themselves and the walls of the vessel and the borehole thereby becoming comminuted by attrition.

Our compositions are useful for the purposes of blasting. Accordingly we provide in a method of blasting wherein an ammonium nitrate fuel oil composition is detonated in a borehole the improvement wherein the ammonium nitrate present in said composition comprises at least two types of ammonium nitrate prills and wherein between 15 percent w/w and 90 percent w/w of the ammonium nitrate prills contained therein have an oil porosity as hereinbefore defined of more than 5 g oil per 100 g ammonium nitrate and between 10 percent w/w and 85 percent w/w of the ammonium nitrate prills have an oil porosity of less than 2 g oil per 100 g ammonium nitrate.

The advantages of using blasting agent systems according to this invention are higher loading densities, hence higher power with consequent savings in drilling operations and secondary blasting. Moreover, because of the higher free flowing bulk density of the ammonium nitrate prills economies may be made in packaging and transport of the same. A further advantage is that the A.N.F.O. compositions which are the subject of this invention are well suited for underground mining operations, such as ring blasting, in which it is necessary to load up-holes with a blasting agent.

The following examples illustrate the invention, but should not be construed as limiting.

In the drawing FIGS. 1 and 2 are graphs of loading density versus weight ratio for various mixtures of ammonium nitrate prills.

Example 1

Table I gives details of the physical characteristics of ammonium nitrate prills which are examples of high porosity ammonium nitrate prills. ##SPC1##

The oil porosity represents the weight of fuel oil in grams which is absorbed by 100 grams of the ammonium nitrate prills. It is determined by adding to 10 g of ammonium nitrate prills in a small jar an amount of fuel oil such that the prills absorb only a portion of it. In most instances 1 g of fuel oil is added; however, with prills known to have a high oil porosity it is desirable to add more oil, for example, 1.5 g. The prills and oil are allowed to remain in contact for 15 minutes at room temperature with occasional gentle tumbling to ensure adequate mixing. Pieces of absorbent filter paper are put into the jar to absorb the excess fuel oil. They are then removed without removing any of the ammonium nitrate particles and the jar and contents are weighed. The oil porosity is calculated according to the formula

Increase in weight/Weight of ammonium nitrate prills × 100.

The hardness value is obtained by subjecting sixty prills selected at random from the sample under test and crushing these individually by an apparatus in which a crushing force is applied by an electrical device to the prill. The current required to break the prill is recorded, it is then interpreted in terms of grams load applied by means of a suitable calibration graph and this represents the hardness of the particular prill tested. The mean hardness of the sixty prills selected is quoted in Table I as hardness.

In order to obtain the free flowing bulk density figure the sample is poured into a 1,000 ml. measuring cylinder and the level in the cylinder adjusted to the 1,000 ml mark with the minimum of disturbance. The ammonium nitrate prills in the cylinder are then poured out into another vessel and poured back into the cylinder to check that the 1,000 ml mark is reached. The contents of the cylinder are weighed and the free flowing bulk density expressed as g/ml.

The ammonium nitrate prills described in Table I were taken from commercially available ammonium nitrate prills and are suitable as the high porosity components of A.N.F.O. compositions which are the subject of this invention.

Example 2

Table II gives details of the physical characteristics of ammonium nitrate prills which are examples of low porosity ammonium nitrate prills. ##SPC2##

The oil porosity, hardness, free flowing bulk density were all determined by the methods described in Example 1.

The ammonium nitrate particles described in Table II were taken from commercially available samples; samples LP -- 1, LP -- 2 and LP -- 3 were all fertiliser grade ammonium nitrate prills. They are all suitable as the low porosity components of A.N.F.O. compositions which are the subject of this invention.

Example 3

Mixtures of ammonium nitrate prills of the types HP -- 4 and LP -- 1 were pneumatically loaded by an N.V.E. loader, previously described, into a glass pipe of 2 inches internal diameter with an end closed with a steel plate, to simulate a borehole, through a flexible hose 30 ft. long and three-fourths in. internal diameter. The air pressure was 40 to 50 p.s.i. and the stand-off distance was 5 ft.

The loading density of the various mixtures tested is plotted as a continuous line vs. weight ratio of HP -- 4 : LP -- 1 on the graph shown in FIG. 1. The broken line represents the calculated composite loading density if the two types of prill had been loaded separately into the same borehole.

For the conditions used in this example and the types of ammonium nitrate prills used, it is desirable that the proportion of low-porosity prills should be at least equal to the proportion of high porosity.

Inasmuch as there is a preferred limitation imposed by the overall oil porosity of the mixture at 2 percent oil, this imposes a preferred upper limit of 82 parts of low porosity prills with 18 parts of high porosity prills in the A.N.F.O. composition formulated from HP -- 4 and LP -- 1.

Example 4

Example 3 was repeated but the stand-off distance was increased to 6 ft.

FIG. 2 shows the plot of the loading densities vs. weight ratio of HP -- 4 and LP -- 1 as a continuous line. The broken line again represents the composite loading density.

Under these conditions, the desirable compositions are between 80 : 20 and 18 : 82 high porosity : low porosity prills.

Example 5

The following A.N.F.O. compositions according to this invention and set out in Table III were formulated using ammonium nitrate prills exemplified in Examples 1 and 2. The two forms of ammonium nitrate prills were mixed, the appropriate amount of diesel oil added followed by the other ingredients. ##SPC3##

The loading densities quoted in Table III were obtained by loading a 2 inch steel tube with an "Anoloder" previously described, operating at 85 - 90 p.s.i. The stand-off distance was 5 ft. The loading density obtained under the same loading conditions with a conventional A.N.F.O. composition (94 percent HP -- 4 and 6 percent diesel oil) was 0.86 g/ml.

Example 6

The following A.N.F.O. compositions as set out in Table IV were formulated by blending the two forms of ammonium nitrate prills, adding the appropriate amount of diesel oil and mixing thoroughly. The loading densities quoted in Table IV were obtained by loading a 2 inch steel tube with an "Anoloder" as previously described and operating at 40 to 50 psi. The stand off distance was 5 feet. ##SPC4##

Example 7

A mixture of ammonium nitrate prills consisting of 50 parts of HP 6, 20 parts of HP 2, 15 parts of LP 6 and 9 parts of LP 1 was prepared and mixed thoroughly with 6 parts of diesel oil. When the mixture was loaded into a tube by the procedure described in Example 6 a loading density of 0.95 g/ml was obtained.

Example 8

This example demonstrates the sensitivity to detonation of compositions according to our invention, prepared as in Examples 5 and 6. A cardboard cylinder 4 inches in diameter and 4 inches long was filled with the composition to be tested and placed on a steel block. An 18 inches length of a detonating fuse was placed between the filled cylinder and the steel block to act as a detonation indicator, the end of the detonating fuse being placed slightly off centre to form a chord with the circular end of the bottom of the cylinder. The appropriate number of No. 8 aluminum plain detonators to be used and one No. 8 aluminum detonator were strapped together with adhesive tape to form a primer and inserted centrally in the top of the cylinder so that the open end of the plain detonators was just covered with the composition to be tested. The primer was then fired by the electric detonator. The detonating fuse was then inspected and detonation of the charge was considered to have been initiated if the detonating fuse was found to have fired. If the detonating fuse was found not to have fired the number of plain detonators was increased in a further trial until the detonating fuse fired. In Table V there is set out the compositions used, using the designations of the samples as in Examples 5 and 6, and the number of detonators required for initiation of the composition.

TABLE V

Sample Detonators required No. for initiation A 5 E 7 F 10 G 3 H 7 I 10 L 15 P 15 R 3

for the purposes of comparison attempts were made to detonate a composition consisting of 94 parts of ammonium nitrate prills designated LP 1 and 6 parts of diesel oil, but these attempts were unsuccessful, even when relatively large amounts of pentolite primer, approximately equal to 30 detonators, were used.

Example 9

A hole 2 inches in diameter and 50 feet long was bored upwards in the roof of a mine. The hole was bottom primed with detonators, a train of detonating fuse run down the length of the hole and the hole was then charged, using an "Analoder" operating at 85 to 90 psi at a stand-off distance of 5 feet with an A.N.F.O. composition. The composition used was that designated as Sample A in Example 5. The composition was detonated successfully by conventional means. Similar results were obtained when Sample A was replaced by Sample F, Sample G or Sample R of Example 6.