Note: Descriptions are shown in the official language in which they were submitted.
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Title: STABILIZED AMMONIUM NITRATE EXPLOSIVES
PRODUCING DECREASED LEVELS OF AMMONIA
FIELD OF THE INVENTION
This invention relates to the field of explosive
compositions which produce decreased levels of ammonia gas. In one
embodiment of the invention, the explosive composition comprises
an inorganic oxidizing salt which includes ammonium (such as in the
form of ammonium nitrate) and a carbonaceous fuel.
BACKGROUND OF THE INVENTION
Explosive compositions comprising ammonium nitrate
have been widely used throughout the world for many years. As
ammonium nitrate is not readily detonatable in of itself, it is typically
15 mixed with organic material in order to obtain a mixture which is
detonatable. Examples of such mixtures include ammonium nitrate
combined with a liquid carbonaceous fuel, which are known as
ammonium nitrate/fuel oil (ANFO) explosive compositions; water-
in-oil emulsion explosive compositions; and, water gel explosive
20 compositions.
Such explosive compositions are typically used in mines
where large amounts of the explosive composition are loaded into a
plurality of boreholes. For example, the explosive composition may be
loaded into from about 10 to 15 boreholes to more than about 100
25 boreholes. Such loading may take a period of days. Typically, the
explosive composition may be kept in a borehole anywhere from one
hour up to about fourteen days or more prior to being detonated.
After being drilled, a borehole may remain dry for an
extended period of time. However, in some cases, water will
30 accumulate in boreholes. One source of water is rain and melted snow
in the case of boreholes which are exposed to the elements, such as in a
surface mine. However, even in underground mining, water may
accumulate in boreholes from the inflow of ground water.
Ammonium nitrate is readily soluble in water. Thus, explosive
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compositions containing ammonium nitrate may be adversely affected
by water penetration and water absorption. For example, if an ANFO
explosive composition is loaded into a wet borehole or a borehole into
which water subsequently seeps prior to detonation, then the ANFO
explosive composition may deflagrate or, in fact, fail to detonate.
Various approaches have been taken by those skilled in
the art to improve the water resistance of ANFO explosive
compositions. One example of this is United States Patent No.
5,480,500. However, despite these approaches, some of the
10 ammonium nitrate may be solubilized by water in a borehole.
Another approach which has been used to prevent water
damage to explosive compositions include putting the explosive
composition in water proof packaging. Borehole liners may also be
used where bulk loaded explosive compositions are used. However,
15 even in a well designed blast, not all of the ammonium nitrate will
react to form gaseous compounds. Accordingly, even if borehole liners
or water proof packaging is used, after detonation, some ammonium
nitrate will be present on the walls or the floor of a mine shaft.
Elevated ammonia levels have been found at mine sites,
20 particularly in underground mines, where ammonium nitrate based
explosive compositions have been used. This is a particular problem
in underground mines where the accumulation of noxious or toxic
gases can pose a serious threat to the health and safety of mine
workers. The production of elevated levels of ammonia have been
25 particularly noted in an underground mine after the explosive
composition has been detonated regardless of whether the explosive
composition is bulk loaded or is prepackaged.
SUMMARY OF THE PRESENT INVENTION
In an aqueous solution, ammonium nitrate dissociates to
produce ammonium and nitrate ions. When the pH of such an
aqueous solution is raised above about 7, ammonium ions react to
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form ammonia which evolves from the aqueous solution. However,
when ammonium nitrate in an explosive composition is dissolved in
water, it produces an aqueous solution having a pH from about 3.5 to
about 5.5 - 6Ø Accordingly, the solubilization of ammonium nitrate
5 does not, in and of itself, result in the production of ammonia.
It has been found that the production of ammonia is
related to the proximity of cement. Cement is commonly used to close
open areas, to smooth walls, or to provide support columns in
underground mines. When exposed to water calcium hydroxide may
10 be leached from the cement to produce a basic solution. If any
unreacted ammonium nitrate is also present, at least some of this
ammonium nitrate will also be dissolved.
Accordingly, ground water in the vicinity of concrete may
solubilize both calcium hydroxide and ammonium nitrate. If too
15 much calcium hydroxide is solubilized, then the pH of the aqueous
solution will be raised above about 7 and ammonia will be produced.
In accordance with one embodiment of this invention, an
explosive composition is provided comprising an inorganic oxidizing
salt including an ammonia precursor (such as ammonium nitrate), a
20 carbonaceous fuel and a water soluble acid. The acid solubilizes at a
rate to maintain an aqueous solution of the ammonium at a pH lower
than the pH at which ammonium reacts to form ammonia.
In accordance with another embodiment of the invention,
a method of reducing the production of ammonia gas subsequent to
25 the detonation of an explosive composition. The method comprises
the steps of providing an inorganic oxidizer salt including ammonium
and a carbonaceous fuel; selecting a water soluble acid, the solubility of
the acid and / or the dissociation constant of the acid selected to
maintain the pH of an aqueous solution produced by the water below
30 the level at which ammonia is released; and, loading an explosive
composition comprising the inorganic oxidizer salt, the carbonaceous
fuel and the water soluble acid into a borehole.
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The explosive composition may be an ANFO explosive
composition, a water gel explosive composition or an emulsion
explosive composition. Preferably, the inorganic oxidizer salt
comprises ammonium nitrate and, more preferably, it consists
essentially of ammonium nitrate. Preferably, the carbonaceous fuel
comprises a fuel oil.
If the explosive composition comprises an ANFO
explosive composition, then the water soluble acid is preferably in a
solid state. Accordingly, the acid may be added as a powder to the
10 mixture of ammonium nitrate and fuel oil. Alternately, the water
soluble acid may be incorporated into the ammonium nitrate during
the formation of, for example, ammonium nitrate prills.
Preferably, the water soluble acid has a solubility in water
comparable to the solubility of ammonium nitrate. More preferably, it
15 has a solubility in water greater than the solubility of ammonium
nitrate in water. The acid is preferably an organic acid, and more
preferably, a weak organic acid.
The acid may be incorporated into the explosive
composition in an amount from about 0.5 to about 10 wt. %, based
20 upon the total weight of the explosive composition, more preferably
from about 1.0 to about 7.0 wt % and, most preferably, from about 1.5 to
about 4.0 wt %.
DESCRIPTION OF PREFERRED EMBODIMENT
The explosive composition of the present invention
comprises an explosive mixture of a carbonaceous fuel, an inorganic
oxldizing salt including an ammonia precursor and a water soluble
acld.
The explosive composition may be in the form of a water-
30 in-oil emulsion explosive composition, a water gel explosive
composition, an ANFO explosive composition or any other known
explosive composition prepared from inorganic oxidizing salts and
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carbonaceous fuels. Preferably, the explosive composition comprises
an ANFO explosive composition and the description of the preferred
embodiment will be based upon ANFO explosive compositions.
However other explosive compositions, including water gels and
5 water-in-oil emulsion explosive compositions may be prepared
including a water soluble acid as described herein to produce explosive
compositions having a reduced production of ammonia gas.
The carbonaceous fuel may be selected from any fuel
known in the art. The fuel may be a solid (e.g. a wax) or a liquid (e.g.
10 fuel oil, heating oil, diesel fuel, jet fuel, kerosene, mineral oil,
saturated fatty acids such as lauric acid and stearic acid, alcohol such as
cetyl alcohol, corn oil, soybean oil and the like) or a mixture of solid
and liquid fuels. Such fuels may also be supplemented with fuel-
soluble ingredients such as glucose, fructose, waxes, such as
15 microcrystalline wax, paraffin wax, petroleum wax and the like.
Preferably, the carbonaceous fuel comprises fuel oil, such as No. 2 fuel
oll.
The inorganic oxidizing salt may be any inorganic
oxidizing salt known in the art which includes an ammonia precursor.
20 Ammonia precursors include those compounds which, on
solubilization in water at an elevated pH, produce ammonia. The
ammonia precursor preferably comprises ammonium. The inorganic
oxidizing salt preferably comprises ammonium nitrate. The
ammonium nitrate, in the case of an ANFO explosive composition,
25 may be in any known solid form used in the art such as prills,
granules, pellets and fines as well as cast or powdered ammonium
nitrate. Particulate ammonium nitrate suitable in ANFO explosive
compositions are known in the art.
A portion of the inorganic oxidizing salt may comprise
30 other known inorganic oxidizing salts including alkali metal nitrates
and prechlorates (such as sodium nitrate and potassium nitrate) or
alkaline earth metal nitrates and perchlorate (such as calcium nitrate,
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magnesium nitrate and barium nitrate). Such additional components
may be added in and about from 0 to about 30 wt. % based upon the
weight of the ammonium nitrate.
The amount of inorganic oxidizing salt and carbonaceous
5 fuel which is incorporated into the explosive composition are known
in the art. If the explosive composition is an ANFO explosive
composition, it may comprise from about 2 to about 10 wt %
carbonaceous fuel based on the weight of the mixture of inorganic
oxidizer salt and carbonaceous fuel. Preferably, the explosive
10 composition contains sufficient carbonaceous fuel so that the
explosive composition is essentially oxygen balanced, taking into
consideration the total oxidizing salts, fuel, sensitizers and other
additives present in the explosive. Preferably, the blend has an oxygen
balance more positive than about minus 25% and, preferably, in the
15 range from about minus 10 to plus 10 %.
If there is water in the vicinity of the blast site, then some
of the unreacted ammonium nitrate will tend to be solubilized. In
pure water, ammonium nitrate will form a weak acidic solution (eg. a
pH from about 3.0 to about 6.0). However, in some environments,
20 such as underground mines, cement is commonly used to close open
areas, to smooth walls, or to provide support columns. Water, such as
ground water, may leech calcium hydroxide from the cement.
Accordingly, the water which solubilizes the unreacted ammonium
nitrate may have a substantially higher pH and, may be basic.
25 Alternately, the water containing the ammonium nitrate may pool on
the floor adjacent to concrete and subsequently leech calcium
hydroxide from the cement thus raising the pH of the aqueous
ammonium nitrate solution. As the pH of the solution rises,
ammonium ions react to form ammonia which is evolved from the
30 solution. Such a reaction tends to occur at a pH of about 7 or higher.
In accordance with the instant invention, a water soluble
acid is associated with the ammonium nitrate so as to maintain the pH
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of such an aqueous solution of ammonium nitrate below the level at
which the aqueous solution will evolve ammonia (i.e. preferably less
than about 7).
The acid is preferably so associated with the ammonium
nitrate so that, subsequent to an explosion, acid will be solubilized
together with ammonium nitrate in any water which is present at the
blast site. For example, in the case of an ANFO explosive composition,
the acid may be incorporated as a solid ingredient in the ANFO
explosive composition (such as a powder or crystals). The acid is
10 preferably added after the addition of the carbonaceous fuel to the
ammonium nitrate. In such an embodiment, the acid is preferably
mixed to form a generally homogenous mixture so as to have an even
concentration of acid throughout the ANFO explosive composition.
In an alternate embodiment, the acid may be incorporated into the
15 liquid from which the ammonium nitrate particles are produced. For
example, a liquid acid solution may be mixed with the ammonium
nitrate solution which is introduced into a prilling tower. The
ammonium nitrate prills formed in this manner will have the acid
incorporated therein.
The amount of acid which is incorporated is preferably
sufficient to maintain the pH of any aqueous solution which may be
formed below about 7 so as to prevent the subsequent evolution of
ammonia. The amount of acid which may be added will vary
depending upon the solubility of the acid in water, the dissociation
25 constant of the acid and other external physical influences such as
temperature.
Preferably, the acid is a weak acid (eg. such as in the case of
citric acid, it may form a solution having a pH from about 1 to about 2).
In such cases, the amount of acid which is incorporated into the
30 explosive composition may vary from out 0.5 to about 10 wt. %, based
upon the total weight of the explosive composition, preferably from
about 1 to about 7 wt. %, more preferably from about 1.5 to about 4 wt.
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% and, most preferably, about 2%. Further, the acid is preferably in a
solid state.
The water soluble acid may be an inorganic acid (such as
sulfamic acid) or an organic acid, such as citric acid, fumaric acid or
5 maleic acid. Preferably, the acid is an organic acid and, most preferably,
the acid is citric acid. An advantage of using an organic acid is that the
acid contributes to the relative energy of the explosive composition.
Accordingly, to the extent that the ammonium nitrate reacts during
the explosion, the citric acid will be available as additional fuel.
Ammonium nitrate is very soluble in water. The
solubility of ammonium nitrate has been measured at 118.3 grams per
100 millimetres of water at 0 degrees centigrade. Citric acid and maleic
acid are both very soluble in water. Citric acid has been measured to
have a solubility in cold water of 133 grams/100 millimetres of water.
15 Preferably, the acid has a solubility comparable to that of the
ammonium nitrate and, more preferably, has a solubility greater than
that of the ammonium nitrate. This is especially so if the acid is a
weak acid (e.g. the acid will produce an aqueous solution having a pH
from about 1 to about 6). It is to be understood that if a stronger acid is
20 used, then the acid may have a lower solubility to maintain the pH of
an aqueous solution of ammonium nitrate below about 7Ø
The explosive composition according to this invention
may be made by any means known in the art provided that the acid is
sufficiently associated with the ammonium nitrate in the explosive
25 composition that at least a portion of the acid will remain in proximity
to the unreacted ammonium nitrate to be solubilized therewith.
Preferably, the concentration of the ammonium nitrate and the acid in
the explosive composition is relatively constant.
It will be appreciated by those skilled in the art that
30 various modifications to the invention may be made. For example, if
the explosive composition is in the form of an emulsion explosive
composition, then the acid may be added as a dry ingredient during the
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manufacturing process. Alternately, the acid may be added to the
aqueous ammonium nitrate solution (thus forming a low pH
ammonium nitrate solution) during the manufacturing process.
5 Examples
Comparative Example 1:
An ANFO explosive composition comprising 94 wt. %
10 ammonium nitrate and 6 wt. % fuel oil was prepared. 89.5 wt. % of
this ANFO explosive composition was placed in a tightly sealed
container with 10 wt. % cement and 0.5 wt. % water. This mixture was
stored at 20~ C for four hours. At the end of this period, the seal on the
container was released and checked for ammonia. At the end of the
15 four hour period, a strong ammonia aroma was readily detected.
Example 2:
Comparative example 1 was repeated except that 2 wt. %
20 citric acid was added to the ANFO explosive composition. No
ammonia was detected during the four hour test period. The mixture
was again tested after a twenty-four hour period and no ammonia was
detected.
25 Example 3:
Example 2 was repeated except with the use of 3% citric
acid. No ammonia was detected during the four hour test.
30 Example 4:
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Example 1 was repeated except that the mixture which was
sealed in the container comprised 83 wt. % of the ANFO explosive
composition, 15 wt. % cement and 2 wt. % water. The ANFO
explosive composition contained 2 wt. % citric acid. Despite the
5 increased amount of cement and water in the container, no ammonia
was detected during the twenty-four hour test period.
Example 5:
Example 4 was repeated except that the mixture placed in
the containers had increased amounts of water. In one case, the
mixture comprised 82 wt. % ANFO, 15 wt. % cement and 3 wt. %
water. In the other case, the mixture contained 81 wt. % ANFO, 15 wt.
% cement and 4 wt. % water. In both cases, no ammonia was detected
15 during the twenty-four hour test period.
The foregoing experiments demonstrate that the addition
of citric acid to an ammonium nitrate fuel oil explosive composition
can eliminate the amount of ammonia produced by an aqueous
solution of ammonium nitrate and hydroxide leeched from the
20 cement. The exact amount of citric acid which is required varies
depending upon the amount of hydroxide ions present in the aqueous
solution. However, additions of about 2 wt. % citric acid appear to
provide a sufficient acidic environment to prevent evolution of
ammonia even with large amounts of hydroxide ions.
Example 6
An ANFO composition comprising 92.3 wt. %
ammonium nitrate, 5.7 wt. % fuel oil and 2.0 wt. % citric acid was
30 prepared. The explosive composition had a bulk density of 0.81 g/cc.
The explosive was loaded into a borehole having a 75 mm diameter.
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The ANFO explosive composition was detonated with a one pound
primer. A velocity of detonation of 3,750 m/s was measured.
The ANFO explosive having the same composition was
loaded into a 35 mm borehole. The loaded density was measured at
5 0.89 g/cc. Upon detonation using a high strength cap, a velocity of
detonation of 2,530 m/s was detected.
