Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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LOW-DENSITY GRANULAR BLASTING AGENT FOR USE IN MINING
1. State of the Art
The present invention refers to the development of a low density granular
Oxidizing
Agent whose ingredients do not segregate and that when mixed with combustible
elements, is transformed into a low density granular Blasting Agent suitable
for using in
open pit and underground mining applications in order to control rock
fragmentation and
damage.
The most used explosives in the mining industry contain ammonium nitrate (AN)
as their
main oxidizing ingredient due to its low cost, safe handling and ease of
transportation
and storage.
Mixtures of prilled ammonium nitrate and fuel oil, commonly named ANFO, are
widely
used in open pit and underground blasting operations. ANFO, in turn, has been
mixed
with other industrial explosives such as "explosive emulsions" (to produce the
so called
Heavy ANFOs) in order to modify the performance of the resulting mixture and
adapt it
to the operation requirements.
Damage control in blasting environments requires in many operational instances
the use
of explosives developing lower energies than the one liberated by the above-
mentioned
industrial products, being density reduction by the addition of a diluent
agent one of the
most accepted practices for the purpose of reducing the energy.
The density of an explosive mixture plays a very important role in the energy
and
pressure delivered during detonation and consequently, in the results of rock
fragmentation and/or wall control; reason why a great effort in time and
resources has
been placed to reduce the density of explosive mixtures, particularly mixtures
comprising
ANFO prills.
A typical option to reduce density of prilled ANFO is to mix it with diluents
such as
sawdust, bagasse, silica or plastic microbubbles, volcanic rock, rice hulls,
expanded
polystyrene and a series of similar products. There is a limit below which it
is not
convenient to dilute a mixture containing ANFO due to the risks of detonation
failure;
said limit shall depend in part on the type of diluent used. All these
mixtures suffer as
well from a serious problem: the unavoidable segregation of the diluent
ingredients while
they are being mixed and/or loaded into the hole, as a result of the
difference in specific
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weights between ANFO and the diluent itself, which only increases the risk of
detonation
failures.
Some examples of low-density explosive mixtures developed for damage control
are
mentioned below.
Patent 246 457 to Gotz et al., published in East Germany on June 10, 1987,
introduces
an explosive mixture comprising 10% to 80% by volume of ammonium nitrate
treated
with 0% to 1% by volume of diesel oil and 90% to 10% by volume of foamed
polystyrene.
US Patent 4,957,569 issued on September 8, 1990 in the name of Waldock,
introduces
an explosive emulsion that comprises various salts in the oil-dispersed phase,
to which
low density diluents such as expanded polystyrene, sawdust, perlite and
vermiculite,
have been added.
US Patent 6,955,731 B2 issued in the name of WALDOCK refers to mixtures of an
emulsion and ANFO known in the art as Heavy ANFOs, to which rice hulls have
been
added in order to reduce density of the mixture and increase sensitivity of
the explosive.
Patent 1,601,972, published in Russia on July 9, 1995, introduces an explosive
mixture
that contains ammonium nitrate and polystyrene, the latter being expanded in a
hot
aqueous solution of ethylene glycol as an inherent part of the manufacturing
process.
Patent US 6,425,965 B1 of July 30, 2002 of G. Silva, includes a novel concept
by which
a reactive nature is conferred to a diluent agent without losing its low
density
characteristics, thus allowing its use as a reactive diluent agent in the
typical explosives
employed in blasting operations, particularly of ANFO in prilled form.
2. Description of the Invention
2.1 Overview
The present invention may be summarized as the development of a low density
granular
Oxidizing Agent (PAN) to which explosive properties are conferred through the
simple
mixture thereof with combustible ingredients, thereby producing a low density
granular
Blasting Agent (PANFO) having many applications in the mining industry.
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The source of the oxidizing ingredient consists in a concentrated aqueous
solution of
ammonium nitrate, which is absorbed by expanded perlite granules, an inert
mineral of a
very low density and with a high liquid absorption and retention capacity.
In a first manufacturing stage, the expanded perlite is soaked in the ammonium
nitrate
solution until ensuring that all its pores are completely saturated with said
solution.
Precipitation of the ammonium nitrate crystals in the perlite shall start as a
result of the
solution cooling below its saturation temperature and by the evaporation
process of
water from the solution. The granular product that we have named PAN shall be
obtained upon precipitation of said crystals within the pores and on the
surface of perlite
and when residual moisture has been removed.
Thus, PAN consists of an oxidizing salt mainly contained within the pores of
inert
material granules. As such, for handling, storage and transportation purposes,
PAN
should be classified as an Oxidizing Agent, in the same way as prilled
ammonium nitrate
(AN) used for manufacturing ANFO.
PAN granules may conveniently be treated with typical combustible ingredients
such as
gas oil, fuel oil, mineral or vegetable oils and others, to originate the
product we have
named PANFO, a low density granular explosive product that for regulatory
purposes
should be classified as a Blasting Agent, in the same way as prilled ANFO.
The reactive nature and low density of PANFO makes it an ideal agent to be
used by
itself, in blasting operations requiring low energy products, or else as a
diluent in
mixtures with ANFO and explosive emulsions in order to reduce density and
detonation
pressure in the resulting mixture, both these parameters having the greatest
importance
for fragmentation and damage control.
When used as a diluent agent of ANFO, the reactive nature of PANFO allows
dilution of
the mixture to values that are not possible with other diluents without
running the risk of
detonation failure, whether the latter is a result of requiring a higher
degree of dilution,
the unavoidable segregation of the selected diluent or of a combination of the
above
two. In other words, the use of PANFO as a diluent added to ANFO allows
working in a
density range starting from ANFO's nominal density (850 kg/m3) to the density
of
PANFO itself (250-300 kg/m3) without any risk of generating detonation
failures due to
an excessive dilution and/or segregation of the components.
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2.2 Detailed Description of the Invention
The main purpose of the present invention is the development of PANFO, a low
density
granular explosive that for handling, storing and transportation purposes
should be
classified as a Blasting Agent. The manufacture of PANFO would comprise two
stages,
an initial stage where two ingredients are mixed to form PAN, followed by a
second
stage that would comprise the addition to PAN, preferably at the moment of
unloading it
into the blastholes, of a third ingredient which would give origin to PANFO.
The three preferred basic components of PANFO consist then in expanded perlite
particles, of an oxidizing ammonium nitrate aqueous solution and of a liquid
combustible
of the fuel oil type. Perlite is an inert material obtained from siliceous
rocks, its more
distinctive characteristic with respect to similar volcanic rocks being its
capacity of being
expanded when subjected to high temperatures, whereby it will become an
extremely
light and absorbent product. Owing to its high absorption capacity, perlite is
commonly
used to control spills (oil, water), deodorize liquid effluents and in its
granular form, as a
carrier for pesticides, herbicides and similar liquid substances.
In the present invention, expanded perlite is used in granular form, with a
preferred
particle size between 2 mm and 100 mm with densities going between 50 kg/ms
and 350
kg/m3, its preferred density being about 100 kg/cm3, because at said value
perlite
preserves an adequate liquid absorption and retention capacity while
maintaining
sufficient mechanic strength.
From the various Oxidizing Agents, ammonium nitrate is the most used in the
explosives
industry for its cost, safety and availability. The ammonium nitrate aqueous
solution
typically used during prill manufacture is a 96% concentrated solution with a
saturation
temperature of 125 C. However, solutions at a lower concentration and
saturation
temperature are transferred and used for different purposes, such as the case
of
solutions having 83% by weight ammonium nitrate with a saturation temperature
that
fluctuates about 65 C. This last concentration has proved convenient in PAN
manufacture due to its relatively high ammonium nitrate contents and to its
low
saturation temperature that makes handling thereof easier during operations.
In the
present invention, it is convenient to work with an oversaturated oxidizing
solution at
high temperatures, so as to ensure the existence of sufficient ammonium
nitrate crystals
to fill all the pores of the expanded perlite, increase crystallization
percentage by cooling
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and as a result, reduce percentage of the crystallization generated by
evaporation of
water from the solution.
The absorption of the oxidizing solution in perlite occurs practically upon
contact.
Crystallization of the ammonium nitrate salts within perlite porous spaces
shall occur as
a result of the natural cooling process and the evaporation of water from the
solution.
The more concentrated the solution, more crystals will be precipitated by
cooling and
less water will be necessary to evaporate in order to complete crystallization
and drying.
The ammonium nitrate crystals thus formed will remain trapped within perlite
porosities
and in a thin surface layer without any possibility of being segregated unless
physical
destruction of the assembly is induced. Perlite acts then as a carrier for
ammonium
nitrate salts giving rise to PAN, a granular product whose final density shall
depend to a
great extent on the porosity degree of the perlite used.
To reduce crystallization of ammonium nitrate on perlite surface and prevent
the
generation of a thicker coating layer that increases PAN density, it is
convenient to
facilitate drainage of the excess solution, by excess being understood that
amount that
exceeds the retention capacity of perlite granules. To this effect it is
recommended to
work with an oversaturated solution at a temperature several degrees above the
saturation temperature that corresponds to concentration of the solution. Said
oversaturated solution may or may not have additional dissolved salts such as
sodium
nitrate and calcium nitrate.
Once the solution has been absorbed, NA crystals have precipitated and the
granular
mixture has been dried, density will be increased from the approximately 100
kg/m3
which correspond to the untreated perlite to about 250-320 kg/m3 corresponding
to PAN.
Due to the inert composition of perlite, PAN should be classified for storage,
handling
and transport purposes as an Oxidizing Agent. Furthermore, after adding the
combustible ingredient that would give origin to PANFO, this should be
classified in turn
as a Blasting Agent, in the same way as standard prilled ANFO.
Due to fewer storage, handling and transportation restrictions associated with
PAN, it is
convenient that the addition of combustible required to transform it into
PANFO, be
conducted moments before it is downloaded into the blastholes, in a similar
fashion as
with the ammonium nitrate prills used to produce ANFO.
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There are two ways of reducing PAN or PANFO density, namely: using a less
porous
perlite or else using a more diluted ammonium nitrate solution such that not
sufficient
crystals form within the perlite pores. However, the use of oversaturated
solutions is
preferred so that crystallization is mainly produced by a cooling action and
not by
evaporation, thus reducing drying times and making the manufacturing stage
easier.
Liquid combustibles typically used in industry, such as fuel oil or gas oil
are
recommended; however any other liquid combustible that may be absorbed by PAN
or
dissolved in the ammonium nitrate aqueous solution would also be adequate. For
example, it has experimentally been confirmed that dissolution of sugar in the
oxidizing
solution of ammonium nitrate would give rise to precipitation of both crystals
in perlite.
Although feasible, said alternative is not advantageous because in so doing we
will end
up working with a Blasting Agent instead of with an Oxidizing Agent such as
PAN.
Addition of the combustible agent to PAN must be carried out at a weight ratio
so to
produce a resulting PANFO balanced in oxygen. For that purpose approximately
6% of
the weight corresponding to precipitated ammonium nitrate crystals must be
absorbed
through the pores. Due to the high absorption capacity of liquid combustible
characterizing perlite, part of said percentage may not be in an intimate
contact with the
ammonium nitrate crystals, thus tending to promote formation of toxic nitrogen
gases. If
possible, it is recommended to carry out trials with the available products in
order to
determine the fuel percentage that will prevent a development of said highly
toxic gases.
If it is not possible to carry out trials, it is recommended to increase
combustible contents
to 6% of the total weight, that is the weight of PAN and not only of the
nitrate crystals;
this will result in a PANFO that is slightly richer in combustible but without
the capacity of
generating the undesirable nitrogen gases.
It is convenient to proceed with a drying stage of PAN, not only to ensure
crystal
precipitation but also to remove residual moisture that may later affect PANFO
initiation
sensitivity and performance. Of the several types of dryers, the most
efficient for treating
said granular product are fluidized bed dryers that due to the excellent
contact between
air and PAN particles offers the best possible energetic exchange for moisture
removal.
However, PANFO irregular grain size distribution may reduce their efficiency
and make
them susceptible of generating an uneven drying. Belt dryers with
countercurrent hot air
flows are another valid alternative.
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Neither PAN nor PANFO have resistance to the damaging effects of water
dissolving the
ammonium nitrate crystals and desensitizing the product. However, there are
ways of
conferring a certain water resistance to the product, the most practical being
mixing
PANFO with an explosive emulsion at a ratio that ensures an adequate
protection. Said
ratio shall be a function of the degree of protection sought, and will have to
be
established in the field. Furthermore, the addition of certain patented
products based on
guar gums and other powdery components, such as that used in WR-ANFO
(registered
trademark) that consists of standard ANFO mixed with guar gum), would also
provide a
certain degree of protection from water damage.
If necessary, PANFO may be mixed with high explosive fines to provide higher
initiation
sensitivity and a performance that complies with user's requirements. Said
compounds
include pentaerythrol tetraamine (PETN), cyclo-1,3,5-trimethylene-2,4,6-
trinitramine
(RDX), trinitrotoluene (TNT), nitroguanidine, cyclotetramethylene
tetranitramine (HMX),
as well as the typical products known in the mining industry, particularly
prilled ANFO
and explosive emulsions.
3. Detailed example of PANFO manufacturing
An example of a two-stage manufacturing process of PANFO will be described
next.
Manufacturing of PAN followed by drying, and Manufacturing of PANFO as the
final
result
For the first stage, a granular expanded perlite having a 3 mm to 5 mm average
particle
size and a density of 0,1 g/cm3, was mixed with a 96% ammonium nitrate
concentrated
solution at 125 C saturation temperature. Once the solution was absorbed by
the perlite,
the excess of saturated ammonium nitrate solution was drained. The resulting
wet PAN
granules were dried using a countercurrent hot air flow belt dryer. The final
bulk density
of PAN was measured at 0.32 g/cm3.
In the second stage, PAN was treated by adding fuel oil in an amount
equivalent to a 6%
weight basis of the ammonium nitrate contained within PAN, resulting in PANFO
having
a density of 0.37 g/cm3. The mixing process of PAN and fuel oil was done right
before
loading it into the blastholes.
The velocity of detonation (VOD) was measured, resulting in a value of 1,937
m/sec.
Comparing this result to the 3.500 to 5.500 m/sec VOD values produced by
typical
commercial explosives used in the industry, a substantial and highly
convenient lower
value is obtained.
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3.1 Experimental Testing
In addition to the example previously described, a number of experimental
tests were
conducted, whose results clearly show the potential the product has for
optimizing
blasting operations in the mining industry.
The following table summarizes VOD testing performed on PANFO prepared using
100
kg/m3 expanded perlite (0.1 g/cm3), ammonium nitrate (AN) solution and
approximately
6% to 10% fuel oil, at various densities, diameters and initiation conditions.
Test Diameter Density Initiator Fuel VOD
# (mm) g/cm3 (g) (%) m/sec
1 75 0.34 220 10% Fuel 1850
Oil
2 50 0.32 220 10% Fuel 1770
Oil
3 75 0.26 220 10% Fuel 1490
Oil
4 50 0.31 220 Sugars 1500
50 0.20 220 10% Fuel 1230
Oil
6 100 0.36 220 Urea 1485
7 50 0.30 220 10% Fuel 1450
Oil
8 75 0.31 220 10% Fuel 1670
Oil
9 50 0.35 30 8% Fuel Oil 1400
50 0.35 30 8% Fuel Oil 1600
11 50 0.28 25 10% Fuel 1790
Oil
12 50 0.37 40 6% Fuel Oil 1937
13 50 0.29 40 6% Fuel Oil 1700
14 50 0.29 40 6% Fuel Oil 1830
50 0.31 20 6% Fuel Oil 1711
In reference to the above experimental results, the following comments are
relevant:
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1. Velocity of Detonation (VOD) records fall within a 1500 to 1900 m/sec range
value,
considerably lower than those generated by commercial explosives commonly used
in
industry, (most typically between 3.500 to 5.500 m/sec).
2. Tests #13, 14 and 15, with a 6% fuel oil content (corresponding to an
oxygen-
balanced mixture), showed a better performance than when a 10% fuel oil
content was
used, as in Tests # 7 and 8. Although these are preliminary results, they
indicate that it
would not be necessary to use a greater percentage of fuel oil to compensate
for its
eventual absorption within the perlite.
3. It is possible to detonate PANFO in 50 mm diameters at a density of 0.20
g/cm3 using
a 0.10 g/cm3 perlite. However, in Test # 5, the observed velocity of
detonation (VOD),
both its numeric value (1230 m/sec) and its depicted record, indicated a
marginal
detonation front propagation. For this reason, to ensure a proper detonation
propagation
it is deemed convenient to manufacture PANFO at densities greater than 0,25
g/cm3, at
least when it is manufactured with a perlite having a density greater than
0.10 g/cm3.
4. According to Test # 4, the fuel oil used as the combustible ingredient may
be replaced
by sugars dissolved in the ammonium nitrate solution. However, this implies
premixing
the ingredients until obtaining a product (PAN + Sugar) that would classified
as a
Blasting Agent and not as an Oxidizing Agent (PAN), which would hinder
logistics with
regard to handling, transportation and storage thereof.
5. According to Test # 6, ammonium nitrate may be replaced by urea, however,
the VOD
record obtained indicated a partial and delayed detonation of the mixture in
spite of
having been conducted in a 100 mm diameter at 0.36 g/cm3 density. This means
that
when mixed with said saline solution, the resulting product has low
sensitivity to initiation
and is prone to a detonation failure.
