Note: Descriptions are shown in the official language in which they were submitted.
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EXPLOSIVE FOR ROCK BREAKING
BACKGROUND OF THE INVENTION
This invention relates generally to the breaking of rock and more particularly
is
concerned with a propellant which can be used for the breaking of rock and
with a
method of manufacturing a propellant of this type.
As used herein the word "rock" includes rock, ore, coal, concrete and any
similar
hard mass, whether above or underground, which is difficult to break or
fracture. It is
to be understood that "rock" is to be interpreted broadly.
A number of techniques have been developed for the breaking of rock using non-
explosive means. These include a carbon dioxide gas pressurisation method
(referred to as the Cardox method), the use of gas injectors (the Sunburst
technique), hydrofracturing and various methods by which cartridges containing
energetic substances pressurise the walls or base of a sealed drill hole to
produce
penetrating cone fractures (known as PCF).
These techniques may be an order of magnitude more efficient than conventional
blasting in that they require approximately 1/10 of the energy to break a
given
amount of rock compared to conventional blasting using high explosives. The
lower
energy reduces the resulting quantity of fly rock and air blast and to an
extent allows
the rockbreaking operation to proceed on a continuous basis as opposed to the
batch-type situation, which prevails with conventional blasting.
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Most non-explosive rockbreaking techniques rely on the generation of high gas
pressures to initiate a tensile fracture at the bottom of a relatively short
drill hole.
When the propellant is initiated, combustion products are produced. Depending
on
the combustion process the combustion products may be toxic or harmful to the
health of persons in the vicinity thereof. Another factor which is to be taken
into
account is that incomplete or imperfect combustion may lead to a reduction in
the
amount of energy generated by the combustion process. Related to this aspect
is
the burning rate of the propellant. If the propellant burns too slowly then
the rate at
which energy is released is reduced and the ability of the propellant to
initiate rock
fracture is also reduced.
On the other hand if the propellant burns too fast then it may burn at a
temperature
which is so high that the propellant effectively functions as an explosive
and, for
safety reasons, the propellant must then be classified as an explosive.
Classification
in this way places severe restrictions on the storage, handling and use of the
propellant.
SUMMARY OF INVENTION
The invention provides a propellant composition which includes a propellant
material
and an oxygen source material.
The propellant composition may be in particulate form and each particle in the
particulate composition may include propellant material and oxygen source
material.
Each particle may include a granule of propellant material which is coated,
preferably
only partly, with the oxygen source material.
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In a different form of the invention each particle is a mixture at least of
the propellant
material and the oxygen source material.
The propellant material may be of any appropriate type which is known in the
art and
preferably is nitrocellulose which may be provided, as indicated, in granular
form.
The oxygen source material may be of any suitable kind and for example may be
ammonium nitrate.
Alternatively the propellant composition is in liquid form. A liquid has the
advantage,
over particulate material, that it is essentially non-compressible.
Consequently, when
initiated, a greater expansive force is generated than for a particular
material. The
liquid may include nitrocellulose as propellant and, optionally, nitro
glycerine as an.
enhancer. The liquid propellant may be a single, double or compound base type.
The oxygen source material may be ammonium nitrate or any similar material.
The invention also provides a method of manufacturing a propellant composition
which includes the step of mixing a granular propellant material with an
oxygen
source material thereby to coat the propellant granules, at least partly, with
a layer of
oxygen source material.
The oxygen source material may be ammonium nitrate or any equivalent material.
The oxygen source material may be coated onto the propellant granules using
any
appropriate technique and preferably use is made of a technique which is
similar to
that use for cladding diamond particles with metal as developed by companies
such
as De Beers Industrial Diamonds or Boart.
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It is also possible to waterproof the propellant using any appropriate
technique which,
for example, coats the propellant granules with a waterproof or water
resistant film or
layer. Use may for example be made of a film, eg. of nitrocellulose, which is
coated
onto the granules using an appropriate technique such as chemical vapour
deposition.
In a different form of the invention there is provided a method of
manufacturing a
propellant composition which includes the step of mixing a particulate
propellant
material with an oxygen source material thereby to form particles wherein each
particle is a mixture of the propellant and the oxygen source material.
The invention also provides rock breaking apparatus which includes a cartridge
which forms an enclosure and a propellant composition of the aforementioned
kind
inside the enclosure.
The cartridge may be made from a malleable material which in this
specification
includes a material which is capable of plastic deformation, without
rupturing, by at
least a predetermined extent eg. of the order of 10% or more.
As used herein "propellant" includes a material such as a propellant, blasting
agent,
gas-evolving substance, explosive or similar means which, once initiated,
generates
high 'pressure jet material typically at least partly in gaseous form.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is further described by way of examples with reference to the
accompanying drawings in which:
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Figure 1 schematically illustrates a method of manufacturing a propellant
composition according to the invention,
Figure 2 is an enlarged cross sectional view of a particle of a
propellant.composition
produced by the manufacturing method shown in Figure 1,
Figure 3 is a view similar to Figure 2 of a different form of a propellant
particle, and
Figure 4 illustrates a cartridge which makes use of a propellant composition
according to the invention for breaking rock.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 1 of the accompanying drawings illustrates, somewhat schematically, a
method according to the invention for manufacturing a propellant composition.
Propellant particles 10 such as nitrocellulose, in granular form, are supplied
to a
mixer 12. The propellant particles are suitably small, for example with an
effective
diameter of between 0,2mm to 1 mm.
An oxygen source material is held in a container 14 and is supplied at a
desired rate,
relatively to the supply rate of the propellant, to the mixer 12. The oxygen
source
material may vary according to requirement and for example is ammonium
nitrate.
The oxygen source material is in liquid form.
The mixer 12 is mechanically agitated to ensure thorough mixing of the
propellant
with the oxygen source material. During the mixing process the various
particles of
the granular propellant are coated to a greater or lesser extent with the
oxygen
source material. This gives rise to composite particles.
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Figure 2 illustrates in cross section a typical composite particle 16 which
includes a
granular core 18 of propellant, ie. nitrocellulose, and an outer coating 20 of
oxygen
source material, ie. ammonium nitrate, which extends over an outer surface of
the
core. The coating 20 preferably does not cover the entire core 18 and leaves
at least
a small section 22 of the core exposed. The thickness of the coating and the
extent
of the coating determine the amount of oxygen source material which is
available, in
relation to the amount of the core propellant.
As an alternative to the aforegoing the ammonium nitrate may be coated onto
the
propellant particles or grains by using techniques which are analogous or
similar to
the techniques developed by De Beers Industrial Diamonds and Boart, and other
companies, for the cladding of diamond particles with metal.
The composite propellantloxygen source material, in granular form, is output
to a
cartridge filling device 26.
When a propellant such as nitrocellulose is ignited, in the absence of
insufficient
oxygen, the resulting combustion process gives rise to carbon monoxide,
nitrates
(NOs) and nitrogen dioxide. These are undesirable and toxic by-products. An
object
of the invention in this respect is to make available additional oxygen by
means of
the source material 20 to allow the combustion process to proceed more fully
so that
the quantity of undesirable combustion products produced by the burning
process is
reduced.
By increasing the quantity of available oxygen on a micro basis, ie. on a
particle by
particle basis, it is possible to produce combustion products such as water,
carbon
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diode and nitrogen which predominate over the aforementioned undesirable by-
products.
It is also highly desirable to prevent the propellant from reacting with water
moisture
which may be present in the atmosphere. The propellant particles may be
waterproofed using any appropriate technique although a preferred route to be
followed, in this connection, is to apply a film or coating of nitrocellulose
lacquer to
the propellant particles using chemical vapour deposition or other appropriate
techniques.
Figure 3 illustrates an alternative form of the invention wherein a composite
propellant particle 30 is produced by the mixer 12 and wherein the particle
includes
propellant particles 32 and oxygen source material particles 34 which are held
in a
matrix 36. The matrix 36 may be any appropriate binder which is combustible or
of
an explosive nature and which is introduced from a source 33 during the mixing
process. Again the intention is to bring into close proximity with each other
particles
of oxygen source material and propellant to ensure that when the burning
process
takes place adequate oxygen is available to effectively complete the
combustion
process.
The situation should be contrasted with what prevails when ammonium nitrate in
particulate form is mixed with granular propellant. If the particles are not
intimately
associated with each other in the manner shown in Figure 2 or Figure 3 then
the
mixture is capable of separating or stratifying, depending on the transport or
storage
conditions, into distinct pockets of propellant and oxygen source material.
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A further benefit which arises from the intimate mixture arrangements shown in
Figures 2 and 3 is that the oxygen source material acts as a deflagratory
agent, ie.
burn agent, which increases the burn rate of the propellant. This increases
the
energy release rate which results from the burning propellant. Despite the
increase
in the energy release rate the ignition temperature is reduced. The propellant
thus
becomes capable of exhibiting properties, from the energy release point of
view,
which are associated with substances normally classified as explosives. It is
believed however that the propellant will still accurately be classifiable as
a propellant
and not as an explosive despite its enhanced properties.
In a variation of the invention the propellant composition is provided in
liquid form
and comprises a mixture of a liquid propellant eg. nitrocellulose, and oxygen
source
material in liquid form, eg. ammonium nitrate. The composition may include an
enhancer, eg. nitro glycerine, and may thus be of a double base type, or have
a
compound base.
Figure 4 illustrates one manner in which the propellant which is produced by
the
method shown in Figure 1, can be used. A hole 50 is drilled into a rock mass
52
from a face 54 using conventional drilling equipment, not shown.
A cartridge 56 is loaded into the hole. The cartridge has a flat base 58 which
engages closely with an end of the hole, and a generally cylindrical side wall
60.
The cartridge forms an enclosure for a propellant material 64 which is
produced by
the method described in connection with Figure 1 and which is loaded into the
cartridge during the step 26 under factory conditions. An initiator 66, of any
appropriate type, is loaded into the cartridge preferably on site.
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Control wires 68 from the initiator to a control unit, not shown, which is
used in a
known manner for initiating the blasting process.
Stemming 70 is placed into the hole 50 from the rock face covering the
cartridge to a
desired extent and is tamped in position.
The propellant 64 is ignited by the firing the initiator 66 and when this
happens high
pressure jet material is released as a consequence of the combustion process.
The
cartridge is designed to contain the expanding high pressure jet material and
is
allowed to deform outwardly, without rupturing, so that the cartridge is
forced into
sealing contact with an opposing surtace of the wall of the hole 50.
The high pressure jet material is initially confined by the cartridge and the
cartridge is
allowed to fracture at a desired point or region which means that the force
which is
released by the combusting propellant is then directed onto a chosen surface
of the
wall of the hole in order to fracture the rock.
The use of a propellant according to the invention in a rock breaking process
of the
type shown in Figure 4 means that the capability of the propellant to release
energy
is increased while the production of harmful by-products is reduced. As noted,
if the
propellant is correctly formulated, the resulting product can still accurately
be
classified as a propellant and not as an explosive.
As has been indicated the propellant composition can be provided in liquid
form. The
liquid is, for all practical purposes, incompressible and thus, when
initiated, produces
a pressure wave which can have a more rapidly rising leading edge than the
particulate composition.
