U.S. patent application number 12/522626 was filed with the patent office on 2011-06-09 for method for gassing explosives especially at low temperatures.
This patent application is currently assigned to Newcastle Innovation Limited. Invention is credited to Gabriel Da Silva, Bogdan Zygmunt Dlugogorski, Eric Miles Kennedy, Mark Stuart Rayson.
Application Number | 20110132505 12/522626 |
Document ID | / |
Family ID | 39608257 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110132505 |
Kind Code |
A1 |
Dlugogorski; Bogdan Zygmunt ;
et al. |
June 9, 2011 |
METHOD FOR GASSING EXPLOSIVES ESPECIALLY AT LOW TEMPERATURES
Abstract
The invention provides a method for gassing an explosive to
sensitise the explosive and/or modify the density of the explosive.
The method comprises reacting at least one oxidiser with at least
one nitrogen containing compound in the explosive to generate
nitrogen gas. The explosive is formulated to effect diffusion of
the oxidiser and/or the compound into contact with each other, the
nitrogen gas being generated by oxidation of the compound by the
oxidiser. The invention extends to the explosive compositions
themselves.
Inventors: |
Dlugogorski; Bogdan Zygmunt;
(Raymond Terrace, AU) ; Kennedy; Eric Miles; (The
Hill, AU) ; Rayson; Mark Stuart; (Shortland, AU)
; Da Silva; Gabriel; (Carlton, AU) |
Assignee: |
Newcastle Innovation
Limited
Callaghan
AU
|
Family ID: |
39608257 |
Appl. No.: |
12/522626 |
Filed: |
January 10, 2008 |
PCT Filed: |
January 10, 2008 |
PCT NO: |
PCT/AU2008/000013 |
371 Date: |
February 24, 2011 |
Current U.S.
Class: |
149/109.4 ;
149/109.6 |
Current CPC
Class: |
C06B 47/145 20130101;
C06B 23/004 20130101 |
Class at
Publication: |
149/109.4 ;
149/109.6 |
International
Class: |
C06B 21/00 20060101
C06B021/00; C06B 43/00 20060101 C06B043/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 10, 2007 |
AU |
2007900069 |
Claims
1. A method for gassing an explosive to sensitise the explosive
and/or modify the density of the explosive, comprising reacting at
least one oxidiser with at least one nitrogen containing compound
in the explosive to generate nitrogen gas, the explosive being
formulated to drive formation of a respective neutrally charged
form of the oxidiser and/or the compound to effect diffusion of the
oxidiser and/or the compound into contact with each other, and the
nitrogen gas being generated by oxidation of the compound by the
oxidiser.
2. A method according to claim 1 wherein the oxidiser and the
nitrogen compound are separated from each other by fuel lamellae in
the explosive and the neutrally charged form of the oxidiser and/or
the compound diffuse across the fuel lamellae into contact with one
another.
3. A method according to claim 1 wherein the explosive is
formulated to effect diffusion of the oxidiser into contact with
the nitrogen compound.
4. A method according to claim 3 wherein the explosive is
formulated for protonation of the oxidiser or the release of at
least one proton from the oxidiser, to effect the diffusion of the
oxidiser.
5. A method according to claim 4 wherein the explosive comprises a
proton donor for protonating the oxidiser.
6. A method according to claim 4 wherein the explosive comprises a
proton acceptor for maintaining pH above a predetermined lower
limit to inhibit crystallisation in the explosive.
7. A method according to claim 3 wherein the explosive comprises a
pH regulating agent which acts as the proton donor and a proton
acceptor for maintaining the pH above a predetermined lower
limit.
8. A method according to claim 1 wherein the explosive is
formulated to have a pH for obtaining the diffusion of the
oxidiser.
9. A method according to claim 8 wherein the explosive comprises a
pH regulating agent which is essentially non-diffusing across fuel
lamellae, for maintaining the pH at a level to control the rate of
diffusion of the oxidiser.
10. A method according to claim 1 wherein the explosive comprises a
proton transfer agent for transferring at least one proton across
fuel lamellae or from one phase to another of the explosive, to
promote the diffusion of the oxidiser and/or the nitrogen
compound.
11. A method according to claim 10 wherein the proton transfer
agent is provided by a pH regulating agent.
12. A method according to claim 10 wherein pH is regulated to delay
the diffusion of the oxidiser and thereby the gassing of the
explosive.
13. A method according to claim 9 wherein the pH regulating agent
comprises one or more compounds selected from the group consisting
of partially or completely deprotonated forms of inorganic acids
and carboxylic acids, and salts thereof.
14. A method according to claim 13 wherein the pH regulating agent
comprises one or more of phosphoric acid, acetic acid, formic acid,
citric acid, tartaric acid, furoic acid, fumaric acid, salicylic
acid, malonic acid, phthalic acid, sulfanilic acid, mandelic acid,
malic acid, butyric acid, oxalic acid, and salts thereof.
15. A method according to claim 10 wherein the proton transfer
agent comprises one or more compounds selected from the group
consisting of inorganic acids, organic acids, carboxylic acids, and
salts thereof, the explosive being formulated such that at least
some of these compounds exist in the explosive in a neutral
form.
16. A method according to claim 15 wherein the proton transfer
agent comprises one or more compounds selected from the group
consisting of alkyl carboxylic acids, acetic acid, formic acid,
phosphoric acid, citric acid, tartaric acid, furoic acid, fumaric
acid, salicylic acid, malonic acid, phthalic acid, sulfanilic acid,
mandelic acid, malic acid, butyric acid, oxalic acid, and salts
thereof.
17. A method according to claim 1 wherein the at least one nitrogen
compound is selected from the group consisting of
NH.sub.3/NH.sub.4.sup.+, ammonium salts, urea, amines, hydrazines,
hydrazides, azides, triazoles, tetrazoles, and derivatives of urea,
amines, hydrazines, hydrazides, azides, triazoles, tetrazoles, and
nitrogen compounds having 3 or more nitrogen atoms for generation
of the nitrogen gas.
18. A method according to claim 1 wherein the nitrogen compound is
selected from the group consisting of NH.sub.3/NH.sub.4.sup.+,
hydrazides, and derivatives of hydrazides.
19. A method according to claim 1 wherein the explosive comprises
NH.sub.3/NH.sub.4.sup.+ and at least one other said nitrogen
compound, the gassing of the explosive including oxidation of the
NH.sub.3/NH.sub.4.sup.+.
20. A method according to claim 19 wherein the other said nitrogen
compound is selected from the group consisting of urea, amines,
azides, hydrazines, hydrazides, tetrazoles, triazoles, and
derivatives thereof.
21. A method according to claim 1 wherein the oxidiser is selected
from the group consisting of hypohalites and hypohalous acids.
22. A method according to claim 1 wherein the temperature at which
the reaction occurs is 40.degree. C. or less.
23. A method according to claim 22 wherein the temperature is
25.degree. C. or less.
24. A method according claim 1 in which the oxidiser is added to
the explosive subsequent to incorporation of the nitrogen compound
in the explosive.
25. A method for gassing an explosive to sensitise the explosive
and/or modify the density of the explosive, comprising reacting at
least one oxidiser with at least one nitrogen containing compound
in the explosive to generate nitrogen gas, the oxidiser and the
compound initially being in different phases of the explosive to
one another, and the explosive being formulated to drive formation
of a respective neutrally charged form of the oxidiser and/or the
compound to effect diffusion of the oxidiser and/or compound into
contact with each other whereby nitrogen gas is generated via
oxidation of the nitrogen compound by the oxidiser.
26. An explosive gassed by a method as defined in claim 1.
27. An explosive comprising at least one oxidiser and at least one
nitrogen containing compound, the explosive being formulated to
drive formation of a respective neutrally charged form of the
oxidiser and/or the compound to effect diffusion of the oxidiser
and/or the compound into contact with each other for oxidation of
the compound by the oxidiser to produce nitrogen gas from the
compound for gassing of the explosive.
28. An explosive according to claim 27 formulated to effect
diffusion of the oxidiser into contact with the nitrogen
compound.
29. An explosive comprising at least one oxidiser and at least one
nitrogen containing compound, the oxidiser and the compound being
in different phases of the explosive to one another, and the
explosive being formulated to drive formation of a respective
neutrally charged form of the oxidiser and/or the compound to
effect diffusion of the oxidiser and/or the compound into contact
with each other for oxidation of the compound by the oxidiser to
produce nitrogen gas from the compound for gassing of the
explosive.
Description
FIELD OF THE INVENTION
[0001] The invention relates to explosive compositions and
particularly to methods of sensitising and/or modifying the density
of explosives by gas bubbles, including emulsion, gel, slurry and
ANFO explosives. The invention finds particular though not
exclusive application in the mining industry.
BACKGROUND OF THE INVENTION
[0002] Prior to detonating, emulsion, gel and heavy ammonium
nitrate-fuel oil (ANFO) explosives require sensitisation, and a
number of technologies exist to perform this task. Originally,
technologies requiring the addition of high explosives were
employed, with examples of such explosives including
trinitrotoluene (TNT), nitroglycerine, nitrocellulose (which
constitutes a major ingredient of smokeless powder), nitrostarch,
nitrocotton, nitroguanidine, hexamethylenetetramine,
ethylenediaminedinitrate, trinitrophenylmethylnitramine, mixtures
of TNT and trimethylenetrinitramine, TNT and pentaerythritol
tetranitrate, and TNT with ethylene dinitramine. Metal particles,
such as those of aluminium, magnesium, boron or silicon, as well as
ferrophosphorous and ferrosilicon, can also be utilised to enhance
the sensitisation by high explosives. However, these methods are
expensive and require sensitising during emulsion or gel
production, normally at the manufacturing plant, necessitating
subsequent transportation of sensitised explosives which, of
course, is highly undesirable. Because of these two considerations,
cost and safety, the use of high explosives for emulsion and gel
sensitisation has now been largely abandoned. Attempts to replace
high explosives with nitrates of aliphatic and phenolic amines (eg.
U.S. Pat. No. 3,431,155, GB Patent No. 1,536,180) also appear to
have met the same fate.
[0003] Another group of technologies used to sensitise explosives
involves entrapping air or adding light particles to slurries,
emulsions and heavy ANFO by physical processes (eg. see U.S. Pat.
No. 3,382,117) as a means to decrease the amount of high-explosive
sensitisers or regulate distribution of explosive strength. U.S.
Pat. No. 3,397,097 for instance is directed to the sensitisation of
gel explosives by this method. To act as sensitisers the voids need
to be small, at least less than 1.6 mm in size and preferably, less
than 100 .mu.m. A shock wave travelling in the sensitised explosive
compresses the voids adiabatically. This raises the local emulsion
temperature above that required to detonate the bulk explosive.
Sensitisation technologies involving the introduction of small
voids can also function as density regulators, to decrease blasting
energy or to distribute explosive strength in a borehole or over a
set of boreholes.
[0004] Glass microspheres (microballoons) have been preferred for
emulsion explosive sensitisation especially at temperatures below
that suitable for chemical gassing based on nitrosation of ammonia
or thiourea (vide infra), in spite of the elevated cost of
microballoons (in the order of AU$2,000/kg), handling difficulty
owing to their low density (<400 kg/m.sup.3, but typically
<200 kg/m.sup.3), and high shipping expenses per unit weight.
However, their advantages include small size (on average 65 .mu.m,
with particle sizes distributed between 10 to 175 .mu.m), provision
of no additional fuel to the sensitised material and ability to be
mixed with explosives over a wide range of proportions. U.S. Pat.
No. 4,737,207, for example, discloses a convenient process to mix
microballoons with emulsion explosives by means of suspensions.
Glass microspheres still find considerable application in
sensitising higher cost packaged explosives, due to their
stability, but are cost inefficient for use in bulk explosives when
compared to low cost chemical gassing technologies (vide
infra).
[0005] Other physical sensitisation technologies that have found
limited application notwithstanding their lower cost, include the
addition of bagasse piths, perlite, vermiculite, pumice, as well as
plastic microspheres (which may be expanded in emulsion by heating
above 85.degree. C., as described in U.S. Pat. No. 6,113,715),
solid foams (e.g., polystyrene-based, U.S. Pat. No. 4,543,137),
liquid foam (after addition to explosive, the foam itself breaks
down with release of bubbles which disperse in the explosive
matrix; European Patent Application No. 514000), puffed rice and
wheat, and more recently expanded popcorn (U.S. Pat. No. 5,409,556)
as well as rice hulls (U.S. Pat. No. 6,995,731).
[0006] Currently, the most industrially important route to
sensitising explosives to detonation comprises the so-called
chemical gassing or foaming, involving the formation of small
bubbles of CO.sub.2, O.sub.2, H.sub.2, NO or N.sub.2 in situ in
emulsions and gel explosives by means of chemical reactions. U.S.
Pat. No. 6,261,393 also provides a brief reference to employing
calcium carbide as a gassing reagent, presumably to generate
C.sub.2H.sub.2, but without providing an example. For the most
part, these gassing technologies allow transport of unsensitised
emulsions and gels, with the addition of bubble generating
chemicals at the time of pumping the emulsions or gels into
blastholes. Chemical gassing was introduced in the late 1960s and
early 1970s to provide alternative means of sensitising gel
explosives and then emulsion explosives (U.S. Pat. No. 3,447,978).
With time, industry has converged on the use of nitrogen gassing
(U.S. Pat. No. 3,886,010), with N.sub.2 formed by reactions between
nitrites and ammonia, thiourea, or other amines in the presence of
catalysts and pH regulators. The known chemical gassing processes
are further reviewed below.
[0007] For instance, U.S. Pat. No. 3,288,658 describes the
application of carbon dioxide to gassing explosive gels in a
process involving mixing of an acid (such as hydrochloric, acetic,
nitric or sulfuric) with a solution of ammonium or alkali metal
carbonate, particularly sodium or potassium bicarbonate, at
temperatures below 50.degree. C. and pH below 6.5. Recently,
CO.sub.2 gassing has been suggested as part of water-resistant ANFO
systems that transform themselves into sensitised slurries in
water-logged blastholes (U.S. Pat. No. 6,261,393). From a chemical
perspective, the process involves a reduction in pH resulting in
the protonation of HCO.sub.3.sup.-, followed by the decomposition
of H.sub.2CO.sub.3 and the evolution of CO.sub.2. This method can
be used to sensitise emulsion explosives provided that acetic acid
or another organic acid of similar pKa is employed as a proton
donor. Unfortunately, the solubility of CO.sub.2 in blasting agents
varies as a function of pressure and the agent's composition,
making the adjustment of emulsion or slurry density with CO.sub.2
particularly difficult to regulate, especially for deep
boreholes.
[0008] Oxygen gassing typically involves the decomposition of
hydrogen peroxide in the presence of catalysts, such as manganese
dioxide, ferric nitrate, potassium iodide, ferrous sulphate,
manganese sulphate, aluminium particles and even coarse sand, at
temperatures in excess of 55.degree. C. (e.g., see U.S. Pat. No.
3,790,415). Other catalysts including carbonates, bicarbonates, and
nitrites as well as oxidisers, such as ferric salts, and oxoanion
oxidisers, such as permanganates, dichromates, peroxysulphates,
hypohalites, can also effectively decompose hydrogen peroxide. The
sensitisation of slurry explosives with lithium, sodium and
potassium peroxides, is described in U.S. Pat. No. 4,081,299.
Oxygen gassing employing a pre-emulsified solution of
H.sub.2O.sub.2, with MnO.sub.2 present in the discontinuous phase
of the emulsion, has been also demonstrated as a viable technology
(U.S. Pat. No. 5,397,399). However, the US Code of Federal
Regulation, 29 CFR 1910.109 Explosives and Blasting Agents excludes
the use of peroxides in emulsion and gel systems (Clark, 1991).
[0009] The application of hydrogen to gassing of slurry and
emulsion explosives is based on the observation that H.sub.2 is
released in reactions involving ammonium salts and alkali metal
borohydrides (ie., lithium, sodium or potassium borohydrides);
e.g., according to
NH4.sup.++BH.sub.4.sup.-.fwdarw.H.sub.3N--BH.sub.3+H.sub.2, at
temperatures in excess of 40.degree. C. A gassing process based on
this reaction is described in U.S. Pat. No. 3,711,345. Safety
concerns relating to the use of alkali metal borohydrides and
hydrogen itself, and possibly the loss of hydrogen from emulsions
owing to rapid diffusion of H.sub.2 through the emulsion matrix,
has prevented further development and implementation of hydrogen
foaming by industry.
[0010] A nitric oxide gassing process involving the nitrosation of
chemical species (substrates) having an enol group, or a
deprotonated enolate form of the enol group, employing a
nitrosating agent such as N.sub.2O.sub.3, ONCl, ONBr, ONSCN, ONI,
nitrosothiourea, nitrosyl thiosulfate, HNO.sub.2, ON.sup.+,
ON.sup.+OH.sub.2 or inorganic nitrosyl complexes, is described in
International Patent Application No. PCT/AU2006/001596). The
substrate used is preferably ascorbic acid or ascorbate (vitamin
C), and the reaction forms O-nitroso products which subsequently
decompose to yield nitric oxide. The reaction rate is pH dependent,
taking approx. 4 min to gas emulsion explosives at 25.degree. C.
and a pH below 3.9. However, nitric oxide may promote the
production of so-called after-blast fumes, although technologies
exist, e.g., based on addition of silicon powder, to alleviate this
factor (U.S. Pat. No. 6,539,870).
[0011] Early attempts to introduce nitrogen gassing to slurry and
emulsion explosives relied on adopting blowing agents (ie., agents
that decompose to N.sub.2 at temperatures above 55.degree. C.) as
described in U.S. Pat. No. 3,713,919, and the oxidation of
hydrazine and its derivatives at temperatures in excess of about
40.degree. C. as described in
[0012] U.S. Pat. No. 3,706,607. Examples of the application of both
technologies to gassing emulsion explosives are described in U.S.
Pat. No. 3,770,522. Chemicals useful as blowing agents include
N,N'-dimethyl- and N,N'-diethyl-N,N'-dinitrosoterephthalamide,
benzensulphonyl hydrazide, azobisisobutyronitrile and
p-tert-butylbenzazide, as summarised in U.S. Pat. No. 4,008,108. A
gassing process involving N,N'-dinitrosopentamethylenetetramine is
also described in U.S. Pat. No. 3,713,919. Gassing was reported for
the reaction of hydrazine monohydrate and other hydrazine
derivatives with various oxidising agents including hydrogen
peroxide, ammonium persulphate and copper(II)nitrate. This gassing
system inherently requires the process be carried out at high
temperatures. This requires that either the gassing be performed
during or immediately after emulsion manufacture, when the mixture
is still hot, or subsequent heating of the explosive if the
explosive is sensitised at a later time. These practical
limitations have prevented the use of this technology. The use of
copper complexes in emulsion explosive systems has also been
abandoned because of safety concerns, similarly to the use of
peroxides as previously mentioned.
[0013] A recent development in nitrogen gassing involves the
application of toxic diazonium salts generated by in-line mixing of
an amine, acid and a nitrite (U.S. Pat. No. 6,027,588). At
temperatures above 35.degree. C., some diazonium salts decompose to
N.sub.2 at rates significantly faster than for a commonly used
foaming system composed of sodium nitrite with sodium thiocyanate
catalyst (vide infra), with fast gassing reported between
50.degree. C. and 70.degree. C.
[0014] Nitrogen gassing via the nitrosation mechanism as introduced
to the field of explosives by U.S. Pat. No. 3,660,181 and U.S. Pat.
No. 3,886,010, has dominated chemical gassing, and emulsion
explosive sensitisation as a whole, over the last 30 years. The
gassing process is initiated by mixing a concentrated solution of
nitrite ion (usually originating from inexpensive NaNO.sub.2, with
other options involving nitrous acid and solutions of potassium and
ammonium nitrites) with slurry or emulsion explosive. Citric or
more frequently-used acetic acid diffusing from emulsion droplets
then protonates nitrite ions to form N.sub.2O.sub.3, which
subsequently transfers back across oil films to react with ammonia
(N.sub.2O.sub.3+NH.sub.3.fwdarw.N.sub.2+NO.sub.2.sup.-+H.sup.++H.sub.2O).
Nitrosation of ammonia constitutes a slow reaction even at around
50.degree. C. and in practice thiourea (or melamine, sulphamic acid
or its salts; Canadian Patent No. 2,239,095) is added to emulsion
to act as a substrate for nitrosation. In this mechanism, ON.sup.+
(from HNO.sub.2) acts as an effective nitrosating agent
(HNO.sub.2+NH.sub.2CSNH.sub.2.fwdarw.N.sub.2+2H.sub.2O+HSCN).
Alternatively, a strong nucleophilic species, such as thiocyanate
(e.g., NaSCN), iodide, bromide, chloride, nitrosothiourea,
nitrosoamines (such as N,N'-dinitrosopentamethylenetetramine, see
U.S. Pat. No. 4,409,044), can serve as catalysts to affect the
nitrosation of ammonia by ON.sup.+ (e.g.,
ONSCN+NH.sub.3.fwdarw.N.sub.2+SCN.sup.-+H.sup.++H.sub.2O).
Optimised formulations designed to operate at temperatures below
25.degree. C. necessitate the use of both thiourea (as a substrate,
added to emulsion) and thiocyanate (as a catalyst, added to
gasser); e.g., see European Patent Application 775,681. The
mechanistic details of the nitrite gassing processes are now
relatively well understood (except, perhaps, for nitrosothiourea,
nitrosoamine and sulphamic acid systems), e.g., da Silva et al.
(2006).
[0015] Since the development of nitrite gassing, a number of
important advances have been introduced to optimise the gassing
technology, especially to accelerate the gassing rate and make the
process operate at lower temperatures and allow formation of gas
bubbles of less than 100 .mu.m in size. They include the use of
microemulsions (European Patent Application No. 775,681), Lewis
acids such as zinc nitrate to facilitate the protonation of nitrite
ions (U.S. Pat. No. 6,855,219), premixing nitrite and thiourea
prior to adding to emulsion (U.S. Pat. No. 6,165,297, Canadian
Patent No. 2,239,095), nitrite as powder (U.S. Pat. No. 4,997,494),
pre-emulsified gasser (U.S. Pat. No. 4,875,951, suggested
independently in PCT/US88/03354), calcium and strontium accelerants
(U.S. Pat. No. 6,022,428), formation of small bubbles with organic
additives (Canadian Patent No. 2,040,751), fluorosurfactants (U.S.
Pat. No. 4,594,118) or high pressure (U.S. Pat. No. 4,676,849).
Technologies combining nitrite gassing with other methods for
regulating emulsion density and/or sensitivity have also been
developed, particularly for specialised applications, such as shock
resistant explosives (microballoons/nitrite, U.S. Pat. No.
5,017,251), porosity-modified ANFO (U.S. Pat. No. 5,240,524) and
enhanced sensitivity explosives (high explosives/nitrite, U.S. Pat.
No. 4,221,616).
[0016] However, in spite of its versatility and low cost (in the
order of AU$6/tonne), nitrite gassing cannot be effectively
implemented for sensitising emulsions at sub or near zero (ie.,
<0.degree. C. or .about.0.degree. C.) temperatures, with the
rate of gassing declining substantially below 25.degree. C.
Concerns have been also raised over the health and environmental
risks associated with the use of nitrite salts, and over safety
implications of pre-mixing of nitrite and thiourea prior to adding
the mix to the emulsion. Furthermore, the identification of the
common accelerant thiourea as a possible human carcinogen means
that even chemical gassing at moderate temperature (ca. 25.degree.
C.) using this compound may be phased out.
SUMMARY OF THE INVENTION
[0017] In an aspect of the invention there is provided a method for
gassing an explosive to sensitise the explosive and/or modify the
density of the explosive, comprising reacting at least one oxidiser
with at least one nitrogen containing compound in the explosive to
generate nitrogen gas, the explosive being formulated to effect
diffusion of the oxidiser and/or the compound into contact with one
another, the nitrogen gas being generated by oxidation of the
compound by the oxidiser.
[0018] Typically, the oxidiser and/or the nitrogen compound diffuse
across fuel lamellae in the explosive into contact with one
another. In at least some embodiments, essentially only the
oxidiser diffuses across the fuel lamellae for reacting with the
oxidiser.
[0019] The gassing of the explosive can be carried out over a range
of temperatures and is particularly suitable for gassing emulsion
explosives, though not solely, at low temperatures including near
or below 0.degree. C.
[0020] Typically, the amount of nitrogen gas generated is
determined by the quantity of the oxidiser added to the explosive.
The oxidiser is normally added in the form of a composition
(gasser) containing the oxidiser. The gasser can be a solution of
the oxidiser.
[0021] The terms "nitrogen containing compound" and variations
thereof such as "nitrogen compound" and the like in the context of
the invention are taken to mean any compound containing nitrogen
that is capable of producing nitrogen gas via reaction with the
oxidiser.
[0022] The term "oxidiser" in the context of the invention is to be
taken to mean a substance that removes one or more electrons from
the nitrogen compound to produce the nitrogen gas from the nitrogen
compound and/or species derived from the compound. This is
distinguished from inorganic oxidiser salts (such as ammonium
nitrate) which form part of prior art base explosive wherein the
term refers to the presence of oxygen in the salts. In the present
invention, the oxidiser is generally added to the explosive
following the manufacture of the explosive, normally when the
explosive is ready to be gassed.
[0023] The explosive can be formulated for protonation of the
oxidiser to effect the diffusion of the oxidiser.
[0024] In at least some embodiments, the explosive can comprise a
proton donor to control the rate at which the oxidiser diffuses to
react with the nitrogen compound, thereby controlling the gassing
rate. Alternatively, or as well, the pH of the gasser can be
adjusted to alter or further influence the rate of oxidiser
diffusion. For example, the pH of the gasser can be raised via the
addition of an alkaline substance to delay the onset of the gassing
reaction to allow more time for mixing of the gasser into the
explosive.
[0025] In addition, the explosive can comprise a proton acceptor
for maintaining pH above a predetermined lower limit or within a
predetermined pH range to inhibit crystallisation in the
explosive.
[0026] A pH regulating agent such as a pH buffer which can act as
both the proton donor and proton acceptor can be used. A pH
regulating agent can also act as a proton transfer agent for
transporting protons across fuel lamellae or into another phase of
the explosive for increasing the rate of diffusion of the oxidiser
and/or the nitrogen compound. In the broadest sense, a proton
transfer agent is to be taken to encompass an agent that can act to
transfer at least one proton across fuel lamellae and/or into
another phase of an explosive for release of the proton.
[0027] Hence, in one or more embodiments of the invention, the
explosive can comprise a proton transfer agent for transferring one
or more protons across fuel lamellae of the explosive or more
generally, across phases of the explosive, to increase the rate of
diffusion of the oxidiser and/or the nitrogen compound.
[0028] It will be understood that the invention is not limited to
the particular nitrogen compound(s) utilised in the explosive and
any suitable nitrogen compound can be used.
[0029] In at least one form, the nitrogen gas is generated by
oxidation of ammonia/ammonium cation (NH.sub.3/NH.sub.4.sup.+) by
the oxidiser. Typically, though not exclusively, the oxidiser
diffuses to the NH.sub.3/NH.sub.4.sup.+. In this embodiment, the
explosive can comprise a proton donor for protonating the oxidiser
to promote the diffusion of the oxidiser into contact with the
NH.sub.3/NH.sub.4.sup.+ as indicated above. Desirably, the
explosive will also include a proton acceptor to maintain pH above
a predetermined lower limit to inhibit crystallisation in the
explosive. Again, a pH regulating agent that acts as both the
proton donor and the proton acceptor can be employed.
Alternatively, the pH can be elevated to increase the concentration
of NH.sub.3 (which is in equilibrium with NH.sub.4.sup.+), to
promote diffusion of the NH.sub.3 e.g., across fuel lamellae. In
this instance, the oxidiser may be essentially non-diffusing in the
context of the invention.
[0030] In another form, the nitrogen gas is generated by oxidation
of an amine (eg., a primary amine), with the oxidation of
NH.sub.3/NH.sub.4.sup.+ occurring as a side reaction. In this
instance the majority of nitrogen gas is generated by reaction of
the oxidiser with the amine. In general, the reaction between an
oxidiser and a primary amine shows a higher selectivity toward the
production of nitrogen gas than the reaction with
NH.sub.3/NH.sub.4.sup.+. Thus reactions with a primary amine afford
a reduction in the amount of oxidiser required to produce a desired
amount of gas in an explosive. Similarly, the explosive in this
embodiment may contain a pH regulating agent and/or proton transfer
agent for protonating the oxidiser to promote the diffusion of the
oxidiser through the fuel lamellae of the explosive.
[0031] In another form, the nitrogen gas is generated by oxidation
of a nitrogen compound containing a hydrazine (--NH--NH.sub.2)
group to produce the nitrogen gas, with the oxidation of
NH.sub.3/NH.sub.4.sup.+ occurring as a parallel side reaction. In
this instance, the nitrogen gas is primarily generated by the
reaction between the oxidiser and the hydrazine, with a lower level
of nitrogen gas generated by the parallel side reaction between the
oxidiser and NH.sub.3/NH.sub.4.sup.+. In general, at temperatures
less than 40.degree. C. hydrazine chemicals are essentially
non-diffusing across fuel lamellae, in that the amount of gas
produced from oxidiser molecules diffusing across the fuel lamellae
to the hydrazine is substantially greater than the amount of gas
produced from the hydrazine diffusing across the fuel lamellae to
the oxidiser. In comparison to the above embodiment involving the
oxidation of NH.sub.3/NH.sub.4.sup.+ and an amine, reduced amounts
of oxidiser and pH regulator agent can be utilised to achieve
sensitisation of and/or density modification of the explosive.
[0032] In yet another form, the nitrogen gas is generated by
oxidation of a nitrogen rich compound having 3 or more nitrogen
atoms for generation of the nitrogen gas, with the oxidation of
NH.sub.3/NH.sub.4.sup.+ occurring as a parallel side reaction.
Examples of suitable such nitrogen rich compounds include those
containing a tetrazole or triazole ring. In this instance, the
nitrogen gas is primarily generated by reaction of the oxidiser
with the nitrogen compound, with a lesser amount of gas being
produced by the oxidation of the NH.sub.3/NH.sub.4.sup.+. The
presence of double bonds between nitrogen atoms of triazoles,
tetrazoles, and like such nitrogens compounds again reduces the
amount of oxidiser required to produce the desired amount of
nitrogen gas required to achieve sensitisation and/or density
modification of the explosive.
[0033] In another aspect of the invention there is provided an
explosive comprising NH.sub.3/NH.sub.4.sup.+ and/or at least one
other nitrogen containing chemical, and at least one oxidiser for
reaction with the NH.sub.3/NH.sub.4.sup.+ and/or the nitrogen
containing chemical to generate nitrogen gas for gassing the
explosive.
[0034] In another aspect of the invention there is provided a
method for gassing an explosive to sensitise the explosive and/or
modify the density of the explosive, comprising reacting at least
one oxidiser with at least one nitrogen containing compound in the
explosive to generate nitrogen gas, the oxidiser and the compound
initially being in different phases of the explosive to one
another, and the oxidiser and/or compound diffusing into contact
with each other whereby nitrogen gas is generated via oxidation of
the nitrogen compound by the oxidiser.
[0035] In another aspect there is provided an explosive gassed by a
method of the invention.
[0036] In yet another aspect of the invention there is provided an
explosive, comprising at least one oxidiser and at least one
nitrogen containing compound, the explosive being formulated for
effecting diffusion of the oxidiser and/or the compound into
contact with each other for oxidation of the compound by the
oxidiser to produce nitrogen gas from the compound for gassing of
the explosive.
[0037] In still another aspect of the invention there is provided
an explosive, comprising at least one oxidiser and at least one
nitrogen containing compound, the oxidiser and the compound being
in different phases of the explosive to one another, and the
explosive being formulated for diffusion of the oxidiser and/or the
compound into contact with each other for oxidation of the compound
by the oxidiser to produce nitrogen gas from the compound for
gassing of the explosive.
[0038] In methods embodied by the invention, the rate of the
overall gassing process is governed by the rate of transfer of the
diffusing reagent and/or the nitrogen compound. In the instance
that both the nitrogen compound and the oxidiser diffuse, the rate
of gassing is limited by the combined rate of diffusion of both
these reagents. However, the rate of oxidiser diffusion typically
far exceeds that of most nitrogen compounds at low temperature, and
as such the vast majority of nitrogen compounds can be considered
as essentially non-diffusing under these conditions.
[0039] As will be understood, the gassing of the explosive by the
generated nitrogen gas may be achieved by simply allowing the gas
to foam the explosive. The gassing of the explosive may also, or
alternatively, involve stirring the explosive during at least a
part of the gassing process to achieve substantially even
distribution of the nitrogen gas bubbles essentially throughout the
explosive. Distribution of the gas bubbles throughout the explosive
may also be achieved by pumping of the emulsion explosive.
[0040] Methods embodied by the invention are particularly suitable
for gassing emulsion explosives. However, it will be understood
that at least some embodiments can be utilised to gas forms of
explosives other than emulsion explosives, such as gel, slurry and
ANFO explosives, and the invention expressly extends to such
further explosives.
[0041] Methods as described herein provide technologies for
sensitisation and/or density modification of explosives at low
temperature as an alternative to conventional low temperature
gassing methods such as those involving the use of expensive glass
microspheres. Advantageously, at least some gassing methods
embodied by the invention require no heating of the explosive prior
to mixing with the oxidiser and/or substrate to above ambient
temperatures, even temperatures of 0.degree. C. or below, affording
a substantial advance in the art. Moreover, various forms of
methods embodied by the invention appear to be comparable in cost
to nitrite-based gassing, which for the previous three decades has
been the preferred technology for sensitisation and/or density
modification of explosives.
[0042] Furthermore, at least some embodiments involving the use of
hydrazine and/or derivatives thereof address a number of
deficiencies of prior art nitrogen gassing methods employing these
substrates. In particular, one or more embodiments as described
herein may allow an emulsion explosive to be gassed to a
pre-determined density at a controlled rate, by mixing one or more
reagents such as the oxidiser into the emulsion after its
manufacture, rather than during the manufacturing process as is the
case in U.S. Pat. No. 3,706,607. This allows emulsion explosives to
be manufactured in bulk and transported safely to mine sites before
being sensitised to detonation, reducing the risk of accidents
associated with transport of explosive materials. Further, one or
more embodiments of the invention provide methods that allow
explosives formulated with nitrogen compounds to be gassed over a
wide temperature range by adjusting the pH of gasser solutions,
removing the need for heating or cooling of the explosives to
achieve desired gassing times.
[0043] Throughout this specification the word "comprise", or
variations such as "comprises" or "comprising", will be understood
to imply the inclusion of a stated element, integer or step, or
group of elements, integers or steps, but not the exclusion of any
other element, integer or step, or group of elements, integers or
steps.
[0044] Any discussion of documents, acts, materials, devices,
articles or the like which has been included in this specification
is solely for the purpose of providing a context for the present
invention. It is not to be taken as an admission that any or all of
these matters form part of the prior art base or were common
general knowledge in the relevant field of technology as it existed
anywhere before the priority date of this application.
[0045] The features and advantages of the invention will become
further apparent from the following detailed description of
embodiments of the invention.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0046] FIG. 1 is a graph showing gassing of an emulsion explosive
in the absence of a pH regulating agent/proton transfer agent;
[0047] FIG. 2 is a graph showing gassing of emulsion explosives in
the presence of a pH regulating agent/proton transfer agent;
[0048] FIG. 3 is a graph showing gassing of an emulsion explosive
containing a primary amine substrate in the presence of a pH
regulating agent;
[0049] FIG. 4 is a graph showing gassing of an emulsion explosive
containing a cyclic hydrazide substrate in the presence of a pH
regulating agent capable of transferring protons from the emulsion
to the gasser;
[0050] FIG. 5 is a graph showing gassing of an emulsion containing
a cyclic hydrazide compound in the presence of a non diffusing pH
regulating agent;
[0051] FIG. 6 is a graph showing a comparison of gassing between
emulsions containing diffusing and non-diffusing pH regulating
agents;
[0052] FIG. 7 is a graph showing gassing of an emulsion containing
a monohydrazide with non-diffusing pH regulating chemical;
[0053] FIG. 8 is a graph showing a comparison of gassing between
emulsions at different pH with a non-diffusing pH regulating
agent;
[0054] FIG. 9 is a graph showing a comparison of gassing between
examples containing a non-diffusing pH regulating agent with and
without an additional proton transfer agent;
[0055] FIG. 10 is a graph showing the gassing of an emulsion with a
monohydrazide and diffusing pH regulating agent;
[0056] FIG. 11 is a graph showing the gassing of an emulsion with a
monohydrazide and diffusing pH regulating agent;
[0057] FIG. 12 is a graph showing the gassing of an emulsion
containing a dihydrazide and a non-diffusing pH regulating agent;
and
[0058] FIG. 13 is a graph showing the gassing of an emulsion
containing a tetrazole and a diffusing pH regulating agent.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0059] The invention relates to methods for gassing an explosive to
sensitise the explosive to detonation and/or modify the density of
the explosive, comprising reacting at least one oxidiser with at
least one nitrogen containing compound in the explosive to generate
nitrogen gas from the compound. Methods embodied by the invention
find application in the gassing of emulsion explosives and
particularly water-in-oil emulsion explosives, and this type of
emulsion is primarily exemplified below. However, it will be
understood that embodiments of methods of the invention also have
application to other explosives including the gassing of
melt-in-oil emulsion, gel, slurry and ANFO explosives (commonly
referred to as heavy ANFO and which comprises emulsion explosives
in combination with ammonium nitrate-fuel oil).
[0060] A typical emulsion explosive consists of (i) a discontinuous
phase of a solution of nitrate salts, such as ammonium nitrate in
water, and (ii) a continuous fuel phase including an emulsifier
such as poly-isobutylene succinic anhydride (PiBSA) and a
carbonaceous fuel, such as diesel, paraffin or bio oils.
[0061] Explosive compositions embodied by the invention will
normally include: (i) at least one nitrogen containing compound
(also known as the reductant(s) or substrate(s)) which is capable
of producing nitrogen gas via reactions with the oxidiser(s), and
(ii) at least one oxidiser for removing electrons from the
reductant(s) to produce the nitrogen gas, with the oxidiser being
added to the emulsion to affect sensitisation and or density
modification at a point in time after the emulsion has been
constructed. In one or more embodiments of the invention the
substrate is normally added to the discontinuous phase of the
explosive (in the case of an emulsion) during the manufacture of
the base explosive, prior to subsequent addition of the oxidiser.
The oxidiser is normally added to the explosive after manufacture
and forms its own discrete droplets (phase) within the emulsion.
Therefore, either the oxidiser or nitrogen compound must diffuse
through fuel lamellae that separate their respective phases in
order to come into contact with each other and react.
[0062] Reactants (i) and (ii) will normally be selected to react
rapidly with one another, even at sub-zero temperatures. In
addition, at least one of either the reductant or the oxidiser will
be capable of diffusing, or the explosive will be formulated to
allow diffusion, of the reductant or oxidiser through oil or fuel
films (fuel lamellae) separating droplets of the reductant(s) and
the oxidiser. As charged molecules are insoluble in the fuel film,
only neutral molecules are capable of diffusing between gasser and
emulsion droplets.
[0063] The substrate can be any compound containing at least one
nitrogen atom in a form capable of being oxidised to produce
nitrogen gas. Such compounds include, but are by no means limited
to ammonia/ammonium cations, ammonium salts, and any alkyl, aryl
and cyclic compound containing at least one or more of a
combination of the following functional moieties; amines including
primary, secondary and tertiary amines, hydrazines, hydrazides
including dihydrazides, hydrazine hydrate and hydrazine dichloride,
azides, triazoles and/or tetrazoles, and derivatives, and salts
thereof.
[0064] Yet further nitrogen compounds that may be used in one or
more embodiments of the invention include azo compounds
(R--N.dbd.N--R) such as dimethyldiazine, azobenzene and
azodicarboxamide, imidates such as ethyl formimidate hydrochloride,
polyamines such as ethylene diamine, methylamine hydrochloride,
parbamates such as methyl carbamate, cyanides, nitriles, azides
such as ethyl azidoacetate, guanidines and salts thereof such as
guanidine nitrate and guanidine carbonate, hydrazones such as
benzaldehyde semicarbazone, hydroxylamines such as
N-methylhydroxylamine hydrochloride, and imines such as formamidine
acetate salt.
[0065] The oxidiser is typically able to diffuse from the gasser
droplets to the emulsion phase, or can be made able to diffuse by
accepting or releasing a proton/protons. Examples of such oxidisers
include hypohalous acids such as hypochlorous and hypobromous acids
and their corresponding alkali metal or alkali earth metal salts,
N-halosulfonamides, such as N-chlorobenzenesulfonamide,
N-chlorotoluenesulfonamide and their salts (Chloramine B and T
respectively for example), N-halosuccinamides, such as
N-chlorosuccinamide and N-bromosuccinamide and their salts, chloric
and bromic acids and their salts, N-oxides such as
N-methylmorpholine N-oxide, trimethylamine N-oxide or pyridine
N-oxide as well as any alkali or alkali earth metal manganates.
[0066] In at least some embodiments, the oxidiser consists of a
solution containing one or more of the compounds comprising any
alkali metal and alkali earth metal hypochlorites, hypobromites or
hypoiodates, known as hypohalites. Such oxidisers are capable of
reacting rapidly with nitrogen compounds at low temperature to
produce nitrogen gas. Furthermore, hypohalites are weak bases in
solution, meaning that they can accept a proton to form
corresponding hypohalous acids, which, as neutral molecules, are
able to diffuse rapidly through fuel lamellae separating emulsion
and droplets of gasser solution.
[0067] Hypohalous acids (e.g. HOCl, HOBr) exist in equilibrium with
the corresponding hypohalite anion, with the relative concentration
of each species being determined by the pH of the solution. At low
pH (pH<7) the hypohalite exists predominantly in the protonated
form, whist at high pH (pH>9) the majority exists as hypohalite
anions. It is evident that in order for hypohalite gassers to
diffuse through fuel lamellae and react with nitrogen compounds,
the pH of the gasser must be sufficiently low to increase the
concentration of the hypohalous acid to a level that allows an
adequate rate of gasser diffusion. Thus the pH of the gasser is a
relevant factor in determining the rate of gassing using hypohalite
oxidisers, with gassers of low pH having significantly faster
gassing times than those with high pH.
[0068] In order to ensure efficient use of hypohalite oxidisers,
the pH of the gasser solution should be continuously lowered (or at
least remain constant at a low value) throughout the gassing
process. This can occur via the transfer of protons (hydrogen
cations) from the emulsion phase to the gasser and ensures that the
entire contents of the gasser can participate in the gassing
reaction. If proton transfer does not occur, hypohalite anions will
be trapped in the gasser droplets and unable to react with nitrogen
containing compounds in the emulsion. Proton transfer in an
emulsion explosive is generally relatively slow, unless a proton
transfer agent is included in the emulsion formulation.
[0069] In a preferred embodiment of the invention, the emulsion
formulation will contain a pH regulating agent capable of
transferring protons from the emulsion phase to the gasser, also
known as a proton transfer agent. In general, the term "a proton
transfer agent" encompasses a weak acid or base that when
protonated, forms a neutral molecule which can diffuse rapidly
through fuel lamellae separating emulsion and gasser droplets. Such
molecules dissociate when they reach the gasser droplets effecting
the release of protons in the gasser and as such, are able to
transfer protons from the emulsion to the gasser droplets. Examples
of proton transfer agents well suited to the present invention
include the carboxylic acids such as acetic acid and formic acid,
which exist in equilibrium with their deprotonated anions, acetate
and formate. There are many other suitable compounds that can be
used as proton transfer agents, provided that the pH of the
emulsion phase is selected to ensure that a significant proportion
of the compound exists in its protonated from. Examples of such
substances include citric, tartaric, furoic, fumaric, salicylic,
malonic, phthalic, sulfanilic, mandelic, malic, butyric and oxalic
acids, and their salts. Typically, the proton transfer agent will
be selected such that it does not react with the oxidiser used.
[0070] The action of a proton transfer agent with a hypohalite
gasser is described below in the following steps, with reference to
the acetic acid/hypochlorite system, which is used in various
examples of the present invention: [0071] 1. Acetic acid present in
the discontinuous phase of the emulsion diffuses through the fuel
film to the gasser. [0072] 2. The acetic acid is deprotonated by
hypohalites present in the gasser, to produce hypohalous acid and
acetate as shown in the following reaction. [0073] 3. Hypochlorous
acid diffuses to the emulsion, where it reacts with nitrogen
containing chemicals to produce nitrogen gas. [0074] 4. Protons
produced in the gassing reaction cause the formation of more acetic
acid from acetate ions, and the process continues until either the
proton transfer agent or the hypochlorite is completely
consumed.
[0075] The amount of proton transfer agent(s) used should be
sufficient to transfer enough protons to protonate substantially
all of the oxidiser molecules, thus ensuring that the entire
contents of the gasser droplets are able to participate in the
gassing reaction. The amount of pH regulating agent used also
depends on how many protons are produced in the gassing reaction,
and should desirably be sufficient to consume the majority of
protons produced in the reaction, thereby maintaining the emulsion
at an essentially constant pH. Likewise, in the case that the
gassing reaction consumes protons, sufficient pH regulating
agent(s) should be added to ensure that enough protons are
available for the reaction to reach completion and that the
emulsion pH does not rise significantly.
[0076] In another embodiment of the invention, at least one pH
regulating agent is added to the gasser to either accelerate or
slow the gassing process. For example, raising the pH of a
hypohalite gasser with a soluble alkali metal hydroxide such as
sodium hydroxide slows the gassing process, as the added hydroxide
must be neutralised by protons transferred from the emulsion before
gassing can occur. Similarly, the addition of small amounts of an
acidic substance, such as sulfuric, hydrochloric, nitric or acetic
acid (amongst others) to hypohalite gassers increases the
concentration of hypohalous acid, thereby accelerating the gassing
process.
[0077] At temperatures in excess of 35.degree. C., gassing using
the above method can be very rapid. In order to slow the gassing
process at higher temperatures, the proton transfer agent may be
replaced with a non-diffusing pH regulating agent. This produces
slow gassing rates, as proton transfer to the gasser is greatly
reduced, allowing the invention to be used at higher temperatures.
In the case that such an emulsion formulation needs to be gassed at
a low temperature, a proton transfer agent such as acetic acid can
be added to the emulsion prior to the addition of the gasser to
increase the gassing rate.
[0078] The above mechanisms apply to the use of all nitrogen
compounds and oxidisers as described herein. Further details of the
invention will now be discussed in relation to four industrially
important classes/groups of nitrogen compounds. However, it will be
understood that the invention is not limited to the use of these
nitrogen compounds.
Class 1
[0079] In one form, the substrate (nitrogen compound) for the
oxidiser will typically be the ammonium cation (NH.sub.4.sup.+) and
ammonia (NH.sub.3) which exist in equilibrium in ammonium solutions
as follows:
NH.sub.3+H.sup.+.revreaction.NH.sub.4.sup.+ [2]
[0080] The ammonium cation and thereby ammonia can be provided by
the addition of one or more ammonium salts to the explosive.
Examples of suitable ammonium salts include, but are not limited
to, ammonium nitrates, sulfates, phosphates, sulfides, chlorides,
bromides, fluorides, iodides, perchlorates, periodates, acetates,
citrates, and tartarates. Typically, the ammonium salt will be
ammonium nitrate or ammonium perchlorate, which are commonly
employed in explosive compositions. In the case that these salts
are already present in the explosive, additional nitrogen
containing chemicals are not required.
[0081] The oxidiser will normally diffuse through the fuel lamellae
to reach the solution containing ammonium salt(s) and/or other
nitrogen containing chemicals as described above. Such oxidisers
include but are not limited to hypohalous acids such as
hypochlorous and hypobromous acids and their corresponding alkali
metal or alkali earth metal salts, N-halosulfonamides, such as
N-chlorobenzenesulfonamide, N-chlorotoluenesulfonamide and their
salts (Chloramine B and T respectively), N-halosuccinamides, such
as N-chlorosuccinamide and N-bromosuccinamide and their salts,
chloric and bromic acids and their salts, N-oxides such as
N-methylmorpholine N-oxide, trimethylamine N-oxide or pyridine
N-oxide, and alkali or alkali earth metal manganates.
[0082] Alternatively, the pH of the solution of ammonium can be
increased by the addition of a proton donor to elevate the
concentration of NH.sub.3 to achieve diffusion of the NH.sub.3 (a
neutral species) through the fuel lamellae. The increase in
NH.sub.3 concentration occurs because at higher pH, fewer protons
are available, and the equilibrium of reaction [2] shifts to the
left, increasing the concentration of NH.sub.3 and thereby the
NH.sub.3 concentration gradient across the fuel lamellae.
[0083] The oxidation of ammonia to nitrogen gas requires the
transfer of 6 electrons between ammonia and the oxidiser as
indicated in the following half reaction:
2NH.sub.4.sup.+.fwdarw.N.sub.2.uparw.+8H.sup.++6e.sup.-(main
reaction) [3]
[0084] However, the inventors have observed in Examples 2.1 and 2.2
described below, that to reach the pre-determined emulsion
explosive density, about 11 electrons are consumed per mole of
nitrogen evolved. Without wishing to be bound by theory, it is
hypothesised that some of the NH.sub.4.sup.+ oxidises to nitrate
ions via a parallel side half reaction as follows:
NH.sub.4.sup.++3H.sub.2O.fwdarw.NO.sub.3.sup.-+10H.sup.++8e.sup.-
(side reaction) [4]
[0085] It is further believed by the inventors that under the
conditions of reactions [3] and [4], approximately 75% of the total
NH.sub.4.sup.+ consumed is oxidised in the main reaction and the
remaining 25% in the side reaction.
[0086] Relevantly, reactions [3] and [4] produce protons, leading
to a significant drop in pH. Although protons are normally consumed
by the decomposing oxidisers, the present inventors have further
found that in the case of NH.sub.3/NH.sub.4.sup.+ this consumption
is limited to three protons per molecule of evolved nitrogen gas
for at least some types of oxidisers, such as alkali metal and
alkali earth metal hypohalite salts, which are particularly
effective in oxidising NH.sub.3/NH.sub.4.sup.+. This means that the
reaction between such oxidisers and NH.sub.3/NH.sub.4.sup.+ leads
to a net increase in the number of protons, or, in other words,
protons are produced in the reaction. This is demonstrated in
reactions [7] and [8]. Specific examples of suitable oxidisers
which can be used include, but are not limited to, lithium, sodium,
potassium, calcium and barium hypohalites (eg., hypochlorites,
hypobromites and hypoiodites). Examples of the relevant half
reactions are as follows:
HClO+H.sup.++2e.sup.-.fwdarw.Cl.sup.-+H.sub.2O [5]
HBrO+H.sup.++2e.sup.-.fwdarw.Br.sup.-+H.sub.2O [6]
[0087] Combining half reaction [3] with [5] and [4] with [6]
provides examples of the overall main and side reactions operating
during the oxidation of NH.sub.4.sup.+ by hypochlorites as
follows:
2NH.sub.4.sup.++3HClO.fwdarw.N.sub.2.uparw.+5H.sup.++3H.sub.2O+3Cl.sup.-
(main reaction) [7]
NH.sub.4.sup.++4HClO.fwdarw.NO.sub.3.sup.-+6H.sup.++H.sub.2O+4Cl.sup.-
(side reaction) [8]
[0088] For hypohalite and chloramine-type oxidisers, the reduction
of the apparent pH to below unity is possible in the discontinuous
phase of the emulsion. In the presence of unreacted oxidisers this
build-up of acidity, if left, may lead to emulsion crystallisation
as a result of the formation of ammonium nitrate crystals leading
to a reduction in emulsion stability. To preclude this, a
commensurate amount of a proton acceptor such as sodium acetate is
added to the discontinuous phase of the emulsion explosive to
prevent pH from decreasing substantially as demonstrated in Example
3 below. The anion of weak acids exists in equilibrium in solution
and as such, can also regulate pH by accepting protons from
solution to avoid a decrease in pH beyond desired levels in at
least some methods embodied by the invention. Further examples of
such pH regulating agents include, but are not limited to, salts of
other weak acids including food acids such as citric and tartaric
acids. Additional examples of pH regulating agents useful in
embodiments of the invention include furoic, fumaric, salicylic,
malonic, phthalic, sulfanilic, mandelic, malic, butyric and oxalic
acids, as well as their salts.
Class 2
[0089] Another group of nitrogen compounds suitable for use in the
present invention are primary amines. A primary amine is a chemical
species with the structure R--NH.sub.2, where R represents any
alkyl, aryl or cyclic substituent. Any such amine can be used as a
constituent of the explosive in embodiments of the invention
described herein. The amine can contain one or more amino groups,
for example NH.sub.2--R--NH.sub.2, and can incorporate one or more
heteroatoms such as O, S or N, and the use of such compounds is
specifically encompassed. Specific examples of amines which may
find use in embodiments of the invention include but are not
limited to methylamine, ethanamide, ethanolamine, trisamine,
aniline (aminobenzene), urea and thiourea. The oxidation of amines
is analogous to that of ammonia/ammonium cations requiring the
removal of 6 electrons per molecule of nitrogen gas produced, as
shown in the following half equation.
2RNH.sub.2+2H.sub.2O.fwdarw.6e.sup.-+N.sub.2+2ROH+6H.sup.+ [9]
[0090] Combining reactions [5] and [9] provides an example of the
oxidation of amines by hypochlorites as follows:
2RNH.sub.2+3HClO.fwdarw.N.sub.2+2ROH+3Cl.sup.-+3H.sup.++H.sub.2O
[10]
[0091] The use of urea in particular finds widespread applications
in nitrosation based gassing technologies, which are currently used
to sensitise emulsion explosives. Unlike the nitrosation of urea
that occurs only at high temperatures (>40.degree. C.), the
oxidation of urea (and other amines) proceeds rapidly at low
temperatures (<20.degree. C.), thus allowing the present
invention to sensitise emulsions designed for nitrosation of urea
to be gassed at low temperature, removing the need to heat the
emulsion prior to gassing. The use of urea to generate nitrogen gas
with hypohalite oxidisers to sensitise and/or modify the density of
an explosive is demonstrated in Example 3.1.
Class 3
[0092] Another group of nitrogen compounds with application in the
present invention are compounds containing a hydrazine group,
--NHNH.sub.2 and derivatives thereof. A particular class of
hydrazines better suited to practical uses are hydrazides, which
have substantially improved safety properties compared to
hydrazines. For this reason, hydrazides are of particular
importance. However, it should be understood that the invention
extends to all compounds and classes of compounds containing an
--NHNH.sub.2 group and derivatives thereof.
[0093] A hydrazide (including the sulfonyl hydrazides) is a
chemical species that contains either the --C(.dbd.O)--NH--NH.sub.2
or --S(.dbd.O).sub.2--NH--NH.sub.2 group in its chemical structure,
i.e., it contains a carbonyl or sulfonyl group bonded to the
hydrazine group Any optionally substituted suitable alkyl, aryl and
cyclic hydrazides can be used as constituents in embodiments of
methods of the invention described herein. The hydrazide can
contain one or more hydrazide groups (ie., polyhydrazide), such as
di- or trihydrazides, and can also contain various combinations of
amino groups and hydrazine groups, for example, semicarbazide and
aminoguanidine. Hydrazides useful in the invention may also
incorporate one or more heteroatoms such as O, S or N, examples of
which include pyridinic nitrogen as in isonicotinic acid hydrazide,
and the use of all such hydrazides is expressly encompassed.
Specific examples of hydrazides which may find use in embodiments
of the invention include, but are not limited to, acetic acid
hydrazide, formic acid hydrazide, oxalic acid dihydrazide, maleic
acid hydrazide (3,6-dihydroxy pyridazine), succinic acid
dihydrazide, semicarbazide, aminoguanidine, isonicotinic acid
hydrazide, benzoic acid hydrazide, o-, m- and p-hydroxybenzoic acid
hydrazide and o-, m- and p-methylbenzoic sulfonyl hydrazides.
Further hydrazides suitable for use in embodiments of methods of
the invention are described in U.S. Pat. No. 3,706,607, the
contents of which is incorporated herein in its entirety by
reference.
[0094] Without wishing to be bound by theory, it is believed by the
inventors that the main oxidation reaction of the hydrazide
requires the transfer of 4 electrons per molecule of nitrogen gas
produced during hydrazide foaming, as illustrated by the following
generic examples of oxidation of the hydrazide by three groups of
oxidisers found to be particularly effective, namely hypohalites,
halites and permanganates:
R--C(.dbd.O)--NH--NH.sub.2+2HOCl.fwdarw.RCOOH+N.sub.2.uparw.+2H.sup.++H.-
sub.2O+2Cl.sup.- [11]
R--C(.dbd.O)--NH--NH.sub.2+HOBrO.fwdarw.RCOOH+N.sub.2.uparw.+H.sup.++H.s-
ub.2O+Br.sup.- [12]
R--C(.dbd.O)--NH--NH.sub.2+MnO.sub.4.sup.-+4H.sup.+.fwdarw.RCOOH+N.sub.2-
.uparw.+3H.sub.2O+Mn.sup.3+ [13]
wherein R denotes an optionally substituted alkyl, aryl or cyclic
group, with or without one or more heteroatoms present in their
structure. It is noted that reaction [13] consumes protons, and if
these are not available, the oxidation of the hydrazide by
permanganates produces MnO.sub.2 rather than Mn.sup.3+, as
indicated below:
3(R--C(.dbd.O)--NH--NH.sub.2)+4MnO.sub.4.sup.-+4H.sup.+.fwdarw.3RCOOH+3N-
.sub.2.uparw.+5H.sub.2O+4MnO.sub.2.dwnarw. [14]
[0095] However, it is noted that permanganate is essentially
non-diffusing limiting its use in methods described herein.
[0096] Side reactions may lead to production of side products
including dimers, such as
R--C(.dbd.O)--NH--NH--NH--NH--C(.dbd.O)--R or
R--C(.dbd.O)--NH--NH--C(.dbd.O)--R, and amides, such as
R--C(.dbd.O)--NH.sub.2. However, the selectivity of the oxidation
reactions to produce N.sub.2 during oxidation of the hydrazide is
substantially higher than that for the direct oxidation of
NH.sub.3/NH.sub.4.sup.+. Reactions [7]-[14] show that control of
the availability of protons (ie., by regulating pH) during the
oxidation process, is dependent on the oxidiser used.
[0097] The hydrazine gassing method described in U.S. Pat. No.
3,706,607 typically operates at temperatures of at least between
32.degree. C. (90.degree. F.) and 54.degree. C. (130.degree. F.),
and in practice can require temperatures up to 71.degree. C.
(160.degree. F.) to sensitise emulsion explosives, as used in U.S.
Pat. No. 3,770,522. In contrast, one or more embodiments of the
invention enable nitrogen gassing employing hydrazides and other
nitrogen compounds as the substrate at temperatures of 0.degree. C.
or less as described above.
[0098] The use of hydrazides as a source of nitrogen for nitrogen
based chemical gassing is demonstrated extensively in Examples 4
through 6.
Class 4
[0099] Another group of nitrogen compounds well suited to use in
methods embodied by the invention are those known as "nitrogen
rich" compounds. These compounds contain a high percentage of
nitrogen, which reduce the amount of substrate required to release
the desired amount of nitrogen gas. Often, nitrogen rich compounds
contain tetrazole and/or triazole rings, and derivatives thereof,
such as those used as propellants and as gas generators in
automobile air bags, including 5-aminotetrazole,
bis(aminotetrazolyl)tetrazine, bisguanidinium azotetrazole and
bitetrazole. Such compounds contain nitrogen-nitrogen double bonds,
which reduce the amount of oxidiser required to liberate a given
amount of nitrogen gas compared to other nitrogen compounds. The
triazoles include the various triazole tautomers, such as the 1H
and 2H 1,2,3 triazole tautomers and the 1H and 4H-1,2,4 tautomers.
Triazole derivatives include mono, di and trisubstituted molecules
containing any alkyl, aryl, sulfonyl, nitro, thio or amino
substituents. Specific examples include but are by no means limited
to 1,2,4-triazole, 1H-1,2,3-triazole, 1,2,4-triazole-3-carboxylic
acid, 3-amino-1,2,4-triazole, 1H-1,2,4-triazole-3-thiol,
3,5-diamino-1,2,4-triazole, and their alkali metal and alkali earth
metal salts.
[0100] Tetrazoles that can be used include the 1H and 2H tautomers,
and their mono or di-substituted derivatives, which can include
species substituted at the 1 and 2 positions in the tetrazole ring,
or the 5 position corresponding to the carbon atom. Substituents
can include one or more or a combination of any alkyl, aryl,
sulfonyl, nitro, amino or thio groups. Specific examples of
tetrazoles include but are by no means limited to
5-amino-1H-tetrazole, bitetrazole, 5-methyl-1H-tetrazole,
5-phenyl-1H-tetrazole, 1H-tetrazole-5-acetic acid and
5-methylthio-1H-tetrazole, and their alkali metal and alkali earth
metal salts.
[0101] The oxidation of the tetrazole ring requires the removal of
four electrons to produce two molecules of nitrogen gas. However,
various substituents present in common tetrazoles can also be
oxidised to produce nitrogen gas, increasing the yield of nitrogen
to greater than two molecules per molecule of tetrazole. Such
substituents also increase the oxidiser requirement, with the level
of increase determined by the nature of the substituent(s). The
oxidation half equation for a common tetrazole derivative,
5-amino-1H-tetrazole is shown below.
CN.sub.5H.sub.3+2H.sub.2O.fwdarw.7e.sup.-+CO.sub.2+2.5N.sub.2+7H.sup.+
[15]
[0102] Combining reactions [15] and [6] yields the overall reaction
between 5-aminotetrazole and sodium hypobromite.
2CN.sub.5H.sub.3+7HOBr.fwdarw.5N.sub.2+2CO.sub.2+7Br.sup.-+7H.sup.++3H.s-
ub.2O [16]
[0103] Reaction [16] shows that CO.sub.2 is produced at a ratio of
one molecule of CO2for every two and a half molecules of nitrogen
gas. Without wishing to be bound by theory, it is believed that the
majority of CO.sub.2 generated remains dissolved in the aqueous
phase of the emulsion as bicarbonates (i.e., HCO.sub.3), provided
that the pH of this phase is sufficiently high, as is generally the
case in embodiments of the invention. It is evident that just 1.4
moles of hypohalite oxidiser are required per mole of nitrogen gas
produced for 5-aminotetrazole, affording a substantial reduction in
the oxidiser consumption compared to other nitrogen containing
compound.
[0104] The particular nitrogen compound employed will depend on
cost considerations, solubility, the amount of oxidiser required
for its oxidation and its ability to diffuse through fuel lamellae
of the explosive.
[0105] The nitrogen compound will normally be added to the
discontinuous phase of the explosive emulsion at a concentration up
to about 0.1 M and more preferably, in a range of from 0.01 to 0.08
M in explosive formulations that do not rely on direct oxidation of
NH.sub.3/NH.sub.4.sup.+. The precise concentration of nitrogen
chemical depends on (i) the type of nitrogen chemical employed and
in particular, the number of moles of nitrogen gas that can be
evolved per mole of substrate, (ii) the target density of the
gassed explosive, (iii) the need to target the oxidation of the
nitrogen compound rather than NH.sub.3/NH.sub.4.sup.+, with higher
concentrations promoting reaction with the nitrogen compound rather
than NH.sub.3/NH4.sup.+, and (iv) the temperature at which foaming
occurs. For example, in explosives in which the nitrogen gas is
produced essentially only from the oxidation of a hydrazide at
20.degree. C., to achieve the final density of 1.05 g/cm.sup.3 of a
typical ammonium nitrate emulsion displaying ungassed density of
1.33 g/cm.sup.3, a concentration of monohydrazide of around 0.021 M
is required, assuming 70% yield of the monohydrazide to nitrogen
gas. However, for processes that rely on simultaneous or
consecutive generation of nitrogen gas both from oxidation of
NH.sub.3/NH.sub.4.sup.+ and oxidation of the hydrazide, a more
preferable range is from 0 M to 0.1 M for monohydrazides and 0 M to
0.05 M for dihydrazides. Similarly, for higher substituted
hydrazides, tetrazoles or triazoles containing multiple nitrogen
atoms, the preferred concentration is from 0 to 0.2/n M, where n is
the number of nitrogen atoms available in each molecule of the
nitrogen compound.
[0106] For gassing systems relying only on the direct oxidation of
NH.sub.3/NH.sub.4.sup.+, the minimum concentration of
NH.sub.4.sup.+ will usually be about three times higher than the
equivalent concentration of other nitrogen compounds. This does not
present any difficulties as present-day emulsion explosives
containing ammonium nitrate are typically characterised by
concentrations of NH.sub.4.sup.+ in the order of about 13 M.
[0107] The amount of gas produced to regulate the density of
emulsion can be varied as a function of the emulsion location in a
borehole. In such applications, an excess of the substrate (eg.,
hydrazide or other nitrogen compounds) can be used in the
discontinuous phase of the emulsion, and the amount of nitrogen gas
released controlled by adjusting the amount of oxidiser mixed in
the emulsion.
[0108] Any suitable oxidiser can be used to oxidise hydrazides and
other nitrogen compounds to nitrogen as described herein. Examples
of representative oxidisers include, but are not limited to
hypohalites such as hypochlorites, hypobromites and hypoiodites,
chlorites, bromites, chlorates, bromates, iodates, perchlorates,
perbromates, periodates, permanganates, manganates, ferrates,
selenates, ruthenates, perborates, peroxodisulphates, and
peroxomonosulphates of ammonium, and corresponding acids, and
alkali metal and alkali earth metal salts of the foregoing (e.g.,
sodium chlorite, sodium bromite, and lithium, sodium, potassium,
magnesium and calcium hypobromite), and organic cations, e.g.
benzyltriethylammonium permanganate. Further examples include, but
are not limited to, hydrogen peroxide, inorganic and organic
peroxides, organic nitrates, such as benzoyl or peracetyl nitrate,
salcomine, chloramine T and B, nitrodisulfonates, N-oxides, such as
N-methylmorpholine N-oxide, trimethylamine N-oxide and pyridine
N-oxide, sodium dichlorocyanurate, acids such as peroxodisulfuric
acid, peroxomonosulfuric acid, trichloroisocyanuric acid,
hypochlorous acid, iodic acid, selenious acid, vanadic acid, and
salts of Cu(II), Fe(II), Mn(III), Co(III), Ti(IV), Cr(VI),
VO.sub.2.sup.+, lead oxide, manganese dioxide, N-halosulfonamides
such as N-chlorotoluenesulfonamide and N-chlorobenzenesulfonamide,
N-halosuccinamides such as N-bromosuccinamide and
N-chlorosuccinamide, iodine monochloride, iodine bromide,
ferricyanide, and dimethyl sulphoxide and its co-oxidants, and
salts of the foregoing including their alkali metal and alkali
earth metal salts. It is noted that the US Code of Federal
Regulation, 29 CFR 1910.109, prohibits any addition or use of
chlorates and peroxides in blasting agents (Clark, 1991).
[0109] The use of oxidisers which diffuse or can be made to diffuse
through the fuel lamellae separating the gasser and the nitrogen
compound(s) are typically employed, with the application of
non-diffusing oxidisers being limited to those instances in which
the nitrogen compound can diffuse rapidly across the fuel lamellae
to the gasser at low temperature. Thus the oxidisers most suitable
for use in the invention include for example hypohalous acids such
as hypochlorous, hypobromous and hypoiodic acids and their
corresponding alkali metal or alkali earth metal salts,
N-halosulfonamides, such as N-chlorobenzenesulfonamide,
N-chlorotoluenesulfonamide and their salts (Chloramine B and T
respectively), salcomine, N-halosuccinamides, such as
N-chlorosuccinamide and N-bromosuccinamide and their salts, chloric
and bromic acids and their salts, N-oxides such as
N-methylmorpholine N-oxide, trimethylamine N-oxide or pyridine
N-oxide as well as alkali or alkali earth metal manganates.
[0110] Some oxidisers such as hydrogen peroxide and peroxosulfates,
display kinetic limitation to oxidation requiring the use of
catalysts (eg., sodium tungstate), which is expressly encompassed
by the invention. Examples of Cr(VI) oxidisers include, but are not
limited to, chromium oxide, chromates, dichromates, including
pyridinium dichromate, chromic acid, dipyridine chromium oxide and
pyridinium chlorchromate.
[0111] The oxidation of NH.sub.3/NH.sub.4.sup.+ will typically
involve the use of strong to very strong oxidisers, including, but
not limited to, hypochlorites, hypobromites, hypoiodites,
chlorites, chloramine T and B, N-bromosuccinamide and
N-chlorosuccinamide, permanganates and peroxosulphates. In gassing
systems involving nitrogen chemicals in addition to NH.sub.4.sup.+,
these oxidisers will oxidise both nitrogen containing chemicals
and, albeit often at a slower rate, NH.sub.3/NH.sub.4.sup.+. Weaker
oxidisers will generally only oxidise nitrogen compounds rather
than NH.sub.3/NH.sub.4.sup.+.
[0112] The amount of the oxidiser used in preferred embodiments of
the invention will depend, inter alia, on: (i) target density of
the foamed explosive composition; (ii) the selectivity of the
oxidiser to produce nitrogen gas in reactions with
NH.sub.3/NH.sub.4.sup.+ and other nitrogen compounds, in the
presence of side reactions; (iii) the concentration of the active
species in the oxidiser; (iv) the need to oxidise
NH.sub.3/NH4.sup.+ or the hydrazide; (v) the number of electrons
withdrawn from substrate by each molecule of the oxidiser, i.e.,
the change in the oxidation state; (vi) the presence of catalyst
(since in the process of its activation, an oxidiser may accept an
electron from a catalyst rather than from the substrate); and (vii)
the temperature of foaming. If an oxidiser is added to an emulsion
explosive as a gasser solution, it is preferred that the mass of
that solution is less than 4% w/w of the emulsion, with the content
of the active species in the solution of between 5% w/w and 50%
w/w.
[0113] For example, to foam an emulsion that contains 0.015 M
maleic hydrazide with sodium hypochlorite in which the hydrazide is
essentially entirely consumed, 2.times.0.015/0.8=0.038 mol sodium
hypochlorite (NaOCl)/L emulsion, or 0.038/1.33=0.028 mol/kg
emulsion would be required. The factor of 0.8 corresponds to
efficiency of gassing and the factor of 2 signifies the ratio of
number of electrons required by a molecule of hydrazide [11] to
that provided by each molecule of the oxidiser [5]. Taking into
account the molecular weight of NaOCl and the concentration of
NaOCl in commercially available solutions of 12.5% w/w, the
calculation becomes 0.028.times.(22.99+35.5+16)/0.125=17 g/kg
emulsion. By selecting a different oxidiser, and/or using more
concentrated solutions, the amount of gasser may be optimised to
less than 0.8% w/w of the emulsion.
[0114] Normally, the species of a nitrogen compound foaming system
that is included in the discontinuous phase of the emulsion will be
at a relatively low concentration. This means that the diffusion of
this species through the fuel lamellae will be slow, even if the
species itself is soluble in the fuel and displays high diffusivity
in that phase. For faster gassing, the second species of the
foaming system (i.e., the oxidiser) that is added to the emulsion
should desirably be able to diffuse rapidly through the fuel
lamellae.
[0115] The nitrogen compound will generally be dissolved in the
discontinuous phase of the emulsion explosive, and diffusion of the
selected oxidiser through the fuel lamellae of the explosive can be
achieved in a number of ways. For example, the diffusion of the
oxidiser can be accomplished by: [0116] (i) Selection of an
oxidiser that diffuses readily under the gassing conditions, such
as chloramine T or B, or N-oxides. This is illustrated for
chloramine T in Example 2.1; [0117] (ii) Selection of an oxidiser
whose acid form exhibits a high pKa value, such as hypochlorites,
hypobromites, hypoiodites, manganates or bromites. Anions of these
salts can be readily protonated owing to proton transfer from the
discontinuous phase of the emulsion, e.g., by diffusion of a pH
regulating agent such as a weak acid like acetic or formic acid. In
this case, the pH of the discontinuous phase and the type of the pH
regulating agent should be selected to accelerate the gassing
process. Specifically, the pH regulating agent itself should be
able to diffuse through the fuel lamellae (this point is
demonstrated by comparing the measurements of Examples 4.1 and 4.2
below).
[0118] Methods described herein have application in the gassing of
emulsion explosives (including melt-in-oil emulsions) of explosive
emulsion, gel, slurry and heavy ANFO compositions. For example, the
gasser can be pre-emulsified or delivered to emulsion in the form
of a micro-emulsion, a Lewis acid catalyst may be added to make an
oxidiser diffuse through fuel lamellae or for instance, an oxidiser
can be added as a powder, as described for nitrite gassing in U.S.
Pat. No. 4,875,951, European Patent Application No. 775,681, U.S.
Pat. No. 6,855,219 and U.S. Pat. No. 4,997,494 respectively, the
contents of which are incorporated herein in their entirety by
cross-reference.
[0119] Water-in-oil emulsion explosives can be any emulsion
comprising a discontinuous phase of an aqueous oxidising solution
of inorganic salts dispersed in a continuous phase of an organic
fuel in the presence of one or more emulsifying agents. Such
emulsion explosives are well known in the art.
[0120] The oxidising salt can, for example, be selected from
ammonium, alkali metal and alkaline earth nitrates, perchlorates
and mixtures of the foregoing. Typically, the oxidising salt will
comprise at least about 50% w/w of the emulsion explosive
composition, more preferably at least about 60%, 70% or 80% w/w and
most preferably, at least about 90% w/w of the explosive. In a
particularly preferred embodiment, the oxidising salt will be
ammonium nitrate alone or in combination with sodium nitrate,
potassium nitrate, calcium nitrate and/or ammonium, alkali metal
and alkaline earth metal perchlorates.
[0121] Proton transfer agents used herein can comprise one or more
compounds selected from the group consisting of inorganic acids,
organic acids, carboxylic acids, and salts thereof, the explosive
being formulated such that at least some of these compounds exist
in the explosive in a neutral form. More particularly, these agents
can comprise one or more compounds selected from the group
consisting of alkyl carboxylic acids, acetic acid, formic acid,
phosphoric acid, citric acid, tartaric acid, furoic acid, fumaric
acid, salicylic acid, malonic acid, phthalic acid, sulfanilic acid,
mandelic acid, malic acid, butyric acid, oxalic acid, and salts
thereof.
[0122] Non-diffusing pH regulating agents as described herein can
comprise one or more compounds selected from the group consisting
of partially or completely deprotonated forms of inorganic acids
and carboxylic acids, and salts thereof. In particular, these
agents can comprise one or more of phosphoric acid, acetic acid,
formic acid, citric acid, tartaric acid, furoic acid, fumaric acid,
salicylic acid, malonic acid, phthalic acid, sulfanilic acid,
mandelic acid, malic acid, butyric acid, oxalic acid, and salts
thereof.
[0123] A melt-in-oil emulsion explosive can be any such explosive
containing little or no water in its formulation, and may solidify
once the temperature decreases below the solidification point of
the melt, which usually lies between 70.degree. C. and 130.degree.
C. Normally, the melt-in-oil emulsion contains ammonium nitrate and
at least one other chemical added to decrease the melting point of
ammonium nitrate. Melt-in-water emulsion explosives are also well
known in the art (e.g., U.S. Pat. No. 4,790,891 and U.S. Pat. No.
4,676,849).
[0124] So-called heavy ANFO mixtures form an important part of
industrial explosives consumption. In heavy ANFO, emulsion
explosives are typically mixed with ANFO, that is, with porous
solid prilled ammonium salts with oil present in the porous space
of the prills. Heavy ANFO may be suitably gassed by the methods
described in this invention, and the use of any such methods to
sensitise and/or modify the density this type of explosive is
specifically encompassed.
[0125] Emulsifiers commonly used in emulsion explosive compositions
include sorbitan monooleate (SMO), polyisobutane succinic
anhydrides (PiBSA) and amine derivatives of PiBSA, and conjugated
dienes and aryl-substituted olefins. U.S. Pat. No. 6,800,154 and
U.S. Pat. No. 6,951,589 provide a particularly comprehensive
summary of suitable emulsifiers which may be used, the contents of
which are incorporated herein by reference in their entirety.
[0126] The fuel used in the explosive can also be any fuel commonly
utilised in explosive compositions. Examples of fuels that can be
utilised include, but are not limited to, paraffinic, olefinic,
napthenic, and paraffin-napthenic oils, animal oils, vegetable
oils, synthetic lubricating oils, hydrocarbon oils in general and
oils derived from coal and shale, as described in detail in U.S.
Pat. No. 6,951,589, the contents of which is also incorporated
herein by reference in its entirety.
[0127] Although gel explosives were utilised by industry prior to
emulsion explosives, their use has declined in preference to
emulsion explosives and heavy ANFO. Nevertheless, gel explosives
are well known to the skilled addressee. Examples of gel explosives
are for instance described in U.S. Pat. No. 3,382,117, U.S. Pat.
No. 3,660,181, U.S. Pat. No. 3,711,345 and U.S. Pat. No.
3,713,919.
[0128] Typically, the gassing of an explosive as described herein
will be achieved at ambient temperature although heating of the
explosive to assist diffusion of the oxidiser and/or nitrogen
compound is not excluded. The gassing may for example be carried
out at temperatures of about 70.degree. C. or below, or at a
temperature of 60.degree. C., 50.degree. C., 40.degree. C.,
30.degree. C., 25.degree. C., 20.degree. C., 10.degree. C.,
5.degree. C. or 0.degree. C. or below, or in any range of from
0.degree. C. or below up to about 70.degree. C. As will also be
understood, the gassing may be carried at any specific temperature
or within any specific range of temperatures within the particular
ranges specified above (eg., 35.degree. C., 15.degree. C.,
14.degree. C. or 13.degree. C., or eg., from 0.degree. C. or below
up to about 15.degree. C.), and all such specific temperatures and
temperature ranges are expressly encompassed.
[0129] An example of an embodiment of the invention is as follows.
Add to an emulsion explosive composition containing a hydrazide
compound (eg., acetyl hydrazide, maleic hydrazide etc., at a
concentration between 0.01 and 0.1 M) and a pH regulating agent
(eg., sodium acetate, approx 0.2 to 1% of the composition) with the
pH of the emulsion between 4.5 and 6, a solution of sodium
hypochlorite (the solution containing 10-20% NaOCl) and mix this
solution into the emulsion for 5-15 s to disperse it and gas the
emulsion. The invention will now be described below by reference to
a number of further non-limiting Examples.
EXAMPLE 1
1. Experimental Protocol
1.1 Emulsions
[0130] In all Examples, the discontinuous phase of explosive
emulsions consisted of a super-saturated solution of ammonium
nitrate, containing approximately 400 g of ammonium nitrate per 100
g of water. In Emulsions A and B, the continuous phase consisted of
a mixture of diesel fuel and a PiBSA emulsifier, such that the
ratio of fuel to emulsifier was approximately 70 parts of diesel
fuel to 30 parts of PiBSA, by mass. The phase ratio for these
emulsions was 87 parts discrete phase to 13 parts continuous phase
on a volumetric basis. This corresponded approximately to 91.5
parts of discrete phase to 8.5 parts of continuous phase on a mass
basis. In Emulsions C-K, the continuous phase consisted of a
mixture of diesel fuel and PiBSA emulsifier, such that the ratio of
fuel to emulsifier was 80 parts diesel to 20 parts PiBSA by mass.
The ratio phase ratio for Emulsions C-K was 94 parts of discrete
phase to 6 parts of continuous phase on a mass basis. The above
compositions conform to those well known in the art, such as those
described in U.S. Pat. No. 4,409,044, U.S. Pat. No. 3,447,978 and
U.S. Pat. No. 3,770,522.
[0131] The discontinuous phase was prepared by dissolving ammonium
nitrate in water at a temperature of 75.degree. C. Various
combinations of pH regulating agents, such as sodium acetate,
sodium hydroxide and tri-sodium citrate were then added to control
the pH of the system if required in a particular emulsion. Some of
these chemicals also serve as proton transfer agents during the
gassing process. Nitrogen containing compounds such urea,
5-aminotetrazole, and various hydrazides/dihydrazides were also
added to some emulsions to improve the efficiency of the gassing
process. The apparent pH of the solution was measured using a pH
probe (Hanna pH 213 meter with HI 1131B pH probe) prior to addition
to the continuous phase. Table 1 lists emulsions used in the
examples described below.
TABLE-US-00001 TABLE 1 Composition of emulsions A B C D E F G H I J
K Discontinuous Phase Ammonium nitrate (g) 1000 1000 600 600 600
400 600 600 600 600 600 Water (g) 250 250 150 150 150 100 150 150
150 150 150 Sodium acetate (g) -- 14.83 4.96 3.29 -- -- -- 3.28
3.29 -- 3.28 Tri-sodium citrate (g) -- -- -- -- 5.87 4.01 5.88 --
-- 5.87 -- Sodium dihydrogen -- -- -- -- -- -- 0.24 -- -- -- --
citrate (g) Maleic hydrazide (g) -- -- -- 1.13 1.14 -- -- -- -- --
-- Succinic dihydrazide (g) -- -- -- -- -- -- -- -- -- 0.73 --
Acetyl hydrazide (g) -- -- -- -- -- 0.58 0.83 0.83 0.83 -- --
5-Aminotetrazole (g) -- -- -- -- -- -- -- -- -- -- 4.47 Urea (g) --
-- 1.01 -- -- -- -- -- -- -- -- Sodium hydroxide -- -- 0.57 8.25
7.94 4.0 -- 1.01 0.75 7.84 3.26 50% solution (g) Apparent pH 3.85
6.05 5.5 6.29 6.24 6.29 5.0 5.5 5 6.24 B 5.5 5.5 Continuous Phase
Diesel oil (g) 81.76 82.96 38.65 38.97 39.12 26.01 38.72 38.53
38.57 39.07 38.90 PiBSA (g) 35.26 35.67 9.66 9.74 9.78 6.5 9.68
9.63 9.64 9.77 9.73 Total mass (g) 1367.0 1383.5 804.9 811.4 813.8
541.1 805.4 803.3 803.1 813.3 809.6
[0132] Emulsions were constructed by slowly adding the above
solution containing ammonium nitrate to the continuous phase at a
temperature of approximately 75.degree. C. The two phases were
mixed thoroughly using an overhead stirrer (IKA Eurostar Digital)
operating at approximately 300-450 rpm. Once the two phases had
been completely combined, the mixing speed was increased to
approximately 500-600 rpm and maintained at this speed until the
emulsion had reached a suitable viscosity.
1.2 Preparation of Gassers
[0133] Five types of gasser chemicals were used, including lithium
and calcium hypochlorites (LiOCl and Ca(OCl).sub.2, respectively),
chloramite T, sodium hypochlorite (NaOCl) and sodium hypobromite
(NaOBr). Lithium and calcium hypochlorites as well as chloramine T
were obtained as solids, whilst sodium hypochlorite and hypobromite
were obtained as solutions. The solids were weighed into a 20
cm.sup.3 beaker and distilled water was then added to affect the
dissolution. The concentration of the gasser solution was generally
between 10 and 20%; although higher concentration could have been
possible. To accelerate the dissolution of the substance, the
beaker was heated lightly (less than 50.degree. C.), and then the
solution was cooled down prior to use. In the case of sodium
hypochlorite and hypobromite, the gassing reactant was present in a
solution of approximately 10-12% by mass. Sodium hypobromite
contained a small percentage (1-5%) of sodium hydroxide for
enhanced shelf life, whilst some examples utilised additional
sodium hydroxide to delay the onset of the gassing process. For
examples utilising additional sodium hydroxide in the gasser, stock
solutions were made by adding the required mass of 50% sodium
hydroxide solution to a known mass of sodium hypochlorite solution.
The desired mass of solution for use in examples was drawn into a
syringe, confirmed by weighing, and used without alteration.
1.3 Gassing Procedure
[0134] Gassing experiments were conducted using 230.+-.2 g of
emulsion, allowing each batch of emulsion of Table 1 to serve three
to five experiments. The gasser solution was mixed with the
emulsion for 5-15 s at 300 rpm with an overhead stirrer to ensure
an even distribution of the gasser throughout the emulsion. In
examples where acetic acid was added to the emulsion prior to
gassing, the desired amount of acid was mixed in the emulsion for
15 s, before adding the gasser after a further 30 s had elapsed.
The emulsion was then transferred to a container of known volume,
such that the emulsion occupied completely the volume within the
container. The container was levelled at the top and weighed at
regular intervals as the gassing progressed in order to observe the
density change and the rate of gassing.
[0135] For experiments conducted at temperatures other than
ambient, the emulsion temperature was adjusted by means of a
constant-temperature refrigerated and heated water bath before
addition of the gasser solution. The container employed in the
experiment was also placed in the water bath before handling. Once
the gasser solution had been added to the emulsion, the container
was returned to the water bath at all times when not being
weighed.
EXAMPLE 2
[0136] 2.1 Oxidation of Ammonia in Emulsion without Proton Transfer
Agent
[0137] This example demonstrates that ammonia/ammonium ions can be
directly oxidised by various oxidisers in order to produce nitrogen
gas to sensitise and/or modify density of an emulsion or gel
explosive. It also demonstrates the influence of the gasser
composition and pH on the rate of gassing and indicates the
desirability of appropriate pH selection to achieve required
sensitisation times.
[0138] The method utilised involved gassing of Emulsion A (with no
pH control or proton transfer agent) using lithium hypochlorite,
sodium hypobromite and chloramine T, at 30.degree. C. Table 2 shows
the composition of gassing solutions and completion time for each
example.
TABLE-US-00002 TABLE 2 Gasser solutions for Example 2.1 Mass (g)
Completion Time* Experiment 2.1a Lithium hypochlorite (35% active)
1.53 5 min Distilled water 10.17 Experiment 2.1b Sodium hypobromite
(10-20% solution 6.56 18 min of NaOBr, contains 1-5% NaOH)
Experiment 2.1c Chloramine T 1.59 2 min Distilled water 11.55
*Completion time is the time taken for the emulsion to reach 90% of
its final density
[0139] The results are plotted in FIG. 1 and show that the reagent
producing the fastest gassing rate was chloramine T (2 min),
followed by lithium hypochlorite (5 min). The reaction with sodium
hypobromite was slower taking approximately 18 min to reach
completion. No attempt was made to optimise the amount of gasser to
regulate the emulsion density to a set level, say 1.05 or 1
g/cm.sup.3. Lower densities could have been achieved in this
example, and in other examples described below, simply by mixing in
larger amounts of the gassers, and in the case of other examples,
adjusting the amounts of pH agents and/or nitrogen compounds.
Similarly adding smaller amounts of gasser would produce a smaller
density change, and as such the invention can produce emulsion
explosives of a density required for practical application, i.e.
from 1.37 g/cm.sup.3 to less than 1 g/cm.sup.3. Hence, one or more
embodiments as described herein can be applied to modify the
density and/or sensitise commercial explosives, with the level of
density change controlled by the amount of gasser added.
[0140] The example also demonstrates that the nature of the gassing
chemical is an important factor that influences the rate of
gassing. Chloramine T exists as a neutral molecule in solution and
is therefore able to diffuse readily through the fuel lamellae from
the gasser to react with the nitrogen compound in the emulsion
phase. As a result, chloramine T shows fast gassing despite the
absence of a proton transfer agent.
[0141] In contrast, the diffusion of hypohalites is determined by
the pH of the gasser solution. Hypohalite anions are weak bases and
are capable of accepting a proton to form hypohalous acids, which
are able to diffuse rapidly through fuel lamellae. Hypohalous acids
(e.g., HOCl, HOBr) exist in equilibrium with the corresponding
hypohalite anion, with the relative concentration of each species
being determined by the pH of the solution and pKa of the gasser.
Normally, pH and pKa are reported in literature under conditions
corresponding to those of infinitely dilute solutions. Embodiments
of this invention involve the use of concentrated solutions of
electrolytes. In such solutions, both pH and pKa usually differ
from their values in the limit of infinite dilution. For this
reason, the meaning of pH in the context of the invention is that
of the apparent (i.e., the measured pH), rather than the actual pH.
At low pH (pH<7) the hypohalite exists predominantly in the
protonated form, whist at high pH (pH>9) the majority exists as
hypohalite anions. It is evident that in order for hypohalite
gassers to diffuse through fuel lamellae and react with nitrogen
compounds, the pH of the gasser should be sufficiently low to
increase the concentration of the hypohalous acid to a level that
allows an adequate rate of gasser diffusion.
[0142] To ensure efficient use of hypohalite oxidisers, the pH of
the gasser solution should be continuously lowered (or at least
remain constant at a low value) throughout the gassing process.
This can occur via the transfer of protons (hydrogen cations) from
the emulsion phase to the gasser and ensures that the entire
contents of the gasser can participate in the gassing reaction. If
proton transfer does not occur, hypohalite anions will be trapped
in the gasser droplets and unable to react with nitrogen containing
compounds in the emulsion. Proton transfer in an emulsion is slow,
unless a proton transfer agent is included in the emulsion
formulation.
[0143] It can be seen from this example that the rate of gassing is
dependent on the pH of the gassing solution. The pH of distilled
water used to dissolve lithium hypochlorite is lower than the pKa
of hypochlorous acid (pKa.about.7.5), and as such the gasser
solution contains a significant concentration of hypochlorous acid.
It can also be seen from FIG. 1 that the gassing rate with this
chemical is reasonably fast, despite the absence of a proton
transfer agent. In contrast, the sodium hypobromite solution has a
very high pH due to the addition of sodium hydroxide, which causes
the initial concentration of hypobromous acid in the gasser to be
very low. For this reason the gassing rate of the sodium
hypobromite experiment (Example 2.1b) is slower than that of
lithium hypochlorite (Example 2.1a). It is evident that the
efficiency of the reaction was much lower than that predicted from
theory, which may be attributed to side reactions that do not
result in the production of nitrogen gas, such as the formation of
nitrate ions.
2.2 Oxidation of Ammonia in Emulsion with a Proton Transfer
Agent
[0144] This example demonstrates the use of a proton transfer agent
to accelerate the diffusion of hypohalite oxidisers added to the
emulsion to react with ammonia/ammonium ions, at temperatures from
12 to 30.degree. C.
[0145] The particular proton transfer agent used in this example is
sodium acetate. Sodium acetate is a weak base in solution, which
makes it capable of accepting a proton to form acetic acid. Acetic
acid (CH.sub.3COOH) is a neutral molecule and is able to diffuse
from the emulsion phase to gasser droplets, allowing it to transfer
protons from the emulsion phase to the gasser. Hypohalites are
stronger bases than acetate, allowing them to deprotonate acetic
acid that has diffused to the gasser droplets to produce hypohalous
acids. (e.g. CH.sub.3COOH+OCl.fwdarw.CH.sub.3COO--+HOCl). The
hypohalous acid produced undergoes rapid diffusion to the emulsion
phase where it can react with a nitrogen-containing chemical.
[0146] In this example, the aforementioned mechanism is used to
react ammonia/ammonium ions with lithium hypochlorite and sodium
hypobromite. Emulsion Composition B (Table 1) was gassed using
lithium hypochlorite at 10 and 30.degree. C. and sodium hypobromite
at 23.degree. C. and 30.degree. C. The emulsion contained
approximately 1% sodium acetate to control the emulsion pH and act
as a proton transfer agent. Table 3 shows the composition of gasser
solution used in each experiment.
[0147] As illustrated in FIG. 2 and Table 3, the gassing rate at
30.degree. C. was fast for both reagents studied, with the gassing
process essentially completed in less than 3 min. This is
considerably faster than the previous emulsion that did not contain
a proton transfer agent. There appears to be only a small
temperature dependence on the oxidation by lithium hypochlorite,
with the reaction at 12.degree. C. taking less than 2 min longer to
reach completion compared to 30.degree. C. There is also little
difference in the gassing rate of sodium hypobromite between
23.degree. C. and 30.degree. C. No crystals were present in the
emulsions after the completion of the gassing reactions.
TABLE-US-00003 TABLE 3 Gasser solutions for Example 2.2 Mass (g)
Completion Time* Experiment 2.2a (30.degree. C.) Lithium
hypochlorite (35% active) 2.29 1 min Distilled water 10.33
Experiment 2.2b (30.degree. C.) Sodium hypobromite (10-20% soln,
13.12 1.5 min contains 1-5% NaOH) Experiment 2.2c (12.degree. C.)
Lithium hypochlorite (35% active) 2.30 3 min Distilled water 11.24
Experiment 2.2d (23.degree. C.) Sodium hypobromite (10-20% soln,
13.21 2.5 min contains 1-5% NaOH) *Completion time is the time
taken for the emulsion to reach 90% of its final density
EXAMPLE 3
3.1 Oxidation of Primary Amine
[0148] This example demonstrates the oxidation of compounds
containing a primary amine group to produce nitrogen gas. Primary
amines exhibit higher selectivity compared to ammonia, reducing the
amount of gasser required to form a desired amount of gas. Urea is
shown in this example as it finds widespread application in
nitrosation based gassing technologies. Unlike nitrosation
reactions, which require high emulsion temperatures, the oxidation
of urea occurs rapidly at ambient temperatures. This allows an
emulsion designed for high temperature nitrosation gassing to be
sensitised or have its density modified using urea oxidation if the
emulsion temperature falls, allowing greater flexibility in
blasting operations.
[0149] The example uses Emulsion C, which contains approximately
0.6% sodium acetate at pH 5.5 to act as a proton transfer agent.
The gasser used is a sodium hypochlorite (NaOCl) solution
containing approximately 10.5% NaOCl by mass. The experiment was
performed at 20.degree. C., and follows the same mechanism as
Example 2.2, in which a proton transfer agent is used to facilitate
the diffusion of a hypohalite oxidiser to the emulsion where it can
react with a nitrogen-containing chemical to produce nitrogen gas.
Table 4 list the composition of the gasser used in this
example.
TABLE-US-00004 TABLE 4 Gasser solutions for Example 3.1 Experiment
3.1 (20.degree. C.) Mass (g) Completion Time* Sodium hypochlorite
(10.5%) 7.01 4 min *Completion time is the time taken for the
emulsion to reach 90% of its final density
[0150] It can be seen from FIG. 3 that there is an induction period
of approximately 80 s in which little or no gassing occurs,
followed by a period of rapid gassing, with the final completion
time being approximately 4 min. The initial lag occurs as it takes
some time for the pH of the gasser to be lowered enough to produce
HOCl, and for the HOCl to diffuse from the gasser to the emulsion.
This short lag may be desirable, as it allows the gasser to be
mixed thoroughly into the emulsion before the production of gas
commences, thus reducing the likelihood that gas is inadvertently
separated out of the emulsion before it enters a borehole. Table 5
provides a comparison between the amount of gasser required for
urea and ammonia gassing. It can be seen that the amount of
oxidiser required to produce gas from primary amines is slightly
less than that for ammonia/ammonium ions.
TABLE-US-00005 TABLE 5 Comparison of gasser requirements of urea
and ammonia based gassing Nitrogen Chemical Moles Hypochlorite/Mole
of Gas Ammonia/ammonium ions 5.16 Urea 4.36
EXAMPLE 4
[0151] 4.1 Cyclic Hydrazide Compound with Diffusing Buffer
[0152] This example demonstrates the use of a cyclic hydrazide
compound, in conjunction with hypohalite oxidisers and a diffusing
buffer/proton transfer agent to sensitise and/or modify the density
of an emulsion at low temperature. The use of a hydrazide reduces
the amount of oxidiser compared to urea and ammonia, as the
reaction requires the removal of four electrons per molecule of gas
rather than six. The example also demonstrates that raising the pH
of hypohalite oxidiser solutions prior to gassing enables an
emulsion to be gassed in a controlled manner over a wide
temperature range.
[0153] Emulsion D (Table 1), containing maleic hydrazide (i.e.,
3,6-dihydroxy pyridazine) and sodium acetate at pH 6.3 was gassed
with sodium hypochlorite at 5.degree. C., 18.degree. C. and
40.degree. C. using 0, 2 and 5% additional sodium hydroxide (NaOH)
in the respective gassers. Sodium hydroxide is a strong base, and
is effective at raising the pH of gasser solutions to control the
gassing rate over a wide temperature range. The gassing rate is
reasonably fast in all experiments ranging from approximately 6 min
at 40.degree. C. to 15 min at 5.degree. C. The gasser composition
is shown in Table 6.
TABLE-US-00006 TABLE 6 Gasser solutions for Example 4.1 Mass (g)
Completion Time* Experiment 4.1a (5.degree. C.) Sodium hypochlorite
solution 10.5% 4.69 15 min Experiment 4.1b (18.degree. C.) Sodium
hypochlorite solution 10.5%, 4.88 11.5 min 2% NaOH Experiment 4.1c
(40.degree. C.) Sodium hypochlorite solution 10.5%, 5.22 6 min 5%
NaOH *Completion time is the time taken for the emulsion to reach
90% of its final density
[0154] It can be seen that despite a large difference in the
temperatures used in each experiment that the gassing times are
quite similar. Sodium hydroxide (NaOH) was added to the gasser to
increase the pH of the gasser solution. Added NaOH must be
neutralised by protons transferred from the emulsion phase before
hypohalite oxidisers can diffuse and so can be used to reduce the
rate of gassing, which is especially useful at temperatures above
30.degree. C. at which proton transfer occurs quite rapidly.
Without the addition of NaOH the gassing at 40.degree. C. would be
extremely fast, and there would be a large difference in the
gassing times of each experiment. It is noted that due to the
relatively high pH of the emulsion that the concentration of
ammonia in the emulsion is significant. A small portion of the
gassing can be attributed to the diffusion of ammonia from the
emulsion to the gasser, which is demonstrated in Example 5.1-5.2.
The results for Example 4.1 are shown in FIG. 4.
4.2 Cyclic Hydrazide Compound with Non-Diffusing Buffer
[0155] This example demonstrates that a non-diffusing buffer may be
used to produce slower gassing times at high temperatures, and that
without a proton transfer agent the gassing rate at low temperature
is significantly slower.
[0156] Emulsion E, containing maleic hydrazide, was gassed with
sodium hypochlorite containing 0 and 2% NaOH at 18.degree. C. and
5% NaOH at 40.degree. C. The emulsion contained approximately 0.7%
tri-sodium citrate acting as a pH buffer. The apparent pH of the
discontinuous phase was 6.25. The composition of the gasser used in
each experiment is shown below in Table 7, and the results are
shown in FIG. 5.
TABLE-US-00007 TABLE 7 Gasser solutions for Example 4.2 Mass (g)
Completion Time* Experiment 4.2a (18.degree. C.) Sodium
hypochlorite solution 10.5% 4.69 40 min Experiment 4.2b (18.degree.
C.) Sodium hypochlorite solution 10.5%, 4.88 48 min 2% NaOH
Experiment 4.2c (40.degree. C.) Sodium hypochlorite solution 10.5%,
5.22 18 min 5% NaOH *Completion time is the time taken for the
emulsion to reach 90% of its final density
[0157] The gassing times of this emulsion containing a tri-sodium
citrate buffer are considerably slower than those of the emulsion
with the same pH and acetate buffer, taking 12 min longer at
40.degree. C. and up to 36 min longer at 18.degree. C. This occurs
as at the emulsion pH used (6.25) citrate molecules are
deprotonated and unable to transfer protons from the emulsion to
the gasser. Because proton transfer occurs so slowly, most of the
hypohalite remains deprotonated and is unable to diffuse from the
gasser to the emulsion, resulting in slow gassing times. Acetate,
on the other hand is protonated to a small extent at pH 6.25, and
can be easily protonated during the gassing process. This allows it
to transfer protons from the emulsion to the gasser to protonate
hypohalite anions that diffuse rapidly into the emulsion phase,
thereby producing significantly faster gassing times that the
non-diffusing citrate buffer. A comparison of Examples 4.1c and
4.2c (40.degree. C. with 5% additional NaOH) is shown in FIG.
6.
EXAMPLE 5
[0158] 5.1 Hydrazide Compound with Non-Diffusing Buffer
[0159] This example demonstrates the use of a hydrazide compound,
acetyl hydrazide in conjunction with a hypohalite oxidiser to
produce controlled gassing of an emulsion between 20 and 40.degree.
C.
[0160] Emulsion F, containing acetyl hydrazide and a tri-sodium
citrate buffer at pH 6.3 was gassed at 20 and 40.degree. C. with
sodium hypochlorite (NaOCl) containing 0 and 5% additional NaOH in
the respective gassers. Similarly to Example 4.2, the gassing rate
at 40.degree. C. is considerably slower than with a diffusing
buffer, with a completion time of 10 min, whilst the gassing rate
at 20.degree. C. is very slow taking 30 min. It is evident that a
proton transfer agent is desirable for fast gassing below
30.degree. C. However a non-diffusing buffer such as tri-sodium
citrate is useful at higher temperatures to ensure that the gassing
process is not too fast. Table 8 shows the composition of the
gasser used in this example, whilst the results are shown in FIG.
7.
TABLE-US-00008 TABLE 8 Gasser solutions for Example 5.1 Mass (g)
Completion Time* Experiment 5.1a (20.degree. C.) Sodium
hypochlorite solution 10.5% 4.68 30 min Experiment 5.1b (40.degree.
C.) Sodium hypochlorite solution 10.5%, 5.21 10 min 5% NaOH
*Completion time is the time taken for the emulsion to reach 90% of
its final density
[0161] It can be noted that the gassing times for acetyl hydrazide
are slightly faster than those of maleic hydrazide (Example 4.2),
with the gassing times of acetyl hydrazide being 5 min faster at
40.degree. C. and 10 min faster at 20.degree. C. This is most
likely due to the production of acetic acid from the oxidation of
the hydrazide, which is able to transfer protons from the emulsion
to the gasser to protonate hypochlorite anions, accelerating the
gassing process.
5.2 Hydrazide Compound with Non-Diffusing Buffer at Lower pH
[0162] This example demonstrates gassing with a hydrazide
containing compound using hypohalite oxidisers in an emulsion with
a non-diffusing buffer at pH 5. The example demonstrates that
ammonia can diffuse to the gasser when the pH of an emulsion
containing ammonium ions is greater than 6, whilst at pH values
near 5 ammonia diffusion is substantially slower. It also
demonstrates that a pH transfer agent such as acetic acid may be
added to the emulsion immediately prior to gassing to increase the
gassing rate.
[0163] Emulsion G containing acetyl hydrazide and a citrate buffer
at pH 5 was gassed with sodium hypochlorite at 5 and 20.degree. C.
Acetic acid was added to the emulsion immediately prior to the
addition of sodium hypochlorite to accelerate the gassing at
5.degree. C. and in Experiment 5.2b at 20.degree. C. The
composition of the gasser used in this example is shown below in
Table 9.
TABLE-US-00009 TABLE 9 Gasser solutions for Example 5.2 Mass (g)
Completion Time* Experiment 5.2a (20.degree. C.) Sodium
Hypochlorite 10.5% 4.69 35 min Experiment 5.2b (20.degree. C.)
Sodium Hypochlorite 10.5% 4.68 3.5 min Glacial Acetic Acid 0.26
Experiment 5.2c (5.degree. C.) Sodium Hypochlorite 10.5% 4.68 4 min
Glacial Acetic Acid 0.26 *Completion time is the time taken for the
emulsion to reach 90% of its final density
[0164] FIG. 8 provides a comparison between emulsions of pH 6.25
and pH 5, containing the same concentration of non-diffusing
buffer. It is apparent that the higher pH emulsion has a faster
gassing rate, despite the reduced driving force for proton
transfer, which should in theory make it slower. However, at high
pH, ammonium ions are deprotonated to form ammonia, which is
capable of diffusing from the emulsion phase to the gasser, where
it may react to form nitrogen gas and/or generate protons to allow
hypohalite diffusion. Therefore, when a non-diffusing buffer is
used a high pH emulsion exhibits a faster gassing rate than a low
pH emulsion.
[0165] FIG. 9 is a plot of the results for this example, showing
comparison between experiments with and without a proton transfer
agent. It can be seen that emulsions with a proton transfer agent
added prior to gassing exhibit fast gassing rates, even at
relatively low temperatures. Acetic acid is able to diffuse to the
gasser causing the pH of the gasser to fall. This causes
hypochlorite anions to be protonated to form hypohalous acids,
which can diffuse rapidly to the emulsion and react with hydrazides
(and other nitrogen containing chemicals) to produce nitrogen
gas.
5.3 Hydrazide with Diffusing Buffer/Proton Transfer Agent
[0166] This example demonstrates the performance of an emulsion
containing a hydrazide and diffusing buffer.
[0167] Emulsions H and I containing acetyl hydrazide and sodium
acetate at pH 5.5 were gassed with hypohalite oxidisers at
temperatures ranging from 5.degree. C. to 40.degree. C. Sodium
hypochlorite was used at 5.degree. C., 20.degree. C. and 40.degree.
C., whilst calcium hypochloite and sodium hypobromite were used at
20.degree. C. Additional acetic acid was added to the emulsion in
one of the 5.degree. C. experiments to accelerate the gassing rate.
Table 10 shows the composition of the gasser employed in each
experiment.
TABLE-US-00010 TABLE 10 Gasser solutions for Example 5.3 Mass (g)
Completion Time* Experiment 5.3a (20.degree. C.) Sodium
hypochlorite (10.5%) 4.68 5.5 min Experiment 5.3b (5.degree. C.)
Sodium hypochlorite (10.5%) 4.68 43 min Experiment 5.3c (5.degree.
C.) Sodium hypochlorite (10.5%) 4.68 3.5 min Glacial acetic acid
0.25 Experiment 5.3d (40.degree. C.) Sodium hypochlorite (10.5%),
5.21 5 min 5% NaOH Experiment 5.3e (20.degree. C.) Sodium
hypobromite (10-12%) 7.47 27 min contains 1-5% NaOH Experiment 5.3d
(20.degree. C.) Calcium hypochlorite (70%) 0.64 1 min Distilled
water 5.01 *Completion time is the time taken for the emulsion to
reach 90% of its final density
[0168] The results for these experiments are shown in FIG. 10 and
FIG. 11. The gassing rate of the emulsion was fast with sodium
hypochlorite at 20.degree. C., being complete within 6 minutes.
Little gassing occurred within the first 1.5 min, which allowed the
gasser to be mixed into the emulsion without the possibility of gas
loss. Similar results were obtained at 40.degree. C. utilising 5%
of additional NaOH in the gasser to slow the gassing rate at this
temperature. The gassing rate at 5.degree. C. was considerably
slower taking 43 min to reach completion. However, the addition of
a small amount of acetic acid to the emulsion prior to gassing
enabled a completion time of 3.5 min to be achieved.
[0169] Gassing with sodium hypobromite was also slow at 20.degree.
C., due to the presence of additional NaOH in the gasser, whilst
gassing with calcium hypochlorite was extremely fast, as it has a
relatively low pH. As such, a significant proportion of the
hypochlorite is protonated before the gasser is added to the
emulsion and thus, the hypochlorite may diffuse rapidly from the
gasser producing very fast gassing times.
EXAMPLE 6
[0170] 6.1 Dihydrazide with Non-Diffusing Buffer
[0171] This example demonstrates the oxidation of a compound
containing two hydrazide groups (i.e., a dihydrazide) to generate
gas in an emulsion containing a non-diffusing buffer. The use of a
dihydrazide allows for a smaller amount of substrate to be used
compared to the corresponding mono-hydrazide, as each molecule
contains four available nitrogen atoms rather than two.
[0172] Emulsion J containing succinic dihydrazide and a citrate
buffer at pH 6.25 was used in this experiment. The emulsion was
gassed at 20.degree. C. and 40.degree. C. with sodium hypochlorite,
with 5% additional sodium hydroxide added for the 40.degree. C.
experiment. The composition of the gasser is shown below in Table
11.
TABLE-US-00011 TABLE 11 Gasser solutions for Example 6 Mass (g)
Completion Time* Experiment 6.1a (20.degree. C.) Sodium
hypochlorite solution 10.5% 4.68 20 min Experiment 6.1b (40.degree.
C.) Sodium hypochlorite solution 10.5%, 5.21 10 min 5% NaOH
*Completion time is the time taken for the emulsion to reach 90% of
its final density
[0173] The results are plotted below in FIG. 12. The emulsion
gassed within a reasonable time at both 20 and 40.degree. C., with
completion times of 20 and 10 min respectively in each experiment.
There is little difference between the gassing rate of an emulsion
containing a dihydrazide and that of a mono-hydrazide, making the
use of a dihydrazide an effective way of reducing the amount of
substrate required to produce the desired amount of gas.
EXAMPLE 7
[0174] 7.1 Nitrogen Rich Compound with Diffusing Buffer/Proton
Transfer Agent
[0175] This example demonstrates the oxidation of nitrogen rich
compounds with hypohalite oxidisers to produce nitrogen gas in an
emulsion containing a diffusing buffer/proton transfer agent. A
tetrazole was utilised in this experiment. Tetrazoles require a
smaller amount of oxidiser per mole of gas produced compared to
hydrazides and amines.
[0176] In particular, the example utilised an emulsion explosive
containing 5-aminotetrazole (5AT) and sodium acetate at pH 5.5 to
produce fast and efficient gassing with sodium hypochlorite and
hypobromite. The experiment with sodium hypochlorite was performed
at 20.degree. C. whilst that for sodium hypobromite was performed
at 30.degree. C. The composition of the gassers used in the example
is shown in Table 12.
TABLE-US-00012 TABLE 12 Gasser solutions for Example 7 Mass (g)
Completion Time* Experiment 7a (20.degree. C.) Sodium hypochlorite
solution 10.5% 3.42 18 min Experiment 7b (30.degree. C.) Sodium
hypobromite solution (10-12%), 5.49 5 min 1-5% NaOH *Completion
time is the time taken for the emulsion to reach 90% of its final
density
[0177] The results for this example are shown in FIG. 13. It can be
seen that the use of a tetrazole compound as the source of nitrogen
for hypohalite based chemical gassing allows a significant
reduction in the amount of oxidiser required to produce the desired
density change. It is apparent however that in the experiment with
sodium hypochlorite there was a significant difference in the
theoretical gasser requirement of 1.4 moles of NaOCl per mole of
gas and the observed value of 2.1. This may be attributed to the
oxidation of ammonia, which occurs in a parallel side reaction
requiring a larger amount of oxidiser per mole of gas. Sodium
hypobromite however appears to exhibit a high selectivity toward
the oxidation of 5AT, with just 1.3 moles of oxidiser required per
mole of gas produced, affording a substantial reduction in the
amount of oxidiser required. Table 13 shows a comparison of the
average oxidiser requirement at 20.degree. C. for each nitrogen
compound studied. Data refers to experiments with NaOCl unless
otherwise stated.
TABLE-US-00013 TABLE 13 Oxidiser requirement for different nitrogen
compounds Nitrogen Compound Moles NaOCl/Mole of Gas
Ammonia/Ammonium Ions 5.2 Urea 4.4 Maleic Hydrazide 2.3-2.7 Acetyl
Hydrazide 2.5-2.7 Succinic Dihydrazide 2.7 5-Aminotetrazole 2.1
5-Aminotetrazole (NaOBr) 1.3
Discussion
[0178] Examples 1-7 demonstrate combinations of oxidisers, pH
regulating agents and substrates (e.g., ammonia/ammonium ions,
amines, hydrazides, and high nitrogen compounds) to sensitise
and/or control density of emulsion explosives and gels. Hypohalite
oxidisers such as lithium, sodium and calcium hypochlorites and
hypobromites, along with chloramine T and other oxidisers that
diffuse across fuel lamellae may be deployed to sensitise an
emulsion explosive containing a pH regulating agent such as sodium
acetate. Emulsions can be sensitised rapidly using this method,
with sensitisation times as low as 1 min at room temperature and 5
to 10 min at below 0.degree. C. Ammonium nitrate crystals were
present in emulsion explosives gassed with hypohalites and
chloramine T that did not contain a pH regulator agent.
[0179] Nitrogen containing chemicals including amines, hydrazides,
and high nitrogen compounds such as triazoles and tetrazoles may be
incorporated into the discontinuous phase of the emulsion to reduce
the quantities of oxidisers and pH controlling agents required for
sensitisation and/or density modification to occur. This method may
be implemented with or without a pH regulating agent. However,
sensitisation times observed with small quantities of a pH
regulating agent, such as sodium acetate in the emulsion, were
significantly faster than those without. Similarly, the addition of
a pH-regulating agent to a gassing solution can be used to
influence the rate of gassing. Adding alkaline substances to the
gasser results in slower gassing times, whilst adding acidic
substances results in fast gassing. In addition, if a pH regulating
agent is not present in the emulsion composition, or is
non-diffusing, a suitable agent such as acetic acid may be added to
the emulsion immediately prior to gassing to give fast
sensitisation times. In many instances it is possible to manipulate
the above variables (notably pH of emulsion and gasser, and the use
of proton transfer agents) to provide an initial delay of 1-2 min
or longer between the addition of the gasser and the onset of the
gassing reaction, with fast gassing ensuing from this point. Such a
delay allows the gasser to be safely mixed into the emulsion
without releasing gas from the gassing reaction and avoids
coalescence of gas bubbles reducing bubble size.
[0180] It will be readily apparent to those skilled in the art that
other nitrogen compounds, oxidants and pH regulating agents may be
employed to achieve the desired level of nitrogen gassing.
[0181] Accordingly it will be further understood that numerous
variations and/or modifications may be made to the invention
without departing from the spirit or scope of the invention as
broadly described. The present embodiments are, therefore, to be
considered in all respects as illustrative and not restrictive.
LITERATURE REFERENCES
[0182] 1. 29 CFR 1910.109, Explosives and Blasting Agents, US
Department of Labor, http://www.osha.gov. [0183] 2. Clark P K,
Interpretation of 29 CFR 1910.109 Relative to Peroxides and
Chlorates in Blasting Agents, Slurries, and Emulsions, OSHA, USA,
1991. [0184] 3. da Silva G, Dlugogorski B Z, Kennedy E M (2006) "An
experimental and theoretical study of the nitrosation of ammonia
and thiourea", Chemical Engineering Science 61(10), 3186-3197;
"Reaction and mass transfer effects during the foaming of
concentrated water-in-oil emulsions by the nitrosation of
thiourea", AIChE J 52(4), 1558-1565.
* * * * *
References