Method of reducing nitrogen oxide fumes in blasting

This invention relates to a method of reducing the formation of toxic nitrogen oxides in after-blast fumes by using an emulsion blasting agent that has an appreciable amount of urea in its discontinuous oxidizer salt phase.

The present invention relates to an improved method of blasting with 
water-in-oil emulsion blasting agents (hereafter referred to as "emulsion 
blasting agents"). More particularly, the invention relates to a method of 
reducing the formation of toxic nitrogen oxides (NO.sub.x) in after-blast 
fumes by using an emulsion blasting agent that has an appreciable amount 
of urea in its discontinuous oxidizer salt solution phase. 
The emulsion blasting agent used in the method of the present invention 
comprises a water-immiscible organic fuel as a continuous phase, an 
emulsified inorganic oxidizer salt solution as a discontinuous phase, an 
emulsifier, gas bubbles or an air entraining agent for sensitization, and 
urea in an amount from about 1% to about 30% by weight of the composition 
for reducing the amount of nitrogen oxides formed in after-blast fumes. 
BACKGROUND OF THE INVENTION 
Emulsion blasting agents are well-known in the art. They are fluid when 
formed (and can be designed to remain fluid at temperatures of use) and 
are used in both packaged and bulk forms. They commonly are mixed with 
ammonium nitrate prills and/or ANFO to form a "heavy ANFO" product, having 
higher energy and, depending on the ratios of components, better water 
resistance than ANFO. Such emulsions normally are reduced in density by 
the addition of air voids in the form of hollow microspheres, other solid 
air entraining agents or gas bubbles, which materially sensitize the 
emulsion to detonation. A uniform, stable dispersion of the air entraining 
agent or gas bubbles is important to the detonation properties of the 
emulsion. Gas bubbles, if present, normally are produced by the reaction 
of chemical gassing agents. Sensitization also can be obtained by 
incorporating porous AN prills. 
A problem associated with the use of emulsion blasting agents in mining 
blasting operations is the formation of nitrogen oxides, a yellow 
orange-colored smoke, in the gasses produced by the detonation of the 
emulsion blasting agent. These gasses will be referred to herein as 
"after-blast fumes." Not only is the formation of nitrogen oxides a 
problem from the standpoint that such fumes are toxic but also these fumes 
are visually and aesthetically undesirable due to their yellow/orange 
color. Many efforts have been made to eliminate or reduce the formation of 
such fumes. These efforts typically have been directed at improving the 
quality of the emulsion blasting agent and its ingredients to enhance the 
reactivity of the ingredients upon initiation. Other efforts have focused 
on improving blast pattern designs and initiation schemes. Still other 
efforts have focused on improving the borehole environment by dewatering 
or using a more water resistant emulsion blasting agent. 
It surprisingly has been found in the present invention that the formation 
of nitrogen oxide fumes can be reduced considerably by adding urea, in an 
amount from about 5% to about 30%, by weight of the composition, to the 
oxidizer salt solution discontinuous phase of the emulsion or in dry form 
or both. The urea apparently reacts chemically with any nitrogen oxides 
that may form as products of the detonation reaction to convert such 
oxides to nitrogen (N.sub.2), water and carbon dioxide. 
Additional advantages are realized by using urea to reduce nitrogen oxides 
in after-blast fumes. The use of urea in the oxidizer salt solution has 
been found to increase the critical diameter of the resulting emulsion 
blasting agent. Consequently, the emulsion blasting agent is more 
compatible (less reactive) with down-hole detonating cord that otherwise 
can cause a pre-detonation reaction to occur when the detonating cord is 
initiated. (The detonating cord leads to a booster located in the bottom 
of the borehole or a series of boosters spaced within the explosives 
column.) This pre-reaction itself can contribute to the formation of 
nitrogen oxides in after-blast fumes. 
Another advantage is that the cost of using urea is considerably less than 
the costs of using plastic microballoons or sensitizing aluminum 
particles, which both have been used previously in an effort to improve 
the quality or reactivity of the emulsion blasting agent and its 
ingredients. Moreover, urea is more effective in chemically reducing 
nitrogen oxide after-blast fumes than these more costly alternatives. 
By using urea, which is a fuel, in the oxidizer salt solution, less organic 
fuel can be used in the continuous organic fuel phase to achieve oxygen 
balance, particularly in emulsion blends containing ANFO or AN prills. 
This also appears to contribute to the reduction of after-blast nitrogen 
oxide fumes. Another advantage is that urea can extend or replace some or 
all of the water required in the oxidizer salt solution to result in a 
more energetic blasting agent. 
Urea has been used or suggested for use in water-bearing blasting agents of 
the emulsion or water-gel type and in ANFO blasting agents. For example, 
U.S. Pat. No. 5,159,153 discloses the use of urea in the oxidizer salt 
solution phase of an emulsion blasting agent for purposes of stabilizing 
the blasting agent against thermal degradation in the presence of reactive 
sulfide and pyrite ores. U.S. Pat. No. 4,338,146 discloses the use of urea 
as an additive in a cap-sensitive emulsion explosive in an amount of less 
than 5% by weight. U.S. Pat. No. 4,500,369 discloses the use of urea in an 
emulsion blasting agent to lower its crystallization temperature. U.S. 
Pat. No. 3,708,356 discloses the use of urea to stabilize ANFO against 
reaction with pyrite ores. These patents do not suggest, however, the use 
of urea for the purposes described herein. 
SUMMARY OF THE INVENTION 
The invention comprises a method of reducing the formation of nitrogen 
oxides in after-blast fumes resulting from the detonation of an emulsion 
blasting agent. The method comprises using an emulsion blasting agent 
having an emulsifier; a continuous organic fuel phase; and a discontinuous 
oxidizer salt solution phase that comprises inorganic oxidizer salt, water 
or a water-miscible liquid and urea present in an amount from about 5% to 
about 30% by weight of the agent. This method particularly works well with 
blasting patterns that use detonating cord downlines in blasting areas 
that are susceptible to NO.sub.x formation and also provides a way to 
reduce the amount of water (that does not contribute energy to the 
blasting agent) and organic fuel (which may increase the formation of 
nitrogen oxides) required in the blasting agent composition.

DETAILED DESCRIPTION OF THE INVENTION 
As indicated above the addition of urea to an emulsion blasting agent, by 
adding it to the oxidizer salt solution phase thereof or as a dry 
ingredient or both, significantly reduces the amount of nitrogen oxides 
formed in the detonation reaction between the oxidizer and fuel in the 
blasting agent. Apparently, the urea reacts with any nitrogen oxides that 
formed to convert them to N.sub.2, H.sub.2 O, and CO.sub.2 according to 
the following reaction: 
EQU urea.fwdarw..NH.sub.2 +.NCO 
EQU .NH.sub.2 +NO.fwdarw.N.sub.2 +H.sub.2 O 
EQU .NCO+NO.fwdarw.N.sub.2 +CO.sub.2 
Further, as mentioned, the urea-containing emulsion blasting agent also is 
less pre-detonation reactive to detonation cord downline, and this helps 
further reduce the amount of nitrogen oxides formed. Preferably the urea 
is dissolved in the oxidizer salt solution prior to the formation of the 
emulsion blasting agent, although it could be added separately to the 
emulsion blasting agent in a powder or prill form. As low as about 5% 
dissolved or dispersed urea can have a dramatic effect on nitrogen oxide 
reduction. In practice, larger amounts are advantageous and urea levels up 
to about 30% are feasible. The degree of effectiveness generally is 
proportional to the amount of urea employed. However, for reasons of 
optimizing oxygen balance, energy and effectiveness, the preferred range 
is from about 5 to about 20% urea. 
The immiscible organic fuel forming the continuous phase of the composition 
is present in an amount of from about 3% to about 12% and preferably in an 
amount of from about 3% to less than about 7% by weight of the 
composition, depending upon the amount of ANFO or AN prills used, if any. 
The actual amount used can be varied depending upon the particular 
immiscible fuel(s) used, upon the presence of other fuels, if any, and the 
amount of urea used. The immiscible organic fuels can be aliphatic, 
alicyclic, and/or aromatic and can be saturated and/or unsaturated, so 
long as they are liquid at the formulation temperature. Preferred fuels 
include tall oil, mineral oil, waxes, paraffin oils, benzene, toluene, 
xylenes, mixtures of liquid hydrocarbons generally referred to as 
petroleum distillates such as gasoline, kerosene and diesel fuels, and 
vegetable oils such as corn oil, cotton seed oil, peanut oil, and soybean 
oil. Particularly preferred liquid fuels are mineral oil, No. 2 fuel oil, 
paraffin waxes, microcrystalline waxes, and mixtures thereof. Aliphatic 
and aromatic nitrocompounds and chlorinated hydrocarbons also can be used. 
Mixtures of any of the above can be used. 
The emulsifiers for use in the present invention can be selected from those 
conventionally employed, and are used generally in an amount of from about 
0.2% to about 5%. Typical emulsifiers include sorbitan fatty esters, 
glycol esters, substituted oxazolines, alkylamines or their salts, 
derivatives thereof and the like. More recently, certain polymeric 
emulsifiers, such as a bis-alkanolamine or bis-polyol derivative of a 
bis-carboxylated or anhydride derivatized olefinic or vinyl addition 
polymer, have been found to impart better stability to emulsions under 
certain conditions. 
Optionally, and in addition to the immiscible liquid organic fuel and the 
urea, solid or other liquid fuels or both can be employed in selected 
amounts. Examples of solid fuels which can be used are finely divided 
aluminum particles; finely divided carbonaceous materials such as 
gilsonite or coal; finely divided vegetable grain such as wheat; and 
sulfur. Miscible liquid fuels, also functioning as liquid extenders, are 
listed below. These additional solid and/or liquid fuels can be added 
generally in amounts ranging up to about 25% by weight. 
The inorganic oxidizer salt solution forming the discontinuous phase of the 
explosive generally comprises inorganic oxidizer salt, in an amount from 
about 45% to about 95% by weight of the total composition, and water 
and/or water-miscible organic liquids, in an amount of from about 0% to 
about 30%. The oxidizer salt preferably is primarily ammonium nitrate, but 
other salts may be used in amounts up to about 50%. The other oxidizer 
salts are selected from the group consisting of ammonium, alkali and 
alkaline earth metal nitrates, chlorates and perchlorates. Of these, 
sodium nitrate (SN) and calcium nitrate (CN) are preferred. When higher 
levels of urea, 10-15% by weight or more, are dissolved in the oxidizer 
solution phase, solid oxidizer preferably should be added to the formed 
emulsion to obtain optimal oxygen balance and hence energy. The solid 
oxidizers can be selected from the group above listed. Of the nitrate 
salts, ammonium nitrate prills are preferred. Preferably, from about 20% 
to about 50% solid ammonium nitrate prills (or ANFO) are used, although as 
much as 80% is possible. 
Water preferably is employed in amounts of from about 1% to about 30% by 
weight based on the total composition. It is commonly employed in 
emulsions in an amount of from about 9% to about 20%, although emulsions 
can be formulated that are essentially devoid of water. With higher levels 
of urea, such as 15% or more, the compositions can be made anhydrous. 
Water-miscible organic liquids can at least partially replace water as a 
solvent for the salts, and such liquids also function as a fuel for the 
composition. Moreover, certain organic compounds also reduce the 
crystallization temperature of the oxidizer salts in solution. Miscible 
solid or liquid fuels in addition to urea, already described, can include 
alcohols such as sugars and methyl alcohol, glycols such as ethylene 
glycols, amides such as formamide, amines, amine nitrates, and analogous 
nitrogen-containing fuels. As is well known in the art, the amount and 
type of water-miscible liquid(s) or solid(s) used can vary according to 
desired physical properties. As already explained it is a particular 
advantage of this invention that substantial urea lowers the 
crystallization point of the oxidizer solution. 
Chemical gassing agents preferably comprise sodium nitrite, that reacts 
chemically in the composition to produce gas bubbles, and a gassing 
accelerator such as thiourea, to accelerate the decomposition process. A 
sodium nitrite/thiourea combination produces gas bubbles immediately upon 
addition of the nitrite to the oxidizer solution containing the thiourea, 
which solution preferably has a pH of about 5.5. The nitrite is added as a 
diluted aqueous solution in an amount of from less than 0.1% to about 0.4% 
by weight, and the thiourea or other accelerator is added in a similar 
amount to the oxidizer solution. In addition to or in lieu of chemical 
gassing agents, hollow spheres or particles made from glass, plastic or 
perlite may be added to provide density reduction. 
The emulsion of the present invention may be formulated in a conventional 
manner. Typically, the oxidizer salt(s), urea and other aqueous soluble 
constituents first are dissolved in the water (or aqueous solution of 
water and miscible liquid fuel) at an elevated temperature or from about 
25.degree. C. to about 90.degree. C. or higher, depending upon the 
crystallization temperature of the salt solution. The aqueous solution, 
which may contain a gassing accelerator, then is added to a solution of 
the emulsifier and the immiscible liquid organic fuel, which solutions 
preferably are at the same elevated temperature, and the resulting mixture 
is stirred with sufficient vigor to produce an emulsion of the aqueous 
solution in a continuous liquid hydrocarbon fuel phase. Usually this can 
be accomplished essentially instantaneously with rapid stirring. (The 
compositions also can be prepared by adding the liquid organic to the 
aqueous solution). Stirring should be continued until the formulation is 
uniform. When gassing is desired, which could be immediately after the 
emulsion is formed or up to several months thereafter when it has cooled 
to ambient or lower temperatures, the gassing agent and other advantageous 
trace additives are added and mixed homogeneously throughout the emulsion 
to produce uniform gassing at the desired rate. The solid ingredients, if 
any, can be added along with the gassing agent and/or trace additives and 
stirred throughout the formulation by conventional means. Further handling 
should quickly follow the addition of the gassing agent, depending upon 
the gassing rate, to prevent loss or coalescence of gas bubbles. The 
formulation process also can be accomplished in a continuous manner as is 
known in the art. 
Reference to the following tables further illustrates this invention. 
It has been found to be advantageous to pre-dissolve the emulsifier in the 
liquid organic fuel prior to adding the organic fuel to the aqueous 
solution. This method allows the emulsion to form quickly and with minimum 
agitation. However, the emulsifier may be added separately as a third 
component if desired. 
Table I contains a comparison of two emulsion blasting agent compositions. 
Example A contains no urea and Example B is similar to Example A except 
that Example B contains 6.59% urea by weight. The urea-containing 
composition, Example B, had a much higher minimum booster (MB) but also a 
higher detonation velocity (D). Example A also contained an additional 
1.3% fuel oil since no urea was present. The total water content in 
Example A is 12.86%, compared to 9.86% in Example B. 
Table II compares theoretical energy and gas volume calculations of the 
examples in Table I. This table shows that urea has sufficient fuel value 
to eliminate part of the fuel oil in Example A. 
Table III compares the detonation and fume results of Examples A & B from 
Table I, both with and without the presence of detonating cord downline. 
In all instances, the examples were tested underwater in 150 mm PVC pipe. 
The fume production from both examples without detonating cord was good, 
with Example A producing a wisp of yellow/orange smoke indicating the 
presence of nitrogen oxides. Example B produced no observable nitrogen 
oxide fumes. The differences were more dramatic when the examples were 
initiated with 25 grain detonating cord downline that led to a primer in 
the bottom of the PVC pipe. Example B, which contained urea, demonstrated 
a significant reduction in after-blast nitrogen oxide (yellow/orange) 
fumes. The qualitative smoke rating ranges from 0 (no observable fumes) to 
5 (heavy, pronounced yellow/orange smoke). 
Table IV provides further comparative examples. Table V shows a composition 
having a higher level of urea, and this composition shot well in a field 
application, producing good energy with no observed post-blast nitrogen 
oxide fumes. 
While the present invention has been described with reference to certain 
illustrative examples and preferred embodiments, various modifications 
will be apparent to those skilled in the art and any such modifications 
are intended to be within the scope of the invention as set forth in the 
appended claims. 
TABLE I 
______________________________________ 
A B 
______________________________________ 
Oxidizer Solution 1 63.8 -- 
Oxidizer Solution 2 -- 65.9 
Fuel Solution 4.8 4.0 
AN Prills 30.0 30.0 
Fuel Oil 1.3 -- 
Gassing Agent 0.1 0.1 
Results at 5.degree. C. 
Density (g/cc) 1.18 1.20 
D, 150 mm (km/sec) 4.5 5.5 
125 mm 4.4 5.5 
100 mm 4.1 4.9 
75 mm 3.7 3.3 
MB, 150 mm, Det/Fail (g) 
4.5/2.0 18/9 
______________________________________ 
Gassing 
Oxidizer Solution 1 
AN NHCN.sup.1 
H.sub.2 O 
Agent HNO.sub.3 
______________________________________ 
66.8 15.0 17.9 0.2 0.1 
Fudge Point: 57.degree. C. 
Specific Gravity: 1.42 
pH: 3.73 at 73.degree. C. 
______________________________________ 
Gassing 
Oxidizer Solution 2 
AN Urea H.sub.2 O 
Agent HNO.sub.3 
______________________________________ 
74.7 10.0 15.0 0.2 0.1 
Fudge Point: 54.degree. C. 
Specific Gravity: 1.36 
pH: 3.80 at 73.degree. C. 
______________________________________ 
Fuel Solution 
SMO Mineral Oil 
Fuel Oil 
______________________________________ 
16 42 42 
Temperature: 60.degree. C. 
______________________________________ 
.sup.1 Norsk Hydro CN: 79/6/15: CM/AN/H.sub.2 O 
TABLE II 
______________________________________ 
A B 
______________________________________ 
AN 42.62 49.24 
NHCN 9.57 -- 
Urea -- 6.59 
Water 11.42 9.86 
Gassing Agent 0.12 0.14 
Nitric Acid 0.06 0.07 
SMO 0.77 0.64 
FO 2.02 1.68 
Mineral Oil 2.02 1.68 
AN Prills 30.00 30.00 
FO 1.30 -- 
Oxygen Balance (%) -1.49 -2.32 
N (Moles Gas/kg) 42.35 44.26 
Q Total (kcal/kg) 734 698 
Q Gas (kcal/kg) 701 689 
Q Solid (kcal/kg) 34 8 
Q/880 0.83 0.79 
A (kcal/kg) 729 697 
A/830 0.88 0.84 
______________________________________ 
TABLE III 
______________________________________ 
A B 
______________________________________ 
Results at 25.degree. C. 
4.7 5.0 
D, 150 mm PVC (km/sec) 
4.5 4.9 
4.7 5.0 
Smoke Rating 0-0.5 0 
0-0.5 0 
0-0.5 0 
D, 150 mm PVC (km/sec) 
4.1 4.8 
25 Grain Cord Traced 4.0 4.5 
-- 4.9 
Smoke Rating 3 0-0.5 
3 1 
3 0.5 
______________________________________ 
TABLE IV 
______________________________________ 
A B 
______________________________________ 
AN 37.48 32.85 
H.sub.2 O 8.80 5.56 
Urea -- 7.87 
Emulsifier 0.66 0.66 
Mineral Oil 0.33 0.33 
Fuel Oil 2.28 2.28 
K15 Microballoons 0.45 0.45 
ANFO 50.00 -- 
AN Prills -- 50.00 
Oxygen balance (%) -3.89 -0.54 
N (moles/kg) 43.81 43.65 
Q Total (kcal/kg) 756 742 
D, 150 mm (km/sec) 3.5 3.4 
3.6 3.3 
3.4 3.4 
3.7 3.5 
3.5 3.3 
Smoke Rating 5 1 
5 1 
5 1 
5 1 
5 1 
______________________________________ 
TABLE V 
______________________________________ 
AN 34.15 
H.sub.2 O 6.46 
Urea 14.54 (9.00 as Dry Additive) 
Emulsifier 0.54 
Mineral Oil 0.70 
Fuel Oil 2.11 
K15 Microballoons 
0.50 
AN prills 40.00 
Added Fuel Oil 1.00 
Oxygen balance (%) 
-10.82 
N (moles/kg) 43.45 
Q Total (kcal/kg) 
645 
______________________________________