CA1081965A - Foamed and thickened explosive compositions having improved stability - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A thickened, aqueous slurry explosive composition is provided which contains a sensitizing amount of finely dispersed entrapped gas bubbles and which retains density and gas bubble stability for extended periods. Use in the com-position as a thickener, of a mixture of an unmodified guar gum and a hydroxypropyl-modified guar gum provides a compo-sition which resists migration or collapse of entrained gas or air even when subjected to shear conditions as in pumping or packaging. The composition may also contain an amount of additional sensitizer material.

Description

lO~ S

This invention relates to gelled or thickened aqueous slurry explosive blasting compositions of improved stability. More particularly, the invention relates to thickened slurry explosives containing sensitizing air or gas bubbles which resist migration or coalescence.
Slurry explosive compositions are usually formulated from water, at least one oxygen-supplying salt, fuel, a thickener and optionally, a thickener crosslinker. The oxygen-supplying salt most frequently used is ammonium nitrate, often accompanied by sodium and/or calcium nitrate, and less often by other nitrate salts, or perchlorates, or chlorates. The ~uel may be liquid or solid particulate, and may be soluble or insoluble in water. Both natural gums and synthetic materials may be employed as thickeners, while borates, chromates and anti-monates are frequently employsd as crosslinking agents.
Slurry explosive compositions may also contain self- ~ `
explosive sensitizers ~uch as particulate TNT which are in-soluble in water. Paint-fine aluminium is also used for sen-sitizing purposes. These sensitizers can contribute all or part of the fuel required for the composition. Slurry explosives also rely frequently for their sensitivity on the presence of small air or gas bubbles within the matrix to create "hot spots"
for shocX initiation. These bubbles reduce the density of the slurry from the unaerated range of 1.4 to 2.0, to as low as 0.5.
The presence and efficiency of air or gas bubbles is particularly important in large-diameter slurries which contain

- 2 -~1)8~965 no self-explosive sensitizer and thus rely nearly completely on the bubbles for generation of initiating reaction sites following passage of the shock from a primer charge, It is believed that there is an optimum size for the bubbles and that they must also be stabilized to resist size changes and loss from the slurry. Bubble loss and coalescence can usually be controlled by achieving a sufficiently high viscosity in the -' slurry using thickening and crosslinking agents. However, the viscosity of a composition does not generally affect the rate of gas transport through the liquid phase of the slurry so bubble sizes can still change by diffusion of dis-solved air or gas in a mechanism known as "Ostwald ripening".
The average size of the bubbles increases with time because of this phenomenom, resulting in lower velocities of detonation and eventually complete failure of the composition to detonate.
A particularly severe condition for bubble stability -~
occurs during the pumping of an aerated explosive into packages ' or boreholes. The high shear generated within the pump will frequently de~ensitize the explosive by coarsening the bubble stru^ture or even causing the bubbles to migrate completelyout of the slurry.
It is often desirable to prepare a prethickened liquor of wataE and some dissolved salts which liquor is used for the manufacture of an explosive slurry by the addition `,~
thereto of other ingredients. This procedure is used to save time during the mixing procedure because hydration , ~ ;

9~5 of the thickener can be slow in a fully formulated slurry.
Natural guar gum is the thickener of choice for slurry explosive liquors and compositions. The thickened liquors are normally stored at their temperature of use which is often above 50C., leading, particularly with natural guar gum, to degradation of the thickening agent and lowering of viscosity with time.
Hence more natural guar thickener may have to be added at inter-vals, which addition is difficult to incorporate efficiently and also frequently inconvenient logistically.
Calcium nitrate-containing compositions present special thickening problems because of the effect of the calcium ion on natural guar gum. In thepresence of large amounts of Ca2~, the rate of hydration of the guar is retarded, and when fully thickened, the viscosity of the solution is very high. This can cause problems in mixing and pumping the slurry. If the thickener level is lowered to accommodate the processing re-quirements, the final gel strength and water resistance after ` ;~
crosslinking is frequently unacceptable. It is therefore par-ticularly important with C~ containing compositions to choose a thickener which can reconcile these conflicting re~uirements.
Problems are also sometimes encountered with the efficient crosslinking of thickened slurries. If crosslinking is too slow with a bulk slurry when pumped into a water-filled borehole, losses of the explosiva into rock fissures or leaching of the soluble ingredients, can occur, thus leading to failure of the explosive. If crosslinking is too fast, it is usuaIly .

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1~8~965 impossible to mix in the crosslinker homogeneously. Some areas of the slurry are not crosslinked, leading to a water leaching problem, while other parts contain concentrated pockets of cross-linker, which can lead to syneresis and chemical degradation of the slurry. The thickener level is often limited in range, being a compromise between ease of mixing and pumping, and ;
having sufficient viscosity to suspend the ingredients and prevent the escape of air bubbles. Also the crosslinker level is often limited in range because sufficient needs to be added to give a satisfactory water resistant gel, but not enough to lead to syneresis. Hence the possibility of being able to select a thickener, or blend of thickeners, to give a convenient cross-linking rate can be most advantageous.
It has been discovered that the use of a blend of natural or unmodified guar and hydroxypropyl-modified guar as the thickening agent in explosive slurries provides many ad-vantages over the use of unmodified guar alone. While the discovery has particularly utility for use in slurry explosives devoid of any self-explosive sensitizer and used in large diameter bulk-loaded boreholes, use of the blended thickeners also has application in slurries containing insoluble explo-sive sensitizers and/or small amounts, up to ten percent by weight, of a soluble sensitizer and used in smaller charges.
Guar gum is classified in chemical terms aq a galactomannan, or a high molecular weight carbohydrate polymer or polysaccharide made up of mannose and galactose units linked , .. :

~(~8~L9~;5 together in the manner shown in the structural formula below.

HO RO _ i I 0~ H / I ~ H
E ~ /~ )H
H H H
O
~H2 CEI20H 1H2 CH20H
~ ~l \ 0 H ~ o ~ ~ 0 H~

With hydroxypropyl-modified guar, R is -CH2CH(CH3)0H, or polymers thereof, and the number of moles per hexose unit of the substituted R may vary from about 0.1 to about 3. The particular hydroxypropyl guar used in the examples hereinafter ;
had a molecular substitution ratio of about 0 4 . The molecular weight of guar gum can vary from about 50,000 to 250,000 depending on the method and conditions of refining and modification.
The following Examples and Tables further illustrate the improved bubble stability of the explosive slurry compo-sitions of the invention. In all examples the proportions of ;ngredients is expressed as percent by weight unless other-wise indicated.

A typical aqueous slurry explosive composition used in large diameter charges was prepared comprising 13% water, 76% salts, 10% fuel comprising: 6% sulphur, 3.5% fuel oil, .
0.5% carbonaceous material, and 0.6% thickener. Four separate ~ -,: .: . ~

~81965 mixes were prepared using various thickener combinations as shown in Table I, below, and each mix was passed through a typical mechanical pump used in slurry operations. The den-sities of the mixes were measured at various time intervals, the : .
results being shown in Table I.

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From the above data it is seen that as little as 1 part in 6 of hydroxypropyl~ guar will stabilize density and bubble structure during pumping The third experiment was particularly severe, since the temperature fell well below the liquor fudge point of 40C.

Eight batches of explosive slurry similar to those described i~ Example 1 were prepared using a 1:1 blend -of straight guar gum and hydroxypropyl-modified guar gum, while eight indentical batches were prepared using straight .
guar gum alone. The sixteen slurries were stored overnight at 50C. in an uncrosslinked state. The results are shown in Table II
TABLE II
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As made After S~oraqe Change Original mean Mean density in bubble density rise structure . .. ..
Blended guar 1.10 0.02 Little change, bubbles slightly laxger but even . distribution maintained Straight guar alone 1.14 0.07 Greater change, bubbles much larger on average and less even distribu-9~s The explosive slurries of Example 2 were cross-linked and stored for 1 week. Six inch diameter cartridges of each slurry type were then detonated at about 5C. un-confined using a 1 lb. pentolite primer. Of the mixed guar cartridges, five detonated with velocities of 3 km/sec. or greater, while only two cartridges made with straight guar alone gave such useful velocities of detonation. The greater reliability of explosive slurries made using a blend of straight guargum and hydroxypropyl-modified guar gums was thus demonstrated.

A typical liquor used in formulating aslurry explosive was prepared comprising 43% by weight ammonium nitrate, 52% by weight calcium nitrate and 5% by weight ~ ~
water. To thicken quantities of this liquor, various guar ~ -gum combinations were added as shown in Table III and the viscosities of the mixture were measured at the various time intervals shown in the Table. The guar thickeners were ~ `
hydrated at 50C. for 1 hour ~nd stored at 70C. After one ~ ~
week the samples were crosslinked by adding 0.5% by weight ~- ; ;
of sodium dichromate at 45C. and the gel strengths were measured using a viscometer at 5 rpm with Helipath stand and 0.195" T-bar - 10 - ' ~::

-~(~8~5 TABLE III

G u a r Viscosity cps Gel Natural Hydroxy- ¦ Strength propyl 1 hour 1 day 7 days . _ . .,1.0% _ 250 86,200 71,000 41.8 0.8% 0,2% 1000 90,500 89,000 66.9 0.5% 0.5% 2700 79,400 87,000 59.2 _ 1.0% 7000 30,600 30,000 _ Examination of the results if Takle III shows that natural guar alone hydrates very slowly and tends to produce lowered viscosity after only 7 days. Gel strength is low. -The use of modified guar alone produces faster hydration but low final viscosity. The blended guaxs produce excellent retention of viscosities and high gel strength.
EX~MPLE 5 The rate of crosslinking of slurries such as are described in Examples 1-3 can be varied by choosing thickened blends having different properties of straight and hydroxy-propyl-modified guar gums. The examples tabulated below were crosslinked using 0.5% of a solution of sodium dichromate (65%) in water (35%, by weight) and the time interval measured until a self-supporting gel was obtained:
TABLE IV

. _ _ . .
Guar Blend Gellinq tim 1.0% straight guar alone 5 1/2 mins.
0.8% " " + 0.2% HP guar 12 1/2 "
0.5% " " + 0.5% " " 20 " ~-- 11 - . , ~ :

~L~819~5 The blended guar thickener used in the slurry explosive compositions of the present invention is present in an amount of from 0.2% to 2 0%, pre~erably from 0 4% to 1.4% by weight of the total slurry composition. The blend comprises from 15 to 85 parts by weight of unmodified guar to 85 to 15 parts by weight of hydroxypropyl modified guar.
A small amount, up to 1.0% by weight of the total composition, of crosslinker may also be present Slurry explosive compo-sitions employing the blended guar thickener may contain up to 76% by weight of inorganic salts, up to 25% by weight of water ~ -up to 30% by weight of sensitizer and up to 40% by weight of fuel. Up to 27% of the inorganic salt may comprise calcium nitrate An aqueous slurry explosive composition typical of the type used in large diameter blasting charges was pre-pared comprising 62.3% ammonium nitrate, 13.4% sodium nitrate, ;
0.3% zinc nitrate, 13.2% water, 3.5% fuel oil, 6.0% sulphur, 0.5% sodium lignosulphonate, 0.6% thickener, 0.060/Oferrous sulphate and 0.14% sodium nitrite (gassing agent), all quanti-ties being percent by weight. This composition containing various thickener combinations as shown in Table V below, was passed through a L.5 cm diameter x 25 cm long nozzle at the end of a laboratory scale blowcase having 10 cm bore. Before each pass the blowcase was filled with the slurry. Blowcase extrusions simulate the shear effect on slurry when it is pump-ed through hoses to fill boreholes or during cartridging operations. The results are recorded in Table V below.

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~1965 The results in Table V demonstrate that slurry explo-sives thickened by means of a blend of unmodified guar and hydroxypropyl modified guar maintained density and bubble structure and also showed reduced bubble growth when compared to other thickening systems. It can be noted that the use of unmodi~ied guar alone or with starch provided little density or bubble stability. In all cases, viscosities were measured using Brookfield RVT viscometer with No, 6 spindle at 20 rpm.

A slurry explosive composition similar to that of Example 6 but devoid of sulphur was manufactured using standard ~-slurry mixing procedures. Various batches containing different ~
, thickener combinations were examined and fired after periods ~-of storage. The results are recorded in Table VI, below: `

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--` lL0~31965 The results shown in Table VI demonstrate that the use of mixed guars provide good density and bubble sta-bility when stored uncrosslinked at 50C. It can be noted that the sample containing only unmodified guar failed to shoot after 8 days storage, due chiefly to poor bubble sta-bility and a resultant loss in sensitivity.
To demonstrate the improved effect of the use of a mixed guar thickening system in slurry explosive composi-tions also containing sensitizing material including self-explosive sensitizers, a series of sensitizer-containing compositions were prepared and tested. Suitable additional sensiti~ers compatible with the mixed thickener system of the invention comprise, for example, particulate organic -explosives such as PETN, T~T and the like, particulate light metals such as air atomized aluminium, organic nitrates, such as alkanolamine nitrates and lower alkyl-amine nitrates. Generally from 10% to 30% of sensitizer -~
may be employed. ~ -Two TNT-sensitized compositions were prepared having the following ingredi~nt mix, the quantities shown being percent by weight:

Composition Composition "A" "B"
Ammonium nitrate 44.3 44.8 Sodium nitrate 8.7 8.8 Zinc nitrate 0.3 0.3 Water 14.0 14.1 Pelleted TNT 20.0 20.0 Atomized aluminium lO.0 10.0 Ammonium lignosulphate 0.5 ': ; , - : .

1~8196S

Composition Composition "A" "B"
_ Thickener 0,5 0,5 Ethylene glycol 1.6 1,6 Sodium nitrite 0.1 0.1 :~
Both compositions were tested in blowcase tests as detailed in Exampl.e 6. The results are recorded in :
Table VII, below~

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, From Table VII it can be seen that the use of the mixed guar thickener system provides particular stability and resistance to shear (during extrusion) as evidenced by density and bubble structure results It can be noted that Composition B, devoid of any lignosulphonate had somewhat poorer bu`bble stability after blowcasing; however, the use of mixed guars (Test 4) even in the absence of lignosulphonate retained stability.

The following slurry explosive composition useful in large diameter blasting charges was prepared by standard mixing methods, tha quantities shown being percent by weight:
Ammonium nitrate 56.9~!
Calcium nitrate 23.0 Zinc nitrate 0.3 Water 9.8 ~
Ammonium lignosulphonate 0.5 ~.
Hydroxypropyl guar 0 2 Unmodified guar 0.3 Starch 0.5 ~ -Fuel oil 5.2 Ferrous sulphate 0.06 .
Atomized aluminium 3.0 ;:~
Sodium nitrite 0.14 Sodium dichromate 0.1 This composition additionally containing two different types of low viscosity guar as shown in Table VIII below, were subjected to blowcase testing as described heretofore. The ~.
results are given in Table VIII, below: . :

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'' ; ,, ' . , ' ' ', . ~, , T A B L E VIII
, Thickener /O/W Test 1 ~ Test 2 .
Unmodified Guar 0.2 _ Hydroxypropyl Guar _ O 2 (low viscosity) Blowcase (Viscosity at start 31,000 cps 33,500 cps experiment(Temperature 10 passes ( C at start 44 50 (Density at start 1.00 1.03 (Bubble structure ( at start Fine, even Fine, even (Temperature C
(after 10 passes 35 37 (Density after ( lO passes 1 03 l.01 (Bubble structure (after 10 passes Coarse, few Unchanged from fine bubbles, before blow-many large casing bubbles ca 300J~
. _ . _ _ ~, .
After storage (Density 1.10 1 08 for 12 days ( at ambient, (Shooting at DET with 40 g. DET with 9 g crosslinked (18-19C pentolite at Detaprime at slurry in 3" ( -3.8 km/s 3 8 km/s diameter ( Fail with 20 g Fail with 5 g cartridges ( pentolite Detaprime (Shooting at (5C Fail with DET with 80 g ( 160 pentolite pentolite at -( 3.5 km/s ( Fail with 40 g ( pentolite . . _ .. __ .
*Brookfield RVT viscometer, spindle No. 6 at 20 rpm.
From Table VIII, Test 1, it is seen that bubble structure is substantially coarsened in the absence of modified guar Also, sensitivity after storage is improved when some modi~ied guar is employed.

.

f;5 A slurry explosive composition sensitized with ethanolamine nitrate, a soluble organic explosive salt, was prepared using standard mixing procedures and comprising the following ingredients, the amounts shown being in percent by weight. ~:
Ammonium nitrate 63.8 Sodium nitrate 13.0 Zinc nitrate 0.3 Ethanolamine nitrate 6.0 Water 11.1 Fuel oil 4.6 Ammonium lignosulphonate0.5 ~ :
Thickener 0.5 Sodium nitrite 0.1 Sodium dichromate 0.1 Samples of the composition containing different ~ :
thickening systems were blowcase tested as described hereto-fore. The results are shown in Table IX, below:

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9~5 From the results in Table IX, it is seen that in the absence of a mixed guars thickening system, density was unstable. After storage, a density rise occurred in the absence of any hydroxypropyl guar, demonstrating an un-stable bubble structure.

A slurry explosive composition sensitized by means of finely divided aluminium was prepared using standard procedures and comprised the following ingredients, the amounts shown being percent by weight:
Ammonium nitrate 65 0 Sodium nitrate 7.6 Water 14.0 Sodium acetate (buffer) 0.1 Gilsonite 1.1 Atomized aluminium 3.8 Paint-fine aluminium 3.8 Ethylene glycol 3.8 Thickener 0.8 Pot. pyroantimonate 0.04 Samples of the composition containing different thickener systems were subjec-ted to blowcase testing as described heretofore. The results are given in Table X, below:

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X, , ~ ,, ,1 ~ ~ o ~ ~ ~ .
1~ ¦ ~J ~ I ~ `1 ~1 t~ E~l N

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.,1 ~ In . ~ ~ ~ I h ~ C) u u~ ~ a O . `.
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h h ~ ,1 ~ ~9 ~ h ~ o .` ` .
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_I h h h 1~ ~q ~ O o . - -O ~ ~ O O O O ~ ~
,Y ~4 4~ ~ ~ ~ ~ ~ 0 u~ ..
U ~ o a) R ~ ,~
E~ ~ ~ ~ ~ ~ ~ .,J ~q ~1 X
O ~ 1 ~ U ~
Q h 0 ~ n ~n ~ ~ 3 rl :, ~3 ~ S~ O ~1 ~a O Q
~ au o a) a~ ~ l ' P m ~ ~ a ~ O ~I h ,~
_. _ , _ . :
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~O~lg65 The results in Table X show improved stability of the density of the compositions with the use of a mixed guar thickener system.

DONALD G. BALLANTYNE
PATENT AGENT

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Claims (10)

The embodiments of the invention in which exclusive property or privilege is claimed are defined as follows:

1. In a slurry explosive composition containing water, inorganic oxidizer salt, fuel, thichener, and a sen-sitizing amount of finely dispersed entrapped gas bubbles, the improvement comprising a thickener consisting essentially of a combination of unmodified guar gum and hydroxypropyl-modified guar gum, which thickener provides density stability through the control of gas bubble migration and collapse.

2. An explosive composition as claimed in Claim 1 also containing an additional sensitizer selected from parti-culate organic explosives, finely divided light metal, alkanol-amine nitrate and lower alkylamine nitrates.

3. An explosive composition as claimed in Claim 1 also containing up to 10% by weight of a soluble organic nitrate sensitizer.

4. An explosive composition as claimed in Claim 1 wherein the thickener comprises from 0.2% to 2.0% by weight of the total composition in the ratio of from 15 to 85 parts by weight of unmodified guar to 85 to 15 parts by weight of hydroxypropyl-modified guar.

5. An explosive composition as claimed in Claim 4 wherein the thickener comprises from 0.4% to 1.6% by weight of the total composition.

6. A thickened and cross-linked water-bearing explosive slurry composition comprising up to 76% by weight of inorganic oxygen-supplying salt, up to 25% by weight of water, from 10% to 31% by weight of soluble organic nitrate sensitizer, up to 40% by weight of fuel, from 0.2% to 2.0%
by weight of a thickener comprising a mixture of from 15 to 85 parts by weight of unmodified guar gum to 85 to 15 parts by weight of hydroxypropyl-modified guar gum and up to 1.0% by weight of thickener cross-linker.

7. An explosive composition as claimed in Claim 6 wherein up to 27% of the total amount of inorganic oxygen-supplying salt comprises calcium nitrate.

8. An explosive composition as claimed in Claim 2 wherein the particulate organic sensitizer is trinitrotoluene.

9. An explosive composition as claimed in Claim 2 wherein the light metal is alumnium.

10. An explosive composition as claimed in Claim 2 wherein the alkanolamine nitrate is ethanolamine nitrate,