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Influence of an emulsifier on the pressure desensitization of emulsion explosives
Mineral
Journal of University of Science and Technology Beijing Volume 13, Number 2, April 2006, Page 102
Influence of an emulsifier on the pressure desensitization of emulsion explosives Yinjun Wangl-”,Xuguang Wang”,and Shilong Yan3’ 1) Civil & Environmental Engineering School. University of Science and Technology Beijing. Beijing 100083, China
2) Beijing General Research Institute of Mining and Metallurgy, Beijing 1ooo44, China 3) Chemical Engineering Department. Anhui University of Science and Technology. Anhui 232001, China (Received 2005-04-06)
Abstract: The desensitization degree of emulsion explosives (EE) was calculated with the peak pressure of explosion shock waves tested in water. To an explosive, the less the desensitization degree, the better the compression resistance, so the compression resistance of an explosive can be compared and analyzed quantificationally with the desensitization degree. The influence of an emulsifier on the pressure desensitization of EE was studied, including the content and category of emulsifiers. Three kinds of emulsifiers (Span-80, compound emulsifier, and T-152) were used in the tests. The experimental results show that both the content and category of emulsifiers make a great effect on the pressure desensitization of EE. The desensitization degree of EE reduces with the emulsifier content being increased, but there is an optimal content of an emulsifier for the compression resistance of EE. While the content of Span-80 reaches 4wt%, the desensitization degree of EE becomes a minimal value, and augments somewhat if the emulsifier content is increased more. That is to say, the compression resistance of EE becomes the highest while the content of Span-80 is 4wt%, and the compression resistance will decline if the content of Span-80 is increased more. The compression resistance of the explosive emulsified by compound emulsifier is the highest among all the explosives, when the content of the whole components and manufacturing engineering are kept invariable. Key words: emulsion explosive; emulsifier; pressure desensitization; shock wave; compression resistance
[This work was,financiall~ supported by the National Natural Science Foundation of China (N0.50574004).]
1. Introduction During millisecond blasting, shock waves or stress waves generated by the first initiated charges will propagate to the delay charges and make a shock pressure on them just before they are initiated. If the blasthole spaces between the adjacent rows are close or the compression resistance of an explosive is low, it is possible that the explosive power of delay charges descends. This phenomenon is called pressure desensitization, always in two forms of half-fire and misfire. As soon as it happens, it brings great harm to blast work. For example, it always delays a project, lowers blasting effect and is also a safety hazard. So it is necessary to study the pressure desensitization of an emulsion explosive. Toshio Matsuzawa er al. studied the explosive property of an emulsion explosive subjected to dynamic pressure, and it shows that bubbles in the explosive as hot-spots make a great effect during the initiating explosives [ I]. S.L. Nie [2] performed a numerical simulation on the process of volume recovering of pressed Corresponding author: Yinjun Wang. E-mail: yjwang0281@ 163.com
bubbles in emulsion explosives. The relation of desensitization of water gel or emulsion explosive to delay time was studied [3]. Two concepts of critical deadpressing space and critical space were put forward based on experiments and practices [4]. It was introduced in both U.S. patent [5] and in other studies [6] that the compression resistance of an emulsion explosive could be improved if the emulsifier content was increased. The influence of an emulsifier on the compression resistance of a mine permitted emulsion explosive was studied [7]. In this paper, the influence of emulsifier’s content and category on the pressure desensitization of emulsion explosives was studied quantificationally and the mechanism of pressure desensitization was analyzed.
2. Experimental method The experiments were performed in a cistern with a diameter of 5.5 m and a water depth of 3.62 m, shown in Fig. I(a). An explosive charge was placed under water at 2/3 of the water depth in axes of the cistern,
Y.J. Wang et al., Influence of an emulsifier on the pressure desensitization of emulsion explosives
which can eliminate the boundary effect of water surface and bottom.
103
tiated at the time of about 10 min after a primary charge exploded. The intensity of shock waves of a primary charge was adjusted by changing the space between the primary and secondary charges. Experimental details were presented in reference [ 111.
3. Test results and analysis 3.1. Test results
Fig. 1. Photographs of the experimental apparatus: (a) the experimental cistern; (b) primary and secondary charges fixed together in an iron stand; (c) a secondary charge with a cap in its center fued in the iron stand in front of a pressure sensor.
A primary charge was made of 3 g RDX, and a secondary charge was made of 10 g emulsion explosive, initiated by a No.8 industrial cap respectively. The two charges were fixed in the middle of an iron stand, with their cores in the same horizontal line (shown in Fig. l(b)). Then the iron stand was placed in the cistern, and the primary charge was initiated. The secondary charge, which was already impacted by shock waves of the primary charge, was taken out and a No.8 cap was inserted in it. It was retightened on the iron stand (shown in Fig. l(c)) and put into the cistern again to be initiated. At a distance of 20 cm from the secondary charge, a pressure sensor caught pressure signals while the secondary charge exploded in water. The period from the moment when a primary charge was initiated to that a secondary charge exploded is defined to be the waiting time, which is the same for every secondary charge with each other. And it was about 10 min and measured by a stopwatch in the experiments, that is to say, the secondary charge was ini-
Two groups of emulsion explosives were used in the experiment, including four kinds in the first group and two kinds in the second. Components of the matrix (except emulsifier) are the oxidizer, water, and the reductant. The content of them is 83.5wt%, 11wt%, 3.5wt% respectively. After the emulsion matrix is completed, the hollow glass microballoons (GMB) are mixed with it while its temperature lowers to about 5O-6O0C, and the content of GMB is 2.5wt%. With the content of other components and manufacturing engineering keeping invariable, only the content of Span-80 in the four explosives of the first group was increased, and it was 2wt%, 3wt%, 4wt%, 5wt% respectively, and the corresponding serial number of the explosives was 1#, 2', 3#, 4'. The matrix components of the two explosives in the second group were nearly the same with 2' explosive, the only difference was the emulsifier category, and the corresponding serial number was 5#, 6'. Actually, 2#, 5' and 6' explosives were emulsified'by Span80, compound emulsifier, and T- 152, respectively. Five space points for every explosive were chosen for testing, and three tests at every space point were performed. The average value of the three test results was used to analyze the desensitization of a secondary charge compressed by shock waves at the point. One of the most important parameters of explosion shock waves is peak pressure, and it is also gauged easily, so it was used to analyze the test results. Shock waves of unpressed explosives and a No.8 cap were tested under the conditions described above. The test results of unpressed 3' explosive is shown in Table 1, and the average value of peak pressure is 1.968 V. And peak pressures of a cap, unpressed 1#, 2#, 4#, 5' and 6' explosives were obtained in the same way, they are 0.645, 1.872, 1.827, 1.915, 2.007 and 1.950 V respectively. The test results of explosion shock waves of 3' explosive after compression by the primary charge's shock waves in water is shown in Table 2. Due to space constraints, the results of other explosives are not listed in this article.
3.2. Calculation of desensitizationdegree The desensitization degree of 3' explosive was cal-
J. Univ. Sci. TechnoL BeGing, Vol.13, N0.2, Apr 2006
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culated with the data in both Tables land 2, based on equation (1): di = (P-PJ/(P-p)
(l) where di is the desensitization degree; P,PL and p are
the peak pressure of shock waves of an unpressed secondary charge, a desensitized secondary charge and a cap respectively, MPa; L is the space between the primary and secondary charges, cm; and i the serial number of explosives.
Table 1. Test results of explosion shock waves of unpressed 3”explosives Serial number
Peak voltage i V
Period i ms
Rise time i ps
0414002 0414003 04 15026 0415027 Average value
2.000 1.851 1.971 2.048 1.968
51.m 5o.m 52.0336 52.2581 5 1S663
20.00 21.82 22.73 20.91 20.9 1
Fall time 1 ps 107.27 110.91 106.36 112.73 108.00
Note: Peak voltage is the top value of a shock wave signal shown in an oscillograph; Period is the air bubble pulsing period; Rise time and Fall time are the time from 0 to top pressure and from top pressure to 0 of shock waves respectively.
Table 2. Test results of explosion shock waves of desensitized 3’ explosives Serial number
Pressed spaceslcm
Time
Peak pressureN
Peridms
Rise timeips
Fall timeips
0415023 04 15025 Average value 0414006 0414008 0414010 Average value
8 8
9’ 25” 9’ 27“
1.275 1.492 1.384 1.568 1.999 1.665 1.744 2.001 1.774 1.902 1.892
44.3936 50.3954 47.3945 50.0727 5 1.1636 50.7272 50.6550
23.64 21.82 22.73 19.09 19.09 20.00 19.39 19.09 20.00 20.91 20.00 19.09 20.00 20.00 19.70 19.09 19.09 20.00 20.00
102.73 104.55 103.64 114.55 1 12.73 108.18 111.82
0413013 0414016 0414018 Average value 0415002 0415004 0415006 Average value 041501 1 0415015 0415017 Average value
-
-
10 10 10
9 ’ 32” 9‘ 38” 9‘ 39”
-
-
15 15 15
9 ’ 39” 9 ’ 45” 9 ’ 40”
-
-
20 20 20
9’ 29” 9 ’ 37” 9’ 45”
-
-
30 30 30
9’ 27” 9‘ 32” 9 ’ 30“
-
-
1.558 2.005 1.911 1.825 1.87 I 2.054 1.963 1.963
50.6 181 5 1.7090 5 1.2933 5 1.0250 5 1.7090 5 1.7090 5 1.4909 5 1.6363 5 1.1636 50.9683 51.8581 5 1.3300
1 17.27 177.27 114.55 112.73 I 12.73 106.36 11 1.82 133.94 113.64 166.36 205.45 161.81
Note: Time is the waiting time of a secondary charge; Pressed spaces refers to the space between the primary and secondary charges.
Because both the primary and secondary charges were initiated by a No.8 industrial electric detonator, whether the secondary charge was desensitized or not, its explosion shock waves involved contribution of a cap, namely P and PLinvolved p. When a secondary charge was dead-pressed, then P,=p, now di= 1 from equation (1); while a secondary charge was unpressed, P,=P, then di=O. Waveform a, b and c in Fig. 2 are of shock waves of an unpressed charge, a desensitized charge and a cap respectively under the same test conditions. In order to compare them, they are superposed with each other. Obviously, the shock wave peak pressure of the desensitized charge is higher than that of a cap and lower than that of an unpressed charge. As the denominator of equation (l), (P-p) is the difference
between shock wave peak pressures of an unpressed charge and a cap; the numerator (P-P,) is the difference between shock wave peak pressures of the unpressed and desensitized charge, shown in Fig. 2. The actual peak pressure of shock waves can be calculated from the following equation [ 121: p=- U K.S
where P is the original peak pressure of shock waves in water, MPa; U the peak voltage recorded by data acquisition instrument, mV; K the plus of preamplifier (or constant-source); S the sensitivity of pressure sensor, mV-MPa-’.
Y.J. Wung et al., Influence of an emulsifier on the pressure desensitizationof emulsion explosives
Under the same experimental conditions, the values of K and S are constant, thus equation (2) is substituted into equation (l), then we derive equation (3) by cancellation of the numerator and denominator of equation (1):
It is known from Table 1 that U is 1.968 V and u is 0.645 V. The values of U,u, and all the U, (average values) in Table 2 are substituted into equation (3), then the desensitization degrees of 3' explosive at every space point are calculated, and the desensitization degrees of other explosives are calculated in the same way. v
I
50
*
I
100
I
I
150
200
2
Time I ps Fig. 2. Oscillogramsof shock waves.
105
3.3. Analysis of the results Because 2', 5' and 6' explosives differed in components from each other only with the category of emulsifiers, 2' explosive was also organized into group two, so it belongs to both the first and second groups. The dependence of desensitizationdegree dj on pressed space L is shown in both Figs. 3(a) and (b) based on the data of desensitizationdegree of all the explosives. As described above, when the desensitizationdegree is 0, explosives explode normally, and while it is 1, explosives are dead-pressed, so it is usually between 0 and 1. The closer it is to 0, the less the desensitization degree, the better the compression resistance of an explosive, and the lower the curve's position in Fig. 3. In reverse, the higher the curve's position, the worse the compression resistance. In both Figs. 3(a) and (b), all the curves show that there is a transition stage for emulsion explosives in the process from being desensitized to being dead-pressed. And it can be divided into two ranges according to pressed space, namely short space range (d15 cm) and long space range (> 15 cm). In the short space range, the desensitization degree declines very soon, while in the long space range it descends slowly. It is concluded from the relative position of curves in Fig. 3(a) that 1' explosive is desensitized most easily, and the compression resistance of 3' explosive is the highest among group one. And in Fig. 3(b), '5 explosive's compression resistance is the highest among the explosives in group two.
Fig. 3. Relationship of the desensitizationdegree with pressed spaces: (a) 1' to 4"; (b) 2", 5*, 6'.
Based on Fig. 3(a), four pressed spaces of 10,15,20, 30 cm were chosen to make relation curves of desensitization degree to emulsifier content, shown in Fig. 4(a). All of the curves in Fig. 4(a) show that desensitization degree reduces evidently, and therefore the compression resistance improves, when the content of Span-SO is increased from 2wt% to 3wt%. Except the curve of 20 cm, the other three curves reach the minimum value while the content of Span-SO reaches 4wt%. Both Figs. 3(a) and 4(a) show that the compression resistance of emulsion explosives improves evidently with the emul-
sifier content being increased, but it will descend if the emulsifier content is increased further than a certain value. The above analysis shows that the compression resistance of 3' explosive is the highest among the first group and 5' explosive is the highest among the second group. Now, to compare the compression resistance of 3' and 5' explosives, the relation curves of 3' and 5" explosives' desensitization degree to pressed space are drawn in the same coordinate system (Fig. 4(b)). Ac-
J. Univ. Sci TechnoL Beijing, voL13,N0.2, Apr 2006
I06
cording to the position of relation curves, the compression resistance of 5’ explosive is higher than that of 3’ explosive evidently. Because 3’ and 5’ explosives are emulsified by Span-80 and compound emulsifier respectively, it is concluded that the compression resistance of the explosive emulsified by compound emulsi-
fier with 3wt% is better than that of the explosive emulsified by Span-80 at its optimal content. It shows that, to obtain a high compression resistance for an emulsion explosive, the optimal emulsifier content is different from different categories of emulsifiers.
1 .O
8
30.8 -0
.-5
0.6
.-3 0.4 -z
0.8 -
lOcm * 15cm A 2Ocm 30 cm
-c
w 2
(b) rn
@
3#explosive S#explosive
-
In
0 K
g 0.2
n
fi
“I
2 3 4 5 Content of emulsifier A / wt%
6
5 Pressed spaces / cm
Fig. 4. Curves of the relationship of desensitization degree to emulsifier content (a) and pressed space (b).
3.4. Analysis of the pressure desensitization mechanism It is known that sensitizer particles are easily broken by external pressure to make the sensitivity of initiation and the explosive power of emulsion explosives decline. The influence of an emulsifier on the pressure desensitization of emulsion explosives indicates that the decline of explosive power under external pressure is related not only to the sensitizers but also to the emulsion granules. Because the breaking of sensitizer particles is of direct correlation to external pressure, but has no relation to emulsifiers, and the content and category of emulsifiers relate with the granule size of disperse phase and the thickness and intensity of interfacial membranes of emulsion, the mechanism of pressure desensitization should be analyzed based on both the granule size of disperse phase and interfacial membranes besides the breaking of sensitizer particles. In a study[ 131, it was discovered that, during the extracting of aniline from aqueous solutions using emulsion liquid membranes, the stability of emulsion liquid membrane improves with the emulsifier content being increased and the membrane is broken obviously while the emulsifier content is low. And this has been a common sense notion in the field. Sensitizer particles in an emulsion explosive, including hollow glass microballoons and expanded perlite, are in continuous oil phase, surrounded by the interfacial membranes of oil and water phase. Affected by the surfaces of sensitizer particles, the intensity and thickness of the membranes with direct contact of sensitizer particles are lowered and thinned, resulting in broken membranes. When impacted by shock waves, the membranes close to sensitizer particles rupture first, then two or more granules are amalgamated into a bigger one in size, causing
ammonium nitrate crystallize partially in it. The bigger and broken emulsion granules exist in the circumference of sensitizer particles, enfolding a sensitizer particle just like a ‘tiny egg’ with a core of sensitizer particle and ‘egg white’ of the broken emulsion granules. Out of the ‘tiny eggs’ we get normal emulsion that has not broken. The broken emulsion granules in ‘tiny eggs’ mixed with the crystal of ammonium nitrate are not divided into two phases entirely and are unstable in capability. As time goes on, the interfacial membranes in ‘tiny eggs’ rupture even more, and the crystallization of ammonium nitrate continues, until the emulsion explosive lose initiation sensitivity to a No.8 industrial electrical detonator. While the shock wave pressure is not very high, the whity ‘tiny eggs’ can be seen with unaided eyes. The diameter of a ‘tiny egg’ relates with the intensity of explosion shock waves of a primary charge, surface and size of sensitizer particles, as well as the size of emulsion granules and the intensity of interfacial membranes. Because the contact area between the oxidizer and reductant in the broken emulsion granules of ‘tiny eggs’ reduces, the initiation and propagation of detonation becomes difficult. So, even though the sensitizer particles are not broken, it is inevitable that the sensitization and explosive power of emulsion explosives descends. Actually, the sensitizer particles must have been broken to some extent, thus the temperature of hotspot created by them falls. It is these two aspects together that cause the desensitization of emulsion explosives after impacted by shock waves. According to Trable Rules [ 141, while an emulsifier is in low content, lowering effect of surface tension is in direct ratio to its content. But surface tension will not lower if the emulsifier content increases more after
Y.J. Wang et aZ., Influence of an emulsifier on the pressure desensitization of emulsion explosives
saturation adsorption at a certain temperature. The rule provides a theoretical support for the experimental results in this paper. It indicates that when the emulsifier content is low, the emulsion granule size is appreciably big and the intensity of interfacial membranes is low. With the emulsifier content being increased, the emulsion granule size reduces and the intensity of interfacial membranes grows. Thus, the compression resistance of emulsion explosives improves when the content of Span-80 is increased from 2wt% to 4wt%, whereas if the content of Span-80 exceeds 4wt% it no longer improves, even declines somewhat, because emulsifier Span-80 in interfacial membranes has attained saturation. According to general opinion, the emulsion with compound emulsifier is not broken easily, because the granule size of dispersed phase is small and interfacial membranes are dense and strong. This is the reason why the compression resistance of 5' explosive is the highest among all the explosives.
4. Conclusions (1) There is an optimal emulsifier content for the
compression resistance of an emulsion explosive, and for Span-80 it is 4wt%. However, in view of the production cost, the content of Span-80 should be around 3wt%, because the emulsion explosive has already shown a very high compression resistance at this content. (2) The compression resistance of the emulsion explosive with compound emulsifier is the highest among all the explosives. For compression resistance, the optimal emulsifier content is different from different kinds of emulsifiers.
(3) The content and category of emulsifiers affect the granule size of dispersed phase and the thickness and intensity of interfacial membranes. The smaller the granule size of dispersed phase and the higher the intensity of interfacial membranes, the higher the compression resistance of emulsion explosive. The breaking of emulsion granules around sensitizerparticles is a very important cause of the pressure desensitization of
107
emulsion explosive.
References [l] Toshio Matsuzawa and Masaharu Murakami, Detonability of emulsion explosives under dynamic pressure, J. Ind. Explos. SOC. (Japan), 43(1982), No.5, p.317. [2] S.L. Nie, Pressure desensitization of a gassed emulsion explosive in comparison with micro-balloon sensitized emulsion explosives, [in] Proc. 3th Annual Symposium on Explosives and Blasting Research, LasVegas, Nevada, USA, 1997, p.2. [3] L.L. Yan, Y.J. Wang, and Y. Liu, Research on the relationship between water-based explosive desensitization and delay time under dynamic, [in] 7th Int. Symposium on Rock , Fragmentation by Blasting, Beijing, 2002, p.88. [4] Y.J. Wang, X.G Wang,Y.L. Yu,et al., Experimental study on two kinds of water-bearing explosives, Eng. Blast. (in Chinese), 9(2003), No.4, p.67. [5] Mullay, John J, Sohara, et al., High Emulsifier Content Explosives, the USA Patent, No.4933028, June 12, 1990. [6] S.B. Zhang, A kind of emulsion explosives resisted pressure desensitization, Explos. Muter. (in Chinese), 22( 1993), No.3, p.35. [7] S.L. Yan, S.B. Zhang, and Y. Liu, Study on the desensitization of the coalmine permitted emulsion explosives under dynamic pressure, J. Anhui Univ. Sci. Technol. (Natural Science)(in Chinese), 23(2003), No.3, p. 27. [8] J.L. Wang, D.K. Zhao, W. Guo, etal., Determination of the reasonable depth of explosives in water to measure under water explosive energy, Chin. J. Explos. Propellants (in Chinese), 25(2002), No.2, p.30. [9] Z.H. Chen, New Testing Technique of Industrial Explosive (in Chinese), Coal Industrial Press, Beijing, 1982. [lo] L. B a n g and D.L. Wang, Study on elimination of the boundary effects in underwater explosion testing, Explos. Muter. (in Chinese), 24(1995), N0.2, p.1. 1I] Y.J. Wang, S.L. Yan, and X.G Wang, Application of oscillograph on explosives desensitization under dynamic pressure, J. China Coal SOC. (in Chinese), 29(2004), No.5, p. 559. 121 P. Cole and Y.J. Luo, Underwater Explos. (in Chinese), Defence Industry Press, Beijing, 1965, p.137. 131 M. Yu, Li, Q.S. Yan, Q.H. Tang, et al., Extraction of aniline from aqueous solutions using emulsion liquid membranes, J. Chem. Ind. Eng. (in Chinese), 54(2003), No.6, p.836. 141 P. Becher, Theory and Practice of Emulsion (in Chinese), Science Press, Beijing, 1978.
Journal of University of Science and Technology Beijing Volume 13, Number 2, April 2006, Page 102
Influence of an emulsifier on the pressure desensitization of emulsion explosives Yinjun Wangl-”,Xuguang Wang”,and Shilong Yan3’ 1) Civil & Environmental Engineering School. University of Science and Technology Beijing. Beijing 100083, China
2) Beijing General Research Institute of Mining and Metallurgy, Beijing 1ooo44, China 3) Chemical Engineering Department. Anhui University of Science and Technology. Anhui 232001, China (Received 2005-04-06)
Abstract: The desensitization degree of emulsion explosives (EE) was calculated with the peak pressure of explosion shock waves tested in water. To an explosive, the less the desensitization degree, the better the compression resistance, so the compression resistance of an explosive can be compared and analyzed quantificationally with the desensitization degree. The influence of an emulsifier on the pressure desensitization of EE was studied, including the content and category of emulsifiers. Three kinds of emulsifiers (Span-80, compound emulsifier, and T-152) were used in the tests. The experimental results show that both the content and category of emulsifiers make a great effect on the pressure desensitization of EE. The desensitization degree of EE reduces with the emulsifier content being increased, but there is an optimal content of an emulsifier for the compression resistance of EE. While the content of Span-80 reaches 4wt%, the desensitization degree of EE becomes a minimal value, and augments somewhat if the emulsifier content is increased more. That is to say, the compression resistance of EE becomes the highest while the content of Span-80 is 4wt%, and the compression resistance will decline if the content of Span-80 is increased more. The compression resistance of the explosive emulsified by compound emulsifier is the highest among all the explosives, when the content of the whole components and manufacturing engineering are kept invariable. Key words: emulsion explosive; emulsifier; pressure desensitization; shock wave; compression resistance
[This work was,financiall~ supported by the National Natural Science Foundation of China (N0.50574004).]
1. Introduction During millisecond blasting, shock waves or stress waves generated by the first initiated charges will propagate to the delay charges and make a shock pressure on them just before they are initiated. If the blasthole spaces between the adjacent rows are close or the compression resistance of an explosive is low, it is possible that the explosive power of delay charges descends. This phenomenon is called pressure desensitization, always in two forms of half-fire and misfire. As soon as it happens, it brings great harm to blast work. For example, it always delays a project, lowers blasting effect and is also a safety hazard. So it is necessary to study the pressure desensitization of an emulsion explosive. Toshio Matsuzawa er al. studied the explosive property of an emulsion explosive subjected to dynamic pressure, and it shows that bubbles in the explosive as hot-spots make a great effect during the initiating explosives [ I]. S.L. Nie [2] performed a numerical simulation on the process of volume recovering of pressed Corresponding author: Yinjun Wang. E-mail: yjwang0281@ 163.com
bubbles in emulsion explosives. The relation of desensitization of water gel or emulsion explosive to delay time was studied [3]. Two concepts of critical deadpressing space and critical space were put forward based on experiments and practices [4]. It was introduced in both U.S. patent [5] and in other studies [6] that the compression resistance of an emulsion explosive could be improved if the emulsifier content was increased. The influence of an emulsifier on the compression resistance of a mine permitted emulsion explosive was studied [7]. In this paper, the influence of emulsifier’s content and category on the pressure desensitization of emulsion explosives was studied quantificationally and the mechanism of pressure desensitization was analyzed.
2. Experimental method The experiments were performed in a cistern with a diameter of 5.5 m and a water depth of 3.62 m, shown in Fig. I(a). An explosive charge was placed under water at 2/3 of the water depth in axes of the cistern,
Y.J. Wang et al., Influence of an emulsifier on the pressure desensitization of emulsion explosives
which can eliminate the boundary effect of water surface and bottom.
103
tiated at the time of about 10 min after a primary charge exploded. The intensity of shock waves of a primary charge was adjusted by changing the space between the primary and secondary charges. Experimental details were presented in reference [ 111.
3. Test results and analysis 3.1. Test results
Fig. 1. Photographs of the experimental apparatus: (a) the experimental cistern; (b) primary and secondary charges fixed together in an iron stand; (c) a secondary charge with a cap in its center fued in the iron stand in front of a pressure sensor.
A primary charge was made of 3 g RDX, and a secondary charge was made of 10 g emulsion explosive, initiated by a No.8 industrial cap respectively. The two charges were fixed in the middle of an iron stand, with their cores in the same horizontal line (shown in Fig. l(b)). Then the iron stand was placed in the cistern, and the primary charge was initiated. The secondary charge, which was already impacted by shock waves of the primary charge, was taken out and a No.8 cap was inserted in it. It was retightened on the iron stand (shown in Fig. l(c)) and put into the cistern again to be initiated. At a distance of 20 cm from the secondary charge, a pressure sensor caught pressure signals while the secondary charge exploded in water. The period from the moment when a primary charge was initiated to that a secondary charge exploded is defined to be the waiting time, which is the same for every secondary charge with each other. And it was about 10 min and measured by a stopwatch in the experiments, that is to say, the secondary charge was ini-
Two groups of emulsion explosives were used in the experiment, including four kinds in the first group and two kinds in the second. Components of the matrix (except emulsifier) are the oxidizer, water, and the reductant. The content of them is 83.5wt%, 11wt%, 3.5wt% respectively. After the emulsion matrix is completed, the hollow glass microballoons (GMB) are mixed with it while its temperature lowers to about 5O-6O0C, and the content of GMB is 2.5wt%. With the content of other components and manufacturing engineering keeping invariable, only the content of Span-80 in the four explosives of the first group was increased, and it was 2wt%, 3wt%, 4wt%, 5wt% respectively, and the corresponding serial number of the explosives was 1#, 2', 3#, 4'. The matrix components of the two explosives in the second group were nearly the same with 2' explosive, the only difference was the emulsifier category, and the corresponding serial number was 5#, 6'. Actually, 2#, 5' and 6' explosives were emulsified'by Span80, compound emulsifier, and T- 152, respectively. Five space points for every explosive were chosen for testing, and three tests at every space point were performed. The average value of the three test results was used to analyze the desensitization of a secondary charge compressed by shock waves at the point. One of the most important parameters of explosion shock waves is peak pressure, and it is also gauged easily, so it was used to analyze the test results. Shock waves of unpressed explosives and a No.8 cap were tested under the conditions described above. The test results of unpressed 3' explosive is shown in Table 1, and the average value of peak pressure is 1.968 V. And peak pressures of a cap, unpressed 1#, 2#, 4#, 5' and 6' explosives were obtained in the same way, they are 0.645, 1.872, 1.827, 1.915, 2.007 and 1.950 V respectively. The test results of explosion shock waves of 3' explosive after compression by the primary charge's shock waves in water is shown in Table 2. Due to space constraints, the results of other explosives are not listed in this article.
3.2. Calculation of desensitizationdegree The desensitization degree of 3' explosive was cal-
J. Univ. Sci. TechnoL BeGing, Vol.13, N0.2, Apr 2006
104
culated with the data in both Tables land 2, based on equation (1): di = (P-PJ/(P-p)
(l) where di is the desensitization degree; P,PL and p are
the peak pressure of shock waves of an unpressed secondary charge, a desensitized secondary charge and a cap respectively, MPa; L is the space between the primary and secondary charges, cm; and i the serial number of explosives.
Table 1. Test results of explosion shock waves of unpressed 3”explosives Serial number
Peak voltage i V
Period i ms
Rise time i ps
0414002 0414003 04 15026 0415027 Average value
2.000 1.851 1.971 2.048 1.968
51.m 5o.m 52.0336 52.2581 5 1S663
20.00 21.82 22.73 20.91 20.9 1
Fall time 1 ps 107.27 110.91 106.36 112.73 108.00
Note: Peak voltage is the top value of a shock wave signal shown in an oscillograph; Period is the air bubble pulsing period; Rise time and Fall time are the time from 0 to top pressure and from top pressure to 0 of shock waves respectively.
Table 2. Test results of explosion shock waves of desensitized 3’ explosives Serial number
Pressed spaceslcm
Time
Peak pressureN
Peridms
Rise timeips
Fall timeips
0415023 04 15025 Average value 0414006 0414008 0414010 Average value
8 8
9’ 25” 9’ 27“
1.275 1.492 1.384 1.568 1.999 1.665 1.744 2.001 1.774 1.902 1.892
44.3936 50.3954 47.3945 50.0727 5 1.1636 50.7272 50.6550
23.64 21.82 22.73 19.09 19.09 20.00 19.39 19.09 20.00 20.91 20.00 19.09 20.00 20.00 19.70 19.09 19.09 20.00 20.00
102.73 104.55 103.64 114.55 1 12.73 108.18 111.82
0413013 0414016 0414018 Average value 0415002 0415004 0415006 Average value 041501 1 0415015 0415017 Average value
-
-
10 10 10
9 ’ 32” 9‘ 38” 9‘ 39”
-
-
15 15 15
9 ’ 39” 9 ’ 45” 9 ’ 40”
-
-
20 20 20
9’ 29” 9 ’ 37” 9’ 45”
-
-
30 30 30
9’ 27” 9‘ 32” 9 ’ 30“
-
-
1.558 2.005 1.911 1.825 1.87 I 2.054 1.963 1.963
50.6 181 5 1.7090 5 1.2933 5 1.0250 5 1.7090 5 1.7090 5 1.4909 5 1.6363 5 1.1636 50.9683 51.8581 5 1.3300
1 17.27 177.27 114.55 112.73 I 12.73 106.36 11 1.82 133.94 113.64 166.36 205.45 161.81
Note: Time is the waiting time of a secondary charge; Pressed spaces refers to the space between the primary and secondary charges.
Because both the primary and secondary charges were initiated by a No.8 industrial electric detonator, whether the secondary charge was desensitized or not, its explosion shock waves involved contribution of a cap, namely P and PLinvolved p. When a secondary charge was dead-pressed, then P,=p, now di= 1 from equation (1); while a secondary charge was unpressed, P,=P, then di=O. Waveform a, b and c in Fig. 2 are of shock waves of an unpressed charge, a desensitized charge and a cap respectively under the same test conditions. In order to compare them, they are superposed with each other. Obviously, the shock wave peak pressure of the desensitized charge is higher than that of a cap and lower than that of an unpressed charge. As the denominator of equation (l), (P-p) is the difference
between shock wave peak pressures of an unpressed charge and a cap; the numerator (P-P,) is the difference between shock wave peak pressures of the unpressed and desensitized charge, shown in Fig. 2. The actual peak pressure of shock waves can be calculated from the following equation [ 121: p=- U K.S
where P is the original peak pressure of shock waves in water, MPa; U the peak voltage recorded by data acquisition instrument, mV; K the plus of preamplifier (or constant-source); S the sensitivity of pressure sensor, mV-MPa-’.
Y.J. Wung et al., Influence of an emulsifier on the pressure desensitizationof emulsion explosives
Under the same experimental conditions, the values of K and S are constant, thus equation (2) is substituted into equation (l), then we derive equation (3) by cancellation of the numerator and denominator of equation (1):
It is known from Table 1 that U is 1.968 V and u is 0.645 V. The values of U,u, and all the U, (average values) in Table 2 are substituted into equation (3), then the desensitization degrees of 3' explosive at every space point are calculated, and the desensitization degrees of other explosives are calculated in the same way. v
I
50
*
I
100
I
I
150
200
2
Time I ps Fig. 2. Oscillogramsof shock waves.
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3.3. Analysis of the results Because 2', 5' and 6' explosives differed in components from each other only with the category of emulsifiers, 2' explosive was also organized into group two, so it belongs to both the first and second groups. The dependence of desensitizationdegree dj on pressed space L is shown in both Figs. 3(a) and (b) based on the data of desensitizationdegree of all the explosives. As described above, when the desensitizationdegree is 0, explosives explode normally, and while it is 1, explosives are dead-pressed, so it is usually between 0 and 1. The closer it is to 0, the less the desensitization degree, the better the compression resistance of an explosive, and the lower the curve's position in Fig. 3. In reverse, the higher the curve's position, the worse the compression resistance. In both Figs. 3(a) and (b), all the curves show that there is a transition stage for emulsion explosives in the process from being desensitized to being dead-pressed. And it can be divided into two ranges according to pressed space, namely short space range (d15 cm) and long space range (> 15 cm). In the short space range, the desensitization degree declines very soon, while in the long space range it descends slowly. It is concluded from the relative position of curves in Fig. 3(a) that 1' explosive is desensitized most easily, and the compression resistance of 3' explosive is the highest among group one. And in Fig. 3(b), '5 explosive's compression resistance is the highest among the explosives in group two.
Fig. 3. Relationship of the desensitizationdegree with pressed spaces: (a) 1' to 4"; (b) 2", 5*, 6'.
Based on Fig. 3(a), four pressed spaces of 10,15,20, 30 cm were chosen to make relation curves of desensitization degree to emulsifier content, shown in Fig. 4(a). All of the curves in Fig. 4(a) show that desensitization degree reduces evidently, and therefore the compression resistance improves, when the content of Span-SO is increased from 2wt% to 3wt%. Except the curve of 20 cm, the other three curves reach the minimum value while the content of Span-SO reaches 4wt%. Both Figs. 3(a) and 4(a) show that the compression resistance of emulsion explosives improves evidently with the emul-
sifier content being increased, but it will descend if the emulsifier content is increased further than a certain value. The above analysis shows that the compression resistance of 3' explosive is the highest among the first group and 5' explosive is the highest among the second group. Now, to compare the compression resistance of 3' and 5' explosives, the relation curves of 3' and 5" explosives' desensitization degree to pressed space are drawn in the same coordinate system (Fig. 4(b)). Ac-
J. Univ. Sci TechnoL Beijing, voL13,N0.2, Apr 2006
I06
cording to the position of relation curves, the compression resistance of 5’ explosive is higher than that of 3’ explosive evidently. Because 3’ and 5’ explosives are emulsified by Span-80 and compound emulsifier respectively, it is concluded that the compression resistance of the explosive emulsified by compound emulsi-
fier with 3wt% is better than that of the explosive emulsified by Span-80 at its optimal content. It shows that, to obtain a high compression resistance for an emulsion explosive, the optimal emulsifier content is different from different categories of emulsifiers.
1 .O
8
30.8 -0
.-5
0.6
.-3 0.4 -z
0.8 -
lOcm * 15cm A 2Ocm 30 cm
-c
w 2
(b) rn
@
3#explosive S#explosive
-
In
0 K
g 0.2
n
fi
“I
2 3 4 5 Content of emulsifier A / wt%
6
5 Pressed spaces / cm
Fig. 4. Curves of the relationship of desensitization degree to emulsifier content (a) and pressed space (b).
3.4. Analysis of the pressure desensitization mechanism It is known that sensitizer particles are easily broken by external pressure to make the sensitivity of initiation and the explosive power of emulsion explosives decline. The influence of an emulsifier on the pressure desensitization of emulsion explosives indicates that the decline of explosive power under external pressure is related not only to the sensitizers but also to the emulsion granules. Because the breaking of sensitizer particles is of direct correlation to external pressure, but has no relation to emulsifiers, and the content and category of emulsifiers relate with the granule size of disperse phase and the thickness and intensity of interfacial membranes of emulsion, the mechanism of pressure desensitization should be analyzed based on both the granule size of disperse phase and interfacial membranes besides the breaking of sensitizer particles. In a study[ 131, it was discovered that, during the extracting of aniline from aqueous solutions using emulsion liquid membranes, the stability of emulsion liquid membrane improves with the emulsifier content being increased and the membrane is broken obviously while the emulsifier content is low. And this has been a common sense notion in the field. Sensitizer particles in an emulsion explosive, including hollow glass microballoons and expanded perlite, are in continuous oil phase, surrounded by the interfacial membranes of oil and water phase. Affected by the surfaces of sensitizer particles, the intensity and thickness of the membranes with direct contact of sensitizer particles are lowered and thinned, resulting in broken membranes. When impacted by shock waves, the membranes close to sensitizer particles rupture first, then two or more granules are amalgamated into a bigger one in size, causing
ammonium nitrate crystallize partially in it. The bigger and broken emulsion granules exist in the circumference of sensitizer particles, enfolding a sensitizer particle just like a ‘tiny egg’ with a core of sensitizer particle and ‘egg white’ of the broken emulsion granules. Out of the ‘tiny eggs’ we get normal emulsion that has not broken. The broken emulsion granules in ‘tiny eggs’ mixed with the crystal of ammonium nitrate are not divided into two phases entirely and are unstable in capability. As time goes on, the interfacial membranes in ‘tiny eggs’ rupture even more, and the crystallization of ammonium nitrate continues, until the emulsion explosive lose initiation sensitivity to a No.8 industrial electrical detonator. While the shock wave pressure is not very high, the whity ‘tiny eggs’ can be seen with unaided eyes. The diameter of a ‘tiny egg’ relates with the intensity of explosion shock waves of a primary charge, surface and size of sensitizer particles, as well as the size of emulsion granules and the intensity of interfacial membranes. Because the contact area between the oxidizer and reductant in the broken emulsion granules of ‘tiny eggs’ reduces, the initiation and propagation of detonation becomes difficult. So, even though the sensitizer particles are not broken, it is inevitable that the sensitization and explosive power of emulsion explosives descends. Actually, the sensitizer particles must have been broken to some extent, thus the temperature of hotspot created by them falls. It is these two aspects together that cause the desensitization of emulsion explosives after impacted by shock waves. According to Trable Rules [ 141, while an emulsifier is in low content, lowering effect of surface tension is in direct ratio to its content. But surface tension will not lower if the emulsifier content increases more after
Y.J. Wang et aZ., Influence of an emulsifier on the pressure desensitization of emulsion explosives
saturation adsorption at a certain temperature. The rule provides a theoretical support for the experimental results in this paper. It indicates that when the emulsifier content is low, the emulsion granule size is appreciably big and the intensity of interfacial membranes is low. With the emulsifier content being increased, the emulsion granule size reduces and the intensity of interfacial membranes grows. Thus, the compression resistance of emulsion explosives improves when the content of Span-80 is increased from 2wt% to 4wt%, whereas if the content of Span-80 exceeds 4wt% it no longer improves, even declines somewhat, because emulsifier Span-80 in interfacial membranes has attained saturation. According to general opinion, the emulsion with compound emulsifier is not broken easily, because the granule size of dispersed phase is small and interfacial membranes are dense and strong. This is the reason why the compression resistance of 5' explosive is the highest among all the explosives.
4. Conclusions (1) There is an optimal emulsifier content for the
compression resistance of an emulsion explosive, and for Span-80 it is 4wt%. However, in view of the production cost, the content of Span-80 should be around 3wt%, because the emulsion explosive has already shown a very high compression resistance at this content. (2) The compression resistance of the emulsion explosive with compound emulsifier is the highest among all the explosives. For compression resistance, the optimal emulsifier content is different from different kinds of emulsifiers.
(3) The content and category of emulsifiers affect the granule size of dispersed phase and the thickness and intensity of interfacial membranes. The smaller the granule size of dispersed phase and the higher the intensity of interfacial membranes, the higher the compression resistance of emulsion explosive. The breaking of emulsion granules around sensitizerparticles is a very important cause of the pressure desensitization of
107
emulsion explosive.
References [l] Toshio Matsuzawa and Masaharu Murakami, Detonability of emulsion explosives under dynamic pressure, J. Ind. Explos. SOC. (Japan), 43(1982), No.5, p.317. [2] S.L. Nie, Pressure desensitization of a gassed emulsion explosive in comparison with micro-balloon sensitized emulsion explosives, [in] Proc. 3th Annual Symposium on Explosives and Blasting Research, LasVegas, Nevada, USA, 1997, p.2. [3] L.L. Yan, Y.J. Wang, and Y. Liu, Research on the relationship between water-based explosive desensitization and delay time under dynamic, [in] 7th Int. Symposium on Rock , Fragmentation by Blasting, Beijing, 2002, p.88. [4] Y.J. Wang, X.G Wang,Y.L. Yu,et al., Experimental study on two kinds of water-bearing explosives, Eng. Blast. (in Chinese), 9(2003), No.4, p.67. [5] Mullay, John J, Sohara, et al., High Emulsifier Content Explosives, the USA Patent, No.4933028, June 12, 1990. [6] S.B. Zhang, A kind of emulsion explosives resisted pressure desensitization, Explos. Muter. (in Chinese), 22( 1993), No.3, p.35. [7] S.L. Yan, S.B. Zhang, and Y. Liu, Study on the desensitization of the coalmine permitted emulsion explosives under dynamic pressure, J. Anhui Univ. Sci. Technol. (Natural Science)(in Chinese), 23(2003), No.3, p. 27. [8] J.L. Wang, D.K. Zhao, W. Guo, etal., Determination of the reasonable depth of explosives in water to measure under water explosive energy, Chin. J. Explos. Propellants (in Chinese), 25(2002), No.2, p.30. [9] Z.H. Chen, New Testing Technique of Industrial Explosive (in Chinese), Coal Industrial Press, Beijing, 1982. [lo] L. B a n g and D.L. Wang, Study on elimination of the boundary effects in underwater explosion testing, Explos. Muter. (in Chinese), 24(1995), N0.2, p.1. 1I] Y.J. Wang, S.L. Yan, and X.G Wang, Application of oscillograph on explosives desensitization under dynamic pressure, J. China Coal SOC. (in Chinese), 29(2004), No.5, p. 559. 121 P. Cole and Y.J. Luo, Underwater Explos. (in Chinese), Defence Industry Press, Beijing, 1965, p.137. 131 M. Yu, Li, Q.S. Yan, Q.H. Tang, et al., Extraction of aniline from aqueous solutions using emulsion liquid membranes, J. Chem. Ind. Eng. (in Chinese), 54(2003), No.6, p.836. 141 P. Becher, Theory and Practice of Emulsion (in Chinese), Science Press, Beijing, 1978.