Cite this paper: Vietnam J. Chem., 2020, 58(3), 380-385 Article DOI: 10.1002/vjch.202000003
Several explosion behaviours of the emulsion explosives sensitized by cenospheres
Dam Quang Sang*, Nguyen Tuan Anh
Le Quy Don Technical University, 236 Hoang Quoc Viet, North Tu Liem, Hanoi 10000, Viet Nam Submitted January 22, 2020; Accepted April 15, 2020
Abstract
The paper presents the influence of contents and particle sizes of cenospheres (a product extracted from fly ashes of thermal power plants) on the velocity of detonation (VOD) and brisance of emulsion explosives. Experimental results show that cenospheres are hollow balloons with 95 % of the particles in a range of 60-270 m, and their main chemical compositions are SiO2 and Al2O3. With cenosphere contents in a range of 10-15 % over 100 % of the emulsion matrix, the VOD reaches approximately 4000 m/s at 31.38 mm of charge diameter and the brisance is in a range of 14-16 mm.
In addition, explosives, sensitized by cenospheres smaller than 160 m, have VOD and brisance are better than which using cenospheres greater than 160 m or unsieved cenospheres. Emulsion explosives sensitized by 10 % of unsieved cenospheres have maximum VOD of 5409 m/s, explosion heat of 999 cal/g, and work capability of 85 % compared to TNT.
Keywords. Emulsion explosive, cenosphere, fly ash, velocity of detonation, brisance
1. INTRODUCTION
Emulsion explosives (EMX) are commercial explosives invented in 1969.[1] Currently, they are widely used over the globe due to advantages such as water resistance, high velocity of detonation (VOD), small critical diameter, quite safe, etc.
Compositions of emulsion explosives include the water phase, oil phase, and sensitizer. The aqueous phase is a concentrated solution of oxidized salts such as ammonium nitrate, sodium nitrate, and calcium nitrate. The oil phase is a mixture of mineral oil, wax, and emulsifiers.[2] The aqueous and oil phases form a "water-in-oil" emulsion called emulsion matrix. Different sensitizers are added into the emulsion matrix due to the fact that it is not detonated by detonators or primers. Some commonly used sensitizers are chemical gaseous bubbles,[3]
perlite,[4] glass microspheres,[5,6] magnesium hydrides,[7] polymeric microspheres.[8,9] Chemical gas bubbles are inexpensive but lead to instability of explosives. Meanwhile, glass or polymeric microspheres make explosives to be more expensive.
In 2005, Sil'vestrov and his colleagues used cenospheres extracted from fly ashes of Novosibirsk thermal power plant as a sensitizer for emulsion explosives.[10] He indicated that the sensitizing effect of cenospheres is between air bubbles and glass microspheres. However, his emulsion explosives,
based on ammonium nitrate and industrial oil, are not sensitive to detonators. On the other hand, the research results did not present other important explosive characteristics such as brisance, the heat of explosion or work capability.
The purpose of this study is to determine the properties and to assess the possibility of using cenospheres in Vietnam as a sensitizer for EMX detonated with detonators. Thereby, it makes to effectively use available domestic materials and to reduce environmental pollution caused by fly ashes of thermal power plants.
2. MATERIALS AND METHODS 2.1. Materials
Ammonium nitrate (AN), sodium nitrate (SN), wax, emulsifiers sorbitan monooleate (span 80) and polyisobutylene polysuccinimide (T-155) were bought from Xilong chemical company (China).
Cenospheres were bought from Caocuong company of Vietnam.
2.2. Preparation of emulsion explosives
The composition of the emulsion matrix include: AN - 70.2 %, SN - 11 %, water - 12 %, wax - 4.1 %, span - 80-1.5 %, T-155 - 1.2 %. The solution of AN
Vietnam Journal of Chemistry Dam Quang Sang et al.
and SN was prepared in a 500-ml beaker at 95 °C.
The mixture of wax, span-80, and T-155 was mixed in another beaker at 95 °C. Slowly to pour the salt solution into the oil beaker and gradually increase the stirring speed to 1500 rpm and maintain for 5 minutes. During emulsification process, the emulsifying beaker is maintained at 95 °C.
When the temperature of emulsion matrix drops to 90 °C, we added a certain amount of cenospheres and stirred it at a rate of 60 rpm until the mixture became homogeneous.
2.3. Characterization
For cenospheres, granular morphology was observed by using a JEOL JSM-6510LV scanning electron microscope at Institute of Environmental Technology belongs to Vietnam Academy of Science and Technology, particle size distribution was determined by a HORIBA LA-950 apparatus at Institute of Material Chemistry belongs to Academy of Military Science and Technology and chemical composition was analyzed by a Perkin Elmer Optima 7300 V ICP-OES apparatus at Institute of Energy and Mining Mechanical Engineering. The bulk density of cenospheres is determined according to GOST 9758-86. The actual density of cenospheres is determined by mixing cenospheres with industrial oil in a certain mass ratio. Then we measured the density of the received mixture by a mass-volume method. The actual density true of cenospheres is calculated according to the following equation
100
c true
o
m o
x
x
(1) where xc: content of cenospheres, %; xo: content of industrial oil; o: density of industrial oil; m: density of the mixture.
Density of explosives in charges was determined by the mass - volume method. The VOD was measured by an EXPLOMET FO-2000 apparatus.
Explosive charges were made by PVC pipes having 1 mm of thick and 200 mm of long (figure 1, a). The length between the starting and ending points on explosive charges is 120 mm. The brisance was determined according to TCVN 6421:1998 (figure 1, b) with 50 g of explosives. The heat of explosion was measured on a DCA-5 apparatus. The work performance was determined on a pendulum according to TCVN 6424:1998. Explosives were initialized using detonators.
Figure 1: Experiments to determine VOD (a) and brisance (b) of explosives
3. RESULTS AND DISCUSSION 3.1. Characteristics of cenospheres
Figure 2 shows SEM images of the original cenospheres and their fragments. Cenospheres are hollow balloons with different diameters like glass microspheres. However, there may be small
"tumors" on the surface of the cenospheres shell and there are pores in the shell wall. The shell thickness is 6 m. This result is similar to data of cenospheres gotten from a thermal power plant in the Russian Federation.[10]\
Figure 2: SEM images of original cenospheres (a) and their shells (b)
The particle size distribution of cenospheres (figure 3) indicates that 100 % of the particles are in the range of 5-350 m, 95 % of particles are from 60 to 270 m, the average diameter is 138 m.
(a)
(b)
(a)
(b)
Vietnam Journal of Chemistry Several explosion behaviours of the emulsion...
Meanwhile, 3M K20 glass microspheres have a narrower particle size distribution, mainly in the range of 30-90 m[11] and their average diameter is 60 m.
Figure 3: The particle size distribution of cenospheres
Table 1 presents the chemical compositions of Vietnamese cenospheres and reference data of two cenospheres in the world. Obviously, different types of cenospheres contain similar chemical compositions (mainly Al2O3 and SiO2).
Table 1: Main chemical compositions of some types of cenospheres
Compositions Content, %
Exp. Ref. [12] Ref. [13]
SiO2 58.16 52.53 63.1-65.1
Al2O3 29.89 30.01 20.0-26.4
K2O 5.99 1.98 2.3-4.4
Fe2O3 2.51 7.53 4.2-5.1
MgO 1.27 0.32 1.0-2.6
TiO2 0.73 1.79 -
CaO 0.44 1.15 0.9-2.1
The bulk (bulk) and true densities (true) of unsieved, smaller than 160 m, and greater than 160
m cenospheres are shown in table 2. Clearly, compared to K20 3M glass microspheres, the true density of cenospheres is almost 2 times greater.
Table 2: Bulk and true densities of cenospheres
Cenospheres, microspheres bulk, g/cm3
true, g/cm3
Unsieved 0.346 0.407
d < 160 m 0.336 0.399
d 160 m 0.340 0.357
Microspheres 3M K20[11] 0.120 0.200
Thus, cenospheres are quite similar to glass microspheres, so they can be used as a sensitizer for emulsion explosives. However, due to the greater particle size and higher density, they must be used in a greater content to make an equivalent amount of
"hot spots" in the explosive.
3.2. The effect of the weight content of unsieved cenospheres
The explosive samples were prepared so that the content (x) of unsieved cenospheres was from 5 to 20 over 100 % of the emulsion matrix. Internal diameter of explosive charges were 31.38 mm.
Experimental data for density (), VOD (D) and brisance (H) are presented in table 3.
Table 3: Impact of unsieved cenosphere contents on density, VOD, and brisance
x, % , g/cm3 D, m/s H, mm
5 1.254 Fail -
8 1.186 3027 -
9 1.165 3209 13.69
10 1.146 3891 16.48
12 1.110 3954 14.97
15 1.062 4019 14.06
20 0.995 3883 -
It is clear that the density decreases from 1.254 to 0.995 g/cm3 when increasing the content of cenospheres. At 5 % of the weight content, the detonation process fault due to a necessary number of "hot spots" is not enough.[14] When increasing the content of cenospheres, the VOD and brisance increase and reach the maximum values at 15 and 10
%, respectively (figure 4). Continuing to increase the content of cenospheres, these parameters decrease.
This phenomenon is explained by the increase in the number of "hot spots" that increases the chemical reaction rate but also reduces the released reaction heat.
Therefore, the content of cenospheres in a range of 10-15 % ensures values of VOD greater than 3800 m/s and brisance in a range of 14-16 mm for commonly used emulsion explosives.[15] Increasing the cenosphere content reduces the explosion heat and volume of gaseous products and reduces work capability.
Vietnam Journal of Chemistry Dam Quang Sang et al.
Figure 4: The influence of weight contents of unsieved cenospheres on explosion parameters 3.3. The influence of the particle size of
cenospheres
Table 4 shows the density, VOD, and brisance of emulsion explosive samples sensitized by three types of cenospheres: unsieved, greater than 160 m and smaller than 160 m. Cenosphere contents are 10 and 12 % over 100 % of the emulsion matrix.
The diameter of explosive charges was 31.38 mm.
Table 4: VOD and brisance of explosives sensitized by different types of cenospheres
x, % Cenospheres , g/cm3 D, m/s H, mm 10
< 160 m 1.140 3961 15.56 Unsieved 1.146 3891 16.48
> 160 m 1.106 3592 12.67
12
< 160 m 1.103 4350 15.03 Unsieved 1.110 3954 14.97
> 160 m 1.066 3659 13.28 From figure 5, it is clear that, at both 10 and 12
% of contents, VOD of explosives sensitized by cenospheres smaller than 160 m is the highest, by unsieved is the medium, and by greater 160 m is the lowest. However, this effect at 12 % of the content is more pronounced than which at 10 % of the content. The effect of the particle size of cenospheres is explained by the fact that, with the same content, smaller cenospheres are, the higher number of “hot spots” in explosives is. Therefore, cenospheres smaller than 160 m, should be used to obtain high VOD.
The same law is true for the brisance. However, there is a special case at 10 % of the content detailly the brisance of explosives using unsieved cenospheres (16.48 mm) is greater than when using cenospheres with particle size smaller than 160 m
(15.56 mm). This may be due to errors in the measurement method.
3.4. The influence of the charge diameter
The VODs of explosives using 10 % of unsieved cenospheres over 100 % of the emulsion matrix at three different explosive charge diameters are presented in table 5.
Figure 5: The influence of the particle size of cenospheres on explosion parameters Table 5: Dependence of VOD according to
explosive charge diameters
d, mm 38.44 31.38 23.34
D, m/s 4172 3890 3370
Thus, the VOD decreases when the diameter of explosive charges decreases. This is explained by the effect of the Taylor wave.[16] In addition, the above results also indicate that this explosive has a critical diameter of less than 23.34 mm. Therefore the explosive can be used at diameters smaller than 32 mm compared to conventional commercial explosives.
Applying Eyring’s formula[17] expressing the dependence of VOD according to the diameter
1 Ar
D D
d
(2)
3027 3209
3891 3954 4019
3883 13.69
16.48
14.97 14.06
0 3 6 9 12 15 18
2500 3000 3500 4000 4500
6 8 10 12 14 16 18 20 22
Brisance, mm
VOD, m/s
Weight percent, %
Velocity of Detonation Brisance
3961 3891
3592 4350
3954
3659
2000 3000 4000 5000
< 160 mm Unsieved > 160 mm
VOD, m/s
Type of Cenospheres
10%
12%
(a)
15.56
16.48
12.67
15.03 14.97
13.28
5 8 11 14 17 20
< 160 mm Unsieved > 160 mm
Brisance, mm
Type of Cenospheres
10%
12%
(b)
Vietnam Journal of Chemistry Several explosion behaviours of the emulsion...
where D and D are VOD at the certain and infinite diameter respectively; Ar is the width of the chemical reaction zone.
Changing formula (2) to:
1 D D D Ar
d
(3) Thus, when d or 1/d 0 then D D. Therefore, if we express the relationship of D by 1/d, the intersection of the graph received with the vertical axis will be the maximum velocity of detonation D, and the slope of the straight line will be DAr (figure 5).
0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 3000
3500 4000 4500 5000 5500 6000
Experimental Regression Extrapolated
VOD, m/s
1/d, 1/mm
Equation y = a + b*x Adj. R-Square 0.99996
Value Standard Error
D Intercept 5409.39733 7.07271
D Slope -47615.80001 206.24496
Figure 5: The influence of the charge diameter on velocity of detonation
From the graph we receive:
5409 47616
r
D D A
or 5409 m/s 8.8 mm
r
D A
.
So the maximum VOD of emulsion explosive sensitized by 10% of unsieved cenospheres D = 5409 m/s, and the width of the chemical reaction zone Ar = 8.8 mm.
3.5. Heat of explosion and work peformance We measured the heat of explosion and the work capability of emulsion explosives using 10 % of unsieved cenospheres (over 100 % of emulsion matrix). The results showed that the heat of explosion equals 999 cal/g. This value is approximately equal to the explosion heat of TNT (1030 cal/g). The relative work performance of the explosive is 85.1 % compared to TNT. The work capability is not high due to cenospheres are an inert substance reducing the volume of gaseous products.
4. CONCLUSION
Cenospheres separated from fly ashes of thermal power plants have a similar ability to sensitize for emulsion explosives as expensive glass microspheres. However, we should use cenospheres in greater amounts. Smaller cenospheres have a better ability to sensitize than greater and unsieved cenospheres. Explosives sensitized unsieved cenospheres also have quite good explosion characteristics such as VOD, brisance, heat of explosion, and work capability.
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Corresponding author: Dam Quang Sang
Le Quy Don Technical University
236, Hoang Quoc Viet Str., North Tu Liem Dist., Hanoi 10000, Viet Nam E-mail: [email protected]
Tel.: +84- 963261081.
