Influence of physical properties of ammonium nitrate on the detonation behaviour of ANFO

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Abstract

In order to obtain a better understanding of the non-ideal detonation behaviour of ammonium nitrate based explosives, detonation velocities of ANFO (ammonium nitrate and fuel oil) prepared with different kinds of ammonium nitrate (AN) were measured in steel tubes. In this series of test six kinds of AN were used and the influence of the pore diameter, the pore volume and the particle diameter of the AN particle on the detonation velocity of ANFO was investigated.

It was found that the pore diameter and the pore volume had a strong influence on the detonation velocities of ANFO. In the case of ANFO samples which were prepared with AN that had the same pore diameter and the pore volume, when tested the highest detonation velocity (3.85 km/s) was observed when the smallest particle diameter (<0.85 mm) was used. This value corresponded to 75% of the ideal detonation velocity, which was theoretically predicted by the CHEETAH code with the JCZ3-EOS.

The 12 months aging showed the change of the detonation velocities of ANFO and the reaction of ANFO was influenced both by the physical and the chemical properties of AN particles and oil during the storage period.

Introduction

Ammonium nitrate (AN) and AN-based explosives have been widely used as industrial explosives or energetic compositions such as ANFO, emulsion explosives or amatol (AN/TNT) etc. They are explosives known for their non-ideal detonation behaviour. This is shown, for example, by the detonation velocity which does not easily reach theoretically predicted values. Explosives behave non-ideally between the critical diameter (dc) below which a steady detonation wave cannot be sustained, and the minimum diameter (dm) above which the detonation is ideal. AN and AN-based explosives are typical “non-ideal” explosives because they have large values for dm and relatively small values for dc. For most practical conditions they will never reach the ideal behaviour as predicted by thermohydrodynamic theory. Generally the non-ideal detonation behaviour is explained by the relatively low decomposition rate of AN, which causes a wide reaction zone, in combination with lateral heat losses and rarefaction waves which extinguish the decomposition reactions (Cook, 1958).

It is the purpose of this investigation to obtain a better understanding of the non-ideal detonation behaviour of AN-based explosives. A series of experiments were carried out to study the influence of the physical properties of AN on the detonation velocities of ANFO. In the previous paper we reported the influence of the oil absorption capacity of AN on the detonation velocity of ANFO (Miyake & Ogawa, 1998). In this paper the influence of the pore diameter, the pore volume and the particle diameter of the AN particle are of particular interest. Furthermore, ANFO which had been stored at room temperature for 12 months was also tested to see the aging effect. Experimentally observed detonation velocities were compared to predictions made with the thermohydrodynamic CHEETAH code with the JCZ3 equation of state.

Section snippets

Materials

The materials used in this study are listed in Table 1. Six kinds of AN were in a variety of 715 to 760 kg/m3 in a bulk density and the mode pore diameter were in a range of 4 to 15 μm. All samples were prepared in a laboratory by changing the process conditions. Samples a–c have almost the same particle diameter but have different pore diameter and therefore different pore volume. On the other hand, samples d–f have the same physical properties but differ only in particle diameters, which were

Theoretical calculation

The ideal detonation velocity of ANFO has been calculated by the thermohydrodynamic theory describing the state attained behind the detonation front. We used the CHEETAH code in combination with the JCZ3 equation of state to calculate the ideal detonation velocity and Chapman–Jouguet parameters (Miyake & Ogawa, 1998, Fried, Howard, & Souers, 1998).

In the calculation of ideal detonation parameters of ANFO, n-Decane was used as the fuel oil. Based on the detonation calculation of ANFO for each

Experimental

Figure 3 shows the experimental set-up of the detonation velocity measurement of ANFO in a steel tube. The steel tube used was JIS-G3454 equivalent and the inner and outer diameters were 35.5 mm and 42.7 mm, respectively, and the wall thickness was 3.6 mm. The total length of the tube was 400 mm and the bottom of the tube was welded and closed with the same steel. 50 g of emulsion explosive (ρ=1160 kg/m3, D=5.85 km/s, P∼10 GPa) was used for the booster and initiated with a No. 6 electric

Results and discussion

The measured detonation velocities of ANFO are shown in Table 2. In this table the capital letters of the sample name (A, B, . . .) indicate the sample of ANFO prepared with the AN shown in Table 1 with the lower-case letters (a, b, . . .), and the capital letters with # show the 12 month aged ANFO. Two or more shots were carried out in the same conditions for unaged ANFO and one shot for each aged ANFO. A stable detonation was observed in all shots and the indexes of non-ideality of detonation

Conclusions

From the detonation velocity measurement of ANFO prepared with six kinds of AN which had different pore dimensions and particle sizes, the following conclusions can be drawn:

  • For the same particle diameter ANFO:

    • (1) The detonation velocity increased with the decrease of the mode pore diameter, and the highest detonation velocity was observed as 3.50 km/s which corresponded to 70% of the theoretically predicted value by the CHEETAH code with the JCZ3-EOS.

    • (2) The Dobs/Dcal value increased in the

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