Andrzej MARANDA, Radosław SZYMAŃSKI ? Institute of Chemistry, Military University of Technology, Warsaw

Abstract:
This paper presents the measurement results of detonation velocity and critical diameter of two-component mixtures containing ammonium nitrate (V) and organic fuel, and  of three-component mixtures which additionally contain aluminium powder. The effects of the type and composition of organic fuel on detonation parameters of these  mixtures were determined.

Please cite as: CHEMIK 2013, 67, 1, 13-18

Introduction

The first explosives containing ammonium nitrate (V) were produced at the beginning of the 19th century. Two scientists, Grindel and Robin, substituted saltpetre in gun powder with ammonium nitrate. Reise and Millon also reported the explosive properties of this mixture in 1849 [1]. The first patented mixture containing ammonium nitrate and fuel was ammoniakkrut ? a mixture of ammonium nitrate (V) and charcoal in proportion 80/20 [2]. The patent was filed by two Swedish scientists, Ohlsson and Norrbin, in 1867 [3]. At the end of the 19th century, such mixtures began to be used in mining. The examples are: Dynammon ? a mixture of ammonium nitrate (V) and charcoal in the proportions 88/12 or 87/13, and wetter-dynammon ? a mixture of ammonium nitrate (V), potassium nitrate (V) and charcoal in the following proportions by weight: 94/2/4. Dynammon was approved by the Austrian government and applied in mining, including hard coal mining, which was not a good idea because this compound was hazardous in presence of dispersed coal dust in the air and methaneair mixture. The explosive known as aerolite containing the following oxidisers was used in the mining industry in Denmark: ammonium and potassium nitrates (V) and organic fuels: sago flour, oil and resin; whereas in the mining industry in Germany, anilite containing ca. 5% of beet sugar was applied [4].

The increasing interest in the mixtures of ammonium nitrate and fuels was shown during the First World War, when the greater demand on explosives also led to the use of mining explosives during combat actions. At the end of the war, the mixtures of ammonium nitrate (V) and wood flour, flour or peat were produced due to seriously reduced material resources. Dynammon K, that is, a mixture of ammonium nitrate and wood flour (90/10) used in the USSR as rock explosive can be used as an example. The detonation velocity of Dynammon K was 2200 m/s, and a blister in a lead block was 320 cm3 [5]. After the First World War, other patents on ammonium nitratebased explosives containing only ammonium nitrate (V) and organic fuel started to appear. Dehn was among the first people who filed the patents in this field. In his patent from 1923 [6], he remarked on insufficient contact of the components in the mixture patented by Ohlsson and Norrbin and proposed imbuing wood flour with saturated solution of ammonium nitrate (V), and then evaporating the solvent at the maximum temperature of 130oC, grinding the obtained mass and reheating it to remove remaining moisture. Such a prepared explosive was characterised by higher velocity of detonation and lower hygroscopicity than the composition prepared only by mechanical mixing of its components.

There are more patent descriptions than scientific papers. Paper [7] described the determination of the detonation velocity of ammonium nitrate (V) mixtures with nitroguanidine nitrate (V), urea, mannite, pentaerythritol and sorbite, depending on the content of the organic component. For some flammable components, the dependence of critical diameter on their content in the explosive mixture was also determined. However, paper [8] described the tests on the dependence of detonation velocity on the volume ratio of fuel/ ammonium nitrate (V) for explosives containing: sawdust, cellulose, charcoal, peat and woodchips. This paper describes the test results for selected detonation parameters (critical diameter, detonation velocity) of ammonium nitrate (V) explosives containing: sugar (saccharose), dry skim milk, flour, coffee, starch, wood dust and PVC. The two-component mixtures (ammonium nitrate (V) ? fuel) and three-component mixtures (ammonium nitrate (V) ? fuel ? aluminium powder) were subjected to tests.

Experimental part

Testing detonation parameters of two-component mixtures

Granulated ammonium nitrate (V) used in our experiments was produced by Nitrogen Plant Puławy S.A. [Zakłady Azotowe Puławy S.A.], and then powdered in the company Nitroerg S.A. It was subjected to screen analysis using the vibrating screen As200 of Retsch Company. The results are presented in Figure 1.

Also the screen analysis for sugar and dry skim milk was carried out. The experimental results are illustrated in Figures 2 and 3.

On the basis of the comparison of screen analyses of ammonium nitrate (V) with two tested fuels, it was found that sugar was the fuel of more similar granulometric composition to the oxidiser than dry milk whose grain size was smaller than 75 ?m in more than 50% of the analysed specimen. The mixtures containing ammonium nitrate (V), whose homogeneity was verified visually, were prepared from fuel specimens.

Then, critical detonation diameter was determined using the method of conical charge. Two types of cones were used. In a small cone, the measurement was performed in a distance of 290 mm, the diameter of base was 35 mm and of the truncated part of the cone ? 6 mm; whereas for a large cone, these sizes were 350 mm, 50 mm and 30 mm, respectively. As the tested explosive mixtures were initiated with a booster made from plastic explosive of 10 g, the charges were elongated from the bottom using the tubes of 35 mm diameter (the small cone) or 50 mm diameter (the big cone) and the length of 100 mm to eliminate the booster effect on the measurement result. The measurement results are presented in Table 1 and Figure 4.

 

The further series of tests, that is, the measurements of detonation velocity were performed using the technique of ionisation probes in Vinidur tubes having the outer diameter of 50 mm and the wall thickness of 1.8 mm. The charges were initiated with boosters used for determining the critical detonation diameter. The average test results for detonation velocity are presented in Table 2 and Figure 5.

Testing detonation parameters of three-component mixtures

The three-component mixtures contained additionally 1% of aluminium flake powder produced by Benda-Lutz plant. The powder contained 93% of pure aluminium, the water coverage was 5500 cm2/g, and the average size of grains was 65 ?m. The detonation velocity was measured for charges containing 10% of fuel. The tests were performed under analogical conditions as for two-component mixtures. Moreover, the detonation velocity was measured for ammonal containing 11% of aluminium powder. The measurement results are presented in Figure 6.

Discussion about test results

By comparing the dependence of critical diameter on the content of organic fuel for explosive mixtures containing ammonium nitrate and saccharose in the amount of 5÷20% and for the explosives discussed in paper [7] and illustrated in Figure 7, the analogical nature of curve directions was found.

These curves show the minimum value of critical diameter for a specified content of organic fuel. For the mixtures of ammonium nitrate (V), mannite and pentaerythritol, the smallest critical diameter of 26 mm was calculated for the charges containing 5% and 10% of mannite, and 5% of pentaerythritol. For 10% content of crystalline urea in the mixture, the critical diameter was 45 mm. The smallest critical diameter of 20 mm was calculated for the mixtures containing saccharose and it corresponded to 10% content of this fuel. It should be noticed that the critical diameters for the mixtures containing saccharose and guanidine nitrate (V) were almost identical in spite of different chemical natures of these fuels. For 15% content of saccharose in the mixture, the critical diameter was 22 mm and it was the same as the one determined for the mixtures containing 40% and 60% of guanidine nitrate (V).

The tested flammable components had different calorific values (commercial parameter) according to their manufacturers. For example, for flour this value was 1460 kJ/100 g, for dry milk ? 1507 kJ/100 g, and for sugar ? 1700kJ/100 g. Figure 8 illustrates the dependence of critical detonation diameter for the mixtures containing 10% of fuel on the calorific value of fuel.

Despite the fact that the grain size of used fuels was not identical, it could be found that the calorific value of fuel influenced the thermal balance in chemical reactions occurring in the detonation wave. The higher the calorific value of fuel is, the more heat can be transferred to maintain the detonation front, which, consequently, could propagate in the charges of smaller diameters. The measurement results for detonation velocity indicated that the presence of aluminium powder in the mixture had a fundamental importance in the detonation process of such explosives as ammonite. For three-component mixtures containing 1% of metallic additive, considerably higher values of detonation velocity were achieved. The comparison of detonation velocity of the two-component mixture (ammonium nitrate (V)/flour 90/10) and the three-component mixture (ammonium nitrate (V)/flour/aluminium powder 89/10/1) indicated that even a low amount of aluminium powder increased the detonation velocity by over 700 m/s.

Summary

Low price and availability of ammonium nitrate make it the basic component in mining explosives. In wartime, it was also used as the military explosive, and recently it has attracted an increased interest from terrorists. The detonation parameters of ammonium nitrate (V) can be adjusted by changing the fineness of grinding (specific surface in case of bulk and granulated explosives) and by adding different types of flammable components. Fuels cause the shift in oxygen balance of the explosive mixture towards zero value, which increases heat change of chemical reactions occurring in the detonation and Taylor waves.

These tests were mainly conducted for research purposes. They expand the knowledge on the detonation parameters of ammonium nitrate-based explosives. At the same time, they confirm the thesis known for many years that the addition of any component of negative oxygen balance to ammonium nitrate (V) results in obtaining an explosive mixture having the capacity of detonation and thermodynamic parameters considerably higher than in case of just the oxidiser. The substances tested during this project, a part of which were food products, can be used as such components.

Literature
1. Akhavan J.: The chemistry of explosives, The Royal Society of Chemistry, Cambridge, 2004.
2. Military explosives, Department of Army, Washington, 1984.
3. Zgłoszenie patentowe nr 2766, Wielka Brytania, 1867.
4. Marshall A.: Dictionary of explosives, Publ. Blakiston?s Son&Co., Philadelphia 1920.
5. Urbański T.: Chemia i technologia materiałów wybuchowych, tom 2, wyd. MON, Warszawa, 1955.
6. Zgłoszenie patentowe nr 1444594, USA, 1923.
7. Maranda A.: Badanie parametrów detonacyjnych mieszanin azotanu amonu z niektórymi paliwami organicznymi, Biul. WAT, 1989, 37, 8-9.
8. Kozhevnikov V.E.: Detonation of ammonium nitrate and dynammons with and without inert additives, Combustion, Explosion, and Shock Waves, 1999, 35, 3.
9. This paper was presented during the Conference on the Safety of Blasting Operations, Ustroń, 10-12 October, 2012

This scientific work is financed from the Polish science budget for years 2010 ? 2012 as the research project.

Andrzej MARANDA ? (Sc.D., Eng) Professor , graduated from the Faculty of Chemistry at the Warsaw University of Technology in 1971. He is currently working at the Military University of Technology and the Institute of Industrial Organic Chemistry in Warsaw. Research interests: chemistry and technology of explosives, environmental protection. He is an author and a co-author of five monographs, 20 patents and over 500 articles and papers published in national magazines and presented at international conferences.
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Radosław SZYMAŃSKI ? M.Sc., graduated from the Faculty of New Technologies and Chemistry at the Military University of Technology in 2012. Specialisation: Explosives and pyrotechnics.