The determination of the composition of ammonium nitrate emulsions is essential for monitoring the quality criteria of the emulsions as well as their stability during storage in order to predict their shelf life. The investigation of these types of emulsions can be time-consuming because a combination of various methods is usually necessary. With the aid of complementary thermal analysis techniques (TGA and DSC), nearly all the constituents of the emulsion can be quantitatively determined with just a minimum of analytical work. The different methods are presented in this article.
“Water-in-oil” ammonium nitrate emulsions are the main components of emulsion explosives. In comparison with traditional nitroglycerine-based dynamites, these explosives have several advantages. Following their development in 1962 [1, 2], they have become widely used due to their good safety properties (handling, insensitivity to temperature change), their high detonation rate, and their low basic cost.
The emulsion itself is not an explosive. An effective commercial explosive is obtained when glass or plastic microbubbles, which lower the density of the system, and metal particles (optionally) like aluminum powder as high-energy fuel are mixed into the emulsion. The explosive can be prepared in a mobile unit on site before the final explosive is pumped into the borehole [1].
In the emulsion, the aqueous phase consists of a supersaturated solution of ammonium nitrate (AN), which is metastable due to supersaturation. Sometimes, other salts such as sodium nitrate (SN) or calcium nitrate are added.
The aqueous phase with a droplet size of around 1 µm is emulsified in a small volume of hydrocarbon oil, which forms a thin film around the droplets. The dispersed aqueous phase contains approximately 90 mass percent of the liquid fraction and the remaining 10 mass percent corresponds to the oil phase. Ammonium nitrate makes up about 60 to 80 percent of the overall composition and is the major ingredient of the emulsion. An emulsifier enables the formation and stabilization of the emulsion, which consists of two phases with very different polarity [1].
During storage, evaporation of water can cause the ammonium nitrate to crystallize. Crystallization leads to breakdown of the emulsion and loss of the explosive properties. On the other hand, a water content that is too high inhibits detonation.
For this reason, stability tests require accurate and precise results for the water content and the state of the ammonium nitrate with respect to its amorphous or crystalline phases [1]. The contents of ammonium nitrate and hydrocarbon oil are important for monitoring the quality control of the emulsion.
Quantitative analysis of the emulsion can be time-consuming because a combination of different methods is usually necessary to detect and determine a range of chemical compounds [3]. With the aid of complementary thermal analysis techniques (TGA and DSC), only a minimum of analytical work is needed to quantitatively determine nearly all the components in ammonium nitrate emulsions. Besides this, the time-consuming separation of the aqueous and oil phases is not necessary so that no errors can arise from this.
Ammonium nitrate 99.999% (AN standard) and sodium nitrate 99.999% (SN standard) from Sigma–Aldrich were used as standard substances. Two commercial explosive samples used in Slovenia were analyzed. Both were “water-in-oil” emulsions. In Sample 1, ammonium nitrate was the sole oxidizing agent, whereas in Sample 2, some sodium nitrate was added...
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Thermal methods of analysis (TGA and DSC) proved to be suitable for determining the water content and the amounts of ammonium nitrate (AN) sodium nitrate (SN) and organic phase in “water-in-oil” emulsions. The water content was determined by isothermal TGA measurement at 100 °C.
In Sample 1, the ammonium nitrate content was determined by performing ten DSC heating and cooling cycles between 25 and 135 °C. During this experiment, water evaporated and ammonium nitrate crystallized. The ammonium nitrate content was calculated from the enthalpy of the phase transition at 126 °C. The content of the organic phase (oil plus emulsifier) was calculated from the difference to 100%.
In Sample 2, with ammonium nitrate and sodium nitrate in the solution, the content of sodium nitrate corresponded to the weight remaining at 470 °C in the dynamic TGA measurement. The unknown ratio of AN/SN in an emulsion was determined from the liquidus curve of the binary phase diagram, part of which was constructed for this purpose.
The content of ammonium nitrate in this sample was calculated by assuming that the ratio of AN and SN in Sample 2 is equal to that in the aqueous phase. The water content was again determined by an isothermal TGA measurement at 100 °C. The difference to 100% was assigned to the sum of hydrocarbon oil and emulsifier.
Composition of Water-in-Oil Ammonium Nitrate Emulsions using TGA and DSC | Thermal Analysis Application No. UC393 | Application published in METTLER TOLEDO Thermal Analysis UserCom 39