Cause analysis of spontaneous combustion and self-detonation of homemade ammonium nitrate explosives

Abstract

With the strict control of industrial explosives, the easy accessibility of raw materials and simple preparation methods have made homemade explosives (HMEs), especially ammonium nitrate (AN)-based HMEs, widely manufactured, sold, and used by criminals and terrorists. Fertilizers containing chemically modified AN are often employed to produce AN-HMEs. To enhance explosive power and sensitivity, criminals commonly add highly energetic substances such as potassium chlorate (KClO₃) to these fertilizers. However, due to the incompatibility of the added substances, spontaneous combustion and self-detonation often occur. The investigation of such spontaneous combustion and self-detonation is more difficult than that of explosions triggered by fire or flammable materials, as they typically arise naturally during storage without external heat source. The compositional analysis of explosive residues deposited at crime scenes is therefore of significant forensic value for subsequent crime reconstruction. Nevertheless, factors such as crystal phase, thermal properties, component compatibility in explosive mixtures, and the intermediates formed are often overlooked. In this study, an explosion during storage caused by AN-HMEs illegally mixed with KClO₃ was investigated. The composition of the explosive residues was analyzed, and simulated storage experiments were conducted under both open and closed conditions. The phase composition, thermal stability, and compatibility of the explosive mixtures in storage experiments were examined to elucidate the mechanisms of spontaneous combustion and self-detonation. Additionally, attempts were made to explain why the characteristic ClO₃⁻ ion added to the fertilizer was not detected in the residues. The results indicate that although AN and KClO₃ are thermodynamically stable individually, their mixture reduces the thermal stability of AN, revealing their incompatibility. It was demonstrated that under room temperature and closed storage conditions, AN and KClO₃ can form an unstable intermediate, ammonium chlorate (NH₄ClO₃). Subsequently, NH₄ClO₃ was synthesized, characterized, and its crystal structure analyzed via Rietveld refinement. NH₄ClO₃ decomposes at 58 °C, releasing heat that accumulates within the explosives and forms hot spots, thereby initiating detonation. Since the decomposition products are gaseous, ClO₃⁻ was not detected in the explosive residues.