Reactive Ground Testing and a Better Understanding of the Pathway to Detonation due to Heat in Transport and Storage of Bulk Explosives

Stage 1 Reactive Ground Testing

There have been incidents of premature detonations in sulphide containing material. A key finding is that host materials including carbonates and coal are inhibitors to the reaction of sulphide with nitrates. The mechanism of inhibition is pH buffering and isolating the surface of sulphide from reactants. It has been identified that acidity is a key driver of the reaction with a pH of approximately 2 required for a reactivity event.

A series of decomposition experiments confirmed that reactive and hot ground are essentially the same phenomenon with the heating produced either by the reaction of sulphide with nitrates or an external heat source. The results indicate that the key driver of detonation in reactive ground is energy and heat released from the acid induced reaction of sulphide and nitrate with no indications of sulphide being a catalyst or contributor to detonation apart from being a heat source. In addition to sufficient sulphide there must be thermal insulation to provide and retain enough heat to increase the temperature.

A pathway to detonation is postulated whereby the heat travels up the explosive column and results in evaporation of water in the top portion of the explosive. When the temperature reaches approximately 200-300°C the ammonium nitrate (AN) starts to decompose, decreasing the mass of the reacting species.  Decomposition of ammonium nitrate produces copious amounts of white fume primarily consisting of water vapour and nitrogen. If there is a high level of heat energy and large enough mass of AN, the decomposition will produce ammonia. When the temperature exceeds 500°C the ammonia cracks to produce nitrogen and hydrogen.

A possible pathway is:

NH4NO3 ↔ HNO3 + NH3    Equation 1

2NH3↔N2 + 3H2         Equation 2

Hydrogen has an autoignition temperature of approximately 600°C and, when exposed to air, ignition of the hydrogen leads to detonation of the AN.

The reactive ground test method currently used in the mining industry is based on the assumption that sulphides cause violent decomposition and detonation of explosives despite no experimental evidence to support this assumption. It is thought that this assumption is due to investigations into incidences incorrectly attributing hot-ground initiated events to reactive ground.

The current test method has arbitrary measures to determine if samples are reactive. This results in samples that are acidic with no buffering capacity but with harmless trace levels of sulphide to test as reactive. In contrast, samples with high levels of sulphide, which can rapidly oxidise when exposed to air and spontaneous react with explosives, may test as not reactive. A new test method has been validated that provides a definitive reactivity classification based on the reactive sulphide content and the buffering capacity of the host material.

Stage 2 Better Understanding of the Pathway to Detonation due to Heat in Transport and Storage of Bulk Explosives

Research conducted in stage 1, Reactive Ground Testing, was not able to experimentally verify that reactive sulphide is a catalyst or contributor to the detonation of bulk explosives. Reactive sulphide may provide a source of heat from the exothermic chemical reaction of sulphide and nitrates contained in bulk explosive. It was demonstrated that ammonium nitrate (AN) emulsion loaded into a lined hot blast hole will detonate due to heat alone.  The accepted risk of AN emulsion detonating due to events such as this was assumed to be negligible due to the high-water content of the explosive. This was thought to prevent a rise in temperature greater than the boiling point of the material until the water evaporates and also a lack of sensitization.  A review of literature identified that the pathway to heat induced detonation of (AN) is essentially unknown with there being more than 80 proposed AN decomposition reaction (Babrauskas & Legger 2019) (Bauer 2015).

Experimental results and review of incidences of spontaneous detonation identified that:

• Decomposition of ammonium nitrate always occurs before detonation.

• The heat source must be at the top of the explosive column for detonation to occur.
• High levels of heat energy are required.

AN normally decomposes when the entire mass is raised to a temperature greater than 200°C. This results in a white gas cloud consisting of mainly nitrogen and water vapour.

A pathway to detonation was postulated whereby decomposition of AN, due to a high level of heat energy, results in the production of ammonia gas. The ammonia gas is then cracked at high temperature to produce hydrogen. This is a surface reaction. The water content of the explosive increases the rate of hydrogen production by vapour removing heat. The reaction does not produce oxygen. Hydrogen accumulates until exposed to air. Hydrogen in air has an autoignition temperature of approximately 600°C. Ignition of the hydrogen occurs when this temperature is reached which leads to detonation of the AN.  This pathway to detonation was experimentally verified by both detection of hydrogen during high temperature decomposition and detonation.