OFFICIAL in most commercial explosives. When the heat generated by reaction cannot be dissipated temperatures can escalate dangerously, this is known as thermal runaway. The reaction between AN and sulphides has a highly variable induction time and can generate intense heat, potentially leading to premature or unplanned detonation of the explosive charge (Rumball, 1991). This is a serious safety hazard. Hot and reactive ground conditions have caused multiple premature detonation incidents in the industry, with at least 13 known unplanned detonations between 2010 and 2019, resulting in 6 fatalities and 15 injuries. These statistics demonstrate that maintaining explosive safety during extended sleep is vital to prevent future incidents. To mitigate reactive ground risks, explosives manufacturers developed reactive ground tests to identify reactive ground and inhibited explosive formulations that suppress the ANsulphide reaction. These inhibited products substantially delay or prevent the runaway reaction, allowing charges to sleep longer in reactive ground. Even so, current guidelines and industry practice limit routine sleep times in reactive ground to about 7 days. For example, the Australian AEISG Code of Practice for Elevated Temperature and Reactive Ground (AEISG, 2020) details the industry standard isothermal2 reactive ground test and mandates laboratory testing for 4 times the planned sleep duration (up to a 28-day test for a 7-day field sleep time) and generally advises one week as the maximum without special precautions. Beyond one week, mines must perform additional risk assessments in collaboration with the explosive supplier, that may include extended lab tests. Sulphide minerals are almost ubiquitous through metal mines (D. Vaughan, 2017), and iron sulphides in particular are commonly co-located-located with coal worldwide (Swaine, 1984). However, reactive ground was identified in about 1 in 5 mines over the course of their operations. The level of reactivity can vary with geology both between sites or even between mining areas at a mine (Valenta, 2019) (M. Frenzel, 2023). Understanding the level of reactivity requires close cooperation with the geology teams to ensure targeted worst-case sampling. There are some mines or mining areas that won’t be suitable for long sleep times, for example if there are self-heating ores that could exceed the safe temperature over time or if the reactive samples fail the qualification test by overwhelming the inhibitor system in the product. New mining methods made possible by wireless blasting demand longer sleep times still. Orica’s analysis of WebGenTM-enabled mine plans identified the need to increase the bulk explosive sleep time up to around 90 days to deliver the full benefits of safety and productivity. Crucially, no standard test protocol or industry precedent existed to qualify explosives for such long sleep times in reactive ground. Two key gaps had to be addressed: (1) developing a reliable laboratory testing methodology to qualify an inhibited explosive to safely withstand ~3 months in reactive conditions, and (2) formulating an explosive with sufficient inhibitor to pass such a test while still meeting operational, cost and performance needs. 2 Isothermal tests target a set temperature but may not account for heat transfer between the test sample and the apparatus or environment.
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