Reactivity Screening
Reactivity screen is performed in accordance with the AEISG's code of practice to determine sample reactivity with ammonium nitrate.
Testing is also available to assess for the presence of reactive sulphides. This is an important test in case natural inhibitors have been included in the test sample.
qmr test
The QMR test evaluates environmental attributes that may inhibit or accelerate reactivity. If the screening test result is not reactive, it is good to know if that outcome was influenced by the presence of a natural inhibitor that may only be present in local lithologies.
Product selection testing
Sleep time testing is available to evaluate the compatibility of your inhibited explosives with your reactive ground. The product is tested for four times the maximum sleep time up to a maximum of 28 days.
What is reactive ground?
Sulphides are typically present at pyrite, however can be found in other minerals. In the presence of moisture, pyrite will weather, forming reagent products. Water, as humidity or as a solution, is necessary for the reaction to occur. In a cool dry environment, a reaction between ammonium nitrate prill and pyrite may not occur at all.
In a mining environment there will be multiple minerals and chemicals that act to inhibit or catalyse the reactivity of ammonium nitrate and sulphides. While water is necessary for the reaction to occur, too much water can dissolve the reactant products and dissipate heat, slowing or preventing the reaction from occurring. It is important to understand the reactions that are taking place to best apply your test results to your site, however it is prudent from a risk management perspective to assume the worst case environment and mitigate the risk of a reactive ground event via comprehensive risk management.
The complex chemistry of sulphide and nitrate reactions are simplified here as a progression. Reactive ground isn’t as simple as identifying specific minerals. Weathering of minerals, including via bacterial oxidation can result in unexpected chemical products.
This is explained concisely explained by the USBM (1979). Pyrite (FeS2), oxygen and water react to form a positively charged ferrous ion, a negatively charged sulphate ion and a positively charged hydrogen ion.
When weathering occurs, the ferrous iron is oxidised to become a ferric iron. The ferric iron reacts with pyrite, resulting in iron sulphides. This reaction can continue unabated and can be increased by certain bacteria, which will create sulfuric acid and ferric iron. Where pyrite exists it is therefore possible that the following reagents will have accumulated: pyrite, sulfuric acid, ferric and ferrous sulphates. This is the same process that causes acid mine drainage.
The induction stage is when ammonium nitrate is introduced into a reactive environment, it will react with iron sulphides, ferrous ions and sulphuric acid to form nitric oxide and ferric ions. The reaction zone will become more acidic and as the redox reaction is exothermic, the temperature will increase.
In the intermediate stage the primary source of the reagents supporting the reaction are no longer environmental. They are provided by the products of the induction stage reaction. Nitric acid and ferric ions, react with pyrite to create the reagents to catalyse the reaction with the ammonium nitrate.
The final stage is ignition. It is critical that risk management and testing has been sufficient to prevent this happening at your site. In the lead up to ignition, the temperature of the bulk product may exceed 500°C and detonate via the decomposition of Ammonium Nitrate to ammonia gas and nitric acid. Ammonia has much higher vapour pressure than nitric acid and the melt self-acidifies, increasing the heat of reaction with any sulphide at temperatures greater than 150°C. The ammonia gas cracks off hydrogen at approximately 500°C and ignition and/or detonation of hydrogen can cause shock initiation of sensitised bulk products.
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