Understanding main causes of nitrogen oxide fumes in surface blasting

Post-blast nitrogen oxide fumes (NOx) from surface blasting activities have become an important operational issue in Australian Coal mines. Post-blast fumes are a direct product of the detonation process which can be easily identified as the resultant yellow to orange post-blast clouds. There is general agreement that the conditions leading to fumes are associated with fuel deficiencies or incomplete detonation of the explosive product. From a practical perspective this can be due to one or a combination of factors such as explosive product characteristics, confinement effects, ground conditions, inappropriate blast design parameters, explosive product selection, on-bench practices and potential contamination of explosive product in the blasthole. This paper presents the preliminary results of a project funded by the Australian Coal Association Research Program (ACARP). The main objective of this project is to gain a better understanding of the principal causes of post-blast fumes at the operational level so that incidences can be minimised. Results from a comprehensive literature review, an industry survey and analysis of newly established blast registers have indicated that an appropriate matching of product to ground is essential to minimise fume incidences. For this to occur there must be a clear understanding of the potential impact of the characteristics of key components in any given product formulation and ANFO/Emulsion blends. Analysis has confirmed the impact of confinement and ground conditions as well as the potential contamination of product with fine drill cuttings near the stemming region. Trends associated with increases in the likelihood of fume incidences from long sleep times were identified but could not be entirely confirmed with the available data. The analysis on overburden blasts showed that it is fair to assume that the likelihood of high level fume incidences may increase when a product is slept for >10 days and that decisions to have product sleeping for longer periods should be supported by a good understanding of the product characteristics and ground conditions (geology and hydrogeology). Further work is required to build capabilities to verify on-bench product specifications and characteristics including the ability to measure in hole density changes, product moisture, verify AN/Emulsion blending ratios and undertake a more through characterisation of ground conditions. It is also important to extend the range of measurement systems to evaluate the performance of the detonation process in situ. To that end the authors have embarked in the development of instrumentation to better understand the impact of changing characteristics of explosive products in situ; a brief description and preliminary results from this research effort are also discussed in this paper.