Open Cut » Environment
Post blast fume has a pungent, acrid odour, features strong colours ranging from yellow to deep red or purples, and is sufficiently toxic to cause serious personal injury. Project C20016, A method to assess and minimise the potential for fume generation from blasting was undertaken to understand the mechanisms that caused post blast fume and develop a blast fume mitigation strategy. It was identified during the project that the propensity for blast fume increased in three particular scenarios:
· In deep holes;
· In wet ground; and
· In soft ground.
ACARP and three mining companies agreed to fund a subsequent project to investigate the physical and detonation characteristics of bulk explosives in these conditions and subsequently minimise the risk of post blast fume. The main objectives of this project were to:
· Develop laboratory and field procedures to assess fumes generation propensity of bulk explosives currently used in deep holes, soft ground and wet conditions;
· Understand the physical and detonation characteristics of bulk explosives used in deep holes, soft ground and wet conditions through lab scale simulations and field measurements; and to
· Develop solutions to minimise the risk of fumes incidences in deep holes, soft ground and wet conditions.
The performance of a detonation is a function of the explosive quality and the environment in which it is detonated. To evaluate the propensity of specific conditions to result in blast fume, a mobile laboratory was developed. Testing was performed on explosives and explosive precursors to evaluate their quality. Index tests were also performed to measure an explosives' consistency over time.
The propensity for blast fume in the three ground conditions listed was investigated statistically. The Fume Steering Group, a Queensland focused industry working group provided their database of over 5000 blasts for analysis. The increased likelihood for fume in wet ground, soft ground and deep holes was identified.
The effect of wet ground on fume propensity was investigated using scale tests, ground water simulation and detonation testing. The hygroscopic nature of ammonium nitrate prill was observed to wick water through high proportion heavy ANFO blends, compromising the explosive. Detonation testing performed on wetted samples of ANFO linked the propensity of fume to the extent of degradation caused by water.
Numerical simulation was used to model the detonation characteristics of ANFO and heavy ANFO. The rapid expansion of the denser, heavy ANFO detonation was found to result in rapid cooling, creating an environment where the synthesis of NO2 is supported.
The effect of deep ground on fume propensity was investigated using VOD and down hole density measurement. Technical issues prevented the identification of definitive link between depth and fume.
A fume mitigation plan is proposed in consideration of the research conducted in this reported, including the literature review. It is recommended that identifying and quantifying any adverse ground conditions is conducted prior to designing a blast. The optimal explosive product must be used; variations to design were identified to result in an increased fume risk. Two flow sheets are presented that summarise the decision making process to minimise the propensity for fume on a mine site.