Print7.2.2: Fumes
The whole point of ensuring an oxygen balance is to achieve the maximum energy from the reaction and to prevent the generation of toxic gases. Remember, however, that the production of large quantities of gas is key to the efficacy of the explosive to fragment rock. On average 700 - 1000 l of gases/kg of explosive are produced, and these are mostly benign – N, CO2, and water. It is impractical to achieve a perfect mix in the field, shot after shot, and consequently, a small (<4%) toxic component of about 3% CO and 1% NO2, depending on the oxygen balance, is not unusual. The percentage of toxic gases will increase significantly as the oxygen balance deteriorates.
Due to the possibility of a toxic component in a well-designed blast, certain precautions must be taken. The ventilation system in underground mines must be designed to dilute and carry away the gases from the blast, and no one should be allowed back into that part of the mine until sufficient time has passed to eliminate any gases from the blast. Some mines, and particularly smaller ones, make it a practice of blasting after the crew from the last shift of the day has exited the mine and the next shift won’t return until the following day. Other mines don’t have that luxury, and must ensure that a properly designed ventilation system is in place.
Fumes are normally less of a concern in a surface mine. However, it is important to note that fumes can be trapped in a pile of blasted rock, only to be liberated when the pile is loaded out. There have been a few fatalities from this, so this possibility is not to be ignored. The precaution of watering down the pile prior to loading is practiced both underground and in some surface applications. In addition to addressing a potential fume issue, the wetting action suppresses respirable dust.
A significant hazard can develop at surface mines if there is an excess production of NO2 or CO. In the late 1990s, I was part of a team investigating a problem in the Powder River Basin, in which the large blasts were producing thick yellow clouds of NOx – clouds that covered acres, and would drift for miles before dispersing! Fortunately, there were no reported ill effects and no fatalities, and that was lucky. My agency became involved when such clouds from a mine settled in a nearby town, near an elementary school filled with children. In the Basin, the problem was found to be water in the hole primarily, and loss of confinement as a secondary cause.
There was another illustrative problem that we investigated involving CO. These were trench-blasting applications, and there were a few fatalities due to CO poisoning. CO from the blast was trapped in the ground, but over a day or so, the gas migrated along a pipeline and entered a structure (basement), creating a toxic environment in the basement.
Fumes are an expected consequence when explosives are used. Proper procedures can help ensure an oxygen-balanced explosion, but will not guarantee a blast completely free of toxic fumes. Precautions, such as those mentioned here, must be implemented.