According to (Sharma, 2011), misfire means the complete or partial failure of a blasting charge to explode as planned. Misfires are a significant concern in mining, quarrying, and civil construction due to safety risks, production delays, and additional costs. Scientifically, misfires occur when the initiation system or the explosive charge fails due to several factors like poor priming, faulty detonators, moisture ingress, or improper stemming.
Dealing with a misfire is potentially the most dangerous activity that site managers and Shotfirers will be involved in during blasting operations. In the event of a misfire, it is likely that unexploded charges and detonators will be left in the face or in the muckpile. These charges could be detonated if drilled into, if struck by an excavator bucket, wheels or tracks, or if inadvertently fed through a crushing plant. Unexploded charges may also be loaded out accidentally and taken either off site in road vehicles or to site tips. In any of these circumstances there may be a risk of danger to the operator or to the public, particularly from fly-rock in the event of a detonation (Sharma, 2011).
There are many different causes of misfires. According to (Taiwo, 2022), they can be classified into two categories: technical failure and site conditions. For the sake of clarity, we’re just going to mention a few of them.
- Poor initiation system: faulty detonators, damaged shock tubes, or weak electrical connections can disrupt the energy needed for detonation. Defective wiring is a common cause of misfires in electrical blasting.
- Improper handling and storage: mishandling explosives or storing them in unfavorable conditions, such as exposure to moisture, heat, or cold, can degrade their effectiveness. Temperature cycling or unintended use of expired or degraded explosives can also cause explosives to fail.
- Environmental factors: wet ground conditions, water ingress in blast holes, or adverse weather can dampen the explosive material and delay or prevent detonation. Acidic ground water can also degrade explosives.
- Product failure: explosives and initiation products can fail, including dynamic desensitization of explosives and dynamic pressure damage to detonators. The incompatibility of key product constituents and the use of different quality diesel or oil can also cause product failure.
To manage misfires, some measures should be taken, let’s check some of them:
- Re-entry protocolsAfter a blast, allow time for gases and dust to clear before re-entry. With multi-blast conditions, a re-entry period of 30 minutes must be allowed. If the blast initiator failed, allow 30 minutes for pyrotechnic systems and 5 minutes for electronics before re-entry.
- Removing stemming and re-priming: when removing stemming, avoid force if the detonator is close to the explosive. Do not use high-velocity air with electric detonators to prevent static charges. Take special care when removing cartridges, especially where detonators are involved, and never pull detonating cords or detonator leads. Use suitable extracting tools if needed. High-pressure water can be used to flush explosives, desensitizing non-waterproof explosives and dissolving water-soluble ingredients.
- Drilling relieving holes: if it is not possible to explode a misfire by re-firing, a relieving hole should be drilled parallel to the original hole, then charged and exploded. Mark the misfired hole clearly or block it with a wooden plug. Maintain a safe distance between the misfired hole and the relieving hole, depending on the size and length of the misfired hole; when the misfired hole is 50 mm or less in diameter and less than 3 m in length, do not drill the relieving hole closer than 600 mm to the nearest point of the misfired hole. Short-circuit the detonator wires and tie them to a permanent object when an electric detonator is involved so they can be recovered after firing the relieving hole. The relieving hole must be parallel to the misfired hole and be of the same depth.
- Best practices: adequate work area preparation and demarcation before drilling and blasting is critical in preventing misfires. Conduct all blasting activities, including post-blast activities, under risk-based procedures established in the site’s safety management system.
Reference
Sharma, P. D. (2011, December 30). HANDLING OF MISFIRES IN MINES. Mining and Blasting. https://miningandblasting.wordpress.com/2011/12/30/handling-of-misfires-in-mines/
Taiwo, B. O. (2022). Blasting Misfire: A Review of Causes, Economic Effect, Control and Handling Techniques. 3(1).
