Larry Wilson, Group technical manager: Research and Development at AEL Mining Services, discusses the realities and facts of hot hole blasting. “Smouldering or burning coal deposits represent a serious problem in operations associated with the surface mining of underground coal seams. In some parts of South Africa underground coal fires have been burning unabated for years.
“These sub-surface fires present a real challenge to mining operations. Blastholes drilled down into the coal to facilitate blasting operations, however, present a more specific hazard. Despite a range of pre-drilling practices, such holes, whilst initially appearing cool and safe to load with explosives, can still undergo sudden and unpredictable escalations in temperature.
“In the absence of sufficient oxygen, coal is consumed by a fire at a relatively slow rate, but introduce oxygen, perhaps via a drill hole or a crack or fissure and the combustion rate and the burning temperature of the coal can increase significantly. Once hot enough, any explosives placed into the hole can be caused to initiate or ignite. This may be life threatening if there are personnel within range of the blast or its flyrock.
“To help address what might otherwise become an extremely hazardous situation AEL has developed Blast Eye a disposable, downhole temperature monitor that immediately warns personnel in the vicinity of such a ‘hot hole’ of the increasing thermal hazard.
“Standard coal mining practice dictates that during the charging up process, hole temperatures should be measured before any loading of explosives is attempted. All holes hotter than 80°C should be abandoned and not charged. Holes above 60°C must be cooled down by means of water or other chemicals, such as Pyrocool. If the cooling operation is successful, (less than 60°C) such holes can be loaded. Holes initially hotter than 40°C, but below 60°C are designated as hot holes and should be charged last and fired as soon as possible.
“The possibility, that despite all precautions, the reheating of a drill hole might still occur has given mine managers sleepless nights. A reheated drill hole, perhaps by now also charged with explosives, could expose any on-bench loading crew in the vicinity to real danger. Explosive products capable of working safely and effectively above 100°C are urgently required, but their commercial availability may still be years away. The problem of hot holes is here and now.
“The problem is getting worse because more and more coal mining operations are expanding into areas where underground fires exist. The heat, smoke and high temperature gases escaping from the burning coal deposits below can lead to unseen changes in the condition of affected blastholes. In a worst case scenario this can result in premature and uncontrolled detonation of the explosives column. Such an event could take place whilst the operators are still working on the bench.
“Most mines have excellent operating procedures for blasting in hot areas, however, there have been several incidents where primed but not charged holes have detonated prematurely.
“AEL engineers in the R&D department have consulted extensively with their coal mining customers. They have not only contributed risk to assessments, but have conducted cutting edge research into the development of explosives and initiating systems suitable for use on hot benches. As a result, the company has developed explosive formulations that can withstand higher temperatures.
“In addition, the AEL R&D teams have developed and recommended safe hot-hole blasting procedures, as well as providing systems that greatly improve safety on hot benches.
“Blast Eye, itself a product of rigorous field testing, is the latest in a series of developments that will provide continuous monitoring of temperatures within a hot blast hole and can provide an early audible warning if the temperature should exceed predetermined limits.
“The Blast Eye system is simple to use. A robust temperature monitoring device unrolled from Blast Eye is attached to the downhole pentolite booster and lowered into the hole. With the monitoring tip now in position, Blast Eye can be activated by pulling out the detatchable tab. The unit is now on guard. At 60°C, Blast Eye will sound a discontinuous alarm which immediately alerts bench personnel that a once safe hole is now hot. A continuous alarm at 80°C warns the bench crew that the hole is now dangerously hot and the bench must be evacuated.
“It also features both audible and visible confirmation that the unit is functioning correctly, and in addition warns the operator if there is a fault or break in the thermocouple lead line.
“The use of calcium nitrate based explosive formulations or even the use of RDX booster technology can also be vulnerable to hot-hole syndrome. Emulsion explosives contain a range of concentrations of ammonium nitrate combined with various other salts, a urea buffer solution and of course, water. Changing these concentration levels through exposure to an external heat source can influence the minimum burning pressure (MBP) of an emulsion explosive. This can lead to the emulsion becoming detonable if confined.
“MBP values of all emulsion explosives sold in South Africa are measured and the lower the MBP of a product, the more sensitive the product becomes to the influence of heat under pressure. All emulsion explosives sold to surface mining operations have an MBP of more than 70 bar, meaning that the product is safe to be pumped and used in hot hole scenarios.”