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Explosives and Blasting in Mines

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Today’s commercial explosives have resulted from a gradual evolution that began over 600 years ago. Black powder was first used in guns around the 14th centaury. But it was not until the 17th Centaury that this explosive began to replace fire setting as the principle method for loosening rock. As black powder became accepted in the mining Industry, the number of accidents increased and a need for safe explosives and initiating systems emerged.  N G Nair & Mr. M C Sharma At Malanjkhand Copper Project . IN- HOLE DELAY SYSTEMCHARGING & POSITIONING PRIMER

Explosives have been the primary method of breaking and loosening rock since the introduction of black powder and largely through dedication to research and development in safety and quality, have evolved into today’s wide range of safe and cost-effective products. When properly initiated, commercial explosive are rapidly converted into gases at high temperature and pressure. When confined by rock, expanding explosion gases result in extremely high strains in the rock. The energy released during detonation acts equally in all directions but as one would expect, tends to escape through any path(s) of least resistance. Therefore, blast holes should to charged and stemmed so that the gases are confined for sufficient time toprovide optimum breakage, displacement and looseness of the blasted rock. An explosive contains the following ingredients: An oxidizer is a chemical which provides oxygen for the reasons. Ammonium nitrate is b far the most common oxidiaser. Fuel reacts with oxygen to provide heat. Common fuels include fuel oil and aluminums powder. A sensiiser provides voids which acts as hot spots and at which reaction starts during detonation. Semsitisers are generally air or gas in the form of very small bubbles, sometimes encapsulated in glass micro balloons (GNBs). In-hole delay systems provide a predetermined time interval between each down line initiation and detonation of the corresponding explosive charge. A suitable time delay ensues that the initiation signal has reached the detonators within each charge before it or adjacent charges begin to disrupt the surrounding rock mass. This minimizes the risk of downlines being physically damaged or cut off by ground moment during the balst, and permits the use of longer inter-row delays where required for improved blast performance. In general it is recommended that all blast holes should contain two or more primers. This multipoint initiation provides security against disruption of the explosive column by round moment, of propagation failure within the charge because of water penetration, etc. Whether the primary delay in the charge is number N eg 600 ms the insurance delay will be number N1 ie. 650 ms to provide single point initiation under normal circumstances. Charging and Measuring Blast holes.All blast holes should be cleaned immediately after the drilling to ensure they are clear and are drilled to the correct depth. This is particularly important in broken and joined ground which falls in easily. Blast holes should be plugged immediately after drilling to prevent water, rocks and drill cuttings from entering the hole. Drill cuttings and rock may need to be cleared from around the collar to prevent these from falling into the blast hole during charging. All blstholes must have their depth measured and recorded immediately before charging. Short blastholes can lead to over charging toe, and digging problems, while overcharged blast holes cause fly rock hazards, noise, and reduce the effectiveness of the explosives.
 N G Nair & Mr. M C Sharma at Malanjkhand Copper Project.

Blast holes which are too deep must to back filled to the correct depth to prevent excessive vibration and damage drilled are also caused by excessive blast hole depth on upper benches. Bore hole charging, any blocked blast holes should be cleared with a charging pole or steel bar. It may be necessary to bring a drill back to the blast to clear, or re-drill some blast holes. A torch or mirror is often useful to examine the blast hole for blockages, cavity or rough sections before charging. Blasting in Mines - Video ClipsBlast Hole Open Stopes , 

Controlled blasting in an open-pit mine for improved slope stability.A controlled blasting programme (presplitting) was adopted in an open-pit copper mine in order to improve the stability of the resulting cut slopes. The aim was to reduce over break and minimize damage to the final pit walls from production blasts. The use of presplitting allowed considerably higher benches to be adopted with much steeper overall slope angles.