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Chapter 7: Blasting
November 1, 2007 By: Pit and Quarry Staff Pit & QuarryBlasting is an area that has come under very strict scrutiny in the post-9/11 work environment. Blasting is heavily regulated and watched by federal and local agencies.
In terms of processing, blasting is the critical first step in the rock-fragmentation process. Maximum profitability depends largely on good blasting. Consider that drilling and blasting are the first operations performed in any hard-rock quarry operation. Therefore, the results of these operations will affect more down-line activities, such as loading, hauling and crushing, than any other processing operation.
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Blasting should always be viewed in the "global" sense. One should examine not only the effect of changes on the drilling and blasting program, but also how the change will affect the productivity and economics of other down-line cost centers. Blasting should also be viewed in the "local" sense. No other quarry operation has more capacity to cause community dissent than blasting.
All quarry operations should have in place a proper public-relations program designed to communicate to the community that proper safety precautions and procedures are in place with regard to its blasting program.
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Fragmentation
In the 1990s increasing emphasis was placed on the role of fragmentation at the operation. In particular the effect of fragmentation on crushing, load and haul, and run-of-mine leach pad efficiency has received considerable attention. Better predictive techniques have been developed, and computer-aided methods for determining the fragmentation distributions in actual blasts are now available. Fragmentation studies can lead to improved profits at many operations. For example, studies at one operation showed that the same production could be obtained with one less excavator in good digging, when compared to poor digging conditions. This is a result with both capital- and operating-cost implications.
For maximum success it is essential that the mine or quarry carefully design its blasts to achieve the desired results. These designs must be accurately implemented in the field. The blasts must be shot in a safe manner, with the area properly barricaded and all persons removed a safe distance away. Environmental affects such as ground vibration, airblast and fume production must also be controlled.
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Explosives
The past 15 years or so have seen new explosive formulations reach the marketplace, and reductions in the use of some products that have been in use for longer periods. The principal newcomers have been the emulsions, and emulsion-ANFO blends usually called Heavy ANFO, that denotes its greater density than ANFO dry mixes.
Emulsions
The formulation of an emulsion is very similar to that of blasting slurries (water gels). However, the cross-linking agent used to stiffen the slurry is replaced by an emulsifying agent. This water-in-oil emulsifying agent suspends minute droplets of the ammonium nitrate (or a combination of AN with either calcium nitrate or sodium nitrate) oxidizer within the fuel. This yields a very intimate oxidizer and fuel mix that leads to high detonation velocities.
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Emulsions may be bulk loaded, or used in packaged form. Packaged products are usually employed in small hole diameters. They are mechanically sensitized using microballoons to introduce sufficient air into the mix and control the density. Bulk emulsions are used in larger diameters and may be mechanically or chemically sensitized, with chemical sensitization being less costly. Bulk-loaded product fully fills the cross sectional area of the hole and delivers maximum energy to the surrounding rock. This is a characteristic of all bulk-loaded products unless they are intentionally decoupled as is often the case in presplitting. Packaged emulsion will usually result in some decoupling with a reduction in borehole pressures. This generally is not a great problem in small diameter blastholes.
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