It is a well known fact that in blasting the best rock breakage occurs when the detonation velocity (DV) of the explosive matches the speed of sound through the rock being blasted. The explosive ANFO is made from a intimate mixture of ammonium nitrate and fuel oil that because of its low cost provides the preponderance of explosives used in mining and quarrying. In most cases the detonation velocity of ANFO is much lower than the speed of sound through the rough is being blasted resulting in poor breakage.
|An explosive charge is fired in a quarry.|
In our quarry we were mining granitic gneiss that had a speed of sound through the rock of about 22,000 FPS roughly 10,000 FPS higher then the DOV of ANFO. The resulting blast using 12,000 pounds of powder loaded into bore holes that were four inches in diameter by 100 feet deep on a six by six foot pattern that produced too much oversized rock that had to be broken to size using a hydraulic hoe/ram to make the shotrock fit our crusher that had a jaw size of 40 x 48 inches. We were producing over 25% of oversize rock that was costing a fortune to break up into size.
We were able to raise the DOV of the ANFO by placing a thin plastic tube loaded with a slurry explosive having a much higher ROV then ANFO. By extending this to both high velocity explosives through the entire column of ANFO we were able to trick the info into exploding at virtually the same ROV as the slurry. The net result was a much better breakage patterns and the shock rock. Much more of it was able to be passed through the crusher without any further work.