The degree of fragmentation obtained in primary quarry blasting has always been a factor in the speed of digging the muckpile.
The quarry under discussion is fairly new, located in Georgia, and is in the large operation category. Loading is done by three yard shovels, and primary crushing by a 48″ x 60″ jaw crusher preceded by a 5″‘ wide feeder. The stone is a hard granite gneiss, of Los Angeles rating 22, and apparent specific gravity is 2.68.
It became obvious that considerable time was being lost in the removal and handling of large rock from the muck pile. Stopwatch studies showed that as much as one-third of the available shovel digging time was being lost to this cause.
In the course of one year, experiments were conducted on many types of powder. Prills were not tried, as water was present in most holes. It was found that thorough fragmentation became increasingly better as slower powders were tried. In the final analysis, a high ammonia-low nitroglycerine powder having a confined speed of only 9500 ft/sec has given the best results in this hard granite, this being considerably slower than the 20,000 ft/sec ammonia gelatin originally used.
The drilling pattern finally selected as optimum became 10′ burden by 14’ spacing, being the widest ever used at the quarry. With this pattern, the cap arrangement of alternate delays on succeeding rows previously mentioned, and utilizing the slow powders a fragmentation considered excellent has been reached at this site.
First and foremost, production is up and total unit cost/ton is considerably lower. Shovels can now efficiently dig the muck pile, and an increase in trucks loaded per shift of 35 percent has been realized.
Second, operating hours on dipper teeth have increased almost 100 percent. Obviously great wear was formerly incurred in prying and rooting the large boulders from the muck pile.