Drill-to-mill: Efficient drilling and blasting resulting in increased mill throughput at Barrick Goldstrike
Measurable improvements in throughput have been documented at Barrick Goldstrike due to increased ore fragmentation through efficient drilling and blasting techniques.
A decrease in throughput at the autoclave, due to increasingly hard ore, was identified in early 2002. A team of individuals from the open pit and process areas worked closely on a continuous improvement initiative to ensure that it would address the needs of both groups, with the target of actually improving throughput in future years, despite the projected harder ores.
The Barrick Goldstrike open-pit group, along with the process division, began experimenting with novel blasting technology and techniques to improve mill throughput in the spring of 2001. A series of tests were developed to determine if fragmentation and, ultimately, mill throughput could be influenced by optimizing the drilling and blasting program in known hard ore areas. This continuous improvement initiative was given the name drill-to-mill (D2M).
Three specific drilling and blasting areas were targeted for D2M evaluation:
Electronic detonators with firing time accuracy to within one-tenth of one millisecond of the designed time and a full range of firing times (1 to 4000 ms in one ms intervals).
A unique brand of porous prilled ammonium nitrate, differentiated from conventional prills by the addition of plastic micro spheres. The velocity of detonation (VOD) of this product is increased by approximately 2000 fps, or about 12%, and produces a higher borehole pressure.
Reducing the blasthole spacing as well as decreasing the blast hole diameter optimized distribution of explosives within the blast pattern. These modifications allowed for the powder column to be raised in the blastholes and improved the explosives distribution both laterally and vertically.
Improved stemming material and staggered patterns were also evaluated and implemented.
Separate test blasts were conducted using each of the new technologies and techniques listed above and compared to a standard blast, or baseline, that was shot in the same hard ore zone, with very similar geologic characteristics. The three blasts (standard or baseline, electronic detonators and EXPAN, and close spacing and EXPAN) were evaluated using excavator productivity, imaging analysis of stockpiled ore, plant data, and circuit survey samples. The results indicated that:
Excavator productivity was increased by 11% in the electronic blast when compared to the pyrotechnic blast.
Imaging analysis used to determine the particle size distribution for run-of-mine ore found the blast using close spacing and EXPAN had a much finer size distribution than the baseline test or the test combining EXPAN and electronic detonators.
Ore from each blast was campaigned through the mill. Grinding plant performance for each ore campaign was ascertained from the plant operating data and circuit survey samples. The results of the batching indicated that an improvement in throughput of 23 to 53 tons per operating hour could be attributed to the optimized drilling and blasting of the D2M program for the targeted hard ore (Bond work index > 17 kWhr/st).
An overall increase in mill throughput, taking into account all ores, has been estimated at approximately six tons per operating hour, resulting in a value of $1.3 to $1.5 million in 2003 at the autoclave alone. Improvements in throughput at the roaster have been realized, but the amount of improvement has not been proven. Additional costs attributed to the D2M program are in the order of $0.10/ton for the targeted material, or about $600,000 in 2003.
Gains in excavator productivity and mill throughput at the Betze Post operation have been achieved through the use of electronic detonators, high VOD ammonium nitrate, and improved explosive distribution. Excavator and mill throughput gains combined more than offset the additional costs of the electronic detonators, high VOD ammonium nitrate, and increased drilling density. The results of this study demonstrate that by increasing drill and blast costs, the overall costs of the operation have been lowered and revenue increased.