Effects of Double-Primer Placement on Rock Fragmentation at Kevitsa Open Pit Mine

Abstract To address the lack of systematic production-scale evaluation of the effects of double-primer placement on blasting performance, this study presents a comparative analysis of two primer configurations. The double-primer placement consists of two primers positioned 2 m above the borehole bottom and 2 m below the stemming, respectively, with simultaneous initiation, whereas the conventional middle-primer placement employed a primer positioned at the midpoint of the explosive column. Twelve full-scale production blast experiments were conducted at Kevitsa mine with rock fragmentation and shovel loading rate as performance indicators. Theoretical analysis and numerical simulations were performed to gain a better understanding of the underlying mechanisms. The experimental results confirmed that the investigated double-primer placement yields finer rock fragmentation than the conventional middle-primer placement across the full range of investigated sizes. Meanwhile, the double-primer placement increases the shovel loading rate by 10.9%, compared to the middle-primer placement. The theoretical analysis indicates that the double-primer placement generates higher pressure loading due to head-on shock-wave collision and greater energy concentration in the surrounding rock, compared to the middle-primer placement. In addition, three-dimensional numerical simulations on bench blasting show that the double-primer placement produces over 300% larger triaxial tension zones with higher stress magnitudes than the middle-primer placement does near the bench face. Given that triaxial tension is more favorable for rock fragmentation than uniaxial tension, it can be concluded that the larger triaxial tensile area generated by the double-primer placement is a significant factor contributing to achieving finer fragmentation, compared to the middle-primer placement.