Drill and blast in hard rock open-pit mining involves operations which include loading explosives into holes to fragment rocks. For such operations to be optimized, the relevant factors have to be analyzed with a focus on burden and spacing. Burden is described as the minimum distance between a blast hole and the closest free face, while spacing means the distance between two adjacent blast holes in one row. The term fines generation is referred to as the excessive grinding of rock materials in the process of blasting.
A proper balancing of burden and spacing has to be achieved so as to reduce the amount of fines and increase efficiency. The mechanism of fragmentation plays an important role in fines generation. At the moment when explosives are detonated, a high temperature and pressure wave is produced; and there is a lot of compressive stresses exerted on the rock materials (Jia et al., 2022). Such pressure is above the rock compressive strength and results in an area with highly crushed material known as a crushing zone. While some degree of fines is inevitable due to the energy liberation, the size of crushing zone depends on the interaction of explosive energy and rock mass.
Reducing the amount of fines in the blasting is crucial since excessive pulverization leads to adverse effects on the whole mine-to-mill chain. There is additional power consumption and cost required in the further processing of aggregates, and there is a significant ore loss (Gao et al., 2023). Besides, the presence of many fines contributes to dust formation, which creates environmental and health hazards at the workplace. Therefore, limiting the crushing zone and the size distribution of the fragments is a major goal in open-pit mining.
Burden is a key factor determining the effectiveness of rock fragmentation. When the burden is too large, it is difficult for the explosive energy to fracture the rock in the direction of the free face; as a result, energy reflects back and over-pulverizes the rock around the borehole. However, when the burden is reduced optimally, there is sufficient relief and stress waves propagate through the rock effectively in the required direction (Shehu et al., 2021). Thus, reducing the burden to the safe level is the way to eliminate explosion confinement and limit fines generation.
In addition to the burden of blast holes, there is the spacing between them as well as the ratio to the burden. Modification of the ratio regulates tensile action of stress waves in rock masses (Gao et al., 2023). With the appropriate ratio, shock waves from neighboring holes work together and propagate existing fractures in the rock till they meet the free face resulting in even fragmentation. On the contrary, if there is too little spacing, the compressive waves overlap and increase the amount of fines generation.
Summing up, optimization of burden and spacing in hard rock open pit mining is a complex engineering task. With the help of the knowledge about the interaction of explosive energy and rock materials, mining engineers can limit the crushing zone and induce tensile fracturing. Modifying the ratio of spacing to burden according to the geological conditions allows reducing fines significantly.
References
Gao, P., Pan, C., Zong, Q., & Dong, C. (2023). Rock fragmentation size distribution control in blasting: a case study of blasting mining in Changjiu Shenshan limestone mine. Frontiers in Materials, 10. https://doi.org/10.3389/fmats.2023.1330354
Jia, Z., Song, Z., Fan, J., Jiang, J., & Guo, S. (2022). Numerical Simulation Study on Dust Suppression Mechanism of Burning Rock Blasting in Open-Pit Mine. Frontiers in Earth Science, 10. https://doi.org/10.3389/feart.2022.832650
Shehu, S. A., Mohd Hashim, M. H., & Awang Kechik, N. A. H. (2021). PRELIMINARY ASSESSMENT OF THE EFFECTS OF BLAST DESIGN FACTORS ON FRAGMENTATION AT LAFARGE KANTHAN LIMESTONE QUARRY, CHEMOR, PERAK. ASEAN Engineering Journal, 10. https://doi.org/10.11113/aej.v10.16598
