The basic aim of all blasting operations is to fragment the rock mass in a suitable granulometric distribution, and obtain a shot pile formation and bulking of the material. The resulting fragmentation can significantly affect later stages of the operation.
In this regard, successfully loading, crushing and milling the material would reduce energy consumption and, in consequence, the total cost of the operation (Figure 1). For this reason, quantifying the resulting fragmentation through photogrammetric techniques combined with predictive models is a fundamental stage in the continuous improvement process for all mining operations.
The energy released when detonating the explosive charge generates a powerful shockwave that is transmitted to the surrounding rock mass, and is continued by a violent gaseous expansion. Although some of the energy released in the detonation is lost in ineffective work, most is concentrated in two distinct phases: (i) dynamic, dominated by the action of the shockwaves; and (ii) semi-static, dominated by the action of the gaseous expansion in the rock mass.
The high pressures generated by the detonation of the explosive column are transmitted to the rock through shockwaves during the dynamic phase. These propagate radially through the rock mass by means of concentric compression pulses that attenuate as they move away from the load centre.
The dynamic state the rock was subjected to recovers its almost static state once the shockwaves have passed. The static properties of the rock mass begin to manifest themselves more effectively due to the appearance of a semi-static field of voltages corresponding to the pressure of the gases in the walls of the blast hole.
This is how the combination of both phases brings about a very specific granulometric distribution, bulking and shot pile format.
The assessment of blasting fragmentation is a recent, effective advance. The volume of the samples to be screened, according to industrial sampling rules, would imply that this assessment would be physically unfeasible due to the financial, human and time costs involved.
However, new photogrammetric techniques are widespread, both in mining and in the ore processing industry, allowing the aforementioned obstacles to be overcome and offering almost immediate results in a convenient, economical manner.
Photometric analysis software can work with images collected using conventional fixed and mobile video cameras, photo cameras or digital files, plus orthophotos obtained with drones. These use automatic algorithms that identify individual blocks by defining contours. Whenever necessary, the images can be manually re-edited to improve the digitisation of each of the blocks.
The flexibility of the series allows several types of reading in accordance with the requirements of each operation, including accumulated distribution charts, histograms and continuous flows, in order to set out any product quality variations quickly in different time periods (see Figure 2).
The need for continuous improvement in all extractive operations makes it necessary to apply measuring models that can, in the design phase, predict the impact of fragmentation on the subsequent phases of the production cycle. It is therefore possible to, after comparing predictions to the actual results, establish the optimal energy configuration in each blast.
Source: MAXAM Australia