The influence of charge distribution on the grindability of the blasted material

In the Mining Industry, Rock Blasting can be considered as the first phase of comminution. The downstream phases of comminution are mechanical, and consist in consecutive stages of crushing and milling for grain size reduction. The total energy of mechanical comminution depends on grain size and on the resistance of the material to grindability. Usually, the first is quantified by the grains size distribution or one of its KPIs (such as the diameters X50 or X80), while the latter by the Working Index (WI), representing the amount of energy necessary to reduce the size of grains of a determined amount of material to the desired equivalent diameter (usually expressed in kWh/t). It is widely recognized that it is possible to manage the output of a blast in terms of grain size distribution varying specific charge (Powder Factor, P.F.) and charge distribution; specific algorithms such as the KUZ-RAM or the SWEBREC have been created to predict such output. Also, Katzabanis et al. have demonstrated how higher P.Fs. weaken the blasted material by creating micro-fractures that reduce its WI. This paper focuses on a related aspect: the influence of charge distribution on the specific comminution energy in crushing and milling. Small-scale blasts have been performed on 14 limestone blocks with different P.Fs. and charge distribution. For every P.F., charges have been designed to simulate concentraded and distributed geometries. Concentrated charges simulate open-cast blasts with large-diameter holes, with large burdens and spacings. Distributed charges simulate bench blasts with small-diameter holes and reduced burden and spacing. Three control blocks have been fragmented by mechanical means for comparison. For every sample has been determined the grain size distribution, the Crushing Specific Energy and the WI in ball mill. Results confirm the existence of precise correlations between charge distributions and the specific energy of mechanical comminution in crushing and milling. It is finally discussed the opportunity of adjusting charge diameters and distributions in open-cast blasts to improve the output of the comminutions circuits.