The relations between energy input and block and particle size during rock fragmentation.

Abstract

Rock fragmentation can occur by crushing, relative radial motion, release of load, spalling, gas extension of strain wave-induced and pre-existing fractures, shear and in-flight collisions. The relative importance of each mechanism, as yet undefined, depends upon explosive properties, rock properties, blast geometry and initiation sequence. The effect of explosive energy and presence of natural fractures on fragmentation are studied. Design of blasts in a copper porphyry ore body were altered to give powder factors between 0.26 and 0.71 kg/m³ in three different types of rock. The rock units in the ore body for this study are classified according to the size distribution of the blocks formed by natural fractures and discontinuities. Block size distribution curves are produced from the core logs and the ore body is divided into six rock classes. Arbitrarily, a representative size distribution curve is assigned to each class. These curves are used to determine the specific surface area (surface area per unit volume of rock) for each rock class. The specific surface area of blasted rock was measured by photo-analysis and correlated closely with the explosive energy and rock type. The explosive energy is calculated in terms of total energy per unit volume of blasted rock, from the powder factor. The results clearly show the effect of natural block sizes on fragmentation. It is shown that most of useful explosive energy goes into opening the pre-existing fractures. Almost the entire product size is controlled by the natural fractures at energy levels below 2 MJ/m³ (powder factor of 0.5 kg/m³). At higher energy levels of 2.85 MJ/m³ (powder factor of 0.71 kg/m³) about 70 to 80 percent of the surface area of the blasted rock is controlled by the pre-existing fractures. The blasted rock is also analyzed using the scale invariant methods as an alternative to the conventional size distribution analysis. The fractal dimension of each blast at different energy level and rock class is determined. The variations of fractal dimensions depends on the energy levels and the rock conditions. At higher energy levels higher fractal dimensions are obtained and more fractured rock showed higher fractal dimension at the same energy level. The fractal concept can be a useful tool to describe rock masses and fragmenting behavior of different rocks.