An experimental and analytical study of the mechanics of rock particle fragmentation during impact crushing

Comminution of rock in general, and impact crushing in particular, have been treated largely as empirical sciences. The present investigation is intended as a step towards establishing understanding of impact crushing that is based on mechanics. Experiments were conducted using a 'rock gun', to observe in detail the processes of rock fragmentation during impact. Force-time records of the impact process were obtained--it appears that no such records had been obtained previously. The records clearly indicate that tensile 'spalling' due to wave reflections at the free boundaries of the projectile is not a significant mechanism of fragmentation in impact crushing.;Three types of rock and two sizes of rock spheres, and arbitrarily-shaped specimens were tested. High speed movies were recorded of the impact process, and provided valuable insight concerning dynamic propagation of fractures and energy partition during fragmentation. Rock spheres were seen to break into three distinct types of fragments: an 'impact cone' of finely pulverized material developed in the zone of the impact, a series of 'orange-slices' resulting from tensile failure and a large intact fragment approximately conical in shape, based on the rear of the original sphere. The first stage of the experimentally observed fragmentation mechanism was reproduced numerically using an explicit, finite-difference code.;Sieve analysis of the broken particles confirms the existence of two distinct size distributions. The number and location of fractures, and thereby the shape of the fragments, depend primarily on the impact velocity. A low velocity, heterogeneity dominated regime and a high velocity, rate dominated regime can clearly be distinguished. From the results of the present investigation it appears that crushing of rock by impact should be at least as energy efficient as the classical slow compression processes, provided the rock particles can be efficiently accelerated. The use of a 'rock cushion', intended to reduce wear of the impact surfaces in commercial impact crushers, was found to produce a dramatic reduction in the efficiency of the comminution process.