Comminution of raw materials

The collision mechanics of impact and percussion comminution – Part 1

In numerous machines, comminution of primary and secondary raw materials is based on the physical effect of collision. This effect is realized by means of the active principles of impact and percussion. These active principles effect four types of collision either between the raw material particles or with machine components: elastic-plastic, rebounding, penetrating and energy-releasing collisions.

Widening of the theory of restitution to the theory of collision enables its formulation in physical terms. For this purpose, the coefficient of restitution is extended and termed the coefficient of collision. The value range then extends from -1 to ∞. Here the coefficient of collision from -1 to 0 characterizes the penetrating collision, from 0 to 1 the elastic-plastic and rebounding collisions and from 1 to ∞ the energy-releasing collision. Comminution is effected in all collision types apart from in elastic-plastic collisions.

1 Introduction

Primary and secondary materials are often comminuted in machines that utilize the physical effect of collision. The raw material particles are comminuted either as a result of impact or percussion with the work surfaces, the moving or non moving machine walls or between the particles themselves. In the case of impact loading, moving particles impact a non-moved wall (e.g. in impact crushers with centrifugal rotors, VSIC), moving walls impact particles (e.g. in impact crushers with rotor beaters, HSIC) or moving particles impact each other (e.g. in jet mills). In percussion loading, particles are comminuted between two walls. Here, one wall can be fixed in place while the other moves (e.g. in single-shaft hammer crushers and shredders) or the two walls move (e.g. comminution between balls in ball mills). In all cases, collisions occur between particles and work surfaces in the machines or between particles.

Such machines have been built and operated successfully for decades. The processes taking place in the process chamber are known and can be described verbally, but so far they have only been inadequately formulated in physical terms. Design guidelines therefore draw on empirical knowledge and field trials in model machines. Simulation calculations do use physical correlations, they reflect, however, the comminution processes and the kinematics of individual particles only to a limited extent. One of the main reasons is a lack of knowledge about the interactions between particles and work surfaces in the process chamber of the machines, especially the collision processes. The theory of restitution as part of the impulse-momentum theory in classical physics cannot reproduce all processes of interaction between the collision partners, like comminution. The theory of restitution with the central parameter of the coefficient of restitution is therefore widened. For the purpose of differentiation, in the following, instead of “impact” and “coefficient of restitution”, the terms “collision” and “coefficient of collision” are used.


Prof. Dr.-Ing. Dr. h. c. Georg Unland, Technische Universität Bergakademie Freiberg

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