Scope of the Report
This report covers comminution by impact as obtained if solid particles moving freely in a static or moving gas, generally air, impinge or collide with one another or with grinding elements in an impact grinding machine.
H. Rumpf /l.l/ distinguished between four types of stress:
Therefore only the second type of stress (II) describes the type of impact grinding covered in this report. With this type of stress the energy available for comminution is defined by the kinetic energy of the particle immediately before impact, which includes its mass and the relative velocity between the particle and a second body, usually taken to be of infinite mass. The final result, for example the surface area produced during comminution, is independent of the fact, whether the relative movement is mainly obtained by a high speed second body orgrindingelementanda slowly moving particle, or a particle impinging at high speed on the surface of a stationary solid object. Depending on the angle of approach between the two bodies andthe relative movement oft heir centres of gravity, impact and friction dependent attrition occur simultaneously. The velocities of impact applied vary between ten and several hundred meters per second.
The word "impact" is used in the Anglo-Saxon literature also for a special kind ofthe first type of stress (I). If stress is applied at a comparatively low rate between the surface of two rigidbodies one defines this type of stress as: lowcompression /1.2, 1.3/. If however, the grinding mechanism of stamp or ball mills had to be evaluated, for example with single particle crushing tests, different so-called "impact-tests" were used, which consisted of:
- drop weight methods, and
-pendulum impact devices.
A considerable number of different test machines have been developed and an enormous amount of data published, most of which seem to have escaped present knowledge. In order to distinguish these "impact- tests" from tests performed under type of stress II, theyhave often been called "double impact-tests", because two solid surfaces contact the particle to be comminuted during stress loading. The main difference to impact grinding (type of stress II) has therefore to be attributed to the following facts:
1) comminution takes place with the stress being applied between two solid surfaces, and
2) the energy available for comminution depends on the mass of the drop or pendulum weights and their height of fall.
Results obtained under these circumstances are not relevant in impact grinding as described in this report. Information on drop weight and pendulum comminution tests for a variety of materials may be obtained from references /1.2 - 1.16/.
The present report covers therefore those impact grinding processes where particles moving freely in a static or moving gas, impinge or collide with one another or with so-called grinding elements or targets ‘of' much higher mass.
The report deals in chapter 2 with basic theoretical considerations, for example the exchange of energy during impact, the frequency of occurrence of direct central impacts, the maximum stress applied during impact etc.
Chapter 3 covers the test machines and results obtained in single particle impact.
Chapter 4 concentrates on the design and the performance of different impact grinding machines, for example: high speed pin and hammer mills, and fluid energy mills.