Comminution is defined as the division of a solid into smaller parts by overcoming the internal binding forces of the material (at the fracture surfaces) under the action of mechanical forces, whereby an increase in surface area is achieved by neutralizing cohesion on individual surfaces. The process engineering goal of comminution depends on subsequent processing stages, i.e., the further processing of the comminuted material, or the intended use of the comminution products. Comminution serves to prepare subsequent separation processes or chemical processes, as well as to produce commercially standard particle size spectra. By comminution, certain properties of the solids, such as bulk density, flowability, mixability, wettability, filterability, reaction behavior, and others, can be influenced. The result of the comminution depends on the type of machine used and the fracture behavior of the solids. During comminution, the particle size should not only be reduced, but the goal is to generate the most uniform particle sizes and shapes possible, making the material suitable for further processing or use, or the solid should be liberated to such an extent that all mineral components exist freely alongside each other and can be enriched in a subsequent process. The objectives of comminution can therefore be summarized as follows:
The evaluation of the comminution result is carried out by comparing the raw material and the comminuted material. A simple key figure in this regard is the size reduction ratio, which is defined as a quotient. Since maximum particle sizes in the entering and exiting material are not easy to determine, the size reduction ratio based on dK,95 is to be preferred, where the dK,95 values are the respective mesh sizes of sieves through which 95% of the material to be assessed passes. The classification of the working fields of comminution is based on the hardness of the material into:
according to the particle size of the comminuted product and the size reduction ratio in dK:
In comminution machines (crushers and mills), solids are exposed to different types of stress. A fundamental distinction is made between material stress between two solid surfaces and stress at one solid surface. Frequently, the material is subject to combined stresses. Hard and brittle substances are comminuted most effectively by pressure and impact; elastic and tough materials by impact and cutting; and soft materials by friction, impact, and cutting.
Jaw, gyratory, and roller crushers or edge runner mills, for example, work with pressure or pressure and friction. Ball, tube, and vibratory mills realize impact and friction effects. In hammer and pin mills, impact and shear stress act, while friction predominates in disc mills. Based on this, a certain pre-selection can be made for a material; the concrete machine selection and setting must then be carried out experimentally. Since the tests are normally carried out on a small scale, there is a necessity for transfer to large-scale execution. In this process, throughput or the number of machines, power, and other operating parameters must be determined.
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