Compressions, comminution force

Compression is the most usual comminution force for brittle materials. Materials are compressed between two heavy-duty metal surfaces. Crushers based on compaction force are jaw, cone, gyratory, and roll crushers. These are most widely used to reduce the size of coarse rocks and minerals particles. In soUd waste treatment, their use is not widespread, because compression force, in most cases, simply deforms the materials but does not change their size (e.g., metals, paper, plastic, organics). 

In providing an overall picture of the Griffith theory applied to the comminution process, it must be pointed out that the theory requires that a tensile stress should exist across a crack to open it further. While a uniform compressive force can close a crack, a nonuniform one can lead to the occurrence of localized tensile stresses. In a comminution process the particles are subjected to nonuniform loading, and therefore it can be surmised that they normally break in tension and not in compression. However, the tensile component of loading in comminution does not form the major loading component and this contributes towards a lowering of the overall energy efficiency of comminution. 

Forces commonly used in food processes for particle size reduction are compressive, impact, attrition (or shear), and cutting forces. More than one force usually participates in the comminution operation in industrial size-reduction equipment. In particular, crushing rolls use mainly compressive forces, hammer mills are based on impact, disc mills cause particle attrition through shear force application, and rotary knife cutters use cutting forces. 

The comminution chamber consists of an impeller rotating at variable speed, imparting a compression or shear force inside a conical screen. 

In addition to the two mechanisms of comminution and stretching, elastomer and carbon black have to be compacted.Compaction, or massing, is the displacement of entrapped air in the machine by applied compressive force. This requires deformation of elastomer domains to match the shape of the carbon black, followed by relaxation of the elastomer in the deformed state. In another case study, Nakajima and Harrell estimated that about 6% of the energy used in incorporation was required for compaction. 

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