Quantitative measurement of particle segregation mechanisms

Pharmaceutical, food, chemical, mining, and energy industries routinely handle and process materials with different particle sizes. Often the different size materials represent different chemical components. Pharmaceutical mixtures are typically mixtures of fine, active, with relatively coarse, incipient ingredients. Food industry mixtures combine particles with very different sizes and shapes. There is a great tendency for these dissimilar materials to separate during processing and handling. The cause of this separation depends on the size and shape of the particles in the mixture. If the fines are less than one-third the size of coarse particles and free flowing, they may percolate through the coarse matrix of particles resulting in sifting segregation. If the particle shapes are different, then the internal friction angles of individual components may be different, resulting in angle of repose segregation. Preventing segregation of a given material mixture depends on the cause of the segregation. If angle of repose segregation is the predominate mechanism, then segregation mitigation can be accomplished by limiting pile formation. However, if sifting segregation is the predominate mechanism, then limiting pile formation may not limit the segregation if the process equipment is subject to significant inter-particle motion or shear. This paper presents a method of measuring the magnitude of sifting segregation occurring in bulk material. It also includes data that relates bulk material strength to separation tendency. This relationship shows an inverse functionality between the yield strength of the material and the bulk segregation. Finally, the work presented in this paper compares segregation due to sifting and repose angle mechanisms to provide a means of differentiating between these mechanisms. (c) 2005 Elsevier B.V. All rights reserved.