(166d) The Influence of Cohesion on Segregation Mechanisms

Segregation of different size particles is one of the main challenges facing engineers working with powder and granular materials. Mixing unit operation often produce reasonably well mixed materials which are transported to storage vessels for further processing. Once these mixed materials reach these storage vessels segregation results. There are two methods to mitigate segregation in a handling process. The first method is to design process vessels with velocity profiles capable of either blending the segregated material or producing uniform draw velocity patterns to maintain the radial segregation profiles and mixing material in the radial direction once it leaves the vessel. The second method of segregation mitigation is to understand the core segregation mechanisms and design the feed system or mixture to prevent segregation in the first place. Companies producing mixtures of material as a final product do not have control of the feed systems in their customer's plants. Consequently, it is often preferable to design a product to minimize the effects or causes of segregation. The challenge with this approach is the diversity of segregation mechanisms and the lack of knowledge concerning the influence of particle scale forces on segregation tendencies.

Segregation is caused by a variety of mechanisms. Fines may sift through the interstitial pores in a matrix of course and fines particles. This mechanism has received a lot of attention in the literature and some models have been proposed that can predict the magnitude of free flowing materials subject to this mechanism. The primary driving force causing sifting segregation is the difference in particle size and the availability of pores of the right size to be present in the material during shear.

Differences in local frictional forces between particles can lead to the formation of different repose angles as bulk materials form piles in process vessels. The differences in inter-particle friction coefficients cause distinct materials to slide down surfaces at different rates resulting in separation as material flows down the surface of the pile. The primary driving force for angle of repose segregation is the difference between particle friction coefficients and the ability to from piles in process equipment.

Air is carried by bulk materials during free-fall into process vessels. When these free-falling streams impact on pile surfaces a local consolidation of the bulk material occurs squeezing some air from between solids pores during impact. This air has sufficient velocity to entrain small particles which are carried by air currents and deposit in areas of the process vessel where gas velocity drops sufficiently to dislodge entrained particles. The main driving force for this segregation is the hydrodynamic particle size and the local air velocities.

Particles impacting on a pile surface can rebound as they contact the pile and take a trajectory based on coefficients of elasticity and the angle of the local impact. If particles have different rebound coefficients they will have different trajectories after impact resulting in segregation. If materials have the same restitution coefficients but different particle size they may have different rebound characteristics. The larger particles will carry greater momentum and will deform the local pile in such a way as to transfer some of the rebound energy into the pile or rebound surface. This can cause particle size segregation due to a rebound mechanism.

Some of these mechanisms result in similar segregation profiles making determination of segregation cause difficult. In addition, cohesive forces between particles influence the magnitude of segregation of all these mechanisms. However, the effect of cohesion in mitigating segregation is very dependant on the segregation mechanism. This paper suggests four simple models and examines experimental evidence for describing segregation due to these mechanisms. Each model incorporates the influence of cohesion and examines the effect of cohesive forces on segregation mitigation. Models can be combined to achieve the combined effect of several segregation mechanisms on segregation patterns. The influence of cohesion is investigated to produce a mechanistic map that can be use to provide guidance to engineers in designing products to mitigate segregation during handling.