(90b) Granular Mixing in the Flowing Layer in Rotating Cylinders

Granular materials are widely used in industries such as coal gasification, iron & steel, mining, cement, construction, fertilizers and pharmaceuticals. In order to design equipment such as rotary kilns, bins, silos, combustors, hoppers, chutes, hydro-cyclones, etc., in an effective and economical way, a thorough understanding of the various factors governing the flow characteristics of granular materials must be obtained. These needs have already motivated extensive analytical and experimental investigations of the flow of granular materials. Mixing is a central feature of many processes such as in the reduction of iron ore, cement clinkering, coal gasification, food processing, and tablet manufacturing. Granular mixing is studied in various geometries such as rotating cylinders, chute, vibration, shaking, and Couette flows. Mixing is very important in all geometries to transport and produce quality product. However, mixing of granular materials is difficult because commonly used mixing methods can lead to undesired segregation. Ineffective mixing, or the inability to control particle segregation, is always costly in terms of rejected materials, extra blending time, and defective end products. Physical parameters that control the extent of mixing are not well-studied even in a simple geometry such as rotating cylinder. We study granular mixing in a rotating cylinder. A thin layer of particles in the free surface moves at very high velocity, whereas rests of the bed materials are rotated as a solid body rotation. Mixing takes place only in the moving layer and no mixing occurs in the solid bed. Particles in the flowing layer experience a random motion due to inter particle collisions and this random motion causes the particles to diffuse in the transverse direction with respect to flow direction. Results of various parameters affecting the mixing are presented. Granular mixing is found to be dependent on the size of the particles and the speed of rotation of the rotating cylinders. Mixing is found to be very slow in the azimuthal direction when compared to the transverse direction. Mixing is predicted with Lagrangian simulation based theory. Layer thickness and dynamics angle of repose are found independent of cylinder fill fraction for a given size of particles rotated at a given speed of rotation.