Kinetics of Radial Segregation of Different Sized Irregular Particles in Rotary Cylinders

Radial and axial segregation occur in rotary reactors such as kilns, driers, mixers, blenders and ball mills as a result of differences in the size, shape and density of the feed or product. Segregation may also be influenced by design and operating variables of the reactors. Segregation of granular solids and powers often leads to unacceptable product quality and/or poor process efficiency. Understanding the effect of material, design and operating variables is an important step in engineering de-segregation.This paper presents some observations of radial segregation due to size differences of irregularly shaped particulate solids in horizontal rotating cylinders. The kinetics of radial segregation is shown to be zero order for slumping and rolling beds and the rate of segregation is directly proportional to rotational speed and independent of bed depth and percent fines. Additionally, the larger the difference in particle size, the faster the normalized rate of segregation. The scale-up criteria appear to be Froude number and size ratio. The general equation Sn = A · Frb describes the normalized rate of segregation for both spherical and irregular solids. Material characteristics affect the coefficient A; while design and operating parameters (namely cylinder diameter and rotational speed) affect Fr. The experimental approach and simple model presented in this work can be used to characterize segregation and flowability of particulate materials and for engineering de-segregation by the design of lifters and protrusions.