Residence time distribution of particles in a continuous liquid–solid classifier

Experiments were carried out in a continuous classification column of diameter 191 mm and height 1.47 m with water as the liquid and glass beads as the particles. Initial experiments were carried out with narrowly sized particles and somewhat larger tracer particles which could be separated by screening. The main experiments involved a wide distribution of particle sizes, with injected pulses of tracer particles of slightly higher density separated from samples using an organic liquid of intermediate density. The results show that the mixing behaviour is intermediate between plug flow and perfect mixing. Increasing the discharge rate of underflow product stream decreased both the mean residence time and the degree of classification significantly. At relatively low voidage, small particles settled more quickly and large particles more slowly than predicted on the basis of the approach proposed by Lockett and Al-Habbooby (Trans. Inst. Chem. Eng. 51(1973) 281–292) where each particle fraction is assumed to travel as if it were alone in a suspension of the overall solids concentration.