Method and apparatus for classifying viscous slurries

A method and apparatus for classifying viscous slurries in order to sharply separate fine particles from coarse particles suspended therein. At a first classification stage fine particles are centrifugally separated from coarse particles by utilizing a liquid cyclone installation. The coarse particles derived from the first stage are delivered to a second stage where a gravitational wet classification is provided for separating coarse particles at the second stage by gravitational settling thereof with respect to the fine particles which overflow at the second stage. The slurry with fine particles therein separated at the second stage is delivered to a third stage where a further wet classification takes place to separate the fine particles derived from the second stage into coarse and fine particles at the third stage. The fine particles which are separated at the third stage are returned to the first stage while the coarse particles derived from the third stage are combined with at least part of the fine particles derived from the first stage to form the output of the system.