TABLE OF CONTENTS Classification is a method of separating mixtures of minerals into two or more products on the basis of the velocity with which the grains fall through a fluid medium (Heiskanen, 1993). In mineral processing, this is usually water, and wet classification is generally applied to mineral particles which are considered too fine to be sorted efficiently by screening. Since the velocity of particles in a fluid medium is dependent not only on the size, but also on the specific gravity and shape of the particles, the principles of classification are important in mineral separations utilising gravity concentrators. Classifiers also strongly influence the performance of grinding circuits.
When a solid particle falls freely in a vacuum, it is subject to constant acceleration and its velocity increases indefinitely, being independent of size and density. Thus a lump of lead and a feather fall at exactly the same rate.
In a viscous medium, such as air or water, there is resistance to this movement and the value increases with velocity. When equilibrium is attained between the gravitational and fluid resistances forces, the body reaches its terminal velocity and thereafter falls at a uniform rate.
The nature of the resistance depends on the velocity of the descent. At low velocities motion is smooth because the layer of fluid in contact with the body moves with it, while the fluid a short distance away is motionless. Between these two positions is a zone of intense shear in the fluid all...
