Principles of Froth Flotation

Abstract

To achieve selective flotation of mineral particles, whether by their removal in a froth or by the less-used method of agglomeration, specific characteristics of one or more of the mineral species present must be adequately developed. Provided these produce sufficiently marked differences of behaviour in the presence of air, they can then be exploited. If a particle is to be held in a mineralised froth, it must be ground to a fineness at which downward pull of gravity is insufficient to overcome its adhesion to an air-water interface. The usual commercial separation entails the lifting of a heavy metal sulphide away from a relatively light gangue by the agency of air bubbles rising through the pulp. This buoyancy results from adhesion of the particle to a comparatively large bubble. The adhesive force with which a particle clings to the air-water interface is opposed by the gravitational drag due to its mass. For successful exploitation of differences in surface properties most ore minerals must be ground finer than 48–65 mesh. A light mineral such as coal (density circa 1.4) can be floated at 10 mesh, provided the bubble system on which it is borne is developed as a quiet layer of froth. Random changes of direction, acceleration, and collision may tear too large a particle out of its bubble. At the other end of the flotation size-range, the surface characteristics of all particles in the pulp are more similar at very fine sizes. Somewhere below 10μ, and for most ores at about 3μ to 5μ, it becomes increasingly difficult to control and exploit differences in surface properties with the accuracy needed to depress gangue, and float concentrate. Typically, flotation is practised between the limits 60 mesh and 5μ. This is quite apart from any consideration of “break” or of liberation mesh.