FLOTATION OF MINERALS

a proce cf mineral concentration in which the valuable portion of the ore is caused to float and thus separate from the worthle mineral portion or gangue. The method involved is thus exactly opposite to that commonly used in water concentration, in which advantage is taken of differences in specific gravities, the heavier portions sinking, and the lighter portions being washed away. The principles on which the various flotation proce es are based are very complex, the most important being surface tension, wettability, adhesion, adsorption, viscosity, and fiocculation, although colloidal conditions and clectrostatic forces may play a part of minor importance. Surface tension is the contractile force at the surface of a liquid, whereby resistance is offered to rupture. This is due to the fact that the molecules at the surface of a liquid have a greater coherence than the molecules Within the liquid, and therefore the surtace acts as if it were an elastic film. In flotation proce es, this phen omen on is closely a ociated with the wettability of a solid by a liquid, and particles which are not easily wetted tend to float, whilst particles which are casily wetted tend to sink. As an example, it may be mentioned that if a small needle be carefully placed on the surface of water it will float, whereas a piece of gla of the same size, though lighter, will sink. A thin film of oil will a ist the needle to float, and may cause the gla also to float, by reducing the easo with which it is wetted with water. If the needle be above a certain size, it will not float because the force of gravity overcomes the force of surface tension. in mineral separation, it is found that sulphides of the metals, for example, copper pyrites, galena, zinc blende, ., are not easily wetted and thus tend to float, whereas the gangue minerals such as silica, barytes, oxide of iron, ., are easily wetted and tend to sink. A second action which has to be taken into account is connected with the behaviour of oil. On being mixed with water, oil rises to the surface owing to its lower specific gravity and insolubility in water; it also shows a.