Principles of flotation
Flotation is a physico-chemical separation process that utilises the difference in surface properties of the valuable minerals and the unwanted gangue minerals. The theory of froth flotation is complex, involving three phases (solids, water, and froth) with many subprocesses and interactions, and is not completely understood.
The process of material being recovered by flotation from the pulp comprises three mechanisms:
(1) Selective attachment to air bubbles (or “true flotation”).
(2) Entrainment in the water which passes through the froth.
(3) Physical entrapment between particles in the froth attached to air bubbles (often referred to as “aggregation”).
The attachment of valuable minerals to air bubbles is the most important mechanism and represents the majority of particles that are recovered to the concentrate. Although true flotation is the dominant mechanism for the recovery of valuable mineral, the separation efficiency between the valuable mineral and gangue is also dependent on the degree of entrainment and physical entrapment. Unlike true flotation, which is chemically selective to the mineral surface properties, both gangue and valuable minerals alike can be recovered by entrainment and entrapment. Drainage of these minerals occurs in the froth phase and controlling the stability of this phase is important to achieve an adequate separation. In industrial flotation plant practice, entrainment of unwanted gangue can be common and hence a single flotation stage is uncommon. Often several stages of flotation (called “circuits”) are required to reach an economically acceptable quality of valuable mineral in the final product.
True flotation utilises the differences in physicochemical surface properties of particles of various minerals. After treatment with reagents, such differences in surface properties between the minerals within the flotation pulp become apparent and, for flotation to take place, an air bubble must be able to attach itself to a particle, and lift it to the water surface. The following picture illustrates the principles of flotation in a mechanical flotation cell. The agitator provides enough turbulence in the pulp phase to promote collision of particles and bubbles which results in the attachment of valuable particles to bubbles and their transport into the froth phase for recovery.The process can only be applied to relatively fine particles, because if they are too large the adhesion between the particle and the bubble will be less than the particle weight and the bubble will therefore drop its load. There is an optimum size range for successful flotation.
In flotation concentration, the mineral is usually transferred to the froth, or float fraction, leaving the gangue in the pulp or tailing. This is direct flotation and the opposite is reverse flotation, in which the gangue is separated into the float fraction. The function of the froth phase is to enhance the overall selectivity of the flotation process. The froth achieves this by reducing the recovery of entrained material to the concentrate stream, while preferentially retaining the attached material. This increases the concentrate grade whilst limiting as far as possible the reduction in recovery of valuables. The relationship between recovery and grade is a trade-off that needs to be managed according to operational constraints and is incorporated in the management of an optimum froth stability. As the final separation phase in a flotation cell, the froth phase is a crucial determinant of the grade and recovery of the flotation process. The mineral particles can only attach to the air bubbles if they are to some extent water-repellent, or hydrophobic. Having reached the surface, the air bubbles can only continue to support the mineral particles if they can form a stable froth, otherwise they will burst and drop the mineral particles. To achieve these conditions it is necessary to use the numerous chemical compounds known as flotation reagents.