Flotation Principles

Froth flotation is based in physiochemical principles which explain the selective adhesion of some particles to air (hydrophobicity) and the preferential adhesion of water to other solids (hydrophilicity). In general the idea is to obtain a product called concentrate which must contain valuable minerals and other no valuable product called tailings which contain the unwanted minerals present in the raw ore.

The surface chemical changes necessary for froth flotation require varying degrees of agitation and, in some cases even more intensive treatment by scrubbing and attrition. Heating may be also used. The processes are always heterogeneous with the mineral surface one component, and the variability arising from the physical state of the other components, and the intensity and duration of the agitation requirements. The reagents used in pretreatment can be completely soluble, partially insoluble solids or liquids, completely insoluble oils, and gases. The specific types of interactions, ordered approximately according to the intensity of agitation required, are as follows: reactions with soluble reagents, reactions with partially soluble precipitates and emulsions, desorbing flotation reagents of one type before retreatment with other types, altering nonselective collector distributions already on surfaces to obtain selectivity, and removal of preexisting foreign minerals films.

To invoke selective hydrophobicity, substances known as collectors are used. Collectors are typically heteropolar organic substances - they contain both polar and non polar chemical groups. The non polar end is almost always a long chain or cyclic hydrocarbon group that is hydrophobic. The collector must be able to attach to the solid, and it does so through its polar end, which is typically an ionic group termed solidophil group.

Another important component in successful flotation is the frother. Once surface is hydrophobic, a solid particle must be able to attach to an air bubble. While it may be possible to initially obtain solid particle attachment to air bubbles in an agitated liquid under aeration alone, these air bubbles are unstable and quickly break down due to collisions with other bubbles, solid particles and the vessel walls. In addition the bubble size may not be sufficient to effectively carry a solid particle to the surface of the liquid. Consequently additional materials called frothers are added to promote the formation of stable air bubbles under aeration. Frothers like collectors are typically comprised of both polar and non polar end. The non polar hydrophobic end orients them into the air phase.

Additional reagents such as activators, depressants and pH regulators are used in Froth Flotation. Activators may be added to chemically resurface the solid to increase the interaction with collectors that are ineffective alone. Depressants form a polar chemical envelope around the solids particles that enhance hydrophilicity, or selectively prevent interaction with collectors that may induce unwanted hydrophobicity. Dispersants act to break agglomerated particles apart so that single particles interact with collector and air bubbles.

A modifier or conditioner in flotation is considered to be any reagent that does not work as collector, depressant, or frother. Functions performed by modifiers and conditioners include pH adjustment, mineral surface change, precipitation of soluble salts, and reagent concentration change. Powdered activated carbon is a modifier or conditioner since it is used to improve separation of metals by adsorbing excess depressants and other organic reagents, such as collectors, from the solution or mineral surface.