Flotation — Mechanisms and reagents

Industrial growth in the twentieth century required considerable quantities of metals and, as a result, the implementation of processes compatible with the treatment of huge rock masses. One such process, flotation, enables solids to be separated from each other, taking advantage of the differences between their surface properties in an aqueous solution and in the presence of air. Later, this process was applied to solid-liquid separation (precipitate flotation) and ion extraction in solution (ion flotation).

The principle of mineral flotation is as follows: solid particles are suspended by agitation in water, after wet grinding to varying degrees has freed the mineral species to be beneficiated from the gangue. This solid-water mixture (or pulp) is conditioned with a chi-mic reagent calledecollector, whose role is to make the surface of the mineral to be floated hydrophobic, so as to give it a greater affinity for the gas phase than for the liquid phase.

Selective action of the collector is achieved by using modifiers - depressants and activators - that alter its affinity for certain mineral surfaces. This is the differential flotation process, which enables the separation of sulfides, oxides, silicates and salts, for example.

The conditioned pulp is introduced into reactors equipped with aerated agitators (flotation cells), air injectors (flotation columns) or electrodes (electroflotation), which generate air bubbles and disperse them. The hydrophobized particles attach themselves to the surface of the bubbles, which act as a transport vector as they rise towards the free surface of the pulp. The result is a solids-laden supernatant foam, calledécume. The size of the bubbles (and hence the liquid-air interfacial area) and the life of the foam are modulated by the addition of a foaming agent. The entrained liquid is drained by gravity into the foam itself, which is collected by overflow.

We can therefore define several elementary operations during the flotation process:

• conditioning of solid surfaces with adsorption modifiers ;

• adsorption of the collector on the surface of a specific solid ;

• contact between solid particles and air bubbles;

• transport of the bubble-particle assembly to the pulp surface;

• foam formation and recovery.

The collector is a surfactant, a heteropolar organic molecule consisting of at least one hydrocarbon chain and a polar head, which may include one or more easily ionizable salt-forming groups. Depending on whether the charge of the polar head, after dissociation in water,...