Flotation contactless

Contactless flotation happens if two conditions are satisfied a potential well must be present, this well must be sufficiently deep. Under these conditions there is no necessity to use the reagents which destroy the hydrophilic layer. Thus the contactless flotation can be collectorless. However, the addition of another type of reagents, i.e. cationic surfactants, can become necessary (see third peculiarity of micro flotation). 

The basis for the introduction of the notion of contactless flotation was the analogy with the well known phenomena of colloid particle coagulation in the secondary energetic minimum. Due to the predomination of the attractive molecular forces at large distances the particles can form aggregates in which some distance between particles is preserved. Thus, there is no direct contact between particles in this type of aggregation. However the notion of "contact" is not so simple. It is sufficient to point to the fact that a water monolayer remains on the hydrophobic surface. Thus the term "contactless" is maybe not suitable. 

Note, that the bubble surface can be covered by an adsorption layer even without the special addition of surfactant due to contaminations in natural, tap, and industrial waters. Thus, the conclusion that wetting films are stable due to the stabilising effect of molecular forces is at least doubtful for natural waters containing traces of surface active compounds which can destabilise the wetting film and can provide contactless flotation. Collectorless microflotation has been observed for example by Goldman et al. (1974). However, they did not perform any colloid-chemical investigations. 

In recharging the bubble surface, the depth of the potential well formed beyond the limits of the barrier of non-electrostatic repulsion forces is insufficient to ensure the contactless flotation of large particles. Therefore, in the presence of the non-electrostatic component of the disjoining pressure recharging can cause the contactless flotation only if the particles are sufficiently small. 

In the absence or weakness of non-electrostatic stability factors and at excess electrolyte concentration above a threshold, possibilities of contactless flotation should be checked. The particle attachment can take place through heterocoagulation in the secondary minimum of the total-interactions energy curve, if its depth is deep enough. High electrolyte concentration, suppressing electrostatic repulsion, weakness of non-electrostatic factors of stability and the presence of attraction forces caused by molecular attraction or action of the adsorbed surfactant are favourable prerequisites for the appearance of a sufficiently deep potential well. 

The greater the equilihrium thickness of the liquid interlayer in a contactless flotation, the higher is the probability of detachment of particles from the Inibble and the more important is the decrease of the bubble size, which reduces the detaching forces. 

Realisation of microflotation is ensured by opposite signs of the charges of the particle and the bubble and at low electrolyte concentration the effect of electrostatic attraction forces is extended over large distances and the depth of the potential well increases. Since particles and bubbles usually carry negative charges, it is expedient to use cation-active surfactants, which are predominantly adsorbed at bubbles in order to ensure contactless flotation. 

The theory of the transport stage of elementary microflotation at strong surface retardation is confirmed by works of Reay Ratcliff (1975), Collins Jameson (1977) and Anfruns Kitchener (1976, 1977). Numerous confirmations of the possibility of contactless and collectorless microflotation, of the importance of overcoming or removing electrostatic barrier at microflotation and of the possibility of flotation even of hydrophilic particles through adsorption of cationic surfactant on bubble surface are presented in Section 10.5. 

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