Flotation elementary acts

Figure 5.43. Elementary act of separation by flotation hydrophobic particles, hydrophilic particles. The former adhere to ascending air bubbles.

Unlike the original flotation whose elementary act is complicated by an inertia impact and the accompanying deformation of bubble surface, microflotation is completely a colloid chemical process and it can be described in terms of modem colloid chemistry as orthokinetic heterocoagulation (Deijaguin Dukhin, 1960). 

The probability of formation of stable particle-bubble aggregates is determined by the probabilities of its attachment and retention on the bubble. The detachment is affected either by gravity or by inertia. These forces are proportional to the volume of particles, i.e. as the cube of the linear dimension of a particle hence, they are very big for large particles and very small for fine particles. This trivial fact results in radical consequences when analysing the role played by the size of particles in the mechanism of the elementary act of flotation (Derjaguin Dukhin 1960, 1979). 

After the main stages of the elementary act have been considered, preliminary considerations about microflotation control are discussed. Control of microflotation processes is based first of all on selection of optimal bubble size and optimal reagent to manipulate electrostatic interaction. The control is efficient if all stages of the elementary flotation act are optimised long and short range interaction, attachment process detachment prevention. 

Depending on the extent of inertia force effects the hydrodynamic stage of the elementary act of flotation proceeds very different. Four quantitatively different conditions of particle deposition on bubbles can be differentiated depending on the Stokes number St (Dukhin et al.,... 

Dynamic adsorption layers (DAL) influence practically all sub-processes which manifest themselves in particle attachment to bubble surfaces by collision or sliding. Surface retardation by DAL affects the bubble velocity and the hydrodynamic field and consequently the bubble-particle inertial hydrodynamic interaction. It also affects the drainage and thereby the minimum thickness of the liquid interlayer achieved during a first or second collision or sliding. Thus elementary acts of microflotation and flotation is systematically considered in this book for the first time with accoimt of the role of DAL. Extreme cases of weakly and strongly retarded bubble surfaces are discussed which assists to clarify the influence of bubble and particles sizes on flotation processes. 

The flotation of small particles represents an independent scientific problem inasmuch as the transition from coarse to fine grinding may be accompanied by qualitative changes in the mechanism of an elementary flotation act considered as the interaction of a particle with a bubble. 

The traditional treatment of flotation in which the emphasis is on the formation of the bubble-particle aggregate and on the physical chemistry of flotation reagents is insufficient for solution of a number of technological problems of flotation, in particular of flotation technology of small particles (less than 20-40pm in size). As applied to water purification, the behaviour of such small particles is of interest in the elementary flotation act. Since flotation of small particles by small bubbles is a qualitatively new process, it is quite natural to use a special term microflotation (Clarke Wilson 1983). 

Passing from large to small particles, the mechanism of the elementary flotation act changes qualitatively, both in the stages of approach of attachment, as suggested by Derjaguin and Dukhin (1959 and 1960) almost three decades ago. 

Effect of Dynamic Adsorption Layer on the Transport Stage of the Elementary Flotation Act... 

The consideration made is of semi-quantitative nature, and is justified both at moderate and large Reynolds numbers. At moderate Reynolds numbers, a refinement of the theory seems to be premature since the notion of the incomplete retardation of the surface is a hypothesis which needs an experimental check. At large Reynolds numbers, a quantitative consideration of the effect of DAL on the elementary flotation act will prove to be possible generally only after the quantitative theory of DAL has been developed. The given evaluations confirm that the effect of DAL on the transport stage of microflotation is high at large Reynolds munbers and, possibly, also at moderate values. 

In conclusion of this section and in accordance with the results obtained in the preceding sections it can be recalled that in studies of DAL by microflotation methods it is necessary to provide attachment of particles on bubble surfaces. The results are then controlled only by the transport stage of the elementary flotation act. 

Therefore, it is necessary to take into consideration surface roughness which determines the face of such particles. The resistance to thinning of the interfacial film can be drastically reduced if the angles between the faces are sufficiently sharp so that "intrusion" of the liquid interlayer by a sharpened section of the particle surface may take place. Such geometric conditions of the elementary flotation act also drastically facilitates motion against the disjoining pressure of the DL. 

The foregoing discussion of the theory of flotation of small particles involving the influence of the SRHI and surface forces shows that the elementary flotation act may remarkably differ for particles with smooth and rough surfaces thus they must be investigated separately. 

The most important distinction of an elementary flotation act of small particles from a similar act of large particles consists in the fact that transport of particles to a bubble surface controls the flotation kinetics rather than the sticking stage. Therefore the main reserve of intensification of water purification and selective flotation is connected with the increase of the number of collisions of particles with a collective of rising bubbles. 

A number of consecutive stages can be distinguished in an elementary flotation act first, the LRHI between a bubble and a particle, in which the particle moves along the streamlines of liquid flowing around the bubble second, the SRHI which arises after the bubble has approached the particle to a distance of the order of the particle size. 

DAL influences practically all stages of the elementary flotation act. Buoyancy velocity of bubbles of definite size with retarded and non-retarded surfaces can differ from each another by a factor of about 2 (see Fig. 8.2). According to the theory of quasi-elastic (Section 10.1) and inelastic (Section 10.2) collisions, a smaller film thickness h corresponding to the beginning... 

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