Bubble flocculators

Bubbles rising through a suspension create velocity gradients due to their motion. These velocity gradients could be used for flocculation in principle, but early assessments of bubble flocculation indicated that the diffusers which existed at that time created bubbles that were too large to be effective... 

With the advent of flotation as a useful treatment process in water and waste water treatment, made possible by the creation of finer bubbles by dissolved air precipitation, and electrolysis, the concept of bubble flocculation can be reconsidered. In the flotation process in water treatment, flocculation takes place prior to bubble flotation, usually by mechanical (paddle) flocculator with a coagulant salt (such as alum). It seems possible that the flocculation and flotation processes could be combined with the bubbles providing the flocculating power. 

Consequently, if the bubbles can be kept fine enough, useful G values can be generated. The use of bubble flocculators is still in the experimental stage, but there are indications that they may be applicable in practice. 

A colloid is a material that exists ia a finely dispersed state. It is usually a solid particle, but it may be a Hquid droplet or a gas bubble. Typically, coUoids have high surface-area-to-volume ratios, characteristic of matter ia the submicrometer-size range. Matter of this size, from approximately 100 nm to 5 nm, just above atomic dimensions, exhibits physicochemical properties that differ from those of both the constituent atoms or molecules and the macroscopic material. The differences ia composition, stmcture, and iateractions between the surface atoms or molecules and those on the iaterior of the colloidal particle lead to the unique character of finely divided material, specifics of which can be quite diverse (see Flocculating agents). 

Air bubbles becoming physically trapped in the insoluble solids original or flocculated structure... 

Air bubbles being chemically adsorbed to the insoluble solids in their original form or their flocculated structure... 

In the recycle flow pressurization system (Figure 27.10), a portion (15-50%) of the clarified effluent from the flotation chamber is recycled, pressurized, and semisaturated with air in the air dissolving tube. The recycled flow is mixed with the unpressurized main influent stream just before admission to the flotation chamber, with the result that the air bubbles come out of aqueous phase in contact with suspended particulate matter at the inlet compartment of the flotation chamber. The system is usually employed in applications where preliminary chemical addition and flocculation are necessary and ahead of flotation. It eliminates the problems with shearing the flocculated particles since only the clarified effluent passes through the pressurizing pump and the friction valve. It should be noted, however, that the increased hydraulic flow on the flotation chamber due to the flow recirculation must be taken into account in the flotation chamber design. 

Hm has been shown to be always positive, which suggests that, in two-phase systems (such as oil-water), the particles will always be attracted to each other. This means that even air bubbles will attract each other, as is also found from experiments. A linear relation is found between H1216/n and yLD, as expected from Equation Experimental values of Am as determined from flocculation kinetics showed that this agreed with the theoretical relation. 

During analysis, a flocculent light brown precipitate may be produced, which can build up in the flowcell, causing noisy peaks and drifting. A dialyser module could prevent this situation otherwise the flowcell could be scrubbed by forcing an air-bubble through between samples. 

From a manufacturing perspective, the excipients used may be very difficult to handle or process. This may lead to any number of problems during compounding, such as foaming, sedimentation, phase separation, particle flocculation, or formation of bubbles. To determine the cause of these problems, careful consideration and planning must be exercised in engineering a plant to ensure reproducible batches that can be efficiently compounded and filled. It is therefore essential that the behavior of each excipient in the formulation be well understood. 

The interaction between gas bubbles and oil drops has been described as follows (I) absorption of an oil drop to a gas bubble due to precipitation of a bubble on (he oil surface and collision between the drop and bubble (2) entrapment of a gas bubble in a flocculated structure of oil drops as it rises and (3) absorption of bubbles into a flocculated structure as it forms. 

Emulsion flotation is analogous to carrier flotation. Here, small-sized particles become attached to the surfaces of oil droplets (the carrier droplets). The carrier droplets attach to the air bubbles and the combined aggregates of small desired particles, carrier droplets, and air bubbles float to form the froth. An example is the emulsion flotation of submicrometre-sized diamond particles with isooctane. Emulsion flotation has also been applied to the flotation of minerals that are not readily wetted by water, such as graphite, sulfur, molybdenite, and coal [623]. Some oils used in emulsion flotation include mixed cresols (cresylic acid), pine oil, aliphatic alcohols, kerosene, fuel oil, and gas oil [623], A related use of a second, immiscible liquid to aid in particle separation is in agglomeration flocculation (see Section 5.6.4). 

Similarly, in floe flotation the small-sized particles become aggregated into floes that can attach to the air bubbles. Here, polymers may be used to induce the required level of flocculation. Examples include the floc-flotation of coal fines [631] or galena [632],... 

In the absence of 02, Fe is oxidized to Fe2+ and Fe(OH)2 is formed. However, Fe(OH)3 precipitates more easily than Fe(OH)2 and, thus, the injection of 02 facilitates pollutant removal. The injection of gas also helps in the flotation process. Vik et al. [162] have described a process in which H2 is produced at the cathode and Al is oxidized to Al(III) ions at the anode. The OH generation from H2 evolution produces precipitation of the metal hydroxides and also contributes to the coagulation-flocculation process. In addition, the hydrogen gas bubbles result in the flotation of the sludge formed. The principles of such a process are depicted in Fig. 24. 

Equipment types range from simple circular tanks, equipped with rake arms for large thickeners used in effluent treatment, through lamella type thickeners, which are fitted with inclined plates to increase the solids handling capacity, to flotation tanks where particles are caused to rise to the surface of the tank through natural low density or the use of gas bubbles or chemical flocculating agents. 

---