Foam Separation and Froth Flotation

Permeability of a membrane is determined partly by gas diffusivity, but adsorption phenomena can exist at higher pressures. Separation factors of two substances are approximately in the ratios of their permeabilities, = PoA.fPoB- Some data of permeabilities and separation factors are in Table 19.7, together with a list of membranes that have been used commercially for particular separations. Similar but not entirely consistent data are tabulated in the Chemical Engineers Handbook (McGraw-Hill, New York. 1984, pp. 17.16,17.18). 

Foams are dispersions of gas in a relatively small amount of liquid. When they are still on the surface of the liquid from which they were formed, they also are called froths. Bubbles range in size from about 50 pm to several mm. The data of Table 19.8 show densities of water/air foams to range from 0.8 to 24 g/L. Some dissolved or finely divided substances may concentrate on the bubble surfaces. Beer froth, for instance, has been found to contain 73% protein and 10% water. Surface active substances attach themselves to dissolved materials and accumulate in the bubbles whose formation they facilitate and stabilize. Foam separation is most effective for removal of small contents of dissolved impurities. In the treatment of waste waters for instance, impurities may be reduced from a 

Due to very high or low resistivity, particles may be difficult to collect. 

Variable air flow can significantly affect the efficiency adversely. 

A cyclone may be needed upstream from a precipitator to reduce the dust load on the unit. 

In many cases, more than one collection device may be necessary to control dust problems. For example, a cyclone collector may be followed by a baghouse, and then perhaps by an electrostatic precipitator, or the cyclone may be upstream from a wet scrubber, since a single unit may not do a thorough cleaning job. 

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Foam Separation and Froth Flotation 635 Foam Fractionation 635... 

A detailed list of foam sepe ration studies is evailable in a number of reviews Refs- 16.1-1, 16.1-2, 16.3-6, 16.5-13-16.5.15). A compilation of major minerals and other paniculate matter diet have been separated by froth flotation are given in Table 16.5.1. 

Froth flotation (qv) is a significant use of foam for physical separations. It is used to separate the more precious minerals from the waste rock extracted from mines. This method reHes on the different wetting properties typical for the different extracts. Usually, the waste rock is preferentially wet by water, whereas the more valuable minerals are typically hydrophobic. Thus the mixture of the two powders are immersed in water containing foam promoters. Also added are modifiers which help ensure that the surface of the waste rock is hydrophilic. Upon formation of a foam by bubbling air and by agitation, the waste rock remains in the water while the minerals go to the surface of the bubbles, and are entrapped in the foam. The foam rises, bringing... 

Foam Production This is important in froth-flotation separations in the manufac ture of cellular elastomers, plastics, and glass and in certain special apphcations (e.g., food products, fire extinguishers). Unwanted foam can occur in process columns, in agitated vessels, and in reactors in which a gaseous product is formed it must be avoided, destroyed, or controlled. Berkman and Egloff (Emulsions and Foams, Reinhold, New York, 1941, pp. 112-152) have mentioned that foam is produced only in systems possessing the proper combination of interfacial tension, viscosity, volatihty, and concentration of solute or suspended solids. From the standpoint of gas comminution, foam production requires the creation of small biibbles in a hquid capable of sustaining foam. 

The slowly rising foam in a tall column can Be employed as the sorbent for continuous chromatographic separations [Talman and Rubin, Sep. Sci., 11, 509 (1976)]. Low gas rates are also employed in short columns to produce the scumlike froth of batch-operated ion flotation, microflotation, and precipitate flotation. 

Both types of coalescence can be important in the foam separations characterized by low gas flow rate, such as batchwise ion flotation producing a scum-bearing froth of comparatively long residence time. On the other hand, with the relatively higher gas flow rate of foam fractionation, the residence time may be too short for the first type to be important, and if the foam is sufficiently stable, even the second type of coalescence may be unimportant. 

Systems Separated Some of the various separations reported in the literature are listed in Rubin and Gaden, Foam Separation, in Schoen (ed.). New Chemical Engineering Separation Techniques, Interscience, New York, 1962, chap. 5 Lemlich, Ind. Eng. Chem., 60(10), 16 (1968) Pushkarev, Egorov, and Khrustalev, Clarification andDeactiva-tion of Waste Waters by Frothing Flotation, in Russian, Atomizdat, Moscow, 1969 Kuskin and Golman, Flotation of Ions and Molecules, in Russian, Nedra, Moscow, 1971 Lemlich (ed.), Adsorptive Bubble... 

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