Foam Fractionation, Gas-, and Liquid-Assisted Flotation

The enrichment factor, the ratio of the concentration in the collapsed foam to the concentration in the feed, varies considerably as expected, but can be from 10 to several hundred. 

The applications of foam fractionation are many. Its use is primarily to concentrate materials present in low concentration (as low as 10 M/L) in liquids where other processes are not economical. On a commercial scale, this process is used to treat waste water to remove organic waste material and to remove ions from process streams such as the oxyanions of Re(VIII), Mo(VI), Cr(VI), W(VI) and V(V) and the cyanide complex anions or the chloride complexes of Zn(Il), Cd(II), Hg(II), and Au(Ill). In the laboratory, it is used to concentrate proteins such as bovine albumin, cytochrome-c, barley malt, alpha amalyase, and beta casein, as well as proteins and enzymes from various plant and animal systems. 

This is a modified foam fractionation in that not all of the adsorbed compound is on the surface, but the desired compound is trapped in the foam. This is a very efficient process, if the bubbles are small and many. In purge and trap, the ethanol vapors would be adsorbed onto a solid such as silica gel and later desorbed by heating. 

A foam is an irregular dispersion of a gas in a liquid with the largest volume usually being the gas. The surface of the gas bubbles is coated with a thin layer of liquid called the lamella. The thickness of this layer varies from 

000 A down to about 50 A, at which point the layer breaks apart, and the bubble collapses. Layers of 200 to 2,000 A are typical. Once the gas bubbles collect into a foam and press against each other, they assume a dodecahedral shape, the liquid between them starts to drain away. 

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CHAPTER 34 - FOAM FRACTIONATION, GAS- AND LIQUID-ASSISTED FLOTATION... 

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