Foam removal, flotation

In the past a large variety of flotation ceU designs were used. All cells have to ensure bubble generation, collision of the ink particles with the bubbles, transport of the ink-bubble aggregate to the suspension surface, and foam removal. 

Ion Flotation and Foam Separation. Ions and dissolved surfactant molecules can be removed from solutions by the agency of foam. In this case ions are sandwiched in foam films. The scientific basis of these processes is weU understood and successes of metal ion recovery from solutions including U, Pt, Au, as weU as different surfactants (detergents) have been reported in the Hterature. 

Overflow at the rate of 2700 m (713,000 gal) per day from a zinc-concentrate thickener is treated by ion flotation, precipitate flotation, and untrafine-particle flotation [Nagahama, Can. Min. Metall. Bull., 67, 79 (1974)]. In precipitate flotation only the surface of the particles need be coated with collector. Therefore, in principle less collector is required than for the equivalent removal of ions by foam fractionation or ion flotation. 

This is very similar to the flotation procedure described under basic physical treatments. In the case of foam fractionation, not only are the pollutants raised to the surface where they can be skimmed off, but a froth, like beer foam, is produced in which the pollutants become concentrated. The key to the process is the adsorption of the pollutants onto the surface-active agents that cause the froth to form. Sometimes a surfactant is added so that non-surface-active components can be removed. 

Haywood and Riley [14] have described a spectrophotometric method for the determination of arsenic in seawater. Adsorption colloid flotation has been employed to separate phosphate and arsenate from seawater [15]. These two anions, in 500 ml filtered seawater, are brought to the surface in less than 5 min, by use of ferric hydroxide (added as 0.1 M FeC 2 ml) as collector, at pH 4, in the presence of sodium dodecyl sulfate [added as 0.05% ethanolic solution (4 ml)] and a stream of nitrogen (15 ml/minutes). The foam is then removed and phosphate and arsenate are determined spectrophotometrically [16]. Recoveries of arsenate and arsenite exceeding 90% were obtained by this procedure. 

Ferric hydroxide coprecipitation techniques are lengthy, two days being needed for a complete precipitation. To speed up this analysis, Tzeng and Zeitlin [595] studied the applicability of an intrinsically rapid technique, namely adsorption colloid flotation. This separation procedure uses a surfactant-collector-inert gas system, in which a charged surface-inactive species is adsorbed on a hydrophobic colloid collector of opposite charge. The colloid with the adsorbed species is floated to the surface with a suitable surfactant and inert gas, and the foam layer is removed manually for analysis by a methylene blue spectrometric procedure. The advantages of the method include a rapid separation, simple equipment, and excellent recoveries. Tzeng and Zeitlin [595] used the floation unit that was devised by Kim and Zeitlin [517]. 

In flotation of ore the material is first crushed to under 0.1 mm particle size. The particles are mixed with water and form a sol. This sol is called pulp. The pulp flows into a container and air bubbles are passed through (Fig. 7.21). The mineral rich particles bind to the air bubbles by hydrophobic forces and are carried to the surface of the container. A stable foam, also called froth, is formed. With the froth the mineral-rich particles can be skimmed off and removed. 

Mineral flotation is a method for selective separation of mineral components out of polymineral dispersions of ground ores in water (ca. 5-35 vol.% of the solid) by using dispersed gas (usually air) bubbles. The method consists in the different adhesion of hydrophobized and hydrophilic mineral particles to an air bubble. Hydrophobized mineral particles adhere to the air bubble and are carried out as a specifically lighter aggregate to the surface of the mineral dispersion where they form a foam (froth) layer. This foam, called concentrate, is mechanically removed (Fig. 1A). A mineral is hydrophobized by adsorption of a suitable surface-active compound (surfactant, collector) on the surface of the mineral component to be flotated. All other nonhydrophobized particles remain dispersed in the mixture (Fig. IB). 

Trials were conducted on an industrial scale (Fig. 5.5). After fining, the must was pressurized with air in the saturation column (6 bar) and sent (140 liL/h) into the flotation tank (80 hL). An aspiration system collected the flotation foam (60 hL) on the top of the flotation tank. Particles of the foam were removed with a rotary filter and a clear must (35 hL) was obtained. In the following example, two identical tanks, containing exactly the same must, were treated with B20 + Siio + gluten 20 or with B20 + Siio + FG20. (FG, fish glue soluble gelatin B, bentonite Si, silica gel ... 

Foams are dispersions of gas in a relatively small amount of liquid. When they are still on the surface of the which they were formed, they also are called froths. Bubbles range in size from about 50 fim to several mm. The data of Table 20.1 show densities of water/air foams to range from 0.8 to 24g/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 content measured in parts per million to one measured in parts per billion. High contents of suspended solids or liquids are removed selectively from suspension by a process of froth flotation. 

If powdered activated carbons (or other adsorbents) are added to water or wastewater for removing soluble pollutants, the spent carbons are then flocculated and floated by a dissolved air flotation cell (or by an electroflotation cell), in which no foam is produced, the process system is termed nonfoaming adsorption flotation (83). 

Flotation is commonly employed for beneficiation of minerals. Frothers are added to the vigorously agitated mixture. Air bubbles attach to the collector-mineral particles, which then rise and are removed along with the froth or foam. The valuable components are separated from the waste or gangue by preferential floating of one of the components to the top while the other sinks to the bottom. 

As mentioned in Chap. 7, molecular imprinting of polymers15 can also be used to prepare hosts for the removal of the desired optical isomer from racemates. Foam flotation with imprinted polymers has been used in chiral separations.16 Flotation is often used to separate minerals from their ores. The polymers can be reused repeatedly. This may turn out to be the lowest-cost method of resolution. 

Chapters 33 and 34 involve the application of bubbles of gas to effect a separation. In Chapter 33 diffusion of a volatile compound into a gas bubble is the operating principle (purge and trap). Chapter 34 discusses foam fractionation in which the gas bubbles are coated with a surfactant. Dissolved compounds adhere to the surface and are removed as a foam. Later on in Chapter 34 flotation, in which small solid particles are coated such that gas bubbles adhere to them and they are lifted out of the solution, is discussed. 

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