Theoretical foundations and classification of flotation methods:
Theoretical foundations and classification of flotation methods:
Heterophase impurities from water can be isolated using the selective wetting phenomenon that underlies the flotation process. The essence of this process lies in the specific action of molecular forces that contribute to the adhesion of suspended substances with bubbles of highly dispersed air in water and the formation of a foam layer saturated with the extracted substance on the water surface. The elementary act of foam flotation is that when a gas bubble approaches a hydrophobic surface in water, the particles of suspension separating their thin layer become unstable and breaks when a certain critical value is reached. Due to the short duration of contact between the bubble and the particle during their collision (0.001–0.002 s), the probability of sticking is determined by the kinetics of the formation of the contact angle. The mass of flotated particles should not exceed the strength of their adhesion to gas bubbles and the lifting force of the latter. The optimal particle size of the particles to be recovered is in the range of 10 ″ 3 —10-1 cm. Fine particles (particles with a diameter of less than 5-10 microns) float very poorly and impair the extraction of large particles. The intensification of the flotation process is achieved by hydrophobization of the surface of dispersed impurities with reagents, which, selectively adsorbing on the surface of the particles, reduce their wettability, resulting in improved adhesion of particles of impurities to air bubbles. In some cases, flotation treatment of water is combined with treatment with solutions of coagulants and flocculants. In this case, the released A1 (OH) 3 or Fe (OH) 3 adsorb water-polluting impurities, the surface of the resulting micellar aggregates is hydrophobized either by the sorbed substances themselves, such as oils, or by the addition of surfactants. After flotation separation, the hydrophobic precipitate of spent hydroxides occupies a much smaller volume and its humidity is lower than the precipitate separated during sedimentation, which is very important, especially when deactivating water. Anionic, cationic and nonionic surfactants are used as hydrophobic flotation reagents. They are sorbed at the solid – water interface so that the polar groups face the surface of the particles, and the non-polar ones face the water, as a result of which suspended solids are hydrophobized. In the case of flotation of alkaline earth metal salts and basic mineral oxides, higher fatty acids and alkaline soaps are usually used, as well as anionic substances – sodium sulfates of higher alcohols, alkyl and alkylaryl sulfonates containing hydrocarbon chains with 12-18 carbon atoms. In the flotation of quartz and acid minerals, cationic substances are used – higher aliphatic amines and quaternary ammonium salts containing chains with 12 or more carbon atoms. Foaming agents are substances capable of adsorption at the water-air interface and moderately stabilizing the foam. For these purposes, products containing terpene alcohols, monohydric aliphatic alcohols with 6-8 carbon atoms, cresols and xylenols, mono methyl and mono butyl ethers of polypropylene glycols, etc. are used. To control the alkalinity of water, lime, soda, sulfuric or hydrochloric acid are used. By flotation, water can be freed not only from suspended solids, but also from petroleum products, oils and other emulsified liquid substances, as well as from individual ions of substances dissolved in water, such as radioactive ones. In the latter case, the added reagents must form surface-active complexes with the recoverable ions. This process of floatation is used in mdifferent types of water filters for home for domestic use.
When an inert gas is passed through such a system, surface-active complex compounds containing the recoverable impurity accumulate in the form of foam on the surface. If in a solution there are several dissolved substances with different surface activity, then in the flotation process there is the prospect of fractional separation. Researchers have established the promise of decontamination of radioactive waters containing Sr90, Y90, Nb95, Csm, the introduction of solid carriers (iron (III) hydroxides and aluminum, copper ferrocyanide) into them, followed by flotation with the addition of appropriate flotation reagents. Currently, many methods are used to saturate a liquid with air bubbles, among which the following can be distinguished by the size of air or gas bubbles: flotation with the release of air from a solution – vacuum, pressure and airlift installations; flotation with mechanical dispersion of air – impeller, pressureless and pneumatic units; flotation with air supply through porous materials, electroflotation.
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