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a process for separating paniculate solids, primarily minerals, that utilizes differences in the wettability of the particles in water. Hydrophobic particles, that is, particles with poor wettability, adhere selectively to a phase interface (usually the boundary between a gas and water) and separate from hydrophilic particles, that is, particles with good wettability. In flotation, bubbles of a gas or drops of oil adhere to hydrophobic particles and raise them to the surface.
Flotation is one of the principal methods of concentrating ores. It is also used to remove organic matter and suspended solids from water, to separate mixtures, and to accelerate settling processes in various industries, such as the chemical, oil-refining, and food-processing industries. A number of flotation methods are distinguished on the basis of the nature and method of the formation of the phase interface (water-oil-gas) to which the separated components are to adhere.
The first method proposed was bulk-oil flotation, for which W. Haynes (Great Britain) received a patent in 1860. When finely crushed ores are mixed with oil and water, sulfide minerals are selectively wetted by the oil and rise with it to the surface of the water, while rocks, such as quartz and feldspar, remain in the medium. In Russia, bulk-oil flotation of graphite was first carried out in 1904 in Mariupol’ (now Zhdanov, Ukrainian SSR).
The ability of hydrophobic mineral particles to remain afloat at the surface of the water while hydrophilic particles sink was used by A. Nibelius (USA, 1892) and A. Macquisten (Great Britain, 1904) to develop a skin-flotation apparatus, in which hydrophilic particles precipitate out of a thin layer of finely crushed ore at the surface of a stream of water.
With the development of the froth-flotation method, the use of flotation became widespread and the range of applications increased. In this method, particles treated by reagents are carried to the surface of the water by air bubbles to form a foamy layer, the stability of which is regulated by the addition of frothing agents. Various methods have been proposed for the formation of the bubbles. C. Potter (1902) proposed the formation of carbon dioxide by a chemical reaction, while F. Elmore (Great Britain, 1906) suggested releasing a gas from solution by reducing the pressure, that is, vacuum flotation. Other methods included vigorous agitation of the pulp and the forcing of air through fine openings.
In froth flotation, the ore of naturally hydrophobic nonmetallic minerals with low densities, such as sulfur, coal, and talc, is crushed so that the particles measure up to 0.5–1.0 mm in size; the ores of metals are crushed so that the particles measure up to 0.1–0.2 mm. Flotation reagents are added to the pulp to generate or increase differences in the hydration capacity of the minerals being separated and to impart sufficient stability to the foam. The pulp is then directed into a flotation machine. The formation of flotation aggregates (particles and air bubbles) occurs through the collision of minerals with bubbles of air introduced into the pulp and through the formation on the particles of gas bubbles liberated from the solution. Flotation is influenced by the ionic composition of the liquid phase of the pulp, by the gases, particularly oxygen, dissolved in the pulp, by the temperature, and by the density of the pulp. The rock and mineral composition of the minerals to be concentrated is studied to select the appropriate flotation process and reagents and to determine the degree of crushing necessary to ensure sufficiently complete separation of the minerals.
Flotation works best for separating particles with sizes 0.1–0.04 mm. Finer particles separate less well, and particles smaller than 5 microns impair the flotation of larger particles. The detrimental action of micron-sized particles is diminished by specific reagents. Large particles (1–3 mm) break away from the bubbles during flotation and do not float. Therefore, for the flotation of large particles (0.5–5 mm) methods of froth separation have been developed in the USSR in which the pulp is deposited on a froth layer that floats only particles that have been rendered hydrophobic. A fluidized-bed flotation machine with a rising stream of aerated liquid has been developed for this purpose. The process utilizing such a machine is far more productive than bulk-oil and skin flotation.
The method of ion flotation, developed in the 1950’s, seems promising for water purification and for the extraction of components from dilute solution. It can be used to treat industrial effluents, mineralized underground thermal springs, water in mines, and seawater. In this method, individual ions, molecules, finely dispersed precipitates, and colloidal particles interact with collector reagents, usually cationic-type collectors, and are drawn by the bubbles into the froth or film on the surface of the solution.
Finely dispersed bubbles can also be obtained by the electrolysis of water and the subsequent formation of gaseous oxygen and hydrogen (electroflotation). The amount of reagents used in electroflotation is considerably lower, and in some cases reagents are not needed at all.
The widespread use of flotation for concentrating minerals has led to the construction of various flotation machines, with large cells up to 10–30 m3 and high productivity. A flotation machine consists of a successive series of cells with pulp feeding and discharging equipment. Each flotation cell is fitted with an aerating device and a froth skimmer.
In the USSR and abroad, deposits of disseminated ores have been brought into commercial production as a result of flotation, and the comprehensive utilization of minerals has been made possible. Factories can now produce as many as five kinds of concentrates. In a number of cases, flotation tailings are not discarded but instead are used as construction materials and agricultural fertilizers. Flotation is the leading process for the concentration of nonferrous-metal ores. The use of recirculating water has been adopted, reducing the pollution of rivers and lakes.
An important role in the development of the theory of flotation was played by the Russian physical chemist I. S. Gromek, who first formulated the basic concepts of the wetting process in the late 19th century, and the Russian physical chemist L. G. Gurvich, who developed the theory of hydrophobic and hydrophilic states in the early 20th century. The work of A. Gaudin (USA), A. Taggart (USA), I. Wark (Australia), and the Soviet scientists P. A. Rebinder, A. N. Frumkin, I. N. Plaksin, and B. V. Deriagin has had a major impact on the development of the modern theory of flotation.
REFERENCESMeshcheriakov, N. F. Flotatsionnye mashiny. Moscow, 1972.
Glembotskii, V. A., and V. I. Klassen. Flotatsiia, Moscow, 1973.
Spravochnik po obogashcheniiurud. Moscow, 1974.
V. I. KLASSEN and L. A. BARSKII
A process used to separate particulate solids, which have been suspended in a fluid, by selectively attaching the particles to be removed to a light fluid and allowing this mineralized fluid aggregation to rise to where it can be removed. The principal use of the process is to separate valuable minerals from waste rock, or gangue, in which case the ground ore is suspended in water and, after chemical treatment, subjected to bubbles of air. The minerals which are to be floated attach to the air bubbles, rise through the suspension, and are removed with the froth which forms on top of the pulp. Although most materials subjected to flotation are minerals, applications to chemical and biological materials have been reported.