1、FROTH FLOTATIONIntroductionFlotation is undoubtedly the most important and versatile mineral-processing technique, and both use and application are being expanded to treat greater tonnages and to cover new areas. Originally patented in 1906,flotation has permitted the mining of low-grade and complex
2、 ore bodies which would have otherwise been regarded as uneconomic. In earlier practice the tailings of many gravity plants were of a higher grade than the ore treated in many modern flotation plants. Flotation is a selective process and can be used to achieve specific separations from complex ores
3、such as lead-zinc, copper-zinc, etc. Initially developed to treat the sulphides of copper, lead, and zinc, the field of flotation has now expanded to include the oxidized minerals and nonmetallic, including-fine coal.Principles of FlotationThe theory of froth flotation is complex and is not complete
4、ly understood. The subject has been reviewed comprehensively by Klassen and Mokrousov and by Glembotskii et al. and will only be briefly dealt with here. Froth flotation utilizes the differences in physic-chemical surface properties of particles of various minerals. After treatment with reagents, su
5、ch differences in surface properties between the minerals within the flotation pulp become apparent and, for flotation to take place, an air-bubble must be able to attach itself to a particle, and lift it to the water surface (Fig.12.1). The process can only be applied to relatively fine particles,
6、as if they are too large the adhesion between the particle and the bubble will be less than the particle weight and the bubble will therefore drop its load. In flotation concentration, the mineral is usually transferred to the forth, or float fraction, leaving the gangue in the pulp or tailing. This
7、 is direct flotation as opposed to reverse flotation, in which the gangue is separated into the float fraction. The air-bubbles can only stick to the mineral particles if they can displace water from the mineral surface, which can only happen if the mineral is to some extent water repllent or hydrop
8、hobic. Having reached the surface, the air-bubbles can only continue to support the mineral particles if they can form a stable forth, otherwise they will burst and drop the mineral particles. To achieve these conditions it is necessary to use the numerous chemical reagents known as flotation reagen
9、ts. The activity of a mineral surface in relation to flotation reagents in water depends on the forces which operate on that surface. The forces tending to separate a particle and a bubble are shown in Fig.12.2. The tensile forces lead to the development of an angle between the mineral surface and t
10、he bubble surface. At equilibrium,S/A=S/W+W/AcosFROTH FLOTATIONCollectors All minerals are classified into non-polar, or polar types according to their surface characteristics. The surfaces of non-polar minerals are characterized by relatively weak molecular bonds. The minerals are composed of coval
11、ent molecules held together by van der Waals forces, and the non-polar surfaces do not readily attach to the water dipoles, and in consequence are hydrophobic. Minerals of this type, such as graphite, sulphur, molybdenite, diamond, coal, and talc, thus have high natural floatabilities with contact a
12、ngles of between 60 and 90. Although it is possible to float these minerals without the aid of chemical agents, it is universal to increase their hydrophobicity by the addition of hydrocarbon oils or frothing agents. Creosote, for example, is widely used to increase the floatability of coal. Use is
13、made of the natural hydrophobicity of diamond in grease tabling, a classical method of diamond recovery which is still used in many plants. The pre-concentrated diamond ore slurry is passed over inclined vibrating tables, which are covered in a thick layer of petroleum grease. The diamonds are attra
14、cted to and become embedded in the grease because of their water-repellency, while the water-wetted gangue particles are washed off the table. The grease is skimmed off the table either periodically, or continuously, and placed in perforated pots(Fig.12.3), which are immersed in boiling water. The g
15、rease melts, and runs out through the perforations, and is collected and re-used, while the pot containing the diamonds is transported to the diamond-sorting section. Minerals with strong covalent or jonic surface bonding are known as polar types, and exhibit high free energy values at the polar sur
16、face. The polar surfaces, react strongly with water molecules, and these minerals are naturally hydrophilic. The polar group of minerals have been subdivided into various classes depending on the magnitude of polarity, which increases from groups 1 to 5(Table 12.1)Minerals in group 3 have similar degrees of polarity, but those in group3(a) can be rendered hydrophobic by sulphidisation of the mineral surfac
