Flotation

Flotation




Separation of mineral by flotation process is based on the hydrophobicity of mineral particles. Certain minerals are naturally hydrophobic for example graphite and elemental sulphur. Hydrophobic means water fearing that is water molecules do not adhere to hydrophobic surfaces. Hydrophobic substances/ particles in turn are aero phallic meaning that they seek to adhere to air bubbles. This property of substances/mineral particles is taken advantage in the process of flotation to recover hydrophobic particles into froth phase. This is achieved by keeping all the particles in suspension in a reactor/stirred tank and passing air bubbles through the suspension of particles in water media. Hydrophobic particles on encountering air bubbles attach to the surface of air bubbles which travel up wards as air bubbles are lighter than the ore particle suspension in water media. The mineral particles attached to the air bubbles form a froth phase over the suspension.




However the froth phase is not stable and the bubbles break very fast leaving the hydrophobic mineral particles back into the suspension. Native floatability is the property of minerals that are naturally hydrophobic such as graphite, elemental sulphur etc. To keep the froth phase more stable certain reagents called frothers are added to the ore suspension in small quantities. Pine oil, eucalyptus oil, methyl isobutyl carbinol, cresylic acid are some of the known frothers used in the flotation process. A frother molecule has hydrocarbon chain which is hydrophobic with hydrophilic functional group such as OH that does not ionize.




Minerals with hydrophilic surfaces can be induced surface hydrophobicity by facilitating selective adsorption of certain reagents called collectors. Collectors are molecules with hydrocarbon chains with a functional group, often ionisable. Selectivity of a collector for a mineral or a group of minerals is due to the functional group and its interaction with the mineral surface. For example xanthates are very selective for sulphide minerals such as chalcopyrite, galena, sphalerite, pyrite etc.




Selectivity of mineral separation is some times achieved by deploying a class of reagents called depressants. Depressants are hydrophilic molecules with functional groups that interact with the mineral surface selectively. Depressants selectively adsorb on gangue minerals that might otherwise respond to the collectors. Sodium silicate is generally used to depress silica or silicate minerals in the soap flotation of minerals such as apatite or fluorite etc. Sodium cyanide is a known depressant for sphalerite and pyrite during galena flotation. Potassium dichromate is a known depressant during chalcopyrite flotation.




Activators are reagents that enhance/ facilitate the adsorption of ions (collector ions for example) that induce surface hydrophobicity to the mineral surface. Copper ions activate sphalerite and facilitate the adsorption of xanthate collector ions. Copper xanthate is more insoluble than zinc xanthate. Copper sulphate is used to activate sphalerite by copper ions.




The interaction of flotation reagents with mineral surfaces is highly dependant on pH of the flotation pulp. Several reagents are used to modify the pH of the flotation pulp. Sodium carbonate, sodium hydroxide and lime are used to adjust the pH of flotation pulp in the alkaline range. Sulfuric acid is used to adjust the pH of flotation pulp in the acidic range.



[1] Which mineral shows native flotability?




[2] Which part of the collector ion imparts surface hydrophobicity to mineral surface?




[3] To which class of flotation reagents sodium cyanide belongs?




[4] Name some pH modifiers?




[5] Which reagent is used to activate sphalerite in industrial process?

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