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Surface chemistry influences all heterogeneous reaction systems and it is important in froth flotation, mainly to the existence of solid-liquid systems. Froth flotation is a process employed to separate valuable minerals (e.g. galena, chalcopyrite, and sphalerite) from undesired particles usually called gangue. Flotation is used extensively throughout mineral processing including after the ore has been reduced to a very small particle size by crushing and grinding operations.
When a mineral is placed in water the interfacial regions of the two phases alter to accommodate the new environment. A layer is established at the solid liquid interface which balances the charge in the system. This affects the behaviour of the mineral surface and its interaction with chemical reagents a factor that is important to surface chemical separation processes. This layer has electric properties and the surface charge on the mineral in a mineral-system is done by several mechanisms: polarization by external source, dissolution of ions from minerals, and rearrangement of the mineral surface.
For sulphides, the potential determining ions are difficult to determine as the surface charge is many times less than for oxides. This is due to limited hydration of mineral surfaces resulting from the inability of sulphides to form appropriate links. So, sulphide mineral surfaces are less hydrophilic and for that reason have a low charge. Their surfaces are influenced by relatively minor surface changes such as addition of surface active reagent. This leads to the possibility of effective sulphide mineral separation from oxide and silicates with only small reagent additions (e.g. collectors) as is done industrially.
There two mechanisms by which particles can be recovered by froth flotation: direct attachment to air bubbles and entrainment in the froth phase. The first is a selective process that happened as consequence of differences in wettability. Although this a selective process, unwanted particles can be recovered by this mechanism due to the presence of fine inclusions in the valuable particles. The other mechanism is a non-selective process resulting from the movement of fine particles with the water which reports to the froth phase. This is an inherent problem in froth flotation. It is believed that the rate at which valuable particles reports to the froth phase is directly related to the mass rate of froth water. Entrainment is minimized by employing several stages of cleaning flotation in order to reduce the presence of impurities in the final product. Fine particles are transported into the froth phase by entrainment and slime coatings on surfaces of valuable minerals.
Without chemical reagents flotation can not be practiced. They are grouped in four categories; collectors, depressants, modifiers and frothers. The first ones make possible to obtain hydrophobic particles (e.g. xanthates). The second ones trends to minimize the floatability of specific minerals that must no float (e.g. sodium cyanide). The third ones change surface mineral properties, thus ones can promote floatability (e.g. copper sulphate) or change polarity (e.g. lime). The last category comprises the reagents that must modify the surface tension of the minerals to be floated, in other words, permit transportation of valuable mineral to the froth phase (e.g. pine oil).
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