Flotation reagent-collectors
In mineral processing plant,a number of
organic and inorganic reagents are used in flotation and auxiliary processes to
achieve separation, including collectors, frothers, extenders, activators,
depressants, deactivators, flocculants, and dispersants. Collectors, frothers,
and extenders are surfactants added to impart hydrophobicity to the minerals
and to make selective adsorption of the collector possible or to eliminate
interference to flotation by various dissolved or colloidal species. We discuss
collectors this time.
Collectors
The
primary role of the collector is to adsorb selectively, which imparts
hydrophobicity to particles of the mineral to be floated. If it is to have the
dual ability to adsorb and to impart hydrophobicity, the collector molecule
must contain at least two functional parts, a nonpolar group of sufficient
hydrophobicity and a polar or ionic group that will be electrostatically or
chemically reactive toward species on the mineral surface. The nonpolar part of
a collector used for flotation of oxides is usually a longchained hydrocarbon
(10 to 18 CH, CH2, and CH3 groups); short-chained hydrocarbons (2 to 5 CH2 or CH3 groups) are used for flotation of
sulfides. The polar group is usually anionic sulfate, sulfonate, phosphate,
carboxylate, oxime or thiocarbonate (xanthate), cationic amine, or nonionic
oximes.
Examples
of collectors used in froth flotation are given in Tables 1 through 4. Ethyl
xanthate is used for flotation of galena, sphalerite, and pyrite; oleic acid is
used for flotation of phosphates and hematite; and dodecylamine is used for
flotation of quartz, potash, and feldspars. Collection by these surfactants
depends on their properties, such as ionization constant, solubility, critical
micelle concentration, and emulsifying power. Any surfactant must be
solubilized or dispersed properly so that it can distribute itself in the pulp
and contact the mineral surface with minimum use of mechanical energy. However,
note that a highly soluble surfactant has a low tendency to depart from the
solution and adsorb on interfaces.
The
tendency to form micelles also influences the utility of the surfactant for
flotation. Micelles are aggregates of surfactants with hydrocarbon chains
oriented toward the interior of the aggregates and the polar or ionic part
oriented to be in contact with the water (Figure 8.7). Each surfactant forms
micelles when its bulk concentration reaches a particular value known as the
“critical micelle concentration” (CMC). Above the CMC, important properties of
the surfactant solutions undergo a marked change. For example, surface tension
of water decreases with the addition of a surfactant but only up to the CMC.
Above the CMC, surface tension remains approximately constant, suggesting that
the activity or concentration of the surface-active monomer species is constant
above the CMC and that the micelles themselves are not surface-active.
Solubility and CMCs of the most commonly used surfactants are given in Tables
5, 6, and 7.
Surfactants
can form salts with the dissolved species of the mineral and other additives,
and solubility of these salts can also have a major influence on the extent of
flotation obtained. Solubility products of various metal carboxylates and
xanthates are given in Tables 8 and 9. Good correlation exists between the
flotation and precipitation properties of surfactants. In many systems,
precipitation can also be expected to occur on the mineral surface and lead to
good flotation. An example is shown in Figure 2, where the onset of flotation
can be seen to correlate well with the onset of precipitation calculated using
data for bulk-solution chemical equilibria. Note that excessive collector loss
is possible if collector precipitation occurs exclusively in the bulk (because
of a rate of metal ion dissolution and diffusion through the interface that is
faster than the rate of diffusion of collector to the particle), because bulk
precipitates are not potent collectors. Oxime-type reagents can act as very
good collectors for problematic minerals because of their ability to chelate
with the metallic surface species. Thus, hydroxamate and salicylaldoxime can
adsorb on chrysocolla or tenorite (CuO) and result in their flotation.
Potential for the use of oximes is related directly to their solubility. Also,
bulk precipitation as well as detachment of the surface chelate from the
particle can interfere with flotation, as bulk chelates are incapable of
causing collection.
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