The flotation of sylvite (KCl) and also some accessory minerals (e.g. kieserite, langbeinite) from crude potash salts are processes used to a large extent all over the world. These separations are made in brines which are in an approximate solubility equilibrium with the mineral phases of the crude potash salts and thus contain concentrations of the crystal ions (K+, Na+, Mg²+, Cl-, SO²4- etc.) in the order of some moles or hundreds of grams per liter, respectively. Hence it follows that the solubilities of the collectors are lower by several orders of magnitude in comparison to those in normal aqueous solutions. A further consequence is the nearly complete compression of the electrical double layers at the mineral surfaces. Under these circumstances the mechanisms of collector adsorption exhibit some characteristics which will be discussed by significant examples.
Flotation Separation of Sylvite (KCl) and Halite (NaCl) with n-Alkylammonium Salts and n-Alkyl Sulfates
For the separation of sylvite and halite by flotation n-alkylammonium salts having chain lengths between ca. C12 and C20 have been the standard collectors for decades because they ensure a good separation efficiency with a relatively low amount of reagents. But for a long time it has also been known that this separation – somewhat less effective, however – is feasible with n-alkyl sulfates.
It is noteworthy that sylvite and halite, which crystallize in the same lattice structure, show quite a different adsorption response with regard to these two collector groups and also other ones. There is no analogon in the field of minerals which have a low solubility. Here, minerals having similar structures show a similar adsorption and flotation behavior, too, as e.g. in the case of carbonates and silicates. Therefore, the adsorption and flotation behavior of easily soluble salt minerals encouraged several authors to theoretical considerations. Simplifying, their results can be classified into the two groups “structure theories” and “hydration theories”. Some time ago the author of this paper analysed these theories and presented his position based on own extensive investigations. It will be briefly discussed in the following completed by some more recent results.
In brines the adsorption of n-alkyl-ammonium ions as well as n-alkyl sulfate ions on sylvite to an extent which leads to surface coverages necessary to flotation only occurs in the range of their limits of solubility or their exceeding, respectively. This is illustrated in Figure 1 by the adsorption isotherms of n-alkylammonium chlorides of different chain lengths on KCl (200 – 250 µm) at 25°C both in saturated KCl-solution (Fig. 1a) and also saturated KCl-NaCl- solution (Fig. 1b). In all sylvinite flotation processes used in practice the solubility limit of collectors is exceeded or at least approximately reached.
If these concepts, which were discussed briefly and hold for ion hydration in di-lute solutions, are applied to the flotation of halide minerals two aspects should be taken into consideration: namely, the admissibility of their transmission to:
a) saturated solutions of the halides and
b) to the conditions on the solid surfaces of the halide minerals.
It has to be considered that with increasing electrolyte concentration the water structure is disturbed more and more and gets lost finally. But there is no reason why the concepts should not be qualitatively true also for brines with respect to the intensity of the interactions between the ions and their adjacent water molecules. This assumption is in harmony with the trend of the viscosity curves, shown in Figure 2, for increased concentrations. When regarding at last the interactions between the ions on the surface of the halide minerals and their adjacent water molecules it could not be understood that they should be principally of a different nature from those within the liquid phase.
From the foregoing, therefore, the conclusion can be drawn that the positive near-hydration of the Na+ ions on the NaCl-surfaces prevents the interactions between the Cl- ions of the surface and the polar groups of the n-alkylammonium ions (Figure 3) as well as between the Na+ ions and the n-alkyl sulfate ions within the meaning of adsorption and thus the formation of stable collector films.
On the other hand, on KCl a near-hydration barrier does not exist and, therefore, stable collector films can form (Figure 4).
