Alunite has been described both by the chemical formula [KAl3(SO4)2(OH)6] and by the chemical formula [K2SO4·Al2(SO4)3·2Al2O3·6H2O]. The latter gives an erroneous impression that water is structurally associated with the mineral. The former representation is therefore, to be preferred. It is more in conformity with the crystal structure of alunite. The structure of alunite is similar to that of crandallite which is rhombohedral. The structure can be visualized as sheets of aluminum octahedra with each aluminum atom surrounded by four hydroxyl groups and two oxygen atoms. Each of the four hydroxyl groups is shared with one other octahedron. Sulfate tetrahedra link the octahedra together forming a basal plane. The sulfate tetrahedra are somewhat distorted. This is revealed from the infrared spectrum of the mineral. The spectrum matches the spectrum given in the literature, and does not show any peaks due to quartz thus indicating that the sample of alunite is very pure. The peaks between 1230-1085 cm-1 are due to the V3 vibrations of the SO4= ion. The existence of more than one peak for V3 vibrations indicate that the SO4= tetrahedron is somewhat distorted. Also seen in the infrared spectrum are the sharp bands at 3508-3482 cm-1 which may be assigned to the stretching vibrations of the O-H group. The sharpness of the band indicates that these bands are due to the hydroxyl group attached to a metal cation. The appearance of a doublet suggests that no hydrogen bonding is present. Thus, it may be concluded that the O-H group is mostly present as a separate entity and not as a part of water molecule. However, there is a medium broad band at 1632 cm-1. It is due to the bending mode of the O-H group in water. Some adsorbed water is thus, present in the alunite mineral sample.

Relatively weak bonding exists between the potassium and six oxygen atoms and six hydroxyl groups. It is the main force holding the octahedral sheets together. Consequently, the structure tends to have basal cleavage along the (0001) plane. The distance from the center of the potassium atoms in the hexagonal ring to the oxygen atoms and hydroxyl groups is about 2.82 A° and 2.87 A° respectively. Also a = 6.97 A° and c = 17.27 A°.

It is to be expected that during grinding and subsequent contact with water potassium will be preferentially leached out as compared to aluminum. The solubility values reported by Andreev in fact, show a preferential dissolution of potassium thus exposing relatively more aluminum sites at the surface.

Aqueous Chemistry of Alunite

Alunite is a semisoluble salt type mineral and when brought in contact with water it will extensively react with water to yield various aluminum and potassium species. The equilibrium abundance of these species has been calculated as a function of pH by using the available equilibrium data which include the following reactions.

Each of the above equilibria gives a nonlinear equation. These along with appropriate mass balance equations form a set of simultaneous equations which have been solved by Newton-Raphson’s method to obtain the concentrations of the various species as a function of pH. Calculations have been made both by assuming that Al(OH)3(s) is formed, in which case equilibrium Equation 12 has been taken into account, and also by neglecting the formation of Al(OH)3(s) and therefore not considering equilibrium Equation 12. The pH-concentration diagrams for both the cases are given. It is seen that the natural pH of the alunite-water suspension in case there is an incipient Al(OH)3(s) formation is 4.8 A few experiments were performed wherein dry ground alunite (0.2 gm) was directly brought in contact with water (10 ml) whose pH had been adjusted to desired values. The suspension was kept overnight and its final pH was recorded. No effort was made to exclude atmospheric CO2. Table 2 gives the recorded values. It is seen from the table that the final pH of the alunite suspension in water was always 4.5 no matter what the initial pH was. The final value of pH (4.5) is very close to the pH 4.8 of Al(OH)3 formation predicted by the pH concentration diagram given in Figure 5. The agreement is fairly good and indicates that the dry ground alunite when brought in contact with water for a long time reacts with water to form Al(OH)3(s) on its surface.

Point of Zero Charge (PZC): It is to be expected from the observations made in the previous section that the point of zero charge of alunite would be located near pH 4.5-4.8. However, this expectation is not realized in practice. Using NaNO3 as inert electrolyte the zeta potentials of alunite were measured as a fundtion of pH. The concentration of the inert electrolyte was kept constant at 1.0×10-2M, 2.0×10-2M and 1.0×10-3M. The pH-zeta potential isotherms are shown. The curve for 1.0×10-2M is not shown in the figure for the sake of clarity. The curves cross the line of zero zeta potential at pH 7.2 ± 0.15 which therefore, is the point of zero charge of alunite as determined experimentally. It is also seen from the figure that for a constant electrolyte concentration the zeta potential curve first steeply rises as the pH is decreased below PZC and then levels off to give a plateau. Similar behavior is noticed in the alkaline region except that the zeta potential is negative. As the concentration of the inert electrolyte is increased the numerical value of the zeta potential decreases for a particular pH, which obviously reflects a decrease in the double layer thickness.

The point of zero charge also appears to be identified from the flotation response of alunite as a function of pH using anionic and cationic collectors. The results for two anionic collectors- sodium dodecyl sulfate (SDS) and high mano lauryl phosphate (HMLP) and one cationic collector – dodecyl amine (DDS). Since the amine is a cationic collector adsorption and flotation is expected to occur in alkaline media above the PZC if adsorption is predominantly coulumbic. On the other hand the anionic collectors should adsorb and promote flotation in more acidic solution below the PZC. The results shown in Figure 8 indicate that the PZC lies between pH 6.5-7.1. This agrees very well with the value determined by the electrokinetic experiments. Thus, the experimentally determined value of the point of zero charge is near pH 7 and is much different from the value expected from the pH-concentration diagram shown in which Al(OH)3(s) is presumed to form. However, the experimental PZC agrees very well with the PZC value of 6.7 expected from the pH concentration diagram shown, which ignores the formation of Al(OH)3(s). This observation is interesting since the alunite sample for the electrokinetic experiments was dry ground and then washed with distilled water several times to remove the colloidal matter afterwards, being aged for 15 days. It appears that in the process of washing most of the Al(OH)3 formed is removed along with the colloidal matter and the subsequent aging does not form an appreciable amount of Al(OH)3. Therefore, the surface is best described by ignoring the formation of Al(OH)3(s). This reasoning is somewhat supported by the observations made by R.W. Smith earlier on the aging of aluminum hydroxy complexes. It was then noticed that the aging is a very slow process and Al(OH)3(s) is not completely formed even after 30 days.