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The solution of the problem of beneficiation of cassiterite from its primary ore deposits is of fundamental importance to Bolivia. The process today involves a gravimetric treatment of the ore followed by sulphide flotation. During the last year, a cassiterite flotation stage has been added in several plants to treat the slime tailings. Although cassiterite flotation is already economical for the slime tailings and the mechanisms of the process are understood, it is felt that further research could improve the results.
Experimental Materials and Methods
Selected samples of quartz and cassiterite from the Bolivian San Jose and Unificada mines respectively were used throughout this investigation. The samples were up-graded by means of gravimetric and magnetic processes and were washed with conductivity water. The specimens were dry ground in a porcelain mortar and screened. The concentrated cassiterite contained 75.2% tin.
For surface charge determinations a fraction of approximately 5 microns in size was obtained by sedimentation. The measurements of the electrophoretic mobilities of quartz and cassiterite in conductivity water were done with a zetameter, from the Zeta Meter Inc. The convertion of the electrophoretic mobility data to zeta-potential values provided information upon which the interaction of the NaCl, H2SO4 and HNO3 acids with the cassiterite was studied.
As the A-22 concentration is increased, the cassiterite recovery is also increased. For 3.0 x 10 -6 M. and 8.0 x 10 -6 M. of A-22 addition, 67% and 90% of cassiterite floated respectively. Maximum flotation was achieved when the collector concentration was increased to 1.5 x 10 -5 M. and 1.5 x 10 -4 M. in the pH range from 1.0 to 4.7, and from 1.0 to 8.0 respectively.
When H2SO4 was used, only 67% recovery was obtained, whereas with HCl 91% of cassiterite floated. The flotation of quartz was not affected by the use of HCl or H2SO4 as pH regulators.
In the alkaline media at pH 9, quartz denoted good flotation response only at considerably high concentrations of NaCl, but then it dropped again to about 50% at pH 11. However, by comparing the flotation response of quartz and cassiterite it becomes clear that optimum selectivity is achieved for pH values of 2.5, 5 and 7.
The influences of Ca+², Fe+² and Fe+³ ions on the flotation response of quartz and cassiterite as a function of A-22 and at a constant pH 5 were studied. For this group of curves the NaCl concentration was kept constant at 10-¹ M.
Zeta-potential data was obtained for cassiterite in conductivity water by using HNO3, HCl, H2SO4, and NaOH as pH regulators. Data was also collected for various additions of NaCl salts using H2SO4 and NaOH for pH adjustment.
Flotation of Cassiterite in the Acid Media
When the zeta-potential curves were run with H2SO4 and HNO3 acids as pH regulators, the PZC lay at 3.7. However, when the curve was run with HCl, the PZC of the oxide occured at pH 3.1. The changes in magnitude of the zeta-potential for pH values less than their PZCs appeared to be related in each curve, to the surface affinity exhibited by the NO3-, Cl-, and the SO4= species. Accordingly, Cl- adsorbs more than NO3-, and a greater adsorption is manifested by SO4= species.
When HCl was used as pH regulator, 92% of the cassiterite floated as compared to 67% when H2SO4 was used. This increased flotation shows that Cl- ion as compared to SO4= species, increases the surface activity of the oxide, thus favoring the collector adsorption. The adsorption of A-22 occurs upon substitution of the Cl- ion initially adsorbed on the surface by the collector ion. The ability of Cl- ions to activate cassiterite flotation when SO4= species are adsorbed on its surface is considerably decreased.
Flotation of Cassiterite in Alkaline Media
When cassiterite was floated with 3.0 x 10 -6 M. of A-22, only 48% recovery was obtained at pH 5.0; at pH 7.0, 12%, and finally at pH 9.0, this oxide did not float. However, at these pH values in the presence of NaCl, optimum flotation response was attained when this salt was added. This flotation data shows that cassiterite recovery is increased as the ionic strength of the solution is also increased. Finally at 10-¹ M. addition of NaCl yielded complete flotation. For pH values greater than the pH of the PZC the subsequent decrease in negative potential values of cassiterite becomes controlled by the preferential adsorption of Na+ ions at the double layer, when NaOH is used as pH regulator. The Na+ ions replace the ionized hydrogen ions of the OH groups of the surface thus causing an increase in the pH of the solution.
Under these conditions the oxide surface probably acquires new characteristics by which the increased population of Na+ ions at the double layer causes a decrease in surface charge thus preventing the surface OH groups from being dissociated. The surface charge deficiency would be overcome by the presence of Cl- and OH ions which may also be adsorbed to some extent.
Flotation of Cassiterite in the Presence of Metal Ions
The zeta-potential values of cassiterite are affected in varying degrees of intensity by the properties of metal ions in solution, like hydrolyzation pH and concentration. The interfacial effects of the metal ions such as Ca+² and Fe+², are to bring zeta-potential to zero, or at best, to form a loose attachment at the surface at the pH values studied. Fe+³ ion below pH 6, changes the sign of zeta -potential due to preferential adsorption of these ions, since below pH 6 the Fe+³ specie is present in its hydrolyzed form. Above this pH value Fe(OH)3 precipitate is formed.
Selective flotation of cassiterite from quartz can be achieved in the presence of NaCl in an alkaline media even at high concentrations of the Ca+², Fe+² and Fe+³ metal ions, thus improving previous successful selective flotation conducted in a very acid media in spite of the corresponding practical and economical disadvantages.