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The role of dissolved metal ionic species in the phosphonic acid flotation of cassiterite Senior, Geoffrey David


The techniques of X-ray photoelectron spectroscopy (XPS), secondary ionization mass spectroscopy, chemical abstraction analysis and microelectrophoresis were used to confirm the existence of iron films on cassiterite grains recovered from the tin concentrator of Renison Limited (Australia). All analyses indicated the level of surface iron contamination to be far in excess of that characteristic of the bulk mineral lattice. No evidence was found in XPS analyses to support the contention that this iron represented lattice iron that had accumulated on the surface due to migration through the lattice, as has been suggested to be the case for certain sulphide minerals. Instead, all of the experimental data supported the hypothesis that the surface film was the result of previous adsorption of hydrous ferric oxide sols onto the cassiterite surface in the plant environment but which had undergone fundamental structural changes when the sample was dried. Specifically, the presence of the iron film did not change significantly the electrokinetic properties of cassiterite, as is known to be the case for other insoluble oxide minerals in the presence of freshly precipitated hydrolyzed metal ionic species. To reconcile these observations, it was shown independently that drying, both thermally and under vacuum, caused the charge reversal phenomena related to hydrolyzed metal ion adsorption either to diminish or disappear. In the presence of styryl phosphonic acid, the iron contaminated cassiterite exhibited a microflotation response that corresponded remarkably with the actual flotation observed in the plant from which it was taken. A critical examination of the literature revealed that this behaviour was indicative of that reported for other cassiterites in both batch and plant flotation of ores. Most notably the mineral floated strongly between pH 4.0 and 7.5. However, after acid leaching, the pH of maximum recovery moved to more acidic values around pH 2.0 and the mineral floated much less strongly in the slightly acidic to alkaline range. This latter behaviour correlated with a large body of published data for which it was known that the cassiterite used was either a high purity sample, not exposed to solutions containing dissolved metal ionic species, or had been acid cleaned prior to test work. Conversely, results from fundamental studies, where this was known not to be the case, exhibited remarkable consistency with the data obtained for the untreated cassiterite sample. The principal influence of the iron species adsorbed onto the surface of cassiterite was shown to be flotation activation in the pH range where the adsorbed species were not extensively solubilized. In the acidic range below pH 4.0, solubilization of the surface iron entities occurred which caused a severe flotation depressant effect. This solubilization was promoted in the presence of sodium fluoride, a known complexing agent for iron in acidic solutions, and the phosphonic acid. In the latter case, evidence was found for the formation of a soluble iron phosphonate complex. Independent confirmation was also sought and obtained from light scattering photometry to verify the strong affinity of phosphonic acids for hydrous ferric oxide species. Other ionic entities, such as those of magnesium and calcium were found to interact far less strongly. On the basis of this research, it has proven possible to provide a rational interpretation of all previously existing data pertaining to cassiterite flotation with phosphonic acids, for which reasonable sample description and preparation information exist. Such a reconciliation has not previously been provided, either through studies on the effects of cassiterite mineral lattice impurities or other work with dissolved metal ionic species. Furthermore, a significant link has been established between the results of fundamental analyses and industrial related observations. In particular, commercially successful practices, such as fluoride pretreatment of ore slurries prior to cassiterite flotation, now can be interpreted successfully in terms of fundamental ion solution chemistry. The evidence is that such processes involve the solubilization under acidic conditions of adsorbed iron hydroxy sols from the surface of minerals prior to cassiterite flotation. A bulk rejection of the complexed metal ionic species is then made in the water split of the desliming circuit that precedes flotation. The solution chemical evidence obtained supporting such a mechanism is considerable and no finding, in this work or elsewhere, refutes it.

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