Influence of Surface Effects on the Electrostatic Separation of Zircon and Rutile

Electrostatic separation is the final separation technique employed in the recovery and upgrade of rutile and zircon feedstocks. This dry separation technique is dependent on the difference in resistivities of the rutile and zircon. Even though both minerals have fairly high resistivities (Kelly and Spottiswood, 1989a), the differences in the resistivities are large enough to ensure separation. Of the two minerals rutile has a lower resistivity and is therefore regarded as the conducting mineral. After the initial electrostatic separation between rutile and zircon, the non-conducting zircon concentrate still contains some rutile (approximately 5% by mass). This concentrate is then acid washed to remove impurities, (for example, oxides or hydroxides (Gerson, 2006), and thus increasing the difference in the resistivities between the rutile and the zircon. After drying, this zirconrich stream is again subjected to an electrostatic separation stage to remove as much of the rutile as possible; during this last separation step it is found that there are zircon losses to the conducting stream. This begs the question whether the acid wash is effective in increasing the difference in resistivity of the two minerals; thus whether it ultimately improves the separation of the minerals. The initial investigation examined the influence of the neutralization pH of the acid wash on the resistivity of the rutile and zircon (Venter and Vermaak, 2006). It was found that zircon was not very sensitive to the neutralization treatment below a pH of 8 (Figure 1). Above a pH of 8 there was a decrease in the resistivity of zircon. It is not expected that a pH as high as 8 will be reached during neutralization of the acid wash thus the focus is in the lower pH range. The rutile, on the other hand, showed sensitivity to change in pH over the entire range of pH values (pH 3–10). These results suggested that the lower the pH during pretreatment, the more efficient the subsequent electrostatic separation stage ought to be. The zircon losses to the conductors also pose the question whether there are some zircon particles that are less resistive than the bulk zircon—zircon particles reporting to the conductor fraction. For this purpose an investigation was performed on the feed stream to the final electrostatic separator stage (after the acid wash) to investigate possible differences in the surface properties of zircon particles reporting to the conductor and the non-conductor fractions (Bruwer et al., 2007). It was found by employing X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) that, although not all of the surface impurities where removed during the acid treatment, there are no detectable differences between the zircon in the conductor and non-conductor fractions These results were verified by use of resistivity measurements that indicated that there is no discernible difference in the resisitivities (approximately 2.85 × 1012 Ω.cm for both) of the zircon in the two streams. Thus it seems that the cause of the zircon losses to the conductors is not material related. The other point of interest is the sensitivity of the rutile to a change in the pH. From the previous investigation (Venter and Vermaak, 2006) it was apparent that the change in the resistivity is not due to the removal of the surface impurities. The initial XPS investigation indicated that the surface species are, however, modified. Over the years there have been many investigations into the interaction of different species with TiO2 (Diebold, 2003), most of these on TiO2 films. From these studies it is evident that rutile can interact with many different species over a wide range of conditions. Rutile also has some catalytic properties which mean that a wide range of species can adsorb and also be modified on its surface. The aim of this study is twofold: (i) to investigate if the effect of particle shielding due to machine operating conditions is the cause of the zircon losses, and (ii) to understand the mechanism by which rutile is rendered less VENTER, J.A., VERMAAK, M.K.G., and BRUWER, J.G. Influence of surface effects on the electrostatic separation of zircon and rutile. The 6th International Heavy Minerals Conference ‘Back to Basics’, The Southern African Institute of Mining and Metallurgy, 2007.