Impact of gypsum supersaturated solution on surface properties of silica and sphalerite minerals

In some sulphide mineral flotation operations, the process water contains high concentrations of calcium and sulphate ions that exceed the solubility limit of gypsum. It has been speculated that the gypsum supersaturated process water would lead to precipitation of gypsum which could coat on mineral surfaces by either nucleation or coagulation, resulting in reduced flotation recovery and selectivity. In this study, a laboratory prepared gypsum supersaturated solution is used to represent the gypsum supersaturated process water, the effect of gypsum supersaturated solution on the surface properties of silica and sphalerite minerals was investigated using zeta potential distribution measurements, scanning electron microscope (SEM), X-ray photon spectroscopy (XPS), Auger electron spectroscopy (AES), and quartz crystal microbalance with dissipation (QCM-D). Our results show that silica and sphalerite minerals carry identical surface charge (−10 mV of zeta potential) in the gypsum supersaturated solution at pH 10 although they are charged differently in simple electrolyte solution at the same pH. Needle shape gypsum precipitates are found in both silica and sphalerite minerals systems conditioned with gypsum supersaturated solution. The gypsum precipitates do not grow on the minerals surfaces but form in the bulk gypsum supersaturated solution. The heterocoagulation between the examined minerals and gypsum particles is insignificant in the gypsum supersaturated solution. It is the high calcium concentration in the gypsum supersaturated solution that has significant effect on the surface properties of silica and sphalerite minerals. The zeta potentials of silica and sphalerite in a 800 ppm calcium solution (similar to the calcium concentration in the gypsum supersaturated solution) are similar to those measured in the gypsum supersaturated solutions. Both silica and sphalerite minerals surfaces are indiscriminately coated with calcium. The surface coating of calcium results in the identical surfaces between silica and sphalerite minerals, and ultimately causes problems for the flotation separation of silica and sphalerite.     

Impact of gypsum supersaturated water on the uptake of copper and xanthate on sphalerite

Gypsum supersaturated process water has been shown to have an adverse effect on the flotation of sphalerite minerals. This study probes the mechanism of such effect by determining the uptake of copper and xanthate on sphalerite in gypsum supersaturated water using zeta potential distribution measurement, atomic absorption spectroscopy, UV–visible spectroscopy and quartz crystal microbalance with dissipation. Our results indicate that the supersaturation of water by gypsum retards the adsorption of copper and hence the sequence xanthate adsorption on sphalerite. The retardation of copper and xanthate adsorption on sphalerite is also observed by the addition of 800 ppm calcium. The adsorption of calcium is identified to compete with copper species for the reactive surface sites of the sphalerite, resulting in the reduction in copper and xanthate uptake and hence flotation recovery of sphalerite.