Surface Energy Heterogeneities and Hydrophobic Interactions - New Insights to understand Flotation

Flotation is a heterocoagulation separation process first described in 1877 by a patent issued in Dresden, Germany. Since then it has become to be the most important and most variable separation process in the beneficiation of minerals. Especially the modern fine grained and polymetallic deposits call for a continuation in process development and even more so in better understanding the flotation process which is based on the selective hydrophobization/hydrophilization of minerals. This paper will present novel insights on the hydrophobization of various mineral particles, i.e. silicates, semi-soluble salt type minerals, metal oxides and sulfides with different collector molecules, which are the surfactants used to selectively hydrophobize minerals in flotation. We investigate the effect of the collectors on the surface energy distribution which is determined with inverse gas chromatography. With this method it is possible to assess the wettability of particles without the difficulties encountered with the conventional sessile drop and penetration methods. By applying inverse gas chromatography we can show that it is due to the collector adsorption the reduction of the highly energetic moieties which only make up less than ten percent of the particle surface. Furthermore we can present the effect of collector adsorption on the different surface energy components, i.e. disperse and specific interactions. By calculating the free energy of interaction between a particle and a bubble immersed in water using the complex surface energy information we can find a good correlation to the flotation response, i.e. recovery determined with classic microflotation experiments in the Hallimond tube. Further insight into the surface heterogeneity regarding the wettability is presented with respect to investigations on planarized mineral samples using the colloidal probe technique in atomic force microscopy in the liquid environment. By assessing force distance spectra on various surface sites with and without the adsorption of collectors we find the well described but so far not well understood long range attractive hydrophobic interactions. Based on the results presented we will critically discuss the different concepts of long range hydrophobic interactions in the context of a fundamental model which describes the floatability of minerals. The minerals used are: quartz, apatite, magnetite and pyrite. The collectors assessed are cationic and different anionic surfactants.