Hydrophobic behavior of fluorite surface in strongly alkaline solution and its application in flotation

Abstract Normally fluorite was collected by fatty acid collectors at pH 5–11, rendering the surface to be hydrophobic. In this study, we found that fluorite surfaces are naturally hydrophobic at high pH without any surfactant, which is beneficial for the selective flotation separation of fluorite from calcite. In a flotation test, fluorite recovery showed a decreasing trend at alkalescent pH but increased at high pH. Sum-frequency generation spectroscopy spectrum confirmed that the orientation and hydrogen bonding structure of the water molecules are dramatically changed at fluorite/water interface. Moreover, a new signal peak, wavenumber of 3657 cm-1, appeared under strongly alkaline conditions; this peak is normally attributed to free OH stretching region and indicates a hydrophobic surface. Meanwhile, the results of contact angle and atomic force microscopy measurements show that the hydrophobicity obviously increases at pH 13. Therefore, the surface structure of fluorite must undergo some significant changes, which will play an important role in the flotation of fluorite.