Investigation On The Relationship Between Anisotropic Crystal Surface Characteristics And Flotation Behaviors Of Three Calcium-bearing Minerals

To separate calcium-containing minerals from each other by means of flotation still remains a thorny problem in the field of mineral processing. Generally, the exposed mineral surfaces in the flotation slurry are both cleavage planes because of the crushing and grinding of the mineral, and expressed surfaces in its crystal morphologies. Most calcium minerals, such as scheelite, calcite and fluorite, show a remarkable variety of habits in nature crystals and hence exhibit several anisotropic exposed surfaces which may possess different physicochemical characteristics. The investigations of anisotropic surface properties of these calcium mineral crystals may have academic guidance meaning for the exploration and interpretation of the mechanisms of the interaction between cationic/anionic collectors and calcium minerals, and for the achievement of selective flotation separation of these minerals.In this work, Materials Studio (MS)5.0modeling package was employed to build a series of surface slabs of scheelite, calcite and fluorite crystals. The anisotropic densities of surface broken bonds and surface energies of three mineral crystals were calculated to analyze the relationship between them. With the aid of the calculation results, the most commonly cleavage and exposed surfaces of the morphologies of three minerals were predicted and confirmed by the XRD observations. The micro-topographies of these exposed surfaces were observed using AFM, while their wettability and surface free energies were investigated by means of contact angle measurements. The mechanisms and differences of the adsorption behaviors of oleate and dodecylamine on the different exposed surfaces of each mineral crystal were comprehensively investigated by molecular dynamics (MD) simulations, zeta potential measurement, AFM observation, contact angle measurement, and flotation test, based on which the differences in the interaction of these two types of collectors with three calcium-bearing minerals were analyzed and interpreted. The major findings of this study are as follows:1. The positive correlations between surface broken (or dangling) bond densities and surface energies of three mineral crystals is established in this study, indicating that the former may be one of the most important factors that determine the latter. The cleavage is more readily generated between two surface layers with charge neutrality and smaller broken bonds density and larger interlayer spacing. The surfaces with lower surface energies are expected to be largely expressed in the stable crystal morphologies.{112} and{001} surfaces for scheelite,{214},{108} and{104} ones for calcite,{100} and{111} for fluorite are the most commonly exposed surfaces for the three minerals, which were confirmed by the experimental observations based on XRD.2. AFM observations show that the roughness Ra of{112} cleavage surface is much lower than{001} surface, indicating a much easier cleavage along the former than the latter, which might be attributed to their different interlayer spacings. A strong growth lines along [441] PBCs are found in the{018} form of calcite crystal, and the{104} cleavage surface shows a distinct reconstruction and dissolution behaviors after exposed to the air and distilled water, respectively. Remarkable growth lines parallel to the crystal edge are observed for {100} fluorite surface, while{111} cleavage surface exhibits steps with height of an integral multiple of0.315nm (corresponding to one F--Ca2+-F-triple layer). While exposed to distilled water, the{100} surface exhibits laddering dissolution behaviors, but the change of{111} surface topography was not obvious, which can be explained by the surface broken bond density and dipole moment perpendicular to the surface. The wettability of commonly exposed surfaces of each mineral is in direct proportion to the surface broken bond density and adhesion work of water on the surface. Surface free energy is positively correlated with surface broken bond density for each mineral crystal.3. MD simulations of the adsorption of oleate on the commonly exposed surfaces of each mineral show that the highest adsorption energy is released by the bridged binding conformation of the oleate molecule near two surface calcium atoms, and then the bidentate, the lowest for unidentate one. The adsorption density of oleate molecules adsorbed at a completed monolayer on each commonly exposed surface is calculated based on the cross-sectional area per oleate molecule vertically adsorbed on the surface and surface unit cell area. The oleate molecules interact more strongly and are arranged more compactly on a certain surface with less hydrophilic mineral surfaces uncovered, resulting in a larger water contact angle, a more hydrophobic surface, which are well consist with the results of AFM observations and contact angle measurements. After interacting with the oleate solution at a concentration below1×10-4mol/L, the contact angle values and flotation recoveries both follow the order:fluorite> calcite> scheelite, which can be interpreted by the adsorption strength and density of oleate molecules and the bond energy of the broken bonds on the most commonly exposed surfaces of these minerals.4. MD simulations show that the driving force for the adsorption of DDA molecule on calcium mineral surface are electrostatic attraction between their nitrogen anion with surface calcium cation, and hydrogen bonding between the hydrogen of the-NH2group and surface oxygen (fluorine). Surface charge properties exert a decisive effect on the adsorption behaviors of DDA species on calcium minerals. The results of zeta potential measurements indicate that the scheelite particles are negatively charged under alkaline to neutral pH conditions, which facilitate the co-adsorption of DDA cationic species and molecules, while the adsorptions of these cations on the positively charged calcite and fluorite are not favorable. Compared to those on calcite, the DDA molecules interact more strongly and are arranged more compactly on fluorite surface, resulting in a better flotation recovery of the latter, while the co-adsorptions of DDA molecules and cation species lead to the best flotation performance of scheelite among three minerals.5. From the viewpoints of wettability and adsorption enengy, it is expected that the hydrophobicity among these three minerals will be to some extend magnified by increasing the exposure ratio of scheelite {001} surface and meanwhile decreasing the exposure of calcite{018} and fluorite{100} surfaces in the flotation pulp whether using oleate or DDA as collector, which will be in favor of the selective flotation separation of these minerals.