Molecular Dynamics Simulation Of Muscovite Flotation System

Flotation is an effective method for selective separation of medium and fine mineral particles. Current primary efforts about flotation are always concentrated on flotation response. Understanding the adsorption of surfactant molecules at the solid/liquid interface is a crucial step toward modeling industrial flotation process. Molecular dynamics (MD) simulation is a useful tool that can be used to provide detailed information and fundamental understanding on issues such as surface potential, surfactant/macromolecule adsorption, and mineral floatability. The mica is a representative silicate mineral, its (001) surface is structurally similar to the dominant surfaces of many clay minerals. Because of amphiphilic character and relatively high solubility, alkyl ammonium salts with long alkyl chains have generally been employed as flotation collectors. Different systems including collector/muscovite, water/muscovite and collector/water/muscovite are constructed to investigate the dynamics and structure of the solid surface by means of molecular dynamics simulations.The adsorption of alky amines with different chain lengths on the muscovite (001) surface have been performed using MD simulations with PCFF_phyllosilicates force field. Quantifying the strength of collector-mineral surface interactions with the interaction energy between ammonium and the muscovite (001) surface, results showed R-NH3+have stronger interaction energy than R-N(CH3)3+, the mobility of the R-N(CH3)3+on the surface was higher than that of R-NH3+. Furthermore, the interaction of the surfactant with the surface was determined by head groups, hydrocarbon chain length could not affect the interaction of the surfactant with the surface, the hydrocarbon chain length decided the association strength of hydrocarbon chain forming aggregates.The adsorption of water molecules on the muscovite surface was studied, atomic density profiles, radical distribution function, hydrogen bond profiles, density field, mean squared displacement, and hydration energy were calculated. Results showed, at a surface with water coverage larger than 1, the density of water molecules in the first three layers near the muscovite/water interface would be stable with the increase of water molecules; water molecules closed to the solid/water interface represented much more ordering than those far from the surface; "solid effect" of muscovite on the water molecules was stressed by atomic density profiles and hydrogen bond profiles. It was found that surface K+ ions have poor mobility, especially in the z direction, and the stability of [Si4Al2]-K+ structure was proven by density field of K+ ions. The calculated hydration energies between muscovite and water were in good agreement with available experimental data. Compared with interaction energy between collectors and the muscovite surface, results confirmed that ammoniumions have thermodynamic advantages to resist the hydration layer sufficiently for effective flotation to occur.Monolayer adsorption of aqueous dodecylamine surfactants on the muscovite (001) surface was investigated by means of molecular dynamics simulations. Three systems including different number of surfactant molecules have been designed to examine the effect of surface coverage on the microscopic structure of the mica/water interface. In all surface coverage, hemimicelle structures formed at the muscovite/water interface, with their head groups directed to the surface and the hydrocarbon tails oriented toward the solution. The modified mica surface obtained a hydrophobic character, the hydrophobicity became stronger with the increasing surface coverage. Results from density field showed adsorption sites for water molecules in the first two layers near the muscovite surface. Hydrogen bonds between bridging oxygen atoms and surfactant head groups were also investigated, and we observed that they were influenced by interfacial water molecules. Furthermore, surfactants formed hemimicelle structure on the mica surface with a thickness roughly less than that of the molecule chain length, which was in good agreement with experimental AFM data.In the last, Molecular dynamics simulations employing PCFF_phyllosilicates force field have been carried out to explore the adsorption of primary ammonium collectors with different alkyl chains in the muscovite/water interface. Results showed all primary ammonium ions coated the muscovite surface, they formed a monolayer coating on the surface, irrespective of the alkyl chain length. By computing interaction energy between the ammonium ion and mica surface, we found primary ammonium ions have consistent interaction with the muscovite surface in the water environment. Atomic density profiles for water molecules on muscovite surface were calculated, we reproduced an important conclusion with theoretical calculation that hydrophobic force for long alkyl chains was stronger than that for short chains. We found ammonium ions formed a relatively orientationally ordered structure with respect to the muscovite surface. The simulation revealed hydrogen bonds between the head groups and the bridging oxygen atoms were affected by water molecules around head groups, however, head groups still constrained themselves above the hexagonal rings of the muscovite surface.