Atomistic simulation techniques for modelling inorganic/organic interface and flotation collector design

Sammanfattning: The choice of collector molecules in flotation process of mineral separation is invariably determined by tedious trial and error experimentation with inherent mounting expenditure. Here, we seek the help of atomistic simulation techniques, to help and judge the suitable collector molecule for selective mineral separation in flotation process, before actually conducting the experiment. This was the domain where we can attempt theoretically to determine the selective collector while considering different molecular structures of collectors having various head groups that attribute specific interactions with different exposed mineral surfaces in their pure or hydroxylated or activated form. By this we can have insight of the structure of heteropolar collector molecule in flotation separation through electrostatic, stereochemical and geometrical matching with active adsorption sites underlying the molecular recognition mechanisms. The quantitative aspect of adsorption is also obtained while comparing the adsorption energy values. Static energy minimisation code METADISE has been used to construct and describe predominant surfaces of quartz and wollastonite crystals structure in atomic scale. The potential parameters used in simulation predict the crystal cell parameters satisfactorily. The stability of surfaces is compared by surface energy calculations. Seven predominant surfaces of scarcely floatable wollastonite have been modelled and their calculated surface energy corresponds well with their preferred morphological domination. Surfaces are identified having four-fold and three-fold coordination of surface silicon. Three fold surface silicons are stabilized by addition of hydroxyl ion on it and proton on surface oxygen. Stable surfaces thus obtained are subjected to surface Ca2+ replacement by 2H+ by transforming 2O2- to 2OH-. Surface energy and reaction energy values indicate wollastonite surface stabilized to a great extent by adsorbing water in dissociated form and Ca2+ replacement is energetically favourable in acidic condition up to few layers from the surface. Atomistic simulation techniques are used to simulate surface structure and adsorption behavior of wollastonite mineral in the presence of molecular and dissociated water, formic acid and methylamine. A comparison of surface energies revealed that all the surfaces become stabilized in the presence of added molecules but the presence of methylamine decreased the surface energy to lower values. Adsorption of dissociated water is preferred by {100} and {102} surfaces, while {001} surface preferred methylamine adsorption as these show highly negative adsorption energies. In terms of molecular adsorption, the preferred adsorption sequence for all the surfaces is methylamine > formic acid > water without considering co-adsorption. For {100} and {102} surfaces, the adsorption energy values of carboxylic acid and amine are more negative than that of water and therefore we conclude that both carboxyl and amine head group molecules adsorb preferably on wollastonite. Our simulation verify usability of carboxylic acid head group as widely used collectors for wollastonite flotation and at the same time it predict use of amine head group collectors as possible modifier which correspond well with our experimental findings. Surface energies of quartz are calculated and unsaturated surface sites are identified by using the same code. Hydroxylation is carried out in order to satisfy full coordination of surface sites. It revealed that quartz surfaces are most stabilised when they adsorb water in dissociated form justifying hydroxylated quartz surface prevalence in nature. Water, formic acid, and methylamine adsorption calculations are carried out on both pure and hydroxylated quartz surfaces. Relative adsorption energies suggest that both formic acid and methylamine adsorb preferably than water on pure quartz surface. In case of adsorption on hydroxylated quartz methylamine shows great deal of adsorption preference than water and formic acid, which match with flotation practice of quartz by cationic amine collectors. Relaxed atomic positions of surface atoms have been studied for ¦Á-Fe2O3 (0001), (10¨©2), (10¨©0), (10¨©1), (01¨©2), (11 0) and the inverse spinel Fe3O4 (001), (011), (101), (110) and (111) surfaces. Water is added on these surfaces both as molecular and dissociated forms and corresponding surface configuration and relative adsorption energies and calculated. Water adsorption is also carried out on hydroxylated surfaces. Formic acid adsorption is studied both on pure and hydroxylated surfaces. The aim of the present work is to determine the most important faces of each iron oxide by calculating their surface energies, to discuss their surface structure and the effects of relaxation under vacuum conditions. The results are discussed and where possible, compared with experimental data.