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 computational solid chemistry techniques, namely the 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. Simulations were performed to study the adsorption behaviour of wollastonite in the presence of molecular and dissociated water, and two widely used collector head groups, i.e., carboxylic and amine groups, on three morphologically predominant Miller indexed surfaces. Surface reconstruction were carried out to make the surface free of lone oxygen and surface cuts terminating with fully coordinated silicon were chosen and used for most of the simulation work for adsorption of molecules. The hydroxylation was carried out on those surfaces where low coordinated silicon was made to saturate by bonding with hydroxyl group and the subsequent charge neutralization was maintained by adding proton on single coordinated surface oxygen. A comparison of surface energies revealed that all the surfaces become stabilized in the presence of added molecules but methylamine decreased surface energy to lowest values. Adsorption of dissociated water is preferred by {100} and {102} surfaces, while {001} preferred to adsorb methylamine as these show highly negative adsorption energies. In terms of pure molecules adsorption, the preferred adsorption sequence for all the surfaces is methylamine > methanoic acid > water. Since the {100} and {102} surfaces are the two most predominant and the difference in adsorption energy values are not significant, we conclude a consideration of collector molecules possessing long alkyl chain with head groups considered here cannot adsorb and render hydrophobicity to wollastonite in pure form if no surface activation is carried out, which correspond well with the reported 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, methanoic acid, and methylamine adsorption calculations are carried out on both pure and hydroxylated quartz surfaces. Relative adsorption energies suggest that both methanoic 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 methanoic acid, which match with flotation practice of quartz by cationic amine collectors.