Hydration of magnesite and dolomite minerals: new insights from ab initio molecular dynamics

Magnesite (MgCO3) and dolomite [(Mg,Ca)(CO3)(2)] are the main minerals involved in the production of magnesium metal, which is considered as a critical raw material in the EU and USA. In this study, we investigated the hydration mechanisms of (1014) magnesite and dolomite surface by static density functional theory relaxations and ab initio molecular dynamics simulations. For both minerals, the dissociative adsorption of water was unfavored compared to the molecular adsorption, which displayed an adsorption energy of 101.2 kJ mol(-1) and 94.2 kJ mol(-1) for magnesite and dolomite, respectively. In the case of dolomite, the adsorption of water was slightly favored on the surface calcium ions compared to magnesium ions, which was attributed to stronger interactions between the hydrogen atoms of water and the oxygen atoms of the surface. Then, the coverage was gradually increased to reach a monolayer: the adsorption energies for magnesite displayed a slight increase with increasing coverage, whereas dolomite exhibited no sensitivity to water coverage. Using ab initio molecular dynamics simulations conducted at 300 K, we showed that, beyond the first layer, water displayed a clustering behavior induced by the intermolecular interactions, while the isosteric enthalpies of adsorption continuously decreased with increasing coverage, due to the screening effect of deeper water layers. When more water molecules were added, until reaching the complete hydration of magnesite and dolomite surfaces, a refined water structuration emerged: a significant layering effect was exhibited, due to highly-organized layers, until 8-10 A from the surface, which therefore corresponded to the distance at which the surface had no more effect on water layers. Overall, magnesite exhibited a larger affinity for water than dolomite, including adsorption energies and structuration of water, which provides new insights in the understanding of the fine surface mechanisms involved in the subsequent flotation process.