Structure of the Interface between Lithium-Rich Spodumene and Saltwater by Density Functional Theory Calculations and Molecular Dynamics Simulations

The surface chemical properties of spodumene, a pyroxene mineral typically found in lithium-rich pegmatites, are key to improve its recovery by means of flotation. Without this information, it is very difficult to find a collector that prefers the spodumene surface over that of associated aluminosilicates. The crystal structure of spodumene includes chains of Si-centered tetrahedra and Al-centered octahedra, with Li occupying cavities between the polyhedra. In this work, density functional theory (DFT) calculations are used for a small group of atoms of the mineral crystal to determine the termination of the (110) surface, the weakest plane, and then to determine the most energy-favorable deprotonation reaction. The excess charge generated by the removal of one hydrogen in the deprotonation reaction is distributed over the surface atoms of the spodumene cluster. Then, molecular dynamics simulations are used to study the adsorption of water and alkali and alkaline-earth metal cations from concentrated solutions on the surface (110) of a large slab of spodumene, both neutral and negatively charged. Results include density profiles of adsorbed cations, orientation profiles of water molecules close to the mineral surface for different cations, and the distance at which the mineral surface becomes neutral or reverses the charge. The new results on the surface chemistry of spodumene should allow reinterpretation of available data from spodumene flotation.