Affinities of Solvated Rare Earth Cations with Collectors and Mineral Interfaces: A Density Functional Theory Investigation

Rare earth mineral flotation collectors are required to have a combination of high affinity and selectivity for adsorption to target mineral surfaces, especially as rare earth ore deposits become increasingly complex. This study uses density functional theory to computationally model the affinity and selectivity of a series of candidate functional groups for targeted flotation of monazite (LnPO4) mineral in the presence of clay and silicate rich gangue. A composite approach is employed is whereby collector screening is based on in-solution models of candidate functional groups bound to key metal cation hydroxyl species, while relavent surface models of monazite and kaolinite minerals with pertinent cation species are considered in separate models to qualify the efficacy of collector binding. According to these models, phosphinate has the best overall affinity for La3+ cations across various speciations and metal-to-ligand ratios, while the phosphonate and diester phosphorate functional groups have best overall selectivity. Two variations of surface models were implemented to investigate the propensity for dissolved [La3+(OH)x]3−x species to aggregate on monazite surfaces compared to kaolinite surfaces in addition to the potential competition from potentially inhibiting species of [Al3+(OH)x]3−x and [Ca2+(OH)x]2−x (for hydroxylated species corresponding to x = 0, 1 or 2). Surface adsorption results predict that aqueous [La3+(OH)x]3−x species should preferentially form active sites for collector coordination on the monazite surface.