Molecular Recognition at Mineral Interfaces: Implications for the Beneficiation of Rare Earth Ores

Abstract

Ce-bastnäsite is the single largest mineral source for light rare-earth elements. In view of the growing industrial importance of rare-earth minerals, it is critical to develop more efficient methods for separating the valuable rare-earth-containing minerals from the surrounding gangue. In this work, we employ a combination of periodic density functional theory (DFT) and molecular mechanics (MM) calculations together with the de novo molecular design program HostDesigner to identify bis-phosphinate ligands that preferentially bind to the (100) Ce-bastnäsite surface rather than the (104) calcite surface. DFT calculations for a simple phosphinate ligand were employed to qualitatively understand key behaviors involved in ligand-metal, ligand-solvent, and solvent-metal interactions. These insights were then used to guide the search for flexible, rigid, and semirigid hydrocarbon linkers to identify candidate bisphosphinate ligands with the potential to bind preferentially to Ce-bastnäsite. Among the five most promising bis-phosphinate ligands suggested by theoretical studies, three ligands were synthesized and their adsorption characteristics to bastnäsite (100) interfaces were characterized using vibrational sumfrequency (vSFG) spectroscopy, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and isothermal titration calorimetry (ITC). The efficacy of the selective interfacial molecular binding was demonstrated by identifying a bis-phosphinate ligand capable of providing an overall higher surface coverage of alkyl groups relative to a monophosphinate ligand. The results highlight the interplay between adsorption binding strength and maximum surface coverage in determining ligand efficiency to render the mineral surface hydrophobic. DFT calculations further indicate that all tested ligands have higher affinity for Ce-bastnäsite than for calcite. This is consistent with the ITC data showing stronger adsorption enthalpy to bastnäsite than to calcite, making these ligands promising candidates for selective flotation of Ce-bastnäsite.

A Molecular-Scale Approach to Rare-Earth Beneficiation: Thinking Small to Avoid Large Losses

“…The vSFG spectra probing the C-H stretching region for the three ligands studied at the bastnäsite-[10 0]-water interface are shown in Figure 3 A at surface saturation. In all cases, the signals from the ligand-adsorption species were much weaker compared with measurements on ligands with alkyl tails ( Sutton et al., 2020 ; Wanhala et al., 2019 ). The weak signal qualitatively suggests that the ligands are arranged such that the transition dipoles are oriented more parallel to the interfacial plane.…”

Origins of Clustering of Metalate–Extractant Complexes in Liquid–Liquid Extraction

“…[36][37] These interactions can vary with the nature (identity) of the metal in the organic phase. Thus, while highly selective extractants are being developed for LLE from a coordination chemistry perspective, [38][39][40][41] the effects of long-range interactions need to be considered. 2or PtCl 6 2-) affects the structure of the organic phase containing the extractant molecules (represented in brown squiggly lines).…”

Unlocking the computational design of metal–organic cages

“…36 Their approach is implemented in their free software, HostDesigner, 79,80 which has been used for designing metal-ion binders to MOCs. 77,[81][82][83][84][85][86][87] Their structure-based algorithms work by docking ''linking fragments'' (from a library) between the target ''complex fragments'', then covalently connecting those fragments, which produces new molecules in active conformations. All possible connection points, conformations, linking isomers and stereoisomers are explored in the assembly algorithm (e.g., the rotations and translations of the two fragments in Fig.…”