am9b22902_si_001.pdf (1.41 MB)
Molecular Recognition at Mineral Interfaces: Implications for the Beneficiation of Rare Earth Ores
journal contribution
posted on 2020-03-31, 11:39 authored by Jonathan E. Sutton, Santanu Roy, Azhad U. Chowdhury, Lili Wu, Anna K. Wanhala, Nuwan De Silva, Santa Jansone-Popova, Benjamin P. Hay, Michael C. Cheshire, Theresa L. Windus, Andrew G. Stack, Alexandra Navrotsky, Bruce A. Moyer, Benjamin Doughty, Vyacheslav S. BryantsevCe-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 bis-phosphinate
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 sum-frequency (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.