Spectroscopic Insights into the Ligand Adsorption for Improved Foam Flotation of Monazite

Rare earth elements (REE), a group of seventeen chemical elements including fifteen lanthanides as well as yttrium and scandium, have been extensively used in most modern technologies, such as batteries, super alloys, sensors, computer hard drives, X-Ray and medical imaging. Monazite, (Ce, La, Nd, Th)PO₄, is a key mineral for light REE, requiring selective foam (froth) flotation to separate REE ores from undesired ores (gangue). Collectors, typically surfactants, have been used in the flotation process to increase monazite recovery through selectively adsorbing on naturally present mineral surfaces, making them hydrophobic. Although the use of collectors in REE flotation has been extensively reported, there is a need to improve selective separations from various feedstocks, including tailings containing depleted amounts of REE. This motivates our work to conduct fundamental studies focusing on the surface chemistry and ligand adsorption mechanism. In the present work, we use spectroscopy methods such as UV-visible (UV-vis) spectroscopy, sum frequency generation (SFG) vibrational spectroscopy, and infrared spectroscopy (IR) to quantify the ligand adsorption onto the model monazite surface and correlate it with results of flotation experiments of the real ore. It benefits the efficient ligand selection for using it as collector to float the rare earth minerals. The effect of parameters such as pH, mixing time on ligand adsorption is studied in the experiments with model monazite. In addition, density functional theory and ab initio molecular dynamics simulations were employed to investigate possible binding position for ligands adsorption on the monazite surface. Lastly, the monazite recovery was tested in the presence of selected ligands using a lab-scale microflotation device.