The influence of metal ions on the adsorption of octyl hydroxamic acid in the flotation of bastnaesite and the adsorption mechanism

The influence mechanism of metal ions on the adsorption of octyl hydroxamic acid (OHA) by bastnaesite in a flotation system was studied by flotation experiments, adsorption capacity tests, X-ray photoelectron spectroscopy (XPS) characterization, and density functional theory (DFT) study. From the experimental results, it can be seen that the flotation and the adsorption capacity tests showed that the single metal ions Ca2+, Ba<sup>2+</sup>, and Fe<sup>3+</sup> promoted the flotation recovery of bastnaesite, and the order of their ability was Ba<sup>2+</sup>>Fe<sup>3+</sup>>Ca<sup>2+</sup>. The addition of mixed metal ions inhibited the adsorption of OHA on the surface of bastnaesite, resulting in a decrease in the flotation recovery of bastnaesite. The XPS analysis showed that the N concentration on the surface of bastnaesite after single metal ion action was significantly higher than that without metal ion action, while the N concentration on the surface of bastnaesite after mixed metal ion action was significantly lower than that without metal ion action. When Ca<sup>2+</sup>, Ba<sup>2+</sup>, and Fe<sup>3+</sup> ions were added, new peaks appeared at 401.24 eV, 397.68 eV, and 397.19 eV in the N1s fitting peaks, respectively, which may be caused by the formation of N-M (OH) chemical bonds between M(OH)<sup>+</sup> (M: Ca<sup>2+</sup>, Ba<sup>2+</sup>, Fe<sup>3+</sup>) and nitrogen atoms in OHA. This indicated that metal ions Ca<sup>2+</sup>, Ba<sup>2+</sup>, and Fe<sup>3+</sup> form metal chelated with nitrogen atoms in OHA. The DFT study on the (100) surface of bastnaesite also showed that the doping of metal ions showed a stronger spontaneity and enhanced adsorption capacity due to the decrease of adsorption energy between OHA and bastnaesite surface. There were two adsorption sites of bastnaesite, one is that the Ce atom adsorbs two oxygen atoms of OHA to form a five-membered chelate, and the other adsorption site was that O-M(OH) on bastnaesite interacts with OHA; Therefore, the configuration of the oxygen atom adsorption site is O-M-O-N bond, during which M(OH)<sup>n+(n:1,2)</sup> loses OH<sup>-</sup>, and the N-O-H chemical bond on OHA loses H<sup>+</sup> to form an H<sub>2</sub>O molecule, a typical dehydration reaction, so the adsorption is more stable. Therefore, the surface of bastnaesite has two adsorption forms with OHA, one is R-NH-O-M-CeFCO<sub>3</sub> and R-NH-O-CeFCO<sub>3</sub>.