The floatability dependence of monazite, xenotime, and zircon on pH and temperature using ionic liquids as collectors

Ionic liquids (ILs) have recently been reported as promising collectors for rare earth minerals (REMs), but how they improve REM floatability is still not well understood. In this study, six types of acid-base ILs—tetraethyl-, tetrabutyl-, tetraoctyl-ammonium mono-(2-ethylhexyl) 2-ethylhexyl phosphonate and tetraethyl-, tetrabutyl-, tetraoctyl-ammonium di(2-ethylhexyl) phosphate—were investigated to understand how the cationic and anionic moieties of these compounds as well as the conditioning temperature influence REMs (i.e., monazite and xenotime) and zircon floatabilities. Flotation experiments were conducted at pH 5, 7, and 9 under both ambient conditions and 60 °C to assess the performance of the ILs. Zeta potential measurements coupled with Fourier transform infrared spectroscopy (FTIR) characterization of minerals were also done to identify the adsorption mechanism of ILs on mineral surfaces. The results showed that tetraoctyl-ammonium di(2-ethylhexyl) phosphate effectively separated monazite from xenotime and zircon via reverse flotation, particularly at pH 9, with conditioning at 60 °C. These findings suggest that the structural characteristics of ILs and their interactions with mineral surfaces, influenced by pH and temperature, play a significant role in enhancing the floatability of REMs. Furthermore, the study demonstrated that using ILs as collectors combined with elevated temperatures during the conditioning stage significantly enhances selectivity. This suggests the potential for effectively separating monazite from xenotime by froth flotation using ILs as collectors without the need for depressants. This innovative method could greatly benefit the REM processing field, particularly in flotation technology.