Probing Surface Characteristics of Rare Earth MineralsUsing Contact Angle Measurements Atomic Force Microscopy and InverseGas Chromatography

摘要

Rare earth minerals (REMs) such as bastnaesite, monazite, and xenotime are of considerable significance since they are the main commercial sources for rare earth elements (REEs) with cutting-edge applications. Fundamental understanding of surface properties of REMs is essential to identify the reactions taking place at different interfaces to develop more robust technologies for the recovery of REEs. The goal of this study is to provide a comprehensive investigation on the surface energy characteristics of bastnaesite and xenotime, as the primary sources of light and heavy rare earth elements, respectively. Crystal’s orientation of REMs was identified using surface X-ray diffraction analysis, whereas the morphology and elemental composition were characterized using scanning electron microscopy and energy dispersive spectra analyses. Wettability of REMs was studied using sessile drop contact angle measurement technique, and the surface energy and its constituents were evaluated using Fowkes, van Oss–Chaudhury–Good, Owens–Wendt–Rabel–Kaelble, Zisman, and Neumann models. Atomic force microscopy (AFM) was usedto compare the local surface properties and work of adhesion of REMsby analyzing the force profile between the mineral surfaces and an-type silicon tip. Inverse gas chromatography (IGC) was employedto study the surface energy heterogeneity of REM powders and evaluatethe dispersive and Lewis acid–base interactions. Results indicatedthat the dispersion forces have a larger contribution to the surfaceenergy of both REMs in comparison with the polar interactions. Thesurface energy values obtained using contact angle measurements werelower than those obtained using IGC, however, the IGC results seemedto be closer to reality since the contact angle results showed a strongdependence on probe liquids, roughness, and local properties of thesurfaces. Contact angle measurements and AFM analysis indicated thatbastnaesite had higher hydrophobic character, whereas the IGC analysisrevealed that the surface energy of xenotime was lower than that ofbastnaesite at higher surface coverages. Despite the shortcomingsof each method, results showed that a combination of these techniquescould provide a deeper understanding of surface energy and wettingbehavior of minerals.