Probing Surface Characteristics of Rare Earth Minerals Using Contact Angle Measurements, Atomic Force Microscopy, and Inverse Gas Chromatography

Abstract

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 used to compare the local surface properties and work of adhesion of REMs by analyzing the force profile between the mineral surfaces and a n-type silicon tip. Inverse gas chromatography (IGC) was employed to study the surface energy heterogeneity of REM powders and evaluate the dispersive and Lewis acid–base interactions. Results indicated that the dispersion forces have a larger contribution to the surface energy of both REMs in comparison with the polar interactions. The surface energy values obtained using contact angle measurements were lower than those obtained using IGC, however, the IGC results seemed to be closer to reality since the contact angle results showed a strong dependence on probe liquids, roughness, and local properties of the surfaces. Contact angle measurements and AFM analysis indicated that bastnaesite had higher hydrophobic character, whereas the IGC analysis revealed that the surface energy of xenotime was lower than that of bastnaesite at higher surface coverages. Despite the shortcomings of each method, results showed that a combination of these techniques could provide a deeper understanding of surface energy and wetting behavior of minerals.

Surface energy evaluation of casting and nanofiber polyurethane films by using different models

“…In fact, the competition between solidliquid adhesive and cohesive interaction determine the contact angle values. 8…”

“…The chemical heterogeneity is due to the existence on the surfaces of some functional groups, hydrophilic/hydrophobic sites, impurities, or orientation of crystals. 8 Rough surfaces have also a significant influence on the wetting behavior of fluids, due to the fact that this surface provides an additional interfacial area for the spreading liquid, resulting in the increase of the surface energy. 2,9 Surface energy arises from the forces between atoms or molecules at the surface and also from the interaction between the molecules and the substrate.…”

“…The main factors that affect the wettability of a material surface include surface chemical composition/heterogeneity, surface roughness, and surface free energy. The chemical heterogeneity is due to the existence on the surfaces of some functional groups, hydrophilic/hydrophobic sites, impurities, or orientation of crystals 8 . Rough surfaces have also a significant influence on the wetting behavior of fluids, due to the fact that this surface provides an additional interfacial area for the spreading liquid, resulting in the increase of the surface energy 2,9 .…”

Evaluation of the surface properties of 4-(Decyloxy) benzoic acid liquid crystal and its use in structural isomer separation

“…It is not possible to analyse high molecular weight and non-volatile materials such as LCs, polymers, composites, etc. in conventional GC [11][12][13][14][15]. Therefore, IGC-ID is a simple, low cost, high efficiency, and high accuracy technique developed to analyse such substances.…”

Lead sulfide scaling in multiphase systems and co-precipitation in the presence of calcium carbonate

“…Whilst sessile drop measurements provided a crude measure of PbS wettability, the heterogeneous distribution of hydrophobic sites upon the PbS [001] crystal plane results in the characterization of PbS mineral throughout this work as 'partially hydrophobic'. 25 Irreversible attachment of partially hydrophobic PbS particles occurs at the o-w interface, where the free energy of desorption for a 50 nm radius PbS particle of at an oil-water interface (γow = 50 mN m −1 ), exhibiting a water contact angle (θw) of 49°, results in a free energy of ∆ =1.3 x 10 -16 J. This is much larger than the thermal energy KbT (5 × 10 −21 J at 95° C), demonstrating the interfacial stability of the nanoparticles even at high temperatures.…”