A Molecular-Scale Approach to Rare-Earth Beneficiation: Thinking Small to Avoid Large Losses

Emerging Trends of Computational Chemistry and Molecular Modeling in Froth Flotation: A Review

Froth flotation is the most versatile process in mineral beneficiation, extensively used to concentrate a wide range of minerals. This process comprises mixtures of more or less liberated minerals, water, air, and various chemical reagents, involving a series of intermingled multiphase physical and chemical phenomena in the aqueous environment. Today's main challenge facing the froth flotation process is to gain atomic-level insights into the properties of its inherent phenomena governing the process performance. While it is often challenging to determine these phenomena via trial-and-error experimentations, molecular modeling approaches not only elicit a deeper understanding of froth flotation but can also assist experimental studies in saving time and budget. Thanks to the rapid development of computer science and advances in high-performance computing (HPC) infrastructures, theoretical/computational chemistry has now matured enough to successfully and gainfully apply to tackle the challenges of complex systems. In mineral processing, however, advanced applications of computational chemistry are increasingly gaining ground and demonstrating merit in addressing these challenges. Accordingly, this contribution aims to encourage mineral scientists, especially those interested in rational reagent design, to become familiarized with the necessary concepts of molecular modeling and to apply similar strategies when studying and tailoring properties at the molecular level. This review also strives to deliver the state-of-the-art integration and application of molecular modeling in froth flotation studies to assist either active researchers in this field to disclose new directions for future research or newcomers to the field to initiate innovative works.

Surface chemistry considerations of gangue dissolved species in the bastnaesite flotation system

Inefficient flotation of bastnaesite remains a challenge in the production of rare earth elements. This study aimed to investigate the dissolution and adsorption behaviour of species that are commonly released into bastnaesite flotation pulp from Ca/Ba-bearing gangue minerals. The influence and corresponding mechanisms on the bastnaesite mineral surface and collectors, namely sodium oleate (NaOL), were evaluated experimentally based on micro-flotation, zeta potentials, in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and X-Ray photoelectron spectroscopy (XPS) analyses. The flotation recovery of bastnaesite significantly decreased from ∼95% to ∼25%, ∼15%, ∼80%, ∼25% when exposed to calcite, fluorite, barite, and mixed dissolved species, respectively. The zeta potential of bastnaesite was pH sensitive, indicating that H+ and OH- determine the surface potential of bastnaesite. Solution chemistry analyses revealed that the presence of the dissolved species differed at various pH values. In situ ATR-FTIR demonstrated the different effects of the dissolved species from calcite, fluorite, and barite on collector adsorption. The former two dissolved species mainly depressed the chemisorption of the NaOL monomers (RCOO‒), whereas calcite also affected the physical adsorption of the oleic acid molecular dimer (RCOOH·RCOO‒). Moreover, the barite dissolved species only affected the physical adsorption of the NaOL species. The results of XPS analysis revealed that dissolved species from these three gangues could pre-adsorbed onto bastnaesite and affected the interaction with the collector. Density functional theory calculations were employed to provide further theoretical insights into the interactions between the dissolved species from calcite, fluorite, and barite and NaOL.

Improving Rare-Earth Mineral Separation with Insights from Molecular Recognition: Functionalized Hydroxamic Acid Adsorption onto Bastnäsite and Calcite

Enhancing the separation of rare-earth elements (REEs) from gangue materials in mined ores requires an understanding of the fundamental interactions driving the adsorption of collector ligands onto mineral interfaces. In this work, we examine five functionalized hydroxamic acid ligands as potential collectors for the REE-containing bastnäsite mineral in froth flotation using density functional theory calculations and a suite of surface-sensitive analytical spectroscopies. These include vibrational sum frequency generation, attenuated total reflectance Fourier transform infrared, Raman, and X-ray photoelectron spectroscopies. Differences in the chemical makeup of these ligands on well-defined bastnäsite and calcite surfaces allow for a systematic relationship connecting the structure to adsorption activity to be framed in the context of interfacial molecular recognition. We show how the intramolecular hydrogen bonding of adsorbed ligands requires the inclusion of explicit water solvent molecules to correctly map energetic and structural trends measured by experiments. We anticipate that the results and insights from this work will motivate and inform the design of improved flotation collectors for REE ores.

Recovery of Rare Earth Oxides from Flotation Concentrates of Bastnaesite Ore by Ultra-Fine Centrifugal Concentration

Historically, the ability to effectively separate carbonate gangue from bastnaesite via flotation has frequently proven to be challenging without sacrificing significant rare earth oxide (REO) grade or recovery. However, in light of the fact that the rare earth bearing minerals often exhibit higher specific gravities than the carbonate gangue, the possibility exists that the use of gravity separation could be used to achieve such a selective separation. This however is complicated by the fact that, in cases such as this study when the liberation size is finer than 50 microns, most traditional gravity separation methods become increasingly challenging. The aim of this study is to determine the applicability of centrifugal concentrators to beneficiate ultra-fine (UF) bastnaesite and calcite bearing flotation concentrates. By using a UF Falcon, it was possible to achieve initial gravity REO recoveries exceeding 90% while rejecting on the order of 25% to 35% of the total calcium from an assortment of rougher and cleaner flotation concentrates. Additionally, when additional stages of cleaner UF Falcon gravity separation were operated in an open circuit configuration, it was possible, from an original fine feed of 35 microns containing 50.5% REO and 5.5% Ca, to upgrade up to approximately 59% REO and 2.0% calcium. While not the goal of this study, these results also support previous limited data to suggest that UF Falcons are potentially capable of treating a wider range of materials than they were originally designed for, including feeds rich in heavy mineral content.