A review of reagents applied to rare-earth mineral flotation

A review of greener approaches for rare earth elements recovery from mineral wastes

The use of rare earth elements (REE) in many various fields, including high-tech products, increases the demand for these materials day by day. The production of REE from primary sources has expanded in response to increasing demand; however, due to its limited, a more sustainable supply is also started to offer for the REE demand by using secondary sources. The most commonly used metallurgical method for REE recovery is hydrometallurgical processes. However, it has some disadvantages, like pyrometallurgical methods. In the review, studies of the environmental impacts of REE production from primary sources and life cycle assessments of products containing REE were investigated. According to the results, it has been seen that those studies in the literature in which hydrometallurgical methods have changed to more environmentally friendly approaches have begun to increase. In this review, mine wastes, which are secondary sources, were defined, conventional methods of recovery of rare earth elements were discussed, greener approaches to the recovery of REE from these sources were comprehensively examined and studies in the literature were evaluated. Furthermore, it was stated that there are limited studies on green approaches and REE recovery from mineral wastes and that this field is developing with an emphasis on the current outlook and future perspectives.

A new concept of biocatalytic synthesis of acrylic monomers for obtaining water-soluble acrylic heteropolymers

Rhodococcus strains were designed as model biocatalysts (BCs) for the production of acrylic acid and mixtures of acrylic monomers consisting of acrylamide, acrylic acid, and N-alkylacrylamide (N-isopropylacrylamide). To obtain BC strains, we used, among other approaches, adaptive laboratory evolution (ALE), based on the use of the metabolic pathway of amide utilization. Whole genome sequencing of the strains obtained after ALE, as well as subsequent targeted gene disruption, identified candidate genes for three new amidases that are promising for the development of BCs for the production of acrylic acid from acrylamide. New BCs had two types of amidase activities, acrylamide-hydrolyzing and acrylamide-transferring, and by varying the ratio of these activities in BCs, it is possible to influence the ratio of monomers in the resulting mixtures. Based on these strains, a prototype of a new technological concept for the biocatalytic synthesis of acrylic monomers was developed for the production of water-soluble acrylic heteropolymers containing valuable N-alkylacrylamide units. In addition to the possibility of obtaining mixtures of different compositions, the advantages of the concept are a single starting reagent (acrylamide), more unification of processes (all processes are based on the same type of biocatalyst), and potentially greater safety for personnel and the environment compared to existing chemical technologies.

Advances in bio/chemical approaches for sustainable recycling and recovery of rare earth elements from secondary resources

Rare Earth Elements (REEs) are indispensable in the growing smart technologies, such as smart phones and electronic devices, renewable energy, new generation of hybrid cars, etc. These elements are naturally occurring in specific geological deposits (bastnäsite, monazite, and xenotime), primarily concentrated in the regions of China, Australia, and the USA. The extraction and processing of REEs and the mismanagement of secondary REE resources, such as industrial waste, end-of-life materials, and mining by-products, raise major environmental and health concerns. Recycling represents a convincing solution, avoiding the necessity to separate low-value or coexisting radioactive elements when REEs are recovered from raw ore. Despite these advantages, only 1 % of REEs are usually recycled. This review overreached strategies for recycling REEs from secondary resources, emphasizing their pivotal role. The predominant approach for recycling REEs involves hydrometallurgical processing by leaching REEs from their origins using acidic solutions and then separating them from dissolved impurities using techniques like liquid-liquid extraction, membrane separation, chromatography, adsorption, flotation, and electrochemical methods. However, these methods have notable disadvantages, particularly their over requirements for water, reagents, and energy. Biohydrometallurgy introduces an innovative alternative using microorganisms and their metabolites to extract REEs through bioleaching. Other investigations are carried out to recover REEs through biological strategies, including biosorption, affinity chromatography with biological ligands, bioflotation employing biological surfactants, and bioelectrochemical methods. However, biohydrometallurgical processes can also be relatively slow and less suitable for large-scale applications, often lacking specificity for targeted REEs recovery. Overcoming these challenges necessitates ongoing research and development efforts to advance recycling technologies.

A review of flotation reagents for bastnäsite-(Ce) rare earth ore

Given the indispensability and immense value of rare earth elements for scientific and technological advancements in the 21st century, extracting high-quality rare earth resources from nature has become a global priority. Bastnäsite-(Ce) is one of the known rare earth minerals with high rare earth content and wide distribution, which occupies a pivotal position in human life and high-end production activities, making its efficient development and utilization crucial. In recent years, research on separating bastnäsite-(Ce) from gangue minerals has focused on the flotation process, with flotation reagents playing a critical role in achieving effective separation. This paper provides a detailed summary of current research on the behavior of bastnäsite-(Ce) flotation agents on minerals, their interaction with mineral surfaces during flotation separation, and outlines future prospects for further research.

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.

The use of pracaxi oil collector in the selective flotation of xenotime from silicates

Rare earth elements have played a key role in technological advancement, attracting great visibility in the global commodity market. Xenotime, a heavy rare earth resource (YPO₄), can be found associated with granitic rocks, as in the Pitinga deposit, located in the Brazilian Amazon region, where the main gangue minerals are quartz, microcline and albite. This research investigates the application of a new collector produced from pracaxi oil, an Amazon oil abundant in Brazil, in the selective flotation between xenotime and its main gangue minerals. The study conducted the synthesis and characterization of the collector, the chemical, mineralogical and surface characterization of minerals, as well as the evaluation of collector adsorption and flotability via microflotation tests, zeta potential measurements, surface tension determination and XRD, WDXRF, ICP-MS, FTIR and XPS analyses. The pracaxi collector was found to be mainly composed of oleic acid (56.2%), linoleic acid (14.1%) and behenic acid (10.6%), in addition to exhibiting a critical micelle concentration (CMC) of approximately 150 mg/L. Microflotation tests indicated that the best condition for selective recovery of xenotime occur at alkaline condition (pH 9.0), presenting selectivity of approximately 90% with collector concentration of 10.0 mg/L. The zeta potential data confirmed a selective adsorption of pracaxi collector onto xenotime, with an increase in the surface charge from -30 mV to -68 mV, whereas no significant changes were detected in the silicates. The FTIR spectra showed the appearance of a band at 1545 cm^-1 on the surface of the xenotime after collector adsorption, which indicates, along with the zeta potential data, the chemical nature of the adsorption. The presence of small amounts of iron in the lattice structure of silicate gangues can act as an activator, and therefore, may be responsible for the small flotability of these minerals. The performance of the pracaxi oil collector presented in this study indicates the great potential of this Amazonian oil for application in the selective flotation of xenotime ores found in the region.

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.

Behavior and Mechanism of a Novel Hydrophobic Collector in the Flotation of Bastnaesite

In order to improve the recovery of rare earth elements, finding a collector with a strong selectivity ability had become the focus of research. In this paper, phenylpropyl hydroxamic acid (PHA) was used as a new hydrophobic surfactant collector for the separation of bastnaesite from calcite, and salicylic hydroxamic acid (SHA) was used as a reference collector. The results of a single mineral flotation test with SHA show that the reagent has good collection performance and selectivity. In addition, Zeta potential measurements and FTIR analysis show that PHA is adsorbed on the surface of bastnaesite by chemical adsorption, and the surface state of bastnaesite changes after PHA treatment. By XPS analysis, PHA interacts with Ce, and forms a Ce–O bond with Ce. It is speculated that the hydroxamic acid forms a five-element-chelated hydroxamic group with Ce on bastnaesite surface, so as to improve the hydrophobicity of bastnaesite, and make bastnaesite float more easily out of the pulp. According to DFT calculation, PHA has better adsorption capacity and stronger hydrophobicity than SHA, and shows superior electronic group capacity and chemical reactions that promote its flotation performance.

Recent Progress of Rare Earth Doped Hydroxyapatite Nanoparticles: Luminescence Properties, Synthesis and Biomedical Applications

Hydroxyapatite nanoparticles (HAP NPs) are host materials and can be modified with various substrates and dopants. Among them, rare earth (RE) ions doped HAP NPs have gathered attention due to their unique physicochemical and imaging properties. Compared to other fluorescence probes, RE-doped HAP NPs display advantages in high brightness, high contrast, photostability, nonblinking, and narrow emission bands. Meanwhile, their intrinsic features (composition, morphology, size, crystallinity, and luminescence intensity) can be adjusted by changing the dopant ratio, synthesizing temperature, reaction time, and techniques. And they have been used in various biomedical applications, including imaging probe, drug delivery, bone tissue engineering, and antibacterial studies. This review surveys the luminescent properties, fluorescence enhancement, synthetic methods, and biocompatibility of various RE-doped HAP NPs consolidated from different research works, for their employments in biomedical applications. For this literature review, an electronic search was conducted in the Pubmed, Web of Science, Google Scholar, Scopus and SciFinder databases, using the keywords: hydroxyapatite, rare earth, lanthanide, fluorescence, and imaging. Literature searches of English-language publications from 1979 with updates through April, 2022, and a total of 472 potential papers were identified. In addition, a few references were located by noting their citation in other studies reviewed. STATEMENT OF SIGNIFICANCE: Hydroxyapatite nanoparticles (HAP NPs) have a broad range of promising biological applications. Although prospective biomedical applications are not limited to rare earth-doped hydroxyapatite nanoparticles (RE-doped HAP NPs), some cases do make use of the distinctive features of RE-elements to achieve the expected functions for HAP families. This review surveys the luminescent properties, synthetic methods, and biocompatibility of various RE-doped HAP NPs consolidated from different research works, for their employments in biomedical applications, including imaging probe, drug delivery, bone tissue repair and tracking, and anti-bacteria. Overall, we expect to shed some light on broadening the research and application of RE-doped HAP NPs in biomedical field.

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.

The study on the effect of flotation purification on the performance of α-hemihydrate gypsum prepared from phosphogypsum

Phosphogypsum (PG) is a massive industrial solid waste. In this paper, PG was purified by flotation method, and α-hemihydrate gypsum (α-HH) was prepared by the autoclaving method. The morphology of α-HH was adjusted by adding different doses of Maleic acid and Aluminium sulfate. The results showed that after flotation purification, the impurity content in PG was significantly reduced, the soluble phosphorus content decreased from 0.48 to 0.07%, the PG purity increased from 73.12 to 94.37%, and the PG whiteness risen from 19.4 to 40.5. Then the performance of α-HH prepared from PG before and after purification was compared. Fixing the amount of aluminium sulfate at 0.2 wt%, the reaction temperature at 140 °C, and the reaction time at 120 min, the average length/diameter ratio of α-HH crystals decreased from 7.2 to 0.6 as the amount of Maleic acid increased from 0 to 0.17 wt%. When the amount of Maleic acid was 0.13 wt%, the α-hemihydrate gypsum reached the best mechanical properties. The mechanical strength of high strength gypsum prepared from PG concentrate was significantly better than that of raw PG, indicating that flotation purification can effectively improve the performance of PG. In this study, a new method of PG purification and resource utilization was proposed.

Flotation of Smithsonite From Quartz Using Pyrophyllite Nanoparticles as the Natural Non-toxic Collector

The use of natural hydrophobic mineral nanoparticles as a collector in froth flotation has recently attracted the attention of researchers. In this article, the separation performance and mechanism of pyrophyllite nanoparticles (PNPs) on smithsonite and quartz flotation system were investigated using the method of flotation, zeta potential, contact angle, and scanning electron microscope (SEM)/energy disperse spectroscopy (EDS). The results of single mineral flotation showed that the difference in flotation recovery between smithsonite and quartz was large for NaOL, DDA, and PNP collectors in the acidic pH range, the largest of which was the PNP system. At pH 6, the optimal dosage of PNPs was 1,000 mg/L. Separation of mixed minerals of smithsonite and quartz using a PNP collector provides the optimum concentrate index (Zn grade 50.84% and Zn recovery 85.36%). According to the results of zeta potential measurement, PNPs and quartz were negatively charged, and the surface of smithsonite was positively charged at pH 6. This provided conditions for smithsonite to selectively adsorb PNPs due to different electrostatic forces. Selective adsorption of PNPs in the smithsonite/quartz flotation system was directly observed by SEM/EDS detection. Hydrophobic PNPs were adsorbed on the surface of hydrophilic smithsonite to make it hydrophobic, and the surface of quartz remained hydrophilic. This is the mechanism for separating smithsonite and quartz using PNPs.

The pH dependent surface charging and points of zero charge. IX. Update

of the points of zero charge (PZC) and isoelectric points (IEP) of various materials published in the recent literature and of older results overlooked in the previous compilations. The roles of experimental conditions, especially of the temperature, of the nature and concentration of supporting electrolyte, and of the type of apparatus are emphasized. The newest results are compared with the zero points reported in previous reviews. Most recent studies were carried out with materials whose pH dependent surface charging is already well-documented, and the newest results are consistent with the older literature. Isoelectric points of Gd(OH)₃, Sm(OH)₃, and TeO₂ have been reported for the first time in the recent literature.

Surface Mechanism of Fe3+ Ions on the Improvement of Fine Monazite Flotation With Octyl Hydroxamate as the Collector

Froth flotation of fine minerals has always been an important research direction in terms of theory and practice. In this paper, the effect and mechanism of Fe³⁺ on improving surface hydrophobicity and flotation of fine monazite using sodium octyl hydroxamate (SOH) as a collector were investigated through a series of laboratory tests and detection measurements including microflotation, fluorescence spectrum, zeta potential, and X-ray photoelectron spectroscopy (XPS). Flotation tests have shown that fine monazite particles (−26 + 15 μm) cannot be floated well with the SOH collector compared to the coarse fraction (−74 + 38 μm). However, adding a small amount of Fe³⁺ to the pulp before SOH can significantly improve the flotation of fine monazite. This is because the addition of Fe³⁺ promotes the adsorption of SOH and greatly improves the hydrophobicity of the monazite surface. This can result in the formation of a more uniform and dense hydrophobic adsorption layer, as shown by the fluorescence spectrum and zeta potential results. From the XPS results, Fe³⁺ reacts with surface O atoms on the surface of monazite to form a monazite–O_surf–Fe group that acts as a new additional active site for SOH adsorption. A schematic model was also proposed to explain the mechanism of Fe³⁺ for improving surface hydrophobicity and flotation of fine monazite using octyl hydroxamate as a collector. The innovative point of this study is using a simple reagent scheme to float fine mineral particles rather than traditional complex processes.

Determination and modulation of the typical interactions among dispersed phases relevant to flotation applications: A review

Flotation is a process involving multi-components, multi-scales, and gas-liquid-solid three phases, where the material separation is achieved based on the difference in surface hydrophobicity of various constituents. In a flotation system, fluids are usually regarded as the continuous phase, while the dispersed phases refer to scattered particles, bubbles, and droplets with low solubility as a dispersion that is surrounded by the aqueous environment. Fundamentally, the interactions among dispersed phases exist throughout the flotation process, and play distinct roles during different periods. For example, the liquid collector-solid, solid-solid, bubble-bubble and gas bubble-solid interactions are closely associated with the particle surface modification, particle behavior, bubble size evolution and separation in flotation, respectively. Therefore, the influences of each stage are all worthy of concern, and should be spared sufficient attention, which requires to formulate a horizontal writing structure. In this review, instead of summarizing all available characterization techniques or measurements, certain typical examples or methods were consciously chosen to perform analysis or comparison, aiming to summarize recent studies on the determination and modulation of dispersed phase interactions. The determination on the interactions among dispersed phases is helpful for fundamentally understanding the microcosmic process connotations, and their modulation contributes to firmly providing macroscopic optimization schemes for practical applications. By integrating some typically available theoretical calculations and experimental measurements related to the dispersed phase interactions, the present article is devoted to revealing the influential factors, finding out the current challenges or knowledge gaps, and affording certain references or suggestions for future investigations.

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

Separating rare-earth-element-rich minerals from unwanted gangue in mined ores relies on selective binding of collector molecules at the interface to facilitate froth flotation. Salicylhydroxamic acid (SHA) exhibits enhanced selectivity for bastnäsite over calcite in microflotation experiments. Through a multifaceted approach, leveraging density functional theory calculations, and advanced spectroscopic methods, we provide molecular-level mechanistic insight to this selectivity. The hydroxamic acid moiety introduces strong interactions at metal-atom surface sites and hinders subsurface-cation stabilization at vacancy-defect sites, in calcite especially. Resulting from hydrogen-bond-induced interactions, SHA lies flat on the bastnäsite surface and shows a tendency for multilayer formation at high coverages. In this conformation, SHA complexation with bastnäsite metal ions is stabilized, leading to advanced flotation performance. In contrast, SHA lies perpendicular to the calcite surface due to a difference in cationic spacing. We anticipate that these insights will motivate rational design and selection of future collector molecules for enhanced ore beneficiation.