Sustainability of the Rare Earth Resources, Technology Practices, and Industry

Dear Colleagues,

Rare earth resources constitute both lighter and heavier types of metals, and they have attracted considerable attention in the past century due to their large number of applications. Rare earth deposits in natural sources across the world are limited, with the majority of the content of total rare earths (REs) located in a small number of countries, such as China—unlike other metals, which are more available and generally present in various countries. Therefore, considering the scenario of the existence of and growing global demand for rare earth metals for everyday needs, the recycling of secondary sources, including end-of-life vehicle batteries, waste permanent magnets, spent catalysts, and e-waste, among others, is emerging as an area of interest.

The process of metallurgy, including pyro- and hydrometallurgy, is a significant approach to treating minerals and secondary sources containing rare earth resources. In addition, researchers have also successfully used bio-hydrometallurgy techniques such as bioleaching, biosorption, biomineralization, and bioprecipitation. The real challenge lies in the development of a sustainable process that can help to mitigate the issues of environmental concern caused by emissions and/or discharges, ensuring a circular economy and attaining a high extraction efficiency, clean separation of REEs, high selectivity of the adopted process, and zero waste yield. As a result, there have been advancements in technologies such as ultrasonication, microwave treatment, baking, surface activation, roasting, mechanical treatment including ball milling, and the adoption of green reagents over classical approaches at the upstream stages. At the downstream stages, by contrast, the use of novel solvent reagents, green reagents (ionic liquids), and ion exchangers; the adoption of membrane separation studies; and the development of noble sorbent material(s), biomaterials, and nanomaterials/composites for clean separation/recovery have had notable success in the extraction and/or recovery of rare earth metals from both natural and secondary sources. In addition, products extracted through the rare earth process have usually been used to develop rare earth compounds, mostly oxides, but more recently, researchers have engaged in synthesizing nano-based compounds/metals/composites out of these sources through various nanosynthesis routes. These adopted processes certainly contribute comprehensively to rare earth processing; nevertheless, more research in this area is required.

Rare-earth-based permanent magnets are in high demand for a wide variety of industrial applications; for instance, in electronic devices, permanent magnets have endorsed a continuous reduction in the size of sensors, cameras, tablets, and cellphones. Larger quantities of RE permanent magnets (NdFeB magnets) are employed in advanced medical and mechanical tools, such as magnetic resonance imaging (MRI) machines, electric fueled vehicles, electric bicycles, speakers, and magnetic separators. In recent years, the production of these magnets has grown steadily due to their potential applications. The (BH)max value of permanent magnets is a crucial parameter to apply in the aforementioned applications. This is mainly dependent on the magnetic properties of magnetic powders. Magnetic powders are the starting material for bulk magnets, and they are produced through diverse techniques such as sintering, hot deformation, and polymer bonding. The magnetic properties of powders depend on the phase purity, crystal structure, and morphology, which can usually be modified during manufacture. Conventional research procedures such as powder metallurgy, rapid quenching, melt spinning, high-energy ball milling, and surfactant-assisted ball milling show disadvantages because they do not yield a uniform powder structure, and there is therefore the risk of contamination of the final product, in addition to the usually non-environmentally friendly processes. Further, the doping limitations with other elements must be considered, as well as the difficulty of producing powders with a size smaller than 3 µm. Thus, to overcome the current disadvantages, the bottom-up approach using wet chemical methods has gained traction in the field.

Considering the lower abundance and scarcity of REs, the fabrication of permanent magnets with a lower amount of REs is pivotal for the industry and an urgent challenge that researchers must tackle. With the help of the aforementioned advantages of the wet chemical methods, however, we will have the chance to reduce the RE amounts in the magnet in permanent magnetic powders and use other, lower-cost elements. Following this substitution, sintering the powders will result in the bulk magnet, and in the sintering process, there will also be the chance to add other elements which can increase the magnetic properties of the final magnets. These routes urgently need to be explored to prepare magnets with lower amounts of REs that maintain the excellent properties of RE-based magnets.

Prof. Dr. Rajesh Kumar Jyothi
Dr. Pankaj Kumar Parhi
Dr. Thenepalli Thriveni
Dr. Rambabu Kuchi
Guest Editors

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• hydrometallurgy
• bio-hydrometallurgy pyrometallurgy
• rare earths
• leaching
• bioseparation process
• solvent extraction
• nanotechnology
• adsorption
• precipitation electroprocess
• rare earth magnets
• magnet recycling