Low Temperature Rare Earth Alloy Formation in Molten Metal

Abstract

Rare earth element (REE) processing, both separation stages and metal production, is chemically intensive. Conventional methods for reducing REEs include molten salt or fused salt electrolysis, and metallothermic reduction. During molten salt electrolysis, separation is achieved by the formation of a “low temperature” (~1100°C) REE alloy employing the use of a liquid cathode such as Zn or Cd [1]. The molten REE metal which is less dense, rises to the surface and is subsequently separated [1]. In metallothermic reduction, conproportionation is used to effectively reduce REEs such as La, Ce, Pr, and Nd at 1100°C using reactive metals such as Na, Li, Al, K, Ca, and Fe [2]. Separation of REEs through solvent extraction or ion exchange is challenging due to nearly equivalent chemical properties and lack of diversity in redox state across the series. Separation is limited to very small separation factors which require up to hundreds of stages to achieve pure products [3]. While these conventional methods are used to process REEs, an alternative was reported involving the amalgam formation for the reduction and separation of REEs [4]. This work was prevalent during the 1940s through the 1960s in nuclear materials processing work, but left dormant for the past half century [4]. The advantages of using liquid metal electrodes such as Hg over solid electrodes is the high hydrogen evolution reaction (HER) overpotential and the reduced reduction potential through alloy formation, allowing reduction from aqueous solution. Previous work showed that REEs formed amalgams over one volt more positive than the standard reduction potential [5]. However, Hg is toxic and thus not suitable for new chemical processes. To utilize alloy formation as a processing route, there is a need for a material substitute with similar characteristics but a nontoxic nature. The promising nature of Ga and Ga alloys have made it an attractive substitute for mercury electrodes. Similar to mercury, Ga and Ga alloys possess a high HER overpotential, renewable surface, and a low vapor pressure [6]. This research investigates the reduction of REEs using liquid Ga as cathode material. The kinetics involving the formation of amalgams with rare earths are studied, as well as the reduction or entrainment of REEs in Ga oxides.In this work REE elements Pr and Nd are reduced at the cathode which comprises a Ga pool electrode with a submerged copper contact. Ga-REE alloys, and entrainment of REEs into a solid dark mass via Ga oxidation were observed to have been formed with each element. This was corroborated through data analysis. This work is part of an effort to process critical materials such as Pr and Nd from magnets, coal fly ash and ores. The setup was designed to operate slightly above room temperature (slightly above the Ga melting point). During electrolysis in the presence of Pr and Nd, two phases are formed. We observe that the shiny Ga surface turns dull during the electrochemical process above ~-1.3 V vs Ag/AgCl in lithium acetate electrolyte. When the electrochemically formed alloy is leached in dilute HCl, analysis shows REEs in the leachate, H2 gas production (water reduction), and a lower REE concentration in the electrolyzed solution. In similar experiments with a stirred Ga pool, a dark grey solid mass is formed. The mass appears to form due to the oxidation properties of Ga in aqueous solution with the reduced REE present. When filtered and analyzed, this mass is rich in REE containing over 50 wt% of REEs from the feed solution with higher concentrated feed solution, and over 90 wt.% of REEs from coal fly ash surrogate solutions. These preliminary results show some encouraging new routes to REE processing.