Beitrag zur Entwicklung eines energieeffizienten Elektrolyseverfahrens für Neodym in geschmolzenen Chloriden

Abstract

Neodym ist ein Hauptbestandteil von Neodym-Eisen-Bor-Magneten, den derzeit leistungsfähigsten Magneten. Diese sind essenziell für Hochtechnologieanwendungen wie Elektrofahrzeuge, Windkraftanlagen und IT-Bauteilen. Da Neodym wie alle anderen Seltenerdmetalle größtenteils in China produziert wird, müssen in Europa neue Quellen für Neodym erschlossen werden, um die Importabhängigkeit zu mindern. Der nachhaltigste Weg ist das Recycling von End-of-Life Magneten. Mehrere Verfahren wurden bereits getestet, besonders bei der Umwandlung von Neodymsalzen in metallisches Neodym besteht jedoch noch großer Forschungsbedarf. Aus diesem Grund beschäftigt sich diese Arbeit mit der Entwicklung eines nachhaltigen Elektrolyseprozesses zur Neodymgewinnung. Um den Energieverbrauch in der Elektrolyse möglichst gering zu halten, wurde eine Schmelze aus Kaliumchlorid, Lithiumchlorid und Neodymchlorid bei ca. 500 °C als Elektrolyt verwendet, statt über 1000 °C in der industriellen Anwendung. In den Experimenten stellte sich heraus, dass die Abscheidung von Neodym auf einer inerten Wolframkathode nicht ohne weiteres möglich ist. Es entsteht stattdessen feinkristallines Neodym-Metall innerhalb des Elektrolyten, das nicht extrahiert werden kann. Im weiteren Verlauf der Arbeit wurde daher der Mechanismus der Abscheidung aufgeklärt und gezeigt, dass die Elektrolyseergebnisse maßgeblich durch Neodym-Intermediate beeinflusst wird. Mit diesen Erkenntnissen ist eine gezielte Weiterentwicklung der Elektrolyse möglich, um Neodym bei vergleichsweise niedrigen Temperaturen zu gewinnen.

Neodymium, a metal of the group of rare earth metals, is a main constituent in neodymium-iron-boron magnets, the most powerful magnets today. These magnets are essential for several high technology applications such as electric vehicles, wind generators or IT components. Most neodymium is currently produced in China, around 65 %. The European industry depends for nearly 100 % on imports of neodymium and magnet alloys. Thus, new reliable sources for neodymium must be established for a sustainable supply in Europe. Rare earth mining in Europe is discussed as well as recycling of neodymium from magnets in end-of-life products. Mining however requires, years of exploration refinement steps not readily available in Europe and uncertain economic competitiveness. Recycling is advantageous in availability of scrap as raw material and reduced separation efforts due to the lower variety of elements compared to ores. Several research projects already concern different recycling routes for rare earth metals from magnets, reaching from reuse of magnetic material to the recovery of pure neodymium. Since recycled magnet alloy is prone to oxidation and can lose performance, processes for reduction of neodymium to the metallic state are necessary. In industrial primary production, high temperature electrolysis of Nd2O3 at above 1000 °C is common. This process requires high energy input, generates serious emissions and is currently almost exclusively available in China. A lot of research is needed to install respective electrolysis processes feasible in Europe. This dissertation investigates the electrolysis process necessary to recover neodymium metal from neodymium chloride in molten salt electrolysis. Scope is the development of a robust and energy efficient electrolysis process. Neodymium chloride is produced in various hydrometallurgical and pyrometallurgical processes in recycling and primary production. The electrolysis experiments were performed in a furnace under inert conditions. To keep the temperature and energy demand to a minimum, a low-melting eutectic mixture of 55 g LiCl and 45 g KCl with 4 g NdCl3 was used as electrolyte. Neodymium was electroplated on a tungsten or iron cathode while graphite was used as anode material and a Pt rod for reference potential. At first, the reduction reaction of neodymium was investigated by cyclic voltammetry and potentiostatic electrolysis. The experiments showed the reduction of Nd3+ to Nd metal via intermediate formation of Nd2+ below -2.05 V vs. Pt. The current was visibly instable and in subsequent galvanostatic experiments, a short circuit occurred between working and reference electrode. This indicates a reaction different to a smooth electrodeposition. Product analysis showed the formation of fine metallic neodymium particles enclosed in the solidified electrolyte after completion of the experiments. This kind of neodymium product cannot be extracted easily for use as a precursor in magnet production. By postulating the mechanism for the electrodeposition including the observed changes during the process, an adaption towards usable neodymium metal was focus of the proceeding work. Intermediate Nd2+ in electroreduction becomes more stable when Nd metal from initial reduction is present and Nd2+ reacts in a disproportionation reaction instead of an electrodeposition reaction. To recover usable neodymium product from electrolysis, the mechanism was altered by addition of KF salt to destabilize Nd2+ versus Nd3+ and enable direct electrodeposition. Due to significant increase in melting temperature and according energy need, the maximum amount of KF to replace KCl was 10 %. At these conditions, no change in mechanism was achieved and neodymium was still received as fine particles in the electrolyte. Also remelting of the resulting product at 1100 °C to separate Nd metal and electrolyte salt proved to be ineffective. The small amount of metal and the high surface area of the fine particles caused the reaction of Nd to NdOCl at interim exposure to air moisture caused by transfer from the furnace. In general, the recovery of neodymium metal from electrolysis in chloride melts at 500 °C is possible. However, the intermediate formation of Nd2+ influences the reduction mechanism over time and prohibits the deposition of smooth metal. With the achieved results, further development of neodymium electrolysis is possible in the future to achieve energy efficient recovery of high-purity neodymium metal.