A Comparison between Two Cell Designs for Electrochemical Neodymium Reduction Using Numerical Simulation
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re-earth metal neodymium is a key component of strong permanent magnets, which are essential for the production of modern electronics and wind energy devices. In 2002, approximately 95 pct[1] of the worldwide neodymium supply was provided by China. There, more than 95 pct[2] of the neodymium metal is reduced electrochemically by the electrolysis of Neodymium oxide dissolved in fused fluoride salts. This can be explained by the great advantages the process has over alternative methods such as metallothermal reduction or electrochemical reduction in fused chlorides.[1,2] In industrial processes, the electrolyte consists of 10 to 20 wt pct LiF and 80 to 90 wt pct NdF3 and the process temperature is about 1323 K (1050 C).[2,3] Depending on process parameters, especially the anode current density and the temperature, carbon monoxide or carbon dioxide is formed at the anode and provides stirring within the cell. According to Faraday’s law, the TIM HAAS and HERBERT PFEIFER are with the Department of Industrial Furnaces and Heat Engineering, RWTH Aachen University, Kopernikusstraße 10, 52074 Aachen, Germany. Contact e-mail: [email protected] SIMON HILGENDORF, HANNO VOGEL, and BERND FRIEDRICH are with IME Process Metallurgy and Metal Recycling, RWTH Aachen University, Intzestrasse 3, 52062 Germany. Manuscript submitted July 5, 2016. Article published online May 8, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS B
productivity of the electrolysis cell is directly proportional to its current. It needs to be considered, though, that increasing the total cell current also raises the current density at both electrodes. By this, the ion transport through the thin diffusion layer may become the reaction rate determining phenomenon, which would cause the emission of environmental hazardous carbon fluorides at the anode[4] and the electrochemical reduction of product impurities at the cathode. The emission problem is particularly critical, because the solubility of Neodymium oxide in the electrolyte is merely about 2 to 4 wt pct.[5] Facing this problem, efficient mixing is important to homogenize the electrolyte and enhance the dissolution of added oxide. However, the mixing must not be too violent, as reduced neodymium droplets might be transported to the anode and reoxidize. This causes a loss in current efficiency. The problem might be overcome by a broader distance between the electrodes. However, this results in a high specific energy as the cell voltage increases. In order to have a productive and efficient cell without the drawbacks mentioned previously, detailed knowledge of the fluid flow distribution is crucial. To this day, the standard 3-kA cell consists of a vertically inserted, perpendicular tungsten rod connected as the cathode, which is surrounded by a graphite hollow cylinder used as the anode. Owing to the highly corrosive electrolyte, the crucible is made of graphite as
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well. The reduced metal is liquidly collected in a tungsten crucible placed beneath the cathode.[6] Since the tungsten cr
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