The Failure Mechanism And Protection Of Graphite Anode In Molten Salt Electrolysis Of Neodymium

Rare earth metal Nd is an important raw material for preparing superior magnetic NdFeB permanent magnets, not only the market demand is growing for the metal Nd, but also users put forward higher requirements to the quality of Nd. Commonly, the dominant technology of large-scale smelting production of neodymium adopts graphite as the anode and tungsten rod as the cathode to electrolyze Nd in high-temperature molten salt(1000~1100?) contained in a graphite tank, and then collect and cast molding in the graphite tank bottom. This technology benefits from properties of graphite, including excellent electrical and thermal conductivity,strong resistance to molten salt corrosion and thermal shock,as well as ease of processing. However, as a consumable anode, graphite electrode consumption in electrolysis is about 633kg/t Nd,accounting for about 25% of the electrolytic production cost. Besides, the graphite consumed will increase the carbon content of NdFeB, which can greatly reduce the intrinsic coercive force of magnet and contaminate product. Therefore, the important and urgent issue in production is to control premature failure of the graphite anode in molten salt electrolysis process of rare earth oxides and thus to reduce carbon pollution. By investigating on-site the consumption of graphite anode in the molten salt electrolysis of neodymium, the study analyzed its failure mechanism. According to that, the study has designed temperature measuring device and molten salt evaporation loss measuring device in the high temperature electrolysis cell, and explored the protective performance of borate and SiC impregnation by experiment research.The main results obtained are as follows:(1)At high temperature(1000~1150?), premature failure of the graphite anode used in fluoride molten salt electrolysis of neodymium appears, which results from oxidation reaction of porous graphite with reactive oxygen species at high temperature and the chemical erosion by fluoride. The oxidation reaction of graphite promoted by the high temperature plays the dominant role in its failure. On the other hand, the flowing and rolling of molten salt brings about erosion and sufficient oxygen supply makes the molten salt liquid surface damaged worst. Though the chemical erosion by fluoride is less harmful, it can promote oxidation and be facilitated by oxidation. The two factors together lead to the reaction inside the hole of graphite and make the graphite particles flaking into the molten salt.(2)In the preparation of Nd by metal molten salt electrolysis, the graphite anode have been in a state of high temperature molten salt, so the electrolysis process parameters-temperature has an important influence on its failure. Therefore, according to the high-temperature environment, the study has designed the temperature measuring device in the high-temperature molten salt electrolysis cell and put forward the method of temperature measurement.(3)In the production of Nd metal(molten salt electrolysis) process,in order to control strong electrolyte composition changes caused by intense electrolyte volatilization under high temperature, the study has designed the molten salt volatilization loss measuring device and proposed the molten salt sampling plan as well as the analysis method of molten salt composition.(4)Borate impregnation can fill the pores and cover the surface of graphite. B2O3 formed at high temperature can isolated the graphite from oxidation and erosion of molten salt. However, the protection effect of borate can only last 1h, after that the borate at the surface and pores of graphite volatilizes and reacts itself due to the high temperature.(5)From thermodynamics calculation,Si and C can generate SiC under lower than 1200?in the surface of graphite anode.As the temperature decreases, it is more prone to produce SiC. However, the experiment confirmed that SiC is not generated in the surface of graphite. It is shown that SiC has spontaneous reaction tendency in thermodynamics, whereas kinetic factor can limit its synthetic speed.