| Rare earth metal plays an important role in the new and sophisticated technology, and has become an indispensable strategic resource. With the rapid development of science and technology, the demand for high strength, high conductivity and high purity rare earth metals increases dramatically. China is rich in rare earths, but the import quantum for high purity rare earth metals is quite large every year. The method of preparing high purity rare earth metals is poor. The large energy consumption, high cost and low purity are also the problems. In order to solve the above problems, it is important to develop new and own intellectual property purification methods for fully utilizing the rare earth resources.As the demand for high purity rare earth metals is urgent for our country, especially in the research of magnetostriction materials and magnetic refrigeration materials, this dissertation focuses on the high purification of gadolinium (Gd) and terbium (Tb). Firstly, the interations between Gd, Tb and impurities, including physical forces, adsorption & desorption heat and dynamic balance parameters, were calculated. Based on the physico-chemical properties, we developed several new multi-methods to investigate the high puritification production. Secondly, the effects of solid solution, phase transformation and electromagnetic driving force were all discussed. Numerical simulation described the purification mechanism and the migration process of impurities. The purification mechanism, the removal of impurities and the migration behavior of oxygen in metal were all discussed, which provided complete theoretical and experimental basis for the development of more effective purification technology. Primary conclusions are listed as follow:(1) Based on the traditional plasma arc melting technology, we put hydrogen into the melting process. Hydrogen plasma can dramatically improve the purification degree. The final purities are 99.9840% and 99.9539%, respectively. Some metallic impurity elements can be removed to below 10 ppm, and the non-metallic impurities contents (oxygen and nitrogen) are also under 10 ppm. Experimental results show that the activated hydrogen atoms, dissociated and ionized in high temperature plasma-arc, are supposed to be the cause for the wonderful refining effect.(2) A self-consistent model of free-burning arc by using FAST-2D has been developed. The solid-liquid equilibrium system in the crucible has also been simulated based on the Stefan theory. The temperature field presents a typical bell shape, and there is a steep gradient from cathode to anode. The temperature of arc near anode spot region can be higher than 10,000 K. A high velocity core exists in the arc column, typically a couple of 400 m/s. The turbulence thermal-flow can carry the evaporative impurity atoms to the gaseous boundary layer. The maximum temperature in the crucible is higher than 5000 K, at the same time, continuous stirring of the liquid field can accelerate the impurity volatilization process.(3) A gas-solid two step way has been used to purify the usual commercial Gd and Tb metals. The rare earth metals can be puritied from 2N to 4N purity degree. The contents of some metallic impurities can be reduced to below 1 ppm. After the solid deoxidation process, the final oxygen contents in Gd and Tb are respectively 3.6 ppm and 1.8ppm. Impurity removal pattern and the pinning effect for each other during the purifying process are all investigated.(4) By using the high chemical reactivity of hydrogen atoms, a new hydrogen in-situ refining method has been developed to remove the interstitial impurities O and N from Gd metal. Hydrogen atoms can dissolve in Gd basic metal, and cause large lattice distortions. During the high vacuum annealing process, active hydrogen atoms interact with non-metallic impurities, and the removal of O and N in Gd can be achieved by the evaporation of H2O and NH3. Based on the plenty solid solution hydrogen, the removal degree can be up to 80%.(5) As one of the most difficult removal impurities in rare earth metals, oxygen migration process and oxidation process have attracted extensive attention. 18O2 isotope has been used to track the oxidation mechanism about Tb in this work.18O exhibits discrete and localized corrosion at begining, the nucleation firstly appears near defects. As time going on, the crystal nucleuses grow along with the grain boundaries, which is called the intergranular corrosion. The nucleuses grow and connect with each other, finally becoming the oxide layer. The calculated diffusion coefficients for 18O in Tb at a simulated natural atmosphere is between 1.4×10-19 and 1.22×l0-18 m2/s. |
