Migration Laws of Impurities in Purification Process of Yttrium Metal by Plasma Zone Melting

Metal Y was purified by plasma zone melting, and the migration laws of Al, Si, Fe, Ni, Cu, and Mo impurities during the zone melting purification process were obtained. Calculation results show that the equilibrium distribution coefficients of the Al, Si, Fe, Ni, Cu, and Mo impurities in metal Y are 0.2173, 0.2201, 0.5065, 0.1586, 0.1742, and 0.8576, respectively. Because all equilibrium distribution coefficients are less than 1, the solubility of impurities in the liquid phase is greater than that in the solid phase. Therefore, theoretically, with the movement of molten zone, the Al, Si, Fe, Ni, Cu, and Mo impurities will be concentrated in the tail side of Y metal ingot, namely last-to-freeze zone. Experiment results demonstrate the correctness of the theoretical calculation. Besides, the internal relationship between the zone melting times and the impurity migration was investigated. Results show that with the increase in the zone melting time from 5 to 10, the concentration degree of impurities at the tail side of Y metal ingot is increased, i.e., the removal ratio is increased. After 10 times of zone melting, the removal ratios of Al, Si, Fe, Ni, Cu, and Mo impurities are 45.71%, 61.54%, 33.98%, 64.15%, 52.14%, and 46.28%, respectively. Because the saturated vapor pressure of the abovementioned impurities is similar to that of metal Y, impurities are difficult to be removed by the common methods. The investigation of plasma zone melting proposes a new research direction for the preparation of high purity Y.