| Description |
A two-step chlorination process was developed to purify cerium metal by volatilizing select impurities. Metal Ce was melted together with impurities Al, Fe, Ga, Ta, and U at 0.1 to 0.4 wt% each. The first step to this purification process is hydriding, and the second step is simultaneous chlorination and volatilization of metal chlorides. The hydriding reaction was found to begin at temperatures as low as 50°C and completed after 20 min. X-ray diffraction confirms that all the cerium metal converted to cerium hydride, and ultraviolet photoelectron spectroscopy found that CeH3 was produced. Oxygen contamination between 0.4 and 0.7 wt% was found in the hydride, which is concluded to be a limiting factor for full conversion to chloride. The chlorination step used anhydrous Cl2 gas to react with small samples of hydride powder. Conversion to chloride increases with increasing Cl2 concentration (balanced with Ar), increasing temperature (up to 700oC), and decreasing particle size. Using 99.5% Cl2, the maximum conversion to chloride was measured to be 92.1%. Using ultra-high purity Cl2 resulted in an increase in conversion to near 100%. The formation of cerium oxide was concluded to be the cause of incomplete conversion to chloride, attributed to O2 present in the lower purity Cl2. The decontamination via volatilization occurs simultaneously with chlorination. The decontamination factor (DF) for each impurity was measured using ICP-MS. It was found that the mixture of metals was not homogeneous, and concentration measurements have significant errors, which makes calculating an accurate DF value difficult. Ta was omitted because of the need to use HF and also because TaCl5 has the highest vapor pressure of all the impurities. The DFs found during the chlorination study using 99.5% Cl2 were very sporadic and yielded low results for all the impurities. Adding a hightemperature step significantly increased decontamination in Fe and Ga. The only way to achieve high U decontamination is to combine the use of UHP Cl2 and high temperatures. Using UHP Cl2 at 700°C yielded DF values averaging 1.9, 9.9, 14.0, and 5.0 for Al, Fe, Ga, and U, respectively. |
