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HAFNIUM
Natural Abundance, Stable Isobars Hf174, 0.163%, Yb174 Hf176, 5.21%, Yb176 Hf177, 18.56%, aLu176 Hf178, 27.10%, bTa180 Hf179, 13.75%, cW180 Hf180, 35.22% The only charge material which has been used in the separation of hafnium isotopes is anhydrous hafnium tetrachloride. The average charge consists of 350 g HfCl4 in a style X-5 stainless steel charge bottle and is prepared by passing a mixture of chlorine gas and carbon tetrachloride vapor over hafnium oxide which is heated to a temperature of 500-550ºC. Relatively pure hafnium oxide is obtained from the Y-12 Plant as a by-product from the production of pure zirconium oxide. The major impurities found in this product are approximately 1% Zr and 0.2% Ti. Hafnium oxide in a Vycor boat is placed in a Pyrex tube 10 cm in diameter by 2 m in length. The inlet end of the tube is reduced to 1 cm and connected to a 500-ml flask which is connected to a cylinder of chlorine gas. The flask is heated with an electric heating mantle and fitted with a separatory funnel which permits a drop-by-drop feed of carbon tetrachloride. An electric tube furnace placed near the inlet end serves to heat approximately one-third of the Pyrex tube. The apparatus is placed near a fume hood in such a manner that the exhaust end of the Pyrex tube projects well into the hood for removal of gaseous reaction products as well as unreacted chlorine and carbon tetrachloride. During operation the carbon tetrachloride is dropped slowly into the heated flask where it vaporizes and, along with chlorine gas from the cylinder, is swept over the heated hafnium oxide. As soon as HfCl4 is formed it sublimes from the hot end of the tube and is collected in the cool zone as a light fluffy powder. Periodically the HfCl4 is removed from the tube and quickly transferred to sealed containers for storage. The efficiency of the reaction is improved by stirring the hafnium oxide every hour or so to expose a fresh surface. Loosely plugging the exhaust end of the reaction tube with glass wool serves to exclude atmospheric moisture from the tube and prevents hydrolyzation of the HfCl4. When the reaction is complete, nitrogen is used to sweep the tube free of any unreacted chlorine. Approximately 700 g of HfCl4 per day can be prepared by this method. Due to the relative scarcity of high purity hafnium, the unresolved charge material is recycled and recovered. The recovery of hafnium consists of washing the calutron components in nitric acid, precipitating hafnium hydroxide with ammonia, removing copper by electrolysis from the nitric acid solution, reprecipitating hafnium hydroxide with ammonia, precipitating impurities from hydrochloric acid with hydrogen sulfide, extracting iron with diethyl ether, and finally precipitating with ammonium hydroxide. The purified hafnium hydroxide is converted to hafnium oxide by slowly heating to 800°C. Elemental hafnium has a low order of toxicity; however, the finely divided metal forms an explosive mixture in air. Hydrolysis of HfCl4 to form hydrogen chloride and hafnium oxychloride presents a toxicity hazard. An additional hazard is phosgene, which is produced by the chlorination reaction. Safe handling of HfCl4 requires the intelligent use of rubber gloves, safety glasses, and a fume hood with good exhaust ventilation. aLu176, natural abundance 2.6%, is radioactive with a half-life of 4.6 × 1010 years. bTa180, natural abundance 0.012%, is radioactive with a half-life of > 107 years. cW180, natural abundance 0.135%, is radioactive with a half-life of 3 × 1014 years. |
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