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TANTALUM
Natural Abundance, Stable Isobars aTa180, 0.0123%, Hf180 Ta181, 99.9877%, bW180 Tantalum pentachloride is the only charge material which has been used in the separation of tantalum isotopes. The average charge consists of 500 g TaCl5 in a style X-5 stainless steel charge bottle. Tantalum forms several compounds with chlorine but the most important, from a calutron separation standpoint, is the pentachloride. Tantalum pentachloride may be prepared by the action of chlorine, phosphorus pentachloride, or sulfur monochloride on tantalum oxide and by the action of chlorine on tantalum metal. The latter method is preferred at ORNL since it permits use of scrap tantalum calutron filaments and because of the simplicity of preparations. The scrap filaments should be free of tungsten and hafnium since these two elements form volatile chlorides and each has an isotope of mass 180. The chlorination reaction is carried out in a Vycor reactor tube 80 cm long and 7.5 cm in diameter heated by an electric tube furnace. The gas inlet end of the reactor tube is constricted to a diameter of 10 mm and terminates in a Pyrex ball joint. Chlorine and nitrogen cylinder gases are fed through a glass T-tube into a gas-washing bottle containing concentrated sulfuric acid, then into the reactor through a ball-joint connection, A rubber stopper fitted with a short piece of tubing is inserted into the downstream end of the reactor, and exhaust gases are led to a fume hood through a connecting length of rubber tubing. About 300 g of tantalum metal in the form of used filament scrap is placed in a quartz combustion boat and inserted in the heated end of the reactor. While the furnace is heating gradually to 300ºC, nitrogen is passed through the reactor. Before chlorine is admitted to the reactor, any moisture evidenced in the cold end of the tube is driven off by flame treating. When the system is thoroughly dry, chlorine is admitted the nitrogen flow is discontinued, and the temperature is raised to 625ºC. The rate of reaction can be controlled by the flow of chlorine. At a high flow rate the tantalum actually burns and becomes incandescent. Tantalum pentachloride is formed immediately and transported to the cool end of the tube, where it accumulates. The product is periodically raked out of the reactor and stored in sealed bottles. When all of the tantalum is converted, the tube and contents are allowed to cool under a flow of chlorine gas. The tube is then flushed with nitrogen for 15 min and any remaining TaCl5 is recovered from the reactor. Tantalum pentachloride decomposes in moist air and evolves hydrogen chloride. These fumes are very corrosive and poisonous, and precautions should be taken to prevent their contact with the eyes, nose, and upper respiratory tract. Adequate ventilation and personnel protective equipment, including a respirator and rubber gloves, should be used when working with TaCl5. aTa180 is radioactive with a half-life of > 107 years. bW180, natural abundance 0.135%, is radioactive with a half-life of 3 × 1014 years. |
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