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RHENIUM
Natural Abundance, Stable Isobars Re185, 37.07%, Os187 Re187, 62.93% Rhenium heptoxide and rhenium pentachloride have been used as charge material in the separation of the isotopes of rhenium. Both compounds can be used; however, Re2O7 is preferred since the charge consumption rate is lower. The usual charge consists of 150 g Re2O7 in a style S-12 stainless steel charge bottle. Rhenium heptoxide is prepared by burning the powdered metal in oxygen. The metal ignites at about 400° C and burns violently until conversion to rhenium trioxide is complete. Additional heat and oxygen serves to complete the oxidation to the yellow Re2O7. All equipment used for the conversion must be extremely clean and dry since any organic matter will reduce Re2O7 to a lower oxide, and moisture will hydrolyze the oxide to perrhenic acid. The flow rate of oxygen must be carefully controlled since too rapid a flow will carry Re2O7 through the exhaust traps in the form of a white smoke. Too low a flow rate, combined with the rapid burning of rhenium metal, may create a partial vacuum within the reactor causing the exhaust trap solution to flow into the reactor. The reactor used for the preparation of Re2O7 consists of a Pyrex tube 7.6 cm in diameter by 50 cm in length having a large ball joint on each end. A gas-washing bottle containing concentrated sulfuric acid is attached to the inlet end of the reactor. The exhaust end of the reactor is attached to a U-trop with the lower two-thirds of the trap immersed in a cold-bath of carbon tetrachloride and dry ice. Following the cold trap and connected by ball-and-socket joints are, in order, an empty trap, a sulfuric acid trap, another empty trap, and an ammonium hydroxide trap. The entire apparatus is fabricated of glass and is held together with clamps. An electric tube furnace 45 cm in length is used to heat the reactor. Approximately 125 g powdered rhenium metal contained in a Pyrex boat is inserted into the reactor. Cylinder oxygen feeds through the sulfuric acid wash bottle, to remove moisture, and passes into the reactor as the heat is gradually increased to 400° C. Careful attention must be given to the oxygen flow rate as the temperature approaches 400° C. After the oxidation to rhenium trioxide appears complete, the temperature is increased to 450° C. Volatile Re2O7 is formed, sublimes, and condenses in the cold U-trap. Upon completion of the reaction, the apparatus is allowed to cool and the hygroscopic Re2O7 is quickly transferred from the U-trap directly into the charge bottle. Due to the high cost of rhenium metal it is necessary to recycle and recover the unresolved charge material. The source, receiver, and liner are washed in a 10% solution of sodium hydroxide containing a few percent hydrogen peroxide. The wash solution is acidified to a pH of 2.0 with sulfuric acid, digested, and filtered. The solids are washed with water and held for additional treatment. The filtrate is combined with washings from the sodium hydroxide precipitation, and the solution is neutralized with hydrochloric acid. An additional amount of hydrochloric acid is then added to bring the solution up to 10% hydrochloric acid. The solution is heated to 70° C and saturated with hydrogen sulfide for four hours in a pressure bottle. The rhenium sulfide precipitate is allowed to settle the supernatant is decanted, and the precipitate washed by decantation, using a 10% hydrochloric acid solution saturated with hydrogen sulfide. Rhenium sulfide is collected on a fritted glass funnel and the filtrate is again saturated with hydrogen sulfide. When no additional precipitate is formed with hydrogen sulfide, sodium thiosulfate is added and the solution boiled for 15 min. Two thiosulfate precipitations usually remove all the rhenium and the last filtrate can be discarded. Use of ammonium hydroxide and nitric acid should be avoided in the initial phases of the procedure since ammonia will complex copper and nickel, thus preventing their removal in the hydroxide precipitation, and nitric acid interferes with the hydrogen sulfide precipitation. The combined rhenium sulfide precipitate is carefully dissolved in 10% ammonium hydroxide containing a few percent of hydrogen peroxide, filtered, and evaporated to dryness. The residue is dissolved in water producing a blue-colored solution which indicates that rhenium is in a reduced valence state. The solution is oxidized by the addition of a small amount of ammonium hydroxide and hydrogen peroxide. A colorless solution of ammonium perrhenate results and is evaporated to crystallization. The crystals are dried and reduced to metal by gradually heating to 900° C in a flow of hydrogen. The solution remaining from the crystallization is evaporated to dryness and also reduced to metal; however, the purity of this metal is somewhat lower than that produced from the crystals. After reduction, the metal is washed with water, vacuum dried, and stored for future use. Solids from the initial filtration are combined with other salvage solids and placed in a large bottle containing 10% sodium hydroxide. Live steam is passed through the solution while hydrogen peroxide is slowly added from a burette. The resulting leach solution is filtered and processed as above. Rhenium and its compounds are not considered toxic. aRe187 is radioactive with a half-life of 5 x 1010 years. |
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