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GALLIUM
Natural Abundance, Stable Isobars Ga69, 60.5%, None Ga71, 39.5% Gallium oxide, gallium chloride, and gallium iodide have been used as charge material for separation of gallium isotopes. Gallium iodide is the preferred charge since it provides adequate vapor pressure within the normal temperature range of the calutron source units. The average charge consists of 150 g GaI3 in a style S-16 stainless steel charge bottle. Gallium iodide can now be obtained from commercial vendors, but several years ago gallium chloride was the only halide salt available commercially, and it was necessary to convert it to iodide. Gallium chloride was dissolved in water, and gallium hydroxide was precipitated with ammonium hydroxide at pH 7.0. Washing removed the chloride ion, and the hydroxide was dissolved in hydriodic acid. The Gal 3 solution was evaporated to incipient crystallization, transferred in an evaporating dish to a vacuum desiccator, allowed to cool under reduced pressure, and then transferred to an outgassing apparatus. At a temperature of 250ºC and a pressure of 40 µ, GaI3 sublimed from the heated zone and collected in the cool zone of the reactor tube. The sublimed anhydrous GaI3 was stored in sealed containers until used as charge material. Due to its high cost, the unresolved gallium charge material is recovered from calutron components. Source, receiver, and liner parts are washed first with water to dissolve GaI3, and then in nitric acid to dissolve the metal, oxide, and other forms not soluble in water. Since the two wash solutions differ considerably in content of gallium and contaminants they are processed separately. The water wash solution, which contains considerable iron but very little copper, is acidified with nitric acid and filtered to remove any solid material. The filtered solution is evaporated to approximately one-fourth its original volume and the pH adjusted to 11.0 with sodium hydroxide. At this pH, iron hydroxide precipitates and gallium remains in solution as the soluble sodium gallate. The precipitate is washed several times with dilute sodium hydroxide solution by decantation, filtered, and then discarded. The combined filtrates and washing are adjusted to pH 4.5 with hydrochloric acid, and gallium is precipitated as the hydroxide. After filtering, the gallium hydroxide is ready for conversion to the iodide as described above. The nitric acid wash solution is filtered to remove solids, and these are combined with solids from the water wash solution. The filtrate, which contains a small quantity of iron and a larger amount of copper, is adjusted to pH 11.0 with sodium hydroxide which precipitates the iron copper as hydroxides. Washing the precipitate with dilute sodium hydroxide, filtering, and precipitating gallium hydroxide is carried out as described above. The combined solids from both wash solutions are ignited and fused in a mixture of 60% potassium pyrosulfate and 40% potassium bisulfate. After cooling, the fused mass is digested in hot water and filtered. The solids are washed with dilute sodium hydroxide and discarded. Filtrates and washings are combined and treated as above to recover gallium hydroxide. Upon completion of the gallium isotope separation series, gallium is converted to the metal for storage. The gallium hydroxide is dissolved in sodium hydroxide and the solution is electrolyzed at 5 v and 2 amp. A platinum anode and a carbon rod cathode are used. Gallium metal plates out on the carbon electrode as long as the temperature of the electrolyte remains below 29.75ºC, which is the melting point of gallium. When the temperature exceeds the melting point, however, the metal drips off the cathode and tends to dissolve in the sodium gailate solution. To prevent this a small plastic funnel is fitted snugly over the bottom of the cathode to catch the liquid gallium and keep it cathodic. Gallium and its compounds have a low order of toxicity. |
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