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LEAD
Natural Abundance, Stable Isobars
Pb204, 1.4%, Hg204
Pb206, 25.2%
Pb207, 21.7%
Pb208, 51.7%
Lead chloride, lead bromide, lead iodide, and lead tetraethyl have been used as charge material in the separation of lead isotopes. Lead chloride is the preferred charge material since a temperature of only 475° C is required to provide sufficient vapor pressure for satisfactory calutron operation. The fact that the compound is not hygroscopic is an additional advantage in that it permits rapid attainment of an operating vacuum in the calutron. The usual charge consists of 500 g PbCl2 in a style C-16 graphite charge bottle.
Lead chloride is prepared by dissolving lead metal shot in nitric acid, filtering, and treating the filtrate with hydrochloric acid to precipitate PbCl2. The PbCl 2 precipitate is washed with ice water to remove nitric acid. The PbCl2 filtrate is combined with the wash solutions and adjusted to pH 7 using ammonium carbonate which precipitates as lead carbonate the small quantities of lead remaining in the solution. The lead carbonate is washed with water, dissolved in a minimum of dilute nitric acid, and precipitated with hydrochloric acid. The resulting PbCl2 is combined with the chloride previously separated, and the salt is outgassed at 450° C for four hours. The dried PbCl2 is placed in a nickel reactor and fused under an inert atmosphere using induction heating (mp, 501° C). The fused crystalline mass is broken out of the reactor and bottled.
Two samples of lead obtained as by-products from uranium and thorium ore processing, one enriched in Pb2 06 and the other in Pb208, were used as feed material for special separations of lead. Radiogenic lead enriched in Pb206 was the end product of the radioactive decay of U238; lead enriched in Pb208 was the end product of the radioactive decay of Th232. Greater than usual care was exercised in handling these materials because of the presence of moderate radioactivity. Trace amounts of Pb210, Bi210, and Po210 were found in the Pb2O6 sample, and Pb210, Bi2lO, Po210, Ra226 , Ra228, Ac228, and Th228 were present in the Pb208 feed.
Approximately 30 kg of lead concentrates in the form of mixed oxide, sulfide, and elemental lead enriched in Pb206 was received with the following isotopic analysis: Pb204 , < 0.2%; Pb206 , 87.8%; Pb207, 8.9%; Pb208 , 3.3%. The calutron charge compound PbCl2 was prepared by dissolving this lead mixture, a little at a time, in hot concentrated nitric acid. Upon cooling, lead nitrate crystallized out of solution. The lead nitrate crystals were dissolved in water, filtered, and treated with hydrochloric acid in the manner described above.
Approximately 13.5 kg of radiogenic Pb208was obtained in the form of a damp sulfate cake with the following isotopic analysis: Pb204, 0.024%; Pb206, 25.58%; Pb2O7, 1.78%; Pb208 , 72.62%. To prepare the PbCl2 calutron charge compound from this material, it was first slurried in water and mechanically stirred with solid ammonium carbonate for four hours. Very slightly soluble lead sulfate was converted by metathesis to insoluble lead carbonate. The lead sulfate and carbonate solid were separated from the liquid slurry by filtration, and lead carbonate was separated from the unconverted sulfate by dissolving in dilute nitric acid. The unconverted lead sulfate was re-treated with ammonium carbonate. Conversion of the soluble lead nitrate to PbCl2 was carried out as described above.
Lead and its compounds are cumulative poisons, repeated small doses being as dangerous as a single large dose. The compounds constitute a greater hazard than the element, and breathing of the dust is more conducive to lead poisoning than ingestion of the dust. All efforts should be made to keep dusting to a minimum, and the use of a fume hood with good exhaust ventilation or use of a respirator is recommended when dusting is encountered. Rubber gloves should be used and good hygienic practices should be followed when handling lead and its compounds. An even greater degree of care should be exercised when handling radiogenic lead contaminated with varying amounts of radioactive isotopes.
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