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POTASSIUM
Natural Abundance, Stable Isobars
K39, 93.08%, Ar40
aK40, 0.0119%, Ca40
K41, 6.91%
Potassium chloride, potassium bromide, potassium iodide, and massive potassium metal have been used as charge material in the separation of the isotopes of potassium. In earlier separations using a low temperature source unit, heat limitations confined the choice of charge material to potassium iodide and potassium metal. Since development of the medium temperature source unit, M-16, any of the above materials can be used satisfactorily; however, experience has established fused KCl as the best charge material. The usual charge consists of 120 g KCl in a style S-16 stainless steel charge bottle.
Potassium chloride usually is obtained from commercial sources and requires no special processing prior to use. One special charge, which had been enriched in K40 by reactor irradiation, was received as a KCl solution containing 0.19% K40 and was prepared for the calutron by precipitating the perchlorate and carefully decomposing it to KCl at 650° C.
Although unresolved potassium charge material is not usually recycled and recovered, this procedure is used with enriched feed materials. The calutron components are washed with dilute hydrochloric acid. The wash solution is filtered to remove solids, and these solids are washed and discarded. Hydrogen sulfide is then passed through the filtered wash solution for 30 min. After settling, the precipitate is separated by decanting and washed with three portions of dilute hydrochloric acid saturated with hydrogen sulfide. The precipitate is filtered on paper, washed, and discarded. The combined filtrates and wash solutions are adjusted with ammonium hydroxide to a pH of 9.0 and saturated with hydrogen sulfide. Again the precipitate is washed by decantation, filtered, and discarded. The solution is then acidified with hydrochloric acid and boiled to agglomerate sulfur, which is removed by filtration, washed, and discarded. Dilute barium chloride solution is added to remove any sulfate which forms by air oxidation of the sulfide. Excess barium is removed by the addition of ammonium carbonate. Both barium sulfate and barium carbonate precipitates are removed by filtration, washed, and discarded.
The solution is then evaporated to incipient dryness, and the ammonium salts destroyed by digestion with aqua regia. It is imperative to destroy all ammonium salts in order to prevent the formation of potentially explosive ammonium perchlorate later in the process. After removal of ammonium salts, the solution is evaporated to dryness. The potassium salt is dissolved in water and filtered to remove any insolubles, and these insolubles are washed and discarded. The filtered solution is concentrated by evaporation and cooled before adding an excess of perchloric acid.
The mixture is chilled in a refrigerator and, while still cold, is filtered through a sintered glass funnel. The potassium perchlorate precipitate is transferred to a quartz dish, covered with a platinum lid, and slowly heated to 650° C. Decomposition begins in the 400° to 450° C range at which time the potassium perchlorate liquefies and has a tendency to spatter and creep. After ignition, the KCl is cooled, dissolved in water, and filtered to remove a small amount of silica leached from the quartz dish. The filtered solution of KCl is then evaporated to dryness in a Pyrex beaker, heated to 300° C, cooled, and stored in sealed bottles.
Although the industrial hazards pertaining to potassium and its compounds are few, the oxide and hydroxide are extremely caustic and will cause burns on the skin. Since potassium metal is pyrophoric, heat and all oxidizing conditions should be avoided. The greatest potential hazard in the above method of potassium recovery is the use of perchloric acid. This acid, plus heat and in the presence of ammonium salts or readily oxidizable substances such as organics, can cause violent explosive conditions. Safety precautions for the handling of potassium and its compounds include the use of safety glasses or face shields, rubber gloves, and respirators, particularly if dusting occurs.
aK40 is radioactive with a half-life of 1.3 x 109 years.
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