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SAMARIUM
Natural Abundance, Stable Isobars
Sm144, 3.16%, bNd144
aSm147, 15.07%, Nd148
Sm148, 11.27%, cNd150
Sm149, 13.84%, Gd152
Sm150, 7.47%, Gd154
Sm152, 26.63%
Sm154, 22.53%
Samarium dichloride and fused samarium trichloride have been used as charge material for the separation of samarium isotopes. The trichloride is the preferred charge since its vapor pressure is higher than that of the dichloride. The average charge consists of 150 g SmCl3 in a style C-16 graphite charge bottle.
Rare earth oxides, with the exception of cerium, can be converted to the anhydrous trichloride by using various chlorinating agents either alone or in the presence of a reducing agent. Heating samarium oxide in an excess of ammonium chloride is the procedure preferred by ORNL Charge Laboratory because of the simplicity of the operation, its adaptability to a large scale process, and the favorable economic considerations. The presence of more than the stoichiometric amount of ammonium chloride effectively eliminates hydrolysis of the chloride and prevents formation of a basic salt. Excess ammonium chloride is volatilized from the anhydrous SmCl3 by heating in a vacuum at 450ºC.
A typical conversion consists of mixing 150 g samarium oxide with 300 g ammonium chloride in a 2000 ml Vycor dish and heating the mixture over a gas flame, stirring frequently to maintain a homogeneous mixture. Heating and stirring are continued until an aliquant of the mixture is completely soluble in water. (This simple test proves quite effective in determining completion of reaction since the rare earth chloride is soluble, while the oxide and basic salts are not.) Approximately one hour is required for the conversion of 150 g of samarium oxide to SmCl3.
The anhydrous SmCl3 is a light fluffy powder and in order to get the required amount into the charge bottle it is necessary to press the material into a dense cake. Compression using several thousand psi produces a more compact form which serves well as a calutron charge material.
Due to the high cost of samarium and most rare earths, the unresolved charge materials are recycled and recovered. The source, receiver and liner parts are washed in dilute nitric acid. The solution is filtered, made basic with ammonium hydroxide, and the insoluble hydroxides are allowed to settle. Washing the precipitate with 5% ammonium hydroxide and decanting is repeated until the supernatant is practically colorless, which indicates that nearly all of the nickel and copper have been removed as the soluble ammonia complexes. The precipitate is then filtered, washed with water, and dissolved with a minimum of dilute hydrochloric acid. Samarium is precipitated from this solution as the oxalate using solid oxalic acid. Optimum conditions for the quantitative precipitation of samarium are found to be a pH in the range of 2-4, use of an excess of oxalic acid crystals, use of a fine frit glass funnel for filtering, and washing with dilute oxalic acid solution to avoid peptization. All oxalate filtrates are allowed to stand three days and any additional small amount of samarium oxalate which settles out is recovered before discarding the solution.
The samarium oxalate is dried and ignited to the oxide at 800ºC. Usually a small amount of iron from the liner wash solution is carried through the procedure and appears as a contaminant in the oxide. This small amount of iron is removed by dissolving the oxide in concentrated nitric acid and repeating the hydroxide and oxalate purification steps.
In the chemical recovery of rare earth elements it is important to note that the hydroxides show a decrease in solubility with increased atomic number, while the solubility of oxalates increases with increased atomic number. Additional small quantities of rare earth elements may be recovered by reprocessing the ammonia and oxalate filtrates, the amount depending upon the particular rare earth involved.
Samarium, like other rare earths, is not considered toxic; however, a fume hood with good exhaust ventilation is recommended, particularly during chlorination with ammonium chloride.
aSm147 is radioactive with a half-life of 1.3 × 1011 years.
bNd144 , natural abundance 23.83%, is radioactive with a half-life of ~2 × 1015 years.
cNd150, natural abundance 5.63%, is radioactive with a half-life of > 1016 years.
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