Patent GB-865011-A (GB)
Title: Process for separation of protactinium from a solution of neutron-irradiated thorium
IPC: C01F15/00
Filed: 1957-07-31
Abstract: 865,011. Separation of protactinium and uranium-233. UNITED STATES ATOMIC ENERGY COMMISSION. July 31, 1957 [Aug. 7, 1956], No. 24248/57. Drawings to Specification. Class 39(4). A nitricacid solution of neutron-irradiated thorium is contacted with an organic solution of a trialkyl phosphate, followed by scrubbing the organic phase with as aqueous solution of an inorganic nitrate, in order to separate protactinium and fission products, which remain in the aqueous phase, from thorium and uranium-233, which pass into the organic phase, the combined extraction and scrubbing operation being carried out under nitrate ion deficient conditions, i.e. the total nitrate ion content of the feed solution and the scrubbing solution is less than the amount required to form normal nitrates with the cations present in these solutions. Irradiated thorium metal encased in aluminium may first be treated with a caustic-sodium nitrite solution to remove the aluminium and then with aqueous nitric acid to dissolve the thorium; alternatively, both metals - may be dissolved simultaneously in aqueous nitric acid containing small amounts of fluoride and mercuric ions. A nitrate ion deficient condition in the feed solution may be obtained by evaporating and digesting before extraction; this also converts silica present to a stable form and prevents extraction of ruthenium into the organic phase. A nitrate ion deficient aqueous solution of sodium or aluminium nitrate may be used as the scrubbing solution; also present may be phosphate ions and ferrous ions (to prevent extraction of chromium). The organic extractant may be a solution of tri-n-butyl phosphate in a kerosene fraction. The kerosene can be purified by treatment with chromyl chloride or by treatment with fuming sulphuric acid, followed by contacting with silica gel; the tributyl phosphate may be purified bywashing with alkali solution. The liquid phases are contacted, preferably in countercurrent, in any type of solvent extraction apparatus, for example pulse columns. The protactinium in the separated aqueous phase may be allowed to decay to uranium-233 and then extracted as previously, or may be adsorbed on silica gel and washed off with aqueous. oxalic acid; another method is to acidify the solution to remove the nitrate ion deficiency and then extract with an organic solution of a trialkyl phosphate or a diisoalkyl carbinol, the protactinium being returned to water solution by extraction with slightly acidic water or a solution of sodium fluoride; alternatively protactinium may be carried out of solution by precipitating aluminium chromate. The organic extract containing uranium-233 and thorium is contacted with dilute aqueous nitric acid, thorium being transferred to the aqueous phase, which may be concentrated and thorium precipitated as oxalate; uranium-233 remains in the organic phase from which it may be removed by contact with slightly acidified water. Further purification may be effected by passage through a cation exchange resin column, followed by elution of the uranium with an acidic acetate or citrate solution. Especially suitable resins are nuclear sulphonated aromatic hydrocarbon polymers, for example as described in U.S.A. Specification 2,366,007 and 2,204,539. The separation process is described in detail with reference to a diagrammatic flow sheet.