Purification of Thorium

Thorium concentrate produced by the processes described in Sec. 8 is too impure to be used as nuclear fuel. Especially objectionable impurities, which frequently are associated with thorium in its ores, are neutron-absorbing rare earths and uranium, the latter because it would dilute isotopically U formed in thorium during subsequent neutron irradiation. The objective of thorium purification is removal of these and other impurities to concentrations below a few parts per million. 

Solvent extraction with TBP has become the standard procedure for purifying thorium, just as for uranium. Processes used in different countries differ, however, in details such as the solvent used to dilute TBP, its concentration, and the means used to strip thorium and coextracted uranium from TBP. Table 6.20 summarizes the main features of processes used for purification of thorium on an industrial scale in the principal thorium-producing countries. Wylie [W5] gives more detail on early pilot-plant thorium-purification runs. Most of the published U.S. work on thorium purification on an industrial scale deals with irradiated thorium rather than natural this will be described under the Thorex process, in Sec. 5 of Chap. 10. 

In the second contacting unit, thorium is stripped from the rich solvent by 0.1 A HNO3 uranium is scrubbed from the thorium product by additional solvent. 

Uranium in solvent leaving the second contacting unit is stripped into an aqueous phase by 5% sodium carbonate solution. Stripped solvent is washed and reacidified with 4 N nitric acid for recycle to the process. 

At the high thorium and nitric acid concentrations used in this flow sheet, two solvent phases may form, one rich in thorium and TBP and the other, lean. Callow [C2] states that formation of the two solvent phases does not interfere with operation of a mixer-settler cascade, whereas difficulty would be experienced with a pulse column. Conditions at which a 

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Table 12.6 contains, in a simplified way, the composition, location, and treatment of main thorium ores. The purification of thorium by TBP extraction is illustrated in Fig. 12.4. This purification takes place after the dissolution of Th in nitric acid, generally from a hydroxide cake. When the Th is dissolved in sulfuric acid, purification is achieved by extraction with long-chain alkylamines. 

Final purification of uranium 65-2.3.2 Final purification of thorium... 

The principal steps in producing refined thorium compounds from thorium-bearing ores are concentration of thorium minerals, extraction of thorium from concentrates, purification or refining of thorium, and conversion to metal or the thorium compound finally wanted. This section describes the concentration of monazite, the principal source of thorium in the past the extraction of thorium from monazite and the recovery of thorium from leach liquors by solvent extraction. Purification of thorium is described in Sec. 9 and conversion in Sec. 10. 

Table 6.20 Examples of purification of thorium on an industrial scale by solvent extraction with TBP... 

June 18, 1957 Solvent Extraction Process for Purification of Thorium F.H. Spedding A. Kant... 

Solvent extraction is now coming into use as a method for the final purification of thorium from uranium and other impurities, to the stringent specification imposed when the element is used for nuclear purposes (Chapter 4).3o... 

Audsley, a., Jamrack, W. D., Oldbury, A. E. and Wraxs, R. A. Recmtly developed processes for the extraction and purification of thorium. Proc. 2nd Int. Conf. on the Peaceful Uses of Atomic Energy, Geneva, 1958. Paper 152. 

A successful process has been developed by the National Chemical Laboratory using cellulose phosphate as a cation-exchange material for the purification of thorium from rare earth elements. Monazite sand is broken with sulphuric acid and extracted with water to give a solution of thorium and rare earth sulphates and phosphates. This is first treated with metallic iron or aluminium to reduce the ferric iron impurity to the ferrous condition. The solution is then fed through a colunm of cellulose phosphate to absorb the thorium. Some of the thorium is present in solution as a cationic phosphate complex, rather than as simple thorium cations, but both forms are retained by the column to a high degree. Rare earth elements, which predominate in the feed solution, are not appreciably absorbed, and the ratio of thorium to rare earths is increased to about 450. ... 

Crystallisation was one of the earliest methods used for separation of radioactive microcomponents from a mass of inert material. Uranium X, a thorium isotope, is readily concentrated in good yield in the mother liquors of crystallisation of uranyl nitrate (11), (33), (108). A similar method has been used to separate sulphur-35 [produced by the (n, p) reaction on chlorine-35] from pile irradiated sodium ot potassium chloride (54), (133). Advantage is taken of the low solubility of the target materials in concentrated ice-cold hydrochloric acid, when the sulphur-35 as sulphate remains in the mother-liquors. Subsequent purification of the sulphur-35 from small amounts of phosphorus-32 produced by the (n, a) reaction on the chlorine is, of course, required. Other examples are the precipitation of barium chloride containing barium-1 from concentrated hydrochloric acid solution, leaving the daughter product, carrier-free caesium-131, in solution (21) and a similar separation of calcium-45 from added barium carrier has been used (60). 

This article presents a general discussion of actinide metallurgy, including advanced methods such as levitation melting and chemical vapor-phase reactions. A section on purification of actinide metals by a variety of techniques is included. Finally, an element-by-element discussion is given of the most satisfactory metallurgical preparation for each individual element actinium (included for completeness even though not an actinide element), thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, and einsteinium. 

Since the radioactivity of thorium salts is smaller than that of the minerals, B. B. Boltwood (93) thought that some of the radiothorium must have been lost during the purification process. On the assumption that radiothorium was formed directly from thorium, he computed that the half-life period of the former ought to be at least six years, whereas... 

Yaftian, M.R. Hassanzadeh, L. Eshraghi, M.E. Matt D. Solvent extraction of thorium (IV) and europium (III) ions by diphenyl-N,N-dimethylcarbamoylmethylphosphine oxide from aqueous nitrate media, Sep. Purif. Tech. 31 (2003) 261-268. 

Govindan, R Palamalai, A. Vijayan, K.S. Raja, M. Parthasarathy, S. Mohan, S.V. Subba Rao, R.V. Purification of 233U from thorium and iron in the reprocessing of irradiated thorium oxide rods, J. Radioanal. Nucl. Chem. 246 (2000) 441 144. 

A similar scheme has been used for example, for the recovery on anion exchanger of uranium as a nitrate complex from its mixture with thorium [17, p. 317], or for the purification of nickel from calcium on cation exchanger[18]. 

Zuo Y, Chen J, li DQ (2008) Reversed micellar solubilization extraction and separation of thorium (IV) from rare earth (iii) by primary amine in ionic liquid. Sep Purif Technol 63 684-690... 

The establishment of a nuclear power industry based on fission reactors involves the production of a number of materials that have only recently acquired commercial importance, notably uranium, thorium, zirconium, and heavy water, and on the operation of a number of novel chemical engineering processes, inciuding isotope separation, separation of metals by solvent extraction, and the separation and purification of intensely radioactive materials on a large scale. This text is concerned primarily with methods for producing the special materials used in nuclear fission reactors and with processes for separating isotopes and reclaiming radioactive fuel discharged from nuclear reactors. 

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