Purification, of actinide metals,

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. 

Pulse ultrasonic relaxation method, 32 18 Pump-and-probe techniques, 46 137 Purification, of actinide metals, see Actinide, metals, purification XjPj Purified protein, 36 94 Purple acid phosphatases, 40 371, 376, 43 362, 395-398, 44 243-245 biological function, 43 395 homology, 43 397... 

Reductive nitrosylation, transition metal nitrosyl complexes, 34 296-297 ReFejSj cluster, 38 41-43 self-assembly system, 38 41-42 Refining, of actinide metals, see Actinide, metals, purification Refractory compounds heat treatment of solids, 17 105-110 crystal growth, 17 105, 106 decomposition, 17 107,-110 spheroidization, 17 106, 107 preparation of, using radio-frequency plasma, 17 99-102... 

In the following, methods for preparation, purification and characterization of actinide metals are reviewed. Properties are presented, the theoretical interpretation of which underlines the special nature of the actinides in comparison with d or 4f (lanthanide) transition metals. 

Preparation Methods. Actinide metal preparation is based on methods known or developed to yield high purity material by metallothermic reduction or thermal dissociation of prepurified compounds. Electrolytic reduction is possible from molten salts, but not from aqueous solutions. Further purification of the metals can be achieved by electrorefining, selective evaporation or chemical vapour transport. 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

In this chapter, preparation and purification methods are reviewed. In view of the expected role of 5 f electrons in the metallic bond of actinides, methods for the preparation of metals have been particularly studied. There has also been important progress in the preparation of simple binary compounds. Special emphasis has been given to the growth of single crystals, particularly needed for the most refined physical techniques. 

The preparation of larger quantities of high purity actinide metals is being based increasingly on separation or purification via evaporation of the actinide metal In these methods, actinide compounds (oxides or carbides) are reduced by metals forming nonvolatile oxides or carbides under conditions where the actinide metals can be volatilized ... 

Efficient purification is achieved by selective evaporation and condensation. This technique is applicable to actinides of medium volatility i.e. Am or Cm The volatile impurities are eUminated by selective condensation of the actinide metal, less volatile impurities are left in the crucible. The efficiency of this refining method is determined by the relative evaporation ratio, which for two elements A and B equals the ratio of their activities at a given temperature. 

The control of the actinide metal ion valence state plays a pivotal role in the separation and purification of uranium and plutonium during the processing of spent nuclear fuel. Most commercial plants use the plutonium-uranium reduction extraction process (PUREX) [58], wherein spent fuel rods are initially dissolved in nitric acid. The dissolved U and Pu are subsequently extracted from the nitric solution into a non-aqueous phase of tributyl phosphate (TBP) dissolved in an inert hydrocarbon diluent such as dodecane or odourless kerosene (OK). The organic phase is then subjected to solvent extraction techniques to partition the U from the Pu, the extractability of the ions into the TBP/OK phase being strongly dependent upon the valence state of the actinide in question. 

The PUREX process exploits two features of U chemistry (1) the UC>22+ ion is the thermodynamically most stable form of U in aqueous solution both Pu022+ and Np022+ are easily reduced to Pu4+ and Np02+ under similar conditions (vide infra) and (2) in general, the actinide MC>22+ ions can be extracted from nitrate solutions into non-polar organic solvents [75] such as the phosphate esters, e.g. TBP. Since most other metal ions are not extracted under similar conditions, solvent extraction provides a convenient route for the purification of U and Pu from practically all other metals. Np can also be rendered extractable by manipulation of its oxidation state. Similarly, U can be separated from Pu by the selective reduction of Pu(IV) to Pu(III), rendering it inextractable into TBP/OK. 

On the topic of lanthanide/actinide separation, few reviews have dealt in detail with the most difficult aspect of this field, separation of the lanthanides from the trivalent transplutonium actinides. Jenkins (1979,1984) reviewed ion exchange applications in the atomic-energy industry. Relatively short sections of these reviews dealt with the separation of the trivalent metal ions. Symposium volumes entitled Actinide Separations (Navratil and Schulz 1980) and Lanthanide/Actinide Separations (Choppin et al. 1985) are collections of papers from several authors covering all aspects of lanthanide/actinide separation, some of which deal with the purification of the trivalent metal ions. 

Tertiary amines are poor extractants for lanthanides and actinides from dilute nitrate media, but extract these metal ions strongly from concentrated nitrate solutions of low acidity (as was true of TBP). Similar observations have been made for extraction from chloride media. Figure 1 indicates that for 30% Alamine 336/xylene/ll M LiCl group separations are good, some interactinide separations are possible, but lanthanide separation factors are small. Weaver briefly discusses the application of the TRAMEX (tertiary amine extraction) process for the purification of... 

All of the actinide elements are metals with physical and chemical properties changing along the series from those typical of transition elements to those of the lanthanides. Several separation, purification, and preparation techniques have been developed considering the different properties of the actinide elements, their availability, and application. Powerful reducing agents are necessary to produce the metals from the actinide compounds. Actinide metals are produced by metallothermic reduction of halides, oxides, or carbides, followed by the evaporation in vacuum or the thermal dissociation of iodides to refine the metals. 

The salt purification process is illustrated in Fig. XXIV-9. A fraction of the molten salt is removed from the electrolysis cell and is placed in contact with lithium-rich liquid cadmium. By the exchange reaction between Li and salt-borne TRU and the fission products, the less stable species in the molten salt are transferred to the liquid Cd. Generally, U and TRU are less stable than the rare earth metals and are first transferred to the liquid Cd. The Li concentration in the liquid Cd must be increased to decrease the contamination of the molten salt by TRU. Then, concentration of the fission products is also increased in the liquid Cd. After a forward reductive extraction process, the decontaminated salt with the salt-borne fission products passes through zeolite beds that replace nearly all of the alkali, alkaline earth, and rare earth metals with K and Li by ion exchange. The residual actinides in the molten salt are also adsorbed in the zeolite. The molten salt leaving the zeolite is free of actinides and fission product ions. The purified salt is mixed with an oxidizer such as CdCb and is contacted with liquid Cd that contains U and TRU by the forward reductive extraction process. CdCb will contain U and TRU to be oxidized. U and TRU are transferred to the molten salt from the liquid Cd. The molten salt with U and TRU is recycled to the electrolysis cell. The liquid metal is also recycled to the forward reductive extraction process. 

Purification methods vary in complexity with the chemical and physical characteristics of the actinide metal and with the quantity of material being processed. The principal refining processes, and their applicability, can be summarized as follows ... 

The first approach to the synthesis of actinide tetracyclopentadienyls, Cp4Th (Fischer and Tribner 1962) and CP4U (Fischer and Hristidu 1962), was made by reacting the actinide tetrachlorides and KCp in benzene. These compounds, due to their low solubility, required purification by Soxhlet extraction. Some other alternative synthetic approaches to tetracyclopentadienyl derivatives included the reaction of the metal tetrafluorides with dicyclopentadienyl magnesium in the absence of solvent (Reid and Wailes 1966) and, in the case of M = U, the reaction of uranium tetradiethylamide and freshly distilled cyclopentadiene (Paolucci et al. 1985a) ... 

Electrochemical methods. Hie electrolysis of dilute sulfuric acid solutions with a mercury cathode results In the quantitative deposition of Cr, Fe, Co, Nl, Cu, Zn, Qa, Oe, Mo, Rh, Pd, Ag, Cd, In, Sn, Re, Ir, Pt, Au, Hg, and T1 In the cathode. i Arsenic, selenium, tellurium, osmium, and lead are quantitatively separated from the electrolyte, but are not quantitatively deposited In the cathode. Manganese, ruthenium, and antimony are Incompletely separated. Uranium and the remaining actinide elements, rare earth elements, the alkali and alkaline eeu th metals, aluminum, vanadium, zirconium, niobium, etc. remain In solution.Casto and Rodden and Warf— have reviewed the effects of many variables In the electrolytic separation of the above-named elements from uranium. According to Rodden and Warf optimum conditions for the purification of uranium In sulfuric acid solutions with a mercury cathode are electrolyte volume,... 

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