Preparation 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. 

The transeinsteinium actinides, fermium (Fm), mendelevium (Md), nobelium (No), and lawrencium (Lr), are not available in weighable ( ng) quantities, so these elements are unknown in the condensed bulk phase and only a few studies of their physicochemical behavior have been reported. Neutral atoms of Fm have been studied by atomic beam magnetic resonance 47). Thermochromatography on titanium and molybdenum columns has been employed to characterize some metallic state properties of Fm and Md 61). This article will not deal with the preparation of these transeinsteinium metals. 

Metallothermic reduction of an actinide halide was the first method applied to the preparation of an actinide metal. Initially, actinide chlorides were reduced by alkali metals, but then actinide fluorides, which are much less hygroscopic than the chlorides, were more 

The vapor pressures at 1473 K of a few of the actinide elements and other materials of interest are given in Table III. All of the actinide (An) elements through einsteinium can be obtained by this process  

This method has also been successfully adapted to the pilot-plant and industrial-scale production of Th, U, Np, and Pu metals. In these cases, calcium metal is preferred as the reductant, and the reaction is  

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Sedimentation equilibrium technique, 19 256 Sedimentation velocity technique, 19 256 Sediments, arsenic in marine samples, 44 149, 162-164, 169, 181 [Se U ] cations, 35 297-298 Selective vaporization, for preparation of actinide metals, 31 12-13, 26 Selenide, production, 38 82 Selenium... 

Purity. The use of evaporation methods for the preparation of actinide metals reduces the number and quantities of impurities. Nevertheless, possible chemical contaminations from reactions with reducing agents, container vessel or crucible material or with constituents of the atmosphere as well as the accumulation of products of radioactive decay have to be taken into account. 

Preparation of base metals by coupled reduction with platinum group metals. Very pure metals of the alkaline- earth, lanthanide and actinide series can be prepared from their oxides (or fluorides) through coupled reduction by pure hydrogen in presence of platinum group metals. According to a precursory paper on this subject (Berndt et al. 1974), the preparation scheme of Li, Ca, Sr, Ba, Am and Cf was described. As an example, Ca can be obtained by synthesis of a Pt compound, followed by its vacuum decomposition and recovery by distillation of the more volatile base metal ... 

This process is particularly useful for the preparation of pure plutonium metal from impure oxide starting material (111). It should also be applicable to the preparation of Cm metal. Common impurities such as Fe, Ni, Co, and Si have vapor pressures similar to those of Pu and Cm metals and are difficult to eliminate during the metallothermic reduction of the oxides and vaporization of the metals. They are eliminated, however, as volatile metals during preparation of the actinide carbides. 

Proceeding from thorium to plutonium along the actinide series, the vapor pressure of the corresponding iodides decreases and the thermal stability of the iodides increases. The melting point of U metal is below 1475 K and for Np and Pu metals it is below 975 K. The thermal stabilities of the iodides of U, Np, and Pu below the melting points of the respective metals are too great to permit the preparation of these metals by the van Arkel-De Boer process. 

V. Specifics of Actinide Metal Preparation A. General Comments... 

All subsequent preparations of Cf metal have used the method of choice, that is, reduction of californium oxide by La metal and deposition of the vaporized Cf metal (Section II,B) on a Ta collector 10, 30, 32, 45, 91, 97, 120). The apparatus used in this work is pictured schematically in Fig. 16. Complete analysis of Cf metal for cationic and anionic impurities has not been obtained due to the small (milligram) scale of the metal preparations to date. Since Cf is the element of highest atomic number available for measurement of its bulk properties in the metallic state, accurate measurement of its physical properties is important for predicting those of the still heavier actinides. Therefore, further studies of the metallic state of californium are necessary. 

Table XI gives the room-temperature, atmospheric pressure crystal structures, densities, and atomic volumes, along with the melting points and standard enthalpies of vaporization (cohesive energies), for the actinide metals. These particular physical properties have been chosen as those of concern to the preparative chemist who wishes to prepare an actinide metal and then characterize it via X-ray powder diffraction. The numerical values have been selected from the literature by the authors.

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... 

Fig. 1. Double glove-box system for the preparation and refining of actinide metals 1. RF heating system 2. Water cooled quartz vacuum furnace 3. Box filled with circulating nitrogen 4. -I- 5. Argon in and out filters filter 6. Stainless steel box filled with circulating argon 7. Vacuum lock chamber 8. Pump...

Previous results of the preparation chemistry of actinide elements have been reviewed in detail by F. WeigeF In the following chapter, the actual state of the possibihties for the preparation and refining of actinide metals will be described the principles and trends in synthesis and crystal growth of (simple) actinide compounds will be shown. 

The compounds with elements of the platinum group can be prepared by direct synthesis, which requires the availabiUty of actinide metals. Such intermetallics can, however, also be obtained by coupled reduction of actinide oxides with hydrogen in the presence of finely divided noble metals ... 

Protactinium tetrafluoride, like the other actinide tetrafluorides, possesses the 8-coordinate UF4-type of structure (Table III) but no bond distances are available. It is easily the most stable tetravalent halide of protactinium and can be handled in the atmosphere, at least for a limited period, without hydrolysis or oxidation occurring. As mentioned earlier it is the usual starting material for the preparation of protactinium metal. Tetrafluoride hydrates have not been fully characterized, but a mixed fluorosulfate, PaF2S04 2H20 can be precipitated from aqueous solution (131). Protactinium tetrafluoride is soluble in aqueous ammonium fluoride solutions, for which some spectral properties have been recorded (4, 83). 

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. 

Californium is the heaviest actinide for which data like the enthalpy of sublimation have been determined directly with bulk quantities of about 2 mg of pure metal. Due to the limited availability of the heaviest actinides down to the one-atom-at-a-time scale, the preparation of the metals becomes an integral part of an experiment for studying the metals. Unusual experimental approaches like the measurement of partial pressures of the actinide under study over an alloy, studies of diffusion of actinide atoms in metals, and adsorption studies of actinide atoms onto metal surfaces by thermochromatography have been reported. 

Uranium tetrachloride [10026-10-5], UCl, has been prepared by several methods. The first method, which is probably the best, involves the reduction/chlorination of UO [1344-58-7] with boiling hexachloropropene. The second consists of heating UO2 [1344-57-6] under flowing CCl or SOCI2. The stmcture of the dark green tetrachloride is identical to that of Th, Pa, and Np, which all show a dodecahedral geometry of the chlorine atoms about a central actinide metal atom. The tetrachloride is soluble in H2O, alcohol, and acetic acid, but insoluble in ether, and chloroform. Industrially the tetrachloride has been used as a charge for calutrons. 

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