Reduction of actinide oxides

B. Metallothermic Reduction of Actinide Oxides Followed by Distillation... 

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 ... 

To obtain and stabilize the actinides under study in the elemental/metallic state, the reduction of actinide oxides with lanthanum metal and the desorption of actinide atoms from metals like tantalum, titanium, and zirconium have been applied successfully. 

Carbides of the Actinides, Uranium, and Thorium. The carbides of uranium and thorium are used as nuclear fuels and breeder materials for gas-cooled, graphite-moderated reactors (see Nuclearreactors). The actinide carbides are prepared by the reaction of metal or metal hydride powders with carbon or preferably by the reduction of the oxides uranium dioxide [1344-57-6] UO2 tduranium octaoxide [1344-59-8], U Og, or thorium... 

Dioxides are known for all the actinides as far as Cf. They have the fee fluorite structure (p. 118) in which each metal atom has CN = 8 the most common preparative method is ignition of the appropriate oxalate or hydroxide in air. Exceptions are Cm02 and Cf02, which require O2 rather than air, and Pa02 and UO2, which are obtained by reduction of higher oxides. 

Newton, T. W. "The Kinetics of Actinide Oxidation Reduction Reactions" of Erda Critical Review Series TID - 26506, 1975. 

The metallothermic reduction of an oxide is a useful preparative method for an actinide metal when macro quantities of the actinide are available. A mixture of the actinide oxide and reductant metal is heated in vacuum at a temperature which allows rapid vaporization of the actinide metal, leaving behind an oxide of the reductant metal and the excess reductant metal, in accord with the following equations ... 

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. 

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. 

The vapour pressure ratio of actinides to noble metals is also the basis of the actinide metal preparation by thermal dissociation of intermetallic compounds. Such intermetallic compounds of An and noble metals can be prepared by hydrogen reduction of a mixture of an An oxide and a finely divided noble metal (Pt, Ir.. in the absence of noble metals, hydrogen reduction of An oxides is impossible. Am and Cm metals have been obtained by thermal dissociation of their intermetallic compounds with Pt and Ir High purity Th and Pa, the least volatile actinide metals, can be prepared by thermal dissociation of their iodides, which form readily by reaction of iodine vapour with car-... 

Well defined oxide phases can be obtained by thermal dissociation of oxalates, by controlled oxidation of compounds or actinide saturated ion exchangers or by reduction of higher oxides with hydrogen. Thermal dissociation of compounds often results in oxides of low density high (almost theoretical) density oxides can be prepared in sol-gel processes. 

Semiempirical calculations of free energies and enthalpies of hydration derived from an electrostatic model of ions with a noble gas structure have been applied to the ter-valent actinide ions. A primary hydration number for the actinides was determined by correlating the experimental enthalpy data for plutonium(iii) with the model. The thermodynamic data for actinide metals and their oxides from thorium to curium has been assessed. The thermodynamic data for the substoicheiometric dioxides at high temperatures has been used to consider the relative stabilities of valence states lower than four and subsequently examine the stability requirements for the sesquioxides and monoxides. Sequential thermodynamic trends in the gaseous metals, monoxides, and dioxides were examined and compared with those of the lanthanides. A study of the rates of actinide oxidation-reduction reactions showed that, contrary to previous reports, the Marcus equation ... 

Rabideau have also reviewed the kinetics of actinide oxidation-reduction reacticxis. C.3 Disproportionation Reactions... 

When the quantity of the material that can be used is not of concern, classical methods of preparing the oxides are used. Thus, one may use the approach of precipitating an oxalate, and after washing and drying, calcine in air to obtain the oxide. Oxides of specific stoichiometry would be obtained by subsequent experimental treatment (e.g., hydrogen reduction of Am02 to obtain the sesquioxide). However, in many cases microtechniques are necessary for the preparation of actinide oxides. In some situations these techniques may require novel approaches to the preparation and study of the oxides. Such techniques may also limit the depth and accuracy of the study. 

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. 

AnXe, and some representative data for these are given in Table XII. The thermal stability of the halides toward reduction of higher oxidation state actinides decreases with increasing atomic number of the halogen. 

With Ta as the reductant, the actinide starting material may instead be the carbide, previously prepared by the carbo-reduction of the oxide. This process is an attractive alternative for producing Pu and Cm because in the preparation of the carbide from the oxide a large number of impurities are eliminated by vaporization. Table 19.4 presents a brief summary of commonly used preparation methods. 

In processes involving oxidation and reduction of actinide species, slow kinetics are quite common. Generally, reactions that involve the formation or rupture of the strongly covalent metal-to-oxygen bonds in the ions MOj and MOJ are slow. On the other hand, reactions involving only an electron transfer, besides relatively minor adjustments of the hydration shells, are apt to be fast and readily reversible. As is often found in kinetic measurements, however, the rates may differ widely between reactions which are seemingly analogous. 

Table 6 presents a summary of the oxidation—reduction characteristics of actinide ions (12—14,17,20). The disproportionation reactions of UO2, Pu , PUO2, and AmO are very compHcated and have been studied extensively. In the case of plutonium, the situation is especially complex four oxidation states of plutonium [(111), (IV), (V), and (VI) ] can exist together ia aqueous solution ia equiUbrium with each other at appreciable concentrations. 

Barium reduces the oxides, haUdes, and sulfides of most of the less reactive metals, thereby producing the corresponding metal. It has reportedly been used to prepare metallic americium via reduction of americium trifluoride (13). However, calcium metal can, in most cases, be used for similar purposes and is usually preferred over barium because of lower cost per equivalent weight and nontoxicity (see Actinides and transactinides). 

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