Uranium tetrafluoride reduction

Reduction of uranium tetrafluoride by magnesium metal has been described in detail (40,53). It is often referred to as the Ames process, since it was demonstrated at the Ames Laboratory in early 1942. The reaction is very exothermic and the reduction process is carried out in a sealed bomb due to... 

Bismuth pentafluoride is an active fluorinating agent. It reacts explosively with water to form ozone, oxygen difluoride, and a voluminous chocolate-brown precipitate, possibly a hydrated bismuth(V) oxyfluoride. A similar brown precipitate is observed when the white soHd compound bismuth oxytrifluoride [66172-91 -6] BiOF, is hydrolyzed. Upon standing, the chocolate-brown precipitate slowly undergoes reduction to yield a white bismuth(Ill) compound. At room temperature BiF reacts vigorously with iodine or sulfur above 50°C it converts paraffin oil to fluorocarbons at 150°C it fluorinates uranium tetrafluoride to uranium pentafluoride and at 180°C it converts Br2 to bromine trifluoride, BrF, and bromine pentafluoride, BrF, and chlorine to chlorine fluoride, GIF. It apparently does not react with dry oxygen. 

Fluorides are nonhygroscopic, and their melting points are higher than those of the corresponding chlorides. Besides, the fluoride reduction reactions are considerably more exothermic. The prime examples of the use of fluorides as intermediates are the reduction of uranium tetrafluoride by calcium or magnesium the reduction of rare earth fluorides by calcium, reduction of beryllium fluoride by magnesium and the reduction of potassium tantalum double fluoride by sodium. 

Magnesium can be obtained in high purity at low cost. It is the preferred reducing agent whenever feasible. The reduction of uranium tetrafluoride by magnesium involves the reaction... 

The reduction of uranium tetrafluoride by calcium, in accordance with the reaction... 

Excer A process for making uranium tetrafluoride by electrolytic reduction of a uranyl fluoride solution, precipitation of a uranium tetrafluoride hydrate, and ignition of this. 

Metallic uranium can be prepared from its oxides or hahdes by reduction at high temperature. Uranium dioxide, UO2, or other oxides such as UO3 or UsOs may be reduced to uranium metal by heating with carbon, calcium or aluminum at high temperatures. Similarly, uranium tetrafluoride or other halides can be reduced to metal by heating with sodium, potassium, calcium, or magnesium at high temperatures. Alternatively, uranium tetrafluoride mixed with fused alkali chlorides is electrolyzed to generate uranium metal. 

M. H. West, M. M. Martinez, J. B. Nielson, D. C. Court, and Q. D. Appert, Synthesis of Uranium Metal Using Easer-Initiated Reduction of Uranium Tetrafluoride by Calcium Metal, LA-12996-MS, Los Alamos National Laboratory, N.M., 1995. 

Uranium Tetrafluoride, Uranous Fluoride, UF, is the chief product obtained when the metal is acted upon by fluorine. It may be prepared by the action of hydrogen fluoride on urano-uranic oxide or on uranous oxide or by reduction of a solution of uranyl fluoride with stannous chloride. It is also formed with uranium hexafluoride when the pentachloride is acted upon by fluorine at —40° C. thus ... 

Uranium tetrafluoride is a key intermediate in the production of thermal reactor fuels. It may be prepared directly from uranyl solutions by reduction of the U to U and addition of HF to precipitate UF4. A number of processes have been developed to produce UF4 by this wet route, which may be used to produce UF4 at the ore processing site. These employ iron, S02/Cu or electrolysis for the reduction step, the latter being preferred since it introduces no contaminants into the solution. The reduction of u in various media has been studied to assess the effect of complexation on the reduction reaction. The standard potential for the reduction of U02 to in 1 M HCIO4 has been given as +0.32 V. " The overall formation constants of fluoride complexes in 1 M NaCl were found to be log 82= 13.12, logj83 = 17.46 and log/84 = 21.8. Although wet processes have been developed as a short cut to UF4, the most widely used process at present involves dry processing. 

Derivation Finely ground ore is leached under oxidizing conditions to give uranyl nitrate solution. The uranyl nitrate, purified by solvent extraction (ether, alkyl phosphate esters), is then reduced with hydrogen to uranium dioxide. This is treated with hydrogen fluoride to obtain uranium tetrafluoride, followed by either electrolysis in fused salts or by reduction with calcium or magnesium. Uranium can also be recovered from phosphate sand. 

It is on occasion necessary to prepare uranium tetrafluoride from uranium hexafluoride, particularly when dealing with material highly enriched in U285. The conversion of uranium hexafluoride to the tetrafluoride has has therefore attracted considerable study, and the present situation is summarized by Smiley and Brater 80). Uranium hexafluoride can be reduced with hydrogen. The thermodynamic equilibrium, even at room temperature, is in favor of the formation of uranium tetrafluoride, but the reaction requires a high energy of activation to inaugurate reduction. The... 

Since a single furnace is used for three dryway operations, calcination, hydrogen reduction and hydrofluorination, the transfer of solids is minimized. The initial charging of trays and loading these into the furnace is relatively free from dust hazard since the ammonium di-uranate feed material is damp. Special precautions are needed, however, for removal of the final uranium tetrafluoride product from the trays, since it is a dry dusty powder and tends to adhere to the trays to some extent. This operation is therefore carried out in a small glove box , i.e. in a sealed and vented... 

Harper, J. and Williams, A. E. Factors Influencing the Magnesium Reduction of Uranium Tetrafluoride. I.M.M. Symposium on the Extraction Metallurgy of Some of the Less Common Metals, London (1956). 

Wilhelm, H. A. The preparation of uranium metal by the reduction of uranium tetrafluoride with magnesium. Proc. 1st Int. Corf, on the Peaceful Uses of Atomic Energy, Geneva, 1955. Paper 817. 

Preparation of uranium metal. As discussed previously, some nuclear power plant reactors such as the UNGG type have required in the past a nonenriched uranium metal as nuclear fuel. Hence, such reactors were the major consumer of pure uranium metal. Uranium metal can be prepared using several reduction processes. First, it can be obtained by direct reduction of uranium halides (e.g., uranium tetrafluoride) by molten alkali metals (e.g., Na, K) or alkali-earth metals (e.g.. Mg, Ca). For instance, in the Ames process, uranium tetrafluoride, UF, is directly reduced by molten calcium or magnesium at yoO C in a steel bomb. Another process consists in reducing uranium oxides with calcium, aluminum (i.e., thermite or aluminothermic process), or carbon. Third, the pure metal can also be recovered by molten-salt electrolysis of a fused bath made of a molten mixture of CaCl and NaCl, with a solute of KUFj or UF. However, like hafnium or zirconium, high-purity uranium can be prepared according to the Van Arkel-deBoer process, i.e., by the hot-wire process, which consists of thermal decomposition of uranium halides on a hot tungsten filament (similar in that way to chemical vapor deposition, CVD). 

The light actinide metals (Th, Pa, and U) have extremely low vapor pressures. Their preparation via the vapor phase of the metal requires temperatures as high as 2375 K for U and 2775 K for Th and Pa. Therefore, uranium is more commonly prepared by calciothermic reduction of the tetrafluoride or dioxide (Section II,A). Thorium and protactinium metals on the gram scale can be prepared and refined by the van Arkel-De Boer process, which is described next. 

Production of metallic uranium from its hexafluoride in a conventional nuclear fuel technology is a two-stage process. The first stage is an exothermic reduction of hexafluoride to tetrafluoride ... 

The process of calcination of oxides, hydrogen reduction of oxides to lower oxides, hydrofluorination and hydrochlorination of oxides, etc., can often be carried out by conventional chemical engineering methods. However, the high standards of purity usually required for rare metal extraction at least necessitate novel materials of construction. In addition, a considerable development and pioneering effort has been devoted to the improvement of these techniques, particularly applied to the intermediates in the production of uranium metal, uranium trioxide, dioxide and tetrafluoride. It is possible, therefore, that the resulting processes can be more widely employed in the rare metal extraction field in the future. 

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