Production of Uranium Metal

Production of uranium metal suffidently pure for use in nuclear reactors is difficult. Uranium forms very stable compounds with oxygen, nitrogen, and carbon, and it reduces the oxides of many common refractories. Methods that yield uranium metal at temperatures below its melting point result in a fine powder that oxidizes rapidly in air and is difficult to consolidate into massive metal. Uranium cannot be deposited electrolytically from aqueous solution. It is not practical to purify uranitun by distillation because of its very high boiling point, 3900° C. Any nonvolatile impurities introduced into uranium during production will remain in it during subsequent operations and contaminate the final product. 

Four methods that have been used to produce utaniiun metal are 

Qectrolysis. Qectrdysis of KUFj or UF4 dissdved in a molten mixture of 80 percent CaQj and 20 percent NaQ was the method used by the Westinghouse Electric Company to produce the first pure uraniiun metal for the Manhattan Project [S4]. Because electrdysis was carried out below the melting point of uranium metal, IISO C, the crude metal contained salt that had to be leached with water and had to be remelted before acceptably pure metal could be obtained. By 1943 this method was superseded by the less costly reduction of UF4 by magnesium, to be described later. 

Possible leductants. Elements that might be considered for reducing UO2, UF4, or UCI4 to metallic uranium are hydrogen, sodium, magnesium, or calcium. Carbon is impractical because of formation of uranium carbide, and aluminum is undesirable because it forms an intermetallic compound with uranium. Sodium, magnesium, and calcium do not do this. 

To show which combinations of uranium compound and reductant are thermodynamically favorable, the ftee nergy change in reducing UOj, UF4, or UG4 by hydrogen, sodium, calcium, or magnesium has been evaluated in Table 5.29. These data are for a temperature of 

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Fluorides. Uranium fluorides play an important role in the nuclear fuel cycle as well as in the production of uranium metal. The dark purple UF [13775-06-9] has been prepared by two different methods neither of which neither have been improved. The first involves a direct reaction of UF [10049-14-6] and uranium metal under elevated temperatures, while the second consists of the reduction of UF [10049-14-6] by UH [13598-56-6]. The local coordination environment of uranium in the trifluoride is pentacapped trigonal prismatic with an 11-coordinate uranium atom. The trifluoride is... 

Uranium dioxide occurs in mineral uraninite. Purified oxide may be obtained from uraninite after purification. The commercial material, however, also is recovered from other uranium sources. Uranium dioxide is obtained as an intermediate during production of uranium metal (See Uranium). Uranyl nitrate, U02(N03)2, obtained from digesting the mineral uraninite or pitchblende with concentrated nitric acid and separated by solvent extraction, is reduced with hydrogen at high temperatures to yield the dioxide. 

The production of uranium metal from purified uranium dioxide ore consists of the following steps ... 

Table 5.29 Free-energy diange in production of uranium metal...

Production of Uranium Metal by Reduction of UF4 with Magnesium... 

In the production of uranium metal by reaction of UF4 with Mg, a 5 percent excess of magnesium is added to ensure complete reaction of UF4. What is the minimum temperature to which the reactants should be preheated to ensure complete melting of the products at 1536 K The enthalpy change of magnesium between 298 and 1536 K is 41.57 kcal/g-mol. 

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. 

Grainger, L. Production of uranium metal. Proc. 1st Int. Conf. on the Peaceful Uses of Atomic Energy, Geneva, 1955. Paper 407. 

PRODUCTION OF URANIUM METAL BY ELECTROLYSIS Uranium metal was produced in the early 1930 s by the electrolysis of the fluoride in a molten salt bath, at the U.S. Westinghouse Electric... 

Buddery, j. H. and Hedger, H. J. Production of uranium metal by electrolysis. Progress in Nuclear Energy, Ser. V, Metallurgy and Fuels, vol. 1. Pergamon Press (1956). 

The formation of the halides of Ca and Mg are favored over the formation of the halides of uranium at all temperatures up to 1,500°C therefore, the reaction of a uranium tetrahalide with Ca° or Mg° will proceed exothermically to produce uranium metal. However, UCI4, UBr4, and UI4 are deliquescent and air-reactive practicality dictates that the production of uranium metal involves the reaction of UF4 with either Mg° or Ca° ... 

The methods of producing plutonium metal are similar to those used in the production of uranium metal. The best reactants are PUF4 and either Ca or Mg . PuFs can also be reduced by the alkaline earth metals imder an argon atmosphere ... 

More then 150 minerals containing uranium are known. Typically, however, uranium ores contain only about 0.1% uranium. In the commercial production of uranium metal, the ore is crushed, concentrated, roasted, and in most cases leached with sulfuric acid in the presence of an oxidizing agent such as manganese dioxide or chlorate ions to convert all of the uranium to uranyl sulfato complexes. Carbonate leaching is used to extract uranium from ores containing minerals such as calcite. The recovery of uranium... 

The production of uranium metal usually involves the reduction of UF4 with magnesium at 700°C. The metal may be refined by molten-salt electrolysis followed by zone melting. Because of the low melting point of uranium, the van Arkel process is not as feasible as for thorium and protactinium. 

One of the first jobs to be set into motion when the UK atomic programme began was the production of uranium metal from its ore, and an old wartime factory at Springfields near Preston, in Lancashire, was converted for the purpose. The steps in the production of fuel rods are shown in the graphic in Figure 2.5. 

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