Reduction of uranium oxide

Preparation of Uranium Metal. Uranium is a highly electropositive element, and extremely difficult to reduce. As such, elemental uranium caimot be prepared by reduction with hydrogen. Instead, uranium metal must be prepared using a number of rather forcing conditions. Uranium metal can be prepared by reduction of uranium oxides (UO2 [1344-59-8] or UO [1344-58-7] with strongly electropositive elements (Ca, Mg, Na), reduction of uranium halides (UCl [10025-93-1], UCl [10026-10-5] UF [10049-14-6] with electropositive metals (Li, Na, Mg, Ca, Ba), electro deposition from molten... 

Cerous bromide [14457-87-5] CeBr, and praseodymium bromide [13536-53-3] PrBr, are claimed to be suitable for a molten salt bath used for the reduction of uranium oxide by magnesium (16). PrBr is claimed to be alight filter in a cathode ray tube (17). 

Figure 4.21 The equilibrium concentrations of HF in a mixture with H20 for the reduction of uranium oxide by hydrogen fluoride versus temperature.

Production of Pure Metallic Uranium by Carbothermic Plasma-Chemical Reduction of Uranium Oxides... 

Carbothermic Plasma-Chemical Reduction of Uranium Oxide (U3O8). Analyze the stoichiometry of the carbothermic reduction of U3O8 (7-24). Explain why the ratio of molar fractions of CO2 and CO in products of the process is not fixed. Find out the relation between the molar fractions of CO2 and CO in the products as a function of initial composition of the solid mixture U3O8-C. Explain why the carbothermic reduction process of UO2 (7-23) assumes only CO in products, while that of U3O8 (7-24) expects formation of CO and CO2. 

Because of the commercial importance of uranium, a number of methods for generating finely divided chemically reactive uranium metal have been developed. Pyrophoric uranium metal powders have been prepared by thermal decomposition of uranium amalgam [21-23] or uranium hydride [24, 25]. Many methods have involved reduction of uranium oxides [26]. Other methods employed are melt electrolysis [26] and potassium reduction of (i/ -C6H5)4U [27]. 

Using that approach, one might achieve as much as 75% filling of the pore space before the surface sealed off This would allow one to incorporate about 56 vol % carbide into the foam. Another route to incorporating uranium as uranium carbide is the earbothermal reduction of uranium oxide by carbon. This will result in at most about 30 vol % UC2 in the foam. 

To leave this framework keeping the screen of the models, we will consider the reduction of uranium oxide UsOg according to ... 

Preparation. Uranium metal may be prepared by several methods the reduction of uranium oxides with carbon In an arc-melting furnace reduction of uranium oxides with magnesium, aluminum, calcium or calcium hydride the reduction of uranium halides with alkali or alkaline-earth metals electrolytic reduction of uranium halides and the themal decomposition of uranium Iodide. 

Calcium metal is an excellent reducing agent for production of the less common metals because of the large free energy of formation of its oxides and hahdes. The following metals have been prepared by the reduction of their oxides or fluorides with calcium hafnium (22), plutonium (23), scandium (24), thorium (25), tungsten (26), uranium (27,28), vanadium (29), yttrium (30), zirconium (22,31), and most of the rare-earth metals (32). 

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... 

For the SOFC, for example, the use of uranium oxide has been studied but not adopted. The onset of current cut-off by concentration polarisation can be delayed, so that the maximum current obtainable is increased, a significant reduction of irreversibility. SOFC cathodes now almost universally use a coating of lithium strontium manganate, a joint electron/ion conductor. 

Oxidation-reduction conditions are important in the geologic transport and deposihon of uranium. Oxidized forms of uranium (U[VI]) are relatively soluble and can be leached from the rocks to migrate in the environment. When strong reducing conditions are encountered (e.g., presence of carbonaceous materials or H2S), precipitahon of the soluble uranium will occur. 

Pyrophoric Uranium.—It has already been mentioned (p. 278) that uranium, in a very finely divided condition, takes fire on exposure to air. The black powder obtained by reduction of the oxide by means of magnesium exhibits this property. Chesneau observed that the sparks detached from uranium by friction with hard steel would ignite mixtures of methane and air, as well as such inflammable liquids as alcohol and benzene, and he concluded that their temperature could not be below 1000° C. The jjroduct obtained by Eeree (see p. 279) by heating uranium amalgam in vacuo burned spontaneously in the air. Alloys of uranium and iron containing more than 20 per cent, of uranium are pyrophoric, the activity increasing with increase in uranium content. 

The temperature must not exceed 400°C, to prevent the formation of U3O8. The nitrous gases produced are processed to nitrie aeid, whieh is recycled. The subsequent reduction of uranium(VI) oxide to uranium(IV) oxide with hydrogen at 500°C also proceeds in the fluidized bed furnace. 

Uranium(IV) oxide is the starting material for uranium(lV) fluoride production in which uranium(lV) oxide is generally reacted with anhydrous hydrogen fluoride. This difficult to carry out exothermic reaction proceeds either in a fluidized bed, in moving bed reactors, or in screw-reactors. To achieve as complete as possible reaction in fluidized bed reactors, two fluidized bed reactors are connected in series. Screw-reaetors are also preferably connected in series. In moving bed reactors the reduction zone and the hydrofluorination are arranged above one another in a plant. The uranium(IV) oxide produced by the reduction of uranium(VI) oxide with hydrogen is very reactive and is eompletely reaeted with HF at temperatures between 500 and 650°C to uranium(lV) fluoride. 

The carbothermic reduction processes are usually strongly endothermic and require high temperatures. For example, carbothermic reduction of uranium (U), boron (B), zirconium (Zr), niobium (Nb), and tantalum (Ta) from their oxides requires 2000 000 K and, therefore, application of thermal plasma. In most plasma-chemical carbothermic reduction processes, an arc electrode is prepared from well-mixed and pressed oxide and carbon particles. The arc provides heating of the mixture, stimulating the reduction process on the electrode. Carbon oxides leave the electrode, finalizing the reduction process. 

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 most widely employed method for plutonium reprocessing used today in almost all of the world s reprocessing plants is the Purex (plutonium-uranium reduction extraction) process. Tributylphosphate (TBP) is used as the extraction agent for the separation of plutonium from uranium and fission products. In effecting a separation, advantage is taken of differences in the extractability of the various oxidation states and in the thermodynamics and kinetics of oxidation reduction of uranium, plutonium, and impurities. Various methods are in use for the conversion of plutonium nitrate solution, the final product from fuel reprocessing plants, to the metal. The reduction of plutonium halides with calcium proved to be the best method... 

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