Process for obtaining uranium trioxide by direct thermal denitra

Details Process for obtaining uranium trioxide by direct thermal denitra Process for obtaining uranium trioxide by direct thermal denitra

1996-02-28

1997-05-06

Miller, Edward A.

Chemistry of inorganic compounds

Radioactive

Uranium compound

423 19, C01G 4301

Patent

active

056280483

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for obtaining uranium trioxide from uranyl nitrate by the thermal decomposition of hexahydrated nitrate.
In its fuel cycle, uranium can pass through the compound UF.sub.4, which is used for two purposes. Uranium tetrafluoride makes it possible to produce metal uranium for the supply of natural uranium reactors. It also makes it possible to obtain the hexafluoride UF.sub.6, which is the supply material for gaseous diffusion or ultracentrifuging-based enrichment plants.
The conversion of uranium into UF.sub.4 requires a purification stage because the uranium concentrates supplied to the refining plants still contain many impurities. During this purification stage, the uranium concentrate is dissolved in nitric acid in order to give an impure uranyl nitrate, which is purified by tributyl phosphate (TBP). The TBP makes it possible to extract pure uranyl nitrate from the acid solution.
Once in the possession of pure uranium in the nitrate state in aqueous solution, its solid form must be restored in order to transform it, in accordance with the dioxide needs, into tetrafluoride or metal. As the uranium is at valency VI, the necessary intermediate will be the trioxide UO.sub.3. By reduction it is then possible to pass to UO.sub.2 and by hydrofluoric acid action UF.sub.4 is then obtained.
There are two means for passing from the uranyl nitrate in solution to the oxide UO.sub.3. A first means consists of adding ammonia in order to obtain by precipitation ammonium diuranate (ADU), which is thermally decomposed into UO.sub.3. A second means consists of thermally decomposing the uranyl nitrate after dehydration.
The oxide UO.sub.3 does not have the same properties when obtained by the first or second means. The first means leads to a very reactive oxide, i.e. having a specific surface of approximately 15 m.sup.2 /g. The second means leads to an only slightly reacting oxide, whose specific surface is 1 to 2 m.sup.2 /g.
The compound UF.sub.4 can be obtained from the oxide UO.sub.3 by different processes. One particularly interesting process is that using a fluid bed or FB furnace. This furnace, which combines a reduction furnace and a hydrofluorination furnace, makes it possible to successively perform two reactions. Firstly the oxide UO.sub.3 reacts hot with the thermally cracked ammonia in order to be reduced to the oxide UO.sub.2, which is then contacted with counter-current-injected, gaseous hydrofluoric acid. On leaving the furnace the compound UF.sub.4 is obtained. The particular arrangement of the top or head of the hydrofluorination furnace makes it possible to bring about a displacement of the equilibrium reaction by progressively lowering the temperature and consequently the hydrofluoric acid is integrally absorbed. Among the advantages of this furnace reference can be made to the elimination of the hydrofluoric acid excess, which will no longer appear in the effluents.
The FB furnace is an effective apparatus provided that it is supplied with the most reactive possible oxide UO.sub.3. A uranium trioxide having a specific surface between 12 and 15 m.sup.2 /g, on leaving the furnace, gives a product having satisfactory characteristics. Typically this product will have the following composition: 96% UF.sub.4, 2% UO.sub.2 and 2% UO.sub.2 F.sub.2. When treated in a flame reactor in the presence of fluorine, it will give a uranium hexafluoride having a satisfactory quality.
Thus, the FB furnace must be supplied by an oxide UO.sub.3 of adequate reactivity. For example, an oxide UO.sub.3 with a specific surface of 5 m.sup.2 /g leads to the production of a uranium hexafluoride which does not comply with industrial standards. Under these conditions the FB furnaces have hitherto been supplied by oxide UO.sub.3 obtained by the first means, i.e. that using ammonia for obtaining ammonium diuranate by precipitation.
However, this solution suffers from a number of disadvantages. On the one hand it requires the consumption of ammonia. On the other the precipitation reaction also sup

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patent: 4687601 (1987-08-01), Bachelard et al.
"Production of Uranium Dioxide by Flame Denitration", W. H. Hedley et al., ol. 3, No. 1, Jan., 1964, I&EC Process Design and Development, pp. 11-14.
Patent Abstracts of Japan, vol. 7, No. 28 (C-149), 4 Feb. 1983 & JP A 57 183327 (Mitsubishi Kinzoku).
Proceedings of the International Symposium on Fluidization, 6 Jun. 1967 Eindhoven, pp. 769-781, Rodrigo Otero, "Fluidized Bed Calcination of Uranyl Nitrate Solutions".

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