Patent Number: 051397099
Section: summary

BACKGROUND OF THE INVENTION Uranium dioxide for the manufacturing of current light water reactor fuel is currently produced from the conversion of UF.sub.6, mainly based either on a dry- or a wet-conversion process. Several routes of the dry-conversion process have been revealed so far, and chemical procedures involved in those routes are similar. UF.sub.6 is usually pyrohydrolyzed with steam to form UO.sub.2 F.sub.2 powder which is reduced to UO.sub.2 directly by a hydrogen-steam mixture, or is calcined in air to U.sub.3 O.sub.8 first and then reduced to UO.sub.2 with a hydrogen-steam gas. In the wet-conversion process, vaporized UF.sub.6 is hydrolyzed with water to form an aqueous UO.sub.2 F.sub.2 -HF solution, from which ammonium diuranate (ADU) or ammonium uranyl carbonate (AUC) is precipitated with ammonia water or ammonium carbonate, respectively. After filtration, ADU or AUC is calcined to UO.sub.3, which is then reduced to UO.sub.2 with a hydrogen-steam gas. According to the chemical compositions of the precipitates, it is called an ADU process or an AUC process. It is recognized that the UO.sub.2 powder produced from the wet-ADU process possesses excellent powder characteristics required for pelletizing and sintering, and gives good microstructure to the sintered pellet. Although the ADU process is widely used currently, it is plagued by some inherent drawbacks. For example, in the conventional ADU process, such as that disclosed in the U.S. Pat. Nos. 3,394,997 and 3,998,925, UF.sub.6 is hydrolyzed with water to form an aqueous solution containing 100 to 200 g/l of uranium and 0.4 to 0.8 mol/l of hydrogen fluoride. As ADU is precipitated from this solution, a pasty slurry is obtained and several tens of liters of the fluoride-containing liquid filtrate is thus generated for the production of 1 kg UO.sub.2. This introduces a serious problem of liquid waste disposal to the conventional ADU process. Moreover, because ADU is a kind of slimy cake, the process also involves a complicated filtration operation. After a series of studies on the formation of ADU, it was found that ADU is formed simultaneously as soon as fine droplets of a concentrated solution of an uranyl compound are introduced into an ammonia gas stream, and the fluorine content of the UO.sub.2 powder consequently produced using uranyl fluoride solution as a feed can be lower than 50 ppm. Therefore, instead of being precipitated from a dilute solution of uranyl compounds with ammonia water, ADU is prepared in particle form directly by introducing atomized droplets of a concentrated solution of uranyl compound into an ammonia gas stream in the novel process disclosed herein. The generation of the fluoride-containing liquid filtrate in converting uranyl fluoride to UO.sub.2 is thus avoided, and filtration operation is no longer necessary. The process is thus greatly simplified. SUMMARY OF THE INVENTION It is the object of the present invention to provide a process generating no liquid filtrate and involving no tedious filtration operation for converting UF.sub.6 or uranyl compounds to UO.sub.2 via ADU. Due to the simple process variables involved in this new process, the UO.sub.2 powder produced inherently possesses much higher consistency in quality than those produced with the conventional wet-ADU process. To achieve its object, this invention provides a process for converting UF.sub.6 to UO.sub.2 powder comprising the steps of (a) pyrohydrolyzing UF.sub.6 with steam to obtain UO.sub.2 F.sub.2 powder; (b) dissolving the said UO.sub.2 F.sub.2 powder in water to form an aqueous uranyl solution; (c) atomizing the said aqueous solution into a gas stream of ammonia gas or ammonium hydroxide to prepare wet ADU particles; (d) drying and calcining the said ADU particles directly to UO.sub.3 or U.sub.3 O.sub.8, or their mixture; (e) reducing the said calcined particles to UO.sub.2 with hydrogen or hydrogen-steam gas. Accordingly, the present invention also provides a process for converting other uranyl compounds which form ADU with ammonium hydroxide, such as uranyl nitrate, uranyl sulfate, uranyl chloride, and etc., to UO.sub.2, comprising dissolving the uranyl compound in water as the first step and the foregoing steps of (c) to (e), whether or not additional metal species is incorporated into the aqueous solution of uranyl compound. Instead of precipitating ADU from diluted uranyl solution with ammonia water in the conventional wet-ADU process, in the present invention, ADU is made by reacting gaseous ammonia or ammonium hydroxide vapor with a rather concentrated solution of uranyl compound. Basically, uranyl solution of any concentration can be used to prepare the ADU powder directly with the present invention, but only from those with high uranium concentration can the ADU be obtained as a divided wet particle rather than a slimy slurry. Nevertheless, dry ADU particles can be obtained directly in all cases by heating the wet ADU particle before settling, just as is usually done in spray drying. The particle thus obtained is of easy-easy-handling and free flowing. No filtration operation is involved and no liquid filtrate is generated in the present invention. The only liquid effluent coming out of the ADU preparation is a limited amount of water condensate recovered in the drying of ADU which it is free from uranium and is re-usable in the dissolution of UO.sub.2 F.sub.2 powder. DESCRIPTION OF THE INVENTION In conducting this invention, UF.sub.6 in a cylinder is vaporized by heating in a water bath. The vapor is then introduced to a tube reactor, where it is pyrohydrolyzed with steam to carry out the reaction: EQU UF.sub.6 +2H.sub.2 O=UO.sub.2 F.sub.2 (HF).sub.n +(4-n)HF (1) With a careful control of the flow rates of UF.sub.6 and steam, finely divided UO.sub.2 F.sub.2 powders are obtained and collected at the bottom of the reactor. HF gas produced in reaction (1) may be neutralized in an alkali scrubber or recovered as a by-product after passing through a sintered-metal filter assembly. The UO.sub.2 F.sub.2 powder obtained is dissolved in de-ionized water to prepared UO.sub.2 F.sub.2 solution. The solution is then atomized to form very small liquid droplets with an atomizer, such as: an impingement type nozzle, or a single-fluid nozzle, or a double-fluid nozzle, or an ultra sonic atomizer, on the top of a spray column. To the bottom of the column, ammonia gas is introduced to react with the liquid droplets of UO.sub.2 F.sub.2 as follows: ##EQU1## As shown, ammonia gas is absorbed by water in the droplets to form NH.sub.4 OH, which then reacts with UO.sub.2 F.sub.2 to form ADU following reaction (3). It is generally recognized that there are four types of ADU, i.e., type I, II, III, and IV, with the value of x expressed in reaction (3) equal to 0, 1/3, 1/2, and 2/3, respectively. Except type I, all other types of ADU may be included in the product of the present invention with their molar ratios depending on the operating conditions, such as UO.sub.2 F.sub.2 concentration in feed solution; the pressure of ammonia gas; the drop size of the aqueous solution of UO.sub.2 F.sub.2 ; and the resident time. Generally, a high pressure of ammonia gas is good for the formation of the high type ADU. However, too high a pressure may cause some operational troubles. A small droplet size of the uranyl compound solution will increase the rate of the formation of ADU, and will be good for the formation of the high type ADU too. However, a droplet too small in size may cause some problems in separating ADU from gas stream. Additional heat may be applied to increase the reaction temperature to facilitate the formation of ADU, and to accomplish a quick removal of the moisture from the ADU product. Ammonium uranyl fluoride (AUF) is a precursory product in the reaction, and it may exist in the ADU product, when a feed solution having a very high concentration of uranium is used, or when the ammoniation is not sufficiently done. Nevertheless, the presence of AUF in the ADU mixture will not give any trouble in converting all the uranium species to UO.sub.2, since AUF, as well as ADU, is also decomposed to form uranium oxide on calcining. The formation reaction of ADU is exothermic, therefore, part of the water in the liquid UO.sub.2 F.sub.2 droplets is vaporized during the formation of ADU. Meanwhile, some of the water becomes a constituent part of ADU. Therefore, a feed solution of UO.sub.2 F.sub.2 having an uranium concentration higher than 500 g/l, or preferably higher than 600 g/l, will give wet finely divided ADU particles in the reaction without applying additional heat. When a less concentrated feed solution is used, the ADU product obtained is no longer divided particles, but is paste-like. Nevertheless, the stream of the wet ADU particles can be heated before settling, so as to remove the moisture to obtain a free flowing dry powder, directly, just as the way usually done in a spray drying. It is preferrable, for simplifing the operation, to carry out the drying and the calcining steps together in a drying-and-calcining step at 300.degree. to 750.degree. C., or preferably at 400.degree. to 600.degree. C. Ammonium fluoride vaporized in the step is separated from water vapor by condensing it at a temperature ca. 105.degree. C. The ammonium fluoride thus recovered is free from uranium, and is readily a valuable resource of fluorine. The water vapor is condensed and recycled for the dissolution of UO.sub.2 F.sub.2 powder. Clearly, no liquid filtrate is generated, and no complicated filtration operation is involved in the preparation of ADU, if the method of the present invention is used. Produced under an atmosphere of nitrogen gas, the calcined product is essentially UO.sub.3, which is then reduced to UO.sub.2 in a reduction furnace with a hydrogen-steam mixture at 500.degree. to 850.degree. C., or preferably at 550.degree. to 650.degree. C. In the reduction furnace, the residual fluoro species interact with steam to form HF and then leave the product. The UO.sub.2 thus obtained is a finely divided powder having low fluorine content, high activity, and good sinterability. Besides uranyl fluoride, other uranyl compounds such as uranyl nitrate, uranyl chloride, uranyl sulfate, and etc., can also be used to prepare ADU with the present method. Furthermore, the present invention is also applicable to the preparation of mixed metal oxides containing uranium, for instance, the mixed oxide of uranium and gadolinium can be made, if a solution containing uranyl nitrate and gadolinium nitrate is used as the feed solution. The following examples illustrate the present invention. It is understood that they are only exemplary and do not limit the scope of the present invention.