Patent Number: 053902181
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT As the result of applying himself to research for attaining this object, the present inventor has found that, as a method of obtaining a easily press moldable soft dry gel particle, that one can prevent the gel particle becoming a particle in the hard glass state by sufficiently washing the gel particle using an organic solvent miscible with water such as alcohol to substitute water therewith and thereafter removing the organ ic solvent used, and that the press moldability can be improved by moistening again the dry heat treated gel, and has invented the present invention based on this knowledge. According to the drying method of the present invention, a low density of soft gel particle as below 29% T.D. can be obtained while the density of dry gel particle obtained by the prior drying method is about 35% T.D. And, according to the method of the present invention in which a dry gel particle is moistened again before press molding, the water content in the moistened gel particle plays the part of lubricant which is added for the purpose of fluidity and adhesive in case of ordinary powder molding, and further plays a part of caking agent, and the moldability can be improved. The present invention will be further explained with Examples by which the present invention is not limited. EXAMPLE 1 [Preparation and gelation of nuclear fuel sol] A sol of 1.0 mol/1 in thorium concentration was prepared by a method described in Literature 6 in which ammonia gas is added to a heated aqueous solution of thorium nitrate to obtain a sol of colloidal thorium oxide. This sol was divided into droplets of 0.3 mm in diameter using a vibration nozzle in air and was gellated in ammonia gas and continuously in ammonia water to obtain microspherical gel particles in which ammonium nitrate contained was washed out with dilute ammonia water. [Drying of gel particle] The water content in the gel particle was substituted with isopropyl alcohol to below 1 wt. % of balance water content concentration in isopropyl alcohol and then the gel particle was separated from isopropyl alcohol. And, after removing most of isopropyl alcohol by vacuum vaporization, the gel particle was dried in air of 80.degree. C. The water content in gel particle in this stage was 10% by weight. This particle was heated in air in 450.degree. C. for 3 hours to make the water content about 0%. The density of this particle was about 29% T.D. This dry particle was used as a raw material for press moldability. [Moistening of particle - Press molding - Sintering] The particles once dried were moistened to each value described in Table 2 (density of thorium oxide pellet -unit : T.D. ) using a thermo-hygrostat. TABLE 2 ______________________________________ Pressure Water content of moisture added (Wt %) (MPa) 0 5 10.about.12 15.about.16 ______________________________________ 150 29.4 33.5 33.7 84.6 94.1 93.8 200 32.5 38.7 90.1 96.8 300 37.6 [43.4]*.sup.2 42.1 39.2 [94.9]*.sup.1 [97.6]*.sup.2 96.7 96.0 400 42.1 98.6 500 [45.8]*.sup.2 43.4 [97.8]*.sup.2 98.6 ______________________________________ *.sup.1 : For comparison the weight of water content of moisture added wa left out of account in the calculation of green density. *.sup.2 : [ ] shows one cracked. The die used for the press mold is 7 or 10 mm in diameter and slightly tapered for easily drawing out therefrom. An alcohol solution of stearic acid was used as a lubricant for dies. It was press molded within the range of 0.about.15-16% in water content added for moistening and the range of 150-500 MPa. This green density is shown in the upper column of Table 2. This green pellet was heated to 500.degree. C. in moistened air and then to 1300.degree. C. in air and, after keeping for 3 hours, was cooled. The density of sintered pellet so obtained is shown in the lower column of Table 2. EXAMPLE 2 A dry gel particle of about 25% T.D. in density and 3% in remained water content was prepared according to the same method as Example 1 exept using uranium for 10% of thorium. This particle was moistened to about 15% water content and press molded under 300 MPa. This green pellet was 40% T.D. in density. After sintering under the same condition as Example 1, it was reduced in a mixed gas of argon and hydrogen at 1300.degree. C. to obtain a pellet of mixed thorium and uraniumm oxides (Th.sub.0.9 U.sub.0.1 O.sub.2) of 97% T.D. in density. The dry gel particle obtained by the process of the present invention is not only low in density and soft but also is so excellent in sinterability that it can be sintered to a high density of particle even in case of sintering as it is. Therefore, even a low density of green pellet as below 40% T.D. press molded under a low pressure as 300 MPa is so effective that it can be sintered to a high density of pellet as above 95% T.D. at a low temperature as 1300.degree. C. And it is effective that it can be press molded to a high density of pellet, comparing with the case of 0% in water content under the same pressure, even without adding an additive, according to the process of the present invention in which, after moistening again a dry gel particle to a water content of about 10-15%, it is press molded (refer to the case of below 300 MPa in Table 2). And the water content of moisture plays a part of caking agent which prevents the occurrence of crack on mold. In Table 2, a remarkable effect is recognized in case of 300 MPa and 500 MPa. Even in low pressure in which such effect is low finally a high density of sintered pellet can be obtained.