Thorium nitrate, calcination

The oxalates obtained above, alternatively, are digested with sodium hydroxide converting the rare earth metals to hydroxides. Cerium forms a tetravalent hydroxide, Ce(OH)4, which is insoluble in dilute nitric acid. When dilute nitric acid is added to this rare earth hydroxide mixture, cerium(lV) hydroxide forms an insoluble basic nitrate, which is filtered out from the solution. Cerium also may be removed by several other procedures. One such method involves calcining rare earth hydroxides at 500°C in air. Cerium converts to tetravalent oxide, Ce02, while other lanthanides are oxidized to triva-lent oxides. The oxides are dissolved in moderately concentrated nitric acid. Ceric nitrate so formed and any remaining thorium nitrate present is now removed from the nitrate solution hy contact with tributyl pbospbate in a countercurrent. 

The acidic and basic properties are dependent upon the preparation and activation methods. Thorium oxide is usually prepared from aqueous solution of thorium nitrate or chloride by precipitation with aqueous ammonia followed by washing, and calcining. In some cases, Th02 is prepared by thermal decomposition of thorium nitrate or thorium oxalate. The acidic and basic properties of the Th02 prepared from ThCU are distinctly different from the other Th02. The catalytic activities of Th02 prepared by different methods for 1-butene isomerizaton and 2-butanol dehydration are summarized in Table 3.5.The ThOz prepared from cholride completely lacks the measur-... 

A third method for the preparation of slurry oxide is the thermal decomposition of thorium formate [28]. In this procedure, thorium nitrate in solution is decomposed on adding it to concentrated formic acid at 95°C [29,30]. The precipitated thorium formate is washed free of excess acid and decomposed by calcination at 500 to 800 C. The oxide from the formate procedure is similar in its slurry behavior to that produced by thorium oxalate thermal decomposition however, less is known about its handling characteristics. Because of this, the oxalate preparation method is preferred at the present time. 

Effect of preparation variables on the particulate properties of thorium oxide. The effects of thorium oxalate precipitation temperature, calcination temperature, and calcination time on oxide properties were initially investigated by Allred, Buxton, and McBride [42]. Oxalate was precipitated at 10, 40, 70, and 100°C from a 1 M thorium-nitrate solution by dropwise addition of oxalic-acid solution and vigorous stirring. The precipitates were fired at 400°C for 16 hr and successively at 500, 650, 750, and 900°C for 24 hr. Electron micrographs of the oxide products showed particles of the approximate size and shape of the original oxalate particles from which they were formed. The particles of oxide prepared from 10°C-precipitated material were approximately 1 micron in size and appeared quite uniform those from the 40°C material were 1 to 2 microns in size and less uniform. A marked increase in particle size was observed for the oxide particles prepared from the 70°C- and 100°C-precipitated materials, which were... 

The principal source of thorium is monazite (p. 425), a phosphate of cerium and lanthanum with up to 15% of thoria. It is dissolved in concentrated sulphuric acid and the thorium phosphate precipitated with magnesium oxide. The washed phosphate heated with sodium carbonate gives crude thoria, ThOg, which is converted to the soluble oxalate and separated from the insoluble oxalates of cerium and lanthanum. After ignition to oxide the nitrate is made, purified by recrystallisation, and again calcined to thoria. 

There are several ways of coprecipitating UO2 and Th02 (Belle and Berman 1984 Radford and Bratton 1975). AECL s experience in coprecipitation has focused on the addition of ammonia to nitrate solutions. In this process, uranium and thorium are dissolved into a nitrate solution to form U02(N03)2 and Th(N03)4. Ammonia is added to the solution to precipitate (NH4)U207 (ammonium diuranate (ADU)) and Th(OH)4 (thorium hydroxide). The precipitate is calcined to form blended UO2 and Th02 powder, which is subsequently processed into fuel pellets. The micro structure and quality of pellets made by this process are generally very good. 

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