Thorium oxalate, decomposition

Oxide products from thorium oxalate decomposition are normally characterized by their behavior as slurries. In addition, they have been characterized by means of electron micrograph pictures, their nitrogen adsorption surface areas, particulate properties as measured by sedimenta-... 

Thorium oxide, has the highest melting point of the usual ceramic materials (3390°C). It is used to form ceramics, Th02, as the so-called meta-Th02, freshly prepared by low temperature decomposition of thorium oxalate it is fairly soluble in acids and tends (especially in the presence of nitrate ions) to form colloidal solutions which can be dried to form stable gels that can be sintered to give high-density ceramic bodies. 

The compound also can be prepared by many other methods including thermal decomposition of thorium oxalate, hydroxide, carbonate, or nitrate. Heating thorium metal in oxygen or air, and hydrolysis of thorium halides also yield thorium dioxide. 

Thorium oxalate yielded [94] the carbonate below 593 K, but reactions at higher temperature yielded Th02 directly. The rate controlling process changed across the temperature interval studied (573 to 623 K). The decomposition of cadmium oxalate at 620 K may be represented [95] as ... 

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-... 

Preparation of thorium oxide. The principal method of preparing thorium oxide for use in aqueous slurries has been the thermal decomposition of the oxalate. Thorium oxalate, precipitated from thorium nitrate solution, is crystalline, easy to wash and filter, and the oxide product is readily dispersed as a slurry. In addition, the oxide particle resulting from oxalate thermal decomposition retains the relic structure of the oxalate, and hence the particulate properties are determined by the precipitation conditions. The mechanism by which the thermal decomposition takes place has been quite widely investigated [24-26]. The following is proposed by D Eye and Sellman [26] for the thermal decomposition ... 

A satisfactory oxide has also been prepared by the hydrothermal decomposition of thorium oxalate as an aqueous slurry in a closed autoclave at... 

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. 

Pig. 4-5. Relationship between average crystallite size and specific surface area for thorium oxide prepared by the thermal decomposition of thorium oxalate (400 to 900°C firings). 

Oxides from the hydrothermal decomposition of thorium oxalate. Oxides prepared by the hydrothermal decomposition of the oxalate [27] at 300°C in a closed autoclave were found to be markedly different in their characteristic properties from the thermally prepared materials. The precipitation temperature of the oxalate had no effect on the final shape or size, and all evidence of the original oxalate structure had disappeared. Sedimentation particle-size analyses indicated particle sizes between 0.5 and 1 micron. 

Fig. 4-6. Effect of oxalate decomposition method on thorium oxide surface area. (Prepared from oxalate precipitated at 100°C.)...

The decomposition products identified following reaction are not necessarily the primary compounds which result directly from the rate limiting step. Particularly reactive entities may rapidly rearrange before leaving the reaction interface and secondary processes may occur on the surfaces of the residual material which often possesses catalytic properties. The volatile products identified [144] from the decomposition of nickel formate were changed when these were rapidly removed from the site of reaction. The primary products of decomposition of thorium formate were identified [17] as formaldehyde and carbon dioxide, but secondary processes occurring on the residual thoria yielded several additional compounds. The oxide product similarly catalysed interactions between the primary products of decomposition of zinc acetate [145]. During the decomposition of rare earth oxalates, carbon monoxide disproportionates extensively to carbon dioxide and carbon [81,82]. 

Thorium (IV) oxide is obtained by thermal decomposition of thorium oxide hydrate (which is precipitated with ammonia) or salts of oxygen-containing acids. The nitrate and oxalate are especially suitable as starting materials, while the sulfates give off the last traces of SO3 only with difficulty. 

Fig. 4-4, Particle shapes of thorium oxide prepared from oxalate thermal decomposition.

Characteristic Properties of Thorium Oxide from Oxalate Thermal Decomposition... 

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