Surface area changes during the calcination of ammonium uranate
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Date
1974-09
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Publisher
Australian Atomic Energy Commission
Abstract
The changes in specific surface area observed during the calcination of ammonium uranate in various atmospheres have been studied using constant rate of heating, isothermal and isochronal experiments. X-ray diffraction, thermogravimetric analysis (TGA) and differential thermal analysis (DTA) results have been used to correlate these changes with the stages of reaction which occur during the calcination. During dehydration (20 to 200ºC) AU 'Type II' remains the major phase. Thermal decomposition (200 to 350ºC) produces amorphous U03 and 3-U03 which retain ammonia in their structure. These are subsequently converted to U308 by a self-reduction process (350 to 450ºC). Reduction in a hydrogen atmosphere takes place between 450 and 510ºC to produce U02. In an inert gas or air atmosphere 3-UO and UO persist to much higher temperatures, and the presence of an oxidising atmosphere may completely eliminate the self-reduction reaction. The observed specific surface area increases during dehydration and decomposition and reaches a maximum between 400 and 450ºC. The major increase in the surface area is attributed to the opening up of internal porosity, formed during decomposition, by stress-induced cracking of the particles. The internal stresses are generated by the nucleation and growth of phases which have different specific volumes from that of the parent solid. The maximum surface area was observed if self-reduction occurred. The observed decrease in specific surface area at higher temperatures is due to sintering.
Description
Keywords
Ammonium compounds, Uranates, Calcination, X-ray diffraction
Citation
Woolfrey, J. L. (1974).Surface area changes during the calcination of ammonium uranate (AAEC/E329). Lucas Heights, NSW: Australian Atomic Energy Commission.