BEHAVIOUR OF URANIUM DURING PHOSPHATE ORE CALCINATION.

During industrial calcination of phosphate rock, uranium is mobilized with changes both in location and oxidation state. Fission track micromapping of uranium under the microscope has been employed to study the precise distribution of uranium within the samples. Oxidation state ratios of U(IV) to U(VI) were determined by chemical separation followed by delayed neutron activation. The calcination process was studied both in the laboratory and in a full-scale production kiln. It is shown that during calcination first carbonate-fluorapatite loses CO//2, and recrystallizes to fluorapatite. The recrystallization process intensifies with increasing temperature. Around 600 degree C all the uranium is oxidized to the hexavalent state. Migration of uranium in the apatite fragments initiates at 800 degree C. On increase of temperature to around 900 degree C, it forms uranium-rich phases in which it reaches 1-2%.