Behaviour of uranium during phosphate ore calcination

Abstract

During industrial calcination of phosphate rock, uranium is mobilised 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(V1) 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 of the Oron plant (Israel). The phosphate ore contains about 100 parts of of uranium, and U(IV) comprises 3540% of the total U present. The uranium which was not extractable in weak acids, was evenly distributed in the apatite rock components. The study revealed that the following changes occur during the calcination process: (a) carbonate-fluorapatite loses COZ, and recrystallises to fluorapatite. The recrystallisation process intensifies with increasing temperature. (b) Around 600°C all the uranium is oxidised to the hexavalent state. (c) Migration of uranium in the apatite fragments initiates at 800°C. On increase of temperature to around 9oo°C, it forms uraniumrich phases in which it reaches 1-2 %. The uranium-rich phases exhibit different solubilities in dilute acids, in some cases enabling its preferential leaching from the rock. Other U-rich phases are extremely insoluble (posslbly fluorite, Ca-silicates) even in phosphoric acid, lowering extraction efficiency. Extraction yields can be improved by addition of salts (e.g. NazC03) which prevent the formation of insoluble U-rich phases.