Study of Neodymium Oxalate Precipitation in a Continuous Mixed Suspension Mixed Product Removal

Oxalic precipitation is used in the nuclear industry. To facilitate the development of experimental methods and data acquisitions, actinides are often simulated using lanthanides, thereby gaining experience in harmless conditions. Precipitation reactions being highly sensitive to many operation parameters, modelling appear to be a very effective tool to predict the evolutions of the system subjected to various operating conditions, especially in nuclear environment in which experiments are limited. The aim of this article is to describe the modelling approach and its application to the neodymium oxalate precipitation in a continuous MSMPR (Mixed Suspension Mixed Product Removal). The model takes into account the kinetic laws of primary nucleation, crystal growth and agglomeration. Thermodynamic effects are taken into account through activity coefficients which are calculated using the Bromley model. For the nucleation study, experimental runs have been performed in a specific device that allows a micromixing time less than a millisecond. The homogeneous nucleation rate follows the Volmer-Weber equation. The crystal growth rate is first order with respect to the supersaturation and controlled by the surface integration into the crystal lattice according to a screw dislocation mechanism. The agglomeration kernel has been found to be independent of the crystal size. The particle sizes predicted for two different mean shear rates are in good agreement with the experimental measurements.