Modelling of neodymium oxalate precipitation by the method of classes

Oxalic precipitation is usually used in nuclear industry to process radioactive wastes and recover actinides from a multi-component solution. 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 under various operating conditions, especially in nuclear environment in which experiments are limited. The aim of this article is to describe the modelling approach of neodymium oxalate precipitation in a continuous MSMPR (Mixed Suspension Mixed Product Removal). Primary nucleation, crystal growth and agglomeration are taken into account in the model. Thermodynamic effects are modelled 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 micro-mixing 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 population balance of the particulate suspension is solved by the method of classes. The particle sizes predicted by the model are in good agreement with the experimental measurements. (authors)