• Giraud, Martin

Abstract

Up to now, the influence of the physico-chemical properties of the elementary particles on the flowability of bulk powders is still unclear. However, understanding the flow behavior of powders is of upmost importance for optimizing and operating process steps such as transport, storage or various unit process operations involving powders. In particular, the MiMas (Micronized Masterblend) process, implemented in MELOX plant for the preparation of MOX nuclear fuel, handles granular materials, mainly through mixing and grinding steps. In this study, various ceramic powders, essentially alumina, zirconia and yttria powders, have been finely characterized in terms of particle properties and rheological behavior with a powder shear tester. A model, involving the dimensionless granular Bond number to predict the macroscopic flow behavior of a polydispersed powder from its elementary particle properties has been extensively studied. In particular, this model was shown to predict successfully the flowability of binary and ternary mixtures as well as grinded powders according to the particles’ properties and as a function of the operating conditions, such as the composition of the blends or the grinding time. The characterization of grinded and co-grinded powders obtained in a ball mill under various conditions also allowed specifying the influence of various operating parameters, such as the grinding time, the rotating speed or the filling ratio of the vessel. These results were particularly useful for identifying the dimensionless parameters to take into account for the scale up of the grinding equipment. Finally, a methodology for quantifying the homogeneity of powder mixtures, shaped in pellets, at various scales has been developed in order to compare several mixing and co-grinding processes.

Topics

• impedance spectroscopy
• grinding
• ceramic