Physical Modeling of Spinel Crystals Settling at Low Reynolds Numbers

The crystallization of large octahedral crystals of spinel during the high-level waste (HLW) vitrification process poses a potential danger to electrically heated ceramic melters. Large spinel crystals rapidly settle under gravitational attraction and accumulate in a thick sludge layer that may partially or completely block the glass discharge riser of the melter. The settling of single particles of different sizes and the motion of hindered settling front of different particle volume fraction suspensions were studied in stagnant, transparent-silicone oils at room temperature to help predict the settling behavior of spinel crystals in the riser. The dimensions and terminal settling velocities of single particles were measured using an optical particle-dynamics-analyzer. The data yielded an experimental shape factor for glass beads that differed only 0.73% compared to the theoretical shape factor for a perfect sphere. The experimental shape factor for the spinel crystals was smaller than that of the beads given the larger drag force caused by the larger surface area to volume ratio of the octahedral crystals, but matched the theoretically predicted value to within 10%. In the hindered settling experiments, both the glass bead and spinel suspensions were found to follow the predictions of the Richardson-Zaki equation with higher particle volume fractions settling at a slower rate. Particle concentration profiles obtained from color threshold analysis (CTA) indicated that for a given volume fraction the rate of clarification increases with an increase in settling vessel angle with respect to the vertical as predicted by the Ponder, Nakamura and Kuroda (PNK) model. The Stokes', Richardson-Zaki and PNK equations can adequately predict the accumulation rate of spinel crystals in the vertical or inclined glass discharge riser of HLW melters.

The Journal of Young Investigators is not affiliated with the US Department of Energy. This paper was written by a student intern with the Department of Energy and does not constitute a declarative position of either the Department of Energy or the Journal of Young Investigators.
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