(78f) Characterization of Density of Cohesive Granular Materials Using An X-Ray CT Scanner

The tendency of powders to flow has a significant effect on a large number of industrial applications. Different experimental techniques to measure density of powders show dramatic differences in the results. We propose a reproducible procedure to measure density using a CT scanner. We performed tap density measurements using an acrylic cylinder containing different cohesive powders in an electromechanized shaker. After tapping, the cylinder was placed inside a new X-ray CT scanner Skyscan -1172. The image reconstruction and the analysis were performed with the Skyscan's NRecon and CTAn softwares. We compared measurements of the tap density using the microCT scanner to other more traditional methods to measure tap density. Results show a significant overestimation of the tap density using conventional methods: The bulk density shows a linear behavior as a function of time, and frequency of tapping. The percent of change in the slope between conventional methods to measure tap density and microCT scanner measurements varies from 33.33 percent at 3 HZ to 54.54 percent differences at 10 Hz.

Tap density as a function of frequency of tapping, and level of cohesion of the powder were computed. Our results show that after a transient the tap density is independent of the frequency of tapping. Density variations as a function of the height of the cylinder were also analyzed using the micro CT scanner. Our results show that, in general, as the height increases, the density of the powders decreases. For the least cohesive powders there is a density difference of 22.40% for a height difference of 1.45cm for a tapping frequency of 3 Hz. For the highly cohesive powders the density difference is not as pronounced; (it is 5.2% for the same conditions.) We plotted the high (tap density) to low density value for different cohesive powders as a function of the flow index. We observed that the higher the ratio between the “high” and the “low” , the more cohesive the powder is and the poorer is their flowability.