The effect of interstitial and circumambient gas in fine powders on the scaling up of powder-handling apparatus as illustrated by ball mill operation

Using the gas—solid interaction number Ng, it is possible to predict the circumstances under which a fine powder will exhibit increased porosity and high mobility. This increased mobility generally favours the handling of powders. The lower the Ng-number the better the mobility and the better the process efficiency. For the case of milling in a ball mill, this is further analyzed in steps: first, the gas encapsulation and the ball behaviour are considered, both in dependence on the Ng-number. Next, the milling effect of single falling or cascading balls is determined from milling experiments on various ball loads. On the basis of the Ng-number, it is assumed that scaling up has similar effects to increasing the gas viscosity on the mobility of the powder during handling. With this assumption and from the above-mentioned results, the scaling up of ball mills is considered. It follows that scaling up rules at low Ng-numbers (Ng <20) must be different from those at high Ng-numbers as they are reported in the literature.