Current scale-up equations are discussed critically. The ratio: rotational mass flow of media to axial mass flow of ore#x2014;a concept recently introduced#x2014;has been refined and extended. It can account for similar breakage in scaled mills by the trade-off between ore/media encounter frequency and average impact energy with specific energy constant. Further consequences are that in scaled mills the average number of revolutions during the residence of an element of ore in the mill, and ore flow path tortuosity, both decrease strongly while axial flow increases very strongly as mill diameter increases. These factors can result in capacity limitations when mill diameters reach a critical range, especially for coarser feed sizes of certain ores. Autogenous mills can operate satisfactorily at relatively large diameters because their large active volumes are occupied either completely or mostly by ore, resulting in higher values of nominal residence time and flow path tortuosity.
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