**A)** Total Apparent Volumetric Charge Filling – including balls and excess slurry on top of the ball charge, plus the interstitial voids in between the balls – expressed as a percentage of the net internal mill volume (inside liners).

**B)** Overflow Discharge Mills operating at low ball fillings – slurry may accumulate on top of the ball charge; causing, the Total Charge Filling Level to be higher than the Ball Filling Level. Grate Discharge mills will not face this issue.

**C)** This value represents the Volumetric Fractional Filling of the Voids in between the balls by the retained slurry in the mill charge. As defined, this value should never exceed 100%, but in some cases – particularly in Grate Discharge Mills – it could be lower than 100%. Note that this interstitial slurry does not include the overfilling slurry derived from the difference between the Charge and Ball Filling.

**D)** Represents the so-called Dynamic Angle of Repose (or Lift Angle) adopted during steady operation by the top surface of the mill charge (“the kidney”) with respect to the horizontal. A reasonable default value for this angle is 32°, but may be easily “tuned” to specific applications against any available actual power data.

The effect of change of feed size, or of return of a classified fraction, is obtained by direct experiment. This is a more time-consuming method, and its success still depends on the consistency of scale-up factors, but it is inherently a more informative test and will be the method considered here. Such tests usually try to obtain a close duplication of conditions in the test mill to those in the production mill, in everything except mill size. It is advantageous to use as large a mill diameter as feasible considering the expense of the test system and handling the larger quantities of material involved for a larger mill. If the feed under investigation contains large material, a direct duplication test requires a big enough mill diameter to handle the particle and ball sizes involved.