The "Critical Speed" for a grinding mill is defined as the rotational speed where centrifugal forces equal gravitational forces at the mill shell's inside surface. This is the rotational speed where balls will not fall away from the mill's shell. Result #1: This mill would need to spin at RPM to be at 100% critical speed.
The critical speed of a rotating mill is the RPM at which a grinding medium will begin to “centrifuge”, namely will start rotating with the mill and therefore cease to carry out useful work.
The ideal mill speed is usually somewhere between 55% to 75% of critical speed.
The percent of critical speed is the ratio expressed as a percentage of the actual mill speed and the Theoretical Critical Speed of that mill. The critical speed of a rotating mill is the RPM at which a grinding medium will begin to "centrifuge", namely will start rotating with the mill and therefore cease to carry out useful work.
The mill was rotated at 50, 62, 75 and 90% of the critical speed. Six lifter bars of rectangular cross-section were used at equal spacing. The overall motion of the balls at the end of five revolutions is shown in Figure 4.
www.911metallurgist.com/blog/ball-mill Learn about Ball Mill Critical Speed and its effect on inner charge movements. The effect of Ball Mill RPM s
The point where the mill becomes a centrifuge is called the "Critical Speed", and ball mills usually operate at 65% to 75% of the critical speed. Ball Mills are generally used to grind material 1/4 inch and finer, down to the particle size of 20 to 75 microns.
As the mill starts, grinding action and throughput increases. However, after reaching a critical speed, the mill charge clings to the inside perimeter of the mill. Under this conditions, the grinding rate is significant reduced or stopped. All mills must operate less than Critical Speed
There exists a speed of rotation the "critical speed" at which the contents of the mill would simply ride over the roof of the mill due to centrifugal action. The critical speed rpm is given by: nC = 42.29/ d, where d is the internal diameter in metres.