Question

Which of the following is true, when operating speed is less than critical speed?

• A. Bold grinding
• B. Effective grinding
• C. No grinding
• D. Best grinding

Hint:

In grinding mills, operate near or slightly below the critical speed for best results. Too far below, and no grinding will occur.

Answer 1

The Correct Option is C

In ball mills and similar grinding equipment, the critical speed is the speed at which the centrifugal force on the mill's contents equals the gravitational force, causing the media to stick to the walls rather than fall and grind.
The formula for critical speed is:
\[ N_c = \frac{1}{2\pi} \sqrt{\frac{g}{R - r}} \]
Where:
$N_c$ = critical speed
$g$ = acceleration due to gravity
$R$ = radius of the mill
$r$ = radius of the ball
When the operating speed is less than the critical speed, the grinding media does not rise to a sufficient height to fall and cause impact. Instead, it rolls along the mill wall, leading to very little or no effective grinding action.
Hence, the correct response is no grinding, because below critical speed, the energy and motion required for particle size reduction are insufficient.
Other options like "bold grinding", "effective grinding", or "best grinding" are incorrect because proper grinding only begins when the speed approaches or slightly exceeds the critical speed.