Question

When a ball mill rotates at a speed higher than the critical speed, what happens to its efficiency?

• A. It increases continuously
• B. It remains constant
• C. It decreases
• D. It becomes maximum

Hint:

\textbf{Above critical speed = Balls stick to wall = No grinding = Low efficiency}.

Answer 1

The Correct Option is C

Concept: In a ball mill, grinding occurs due to the impact and attrition of balls falling on the material. The efficiency depends on the rotational speed relative to the critical speed.
Step 1: Understanding critical speed.
Critical speed is the speed at which the centrifugal force equals gravitational force, causing the balls to cling to the mill wall.
Step 2: Behavior above critical speed.
When the mill rotates above critical speed, the balls are carried along the wall and do not fall onto the material. Hence, no effective grinding occurs.
Step 3: Effect on efficiency.
Since impact action is lost, the grinding efficiency decreases significantly.
Conclusion:
Thus, the efficiency decreases when operating above critical speed.