Question

When a ball mill rotates at a speed higher than the critical speed, its efficiency is:

• A. minimum
• B. maximum
• C. optimum
• D. hundred percent

Hint:

Efficiency of a ball mill decreases when the speed is higher than the critical speed, as the grinding media do not fall back to crush the material.

Answer 1

The Correct Option is A

Step 1: Understanding the critical speed of a ball mill.
The critical speed of a ball mill is the speed at which the centrifugal force is equal to the gravitational force, causing the balls to stay at the top of the mill and not fall back. At speeds higher than the critical speed, the balls are carried to the top and do not effectively crush the material, thus the mill's efficiency is reduced.

Step 2: Explanation of options.
- (A) When the speed is above the critical speed, the efficiency is lowest. - (B) and (C) Efficiency does not increase above the critical speed; it actually decreases. - (D) There is no such thing as "hundred percent" efficiency at this point.

Final Answer: \[ \boxed{\text{A) minimum}} \]