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: Define Critical Speed of a Ball Mill.
The critical speed of a ball mill is reached when the centrifugal force equals the gravitational force. At this speed, the balls remain at the top of the mill, impeding their return. Operating above critical speed causes balls to adhere to the top, reducing material crushing effectiveness and mill efficiency.

Step 2: Evaluate Options.
- (A) Efficiency is at its lowest when the speed exceeds the critical speed. - (B) and (C) Efficiency does not improve beyond the critical speed; it declines. - (D) Achieving "hundred percent" efficiency at this juncture is not possible.

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