Step 1: Understanding the Question:
The question asks about the effect on the grinding efficiency of a ball mill when its rotational speed exceeds a specific threshold known as the "critical speed".
Step 2: Key Formula or Approach:
The concept of critical speed (\(N_c\)) is central here. It is the theoretical rotational speed at which the centrifugal force acting on the outermost layer of grinding balls exactly balances the force of gravity. At this speed, the balls are just about to be pinned to the inner wall of the mill.
Step 3: Detailed Explanation:
The grinding action in a ball mill primarily occurs from the impact and attrition of balls (the grinding media) as they are lifted by the rotating mill and then cascade or cataracting down onto the material being ground.
Below Critical Speed: The balls are lifted and then tumble down, causing effective grinding. The efficiency is highest at a certain percentage (typically 65-75%) of the critical speed.
At Critical Speed: The outermost balls are theoretically carried around in a full circle, clinging to the wall.
Above Critical Speed: When the speed is higher than critical, the centrifugal force significantly outweighs gravity. This causes all the balls to be pinned against the inner lining of the mill, a phenomenon called "centrifuging". The balls rotate with the mill but do not fall. As a result, there is no tumbling, no impact, and virtually no grinding action.
Step 4: Final Answer:
Since the grinding action ceases when the speed is higher than the critical speed, the efficiency of the ball mill drops drastically and essentially decreases to near zero. Therefore, option (C) is correct.