Actual operating speed of a ball mill may vary from 65 to 80% of the critical speed. Which of the following duties would require the ball mill to be operated at maximum percentage of critical speed?

• Wet grinding in low viscous suspension.
• Wet grinding in high viscous suspension.
• Dry grinding of large particles (upto 1.25 cms).
• Dry grindng of large particles in un-baffed mills.