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

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Mechanical Operations
Rod mills employed for grinding

Employ steel rods of 2-12 cms diameter extending over full length of the mill
All of these
Is useful for handling sticky materials
Employ a steel shell having L/D ratio of 1.5 to 3.0

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Mechanical Operations
During agitation of liquids, the

All of these
Power number becomes independent of impellers Reynolds number at high Reynolds number, but is dependent on the geometry of the impeller
Froude number is used to account for the effect of surface (e.g., the centre vortex) on the power number
Froude number is independent for the curves between power number and Reynolds number in baffled system

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