Mechanical Operations
Work index is defined as the

Gross energy (kWh/ton of feed) needed to reduce very large feed to such a size that 80% of the product passes through a 100 micron screen
Energy needed to crush one ton of feed to 100 microns
Energy needed to crush one tonne of feed to 200 microns
Energy (kWh/ton of feed) needed to crush small feed to such a size that 80% of the product passes a 200 mesh screen

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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?

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

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