The rotational speeds of the ball mills are kept at ______ % of the critical speed, with the lower values for wet grinding in viscous suspension.

Correct option is C

· Critical Speed of Ball Mills:
· The critical speed is the speed at which centrifugal forces equal gravitational forces on the mill's interior.
· For efficient grinding, ball mills operate at 65-85% of the critical speed, ensuring proper cascading and grinding action.