What is the critical rotation speed in revolutions per second, for a ball mill of 1.2 m diameter charged with 70 mm dia balls ? / Mechanical Operations / By admin What is the critical rotation speed in revolutions per second, for a ball mill of 1.2 m diameter charged with 70 mm dia balls ? A. 0.5 B. 1.0 C. 2.76D. 0.66