### D diameter of ball mill in m D 450 mm 045 m R

D = diameter of ball mill in m D = 450 mm = 0.45 m ∴ R = 0.225 m Diameter of ball = 25 mm = 0.025 m ∴ r = 0.0125 m g = 9.81 m/s 2 N c = 1 2 π 9.81 0.225 – 0.0125 = 1.08 r.p.s. = 1.08 × 60 = 64.88 r.p.m. ≈ 65 r.p.m. Critical speed = 65 r.p.m. … Ans. Example 2.4 : A pair of rolls is to take a feed equivalent to sphere 38 mm in diameter and crush them to sphere having a diameter of 12.7 mm.